TiO2 Nanowires-Supported Sulfides Hybrid Photocatalysts for Durable

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TiO2 Nanowires-Supported Sulfides Hybrid Photocatalysts for Durable Solar Hydrogen Production Ping-Yen Hsieh, Yi-Hsuan Chiu, Ting-Hsuan Lai, Mei-Jing Fang, Yu-Ting Wang, and Yung-Jung Hsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17858 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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ACS Applied Materials & Interfaces

TiO2 Nanowires-Supported Sulfides Hybrid Photocatalysts for Durable Solar Hydrogen Production Ping-Yen Hsieh,1 Yi-Hsuan Chiu,1 Ting-Hsuan Lai,1 Mei-Jing Fang,1 Yu-Ting Wang,1 and Yung-Jung Hsu1,2,*

1Department

of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

2Center

for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan

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Abstract As the feet of clay, photocorrosion induced by hole accumulation has placed serious limitations on the widespread deployment of sulfides nanostructures for photoelectrochemical (PEC) water splitting. Developing sufficiently stable electrodes to construct durable PEC systems is therefore the key to the realization of solar hydrogen production. Here, an innovative charge transfer manipulation concept based on the aligned hole transport across the interface has been realized to enhance the photostability of In2S3 electrodes toward PEC solar hydrogen production. The concept was realized by conducting compact deposition of In2S3 nanocrystals on the TiO2 nanowires array. Under PEC operation, the supporting TiO2 nanowires functioned as an anisotropic charge transfer backbone to arouse aligned charge transport across the TiO2/In2S3 interface. Because of the aligned hole transport, the TiO2 nanowires-supported In2S3 hybrid nanostructures (TiO2-In2S3) exhibited improved hole transfer dynamics at the TiO2/In2S3 interface and enhanced hole injection kinetics at the electrode surface, substantially increasing the long-term photostability toward solar hydrogen production. The PEC durability tests showed that TiO2-In2S3 electrodes can achieve nearly 90.9 % retention of initial photocurrent upon continuous irradiation for 6 h, whereas the pure In2S3 merely retained 20.8 % of initial photocurrent. This double-gain charge transfer manipulation concept is expected to convey a viable approach to the intelligent design of highly efficient and sufficiently stable sulfides photocatalysts for sustainable solar fuel generation.

Keywords: photocorrosion, In2S3, CdS, solar hydrogen production, interfacial charge dynamics


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1. Introduction Utilization of solar energy to produce storable clean fuels, e.g. hydrogen, is a challenging but important research subject owing to the increasing energy demand and growing environmental concerns.1-3 Photoelectrochemical (PEC) solar hydrogen production from water splitting is a practical means of accessing renewable hydrogen. Since the pioneering work by Honda and Fujishima,4 PEC water splitting on TiO2 electrodes has been widely examined and continues to lead the way toward the realization of hydrogen society.5,6 The utility of TiO2 as photoelectrodes has however been hindered by the large bandgap energy which restricts its photon absorption to merely UV region.7 Much effort has thus been spent in order to extend the photoresponse of TiO2 into the visible region of solar spectrum. Typical strategies include elemental doping,8 molecular sensitization,9,10 plasmonic metal particles decoration11,12 as well as modification with other semiconductors.13 Among the different approaches, introducing narrow-bandgap semiconductors is particularly appealing for endowing TiO2 with visible photoactivity because the resultant hybrid structures may benefit from the feasibility of band alignment engineering. With the unique optical property and favorable band structure, chalcogenide semiconductors, particularly CdS and CdSe, have ushered the successful cooperation with TiO2 for realizing PEC solar water splitting.14,15 Nevertheless, the biological toxicity of Cd element and severe photocorrosion problem remain a poential threat to their widespread operations. The search for non-toxic, sufficiently stable, and visible-responsive semiconductor modifiers is therefore a critical step toward the practical use of TiO2-based photoelectrodes for solar hydrogen production. Compared to CdS and CdSe, In2S3 shares similar band structure but poses less environmental and health concerns. In2S3 has been reported to exist in three different crystallographic forms, including α-In2S3, β-In2S3, and γ-In2S3. By virtue of the high photosensitivity, narrow bandgap and non-toxicity essence, the three polymorphs of In2S3 have been prosperously explored as alternative photoactive components to CdS in a broad spectrum of optoelectronic fields.16-19 In particular, 3 ACS Paragon Plus Environment


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β-In2S3 has peculiar defect structure that can generate distorted electric field to promote charge separation, a key step in photocatalytic energy conversion.20 Despite these intriguing attributes, the operation of In2S3 photocatalysts is still hindered by significant photocorrosion associated with the accumulation of photoexcited holes.21 Under light illumination, sulfides nanostructures are susceptible to photocorrosive oxidation with oxygen and holes in aqueous electrolyte. Inevitable decomposition into soluble sulfates then concomitantly occurs to deteriorate the photocatalytic performance.22 A great deal of endeavor has been given to resolve the photocorrosion issue by passivating the sulfides surface with specific corrosion-resistant materials. Such a surface passivation effect may either prevent direct contact of sulfides nanostructures with electrolyte23 or mediate the hole transfer kinetics at the sulfides surface,24 conducing to increased durability for extended photocatalytic operation. For example, decoration with a thin polymer layer may protect sulfides nanostructures from photocorrosion since polymer acts as a shield layer suppressing the undesirable substitution reaction with electrolyte.25,26 Besides, inorganic modifiers such as ZnO,27 HfO2,28 and graphene29 may also function as a protection layer to retard the photocorrosion reaction for sulfides nanostructures. On the other hand, introducing staggered band alignment at the sulfides surface by means of surface modification has also proven useful for addressing the photocorrosion issue.30-33 Typical examples include MoS2-deposited CdS,34,35 polyaniline-hybridized CdS,36 and graphene quantum dots-modified CdSe,37 in which the hole transfer kinetics at interface can be mediated to ease hole accumulation and therefore improve the photocorrosion resistance. The surface passivation approach however can be a double-edged sword as the introduced modifiers may mantle the entire photocatalyst surface to reduce the number of accessible active sites. Clearly, more breakthrough strategies are required to resolve the photocorrosion issue in order to advance the practical use of the inherently unstable yet essentially appealing sulfides photocatalysts. In this work, we presented an innovative charge transfer manipulation approach to the enhancement of photostability for In2S3 nanostructures toward PEC solar hydrogen production. The approach was based on the compact deposition of In2S3 nanocrystals on the TiO2 nanowires array, which was achieved by using a facile chemical bath deposition method.20 The TiO2 4 ACS Paragon Plus Environment

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nanowires-supported In2S3 hybrid nanostructures (denoted as TiO2-In2S3) were used as photoanode to construct a PEC cell for performing solar hydrogen production. For TiO2-In2S3, the band alignment at interface may promote electron transfer from In2S3 to TiO2 while boost hole transfer from TiO2 to In2S3,38,39 creating a charge transfer scenario beneficial for PEC reactions. Under PEC operation, the supporting TiO2 nanowires functioned as an anisotropic charge transfer backbone to enable unidirectional charge transfer along the longitudinal direction.40-43 As driven by the built-in potential, charge transport across the TiO2/In2S3 interface can be aligned to converge with the longitudinal charge transfer of TiO2. Because of the aligned hole transport, the hole transfer dynamics at the interface and hole injection kinetics into the electrolyte can be facilitated, prohibiting undesired hole trapping and hole accumulation events to ameliorate the photocorrosion resistance. The interfacial charge dynamics and hole injection kinetics of the electrodes were investigated with time-resolved photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The PEC durability tests showed that TiO2-In2S3 electrodes can achieve nearly 90.9 % retention of initial photocurrent upon continuous irradiation for 6 h. Despite slight instability, a nearly 100 % of Faradic efficiency for solar hydrogen production can be attained, illustrating the promise of TiO2-In2S3 as competent electrodes to various PEC applications. In contrast, the pure In2S3 electrodes showed a vast photocurrent decrease, merely retaining 20.8 % of initial photocurrent during the same illumination period. The current charge transfer manipulation approach is generally viable and can be applicable to achieve noticeable photostability improvement for other chalcogenide semiconductors photocatalysts such as CdS

2. Experimental section 2.1. Chemicals. All chemicals, including titanium n-butoxide (Ti[O(CH2)3CH3]4), hydrochloric acid (HCl), indium(III) chloride (InCl3) and thioactamide (C2H5NS), were analytical grade and used as received. Deionized water was used as the aqueous solvent in the synthesis.



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2.2. Preparation of TiO2 nanowires arrays. A typical hydrothermal method was used to grow rutile TiO2 nanowires arrays on the fluorine-doped tin oxide (FTO) coated glass substrates.44 Firstly, the FTO substrates were cleaned by sonification with water and ethanol alternately. On the other hand, 40 mL of concentrated HCl (≥37 wt%) was diluted using 40 mL of deionized water, followed by mixing with 1.33 mL of Ti[O(CH2)3CH3]4 in a beaker under steadily stirring. The resultant clear solution was transferred to a Teflon-lined stainless-steel autoclave (100 mL in volume), in which six pieces of FTO substrates were placed vertically in the solution. The autoclave was sealed and heated in an oven at 150 oC for conducting hydrothermal reaction for 5 h. The substrates on which the TiO2 nanowires arrays were grown were rinsed with deionized water, dried under ambient conditions and then annealed in air at 550 oC for 3 hr.

2.3. Chemical bath deposition of In2S3 and CdS. The deposition of In2S3 nanocrystals on TiO2 nanowires was conducted by using a typical chemical bath deposition method20 with slight modifications. Briefly, InCl3 (0.422 g, 1.91 mmol) and C2H5NS (0.162 g, 2.16 mmol) powders were dissolved in deionized water (30 mL) in a beaker (50 mL in volume). The FTO substrate on which TiO2 nanowires were grown was placed vertically in the solution with the glass side attaching to the beaker wall. After stirred at 90 oC for 2 h, the substrate was taken out, cleaned with deionized water, and subjected to annealing treatment at 350 oC for 2 h under nitrogen atmosphere.45 In this work, three concentrations of In and S precursors (0.64 mmol InCl3 vs. 0.72 mmol C2H5NS, 1.91 mmol InCl3 vs. 2.16 mmol C2H5NS, and 3.82 mmol InCl3 vs. 4.32 mmol C2H5NS) were used for In2S3 deposition to prepare for TiO2-In2S3 with increasing In2S3 contents. The samples were respectively denoted as TiO2-In2S3-1, TiO2-In2S3-3, and TiO2-In2S3-6. For comparison reason, pure In2S3 nanocrystal films were also prepared by conducting the same chemical bath deposition on FTO substrates. Furthermore, a similar chemical bath deposition method was adopted to deposit CdS nanocrystals on TiO2 nanowires. In a typical synthesis, Cd(NO3)2•4H2O (1.177 g, 6.36 mmol) and C2H5NS (0.325 g, 4.32 mmol) powders were dissolved in deionized water (30 mL) in a beaker (50 6 ACS Paragon Plus Environment

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mL in volume), followed by performing the same chemical bath deposition reaction described above. Pure CdS nanocrystal films were also prepared for performance comparison with TiO2-CdS. 2.4. Characterizations. The instrumentation setups for property characterizations can be referred to from our previous works.37,46 The microstructural features of the samples were characterized with scanning electron microscope (SEM, JEOL, JSM-6500F) and high-resolution transmission electron microscope (TEM, JEOL, JEM-ARM200FTH). The crystallinity and compositions were examined using selected-area electron diffraction (SAED, an attachment to TEM), X-ray diffraction (XRD, Bruker, D2 Phaser), and energy dispersive spectrometry (EDS, an accessory of TEM). The steady-state PL spectra were recorded on a fluorescence spectroscope (Horiba, iHR320), in which a He-Cd laser was used as the excitation source (λex = 320 nm). The time-resolved PL spectra were obtained in a customized single photon counting system, where a sub-nanosecond pulsed diode (λ = 320 nm, PicoQuant, PLD 320) with the FWHM less than 600 ps was installed. The data were acquired by recording the repetitively emissive photons from TiO2 component at λem = 420 nm. The UV-visible diffuse reflection absorption spectra were recorded on a spectrophotometer (Hitachi, U3900H) by use of an integrating sphere. The PEC analyses were carried out in a three-electrode cell, which contains a Pt foil counter electrode, an Ag/AgCl (saturated KCl) reference electrode, and the Na2S/Na2SO3 electrolyte. The working electrode was fabricated by affixing a copper cable to the conductive part on the FTO substrate surface. The contact was jointed using silver conducting paint and then sealed with epoxy. All the PEC data including I-V curves, I-t curves and Nyquist plots were obtained on a potentiostat (Autolab, PGSTAT204) under AM 1.5G illumination (100 mW/cm2) produced by a solar simulator (Newport, LCS-100, 94011A). The Nyquist plots were recorded at open-circuit potential by applying an AC signal of 10 mV over the frequency ranging from 100 kHz to 0.001 Hz. The incident-photon-to-current-conversion-efficiency (IPCE) measurements were carried out under monochromatic excitation provided by a monochromator (Horiba, Tunable PowerArc, 0.2 m, 1200 gr/mm, dispersion = 2 nm). The amount of hydrogen produced in the PEC cell was examined by collecting the evolved gas with an air-tight syringe. The sampled gas was injected into a gas chromatography spectrometer (Bruker SCION, 463-MS) for 7 ACS Paragon Plus Environment


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hydrogen quantification. The ultraviolet photoelectron spectroscopy (UPS) data were recorded on an X-ray photoelectron spectrometer (ULVAC-PHI, PHI 5000 VersaProbe II) using He I as the excitation source. Inductively coupled plasma-mass spectrometer (ICP-MS, Agilent, 7500ce) was used to measure the catalyst loading on the electrode.

3. Results and Discussion 3.1. Structural and compositional investigations. Electron microscopy was first utilized to observe the microstructures of the samples. Figure 1(a) shows the SEM morphology for TiO2 nanowires array prepared from the typical hydrothermal reaction. Albeit partially aligned, these nanowires were spatially separated, which guaranteed adequate space for the subsequent compact growth of In2S3 nanocrystals. Upon chemical bath deposition, the TiO2 nanowires surface was covered by a compact layer of In2S3 nanocrystal film. Here, three precursor concentrations were employed to deposit an increasing content of In2S3 on the TiO2 nanowires surface. As Figure 1(b)-(d) present, the deposited In2S3 displayed structural characteristics gradually varying with increasing precursor concentration, from a surface-covered, fluffy layer at low precursor concentration (TiO2-In2S3-1) to a totally-capped, flake-like film at high precursor concentration (TiO2-In2S3-6). A corresponding thickness growth with increasing precursor concentration can also be identified from the cross-sectional SEM images shown in Figure S1 (supporting information). For TiO2-In2S3-1, the grown In2S3 merely covered the TiO2 nanowires surface, showing an entire structural thickness fairly close to the initial TiO2. As to the TiO2-In2S3-3, the deposited In2S3 however infilled the interstices between individual nanowires and even overgrew to cap the entire nanowires array, giving rise to a substantial thickness increase to 0.863 µm. A drastic thickness increase to 1.355 µm was observed for TiO2-In2S3-6, reflecting the structural dominance for the overgrown In2S3 at the high precursor concentration condition. For comparison purpose, pure In2S3 nanocrystal films were also deposited on FTO substrate by using the same chemical bath deposition method. A gradual thickness increase with increasing precursor concentration was also noticed from Figure S2 (supporting information). 8 ACS Paragon Plus Environment

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Figure 1. SEM images of (a) pristine TiO2, (b) TiO2-In2S3-1, (c) TiO2-In2S3-3, and (d) TiO2-In2S3-6.

The detailed crystallographic structures and compositional information of TiO2-In2S3 were learned by TEM, SAED, EDS and XRD analysis. In Figure 2(a), the cross-sectional TEM image clearly displayed high coverage density of the deposited In2S3 for TiO2-In2S3-3. The inserted SAED patterns comprised well-resolved symmetric dots ascribable to single crystalline TiO2 and weak dot-like spots indicative of highly crystallinity of In2S3. The high-resolution TEM image of Figure 2(b) revealed two distinct sets of lattice fringes associated with rutile TiO2 and β-In2S3. Noticeably, In2S3 nanocrystals were deposited on the TiO2 nanowires surface with compact interface, which is propitious for facilitating interfacial charge transfer.47,48 In Figure 2(c) and Figure 3(a), the TEM-EDS elemental mapping data and XRD analyses further verified the growth of β-In2S3 on the rutile TiO2 upon the chemical bath deposition reaction. The pure In2S3 nanocrystal films were also characterized by XRD, showing diffraction patterns assignable to the single-phase β-In2S3 (Figure S3(a), supporting information).



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Figure 2. (a) Typical cross-sectional TEM image along with SAED patterns, (b) high-resolution TEM image taken at the marked region of (a), and (c) TEM-EDS elemental mapping data for TiO2-In2S3-3. In (b), the compact interface was highlighted with a dashed line. 3.2. Optical properties and interfacial charge dynamics. The UV-visible absorption and PL spectroscopy were used to study the optical properties of the samples. In Figure 3(b), the pristine TiO2 nanowires showed a prominent absorption onset at 400 nm, which corresponded well to the reported bandgap energy of 3.1 eV.49 For the three TiO2-In2S3 hybrid nanostructures, the absorption edge was found to extend to the visible region, presumably due to the narrow bandgap of the deposited In2S3. A slight redshift of absorption onset was noticed for TiO2-In2S3 with increasing In2S3 content. Similar phenomenon was also evidenced for pure In2S3 nanocrystal films (Figure. S3(b)), which was ascribable to the larger characteristic size for the grown In2S3 at higher precursor 10 ACS Paragon Plus Environment

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concentration.50 Figure 4(a) presents the steady-state PL spectra for the four TiO2-based nanowires samples, which all exhibited a single emission band associated with the excitonic band edge emission of TiO2. As compared to pristine TiO2, the three TiO2-In2S3 displayed substantially depressed PL intensity, signifying effective charge separation as a result of the interfacial band alignment. For TiO2-In2S3, the photogenerated holes of TiO2 would preferentially transfer to In2S3 due to the more positive valence band level of TiO2 (+2.90 V vs. NHE) than that of In2S3 (+1.50 V vs. NHE).51 Such an interfacial hole transfer process largely reduced the possibility of electron-hole recombination for TiO2, resulting in the suppression of PL emission as observed. The increasingly depressed PL intensity for TiO2-In2S3 with increasing In2S3 content further suggested an increased number of the delocalized holes from TiO2. Note that these delocalized holes could either transfer to In2S3 for promoting effective charge separation or were trapped at the interface resulting in depletion of available charge carriers. To acquire much insightful information on the hole transfer dynamics, time-resolved PL spectra were further obtained. Note that pure In2S3 had a bandgap energy (2.42 eV for In2S3-1, 2.29 eV for In2S3-3, 2.11 eV for In2S3-6) distinct from pristine TiO2 (3.13 eV for TiO2), signifying the different PL emission position between In2S3 (beyond 510 nm) and TiO2 (centered around 420 nm). For the time-resolved PL spectra, the data were obtained by recording the repetitively emissive photons at 420 nm, a region where pristine TiO2 showed significant PL emission while pure In2S3 barely exhibited appreciable PL band. Because the recorded data were not interfered by In2S3 component, the fate of charge carriers produced within TiO2 can be precisely interpreted and its variation with the introduction of In2S3 can be realized in terms of interfacial charge transfer dynamics. The recorded carrier dynamics spectra in Figure 4(b)-(d) were fitted and analyzed using a biexponential function to better interpret the data. Also included in Figure 4(b)-(d) were the carrier dynamics profiles for TiO2-mixed In2S3 samples (denoted as TiO2/In2S3), which were prepared by simply mixing TiO2 nanowires with In2S3 nanocrystals suspension. Here, the TiO2/In2S3 sample represented the control group where the introduced In2S3 was in arbitrarily loose contact with TiO2 surface. As shown in Figure S4 (supporting information), the In2S3/TiO2 sample was characteristic of random distribution of 11 ACS Paragon Plus Environment


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aggregated In2S3 nanocrystals at the vicinity of TiO2 nanowires without intimate contact. The spectral comparison between TiO2-In2S3 and TiO2/In2S3 can then disclose the pivotal role of compact interface in mediating the hole transfer dynamics. As summarized in Table S1 (supporting information), all the dynamics profiles were composed of a slow (τ1) and a fast (τ2) carrier lifetime components that can be related to the radiative and nonradiative relaxation pathways, respectively.52 An intensity-weighted carrier lifetime () was computed to offer an overall comparison. For TiO2-In2S3, the average carrier lifetime was found to decrease with increasing In2S3 content, with the TiO2-In2S3-6 showing the shortest of 3.82 ns. This observation was consistent with the stead-state PL data, suggesting the most increased number of delocalized holes for TiO2-In2S3-6. The drastic increase of amplitude contribution from the fast relaxation component (A2 = 88.2 %) however pointed out the prevalence of hole trapping event for TiO2-In2S3-6. Because the thickness of the overgrown In2S3 was much larger than the effective hole diffusion length (around 619 nm of β-In2S3),53 the delocalized holes were inclined to be trapped and recombined with the localized electrons, giving rise to much faster carrier relaxation to increase the amplitude contribution. Note that this trap state-related relaxation was detrimental to the extended use of In2S3 as photocatalyst because the trapped holes may induce undesirable photocorrosion problem, which will be later illustrated in the PEC measurements. Further insightful information can be learned from the spectral comparison between TiO2-In2S3 and TiO2/In2S3. For TiO2-In2S3-1 and TiO2-In2S3-3, the recorded carrier lifetime was larger than the value of the corresponding control group. For example, TiO2-In2S3-1 showed an average carrier lifetime of 5.05 ns, whereas TiO2/In2S3-1 had an average carrier lifetime of 3.52 ns. For TiO2-In2S3-1, the aligned hole transport across the compact TiO2/In2S3 interface ensured efficient interfacial hole transfer, which suppressed the possible hole trapping event to extend the carrier lifetime. On the contrary, TiO2/In2S3-1 was believed to have many structural imperfections at interface, for example, inter-particle boundary and arbitrarily loose contact of In2S3 with TiO2, which encouraged nanoradiative hole trapping processes to reduce the carrier lifetime. The observation that TiO2/In2S3-1 displayed significantly enhanced amplitude contribution from the fast relaxation term (A2 = 84.4 %) could validate the proposition. Similar 12 ACS Paragon Plus Environment

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contention was considered for TiO2-In2S3-3 and TiO2/In2S3-3, manifesting more efficient interfacial hole transfer dynamics for TiO2-In2S3-3 as a result of the aligned hole transport across the compact interface. The difference in carrier dynamics however became almost negligible for TiO2-In2S3-6 and TiO2/In2S3-6, reflecting a similar hole transfer scenario for the two samples. Based on the relatively high A2 value (A2 = 88.2 % for TiO2-In2S3-6 and A2 = 90.1 % for TiO2/In2S3-6), we considered that nonoradiative hole trapping processes were prevalent for both TiO2-In2S3-6 and TiO2/In2S3-6.

Figure 3. (a) XRD patterns and (b) UV-visible diffuse reflectance spectra for pristine TiO2 and the three TiO2-In2S3. In (a), the patterns of FTO substrate and standard rutile TiO2 and β-In2S3 were also included.



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Figure 4. (a) Steady-state PL spectra for pristine TiO2 and the three TiO2-In2S3. (b)-(d) Time-resolved PL spectra for the three TiO2-In2S3 and the corresponding TiO2/In2S3.

3.3. PEC performance toward hydrogen production. To evaluate the performance of solar hydrogen production, the samples were fabricated as photoanode and examined in a PEC cell. Note that during the PEC operation, Na2S/Na2SO3 was used as the sacrificial electrolyte in order to ensure effective hydrogen production in the cell. The use of sacrificial reagent was also to highlight the inherent instable nature of sulfides nanostructures in the PEC reaction even when sacrificial reagent is present. Figure 5(a) shows the typical linear-sweep voltammograms (I-V curves) recorded on pristine TiO2, pure In2S3 and the three TiO2-In2S3 electrodes under AM 1.5G illumination. It can be seen that the three TiO2-In2S3 electrodes surpassed pristine TiO2 and pure In2S3 in photocurrent generation. This superiority emanated from the effective charge separation caused by the interfacial band alignment as well as the extended light absorption enabled by the deposited In2S3. Among the three TiO2-In2S3 electrodes, the TiO2-In2S3-6 exhibited the highest photocurrent density of 1.74 mA/cm2 at 0 V vs. Ag/AgCl, indicating its most adequate use for PEC solar hydrogen production. 14 ACS Paragon Plus Environment

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However, the otherwise decreased photocurrent density at the high applied potential raised the instability concern for TiO2-In2S3-6. To examine the steady photoresponse of the electrodes, the chronoamperometry scans (I-t curves) were carried out under chopped light irradiation. As Figure 5(b) shows, all the electrodes were prompt in photocurrent generation during the repeated cycles of light on/off operations. The three pure In2S3 electrodes displayed obvious photocurrent decay over time, reflecting the occurrence of severe photocorrosion even though sacrificial reagent was used.35 This observation also illustrates the bottleneck of pure sulfides electrodes for practical use in PEC applications. The gradual photocurrent decay was also noticed for TiO2-In2S3-6, suggesting that photocorrosion also took place on TiO2-In2S3-6 despite the high photocurrent generation. In contrast, both TiO2-In2S3-1 and TiO2-In2S3-3 exhibited little photocurrent decay during the same illumination period. The long-term photostability of these electrodes was further evaluated by monitoring the photocurrent variation under continuous illumination for 6 h. As can be observed from Figure 5(c), the photocurrent density of TiO2-In2S3-6 electrode decreased significantly, from 1.13 mA/cm2 to 0.16 mA/cm2 after 6 h of illumination. This photocurrent drop (86.2 %) was comparable to the extent of photocurrent decay for the corresponding pure In2S3 (around 92.2 % decay for In2S3-6). According to the time-resolved PL data, hole trapping processes were considered prevalent for TiO2-In2S3-6, which may induce photocorrosion to deteriorate the long-term photostability. On the other hand, the photocurrent density on TiO2-In2S3-1 and TiO2-In2S3-3 respectively diminished by 9.1 % and 19.9 % over 6 h, which was substantially better than that attained from pure In2S3 (79.2 % decay for In2S3-1 and 68.8 % decay for In2S3-3). The efficient hole transfer dynamics as a result of the aligned hole transport across the compact interface may account for the largely improved photostability for TiO2-In2S3-1 and TiO2-In2S3-3. The turnover number (TON) of the electrodes toward hydrogen evolution can be estimated by the equation TON =

𝐶 54-57 2𝐹𝑛,

where C is the electric charges (coulomb) participated in the hydrogen

evolution reaction, F is the Faraday constant (96,485 coulomb/mole) and n is the catalyst loading (mole) on the electrode. Here, the C was determined from the recorded photocurrent by assuming a 15 ACS Paragon Plus Environment


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100 % of Faradic efficiency, and n was obtained from ICP-MS analysis. As summarized in Table S2 (supporting information), the three TiO2-In2S3 electrodes all exhibited a substantially higher TON than the corresponding pure In2S3. For instance, the calculated TON of TiO2-In2S3-3 after 6 h of continuous illumination was 104.0, almost double the value of In2S3-3 (TON = 50.1), reaffirming much improved photostability for the current TiO2-In2S3. To further explore the mechanism behind the improved photostability, EIS analysis was conducted on TiO2-In2S3-3 to understand the hole transfer kinetics at the interface between electrode and electrolyte. As Figure 5(d) shows, compared with pristine TiO2 and In2S3-3, the TiO2-In2S3-3 electrode displayed a more compressed arc characteristic of enhanced charge transfer kinetics. It should be emphasized that the EIS analysis was conducted in a PEC system using n-type TiO2-based phtotoelectrodes as the photoanode. As the minority carriers, the photogenerated holes at the photoanode surface were rate-determining charge carriers dominating the redox reaction at the electrode/electrolyte interface. As a result, the resistance component at the photoanode/electrolyte interface recorded from EIS analysis can be directly associated with the hole injection kinetics from photoanode to electrolyte. To interpret the Nyquist plots, a specific equivalent circuit composed of two serial RC elements along with a corresponding constant phase element (CPE) standing for the non-ideal capacitance behavior of the electrode was used for data fitting.58-60 As shown in the inset of Figure 5(d), the equivalent circuit contained an overall series resistance Rs, a charge transfer resistance inside the photoanode Rsc, and a charge transfer resistance at the electrode/electrolyte interface Rct. The Rct component was much significant because it represents the hole injection resistance from electrode surface to electrolyte, which has been regarded as the most determinant factor dictating the PEC performance. As noticed from Table S3 (supporting information), the TiO2-In2S3-3 had a much smaller Rct value than pristine TiO2 and In2S3-3 did, suggesting enhanced hole injection kinetics at the interface between electrode and electrolyte. This enhancement appeared to be related to the aligned hole transfer across the compact TiO2/In2S3 interface, which mediated hole diffusion to the electrode surface for expediting hole injection to the electrolyte. 16 ACS Paragon Plus Environment

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Figure 5. (a) I-V curves of pristine TiO2, pure In2S3 and the three TiO2-In2S3 recorded under darkness and AM 1.5G illumination. (b) The corresponding I-t curves recorded at 0 V vs. Ag/AgCl under chopped light irradiation. (c) Photocurrent stability tests for pure In2S3 and the three TiO2-In2S3 under continuous illumination for 6 h. (d) Nyquist plots measured on pristine TiO2, pure In2S3 and TiO2-In2S3. The equivalent circuit used for data fitting was shown as inset.

To exemplify the use in practice, solar hydrogen production by employing TiO2-In2S3 as the photoanode was further tested. Figure 6(a) presents the hydrogen evolution data collected in the PEC cell by pristine TiO2, In2S3-3 and TiO2-In2S3-3 electrodes. Noticeable hydrogen bubbles can be recognized on the counter electrode surface, signifying effective hydrogen production from the current PEC configuration. The hydrogen evolution rate by using TiO2-In2S3-3 was 20.8 μmol/h, nearly 4-fold increase over pristine TiO2 and In2S3-3 (around 4.2 μmol/h). Importantly, the obtained hydrogen evolution rate was rather approximate to the ideal value estimated from 100 % of Faradaic efficiency. This demonstration corroborates that the photocurrent production from TiO2-In2S3-3 totally emanated from the hydrogen production reaction, rather than other undesirable side redox reactions. The decreased hydrogen production rate of TiO2-In2S3-3 at longer illumination 17 ACS Paragon Plus Environment


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time resulted from the otherwise slight instability as a result of the exposed In2S3 in the PEC reaction. To further understand the photoactive wavelength region for the electrodes, the incident-photon-to-current-conversion-efficiency (IPCE) data were collected. As shown in Figure 6(b), the three electrodes all exhibited IPCE spectral features complying with the corresponding absorption spectra, suggesting that the harvested photons can mostly participate in hydrogen evolution reaction. Compared with pristine TiO2 and In2S3-3, the TiO2-In2S3-3 electrode displayed much higher IPCE values across the UV to visible regions, in accordance with the PEC data. The photoactivity enhancement at UV region mainly resulted from the effective charge separation caused by the interfacial band alignment. On the other hand, the increased visible photoactivity was closely related to the extended light absorption enabled by the deposited In2S3. Along with the improved hole transfer dynamics and enhanced hole injection kinetics, the TiO2-In2S3-3 electrode exhibited noticeable photoactivity with increased durability toward solar hydrogen production.

Figure 6. (a) Experimental vs. calculated hydrogen production data for pristine TiO2, In2S3-3 and TiO2-In2S3-3 recorded at -0.48 V vs. Ag/AgCl (+0.49 V vs. RHE) under AM 1.5G illumination. Inset is the picture of the PEC cell, in which WE, CE, RE, GC and Ar respectively represent working, counter, reference electrodes, gas chromatography spectrometer and argon purging. The datum points denote the experimental values, while the dashed lines represent the calculated results. (b) shows the corresponding IPCE spectra.

3.4. Band structure and plausible mechanism. We proposed a plausible charge transfer mechanism to pinpoint the reasoning behind the distinctive PEC properties for the three TiO2-In2S3 18 ACS Paragon Plus Environment

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electrodes, especially the photostability difference between TiO2-In2S3-3 and TiO2-In2S3-6. To embody the electronic band structure, UPS measurements were carried out on the constituent components of the electrodes, i.e. pristine TiO2, In2S3-1, In2S3-3 and In2S3-6. In Figure 7(a), the UPS spectra of the different components displayed distinct secondary-electron cut-off energy at 16.12, 16.45, 16.43 and 16.19 eV. Subtracting the cut-off energy from the photon energy (hʋ = 21.22 eV) gave rise to the work function value of the component (EF). On the other hand, the UPS spectra at the valence band distribution region provided the energetic information regarding the valence band potential apart from the Fermi level (Evb-EF). As shown in Figure 7(b), the Evb-EF value of TiO2, In2S3-1, In2S3-3 and In2S3-6 was respectively determined as 3.36, 1.60, 1.64 and 1.56 eV. Adding the work function gave rise to the value of valence band potential vs. vacuum level (Evb), which was -8.14, -6.37, -6.43 and -6.59 eV for TiO2, In2S3-1, In2S3-3 and In2S3-6, respectively. By further adding the optical bandgap (3.13 eV for TiO2, 2.42 eV for In2S3-1, 2.29 eV for In2S3-3, 2.11 eV for In2S3-6, as determined from the absorption spectra), the conduction band potential vs. vacuum level (Ecb) can be respectively computed as -5.01, -3.95, -4.14 and -4.48 eV. On the basis of the above results, the band structures of the three TiO2-In2S3 hybrid nanostructures can be delineated, which appeared to be a type-II staggered band alignment. Here, we took TiO2-In2S3-3 as the representative example to specify the charge transfer scenario. As depicted in Figure 7(c), upon light irradiation, the photoexcited electrons of In2S3 would preferentially transfer to TiO2 because of the more negative Ecb level of In2S3. As driven by the internal photovoltage, the separated electrons then passed through the external circuit to the counter electrode for conducting hydrogen evolution. Meanwhile, the less positive Evb level of TiO2 drifted the photogenerated holes in opposite direction from TiO2 to In2S3. The delocalized holes then diffused to In2S3 surface and reacted with the redox couple of electrolyte. These highly oxidative holes can however breed photocorrosive decomposition upon the vulnerable In2S3, which deteriorated the durability of the entire PEC cell. Note that the nanostructural features of the samples and the relatively low conductivity of the supporting FTO substrate may cause inevitable inhomogeneous charging during the UPS measurements. Such a localized charging effect can induce binding energy shift for the 19 ACS Paragon Plus Environment


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ejected electrons, which was accountable for the deviation of the obtained Ecb of TiO2 (-5.01 V vs. vacuum) and In2S3 (-4.14 V vs. vacuum) from the theoretical value (-3.92 V vs. vacuum for TiO2 and -3.02 V vs. vacuum for In2S3).61,62 Despite the deviation, the difference of the obtained Ecb level between TiO2 and In2S3 (0.53~1.06 V) was approximate to the values reported in the literature (around 0.5~1.2 V),20,38,51,63,64 suggesting that the proposed type-II staggered band alignment for the current TiO2-In2S3 was essentially valid. Much importantly, because pristine TiO2 showed vigorous hydrogen evolution in the PEC cell, the actual Ecb level of TiO2 should be cathodic enough to drive hydrogen evolution reaction. As evidenced from the time-resolved PL spectra and EIS data, both TiO2-In2S3-1 and TiO2-In2S3-3 exhibited improved hole transfer dynamics at interface and enhanced hole injection kinetics at electrode surface, substantially increasing their long-term photostability toward solar hydrogen production. As to the TiO2-In2S3-6, the prevalence of hole trapping event as a result of the overgrown In2S3 however effectuated significant photocorrosion, causing rapid photoactivity decay as observed. In the current study, the structural backbone of TiO2 nanowires was deemed as the paramount element for arousing the aligned hole transport across the compact interface and thereby achieving the increased PEC durability. This charge transfer manipulation concept may supply alternative approach to circumventing the photocorrosion issue for sulfides photoelectrodes, which has been studied for decades but very little progress was made. To demonstrate the generality of this concept, the TiO2 nanowires arrays were used as structural backbone for further chemical bath deposition of CdS, a widely explored photocatalyst paradigm for solar hydrogen production. As Figure S5 (supporting information) shows, the resultant TiO2 nanowires-supported CdS hybrid nanostructures shared similar microstructural features to TiO2-In2S3-3, with the deposited CdS infilling the nanowire interstices and slightly capping the top surface. In Figure S6 (supporting information), the long-term photostability test revealed that the photocurrent retention of TiO2-CdS can be largely promoted, which showed nearly 100 % of initial photoactivity upon 6 h of PEC operation. On the contrary, the pure CdS only retained 74.9 % of photocurrent under the same experimental conditions. This demonstration further highlights the versatility of the general, viable charge transfer manipulation approach towards achieving both 20 ACS Paragon Plus Environment

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highly active and sufficiently stable sulfide-based photoelectrodes for durable solar hydrogen production.

Figure 7. UPS spectra for pristine TiO2, In2S3-1, In2S3-3 and In2S3-6 at (a) secondary-electron cut-off region and (b) valence band distribution region. (c) Plausible band structure and charge transfer mechanism for TiO2-In2S3-3.

4. Conclusions In conclusion, an innovative charge transfer manipulation concept based on the aligned hole transport across the compact interface has been realized to enhance the photostability of In2S3 toward PEC solar hydrogen production. Time-resolved PL and EIS data corroborated that the hole transfer dynamics across the TiO2/In2S3 interface and hole injection kinetics to electrolyte can be facilitated for TiO2-In2S3-1 and TiO2-In2S3-3, prohibiting undesired hole trapping and hole accumulation events to ameliorate the photocorrosion resistance. The PEC durability tests showed that TiO2-In2S3-1 electrodes can achieve over 90 % retention of initial photocurrent upon 21 ACS Paragon Plus Environment


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continuous irradiation for 6 h, substantially better than that attained from pure In2S3 (around 20 % retention). Despite slight instability, a nearly 100 % of Faradic efficiency for solar hydrogen production can be gained, illustrating the promise of TiO2-In2S3 as competent electrodes to various PEC applications. The current charge transfer manipulation approach is generally viable and can be applicable to achieve noticeable photostability improvement for other chalcogenide semiconductors photocatalysts such as CdS.

Associate Content Supporting Information SEM images, XRD patterns, UV-visible diffuse reflectance spectra, photocurrent stability tests, fitting data of time-resolved PL spectra and Nyquist plots, and TON data of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information

Corresponding Author *E-mail: [email protected] (Y.-J. Hsu) Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the Ministry of Science and Technology (MOST) of Taiwan under grants MOST 106-2113-M-009-025 and MOST 107-2113-M-009-004. Y.-J. Hsu also 22 ACS Paragon Plus Environment

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acknowledges the budget support from the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan. References 1.

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TiO2 Nanowires-Supported Sulfides Hybrid Photocatalysts for Durable

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