Novel 3-D nanoporous graphitic-C₃N₄ nanosheets with heterostructured modification for efficient visible-light photocatalytic hydrogen production

Abstract

Phosphorus-doped g-C₃N₄ (P-C₃N₄) nanosheets with unique 3-D nanoporous structures are synthesized for the first time in this work; such functional porous architectures coupled with BiPO₄ nanorods can exhibit superior photocatalytic activity for hydrogen production. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV-vis diffusive reflectance spectroscopy have been employed to characterize the heterostructured photocatalytic materials. The as-prepared composite photocatalyst show outstanding activity for photocatalytic hydrogen production under visible light (λ > 420 nm). The composite photocatalyst with 3.0 wt% BiPO₄ shows an optimum photocatalytic activity with a H₂-production rate of 1110 μmol h⁻¹ g⁻¹. The enhanced photocatalytic activity for P-C₃N₄ coupled with BiPO₄ comes from the high migration efficiency of photoinduced electrons on the interface of P-C₃N₄ and BiPO₄.

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Introduction

Polymeric graphitic carbon nitride (i.e. g-C₃N₄) materials have attracted much attention in recent years because of their interesting electronic properties and promising catalytic activities.¹ Considering the high thermal and chemical stabilities together with their semiconductor properties and low cost of mass production, g-C₃N₄ based materials are potentially close to being ideal candidates for sustainable solar energy conversion systems.² With a narrow band gap of 2.7 eV, these polymeric semiconductors have been extensively explored as attractive photocatalysts for hydrogen reduction via solar photocatalytic water splitting.³ Although the carbon nitrides have great applied potentials, the photocatalytic activity of pristine g-C₃N₄ is still deficient and the apparent quantum efficiency is only 1.5%. With this in mind, it is one of fundamental interest to further promote the photocatalysis performance of such material sunder visible light irradiation.

To overcome this problem and enhance the photocatalytic performance of the carbon nitride system, a series of research strategies have been developed, including (i) design and synthesis of favourable nanostructures of g-C₃N₄ such as nanoporous structures⁴ and low-dimension nano architectures (e.g. nanorods,⁵ nanotubes⁶ and nanosheets⁷), and (ii) resorting to heterostructure modification of g-C₃N₄ such as chemical element doping (e.g. Boron,⁸ Iron⁹ and Sulfur¹⁰) and synergistic component coupling (e.g. TiO₂,¹¹ graphene¹² and Ag₃PO₄ (ref. 13)). First of all, the well-defined nanostructured materials especially allow for enhancing the efficiency of energy conversion by increasing the material surface area and shorting the bulk-to-surface distances. Furthermore, heterostructured modification often improves the photocatalytic activity of photocatalysts, when the recombination of photogenerated electrons and holes is effectively decreased, thus increasing the quantum efficiency.

In this investigation, we demonstrate unique 3-D nanoporous g-C₃N₄ nanosheets endowed with heterostructured modification by the binary phosphorus doping and BiPO₄ coupling, as a new complex catalyst system for visible-light photocatalytic hydrogen production. The novel complex catalyst system is designed on the basis of the following considerations: (i) the 3-D nanoporous structures of nanosheets will provide the maximum accessibility for active species to the catalysts to ensure an effective mass transfer of reactants and/or products.¹⁴ (ii) Phosphorus-doped g-C₃N₄ has showed remarkably enhanced photocurrent since the anticipated change of electronic structure for the g-C₃N₄.¹⁵ (iii) The new type of BiPO₄ oxyacid salt proved a high photocatalytic activity photocatalyst with high position of valence band and high separation efficiency of electron–hole pairs.¹⁶ With these merits, we testify that visible-light photocatalytic hydrogen production with high efficiency can be designed on the basis of this hybrid nanomaterial.

Experimental

Materials synthesis

In a typical hydrothermal synthesis progress, 20 mL amino cyanide (CH₂N₂) solution (50 wt% in H₂O) was mixed with a certain quality of sodium di-hydrogen phosphate (NaH₂PO₄) in the presence of 20 mL diluted nitric acid solution, the mixed solution was ultrasound for 30 min and then transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 483 K for 4 h in an oven. After the autoclave was cooled down to room temperature, the sample was collected and calcined at 823 K for 4 h to obtain the finally phosphorus doping 3-D nanoporous graphene-like C₃N₄ nanosheets (P-C₃N₄). For the synthesis of BiPO₄ coupling 3-D nanoporous g-C₃N₄ nanosheets (P-C₃N₄/BiPO₄), bismuth nitrate (Bi(NO₃)₃·5H₂O) was a forehand dissolved into the diluted nitric acid solution without other procedures changed. An estimate of the BiPO₄ content in the nanoporous P-C₃N₄ nanosheets were obtained by changing the amounts of NaH₂PO₄ and Bi(NO₃)₃·5H₂O. For example, the P-C₃N₄/BiPO₄-1, indicated that 1 wt% BiPO₄ has been coupled with P-C₃N₄ nanosheets.

Materials characterization

The surface morphologies of the as-prepared samples were observed by thermal field emission environment scanning electron microscopy (FE-SEM, FEI, Quanta 400), and transmission electron microscopy (TEM, JEM-2010HR). The compositions and structures of the products were analyzed by X-ray diffraction (XRD, D8 ADVANCE X-ray diffractometer, CuKα radiation l = 0.15418 nm) with a scanning rate of 10° min⁻¹ in the 2-theta range from 10° to 70°. A Shimadzu spectrophotometer (model 2501 PC) equipped with an integrating sphere was used to record the UV-vis diffuse reflectance spectra of the samples. The X-ray photoelectron spectroscopy (XPS) was performed with a VG ESCALAB250 surface analysis system using a monochromatized Al Kα X-ray source (300 W, 5 mA, and 15 kV). The BET-specific surface areas were measured by N₂ adsorption at liquid N₂ temperature in an ASAP 2010 analyser.

Photocatalytic experiments

Photocatalytic water splitting was carried out in a LabSolar H₂ photocatalytic hydrogen evolution system (Perfectlight, Beijing) including a 300 W Xe lamp (PLS-SXE300, Beijing Trusttech). In a typical reaction, 50 mg of the as-prepared samples was dispersed in a Pyrex glass reactor containing 100 mL of Na₂S (0.1 M) solution. Then the system was sealed and vacuumized to keep the pressure as 0.1 MPa. Afterwards, a circular cooling water system was turned on and the reactor was irradiated with Xe lamp (300 W) under magnetic stirring. The gases evolved were analyzed on line with a gas chromatograph (GC-7900, TCD, with N₂ as carrier gas) after 1 h of illumination. The reaction was continued for 5 h.

Results and discussion

The XPS result gives preliminary evidence of a P heteroatom in the modified g-C₃N₄ nanosheets as well as the degree of doping (Fig. 1), which is similar to that of previously reported P-doped carbon nitride solid (P possibly replace C position).¹⁵ The averaged content of phosphate was estimated to be about 0, 0.85 and 1.7% with the increase of P-doped samples from pure g-C₃N₄ to P-C₃N₄ and P-C₃N₄/BiPO₄. Particularly, when it comes to P-C₃N₄/BiPO₄ (3 wt%), the binding energy exhibits a positive one as compared to the P-C₃N₄ (131.6 vs. 131.5 eV), these results show that the interaction between BiPO₄, P and g-C₃N₄ are not simply physical adsorption.^16,17 The XPS result indicates that the C/N ratio of pure-C₃N₄, P-C₃N₄ and P-C₃N₄/BiPO₄ (3 wt%) is 0.782, 0.781 and 0.778, respectively, which is close to the expected value (0.750) of the theoretically predicted C₃N₄ empirical stoichiometry. This result further suggests that P possibly replace C position in the heterostructured modified g-C₃N₄ samples as reported by literature.¹⁵

Fig. 1 (A) XPS spectrum of phosphors recorded from pristine surface of pure-C₃N₄, P-C₃N₄ and P-C₃N₄/BiPO₄ 3 wt%, and (B) molecule structure model of P-C₃N₄ constitution unit.

The microstructures of P-C₃N₄ nanosheets were first investigated by TEM. The TEM image reveals that the spatially 3-D P-C₃N₄ were stacked up by 2-D nanosheets layer by layer in three or more layers (Fig. 2(A)). From the edges of P-C₃N₄ nanosheets, it is clearly to see the smooth surface, mesoporous structure and a rolling up edges, which clearly showed that the thickness of single P-C₃N₄ layer was about 30 nm (Fig. 2(B)). The SEM images of phosphorus doped graphene like carbon nitride samples were further presented in Fig. 3. It can be seen that the P-C₃N₄ are pages-like overlapped by means of larger integrated nanosheets and well associated assembled by smaller fractured nanosheets from a front (Fig. 3(A)) and aside view (Fig. 3(B)), respectively. The flat surface and porous structure of nanosheets were more clearly identified from the high-magnification SEM images (Fig. 3(C) and (D)). Specially, it is interesting that the space gaps between layers hinting at the confined sites may gave chance to those nano-sized species, such as nanorods and nanoparticles, grow in or out directly. We can grandly call these space gaps as “growth sits”. Furthermore, nitrogen adsorption–desorption measurement (see Fig. S1†) show that the BET surface areas and total pore volumes of the porous g-C₃N₄ nanosheets were 45.72 m² g⁻¹ and 0.183 cm³ g⁻¹, respectively. The pore size distribution (inset of Fig. S1†) indicates that the sample has a satisfactory nanoporous texture (10–150 nm).

Fig. 2 Typical TEM images of P-C₃N₄ nanosheets in different magnification.

Fig. 3 Typical SEM image of the P-C₃N₄ nanosheets from different viewpoints. (A) Front and (B) side, (C and D) high-magnification image from (A) and (B).

Fig. 4 shows the contrastive TEM images of P-C₃N₄ and P-C₃N₄/BiPO₄ (3 wt%) samples. The TEM images of P-C₃N₄ clearly show a hierarchical and inter-connected 3-D nanoporous architecture and the pore size is in the range from 10 to 100 nm (Fig. 4(A) and (B)). On the other hand, well-defined and fairly long nanorods, whose average diameter is about 200 nm and whose surface is distinctly enwrapped with smooth, flayers and gauze-like P-C₃N₄ nanosheets, could be obtained in the cases of P-C₃N₄/BiPO₄ nanocomposites (Fig. 4(C)). The results well indicated that BiPO₄ nanorods were distributed into P-C₃N₄ nanosheets in the composite samples, there are no apparent aggregation of BiPO₄ which means the formation of interfaces between BiPO₄ and P-C₃N₄. It is also observed that the P-C₃N₄ still has the mesoporous nanosheets structure after coupling with BiPO₄ nanorods. Fig. 4(D) shows the HRTEM images of the interfaces between BiPO₄ nanorods and P-C₃N₄ nanosheets. From the HRTEM images we can infer: (i) on the basis of the lattice spacing of 0.303 nm was correspond to the (200) planes of BiPO₄ (shown in Fig. S2†), indicating that the nanorods in Fig. 4(C) were exactly BiPO₄ nanorods. (ii) The existence of some BiPO₄ nanorods were between P-C₃N₄ layers, apparently, the bright portions (in the yellow line) were attributed to the pore of P-C₃N₄ nanosheets and the dark parts on BiPO₄ nanorods were refer to the existence of P-C₃N₄, all these demonstrate that there are at least one layer of P-C₃N₄ nanosheet covered on the surface of BiPO₄ nanorods. (iii) The natural existed of “growth sits” on 3-D porous nanosheets architectural structure, which may efficient avoid strongly aggregation of BiPO₄ nanorods during the synthesised process. The TEM image of comparing experiment pure BiPO₄ nanorods was showed in Fig. S3,† whose length (about 600 nm) is much shorter than that in P-C₃N₄/BiPO₄ (about 2000 nm). Taken together, a novel sandwich-like heterocrystals have been obtained by a simple one-step hydrothermal synthesis progress. In addition, as the lattice and energy level between C₃N₄ and BiPO₄ is match well, the heterojunctions could promote the transfer and separation of photoexcited electron–hole pairs under visible light irradiation.

Fig. 4 Contrastive TEM images of the P-C₃N₄ (A and B) and P-C₃N₄/BiPO₄ 3 wt% (C and D) samples.

The XRD pattern of P-C₃N₄ nanosheets and P-C₃N₄/BiPO₄ composited photocatalysts were shown in Fig. 5(A). From XRD pattern, two peaks at around 13° and 27.4° of P-C₃N₄ were corresponding to in-plane structural packing motif of tri-s-triazine which is indexed as (100) peak and interlayer graphitic packing motif of aromatic segments which is indexed as the (002) peak for graphitic materials,¹ respectively. These two feature diffraction peaks of g-C₃N₄ suggest that the original atomic structure is largely retained, which the evidence is denoted as graphitic 3-D P-C₃N₄ with layer by layer structure. The P-C₃N₄/BiPO₄ samples with different proportion (defined as BiPO₄ : P-C₃N₄ wt% discriminated by the mass of initial reaction precursors) were synthesized. The diffraction peaks of P-C₃N₄/BiPO₄ compounds clearly revealed the cubic phase of BiPO₄ (JCPDS no: 15-0766) and crystalline P-C₃N₄ peak. All the P-C₃N₄/BiPO₄ samples exhibit diffraction peaks corresponding to both g-C₃N₄ and BiPO₄, and no other impure peaks can be observed, suggesting a two-phase composition of g-C₃N₄ and BiPO₄ in these composites. Combining with TEM analysis the two-phase materials connected so closely that they advantageously formed heterojunction structure which is proved that could decrease the recombination rate of photogenerated electrons and holes, and contribute to promote the utilization efficiency of photo-generated electrons.¹⁸ As we all know, the improvement in mobility of the charge is important to photocatalysts. Fig. 5(B) shows the result of UV-visible diffuse reflectance (DRS) spectra of P-C₃N₄ and P-C₃N₄/BiPO₄ composited photocatalysts. It has been reported that the absorption edge of the pure BiPO₄ occurred at about 330 nm.^19,20 However, when BiPO₄ loaded on the inter-surface of 3-D P-C₃N₄ nanosheets, the ability of light absorption was enhanced in all of the wavelength range from 200 to 700 nm. Interestingly, it is also found that the absorption of the P-C₃N₄/BiPO₄ composites increased upon the addition of BiPO₄ nanorods. These results may be attributed to two major reasons. First of all, the interaction of heterojunction between BiPO₄ and P-C₃N₄ effectively accelerates the separation of electron–hole pairs account for the band-gap transition of photo-generated electrons, and then enhanced the absorption in the visible-light region. Secondly, as a colour change of the composites samples, which become darker, that is, from pale yellow to grey, along with a higher amount of BiPO₄ was introduced into the 3-D P-C₃N₄ nanosheets would also cause the absorbance enhanced.

Fig. 5 (A) XRD pattern and (B) UV-vis diffuse-reflectance spectra of all the synthesised samples. a: Pure mpg P-C₃N₄, b: P-C₃N₄/BiPO₄-1, c: P-C₃N₄/BiPO₄-3, d: P-C₃N₄/BiPO₄-5, e: P-C₃N₄/BiPO₄-7.

It has been investigated that BiPO₄ possesses a wide band gap which limits its photo-absorption to only the UV region, normally accounting for ca. 5% of the total sunlight, which is undoubtedly important to develop photocatalytic materials that harvest a wide range of visible photons.²⁰ In addition, there are few research has applied bismuth phosphate as a photocatalyst using in water splitting for producing hydrogen. In this study, the BiPO₄ nanorods were successfully growing in the inter space of 3-D mesoporous P-C₃N₄ nanosheets, and the novel 3-D structure composited photocatalysts were first detected the performance of visible light photocatalytic activity in direct water splitting into H₂.

Fig. 6(A) shows a standard curve obtained by plotting the peak area, which was detected by a Labsolar H₂ photocatalytic hydrogen evolution system (Perfect light, Beijing) with gas chromatography (GC-14C, Shimadzu, Japan, TCD), against the known number of hydrogen counts, such as 0, 100, 200, 300, 400, 500 μmol per tube. The line has a slope of 9.015 × 10⁻⁴ and corresponding coefficient of determination R² of 0.998. As the curve was passing through the origin point, the H₂ content can be calculated as the following standard equation:

Y_H2 (μmol) = 0 + bX = 9.015 × 10⁻⁴X (peak area)

here, we take P-C₃N₄/BiPO₄ (3 wt%) as a concrete case to experiment the hydrogen gas producing by irradiating with visible light during the five hours. Fig. 6(B) gave out the actual hydrogen gas testing chart detected by gas chromatography. The exact location of the detected hydrogen peaks was consistent at time of 1.23 min and the peak areas were gradually increased with the time prolonged. This measured experimental results also demonstrated the stability of the testing system. Fig. 6(C) presents a comparison of the pure 3-D P-C₃N₄ and all the synthesised BiPO₄ coupling samples. As can be seen from this figure, the BiPO₄ content has a significant influence on the photocatalytic activity of P-C₃N₄ and all the samples exhibit very stable activity during the 5 h irradiation. Obviously, the amount of H₂ production over P-C₃N₄/BiPO₄ (3 wt%) was much more than that of the other samples. From Fig. 6(D), it can be seen that the H₂ production rate of P-C₃N₄ sample was 676 μmol h⁻¹ g⁻¹, however, when the content of BiPO₄ was increased to 3 wt%, the H₂ production rate reached the highest value of 1110 μmol h⁻¹ g⁻¹. In this regard, the photocatalytic activity of sample P-C₃N₄/BiPO₄ (3 wt%) exceeds that of pure P-C₃N₄ by a factor of 1.64, and the H₂-production rate is significantly greater than that of most C₃N₄-basis semiconductor photocatalysts. The present results also showing that a suitable loading content of grapheme is crucial for optimizing the photocatalytic activity of P-C₃N₄/BiPO₄ 3-D nanocomposites.

Fig. 6 (A) Standard curve for the quantitative estimation of hydrogen gas; (B) the actual hydrogen gas testing chart detected by gas chromatography; (C) the time course of H₂ evolution under visible irradiation over all the synthesised samples; (D) comparison of the visible-light photocatalytic activity of samples for H₂ production using 10% methanol aqueous solution as a sacrificial reagent and 0.3 wt% Pt as a co-catalyst; light source: Xe lamp (300 W) with an optical filter (λ > 420 nm). a: Pure mpg P-C₃N₄, b: P-C₃N₄/BiPO₄-1, c: P-C₃N₄/BiPO₄-3, d: P-C₃N₄/BiPO₄-5, e: P-C₃N₄/BiPO₄-7, f: pure-BiPO₄.

In corresponding experiment, no hydrogen was detected when BiPO₄ nanorods were used as the photocatalyst with Pt as a cocatalyst under visible light irradiation, suggesting that the bare BiPO₄ without P-C₃N₄ nanosheets is likely not active for photocatalytic H₂ production under the experimental conditions studied (shown in Fig. 6(C) and (D)). On the basis of the above results, a photocatalytic mechanism of the P-C₃N₄/BiPO₄ under visible-light irradiation can be proposed (Fig. 7). Under visible-light irradiation, electrons (e⁻) are excited from the valence band (VB) to the conduction band (CB) and created holes (h⁺) in the VB. Normally, for pure P-C₃N₄, the charges are likely transfer in one of three following ways: (i) injected to Pt nanoparticles located on the P-C₃N₄ nanosheets; (ii) directly onto the surface of P-C₃N₄ nanosheets; (iii) quickly recombine with the h⁺ one the VB. The accumulated e⁻ on the Pt and the CB of P-C₃N₄ can effectively reduce H₂O (or H⁺) to produce H₂, while holes at the VB of P-C₃N₄ accumulated and can react with methanol as a sacrificial reagent. Once BiPO₄ was introduced to the P-C₃N₄ nanosheets, as the reported previously, the highest occupied molecular orbital (HOMO) of g-C₃N₄ was more negative than the conduction band of BiPO₄ (−1.12 V vs. −0.65 V)^3,21 the photo-generated charge on P-C₃N₄ can directly transfer to the CB of BiPO₄ (occurred step (i) or (ii)), eventually the electrons react with the adsorbed H⁺ ions to form H₂, and reducing the probability of electron–hole recombination and resulting in an enhanced photocatalytic activity which driven by band potentials between two semiconductors. Furthermore, the interaction of heterojunction between BiPO₄ and P-C₃N₄ effectively accelerated the charge transfer the two semiconductor would be more spatially smooth, which is fundamental for the improvement of photocatalytic activity. The unique 3-D structure of P-C₃N₄/BiPO₄ allowed the photocatalytic reactions to take place not only on the surface of semiconductor catalysts, but also on the inset space between the layers of P-C₃N₄ nanosheets, effectively increasing the reaction sites.

Fig. 7 Schematic illustration of the charge separation and transformation in the P-C₃N₄ nanosheets/BiPO₄ nanorods system under visible light irradiation.

Conclusions

In this work, we demonstrate unique 3-D nanoporous g-C₃N₄ nanosheets endowed with heterostructured modification by the binary phosphorus doping and BiPO₄ coupling, as a new complex catalyst system for visible-light photocatalytic hydrogen production. Results show that pure BiPO₄ could not split water directly under visible light irradiation. However, comparing the energy levels of C₃N₄ with BiPO₄, it is fortunate to notice that their well-matched overlapping band-structures are quite suitable to construct heterostructures that would bring an effective separation and transfer of photogenerated charges, which would remarkable, enhance the visible-light photocatalytic H₂-production activity of the composited semiconductors. The optimal BiPO₄ content was found to be 3.0 wt%, and the corresponding H₂-production rate was 1110 μmol h⁻¹ g⁻¹. The unique 3-D mesoporous structure of P-C₃N₄ nanosheets effectively increasing the reaction sites to allow the photocatalytic reactions to take place. Those may be an expected opportunity for g-C₃N₄ on element doping and other inorganic semiconductor modification by the novelty one-step hydrothermal synthesis progress.

Acknowledgements

This research was supported by NSF of China (21105030 and 21173088) and the key Academic Program of the 3rd phase “211 Project” of South China Agricultural University. The authors thank the Foundation for High-level Talents in Higher Education of Guangdong Province and Guangdong Natural Science Foundation (S2013040013755), Colleges and Universities in Guangdong Province Science and Technology Innovation Project (2013KJCX0123). Special thanks to Prof. Can Li in State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. This work also partly supported by the State Key Laboratory of Catalysis cooperation project (N-08-08).

Notes and references

1. J. Xu, L. Zhang, R. Shia and Y. Zhu, J. Mater. Chem. A, 2013, 1, 14766–14772.
2. J. Low, S. Cao, J. Yu and S. Wageh, Chem. Commun., 2014 10.1039/c4cc02553a.
3. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
4. S. Yang, W. Zhou, C. Ge, X. Liu, Y. Fang and Z. Li, RSC Adv., 2013, 3, 5631–5638.
5. X.-H. Li, J. Zhang, X. Chen, A. Fischer, A. Thomas, M. Antonietti and X. Wang, Chem. Mater., 2011, 23, 4344–4348.
6. J. Gao, Y. Zhou, Z. Li, S. Yan, N. Wang and Z. Zou, Nanoscale, 2012, 4, 3687–3692.
7. L. Lin, Z. Cong, J. Li, K. Ke, S. Guo and H. Yang, J. Mater. Chem. B, 2014, 2, 1031–1037.
8. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2010, 26, 3894–3901.
9. X. Chen, J. Zhang, X. Fu, M. Antonietti and X. Wang, J. Am. Chem. Soc., 2009, 131, 11658–11659.
10. J. Zhang, J. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti, X. Fu and X. Wang, Energy Environ. Sci., 2011, 4, 675.
11. Y. Zang, L. Li, Y. Xu, Y. Zuo and G. Li, J. Mater. Chem. A, 2014 10.1039/c4ta02082k.
12. K. Dai, L. Lu, Q. Liu, G. Zhu, X. Wei, J. Bai and L. Xuana, Dalton Trans., 2014, 43, 6295–6299.
13. S. Kumar, T. Surendar, A. Baruah and V. Shanke, J. Mater. Chem. A, 2013, 1, 5333–5340.
14. C. Hu, H. Cheng, Y. Zhao, Y. Hu, Y. Liu, L. Dai and L. Qu, Adv. Mater., 2012, 24, 5493–5498.
15. Y. Zhang, T. Mori, J. Ye and M. Antonietti, J. Am. Chem. Soc., 2010, 132, 6294–6295.
16. C. Pan, J. Xu, Y. Wang, D. Li and Y. Zhu, Adv. Funct. Mater., 2012, 22, 1518–1524.
17. A. K.-V. Alexander, V. Naumkin, S. W. Gaarenstroom and C. J. Powell, The U.S. Secretary of Commerce on behalf of the United States of America, 2012, http://srdata.nist.gov/xps/.
18. L. Zhang, H. Cheng, R. Zong and Y. Zhu, J. Phys. Chem. C, 2009, 113, 2368–2374.
19. C. Pan, D. Li, X. Ma, Y. Chen and Y. Zhu, Catal. Sci. Technol., 2011, 1, 1399.
20. Z. Li, S. Yang, J. Zhou, D. Li, X. Zhou, C. Ge and Y. Fang, Chem. Eng. J., 2014, 241, 344–351.
21. C. Pan and Y. Zhu, Environ. Sci. Technol., 2010, 44, 5570–5574.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10038g

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