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. 2020 Mar 18;5(12):6405–6413. doi: 10.1021/acsomega.9b03876

A Novel Tin-Doped Titanium Oxide Nanocomposite for Efficient Photo-Anodic Water Splitting

Manzar Sohail †,*, Nadeem Baig ‡,§, Muhammad Sher ∥, Rabia Jamil †, Muhammad Altaf #, Sultan Akhtar ⊥, Muhammad Sharif ‡,*
PMCID: PMC7114145  PMID: 32258875

Abstract

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Herein, we report the expedient synthesis of new nanocomposite Sn0.39Ti0.61O2·TiO2 flakes using simple sol–gel and calcination methods. In order to prepare this material, first, we generated a polymeric gel using cost-effective and easily accessible precursors such as SnCl4, titanium isopropoxide, and tetrahydrofuran (THF). A small amount of triflic acid was used to initiate THF polymerization. The calcination of the resulting gel at 500 °C produced a Sn–Ti bimetallic nanocomposite. This newly synthesized Sn0.39Ti0.61O2·TiO2 was characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–visible spectroscopy. The photoelectrochemical (PEC) studies were performed for the first time using Sn0.39Ti0.61O2·TiO2 coated over fluorine-doped tin oxide (FTO) under simulated 1 sun solar radiation. The chronoamperometric study of the Sn0.39Ti0.61O2·TiO2/FTO revealed the repeatable and substantially higher photocurrent for the oxygen evolution reaction (OER) when compared to only TiO2. Moreover, the synthesized material exhibited high stability both in the presence and absence of light. The photocatalytic studies suggested that the sol–gel-synthesized Sn0.39Ti0.61O2·TiO2 can be efficiently used as a photoanode in the water-splitting reaction.

1. Introduction

Nanomaterials represent highly privileged class of advanced materials, which become increasingly important1,2 in a diverse range of fields such as energy, catalysis, sensing, drug delivery, and painting technology.3 Although the number of such materials is literarily unknown, still the development of novel and cost-effective new materials for valued application is highly desired. In particular, the preparation of low-cost nanocomposite materials continues to be a major challenge and is crucial for the advancement of chemical processes related to academic research and industrial production.4−6 Photoelectrochemical (PEC) water splitting is among the most promising approaches in artificial photosynthesis and a potential alternative of fossil fuels. PEC splitting of water was first reported by Fujishima and Honda in 1972 using TiO2 as a catalyst.7−9 Since then, a tremendous amount of work has been reported on PEC water splitting with several nanocomposite catalysts. PEC water splitting utilizes a semiconducting photocathode for H2 evolution (HER) and/or a photoanode for O2 evolution (OER) reactions, making it a convenient way to convert solar energy into renewable fuels that can be easily transferred and transported.10 Among these two reactions, OER is more challenging as it requires 4 moles of electrons per one mole of oxygen produced. For the development of commercially viable devices, one of the crucial challenges is the design of a photoanode with higher efficiency and stability. For water-splitting photocatalysts, one of the major challenges is photocorrosion of the electrodes. Some of the photogenerated holes and electrons do not participate in the oxidation/reduction of water and instead initiate the decomposition of the electrode itself.11

One of the most commonly applied photoactive materials is titanium dioxide; it got attention due to its chemical inertness, photostability, nontoxicity, and cost-effectiveness. However, the higher band gap of TiO2 puts a limitation on its PEC application. Due to its high band gap, it absorbs sunlight only in the ultraviolet (UV) region. Natural light consists of just a small portion of UV light (4–5%), while the visible region ranging from 400 to 800 nm is the major part of sunlight.12,13 The photocatalytic effect generated by absorption of the photon results in the creation of the hole–electron (h+/e–) pairs.14 The anatase TiO2 upon exposure to the UV light produces a good photoresponse.15 However, nonmodified anatase TiO2 absorbs only a small fraction of light for the h+/e– pair generation, and also, recombination of h+/e– pairs is fast. Overall, the TiO2 thin-film photoelectrode’s low photocurrent might be due to the lethargic electrons’ movement to the rear of the electrodes, recombination of electron and hole pairs, and poor electrical conductivity.16

More recently, a number of other materials including WO3, Fe2O3, CuWO4, BiVO4, and SnO2 have been used as photoanodes.17−25 However, the photochemical efficiency of the individual aforementioned semiconductors is low because of fast charge recombination occurring at the interface and surface, leading to low charge transfer and thus lowering the energy conversion.

Among various metallic oxides, the SnO2 can be proven as an excellent photoanode material due to its promising features of low cost, intrinsic stability, excellent charge mobility (100–200 cm2 V–1 s–1), and high electron transfer efficiency.26 However, pure SnO2-based photoanodes exhibit low photocatalytic activity even under UV light because of the small surface to volume ratio, low light capture efficiency, and a large band gap. To address these issues, different semiconducting materials with large surface areas such as nanosheets, nanowires, and nanotubes have been used as a photoanode to improve the light-harvesting ability and active surface area for the electrochemical reaction. The porous anodic tin oxide films fabricated by the Sn anodization and annealing have displayed better PEC performance in the visible region.27 As discussed, the SnO2 possessed better conductivity, nearly two orders of magnitude compared to pure TiO2.27 The combination of SnO2 with other nanomaterials displays different band gaps or Fermi levels, which can improve the PEC performance of the composite material. The SnO2 and TiO2 bilayer structures were used to develop effective heterojunction photoelectrodes.17 Similarly, a lot of efforts have been made on the modification of semiconductors such as TiO2 to improve their PEC behavior either by impurity doping or semiconductor coupling.28,29

Recently, numerous heterostructured semiconductor–TiO2 thin films including CdS·TiO2,30−32 ZnO·TiO2,33,34 FeS2·TiO2,35 and SnO2·TiO236 have been developed as photoelectrodes. Among these doped nanocomposites, the Sn- or SnO2-doped TiO2 is getting great attention in photocatalysis.37,38 This system is widely being explored to improve the PEC behavior of hybrid materials for enhanced water splitting. The doping of Sn2+ or Sn+4 is receiving consideration especially for TiO2 as a small mismatch in the lattice of the TiO2 and SnO2 provides good structural stability and compatibility. Sn doping in TiO2 is considered more effective compared to the other dopings due to the rough similarities of the Sn+4 (0.690 Å) and Ti+4 (0.605 Å).39,40 At the interface, the mixed cation nanocomposite SnxTi1–XO2 can facilitate the excitation by modulating the electronic properties and thus provide a better charge separation.41 SnO2 and TiO2 features are different as SnO2 has greater electron mobility and its conduction band is lower than TiO2.42 A combination of SnO2 and TiO2 imparts unique characteristics to the hybrid material. These electronic properties of SnO2·TiO2 hybrid heterojunction can allow the photogenerated electrons in the TiO2 conduction band migrate to SnO2, while photogenerated holes in the SnO2 valence band migrate to the TiO2.43 Various chemical routes are being adopted for tin doping into TiO2 to enhance the photoactivity of the composite.36,44 Ti1 – xSnxO2 nanocrystalline materials were synthesized by Fresno et al. in which Sn4+ doping was reported with both anatase and rutile phases of TiO2. It was observed that the presence of Sn4+ changes the structural and electronic properties. Ti1–xSnxO2 nanocrystals showed better photocatalytic activity for oxidative decomposition of oxidation of methylcyclohexane dye.45 Chua et al. reported the synthesis of thin films of anatase TiO2 with 3.4% Sn4+ doping through aerosol-assisted chemical vapor deposition. Sn4+-doped anatase displayed 10% enhanced photocatalytic efficiency for stearic acid degradation owing to the decreased rate of electron–hole recombination.46 Arunachalam et al. synthesized Sn4+-doped TiO2 porous indium tin oxide electrodes by spray pyrolysis and showed that Sn4+-doped TiO2 electrodes showed better photocatalytic activities with a decreased band gap.47 Chandra et al. synthesized polythiophene over Sn4+-doped TiO2 and studied that polythiophene performed better for LPG sensitivity due to better photocatalytic activity of Sn4+-doped TiO2.48 In this work, we report on the synthesis of a bimetallic Sn0.39Ti0.61O2·TiO2 nanocomposite using a facile and more convenient sol–gel method and for the first time reported its PEC activity for the OER reaction.

2. Results and Discussion

2.1. Structural Characterization and Morphological Studies of the Sn–Ti Nanocomposite

Figure 1 presents the pXRD diffraction pattern of Sn0.39Ti0.61O2.TiO2, which was characterized by ICDD-BD card no. 01-076-8392 for tin–titanium (Sn0.39Ti0.61O2) and ICDD-BD card no. 01-075-2544 for titania (TiO2) in anatase form. The ratio between Sn0.39Ti0.61O2 and TiO2 was found to be 82.5 and 17.5% by a relative intensity ratio (RIR) analysis method (Figure S1). The TiO2 anatase displayed its characteristics XRD peaks (Figure 1A) at 25.5° (101), 38.3° (004), 48.3° (200), 54.1° (105), 55.4° (211), 63.1° (204), 69.1° (116), 70.6° (220), and 74.5° (215).49 Sn0.39Ti0.61O2 was characterized by major peaks present at 26.8° (110), 34.5° (101), and 52.8° (211), while Sn0.39Ti0.61O2·TiO2 was characterized by peaks at 25.3°, 38.3°, and 47.9° (Figure 1B). All other small peaks also belonged to either Sn0.39Ti0.61O2 or anatase TiO2. The crystallite size calculated by the Debye–Scherer method for Sn0.39Ti0.61O2 ranged from 8–11 nm, while for anatase TiO2, it was found in between 9–14 nm. The synthesized photoactive composite Sn0.39Ti0.61O2 has displayed small-sized nanoparticles compared to the anatase TiO2 nanoparticles. Furthermore, it is evident from the substantial reduction of the XRD peak intensity of the photoactive composite Sn0.39Ti0.61O2 compared to the XRD spectrum of the anatase TiO2.

Figure 1.

Figure 1

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XRD patterns of (A) TiO2 anatase and (B) Sn0.39Ti0.61O2.TiO2 nanocomposites

Sn-doped titanium oxide was further investigated by XPS spectroscopy to get the information of bonding composition. The survey scan XPS spectra of the synthesized material revealed the various electronic orbital signatures of Ti 2p3/2, 1/2, Sn 3d5/2, 3/2 O 1s, and C 1s (Figure 2). The carbon signal might be due to residual C produced during calcination. The binding energy of the C 1s main peak was adjusted to 284.8 eV for compensating sample charging, and all other peaks were corrected accordingly (Figure S2). The well resolved two characteristics peaks Ti2p3/2 and Ti2p1/2 and Sn3d5/2 and Sn3d3/2 were observed in the XPS spectra. XPS spectra (Figure 2b) demonstrate two distinguish peaks of the Sn 3d5/2 and 3d3/2 at 486.668 and 495.190 eV, respectively. The binding energy difference between two peaks of Sn was 8.522 eV. Typically, in the XPS spectra of Ti 2p, peaks appear at 458.6 eV (2p3/2)and 464.4 eV (2p1/2). As presented in Figure 2c, the doping of Sn into TiO2 imparted a blueshift at 464.686 eV. The blueshift in the binding energy was 0.404 eV and 0.286 eV, respectively. This change in binding energy was an indication of successful doping of Sn atoms into the TiO2 instead of just being present at the surface. In Sn-doped TiO2, the O 1s spectra’s prominent peak appeared at 532.274 eV. However, O 1s spectra also revealed another peak at 530.624 eV, which indicated the presence of two types of O present in the nanocomposite (Figure S3).

Figure 2.

Figure 2

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XPS spectra of the Sn0.39Ti0.61O2.TiO2: (a) Survey scan and high resolution (b) Sn 3d5/2 and Sn 3d3/2, (c) Ti 2p3/2 and Ti 2p1/2 states.

The surface morphology of the synthesized tin-doped titanium oxide composite was revealed by field-emission scanning electron microscopy (FESEM). SEM images were evaluated at different magnifications as shown by Figure 3a,b. It was clearly observed that the tin-doped titanium oxide composite demonstrated the flake-like morphology with varying sizes and thicknesses.

Figure 3.

Figure 3

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FESEM images of the synthesized Sn0.39Ti0.61O2.TiO2 composite at different magnifications: (a) 500 μm and (b) 2 μm. The composite Sn0.39Ti0.61O2·TiO2 emerged as flakes of varying thicknesses.

Energy-dispersive X-ray spectroscopy (EDX) study was performed to confirm the elemental existence of the synthesized tin-doped titanium oxide nanocomposite as shown in Figure S4. EDX mapping study of the synthesized composite demonstrates the uniform distribution of the Sn in the TiO2 (Figure S4a). The uniform distribution of the Sn, Ti, and O can be seen in Figure S4b–d, indicating the equal ratio of elements in the composite lattice. The mapping of the Sn0.39Ti0.61O2.TiO2 also revealed that the Sn is successfully incorporated into the TiO2. No data was found for the impurity elements, which proved the pure preparation of the tin-doped titanium oxide nanocomposite. The EDX spectrum (Figure S4e) revealed the presence of Sn, Ti, and O in the synthesized Sn0.39Ti0.61O2·TiO2 flakes. Elemental analysis and EDX elemental mapping images show comparable results. ICP elemental mapping analysis shows 40.8% Sn and 16.9% Ti, while EDX shows 39.6% Sn and 16.8% Ti. FESEM examination, EDX mapping, and EDS elemental results altogether confirmed the successful preparation of the Sn0.39Ti0.61O2·TiO2 composite via a sol–gel route.

Transmission electron microscopy (TEM) was carried out to evaluate the surface morphology and structure of synthesized Sn0.39Ti0.61O2·TiO2 flakes at high resolution. Figure 4 shows the TEM image of one of the tin-based titanium oxide flakes along with the electron diffraction pattern. It was clear that the composite was showing flake-like morphology with almost a uniform thickness as judged by the same contrast. The TEM image further revealed that the flake consisted of smaller particles in the range of 10–20 nm. The particle size of 10–20 nm and crystallite size calculated from pXRD demonstrated that particles consisted of 1–2 crystallites. These nanoparticles collected together and consequently shaped the flake-like morphology. These nanoflakes displayed the crystalline nature when selected-area electron diffraction pattern (SAED) was recorded from the flake region (Figure 4b). The SAED pattern showed the ring patterns where each ring was indexed as (started from the inner ring) 110, 101, 211, 022, etc. in agreement with XRD data.

Figure 4.

Figure 4

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(a)TEM image of the Sn0.39Ti0.61O2·TiO2 composite along with the SAED pattern. The nanocomposite composed of individual particles in the range of 10–20 nm. (b) SAED pattern displaying the crystalline structure of the composite. The major reflections are indexed as 110, 101, 211, and 022.

The Brunaur–Emmet–Teller (BET) method was used to determine the surface area, size of pores, and their distribution. The BET surface area found was 120 m2/g with an average pore diameter of 8.28 nm, and a pore volume of 0.000915 cm3/g was obtained. These results are comparable to the high-surface-area anatase reported in the literature.50,51 See Figure S6.

2.2. UV–Visible Study of the Sn0.39Ti0.61O2·TiO2 Composite

The synthesized photoactive material was further investigated by UV–visible spectroscopy. The analysis of UV–visible spectroscopy facilitates the band gap energy estimation of Sn-doped TiO2. For UV–visible analysis, the material was scanned from 200 to 800 nm. The synthesized material demonstrated an absorbance from 550 to 200 nm. The absorbance capability was enhanced as λ was moving from 550 to 200 nm (Figure 5a). From the absorbance curve, the cut off wavelength was found at 395 nm. The band gap energy calculated using eq 1 was 3.14 eV. Furthermore, the band gap energy calculation was done using diffused reflectance spectroscopy (Figure 5b). The band gap energy was further verified by using the Tauc equation and Kubelka–Munk equation.52 The band gap value was calculated at about 3.11 eV. This value is very close to the UV–visible calculated value of 3.14 eV. This band gap was slightly lower than the band gap of pure TiO2 (∼3.2 eV).

Figure 5.

Figure 5

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(a) UV–visible spectra of Sn0.39Ti0.61O2·TiO2 from 200 to 900 nm and (b) Kubelka–Munk plot from diffused spectra of Sn0.39Ti0.61O2·TiO2 for calculation of the band gap.

2.3. PEC Investigation of the Sn0.39Ti0.61O2·TiO2 Composite

For PEC investigation of the synthesized material, the FTO electrode was drop-casted with the Sn0.39Ti0.61O2·TiO2 as a thin film at the surface of the electrode. The PEC behavior of the Sn0.39Ti0.61O2·TiO2/FTO electrode was explored under 1 sun solar simulator light presence and absence by dipping in a 0.5 M Na2SO4 electrolyte. First of all, the LSV was scanned by switching on and off the light simulator, and it was found that the current was enhanced significantly under light and returned back to the background value as the light was turned off. This behavior of the material revealed that Sn0.39Ti0.61O2·TiO2 was photosensitive (Figure 6).

Figure 6.

Figure 6

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LSV curve recorded with the Sn0.39Ti0.61O2·TiO2/FTO electrode for the OER reaction under chopping light mode.

The background current was also enhanced as the potential was increased beyond 0.5 V. Chronoamperometry was applied to observe the reproducibility and stability of the Sn0.39Ti0.61O2·TiO2/FTO electrode for OER. In chronoamperometry, the current density (J) was recorded at a specific time interval by controlling the on/off mode of the light simulator. The sharp spike of the current was observed as the light was switched on, and after a certain time, the current was leveled off. The photocurrent was increased to 210 μA from about 0 μA when the light was turned on, indicating that the OER started when the light struck the Sn0.39Ti0.61O2·TiO2 surface. The photoanodic current density was sharply decreased, close to the background current, when the light was turned off (Figure 7a). However, under the same set of conditions, TiO2 anatase/FTO has displayed a poor photoelectrochemical response. The maximum current displayed by the TiO2 anatase/FTO photoelectrode under light was 0.05 μA (Figure S5), which was extremely lower compared to the Sn0.39Ti0.61O2.TiO2/FTO electrode photocurrent (210 μA) for OER. The photoelectrochemical analysis of the TiO2 anatase/FTO and Sn0.39Ti0.61O2·TiO2/FTO electrodes have revealed that the resultant composite (Sn0.39Ti0.61O2·TiO2), by doping of the Sn into TiO2, has substantially improved the photoelectrochemical performance of the photoelectrode. This behavior of the Sn-doped TiO2 demonstrated that this nanocomposite is highly sensitive toward light.

Figure 7.

Figure 7

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. (a) Chronoamperometery in light chopping mode and (b) stability of the Sn0.39Ti0.61O2·TiO2/FTO response for the OER reaction under dark and light.

The stability of the Sn0.39Ti0.61O2·TiO2/FTO photoanode was also tested under dark and under 1 sunlight illumination as displayed in Figure 7b. The dark current was very stable, while only a slight reduction in photocurrent was observed over more than 30 min of testing time due to O2 evolution and accumulation of O2 bubbles at the Sn0.39Ti0.61O2·TiO2/FTO surface.

2.4. Possible Mechanism of OER on the Sn0.39Ti0.61O2·TiO2 Photoanode

The doping of Sn into TiO2 has provided a better charge separation in the photoactive material. The interface of the TiO2 and SnO2 facilitates the separation of the hole and electrons by providing the alternate positions for them. The mechanism of the charge separation at the interface of TiO2/SnO2 was proposed in Figure 8. The band gap of SnO2 is wider than TiO2. However, it is interesting that the conduction band (CB) of the SnO2 appeared lower than the conduction band of TiO2. Similarly, the valence band (VB) of the TiO2 appeared above the VB of the SnO2. As the light radiations strike on the interface of the TiO2/SnO2, the holes generated during the active state started to migrate from the VB of SnO2 to TiO2. Similarly, the electrons moved from the CB of TiO2 to SnO2 during the excitation state. This phenomenon has provided the effective separation of the electron–hole pair and increased the lifetime by suppressing their recombination (Figure 8). This is according to the literature that the potential difference among SnO2 and TiO2 permitted the facile migration of photoelectrons from the TiO2 to SnO2 CB.53 Moreover, the mixed cation composition Sn0.39Ti0.61O2·TiO2 accelerates the exciton generation and separation, which enhanced the photocurrent.

Figure 8.

Figure 8

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. Schematic illustration of the photoelectrochemical reaction on the photoanode Sn0.39Ti0.61O2·TiO2-coated FTO.

3. Materials and Methods

3.1. Characterization of the Nanocomposite

The XRD patterns were recorded using a Smart Lab X-ray diffractometer (Rigaku, Japan) with Cu-Kα X-ray radiations (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB 250Xi, Thermo Scientific, UK instrument. The adventitious carbon peak that appeared at a binding energy of 284.8 eV was used as a reference. A UV/vis–NIR spectrometer by “Agilent Cary series” was used for determination of optical properties. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were carried out by a field-emission scanning electron microscope (FESEM; Tescan Lyra-3) equipped with a focused ion beam (FIB) and energy-dispersive spectroscopic (EDS) detector. The TEM analysis of the synthesized composite was performed using a transmission electron microscope (TEM; FEI, Morgagni 268, Czech Republic). Selected-area electron diffraction (SAED) patterns in TEM were recorded to evaluate the structure of the nanocomposite and confirm the XRD data. A TEM sample was dispersed in ethanol and deposited onto a Cu grid holding a holey carbon film. SEM and TEM instruments were operated at working potentials of 20 and 80 kV, respectively. For surface area determination, a Micromeritics (ASAP model) BET analyzer (Micromeritics Headquarters, 4356 Communications Drive, Norcross, GA 30093-2901, U.S.) was used. Before the measurement of surface area, the sample was kept under N2 flow at 250 °C for 5 h to remove moisture and other adsorbed gases. Then N2 adsorption/desorption was performed at 77 K using a liquid nitrogen bath.

3.2. Optical Band Gap Determination

The band gap energy was calculated using the following equation

3.2. 1

where E is the band gap energy (eV), h is Planck’s constant (6.626 × 10–34 J/s), C is the light speed (3.0 × 108 m/s), and λ is the cut off wavelength (nm). Furthermore, the band energy calculation was done using diffused reflectance spectroscopy (Figure 5b). The band gap energy was further verified by using the Tauc equation and Kubelka–Munk equation.52 The following Tauc equation (eq 2) was used for the calculation of the band gap energy

3.2. 2

where α is the absorption coefficient, h represents Planck’s constant, A is a constant, ν represents the frequency, Eg represents the band gap, and the n value is 1/2 for the direct allowed transition.

The Kubelka–Munk equation (eq 3) was used for the calculation of the absorption coefficient (α) by using the diffuse reflectance.

3.2. 3

In eq 3, R represents the absolute reflectance, s represents the scattering coefficient, and k represents the molar absorption coefficient. The Kubelka–Munk function F(R) was calculated by using the assimilated diffuse reflectance spectrum that was equivalent to the absorption coefficient (α). The obtained quantity [F(R)hν] 0.5 was plotted against the photon energy (hν) to obtain the band gap energy. The linear part of the curve was extended to obtain the band gap value.

3.3. Synthesis of Sn0.39Ti0.61O2·TiO2

Tin chloride, titanium isopropoxide, triflic acid (TFC), and tetrahydrofuran (THF) were obtained from Sigma-Aldrich and used without any purification. The detailed synthesis procedure of Sn0.39Ti0.61O2·TiO2 along several other new nanocomposites has been reported recently.54 Briefly, tin(IV) chloride and titanium isopropoxide as precursors for the Sn0.39Ti0.61O2·TiO2 composite were added to 25 mL of tetrahydrofuran (THF), and polymerization of THF was initiated by adding 0.5 mL of triflic acid (TFC) as shown in the equation. In a 50 mL round-bottom flask, tin(IV) chloride pentahydrate (0.01 M, 0.043 mL) and titanium isopropoxide (0.01 M, 3.03 mL) were mixed in 25 mL of THF at 25 °C. Then, 0.5 mL of TFC was added to initiate the polymerization of THF. Slow polymerization of THF was allowed to occur with constant stirring at 25 °C for 4 h. After the formation of polymeric gel, whole reaction products were transferred to a crucible and placed in a furnace. The calcination was performed by raising the temperature to 500 °C at a rate of 4 °C per min and held there for 2 h. After pyrolysis, the catalytic material was cooled to room temperature and stored in glass vials.

3.

3.4. PEC Studies of the Nanocomposite

The PEC measurements were carried out by a conventional three-electrode system in 1 M Na2SO4 (pH = 7) as a supporting electrolyte. The working electrode was an FTO glass coated with 200 μL of 2% nafion suspension of the composite Sn0.39Ti0.61O2·TiO2; Pt gauze served as an auxiliary electrode and the standard Ag/AgCl/3 M KCl as a reference electrode (SCE). All the PEC experiments were performed using a Metrohm Autolab potentiostat (PGSTAT302N) instrument. For solar light in the laboratory, an Oriel Sol3A class AAA solar simulator (Newport) with the following specifications were used: power of 100 mW cm–2 (1 sun), IEC/JIS/ASTM-certified, containing a 450 W xenon lamp, Air Mass 1.5G Filter, UV cut off the filter and 2 × 2 in aperture for output beam.

4. Conclusions

A new sol–gel method is introduced for the synthesis of the photoactive catalyst for PEC splitting of water. The Sn0.39Ti0.61O2·TiO2 catalyst was synthesized by utilizing the acid-catalyzed ring-opening polymerization of THF. The attractive features of the synthetic route of the catalyst are its simple methodology and a potential to synthesize several other nanocomposite materials. The Sn0.39Ti0.61O2·TiO2 was characterized using SEM, TEM, EDX, XRD, UV–visible spectroscopy, voltammetry, and chronoamperometry. The structure characterization confirmed the formation of Sn0.39Ti0.61O2·TiO2 with successful doping of Sn+4 into TiO2. The combination of the VB and CB of the SnO2 and the TiO2 in the nanocomposite provided a heterojunction displaying a great capability to harvest natural light for efficient splitting of water. The Sn0.39Ti0.61O2·TiO2 demonstrated good stability under both light and dark conditions. This material may prove to be a good addition as a photoanode for PEC water splitting.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03876.

  • Relative intensity ratio analysis from pXRD, C 1s high-resolution XPS spectrum, O 1s high-resolution XPS spectrum, EDX mapping images and spectrum, chronoamperometery in light chopping mode of TiO2, and BET adsorption–desorption isotherm graph for Sn0.39Ti0.61O2·TiO2 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b03876_si_001.pdf (466.2KB, pdf)

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