Enhanced Solar Water Oxidation Performance of TiO2 via Band Edge

Oct 17, 2017 - We report the rational design and fabrication of earth-abundant, visible-light-absorbing Cu2ZnSnS4 (CZTS) nanoparticle (NP) in situ sen...
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Enhanced Solar Water Oxidation Performance of TiO via Band Edge Engineering: A Tale of Sulfur Doping and Earth-Abundant CZTS Nanoparticles Sensitization Mahesh P. Suryawanshi, Uma V. Ghorpade, Seung Wook Shin, Myeng Gil Gang, Xiaoming Wang, Hyunwoong Park, Soon Hyung Kang, and Jin Hyeok Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02102 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Enhanced Solar Water Oxidation Performance of TiO2 via Band Edge Engineering: A Tale of Sulfur Doping and EarthAbundant CZTS Nanoparticles Sensitization Mahesh P. Suryawanshia, Uma V. Ghorpadea, Seung Wook Shinb, Myeng Gil Ganga, Xiaoming Wangb, Hyunwoong Parkc, Soon Hyung Kangd*, Jin Hyeok Kima** a

Optoelectronics Convergence Research Center and Department of Materials Science and Engineering, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea b Department of Physics and Astronomy and Wright Center for Photovoltaic Innovation and Commercialization, University of Toledo, Toledo, Ohio 43606, USA c School of Energy Engineering, Kyungpook National University, Daegu 41566, South Korea d Department of Chemistry Education and Optoelectronics Convergence Research Center, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea ABSTRACT: We report the rational design and fabrication of earth-abundant, visible-light absorbing Cu2ZnSnS4 (CZTS) nanoparticles (NPs) in-situ sensitized S-doped TiO2 nanoarchitectures for high-efficiency solar water splitting. Our systematic studies reveal that these nanoarchitectures significantly enhance the visible-light photoactivity compared to that of TiO2, S-doped TiO2 and CZTS NPs-sensitized TiO2. Detailed photoelectrochemical (PEC) studies demonstrate an unprecedented enhancement in the photocurrent density and incident photon-to-electron conversion efficiency (IPCE). This enhancement is attributed to the significantly improved visible-light absorption and more efficient charge separation and transfer/transport, resulting from the synergistic influence of CZTS NPs sensitization and S doping, which were confirmed by electrochemical impedance spectroscopy (EIS). Moreover, density functional theory (DFT) calculations supported by the experimental evidences revealed that the gradient S-dopant concentration along the depth direction of TiO2 nanorods led to the bandgap grading from ~ 2.3 eV to 2.7 eV. This S-gradient doping introduced the terraced band structure via upshifting the valence band (VB), which provides channels for easy hole transport from the VB of S-doped TiO2 to the VB of CZTS and thereby enhances the charge transport properties of the CZTS/S-TNRs photoanode. This work demonstrates the rational design and fabrication of nanoarchitectures via band edge engineering for improving the PEC performance using simultaneous earth-abundant CZTS NPs sensitization and S doping. This work also provides useful insight for the further development of different nanoarchitectures using similar combinations for energy harvesting related applications.

KEYWORDS. Solar water splitting, earth-abundant elements, band edge engineering, density functional theory (DFT), nanoparticles sensitization, charge separation and transport properties Introduction. Titanium oxide (TiO2) is a well-studied photoanode material for photoelectrochemical (PEC) water splitting due to its low-cost, earth abundance, suitable band edge positions and excellent stability in acidic electrolytes.1-4 However, the PEC water-splitting performance of TiO2 is limited by its poor light absorption ability in the visible region due to its wide band gap (~3.0 eV for rutile and 3.2 eV for anatase) and usually fast electron-hole recombination due to its high defect density, which provides trap states.5,6 In particular, the wide band gap energy of TiO2 considerably limits its photocatalytic activity to the near ultraviolet (UV) region of the sunlight spectrum, which constitutes a small fraction (~5%) of the sun’s energy compared to that of visible light (45%). Therefore, enhancing the absorption of TiO2 in the visible region has been widely studied. Several approaches have been suggested and successfully employed to enhance the visible-light activity of TiO2 by doping with metals or non-metals,6-13 as well as sensitization with visible-light absorbing nanoparticles (NPs).14-19 Among the different approaches, one effective approach is to modify the electronic properties of TiO2 to improve its optical absorption in the visible region through impurity doping of metals and non-metals.6-13 Metal-doped TiO2-based nanostructures have

been extensively studied, and it has been found that the PEC performance of pristine TiO2 can be enhanced via metal doping.6-10 The doping of metals into TiO2 incorporates additional discrete levels that act as either donor or acceptor levels and can induce visible-light absorption in TiO2. However, metal-doped TiO2 exhibits several drawbacks, such as i) the requirement of an ionimpletion technique,20 ii) thermal instability, and iii) additional impurity levels introduced by metal doping, which act as carrier recombination centers, resulting in a loss of excited charge carriers. Unlike metal doping, the doping of non-metals such as N, S, C, B, P, and F in TiO2 allows the p orbitals of the incorporated ions to mix with the O 2p orbitals of TiO2, which shifts the valence band (VB) of TiO2 upward and thus narrows the band gap.21 Theoretical calculations under the local density approximation have evidenced that the substitutional doping of non-metal elements for the O sites in the TiO2 crystal structure is the most effective among several other non-metals because their p states contribute to the improvement in the visible-light absorption of TiO2 by mixing with the O 2p states.22 Asahi et al.22 reported Ndoped TiO2 with significant visible-light photoactivity for the first time and suggested that N doping in TiO2 is more effective than S doping, since incorporation of S into the TiO2 lattice is difficult

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due to its larger ionic radius. On the other hand, Umebayashi et al.23,24 suggested that the substitution of S at the O sites could significantly modify the electronic structure of TiO2, since S has a larger ionic radius compared to both O and N. Recently, Piskunov et al.21 studied the photocatalytic ability of C-, N-, S-, and Fedoped TiO2 nanotubes using first-principles calculations. They demonstrated that the co-doping of N and S into TiO2 could result in extreme narrowing of the band gap up to 2.2 eV, compared to that of doping with S (2.4 eV) and N (2.5 eV) individually. However, the co-doping of N and S leads to two different point defect levels in the VB of TiO2, which act as recombination sites for charge carriers, whereas N doping into TiO2 introduces a point defect level near the reduction potential. Thus, S doping into TiO2 is the most effective approach to further improve the visible-light activity of TiO2 due to i) the narrow band gap of ~2.4 eV and ii) a higher gap between the positions of the VB and the reduction level, which results in a higher driving force for charge separation and transport. In addition to doping, alternative strategies to increase the visible-light absorption of TiO2 include sensitization with narrow band gap semiconductor NPs mainly due to the broad visible absorption of NPs, which maximizes the absorption over the solar spectrum. Moreover, the formation of type II band alignment between the wide band gap metal oxide and semiconductor NPs is of great advantage for efficient electron-hole separation due to a high driving force from their large band position differences. Thus far, CdS, CdSe, CdTe, PbS, and their alloys sensitized on a TiO2 nanostructures have been reported to show better performances.15-17,25-27 However, few works have investigated the synergistic influence of NPs sensitization and non-metal doping in TiO2. Hensel et al.28 reported the first example of CdSe NPs sensitization and N-doped TiO2 nanoarchitectures for PEC hydrogen generation, which showed an enhanced photocurrent density of up to 2.75 mA/cm2. Subsequently, several other studies have reported the combined effect of NPs sensitization and nonmetal doping, for example, with Cd(Sx,Se1-x)-sensitized N-doped TiO2 heterostructures27 and CdS-sensitized B and S co-doped TiO2.29 However, the obtained photocurrent densities were relatively low. Additionally, the use of heavy (Pb) and highly toxic metals (Cd and Te) greatly limits their potential applications due to environmental and health issues. Recently, earth-abundant, low-cost and environmentally benign Cu2ZnSnS4 (CZTS) NPs have been widely investigated for applications in solar energy harvesting owing to their outstanding merits, such as i) heavy and toxic metal-free NPs (without toxic elements, such as Cd and Pb);30 ii) high absorption coefficient (over 104 cm-1)31; iii) suitable direct optical band gap (1.5 eV)32,33, which is more effective for broadening the light absorption range compared to that of CdS, CdSe, CdTe, etc., as well as good charge transport properties and high mobility34-37; and iv) favorable electronic band alignment for water reduction.38 Despite these outstanding properties, few papers have described the use of CZTS NPs for PEC water splitting, and the obtained photocurrent density is still very low (2.16 mA/cm2 at 1.0 V vs. saturated calomel electrode (SCE)).39 Higher photocurrents (~12.59 mA/cm2 at 0 V vs. SCE) were achieved by using a complicated two-storied 3D CZTS/CdS/ZnO@steel composite nanostructure, which again raises issues due to the use of toxic CdS, as well as the difficult fabrication process for the complicated photoelectrode structure.40 Recently, our group demonstrated the great potential of CZTS for enhanced solar water-splitting performance by designing a novel one-dimensional (1D) nanostructured photoanode based on presynthesized CZTS NPs sensitized on Zn(O1-x,Sx)-passivated TiO2 nanorod arrays (TNRs), which exhibited a remarkably enhanced photocurrent density of 15.05 mA/cm2 at 1.23 V (vs. normal hydrogen electrode (NHE)).41 Despite of such outstanding performance, the fabricated photoanode struggles to achieve high QD loading via ex situ method, since it relies on the infiltration of

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preformed NPs onto the nanostructured photoanode rather than infiltration of the chemical precursors for NPs growth. Among the different in situ infiltration methods, successive ionic layer adsorption and reaction (SILAR) method is the most common method for in situ growth of different metal chalcogenides NPs onto the nanostructured photoanode. Although, SILAR method has been studied widely for the sensitization of binary cadmium chalcogenides onto TNRs, the sensitization of quaternary CZTS NPs on TNRs photoanodes via conventional SILAR method remains sparsely studied, which is mostly likely due to the competitive absorptivities of different metal cations (Cu+, Zn2+, and Sn4+) in a single cationic bath. This makes the synthesis of high quality quaternary CZTS NPs with compositional homogeneity via conventional SILAR method much more challenging. In the view of overcoming the limitations of conventional SILAR method to sensitize quaternary CZTS NPs on TNRs photoanode, we demonstrated a modified single step SILAR approach for in situ growth of high quality CZTS NPs with compositional homogeneity. In this work, we demonstrate, for the first time, the synergistic influence of earth-abundant CZTS NPs sensitization and S doping into 1D TiO2 nanorod arrays (TNRs) for highly efficient PEC water splitting. Moreover, 1D TNRs offer advantages such as i) high surface area for sensitizing narrow band semiconductors, ii) enhanced light scattering for improved absorption, and iii) fast electron transport to the back electrode.1,2,11,12,14 A simple annealing treatment under S atmosphere is employed for doping S into TNRs (S-TNRs), which is a more reliable strategy for introducing selected impurities into solid-state compounds. We further employ a simple and cost-effective modified SILAR approach for the in-situ deposition of CZTS NPs onto S-TNRs (CZTS/S-TNRs) as a visible-light sensitizer. This ingenious type II nanoarchitecture simultaneously allows a large amount of light absorption and a high carrier collection efficiency with a significantly enhanced photocurrent of 8.84 mA/cm2 at 1.23 V (vs. RHE) compared to that of TNRs (0.73 mA/cm2 at 1.23 V (vs. RHE)), S-TNRs (4.71 mA/cm2 at 1.23 V (vs. RHE)) and CZTS sensitized TNRs (CZTS/TNRs) (5.38 mA/cm2 at 1.23 V (vs. RHE)). Moreover theoretical predications performed on the electronic structure of TiO2 and S-doped TiO2 using density functional theory (DFT) calculations were substantiated by a comparison of the obtained experimental results using pristine TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs photoanodes. To the best of our knowledge, this is the first example of the earth-abundant CZTS NPs sensitization of S-TNRs by a modified SILAR process, which shows a remarkable photocurrent and could be used to achieve low-cost, green, stable and high-efficiency solar-driven PEC water splitting. Results and Discussion. Characterization of CZTS NPs sensitized S-doped TNRs photoanode. Figure 1 shows the fieldemission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the hydrothermally grown TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs. The planar FE-SEM image of TNRs reveals the formation of homogenous, dense and vertically aligned nanorod arrays with an average length of ~2 µm and diameter in the range of 80 to 250 nm (Figure 1a). The high aspect ratios and high densities of TNRs may promote enhanced charge transfer/transport and thus contribute to the enhanced PEC performance. No significant morphological difference is observed between TNRs and S-TNRs (Figure 1a and d). In contrast, the FESEM images of CZTS NPs-sensitized TNRs and S-TNRs show the formation of hierarchical nanostructures after sensitizing with CZTS NPs, which confirms that the CZTS NPs are homogenously sensitized onto the surfaces of TNRs and S-TNRs. Clearly, the SILAR processes ensued along the depth of the TNRs and STNRs, owing to the effectiveness of SILAR in homogenously sensitizing the surfaces of high aspect ratio nanostructures. TEM and HR-TEM images further confirm that the TNRs exhibit a very

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sharp rod edge with a smooth surface (Figure 1b) and wellresolved lattice fringes with an interplanar spacing of 0.249 nm (Figure 1c), corresponding to the rutile (101) plane of the TiO2 crystal system.11,41,42 Interestingly, the S-TNRs show a rougher surface, as seen in the SEM and TEM images (Figure 1d and e), which could be due to elemental rearrangement and recrystallization at the edge region in TNRs through annealing in S atmosphere.43 The HR-TEM image (Figure 1f) shows a slightly larger interplanar spacing of 0.252 nm compared to the rutile (101) plane of the TiO2 crystal system, which indicates the partial substitution of O atoms by S atoms and is attributed to the difference in the ionic radius between S2- (0.184 nm) and O2- (0.136 nm).43 Figure 1h and k clearly show that the CZTS NPs are homogenously sensitized onto the TNRs and S-TNRs without aggregation. These TEM images (Figure 1h and k) also show high loadings of CZTS NPs on the TNRs and S-TNRs nanostructures. Moreover, a sharp interface between CZTS NPs and TNRs, as well as between CZTS NPs and S-TNRs, is observed from the TEM and HR-TEM images (Figure 1h, i, k and l), indicating no formation of any intermixed compounds or interfacial reactions during CZTS NPs sensitization using the modified SILAR method. The HR-TEM images (Figure 1i and l) show well-resolved lattice fringes with

lattice spacings of 0.248 nm and 0.325 nm in the TNRs or STNRs regions, respectively, corresponding to the (101) and (110) planes for the rutile TiO2 crystal structure, and a lattice spacing of 0.313 nm in the NPs regions corresponding to the (112) plane of the kesterite CZTS crystal structure.39-41 The diameter of the CZTS NPs is found to be around ~6-8 nm. The HR-TEM images also demonstrate the highly intimate contact between the CZTS NPs and TNRs/S-TNRs, which indicates the possibility of a fast electron transfer rate from the NPs to TNRs/S-TNRs, thereby improving the performance of the CZTS NPs-sensitized PEC devices due to decreased photoexcited charge recombination. Additionally, no changes were observed in the structure and/or lattice in the TNRs and S-TNRs after SILAR sensitization of CZTS NPs, which indicates that the SILAR method is a low-cost, efficient and non-destructive approach for the in-situ deposition of NPs onto 1D nanostructures. Furthermore, the high-angle annular dark field (HAADF) scanning TEM (STEM) image and elemental mapping of the CZTS/S-TNRs sample in Figure 2 confirm the homogeneous distribution of CZTS NPs onto S-TNRs. Uniform signals from Ti, O, Cu, Zn, Sn, and S are observed, which ensures a homogenous coating of CZTS NPs onto S-TNRs.

Figure 1. Structural characterization of the TNRs, S-TNRs, CZTS/TNRs, and CZTS/S-TNRs samples. SEM, TEM and HR-TEM images of (a, b, and c) the pristine TNRs, (d, e, and f) S-TNRs, (g, h, and i) CZTS/TNRs, and (j, k, and l) CZTS/S-TNRs samples. The TEM samples were prepared on carbon-meshed Ni grids by drop casting using NPs dispersed in ethanol.

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Figure 2. High-angle annular dark field (HAADF) STEM image and STEM-EDS mapping of Ti, O, Cu, Zn, Sn, and S elements in the CZTS/S-TNRs sample. To probe the surface chemical states in TNRs after S doping and CZTS NPs sensitization, we employed X-ray photoelectron spectroscopy (XPS) to the TNRs, S-TNRs, and CZTS/S-TNRs samples. High-resolution core level XPS scans of the TNRs, STNRs, and CZTS/S-TNRs samples were recorded and are shown in Figure 3 to simultaneously determine the chemical states of Ti, O, S, Cu, Zn, and Sn. The Ti 2p and O 1s XPS spectra (Figure 3a) of pristine TNRs are in good agreement with those reported for rutile TiO2, with peaks for (Ti 2p3/2) and (Ti 2p3/2), respectively.42,43 In the S-TNRs sample, two distinct broad peaks corresponding to the S 2p state are observed at 161.5 eV and 167.7 eV, indicating that S has different bonding states after doping into TiO2 (Figure 3b). The peak at 161.5 eV

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can be assigned to the formation of Ti-S bonds, since some of the O2- ions in the TiO2 lattice are substituted by S2- ions. According to Umebayashi et al.,23,24 the typical peak of the S 2p state of adsorbed sulfur dioxide (SO2) molecules is located between 166 and 170 eV. In this regard, the peak observed at 167.7 eV in the S 2p state can be ascribed to surface-adsorbed SO2 molecules, which may arise from impurities in the H2S gas or through the decomposition and oxidation of H2S gas during the annealing process. Han et al.44 observed a marked decrease in the S 2p peak at a higher binding energy after the Ar+ sputtering etching of S-doped TiO2 particles, whereas Shin et al.43 observed that the S-doped TiO2 nanotube arrays changed color after PEC testing, which verified that the typical peak of the S 2p state at a binding energy of 167.7 eV was primarily due to the presence of sulfate groups anchored on the surface of Sdoped TiO2. We also observed that the binding energies of the Ti 2p state in the S-TNRs sample is lower, i.e., ~0.3 eV for 2p3/2 and ~0.2 eV for 2p1/2, compared to those of the Ti 2p state in the pristine TNRs sample, which indicates a decrease in the anionic bonding nature of Ti4+ due to the partial substitution of oxygen by sulfur, which is less electronegative. The substitution of O by S in the equilibrium state is thought to be unfavorable due to the larger size of S2- versus O2- and the better thermal stability of the TiO2 system.22 However, S doping can be accomplished at nonequilibrium states, such as at high temperatures and on surface sites.45 High-temperature annealing under S atmosphere results in the formation of O vacancies in TiO2, and the surface state becomes loosely packed and rich in dangling bonds. These nonequilibrium structures enable the substitution of O2- ions in TiO2 by large S2- ions by providing extra energy and sufficient space. These results suggest that S atoms are doped into the bulk phase of TiO2, which is in good agreement with the reported literature.23,24,43 Figure 3c shows the characteristic peaks of Ti, O, Cu, Zn, Sn, and S in the CZTS/S-TNRs sample at their respective binding energies, which are easily distinguishable. The S 2p core level spectrum of the CZTS/S-TNRs sample shows three different peaks at 161.5 eV, 162.4 eV and 167.7 eV. The peak at 162.4 eV indicates that S is present in its sulfide state, which arises from the sensitization of CZTS NPs onto STNRs,36,46 whereas the latter two peaks confirm the doping of S into the bulk of TiO2, as described above. Thus, these results clearly demonstrate the validity of our proposed nanoarchitecture. To determine the crystal structure and possible phase changes after S doping and CZTS NPs sensitization onto TNRs, X-ray diffraction (XRD) patterns were collected for the pristine TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs samples (Figure 4a). The sharp diffraction peaks in the TNRs sample located at 36.10°, 41.26°, and 54.3° correspond to the (101), (111) and (211) planes, respectively, indicating the formation of a tetragonal rutile phase (JCPDS Card No.: 75-1753) in all samples. The XRD pattern of S-TNRs shows the rutile phase, and no changes in the peaks are seen, suggesting that annealing in S atmosphere does not change the overall phase of TNRs. However, additional broad and weak peaks at 28.53°, 47.33° and 56.17° are observed, which correspond to the (112), (220) and (312) planes of the kesterite CZTS phase (JCPDS Card No.: 00-026-0575) in the CZTS/TNRs and CZTS/S-TNRs samples, indicating that the CZTS NPs are successfully sensitized onto the TNRs and STNRs nanostructures.

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Figure 3. XPS characterization of the (a) TNRs, (b) S-TNRs, and (c) CZTS/S-TNRs samples.

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Figure 4. Structural and optical characterizations. (a) XRD patterns and (b) UV-Vis absorption spectra of the TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs samples. We also conducted UV-Vis diffuse reflectance spectroscopy (DRS) measurements to study the optical properties of pristine TNRs, S-TNRs, CZTS/TNRs, and CZTS/S-TNRs (Figure 4b). The pristine TNRs show a light absorption edge at approximately 420 nm, which is consistent with the band gap of rutile TiO2 (3.0 eV).47,48 After S doping, the absorbance in the visible-light region over the wavelength range of 400 to 750 nm is enhanced, suggesting a broad distribution of localized energy levels resulting from the VB upshift. The S-TNRs sample shows an apparent absorption edge at approximately 470 nm, indicating a welldefined energy level formed by VB upshift after S doping. Importantly, the absorption edge is significantly extended to the visible-light region at a maximum wavelength of approximately 730 nm and 790 nm after sensitization of CZTS NPs on pristine TNRs and S-TNRs, respectively, which arises from the strong visible light harvesting by the CZTS NPs that exhibit a band gap energy of 1.5 eV. These studies clearly demonstrate that S doping and CZTS NPs sensitization can fundamentally change the optical properties of pristine TNRs and produce S-TNRs with substantial visible-light absorption with further broad absorption up to a wavelength of approximately 800 nm after CZTS NPs sensitization. This enhancement in the optical properties of TNRs due to the synergistic influence of S doping and CZTS NPs sensitization is further confirmed from the visual appearance (color) of the samples (Figure S1). The color of the TNRs changed from white to brown after S doping and further changed to black after CZTS NPs sensitization, demonstrating that the optical properties of TiO2 in the visible region are significantly enhanced by S doping and CZTS NPs sensitization. Photoelectrochemical (PEC) performance of CZTS NPs sensitized S-doped TNRs nanoarchitecture-based water splitting device. PEC measurements were carried out using a three-electrode PEC system with Pt foil as the counter electrode and Ag/AgCl as the reference electrode in 1 M sodium sulfate (Na2SO4) electrolyte (pH = 6.8). Figure 5a shows the linear sweep voltammograms (LSVs) of the pristine TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs photoanodes under chopped conditions. All the photoanodes show a rapid and reproducible photocurrent response with respect to the on/off cycle, where the steady vertical rising and falling photocurrent densities of the transient photocurrent response indicate decreased electronhole recombination and quick charge transport in all photoanodes (Figure 5a). Figure 5b shows the PEC performance in

terms of the photocurrent density vs. applied potential in the dark and under light illumination for the pristine TNRs, STNRs, CZTS/TNRs and CZTS/S-TNRs photoanodes. A negligible background current is observed under dark conditions. Pristine TNRs exhibit a photocurrent density of ~0.73 mA/cm2 at 1.23 V (vs. RHE), which is due to the limited absorption of visible light (Figure 4b). Furthermore, to choose the proper annealing temperature for S doping, the annealing of TNRs photoanodes under S atmosphere was carried out at different temperatures (400 °C, 450 °C and 500 °C) for 10 min. The corresponding LSVs with and without chopping are shown in Figure S2a and S2b, indicating that the highest photocurrent density of 4.71 mA/cm2 at 1.23 V (vs. RHE) is obtained at an annealing temperature of 450 °C, which is considered to be the optimized temperature. Generally, S doping in TNRs increases the conductivity of TNRs and improves its optical absorption for harvesting part of the visible-light region.43 Additionally, the O vacancies introduced in the TNRs during annealing under S atmosphere trap some of the charge carriers and restrict the recombination of generated charge carriers to some extent.11 Subsequently, the S-TNRs photoanode under identical conditions exhibits an enhanced photocurrent density compared to that of pristine TNRs. Furthermore, the number of SILAR cycles for CZTS NPs sensitization on the TNRs photoanode was optimized by varying the number from 5 to 20 cycles at an interval of 5 cycles. The LSV was measured with and without chopping (Figure S3a and 3b), and 15 SILAR cycles were required to obtain the maximal performance of the CZTS/TNRs photoanode, with the highest photocurrent density of 5.38 mA/cm2 (vs. RHE). The obtained photocurrent density in the CZTS/TNRs photoanode after 15 cycles of CZTS NPs sensitization is significantly higher than that of pristine TNRs (~0.73 mA/cm2 at 1.23 V (vs. RHE)) and is relatively higher than that of S-TNRs (4.71 mA/cm2 at 1.23 (vs. RHE)). This observation could be attributed to an extensive redshift in the absorption edge of pristine TNRs mediated by the sensitization of the narrow band gap, earth-abundant CZTS NPs (Figure 4b). Remarkably, the CZTS/S-TNRs photoanode exhibits a photocurrent density of 8.84 mA/cm2 at 1.23 V (vs. RHE), which is approximately 1.6, 1.9 and 12 times higher than that of the CZTS/TNRs (5.38 mA/cm2 at 1.23 (vs. RHE)), S-TNRs (4.71 mA/cm2 at 1.23 (vs. RHE)) and TNRs (0.73 mA/cm2 at 1.23 V (vs. RHE)) photoanodes, respectively. Interestingly, this substantially en-

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hanced photocurrent density of CZTS/S-TNRs is almost equal to the sum of photocurrents (at 1.23 V (vs. RHE)) from S-TNRs and CZTS/TNRs, which clearly indicates the synergistic effect of S doping and CZTS NPs sensitization. Furthermore, the photocurrent onset potential of TNRs (-0.15 V vs. RHE) is slightly shifted to -0.058 V (vs. RHE) for S-TNRs and gradually shifted to -0.43 V (vs. RHE) for CZTS/TNRs and -0.47 V (vs. RHE) for CZTS/S-TNRs. The origin of onset potential shifting after Sdoping and CZTS NP sensitization onto TNRs could be explained on the basis of different reasons. First, the S-doping in TNRs could effectively diminish the surface defects, thus lowering the recombination between electrons in TNRs and holes in the electrolyte, leading to longer charge carrier life-time. From

another standpoint, the electron transport in CZTS/S-TNRs photoanode must be faster than bare TNRs and S-TNRs photoanodes, due to the better charge separation induced by the formation of a favorable type II band alignment between CZTS NPs and S-TNRs. The saturated photocurrent density of the CZTS/S-TNRs heterostructure photoanode reaches a value of ~ 10.79 mA/cm2 at 1.8 V (vs. RHE), which is notably the first demonstration of the synergistic effects of S doping and earthabundant CZTS NPs sensitization on TiO2. Notably, this value is comparable with highly toxic, heavy NPs-sensitized photoanodes, such as CdS/TiO2 or CdSe/TiO2, and significantly higher than similar CZTS-based photoanodes for solar water splitting (see Table S1 and S3 in the Supporting Information).

Figure 5. PEC performances of the TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs photoanodes. (a) Linear sweeps voltammograms (LSVs) under chopped on/off conditions, (b) LSVs under one sun illumination conditions (100 mW/cm2 with AM 1.5G), (c) photoconversion efficiency as a function of applied bias, and (d) incident photon-to-current conversion efficiency (IPCE) as a function of wavelength. The electrolyte solution was 1 M Na2SO4 (pH 6.8).

Furthermore, the photoconversion efficiency of each photoanode as a function of applied bias for the water-splitting reaction in PEC cells was calculated using Figure 5b via an equation:   

  

 1.23    100 

where, jp is the measured photocurrent density in mA/cm2, V is the applied electrical bias with respect to RHE in V, and P is the incident light power density in mW/cm2. The photoconversion efficiency was plotted vs applied potential (V vs RHE) and is presented in Figure 5c. The pristine TNRs photoanode shows a very low photoconversion efficiency of ~0.37% at -0.233 V (vs. RHE). However, S doping and CZTS NPs sensitization of TNRs

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boosts its efficiency to ~1.34% at -0.233 V (vs. RHE) for the STNRs photoanode and ~1.67% at 0.49 V (vs. RHE) for the CZTS/TNRs photoanode. Moreover, the CZTS/S-TNRs nanoarchitecture results in a maximum photoconversion efficiency of ~3.14% at 0.49 V (vs. RHE), which is significantly higher in comparison to that of pristine TNRs, S-TNRs and CZTS/TNRs photoanodes. To evaluate the PEC water oxidation performance of CZTS NPs-sensitized S-doped TNRs, incident photon conversion efficiency (IPCE) measurements were performed. Figure 5d shows the IPCE spectra of the pristine TNRs, S-TNRs, CZTS/TNRs, and CZTS/S-TNRs photoanodes. The pristine TNRs photoanode has a strong photoresponse in the near-UV region but little photoresponse at > 430 nm. However, S doping has a significant influence on the IPCE of TNRs over the UV region and unprecedented visible-light photoactivity beyond 430 nm. The maximum IPCE of S-TNRs in the visible region is ~20% at 450 nm, which is much higher than that of pristine TNRs (~0.4% at 450 nm). Particularly, this IPCE value of ~20% at 450 nm for Sdoped TNRs is the highest among all reported doped TiO2-based photoanodes without the loading of visible-light semiconductors or co-catalysts (see Table S2). Comparatively, this value is higher than the previously reported values of ~4% for core-shell

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S-doped black TiO2,49 ~10% for N-doped TiO2 nanowires (NWs),48 ~3.5% for H and N co-doped TiO2 NWs,50 and ~1 for H-treated TiO2 NWs.11 In contrast, the CZTS NPs-sensitized TNRs (i.e., CZTS/TNRs) photoanode shows substantial photoactivity over the broad visible region from 400 to 800 nm in addition to a strong photoresponse in the near-UV region. These results clearly confirm that CZTS sensitization improves the visible-light absorption and that photogenerated electrons in CZTS NPs can be transferred to TiO2. While S doping and CZTS NPs sensitization individually influence the visible-light photoactivity of pristine TNRs, the IPCE of TNRs is enhanced significantly over the entire visible region to the near-UV region by simultaneous S doping and CZTS NPs sensitization. The enhancement in IPCE is almost equal to the sum of the IPCEs of S-TNRs and CZTS/TNRs, which is in good agreement with the observed enhancement in the photocurrent density. These results again clearly confirm the significant synergistic influence of S doping and CZTS NPs sensitization of TNRs. Remarkably, the obtained photocurrent density and IPCE for the CZTS NPssensitized S-doped TNRs photoanode in our study is the highest ever reported for NPs-sensitized doped TiO2 photoanodes (see Table S1).

Figure 6. (a) Stability test of the CZTS/S-TNRs photoanode. The chronoamperometry curve was measured under simulated light illumination for 120 min. (b) Detected O2 and H2 gases evolved from CZTS/S-TNRs and the Pt counter electrode, respectively by a gas chromatography. To study the stability of our photoanode, which is the main concern, especially for chalcogenide NPs, time-dependent measurements were carried out on the CZTS NPs-sensitized S-doped TNRs (CZTS/S-TNRs) photoanode under simulated light illumination for 120 min. The chronoamperometry, shown in Figure 6a for the CZTS/S-TNRs photoanode at an applied potential of 1.23 V (vs. RHE) shows only a ~20% drop in the initial photocurrent density after 120 min. Due to the smaller size of CZTS NPs, these NPs normally exhibit a much higher density of surface defects in the form of dangling bonds. Under light illumination, free radicals generated by the dangling bonds in CZTS NPs can interact with oxidized species of the electrolyte, resulting in the partial oxidative decomposition of CZTS NPs. This leads to a decrease in the photocurrent density of the CZTS/S-TNRs photoanode after prolonged light illumination, although the achieved stability of our CZTS/S-TNRs photoanode is comparable to the best QDs/NPs-sensitized photoanodes reported in the literature (see Table S2).

Further, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) taking place on the CZTS/S-TNRs photoanode and the Pt counter electrode, respectively, were confirmed by measuring the gases (O2 and H2) evolved from both electrode using gas chromatography (Figure 6b). The steady O2 and H2 gases evolution rate was measured as a function of time at 1.23 V vs. SCE under 100 mW/cm2 illumination. The O2 and H2 gases were linearly evolved from CZTS/S-TNRs photoanode and the Pt counter electrode, respectively, over a period of 2 h (~ 27 µmol of O2 and ~ 77 µmol of H2). The slight deviation in the O2 evolution rate than the stoichiometry could be attributed to slow kinetics of oxygen evolution in the absence of oxygen evolution catalysts.51 The faradaic efficiency (actual gas evolution rates/calculated amount of photocurrent generation) during the reaction was about 61-88%. The loss in the faradaic efficiency for O2 and H2 probably comes from the unwanted backward reaction with H2 and O2 or the slow kinetics of water oxidation, respectively.51

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Charge dynamics of CZTS NPs sensitized S-doped TNRs photoanode. Electrochemical impedance spectroscopy (EIS) was employed to investigate the electrochemical behavior of the photoanodes. Figure 7a shows the EIS results presented in the form of Nyquist plots. The symbols in Figure 7a represent the obtained experimental data, whereas the solid lines show the fitting of experimentally obtained data to an equivalent RC circuit model shown in Figure S4. The RC circuit model comprises one resistor and 2-RC elements in series representing the bulk photoanode and photoanode/electrolyte, two major interfaces within photoanodes forming impedance elements. The low frequency region in Nyquist plots (Figure 7a) is assigned to the photoanode/electrolyte charge transfer resistance, RCT, alongwith the Helmholtz capacitance (CH), whereas the high frequency arc is designated to the processes occurring in the bulk photoanode as represented by the interfacial charge transfer resistance (RSC), and the space charge capacitance (CSC).52 The impedance measurements of four different photoanodes were carried out using the standard three-electrode configuration under the dark conditions in the frequency range of 0.1 Hz to 10 kHz with an amplitude of ± 10 mV. The fitted parameters are summarized in Table S4. The TNRs photoanode exhibits an Rs of 82 Ω. However, the Rs is found to decrease in the S-TNR photoanode due to the increased conductivity in the TiO2 crystal system after annealing under S atmosphere. In contrast, the values of Rs are higher in the CZTS/TNRs and CZTS/S-TNRs photoanodes compared to those of TNRs and S-TNRs pho-

toanodes due to the series resistance being oriented after the sensitization of the CZTS NPs on the TNRs. Since, the water oxidation takes place at TNRs/electrolyte, the larger RCT and double layer capacitance are observed here. Thus, the pristine TNRs photoanode exhibits a considerably higher a RCT and capacitance (see Table S4) suggesting an unfavorable environment for the charge transfer process at the semiconductor depletion region as well as the photoanode/electrolyte interface. Interestingly, S-doping and CZTS NPs sensitization significantly improved charge transfer characteristics of namely bulk TNRs and TNRs/electrolyte in case of S-doping and TNRs/electrolyte in case of CZTS NPs sensitization. In both cases, RCT decreased and capacitances increased, suggesting the enhanced charge separation and charge transport via favorable type II band alignment between CZTS and TNRs. Thus, CZTS/S-TNRs photoanode exhibited better charge transfer characteristics at the bulk TNRs as well as TNRs/electrolyte among all photoanodes as synergistic influence of S-doping and CZTS NPs sensitization. This implies that this photoanode has better intrinsic conductivity and lower charge transfer resistance among all the studied photoanodes. Thus, it is concluded that S-doping and CZTS NPs sensitization onto TNRs simultaneously promotes charge separation and transport via a favorable type II band alignment between CZTS and S-doped TNRs and thus, significantly improved the water oxidation reaction on the surface of TNRs photoanode.

Figure 7. Electrochemical properties and average photoexcited carrier lifetime. (a) Electrochemical impedance spectra (EIS) and (b) Open circuit photovoltage decay (OCVD) curves of the TNRs, S-TNRs, CZTS/TNRs, and CZTS/S-TNRs photoanodes. To further investigate the electron recombination kinetics, open circuit photovoltage decay (OCVD) spectra were measured and are shown in Figure 7b. Analysis of the photovoltage decay gives the electron lifetimes (τn) via the slope of the photovoltage vs. time plot.53 The calculated τn values are presented in Table S4 and indicate that CZTS/S-TNRs has a relatively longer τn compared to that of pristine TNRs, S-TNRs and CZTS/TNRs, which leads to the highest photocurrent density of the CZTS/STNRs photoanode. A nearly 4-fold enhancement in the τn of the CZTS/S-TNRs photoanode compared to that of the pristine TNRs photoanode can be attributed to the suppression of charge recombination between photogenerated electrons from CZTS NPs with the oxidized species in the electrolyte, owing to the formation of the type II band alignment with S-doped TiO2. Therefore, we conclude that as a result of the higher RCT and longer τn, the CZTS/S-TNRs photoanode shows a significantly

enhanced photocurrent density compared to that of the TNRs, STNRs and CZTS/TNRs photoanodes.

Understanding the charge separation and transfer mechanism in CZTS NPs sensitized S-doped TNRs photoanode. To gain deeper insight into the electron-hole transport mechanism in the CZTS/S-TNRs photoanode, we proposed a model based on the relevant electronic band positions of TiO2, S-doped TiO2 and CZTS. The schematic representation of the model band energy diagram is shown in Scheme 1. It is well known that a suitable semiconductor heterojunction with type II band alignment is highly beneficial for efficient charge separation in a typical n-type photoanode since it suppresses electronhole recombination and improves the transport of photoexcited charge carriers.54 The conduction band (CB) edge of TiO2 has been reported to be located at -0.5 V vs. NHE.55 In contrast, the CB edge of CZTS is close to -0.7 V vs. NHE, which is more

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negative than that of the CB edge of TiO2.56 This leads to the formation of a typical type II heterojunction between TiO2 and CZTS. Upon illumination, the photoexcited electrons in the CB of the narrow band gap CZTS are expected to efficiently inject into the CB of TiO2, and then travel towards the Pt counter electrode (CE) via the external circuit. The electrons at the Pt CE then combine with the hydrogen cations to generate H2. Meanwhile, the photoinduced holes in the VB of TiO2 will separate and transfer quickly to the VB of CZTS. The accumulated holes in VB of CZTS take part in oxidation reactions at the pho-

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toanode/electrolyte. This type II band alignment between CZTS and TiO2 increases the driving force for photogenerated charge carrier separation, thus reduces their recombination and enhances the solar water-splitting performance. Although the sensitization of CZTS NPs onto TNRs enhances the photocurrent density, the VB edge of CZTS lies much above the VB edge of TiO2, which may hinder the easy hole transport from the VB of TiO2 to the VB of CZTS (Scheme 1a).

Figure 8. Total DOS of (a) TNRs and (b) S-TNRs. The S doping concentration of ~ 3.125% was considered during the simulation process.

The distortion of the oxygen sublattice in TiO2 is predicted to increase the VB edge, which can thus considerably enhance the hole transport from TiO2 to CZTS by engineering the VB alignment of TiO2. S doping can cause such lattice disorder and shift the VB edge in TiO2, which can lead to considerably enhanced hole transport from the S-doped TiO2 to the CZTS NPs and thus promote efficient photogenerated electron-hole pair separation. Accordingly, the pristine TNRs, S-TNRs, CZTS/TNRs and CZTS/S-TNRs photoanodes were designed, developed and tested further for their solar water-splitting performance. Although the sensitization of CZTS NPs onto TNRs creates type II band alignment, the obtained PEC performance is comparatively good. However, significant improvements in the PEC performance are achieved when CZTS NPs are sensitized onto S-doped TNRs. The EIS measurements further confirm that the CZTS/S-TNRs photoanode outperforms the CZTS/TNRs, STNRs and TNRs photoanodes in terms of charge transport properties. This again simply implies that the type II band alignment between CZTS and TiO2 hinders hole transport in CZTS/TNRs, while it facilitates hole transport in CZTS/S-TNRs via the engineering of type II band alignment (i.e., upshift of the VB edge in TiO2) through S doping. To shed a light on the electronic band structures of TiO2 and S-doped TiO2, DFT calculations were performed using VASP code57-59 and projector augmented-wave (PAW)60 potentials and the obtained results are presented in Figure 8. Figure 8a shows the density of states (DOS) of pure TiO2, yielding the band gap of ~ 3.07 eV, which is close to the experimentally observed band gap of ~ 3.1 eV. From the calculated DOS (Figure 8b) plots, it is revealed that the S 3p states are partially localized, when S is incorporated into TiO2. These partially localized S 3p states can contribute to the formation of the VB, which leads to the shifting of the VB edge upwards resulting in a narrowing of the band gap energy to 2.28 eV in S-

doped TiO245, assuming that the S-dopants are uniformely distributed into the TiO2 lattice. Further, the band gap energy calculated experimentally from the DRS plot revealed a narrowing band gap energy of ~ 2.7 eV for S-TNRs compared to ~3.1 eV for pristine TNRs (Figure S5). Interestingly, an additional band gap tale at ~ 2.28 eV was also observed in the DRS plot for S-TNRs (Inset of Figure S5). This double band gap behavior in S-TNRs is attributed to gradient distribution of S-dopant concentration into the TiO2 lattice. In our previous reports, it has been evidenced that the S-dopant concentration decreased towards the direction of TiO2 nanotube (TNT) core, which was characterized by the element depth profiles in the TEM measurements.43 This S-gradient doping concentration is due to limitation of S solubility in TNRs lattice or limitation of S diffusion to the TNRs lattice at relatively low sulfurization temperature and relatively short sulfurization time. Based on the theoretically calculated band gap energy and experimentally observed double band gap behavior in S-TNRs, it is concluded that Sgradient doping into TNRs lattice led to the band gap grading from ~ 2.28 eV to 2.7 eV along the depth direction of TNRs. This stepwise narrowing of band gap in S-TNRs would lead to the formation of terraced band structure via upshifting of the VB due to S doping in TiO2, which will induce internal driving force for easy hole transport from the VB of S-doped TiO2 to the VB of CZTS and thereby enhances the hole transport properties of the CZTS/S-TNRs photoanode (Scheme 1b). Moreo-

ver, several other factors are responsible for the enhanced PEC performance of the CZTS/S-TNRs photoanodes, such as i) the shifting of the absorption edge towards the visiblelight region originating from the synergistic effect of S doping and CZTS NPs sensitization; and ii) the formation of a larger surface area for efficient immobilization of CZTS NPs onto S-TNRs, which allows the rapid diffusion of free elec-

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ACS Catalysis

tron transport.61 To be specific, S doping and CZTS NPs sensitization synergistically lower the recombination rate of photogenerated electron-hole pairs, thus facilitating the participation of more charge carriers in solar-driven oxidation and significantly enhancing the solar water-splitting perfor-

mance. Further detailed studies on the interfacial charge transfer mechanism between CZTS NPs and S-TNRs are under investigation.

Scheme 1. Schematic of the Energy Band Structure and Charge Transfer Mechanism in (a) CZTS/TNRs and (b) CZTS/S-TNRs nanoarchitectures Conclusions. In summary, we successfully fabricated earthabundant, visible-light absorbing CZTS NPs-sensitized S-doped TiO2 nanoarchitectures for highly efficient solar water splitting. To the best of our knowledge, we demonstrated for the first time that the synergistic influence of earth-abundant CZTS NPs sensitization and S doping in TiO2 nanostructures significantly enhances the visible-light absorption and thereby the photocurrent density of TiO2 nanostructured photoanodes. A maximum photocurrent density of 8.84 mA/cm2 at 1.23 V (vs. RHE) was achieved for CZTS/S-TNRs, which was higher than those of TNRs, S-TNRs and CZTS/TNRs and can be attributed to the enhanced light absorption and improved charge carrier transfer/transport. Based on the DFT electronic calculations and experimental evidences, the VB edge realignment within the CZTS/S-TNRs nanoarchitecture, was traced back to an upshift in the VB due to S doping in TiO2, resulting in a more favorable type II band alignment between the CZTS NPs and S-doped TiO2. This led to efficient charge separation and facile hole transport properties by providing channels within the CZTS/STNRs nanoarchitectures, compared to CZTS/TNRs, and is expected to contribute to the significantly enhanced photocurrent density in the CZTS/S-TNRs nanoarchitectures. Our work demonstrated a straightforward pathway for the design and fabrication of nanoarchitectures via band energy alignment for PEC hydrogen generation and other solar energy-harvesting nanodevices.

Experimental Section. Rutile TNRs were prepared on a fluorine-doped tin oxide (FTO) glass substrate by a previously reported hydrothermal method.41,42 S doping was achieved by annealing the TNRs under S atmosphere using a conventional two-zone furnace at 450 °C for 10 min. Detailed experimental information on S doping into TiO2 can be found in Ref. 43. Furthermore, S-doped TNRs were sensitized with CZTS NPs by a modified SILAR method following our previously published results.46,62,63 Detailed information on the fabrication of TNRs, S-TNRs, and CZTS/TNRs and CZTS/S-TNRs photoanodes can be found in the experimental section of the Supporting Information.

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedure for photoanode fabrication, characterization methods, PEC measurements, detection of O2 and H2 gases using gas chromatography, and computation details. A schematic representation of fabrication process of different photoanodes. Digital photographs of TNR, S-TNR, CZTS/TNR, and CZTS/S-TNR photoanodes. J-V plots under chopped on/off conditions and under light illumination for TNRs photoanodes annealed under S atmosphere at different temperatures for S doping. J-V plots under chopped on/off conditions and under light illumination for TNRs photoanodes after CZTS NPs sensitization with different SILAR cycles. Equivalent circuit model used to stimulate the

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Nyquist plots. Tauc’s plots of pristine and S-doped TNRs. Tables representing reports based on variety of QDs/NPs sensitized TiO2, different nonmetals doped TiO2, QDs/NPs sensitized doped TiO2 nanostructures-based photoanodes for solar water splitting, and Table containing parameters obtained from the EIS fittings and OCVD curves of the TNRs, S-TNRs, CZTS/TNRs, and CZTS/S-TNRs photoanodes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

*Soon Hyung Kang Email: [email protected], TEL: 82-62-530-2497 **Jin Hyeok Kim Email: [email protected] TEL: 82-62-530-1709, Fax: 82-62-530-1699

Author Contributions The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript. Prof. S. H. Kang and Prof. J. H. Kim contributed equally to this work as corresponding authors.

ACKNOWLEDGMENT This work was supported by a Human Resources Development grant (No. 20164030201310) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government Ministry of Trade, Industry and Energy and was partially supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2016936784).

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