Sulfur doped ceria-titania (S-CeTiO4−x) nanocomposites for enhanced solar-driven water splitting

Sulfur doped ceria-titania (S-CeTiO4−x) nanocomposites for enhanced solar-driven water splitting

Solar Energy 188 (2019) 890–897 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Sulfur dop...

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Solar Energy 188 (2019) 890–897

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Sulfur doped ceria-titania (S-CeTiO4−x) nanocomposites for enhanced solardriven water splitting

T

Muhammad Qamaruddina, Ibrahim Khana,b, Oluwole Olagoke Ajumobic, ⁎ Saheed Adewale Ganiyua,d, Ahsanulhaq Qurashia, a

Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia Center for Integrative Petroleum Research, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia c Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia d Department of Chemistry, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ceria (CeO2) Ceria/Titania Sulfur doping Water splitting Photocurrent

In this work the synthesis of sulfur-doped cerium-titania based nanocomposites is carried out for photoelectrochemical (PEC) water splitting applications. Pristine ceria (CeO2), titania (TiO2) and S-CeTiO4−x nanohybrid have been synthesized via facile hydrothermal technique. Additionally, sulfur doping is performed at 350 °C to achieve sulfidized-CeTiO4−x (S-CeTiO4−x) nanohybrid with improved optoelectronic properties. The energy disperse x-ray spectrometer (EDS) spectra and elemental mapping of S-CeTiO4−x showed the presence of sulfur. The X-Ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD) analysis further confirm sulfur doping and composite formation. Additionally, the XRD patterns of S-CeTiO4−x suggest the anatase phase of TiO2, which is slightly mitigated with sulfidation. The Brunauer–Emmett–Teller (BET) isotherms indicate the average pore size decreases from 20.82 nm to 18.35 nm after sulfidation, which confirms successful sulfur incorporation in the pores. The Kubelka-Munk plots acquired from UV/Vis-diffuse reflectance spectroscopy (DRS) displayed a substantial red shift in the bandgap with sulfidization from 3.00 eV to 2.50 eV. The photoelectrochemical (PEC) water splitting of S-CeTiO4−x photoanode in terms of photocurrent density suggesting more than 3-times increase as compared to pristine TiO2 nanoparticles. These results affirm the photoelctrocatalytic nature of ceriabased nanostructures for PEC water splitting.

1. Introduction Incorporation of distant moieties in the lattice of wide bandgap semiconductor materials significantly alters their optoelectronic characteristics and made them suitable for various intriguing applications. The incorporation can be carried out via various doping techniques. Different materials are useful for incorporation, but the non-metals especially nitrogen, phosphorous and sulfur are considered more suitable due to their abundant availability, low cost and reasonable catalytic properties. In this scenario, we expect the sulfidized products to be more useful in photoelectrochemical (PEC) water splitting applications. The PEC water splitting was initially demonstrated experimentally by Fujishima and Honda (1972) using n-type TiO2; and it is one of the evolving green energy harvesting techniques for renewable hydrogen generation. TiO2 is still regarded an important semiconductor for solar energy conversions due to good chemical stability, efficient oxidizing activity, reasonable photocorrosion resistance, and adequate



photocatalytic reaction. TiO2 existed mainly in two crystalline forms i.e. anatase and rutile with the wide bandgaps of about 3.2 and 3.0 eV, correspondingly. Various studies have been devoted to the advancement of the photocatalytic characteristics of pristine TiO2 with a focus on improving visible light absorption by employing the absorption power of TiO2 via the addition of dopants (Piskunov et al., 2015). The photoactivity of TiO2 can be increased via doping, composite formation, and co-catalysis etc. (Ochiai and Fujishima, 2012; Barreca et al., 2015; Khan and Qurashi, 2018). Both cation and anion dopants are used to reduce the bandgap to a suitable range. Nevertheless, cationic dopants are susceptible to corrosive processes and hence can extensively affect the chemical stability and other photocatalytic properties including electron trapping and the electron/hole recombination sites (Biedrzycki et al., 2014; Wang et al., 2014), thus decrease the overall efficiency of the photocatalyst. On contrary, anion dopants such as sulfur can increase the stability and absorption capacity of the catalyst and hence sulfur-doped TiO2 drawn noteworthy attention in

Corresponding author. E-mail addresses: [email protected], [email protected] (A. Qurashi).

https://doi.org/10.1016/j.solener.2019.05.058 Received 19 February 2019; Received in revised form 29 April 2019; Accepted 22 May 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.

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method followed by calcination at high temperature to get anatase crystal phase, as reported elsewhere (Stallings and Lamb, 2003).

recent times due to their significance in photocatalytic applications (Swierk et al., 2016; Li et al., 2016). The TiO2 structure can be made oxygen deficient and therefore represented as TiO2 − X, where (x) shows oxygen vacancies/deficiency (Padilha et al., 2016). It has been established that oxygen defects could generate additional electron energy levels and reorganized the optical band positions of TiO2. It is demonstrated that new levels occupy position about 0.75–1.18 eV below the conduction band (CB) (Qin et al., 2016; Weng et al., 2016). The nanocomposites of TiO2 such as α-Fe2O3/TiO2 (Khan and Qurashi, 2018; Song et al., 2017); graphene/TiO2 (Zhang et al., 2010); graphene oxide (GO)/reduced-TiO2 (Li et al., 2016); BiOI/TiO2 (Zhang et al., 2016), p-Co3O4/n-TiO2 (Zhang et al., 2016); BiVO4/TiO2 (Resasco et al., 2016) and V2O5/BiVO4/TiO2 (Sun et al., 2014) showed magnificent water splitting capabilities due to sufficient light absorption and electron/hole separation. Though sustaining long-term chemical stability in the electrolyte is always challenging. Therefore, chemically stable support or catalyst should be mixed with pristine TiO2 nanoparticles (NPs) to achieve sufficient chemical and photochemical stability. In this regard ceria (CeO2) can be an effective supplement as it is highly stable and showed sufficient thermochemical water splitting at relatively high temperature (1500 °C) (Le Gal and Abanades, 2012; Zhao et al., 2016; Scirè et al., 2003). CeO2 has broadly explored in the fields of catalyst (Izu et al., 2002), oxygen sensing (Izu et al., 2002) and electrolyte or anode sensible in solid oxide fuel cells (Zhao and Du, 2017; Lee et al., 2014). Like TiO2, CeO2 is also prone to oxygen vacancies and significant due to its fluorite configuration, diffusion of oxygen ions (O2−) crystals is rapid (D’Angelo et al., 2016; Zabilskiy et al., 2015). More recently few researchers successfully employed CeO2 in photocatalytic CO2 reduction (Abdullah et al., 2015), water decontamination (Eskandarloo et al., 2014) and PEC water splitting applications (Fiorenza et al., 2016; Zhao et al., 2015; Manwar et al., 2016). Thus, by combining ceria with suitable photocatalyst could further enhance the chemically stability and cost-effectiveness of the photoelectrodes in PEC water splitting application (Chen et al., 2014). Effect of sulfur doping and CeO2 incorporation on the overall optical-chemical properties of TiO2 is discussed in this manuscript. The SCeTiO4−x nanostructured composite was synthesized by facile hydrothermal and post-annealing methods. The products were analyzed via various morphological and structural techniques. The measured PEC water splitting performance of TiO2, CeTiO4−x and S-CeTiO4−x nanostructures suggest that CeTiO4−x and S-CeTiO4−x distinctly enhanced the water splitting performance of pristine TiO2 NPs by 2- and 3-times, respectively.

(b) Titania-Ceria (CeTiO4−x) nanocomposite Titania-Ceria (TiO2-CeO2) was also synthesized by wet chemistry hydrothermal technique assisted by stirring and calcination. Titania isopropoxide and cerium (III) nitrate hexahydrate precursors were weighed and dissolved in deionized water in the ratio 75 wt% Ti:25 wt % Ce, and stirred at 600 rpm for 1 hr at 50 °C. Ammonium hydroxide (NH4OH) solution was slowly added after 1 h, which acted as a precipitant, followed by 4 h stirring to induce particle nucleation. The resultant solution was transferred to 45 mL autoclave and placed in a hydrothermal oven at 130 °C for 48 h. The gel product was obtained which was repeatedly washed and centrifuged, followed by drying and calcination at 600 °C for 6 h. (c) Sulfidized Titania-Ceria (S-CeTiO4−x) nanocomposite The sulfidization experiment was performed in a specialized tube furnace, which is supported by the temperature and liquid flow controller. 1M carbon disulphide (CS2) dissolved in cyclohexane was used as a sulphur source, which is relatively less toxic as compared to H2S as a sulfiding agent. The flow rate of CS2 was controlled at 1 mL/min. 1 g of CeTiO4−x powder was supplied in a ceramic alumina boat and placed inside the tube furnace at 350 °C. The sulfidization was performed for 6 h. The yellowish CeTiO4−x powder turns blackish upon successful incorporation of sulphur. The sulfidation of CeTiO4−x was confirmed by XPS analysis. 2.2. Material characterizations X-ray diffraction (XRD) was carried out on the calcined CeO2, CeTiO4−x and S-CeTiO4−x powder samples for structural and crystal phase analysis using Rigaku Miniflex II Desktop X-ray diffractometer for 2θ range of 5–80°, sampling step size of 0.03° and 3.00 scan speed. Jobin Horiba Raman spectrometer (iHR320) with charge-coupled device (CCD) detector at a spectrum window of 30–2000 cm−1, laser (green type, 532 nm) intensity of 50% and an exposure time of 15 s was used for Raman spectra. Field Emission Scanning Electron Microscopy (FE-SEM) was employed for surface morphology analysis using Oxford Instrument, X-Max Scanning Electron Microscope at a high voltage of 20 kV and imaging scaling of 500 nm. The surface area and pores were analyzed by using a Micromeritics ASAP 2020 BET analyzer. The samples were degassed at 180 °C for 4 h under a vacuum to eliminate impurities, prior to N2 physisorption measurement. The optical properties were recorded through UV/Vis-diffuse reflectance (DRS) (Shimadzu UV-2450) and photoluminescence (PL) emission spectroscopies (Perkin- Elmer LS55 fluorescence spectrophotometer).

2. Experimental 2.1. Materials and synthesis (a) Pure Ceria (CeO2) and Titania (TiO2) nanoparticles

2.3. Photoelectrochemical water splitting measurements

Ceria (CeO2) nanoparticles (NPs) was synthesized via facile singlestep hydrothermal technique. In the synthetic procedure, 5.01 g of cerium (III) nitrate hexahydrate (Sigma Aldrich, purity 99.8%) were weighed and dissolved completely in deionized (DI) water with continuous magnetic stirring at 600 rpm for 1 h at 25 °C. Potassium hydroxide (KOH) solution (Sigma Aldrich, purity 99.9%) was slowly added after 1 h, which acted as a precipitant and the solution was further stirred to induce particles nucleation. This mixture transferred to 45 mL Teflon-lined steel autoclave and placed in a hydrothermal oven at 180 °C for 24 h. The colloidal solution was obtained, which was repeatedly washed and centrifuged using deionized water to remove the non-reacted species at neutral pH. The washed product was dispersed in ethanol and dried in air overnight to remove unwanted constituents via evaporation. This was followed by drying at 100 °C in an oven and calcined at 600 °C for 6 h at 10 °C/min ramping temperature. Titania isopropoxide is used to synthesize TiO2 NPs by hydrolysis

The photocatalytic performance of TiO2, CeTiO4−x and S-CeTiO4−x nanostructures were measured in 0.5 M sodium sulfate (Na2SO4) (Sigma Aldrich, purity = 99.9%) electrolyte in a standard three-electrode PEC cell. The uniform pastes of as-synthesized samples were deposited over fluorinated tin oxide (FTO) conducting substrate, which acted as photoelectrodes. The Platinum foil served as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. The potential of the photo-electrode were controlled by a potentiostat (Autolab). Photoelectrodes were illuminated by an artificial sunlight simulator (Oriel Newport) equipped with xenon lamp to simulate AM 1.5 illumination (100 mW/cm2). The photoelectrode was prepared by dispersing 10 mg of the sample in 1 mL ethanol containing 0.5% Nafion. After which the paste of the sample was uniformly deposited over FTO via dip coating technique. The photoelectrodes were dried at 50 °C for 4 h. 891

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fluorite phase structure (Zheng et al., 2011). The composite materials CeTiO4−x and S-CeTiO4−x exhibit the characteristic diffractograms of both components at 3: 1 ratio of TiO2 and CeO2. However, there is a slight shift in 2-θ of main corresponding peaks and reduction in the intensity, which is due to the defect introduced by oxygen vacancies in the composite and possible intervention of cerium ions (Ce4+) with larger ionic radius (0.97 Å), which substitutes titanium ions (Ti4+, ionic radius 0.61 Å) in the lattice CeTiO4−X structure, slightly mitigating the overall unit cell (Tsoncheva et al., 2017; Kho et al., 2017). The data obtained from TiO2 (1 0 1), CeO2 (1 1 1) diffraction peaks, and (1 0 1) peak of the ceria-doped TiO2 were used for estimating crystal size of the prepared samples using Scherrer’s equation (Khan et al., 2017) as shown in the electronic supporting information (ESI) as Table S1. The XPS analysis further illustrates the successful doping of sulfur and formation of CeTiO4−x, The recorded high-resolution deconvoluted XPS spectra of Ti(2p), Ce(3d), O(1s) and S(2p) elements present in SCeTiO4−X nanocomposite given in Fig. 2. The XPS survey spectra of the sample predominantly consist of Ce, Ti, S, and O elements besides carbon, which is a standard. The deconvoluted XPS profile of Ti(2p) shows asymmetric characteristic doublets at 465.26 and 459.54 eV ascribed to the core levels of Ti(2p1/2) and Ti(2p3/2), respectively. These doublets are the characteristic of quadrivalence (Ti4+) ions. The doping of S−2 into CeTiO4−x accounts for the minor shift of TiO2 XPS peaks (∼0.5 eV) from the reported Ti4+ XPS binding energies, indicating the variation of electron density around Ti atoms in the TiO2 crystal. Wang et al. (2017) also observed a similar shift in the case of non-metal doping. The peaks in Ce(3d) deconvoluted spectra in Fig. 2c. matched the 3d3/2 (d1) spin–orbit states and 3d5/2 states (d2), correspond to the Ce4+ and Ce3+ cations state, respectively. This affirms that the exposed

(116) (220) (400)

(204)

JCPDS 34-0394

(400)

(105) (311) (211) (311) (222) (222)

(200) (220) (220)

(004)

(200)

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CeO2

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2 () Fig. 1. XRD patterns for TiO2, CeO2, CeTiO4−X, and S-CeTiO4−X nanoparticles.

2.4. Results and discussion Phase and structural analysis of the prepared catalysts were done by XRD as provided in Fig. 1. The observed patterns of TiO2 at 2θ = 25.5, 37.8, 48, 53.8, and 55°, corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) planes, respectively, suggested the anatase form (Sreethawong et al., 2005). The diffraction patterns exhibited by CeO2 were indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (3 3 1) with cubic-

(b) Ce(3d) Peak Intensity(a.u.)

Peak Intensity(a.u.)

(a) Ti(2p)

2p1/2 465.26 eV

d1d2 d1

2p3/2 459.54 eV

d2

457 458 459 460 461 462 463 464 465 466 467

877

887

Binding Energy (eV)

897

907

917

Binding Energy (eV)

2p3/2 161.54 eV Peak Intensity(a.u.)

(c) O(1s) Peak Intensity(a.u.)

d1

d1 d2 d1

1s3/2 529.34 eV

(d) S(2p)

2p1/2 162.63 eV

1s1/2 531.30 eV

528

529

530

531

532

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162

Binding Energy (eV)

164

166

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Binding Energy (eV)

Fig. 2. High-resolution deconvoluted XPS spectra for various elements of S-CeTiO4−x heterostructure. 892

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Volume adsorbed (cm 3/g )

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TiO2

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Relative pressure (P/Po)

Fig. 3. N2 analysis of pure (TiO2 and CeO2) and composite (CeTiO4−x and S-CeTiO4−x).

agglomeration and probable identical morphologies. Therefore, we also performed the EDS and elemental mapping analysis, which identifies the existing of Ce, Ti, O and S as constituent elements in S-CeTiO4−x heterostructure in well-distributed form. The N2 physisorption of pure TiO2, CeO2, and composite samples with or without sulfidation (CeTiO4−x and S-CeTiO4−x) are analyzed by BET and the isotherms are provided in Fig. 3. As shown in Table 1, the surface area of pure TiO2 is higher than pure CeO2 at 116 m2/g and 88 m2/g, respectively. Similarly, the same trend observed for pore size and pore diameter. The surface area of CeTiO4−x and S-CeTiO4−x are the same, while there is a slight reduction in the pore size distribution after sulfidation process as shown in Table 1. In addition, the observed surface area of CeTiO4−x and S-CeTiO4−x is higher than that of CeO2 but lower to TiO2. This observation is expected as the composite contains 75% TiO2 and 25% CeO2, and it shows that the incorporation of ceria and sulfur are successful. The observed isotherms as presented in Fig. 3. show that all the sample exhibit Type IV isotherm with H1 hysteresis loop, with the exception of CeO2, which demonstrates H2 hysteresis loop (Thommes et al., 2015).

Table 1 Textural Properties of the composite and pure metal-oxide. Sample

BET Surface Area (m2/g)

Total Pore Volume (cm3/g)

Average Pore Size (nm)

TiO2 CeO2 CeTiO4−x S-CeTiO4−x

116 88 91 91

0.24 0.13 0.55 0.48

7.3 5.7 20.82 18.35

surface of the catalyst partially oxidized. The partial oxidation results the creation of oxygen vacancies and suitable for the formation of Ce–O–Ti linkage bonds as reported elsewhere (Ding et al., 2016). The fitted deconvoluted O(1s) data shows two peaks (Fig. 2c.), designating the existence of at least two kinds of oxygen species in the S-CeTiO4−x nanocomposite. The located peak at about 529.34 eV demonstrates characteristic crystal lattice oxygen. The peak at 531.30 eV possibly appears from the chemisorption of oxygen as O-H on the surface. Fig. 2d. describes the deconvoluted XPS profile for the S(2p) regions of the S-CeTiO4−x nanocomposite. Two distinct peaks can be observed at 162.63 and 161.54 eV, which can be ascribed to the core levels of S (2p1/2) and S(2p3/2), correspondingly. The values are steady with the study performed by Mullins and McDonald (2007) and they observed that the adsorbed S2− states i.e. 2p1/2 and 2p3/2 have binding energies lower than 163 eV. Furthermore, no evidence of S6+ (binding energy at 169.0 eV) is distinguished. The XPS results concluded that the CeTiO4−x hybrid is successfully developed and sulfur is incorporated in the S-CeTiO4−x heterostructure. The SEM results show that CeO2, CeTiO4−x and S-CeTiO4−x only exhibited high agglomerated nanoparticles morphology (Fig. S1 in ESI), though no clear evidence of multiphase is observed due to high

3. Raman analysis Raman spectroscopy is an important characterization technique to observe the crystallinity and mode of vibrations of the materials. Analysis of titania, ceria, CeTiO4−x and S- CeTiO4−x bands are observed by Raman analysis. Typically, pure anatase-TiO2 exhibits six Raman active modes of vibrations (Maity et al., 2001). As revealed in Fig. 4, the as-synthesized TiO2 shows a characteristic of pure anatase phases corresponding to the stretching (Eg) mode of vibrations in O-TiO observed at 144, 197, and 630 cm−1, while the symmetric (B1g) and 893

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TiO2

Intesnity (a.u.)

CeO2

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Intensity (a.u.)

CeTiO4-x

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Fig. 4. Raman analysis of pure (TiO2 and CeO2) and composite (CeTiO4−x and S-CeTiO4−x) samples.

photooxidation application.

S-TiCeO4-x TiCeO4-x CeO2 TiO2

4. Optical properties

KM

Fig. 5. represents the optical properties of the samples in terms of Tauc’s plots, which was obtained from UV/vis-DRS. The DRS onsets of TiO2, CeTiO4−x and S-CeTiO4−x nanostructures lie at the bandgap values of 3.2, 2.82 and 2.38 eV, respectively. Therefore, it can be established that incorporation of CeO2 and sulfidation significantly enhanced the absorption capacity of S-CeTiO4−x nanocomposite by changing their optical band positions. The absorption is shifted towards the visible light region and this lower bandgap value is appropriate for PEC water splitting process as the bandgap standard value for this reaction is between 1.5 and 2.5 eV. Moreover, Fig. 6 shows photoluminscence spectroscopy (PL). From the spectra, it is evident that the PL emission intensity at ∼528 nm for the S-CeTIO4−x nanocomposites is much lower than TiO2 and CeTiO4−x nanostructures. The highest emission point is also lied at a relatively higher wavelength (528 nm) as compared to the position of TiO2 and CeTiO4−x nanostructures (∼490 nm). Similarly, in comparison with TiO2 nanoparticles, the CeTiO4−x nanocomposite has lower emission intensity. Similar quenching is reported by various research groups and it is well established that the drastic quenching of the emission may probably lead to sufficient charge separation (Xu et al., 2016). Additionally, the shifting towards higher wavelength can be related to good light absorption in the case of S-CeTiO4−x nanocomposites. These results are suggesting that the formation of the S-CeTiO4−x nanostructure is reasonable and can enhance the visible light absorption capacity.

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

Bandgap Energy (eV) Fig. 5. Tauc‘s plots showing the bandgaps of TiO2, CeO2, CeTiO4−x and SCeTiO4−x nanostructures.

asymmetric (A1g) bending vibrations are at 395 and 520 cm−1, correspondingly (Ohsaka, 1980). The characteristic peak of CeO2 is found at 460 cm−1, which is associated with fluorite structured materials. For the composite material (CeTiO4−x) without sulfidation, the characteristic peaks of both materials were present without peak shift, However, there is a reduction in peak intensity for CeO2 main characteristic peak due to the compositional amount (25%) in the composite. The sulfidation effect was noticeable in the composite material due to the reduction in characteristic peak intensities of CeTiO4−x. This implies that sulfur is well introduced to the composite material for the efficient creation of oxygen vacancies that will be necessary for the

5. Photoelectrochemical water splitting performance The PEC performance of the TiO2, CeTiO4−x and S-CeTiO4−x 894

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850.0M

S-CeTiO4-X

750.0M PL Intensity

photoanode was carried out by linear-sweep voltammetry (LSV) and chronoamperometry. Fig. 7(a). demosntrates the LSV analysis under solar illumination. The photocurrent enhanced with the voltage sweep from the onset potential in all the cases. The onset potential of photocurrent is observed at 0.8 V for CeTiO4−x nanocomposite, and the plateau is witnessed due to photocurrent saturation after 0.9 V, which remains steady until 1.2 V. Important decrease in the onset potential is observed in case of S-CeTiO4−x nanocomposite, as it drops to 0.68 V and larger saturated photocurrent achieved at 0.8 V. The chronoamperometric (I-t) results measured under consecutive light and dark cycles with ∼40 s intervals added more evidence of the results discussed above (Fig. 7b). The I-t ON/OFF cycles revealed through light chopping, the photocurrent enahanced to a maximum value under illumination and reduced in the dark at 1.00 V. The maximum photocurrent density under illumination is in the order ∼0.78, 1.30, and 2.34 mA/cm2 for TiO2, CeTiO4−x and S-CeTiO4−x nanostructures, respectively. Fig. 7(c). shows the I-t stability curve acquired from photoanodes under the light with an illumination period of 2000s. In all cases, the curves show an initial drop and then achieved the equilibrium and sustained it till the course of 2000s. The chemical stability against photocorrosion is a very important factor and these photocatalysts showing much potential to overcome this corrosion as suggested by the stability curves.

TiO2 CeTiO4-X

800.0M

700.0M 650.0M 600.0M 550.0M 500.0M 450.0M 400.0M 437.5

468.75

500

531.25

562.5

593.75

Wavelength (nm) Fig. 6. Photoluminescence (PL) emission spectra of TiO2, CeTiO4−x and SCeTiO4−x nanostructures.

6. Mechanism of photoelectrochemical water splitting The possible mechanism for water splitting by S-CeTiO4−X can be attributed to the surface species involved in PEC assisted HER and OER reactions. This is an established fact that these reactions essentially require hole/electron pairs generations under solar light. Yet, the electrons/holes role and the interfacial reactions are different for different species. For OER reactions, valence band holes (VB) are very important, which oxidized the water into oxygen by removing the electron, whereas in for HER, conduction band (CB) electrons becomes critical as they reduced the protons from water into two hydrogen molecules. Doping of sulfur in TiO2 is widely reported for photocatalytic applications, as it enhances the photocatalytic characteristics due to efficient charge (electron/hole) separation (Devi and Kavitha, 2014; Tang and Li, 2008). Therefore, we can assume similar phenomena in our case. Both CeO2 and sulfur doping enhanced the water splitting capabilities as predicted from their bandgap and photocurrent values. CeO2 composite slightly shifts the XRD peak at 22.4–22.5, due to probable substitution/incorporation of heavy cerium ions (Ce3+ and Ce4+) in the TiO2 framework. On the other hand, the sulfur doping caused XRD peaks to shift from 22.4 to 21.5 of ceria and titania due to smaller size S−2. According to literature, the higher PEC performance of ceria-based catalyst can be attributed to the existence of cerium defects (as revealed from XPS), which act as hole traps (Fiorenza et al., 2016), and thus minimize the recombination of electrons and holes probably. The sulfur content also induced positive effects on the PEC performance by (i) growing the structural defects in ceria (CeO2), and (ii) preferring the CeO2 enrichment on the TiO2 surface. The holes are major contributors to water oxidation on the catalyst interface, while the electrons reduced H+ ions into H2 gas at the platinum electrode. 7. Conclusion We successfully synthesized ceria-based titania and doped sulfur by using more environmental friendly CS2 at 300 °C. The samples morphology showing spherical shaped nanoparticles in agglomerated form. The XRD affirm that S-CeTiO4−x showed a slight shift due to the incorporation of sulfur. The BET also showed lowering of pore size from 20.82 to 18.35 nm with sulfidation. The optical properties via UV/visDRS spectra revealed the decrease in the bandgap from 3.2 eV towards the visible range of solar light i.e. 2.5 eV in case of sulfidized product.

Fig. 7. Photoelectrochemical (PEC) performance of TiO2, CeTiO4−x and SCeTiO4−x nanostructures (a) linear sweep voltammograms (LSV) under light (b) Periodic photocurrent density (I) under regular light ON/OFF intervals and (c) The photocurrent-time (I-t) stability spectra at 1.0 V vs RHE.

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The PEC water splitting results established that S-CeTiO4−x showed almost 3-times and CeTiO4−x showed 1.6-times photocurrent density increase as compared to pristine titania. The onset photocurrent potential observed at ∼0.82 V for CeTiO4−x, which shift to 0.40 V in the case of S-CeTiO4−x. The photocatalysts were found to be highly stable and showed significant electrochemical photoresistance until 2000s. These results concluded that sulfidation and proper catalyst selection can be helpful in obtaining significant PEC water splitting.

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