Impact of Bi³⁺ and Cu²⁺ doping on the optical and electronic properties of CsPbBr₃ for photocatalytic toluene oxidation

Marija Knezevic† ^a, Thi-Hieu Hoang†^a, Vien Duong Quach^a, Alba Garzón Manjón^c, David Llorens Rauret^c, Marie Erard^a, Audrey Gayral^a, Mireille Benoit^a, David Berardan^b, Jordi Arbiol^cd, Christophe Colbeau-Justin^a and Mohamed Nawfal Ghazzal*^a
aUniversité Paris-Saclay, UMR 8000 CNRS, Institut de Chimie Physique, 91405, Orsay, France. E-mail: mohamed-nawfal.ghazzal@universite-paris-saclay.fr
^bUniversité Paris-Saclay, UMR 8182 CNRS, Institut de chimie moleculaire, F-91405, Orsay, France
^cCatalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain
^dICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain

First published on 28th July 2025

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Introduction

Metal halide perovskites (MHPs) have renewed the interest in solar energy conversion since they were included in dye-sensitized solar cells and other solar cell technologies.^1,2 MHPs have versatile properties that offer plenty of possibilities to control their optoelectronic properties and light-harvesting capabilities.³ The most commonly reported is the partial anion substitution (Br_1−x/Cl_x or Br_1−x/I_x), which is an excellent way to tune the optical and electronic bandgaps of perovskites, leading to the fine control of photoluminescence, charge-carrier transfer, and photocatalytic properties.⁴ Partial halide substitution leads to an upward shift in the valence band maximum (VBM) and a downward shift in the conduction band minimum (CBM) as both have a contribution from the halide antibonding orbitals.^5,6 Cation substitution has been envisaged mainly to diminish the toxicity of MHPs due to the presence of Pb in their structure.⁷ Partial replacement of Pb²⁺ cations in CsPbBr₃ QDs with Sn²⁺, Cd²⁺, and Zn²⁺ was achieved by post-synthetic cation exchange.^8,9 A partial substitution of Pb leads to either PL enhancement or quenching, depending on the oxidation potential of the metallic cation and whether or not the trap states are introduced.^7,10,11 In addition, the complete replacement of lead through single and bimetallic substitution (Ag, Bi, Sn, and Sb) resulted in the formation of 2D double perovskites (A₂MX₆, A₃M₂X₆, and A₂M¹M²X₆).^12–14 On the contrary, divalent and trivalent cations, such as Mn, Ni, Cu, Al, Ce, and Bi, could affect structural, optical, and electronic properties while retaining either the cubic or orthorhombic crystal structure of MHPs.^{10,11,15–17} The trivalent Bi³⁺ and Pb²⁺ have very close ionic radii and are isoelectronic (6s²). Bi³⁺ was employed to substitute Pb²⁺ and reduce the toxicity of 2D perovskite structures, which have shown enhanced photoactivity, e.g., 0D bismuth-based halide double perovskites such as Cs₃Bi₂I₉,¹⁸ Cs₃Bi₂X₉ embedded in mesoporous silica (Cs₃Bi₂X₉-SBA-15),¹⁹ and Cs₃Bi₂X₉ coupled with graphitic carbon nitride (Cs₃Bi₂X₉-g-C₃N₄).²⁰ Moreover, 2D Cs₃Bi₂X₉ and Cs₃Bi₂X₉-g-C₃N₄-Pt demonstrated interesting activity for H₂ production from pure ethanol and 10% v/v aqueous triethanolamine, respectively.^21,22 From another point of view, Bi-doped hybrid or inorganic perovskites have demonstrated promising optical and electronic properties.^23–25 For example, doping CsPbBr₃ perovskite nanocrystals with Bi³⁺ ions led to photoluminescence quantum yield (PLQY) quenching and a potential change in the bandgap energy.¹⁶ However, this change in bandgap energy has been questioned. In particular, the combination of spectroscopic ellipsometry and transmission spectroscopy suggested that the change in the absorption onset does not rationalize the change in the optical bandgap. Further direct measurement of the bandgap of Bi-doped CsPbBr₃ using angle-resolved photoelectron spectroscopy revealed no change in the valence band energy.²⁶ Upon Bi doping, the photoluminescence (PL) quenching was instead confirmed and associated with the formation of trap states that enhance electron trapping and suppress radiative recombination.²⁶ In the same context, copper, with a smaller ionic radius (73 pm) compared to Pb²⁺ (119 pm), is another potential dopant for MHPs. Doping mixed-halide perovskites with divalent Cu²⁺ ions was reported to increase lattice binding energy and improve the short-range order of the lattice, which was identified as a factor improving both the thermal stability and PLQYs.⁷ The crystal lattice contraction through partial substitution of Pb²⁺ with Cu²⁺ and a reduction in surface energy in two-dimensional CsPbBr₃ perovskite nanoplatelets after Cu doping lead to long-term stability and photostability under ambient conditions. Although Cu-doped all-inorganic perovskites have primarily been studied for their photoluminescence properties, there is limited research on their photocatalytic properties. To the best of our knowledge, only the photocatalytic activity of Cu/CsPbBr₃–Cs₄PbBr₆ nanocrystals (NCs) has been studied.²⁷

Herein, we investigated the effect of CsPbBr₃ doping with bi- and tri-valent cations on its electronic and photocatalytic properties. This choice was motivated by the fact that Bi and Cu doping affect charge carriers differently, which is a determining parameter expected to modify the photocatalytic activity. Indeed, Bi has been reported to quench the PL, whereas Cu doping is more likely to enhance it. Thus, chemical, morphological, structural, optical, and electronic properties were investigated after the doping. Interestingly, varying the valence state of the dopants led to different effects, imparting each perovskite with specific properties. For instance, a drastic change in the morphology and the optical response was observed with Bi doping, whereas the perovskite maintained its morphology once doped with Cu. The charge carrier lifetime was determined using time-resolved laser scanning time-correlated single photon counting (TCSPC) microscopy, suggesting that both Cu and Bi doping could introduce shallow defects, thus affecting the photocatalytic efficiency in toluene oxidation.

Experimental section

Materials

All chemicals were used as received, without further purification or modification. Cesium carbonate (Cs₂CO₃, 99.5%), lead bromide (PbBr₂, 99%), and oleylamine (OAm, 80%–90%) were purchased from Acros Organics. Oleic acid (OA) was obtained from Riedel de Haën. Toluene (99.8%), tetrabutylammonium hexafluorophosphate ((Bu₄N)(PF₆), ≥98%), dichloromethane (DCM, ≥99.8%), 1-octadecene (ODE, 90%), n-hexane (95%), and acetonitrile (CH₃CN, ≥99.9%, for HPLC) were purchased from Sigma-Aldrich.

Synthesis of phase-pure CsPbBr₃

CsPbBr₃ nanocrystals were synthesized following a previously reported procedure.^28,29 Briefly, Cs-oleate was prepared by mixing pre-dried Cs₂CO₃ (0.163 g), OA (1.58 mL), and ODE (3.42 mL) at 120 °C under an argon flow for 1 h with vigorous stirring at 500 rpm. In order to completely dissolve Cs₂CO₃, the temperature was increased up to 130 °C at a rate of 1 °C min⁻¹. Cs-oleate must be preheated to 120 °C before use in the synthesis of CsPbBr₃ to avoid precipitation at room temperature. Simultaneously, PbBr₂ (0.21 g), OA (1.5 mL), OAm (1.5 mL), and ODE (5 mL) were mixed and heated at 120 °C under an argon flow for 1 h. After complete dissolution of PbBr₂, the temperature was raised to 200 °C. The previously prepared and preheated solution of Cs-oleate was then injected (0.75 mL). The reaction was immediately quenched by immersing the flask in an ice/water bath. CsPbBr₃ was dispersed in toluene and centrifuged at 7500 rpm for 20 min. The supernatant was discarded, and CsPbBr₃ NCs were washed three times with n-hexane (total 10 mL) and centrifuged again at 7500 rpm for 10 min. After the removal of aggregates, the elaborated CsPbBr₃ was dispersed in n-hexane.

Synthesis of Bi-doped CsPbBr₃ nanocrystals

The bismuth-doped CsPbBr₃ MHPs were synthesized by adapting the synthetic procedure used for pristine CsPbBr₃. Bi doping was introduced by partially substituting Pb²⁺ with Bi³⁺ to yield Cs₂Pb_1−xBi_xBr₃ compositions, where x = 0.01, 0.02, 0.05, and 0.10, corresponding to 1, 2, 5, and 10 mol% of Bi relative to the total B-site cations (Pb + Bi). The amount of PbBr₂ was fixed at 0.21 g, and the mass of BiBr₃ was varied accordingly to achieve the desired stoichiometric ratios. The variable mass ratio of BiBr₃ and PbBr₂ was mixed with OA (3 mL), OAm (1.5 mL), and ODE (10 mL), and heated at 120 °C, stirring at 500 rpm under argon flow for 1 h. After the complete dissolution of BiBr₃ and PbBr₂, the temperature was increased to 160 °C at a rate of 1 °C min⁻¹. The previously prepared and preheated solution of Cs-oleate was injected (0.75 mL). The reaction was immediately quenched by immersing the flask in an ice/water bath. The purification process using hexane and toluene for the bismuth-doped CsPbBr₃ was identical to that of the pristine CsPbBr₃. The synthesis of bismuth-doped CsPbBr₃ was also carried out at 200 °C, using equivalent amounts of OA (3 mL), OAm (3 mL), and ODE (10 mL).

Synthesis of Cu-doped CsPbBr₃ nanocrystals

The copper-doped CsPbBr₃ was synthesized by mixing a variable molar ratio (Pb:Cu = 1:0.25, 1:0.5, 1:1, 1:1.5) of CuBr₂ and PbBr₂ (0.21 g) with OA (1.5 mL), OAm (1.5 mL), and ODE (10 mL), and heating the mixture at 120 °C under argon flow for 1 h. After the dissolution of CuBr₂ and PbBr₂, the temperature was increased to 200 °C. The previously prepared and preheated solution of Cs-oleate (0.75 mL) was injected. The reaction was immediately quenched by immersing the flask in an ice/water bath. The purification process with hexane and toluene was identical to that of pristine CsPbBr₃.

Characterization techniques

Diffuse reflectance spectra (DRS) were recorded using a UV-vis-NIR Cary 5000 spectrophotometer (Agilent Technologies) equipped with an integrating sphere for diffuse and total reflection measurements. The maximum reflectance was set to 100% using BaSO₄ as a reference in the 200 to 1100 nm wavelength range.

Photoluminescence (PL) emission spectra were recorded using a Fluorolog-3 spectrofluorometer (HORIBA) equipped with a solid-state sample holder. All samples were excited at 400 nm, and emission spectra were monitored over the 450 to 700 nm range.

Results and discussion

We prepared undoped and doped CsPbBr₃ by a hot-injection method. The obtained Cu-CsPbBr₃ nanoparticles demonstrated a similar yellow color with varying PbBr₂/CuBr₂ ratios in the crude solution, i.e., the molar ratio of Pb/Cu was set to 0.25, 0.5, 1.0, and 1.5. Cu doping of MHPs required a significant amount of Cu(II) precursor due to the higher formation energy of Cu–Br compared to Pb–Br, e.g., −14.2 eV for CsCuBr₃, −6.82 eV for CsPb_0.93Cu_0.07Br₃, and −6.56 eV for CsPbBr₃.⁷ Bismuth-doped cesium-lead bromide (hereafter Bi-CsPbBr₃) nanoparticles were first synthesized at 200 °C by a hot-injection method. Higher temperatures were found to induce the reduction of Bi precursors and the formation of bismuth nanoparticles alongside perovskite nanocrystals (Fig. S1). Bismuth reduction occurred at high temperatures in a cesium-free solution in the presence of reductive agents and capping agents essential for controlling the size of MHPs, such as OA and OAm.^30–32 Thus, the synthesis temperature was decreased to 160 °C, and the ratio of OA to OAm was adjusted (1:2) to avoid thermal reduction of bismuth.^33,34 The color of the obtained Bi-CsPbBr₃ nanoparticles changed from yellow (CsPbBr₃) to dark orange (CsPb_0.90Bi_0.10Br₃ – all compositions given in the article are nominal compositions), as the amount of Bi in the crude solution increased (1%, 2%, 5%, and 10%) (Fig. 1).

The absorbance spectra show that the excitonic band remains unaffected upon doping (Fig. 2a). Cu doping results in a blue-shifted emission as the molar ratio of Cu increases, compared with the pristine CsPbBr₃ (Fig. 2b), which is in agreement with previous reports.^7,35 The Kubelka–Munk calculation indicates no significant change in the optical bandgap (Fig. S2), which contrasts with the PL shift to a shorter wavelength (Fig. 2b). Cyclic voltammetry measurements were carried out for all samples, demonstrating changes in the potential energies of the VBM and CBM (Fig. 2c). The bandgap energy of CsPbBr₃ increases from 2.31 eV to 2.50 eV upon Cu doping (Fig. 2d), aligning with the PL results. We attributed this discrepancy to the additional states introduced as shallow trap states in the CBM, which could not be observed in UV-Vis spectroscopy.

Before discussing the Bi doping, we recorded the PL spectra of undoped CsPbBr₃ synthesized at different temperatures (160 °C and 200 °C) and at 160 °C with an excess OA to enhance the solubility of the bismuth precursor (Fig. 3a). A blue shift of 11 nm in the PL maximum was observed when the synthesis temperature was lowered. An additional shift of 6 nm was noted when excess OA was used. Interestingly, the PL spectrum of the MHP synthesized with excess OA exhibited three additional emission peaks with lower intensity. The lowest-energy emission peak at 460 nm could be attributed to biexciton (BX), while the highest-energy exciton at 491 nm could be assigned to the active bright band (DX).^36,37 This PL emission behavior is typically observed in 2D perovskites.^36,37 The PL spectra of Bi-doped CsPbBr₃ displayed a broad emission band covering a large emission range between 420 and 540 nm (Fig. 3b). The band edge emission was found to be composite, consisting of multi-emission peaks centered at 430, 465, 480, 490, and 520 nm, with the intensity decreasing as the Bi content increased. Additional emission peaks appear compared to undoped CsPbBr₃, suggesting a supplementary excitonic phenomenon that could not be assigned at this stage. As the doping ratio increases from 1% to 2%, the peaks at 490 and 520 nm are quenched. At a higher doping ratio (10%), all peaks disappeared, and a new band edge emission appeared at 435 nm. Upon Bi doping, the excitonic absorption band is observed around 540 nm, similar to that of undoped CsPbBr₃. Further increase in the Bi ratio leads to the gradual appearance of a band-tail in the absorbance spectrum, as shown in Fig. 3c. We evaluated the bandgap energy using the Kubelka–Munk function, and all Bi-CsPbBr₃ samples demonstrated bandgap energies in the range of 2.2–2.3 eV (Fig. S2b). The conduction and valence band positions were determined using cyclic voltammetry measurements (Fig. S3). The obtained values, shown in Fig. 3d, are slightly higher than values obtained using the Kubelka–Munk calculation, falling in the range of 2.40–2.46 eV. This discrepancy is due to the presence of multi-emission peaks and the formation of trap states, which may account for the band-tail observed in the 550–700 nm range of the absorbance spectrum.^38,39 Note that the optical bandgap describes the transition of the excitonic band of the electron–hole pair (eqn (1)):

Eopt = Eg + Eb	(1)

where E_b represents the exciton binding energy and E_g represents the electronic bandgap.⁴⁰ The exciton binding energy represents the coulombic interaction between electrons and holes and is typically observed as a smaller bandgap. The difference between the optical and electronic bandgap corresponds to the excitonic states located below the bandgap minimum, indicating that Bi doping introduces shallow trap states below the CBM. It is worth noting that Bi-doped CsPbBr₃ demonstrated an upward shift of the conduction bandgap minimum (CBM), while the valence band maximum (VBM) remains at an energy level similar to that of pristine CsPbBr₃ synthesized at 160 °C. Furthermore, the average bandgap energy estimated from the CV measurements is 0.3 eV higher than that of pristine CsPbBr₃, consistent with previous studies.²⁶

X-ray diffraction (XRD) patterns of each CsPbBr₃ powder obtained using different doping amounts of Cu showed no extra peaks (Fig. 4a) compared with the undoped CsPbBr₃ nanocrystal (Fig. S4), indicating an orthorhombic single-phase material that retains the structural framework of pure CsPbBr₃ crystals, which were further confirmed using transmission electron microscopy (TEM) and fast Fourier transform (FFT) analysis (Fig. S5 and S6). The morphology of Cu-doped CsPbBr₃ was observed by TEM, and the results are shown in Fig. 4b and Fig. S7a–c. TEM images revealed cubic morphologies of Cu-doped CsPbBr₃ nanoparticles, with average sizes of 37, 20, and 21 nm when the ratio between Pb:Cu was 1:0.25, 1:0.5 and 1:1.5, respectively (Fig. S7). The CuBr₂ treatment of CsPbBr₃ at 200 °C did not affect the shape and size of Cu-CsPbBr₃, except at the 1:0.25 doping level, where the particle size was twice that of pristine MHPs (∼20 nm). For the Pb:Cu = 1:1 sample, we could also assign orthorhombic CsPbBr₃ (ICSD: 97851) NPs (Fig. S8a–e). The nanoparticles were oriented along the [101] or [210] zone axes (Fig. S8d and e). Similarly, at a higher doping ratio (1:1.5), the FFT analysis of the image revealed that the NPs adopted an orthorhombic CsPbBr₃ structure. Despite Cu doping, we did not observe significant changes in the d-spacings and angles with respect to the reported CsPbBr₃ crystal structure (Fig. 4b and Fig. S9), which is consistent with the XRD analysis, showing no evidence of any significant change in the lattice parameters of the orthorhombic structure. The Cu-doped CsPbBr₃ nanoparticles maintained the same zone axis orientation. Moreover, energy-dispersive X-ray spectroscopy (EDS) mapping demonstrated a homogeneous distribution of Cs, Pb, and Br atoms (Fig. 4d). Electron energy loss spectroscopy (EELS), combined with principal component analysis (PCA), was applied to the image shown in Fig. 4c, to analyze elemental signal within the sample. Close-up spectra of individual elemental edges are shown in Fig. S10. Note that no Cu signal from the L_3,2 edges was detected, which should appear at 931 and 951 eV. We can hypothesize that the intense signal from Cs might be suppressing the detection of the Cu signal.

X-ray diffraction (XRD) patterns indicate that the perovskite retains its orthorhombic structure upon treatment with a variable ratio of BiBr₃ (Fig. 5a). The synthesis of Bi-doped MHP was performed at 160 °C. Thus, for comparison, TEM images of undoped CsPbBr₃ are shown in Fig. S11. The images show well-dispersed nanocrystals with an average size of 16 nm. The morphology of CsPbBr₃ shows the formation of CsPbBr₃ nanosheets with a limited number of nanocubes.⁴¹ In contrast, we observe that the nanosheets are thinner than the nanocubes, with variable aspect ratios. Higher TEM magnification of Bi-doped CsPbBr₃ indicates that the hot-injection synthesis at 160 °C yields nanocubes of variable sizes, ca. 10, 20, and 40 nm, with no formation of Bi nanoparticles. The shape of the MHP appears to change as the doping ratio increases, leading to the formation of 2D perovskite with variable aspect ratios (Fig. 5b, c and Fig. S12a–h). These 2D perovskites are mainly observed at lower doping ratios (less than 10%). The broad size distribution and the formation of the 2D perovskite rationalize the multiple-emission spectra observed in Fig. 3b. The 2D perovskite comprises varying aspect ratios and retains some 3D perovskite, leading to variable exciton relaxation behavior and translating variable photoluminescence peaks. A reduced dimension or thinner layer results in blue-shifted photoluminescence and a lower bandgap value. The FFT analysis of the image reveals that the NPs adopt an orthorhombic CsPbBr₃ structure (ICSD: 97851), consistent with the XRD patterns (Fig. 5e). Nanoparticles were oriented along the [101] zone axis.

X-ray photoelectron spectroscopy (XPS) surface analysis was performed to demonstrate the presence of Bi and Cu in the doped perovskite. The survey and the high-resolution spectra of the samples indicate the presence of Cs, Pb, and Br in all doped and undoped CsPbBr₃ (Fig. S13 and S14). High-resolution XPS spectra show the presence of oxygen in CsPbBr₃, which could originate from OA binding to the surface, with a signal located at 532.1 eV (Fig. S14). The Cs 3d spectra of doped CsPbBr₃ originate from 3d_3/2 and 3d_5/2 spin–orbit coupling, located at 724.5 and 738.4 eV, respectively (Fig. 6a). The Pb 4f peaks are split into 4f_7/2 and 4f_5/2, located at 138.2 and 143.1 eV, respectively (Fig. 6a). The high-resolution Br 3d spectra are centered at 68.1 and 68.3 eV (Fig. 6a). All peaks are similar to those obtained for Cu-doped perovskites and do not exhibit any significant shift compared to pristine CsPbBr₃ (Fig. S14). XPS analysis confirms the presence of Bi³⁺ as the doping is performed. The Bi 4f_5/2 component is distinguishable at a binding energy 164.3 eV, as shown in Fig. 6b. The Bi 4f region splits into two components, including Bi 4f_7/2, which obviously overlaps with the Cs 4P_3/2 component at 158.9 eV.^11,42 The Cu-doped perovskites exhibit Cu 2p transitions, with two main contributions located at around 931.3 and 936.0 eV, as shown in Fig. 6c. These contributions are likely to translate into contributions that can be attributed to Cu²⁺ species in different coordination.^43,44 The first peak is shifted to lower energy compared to that reported for tetracoordinated Cu. However, the peak centered at ca. 937 eV corresponds to isolated Cu²⁺ in octahedral coordination, consistent with the substitution of the Pb atom into the perovskite structure. The Cu 2p region overlaps with a peak at 928.1 eV, assigned to Cs MMNT.

The photocatalytic activities of Bi- and Cu-doped CsPbBr₃ NCs were evaluated for toluene oxidation under xenon lamp illumination for 3 h (Fig. 7). The photocatalytic activity showed a significant improvement in the oxidation of toluene due to Bi doping (Fig. 7a). We observed enhanced photocatalytic activity with Bi doping of CsPbBr₃, irrespective of the ratio. The photocatalytic efficiency exhibited a maximum oxidation rate at 1% and 10% doping levels. The linear conversion trend of toluene to benzaldehyde during 3 h of illumination indicates the stability of the photocatalyst throughout the reaction. The photocatalytic oxidation rate increased from 171 ppm g⁻¹ min⁻¹ for undoped CsPbBr₃ to 435 and 411 ppm g⁻¹ min⁻¹ for CsPb_0.99Bi_0.01Br₃ and CsPb_0.90Bi_0.10Br₃, respectively (Fig. 7b). We also assessed the photocatalytic properties of Cu-doped CsPbBr₃ NCs. The photocatalytic toluene oxidation improved significantly compared to undoped CsPbBr₃. The photocatalytic activity increased gradually, reaching an optimum at a Pb/Cu ratio of 1:1, with a conversion of 80 × 10³ ppm g⁻¹ after 3 h of reaction. However, excessive doping (1:1.5) led to a drop in the benzaldehyde production, which was inferior to that of the undoped CsPbBr₃ NCs. The rate of toluene photocatalytic oxidation reached 460 ppm g⁻¹ min⁻¹,¹ closely comparable to that of Bi-doped CsPbBr₃ (Fig. 7b).

The photocatalytic oxidation of toluene under UV-Vis illumination produces benzaldehyde as the main product, with benzylic alcohol as an intermediate. The key to successful toluene oxidation is pre-saturation with oxygen, which allows the generation of superoxide anion radicals (O₂˙⁻) in the presence of electrons (eqn (2)). The rate-determining step in toluene oxidation involves C(sp³)–H bond activation of adsorbed toluene on the photocatalyst surface, upon interaction with holes on the MHPs site (eqn (3)).^19,45–48

O2 + e− → O2˙−	(2)

C6H5CH3 + h+ → C6H5CH2˙	(3)

The proposed mechanism of toluene oxidation is given by the following equations:

C6H5CH2˙ + O2 → C6H5CH2OO˙	(4)

C6H5CH2OO˙ + C6H5CH2˙ → C6H5CH2OH + C6H5CHO	(5)

C6H5CH2˙ + O2˙− → C6H5CH2OO−	(6)

	(7)

Photogenerated charge carriers play a major role in toluene oxidation. To investigate the effects of bismuth and copper on these carriers, time-resolved photoluminescence spectroscopy was performed. When CsPbBr₃ was doped with bismuth, its photoluminescence was dramatically quenched. The average PL lifetime decreased from 17.5 ns in undoped CsPbBr₃ (160 °C) to 1.3 ns (Fig. 8a and Table S1). On the other hand, undoped CsPbBr₃ nanosheets prepared under the same conditions exhibited an even longer lifetime of 28.9 ns. Bismuth introduces shallow states just below the conduction band minimum, acting as electron-type trapping states.¹¹ These additional states can trap electrons, leading to PL quenching. Copper doping gradually reduced the photoluminescence lifetime, but an excessive Cu content (Pb:Cu = 1:1.5) led to an increase in lifetime, contrary to earlier reports that show enhancement at low doping and suppression at higher levels (Fig. 8b and Table S2).⁷ The PL lifetime decay profile was fitted using a multiexponential fit, and the average lifetime decreased to 1.4 ns at a Pb:Cu ratio of 1:1. Interestingly, at Pb:Cu = 1:1.5, the lifetime increased to 6.2 ns, although this value remains much lower than that observed for undoped CsPbBr₃.

In summary, low bismuth doping generates shallow traps that capture electrons and facilitate the formation of oxygen radicals, thereby participating in charge carrier separation, which is reflected in the reduction of photoluminescence (PL) lifetime. Consequently, holes remain free for the activation of C(sp³)–H bonds, resulting in the generation of benzylic radicals. However, further doping with bismuth does not yield a beneficial effect on the photocatalytic oxidation of toluene, except at a higher bismuth doping ratio. At elevated doping levels, the effects are expected to be more pronounced due to the reduced PL lifetime, which leads to enhanced photocatalytic activity. Additionally, copper appears to act as an electron collector, although to a limited extent. Indeed, the quenching of PL is interpreted as a result of copper creating shallow traps that capture electrons, thus leaving free holes for the oxidation reaction. Nevertheless, an excessive loading of copper leads to photocatalytic deactivation, which may originate from the formation of deep traps within the band structure that function as recombination sites.

Conclusion

Herein, we achieved Bi and Cu doping of CsPbBr₃ using the hot-injection method. We found that the effect of doping on the optical, electronic, morphological, and photocatalytic activity depends on the nature of the cation used. For Bi doping of CsPbBr₃, only a small amount of BiBr₃ is necessary to initiate the reaction. It is crucial to maintain the synthetic reaction temperature at 160 °C to avoid the Bi reduction and formation of metallic nanoparticles. A reduction in the temperature led to the formation of 2D perovskite nanosheets with varying aspect ratios. However, doping with copper requires a significantly higher amount of CuBr₂ to initiate doping owing to the higher energy required to form the Cu–Br bond compared to the Pb–Br bond. We found that the bandgap energy increased with Bi and Cu doping, indicating the introduction of a trap state. The band-tail in the absorption spectrum of Bi-doped MHP suggests the generation of shallow trap states in the conduction band. Both metals indicate photoluminescence quenching, demonstrating charge transfer from MHP to either the Bi or Cu trap states. We found that the photocatalytic activity toward toluene oxidation improved upon doping. However, the mechanism behind this enhancement varies depending on the cation used. Copper doping exhibited remarkable photocatalytic activity and stability during 3 h reactions. The optimal photocatalytic activity was achieved at a nominal Cu ratio of 0.5 (Pb:Cu = 1:0.5), surpassing that of Bi-doped MHP. Unlike bismuth, which incorporates trap states below the conduction band minimum (CBM), copper influences both the conduction and valence band positions, making the oxidation reaction more favorable.

Author contributions

M. Knezevich and T.-H. Hoang carried out the experiments and investigation process, curated the data, wrote the initial draft, and reviewed and edited the final manuscript. V. D. Quach performed the PEC experiments. A. G. Manjón, D. Llorens Rauret, and J. Arbiol performed the electron microscopy analyses and discussed their results. M. Erard and A. Gayral performed the TRPL measurements and discussed the results. D. Berardan performed the XRD analysis and discussed the result. M. Benoit supervised the HPLC analysis and set up the analytical method. C. Colbeau-Justin supervised the study. M. N. Ghazzal conceptualized and conceived the idea, acquired the financing, supervised the work, contributed to the manuscript structure, and reviewed and edited the final manuscript. The manuscript was written with the help of all the co-authors.

Conflicts of interest

The authors declare that no conflict of interest, financial or otherwise, exists.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information such as TEM, Kubelka-Munk calculations, XRD, Cyclic voltammograms, EELS spectra, XPS and TRPL is available. See DOI: https://doi.org/10.1039/d5nr02442k.

Acknowledgements

MK acknowledges the French “Ministère de l'enseignement supérieur et de la recherche” (MESR) for the PhD Grant. MNG thanks the public grant overseen by the French National Research Agency (ANR) through the IngenCat project (ANR-20-CE43-0014) and the NEXTCCUS project as part of the ERANET-ACT3 call program for the financial support. The authors thank François Brisset for his valuable help in performing the TEM analysis. The authors thank Diana Dragoe for her valuable help in performing the XPS analysis. ICN2 is supported by the Severo Ochoa program from the Spanish MCIN/AEI (grant no.: CEX2021-001214-S) and is funded by the CERCA Programme from the Generalitat de Catalunya. ICN2 acknowledges funding from the Generalitat de Catalunya 2021SGR00457. The authors thank for the support from the project AMaDE (PID2023-149158OB-C43), funded by MCIN/AEI/10.13039/501100011033/ and by “ERDF A way of making Europe”, by the “European Union”. AGM acknowledges the RyC Programme by MCIN/AEI and Ayuda RYC2021-033479-I funded by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. Part of the present work was performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. ICN2 is a founding member of e-DREAM.⁴⁹

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Footnote

† These authors contributed equally.

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