Ag nanoparticle-decorated CoS nanosheet nanocomposites: a high-performance material for multifunctional applications in photocatalysis and supercapacitors

Abstract

A novel Ag/CoS nanocomposite has been successfully synthesized by attaching Ag nanoparticles on the surface of CoS nanosheets with the assistance of light illumination. The photocatalytic activity of the prepared Ag/CoS photocatalysts was evaluated by the decomposition of methylene blue (MB) under visible light. The composites exhibited enhanced photocatalytic activities compared with the pure CoS nanosheets and the 2 wt% Ag/CoS composite showed the best performance, which was 2 times higher than pristine CoS. It was illustrated that the improved photocatalytic activity of Ag/CoS photocatalysts mainly stems from the elevated charge separation efficiency caused by surface plasmon resonance of Ag nanoparticles. Furthermore, the Ag/CoS supercapacitor electrode showed a high capacitance of 370 F g⁻¹ at a current density of 1 A g⁻¹ in 1 M KOH aqueous electrolyte. The electrode exhibited excellent cyclic stability after 1000 cycles at 10 A g⁻¹ current density. This work can be helpful for the exploration of plasmonic photocatalysts with enhanced photocatalytic activity and higher capacitance performance.

---

Introduction

In the past decade, inspired by the layer structure of graphene, two-dimensional (2D) materials have attracted great attention due to their unique dimension-dependent properties, such as their unusual physicochemical properties, high specific surface areas and novel electronic structures.^1–4 Among those 2D materials, transition-metal chalcogenides distinguish themselves by their advantageous electrical and catalytic properties over the bulk materials and traditional nanocrystallites.^5–7 For example, atomically-thick two-dimensional ZnSe crystals show 2 orders of magnitude higher solar water splitting activity compared with their bulk counterparts.⁸ A high mobility of at least 200 cm² V⁻¹ S⁻¹ at room temperature has been reported in a field effect transistor based on a single layer of MoS₂.⁹ As a typical class of transition-metal chalcogenides, cobalt sulfides have been investigated for catalysis, solar cell, lithium ion batteries, and supercapacitors. Owing to its variable valence of Co, cobalt sulfides have different chemical formulas such as CoS, CoS₂, Co₉S₈, Co₃S₄, and Co_1−xS.^10–20 Among these, CoS nanosheets have attracted significant interest in photocatalysis, lithium ion batteries, and supercapacitors. However, due to its inherent drawback, the performance of single-component CoS nanosheets was insufficient, thereby impeding its practical application. Hence, it is indispensable to improve the catalytic and electrochemical activity of CoS nanosheets for high-performance applications.

One effective strategy to overcome the drawback of bare CoS nanosheets is to construct composite system by combining with other materials,²¹ such as graphene and carbon nanotubes.^11,20 In particular, it was well demonstrated that the metal nanoparticles could effectively enhance their photocatalytic and electrochemical activity. With the respect to photocatalysis, the integration of semiconductors with metal nanoparticles can build a Schottky barrier at the interface, which could promote the charge transfer between the semiconductor and metal, and thereby accelerates the photoinduced electron–hole separation.^22–26 Among these NPs, Ag is relatively low-cost and displays forceful local electromagnetic field caused by surface plasmon resonance (SPR), which could improve the separation efficiency of photogenerated electron–hole pairs.^27–30 With respect to supercapacitors, incorporation of metal cations or nanoparticles, especially Ag, Au, Cu, and Ni can improve the specific capacitance of various types of electrodes due to the improved conductivity compared with the bare material.^31–34 Inspired by these results, we envision that the combination of metal nanoparticles and CoS will be a feasible way for the construction of high-performance photocatalyst or electrode for supercapacitor. However, to the best of our knowledge, no work is reported for the fabrication of Ag/CoS composite with multifunctional application in photocatalysis and supercapacitors.

Herein, we report for the first time a novel Ag/CoS nanocomposite by a facile two-step method with its applications as a photocatalyst to decompose MB under visible-light irradiation and electrode material for supercapacitor. The as-prepared Ag/CoS composites presented improved photocatalytic activities due to the SPR effect, which resulted in efficient separation of photoinduced charge carriers. The specific capacitance of the Ag/CoS electrode for supercapacitor can reach 370 F g⁻¹ at a current density of 1 A g⁻¹ and is much higher than that of pure CoS electrode. It is anticipated that our work can provided some inspiration for the fabrication of CoS-based nanocomposites with efficient photocatalytic and capacitive performance.

Experimental section

Materials

All chemicals and reagents used in this work were of analytical grade. Cobalt dichloride (CoCl₂·6H₂O), thiourea (CH₄N₂S), ethylenediamine (En), silver nitrate (AgNO₃) and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Deionized water was used in the experiments.

Synthesis of Ag/CoS nanocomposites

An amount of CoCl₂·6H₂O (0.1903 g) and CH₄N₂S (0.1218 g) were dissolved in 37 mL of ethanol and stirred vigorously for 30 min to form a transparent blue solution. Then 3 mL of En was added to the above solution. After vigorous agitation for 10 min, the solution was transferred into a Teflon-lined autoclave (50 mL) and heated at 180 °C for 12 h. The autoclave was quenched to room temperature after the solvothermal reaction. The obtained precipitates were centrifuged, rinsed with distilled water and ethanol. As above, the powder of CoS nanosheet sample was gained.

The Ag/CoS catalysts were prepared by dispersing 0.1 g of the as-synthesized CoS in 100 mL of water containing different amount of AgNO₃. After exposed to the illumination for 4 h, the suspension was operated by centrifugation to collect the products. The sediments were washed thoroughly with water and dried. The nominal weight ratios of Ag were 1, 2, 5 and 10 wt%, and the samples were labeled as 1%, 2%, 5% and 10% Ag/CoS, respectively.

Characterization

The X-ray diffraction (XRD) patterns were obtained from a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCA PHI500 spectrometer. Catalyst morphology and micro-structure were analyzed by transmission electron microscopy (TEM, JEOL-JEM-2010, 200 kV). A Shimadzu UV-2450 spectrophotometer was used to obtain the UV-vis diffuse reflectance spectra (DRS) of the samples. The specific surface areas for the samples were calculated by the Brunauer–Emmett–Teller (BET) method. Photoluminescence (PL) spectra were measured at room temperature on a Varian Cary Eclipse spectrometer.

Photocatalytic activity

The evaluation of photocatalytic performance for the as-prepared samples was via photodegradation of MB under visible light irradiation. 50 mg of photocatalyst was dispersed into 50 mL of 10 mg L⁻¹ MB solution. The solution was kept stirring in the dark for 30 min to reach the adsorption–desorption equilibrium before the irradiation. An air pump was used to ensure full mixing between the solution and photocatalyst and supply oxygen constantly during the photoreactions. Samples were collected every 1 h and centrifuged to remove the catalyst, then analyzed by an UV-visible spectrophotometer (UV-2450, Shimadzu, Japan) at 664 nm.

Electrochemical measurement

A three-electrode system was used in the measurements, which consisted of a working electrode, a platinum foil counter electrode (2 × 2 cm²), a saturated calomel electrode (SCE) as the reference electrode, and 1 M KOH aqueous solution as the electrolyte. The working electrode was composed of 80 wt% active materials (as-prepared CoS or 2% Ag/CoS sample), 10 wt% each of the conductive agent (acetylene black) and the binder (poly vinylidene difluoride, PVDF). The loading amount of the electrode on nickel foam was around 8 mg cm⁻². Electrochemical measurements were conducted using a CHI 660D electrochemistry workstation. Cyclic voltammetry (CV) measurements were carried out from 0 to 0.4 V vs. SCE at 10, 20, 40 and 80 mV s⁻¹. The galvanostatic charge–discharge characteristics were examined by a chronoamperometry technique.

Results and discussion

Structures and physical properties

XRD patterns were recorded to confirm the crystallographic phase of the as-prepared CoS and Ag/CoS composites. In Fig. 1, the four characteristic diffraction peaks are corresponding to (100), (101), (102) and (110) planes which can be indexed to the hexagonal CoS (PDF 65-3418). The XRD patterns of Ag/CoS nanocomposites are similar to that of CoS. The peaks of the Ag can hardly be detected, probably because the contents of Ag are quite low. Peaks related to other materials are not detected, indicating that the samples are of a high purity.

Fig. 1 XRD patterns of pure CoS and Ag/CoS composites.

To verify the surface chemical compositions of the Ag/CoS products, further XPS studies were conducted. The survey spectrum in Fig. 2a shows the presence of Co, Ag, C and S. The binding energy at 284.68 eV is a calibration of C 1s. The Ag 3d_5/2 peak appears at a binding energy of 367.8 eV (Fig. 2b), and the spin–orbit splitting value of the 3d doublet is 6.0 eV. This binding energy indicates that Ag is of metallic nature.³⁵ As shown in Fig. 2c, the main peaks at 778.2 and 780.1 eV are noticed in the region of Co 2p_3/2, which can be assigned to Co²⁺ in CoS and the spin–orbit splitting value of the 2p doublets are 15.2 and 16.3 eV, respectively.^36,37 Fig. 2d shows the spectrum of the S 2p region, and the two peaks at the binding energies of 161.5 and 162.4 eV are attributed respectively to S 2p_1/2 and S 2p_3/2, indicating that the S species exist as S²⁻ in the product.³⁸ These results indicate most definitely that CoS was successfully synthesized via the solvothermal method.

Fig. 2 XPS spectra of the 2% Ag/CoS sample: (a) survey; (b) Ag 3d; (c) Co 2p; (d) S 2p.

The structure and morphology of the pure CoS and Ag/CoS composites were characterized by TEM analysis. Fig. 3a illustrates that the CoS sample features a 2D irregular sheet-like structure. Fig. 3b–f present that the Ag nanoparticles with sizes of approximately 14–25 nm are clearly attached to the surface of CoS nanosheets, which is a strong evidence for explaining the formation of Ag/CoS composites. Fig. 3d shows the HRTEM image of the 2% Ag/CoS nanocomposite. The lattice spacing of Ag nanoparticle is determined to be 0.236 nm, which corresponding to the (111) crystal plane.³⁵ The distinct fringe space of 0.257 nm can be determined and ascribed to the (101) planar spacing of hexagonal CoS (PDF 65-3418).²⁰ The above results further demonstrate that the Ag particles have modified on the CoS nanosheets to construct Ag/CoS composites. To further verify the morphology of Ag/CoS composites, size distribution of Ag nanoparticles and atomic force microscopy (AFM) analysis were measured (Fig. 4). As shown in Fig. 4a, the average size of Ag particles loaded to the surface of CoS is about 16.2 nm. The AFM image (Fig. 4b) and corresponding height and distance distribution of the CoS nanosheet illustrate that the thickness and size of the CoS sheet are about 5 nm and 100 nm, respectively.

Fig. 3 TEM images of the samples: (a) pure CoS, (b) 1% Ag/CoS, (c) 2% Ag/CoS, (e) 5% Ag/CoS, (f) 10% Ag/CoS; (d) HRTEM image of the 2% Ag/CoS sample.

Fig. 4 (a) Size distribution of Ag nanoparticles; (b) the AFM image of the CoS nanosheet.

N₂ adsorption–desorption isotherms (Fig. 5) of the CoS and 2% Ag/CoS samples were conducted to determine the surface area of the pure and hybrid materials. The specific surface area of the CoS nanosheets and 2% Ag/CoS are calculated to be 8.70 m² g⁻¹ and 29.81 m² g⁻¹, respectively. It can be seen that the surface area greatly improves as the introduction of Ag particles to CoS nanosheets. The improved specific surface area is beneficial for absorbing more active species and reactants on the surface, which favors the improvement of photocatalytic performance.

Fig. 5 Nitrogen absorption–desorption isotherms of CoS and 2% Ag/CoS samples and the inset is pore diameter distribution of the samples.

UV-vis DRS was used to determine the optical response of the synthesized samples and the results are displayed in Fig. 6. It can be obtained that the pure CoS exhibits a continuous absorption band in the range of 200–800 nm. The addition of Ag particles induces the increased light absorption intensity in both the ultraviolet and visible light regions compared to the pure CoS. The enhancement of absorption intensity could be attributed to the SPR effect of silver nanoparticles deposited on the CoS surface.

Fig. 6 UV-vis absorption spectra of various samples.

Photocatalytic activity and mechanism

The photocatalytic activity of the as-prepared Ag/CoS nanophotocatalysts was examined by visible-light degradation of MB molecules in aqueous solution. The adsorption equilibrium image is shown in Fig. 7a. It is noted that MB has reached the adsorption equilibrium within 30 min in the dark. As shown in Fig. 7b, a blank experiment was implemented to prove that the direct photolysis of MB could almost be disregarded. The photocatalytic efficiency shows excellent enhancement with the modification of Ag nanoparticles compared to the pure CoS. With the increase of Ag concentration, the photocatalytic activity of Ag/CoS first increase and then decrease. When the content of Ag is 2%, the optimal photocatalytic activity of Ag/CoS photocatalyst is obtained with the best degradation rate reaching 72.65%. This value is 2 times higher than that of pure CoS. The high photodegradation ability of MB would be attributed to the Ag nanoparticles, which could effectively separate the photogenerated electron–hole pairs by SPR effect. However, the superabundant Ag nanoparticles may overlap the surface of the nanosheets and repress the light absorption, causing a reduction in the number of photogenerated electron–hole pairs.²⁷

Fig. 7 (a) Absorption properties of MB in the dark; (b) the photodecomposition of MB over CoS and Ag/CoS samples; (c) kinetic curves for the MB photodegradation with pure CoS and Ag/CoS samples; (d) the comparison of reaction rate constant, k, obtained from linear fitting.

The photocatalytic degradation kinetics of MB was investigated and the results are shown in Fig. 7c. The pseudo-first-order model was applied in this experiment. The values of the reaction rate constant (K_app) are calculated to be 0.10365, 0.13433, 0.31071, 0.29852 and 0.17441 h⁻¹, corresponding to CoS, 1% Ag/CoS, 2% Ag/CoS, 5% Ag/CoS and 10% Ag/CoS, respectively (Fig. 7d). It can be found that the rate constant of 2% Ag/CoS is about 3 times higher than that of CoS.

The stability of the photocatalysts plays a significant role in the practical application. To estimate the stability of Ag/CoS composites, recycling reactions were performed for 2% Ag/CoS photocatalyst by the photodegradation of MB under visible light irradiation and the results are presented in Fig. 8a. After four recycles, the photocatalytic activity of 2% Ag/CoS was decreased from 71% to 63%. A slight loss of photocatalytic performance can be observed, suggesting that the 2% Ag/CoS composite presents good stability in the MB degradation process. The decrease of the photocatalytic performance may be attributed to the loss of catalyst in the recycling process. Fig. 8b displays the XRD patterns of the used and fresh catalysts. The XRD patterns demonstrate that the catalyst exhibits no obvious difference with that before the reactions. These results illustrate that the Ag/CoS composites are stable.

Fig. 8 (a) Cycling degradation efficiency of the MB over 2% Ag/CoS sample under visible light irradiation; (b) XRD patterns of 2% Ag/CoS before degradation and after degradation for the four cycles.

To investigate the migration and recombination processes of photogenerated electron–hole pairs, PL spectroscopy measurements were performed. Fig. 9a shows that the CoS and Ag/CoS materials exhibit main emission peaks centered at about 536 nm at an excitation wavelength of 215 nm. The Ag/CoS composites display lower PL absorbance intensities than the CoS sample, which means the addition of Ag reducing the recombination of electron–hole pairs. The suppression could be attributed to the SPR formed at the Ag–CoS interface which could prevent electron–hole recombination efficiently. In addition, the order of PL intensity is CoS > 1% Ag/CoS > 10% Ag/CoS > 5% Ag/CoS > 2% Ag/CoS, which agrees well with the results of photocatalytic activity mentioned above. It is worth noting that excessive Ag nanoparticles were added to the surface of CoS nanosheets. It may be responsible for the increase of PL intensity that intensive and overlapping Ag nanoparticles generated electronic mutex and enhanced the recombination rate of photogenerated electron–hole pairs.

Fig. 9 (a) PL spectra of the as-prepared samples; (b) electrochemical impedance spectroscopy of pure CoS and 2% Ag/CoS samples.

The charge transfer properties of CoS and 2% Ag/CoS were further studied by electrochemical impedance spectroscopy (EIS) measurements.³⁹ As shown in Fig. 9b, in the Nyquist plot, the 2% Ag/CoS sample presents a circular radius smaller than pure CoS, which suggests that 2% Ag/CoS has a interfacial charge-transfer resistance lower than pure CoS. The EIS result demonstrates that 2% Ag/CoS composite shows a preferable efficiency in interfacial charge-transfer owing to the Ag nanoparticles.

Electrochemical performance

The electrochemical properties of the pure CoS and 2% Ag/CoS electrodes were investigated for the application of supercapacitor. Typical CV curves of 2% Ag/CoS electrode tested in 1 M KOH electrolyte at various scan rates are shown in Fig. 10a. These CV curves display similar shapes without redox peaks, indicating an ideal capacitive behavior within the tested potential window. As shown in Fig. 10b, the CV of the 2% Ag/CoS electrode reveals a more rectangular shape than that of the CoS electrode at a scan rate of 20 mV s⁻¹, suggesting that the 2% Ag/CoS electrode has higher capacitance performance. Meanwhile, the 2% Ag/CoS electrode shows the higher current response, implying that conductive Ag nanoparticles accelerate electron transport in the CoS nanosheets and also improve the energy density of the electrode.

Fig. 10 (a) CV curves of 2% Ag/CoS electrode at various scan rates; (b) CV spectra of CoS and 2% Ag/CoS electrodes in 1 M KOH solution with a scan rate of 20 mV s⁻¹.

To evaluate the specific capacitance and cycling stability of the Ag/CoS electrode, galvanostatic charge–discharge measurements were conducted. Fig. 11a and b shows the chronopotentiometric curves of CoS and 2% Ag/CoS electrodes at different current densities, respectively. The time durations of charge and discharge cycles for the two samples are both decreased with the current density increasing. The specific capacitance is calculated according to the following equation:

where C (F g⁻¹) is the specific capacitance, I (A) is the discharge current, m (g) is the mass of the active material, Δt (s) is the discharge time and ΔV (V) is the potential change during discharge. The specific capacitance values of 2% Ag/CoS electrode are determined to be 370, 315, 206 and 115 F g⁻¹ at 1, 2, 5 and 10 A g⁻¹. Fig. 11c displays the chronopotentiometric curves of CoS and 2% Ag/CoS electrodes at a current density of 1 A g⁻¹. The specific capacitance of 2% Ag/CoS is 1.2 times higher than that of CoS (318 F g⁻¹) at the discharge current density of 1 A g⁻¹. The enhancement of the specific capacitance possibly attributes to the increscent surface area and the modification of conductive Ag nanoparticles. Compared with other metal nanoparticles-based electrode materials, our Ag/CoS electrodes shown good specific capacitance. For example, Xia et al.³³ designed Ag/MnO₂ hybrid electrode which can deliver a specific capacitance of 293 F g⁻¹. Shao et al.⁴⁰ fabricated the graphene/Ag hybrid thin-film electrode with a specific capacitance of about 210 F g⁻¹. Additionally, comparing with the CoS non-nanosheets electrode, the present CoS nanosheets electrode possesses a higher specific capacitance. For example, Xu et al.⁴¹ synthesized the electrode of irregular CoS particles with a specific capacitance of about 280 F g⁻¹ at a current density of 1 A g⁻¹. The improvement of capacitance performance for the CoS nanosheets may be due to the higher specific surface areas and good electric conductivity and flexibility. The cycling stability of 2% Ag/CoS electrolyte was confirmed (Fig. 11d). The electrolyte exhibits almost no capacitance fading up to 1000 cycles, showing excellent charge–discharge stability.

Fig. 11 Galvanostatic charge and discharge curves of (a) CoS electrode and (b) 2% Ag/CoS electrode at various current densities; (c) charge and discharge curves of CoS and 2% Ag/CoS electrodes at a current density of 1 A g⁻¹; (d) specific capacitance vs. cycle number for 2% Ag/CoS electrode at a current density of 10 A g⁻¹ and the inset is charge and discharge spectra of the first ten cycles.

Conclusions

In summary, a novel Ag modified CoS nanosheets nanocomposite was successfully synthesized with enhanced photocatalytic activity toward photodegradation of MB and good electrochemical performance as electrode for supercapacitor. Among the specimens, the 2% Ag/CoS material displayed the optimal photocatalytic efficiency with the degradation rate of 72.65%. The improved photocatalytic activity for the Ag/CoS nanocomposite could be attributed to the SPR effect of metallic Ag nanoparticles with high efficiency of the photogenerated electron–hole separation and enhanced light harvesting. The specific capacitance of the Ag/CoS electrode could achieve 370 F g⁻¹ at the current density of 1 A g⁻¹ and the electrode exhibited excellent cyclic stability after 1000 cycles. This work can provide experimental insight into the design of CoS-based nanocomposites with enhanced photocatalytic activity and bring new opportunities for supercapacitor with the modification of Ag nanoparticles.

Acknowledgements

This work was supported by the financial supports of National Nature Science Foundation of China (No. 21406091 and 21576121), Natural Science Foundation of Jiangsu Province (BK20140530 and BK20150482), China Postdoctoral Science Foundation (2015M570409), College Natural Science Research Program of Jiangsu Province (13KJB610003), and Key Research Plan of Zhenjiang City (GY2015031).

References

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
2. K. S. Novoselov, V. I. Fal, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim, Nature, 2012, 490, 192–200.
3. Y. F. Sun, S. Gao and Y. Xie, Chem. Soc. Rev., 2014, 43, 530–546.
4. N. Zhang, M. Q. Yang, S. Q. Liu, Y. G. Sun and Y. J. Xu, Chem. Rev., 2015, 115, 10307–10377.
5. Y. F. Sun, Z. H. Sun, S. Gao, H. Cheng, Q. H. Liu, J. Y. Piao, T. Yao, C. Z. Wu, S. L. Hu, S. Q. Wei and Y. Xie, Nat. Commun., 2012, 3, 1057.
6. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147–150.
7. X. Fang, T. Song, R. Y. Liu and B. Q. Sun, J. Phys. Chem. C, 2014, 118, 20238–20245.
8. X. D. Duan, C. Wang, A. L. Pan, R. Q. Yu and X. F. Duan, Chem. Soc. Rev., 2015, 44, 8859–8876.
9. M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275.
10. X. Huang, Z. Y. Zeng and H. Zhang, Chem. Soc. Rev., 2013, 42, 1934–1946.
11. S. J. Peng, L. L. Li, X. P. Han, W. P. Sun, M. Srinivasan, S. G. Mhaisalkar, F. Y. Cheng, Q. Y. Yan, J. Chen and S. Ramakrishna, Angew. Chem., Int. Ed., 2014, 53, 12594–12599.
12. Z. H. Wang, L. Pan, H. B. Hu and S. P. Zhao, CrystEngComm, 2010, 12, 1899–1904.
13. S. M. Liu, J. X. Wang, J. W. Wang, F. F. Zhang, F. Liang and L. M. Wang, CrystEngComm, 2014, 16, 814–819.
14. Q. H. Wang, L. F. Jiao, Y. Han, H. M. Du, W. X. Peng, Q. N. Huan, D. W. Song, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Phys. Chem. C, 2011, 115, 8300–8304.
15. J. L. Gómez-Cámer, F. Martin, J. Morales and L. Sánchez, J. Electrochem. Soc., 2008, 155, A189–A195.
16. Y. N. Ko, S. H. Choi, S. B. Park and Y. C. Kang, Chem.–Asian J., 2014, 9, 572–576.
17. G. Chen, W. Ma, D. Zhang, J. Y. Zhu and X. H. Liu, Solid State Sci., 2013, 17, 102–106.
18. R. C. Jin, J. S. Liu, Y. B. Xu, G. H. Li and G. Chen, J. Mater. Chem. A, 2013, 1, 7995–7999.
19. S. J. Peng, L. L. Li, H. T. Tan, R. Cai, W. H. Shi, C. C. Li, S. G. Mhaisalkar, M. Srinivasan, S. Ramakrishna and Q. Y. Yan, Adv. Funct. Mater., 2014, 24, 2155–2162.
20. S. F. Kong, Z. T. Jin, H. Liu and Y. Wang, J. Phys. Chem. C, 2014, 118, 25355–25364.
21. Q. Z. Wang, Y. B. Shi, L. L. Pu, Y. T. Ta, J. J. He, S. L. Zhang, J. B. Zhong, J. Z. Li and B. T. Su, Appl. Surf. Sci., 2016, 367, 109–117.
22. S. K. Dutta, S. K. Mehetor and N. Pradhan, J. Phys. Chem. Lett., 2015, 6, 936–944.
23. N. Zhang, S. Q. Liu and Y. J. Xu, Nanoscale, 2012, 4, 2227–2238.
24. A. A. Melvin, K. Illath, T. Das, T. Raja, S. Bhattacharyya and C. S. Gopinath, Nanoscale, 2015, 7, 13477–13488.
25. Q. H. Chen, H. L. Liu, Y. J. Xin and X. W. Cheng, Chem. Eng. J., 2014, 241, 145–154.
26. M. M. Gui, W. M. P. Wong, S. P. Chai and A. R. Mohamed, Chem. Eng. J., 2015, 278, 272–278.
27. B. F. Luo, D. B. Xu, D. Li, G. L. Wu, M. M. Wu, W. D. Shi and M. Chen, ACS Appl. Mater. Interfaces, 2015, 7, 17061–17069.
28. H. L. Duan and Y. M. Xuan, Sol. Energy Mater. Sol. Cells, 2014, 121, 8–13.
29. P. Jain and P. Arun, J. Appl. Phys., 2014, 115, 204512.
30. W. L. Yang, L. Zhang, Y. Hu, Y. J. Zhong, H. B. Wu and X. W. Lou, Angew. Chem., Int. Ed., 2012, 51, 11501–11504.
31. Q. Y. Lv, S. Wang, H. Y. Sun, J. Luo, J. Xiao, J. W. Xiao, F. Xiao and S. Wang, Nano Lett., 2016, 16, 40–47.
32. J. Z. Zhang, Y. H. Xu, Z. Liu, W. R. Yang and J. Q. Liu, RSC Adv., 2015, 5, 54275–54282.
33. H. Xia, C. Y. Hong, X. Q. Shi, B. Li, G. L. Yuan, Q. F. Yao and J. P. Xie, J. Mater. Chem. A, 2015, 3, 1216–1221.
34. R. Kumar, A. Agrawal, R. K. Nagarale and A. Sharma, J. Phys. Chem. C, 2016, 120, 3107–3116.
35. J. G. Yu, J. F. Xiong, B. Cheng and S. W. Liu, Appl. Catal., B, 2005, 60, 211–221.
36. F. L. Luo, J. Li, H. Y. Yuan and D. Xiao, Electrochim. Acta, 2014, 123, 183–189.
37. K. Dai, D. P. Li, L. H. Lu, Q. Liu, J. L. Lv and G. P. Zhu, RSC Adv., 2014, 4, 29216–29222.
38. X. M. Li, Q. G. Li, Y. Wu, M. C. Rui and H. B. Zeng, ACS Appl. Mater. Interfaces, 2015, 7, 19316–19323.
39. Y. Liu, Y. Q. Zhan, Y. L. Ying and X. S. Peng, New J. Chem., 2016, 40, 2649–2654.
40. Y. L. Shao, H. Z. Wang, Q. H. Zhang and Y. G. Li, J. Mater. Chem. C, 2013, 1, 1245–1251.
41. L. Xu and Y. Lu, RSC Adv., 2015, 5, 67518–67523.

---