ZnCo2O4

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Designing Co-Pi Modified 1D n-p TiO2/ZnCo2O4 Nanoheterostructure Photoanode with Reduced Electron-hole Pair Recombination and Excellent Photoconversion Efficiency (>3%) AYAN SARKAR, Keshab Karmakar, and Gobinda Gopal Khan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08213 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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The Journal of Physical Chemistry

Designing Co-Pi Modified 1D n-p TiO2/ZnCo2O4 Nano-heterostructure Photoanode with Reduced Electron-hole Pair Recombination and Excellent Photoconversion Efficiency (>3%) Ayan Sarkar1, Keshab Karmakar2 and Gobinda Gopal Khan 3,* 1

Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, BlockJD2, Sector-III, Salt Lake, Kolkata 700106, West Bengal, India 2

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700106, West Bengal, India 3

Department of Material Science and Engineering, Tripura University (A Central University), Suryamaninagar, Tripura West 799022, Tripura, India *

Corresponding Author: G. G. Khan, Email: [email protected]

ABSTRACT The poor visible light absorption, defect mediated charge carrier recombination, slow water oxidation kinetics, and charge transportation limits the performance of TiO2 photoelectrodes for water oxidation. In order to tackle these issues, here, a one-dimensional photoanode is designed by electrodepositing a p-ZnCo2O4 nanolayer on the n-TiO2 nanotubes surface and finally, electrochemically coupling the TiO2/ZnCo2O4 surface with an ultrathin layer of the cobalt phosphate (Co-Pi) catalyst nanoparticles. This typical TiO2/ZnCo2O4@Co-Pi nanoheterostructures exhibit remarkably enhanced the visible light driven photoelectrochemical property with applied bias photoconversion efficiency (ABPE) ~3% at 0.2 V vs. NHE. The TiO2/ZnCo2O4@Co-Pi nano-heterostructures also show enhanced visible light absorption with large photocurrent density ~440% higher than that of the TiO2 nanotubes electrode at 1.2 V vs. Ag/AgCl) and significantly low onset potential for water oxidation. Studies on the transient photocurrent and flat-band potential demonstrate the remarkable improvement in the photogenerated charge carrier separation or reduced recombination because of the favourable band alignment at the hetero-interface. The Co-Pi catalyst further boosts the water oxidation reaction by reducing electron-hole pair recombination through the suppression of the surface trap states. Moreover, Co-Pi also serves as a hole acceptor layer, improving the charge transfer kinetics for an enhanced photoelectrochemical performance. 1. INTRODUCTION Photoelectrochemical (PEC) cell is an efficient device for renewable energy technologies to convert the abundant solar energy to electricity or chemical fuels by most green chemical ways.1 Recently, intense research works have been focused on the development of active solar water splitting electrodes for the PEC cell to produce hydrogen, as hydrogen is considered to the fuel for the future generation.2 In this context, the semiconductors hold a great potential for the photocatalytic as well as the photoelectrochemical water splitting 1 ACS Paragon Plus Environment


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activities, and hence various semiconductors have been extensively studied.3-6 Since the first report of photolysis of water using the semiconductor electrode,7 TiO2 has been considered as an excellent electrode material for the solar water splitting, because of the advantages like the favourable electronic band structure for water splitting with the conduction band minima placed around 0.5 V vs. the standard hydrogen electrode (SHE), high thermal and photochemical stability, strong light absorbance, environmental compatibility, abundance, and relatively low cost.8-12 In spite of the above advantages, TiO2 suffers from the following limitations: the wide band gap of TiO2 (33.3 eV) only absorbs ultraviolet (UV) light, which is only 4% of the total solar spectrum, poor photogenerated charge carrier lifetime or the fast electron-hole pair recombination, and a high overpotential for the water splitting.13 However, considering the light absorption ability and the wide band gap of TiO2, the theoretical solarto-hydrogen efficiency (STH) of anatase and rutile TiO2 lies ~ 1.3% and 2.2%, respectively, under AM 1.5 G visible light illumination. In this context, to boost the PEC performance of the TiO2, various strategies are adopted and among them, surface sensitization is found to work very effectively.11, 14-18 However, by wisely choosing the proper material for the surface sensitization, it is possible to enhance the light absorption capability of the electrode. Along with it, the interfacial band alignment at the material interface might provide a suitable impact on the photogenerated charge carrier separation and reduced carrier recombination. Furthermore, the surface sensitization also helps in suppressing the surface traps/defect states, which act as the recombination centres for the photogenerated carriers. Moreover, the same surface layers can also act as a catalyst to further enhance the PEC performance of the electrodes. Considering the aforementioned benefits of the proper surface sensitization, this work attempts to design a 1D photoanode by electrodepositing a ZnCo2O4 nanolayer on the TiO2 NTs surface and coating it with an ultrathin cobalt phosphate (Co-Pi) nanoparticle catalyst outer layer, by proper surface engineering. Recently, the 1D TiO2 NTs have earned its reputation as an efficient photocatalyst.19-21 Moreover, here the TiO2 NTs backbone provides a large surface area for the photochemical reaction where the unique 1D structure of NTs helps in the easy and unidirectional charge transportation. Thin p-type ZnCo2O4 surface nanolayer is incorporated on the n-type TiO2 NTs by an easy electrodeposition technique and finally the TiO2/ZnCo2O4 (TO/ZCO) NHs is covered with nanoparticles of Co-Pi catalyst (TO/ZCO@Co-Pi) using the electrochemical route. However, there are only a few reports on the PEC water splitting property of p-ZnCo2O422 and Zn-Co oxide catalyst,23 hence, the PEC property of TO/ZCO NHs has not yet been studied, to the best of our knowledge. Moreover, the electrodeposition technique chosen to fabricate the TO/ZCO@Co-Pi NHs electrode is very easy, inexpensive, and scalable compared to the conventional ALD technique. Herein, the p-ZnCo2O4 is chosen as a surface layer on TiO2, expecting of the following advantages (i) suitable energy band position and band gaps of ZnCo2O4 for the water splitting and visible light absorption, respectively, (ii) the formation of a favourable p-n ZnCo2O4-TiO2 nanoheterojunction is expected to cause the expeditious separation of the photogenerated charge carriers, (iii) presence of Co and Zn oxides might also act as oxygen-evolving catalyst (OEC), and (iv) ZnCo2O4 will act as an acceptor for the photogenerated „holes‟ and hence boost the PEC performance by reducing the photocarrier recombination. Furthermore, the 2 ACS Paragon Plus Environment

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incorporation of the Co-Pi nanoparticles on TO/ZCO NHs is anticipated to boost up the PEC performance further, as recently Co-Pi has earned its reputation as an efficient OEC.24, 25 In this work, the as-prepared TO/ZCO NHs are found to exhibit enhanced visible light absorption, better charge carrier generation, separation, and transportation compared to the pristine TiO2 NTs. It is evidenced that the large band bending at the TO/ZCO interface improves the charge carrier separation, collection, and the reduction in electron-hole recombination to enhance the PEC property.26 Furthermore, the PEC performance of the TO/ZCO NHs is found to improve remarkably after the incorporation of Co-Pi catalyst layer, where the applied bias photon-to-current efficiency is found to become above 3% at 0.2 V vs. NHE. Herein, the Co-Pi catalyst nanoparticle layer is found to suppress the surface defects remarkably and hence boost the fast carrier separation and reduced carrier recombination in TO/ZCO NHs. Moreover, the Co-Pi is found to act as the „hole‟ acceptor to facilitate the migration of holes from ZCO to the electrolyte resulting enhanced oxygen generation at the TO/ZCO@Co-Pi NHs photoanode. Therefore, this type of nano-engineering is found to be very effective to significantly enhance the solar water oxidation of the photoelectrodes. 2. EXPERIMENTAL SECTION 2.1 Materials and Reagents

Titanium plate (99.99% pure, 0.25 mm thick, Alfa Aesar), formamide (HCONH2, Loba Chemie), ammonium fluoride (NH4F, Loba Chemie), cobalt(II) sulfate heptahydrate (CoSO4.7H2O, Sigma Aldrich), zinc(II) nitrate hexahydrate (Zn(NO3)2.6H2O, Merck), boric acid powder (H3BO3, Merck), cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O, Sigma Aldrich), potassium hydrogen phosphate (K2HPO4, Merck), potassium dihydrogen phosphate (KH2PO4, Merck). All the reagents used here, are of analytical grade and were used as received. Only Millipore water (Milli-Q) was used throughout the experiment as required. 2.2 Synthesis of TiO2 nanotubes

A highly ordered array of TiO2 nanotubes was synthesized by the anodization technique as described in our previous reports.27, 28 In a nutshell, a thoroughly cleaned Ti and a copper plate of similar dimensions were used as the anode and cathode, respectively in a twoelectrode electrochemical cell, containing an electrolyte of 0.2M NH4F, 95 vol.% formamide, and 5vol.% water. The 20 h long anodization process was conducted at 20C under a dc bias of 25 V. After the accomplishment of the anodization process, the anodized Ti plate was thoroughly rinsed with acetone, ethanol, and water. After drying, the anodized Ti plate was annealed at 400C for 2 h in the air to obtain the high-quality anatase phase of TiO2 NTs arrays. 2.3 Synthesis of TiO2/ZnCo2O4 NHs

To prepare the TiO2/ZnCo2O4 NHs (TO/ZCO NHs), co-deposition of Zn and Co was conducted by electrodeposition technique using a three-electrode electrochemical workstation (CHI660E, CH Instruments) with the as-prepared TiO2 NTs arrays on the titanium plate as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference 3 ACS Paragon Plus Environment


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electrode. An aqueous electrolyte containing 0.3 M zinc nitrate, 0.3 M cobalt sulfate, and 0.3 M boric acid was used to electrodeposit thin layer of ZnCo2O4 on TiO2 NTs sample by using a cathodic voltage of 1 V vs. Ag/AgCl for 160 s at room temperature. After the electrodeposition, the electrode was washed with water and then it was annealed in air at 600C for 2 h. The ZnCo2O4 film was also deposited on conducting FTO substrate for 30 min following the same process. 2.4 Synthesis of TiO2/ZnCo2O4@Co-Pi NHs

Co-Pi catalyst was photoelectrochemically deposited on the TO/ZCO NHs electrode using an electrochemical workstation (CHI660E, CH Instruments) where the NHs sample was used as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The Co-Pi precursor electrolyte was prepared from cobalt nitrate and potassium phosphate buffer with a molar ratio 1:200. The 0.1M potassium phosphate buffer (pH=7) was prepared by mixing 61.5 ml 1 M aqueous potassium hydrogen phosphate and 38.5 ml aqueous 1 M potassium dihydrogen phosphate at room temperature. The TO/ZCO NHs electrode was thoroughly drenched with the precursor solution for 30 min so that the adsorption of Co2+ ions can take place. Afterwards, electrodeposition was conducted at 0.9V vs. Ag/AgCl carried out for 200s under the illumination of the visible light (10 mW.cm-2, wavelength >420 nm). 2.5 Material Characterization

The structure and morphology of the as-prepared nanotubes and nano-heterostructures were studied by the field emission scanning electron microscope (FESEM, FEI Quanta-200 Mark2). The high-resolution transmission electron microscope (HRTEM, JEOL JEM 2100) was employed to elucidate on the inner morphology and crystal structure. Energy dispersive Xray spectroscopy (EDS, EDAX attached to the FESEM) and energy filter transmission electron microscopy (EFTEM) were also employed to probe the constituting elements and their distribution. Grazing incidence X-ray diffraction pattern (GIXRD, Pananlytical X‟Pert Pro diffractometer) of the TO/ZCO sample was obtained using Cu K line (=1.54 Å) source. The selective area electron diffraction (SAED), acquired using the TEM and GIXRD patterns were used for the crystallographic studies. The HRTEM and SAED analyses were performed with the help of Gatan Digitalmicrograph 2. X-ray photoelectron spectroscopy (XPS, Omicron Multiprobe Electron Spectroscopy System) technique was also used to investigate the chemical composition and ionic states of the present elements with Al Kα line (1486.7 eV). The room temperature UV-Vis-NIR absorption spectra of the samples were recorded using the Perkin Elmer Lambda 1050 UV/Vis spectrometer in the range of 800-250 nm while the room temperature photoluminescence (PL, Horiba Fluorolog-3 spectrofluorometer) emission spectra of the samples were acquired by exciting the samples by photons of wavelength 325 nm. 2.6 Electrochemical Characterization

All the electrochemical measurements were performed at room temperature using a software controlled three-electrode electrochemical workstation (CHI 660E, CH Instruments). The 4 ACS Paragon Plus Environment

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sample materials were used as the working electrode while a highly pure platinum wire and an Ag/AgCl (saturated with 1 M aq. KCl) electrode served as the counter and reference electrodes, respectively. An aqueous solution of 0.5 M Na2SO4 was used as the electrolyte throughout all the measurements. A 200 W incandescent lamp (Philips India, with > 420 nm and intensity ~ 10 mW.cm-2, as available with us) was used as the visible light source. The linear sweep voltammetry (LSV) measurements were performed at a scan rate of 100 mV.s-1 from -0.5 V to 1.5 V vs. Ag/AgCl. The electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 1MHz to 1mHz, at 0V vs. Ag/AgCl DC bias with an AC voltage perturbation of 5 mV amplitude. The Mott-Schottky measurements were performed at the frequency of 5 kHz. 3. RESULTS AND DISCUSSION 3.1 Morphology and crystallography of NHs

Figure 1a shows a typical top view FESEM image of the TO/ZCO NHs. Formation of the ZnCo2O4 layer over the vertically grown TiO2 nanotubes of diameters ~100 nm is obvious from the micrograph. The as-prepared arrays of TiO2 NTs is shown in Figure S1 (supporting information) for a comparison between pristine TiO2 NTs and ZnCo2O4 deposited TiO2 NTs. The EDS mappings, given in Figure 1b, c, d, and e confirm the uniform presence of Ti, O, Zn, and Co elements in the TO/ZCO NHs, respectively.

Figure 1. (a) FESEM micrograph of TiO2/ZnCo2O4 NHs. EDS elemental colour mapping of (b) Ti, (c) O, (d) Zn, and (e) Co, present in the TiO2/ZnCo2O4 NHs. (f) GIXRD pattern of as-prepared TiO2/ZnCo2O4 NHs.

From the XRD pattern of TiO2/ZnCo2O4 NHs (Figure 1f), it is found that the TiO2 NTs have anatase phase (JCPDS File No. 21-1272) with preferential orientation along (1 0 1) plane (the XRD pattern of the pristine TiO2 NTs is shown in Figure S2a of the supporting information). The XRD pattern also confirms the presence of ZnCo2O4 which has an FCC structure with Fd ̅ m space group (JCPDS File No. 23-1390) and such ZnCo2O4 is reported as a p-type semiconductor.29, 30 The peaks for Ti in the XRD pattern are arising due to the presence of titanium substrate at the bottom of the NHs sample. However, after the Co-Pi modification, the GIXRD pattern gets dominated by anatase titanium dioxide and cobalt phosphate peaks (Figure S2b, in the supporting information). In this work, Co-Pi nanoparticles are found to be crystalline in nature.

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Figure 2. (a) TEM micrograph of the TO/ZCO@Co-Pi NHs. The red circles enclose some of the Co-Pi nanoparticles and the red contour represents the ZnCo2O4 layer on the nanotube. (b) and (c) the HRTEM micrograph and SAED pattern of the TO/ZCO@Co-Pi NHs, respectively. EFTEM micrographs, showing the presence of elemental (d) Ti, (e) O, (f) Zn, (g) Co, and (h) P, respectively, in the TO/ZCO@Co-Pi NHs. (i) The EFTEM micrograph recorded with all the constituent elements together.

A typical TEM micrograph (Figure 2a) of the TO/ZCO@Co-Pi NHs reveals the tubular structure of TiO2 which forms the backbone of the NHs. From Figure 2a, a large number of tiny spots (few of those have been encircled by the red circles) can easily be found scattered all over the surfaces of the nanotubes and these spots represent the Co-Pi nanoparticles, anchored on the tube surfaces. Moreover, from Figure 2a, the ZnCo2O4 material can also be identified as the darker thin nanolayers of arbitrary contours; one of which has been marked in the red colour. Figure 2b represents the HRTEM micrograph of the TO/ZCO@Co-Pi NHs, showing the presence of lattice fringes of TiO2 anatase (1 0 1) plane, (0 1 1) plane of Co-Pi, and (4 0 0) plane of ZnCo2O4 in the selected section, which are confirmed from the respective measured d-spacings of 0.351, 0.56, and 0.201 nm, respectively. The SAED pattern (Figure 2c) reveals the presence of different crystal planes of anatase TiO2 (such as A(1 0 1), A(1 0 5), A(2 0 4)), ZnCo2O4 (such as ZCO(3 3 1), ZCO(2 2 0)), and Co-Pi (such as Co-Pi(1 1 0), Co-Pi(1 3 1)), in the TO/ZCO@Co-Pi NHs; clearly indicating the crystalline nature of the NHs. The elemental colour mappings, shown in Figure 2d-h, obtained using the EFTEM, confirm the presence of the constituent elements i.e., Ti, O, Zn, Co, and P in the TO/ZCO@Co-Pi NHs, respectively. The EFTEM micrographs also reveal the uniform distribution of the aforesaid elements on all over the TiO2 NT surface in the TO/ZCO@Co-Pi NHs. This also shows the formation of homogeneous NHs. 3.2 XPS analysis of NHs


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Figure 3. (a) XPS survey spectrum. High resolution XPS spectra of (b) Ti 2p, (c) O 1s, (d) Zn 2p, (e) Co 2p, and (f) P 2p, obtained from the TO/ZCO@Co-Pi sample.

XPS is a very powerful tool for finding the presence of the constituent elements of a material as well as for determining their ionic states. The overall XPS spectrum of the TO/ZCO@CoPi NHs is shown in Figure 3a. Figure 3b shows the XPS spectrum of the Ti 2p state consisting of the prominent Ti 2p3/2 and Ti 2p1/2 doublet which is respectively positioned around 458.1 and 463.9 eV, with separation energy E = 5.8 eV. The binding energies associated with the Ti 2p3/2 and Ti 2p1/2 doublet signify the presence of +4 oxidation state of Ti in the NHs.31 The deconvolution of the O 1s XPS spectrum (Figure 3c) indicates that the spectrum is actually a superposition of two peaks located around 529.5 and 531.4 eV. The lower binding energy peak ca. 529.5 eV is accredited to the 2 oxidation state of oxygen in metal oxides. The higher binding energy shoulder peak ca. 531.4 eV generally appears, due to the presence of hydroxyl species coming from the surface adsorption of moisture or due to the oxygen vacancies.28 The quantity (Ti-O), which defines the binding energy difference between the O 1s and Ti 2p3/2 states, can be used to characterize the oxides involving different ionic states of titanium. In this work, the value of (Ti-O) for Ti4+ state is 71.4 eV and this value features the anatase phase of TiO2.32 Figure 3d depicts the core level XPS spectrum of Zn 2p. The Zn 2p3/2 and Zn 2p1/2 peaks are respectively located around 1021.3 and 1044.5 eV with E = 23.2 eV. This indicates the presence of +2 oxidation state of zinc in TO/ZCO@Co-Pi NHs.33 The XPS spectrum of the Co 2p3/2 state, obtained from the TO/ZCO@Co-Pi sample is shown in Figure 3e. Deconvolution of the spectrum produces four peaks, respectively centred around 780.30, 781.10, 785.30, and 791.40 eV. The first two peaks at 780.30 and 781.10 eV are attributed to the main peaks for Co3+ state34, 35 and Co2+ state,36, 37 respectively. The other two peaks associated with the Co 2p3/2 state at high binding energy regions, are the shake-up satellite peaks, where the 785.30 eV satellite peak is generally associated with the Co2+ state37 while the satellite peak at 791.40 eV redefines the 7 ACS Paragon Plus Environment


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presence of Co3+.34, 35 Figure 3f represents the XPS spectrum of the 2p state of phosphorus. The Gaussian fitting of the XPS signal produces the centre around 133eV and this peak appears because of the presence of phosphate in TO/[email protected], 39 3.3 Light absorption and emission property of NHs

Figure 4. (a) The absorption spectra of TiO2 NTs, TO/ZCO NHs, and TO/ZCO@Co-Pi NHs. (b) Room temperature PL spectra of pristine TiO2 NT, TO/ZCO NHs, and TO/ZCO@Co-Pi NHs sample (ex = 325 nm). (c), (d), and (e) respectively represent the Gaussian peak fitted PL spectra of TiO 2 NT, TO/ZCO NHs, and TO/ZCO@Co-Pi NHs.

The room temperature UV-Vis absorption spectra of pristine TiO2 NTs, TO/ZCO, and TO/ZCO@Co-Pi NHs have been shown in Figure 4a. It is observed that the absorption profiles of TO/ZCO and TO/ZCO@Co-Pi NHs almost replicate the absorption profile of pristine TiO2 NTs and it is quite justified as TiO2 serve as the backbone of the NHs. It is to be noted that the values of absorbance for both TO/ZCO and TO/ZCO@Co-Pi NHs are higher than that of the pristine TiO2 NTs in the entire range of the scan, which undoubtedly proves that the NHs possess better light absorption capability than the pristine TiO2 NTs, both in the visible light and UV regions. However, the identical absorption spectrum profiles of the TO/ZCO and TO/ZCO@Co-Pi NHs clearly indicate that the Co-Pi does not take part in the light absorption as reported by others too.39, 40 The band gap energies of the nanostructures estimated from the Tauc‟s plot (Figure S3, in the supporting information) indicate that the asprepared anatase TiO2 NTs is a direct band gap material41-45 with band gap energy of 3.26 eV and the band gap energies of TO/ZCO and TO/ZCO@Co-Pi NHs are 3.32 and 3.34 eV, respectively. Moreover, the absorption edges around 2.48, 2.57, and 2.58 eV for pristine TiO2 8 ACS Paragon Plus Environment

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NTs, TO/ZCO NHs and TO/ZCO@Co-Pi NHs, respectively, can be related to the band tail state, which arises as the mid-band gap states/defect states form a continuum extending to and overlapping with the conduction band edge.46 Now, to get a clear idea about the charge carrier recombination process and light emission property, the PL spectra of the nanostructures were recorded. From the PL emission spectra (Figure 4b), it is evident that the pristine TiO2 NTs exhibit very weak photoemission, which is because of the less recombination of photogenerated electron-hole pairs to generate visible light. However, after the formation of a ZnCo2O4 layer on TiO2 NTs, the as-prepared hybrid nanostructure exhibits enhanced PL emission. When the TO/ZCO NHs is optically excited during the PL measurement, electron-hole (e-h) pairs are generated both in n-type TiO2 and p-type ZnCo2O4 and at no external bias the photogenerated e-h pairs (even after charge separation) cannot get transported out of the material and they recombine radiatively leading to enhanced visible light emission because of the presence of mid-band gap defects. However, interestingly the PL emission intensity of the TO/ZCO NHs is found to quench remarkably after the incorporation of Co-Pi catalyst on the surface of the NHs, which demonstrates the significant reduction of electron-hole pair recombination in the NHs after Co-Pi modification. Therefore, here Co-Pi must be playing a key role, most probably by acting as a hole acceptor47 and thereby facilitating an efficient e-h pair separation reducing the carrier recombination probability which in turn diminishes the PL intensity of the NHs. The results demonstrate that the Co-Pi modification does not ameliorate the photocurrent by means of increasing the light absorbance rather it does so by passivating the surface recombination centres. Here, we have conducted an in-depth study on the defect-mediated PL emission to probe the carrier generation and carrier recombination process more vividly. Figure 4c, d, and e represent the deconvoluted PL spectra of pristine TiO2 NTs, TO/ZCO, and TO/ZCO@CoPi NHs, respectively. The positions of different peaks of the three materials have been tabulated in Table 1. The peak 1, positioned at 380 nm in pristine TiO2 NTs originates from the recombination of the photogenerated e-h pairs, releasing 3.26 eV of energy which matches quite well the band gap energy of anatase TiO248 and we also have found the same value as the band gap energy of pristine TiO2 from the Tauc‟s plot. The peak 2, centred at 411 nm (3.01 eV) for TiO2 might have been originated because of the free exciton recombination.49 Table 1. Summary of the key PL emission peak positions of the three nanostructures, obtained from the deconvolution of the PL spectra

Nanomaterials TiO2 TO/ZCO NHs TO/ZCO@Co-Pi NHs

1 380 373 369

Peak Name/Position (nm) 2 3 3a 411 437 465 407 448 404 445 -

4 496 492 489

5 523 538 536

The emission peak 3, positioned at 437 nm (2.84 eV) may be assigned to the self-trapped excitons located at the TiO6 octahedra, as reported.50 However, there are other reports which suggest that the surface defect states are responsible for the origin of the peak 3. 51, 52 The peak 3a, centred at 465 nm (2.66 eV), is exclusively present in TiO2 and is associated with 9 ACS Paragon Plus Environment


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the surface trap states, owing to incomplete surface passivation.53 A little more elucidation is obtained from other reports which inform that this 2.66 eV emission peak arises because of the shallow and deep surface traps associated with the Ti3+ states which are formed by the Ti4+ ions adjacent to the oxygen vacancies at the surface.54, 55 Therefore, the peak 3a is believed to have originated because of the surface defect states and the exclusive presence of peak 3a in pristine TiO2 NTs arrays further indicates that those surface defects have been subdued by the formation of the ZnCo2O4 layer on TiO2. The emission peak 4 and peak 5 centred at 496 nm (2.5 eV) and 523 nm (2.37 eV), respectively, in TiO2, are most likely to be related to the band tails and different surface oxygen vacancies which further give rise to F and F+ centres, respectively.56, 57 It is quite fascinating to note that peaks 1, 2, and 4 are blueshifted in TO/ZCO NHs and TO/ZCO@Co-Pi NHs compared to that of the pristine TiO2 NTs whereas peak 3 and 5 which are related to surface defects, are red shifted. Here, the blue shifts in the PL peaks can be attributed to the lattice strain that is produced in the host lattice during the formation of the heterojunction.58, 59 Reduction of the crystal size or wall thickness of the nanotubes may also be responsible for the blue-shifts. However, the red shift of peak 3 and 5 may be explained by the inhomogeneity of the surface defect states, because the formation of heterojunction and surface passivation eliminate the shallower surface traps more efficiently, leaving behind the deeper surface traps.60 3.4. Photoelectrochemical properties

Different aspects of the photoelectrochemical properties of the pristine TiO2 NTs, TO/ZCO, and TO/ZCO@Co-Pi NHs electrodes, have been studied in both dark and visible light illumination conditions. The current densities vs. potential (J-V) plots for the TiO2 NTs and TO/ZCO NHs electrodes are shown in Figure 5a. It is evident that compared to the pristine TiO2, both the dark current and photocurrent density have increased after the formation of the TO/ZCO NHs. The integration of Co-Pi on the TO/ZCO NHs further increases the current densities significantly. It is found that, while the photocurrent density in the pristine TiO2 NTs electrode increases negligibly (~9%) with respect to the dark current density. However, here, the large dark current for TiO2 NTs is most probably because of the low shunt resistance due to the presence of the cracks on the surface of the as prepared TiO2 NTs film appeared because of annealing (see Figure S1 in supporting information). The photocurrent densities in the TO/ZCO and TO/ZCO@Co-Pi NHs electrodes increase by 56% and 34%, respectively, at 1 V vs. Ag/AgCl with respect to the corresponding dark current densities. Interestingly, after the introduction of Co-Pi catalysts on TO/ZCO NHs, the dark and photocurrent densities increase by 76% and 52%, respectively, at 1 V vs. Ag/AgCl with respect to the TO/ZCO NHs electrode. This result clearly demonstrates the positive role of Co-Pi catalysts to enhance the PEC activity of NHs based photoelectrode. The onset potentials of photocurrent densities upon the visible light illumination (10 mW.cm-2, wavelength >420 nm) for different photoelectrodes have been determined from the LSV curves by differentiating the respective current densities with respect to the applied potential (dJ/dV), and then finding the respective values of dJ/dV where it attains the value of 0.2 mA.cm-2.V-1 61, 62 From Figure 5b, it is found that the onset potentials for the TO/ZCO and TO/ZCO@Co-Pi electrodes are 1.343 vs. Ag/AgCl (1.54 V vs. NHE) and 1.301 V vs. Ag/AgCl (1.498 V vs. NHE), respectively, which are cathodically shifted compared to that of 10 ACS Paragon Plus Environment

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the pristine TiO2 NTs (2 V vs. Ag/AgCl or 2.197 V vs. NHE). However, the onset potential for the TO/ZCO@Co-Pi electrode again shows a cathodic shift of ~ 40 mV with respect to the TO/ZCO electrode. Therefore, it is evident that the NHs exhibit significantly low onset potential compared to the pristine TiO2 NTs.

Figure 5. (a) LSV plots of different electrodes under dark and visible light illuminated conditions. (b) dJ/dV vs. potential plots of the different electrodes, (c) % vs. applied potential (vs. NHE) plots for the different electrodes.

The PEC water splitting performance of each of the photoelectrodes under an applied voltage upon the light illumination can be determined by calculating the applied bias photonto-current efficiency (ABPE) % using the following equation63

Where Jph is the photocurrent density, V is the applied potential, and Pin is the incident photon power density (10 mW.cm-2). The above equation gives the thermodynamic measure of the efficiency and can be used irrespective of the electrode configuration.64 The applied bias photoconversion efficiency of the different electrodes as a function of the applied voltage is shown in Figure 5c. It is evident that the photoconversion efficiency of the TO/ZCO@Co-Pi NHs electrode increases significantly compared to the TO/ZCO NHs as well as the pristine TiO2 NTs electrodes. The maximum value of the overall applied bias photoconversion efficiency for the TO/ZCO@Co-Pi NHs electrode is measured to be ~3% at 0.2 V vs. NHE, which is considerably higher compared to the different recently reported TiO2 based electrodes as summarized in Table 2. It is worth to note that the photoconversion efficiency enhances ~40% after the surface modification of the TO/ZCO NHs with Co-Pi catalysts.



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Table 2:

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Summary of the photoconversion efficiency of different TiO2 nanostructures based photoelectrodes

Electrode device

Electrolyte and illumination Photoconversion conditions efficiency 9 2 TiO2@BiVO4 Photoanode 0.1 M Na2SO3, 100 mW/cm (AM 1.24% 1.5G) c-Si/TiO2 core–shell nanowire65 0.5 M Na2SO3, 100 mW/ cm2 0.12% (AM 1.5G) Hydrogen-Treated TiO2 Nanowire16 1 M NaOH 100 mW/cm2 (AM ∼1.63% 1.5G) FeS2 Sensitized TiO2 Nanotube11 0.5 wt % NH4F in EG solution 0.41% with 2 vol % water, 100 mW/cm2 (AM 1.5G) Au/photonic crystal TiO2 NTs14 1 M KOH 100 mW/cm2 (AM 1.10% 1.5G) TiO2/g-CN18 0.2 M Na2SO4 solution, 20 0.135% 2 mW/cm (λ > 420 nm) ZnO@TiO2 Core−Shell 0.1 M NaOH, 100 mW/cm2 (AM 0.04% 12 Nanostructures 1.5G) Hydrogenated anodic one- 1M KOH with 1 wt.% EG 0.30% 15 2 dimensional TiO2 solution, 100 mW/cm (AM 1.5G) 8 Sn/TiO2 NW (annealed in H2) 1M aq. KOH, 75 mW/cm2 (AM ~1.2% 1.5G)

Figure 6. (a), (b), and (c) The photo-switching activities of TiO2 NTs, TO/ZCO, and TO/ZCO@Co-Pi NHs electrodes at 0.5, 0.8, and 1 V vs. Ag/AgCl under the chopped visible light.

Figure 6 shows the photo-switching activities of the TiO2 NTs, TO/ZCO NHs and TO/ZCO@Co-Pi NHs at different voltages (vs. Ag/AgCl). It is evident that under the same illumination, TO/ZCO@Co-Pi offers better photo-switching activity compared to TO/ZCO at the same applied bias. However, both TO/ZCO and TO/ZCO@Co-Pi NHs exhibit better photo-switching than the pristine TiO2 NTs. The photo-switching patterns of the pristine TiO2 NTs are quite dissimilar to that of the TO/ZCO and TO/ZCO@Co-Pi NHs. The „spikes‟ in


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the photocurrent density which appear for both the TO/ZCO and TO/ZCO@Co-Pi NHs are absent for the pristine TiO2 NTs. Instead, the photocurrent densities for the pristine TiO2 NTs are found gradually increasing during the light illumination. The phenomenon of the gradual increase in the photocurrent density during the „light on‟ period can be attributed to the thermal liberation of the trapped charge carriers. If electrons get trapped at the shallow trap centres which are placed very near to the conduction band of the pristine TiO2 NTs, then the thermal energy (kT) of the light source and the surroundings will be enough to liberate those electrons from the traps and to send them to the conduction band and there they will contribute in the conduction process.66 It is obvious from Figure 6b and c that upon visible light illumination, the current densities in TO/ZCO and TO/ZCO@Co-Pi NHs suddenly attain a maximum value like a „spike‟ and then they decay exponentially (transient photocurrent decay) before reaching the equilibrium. This phenomenon occurs because of the presence of surface trap states on the surface. The surface states behave as traps which hold charges and give rise to capacitance. Whenever the electrode is illuminated under a suitable positive bias, the holes start accumulating at the semiconductor liquid junction (SCLJ) and during the „light on‟ period, some of those holes get trapped inside the surface states and they might get recombined with the electrons in due course of time according to the nature of the traps. It is reported that the oxygen vacancies and the surface adsorbed O2¯ ions present in oxides also act as hole traps.67,

68

The photo-generated electrons also can be trapped at

different surface states. Moreover, as soon as the light illumination is „switched off‟ it is found that the current density sharply falls and thereafter the current density starts showing an exponential increment and this may be attributed to the dissipation of the accumulated charges.69

Figure 7 (a) The charge q(t) vs. time plot and (b) the current i(t) vs. time plot for the charging of a capacitor through a resistance in a series R-C circuit, which is shown in the inset of Figure 7(a).

However, it may be presumed as if the surface states make a series combination of resistor and capacitor (R-C) circuit and during the „light on‟ period the capacitors start getting 13 ACS Paragon Plus Environment


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charged up, showing an exponential response (see Figure 7a). It is known that charging current of the R-C circuit can be written as; (

)

Where, i0 is the maximum current possible in the circuit and = RC, is known as the time constant. Evidently, with the increase of time, the current decays exponentially with a time constant ( ) (see Figure 7b) (for details see the supporting information). However, the validity of this model depends on the following: (i) a large amount of de-trapping of the trapped charges is not simultaneously happening during the light illumination; this phenomenon happens when the applied bias is considerably high, because high applied bias will produce high electric field and that can effectively reduce the barrier thickness of the trap centre so that the trapped charge can tunnel out of the traps,70, 71 (ii) the rate of impact ionization is considerably less than the rate of charge carrier trapping and (iii) the rate of photon and phonon-assisted depopulation of the trapped charge carriers is also less than the rate of charge carrier trapping.70 However, if the aforesaid factors start dominating over the trapping processes, the exponential nature of the transient current will be lost, as reported previously.17, 27 Now, from Figure 8a the transient photocurrent density can be expressed as (for details see SI), (

)

Where, j(t) and j0 are the modified instantaneous and peak current densities, respectively, and they are written as j(t)= j(t)  jf and j0 = ji  jf (see Figure 8a). Now, after substituting the values of j(t) and j0, the above equation becomes:

The (j(t)  jf)/( ji  jf ) vs. time plots for the TO/ZCO and TO/ZCO@Co-Pi NHs electrodes at different applied potentials are shown in Figure 8b and c. The calculated values of time constants at different applied bias voltages, for both TO/ZCO and TO/ZCO@Co-Pi NHs electrodes have been tabulated in Table 3. Table 3: Calculated values of time constants of TO/ZCO and TO/ZCO@Co-Pi NHs electrodes at different applied potentials.

Potential vs. Ag/AgCl in V

Time constant ( ) in sec TO/ZCO

TO/ZCO@Co-Pi

0.5

4.76

3.55

0.8

5.92

5.16 14

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1.0

8.31

6.92

Figure 8. (a) Current density vs. time plot for the TO/ZCO@Co-Pi NHs electrode recorded at 0.5V vs.Ag/AgCl showing the peak current density, transient photocurrent density decay and the final current density, (b) and (c) respectively demonstrate the (J(t)-Jf)/(Ji-Jf) vs. time plots for the TO/ZCO and TO/ZCO@Co-Pi NHs electrodes at different applied potentials.

Now, at a fixed applied bias, the high value of the time constant indicates the high capacitance, which here signifies the presence of large numbers of trap states. Hence, from Table 3, the lower values of the time constant ( ) for TO/ZCO@Co-Pi NHs than that of the TO/ZCO NHs at different applied bias indicate that the Co-Pi modification on the TO/ZCO NHs significantly reduces the surface trapping states most probably by surface passivation. This result also indicates that the Co-Pi nanoparticle decoration on the surface of TO/ZCO NHs will significantly reduce the charge carrier recombination in this electrode by reducing the trapping states, which again support the earlier PL studies on the NHs electrodes. However, it is to note that for TO/ZCO and TO/ZCO@Co-Pi NHs, the values of the time constant increase with the increase of the applied voltage. It may be believed that with the increase in applied positive voltage under the visible light illumination, the applied positive bias hinders the traps to be easily filled up by charges; as a result, it takes more time to be filled up and hence increases with bias. For a better understanding of the charge transport properties and the electron recombination kinetics of the NHs electrodes, the open circuit potential (Voc) of the electrodes have been recorded under chopped light condition. Open circuit potential (Voc) is the potential at which the Fermi levels of the electrode and the electrolyte coincide, resulting in a zero charge carrier flow which thereby makes a zero current flow. Figure 9a demonstrates the open circuit potential (Voc vs. Ag/AgCl) vs. time behaviours of the TO/ZCO and TO/ZCO@Co-Pi NHs electrodes under dark and illuminated conditions. Whenever the electrodes are illuminated the carrier concentrations in the respective conduction bands get increased because of the electron-hole pair generation and the electrons get transferred to the conduction band of TiO2 from ZnCo2O4 resulting an increase in photocurrent density. This increment in the carrier concentration increases the internal chemical potentials () of electrons and holes inside the electrode and hence Voc increases. When the light is switched off, the electron density in the conduction band of TiO2 decreases rapidly because of recombination, resulting Voc decay. The increase in the value of the Voc (becomes more negative) after light illumination may be ascribed to the fact that holes tend to flow towards the electrolyte from the electrode and the Voc becomes more negative to inhibit this hole flow 15 ACS Paragon Plus Environment


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to make the net current zero. Therefore, upon light illumination, the significant increase in the Voc for the TO/ZCO@Co-Pi NHs compared to TO/ZCO NHs (Figure 9a), indicates a better hole transport property as well as reduced carrier recombination of the electrode after Co-Pi modification. It is evident from Figure 9a that for TO/ZCO NHs the Voc almost gets saturated quickly during the light off intervals. On the other hand, the Voc for TO/ZCO@Co-Pi NHs sample takes quite a longer time to get saturated after the light is switched off and this is quite evident from the nearly exponential decay nature of Voc. However, the above observation again confirms the longer lifetime of photoexcited carriers (reduced recombination rate) in Co-Pi modified electrode.72, 73 As the Co-Pi modification reduces the carrier recombination probability, there exist more photogenerated holes in TO/ZCO@Co-Pi NHs than those in TO/ZCO NHs and the greater number of photogenerated holes give rise to more negative Voc upon light illumination indicating the good flow of holes into the electrolyte. Furthermore, the electron lifetime can be calculated the from the Voc vs. time plot. Considering the change in the Voc (before it becomes almost saturated) after the suspension of the visible light illumination, the electron lifetime ( n) can be calculated as74, 75



(

)

Where kT is the thermal energy and e is the elementary charge. The variation of n over a range of Voc; plotted in Figure 9b, for the NHs electrodes, clearly indicates higher values of n for the TO/ZCO@Co-Pi NHs compared to TO/ZCO NHs. This observation demonstrates that the Co-Pi modification enhances the carrier lifetime in the electrode, which results in the effective reduction of electron-hole pair recombination because of the annihilation of surface trap states.

Figure 9. (a) Open circuit potential (Voc) vs. time plots and (b) calculated electron lifetime vs. open circuit voltage (vs. Ag/AgCl) plots for TO/ZCO and TO/ZCO@Co-Pi NHs. (c) The Nyquist plots for the pristine TiO2 NTs, TO/ZCO and TO/ZCO@Co-Pi NHs electrodes.

The electrochemical impedance spectroscopy (EIS) is one of the key techniques for analysing different electrical quantities such as resistance, impedance, capacitance etc. of the electrodes. Moreover, EIS also provides information about the charge transfer process at the electrode/electrolyte interface. However, the LSV experiment (current-voltage curves in Figure 5a) clearly indicates that the charge transfer rate in the TO/ZCO NHs electrode enhances significantly after Co-Pi modification. The Nyquist plots of TiO2 NTs and the NHs electrodes (Figure 9c) obtained by performing EIS in the 0.5 M aqueous Na2SO4 electrolyte 16 ACS Paragon Plus Environment

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at 0 V vs. Ag/AgCl with a perturbation of 5 mV AC amplitude, show a small arc followed by one large arc, which corresponds to the charge transfer resistance at electrode/electrolyte interface, where the charge transfer rate can be determined from the arc diameter of the Nyquist plot.76, 77 Clearly, the largest arc diameter in the pristine TiO2 NTs corresponds to the high charge-transfer resistance and poor charge transfer. The significant reduction of the arc diameter in TO/ZCO@Co-Pi NHs indicates lower charge-transfer resistance and noteworthy improvement of charge transfer at the electrode/electrolyte interface into the electrolyte to drive oxygen evolution reaction (OER) due to the incorporation Co-Pi, which results enhanced electrochemical performance of the photoelectrode. Moreover, the equivalent circuit corresponding to the Nyquist plots of the NHs electrodes is shown in the inset of Figure 9c. Where, Rct indicates the charge transfer resistance of the outer ZCO or ZCO@CoPi nanolayer and the estimated charge transfer resistance for TO/ZCO and TO/ZCO@Co-Pi NHs are found to be 8.6 and 5.3 cm2, respectively. The charge transfer resistance for pristine TiO2 NTs and as deposited ZCO thin films on FTO substrate was measured to be 13  cm2 and 20 .cm2, respectively (see supporting information). Therefore, this result clearly suggests that the Co-Pi OEC helps in the fast transfer of charge carriers into electrolyte to drive OER.73, 74 The interfacial band bending, caused by the fabrication of nano-heterojunction has a crucial role in the charge carrier separation, collection, and transportation. Therefore, for a better understanding of the interfacial band positions, the flat band potential (Vfb)78 of the pure TiO2 NTs and the ZCO thin films grown on FTO was estimated from Mott-Schottky study (Figure 10a) (see the supporting information). The positive slope of pristine TiO2 NTs (Vfb = -0.26 V vs. NHE)5 indicates its n-type behaviour. Hence, the conduction band minima for pristine TiO2 NTs is expected to be near -0.26 V vs. NHE.5 For ZCO thin film the Vfb value is calculated to be +1.1 V vs. NHE from the second region the

plot (inset of Figure

10a), which shows a negative slope indicating the p-type nature of the as grown ZCO film on FTO. Hence, the valence band maxima for ZCO is expected to be around +1.2 to +1.3 V vs. NHE as the position of valence band maxima for p-type semiconductors is assumed to be 0.10.2 eV (vs. vacuum) below the flat band potential.79 Furthermore, the band gap energy of the TiO2 NTs is estimated to be ~3.26 eV from the UV-Vis absorption study. Previous reports on the p-type ZnCo2O4 show that it has two different band gap energies, first one is found to be varying between 3.4-3.6 eV and the other one is found to be around 2.1 eV.29, 80, 81 It is also reported that for ZnCo2O4, the O 2p and Co 3d orbitals act as the valence band and conduction band, respectively.30, 80, 81 There are two different energy levels introduced by Co 3d orbital, namely high energy level Co 3d-eg orbital, which is not filled and the other one is the partially filled Co 3d-t2g orbital, situated in the mid-band gap.30, 80 Based on the above studies, the schematic of band edge positions and photo carrier generation/transportation of the heterojunction are shown in Figure 10b.



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Figure 10. (a) Mott-Schottky plot for the pristine TiO2 NTs and pure ZnCo2O4 film grown on FTO substrate (inset). (b) Schematic band diagram and charge carrier separation/transport in TO/ZCO@Co-Pi NHs electrode.

Now, the enhanced PEC performance of the Co-Pi modified TO/ZCO NHs could be understood by analysing the charge carrier generation and transfer mechanism outlined in Figure 10b. The improved PEC performance of the TO/ZCO NHs can be attributed to the efficient light absorption by the p-type spinel ZnCo2O4 layer, which generates a large number of electron-hole pairs. With the illumination of visible light, the photo-generated electrons can easily move from the valence band of ZnCo2O4 to the partially filled Co 3d-e2g orbital and also from the partially filled Co 3d-t2g orbital to the empty Co 3d-eg orbital, leaving holes behind.30, 80, 81 The photo-generated electrons at the Co 3d-t2g and Co 3d-eg orbitals of pZnCo2O4, readily migrate towards the n-type core TiO2 NTs, due to the formation of a favourable inversion layer because of the huge interfacial band bending between the n-TiO2 and p-ZnCo2O426 (see Figure 10b). Finally, the photo-generated electrons move towards the Pt counter electrode through Ti substrate. On the other hand, the hole trapped in the valence band of ZnCo2O4 reaches the electrode/electrolyte interface to take part in the water oxidation process. Furthermore, the incorporation of the Co-Pi OEC on ZnCo2O4 gives rise to an expeditious transportation/migration of holes from the valence band of ZnCo2O4 to the Co-Pi, leading to an enhanced electron-hole pair separation, resulting in a remarkable reduction in the recombination of photo-generated charge carriers in the NHs electrode. The photogenerated holes (h+) of the ZnCo2O4 nanolayer react with Co2+/3+ ions in the Co-Pi catalyst and convert them to Co4+ (Co2+/3+ + h+  Co4+).82 Finally, the Co4+ ions again convert into Co2+ after reacting with water molecules generating oxygen (Co4+ + 2H2O  O2 + Co2+).82 Thus the Co-Pi modification further improves the water oxidation kinetics of the TO/ZCO NHs electrode by driving the above mentioned circular reaction.


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4. CONCLUSIONS In summary, the present work presents an effective, easy, and scalable approach for the successful fabrication of a dense array of Co-Pi catalyst incorporated n-p TiO2/ZnCo2O4 nano-heterostructures by the electrochemical route, for the potential application in the visible light photoelectrochemical device. Results indicate that the coating of p-ZnCo2O4 nanolayer on the TiO2 NTs remarkably improves the light absorption performance of the nanoheterostructures and hence the overall PEC property. It is evident that the formation of n-p TiO2/ZnCo2O4 heterojunction introduces large concentration of defect/trap states in the nanoheterostructures electrode. However, because of the dominance of the photogenerated charge carriers over the recombined charge carriers in TiO2/ZnCo2O4 nano-heterostructures, the same also exhibits better PEC performance over the pristine TiO2 NTs. The study indicates that the incorporated Co-Pi layer on the TiO2/ZnCo2O4 nano-heterostructures mainly acts as an oxygen evolving catalyst, which boosts the PEC property by suppressing the trap states and hence effectively reducing the electron-hole pair recombination and facilitating the fast charge carrier separation. Although, the anchoring of Co-Pi catalyst on the NHs has little influence to improve the overall light absorption property of the nano-heterostructures. The Co-Pi modification remarkably improves the applied bias photoconversion efficiency of the n-p TiO2/ZnCo2O4 nano-heterostructures >3%. The studies on the photoluminescence and transient photocurrent density clearly demonstrate that the enhanced solar water oxidation property of the photoanode is because of the reduced charge carrier recombination and good transportation of electron towards the counter electrode through TiO2, due to the favourable nano-junction formation. Therefore, it is evident that the proper nano-engineering of the oxide semiconductor electrodes can remarkably improve their solar energy harvesting efficiency. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website. FESEM micrograph of as prepared TiO2 NTs, XRD and GIXRD pattern of pristine TiO2 NTs and TO/ZCO@Co-Pi NHs, respectively, the calculation of band gap energy and Tauc‟s plots for the NHs electrodes, details of photoelectrochemical studies and LSV plots of different electrodes, the photoconversion efficiencies of the photoelectrodes with respect to the voltage (V) vs. Ag/AgCl reference electrode, details of charge/current calculation for the charging of a capacitor through a resistance in a series R-C circuit, details of the calculation of transient photocurrent density, details of Mott-Schottky analysis of the electrodes and the Nyquist plot of the FTO/ZCO electrode (PDF)



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ORCID Gobinda Gopal Khan: 0000-0003-3040-0155 Ayan Sarkar: 0000-0002-5584-0360 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the start-up research grant (No. SR/FTP/ETA-0142/2014) from Science and Engineering Research Board (SERB), the Government of India. Author G.G.K. is thankful to the Department of Science and Technology (DST), the Government of India, for providing research support through the “INSPIRE Faculty Award” (IFA12-ENG-09). Author K.K. is thankful to the Department of Science and Technology (DST), for the award of “Inspire Fellowship” (2015/IF150237). The authors also sincerely acknowledge Mr. G. Sarkar and Prof. M. Mukherjee of Saha Institute of Nuclear Physics (SINP), Kolkata, India for the XPS measurements. REFERENCES 1. Grätzel, M., Photoelectrochemical Cells. Nature 2001, 414, 338-344. 2. Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983-2002. 3. Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. 4. Schultz, D. M.; Yoon, T. P. Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014, 343, 6174. 5. Miseki, A. K. a. Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. 6. Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432-449. 7. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. 8. Xu, M.; Da, P.; Wu, H.; Zhao, D.; Zheng, G. Controlled Sn-Doping in TiO2 Nanowire Photoanodes with Enhanced Photoelectrochemical Conversion. Nano Lett. 2012, 12, 1503-1508. 9. Zhang, X.; Zhang, B.; Cao, K.; Brillet, J.; Chen, J.; Wang, M.; Shen, Y. A Perovskite Solar CellTiO2@BiVO4 Photoelectrochemical System for Direct Solar Water Splitting. J. Mater. Chem. A 2015, 3, 21630-21636. 10. 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. 11. Xin, Y.; Li, Z.; Wu, W.; Fu, B.; Zhang, Z. Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near-Infrared Light Photoelectrochemical Water Splitting. ACS Sustainable Chem. Eng. 2016, 4, 6659-6667. 20 ACS Paragon Plus Environment

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