Whole-Visible-Light Absorption of a Mixed-Valence Silver Vanadate

Article pubs.acs.org/IC

Whole-Visible-Light Absorption of a Mixed-Valence Silver Vanadate Semiconductor Stemming from an Assistant Effect of d−d Transition Hongjun Dong,†,‡ Gang Chen,*,† Jingxue Sun,*,† Chunmei Li,† Yidong Hu,† and Zhonghui Han† †

Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

‡

S Supporting Information *

ABSTRACT: Wide-light absorption is important to semiconductors exploited in many applications such as photocatalysts, photovoltaic devices, and lightemitting diodes, which can effectively improve solar energy utilization. Especially for photocatalysts, the development and design of new semiconductors that harvest the whole-visible-light region (λ = 400−800 nm) is rarely reported, which is still a tremendous challenge up to now. Here we realize whole-visiblelight absorption up to 900 nm for a semiconductor by means of construction of a mixed-valence Ag0.68V2O5, which results from an assistant effect of d−d transition. Ag0.68V2O5 serving as a photocatalyst obviously exhibits photoelectrochemical and photocatalytic properties. Our results provide a brand-new feasible design strategy to broaden the light absorption of semiconductors and highlight a route to further make the best use of the full solar spectrum.

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INTRODUCTION The technology of semiconductor-based photocatalytic hydrogen production and organic pollutant degradation using solar energy has been considered as one of the most important approaches to solving the world energy and environment crisis.1−5 Upon the discovery of the photoelectrocatalytic property on the TiO2 electrode under ultraviolet (UV) light in 1972,6 30 years of intensive research into widening light absorption was launched until the photocatalyst with visiblelight response was found in 2001.7 In recent years, plenty of researches concentrated mainly on the exploitation of semiconductor photocatalysts responding to visible light (∼43%) owing to the lower proportional UV light (∼8.7%) in the solar spectrum, e.g., black TiO2.8,9 Currently, a variety of methods are applied to broaden the light absorption of semiconductors, such as noble-metal loading,10 construction of a heterojunction,11 and introduction of an impurity12,13 or a defect level.14,15 However, these methods usually only extend light absorption within a small range, in which it is difficult to realize the harvesting of whole visible light, accounting for the greater proportion in the solar spectrum. Therefore, it will be a very significant milestone to develop a feasible design strategy to realize light absorption of a semiconductor covered with the whole-visible-light region in order to take full advantage of solar light. It is well-known that the light absorption of a semiconductor is essentially determined by the electronic structure, which has roots in the electronic transition from the valence band (VB) to the conduction band (CB) based on the traditional energyband theory. As a consequence, adjustment of the energy-band structure should be one of the best choices to improve essentially the light absorption ability of a semiconductor. Ag+ © 2015 American Chemical Society

ion is an effective electronic structure regulator, the hybridized and adjusted effects of which have been investigated in most single-valence silver-based oxide semiconductor photocatalysts, such as Ag2CO3,16 Ag2Nb4O11,17 Ag3PO4,18 Ag2Si2O7,19 AgTaO3,20 Ag2V4O11,21 and so on. We know that V2O5 is a typical layered oxide, in which intercalation reactions easily take place.22 Therefore, here by introducting limited Ag+ ions into V2O5 to construct a mixed-valence silver vanadate Ag0.68V2O5, we expect to achieve a semiconductor with a unique electronic structure. An interesting and desired result is that Ag0.68V2O5 exhibits a broad intense light absorption covering the wholevisible-light region and reaching 900 nm. In addition, a Ag0.68V2O5 semiconductor can serve as a photoelectrode material and a photocatalyst, which show an intense photocurrent response and methylene blue (MB) removal ability under visible light, respectively. We gain insight into the light absorption mechanism resulting from an assistant effect of d−d transition by means of combining the energy-band and crystalfield theories.

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EXPERIMENTAL SECTION

Preparation. Ag0.68V2O5 was synthesized using a typical hydrothermal reaction method. A total of 0.170 g of AgNO3, 0.182 g of V2O5, and 0.1 g of poly(vinylpyrrolidone) (Mr ≈ 1300000) were weighed out and added to a beaker containing distilled water (25 mL) under vigorous magnetic stirring for 30 min at room temperature. Then the suspension was transferred to a 30 mL Teflon-lined stainless steel autoclave (filling volume of 80%) to perform a 12 h hydrothermal reaction at 140 °C. After being cooled to room temperature, the Received: August 30, 2015 Published: November 25, 2015 11826

DOI: 10.1021/acs.inorgchem.5b01976 Inorg. Chem. 2015, 54, 11826−11830

Article

Inorganic Chemistry precipitate was collected by centrifugation, washed with distilled water and ethanol several times in turn, and then dried for 4 h at 60 °C. The final dark-cyan product was obtained. Characterization. The phase of the Ag0.68V2O5 sample was characterized by powder X-ray diffraction (XRD; Rigaku D/max-2000) with Cu Kα radiation at a scanning rate of 5°/min in the 2θ range of 10−90°. The morphology of the sample was characterized utilizing field-emission scanning electron microscopy (FESEM; MX2600FESEM) and transmission electron microscopy (TEM; FEI, Tecnai G2 S-Twin). X-ray photoelectron spectroscopy (XPS) analysis was measured on an American electronics physical HI5700ESCA system with an X-ray photoelectron spectroscope using Al Kα (1486.6 eV) monochromatic X-ray radiation. The peak positions were corrected against the C 1s peak (284.6 eV) of contaminated carbon. UV−vis diffuse-reflectance spectroscopy (DRS) of the sample was recorded on a UV−vis spectrophotometer (PG, TU-1901) at room temperature with BaSO4 as the background at 200−900 nm. Theoretical Calculations. Theoretical calculations are performed based on ab initio density functional theory (DFT). Exchangecorrelation effects were taken into account by using the generalized gradient approximation function of Perdew, Burke, and Ernzerhof. The band structure and density of states (DOS) calculations were performed using the CASTEP code program package, which utilized pseudopotentials to describe electron−ion interactions and represented electronic wave functions using a plane-wave basis set. The kinetic energy cutoff with 340.0 eV, ultrasoft pseudopotential, convergence tolerance with 1.0 × 10−6 eV/atom, and band energy tolerance with 1.0 × 10−5 eV were adopted in the calculations. The Monkhorst−Pack k points were sampled at 7 × 7 × 3 and 2 × 6 × 7 for Ag0.68V2O5 and V2O5, respectively. Photoelectrochemical and Photocatalytic Measurements. The photoelectrochemical characteristics were measured in a CHI604C electrochemical work station using a standard threecompartment cell under UV−vis light. Catalyst-coated FTO glass, a piece of platinum sheet, an Ag/AgCl electrode, and 0.05 M sodium sulfate were used as the working electrode, counter electrode, reference electrode, and electrolyte, respectively. The degradation experiment was carried out with a 0.05 g sample suspended in the MB dye solutions (10 mg/L, 100 mL) in a quartz photochemical reactor. Then it was magnetically stirred for 30 min in the dark to reach adsorption−desorption equilibrium between the dyes and catalyst. The above suspension was exposed to light irradiation provided by a 300 W xenon arc lamp under magnetic stirring (λ > 400 nm). A UV− vis spectrophotometer (PG, TU-1901) monitored the absorbances of the dye solutions at intervals of 1 h. Before measurement, the catalyst was removed from the reactor by centrifugation.

Figure 1b, the V element valence states in the Ag0.68V2O5 sample are identified by XPS. It is worth noting that the XPS spectrum of V exhibits two asymmetric peaks, which implies that V has two different valence states. These two peaks can be further divided into two group peaks, respectively. The peaks at 516.8 and 524.0 eV derive from the V5+ 2p3/2 and V5+ 2p1/2 states in the Ag0.68V2O5 lattice,22 respectively. The peaks at 515.7 and 522.9 eV are ascribed to the V4+ 2p3/2 and V4+ 2p1/2 states,22 respectively. The results suggest that lattice V ions possess both V5+ and V4+ states coexisting in the Ag0.68V2O5 sample. The micromorphology of the Ag0.68V2O5 sample is revealed by FESEM in Figure 2a. It shows that typical two-dimensional

Figure 2. FESEM (a and b), TEM (c), and HRTEM (d) images and the FFT pattern (the inset of part d) of Ag0.68V2O5.

Ag0.68V2O5 nanosheets are obtained without any other shape. Figure 2b further displays that the thickness of a single curly nanosheet is ∼20 nm, which can shorten the migration path of charge carriers perpendicular to the nanosheet direction in favor of enhancing the photocatalytic and photoelectrochemical activity. Furthermore, the TEM image in Figure 2c further demonstrates the formation of this ultrathin nanosheet. Highresolution TEM (HRTEM) of an individual nanosheet is also performed to identify the lattice parameters of the Ag0.68V2O5 sample (Figure 2d). The interplanar spacing d is 0.208 nm, which coincides well with the (−403) lattice plane in the Ag0.68V2O5 crystal. The corresponding fast Fourier transform (FFT) pattern (the inset of Figure 2d) exhibits symmetrical diffraction spots, which indicates the monocrystal characteristic of a single Ag0.68V2O5 nanosheet. The UV−vis DRS spectra of Ag0.68V2O5 and V2O5 as a comparison are measured to explore variations of the light absorption properties (Figure 3a). First of all, we identify that strong absorptions result from the energy-gap transition rather than the impurity level owing to their steep absorption edge.16 Typical single-valence silver vanadates such as Ag2V4O11,21 AgVO3,23 and Ag3VO424 extend merely their absorption edge to ∼600 nm (the band gap is ∼2.0 eV) and slightly increase the light harvest range in the solar spectrum compared with V2O5 (the inset of Figure 3a), which is mainly attributed to the hybridized effect of Ag+ ions because they all have single V5+ ions with a 3d0 electronic configuration. In contrast, it is interesting that the absorption range of Ag0.68V2O5 shifts from ∼560 nm of V2O5 to ∼900 nm of, covering the whole-visiblelight region. Moreover, it displays clearly that coverage of the light harvest for Ag0.68V2O5 almost reaches ∼60% in the solar

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RESULTS AND DISCUSSION Figure 1a presents the XRD pattern of the as-prepared Ag0.68V2O5 sample. All of the diffraction peaks can be wellindexed as a monoclinic Ag0.68V2O5 crystal (JCPDS card no. 74-2407), and no diffraction peaks from the impurities are detected, which indicates that the pure crystalline phase Ag0.68V2O5 product is obtained. In addition, as shown in

Figure 1. (a) XRD pattern of Ag0.68V2O5 and (b) XPS spectrum of V in Ag0.68V2O5. 11827

DOI: 10.1021/acs.inorgchem.5b01976 Inorg. Chem. 2015, 54, 11826−11830

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Inorganic Chemistry

investigate the charge-carrier-transfer behaviors. As shown in Figure 3c, when the monochromatic wavelength light (λ = 700 ± 15 nm) is first used to excite the Ag0.68V2O5 film electrode, no photocurrent response is observed at the present conditions. When light with a monochromatic wavelength (λ = 420 ± 15 nm) is applied to irradiate the Ag0.68V2O5 film electrode, it brings about an obvious photocurrent response. However, the Ag0.68V2O5 film electrode interestingly exhibits a more intense photocurrent response when it is exposed to light irradiation of λ > 400 nm, indicating that photogenerated electron−hole pairs can effectively be separated under whole-visible-light irradiation. All of the above imply that Ag0.68V2O5 may have a special electron-transfer characteristic. Furthermore, the Ag0.68V2O5 sample can also be applied to serve as a photocatalyst to degrade MB dye under visible-light irradiation (λ > 400 nm) and exhibits improved degradation activity compared with V2O5 (Figures S1 and S2). In addition, the cycle degradation kinetic curves of MB solutions in Figure 3d show that Ag0.68V2O5 has a good stability and recyclability. In order to reveal the unique light absorption and the photocurrent and photocatalysis properties presented by the Ag0.68V2O5 sample, the electronic transfer mechanism is investigated in detail based on ab initio DFT and crystal-field theory. Electronic band structure calculations are performed to understand the electronic structure and nature of the band edge wave functions so as to achieve the expected designed interesting electronic properties (see the Supporting Information). As shown in Figure 4, the energy-band distributions of Ag0.68V2O5 and V2O5 confirm their essential indirect transition feature. The CB (3.17 eV) and VB (5.41 eV) distribution widths of Ag0.68V2O5 are wider than those of V2O5 (2.89 and 4.96 eV), which suggests that the former has high delocalization in favor of electron−hole transfer and separation. In addition, the calculated band-gap value (1.53 eV) is evidently decreasing compared with that of V2O5 (1.77 eV). These changes are predictable results that are similar to those most reported for single-valence silver-based photocatalysts,16−21 which mainly

Figure 3. (a) UV−vis DRS spectra of Ag0.68V2O5 and V2O5. The insets are the absorption edge positions of V2O5 (i), typical single-valence silver vanadates (ii), and Ag0.68V2O5 (iii) in the solar spectrum. (b) Plots of (αhν)1/2 versus hν of Ag0.68V2O5 and V2O5. (c) Photocurrent responses of the Ag0.68V2O5 film electrode at different light irradiation. (d) Circle degradation kinetic curves of MB solutions under wholevisible-light irradiation (λ > 400 nm).

spectrum, which is about 2 times more than that of V2O5 and typical single-valence silver vanadates (the inset of Figure 3a). Furthermore, according to the typical calculation method of a crystal semiconductor, the energy gaps of Ag0.68V2O5 and V2O5 are estimated according to their indirect transition feature, as revealed by the following theory calculation results. As shown in Figure 3b, using plots of (αhν)1/2 versus hν, the experimental energy gaps of Ag0.68V2O5 and V2O5 are estimated to be 1.35 and 2.21 eV, respectively. The photoelectrochemical property of Ag0.68V2O5 fabricated into the film electrode is evaluated by the photocurrent response under different light irradiation conditions to

Figure 4. Energy-band diagrams, DOS distribution, and energy-level configuration of V4+ and V5+ ions in the octahedral crystal field of Ag0.68V2O5 (a) and V2O5 (b). 11828

DOI: 10.1021/acs.inorgchem.5b01976 Inorg. Chem. 2015, 54, 11826−11830

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Inorganic Chemistry derive from the hybridization effect of Ag+ ions. However, different from the single-valence silver vanadates such as Ag2V4O11,21 AgVO3,25 and Ag3VO4,26 it is worth noting that the Fermi level of Ag0.68V2O5 exhibits an obvious upshift across the CB, as well as a calculated energy difference between the Fermi level in the CB, and the higher unoccupied band (HUB) above the CB is only 0.35 eV, which implies that it may produce the electronic transition from CB to HUB to result in light absorption under only low-light energy excitation. Furthermore, the DOS in Figure 4a shows that the CB of Ag0.68V2O5 is mainly composed of V 3d orbital, which is the same as that of V2O5 (Figure 4b). By contrast, the VB of Ag0.68V2O5 is mainly composed of the hybridized Ag 4d and O 2p orbitals. Introducing Ag+ ions into V2O5 can lead to the shifting up of the VB top of Ag0.68V2O5 compared with that of V2O5 composed of only the O 2p orbital, which results in narrowing of the band gap to improve the light absorption ability. This hybridization effect of Ag+ ions has been certified in plenty of reported single-valence silver-based photocatalysts.16−21 However, what is noteworthy is that, unlike V2O5, the experimental value (1.35 eV) of Ag0.68V2O5 obtained by the UV−vis DRS spectrum is unusually smaller than the band-gap energy (1.53 eV) predicted by theoretical calculations, which goes against the typical underestimation resulting from DFT calculations, especially for semiconductors. Therefore, it seems that light absorption is the most probable transition from the Fermi level to HUB and not merely that from the VB to CB because the Fermi level passes through the CB. As shown in the UV−vis DRS spectrum in Figure 3b, Ag0.68V2O5 evidently produces two overlapped absorption bands on either side of the dividing line at ∼550 nm. In addition, according to theoretical band-gap variation from V2O 5 to Ag0.68 V2O 5, we can approximately estimate that the actual band-gap value of Ag0.68V2O5 is ∼1.97 eV (see the Supporting Information), which is in accordance with ∼2.0 eV of most single-valence silver vanadates.21,25,26 It is not difficult to understand the above unique properties if we combine the energy-band structure feature with the energy level transition in the crystal-field theory. In the V2O5 crystal structure, V5+ ions are located in an octahedral environment of O2− ions (Figure 4b). After the introduction of limited Ag+ ions into V2O5 to form Ag0.68V2O5, the VO6 octahedra are not changed, but the some of center V5+ ions are transformed into V4+ ions (Figure 4a). VO6 octahedra can lead to strong crystalfield splitting. According to crystal-field theory, the V 3d orbitals split into doubly degenerate eg states containing highenergy dx2−y2 and dz2 orbitals as well as triply degenerate lowenergy t2g states containing dxy, dyz, and dxz orbitals. As for V2O5 and other single-valence silver vanadates, the V5+ 3d0 states are unoccupied because the t2g and eg states are located largely above the Fermi level, which make up the CB and HUB, respectively. Thus, under suitable light irradiation, the V2O5 sample only produces electron transfer from the VB (O 2p state) to CB (t2g state). In contrast, as far as Ag0.68V2O5 is concerned, the V5+ 3d0 state will be converted to the V4+ 3d1 state due to limited Ag+ ion introduction. The triply degenerate t2g state is occupied by one electron, which directly results in the Fermi level passing through the CB. Therefore, the energygap values of 1.35 and 1.97 eV obtained by the UV−vis DRS spectrum of the Ag0.68V2O5 sample should originate from electron transition from the CB (t2g state) to HUB (eg state)

and from the VB (hybridized Ag 4d and O 2p state) to CB (t2g state) under suitable light irridiation, respectively. The photoelectrochemical and photocatalytic mechanisms of Ag0.68V2O5 are unambiguous based on the above discussions. When the monochromatic wavelength light (λ = 700 ± 15 nm) is first used to illuminate the Ag0.68V2O5 film electrode, only electrons in the CB (t2g state) are excited into the HUB (eg state) owing to an energy band gap of 1.35 eV among them. However, it is reported that electrons in the CB below the Fermi level have higher mobility and conductivity similar to free electrons,27,28 so it is difficult to separate the photoexcited electrons and holes. Thus, the photogenerated electron−hole pairs stemming from an interior d−d (t2g−eg) transition in the CB immediately recombine as soon as they are created, resulting in no photocurrent response generation. In contrast, when the monochromatic wavelength light (λ = 420 ± 15 nm) is further applied to irradiate the Ag0.68V2O5 film electrode, the electrons in the VB (hybridized Ag 4d and O 2p states) are excited in addition to those in the CB (t2g state) because of the narrow band gap of 1.97 eV between the VB (hybridized Ag 4d and O 2p states) and the CB (t2g state). The electrons in the CB (t2g state) are quickly excited into the HUB (eg state), and the holes are created simultaneously in the CB (t2g state). Meanwhile, the electrons in the VB (hybridized Ag 4d and O 2p states) are also immediately excited into the CB (t2g state), which can in time combine with the holes created in the CB (t2g state). The final results are that the electrons and holes are generated in the HUB (eg state) and VB (hybridized Ag 4d and O 2p states), respectively, thus bringing about the effective separation of electrons and holes. Therefore, an obvious photocurrent response is produced on the Ag0.68V2O5 film electrode under monochromatic light irradiation (λ = 420 ± 15 nm). Here we have to point out that d−d electron transition resulting from the CB (t2g state) to HUB (eg state) actually plays an important assistant effect for improving the separated efficiency of photogenerated electron−hole pairs. Furthermore, when whole-visible-light irradiation (λ > 400 nm) is used, this assistant effect becomes more distinct, as a result producing a more intense photocurrent response. In summary, Figure 5

Figure 5. Photocatalytic and photoelectrochemical mechanisms of Ag0.68V2O5 under whole-visible-light irradiation (λ > 400 nm).

shows the electron-transfer behavior as well as the photocatalytic and photoelectrochemical reaction mechanisms of Ag0.68V2O5 under whole visible light. As for the photocatalytic process, the oxidation and reduction reactions take place at the VB and HUB of Ag0.68V2O5 in line with step 1, respectively. Correspondingly, the oxidation and reduction reactions at the photoelectrochemical process occur in the VB of Ag0.68V2O5 11829

DOI: 10.1021/acs.inorgchem.5b01976 Inorg. Chem. 2015, 54, 11826−11830

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Inorganic Chemistry

(11) Xiang, Q.; Yu, J.; Jaroniec, M. J. Am. Chem. Soc. 2012, 134, 6575−6578. (12) Chen, X.; Burda, C. J. Am. Chem. Soc. 2008, 130, 5018−5019. (13) Liu, B.; Chen, H. M.; Liu, C.; Andrews, S. C.; Hahn, C.; Yang, P. J. Am. Chem. Soc. 2013, 135, 9995−9998. (14) Guan, M.; Xiao, C.; Zhang, J.; Fan, S.; An, R.; Cheng, Q.; Xie, J.; Zhou, M.; Ye, B.; Xie, Y. J. Am. Chem. Soc. 2013, 135, 10411−10417. (15) Kong, M.; Li, Y.; Chen, X.; Tian, T.; Fang, P.; Zheng, F.; Zhao, X. J. Am. Chem. Soc. 2011, 133, 16414−16417. (16) Dong, H. J.; Chen, G.; Sun, J. X.; Li, C. M.; Yu, Y. G.; Chen, D. H. Appl. Catal., B 2013, 134−135, 46−54. (17) Dong, H.; Chen, G.; Sun, J.; Feng, Y.; Li, C.; Lv, C. Chem. Commun. 2014, 50, 6596−6599. (18) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. Nat. Mater. 2010, 9, 559−563. (19) Lou, Z.; Huang, B.; Wang, Z.; Ma, X.; Zhang, R.; Zhang, X.; Qin, X.; Dai, Y.; Whangbo, M. H. Chem. Mater. 2014, 26, 3873−3875. (20) Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 12441−12447. (21) Shi, H.; Li, Z.; Kou, J.; Ye, J.; Zou, Z. J. Phys. Chem. C 2011, 115, 145−151. (22) Xu, K.; Hu, S.; Wu, C.; Lin, C.; Lu, X.; Peng, L.; Yang, J.; Xie, Y. J. Mater. Chem. 2012, 22, 18214−18220. (23) Konta, R.; Kato, H.; Kobayashi, H.; Kudo, A. Phys. Chem. Chem. Phys. 2003, 5, 3061−3065. (24) Dolgos, M. R.; Paraskos, A. M.; Stoltzfus, M. W.; Yarnell, S. C.; Woodward, P. M. J. Solid State Chem. 2009, 182, 1964−1971. (25) Sang, Y.; Kuai, L.; Chen, C.; Fang, Z.; Geng, B. ACS Appl. Mater. Interfaces 2014, 6, 5061−5068. (26) Li, D.; Duan, X.; Qin, Q.; Fan, H.; Zheng, W. CrystEngComm 2013, 15, 8933−8936. (27) Shein, I. R.; Kozhevnikov, V. L.; Ivanovskii, A. Solid State Sci. 2008, 10, 217−225. (28) Xu, X.; Randorn, C.; Efstathiou, P.; Irvine, J. T. S. Nat. Mater. 2012, 11, 595−598.

and platinum sheet as a counter electrode in accordance with step 2, respectively.

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CONCLUSIONS In summary, we obtained a new mixed-valence Ag0.68V2O5 semiconductor, which exhibits wide-light absorption covering the whole-visible-light region and reaching ∼900 nm. Its unique light absorption, and photoelectrochemical and photocatalytic properties originate from an assistant effect of the interior d−d transition (t2g−eg state). Our results provide a brand-new feasible design strategy to broaden light absorption of semiconductors, which may be extended to obtain other semiconductors that can harvest the broader light region. The work highlights a route to further make the best use of the full solar spectrum.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01976. Additional calculation of the band gap of a semiconductor and other extensive details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+86)-451-86413753. *E-mail: [email protected]. Author Contributions

The authors declare no competing financial interest. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by projects of Natural Science Foundation of China (Grants 21271055 and 21471040) and the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. A. 201410). We acknowledge support by the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Grant QAK201304), Program for Innovation Research of Science in Harbin Institute of Technology (PIRS of HIT B201412), and the Foundation Research Project of Jiangsu Province (The Natural Science Fund BK20150536).

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REFERENCES

(1) Luo, L.; Zeng, Y.; Li, L.; Luo, Z.; Smirnova, T. I.; Maggard, P. A. Inorg. Chem. 2015, 54, 7388−7401. (2) Peng, Y.; Shang, L.; Bian, T.; Zhao, Y.; Zhou, C.; Yu, H.; Wu, L. Z.; Tung, C. H.; Zhang, T. Chem. Commun. 2015, 51, 4677−4680. (3) Bian, T.; Shang, L.; Yu, H.; Perez, M. T.; Wu, L. Z.; Tung, C. H.; Nie, Z.; Tang, Z.; Zhang, T. Adv. Mater. 2014, 26, 5613−5618. (4) Yang, L.; Liu, J.; Chang, H.; Tang, S. RSC Adv. 2015, 5, 59970− 59975. (5) Li, C.; Chen, G.; Sun, J.; Feng, Y.; Liu, J.; Dong, H. Appl. Catal., B 2015, 163, 415−423. (6) Fujishima, A.; Honda, K. Nature 1972, 238, 37−38. (7) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625− 627. (8) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746− 749. (9) Hu, Y. H. Angew. Chem., Int. Ed. 2012, 51, 12410−12412. (10) Ingram, D. B.; Linic, S. J. Am. Chem. Soc. 2011, 133, 5202−5205. 11830

DOI: 10.1021/acs.inorgchem.5b01976 Inorg. Chem. 2015, 54, 11826−11830