Advancing photocatalysis through efficient fabrication of morphology-engineered photonic crystals

Yukai Chen*^a, Qinglian Tan^a, Yiyi Ji^a, Dan Wang^a, Rulin Dong*^a and Baoying Dai*^b
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R. China. E-mail: ykchen@cczu.edu.cn; dongrl@cczu.edu.cn
^bState Key Laboratory of Organic Electronics and Information, Displays and Institute of Advanced Materials (IAM), Jiangsu Key Laboratory for Biosensors, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China. E-mail: iambydai@njupt.edu.cn

First published on 11th July 2025

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1. Introduction

Due to the limited availability of fossil resources, the development of new energy sources has become increasingly urgent for sustainable development. Although emerging energy technologies such as wind and tidal energy have been proposed and have made some progress, their practical applications still face numerous challenges due to limitations in technological maturity, geographical conditions, and economic costs, making their large-scale and efficient utilization difficult. Solar energy, with its numerous advantages such as abundance and renewability, cleanliness, wide distribution, and safety, has attracted significant attention. These unique advantages have driven exploration into solar energy, making the efficient and rational utilization of light energy a hot topic in research. Photocatalytic technology is an effective approach for achieving the efficient conversion of solar energy into chemical energy.¹ In brief, it refers to using photocatalysts (usually semiconductors which can be excited by photons) to carry out photocatalytic reaction under light irradiation.² The photocatalytic process typically involves semiconductor materials that absorb photons of specific wavelengths under light irradiation, which excites electrons to transition from the valence band to the conduction band, thereby generating electron–hole pairs. These excited electrons and holes participate in redox reactions with substances adsorbed on the catalyst surface, enabling specific chemical transformations, such as accelerating water splitting to release hydrogen, enhancing surface catalytic reactions, utilizing hydrogen energy to alleviate energy crisis pressures, or degrading environmental pollutants to benefit humanity.^3,4 With its advantages of high efficiency, environmental friendliness, and sustainability, photocatalytic technology continues to inspire researchers’ enthusiasm.

With the in-depth study of photocatalytic technology, its inherent limitations have gradually emerged. Research indicates that photocatalytic efficiency is constrained by multiple factors, including illumination duration, environmental temperature, catalyst band structure, recombination rate of photogenerated carriers, and catalyst surface morphology. These limitations make it difficult for existing photocatalytic systems to meet the growing demand for efficient solar energy conversion. In terms of material morphology design, significant progress has been made in improving light energy utilization efficiency, laying an important foundation for the practical application of photocatalytic technology. In the realm of light regulation, the limitations of traditional optical materials have become increasingly apparent, failing to meet modern research demands. The emergence of photonic crystals (PCs) has opened new research directions in the field of photocatalysis. Photonic crystals, distinguished by their unique periodic dielectric structures and photonic bandgap (PBG) properties, enable precise control and efficient utilization of light through the modulation of their multiple dimensions. This makes them crucial materials for regulating light propagation and energy conversion. Therefore, by designing the structure of photonic crystals according to the required light wavelength range for photocatalytic reactions, the PBG of photocatalysts can be tuned to effectively match the solar spectrum. As a result, light within the PBG is strongly reflected or scattered, increasing the propagation path and residence time of light around the photocatalyst. This allows the photocatalyst to fully absorb light energy, thereby improving light utilization efficiency. Through the multidimensional structural design (one-dimensional, two-dimensional, and three-dimensional) and functional modification of photonic crystals, the spectral response range, light capture efficiency, carrier separation capability, and photocatalytic activity of semiconductor materials have been significantly enhanced.⁵

In this review, the research progresses on the design and application of photonic crystals in multidimensional structures were presented (Scheme 1). Compared to traditional photolithography and etching methods, this article discusses preparation methods with simplified processes, mild conditions, and high resource utilization efficiency. In the study of photonic crystals, the most common opal structures are typically formed by the self-assembly of colloidal particles, characterized by the ordered stacking of spheres. Research shows that photonic crystals with special morphologies, such as inverse opal structures, exhibit significant advantages in photocatalytic applications. These structures not only generate PBGs over a wider wavelength range but also allow precise tuning of the position and width of the PBG by adjusting structural parameters like pore sizes. Based on this, significant progress has also been made in inverse opal fabrication techniques. The template method is widely used due to its simplicity, while the deposition method reduces preparation errors by simplifying process steps. The sol–gel method addresses the limitations of the deposition method, particularly in improving material uniformity when preparing composites of different semiconductor materials. Through continuous exploration by researchers, a series of structures with special morphologies, such as core–shell and hierarchical flower-like structures, have been successfully fabricated. Finally, this article summarizes the research progress on the unique structures of photonic crystals and strategies for enhancing photocatalytic performance. These include: (1) utilizing localized electromagnetic fields generated by metal plasmon resonance to enhance light absorption and constructing photocatalytic materials through doping modification; (2) fabricating heterojunctions to optimize carrier separation efficiency; (3) employing upconversion technology to convert low-energy photons into high-energy photons, broadening the spectral response range; and (4) developing photothermal synergistic catalytic systems to accelerate reaction kinetics by increasing reaction temperature. These studies provide theoretical references for the future design and application of photocatalytic materials.

2. Multidimensional photonic crystals

It is well known that photonic crystals are artificially engineered microstructured materials characterized by a periodic arrangement of media with different refractive indices in space, exhibiting PBG properties. Depending on the arrangement and stacking of materials, photonic crystals can be classified into one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures (Fig. 1). 1D photonic crystals are structured by alternated layers along one dimension and are commonly applied in Bragg reflectors and one-dimensional photonic crystal fiber gratings. Similarly, 2D photonic crystals are fabricated by interlaced structures along two directions, showing their applications in light-emitting diodes (LEDs) and color display technologies. 3D photonic crystals exhibit prominent applications in thermophotovoltaic cells and photocatalysis. It is worth noting that all three kinds of photonic crystals affect light propagating in any direction, and their difference mainly focuses on the stacking methods.

2.1 One-dimensional photonic crystals

One-dimensional photonic crystals (1D PCs) have garnered significant attention due to their simple structure and unique optical properties, exhibiting excellent light regulation capabilities. As a result, they hold broad application prospects in optical filters, light shielding, and structural coloration. A typical 1D photonic crystal is fabricated by alternately stacking two or more optical materials with different refractive indexes layer by layer along the same direction. The color shade and reflectivity of 1D photonic crystal can be adjusted by modulating the precursor concentration and layer stack number, respectively. The structural color and PBG position can be adjusted by some parameters such as the film thickness (by adjusting the number of periods in the stack or the value of the periods), the thickness of each layer, the layer materials, and the incidence angle of the light. Especially, benefited from the light management of 1D photonic crystal, slow light effect happens inside the photonic crystal. Thus, if semiconductor photocatalyst films are used to fabricate 1D photonic crystal layer by layer, the slow light effect can enhance the photon–matter interaction of the photocatalyst, thereby elevating the photocatalytic performance. For instance, Wang et al.⁶ prepared a 1D photonic crystal films by spin-coating TiO₂ sol–gel and graphene oxide (GO) alternately onto a slide. Since the TiO₂ and GO exhibits different refractive indexes, there exists the slow light effect at certain wavelength. When the slow light region was adjust to the absorption edge of TiO₂, the photocatalytic performance increased dramatically. Using a similar principle, Wei et al.⁷ developed a novel three-layer composite structure: a bottom layer of metal-ceramic, a middle layer of 1D photonic crystal formed by alternating Si and SiO₂ multilayers, and a top layer of yttria-stabilized zirconia (YSZ) heterostructure (Fig. 2a). The 1D photonic crystal structure in the middle layer significantly enhanced light absorption efficiency by constructing a wide PBG, while providing high reflectivity. This unique structure not only effectively separates the solar spectrum but also enables efficient light regulation. Based on this principle, researchers successfully developed a high-efficiency optical filter for solar spectrum applications.⁷

To fully leverage the reflective and refractive advantages of 1D photonic crystals, the Xia's team⁸ developed a cellulose nanocrystal (CNC)/polyethylene glycol (PEG)/graphene oxide (GO) composite film with excellent ultraviolet (UV) blocking performance and transparency. Owing to its periodic structure and high refractive index contrast, this film achieves efficient shielding of long-wave ultraviolet (UVA) and medium-wave ultraviolet (UVB), with shielding efficiencies of 98.3% and 100%, respectively, while maintaining a visible light transmittance of 60.5%. Similarly, Yuan et al.⁹ fabricated a 1D photonic crystal with high refractive index contrast by alternately spin-coating hollow silica and titanium dioxide under mild conditions (Fig. 2b). This multilayer stacked structure generates multiple reflections of light at specific wavelengths, successfully achieving efficient blue light shielding while exhibiting exceptional adhesion and stability. These studies collectively demonstrate the broad application potential of 1D photonic crystal structures in light regulation.

1D photonic crystals are renowned for their high refractive index properties, which enable the efficient refraction of light back into the system, significantly reducing energy loss. This characteristic successfully addresses the issue of excessive thermal loss in traditional receiver window coatings during solar energy absorption, providing a novel solution for improving solar energy utilization efficiency. Inspired by this, researchers¹⁰ innovatively applied a one-dimensional photonic crystal transparent heat mirror coating (TPCHM) to concentrator receivers. The TPCHM structure, constructed by alternately stacking dense and porous zirconia layers, was designed to optimize the refractive index and enhance solar energy utilization. Even at a low solar concentration ratio, the system achieved a 62% improvement in solar-to-thermal energy conversion efficiency compared to an open receiver when operating at 1000 K, and a remarkable 193% increase at 1500 K.

Furthermore, the 1D photonic crystals can be complexified into chiral nematic (Bouligand) structures to expand their applications. For instance, MacLachlan et al.¹¹ reported the synthesis of a chiral nematic mesoporous nanocomposite (metal and metal oxide) through the vacuum-assisted loading of the metal precursors into a chiral nematic mesoporous silica template, following by the high temperature treatment. The prepared films were iridescent owing to the reflection of left-handed circularly-polarized light by the chiral nematic structure. Through this method, the growth of the metal nanoparticles can be contained within the porous silica template. As a result, the prepared CuO photonic crystal showed excellent activity than commercial CuO nanoparticles. Inspired by the structure, similar designs were fabricated for the application of photocatalysis. A black TiO_2−x/Cu_xO with chiral-like structures with Pt single atom deposited on the surface was prepared for visible-light hydrogen evolution.¹² With the structural optimization, the composite film exhibits excellent light harvesting ability in the visible region. Ghazzal et al.¹³ reported macroscopic chiral nematic structured freestanding TiO₂ films through the cellulose nanocrystals (CNCs) biotemplate. Arising from the light scattering induced by the chiral nematic structure, the nanostructured film shows 5.3 times higher performance compared to lamellar TiO₂ in the hydrogen evolution tests.

In general, the applications of 1D photonic crystals primarily include the fabrication of anti-counterfeiting patterns, optical filters, and photocatalytic composite materials. For 1D photonic crystal photocatalytic composites, future development trends mainly focus on the following aspects: (1) increasing the porosity of each layer to add photocatalytic reaction active sites, such as preparing porous thin films; (2) incorporating photocatalytic enhancement methods like atomic doping and heterojunctions into 1D photonic crystals; (3) selectively enhancing specific wavelength regions to match the absorption edges of photocatalysts or the excitation wavelengths of upconversion materials.

2.2 Two-dimensional photonic crystals

To overcome the limitation of one-dimensional ordered structures, which has limited applications, researchers have expanded their focus to two-dimensional (2D) structures. These 2D periodic structures exhibit optical properties in two orthogonal directions, enabling more flexible and precise control of light compared to one-dimensional photonic crystals. As a instance, Wu et al.¹⁴ discovered the potential of 2D materials for photocatalytic hydrogen production. By modifying the edges of 2D Bi₂WO₆ nanosheets, the bandgap was narrowed, and charge separation and reduction were enhanced. In photocatalytic experiments, the hydrogen evolution efficiency significantly increased to 56.9 μmol g⁻¹ h⁻¹. Building on this, the development of 2D photonic crystals not only retains the PBG modulation capabilities of photonic crystals but also leverages the surface advantages of 2D materials, opening new research directions for photocatalyst design.

The emergence of 2D photonic crystals has attracted widespread attention due to their ability to precisely control light propagation and localization through periodically arranged dielectric structures, significantly enhancing the light absorption efficiency and carrier separation capabilities of photocatalytic materials. Torras's group¹⁵ successfully achieved the controlled fabrication of Au-decorated titanium dioxide (TiO₂) 2D photonic crystals using soft nanoimprint lithography and template methods (Fig. 2c). This 2D stacking structure significantly improved the light-harvesting efficiency of TiO₂ semiconductors beyond the ultraviolet region. Leveraging the photon–plasmon effect, the hydrogen production rate reached 8.5 mmol g⁻¹ h⁻¹, marking a significant step forward in the application of 2D photonic crystals for efficient photocatalytic hydrogen production. In the context of CO₂ reduction, a 2D n⁺–p silicon-based photonic crystal (SiPC) was fabricated using mature lithography–etching techniques¹⁶ (Fig. 2d). This structure, composed of a square array of circular dielectric pillars, exhibited significantly enhanced absorption at around 450 nm. Compared to planar silicon wafers, the 2D SiPCs demonstrated higher photocurrent density and catalytic activity. The introduction of Pt nanoparticles further promoted CO₂ reduction on SiPCs, achieving a methane selectivity of 25%.

2.3 Three-dimensional photonic crystals

Due to structural limitations, the preparation of 1D and 2D photonic crystals sometimes needs complicated process. To explore more possibilities and simplify the preparation, three-dimensional (3D) photonic crystals with periodic dielectric structures have emerged as a key solution. These 3D structures can form PBG in all directions, owing greater potential in practical applications. It is worthy noting that in self-assembled 3D structures, the formation of a complete PBG needs high refractive index contrast (>2.8), which is not possible for most semiconductor photocatalysts.¹⁷ Fortunately, the enhancement of photocatalysis does not need a complete PBG. As a result, 3D photonic crystals have become a research hotspot in the field of photocatalysis, injecting new vitality into optical research and driving in-depth exploration by scientists. Simultaneously, 3D photonic crystals offer greater potential for improving the functionality of numerous cutting-edge applications. Owing to their spatial versatility, 3D photonic crystals have given rise to a rich variety of structural types.

The simple structures of three-dimensional (3D) photonic crystals include simple woodpile structures and etched diamond structures. For example, Zheng et al.¹⁸ developed a 3D woodpile photonic crystal (Fig. 3a) by depositing an MoS₂ photocatalyst onto a woodpile template using chemical vapor deposition. The structure consists of multiple layers of mutually perpendicular dielectric rods. Research shows that as the thickness of the deposited MoS₂ film increases, the specific bandgap in the near-infrared region exhibits a red shift. Specifically, for every 1 nm increase in MoS₂ film thickness, the bandgap position shifts by approximately 10 nm. This bandgap modulation significantly enhances the material's utilization of the solar spectrum. In fact, there are more parameters can be adjusted to enhance the photocatalytic performance. The position of the PBG is influenced by multiple factors including the refractive index of materials, the arrangement of periodic structures, and the angle of incident light, while the thickness of photonic crystal layers and surface properties of materials can further affect photocatalytic performance.

With further research, the emergence of special structures such as core–shell and inverse opal photonic crystals has advanced the field of optics. Among these, the core–shell structure, with its dual-layer composition, demonstrates flexible control over material properties. The preparation conditions for SiO₂@Fe₃O₄ core–shell structures were optimized using a template method¹⁹ (Fig. 3b), achieving precise control over material size at a stirring speed of 1000 rpm and a temperature of 60 °C. This size control further influences the optical properties of the photonic crystals, particularly their color appearance. This study provides new insights into color modulation in photonic crystals, offering significant scientific and practical value.

The most notable structural features of inverse opal photonic crystals are their three-dimensional (3D) periodic porous structure and high specific surface area. Therefore, adjusting the pore size and introducing other photocatalytic materials for synergistic doping have become common strategies for photocatalytic regulation. The Xie's group²⁰ successfully prepared a series of inverse opal ZnIn₂S₄ (ZIS) structures with varying pore sizes with a template method. The synthesized inverse opal ZnIn₂S₄ exhibits a unique PBG, and its slow light effect significantly enhances light–matter interactions, thereby improving light utilization efficiency. Among the materials with different pore sizes, ZIS-200 with a pore size of 200 nm demonstrated the most outstanding performance, achieving a hydrogen evolution rate of up to 14.32 μmol h⁻¹.

However, every single photocatalytic material, despite excelling in certain aspects, inevitably have limitations. To overcome this, researchers have explored combining two materials to leverage their complementary advantages and improve light utilization. For instance, TiO₂ is a common photocatalytic material, has a large specific surface area and high photocatalytic activity. However, the high recombination rate of its photogenerated electron–hole pairs hinders the performance. Thus, researchers²¹ deposited BiVO₄ onto an inverse opal TiO₂ framework to create a heterojunction. By tuning the pore size and incident light angle, the slow light effect was activated to enhance the photocatalytic performance. Compared to pure TiO₂, the heterojunction exhibited a seven-fold increase in photocatalytic efficiency. In summary, adjusting the pore size primarily enhances photocatalytic performance by controlling light propagation direction and enhancing multiple refraction and reflection effects.

To further promote the separation of photogenerated carriers and accelerate photocatalytic reactions at the mechanistic level, the conductivity and thermal properties of metals can be explored to significantly enhance photocatalytic effects. Metals, due to their unique conductivity, can serve as electron transport media, increasing the migration rate of photogenerated carriers and reducing electron–hole pair recombination. They also provide active sites, promote reactant adsorption and reactions, alter surface chemical environments, and lower reaction activation energy. Han et al.²² induced the in situ growth of graphdiyne (GDY) on the surface of a photonic crystal Cu/ZnO to construct PC GDY-Cu/ZnO (Fig. 3c). Leveraging the advantages of the photonic crystal structure and the all-solid-state Z-scheme system, the material significantly expanded the visible light response range, improved light utilization efficiency, constructed transport channels to accelerate photoelectron migration, and regulated active site density to enhance the adsorption and activation of CO₂ and H₂O. This resulted in a CO₂ photoreduction rate to CH₄ of 33.67 μmol g⁻¹ h⁻¹ with a selectivity of 93.3%, demonstrating excellent photocatalytic performance.

In addition, metals exhibit surface plasmon resonance (SPR) effects, which enhance light absorption and scattering and generate photogenerated carriers to participate in reactions. Even under high temperatures or harsh conditions, metal doping can improve thermal stability, maintain activity and structural integrity. Xu et al.²³ combined template and photo-assisted reduction methods to successfully prepare an S-type Ag/ZnO/CeO₂ IO composite photocatalyst (Fig. 3d). Experiments confirmed that when Ag nanoparticles were deposited on the ZnO/CeO₂ IO walls, the highly ordered and interconnected framework remained intact. Under visible light irradiation, the Ag/ZnO/CeO₂ IO composite exhibited enhanced photocatalytic degradation activity for RhB dye solution, which is benefited from the synergistic effects of the S-type heterojunction, the SPR effect of Ag nanoparticles, and the specific structure of the inverse opal macroporous material. These synergistic effects significantly improved the material's light absorption capability, leading to a notable enhancement in photocatalytic performance compared to ZnO IO.

3. Preparation methods of photonic crystals

With the increasing importance of photonic crystals in the field of photocatalysis, their demand has also gradually increased. Thus, studying a variety of efficient methods for preparing photonic crystals has become a major challenge in scientific research. Through the continuous attempts and unremitting efforts of researchers, not only simple and efficient preparation schemes for photonic crystals are developed, but also the controllable synthesis technology of the special inverse opal photonic crystal structure are successfully mastered.

3.1 General preparation methods

Traditional methods for preparing photonic crystals mainly include photolithography (apply non-uniform light intensities on dielectric materials) and etching (using masks for protection), whose preparation processes are cumbersome and imprecise, consuming a large amount of resources. In recent years, to break through this bottleneck, researchers have developed methods such as the sol–gel method,²⁴ the solvothermal method,²⁵ the deposition method,²⁶ and the template method,²⁷ which can efficiently prepare photonic crystals.

The sol–gel method, which has become popular in recent years, uses highly reactive compounds as precursors. In the liquid phase, these raw materials are uniformly mixed, and a series of chemical reactions are carried out to form a stable and transparent sol system. Subsequently, the sol particles slowly polymerize to form a three-dimensional network structured gel, and the gel network space is filled with solvents. Finally, the gel is dried and sintered to prepare the desired molecular or nanomaterials. The sol–gel process relies on the condensation and hydrolysis of metal salts or metal alkoxides with various types of solvents. This process can obtain inorganic networks by chemical reactions at low temperatures, which also owns the advantages of simplified conditions and cost-effectiveness. Experimental studies have found that the structural color and optical properties of the multilayer CuO/TiO₂ 1D photonic crystal film²⁸ synthesized by the sol–gel method are affected by the doping of non-metallic elements and rare-earth elements. The film color can be adjusted through dopants, and a red-shift phenomenon in the absorption spectrum occurs with N doping. Subsequently, the Guettaf Temam's team²⁹ used a TiO₂ skeleton to dip-coat in sol solutions containing Al and Ni metals multiple times to ensure full contact, and then calcined at 450 °C to complete the preparation of Al/TiO₂ and Ni/TiO₂ film samples. As the result, the Ni/TiO₂ film had the best degradation performance for methylene blue, up to 93%.

To overcome the problems of the sol–gel method, such as a long preparation cycle and the need for high-temperature calcination, researchers developed the solvothermal method. A special form of the solvothermal method is the hydrothermal method,³⁰ which is widely used because of its advantages such as mild synthesis conditions, a short reaction cycle, high product purity, good crystal growth quality and the ability to precisely control the morphology and size of the product. The mechanism of the hydrothermal method usually refers to using an aqueous solution as the reaction medium in a closed system (such as a high-pressure reactor). The reactants are dissolved in the solvent, and a high-pressure environment is generated inside the system by heating. Chemical reactions are carried out under such high-temperature and high-pressure conditions to prepare nanomaterials. Periodic structures (usually spheres) are formed through the addition of morphology control agents such as polyvinyl pyrrolidone. The superiority of hydrothermal method is the shape-controlling, which is easy to design specific structures such as core–shell structures. Zhou's research group³⁰ synthesized a 3D core–shell structured Bi₂WO₆/BiOCl photocatalyst (Fig. 4a) by the hydrothermal method. It was found that the catalyst had better photocatalytic activity and cycle stability than single samples due to its large specific surface area and pore size. When the addition amount of Bi₂WO₆ was 0.1%, the performance was the best. The photocatalytic degradation efficiency for 2,4,6-trinitrotoluene was 0.2 μmol min⁻¹, and the degradation rate was 90%. After 4 cycles, the degradation rate was still about 80%.

Although some shape-controlling methods can be adopted in sol–gel and solvothermal preparation process, the stability of shape-controlling and the designing ability of various structures are insufficient. The template method is using a template with a specific structure and shape to guide the growth, deposition, or assembly of the target material on its surface or inside, thereby obtaining a material with a specific morphology, size, and structure that is complementary to or restricted by the template structure. In this method, the template plays a key guiding and restricting role. The formation mechanism of the photonic crystal is close to that of the sol–gel method. However, the design of template is more versatile. Most methods such as self-assembly, photolithography and mask method can be used for the preparation of templates. As shown in Fig. 4b, after preparing the PS@FTO template, a TiO₂ inverse opal framework was grown on the template, and finally an inverse opal structured ZnO@TiO₂ nanocomposite film²⁷ was prepared. The piezoelectric photocatalytic constant of this composite film was much higher than that of the pure inverse opal TiO₂ framework. The reason was that the inverse opal structured TiO₂ framework was conducive to the growth of ZnO nanorods and the initiation of redox reactions, and the ZnO nanorods could promote carrier separation, thereby enhancing the piezoelectric photocatalytic activity.

Compared with the template method, the deposition method has a simpler operation process and does not require complex template preparation and removal steps. As an important technology widely used in many fields such as materials science and chemistry, it uses physical or chemical means to transform substances from the gaseous, liquid, or solution state to the solid state and accumulate or precipitate on a specific surface or medium. Usually, the dispersion solvent minimizes the surface area of the colloids by staying in interparticle spaces and bringing the particles closer during solvent evaporation, which owns the advantage of simplicity and stability. Researchers³¹ innovatively introduced planar defects into the CoO_x–TiO₂ inverse opal by the deposition method and prepared a specific three-layer photonic crystal film (Fig. 4c). At the same time, the CoO_x nanocomposite was used for surface modification to activate visible light. This structure reduced the Bragg reflection and broadened the slow-photon spectral range. The photocatalytic performance of the composite material was significantly improved, showing excellent performance in salicylic acid degradation and photocurrent generation, with an enhanced reaction rate and significantly improved charge-separation efficiency.

Among the deposition methods, the chemical deposition method is the most commonly used because of its simple operation and wide application range. The chemical deposition method is using chemical reactions to precipitate substances from solutions or gas phases and deposit them on the substrate surface to form films or solid materials. It mainly includes chemical vapor deposition, which uses gaseous reactants to react under conditions such as high temperature and catalysts to generate solid products and deposit them on the substrate surface. This method has relatively simple equipment, can operate on substrates with complex shapes and precisely control the composition and thickness of the deposition layer. To more precisely control the deposited samples, researchers often combine self-assembly with the chemical deposition method. For example, they synthesized a silver phosphotungstate/polyimide photocatalyst with a complex morphology for research (Fig. 4d).³² The photocatalytic activity of this catalyst was enhanced because the stacking structure of polyimide changed and the POMs-π interaction in the polyoxometalate (POMs) material promoted charge transfer. Under visible light, a degradation reaction occurred along the Z-type pathway and best degradation was achieved.

3.2 Unique structure designs for photonic crystals

The structures of photonic crystals are rich and diverse, and the most common one is the structure formed by the ordered stacking of layers. In these structures, the reflection and refraction of light between the layers can regulate light with specific wavelengths. Its preparation process is mature and it has numerous application fields. The design of photocatalytic materials is a research hotspot in the modern optical field. Breaking the limitations of traditional thinking can lead to the construction of some special and complex structures, such as core–shell structures,³³ porous structures,³⁴ chiral structures,³⁵ etc.

As is well known, in the field of materials science, spherical structures have attracted much attention due to their unique physical properties. From the geometric principle, among common geometric bodies with the same volume, the sphere has a relatively large specific surface area, which means that it can provide a larger contact area for chemical reactions and promote the progress of reactions. At the same time, the sphere also has a high degree of symmetry. This characteristic makes its physical and chemical properties relatively uniform in all directions, which effectively improve the stability and repeatability of the reaction. Based on these significant advantages, in the design and preparation of catalysts, the spherical structure has become an extremely ideal choice. Therefore, most of the common catalysts directly adopt the spherical structure or are ingeniously modified on the basis of the spherical shape to further optimize their performance and better meet the needs of different chemical reactions. Most importantly, spheres can self-assemble into periodic 3D structures through facile methods, solving the problems of powder photonic crystal photocatalysts. As is known to all, the enhancement of photocatalysis by photonic crystal relies on the slow light effect, which has a dependence on the light incident angle.³⁶ In the continuous stirring condition, the light incident angle for powders is shifting, affecting the effectiveness of slow light effect. Fortunately, the isotropic periodic structure assembled by spheres can avoid this drawback. The slow light effect happens in all directions, facilitating the absorption of photons. Wang et al.³⁷ used the methods of droplet microfluidics and colloidal assembly and regulated the mass transfer of silicon monoxide (SiO) vapor to achieve the directional growth of silica nanowires (SiO₂NWs) (Fig. 5a). The morphology of SiO₂NW is controlled by the diffusion reaction of SiO vapor. At a high concentration gradient, the NWs are neatly arranged and grow directionally parallel to the surface of the substrate, while at a low concentration gradient, they grow randomly in a “flower-like” shape. This controllable reaction enables the surface of the 3D spherical photonic crystal (SPC) to be covered with a large number of NWs, which have multi-level characteristics at the micro and nano scales and a high specific surface area. The SiO₂NWs-SPCs coated with FDTS (perfluorodecyltrichlorosilane) are superhydrophobic and superoleophilic, which is convenient for oil–water separation.

In another work, W₁₈O₄₉ nanosheets was uniformly grown on the surface of hollow SiC nanocages³⁸ (Fig. 5b) and the heterojunction formed by their combination shows a sea urchin-like morphology. This morphology enhances the light absorption by scattering light for multiple times within the outside grown needle-like structure. In addition, the large specific surface area of the sea urchin-like morphology provides greater convenience for the contact between the photocatalyst and CO₂ reactants. The electron transfer path of the SiC–W₁₈O₄₉ composite photocatalyst is consistent with that of the S-scheme heterojunction. Moreover, the S-shaped internal electric field at the interface of the one-dimensional composite heterojunction can effectively reduce the recombination rate of photogenerated electrons and holes and accelerate the transfer of electrons. The yield of the CO₂ photocatalytic reduction products of the composite photocatalyst is 3.38 and 3.71 times that of SiC and W₁₈O₄₉, respectively.

The chiral structure is a special geometric structure. Researchers rationally draw inspiration from the spiral shape of the Swiss roll. At the nanoscale, colloidal lithography technology is used to prepare a continuous film of chiral Swiss roll nanoarrays (SRNAs)³⁵ (Fig. 5c). This film has two remarkable characteristics: chiral and polarization-sensitive photocatalytic activity. By reducing the size of the Swiss roll material to the nanoscale, the optical response effect in the visible light range is enhanced. It is found that when the polarization direction of circularly polarized light is consistent with the chirality of the chiral SRNAs, their photocatalytic plasma performance is significantly enhanced benefited from the chiral structure. At the same time, relying on the unique properties of the continuous film and the three-dimensional plasma cavity, the chiral SRNAs can easily achieve a high reuse rate.

Similarly, the conical structure can be duplicated and designed as periodic structures to exert the advantages of photonic crystal. A recent study reported a slanted parabolic pore photonic crystal (spbPore PC) structures with graphitic carbon nitride (g-CN), nickel oxide (NiO), or 6H silicon carbide protective coatings in order to overcome the drawbacks of cubic silicon carbide (3C-SiC) photoelectrodes.³⁹ By the design of the 3C-SiC inverse opal architecture, the inferior light absorption and anodic instability of 3C-SiC photoelectrodes were improved. The maximum photocurrent densities of 9.95 and 11.53 mA cm⁻² were achieved in the [280.5, 600] nm region, respectively, representing the 75.7% and 87.7% of total available solar photocurrent density. Furthermore, the g-CN or NiO coating can form a type-II heterojunction with the 3C-SiC spbPore PC, further facilitating the charge transport and improving the corrosion resistivity.

The most direct way to regulate the morphology of photonic crystals is to control the reaction conditions during the preparation process, such as temperature, pH value, or the reaction environment. Joint research by multiple laboratories has found that by adjusting the pH value (Fig. 6a), the structure of the Ag₄V₂O₇/Ag₃VO₄ (BAVO) heterojunction can be well regulated.⁴⁰ This 0D/1D hierarchical structure helps the separation of photoinduced carriers owing to the close contact between two kinds of photocatalysts and the established built-in electric fields. Under the condition of visible light irradiation, the BAVO heterojunction exhibits strong photocatalytic ability because the internal electric field it establishes promotes the Z-scheme charge transfer path and realizes effective carrier separation. It can completely degrade rhodamine B within 10 minutes, and its reaction rate is 13.1 times, 83.8 times, and 1.9 times that of AgVO₃, Ag₄V₂O₇, and Ag₃VO₄ respectively. Moreover, the BAVO heterojunction can maintain stable performance in multiple cycle tests.

The unique double-layer structure of the core–shell structure is also attractive. Metal–organic framework (MOF) usually has a high specific surface area, which adds rich active sites, and its adjustable pore structure provides convenience for forming various shapes; covalent organic framework (COF) has the advantages of chemical stability and structural tunability. Combining the two into a core–shell structure can comprehensively utilize their advantages. The core–shell structure usually owns a more smooth interface and porous structure of MOF provides longer routes for light scattering. A recently synthesized novel MOF@COF core–shell structured photocatalyst TPTi-0.225³³ successfully reduces the bandgap and enhances the visible light response, promoting the separation of photogenerated carriers. Under visible light, the removal rate of TPTi-0.225 for p-hydroxybenzoate reaches 99% within 2 hours, and the rate constant far exceeds that of TPMA (a COF with a triazine structure and a donor–acceptor structure) and NH₂-MIL-125 (a Ti-based MOF). It has good degradation performance in different pH values and water matrices and can degrade p-hydroxybenzoates into low-toxic products.

In terms of special structures, researchers mostly start with constructing special morphologies or designing photonic crystals with defective structures.⁴² The internal point, line, and surface defects will generate defect states in the PBG, which can capture light of specific frequencies, increase the specific surface area and the length of the interaction path between light and matter, and improve the photocatalytic efficiency and sensing sensitivity, bringing more unique and excellent optical properties. Powar⁴³ studied the defect states of the amorphous In₂TiO₅ photocatalyst, which reduced the bandgap. It was determined that the defect state was Ti³⁺, which led to the generation of oxygen vacancies. These defective active sites play a key role in the photocatalytic reduction of CO₂. Introducing a 2D MoSe₂ nanolayer on In₂TiO₅ can increase the active surface area, which promotes charge separation, is conducive to the accumulation of photoexcited electrons in Ti³⁺ species, stabilizes the CO₂ reduction intermediate, and at the same time, MoSe₂ increases the exciton lifetime and enhances charge separation and transfer. The CH₄ production rate of this material can still reach 67.80% after three cycles. In another report, researchers have fully explored the potential value of the defect state Ti³⁺ and cleverly combined it with oxygen vacancies. The synergistic effect of the two has led to the preparation of a Co₃O₄/Ti³⁺-TiO₂/NiO hollow core–shell heterojunction⁴¹ (Fig. 6b), which has significantly better photocatalytic performance compared with single-crystalline TiO₂. Its hydrogen evolution rate reaches about 2134.63 μmol g⁻¹ h⁻¹, which is 80 times that of single-crystalline TiO₂. It also improves the selectivity of reducing CO₂ to CH₄ and has good stability. In short, the synergistic effect between the Ti³⁺/oxygen vacancies and the double p–n junctions of this heterojunction not only efficiently promotes the separation and transfer of photogenerated carriers to improve the utilization efficiency of solar energy but also reduces the threshold of reactions such as the conversion of H₂O to H₂, enhances the hydrogen evolution reaction, and the increased multiple active sites improve the selectivity for CH₄. At the same time, it also promotes the diffusion of H⁺ and the absorption of H₂O, CO₂, and CO.

The inverse opal framework is a relatively common special structure in photonic crystals. Using the opal structure formed by the self-assembly of colloidal particles such as silica or polystyrene as a template, a photocatalytic material with a three-dimensional periodic porous structure is prepared through a certain method. This periodic structure has a certain degree of symmetry and regularity in three dimensions. The pore size and shape are relatively uniform, and the pores are interconnected with each other, forming a unique three-dimensional porous skeleton. Similar with the opal structure, the inverse opal structure also presents advantages such as the PBG effect and the slow light effect. Additionally, it has more active sites than the opal structure, making the transportation of photogenerated carriers easier. Zheng⁴⁴ found that after soaking the opal structure polystyrene (PS) template with the Au/ZnO precursor solution, the opal template was removed by calcination, and the Au/ZnO-IO photocatalyst could be obtained. This 3D ordered Au/ZnO-IO photocatalyst exhibits excellent photocatalytic activity, improves the degradation performance, and accelerates the water decomposition rate. However, the particularity of the inverse opal structure also brings some problems, such as the pressure difference when the solution enters the interior. Wu³⁴ proposed a solution to this problem by using the template method to prepare a combination of multiple heterojunctions with a unique three-dimensional dendritic porous structure (Fig. 6c). Using SiO₂ as the skeleton, a TiO₂–ZrO₂–SiO₂ three-phase composite material (IOP-S@TZS) inverse opal structure catalyst was loaded. This multi-level branched structure solves the problem that pollutants are difficult to quickly enter the traditional inverse opal structure due to the pressure difference. It also has the synergistic effect of the type I/II heterojunction structure to promote charge separation, and the active free radicals OH and ˙O₂⁻ accelerate the redox reaction. Compared with the TZS powder, the degradation rate of IOP-S@TZS increased by 10% after 30 minutes.

3.3 Preparation methods of inverse opals

Based on the special structural properties of photonic crystals, photocatalytic materials with inverse opal frameworks have attracted much attention due to their excellent optical properties. In recent years, the development of simple, efficient, and controllable preparation methods has become an important research direction in this field. The deposition method is a key technology in the field of material preparation, which mainly includes electrodeposition,⁴⁵ chemical reactions,⁴⁶ and atomic layer deposition.⁴⁷ Electrodeposition relies on the action of an external electric field to reduce metal ions in the solution and deposit them on the electrode surface. Chung⁴⁸ studied the preparation of a composite Cu₂O/Au inverse opal by depositing Cu₂O on an Au inverse opal framework through pulse electrodeposition. Polystyrene (PS) microspheres were self-assembled into a colloidal crystal by vertical electrophoresis, and then Ni adhesion layer and Au were successively deposited in it by electrodeposition. After removing the PS microsphere template, an Au inverse opal structure was obtained. Finally, Cu₂O was deposited on the Au inverse opal by the pulse potential method to prepare different composite samples. The Au inverse opal provides a large specific surface area porous scaffold, making the composite Cu₂O/Au inverse opal have better sensing performance for the sensing response of H₂O₂ compared with planar Cu₂O/ITO and Au inverse opal.

Deposition, on the other hand, relies on chemical reactions or physical absorptions to generate solid-state deposits on the surface of the substrate. Vertical deposition uses gravity and capillary force to make particles orderly arrange and deposit on the surface of the substrate. Niu⁴⁹ used the hydrothermal method to successfully synthesize carbon nanospheres with a particle size of approximately 140 nm. Then, these carbon nanospheres were used as template materials and deposited on the surface of P25 (TiO₂) by the vertical deposition method. Subsequently, the deposited carbon nanospheres were immersed in a TiO₂ precursor solution. Through a series of reactions, an inverse opal TiO₂ photonic crystal structure was successfully prepared. Data show that the final photoelectric conversion efficiency of the dye-sensitized solar cell (DSSC) prepared with carbon nanospheres as template materials reaches 7.02%, which is comparable to that of the DSSC prepared using polystyrene nanospheres as template materials. Compared with the ordinary P25-DSSC, there is an increase of approximately 16% in the photoelectric conversion efficiency. Titanium dioxide photocatalytic materials only respond to ultraviolet light, and ultraviolet light only accounts for a very small part of sunlight. Therefore, the Zhang's group⁵⁰ innovatively proposed to use the vertical deposition method to co-assemble mercaptoacetic acid-modified cadmium sulfide nanocrystals and a P(St-MMA-SPMAP) polymer template to successfully develop an inverse opal structure of cadmium sulfide. The inverse opal structure takes advantage of its own macroporous structure and multiple reactive active sites to accelerate the transportation of substances, enhance the adsorption of molecules and increase the contact area between cadmium sulfide and water. Compared with cadmium sulfide nanoparticles, the inverse opal structure has better hydrogen production performance. By continuously reflecting and refracting the incident light, the light utilization rate of CdS is significantly improved.

Atomic layer deposition (ALD) is a special chemical vapor deposition technology. By alternately introducing gas-phase precursors into the reaction chamber in pulses and allowing chemical adsorption and reactions to occur on the surface of the substrate, a film is deposited layer by layer. The core principle of ALD is to use the “self-limiting” nature of chemical reactions to grow the film layer by layer in units of atomic or molecular layers, ensuring that only one atomic layer is deposited in each reaction. Hedrich⁵¹ used the atomic layer deposition method to prepare TiO₂–Fe₂O₃ inverse opals (Fig. 7a). First, a polystyrene (PS) colloidal template was synthesized. Then, using the ALD technology, TiO₂ was coated on the PS template, and the ALD cycle was repeated until different thicknesses were obtained. After that, the PS template was removed by calcination to obtain TiO₂ IO. Subsequently, Fe₂O₃ and TiO₂ were respectively deposited using ALD to form a TiO₂–Fe₂O₃–TiO₂ multi-layer inverse opal structure (Fig. 7b). The photocatalytic degradation rate of the TiO₂–Fe₂O₃ structure increased by 27% compared with that of the pure TiO₂ IO. Research shows that the catalytic efficiency of the double-layer IO composed of a 16 nm TiO₂ and a 2 nm Fe₂O₃ coating is the best, which is 1.38 ± 0.09 h⁻¹. Depositing a TiO₂ layer on this basis can modify it and enhance the induced crystallization and photocatalytic performance.

Compared with the deposition method, the inverse opals prepared by the sol–gel method have higher uniformity, and the regulation of the types and proportions of components is more precise, especially in some relatively complex systems. The Zhang's group⁵² studied the preparation of inverse opal TiO₂ (Fig. 7c) with a three-dimensional closely packed face-centered cubic structure by the sol–gel method. The outstanding structural properties of TiO₂ are manifested in a specific surface area as high as 91 m² g⁻¹, which is 4.33 times that of powdered TiO₂. At the same time, TiO₂ also has a high pore volume of 0.13 cm³ g⁻¹. Its periodically interconnected macropores are conducive to the separation of photogenerated carriers, significantly improving the photocatalytic efficiency. The rate constant is 9.96 × 10⁻³ min⁻¹, which is 1.81 times that of powdered TiO₂. The study also found that the slow light effect makes the photodegradation efficiency of 200 nm IO TiO₂ reach up to 70.02% at most, and the rate constant is 2.13 times that of porous bulk TiO₂. Aiming at improving the efficiency of degrading pollutants, Yu et al.⁵³ proposed to use the sol–gel method to efficiently prepare a composite photocatalyst of in situ doped carbon nitride quantum dots (CNQD) and inverse opal TiO₂ (Fig. 8a) with a colloidal photonic crystal made of PS as a template. Under sunlight irradiation, it has better degradation performance for pollutants such as phenol, toluene and RhB. Its excellent performance is due to the modification of the CNQD and the inverse opal structure, which increases the specific surface area and the concentration of active sites and reduces the recombination rate of photogenerated electrons and holes.

The template method offers substantial advantages in fabricating photonic crystals, particularly in structural regulation. This approach enables precise control over the periodicity, dimensions, and morphology of inverse opal structures, thereby achieving accurate light manipulation. Material versatility constitutes another key benefit, as the technique demonstrates compatibility with diverse substances including inorganic compounds, organic polymers, and metallic components. Furthermore, it facilitates material compounding and doping for performance optimization through straightforward processes. The preparation process distinguishes itself through operational simplicity and technical accessibility. The relatively uncomplicated procedures lower production costs while enabling scalable manufacturing, effectively reducing technical barriers to implementation. Notably, this methodology supports the construction of specialized architectures. Specific applications include introducing defect states to create optical microcavities, as well as fabricating complex three-dimensional configurations like inverse opal structures through controlled synthesis parameters.

The Pham's group⁵⁶ studied the preparation of multidimensional inverse opals of TiO₂–SiO₂ (TSIO) and TiO₂–ZrO₂ (TZIO) by the template method, and then modified their photocatalytic performance by doping gold nanoparticles (AuNPs). The preparation process is to first self-assemble SiO₂ or polymer spheres, then infiltrate the assembled template with the precursor solution, and finally remove the template by calcination. The results show that the photocatalytic activity of the pure TIO structure is 8.3% lower than that of the TSIO structure. The addition of AuNPs significantly enhances the photocatalytic activity. Among them, the activity of the Au-TSIO sample is the highest, which is 206% and 125% higher than that of the AuNPs-TIO and TIO structures respectively. Researchers also reported the successful preparation of an IL-CuCQDs-F/TiO₂ inverse opal composite material with unique properties bridged by Ti–F bonds⁵⁴ (Fig. 8b) using the template method. It can expand the light absorption range to the visible light region and achieve photoelectric conversion within the inverse opal structure of TiO₂ and efficiently capture CO₂ at the same time. The Ti–F bonding interface of IO-CFTi is conducive to the transmission of photogenerated electrons, thus promoting the reduction process of CO₂. By regulating the pore size of the inverse opal structure, the enhanced visible light absorption and the slow light effect work synergistically, significantly improving the absorption and utilization efficiency of visible light. In addition, the Cu–N bonds and imidazole groups existing on the 3DOM structure of IO-CFTi play a role in promoting the adsorption of CO₂, and its CO generation rate reaches 78.1 μmol g⁻¹ h⁻¹, which is increased by 50.1 times. It is worth noting that not only the pore size but also the refractive index of materials, the arrangement of periodic structures, and the angle of incident light can be adjusted to tune the PBG position. Furthermore, the thickness of photonic crystal layers and surface properties of materials also affect photocatalytic properties.

In order to simplify the fabrication progress of inverse opal structures, a one-step progress method is proposed as co-assembly (Fig. 8c).⁵⁵ In detail, the template material (mostly PS or PMMA microspheres) is dispersed with the goal photocatalyst together to form a uniform solution. Then, the mixture colloid goes through an evaporation induced self-assembly to get the photocatalytic inverse opal structure with microspheres inside. By removing the template material through high temperature sintering, photocatalytic inverse opal structure with regular pores is gained. Compared to traditional methods, this co-assembly method is more facile and stable. For example, a heterostructured three-dimensional porous inverse opal WO₃/TiO₂ film was developed by this single-step, three-phase co-assembly method.⁵⁷ The colloidal template was mixed with water soluble precursors, which enables the simultaneous growth of different metal oxides into a firmly interconnected periodic pore framework. Owing to the in situ contact of WO₃ and TiO₂, uniformly distributed nanoscale type-II heterojunctions were constructed. Consequently, the photoelectrochemical performance of the WO₃/TiO₂ inverse opal film greatly enhanced. Similarly, Au/TiO₂ inverse opal photocatalysts prepared by the co-assembly method were reported.⁵⁸ The sol–gel infiltration minimizes crack formation and the associated deterioration of optical properties of the inverse opal films. To compare the difference of the Au decorating methods, the Au/TiO₂-on sample consisted of a two-component assembly process followed by nanoparticles infiltration and the Au/TiO₂-in comprised a three-component assembly were prepared. In the former case, the Au nanoparticles mainly decorated on the skeleton of the TiO₂ inverse opal. While in the latter case, the Au nanoparticles were enclosed by the TiO₂ skeleton. Benefited from the co-assembly method of TiO₂ inverse opal and the high activity of Au nanoparticles, both samples present better performance than the blank sample.

4. Enhancing the photocatalytic performance by photonic crystals

Efficient utilization of full solar energy has become the mainstream trend in the development of new energy sources nowadays. According to the current research situation, to enhance the photocatalytic performance, some suitable semiconductor materials with good photocatalytic performance are usually selected, and methods such as modification or doping are carried out on them to enhance the photocatalytic performance. However, the light absorption range of semiconductor materials is still limited. The periodic structure of photonic crystals enables the incident light to be more evenly distributed on the catalyst and allows multiple repetitive reflection and refraction behaviors to occur, increasing the interaction time between the light and the catalyst, and at the same time reducing light loss. The specific PBG value can be matched with the absorption spectrum of the photocatalyst, confining the light to the area where the photocatalyst is located. This not only increases the propagation path and residence time of the light but also broadens the light absorption range, thereby improving the photocatalytic efficiency. The slow light effect (or slow photon effect) plays a significant role in photocatalysis. It means the group velocity of light vanishes at certain wavelength (usually the red edge or blue edge of the PBG), leading to the increased photon density and intensity, as well as the increased interaction time between photons and matters.¹⁷ Commonly, the slow light effect leads to an enhanced electric field on the materials with high refraction index (usually photocatalysts, low frequency, red edge), which strengthen the photocatalytic performance directly. However, some researches also reported that the enhanced electric field on the materials with low refraction index (usually pores, high frequency, blue edge) also helps the photocatalysis.^59,60 This can be explained by the loose confinement of the electric field.⁶¹ Since the size of pores is small in photocatalytic photonic crystals, the enhanced electric field in the blue edge will extend to the skeleton to strengthen the light absorption of photocatalysts, leading to the improvement of photocatalysis. Therefore, photonic crystals have become an ideal candidate material structure for enhancing the photocatalytic performance.

4.1 Modifying photonic crystals by metal doping

Although traditional photonic crystals perform excellently in regulating light propagation, their performance still has certain limitations. For example, there are still problems to be solved, such as the width of the PBG, a high recombination rate of photogenerated carriers, and a limited light absorption range. Metal materials have good electrical conductivity, and electrons can move freely inside the metal. Liu⁶² first reported a synthesis method of an Ag-CN IO photocatalyst with well performance (Fig. 9a). With the doping of metallic Ag, the catalyst has a strong H₂ release ability, which is derived from the multiple scattering phenomenon of the IO structure, efficient charge transfer and the SPR effect of Ag nanoparticles.

The wide bandgap of perovskites has always been a challenge that restricts their application in the field of optoelectronics. Previously, silver was often used to improve the optoelectronic properties of perovskites, but silver is expensive. To reduce costs, Tailor's team experimented with using copper instead of silver to synthesize the Cu-Cs₃Bi₂Cl₉ composite material.⁶³ Cu as an interstitial dopant leads to the increase in the lattice spacing, the red shift in the absorption spectrum, the narrowing of the bandgap, the shift in the Fermi level, and the improvement in thermal stability. Photocatalytic experiments show that the CO₂ reduction activity of Cu-Cs₃Bi₂Cl₉ is higher than that of pure Cs₃Bi₂Cl₉, and the yields of CH₄ and CO increase significantly. This benefits from that the introduction of the copper dopant broadens the absorption spectrum, promotes an increase in the generation of carriers, and the longer carrier relaxation process is also conducive to prolonging the carrier recombination lifetime and enhancing the catalytic activity.

Combining the metal with the photocatalytic material might solve the problem of the wide bandgap in the photonic crystal. The Wu's group⁶⁶ combined the hydrothermal method, chemical precipitation method and photochemical deposition method to prepare a ternary system of Au/ZnWO₄/CdS modified with oxygen vacancies. Due to the addition of CdS and Au nanoparticles, the light absorption range of ZnWO₄ (ZWO) has been significantly broadened, and at the same time, it has added more active centers to its hydrogen production process. The combined application of a type-II heterojunction and an Au promoter is a feasible and efficient means to enhance the separation and transfer efficiency of electron–hole pairs and thus effectively promote photocatalytic hydrogen production. The hydrogen production rate of the sample with 5% Au deposited on 10 ZC (10% CdS loaded on ZWO) reaches 5483.62 μmol g⁻¹ h⁻¹, which is 167 times higher than that of the unmodified ZWO and 29 times higher than that of pure CdS.

Due to the influence of the valence state of the metal, when it is doped into the photonic crystal, if the valence state is different from that of the replaced lattice, it will lead to charge imbalance. In order to maintain the electrical neutrality of the crystal, the crystal structure will make corresponding adjustments. One common way is to generate oxygen vacancies. Researchers such as Li⁶⁷ found that doping 1%–5% of Cu²⁺ into Co₃O₄ and forming oxygen vacancies can significantly improve the catalytic performance of the Co₃O₄/N-doped carbon (Co₃O₄/NC) material for the hydrogenation reduction of p-nitrophenol and advanced oxidation reactions. When the molar ratio of NaBH₄ to PNP is 100:1 and the reaction time is 2 min, the activity parameter is 523.3 s⁻¹ g⁻¹. When peroxy-sulfate is used as the oxidant, ¹O₂, as the main reactive oxygen species, can improve the efficiency of the oxidation reaction, and its activity parameter can reach 27 s⁻¹ g⁻¹. By replacing Co³⁺ with Al³⁺, the catalytic performance of the Cu²⁺-doped Co₂AlO₄/NC catalyst is significantly better than that of the Cu²⁺-doped CoAl₂O₄/NC material, which indicates that Cu²⁺ can replace Co³⁺ ions in the Co₃O₄ lattice to form oxygen vacancies.

Doping the transition metal rhenium into the inverse opal TiO_2−x (black TiO₂ obtained by hydrogenation)/SnO₂ (Fig. 9b) can also construct a heterojunction catalyst containing Ti³⁺ and oxygen vacancies (OVs).⁶⁴ This catalyst has a high light capture ability due to the slow light effect of the inverse opal structure and the oxygen-deficient state. At the same time, choosing SnO₂ to construct a heterojunction is very beneficial for improving the separation efficiency of photogenerated carriers, which benefits from the excellent electron migration rate of SnO₂. When this catalyst is used for the photocatalytic reduction of CO₂, the CO production is 16.59 μmol g⁻¹ h⁻¹, which is 1.21 times, 2.14 times, and 7.44 times higher than that when using IO-TiO_2−x/SnO₂, IO-TiO_2−x, and SnO₂, respectively.

Another possible situation is that lattice distortion leads to a change in the connection pattern between atoms, ultimately affecting the optical properties of the catalyst. The Feng's group⁶⁸ prepared a carbon nanotubes (CNTs)/MgAl₂O₄ whisker hybrid material by the catalytic carbon bed sintering method. The study found that when the content of Fe(NO₃)₃·9H₂O increases, the carbon content and the number of MgAl₂O₄ whiskers first increase and then decrease. In CNTs/MAW, iron nanoparticles connect carbon nanotubes and MgAl₂O₄ whiskers to form an interlocking structure. Fe doping causes the bond length of the metal–oxygen bond to become longer, and the accompanying defects provide active sites for the growth of whiskers. The unique interlocking structure enhances the wettability and oxidation resistance of carbon nanotubes. Compared with CNTs/MA, the zeta potential of CNTs/MAW is increased by 24.02%, the contact angle is reduced from 108.0° to 54.5°. In other domains, Liu⁶⁵ studied the regulation of the structural color of the crystal by metal doping and successfully prepared a Fe³⁺-doped SnO₂ inverse opal structure (Fig. 9c) by the template method. Its structure has no obvious change compared with that of pure SnO₂, but its optical properties are improved. Because Fe³⁺ absorbs most of the scattered light, the color saturation is enhanced, and the refractive index is increased. The full spectrum coverage is achieved by doping different amounts of Fe³⁺ with three particle size templates of 346 nm, 391 nm, and 467 nm. Pan's team⁶⁹ embedded Pd nanoparticles into a porous substrate to obtain a new catalyst. This single-metal catalyst does not require a reducing agent and has excellent catalytic reduction ability for nitrate in a wide pH range (3–11), with a N₂ selective reduction efficiency as high as 95%.

Besides, it is worthy noting that the metal-doping strategy can be further incorporated with other enhancement methods. Considering the interfacial stability issues, Likodimos et al.⁷⁰ used the Mo-BiVO₄ and the Ca-BiVO₄ to form a homojunction inverse opal structure for pharmaceutical degradation. By substituting part of NH₄VO₃ by (NH₄)₆Mo₇O₂₄·4H₂O, the Mo element was doped into the BiVO₄ lattice. Similarly, the Ca doping was realized by replacing part of Bi(NO₃)₃·5H₂O with Ca(NO₃)₂·4H₂O. The codoped (Mo, Ca)-BiVO₄ was prepared by combining the two substitutions together. The prepared homojunction owns a stable interface and accelerate the carrier separation, leading to a satisfactory performance. Furthermore, the PBG positions of the inverse opal structures were adjusted by the sizes or PS spheres, which proves that the inverse opal structure whose PBG is consistent with the absorption edge of BiVO₄ presents the best performance. By introducing shallow dopant states in BiVO₄, relatively weak structural distortions but significantly different donor concentrations were achieved.

4.2 Design of heterostructure composites

The most effective way to regulate the bandgap value is to utilize the band differences of different materials at the heterojunction interface, including the offsets in the energy positions of the conduction band and the valence band, thereby inducing the redistribution of electrons and holes near the interface.^71,72 By constructing a heterojunction composite structure, the advantages of two or more photocatalytic materials can be integrated, the band structure can be optimized, and the efficient separation of photogenerated carriers can be achieved, thus maximizing the utilization efficiency of light. At the meantime, the positions of PBG and absorption edge can be tuned to the same position. In the photonic crystal structures, through the assistance of slow light effect, the light-photocatalyst interaction is greatly enhanced, which brings more photoinduced electrons and holes. However, most of these photoinduced carriers will recombine with others if no further actions are adopted, weakening the enhancement of the photonic crystal structures. The introducing of heterojunction exactly solve this problem. By separating photoinduced electrons and holes, more useful carriers take part in the chemical reaction, finally leading to enhanced photocatalysis. For example, Guo et al.⁷³ successfully developed a defect-mediated S-C₃N₄/MnCdS photocatalyst by constructing a Z-scheme heterojunction. This performs excellently in the oxidation reaction of benzyl alcohol and the synergistic hydrogen production process. Experiments show that under visible light, the hydrogen evolution rate of CN/MCS150 (S-C₃N₄ 150 mg) is stably maintained at 1.50 mmoL g⁻¹ h⁻¹. At the same time, in the conversion of benzyl alcohol, the conversion rate exceeds 90%, and the selectivity for the formation of the target product benzaldehyde is as high as approximately 99%. Compared with MnCdS, the performance of CN/MCS150 is improved by 7 times.

Furthermore, the construction of the heterojunction can prolong the lifetime of photoelectrons, promote photodegradation, enrich the microbial community, and enhance the metabolic activity of the biofilm. The Z-scheme heterojunction of gCN/TiO₂ (gCNT) is applied to the degradation system⁷⁴ (Fig. 10a). In this system, through the synergy between active substances (˙OH and ˙O₂) and microbial metabolism, 93.55% of sulfadiazine can be effectively removed, and it still maintains a good removal rate after 10 cycle experiments. In addition, due to the Z-scheme heterojunction, the active substances and residual photoelectrons generated by gCNT stimulate microbial metabolism and improve the removal efficiency.

In the development history of semiconductor technology, the p–n junction and the Z-scheme heterojunction, as two important structures, each play a unique role. As a more traditional and classic structure in the semiconductor field, the p–n heterojunction, with its advantages such as simple structure, mature preparation process, and clear unidirectional conductivity, has laid a solid foundation in many electronic devices and circuits and demonstrated irreplaceable practical value. In order to improve the electron–hole recombination rate of the CdS photocatalyst, the Wang's group⁷⁵ combined it with the monoclinic phase β-AgVO₃ with a narrow bandgap to form a 1D/1D p–n heterojunction (Fig. 10b). The built-in electric field of the p–n heterojunction greatly increases the directional flow speed of electrons, and its hydrogen evolution effect reaches 581.5 μmol produced within 5 h, which is 3.8 times and 273 times higher than that of pure CdS and β-AgVO₃, respectively. Similarly, Yu's team⁷⁶ innovatively explored the in situ reversible assembly method to prepare a p–n type BiOI/Bi₅O₇I composite material combination. By precisely controlling the reaction time or the addition amount of the KI precursor, BiOI@Bi₅O₇I and Bi₅O₇I@BiOI heterojunctions with a large specific surface area and sufficient interfacial electric field to induce the directional movement of charges were successfully prepared. When the optimized heterojunction material is used as a visible light photocatalyst in the reaction, it is found that this material can efficiently reduce CO₂ reactants to CO products, with a yield of 0.46 μmol g⁻¹ h⁻¹. This catalytic performance is approximately 6.6 times higher than that of using BiOI material alone and 15.3 times higher than that of using Bi₅O₇I material alone. Li⁷⁷ proposed to construct a p–n type inorganic symbiotic heterojunction (pn-IIBH) to effectively solve these problems. Using the CuNiAl layered double hydroxide as precursor, a p–n type inorganic symbiotic heterojunction (pn-IIBH)-NiO(Al)/CuO(Ni,Al) was prepared (Fig. 10c). The rate of photocatalytic reduction of CO₂ to CH₄ in H₂O of this heterojunction reaches 86.59 μmol g⁻¹ h⁻¹, the generation rate of O₂ reaches 150.84 μmol g⁻¹ h⁻¹, and the selectivity of CH₄ exceeds 87.13%.

Utilizing the special structure of photonic crystals to construct a heterojunction interface is an effective method to improve the photocatalytic efficiency. Ran et al.⁷⁸ studied and used the solvothermal method to successfully prepare a hollow core–shell structured TiO₂/NiCo₂S₄ Z-scheme heterojunction photocatalyst by in situ growing NiCo₂S₄ nanoparticles on the outer surface of TiO₂ (Fig. 10d). The formation of a Z-scheme heterojunction between TiO₂ and NiCo₂S₄ reduces the charge transfer barrier and generates a strong interaction due to the formation of a large number of heterojunction interfaces, promoting the effective separation and transfer of carriers and eliminating the photocorrosion phenomenon of NiCo₂S₄. Under simulated sunlight conditions, when the molar ratio of the TiO₂/NiCo₂S₄ photocatalyst is 0.3, the optimal hydrogen evolution rate is 8.55 mmol g⁻¹ h⁻¹, which is approximately 34 times that of pure TiO₂ and 94 times that of pure NiCo₂S₄.

The Jiang's group⁷⁹ proposed a novel amorphous–crystalline heterojunction a-CeO₂/SnFe₂O₄ and applied it to the fields of near-infrared photocatalytic degradation and sterilization. By introducing a disorder-π ordered interface into the heterojunction, the built-in electric field is significantly enhanced, which creates favorable conditions for the dissociation of excitons. At the same time, the oxygen vacancies existing in the a-CeO₂/SnFe₂O₄ system regulate the electronic structure of the material, enabling better synergy between exciton dissociation and the built-in electric field. Its photocatalytic degradation removal rate of ciprofloxacin is more than 71%, and it can completely eliminate Staphylococcus aureus. The successful application of the experimental results has opened up a new path for amorphous materials in the optical field.

Although the performance of the Z-scheme heterojunction is better than that of the traditional heterojunction, the photocatalytic efficiency still fails to reach the ideal state. Thus, the Ren's team⁸⁰ constructed a novel double Z-scheme MoO₃/ZnIn₂S₄/black phosphorus quantum dots (MZBP) heterojunction for photocatalytic hydrogen production. This heterojunction exhibits excellent photocatalytic performance, with a hydrogen production efficiency of 5.45 mmol g⁻¹ h⁻¹ under visible light, which is increased to 10 times and 1.6 times compared with the pure ZnIn₂S₄ and MoO₃/ZnIn₂S₄ materials, respectively. At 420 nm, the apparent quantum yield of MZBP is 13.2%, indicating high energy conversion characteristics. Through degradation and radical capture experiments, it was confirmed that ˙O₂⁻ is an important active species in the system. The double Z mechanism endows the MZBP heterojunction with a longer lifetime of photogenerated carriers and a larger redox surface than traditional photocatalysts.

Till now, most heterostructure inverse opal photonic crystals were prepared by two-step methods, preparing a photocatalyst skeleton first and then grow another photocatalyst onto the skeleton. This method might bring the problem of the low adhesion ability. A co-assembling method was introduced to solve this drawback.⁸¹ In this work, in situ incorporation of MoS₂ nanosheets into the TiO₂ inverse opals was implemented by a three-phase co-assembly technique (assembling PS microspheres, TiO₂ and MoS₂ together). This in situ incorporation is much simpler than conventional topdown (impregnation and spin/dip-coating) and bottom-up (chemical vapor, chemical bath, and hydrothermal) deposition methods, providing better adhesion ability. Results prove that the integration of low amounts of MoS₂ nanosheets in the inverse opal skeleton maintains the periodic porous structure intact and enhances the surface area, which results in more efficient antibiotic degradation than benchmark TiO₂ films.

4.3 Coupling of upconversion materials

In terms of solar energy conversion, the successful collection and conversion of low-energy photons into high-energy photons are attributed to the research and development of upconversion technology. This discovery provides a new solution for improving the utilization rate of solar energy and lays an important foundation for in-depth research in the field of photocatalysis. Most traditional upconversion nanoparticles have the phenomenon of quenching, which seriously affects their application in real life. The PBG has the characteristics of suppressing the non-radiative transition of photons, and the metal surface plasmon resonance effect has the function of enhancing the local electromagnetic field, which have become two key factors affecting the upconversion quantum yield. Zhu et al.⁸² utilized the interaction of the dual local electromagnetic fields coupled by PBG and LSPR to increase the fluorescence intensity of TTA-UC by a maximum of 10.74 times, with a quantum efficiency of 11.05%. Under visible light and green light, the degradation efficiency of the upconversion-photocatalytic film for organic compounds increased by 3.29 times and 5.25 times, respectively. This can be attributed to the interaction of photon localization and the dual local electromagnetic fields induced by LSPR, which greatly improves the fluorescence efficiency of TTA-UC, thereby increasing the sensitized photons of CdS. The number of sensitized photons of CdS can also be increased by adjusting the size of CdS.

With similar methods, the Hou's team⁸³ synthesized three different-sized CdS nanocrystals, CdS 393, CdS 405, and CdS 426, with diameters of 3.1, 3.3, and 4.3 nm respectively. At the same time, four kinds of media and four kinds of annihilators were selected for systematic research. The research results show that the methods of introducing media by direct mixing and precisely matching energy levels can improve the photon upconversion performance of CdS nanocrystals, which is manifested in that the triplet energy transfer efficiency can be close to 1. By the combination of CdS 405/3-PCA (phenanthrene-3-carboxylic acid) and polyphenol oxidase, a TTA-UC quantum yield of up to 10.4% was achieved. By the combination of CdS NCs/3-PCA and naphthalene, TTA-UC photons with an energy close to 4 eV were observed, greatly expanding the spectral range. In order to further improve the ability of upconversion materials to capture photons, Fang⁸⁴ combined photonic crystals with AuNPs to prepare a photonic crystal film. Under green light excitation, the blue light intensity of the film containing gold nanoparticles increased by 3.69 times. After the coupling of the photonic crystal and AuNP, the green-to-blue TTA-UC intensity of the composite film increased by 7.68 times compared with the planar film without AuNP. Under visible light, the pseudo-first-order rate constant of the g-C₃N₄@CdS composite film was 3.88 times higher than that of the planar film without doped AuNPs.

Anthracene-based annihilators are highly favored by researchers as a key component of upconversion materials. Jin et al.⁸⁵ introduced that the Fe(III) complex is associated with the anthracene group through π–π interaction, which prolongs the electron lifetime in the excited state and promotes energy transfer. The doublet–triplet energy transfer from the Fe(III) complex to anthracene realizes color upconversion, and the upconversion luminescence quantum yield (Φ_UC) is in the range of 0.003%–0.06%. However, adding anthracene as a medium can increase Φ_UC by 6 times to 0.19%. Moreover, the Fe(III)/9,10-diphenylanthracene (DPA) upconversion pair can initiate the photopolymerization of acrylate, and the singlet excited state of DPA undergoes electron transfer to catalyze the polymerization (Fig. 11a and b). Chua⁸⁶ studied using mixed halide perovskite nanocrystals (CsPbX₃, X = Br/I) as triplet sensitizers, which can transfer the excitation energy to two triplet annihilators, 9,10-diphenylanthracene (DPA) and 9,10-bis [(triisopropylsilyl)ethynyl]anthracene (TIPS-An). The upconversion efficiency can reach 0.172% when using DPA and TIPS-An, which is five times higher than the sum of the efficiencies (0.019% and 0.015%) of the single-receptor systems using these two annihilators alone (Fig. 11c).

With its unique photon conversion characteristics, upconversion technology can achieve the encryption and precise identification of key information in the field of anti-counterfeiting. For example, Meng⁸⁸ developed a film composed of upconversion nanoparticles (M-UCNP) and a double-layer inverse opal photonic crystal (IOPC). In this film system, the luminescent M-UCNP particles are uniformly deposited on the surface of the IOPC structure with a two-photon stop band. In terms of structural color, a color transition from green to blue can be seen at the specular angle of the front side (540-layer) of the film, and the back side (808-layer) shows a non-specular angle scattering color from red to blue under natural light. In terms of luminescence performance, the 808-layer enhances the intensity of the excitation light, and the 540-layer reflects the emitted light of M-UCNP, both of which promote the upconversion luminescence (UCL), endowing the film with excellent night vision ability. Relying on the synergistic effect of the unique dual-mode structural color and UCL performance, triple anticounterfeiting of banknotes is achieved, bringing a new breakthrough to anticounterfeiting technology. In the field of degrading pollutants, Cho⁸⁷ prepared a special device mainly composed of three layers: first, two TTA-UC layers capable of converting green photons through polymer triplet–triplet annihilation upconversion, which can convert low-energy green photons with a wavelength of 532 nm and an energy of 2.33 eV into high-energy blue photons with a wavelength of 425 nm and an energy of 2.92 eV; second, a Pt-modified WO₃ layer with visible light photocatalytic activity; third, a PC layer that can optimize TTA-UC and UV. The combined layered structure of TTA/Pt-WO₃/TTA/PC is conducive to the generation of hydroxyl radicals, and its yield is significantly increased by 38.8 times, thus having a strong degradation effect on a variety of organic pollutants (Fig. 11d).

Besides of the introduction of upconversion materials, another strategy to make more utilization of the solar spectrum is structure tuning. Wong's team⁸⁹ prepared a bio-inspired antireflective surface that emulates the surface architectures of leafhopper-produced brochosomes—soccer ball-like microscale granules with nanoscale indentations. This design is compatible with various materials such as metals, metal oxides, and conductive polymers, exhibiting strong omnidirectional antireflective performance of wavelengths from 250 to 2000 nm. Ge's group⁹⁰ synthesized photonic crystal films by a two-step spraying process, in which many spectral/non-spectral colors can be displayed through the mixing of base colors. This is simply achieved by the stacking of photonic crystal layers with different PBG positions. Based on the same principle, Su et al.⁹¹ fabricated bilayer inverse opal TiO₂@BiVO₄ structures which manifested two distinct PBG peaks arising from different pore sizes in each layer, with slow photons available at either edge of each PBG. Consequently, slow light effect happens at the absorption edge of both TiO₂ and BiVO₄, elevating the photocatalytic properties dramatically.

4.4 Regulation of the photothermal synergistic effect

In most photocatalytic reactions, as the temperature rises, the molecular thermal motion increases, which helps the photogenerated carriers to migrate to the surface of the catalyst more quickly to participate in the reaction.⁹² This effectively reduces the recombination rate of carriers, thus accelerating the reaction rate. In this process, light and heat promote each other and complement each other, leading to the generation of the photothermal synergistic effect. For photothermal materials, part of the absorbed energy is used for the exciting of electrons, and the other part is converted into heat energy, thus triggering the occurrence of the thermal effect. Based on this, researchers⁹³ developed a CdS/GO/polystyrene photonic crystal film with integrated photothermal catalytic properties. The system strategically combines CdS as the photocatalytic component, graphene oxide (GO) for near-infrared light absorption, and polystyrene serving as structural matrix, achieving enhanced utilization across the solar spectrum. Compared with the pure CdS film, the photothermal catalysis of this film is manifested in an increase of 18.3 °C in the working temperature and a 57.8% improvement in the photocatalytic performance. Furthermore, an extend research coated polydopamine (PDA, photothermal material) onto the SiO₂ spheres to gain PDA@SiO₂ core–shell photothermal spheres, which were then self-assembled as a opal photonic crystal with CdSe photocatalysts coated onto the surface (Fig. 12a).⁹⁴ By tuning the PBG position of the photonic crystal, photons with certain wavelength are reflected to the CdSe photocatalyst. In the meantime, the slow light effect helps the absorption of NIR light by PDA shells and enhances its photothermal effect, raising the reacting temperature for photocatalysis. As a consequence, the sample with 20%PDA coating shows a 4.77-fold efficiency than the blank sample, proving the reliability of this design.

ZnIn₂S₄ (ZIV) has the advantages of controllable structure and morphology and stability. However, its hydrogen evolution efficiency is only 367 μmol g⁻¹ h⁻¹, which indirectly indicates that during its photocatalytic process, the electron–hole recombination rate is high, reflecting its poor photothermal conversion ability.⁹⁵ The Zeng's group⁹⁶ added Ni to accelerate electron transfer and reduce the electron–hole recombination rate. Leveraging the superior thermal conductivity of carbon nanofibers (CNFs), Ni-NiO@CNFs/ZIS-Vs was synthesized via a solvothermal method. This composite architecture integrates nickel–nickel oxide heterostructures anchored on CNFs with vacancy-engineered zinc indium sulfide (ZIS-Vs), capitalizing on the synergistic effects between the components. The S-scheme interface between ZIS-Vs and NiO generates many photogenerated carriers with high redox properties. Ni and CNFs serve as electron channels, improving the transmission rate of photogenerated electrons. Both the thermal conductivity of CNFs and the LSPR effect of Ni exhibit obvious photothermal conversion effects, converting low-frequency photons into heat, accelerating interfacial charge transfer, continuously generating heat, and increasing the reaction temperature.

Similarly, Shi et al.⁹⁷ utilized the advantage that Co₃O₄ can absorb Vis-NIR light and continuously generate heat to prepare a Co₃O₄@ZnIn₂S₄ S-type core–shell heterostructure (Fig. 12b). The mechanism of the S-type heterostructure is conducive to the generation of photogenerated carriers, thus realizing photothermal synergistic catalysis to promote the evolution of H₂. The special morphology of the core–shell structure helps to form a unique optical microcavity structure. When illuminated by sunlight, the light will undergo multiple refractions and reflections inside the cavity of the core–shell, increasing the light capture rate and thus enhancing the photothermal effect. Therefore, Li's team⁹⁸ designed a Bi₃O₄Br:Er³⁺@Bi₂O_3−x core–shell S-type heterostructure with a specific structure through innovative means (Fig. 12c). They utilized the upconversion function of Er³⁺, the plasma resonance effect and photothermal effect of Bi₂O_3−x, and constructed a high-quality interface through the sharing of [Bi–O] tetrahedrons and the equivalent layer structure to achieve effective charge transfer. The photocatalytic degradation performance of this heterojunction for bisphenol A under full-spectrum light irradiation and near-infrared light irradiation has been greatly improved compared with BOBE, reaching 2.40 times and 4.98 times respectively.

The color of black substances is due to their absorption of almost all wavelengths of light, and thus they cannot reflect the light back. Taking aniline black as an example, its molecular structure contains a large number of chromophores such as conjugated double bonds. These conjugated structures enable the electrons in the molecules to absorb photons within a wide energy range. Thus, aniline black (AB) and polyvinylidene fluoride (PVDF) were prepared into an inverse opal photonic crystal, and a zinc oxide photocatalyst was loaded to obtain a ZnO/0.5 AB-PVDF IO (Z0.5A) film.¹⁰⁰ The doping of aniline black enhances the near-infrared photothermal effect of the heterojunction. Under the full spectrum, the photocatalytic efficiency of the Z0.5A film is 1.63 times higher than that of the pure ZnO film. Moreover, the photocatalytic efficiency in the microreactor is 5.85 times higher than that in the ordinary reactor. Therefore, through the design of the film and the reactor, the synergistic catalytic promotion effect of slow light and the photothermal effect has been successfully achieved. Cui et al.¹⁰¹ increased the possibility of electron transition by introducing more defects and impurity energy levels, which can absorb more light of different wavelengths and broaden the light absorption range. On this basis, they successfully prepared a black titanium dioxide/indium oxide S-type heterojunction (IO-B-TiO₂/In₂O₃) with a three-dimensional porous inverse opal structure. The defects on the surface of the catalyst provide more active sites, giving it excellent photothermal catalytic performance. The generation rate of CO₂ converted into CO reaches 251.25 μmol g⁻¹ h⁻¹. This generation rate is 8.62 times that of IO-TiO₂ under non-photothermal conditions and 1.55 times that of IO-B-TiO₂/In₂O₃.

When light enters a solution or water, the incident light will be refracted and reflected, resulting in a certain loss of light. To solve this problem, Zhang⁹⁹ proposed floating the catalyst on the water surface to make more full use of the light. They used a combination of hydrothermal in situ growth and ultrasonic treatment to prepare a self-floating Bi₂S₃@Ag₂S heterojunction on the surface of carbon fiber cloth (CC) (Fig. 12d). CC serves as a carrier to convert solar energy into heat energy and realize the circulation of the catalyst. The prepared composite film can degrade tetracycline hydrochloride up to 97.2%. Its stability is affected by superoxide groups, and its performance advantages mainly come from self-floating light absorption and high carrier separation. The addition of Ag₂S not only broadens the absorption spectrum but also enhances the stability of Bi₂S₃. It is found that increasing the temperature can enable the Bi₂S₃@Ag₂S heterojunction to promote the degradation of organic matter in low-temperature wastewater.

5. Summary and outlooks

This review systematically summarizes photonic crystals from the following aspects: first, photonic crystals are classified and analyzed from the perspective of dimensionality. Research shows that one-dimensional (1D) photonic crystals can only regulate light in a single direction, while multidimensional photonic crystals offer the advantage of utilizing sunlight from multiple directions, providing broader development prospects for photocatalytic applications.

Second, the main preparation methods of photonic crystals in recent years and their advantages are systematically summarized. For example, the hydrothermal method excels in precise control of morphology and size, while the template method can fabricate photonic crystals with special structures. This enhances photocatalytic performance and provides an effective approach for exploring novel photonic crystal morphologies. Among these, inverse opal photonic crystals, with their high specific surface area providing abundant active sites and tunable pore sizes enabling precise control of light propagation paths, are discussed in detail to demonstrate their significant potential in the field of photocatalysis. Additionally, the optical properties of photonic crystals and their mechanisms in photocatalysis are thoroughly analyzed. Photonic crystals, with their high refractive index and reflectivity, significantly enhance the absorption efficiency of incident light by semiconductor materials. Simultaneously, by tuning the PBG to match the band structure of the photocatalyst, the recombination rate of photogenerated electron–hole pairs can be effectively reduced.

Finally, how to elevate the performance of photonic crystal enhanced photocatalytic system are explored. For instance, the surface plasmon resonance effect of metal nanoparticles in the visible light region promotes the generation and separation of electron–hole pairs, while metal particles provide more active sites, both contributing to enhanced photocatalytic efficiency. Heterojunction composites optimize the band structure, improving the effective separation efficiency of photogenerated carriers. Research shows that upconversion materials convert low-energy photons into high-energy photons, broadening the light absorption range. Photothermal synergistic catalysis accelerates chemical reaction efficiency by converting light energy into heat, raising the temperature to overcome reaction energy barriers. Thus, employing these methods helps the performance enhancement of photocatalytic photonic crystal structures. Based on precise control of material structure and properties, photonic crystals demonstrate unique advantages in replacing traditional photocatalytic materials, offering new research directions for the development of photocatalytic technology.

However, in the field of photonic crystal-enhanced photocatalysis, despite the increasing number of achievements in recent years, technological development still lags. Many challenges and issues remain to be addressed for the future advancement of photonic crystal assisted photocatalysis:

(1) Beyond the aforementioned special structures, there are still numerous unique shapes of photonic crystals to be explored in this field. These understudied photonic crystal morphologies may provide more possibilities for solving light propagation loss issues, becoming a key direction for breaking through current technological bottlenecks.

(2) Addressing the light propagation losses is also a highly challenging task. Losses are ubiquitous throughout the light propagation process. For example, when incident light undergoes refraction in photonic crystals, a certain degree of scattering loss occurs. Successfully solving this issue would undoubtedly open new avenues for related fields.

(3) The chemical environment plays a crucial role in optimizing electron transfer. By regulating chemical environment parameters such as pH, solvent polarity, and ionic strength, the surface charge distribution and band structure of the catalyst can be effectively adjusted. This promotes the separation and directional migration of photogenerated electron–hole pairs, ultimately leading to a significant improvement in photocatalytic reaction efficiency.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

Financial support from Natural Science Foundation of Jiangsu Province (BK20240982), Natural Science Foundation of China (22202106), China Postdoctoral Science Foundation (2023M731772), Changzhou Leading Innovative Talent Project (CQ20230098), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX25_1652), Changzhou University (ZMF23020017) is gratefully acknowledged. The authors also thank the Analysis and Testing Center, NERC Biomass of Changzhou University.

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