A Simple Fabrication of Sb2S3/TiO2 Photo-Anode with Long Wavelength Visible Light Absorption for Efficient Photoelectrochemical Water Oxidation

An Sb2S3-sensitized TiO2 (Sb2S3/TiO2) photo-anode (PA) exhibiting a high photo-electrochemical (PEC) performance in water oxidation has been successfully prepared by a simple chemical bath deposition (CBD) technique. Herein, the Raman spectra and XPS spectrum of Sb2S3/TiO2 confirmed the formation of Sb2S3 on the TiO2 coatings. The Sb2S3/TiO2 photo-anode significantly shifted the absorption edge from 395 nm (3.10 eV) to 650 nm (1.90 eV). Furthermore, the Sb2S3/TiO2 photo-anode generated a photo-anodic current under visible light irradiation below 650 nm due to the photo-electrochemical action compared with the TiO2 photo-anode at 390 nm. The incident photon-to-current conversion efficiency (IPCE = 7.7%) at 400 nm and −0.3 V vs. Ag/AgCl was 37 times higher than that (0.21%) of the TiO2 photo-anodes due to the low recombination rate and acceleration of the carriers of Sb2S3/TiO2. Moreover, the photo-anodic current and photostability of the Sb2S3/TiO2 photo-anodes improved via adding the Co2+ ions to the electrolyte solution during photo-electrocatalysis.


Introduction
PEC water splitting using semiconductors has been expected to be one of the ideal ways to realize the conversion of solar radiative energy into hydrogen energy [1][2][3]. In the PEC system, water oxidation is considered the kinetic control step of the whole water splitting process due to the sluggish kinetics of PEC water oxidation through photo-anodes (pAs). Therefore, developing a robust, stable, and economical photo-anode is critical for highly efficient solar water-splitting.
PEC water oxidation using n-type TiO 2 semiconductor pAs has gained considerable attention since 1972 due to their chemical and electrochemical stability in oxidation conditions, easy preparation, and noticeable incident light-to-current conversion efficiencies [4]. However, TiO 2 pAs-based PEC water oxidation is limited because of two serious drawbacks. First, TiO 2 has a wide bandgap (3.0-3.2 eV) that solely responds to the ultraviolet fraction of the solar spectrum (accounts for just 5% of solar irradiation) [5,6]. Second, a high recombination probability of electron-hole pairs leads to decreased incident light-to-current conversion efficiency.

of 10
Among these, the sensitization of TiO 2 utilizing chalcogenide-based (Sb 2 S 3 ) catalysts has received significant attention [19][20][21]31,32]. Sb 2 S 3 possesses a suitable bandgap (1.6-1.9 eV) and a high absorption coefficient (105 cm −1 ) in the visible region, which benefits from absorbing the whole visible and near-infrared range of the solar spectrum. Moreover, Sb 2 S 3 has low toxicity and moisture, making it one of the most potential sensitizers to investigate the sensitization application. Due to these phenomena, Sb 2 S 3 /TiO 2 pAs were fabricated by a sensitization process with Sb 2 S 3 as a sensitizer, exhibiting enhanced PEC water oxidation properties.
Meanwhile, different strategies have been noted for preparing the Sb 2 S 3 /TiO 2 pAs, such as atomic layer deposition (ALD), chemical bath deposition (CBD), successive ionic layer adsorption and reaction, etc. However, the costs associated with these physical methods are comprehensive. CBD has been selected in this article due to its low cost and simple procedure [33][34][35][36]. Moreover, several synthetic strategies need subsequent complex steps involving a somewhat complicated procedure for reproducible Sb 2 S 3 /TiO 2 and developing a simple and reproducible fabrication technique for Sb 2 S 3 /TiO 2 pAs synthesis. Herein, we describe a simple, low-cost, straightforward strategy for fabricating Sb 2 S 3 /TiO 2 pAs using a CBD technique. Sb 2 S 3 were coated and deposited on the TiO 2 coated previously squeezed onto the ITO substrate utilizing a commercially available TiO 2 paste (PST-18NR). The Sb 2 S 3 /TiO 2 pAs show a higher PEC execution in water oxidation than TiO 2 pAs due to the charge separation and transportation acceleration phenomena.

Fabrication of TiO 2 /ITO Electrode
In a typical procedure, the TiO 2 pAs was synthesized as follows. Before coating, the FTO-glass substrate (1.0 cm −2 area) was cleaned up using deionized water, acetone, and ethanol by sonication for 15 min. Then TiO 2 paste was squeezed over an ITO-glass substrate by a doctor-blade coater and dried at 80 • C for 15 min. After repeating the procedure twice, the electrode was calcined at 550 • C under an air atmosphere for 2 h to create a TiO 2 film-coated ITO electrode (TiO 2 /ITO).

Fabrication of Sb 2 S 3 /TiO 2 /ITO Electrode
SbCl 3 (787.1 mg, 3.46 mmol) was solved in acetone (3.03 mL); after the Na 2 S 2 O 3 (30.3 mL, 1 mol/L) solution was added, the Sb 2 S 3 suspension was obtained. Then, the TiO 2 /ITO was immersed into the resulting Sb 2 S 3 suspension for 24 h in the refrigerator at 10 • C. The electrode was washed thoroughly with water and air-dried to obtain the Sb 2 S 3 /TiO 2 /ITO electrode.

Measurement
A Shimadzu XRD-6000 diffractometer (Shimadzu International Trade (Shanghai) Co., Ltd., Shanghai, China) with Cu Kα (λ = 0.15406 nm) radiation was utilized to determine the structure of samples and to obtain the X-ray diffraction (XRD) patterns. Raman microspectroscopic apparatus (Horiba-Jobin-Yvon LabRAM HR, Paris, France) was used to obtain the Raman spectra. At room temperature, a silicon reference sample was calibrated with a theoretical position of 520.7 cm −1 . The spectrophotometer (Shimadzu UV-2700, Shimadzu International Trade (Shanghai) Co., Ltd., Shanghai, China) was used to test the UV-visible diffuse reflectance spectra (DRS). An electron probe microanalysis (JED-2300, JEOL, Tokyo, Japan) operating at 10 kV (acceleration voltage) was used to obtain the energy-dispersive X-ray spectroscopic (EDS) data. The XPS spectra were performed using a Thermo Fisher Scientific ESCALAB Xi+ instrument (Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China) and calibrated in reference to C 1 s peak fixed at 284.2 eV.
All PEC measurements were examined using an electrochemical workstation in a single-compartment PEC cell (Shanghai Chenhua Instrument Co., Ltd. Shanghai, China, CHI760E). The prepared electrode was used as the working electrode. An Ag/AgCl electrode was used as the reference electrode, and a Pt wire as the counter electrode. An aqueous 0.1 M phosphate solution (pH 6.0) was used as an electrolyte in the cell compartments and Ar-gas environments. The cyclic voltammogram (CV) was recorded at a scan rate of 50 mV s −1 at room temperature. The output of light intensity was calibrated as 100 mW cm −2 using a spectroradiometer (USR-40, Ushio Shanghai Inc., Shanghai, China). Light (λ > 500 nm, 100 mW/cm 2 ) was irradiated from an irradiation source of a 500 W xenon lamp (Optical Module X, Ushio Shanghai Inc., Shanghai, China) with a UV-cut filter (λ > 500), and a 0.2 M CuSO 4 was used as a liquid filter for cutting the heat ray. The linear sweep voltammograms (LSV) were measured at a scan rate of 5 mV s −1 . A monochromic light with a 10 nm bandwidth was used with a 500 W xenon lamp using a monochromator for incident photon-to-current conversion efficiency (IPCE) measurements.
Photo-electrocatalysis was executed under the potentiostatic conditions at −0.3 V at 25 • C with the illumination of light (λ > 500 nm, 100 mW/cm 2 ) for 1 h. 0.1 mM Co(NO 3 ) 2 ·6H 2 O was dissolved in the electrolyte solution to test the effect of Co 2+ ions on photo-electrocatalysis.

Optimization of Immersion Time
The UV-vis absorption spectrum of Sb 2 S 3 /TiO 2 /ITO electrodes immersing at different times is shown in Figure 1a. No response was observed for the TiO 2 /ITO electrode in the visible region. On the contrary, a dramatic response was observed for Sb 2 S 3 /TiO 2 /ITO electrodes, which are extended over the whole visible region (450-800 nm), demonstrating that the hole-electron pairs can be separated efficiently after sensitizing with Sb 2 S 3 . Figure 1b shows the plots of the immersion times with the absorbance at 460 nm for the Sb 2 S 3 /TiO 2 /ITO electrodes. The absorbance exhibited an increasing tendency with immersion time increased; the highest absorbance was observed when the immersion time was 24 h. After that, the absorbance decreased when the immersion time increased, suggesting that the optimum immersion condition was 24 h, which was employed in the following experiments.

Characterization Structure of Different Samples
The crystalline structures of the ITO, TiO2 and Sb2S3/TiO2 electrodes were characterized by XRD diffraction (Figure 2). Figure 2Ia shows the peaks of the ITO substrate. The diffracted peaks of TiO2/ITO ( Figure 2Ib) and Sb2S3/TiO2/ITO electrodes

Characterization Structure of Different Samples
The crystalline structures of the ITO, TiO2 and Sb2S3/TiO2 electrodes were characterized by XRD diffraction (Figure 2). Figure 2Ia shows the peaks of the ITO substrate. The diffracted peaks of TiO2/ITO ( Figure 2Ib) and Sb2S3/TiO2/ITO electrodes ( Figure 2Ic) were clearly detected at 2θ = 25.1°, 37.7°, 47.8°, 53.9° and 53.9°, respectively (JSPDF number: 89-4921) [5]. No peaks in terms of Sb2S3 were observed for both electrodes. However, it should be noted that two slightly broadened peaks were detected around 2θ = 12 and 55° for the Sb2S3/TiO2/FTO electrode. Raman spectroscopy was carried out for different powders scratching from the electrodes. Both TiO2 ( Figure 2IIb) and Sb2S3/TiO2 (Figure 2IIc) samples exhibited the typical peaks located at 147, 199, 400, 517, and 638 cm −1 , which correspond to the vibrational modes of pure anatase TiO2 [20,37]. Figure 2IIa shows the peak of 289 cm −1 for Sb2S3, which could be assigned to the Sb-S stretching modes [20,31,[38][39][40]. Moreover, this broad peak was observed in Sb2S3/TiO2, while it was not detected for pure TiO2 (Figure 2IIb), confirming the Sb2S3/TiO2 composite formation. The valence state of Sb2S3/TiO2 was investigated utilizing XPS. As shown in the survey scan in Figure 3a, Sb, Ti, S, and O elements were verified. In the high-resolution Ti 2p spectrum (Figure 3b), two peaks at 458.8 and 464.5 eV were observed, assigned to the binding energies of Ti 2p3/2 and Ti 2p1/2 of the TiO2 lattice, respectively [16,17,37]. The XPS spectrum of Sb2S3/TiO2 exhibited two peaks in the S 2p region at 161.4 and 162.4 eV correspond to S 2p1/2 and S 2p3/2 of divalent S in Figure 3c [16,19,41]. The peaks at 529.3 and 538.7 eV in the Sb 3d region for Sb2S3/TiO2, respectively, are assigned the Sb 3+ oxidation state of Sb and are consistent with the earlier-reported values of Sb2S3 [19,42- The valence state of Sb 2 S 3 /TiO 2 was investigated utilizing XPS. As shown in the survey scan in Figure 3a, Sb, Ti, S, and O elements were verified. In the high-resolution Ti 2p spectrum (Figure 3b), two peaks at 458.8 and 464.5 eV were observed, assigned to the binding energies of Ti 2p 3/2 and Ti 2p 1/2 of the TiO 2 lattice, respectively [16,17,37]. The XPS spectrum of Sb 2 S 3 /TiO 2 exhibited two peaks in the S 2p region at 161.4 and 162.4 eV correspond to S 2p 1/2 and S 2p 3/2 of divalent S in Figure 3c [16,19,41]. The peaks at 529.3 and 538.7 eV in the Sb 3d region for Sb 2 S 3 /TiO 2 , respectively, are assigned the Sb 3+ oxidation state of Sb and are consistent with the earlier-reported values of Sb 2 S 3 [19,[42][43][44]. The relatively weak peak at 531.7 can be assigned to the formation of the Sb-O band at the surface, possibly associated with the oxygen atom of TiO 2 .  The FE-SEM observed the morphologies of the TiO2 and Sb2S3/TiO2 electrodes. Figure  4Ia exhibited the top view of the TiO2 electrode at low magnification, in which a continuous uniform and smoothed surface were formed. This would be beneficial to improve the PEC performance of the Sb2S3/TiO2 electrode due to accelerating the water  The FE-SEM observed the morphologies of the TiO 2 and Sb 2 S 3 /TiO 2 electrodes. Figure 4I(a) exhibited the top view of the TiO 2 electrode at low magnification, in which a continuous uniform and smoothed surface were formed. This would be beneficial to improve the PEC performance of the Sb 2 S 3 /TiO 2 electrode due to accelerating the water oxidation reaction at the surface of the electrolyte solution. The FE-SEM images at high magnification ( Figure 4I(b)) showed that the average particle size of TiO 2 was 10 nm. The surface FE-SEM image of the Sb 2 S 3 /TiO 2 electrode at low magnification is shown in Figure 4I(c), where the larger Sb 2 S 3 particles were deposited on the TiO 2 surface. After that, the 60-100 nm Sb 2 S 3 particles were clearly observed in Figure 4I(d). EDX analyses of the Sb 2 S 3 /TiO 2 electrode were taken to confirm the Sb 2 S 3 deposition and complete the chemical composition information. The elemental maps of the EDX for the Sb 2 S 3 /TiO 2 electrode are shown in Figure 4II. The Ti and O mapping signals ( Figure 4II(c,d)) and S and Sb ( Figure 4II(e,f)) were detected. The distribution of Sb 2 S 3 onto the TiO 2 in the Sb 2 S 3 /TiO 2 electrode appeared clearly.

The Optical Properties of Sb2S3/TiO2
The diffuse reflection spectrum of TiO2 and Sb2S3/TiO2 are presented in Figure 5a. It was observed that the Sb2S3/TiO2 absorb the visible light below 650 nm, exhibiting significant redshift in a broad wavelength region compared to TiO2 (395 nm). Tauc plots based on UV-vis DRS are exhibited in Figure 5b. The TiO2 showed a single absorption

The Optical Properties of Sb 2 S 3 /TiO 2
The diffuse reflection spectrum of TiO 2 and Sb 2 S 3 /TiO 2 are presented in Figure 5a. It was observed that the Sb 2 S 3 /TiO 2 absorb the visible light below 650 nm, exhibiting significant redshift in a broad wavelength region compared to TiO 2 (395 nm). Tauc plots based on UV-vis DRS are exhibited in Figure 5b. The TiO 2 showed a single absorption edge of 3.20 eV, corresponding to the previous report relating to anatase TiO 2 [5]. The Sb 2 S 3 /TiO 2 provided two different slops due to the appearance of the new absorption edges, one of the band energies was 1.94 eV, and another was 2.37 eV. The photo-response of the Sb 2 S 3 /TiO 2 was significantly improved in the visible region. This is attributed to the existence of Sb 2 S 3 , which significantly promotes the separation of the electron-hole pairs.

Photoelectrocatalytic Properties
The action spectra of IPCE were recorded at -0.3 V by utilizing TiO2 and Sb2S3/TiO2 electrodes, as shown in Figure 5c. The IPCE value at 400 nm for Sb2S3/TiO2 electrode (7.7%) is 37 times higher than that for TiO2 electrode (0.21%). Further, the onset wavelength for photo-current generation on Sb2S3/TiO2 electrode was 650 nm (1.9 eV), which is noticeably longer than the TiO2 electrode (400 nm, 3.1 eV). The IPCE action spectra consisted of the UV-DRS data for Sb2S3/TiO2 electrode, indicating that the pC was generated based on the bandgap excitation through collateral excitation from Sb 3d orbital to CB.
Several PEC examinations were performed to reveal the intrinsic PEC performances of TiO2 and Sb2S3/TiO2 electrodes towards water oxidation, as shown in Figure 6. Figure  6a showed the CV of the TiO2 (black) and Sb2S3/TiO2 (red) electrodes in a 0.1M phosphate solution (pH = 6.0). Both electrodes did not have photo-anodic currents in the dark condition (dashed line). Contrary to the TiO2 electrode, which showed a 9.1 A/cm 2 at 0.2 V, the Sb2S3/TiO2 electrode generated a stable photo-anodic current of 42.3 A/cm 2 based on water oxidation over 0.6 V, showing 4.6 times higher value than that of the TiO2 electrode. Such remarkable enhancement of pC density for the Sb2S3/TiO2 electrode can be ascribed to the improvement of light absorption in the visible region. The LSV of two electrodes shown in Figure 6b was taken under chopped/simulated chopped visible light irradiation. Interestingly, it can be observed that the photo-responses of anodic and cathodic currents for the Sb2S3/TiO2 electrode (red) were clearly observed above or below -0.5 V, although hardly any pAs current was observed for the TiO2 electrode (black, insert).
The results suggest that the photo-current direction of the Sb2S3/TiO2 electrode can be modified according to the applied potential. When the potential is below -0.5 V, the charges separation based on bandgap excitation has caused upon irradiation with visible light, then the electrons from ITO are rapidly injected into the valence band of Sb2S3 through the conduction band of TiO2. Afterward, the electron acceptor in the conduction band of Sb2S3 is reduced by the electrons to generate a cathodic current. On the other condition of potential above -0.5 V, Sb2S3 absorbs photons to generate electron-hole pairs, the electrons are injected into the electron transport layer (TiO2) and collected by ITO, and the holes oxidize the water at the surface of Sb2S3 to generate an anodic current. As the light was turned on and off, instantaneous positive and negative transient photocurrents

Photoelectrocatalytic Properties
The action spectra of IPCE were recorded at −0.3 V by utilizing TiO 2 and Sb 2 S 3 /TiO 2 electrodes, as shown in Figure 5c. The IPCE value at 400 nm for Sb 2 S 3 /TiO 2 electrode (7.7%) is 37 times higher than that for TiO 2 electrode (0.21%). Further, the onset wavelength for photo-current generation on Sb 2 S 3 /TiO 2 electrode was 650 nm (1.9 eV), which is noticeably longer than the TiO 2 electrode (400 nm, 3.1 eV). The IPCE action spectra consisted of the UV-DRS data for Sb 2 S 3 /TiO 2 electrode, indicating that the pC was generated based on the bandgap excitation through collateral excitation from Sb 3d orbital to CB.
Several PEC examinations were performed to reveal the intrinsic PEC performances of TiO 2 and Sb 2 S 3 /TiO 2 electrodes towards water oxidation, as shown in Figure 6. Figure 6a showed the CV of the TiO 2 (black) and Sb 2 S 3 /TiO 2 (red) electrodes in a 0.1M phosphate solution (pH = 6.0). Both electrodes did not have photo-anodic currents in the dark condition (dashed line). Contrary to the TiO 2 electrode, which showed a 9.1 A/cm 2 at 0.2 V, the Sb 2 S 3 /TiO 2 electrode generated a stable photo-anodic current of 42.3 A/cm 2 based on water oxidation over 0.6 V, showing 4.6 times higher value than that of the TiO 2 electrode. Such remarkable enhancement of pC density for the Sb 2 S 3 /TiO 2 electrode can be ascribed to the improvement of light absorption in the visible region. The LSV of two electrodes shown in Figure 6b was taken under chopped/simulated chopped visible light irradiation. Interestingly, it can be observed that the photo-responses of anodic and cathodic currents for the Sb 2 S 3 /TiO 2 electrode (red) were clearly observed above or below −0.5 V, although hardly any pAs current was observed for the TiO 2 electrode (black, insert). The results suggest that the photo-current direction of the Sb 2 S 3 /TiO 2 electrode can be modified according to the applied potential. When the potential is below −0.5 V, the charges separation based on bandgap excitation has caused upon irradiation with visible light, then the electrons from ITO are rapidly injected into the valence band of Sb 2 S 3 through the conduction band of TiO 2 . Afterward, the electron acceptor in the conduction band of Sb 2 S 3 is reduced by the electrons to generate a cathodic current. On the other condition of potential above −0.5 V, Sb 2 S 3 absorbs photons to generate electron-hole pairs, the electrons are injected into the electron transport layer (TiO 2 ) and collected by ITO, and the holes oxidize the water at the surface of Sb 2 S 3 to generate an anodic current. As the light was turned on and off, instantaneous positive and negative transient photocurrents were observed under chopper illumination at −0.3 V (Figure 6c).
A negligible photo-current for the TiO 2 electrode (black) was observed. However, the photo-current of 30 A/cm 2 for the Sb 2 S 3 /TiO 2 electrode (red) was observed. It is indicated that the charge carriers are very efficiently separated and limited to recombine after Sb 2 S 3 sensitization. The photo-electrocatalysis was conducted under visible light irradiation (λ > 500 nm, 100 mW/cm 2 ) at −0.3 V in a 0.1M phosphate solution (pH = 6.0) for 1 h (Figure 6d). The I-t profile of the Sb 2 S 3 /TiO 2 electrode exhibited a photo-current of 30 A/cm 2 over the period, while for the TiO 2 electrode, almost no photo-current was observed. The PEC performance of hematite and WO 3 photo-anodes for water oxidation can be enhanced by adding Co 2+ ions into the electrolyte solution to act as catalysts at the electrode interface with the electrolyte solution without any deposition [45,46].
The influence of adding Co 2+ ions on the PEC water oxidation for the Sb 2 S 3 /TiO 2 electrode was measured to prove this assumption. The performance of the Sb 2 S 3 /TiO 2 electrode for water oxidation was improved by the presence of 0.1 mm Co 2+ ions in the phosphate buffer solution. It should be noted that the stability of the photo-current for the Sb 2 S 3 /TiO 2 electrode by the presence of Co 2+ ions was remarkably improved, which consequently generated a 2.7 times higher photo-current of 80 A/cm 2 compared to the absence one. Such a lower decay rate (20%) of photo-current for the Sb 2 S 3 /TiO 2 electrode by the addition of Co 2+ ions demonstrates that the stability is ascribed to the acceleration of the water oxidation reaction between the Sb 2 S 3 /TiO 2 surface and the electrolyte solution interface, which significantly reduce the recombination rate of photo-generated carriers.

Results and Discussion
Two main disadvantages are associated with high recombination of electron-hole pairs and a wide bandgap for TiO 2 . Many efforts have been devoted to noble metal loading, semiconductor recombination, doping and sensitization to enlarge the spectral response range due to improving its PEC performance for efficient water oxidation and promoting electron-hole pairs separation. Sb 2 S 3 possesses a suitable bandgap and a high absorption coefficient in the visible region, which benefits from absorbing the whole visible and near-infrared range of the solar spectrum. Thus, the Sb 2 S 3 /TiO 2 were fabricated by a sensitization process with Sb 2 S 3 as a sensitizer, exhibiting enhanced PEC water oxidation properties. Herein, a simple, low-cost and straightforward strategy for fabricating Sb 2 S 3 /TiO 2 using a CBD technique was reported in this paper.

Conclusions
An Sb 2 S 3 /TiO 2 photo-anode was fabricated utilizing a simple and low-cost CBD strategy. XPS and elemental mapping detected Sb and S signals and Ti and O, demonstrating the formation of Sb 2 S 3 and TiO 2 , respectively. The Sb 2 S 3 /TiO 2 photo-anode can utilize visible light below 650 nm for PEC water oxidation, in contrast to utilization below 395 nm for the TiO 2 photo-anode. The IPCE of 7.7% at 400 nm for Sb 2 S 3 /TiO 2 photo-anode was higher than that of 0.21% for TiO 2 photo-anode by 37 times. The Sb 2 S 3 /TiO 2 photo-anode can generate a photo-anodic current and a photo-cathodic current in contrast to generating a photo-anodic current for the TiO 2 photo-anode. The photo-electrocatalytic performance of Sb 2 S 3 /TiO 2 photo-anode would be further improved by adding Co 2+ ions in the electrolyte solution due to low recombination rates of charge carriers and facilitated electron transfers.