Research Article pubs.acs.org/journal/ascecg
High-Efficiency Water-Splitting Solar Cells with Low Diffusion Resistance Corresponding to Halochromic Pigments Interfacing with ZrO2 Yi-Sheng Lai, Hsuan-Heng Lu, and Yen-Hsun Su* Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan S Supporting Information *
ABSTRACT: ZrO2 nanoparticle films coated with halochromic pigments are applied to water-splitting solar cells. On the basis of our results, ZrO2 nanoparticle films coated with methyl orange have remarkable water-splitting properties. Under positive applied voltages and AM 1.5G irradiation, the highest hydrogen gas generation rate (1.8 mL/h·cm2) is measured for ZrO2 nanoparticle films coated with methyl orange in 0.2 M H2SO4 water solution as electrolyte. As the electrolyte is changed from KHCO3 water solution to the H2SO4 water solution, the interface resistance between ZrO2 corresponding to halochromic pigments is reduced (from 107 to 11.7 Ω·m) and the electron diffusion coefficient is raised (821.67%). KEYWORDS: ZrO2, Water splitting, Alternating current impedance, Halochromic pigments
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INTRODUCTION To address the concerns of growing energy requirements and environmental pollution, water-splitting solar cells are a highly attractive option.1−5 Generally, solar light is perennial and widely distributed on the earth. Nontoxic and storable solar energy is helpful in reducing pollution and increasing the energy generation for our plant. Hydrogen gas is a high energy density gas used in fuel cells,6−9 and surface water is a main constituent of Earth. Thus, splitting water to generate hydrogen gas is an excellent choice. ZrO2, with an extraordinarily wide band gap, is necessary for preparing ultrahigh performance water-splitting solar cells.8−13 The band bap of ZrO2 is too wide to absorb the solar light to excite the electrons from the valence band to the conduction band. As such, photosensitizers provide a suitable excited energy level to act as a step for the conduction band of ZrO2, which is necessary. Methyl violet, methylene blue, methyl orange, and Congo red are the candidates that have a suitable excited energy level for the conduction band of ZrO2. To generate hydrogen from water splitting, KHCO3 and H2SO4 solutions are generally used as the electrolyte. The halochromic pigments mentioned tune the excited energy level by difference in pH value.14−16 Thus, these halochromic pigments have wide application prospects in water-splitting experiments. According to the water-splitting circuit, the interface resistance between electrode and electrolyte and the electron diffusion coefficient in electrolyte are involved.17−20 Thus, the electrochemical impedance spectroscopy (EIS) and the Nyquist © 2017 American Chemical Society
plots are applied to investigate the determination of the interface resistance and the calculation of diffusion coefficient. The main ideas of this study are the following: (1) The ZrO2 is used as the semiconductor material for hydrogen generation from water splitting. (2) The energy level of halochromic pigments are tuned by the pH value of electrolyte to fit the conduction band of ZrO2. (3) EIS and the Nyquist plots are applied to investigate the electrochemical analysis and thermodynamic calculation.
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EXPERIMENTAL SECTION
Material. ZrO2 nanoparticles and zirconium dichloride oxide (ZrOCl2·8H2O) were purchased from Sigma-Aldrich. The following samples were analytical grade without further purification: Methyl Violet 2B (methyl violet), Methylene Blue (methyl blue), and Methyl Orange (methyl orange), which are purchased from Acros Organic. Congo red was purchased from Tokyo Chemical Industry (TCI). Potassium bicarbonate (KHCO3) and sulfuric acid (H2SO4) were purchased from Showa Chemical Industry. High-performance liquid chromatography (HPLC) grade ethanol and deionized water were used in all the experiments. Fabrication of a Matrixed Electrode of ZrO2 Nanoparticle Films. ZrO2 nanoparticles (0.3 g) are dispersed in 10 mL of ethanol to prepare the suspension solution. The dipping volume of the suspended solution of ZrO2 nanoparticles on the precleaned indium tin oxide (ITO) glass is 1 mL/cm2. The ZrO2 nanoparticle films are dried in air at room temperature. Then, the the ZrO2 nanoparticle films (1 mL/ Received: April 16, 2017 Revised: June 18, 2017 Published: July 5, 2017 7716
DOI: 10.1021/acssuschemeng.7b01154 ACS Sustainable Chem. Eng. 2017, 5, 7716−7722
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Figure 1. Absorption spectra of methyl violet (a), methylene blue (b), methyl orange (c), and Congo red (d) solution (black line, ethanol solution; blue line, 0.4 M KHCO3 solution; and red line, 0.2 M H2SO4 solution). EIS spectra are fitted to an appropriate equivalent circuit by CH Instruments. The thermodynamic diffusion coefficient of the electrons in the electrolyte is discussed. The experimental settings are as follows: frequency from 100 000 to 1 Hz with an amplitude of 0.7 V.
cm2) are dipped into zirconium dichloride oxide ethanol solution (0.5 g of ZrOCl2·8H2O in 10 mL of ethanol). The ZrO2 nanoparticle films are dried in air before drying further at 150 °C for 12 h. The halochromic pigments are dissolved in ethanol to prepare solutions of halochromic pigments with a standard concentration (2 × 10−5 M). ZrO2 nanoparticle films are soak in these halochromic ethanol solutions for 12 h, and the halochromic pigments are coated on the ZrO2 nanoparticle films. Fabrication of Pt Films. Pt are depositing on the precleaned ITO glass by the ionic reaction deposition sputter method. The sputter time is 1000 s and the sputter current is 20 mA. The thickness of the Pt film on ITO is about 20 nm. The conductivity is about 2.79 (S/m). Characterization. The X-ray diffraction (XRD) patterns of ZrO2 nanoparticles are measured and obtained by a Bruker AXS (D8) X-ray diffractometer with Cu Kα radiation. The high-resolution field emission scanning electron microscopy images are collected on a Hitachi 6700F microscope. The UV−vis absorbance of the halochromic pigments solution are measured by a CT-2200 spectrophotometer. The solar light is simulated by AM 1.5G (1000 W/m2). An electrochemical analyzer (CH Instruments 6273E) is combined with AM 1.5G to carry out the hydrogen generation experiments and to obtain the electrochemical impedance spectra (EIS). Hydrogen Generation from Water Splitting. The watersplitting efficiency for hydrogen generation is carried out with ZrO2 nanoparticle films coated with halochromic pigments under AM 1.5G irradiation in electrolyte. KHCO3 water solution (0.4 M) and H2SO4 water solution (0.2 M) served as the electrolyte for water splitting, respectively. The pH value of the 0.4 M KHCO3 water solution is 9.0. The pH value of the 0.2 M H2SO4 water solution is 1.5. The temperature affected the water-splitting experiments. Thus, the temperature is set at room temperature (25 °C). Electrochemical Impedance Spectroscopy Test. AM 1.5G is used as a simulated solar source in the ESI experiment. The measurements are performed at room temperature at 0.7 V. The
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RESULTS AND DISCUSSION UV−Vis Absorbance Spectrum. The absorption spectra of halochromic pigments in different electrolyte solutions are shown in Figure 1. From Figure 1a, in the ethanol and 0.4 M KHCO3 water solution, the methyl violet has a strong absorption at 587 nm. In the 0.2 M H2SO4 water solution, methyl violet has two strong absorptions at 622 and 423 nm. From Figure 1b, the methylene blue, which is dissolved in ethanol solution, demonstrates two strong absorption peaks at 657 and 293 nm. The methylene blue is dissolved in 0.4 M KHCO3 water solution and 0.2 M H2SO4 water solution, and strong absorption peaks are shown at 667 and 293 nm. From Figure 1c, the methyl orange has the strongest absorption peak at 425 nm in ethanol solution. When methyl orange is dissolved in 0.4 M KHCO3 water solution, the strongest absorption peak shifts to 472 nm, and if it is dissolved in 0.2 M H2SO4 water solution, the strongest absorption peak is at 510 nm. From Figure 1d, the Congo red shows a strong absorption peak at 511 nm in ethanol solution. In 0.4 M KHCO3 water solution, Congo red has a strong absorption peak at 486 nm, and in 0.2 M H2SO4 water solution, it demonstrates a wide absorption range between 200 and 800 nm. According to Figure 1, the halochromic pigments demonstrate different absorption peaks in different electrolytes (at the same concentration). Thus, the energy level of the halochromic pigments are tuned by different electrolytes. 7717
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ACS Sustainable Chemistry & Engineering Photocurrent Density and the Efficiency of the Water Splitting Solar Cells. The efficiency of water splitting follows eq 1: η (%) =
Jop (1.23 − |Eop|) Int
× 100%
(1)
Eop is the applied voltage under optimal operating conditions, the photocurrent density at Eop is Jop, and the intensity of the incident solar light (AM 1.5G) is denoted as Int. The theoretical voltage for electrolyzing water into O2 and H2 is 1.23 V. To investigate the water-splitting efficiency of ZrO2 nanoparticle films coated with halochromic pigments, the experiment is conducted under AM 1.5G irradiation in 0.4 M KHCO3 or 0.2 M H2SO4 water solutions. Under positive applied voltages, the halochromic pigments that are coated on ZrO2 nanoparticle films generate the excited electrons by absorbing the solar energy. The excited electrons transport into the conduction band of ZrO2. Then, the electrons inject into the electrolyte for hydrogen gas generation by a cathode of Pt films. Under negative applied voltages conditions, the excited electrons generated from halochromic pigments inject into the electrolyte directly. The excited electrons reduce the H+ ions into hydrogen gas. Under AM 1.5G irradiation and 0.4 M KHCO3 water solution for electrolyte, the photocurrent density and the watersplitting efficiency of ZrO2 nanoparticle films coated with halochromic pigments are measured and studied. The related results are shown in Figure 2 and Table 1. From Figure 2a, the photoelectric current density of ZrO2 nanoparticle films coated with methyl orange is the highest. The current density Jop is 1.81 A/m2 and the water splitting efficiency is 0.077%. The results of the hydrogen generation of ZrO2 nanoparticle films coated with halochromic pigments are plotted in Figure 2b. From Figure 2b, the hydrogen generation rate of ZrO2 nanoparticle films coated with methyl orange is faster than that of the other ZrO2 nanoparticle films coated with halochromic pigments. A total of 0.43 mL of hydrogen gas is generated in 1800 s from ZrO2 nanoparticle films coated with methyl orange (the sample size is 1 cm2). The hydrogen generation rate of ZrO2 nanoparticle films coated with methyl orange is 0.83 mL/h·cm2. Then, 0.2 M H2SO4 water solution is replaced by 0.4 M KHCO3 water solution for electrolyte. The related results of water-splitting efficiency of ZrO2 nanoparticle films coated with halochromic pigments are studied under AM 1.5G irradiation, positive applied voltages, and 0.2 M H2SO4 water solution for electrolyte, as shown in Figure 3 and Table 2. From Figure 3a, the photoelectric current density of ZrO2 nanoparticle films coated with methyl orange is the highest. Jop is 3.83 A/m2 and the water splitting efficiency is 0.236%. From Figure 3b, 0.9 mL of hydrogen gas is generated by ZrO2 nanoparticle films coated with methyl orange in 1800 s (the sample size is 1 cm2). The hydrogen generation rate of ZrO2 nanoparticle films coated with methyl orange is 1.76 mL/h·cm2. The experimental results from the different electrolytes demonstrate that the ZrO2 nanoparticle films coated with methyl orange have the best water-splitting efficiency and hydrogen generation rate. From Figure 1, the excited-state energy level of methyl orange in 0.2 M H2SO4 water solution is calculated to be −2.18 eV, which is much higher than the conduction band of ZrO2 (−2.5 eV). While methyl orange
Figure 2. Photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticle films coated with halochromic pigments (see the figure legends) on the ITO glass substrate under AM 1.5G irradiation and applied positive voltages in 0.4 M KHCO3 water solution as the electrolyte.
generated the excited electrons by absorbing the light from AM 1.5G irradiation, the excited electrons transfer to the conduction band of ZrO2. The excited electrons migrate to the Pt electrode and inject into electrolyte for hydrogen gas generation. Electrochemical Impedance Spectroscopy (EIS) Analysis. The EIS is modeled by the equivalent electrical analogs, as shown in Figure 4. The model consists of the interface resistance (RS), cell resistance (RI), and effective capacitance (CP). The operating settings for EIS measurements of ZrO2 nanoparticle films coated with halochromic pigments are as follows: AM 1.5G irradiation with a potential bias of 0.7 V and 100 000−1 Hz for the testing range. The related EIS results and Nyquist plots of ZrO 2 nanoparticle films coated with halochromic pigments in 0.4 M KHCO3 water solution are shown in Figure 4. From Figure 4a, ZrO2 nanoparticle films coated with methyl orange show the smallest Nyquist plots, which means the smallest charge transfer resistances between semiconductor and pigment. As seen in the Bode plot and the phase angle plot (log |Z| vs log f, and θ vs log f) in Figure 4b, the ideal capacitor21 is related to the slope value ( log |Z| approaching −1) and the phase angle log f
values (approaching 90°). Thus, the ZrO2 nanoparticle films coated with Congo red is referred to the ideal capacitor. 7718
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Table 1. Water-Splitting Properties of ZrO2 Nanoparticle Films Coated with Halochromic Pigments on the ITO Glass Substrate in 0.4 M KHCO3 as Electrolyte under AM 1.5G Irradiationa pigment coating methyl methyl methyl Congo a
violet blue orange red
J at 1.23 V (A/m2)
Jop (A/m2)
1.23 V − Eop (V)
η (%)
H2 (mL/h·cm2)
2.18 0.91 2.49 0.12
1.09 0.47 1.81 0.02
0.157 0.257 0.425 0.355
0.017 ± 0.005 0.012 ± 0.004 0.077 ± 0.005 0.001 ± 0.002
0.51 0.21 0.83 0.01
Scan range: 0−1.23 V. Scan rate: 50 mV/s.
smallest Nyquist plots. From Figure 5b, the slope value and the phase angle demonstrate that the water-splitting cell of ZrO2 nanoparticle films coated with methyl orange under AM 1.5G irradiation in 0.2 M H2SO4 water solution electrolyte is far from being the ideal capacitor. The electron-transporting phenomena is determined by thermodynamic calculation.17−20 The electron mobility, electrons diffusion lifetime, diffusion coefficient, and average distance are included in the results of thermodynamic calculation, which are shown in Table 3. The electron mobility (μ) is determined by using eq 2 σ = μnq (2) where σ is the electric conductivity (S·m−1), n is the electron concentration, q is the electricity (1.6 × 10−19 C). The diffusion coefficient D is determined by eq 3, kB is the Boltzmann constant (1.38 × 10−23 J·K−1), and T is temperature in kelvin:
D = μkBT /e
(3)
The electrons’ diffusion lifetime (τ) is calculated by the following (eq 4)
τ = μm H+ /e
(4)
where mH+ is the mass of the hydrogen ion. The average diffusion distance ⟨r⟩ is determined by eq 5. r =
Dτ
(5)
The values of electric conductivity are calculated with eq 6 R=
l σA
(6)
where R is the interface resistance, l is the length of the resistance, and A is the area of the resistance. Equation 1 suggests that electron mobility is related to electric conductivity. From Table 3, the ZrO2 nanoparticle films coated with methyl orange have the smallest interface resistance and are capacitive in 0.4 M KHCO3 and 0.2 M H2SO4 water solutions. According to the interface resistance between ZrO2 and the halochromic pigments, the low interface resistance induces the high electron mobility and the high diffusion coefficient of the electrons at the interface. The electron mobility is carried into
Figure 3. Photoelectric current density (a) and hydrogen generation rate (b) of ZrO2 nanoparticle films coated with halochromic pigments (see the figure legend) on the ITO glass substrate under AM 1.5G irradiation and applied positive voltages in 0.2 M H2SO4 water solution as the electrolyte for water splitting.
The EIS results of ZrO2 nanoparticle films coated with halochromic pigments in 0.2 M H2SO4 water solution are shown in Figure 5. From Figure 5a, the semicircle of ZrO2 nanoparticle films coated with methyl orange shows the
Table 2. Water Splitting Properties of ZrO2 Nanoparticle Films Coated with Halochromic Pigments on the ITO Glass Substrate in 0.2 M H2SO4 as Electrolyte under AM 1.5G Irradiationa pigment coating methyl methyl methyl Congo a
violet blue orange red
J at 1.23 V (A/m2)
Jop (A/m2)
1.23 V − Eop (V)
η (%)
H2 (mL/h·cm2)
20.83 3.62 4.95 0.43
2.99 1.35 3.83 0.16
0.725 0.535 0.615 0.297
0.217 ± 0.09 0.072 ± 0.002 0.236 ± 0.08 0.005 ± 0.002
1.37 0.62 1.76 0.07
Scan range: 0−1.23 V. Scan rate: 50 mV/s. 7719
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Figure 4. Nyquist diagram (a) and Bode plots (b) of ZrO2 nanoparticle films coated with halochromic pigments (see the figure legends) under AM 1.5G irradiation, at 0.7 V in a two-electrode configuration in 0.4 M KHCO3 electrolyte. Equivalent electrical analogs for modeling the electrochemical cell of ZrO2 nanoparticle films coated with halochromic pigments in a two-electrode configuration at a potential bias of 0.7 V (RS, interface resistance between electrode and electrolyte; RI, cell resistance; CP, effective capacitance).
Figure 5. Nyquist diagram (a) and Bode plots (b) of ZrO 2 nanoparticle films coated with halochromic pigments (see the figure legends) under AM 1.5G irradiation, at 0.7 V in a two-electrode configuration in 0.2 M H2SO4 electrolyte.
of the halochromic pigments is measured by the cyclic voltammetry methods, and the LUMO states are calculated from the onset absorption peak of the UV−vis spectrum. From Figure 6, it is seen that the excited energy level of the halochromic pigments is higher than the conduction band of ZrO2. The valence band of the ZrO2 is lower than all of the ground state energy levels of the halochromic pigments. According to the characterization results, the mechanism of the hydrogen generation from water splitting is proposed in Figure 7. The electrons are excited to the LUMO state by the halochromic pigments absorbing the incident solar energy (AM 1.5G irradiation). The excited electrons migrate to the conduction band of ZrO2, follow the circuit, and then inject into the electrolyte to
eq 4, from which the electrons’ diffusion lifetime is calculated. The average diffusion distance is calculated by eq 5. In the water-splitting experiments, the ZrO2 nanoparticle films coated with methyl orange have the highest water-splitting efficiency. The EIS results show that the charge transfer resistances between the ZrO2 nanoparticle films and methyl orange are the smallest. The energy level of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for the ZrO2 and the halochromic pigments are shown in Figure 6. The HOMO state 7720
DOI: 10.1021/acssuschemeng.7b01154 ACS Sustainable Chem. Eng. 2017, 5, 7716−7722
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ACS Sustainable Chemistry & Engineering Table 3. Thermodynamic Calculation of Electron Diffusion Activation pigment coating 0.4 M KHCO3
0.2 M H2SO4
methyl violet methylene blue methyl orange Congo red methyl violet methylene blue methyl orange Congo red
R (Ω·m) 223.8 1606.0 107.0 14150.0 26.7 423.2 11.7 12400.0
capacitance (F) 1.76 3.22 1.71 6.25 1.56 2.34 1.46 5.25
× × × × × × × ×
−5
10 10−5 10−5 10−5 10−5 10−5 10−5 10−5
σ (S·m−1)
μ [m2/(V·s)]
D (m2/s)
4.47 0.62 9.35 7.07 × 10−2 37.41 2.36 85.54 8.06 × 10−2
× × × × × × × ×
0.29 0.04 0.60 0.0046 2.42 0.15 5.53 0.01
6.98 9.73 1.46 1.10 5.85 3.69 1.34 1.26
19
10 1018 1020 1018 1020 1019 1021 1018
τ (s) 1.45 2.02 3.03 2.29 1.21 7.66 2.77 2.62
× × × × × × × ×
1015 1014 1015 1013 1016 1014 1016 1013
⟨r⟩ (m) 2.05 2.85 4.28 3.24 1.71 1.08 3.92 3.69
× × × × × × × ×
107 106 107 105 108 107 108 105
Figure 6. Energy level positions of the halochromic pigments in the KHCO3 and H2SO4 water solutions.
voltage. The photocurrent density is 3.83 A/m2 and the hydrogen generation rate is 1.76 mL/h·cm2. Simultaneously, the energy level of LUMO states of halochromic pigments are tuned by 0.4 M KHCO3 and 0.2 M H2SO4 water solutions due to the modulation of the pH value. When the electrolyte is changed from KHCO3 water solution to the H2SO4 water solution, the interface resistance is reduced from 107 to 11.7 Ω· m.
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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/acssuschemeng.7b01154. X-ray diffraction of ZrO2 nanoparticles; morphology of ZrO2 nanoparticles; cyclic voltammograms of methyl violet, methylene blue, methyl orange, and Congo red in electrolyte solutions; Nyquist diagrams and Bode plots of ZrO2 films coated with methyl violet, methylene blue, methyl orange, and Congo red in electrolyte solutions; and AFM image of the Pt film (Figures S1−S14) (PDF)
Figure 7. Scheme of the sandwich structure for ZrO2 nanoparticle films coated with halochromic pigments on the ITO glass substrate under AM 1.5G irradiation for water splitting.
reduce H+ ions into hydrogen gas. The electron holes migrate into the water to oxide the OH− into oxygen gas. The energy difference between the LUMO state energy level of photosensitizer and the conduction energy level of ZrO2 is 0.32 eV.22−25 The methyl orange is able to be tuned to the LUMO state energy level by 0.2 M H2SO4 water solution. Thus, the efficiency of the hydrogen generation is enhanced.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ORCID
CONCLUSIONS ZrO2 nanoparticle films coated with methyl orange demonstrate the highest efficiency of hydrogen generation from water splitting in 0.2 M H2SO4 water solution under positive applied
Yi-Sheng Lai: 0000-0001-5897-8763 Notes
The authors declare no competing financial interest. 7721
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natural azulene: syntheses, halochromism and one-photon and twophoton microlithography. J. Mater. Chem. C 2015, 3, 8495−8503. (17) Kumar, A. V. N.; Joseph, J. Influence of co-electrodeposited gold particles on the electrocatalytic properties of CoHCF thin films. Electrochim. Acta 2014, 139, 88−95. (18) Kniaginicheva, E.; Pismenskaya, N.; Melnikov, S.; Belashova, E.; Sistat, P.; Cretin, M.; Nikonenko, V. Water splitting at an anionexchange membrane as studied by impedance spectroscopy. J. Membr. Sci. 2015, 496, 78−83. (19) Shimizu, K.; Lasia, A.; Boily, J.-F. Electrochemical impedance study of the hematite/water interface. Langmuir 2012, 28, 7914−7920. (20) Diez-Garcia, M. I.; Gomez, R. Investigating water splitting with CaFe2O4 photocathodes by electrochemical impedance spectroscopy. ACS Appl. Mater. Interfaces 2016, 8, 21387−97. (21) Lopes, T.; Andrade, L.; Ribeiro, H. A.; Mendes, A. Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy. Int. J. Hydrogen Energy 2010, 35 (20), 11601−11608. (22) Ma, S.; Song, W.; Liu, B.; Zhong, W.; Deng, J.; Zheng, H.; Liu, J.; Gong, X.-Q.; Zhao, Z. Facet-dependent photocatalytic performance of TiO2: A DFT study. Appl. Catal., B 2016, 198, 1−8. (23) Wang, Z.; Yu, H.; Su, H. The transport properties of oxygen vacancy-related polaron-like bound state in HfOx. Sci. Rep. 2013, 3, 3246. (24) Chen, H. Y. T.; Tosoni, S.; Pacchioni, G. Hydrogen adsorption, dissociation, and spillover on Ru10 clusters supported on anatase TiO2 and tetragonal ZrO2 (101) surfaces. ACS Catal. 2015, 5, 5486−5495. (25) Wolf, M. J.; McKenna, K. P.; Shluger, A. L. Hole trapping at surfaces of m-ZrO2 and m-HfO2 nanocrystals. J. Phys. Chem. C 2012, 116, 25888−25897.
REFERENCES
(1) Green, M. A.; Bremner, S. P. Energy conversion approaches and materials for high-efficiency photovoltaics. Nat. Mater. 2017, 16, 23− 34. (2) Kolesov, V. A.; Fuentes-Hernandez, C.; Chou, W. F.; Aizawa, N.; Larrain, F. A.; Wang, M.; Perrotta, A.; Choi, S.; Graham, S.; Bazan, G. C.; Nguyen, T. Q.; Marder, S. R.; Kippelen, B. Solution-based electrical doping of semiconducting polymer films over a limited depth. Nat. Mater. 2017, 16, 474−480. (3) Yang, J. H.; Cooper, J. K.; Toma, F. M.; Walczak, K. A.; Favaro, M.; Beeman, J. W.; Hess, L. H.; Wang, C.; Zhu, C. H.; Gul, S.; Yano, J. K.; Kisielowski, C.; Schwartzberg, A.; Sharp, I. D. A multifunctional biphasic water splitting catalyst tailored for integration with highperformance semiconductor photoanodes. Nat. Mater. 2017, 16, 335− 341. (4) Raccuglia, P.; Elbert, K. C.; Adler, P. D. F.; Falk, C.; Wenny, M. B.; Mollo, A.; Zeller, M.; Friedler, S. A.; Schrier, J.; Norquist, A. J. Machine-learning-assisted materials discovery using failed experiments. Nature 2016, 533, 73−76. (5) Tsai, H. H.; Nie, W. Y.; Blancon, J. C.; Stoumpos, C. C. S.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G.; Mohite, A. D. Highefficiency two-dimensional Ruddlesden-Popper perovskite solar cells. Nature 2016, 536, 312−316. (6) Bloor, L. G.; Solarska, R.; Bienkowski, K.; Kulesza, P. J.; Augustynski, J.; Symes, M. D.; Cronin, L. Solar-driven water oxidation and decoupled hydrogen production mediated by an electron-coupledproton buffer. J. Am. Chem. Soc. 2016, 138, 6707−6710. (7) Kumari, S.; Turner White, R.; Kumar, B.; Spurgeon, J. M. Solar hydrogen production from seawater vapor electrolysis. Energy Environ. Sci. 2016, 9, 1725−1733. (8) Zhang, J. Q.; Li, L.; Xiao, Z. X.; Liu, D.; Wang, S.; Zhang, J. J.; Hao, Y. T.; Zhang, W. Z. Hollow sphere tio2-zro2 prepared by selfassembly with polystyrene colloidal template for both photocatalytic degradation and h2 evolution from water splitting. ACS Sustainable Chem. Eng. 2016, 4, 2037−2046. (9) Lai, Y.-S.; Su, Y. H.; Lin, M. I. Photochemical water splitting performance of fluorescein, rhodamine B, and chlorophyll-Cu supported on ZrO2 nanoparticles layer anode. Dyes Pigm. 2014, 103, 76−81. (10) Wu, Y. J.; Yue, Z. K.; Liu, A. J.; Yang, P.; Zhu, M. S. p-type Cudoped Zn0.3Cd0.7S/Graphene photocathode for efficient water splitting in a photoelectrochemical tandem cell. ACS Sustainable Chem. Eng. 2016, 4, 2569−2577. (11) Yan, X. D.; Tian, L. H.; Atkins, S.; Liu, Y.; Murowchick, J.; Chen, X. B. Converting CoMoO4 into CoO/MoOx for overall water splitting by hydrogenation. ACS Sustainable Chem. Eng. 2016, 4, 3743−3749. (12) Zhang, J.; Chen, A. S.; Wang, L. H.; Li, X. A.; Huang, W. Striving toward visible light photocatalytic water splitting based on natural silicate clay mineral: the interface modification of attapulgite at the atomic-molecular level. ACS Sustainable Chem. Eng. 2016, 4, 4601−4607. (13) Su, Y. H.; Lai, Y. S. Performance enhancement of natural pigments on a high light transmission ZrO2 nanoparticle layer in a water-based dye-sensitized solar cell. Int. J. Energy Res. 2014, 38, 436− 443. (14) De Meyer, T.; Ensing, B.; Rogge, S. M. J.; De Clerck, K.; Meijer, E. J.; Van Speybroeck, V. Acidity Constant (pK(a)) calculation of large solvated dye molecules: evaluation of two advanced molecular dynamics methods. ChemPhysChem 2016, 17, 3447−3459. (15) De Meyer, T.; Hemelsoet, K.; Van der Schueren, L.; Pauwels, E.; De Clerck, K.; Van Speybroeck, V. Investigating the halochromic properties of azo dyes in an aqueous environment by using a combined experimental and theoretical approach. Chem. - Eur. J. 2012, 18, 8120−8129. (16) Ghazvini Zadeh, E. H.; Tang, S.; Woodward, A. W.; Liu, T.; Bondar, M. V.; Belfield, K. D. Chromophoric materials derived from a 7722
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