Enhanced electrochemical water splitting activity in annealed TiO2 nanoparticles of photoanode

In this paper, we present the electrochemical water splitting characteristics of TiO 2 /FTO electrodes via spin-coating method. By using thermal annealing approach, the TiO 2 nanoparticle (P25) was modified with a more active photocatalyst. The annealed TiO 2 nanoparticle-based photoanode in vacuum shows photocurrent density of 0.27 mA/cm 2 and photoconversion efficiency of (  = 0.22%) at potential of 0.4 V ( vs. RHE ), which are higher than those of annealed TiO 2 nanoparticle-based electrode in air. The improved photoelectrochemical property is attributed to high oxygen vacancy density with more active sides, while TiO 2 nanoparticle was annealed in vacuum (  10  1 torr) with oxide concentration conditions. From this finding, we propose that a thermal annealing process might serve as an approach for fabricating the photoanodes of TiO 2 -based materials consisting of much active photocatalyst


Introduction
In the past, metal oxide (MO) materials and their composites have been attracting much attention from the research community in terms of photocatalytic splitting of water using sunlight [1][2][3][4][5]. Various MO (TiO2, WO3, CuO, Fe2O3, In2O3, and BiVO4) structures and morphology engineering for photoelectrochemical (PEC) devices are of interest to obtain a great product of H2 and O2 gases [6][7][8][9][10]. Even though traditional MOs for splitting water have been known for decades, recently the PEC systems were synthesized by a hybrid of MO and other semiconductors. Among them, crystalline anatase TiO2 [2,11] is distorted octahedral coordination (Oh), which consists of more active sites than other phases (rutile (D2h) and brookite). An efficient PEC water splitting system has been developed due to useful properties such as an ntype semiconductor, high photoconversion efficiency, long-term stability, large density of states in the conduction band derived from the dorbital, inexpensive cost, earth-abundant, environmentally friendly issues for device fabrication [12,13]. In general, electrons photoexcited in the valence band of TiO2 are sufficiently energetic to be transferred to the water for H2 production. However, the high recombination rate of carrier pairs, non-Schottky contact, and large bandgap (ca. 3.2 eV) issues of as-synthesized TiO2 reduce the generated photocurrent of the PEC device [14][15][16][17][18]. Thus, photocatalytic water-splitting using anatase TiO2 for high H2 production has been of interest to scientists. Based on those issues, many efforts to fabricate TiO2-based PEC devices for H2 production were reported such as structure engineering [6,19], noble metal loading/doping [14,5,18], modified oxygen vacancy density [20,21], and enhanced localized surface plasmon resonance [22][23][24] approaches. Among them, the preparation of anatase TiO2 phase with modified oxygen vacancy and large surface area has attracted much attention in terms of increasing the active photocatalytic properties compared to other phases. Because it has been vastly used as an overall efficient photocatalyst, and stable for PEC application [3,14,25,5,9,17,26]. Therefore, many techniques have been done to synthesize anatase TiO2 phase for water splitting application, including hydrothermal/ solvothermal [27,19,28], oxidation [29,19,28], thermal plasma [30,28], electrophoretic deposition [31,32,28], sol-gel [33,34,28], spray pyrolysis [35,28] methods. However, the mechanism of TiO2-based water splitting is not clearly known, there are still many limitations affecting the PEC performance [3,34]. In this work, thus, we consider the preparation of TiO2 nanoparticlebased photoanode using commercial TiO2 nanoparticles (P25) by the spin-coating approach. TiO2 nanoparticle is annealed at a temperature of 450 o C in air/vacuum (10 1 torr) for 100 min. By manipulating annealing conditions, the effect of annealing conditions on the crystalline structure, morphology, optical properties of TiO2 material was investigated. Based on this result, we studied the PEC water splitting characteristics of the TiO2 photoanode using an electrolyte (Kpi phosphate buffer solution, 1.0 M KH2PO4/K2HPO4, pH = 7). This annealing method paves the way for the fabrication of the TiO2 material-based PEC device with a more active photocatalyst.
A mix of 0.73 mg of TiO2 nanoparticles was dissolved into 50 ml ethanol for a 1.8% in weight concentration. This solution was stirred for 30 min at room temperature (RT) for a uniform. This solution was deposited on the FTO substrate by the spin-coating technique (with step 1: a 500 rpm for 5 sec, and step 2: a 3000 rpm for 30 sec) three times. Samples were investigated at an annealing temperature of 450 o C for 100 min in air (TO-Abi sample) and vacuum ( 1 × 10 1 torr, TO-Vac sample). The composition of the TiO2 layer was studied by the energy dispersive Xray (EDS, Hitachi S4800, Japan). The crystalline structure of TiO2 film was measured by the Siemen D5000 X-ray diffractometer using CuKα radiation, λ = 1.546 Å. The optical properties were investigated by a Raman Horiba XploRA Plus, Nicolet iS10, an Agilent Cary 5000 UV/VIS/NIR spectrometer, a Horiba iHR550 spectrometer using a 355 nm pulsed Nd:YAG laser as an excitation source. The photoelectrochemical (PEC) cell with a working area of 0.5 cm × 0.5 cm was made from the TiO2/FTO/glass substrate using epoxy to cover the undesired area. The PEC characteristic was investigated by a three electrode system (a saturated Ag/AgCl electrode as the reference electrode (in a 3.0 M KCl, CHI instruments), a Pt sheet as the counter, and an FTO/TiO2 as the working electrode), an electrolyte analyzer (BioLogic SP300 potentiostat) using a Kpi phosphate buffer solution (1.0 M KH2PO4/K2HPO4, pH = 7) and solar simulation system (PEC-L01 AM 1.5G lam power 100 mW/cm 2 , 10 mV.s 1 ).  The existence of two anatase and rutile phases comes from the mixing phase of the non-completed synthesis process of TiO2 at low temperature (below 500 o C). The number and intensity of XRD peaks of the rutile and anatase phases of both samples are similar.

Results and discussion
The number of XRD peaks of the rutile phase is less than that of the anatase phase, which is attributed to the dominant phase of the anatine crystal at 450 o C. At present, the anatase phase tendency has grown by converting rutile to anatase phase. The phase shift is noncompleted, which is original to form high oxygen vacancy (OV) density of the anatine phase. Because OVs of TiO2 are attributed to reduced energy bandgap of TiO2 structure [20,36,37,21] that might extend the absorption wavelength range of sunlight for improving the efficiency of PEC device. respectively, for the TO-Abi sample, while those elements accounted for 66.92 and 33.08% in weight for the TO-Vac sample, respectively. The oxygen concentration of the TO-Abi sample is higher at 3.34% than that of the TO-Vac sample. We did not obtain the Sn and F elements as well as other contaminations from the FTO substrate. The low oxygen density during the phase process from rutile to anatase causes high OV density of TiO2 structure, especially at the interface between rutile and anatase phases. Due to the effect of defect density and OV density on bandgap energy of TiO2 [20,36,25,21], the role of contamination is very important to manipulating physical properties as well as PEC TiO2 based application. Thus, the generated activated photocatalyst of OV under illumination strongly affects the efficiency of PEC device, further.  Fig. 3(a). As can be seen, there are three first order frequency vibrations at  [38,39], the optical phonon ( opt  ) of anatase TiO2 structure is given by Equ. 1  [25], which are expected to play a crucial role in doping of anatase TiO2 structure [36].   for TO-Abi and TO-Vac samples, respectively [5,44]. The absorption spectrum of both samples indicated the direct transition bandgaps [45]. The absorption edge of the TO-Vac sample shows a slight red shift towards the long wavelength, which is in a agreement with the photocatalyst activity of MO in the visible region. This phenomenon is attributed to the effect of modified energy electron density states (EDS) on the bottom of the conduction band. There is low oxygen density during the annealing process (in vacuum conditions) leading to low EDS, which exhibits a shifting absorption edge to a long wavelength. To confirm that, we measured the photoluminescence (PL) spectrum of both samples in the range of 400-700 nm at RT, as seen in Fig. 3(d). The normalized intensity (ITO) of PL spectra shows a peak at 413 nm (3.0 eV) for the bandgap transition of TiO2, which is consistent with previous reports [5,46]. While one emission peak at 516 nm (2.4 eV) is assigned to the transition band of oxygen vacancies (OVs) [47,5]. The emission intensity (IOV) peak at 516 nm of the TO-Vac sample is higher than that of the TO-Abi sample. However, the obtained PL spectrum is quite different compared to other reported PL spectrum below annealing temperature of 500 o C [47,20,37,46,21,48]. It should be noted that this difference might be understood by different degrees of formation of oxygen vacancies, as well as quantum confinement effect vs. different sizes under synthesis conditions. Interestingly, we have compared the PL quantum efficiency of TiO2 and evaluated the effect of vacuum on the contribution of oxygen vacancy to the spectral PL by the intensity between the PL peaks (IVO/ITO). If the ratio is increasing with high oxygen concentration, it indicates high VO density. Thus, it strongly suppresses the PL intensity due to high carrier recombination and trapping carrier density, which causes reduced photocurrent of the TiO2 film-based PEC device. For detailed reduced quantum yield and VO density, and mechanism of PL emission spectra of TiO2, we can reference other reports, elsewhere [47,20,21].   Faradaic efficiency of both PEC devices was evaluated by following Equ. 7 [4] () (%) 100 Where Eo = 1.23 V is the standard reversible potential, J is the photocurrent density (mA/cm 2 ), VRHE is the bias potential vs. RHE (V), and P is the power density of incident light (mW/cm 2 ). As a result, the conversion efficiency of TO-Vac sample is 0.22%), which is higher than that of the TO-Abi electrode ( = 0.15%). In fact, due to a low oxygen concentration during the annealing process, OV density in anatase TiO2 structure increases, in which if the carrier concentration is sufficient, the change in the concentration of carrier will no longer make the Fermi level increase, but instead strongly alter the dipole distribution in the Helmholtz layer at the interface of photoanode and electrolyte [20,5,21]. Also, when the OV concentration near the TiO2 surface is high, it greatly increases the Schottky barrier height (SBH). Minor carriers are now blocked by the SBH and cannot be transferred to TiO2 nanoparticles to contribute to the PEC current.
Thus, the photoelectrocatalytic performance of the TO-Vac photoanodes was significantly enhanced in comparison with that of the TO-Abi photoanodes.
Besides that, we measured the stability of TiO2/FTO samples in a 1.0 M Kpi electrolyte for 1 min, as seen in Fig. 4(c). The effect of pressure on the stability of

Conclusions
We investigated the synthesis of TiO2/FTO (P25) nanoparticle electrodes by the spincoating method using thermal annealing in air (TO-Abi sample) and in vacuum (TO-Vac sample) conditions. The optical characteristics of TiO2/FTO samples were studied by application. This is further a promising approach to enhance photoelectrochemical water splitting efficiency of anatase crystalline TiO2