Oxygen vacancies confined in Co₃O₄ quantum dots for promoting oxygen evolution electrocatalysis†

Yun Tong *, Hainiao Mao , Yanglei Xu and Jiyang Liu
Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou, China. E-mail: tongyun@mail.ustc.edu.cn

First published on 19th June 2019

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1. Introduction

The massive exploitation of traditional fossil fuels, such as coal and natural gas, as the main energy source will result in serious environmental conservation issues and global energy exhaustion.¹ Therefore, developing sustainable energy storage and conservation technologies, such as metal–air batteries and fuel cells, to produce clean energy is significant.² Electrocatalytic water splitting is considered to be a simple and efficient strategy to convert renewable fuels into clean chemical energy.³ However, as an important half-reaction of water splitting, the oxygen evolution reaction suffers from a bottleneck because of the sluggish kinetics of the four electron transfer process.⁴ Up to now, precious metal based oxides, such as IrO₂ RuO₂, etc. are still regarded as the most efficient OER catalysts.⁵ However, the disadvantages of high cost and terrestrial scarcity limit their widespread applications for industrialized production. Currently, inexpensive and abundant transition metals (e.g., (Mn, Fe, Co, and Ni) oxide (TMO)) have been widely investigated as alternative OER catalysts because transition ions usually exhibit different oxidation states as an avenue to regulate electronic states.⁶ However, pristine transition metal oxides usually show low OER catalytic activity due to their low surface area with few exposed active sites and poor conductivity.

Among the most reported transition metal oxides, cobalt-based oxides have attracted a lot of attention due to their huge potential for remarkable OER catalytic activity. The high OER catalytic activity can be ascribed to their unique Co₄O₄ cubane-like structure. Typically, Co₃O₄ catalysts are the most studied due to their excellent stability during the electrocatalytic water splitting process in alkaline media. However, in actual application, Co₃O₄ catalysts always suffer from a high overpotential, leading to low energy efficiency. As reported in the literature, the catalytic activity of Co₃O₄ depends on the ratio of Co²⁺/Co³⁺, in which more Co²⁺ leads to higher OER catalytic activity.⁷ The tetrahedral Co²⁺ site in the Co₃O₄ framework tends to form cobalt oxyhydroxide (CoOOH) species, which is the catalytic centre for water oxidation. Therefore, increasing the content of Co²⁺ sites can be expected to be an efficient method to improve the catalytic activity. Besides, the low electronic conductivity is another obstacle for Co₃O₄ catalysts to achieve high catalytic performance. Thus, it is essential to explore a method that realizes the synergistic optimization of the active sites and electronic conductivity of Co₃O₄ catalysts to improve their OER catalytic performance.

Defect engineering, such as introducing edge defects at hole sites, twinning boundaries and amorphous boundaries into a non-noble-metal framework, has been considered as a useful method to modulate their electronic structure for improving the electronic conductivity and increasing catalytically active sites, leading to significantly enhanced catalytic activity.⁸ Moreover, oxygen vacancies, as one of the most common types of defects in TMO materials, could weaken the metal–oxygen bonds to implement a more efficient intermediate exchange effect for a greatly enhanced electrocatalytic activity.⁹ For example, introducing abundant oxygen vacancies into perovskite-type catalysts causes the electronic-state transformation with synergistically optimizing active sites and conductivity, resulting in a greatly enhanced catalytic performance.¹⁰ On the other hand, when the dimensions of catalysts reduced from bulk to the nanoscale level more interior active sites will be exposed on the surface to afford high catalytic activity and the confinement effect will lead to electronic-state optimization to improve electronic conductivity.¹¹ Meanwhile, nanoscale-level nanomaterials provide a platform for further surface modification by using various strategies, such as surface/interface treatments and defect engineering.¹² Therefore, confining abundant oxygen vacancies in nanoscale-level Co₃O₄ catalysts for superior catalytic activity will be worth expecting.

In this work, we highlighted a defect engineering strategy via a simple reducing gas treatment to achieve oxygen-defect-abundant Co₃O₄ quantum dots, functioning as advanced OER electrocatalysts. Compared to solvothermal or reducing solvent treatment strategies, the hydrogen gas treatment method to produce oxygen vacancies provides a cleaner system to investigate the relationship between oxygen vacancies and catalytic activity. Coupled with the formation of oxygen vacancies in Co₃O₄ catalysts, the proportion of Co²⁺/Co³⁺ increases, which was confirmed from the X-ray photoelectron spectra (XPS). It indicates the rapid formation of CoOOH active sites to promote the water oxidation process. Moreover, the electronic conductivity measurement shows that oxygen-defect-abundant Co₃O₄ quantum dots exhibit the lowest conductivity compared to oxygen-defect-rare and pristine Co₃O₄ quantum dots. Benefiting from the synergetic optimization of active sites and conductivity, oxygen-defect-abundant Co₃O₄ quantum dots exhibit a lower overpotential (315 mV at 10 mA cm⁻²) along with faster reaction kinetics (Tafel slope of 49 mV dec⁻¹) in alkaline medium. Moreover, after 1000 cycles of cyclic voltammetry (CV), there is only about 7 mV positive shift for achieving a current density of 10 mA cm⁻² of oxygen-defect-abundant Co₃O₄ quantum dots, indicating its good stability. This work provides a general strategy for guiding the rational design of highly efficient non-noble metal TMO electrocatalysts.

2. Experimental section

2.1 The synthesis of pristine Co₃O₄ quantum dots

All the chemicals were of analytical grade and used without further purification. The pristine Co₃O₄ quantum dots were prepared by a simple chemical reaction. In a typical procedure, 200 mg cobalt(II) acetate (C₄H₆CoO₄) was firstly added into a beaker that contains 10 ml benzyl alcohol (C₇H₈O) solution. Then, the mixture solution was continuously stirred for 1 h at room temperature before adding 8 ml of commercial ammonium hydroxide (NH₄OH) solution. After vigorously stirring for another 10 min, the beaker with a reddish-brown solution was heated to 170 °C and maintained at this temperature for another 2 h under continuous stirring. With the reduction of volume, a black suspension was finally obtained. Then, 20 ml of diethyl ether was added into the above reaction solution and the black precipitate was collected by centrifugation. The black samples were further washed with ethanol several times before the next characterization.

2.2 The synthesis of oxygen defect-rich Co₃O₄ quantum dots

In a typical process, 50 mg pristine Co₃O₄ quantum dots were added into a porcelain boat and then it was placed into a tube furnace under a mixed gas atmosphere of 5% H₂/95% N₂. The samples were heated to 170 °C and 200 °C at 3 °C min⁻¹ and maintained at these temperatures for 1 h for preparing different oxygen defect-rich Co₃O₄ quantum dots (Co₃O₄-170; Co₃O₄-200).

2.3 Structural characterization

The crystal structure of different materials was determined by X-ray diffraction (XRD) on a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd) using Cu Kα radiation. The Raman spectra were recorded at ambient temperature with a LABRAM-HR Confocal Laser Micro Raman Spectrometer at 750 K with a laser power of 0.5 mW. The microstructure, morphology and elemental analyses were carried out by transmission electron microscopy (TEM) on a JEOL JEM-2001 electron microscope. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg Kα (hν = 1253.6 eV) as the excitation source. The binding energies obtained from the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.8 eV.

2.4 Electrochemical measurements

The electrochemical performance was firstly tested in a conventional three-electrode system on an electrochemical (CHI660E) workstation (Chenhua, Shanghai, China) at room temperature. The Ag/AgCl (saturated KCl) electrode as the reference electrode, Pt gauze as the counter electrode and the glassy carbon electrode coated with the as-obtained electrocatalysts as the working electrode were used. In a typical process, 4 mg catalysts and 35 μl of Nafion solution (5%, Sigma Aldrich) were dispersed in 1 ml of ethanol solution and then the above solution was sonicated for another 30 min to form a uniform ink. After that, 5 μl of the above as-prepared homogeneous ink was coated onto a glassy carbon electrode and dried for 2 h. Linear-sweep voltammetry was performed at a scan rate of 5 mV s⁻¹ in 1 M KOH solution. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an AC voltage with a 5 mV amplitude in the frequency range from 100 kHz to 0.1 Hz.

3. Results and discussion

In this work, pristine Co₃O₄ quantum dots (denoted as P-Co₃O₄) were synthesized by a simple surfactant-free approach and then calcined at different temperatures to form oxygen-defect-abundant Co₃O₄ quantum dots under a H₂/Ar atmosphere. The morphology and structure of the as-prepared Co₃O₄ nanoparticles were firstly characterized by TEM. The pristine Co₃O₄ catalyst shows the nanoparticle morphology with the particle size ranging from about 1.3 nm to 5 nm (Fig. S2†). Moreover, the high-resolution TEM image in Fig. S3† of an individual pristine Co₃O₄ nanoparticle displays a clear crystal lattice with a spacing of 0.202 nm corresponding to the (400) plane, demonstrating the high crystallinity of pristine Co₃O₄ nanoparticles. After introducing oxygen defects, low-resolution and high-resolution TEM images of the Co₃O₄-200 product in Fig. 1a and b exhibit a similar nanoparticle morphology, indicating the well-maintained quantum dot morphology. In order to confirm the structure and composition of Co₃O₄ products, systematic characterization studies have been performed. Energy-dispersive spectroscopy (EDS) and the corresponding elemental mapping images have been utilized to verify the elemental composition of the as-prepared Co₃O₄-200 product. As shown in Fig. 1c, a homogeneous distribution of cobalt and oxygen elements can be observed in the obtained Co₃O₄-200 product, indicating that there are only two elements of cobalt and oxygen in the Co₃O₄-200 product.

Moreover, the powder X-ray diffraction (PXRD) pattern has been firstly utilized to detect the crystal structure of Co₃O₄ nanoparticles. As shown in Fig. 2a, all the diffraction peaks of the as-obtained Co₃O₄ products can be perfectly indexed to the cubic phase Co₃O₄ (JCPDS No. 421467), demonstrating that the formation of oxygen vacancies did not destroy the long-term structure of Co₃O₄ nanoparticles (Fig. S1† and Fig. 2d). Moreover, in Fig. S4a† the XRD pattern for the products obtained at different temperatures shows that the crystal structure transition will occur in the samples when the hydrogen treatment temperature increases over 200 °C. And on further increasing the hydrogen treatment temperature from 210 °C to 250 °C, the CoO phase can be observed (Fig. S4b†). Therefore, 200 °C is the optimal temperature to generate rich oxygen vacancies and simultaneously maintain the crystal structure. In addition, as shown in Fig. 2b, the high-resolution TEM image of an individual Co₃O₄-200 nanoparticle also exhibits a clear crystal lattice with a spacing of 0.202 nm corresponding to the (400) plane, indicating pure crystalline phases and no crystal structure transition during the evolution of oxygen vacancies in the framework. As Raman scattering is very sensitive to the variation of the microstructure of the nanomaterials, Raman spectroscopy has been performed to explore the structure of Co₃O₄ nanoparticles. Fig. 2c shows the Raman spectrum of the as-prepared Co₃O₄-200, Co₃O₄-170 and p-Co₃O₄ samples. Five Raman active modes at 193.5, 481.4, 530, 618.5 and 690.6 cm⁻¹ have been observed in Co₃O₄-200, Co₃O₄-170 and p-Co₃O₄ samples, in keeping with those of high-purity Co₃O₄ nanoparticles. These five Raman active modes demonstrate that the obtained Co₃O₄ samples exhibit a spinel structure, which corresponds with the results of the XRD pattern.¹³

In order to investigate the chemical composition and assess the valence state of Co₃O₄ products, X-ray photoelectron spectroscopy (XPS) has been performed. Survey-scan X-ray photoelectron spectra in Fig. S5 and 6† show that the elements in the products include Co, O and C elements, which is in accord with the results of EDS. Moreover, as shown in Fig. 3a, the XPS spectra of both pristine Co₃O₄ and oxygen vacancies confined in Co₃O₄ products show six characteristic peaks. In detail, the two peaks located at 779.9 eV and 794.9 eV can be assigned to Co(III), while the two peaks located at 781.6 eV and 797.8 eV can be assigned to Co(II).¹⁴ Besides, the two broad peaks located at about 786.0 eV and 803.7 eV are the shake-up satellites of Co²⁺ (2p_1/2) and Co²⁺ (2p_3/2), respectively.¹⁵ Furthermore, the content of Co(II) species increased when the pristine Co₃O₄ products were calcined in a H₂ atmosphere to form oxygen vacancies. Notably, on further increasing the hydrogen treatment temperature, the Co 2p spectrum of Co₃O₄-250 shows a typical peak shape of CoO, including slight peak shifts and obvious strong satellites (Fig. S7†). Moreover, O 1s spectra provide more evidence of oxygen vacancies in the framework. There are three peaks in Fig. 3b that can be clearly identified, of which the two peaks at 529.8 and 532.1 eV can be assigned to metal–oxygen bonds and surface-adsorbed water molecules, respectively,¹⁶ while the peak located at 531.3 eV can be ascribed to the oxygen atom in the neighbourhood of an oxygen vacancy.¹⁷ Obviously, the proportion of oxygen vacancies of Co₃O₄-200 is larger than those of Co₃O₄-170 and pristine Co₃O₄, demonstrating the formation of abundant oxygen vacancies in the Co₃O₄-200 sample. The electron paramagnetic resonance (EPR) has further been employed to prove the formation of oxygen vacancies. As shown in Fig. 3c, Co₃O₄-200 and Co₃O₄-170 show a symmetrical EPR signal, while the EPR signal for the P-Co₃O₄ is negligible. The obvious resonance line of the paramagnetic phase in the EPR spectra of the Co₃O₄-200 and Co₃O₄-170 products confirmed that there were many Co²⁺ ions present in the Co₃O₄ crystal structure, which suggests that the presence of oxygen vacancies and the content of oxygen vacancies increased from P-Co₃O₄ to Co₃O₄-170 and to Co₃O₄-200. However, a large number of oxygen defects were formed in the Co₃O₄ quantum dots, but the XRD patterns show that there is no obvious phase change, indicating that the main oxygen vacancies were produced on the surface of the Co₃O₄ quantum dots. Moreover, as shown in Fig. 3d, the conductivity of Co₃O₄-200 is higher than those of Co₃O₄-170 and pristine Co₃O₄. As is well-known, a high electrical conductivity of catalysts will facilitate the electron transfer between the interfaces of the current collector and catalyst during the oxygen evolution process. Therefore, compared with Co₃O₄-170 and pristine Co₃O₄, the Co₃O₄-200 sample with better conductivity was expected to show superior OER catalytic activity.

In order to evaluate the OER electrocatalytic performance of pristine Co₃O₄ and oxygen vacancies confined in Co₃O₄, linear scan voltammetry (LSV) experiments have been firstly performed. Fig. 4a shows the IR-corrected LSV curves for the as-synthesized P-Co₃O₄, Co₃O₄-170 and Co₃O₄-200 samples. The Co₃O₄-200 sample showed a current density of 10 mA cm⁻² with an overpotential of 315 mV, which is much smaller than those of P-Co₃O₄ (370 mV) and Co₃O₄-170 (348 mV) catalysts. Moreover, at a specific overpotential of 380 mV, the overpotential of Co₃O₄-200 is calculated to be 144 mA cm⁻², which is certainly higher than those of P-Co₃O₄ (31.8 mA cm⁻²) and Co₃O₄-170 (15.2 mA cm⁻²). In addition, the OER mechanism for the series of Co₃O₄ catalysts was investigated using the corresponding Tafel slopes. As shown in Fig. 4b, the corresponding Tafel slopes of Co₃O₄-200, Co₃O₄-170 and P-Co₃O₄ catalysts are 49, 66, and 72 mV dec⁻¹, indicating that Co₃O₄-200 with more oxygen vacancies exhibits the fastest OER reaction kinetics. Electrochemical impedance spectroscopy (EIS) experiments in Fig. 4c allow further evaluation of the OER kinetics of the as-prepared catalysts. Oxygen vacancies confined in Co₃O₄-200 showed a dramatically smaller interfacial charge transfer resistance in contrast to Co₃O₄-170 and P-Co₃O₄ catalysts, which demonstrated the largely promoted OER kinetics of Co₃O₄-200. Stability is an important factor for assessing the feasibility of a catalyst for practical application. Therefore, the long-term CV cycling test was further performed to evaluate the stability of the as-synthesized Co₃O₄-200 electrode material. The LSV curves before and after 1000 CV tests and long-time chronoamperometric response (Fig. 4d and Fig. S8, 9†) of the as-prepared Co₃O₄-200 and Co₃O₄-170 show superior catalytic stability with a slight degradation in the current density after long-term CV cycling, indicating the superior durability of Co₃O₄-200 in alkaline medium.

The active surface area of the as-prepared Co₃O₄ catalysts has been evaluated using the electrochemical double-layer capacitance (C_dl).¹⁸ As shown in Fig. 5a–c, different CV curves of the P-Co₃O₄, Co₃O₄-170 and Co₃O₄-200 catalysts at different scan rates from 2 to 10 mV s⁻¹ have also been recorded. Co₃O₄-200 and Co₃O₄-170 demonstrate a larger increase in current density than that of the P-Co₃O₄ catalyst with an increase in scan rates. Furthermore, electrochemical double-layer capacitance (C_dl) measurements have been further performed to evaluate the active surface areas of catalysts. As shown in Fig. 5d, the C_dl value of Co₃O₄-200 is calculated to be 58.4 mF cm⁻², which is much higher than those of Co₃O₄-170 (C_dl = 48.7 mF cm⁻²) and P-Co₃O₄ (C_dl = 24.6 mF cm⁻²). Since the active surface area of electrocatalysts is proportional to the C_dl value, the results demonstrate that Co₃O₄-200 is more efficient in enlarging the catalytically active specific surface area in contrast to P-Co₃O₄ and Co₃O₄-170 benefiting from the formation of more oxygen vacancies, and thus better exposure and utilization of electroactive sites. In addition, the formation of oxygen vacancies on the surface of TMOs can also lead to obvious electronic delocalization and enhances the carrier transport for participating in electrocatalytic reactions, narrows the bandgap, modulates the electronic structure for accelerating the charge transfer, and weakens the metal–oxygen bonds to implement a more efficient intermediate exchange effect.¹⁹ Benefiting from the above important parameters, the oxygen-defect-abundant Co₃O₄-200 catalyst has been proved to be a superior electrocatalyst for promoting the oxygen evolution reaction process.

4. Conclusion

In conclusion, we highlighted a facile method to confine abundant oxygen vacancies into low-dimensional Co₃O₄ quantum dots via a simple hydrogen gas treatment. Formation of oxygen vacancies in Co₃O₄ quantum dots provides more efficient Co²⁺ sites in the framework, successfully providing more exposed active sites and better electron transport capacity for superior OER catalytic performance. As a result, the Co₃O₄-200 quantum dots show a greatly enhanced OER catalytic activity with a lower overpotential of 315 mV at 10 mA cm⁻² as well as a smaller Tafel slope of 49 mV dec⁻¹ in alkaline medium. This work provides a general strategy for guiding the rational design of highly efficient non-noble-metal TMO electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21701159), the Zhejiang Sci-Tech University (18062242-Y) and the Scientific Research Project for the Education Department of Zhejiang Province (No. Y201839992).

References

1. (a) J. S. Li, S. L. Li, Y. J. Tang, M. Han, Z. H. Dai, J. C. Bao and Y. Q. Lan, Chem. Commun., 2015, 51, 2710–2713 RSC ; (b) W. H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck and E. Fujita, Chem. Rev., 2015, 115, 12936 CrossRef CAS PubMed ; (c) H. Zhou, F. Yu, Y. Liu, J. Sun, Z. Zhu, R. He, J. Bao, W. A. Goddard, S. Chen and Z. Ren, Energy Environ. Sci., 2017, 10, 1487 RSC .
2. (a) P. L. He, X. Y. Yu and X. W. Lou, Angew. Chem., 2017, 129, 3955 CrossRef ; (b) P. Chen, T. Zhou, S. Wang, N. Zhang, Y. Tong, H. Ju, W. Chu, C. Wu and Y. Xie, Angew. Chem., Int. Ed., 2018, 57, 15471–15475 CrossRef CAS PubMed ; (c) P. Chen, Y. Tong, C. Wu and Y. Xie, Acc. Chem. Res., 2018, 51, 2857–2866 CrossRef CAS PubMed .
3. (a) Y. Xu, M. Kraft and R. Xu, Chem. Soc. Rev., 2016, 45, 3039 RSC ; (b) P. Chen, K. Xu, T. Zhou, Y. Tong, J. Wu, H. Cheng, X. Lu, H. Ding, C. Wu and Y. Xie, Angew. Chem., Int. Ed., 2016, 55, 2488–2492 CrossRef CAS PubMed ; (c) P. Chen, T. Zhou, M. Zhang, Y. Tong, C. Zhong, N. Zhang, L. Zhang, C. Wu and Y. Xie, Adv. Mater., 2017, 29, 1701584 CrossRef PubMed .
4. (a) P. Chen, K. Xu, Z. Fang, Y. Tong, J. Wu, X. Lu, X. Peng, H. Ding, C. Wu and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 14710–14714 CrossRef CAS PubMed ; (b) J. X. Feng, S. H. Ye, H. Xu, Y. X. Tong and G. R. Li, Adv. Mater., 2016, 28, 4698 CrossRef CAS PubMed .
5. (a) J. Park, Y. J. Sa, H. Baik, T. Kwon, S. H. Joo and K. Lee, ACS Nano, 2017, 11, 5500 CrossRef CAS PubMed ; (b) H. G. Sanchez Casalongue, M. L. Ng, S. Kaya, D. Friebel, H. Ogasawara and A. Nilsson, Angew. Chem., Int. Ed., 2014, 53, 7169 CrossRef CAS PubMed .
6. (a) T. T. Liu, Q. Liu, A. M. Asiri, Y. L. Luo and X. P. Sun, Chem. Commun., 2015, 51, 16683 RSC ; (b) P. Z. Chen, T. P. Zhou, M. Chen, Y. Tong, N. Zhang, X. Peng, W. S. Chu, X. J. Wu, C. Z. Wu and Y. Xie, ACS Catal., 2017, 7, 7405 CrossRef CAS ; (c) K. L. Yan, X. Shang, W. K. Gao, B. Dong, X. Li, J. Q. Chi, Y. R. Liu, Y. M. Chai and C. G. Liu, J. Alloys Compd., 2017, 719, 314 CrossRef CAS .
7. (a) G. X. Zhang, J. Yang, H. Wang, H. B. Chen, J. L. Yang and F. Pan, ACS Appl. Mater. Interfaces, 2017, 9, 16159–16167 CrossRef CAS PubMed ; (b) H. Y. Wang, S. F. Hung, H. Y. Chen, T. S. Chan, H. M. Chen and B. Liu, J. Am. Chem. Soc., 2016, 138, 36 CrossRef CAS PubMed .
8. (a) Y. Y. Wang, Y. Q. Zhang, Z. J. Liu, C. Xie, S. Feng, D. D. Liu, M. F. Shao and S. Y. Wang, Angew. Chem., Int. Ed., 2017, 56, 5867 CrossRef CAS PubMed ; (b) M.-Q. Yang, J. Wang, H. Wu and G. W. Ho, Small, 2018, 14, 1703323 CrossRef PubMed ; (c) K. Chatterjee, Y. Matsumoto and O. Dopfer, Angew. Chem., Int. Ed., 2019, 58, 3351–3355 CrossRef CAS PubMed ; (d) G. Yilmaz, C. F. Tan, M. Hong and G. W. Ho, Adv. Funct. Mater., 2018, 28, 1704177 CrossRef ; (e) G. Yilmaz, C. F. Tan, Y.-F. Lim and G. W. Ho, Adv. Energy Mater., 2019, 9, 1802983 CrossRef .
9. (a) Y. Tong, P. Z. Chen, M. X. Zhang, T. P. Zhou, L. D. Zhang, W. S. Chu, C. Z. Wu and Y. Xie, ACS Catal., 2018, 8, 1 CrossRef ; (b) J. Bao, X. D. Zhang, B. Fan, J. J. Zhang, M. Zhou, W. L. Yang, X. Hu, H. Wang, B. C. Pan and Y. Xie, Angew. Chem., 2015, 127, 7507 CrossRef .
10. Y. Q. Guo, Y. Tong, P. Z. Chen, K. Xu, J. Y. Zhao, Y. Lin, W. S. Chu, Z. M. Peng, C. Z. Wu and Y. Xie, Adv. Mater., 2015, 27, 5989 CrossRef CAS PubMed .
11. (a) Y. F. Sun, S. Gao, F. C. Lei and Y. Xie, Chem. Soc. Rev., 2015, 44, 623 RSC ; (b) X. H. Cao, C. L. Tan, X. Zhang, W. Zhao and H. Zhang, Adv. Mater., 2016, 28, 6167 CrossRef CAS PubMed ; (c) C. L. Tan and H. Zhang, Chem. Soc. Rev., 2015, 44, 2713 RSC .
12. (a) Z. Peng, D. S. Jia, A. M. Al-Enizi, A. A. Elzatahry and G. F. Zheng, Adv. Energy Mater., 2015, 5, 1402031 CrossRef ; (b) D. Kufer, I. Nikitskiy, T. Lasanta, G. Navickaite, F. H. L. Koppens and G. Konstantatos, Adv. Mater., 2015, 27, 176 CrossRef CAS PubMed .
13. (a) I. Lorite, J. J. Romero and J. F. Fernández, J. Raman Spectrosc., 2012, 43, 1443 CrossRef CAS ; (b) J. Yang, H. W. Liu, W. N. Martens and R. L. Frost, J. Phys. Chem. C, 2010, 114, 111 CrossRef CAS .
14. (a) Z. J. Wang, B. Li, X. M. Ge, F. W. T. Goh, X. Zhang, G. J. Du, D. Wuu, Z. L. Liu, T. S. Andy Hor, H. Zhang and Y. Zong, Small, 2016, 12, 2580 CrossRef CAS PubMed ; (b) K. Xiang, Z. C. Xu, T. T. Qu, Z. F. Tian, Y. Zhang, Y. Z. Wang, M. J. Xie, X. K. Guo, W. P. Ding and X. F. Guo, Chem. Commun., 2017, 53, 12410 RSC .
15. M. Leng, X. L. Huang, W. Xiao, J. Ding, B. H. Liu, Y. H. Du and J. M. Xue, Nano Energy, 2017, 33, 445 CrossRef CAS .
16. (a) Y. Y. Bu, J. Tian, Z. W. Chen, Q. Zhang, W. B. Li, F. H. Tian and J. P. Ao, Adv. Mater. Interfaces, 2017, 4, 1601235 CrossRef ; (b) X. F. Lu, D. J. Wu, R. Z. Li, Q. Li, S. H. Ye, Y. X. Tong and G. R. Li, J. Mater. Chem. A, 2014, 2, 4706 RSC .
17. F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan and Y. Xie, J. Am. Chem. Soc., 2014, 136, 6826 CrossRef CAS PubMed .
18. (a) Y. Tong, J. Wu, P. Chen, H. Liu, W. Chu, C. Wu and Y. Xie, J. Am. Chem. Soc., 2018, 140, 11165–11169 CrossRef CAS PubMed ; (b) T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, J. Am. Chem. Soc., 2014, 136, 13925–13931 CrossRef CAS PubMed .
19. (a) Y.-C. Zhang, N. Afzal, L. Pan, X. Zhang and J.-J. Zou, Adv. Sci., 2019, 6, 1900053 CrossRef PubMed ; (b) R. Zhang, Y.-C. Zhang, L. Pan, G.-Q. Shen, N. Mahmood, Y.-H. Ma, Y. Shi, W. Jia, L. Wang, X. Zhang, W. Xu and J.-J. Zou, ACS Catal., 2018, 8, 3803–3811 CrossRef CAS ; (c) Y.-C. Zhang, Z. Li, L. Zhang, L. Pan, X. Zhang, L. Wang, A. Fazale and J.-J. Zou, Appl. Catal., B, 2018, 224, 101–108 CrossRef CAS ; (d) Y.-C. Zhang, L. Pan, J. Lu, J. Song, Z. Li, X. Zhang, L. Wang and J.-J. Zou, Appl. Surf. Sci., 2017, 401, 241–247 CrossRef CAS .

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qi00325h

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