CoreâShell Vanadium Modified Titania@Î²-In2S3 Hybrid Nanorod

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Core−Shell Vanadium Modified Titania@β-In2S3 Hybrid Nanorod Arrays for Superior Interface Stability and Photochemical Activity Asad Mumtaz,†,‡,⊥ Norani Muti Mohamed,*,†,‡,⊥ Muhammad Mazhar,§,⊥ Muhammad Ali Ehsan,∥ and Mohamed Shuaib Mohamed Saheed†,‡ †

Centre of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia ‡ Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia § Department of Chemistry, Faculty of Science, University of Malaya, Lembah Pantai, 50603 Kuala Lumpur, Malaysia ∥ Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum & Minerals, Dhahran 34464, Saudi Arabia S Supporting Information *

ABSTRACT: Core−shell rutile TiO2@β-In2S3 and modified V-TiO2@β-In2S3 were synthesized to develop bilayer systems to uphold charge transport via an effective and stable interface. Morphological studies revealed that β-In2S3 was deposited homogeneously on V-TiO2 as compared to unmodified TiO2 nanorod arrays. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry studies verified the presence of various oxidation states of vanadium in rutile TiO2 and the vanadium surface was utilized for broadening the charge collection centers in host substrate layer and hole quencher window. Subsequently, X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectra confirmed the rutile phases of TiO2 and modified V-TiO2 along with the phases of crystalline β-In2S3. XPS valence band study explored the interaction of valence band quazi Fermi levels of β-In2S3 with the conduction band quazi Fermi levels of modified V-TiO2 for enhanced charge collection at the interface. Photoelectrochemical studies show that the photocurrent density of V-TiO2@β-In2S3 is 1.42 mA/cm2 (1.5AM illumination). Also, the frequency window for TiO2 was broadened by the vanadium modification in rutile TiO2 nanorod arrays, and the lifetime of the charge carrier and stability of the interface in V-TiO2@β-In2S3 were enhanced compared to the unmodified TiO2@ β-In2S3. These findings highlight the significance of modifications in host substrates and interfaces, which have profound implications on interphase stability, photocatalysis and solar-fuel-based devices. KEYWORDS: vanadium-doped TiO2 nanorods, β-Indium sulfide, photoelectrochemical, thin film heterojunction, hole quencher window

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INTRODUCTION Solar water splitting to hydrogen for its use as fuel is one of the most promising practices in energy production because both water and sunlight are earth-abundant.1,2 A variety of photoactive materials have been explored and optimized to improve photoelectrochemical (PEC) properties based on the reflections: small band gap semiconductors with wide spectral response, proper conduction/valence band position that satisfies water oxidization and reduction potentials, fast electron/hole pair separation/transportation, stability and efficiency. However, none of the materials could meet all of these requirements. Although TiO2 has a large band gap of 3.2 eV and a small electron mobility of 1 cm2/V/s, which limit its quantum efficiency, it has been widely studied compared to other metal oxide based semiconductors because of its high © 2016 American Chemical Society

resistance to photocorrosion, viability, and natural abundance. One-dimensional (1D) nanostructures, namely, nanorods and nanotubes, can provide a short diffusion length perpendicular to the charge collecting substrate, yielding a low recombination of electron/hole pairs. While many approaches (e.g., doping with heteroatoms such as transition metals,3 nitrogen,4 and carbon5) have been adopted to reduce the band gap, the efficiency of a TiO2 based PEC cell is low, and further research is necessary, particularly in designing new multicomponent nanostructures with improved interface properties for photoelectrodes. In this regard, alternative routes such as plasmonic Received: October 23, 2015 Accepted: February 8, 2016 Published: February 8, 2016 9037

DOI: 10.1021/acsami.5b10147 ACS Appl. Mater. Interfaces 2016, 8, 9037−9049

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ACS Applied Materials & Interfaces assistance, cocatalysis, heterostructures, and sensitization with small bandgap semiconductors on TiO2 are important.6,7 Heterostructure sensitizing multiband gap and multilayer morphological semiconductors require a careful design of surface area, mutual band gap alignment, and alignment with water redox potentials, cropping solar spectrum, electron/hole pair separation, and transportation through the multilayer interfaces and stability of the multilayer photocatalysts. Bilayer structure has attracted increasing attention due to its potential applications in different fields, such as photoconduction, charge storage, and solar hydrogen generation.7 Cho et al. enhanced the performance of PEC using a bilayer anatase/rutile TiO2 nanorods as hierarchical nanostructures which simultaneously provide the advantages of a rapid charge transfer pathway for carrier injection, excellent light trapping sites and an optimized enhanced surface area for increased reaction sites.8 Common methods used for the development of bilayer nanostructures are presynthesis and postsynthesis sensitization.9,10 In presynthesis, sensitizers are synthesized and loaded on the host nanomaterial that has been sensitized using organic linkers, whereas in postsynthesis, sensitizers are directly grown on the first host nanomaterial using successive ionic layer adsorption and reaction (SILAR), chemical bath deposition (CBD), wet impregnation, electrochemical deposition (ECD), and chemical vapor deposition (CVD) techniques. For detailed study, a review on bilayered nanomaterials outlining the n−n, p−p, and p−n junctioned bilayered strategies has been reported.11 Figure 1 describes the hybrid nanocomposite strategies for carrier charge injection, separation and transportation across the interfaces used in the solar cell devices. As shown in Figure 1, Scheme a, represents the energy band diagram of a bilayer strategy in which first (host) layer (indicated by black line) is a substrate semiconductor and the second (guest) layer (green line) could be any of the narrow band gap nanomaterial from II−VI and III−VI semiconductors, especially their sulfides that have enhanced visible range cropping of solar light. An efficient electron injection from sensitizer (2nd/guest) to substrate (1st/host) layer as shown by electron (e−) arrow is expected but the holes produced in the substrate layer will adversely affect the sensitizer layer and gradually the interface properties, resulting in the PEC performance as observed in the literature.7,12 Among the trilayered strategies with the intention for maximum solar cropping, one worth mentioning is where the band gaps of the trilayered nanomaterials mutually strategized. Here, the alignment of the conduction bands are initially in the order of third layer > second layer > first layer for efficient charge transportation (electron injection) as reported in CdSe/CdS/TiO2 (3rd/2nd/1st).7,13,14 One step further in order to improve the stability of the interfaces and to keep the electron hole pair separated from each other, a third layer is proposed to be mutually aligned with the other layers (2nd and first) which could perform better as illustrated in Figure 1, Scheme b. Here, the conduction band of the third layer utilizes the holes of the second layer and avoiding it from being oxidized (by the holes of the first layer) while the holes of valence band of the third layer could be transferred to first layer for efficient charge separation as well as transportation as suggested by Serpone et al.15 In other words, the third layer can be used to protect the second layer from being oxidized by holes of the first layer. In order to realize the synthesis protocols for bi/trilayered strategy physically, the solution-based deposition methods are commonly used for the sensitization of TiO2-based PEC

Figure 1. A bilayer scheme (a) without internal modifications (recombination centers), (b) with trilayered strategy adopted from ref [15] and (c) with internal modifications (recombination centers, surface defects).

photoanodes. However, percent loading of the guest layer upon host layer by the solution-based method is generally low. Multiple steps are required to increase the percent loading and also the charge transfer from sensitizer (2nd, 3rd guest layer) to TiO2 (1st host layer) could be unfavorably affected in the presence of ionic moieties at the guest−host interface. The loss of charges failing to transport through effective interface due to ionic species could also affect the conversion efficiency. A substantial number of reports regarding mono and bimetallic chalcogenides (AL) (A = Zn, Cd, In, etc., and L = S, Se, Te) cosensitized TiO2 photoanode for PEC has been reported.7,16,13,17−19 Where considerable attention has been given to enhance solar light cropping, electron/hole pair generation and alignment of band gaps, little attention has been given to improve the interfacial interaction of the bilayered systems, charge collection from the sensitizer to the TiO2, their separation and transportation. Therefore, keeping these aspects in view, a nanostructured bilayer strategy has been developed by introducing the recombination centers below the conduction band minimum (CB) or at the surface of first (host) layer which could retain the interface properties efficiently as compared to those without the recombination centers. Developing such centers could quench the holes of the sensitizers (2nd/guest layer) at the interface and maintain its photocatalytic properties from being executed by photocorrosion as shown in Figure 1, Scheme c. 9038


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ACS Applied Materials & Interfaces

measurements in a three electrode configuration. The prepared TiO2 and V-TiO2 nanorods layered with crystalline β-In2S3 were used as the working electrodes (8 × 7 mm), Ag/AgCl electrode saturated with 3 M KCl as a reference and a Pt wire as the counter electrode. The electrolyte contains an aqueous solution of 0.25 M Na2S and 0.35 M Na2SO3 at a pH of 11.9. Prior to the measurement, Ar was bubbled through the solution to expel any dissolved oxygen from aqueous electrolyte. The photocurrent density (J) versus potential (V) was recorded in the range of −1.0 to 0.5 V by linear sweep voltammetry at a scan rate of 20 mV/s performed in a standard three electrode configuration. Solar light simulator was used as the light source, whereby its intensity was calibrated with a standard silicon solar cell and controlled at 100 mW/cm2 under AM 1.5 illumination. The stability photoresponse of different photocatalysts was carried out by chronoamperometric (I−t) study under continuous illumination at a bias of 0.5 V vs Ag/AgCl. For more insight at the electrode− electrolyte interface, we performed electrochemical impedance spectroscopy (EIS) for all the samples on the same workstation under the open−circuit condition with the frequency range of 0.1 Hz to 10 kHz at an alternating current (AC) potential amplitude of 10 mV in dark and light conditions. To explore the electronic properties of the photocatalysts, the carrier concentrations were determined quantitatively using the Mott−Schottky plots at a frequency of 1000 Hz in dark.

It is proposed that the recombination resistance could be enhanced with high density charge injection and separation of e−/h+ pairs could be an added advantage in this bilayer system. This bilayer strategy could be a viable and efficient alternative route as compared to a trilayered strategy, which has already been proposed for the purpose Figure 1, Scheme b. In this work, a suitable precursor was synthesized for β-In 2 S 3 sensitization on substrates of TiO2 and modified V-TiO2 (1st layer) nanorod arrays. The aerosol-assisted chemical vapor deposition (AACVD) method was applied for sensitization. The deposition time was optimized for high surface area and efficient charge transfer at the TiO2@β-In2S heterojunction. The effects of the host (first layer TiO2) modification on the surface coverage of β-In2S3 and collection of charges by the host (first layer TiO2) layer upon its modification were investigated. It was found that high percentage loading, homogeneity and improved interface of modified V-TiO2@βIn2S3 heterojunction can be achieved by the AACVD synthesis method. Overall, this study provides a promising route for future work with emphasis on broadening the charge collection window of host layers from the guest layers in bi- and trilayered photoanodes.

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RESULTS AND DISCUSSION Structural and Morphological Studies. The SEM image of the modified V-TiO2 nanorod arrays grown on FTO substrate is presented in Figure 2a that shows the whole surface of the FTO substrate is covered uniformly with V-TiO2 nanorods. The reaction conditions with respect to Ti precursor are optimized for enhanced surface area, and 0.45 mL of tetrabutyl titanate was found to show better nanorods density and surface area as shown in Figure S1. The tips of the TiO2 nanorod arrays are rough compared to its walls, which suggest the preferred tip growth of TiO2 nanorods. Square shaped nanorods with diameters ranging from 30 to 120 nm are observed to be almost perpendicularly grown upon the surface of FTO substrate, suggesting a tetragonal crystal structure as the expected growth habit.23 The cross-sectional view of the sample in Figure 2b shows that the nanorods are vertically aligned with almost 2−3 μm in length. EDX analysis (Figure 2c) confirmed the presence of the respective elements in the proper ratio and the average content of V in TiO2 is found to be 0.75 atom %. However, high intensity Sn peak is observed from the FTO substrate. Elemental distribution analysis confirms that the vanadium is homogeneously distributed throughout the nanorod arrays, as shown in Figure S2. The surface morphologies of TiO2@β-In2S3 nanorod arrays are shown in Figure 3a−f. The spacing within the TiO2@βIn2S3 nanorods (Figure 3a) is more evident as compared to the nondeposited TiO2 nanorods in Figure 2a. It is also observed that as the deposition time increases, the β-In2S3 loading upon TiO2 nanorods also increases, as shown in Figure 3a−d, but after 30 min of β-In2S3 deposition on TiO2, the nanorods seem to be agglomerated as observed in Figure 3e and f. It is also revealed that the surface of the TiO2 nanorod array is covered by β-In2S3 with intraspaced nanorods indicating a high surface area where deposition is favored at the tips of the TiO2 nanorods as observed from the cross section views of Figures 3b,d,f. A reverse trend appears in Figure 3g,h, whereby the βIn2S3 deposition is homogeneous and favored at the tips and walls of the V-modified nanorods, which is also evident from a high-resolution image shown in Figure 3i. More importantly, the modified V-TiO2@β-In2S3 with 30 min loading time

EXPERIMENTAL AND METHODS

Synthesis of TiO2 Nanorod Arrays. In a typical hydrothermal synthesis,16 15 mL of concentrated HCl (36−38 wt %) was added into 15 mL of deionized water. After the mixture stirred for 5 min, 0.45 mL of tetrabutyl titanate (99.99%) was added to the mixture, and the resultant mixture was allowed to stir for 120 min before being transferred to a Teflon lined stainless steel autoclave with a 75 mL capacity. A piece of FTO substrate with dimension 4 × 2 cm was placed into the solution with the conducting side faced downward. The autoclave was then sealed and the reaction was carried out at 170 °C for 10 h. For vanadium modification, 1.52 mM V2O5 was dissolved in the initial mixture followed by the same procedure. The sample was cooled to room temperature before it was taken out and rinsed with deionized water followed by ethanol, and finally, the as-grown TiO2 nanorod arrays were annealed at 400 °C for 1 h with a heating rate of 5 °C/min under Ar atmosphere. Synthesis of TiO2@β-In2S3 Nanorod Arrays. An amount of 0.1 g (0.775 mmol) of indium dithiocarbamate complex was prepared according to our procedure reported previously.20 The complex was dissolved in ethanol/pyridine (20:5 mL) mixture and β-In2S3 was deposited on TiO2 and modified V-TiO2 nanorod arrays using aerosolassisted chemical vapor deposition (AACVD) at 400 °C, as described earlier.21,22 Sample Characterization. The morphology of the synthesized samples was analyzed using a Zeiss Supra55 VP field-emission scanning electron microscope (FESEM). For more internal insight, Zeiss Libra 200 high-resolution transmission electron microscopy (HRTEM) equipped with electron energy loss spectrometer (EELS) was used with fixed parameters of 1 mrad illumination angle and 5 s exposure time. Diffraction spectroscopy was conducted to confirm the presence of respective elements with their finger prints. Structural phases were recorded using Bruker diffractometer with Cu Kα (λ = 0.1540 nm) as the light source. X-ray photoelectron spectroscopy (XPS) measurements were performed in Thermo Fisher Scientific KAlpha setup using a monochromatic Al Kα1 source (1593 eV). Prior to the measurements, the scale of binding energy (BE) was calibrated with Ag and Cu by setting Ag 3d5/2 and Cu 2p3/2 at 368.26 and 932.67 eV, respectively. The absorption study was done by a UV−vis Cary Series (Agilent Technologies) spectrophotometer. Raman vibrational modes were studied using (Horiba Jobin Yvon HR800) monochromatic laser beam Raman spectrophotometer. The laser beam energy was set at 514 nm for the determination of structural features. Photoelectrochemical (PEC) Measurements. Autolab electrochemical workstation (PGSTAT 302N) was used to perform PEC 9039


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range (JCPDS file no. 25-0390)27 as it reflects the tetragonal βIn2S3 phase and no other phase impurities of red α-In2S3 and In2(SO4)328 are observed. Inorganic Crystal Structure Database ICSD [01-073-1366] also confirms the respective diffraction peaks for tetragonal phase as indexed in Figure 3j. XPS analysis was carried out to study the surface and subsurface chemical states of the prepared samples and is shown in Figure 4. The 3d core of In splits into 3d5/2 (445.08 eV) and 3d3/2 (452.28 eV), which are consistent with the values for In3+ (Figure 4A). The S 2p1/2 (161.78 eV) with its shoulder peak at (162.92 eV) is ascribed to sulfur coordinated to In in βIn2S3 prepared samples with its oxidation state of S2− (Figure 3B).29 The Ti 2p3/2 and 2p1/2 of the TiO2 photoelectron peaks are symmetrical in nature and their binding energy positions are at 459.08 and 464.78 eV respectively, thereby revealing the trivial Ti4+ state of the TiO2 nanorods as indicated in Figure 4C(a)7,30 while no obvious Ti3+ peak is observed. The Ti 2p3/2 and Ti 2p1/2 peaks of the V modified TiO2 show a negative shift in its binding energy to 458.98 and 464.68 eV, respectively as represented in Figure 4C(b), which is consistent with the XRD results and may explicate vanadium doping in TiO 2 . Surprisingly as shown in the inset of Figure 4C, a wide peak of V 2p3/2 in the range of V2+ (513.6 eV) and V0 (512.3 eV) is observed which shows that V5+ has been reduced to V2+ and V0, respectively, while 2p1/2 at about 519.9 eV for V0 is also observed.31 Because XPS is a surface characterization technique, there is no peak observed for V4+ 2p3/2 at 516.4 and not even for V2O3 at 517.1 eV.31 The very low intensity signal of V 2p1/2 that appears at 522.8 eV for V4+ may endorse the successful doping of TiO2. Moreover, a negative shift is detected from the unmodified TiO2 (Figure 4D(a,b)) (Ti−O) peak at 530.28 eV to the modified V-TiO2 at 530.18 eV. A corresponding negative shift appears for the hydroxyl peak due to the adsorbed water by TiO232,33 from 532.18 eV (TiO2) to 531.88 eV (modified V-TiO2). Also the ratio of O−H to Ti−O groups at the surface of TiO2 nanorod arrays is found to be 0.2330:1, while this ratio reduced significantly to 0.1097:1 for the modified V-TiO2. These findings may be attributed to the VO functionalization at the surface of V-TiO2 nanorod arrays, suggesting a favorable interface for the transportation of photoexcited electrons at the V-TiO2-β-In2S3 interface. The morphology and structure of the samples were analyzed by high resolution transmission electron microscopy (HRTEM). The tetragonal rutile TiO2 nanorod tip (Figure S4) shows (110) facet with d-spacing of 0.32 nm which is consistent with XRD results and this is the most dominant facet with the lowest formation energy.34,35 Moreover, it is revealed that β-In2S3 is deposited homogeneously on TiO2 nanorod arrays and the d-spacing pattern at high resolution confirms its crystallinity as depicted in Figure 5a,b. Figure 5c shows the tip of a modified V-TiO2 with uniform β-In2S3 thin film coating on its surface to give a V-TiO2@β-In2S3 heterojunction. Figure 5d shows the clear boundaries of the β-In2S3 deposited on modified V-TiO2 and its high resolution also confirms the crystalline morphology (Figure 5f). Figure 5e is the Inverse Fast Fourier Transform (IFFT) of selected region (Figure 5f) of the V-TiO2@β-In2S3. The corresponding facets and their interplanar spacing are in conjunction with the XRD results. For Ti L2,3 edges in electron energy loss spectra (EELS) of Figure 5g, the Ti 3d character splits into two groups; the 3-fold t2g and 2fold eg orbitals, related to the octahedral co-ordination with O atom creating s-type and p-type bonds.36 The t2g-eg crystal field splitting is similar for the modified V-TiO2 but with little blue

Figure 2. (a) Top view and (b) cross-sectional SEM images of modified V-TiO2 nanorods and (c) EDX spectrum with (inset) atomic percent composition.

(Figure 3g) revealed less agglomeration as compared to TiO2@ β-In2S3 with 30 min loading time (Figure 3e). Hence, one could suggest that V modification in rutile TiO2 nanorods increases its roughness factor as well as surface defects as also observed from XPS results which consequently lead to the better surface coverage of modified V-TiO2 by β-In2S3 as compared to TiO2. The EDX spectra of VTiO2@β-In2S3 showing average elemental ratio are depicted in Figure S3 and Table S1. The XRD patterns of TiO2, V-TiO2 and V-TiO2@β-In2S3 are shown in Figure 3j (a−c), respectively. After marking the FTO crystal phases and subtracting it from the spectrum, the remaining peaks are indexed according to the characteristic peaks of the tetragonal rutile TiO2 phase with JCPDS file no. 88-1175. This demonstrates that the rutile crystal structure of TiO2 does not change with V modification.24 Moreover, the inset of Figure 3j shows an apparent shift in the (101) plane to a higher 2θ value from 36.05 to 36.12° for TiO2 and V-TiO2, respectively, that may be due to the lattice shrinkage caused by replacement of Ti4+ (r = 0.74 Å) by V4+ (r = 0.72 Å).25 The ionic radii of V2+ brings insignificant changes in the rutile tetragonal structure of TiO2.26 The diffraction pattern of VTiO2@β-In2S3 shows peaks at 2θ = 23.32, 27.39, 33.18, and 47.66° with diffraction planes of (116), (109), (0012), and (2212) (marked green arrows), respectively. However, the diffraction planes (103) and (112) are not included in spectrum 9040


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Figure 3. SEM images of β-In2S3 thin film deposited on TiO2 for different time durations (a, b) 10, (c, d) 20, and (e, f) 30 min and upon V-TiO2 for (g, h) 30 min, (i) 30 min high-resolution image, (j) XRD pattern of (a) TiO2, (b)V-TiO2, and (c) V-TiO2@β-In2S3.

Optical and Optoelectronic Studies. The Raman scattering vibration modes detected at 236, 450, and 616 cm−1 indicate the presence of rutile structure for both unmodified and vanadium-modified TiO237,41 as given in Figure 6A. The reduced intensity and blue shift at 450 cm−1 of modified V-TiO2 as compared to unmodified TiO2 is shown in the inset of Figure 6A. This is in good agreement with the XRD, XPS, and EELS results. Also the Raman active modes of β-In2S3 synthesized in this study at 400 °C are shown in SI Figure S5. High intensity peaks are observed at 138.6, 180 (with a shoulder at 198), 247 (with a low intensity shoulder at 266), 306, 328, and 368 cm−1. There is no indication of Raman active mode of α-In2S3 in the spectrum as reported in the literature42,43 and only the Raman active modes of β-In2S3 at

shift, which is consistent with the XPS and XRD results. Also the apparent increase in the intensity of Ti L2 edge of V-TiO2 may correspond to the reduction of the density of Ti3+ defects37 and a corresponding decrease in the intensity of Ti L3 edge. It may suggest that V4+ replaces not only Ti3+ defect states but also Ti4+ in modified V-TiO2, while the intensity reduction of Ti L2 edge of TiO2 appears where Ti3+ defects are enhanced.37−39 In addition, for the V L2,3 edges of the modified V-TiO2 (Figure 5h), the V 3d character split40 is related to the host octahedral coordination with O atom in TiO2 nanorod arrays. This suggests different oxidation states of V (i.e., V4+, V2+, and V0), which is in good agreement with the XPS results, while V4+ and V5+ are also distinctly observed for V L2 edge in EELS inferring V doping in TiO2. 9041


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Figure 4. XPS spectra of (A, B) V-TiO2@β-In2S3, (C) Ti 2p of (a) TiO2 and (b) V-TiO2, and insight shows V 2p of vanadium (D) O 1s of (a) TiO2 and (b) V-TiO2.

138.6, 180, 247, and 306 cm−1 were detected to confirm the composition and structure of the β-In2S3 synthesized at 400 °C.37,44−46 The optical properties of the hybrid V-TiO2@β-In2S3, VTiO2, and TiO2 were characterized by absorption and reflectance spectra as shown in Figure 6B and C. It can be seen that the absorption of modified V-TiO2 is improved in the ultraviolet region while extending in the visible region (below ∼600 nm) and sharing the spectral region with the hybrid VTiO2@β-In2S3. The relation between the absorption coefficient and photon energy is given as (F(R)hν)2 = A(hν − Eg). where F(R) is the Kebulka Munk function which is proportional to the absorption coefficient (α), h Planck constant, ν the photon energy, A a constant and Eg the band gap energy of semiconductor. The band gap of TiO2, V-TiO2 and V-TiO2@ β-In2S3 are calculated to be 3.02, 2.96, and 2.3 eV respectively in SI Figure S6. Vanadium modification in TiO2 has enhanced UV and visible absorption and it may stem from the Ti4+ replacement by the V4+ and V5+ dopant defects that are in good agreement with the XRD and EELS results. Moreover, the reflectance spectra (Figure 6C) revealed that the modified VTiO2 (b) and core−shell heterojunction V-TiO2@β-In2S3 nanorod arrays (c) have lower reflectance below 600 and 552 nm, respectively, compared with the pristine TiO2 nanorods. This is probably due to the multiple scattering in V-TiO2 and V-TiO2@β-In2S3 as compared to a single reflection that stems from the 1D TiO2 nanorod arrays.23 The conduction band is an important factor to determine the onset potential and electron injection capability of the guest layer to the host layer in a bilayer strategy. XPS is a commonly used technique to determine the valence band maximum (VBM) in a semiconductor.7,47 The valence band plots are shown in Figure 7A. The VBM of the prepared samples are determined by linear extrapolation method and found to be 2.66, 2.42, 0.75, and 0.47 eV for TiO2, V-TiO2, TiO2@β-In2S3, and V-TiO2@β-In2S3 electrodes, respectively. The positions of the conduction band minimum (CBM) could be determined on the basis of the results of band gap energy and VBM. In Figure 7A, the VBM of TiO2 is found at 2.66 eV which is in

good agreement with the reported value7 and it shows an upward shift to 2.42 eV which may be attributed to the vanadium doping in TiO2, hence creating additional molecular orbitals above the valence band maximum and below the conduction band minimum by narrowing the band gap.26 Figure 7B(a) shows the valence band and conduction band values in the bilayer system. β-In2S3 VBM position appears at −0.75 eV as given in Figure 7B(a), almost near to CBM of the TiO2 in TiO2@β-In2S3, while β-In2S3 VBM shift upward to −0.47 eV in V-TiO2@β-In2S3 in Figure 7B(b). An apparent shift of 0.28 eV in VBM of β-In2S3 is almost in agreement with the shift of 0.24 eV in VBM of TiO2 on V modification. It seems that the conduction band quasi Fermi levels of V-TiO2 are set in equilibrium with the valence band quasi Fermi levels of β-In2S3 as indicated in Figure 7B(b). This equilibrium could be beneficial in a way that holes of VBM in β-In2S3 can quench the photoexcited electrons of splitted molecular orbitals in the CBM of the V-TiO2 at the V-TiO2@β-In2S3 interface. The effect of vanadium modification in rutile TiO2 nanorods and the deposition of β-In2S3 thin film upon TiO2 and V-TiO2 have been investigated to determine the charge injection efficiency from the sensitizer (2nd layer) to the substrate (1st layer) and stabilization of sensitizer layer according to strategy in Figure 1, Scheme c. Figure 8a shows J−V characteristicthat the photocurrent density is boosted from 0.53 to 0.66 mA/cm2 for the unmodified TiO2 to the modified V-TiO2 at 0.5 V vs Ag/AgCl. Also the stability (J−t) curves maintain its photocurrent density for the measured time interval. The onset potential is also an important parameter when dealing with the photocatalysts. The charge injection for V-TiO2 is observed at lower onset potential as compared to TiO2, this may be attributed to the additional molecular orbitals and surface hydroxyl group modifications introduced in the V-TiO2 which is confirmed by the Raman, XRD and XPS studies, as discussed below. It is also observed that in V-TiO2, charge injection begins at low onset potential but its curve increases with much less slope from −0.754 to −0.7 V and leads to the sharp slope beyond that range as compared to TiO2. The gradual slope change in the first step is attributed to the narrowing of band 9042


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Figure 5. HRTEM of TiO2@β-In2S3 (a, b), V-TiO2@β-In2S3 (c, d, f), IFFT of V-TiO2@β-In2S3 (e), EELS of TiO2 (black), and V-TiO2 (red) with L 2,3 edges (g) and V-TiO2 with L 2,3 edges of V (h).

Figure 6. (A) Raman spectra of (a) V-TiO2@β-In2S3, (b) TiO2@βIn2S3, (c) TiO2, and (d) V-TiO2. Inset of magnified (c) TiO2 and (d) V-TiO2 at 450 cm−1. (B) UV−visible spectra of TiO2 (a), V-TiO2 (b) and with V-TiO2@β-In2S3 (c). (C) Reflectance spectra of (a) TiO2, (b) V-TiO2, and (c) V-TiO2@β-In2S3.

gap due to V-TiO2 which consequently leads to the two competing factors: (1) hopping of the electrons from valence band to the splitted molecular orbitals in the conduction band of V-TiO2 and (2) the recombination of electron hole pairs due to recombination centers. Conversely, as long as holes are produced in V-TiO2, its narrow band gap with splitted conduction band molecular orbitals increases the possibility of electron hole pair recombination as compared to the TiO2 and hence the recombination resistance decreases in doped VTiO2. Beyond the range of −0.754 to −0.7 V, a sharp slope and high photocurrent density appear in V-TiO2 until the saturation point is reached as compared to the unmodified TiO2, which may be attributed to the efficient electron injection of V-TiO2, regardless of the fact that the recombination process is still in existence and in competition at whole voltage range for both modified and unmodified TiO2 nanorod arrays. In Figure 8b for the J−V character, it is observed that as the loading time for TiO2@β-In2S3 increases from 10 to 30 min, the photocurrent density is also increased from 0.92 to 1.05

mA/cm2 at 0.5 V vs Ag/AgCl but the photocurrent density for 20 min loading is slightly less. This is evident from the high surface coverage of β-In2S3 at the tip of the TiO2 nanorods as shown in Figure 3c,d and this phenomena is in line with the observation reported in the literature.23 Importantly, the electron collection efficiency by the substrate TiO2 is the same for different β-In2S3 loading times of 10−30 min. The conduction band levels, electron trapping states due to intrinsic defects are common energy states of TiO2 and V-TiO2 but the additional splitted molecular orbitals, VO surface modifications and creation of extrinsic defect states including oxygen vacancies of V-TiO2 promote higher electron injection from the β-In2S3 to V-TiO2@β-In2S3 which is in good agreement with the results of carrier concentration study, J− V characteristics and electrochemical impedance spectroscopy (EIS). Consequently, 1.42 mA/cm2 photocurrent density is observed for V-TiO2@β-In2S3 at 0.5 V vs Ag/AgCl. To the best of our knowledge, this photocurrent value is the highest reported in the literature.12,23 Also in Figure 8c, the J−t 9043


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Figure 7. (A) XPS Valence band edges, (B) valence band alignment in a bilayer strategy (a) TiO2@β-In2S3 and (b) V-TiO2@β-In2S3.

characteristics of the photocurrent density for TiO2@β-In2S3 is different for each deposition time. It is observed that for 10 min TiO2@β-In2S3 the photocurrent density increases in the first few minutes and then decreases rapidly as compared to the 20 min loading TiO2@β-In2S3 but for 30 min loading TiO2@βIn2S3, it remains almost stable with much less slope. Most importantly, regardless of the different deposition times of TiO2@β-In2S3, it is worth noticing that the photocurrent densities gradually decrease and become almost equal in the range of 0.7−0.8 mA/cm2 after 2000 s and they can even reduce more with the passage of time. The stability for TiO2@ β-In2S3 is even more questionable for the given specific time interval, as shown in Figure 8c, as compared to the V-TiO2@βIn2S3. This strategy of vanadium doping in TiO2 seems to be working satisfactorily as the results indicate that the TiO2 defects or electron trapping states in between the valence band and conduction band may be a source of quenching the holes of β-In2S3. The limiting window source of TiO2 is obviously much narrower than modified V-TiO2 that is why for each 10−30 min duration of loading β-In2S3 layer, regardless of different loading times, the photocurrent density becomes almost equal after specific time intervals up to 2000 s because of the deterioration of the interface by the holes from TiO2. This is a state of the art problem faced by devices with narrow band gap II−VI and III− VI semiconductors as shown in Figure 1, Scheme a.7,12,48,49 Alternatively, the photocurrent density for V-TiO2@β-In2S3 appears to be well stabilized at 1.32 mA/cm2 during specific interval. This high stability may be evidence for the significance of broader quenching window, because instead of accepting holes by β-In2S3 from VBM of V-TiO2, the photoexcited electrons from the splitted molecular orbitals below the conduction band of V-TiO2 are being quenched by the holes

Figure 8. Linear sweep voltammetric curves (LSV) left-bottom scale with TiO2 and V-TiO2 shows its J−V characteristic in dark (D) and light (L) (a) and Chronoamperometric curves (CA) top-right scale with TiO2 and V-TiO2 shows its J−t characteristic at 0.5 V vs Ag/AgCl in light (L) (a), β-In2S3 coated on TiO2 for different time durations 10 min (black), 20 min (red), 30 min (green) and V-TiO2 for 30 min (blue) shows its J−V characteristic in light (L) (b) and β-In2S3 coated on TiO2 for different time durations (black) 10, (red) 20, and (green) 30 min; (blue) 30 min on V-TiO2; and (cyan) 30 min β-In2S3 on FTO shows its J−t characteristic at 0.5 V vs Ag/AgCl in light (L) (c).

of the β-In2S3 at the interface and that may give protection from self-oxidation during illumination. Moreover, the presence of V0 may further facilitate charge transfer at the interface by immediate injection of photoexcited electrons into it.50 To confirm the issue and its reliability, β-In2S3 thin film is coated on FTO for 30 min with the same procedure and used in PEC cell with the same protocol. It is observed in Figure 8c (cyan) that after a few minutes, photocorrosion of β-In2S3 initiates and 9044


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Figure 9. Nyquist plots of the photoanode (a) in darkness and (b) in light of (black) V-TiO2@β-In2S3, (blue) TiO2@β-In2S3, (red) V-TiO2, and (green) TiO2 and (insets) corresponding high-frequency regions. (c) Bode plot of the photoanode in light, (black) V-TiO2@β-In2S3, (blue) TiO2@ β-In2S3, (red) V-TiO2, and (green) TiO2. (d) Circuit model.

substrate so that higher exposure to solar light as well as interface stability could be achieved for the sensitizer layer. Efforts in experimental and theoretical study are still needed in order to have better insight of the hole quenching and quasi Fermi level equilibrium at the bilayer interface ofthe photocatalyst as discussed in Figure 1, Scheme c. To have a deeper insight into the charge injection and recombination phenomenon at the bilayer and layered surfaceelectrolyte interface in the prepared photocatalyst, we conducted the electrochemical impedance spectroscopy (EIS) in darkness (Figure 9a) and in light (Figure 9b,c). The transport rate, recombination resistance, and lifetime of charge carriers can be derived on the basis of an equivalent circuit model (Figure 9d), where Rs is the series resistance and Rct is the total charge transport resistance and constant phase element (CPE) alternatively used for space charge region capacitance (C) and its value (n = 0−1) depends upon the smooth surface of the photoanode; one for the highest and zero for the lowest smoothness respectively, including charge transfer across the β-In2S3−V-TiO2/FTO interface and βIn2S3-electrolyte interface of the photoanode. RPt and CPt are the resistance and capacitance of the Pt counter electrode, respectively, while W is the Warburg impedance. In darkness, under open circuit potential (OCP) conditions, (Figure 9a), the curves show two semicircles; the first semicircle is at the high-frequency region related to the charge transfer resistance, and the second semicircle is at the low-frequency region correspond to mass transfer resistance. Low-frequency-region semicircles can be seen clearly from the Figure 9a, while high frequency semicircles are observed in the inset, suggesting the mass transfer to be the dominant process in dark conditions. A third semicircle appears only in the V-TiO2 (red) at very low frequency region, corresponding to the Warburg diffusion which is due to the electrolyte diffusion at the surface of the

its photocurrent density drops rapidly, indicating how the absence of quenched-window could result in photocorrosion of the sensitizer. Moreover, 30 min loading time of β-In2S3 on VTiO2 improved the onset potential (−0.95 V) significantly as compared to TiO2@β-In2S3 (−0.755 V) vs Ag/AgCl. A reduced photocurrent onset and saturated potential is important because it reduces the applied bias required to gain the maximum photocurrent and, hence, increases the efficiency of the hydrogen generation in PEC cell. The solar to hydrogen efficiency (STH) of the photoanodes were calculated by using the equation η = I(1.23 − V )/Pin

where I is the photocurrent density at the measured potential, V is the applied bias with respect to RHE, Pin is the irradiance intensity of the incident light at 100 mW/cm2 (AM 1.5 illumination). The plots of calculated STH efficiency as a function of applied bias are presented in Figure S7. The pristine TiO2 and modified V-TiO2 exhibit an optimal conversion efficiency of 0.30 and 0.34% at −0.42 V vs Ag/AgCl, respectively. Significantly, the V-TiO2@β-In2S3 exhibits the maximum efficiency of 1.04% at a very low bias of −0.63 V vs Ag/AgCl. Also, the TiO2@β-In2S3 shows decreased efficiency of 0.64% at comparatively higher voltage of −0.46 V vs Ag/ AgCl. Vanadium modification substantially enhances the photoconversion efficiency of V-TiO2@β-In2S3 hybrid composite by reducing the current saturation potential and improving the maximum photocurrent. It may also be suggested that photon conversion density of βIn2S3 and the electron injection efficiency from β-In2S3 to VTiO2 may be controlled by the loading time of β-In2S3 and by the surface morphology of the V-TiO2@β-In2S3, respectively. A bespoke synthetic procedure is therefore required which could give a high surface area with broader quenched-window 9045


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substrate through the Schottky barrier. None of the reports have yet revealed the controlled parameters of the path of holes at the interface which is an important point to be explored regarding its stability. In this study, we emphasized the stability of the interface by hole-quencher window used as a sink for holes at the interface. We suggest that the trilayered scheme consisting of the metal oxide/metal oxide heterojunction may be also tried for productive results (Figure 1, Scheme c), as reported recently, in which graphene is used as an efficient hole quencher window at the bilayer interface.54 A possible mechanism can be illustrated on the basis of facts stated in this study. Under solar irradiation, electrons are excited from VBM to the CBM of both V-TiO2 and β-In2S3 with the same amount of holes left in their respective VBM. The band alignment of the V-TiO2@β-In2S3 heterojunction allows an efficient transfer of photoexcited electrons through the Schottky barrier, as shown in Figure 10, as compared to

photoanode. In light under the OCP conditions, Figure 9b shows the semicircles with different diameters and there is no semicircle that appears at high frequency region as shown in the inset. This may suggest charge transfer as the dominant process in light. Normally, the smaller the diameter of the semicircle the lower the charge transfer impedance at the electrode− electrolyte interface.51 Therefore, it has been observed that the recombination resistance of the doped V-TiO2 (red) decreases significantly, emphasizing the introduction of recombination centers successfully as discussed in XRD, XPS and photoelectrochemical (J−V) studies. Efficient charge transfer can be observed at V-TiO2@β-In2S3-electrolyte (black) interface as compared to TiO2@β-In2S3-electrolyte (blue) interface. Moreover, the characteristic peak frequency shifts to lower frequency value in Bode phase plot (Figure 9c) suggesting a longer lifetime of the corresponding heterojunction V-TiO2@β-In2S3 (black) as compared to TiO2@βIn2S3 (blue). Most importantly in Figure 9c, it is observed that efficient charge transfer via a heterojunction depends on the frequency window of the substrate (1st layer). The electrochemical impedance measurements are conducted at a frequency of 1000 Hz in darkness on TiO2, VTiO2, TiO2@β-In2S3, and V-TiO2@β-In2S3 to investigate the influence of V modification in TiO2, sensitization of β-In2S3 upon TiO2 and V-TiO2 on electronic properties. In Figure S8, it is observed that all the samples show a positive slope in Mott− Schottky plots. Importantly, modified V-TiO2, TiO2@β-In2S3, and V-TiO2@β-In2S3 show smaller slopes of Mott−Schottky plot as compared to TiO2, suggesting an increase of donor densities. Carrier densities of these samples were calculated from the slopes of Mott−Schottky plots using the equation

Figure 10. Supposed mechanism for the charge transfer in V-TiO2@βIn2S3 bilayer system.

Nd = (2/eo ,o,)[d(1/C 2)/dV ]−1

TiO2@β-In2S3 heterojunction. From where the electrons are flowing across the circuit to Pt counter electrode and reduce the H+ to H2, but the photoelectrons present in the wide hole quencher window (comprises of splitted molecular orbitals created by the modified V-TiO2, the electron trapping states and defects i.e., oxygen vacancies ref [47]) of modified V-TiO2 CBM are utilized to quench the holes of the β-In2S3 present in its VBM at the interface immediately while this phenomena simultaneously inhibit the holes of the V-TiO2 to move toward the VBM of the β-In2S3 and remain as trapped in buried layer of V-TiO2. Exceptionally, if the holes of V-TiO2 would move to β-In2S3; they would be quenched by its own photogenerated electrons at V-TiO2-β-In2S3 interface to maintain its stability. In other words, the V-TiO2 is not only act as electron transfer media but also act as interface stabilizer. To some extent it also promotes the carrier concentration of the V-TiO2@β-In2S3 due to its visible absorption region provided that the photoexcited electrons in V-TiO2 could across the FTO-V-TiO2 Schottky barrier. Consequently, due to the hole quencher window, the interface stability is maintained at V-TiO2@β-In2S3 heterojunction as compared to TiO2@β-In2S3 heterojunction. On the other hand, holes at the β-In2S3-electrolyte interface are utilized by the sacrificial agents. Moreover, presence of V0 may also facilitate by acting as a permanent sink for the holes of β-In2S3 at the interface. The stability of V-TiO2@β-In2S3 could also be endorsed from Figure 7B(b), where the valence band Fermi level of β-In2S3 is in equilibrium with the conduction band Fermi level of V-TiO2, more significantly interacting with the splitted molecular orbitals created by the modified V-TiO2 (Figure 7Bb) and the electron trapping states/defects i.e.,

where eo is the electron charge, , is the dielectric constant, ,o is the permittivity of vacuum, Nd is the donor density, and V is the applied bias at the electrode. The donor density is calculated to be 1.20 × 1012 cm−3, 8.81 × 1012 cm−3, 1.90 × 1013 cm−3, and 7.88 × 1016 cm−3 for TiO2, V-TiO2, TiO2@β-In2S3, and VTiO2@β-In2S3, respectively. The increased donor density of the modified V-TiO2 from pristine TiO2 effectively demonstrates its visible absorption due to the introduction of splitted molecular orbitals, enhanced defects and oxygen vacancies. Those photogenerated electrons in V-TiO2 molecular orbitals and defects located below the FTO conduction band level are unable to transfer at FTO-V-TiO2 interface, and those in oxygen vacancies are also not involved in water splitting because their energy levels are well below the H2O/H2 reduction potential47 and would be available to work as quenching window source. The substantial increment in the donor density of V-TiO2@β-In2S3 as compared to the TiO2@βIn2S3 is achieved due to the improved electron transport at βIn2S3−V-TiO2 interface by reducing the −OH groups, broader collection efficiency by increasing the lifetime of the charge carriers and improving the carrier concentration by photogenerated electrons in splitted molecular orbitals of modified VTiO2. Recently, efficient charge transfer from CdS sensitizer to the substrate is emphasized, confirming its reliability toward long-term stability through variation in percentage doping of the substrate.52 In addition, a report on the surface modification of the TiO2 substrate by CoO nanoparticles appeared in literature.53 Both the reports emphasized on the higher photon conversion in the sensitizer and electron injection to the 9046


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ACS Applied Materials & Interfaces Notes

oxygen vacancies as indicated in the mechanism here. This window source could be varied with percentage doping of transition metals or with reduced or defected (Ti3+) states within the substrates (TiO2, ZnO, etc.) in bilayer systems. The mechanism of electron hole pair separation in a bilayer strategy is in conjunction with the concept used in Figure 1, Scheme b, of a trilayered strategy, and it provides a viable and smart nanostructure replacement of trilayered systems. This work has successfully demonstrated the optimized surface area of the hybrid V-TiO2@β-In2S3 bilayer system, efficient charge transport at V-TiO2-β-In2S3 interface via VO functionalized surface groups, improved charge collection efficiency by introducing additional substates in V-TiO2, enhanced carrier concentration in V-TiO2@β-In2S3 due to photoexcited electrons of V-TiO2 and β-In2S3, overall enhanced recombination resistance, reduced onset potential and, importantly, the stability of the interface via introducing broader hole quenching window source in the substrate. These features provide a viable and efficient alternative route for sustaining the interface properties in bilayer systems.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work acknowledges the financial assistance from Centre of Innovative Nanostructures and Nanodevices (COINN) and the Centre of Post Graduate Studies (CGS) for providing graduate assistance (GA). We are also thankful to MOR. Nanotechnology and Centralized Analytical Laboratories (CAL) for providing research facilities. MM acknowledges High-Impact Research Grant no. UM. C/625/1/HIR/242. UMRG Grant no. M.TNC2/RC/261/1/1/RP007A/B-13AET.

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(1) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (2) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of Titanium Oxide Nanotube. Langmuir 1998, 14, 3160− 3163. (3) Liu, B.; Chen, H. M.; Liu, C.; Andrews, S. C.; Hahn, C.; Yang, P. Large-Scale Synthesis of Transition-Metal-Doped TiO2 Nanowires with Controllable Overpotential. J. Am. Chem. Soc. 2013, 135, 9995− 9998. (4) Wang, J.; Tafen, D. N.; Lewis, J. P.; Hong, Z.; Manivannan, A.; Zhi, M.; Li, M.; Wu, N. Origin of Photocatalytic Activity of NitrogenDoped TiO2 Nanobelts. J. Am. Chem. Soc. 2009, 131, 12290−12297. (5) Chen, X.; Burda, C. The Electronic Origin of the Visible-Light Absorption Properties of C-, N-and S-Doped TiO2 Nanomaterials. J. Am. Chem. Soc. 2008, 130, 5018−5019. (6) Afzaal, M.; O’Brien, P. Recent Developments in II−VI and III− VI Semiconductors and their Applications in Solar Cells. J. Mater. Chem. 2006, 16 (17), 1597−1602. (7) Luo, J.; Ma, L.; He, T.; Ng, C. F.; Wang, S.; Sun, H.; Fan, H. J. TiO2/(CdS, CdSe, CdSeS) Nanorod Heterostructures and Photoelectrochemical Properties. J. Phys. Chem. C 2012, 116, 11956−11963. (8) Cho, I. S.; Chen, Z.; Forman, A. J.; Kim, D. R.; Rao, P. M.; Jaramillo, T. F.; Zheng, X. Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production. Nano Lett. 2011, 11, 4978− 4984. (9) Qian, S.; Wang, C.; Liu, W.; Zhu, Y.; Yao, W.; Lu, X. An Enhanced CdS/TiO2 Photocatalyst with High Stability and Activity: Effect of Mesoporous Substrate and Bifunctional Linking Molecule. J. Mater. Chem. 2011, 21, 4945−4952. (10) Kameyama, T.; Douke, Y.; Shibakawa, H.; Kawaraya, M.; Segawa, H.; Kuwabata, S.; Torimoto, T. Widely-Controllable Electronic Energy Structure of ZnSe-AgInSe2 Solid Solution Nanocrystals for Quantum Dot-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 29517−29524. (11) Choudhary, S.; Upadhyay, S.; Kumar, P.; Singh, N.; Satsangi, V. R.; Shrivastav, R.; Dass, S. Nanostructured Bilayered Thin Films in Photoelectrochemical Water Splitting−A Review. Int. J. Hydrogen Energy 2012, 37, 18713−18730. (12) Li, Y.; Luo, S.; Wei, Z.; Meng, D.; Ding, M.; Liu, C. Electrodeposition Technique-Dependent Photoelectrochemical and Photocatalytic Properties of an In2S3/TiO2 Nanotube Array. Phys. Chem. Chem. Phys. 2014, 16, 4361−4368. (13) Lee, Y.-L.; Chi, C.-F.; Liau, S.-Y. CdS/CdSe Co-Sensitized TiO2 Photoelectrode for Efficient Hydrogen Generation in a Photoelectrochemical Cell. Chem. Mater. 2010, 22, 922−927. (14) Mumtaz, A.; Mohamed, N. M. Synergetic Effects of II−VI Sensitization upon TiO2 for Photoelectrochemical Water Splitting; A Tri-layered Structured Scheme, In Proceedings of the Third International Conference on Fundamental and Applied Sciences (ICFAS 2014): Innovative Research in Applied Science for a Sustainable Future, June 3−5, 2014, Kuala Lumpur, Malaysia, AIP Publishing: Melville, NY, 2014; pp 637−642.

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CONCLUSION In summary, a strategy has been developed for the synthesis of bilayered nanostructured core−shell V-TiO2@β-In2S3 heterojunction nanorod arrays and tested for their photocatalytic applications. A total loading of 0.75% of vanadium in TiO2 lattice exhibits superior photoelectrochemical properties and the recombination centers produced in V-TiO2 are used as an effective electron donor at the interface, while at the same time the presence of V0 at the interface resists the photocorrosion. The photocurrent density of modified V-TiO2@β-In2S3 at 0.5 V vs Ag/AgCl is 1.42 mA/cm2 (AM 1.5 illumination) and is 2fold that of 0.78 mA/cm2 for TiO2@β-In2S3 and 3-fold that of pristine TiO2. This increased photocatalytic activity and enhanced stability of V-TiO2@β-In2S3 layered nanorod arrays is initiated by the hole-quenching window and the enhanced collection of electrons from the β-In2S3 to the V modified substrate as compared to TiO2@β-In2S3 layered nanorod arrays.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10147. FESEM of TiO2 nanorod arrays, effect of precursor concentration on TiO2, elemental distribution of vanadium in TiO2 nanorod arrays (mapping), EDX spectra of V-TiO2@β-In2S3, HRTEM of TiO2 nanorods arrays, EDX of V-TiO2@β-In2S3, Raman spectrum of βIn2S3, band gap spectrum of V-TiO2@β-In2S3, photoconversion efficiency and Mott−Shottky. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

* [email protected]. Author Contributions ⊥

These authors contributed equally.

Funding

The funding for this project is provided by Centre of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS (UTP), Malaysia. 9047


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