Influence of TiO2’s (101) crystal facet exposure on the photoelectroactivity of TiO2 nanooctahedra/BiVO4 nanocomposite

This work studies a research regarding the effect of exposing (101) crystal facet of anatase TiO2 to photoelectroactivity of photoanode composed of TiO2 nanooctahedra/BiVO4 nanocomposite film. Here, the photoanode was fabricated by depositing the nanocomposite on the surface of FTO via doctor blade technique. In this study, anatase TiO2 nanooctahedra were synthesized via solvothermal method in the presence of hydrazine hydrate as a directing agent. Furthermore, the as-prepared TiO2 nanooctahedra was characterized using X-Ray Diffractometer (XRD), Transmission Electron Microscopy (TEM), Brunauer-Emmelt-Tellers (BET) and UV-Vis diffuse reflectance spectra (DRS). Based on the result, the diffraction peaks revealed characteristic for the pure anatase phase with exposure (101) crystal facet. Additionally, photoelectrochemical response of the photoanode was also evaluated using a three-electrode system and exhibited a significantly high current density value of 0.26 mA/cm2.


Introduction
Crisis energy issues due to the lack of fossil fuels lead to an idea to substitute petroleum based energy with hydrogen based energy by using water as electron and proton sources. Photoelectrochemical (PEC) devices convert water to oxygen and hydrogen using photon and semiconductor as photoanode. Since the photoanode plays an important role in PEC, visible light responsive, high efficiency, and low cost are crucial to the performance of PEC [1,2]. One of the promising semiconductors type-n, titanium dioxide (TiO2), is generally applied, owing to its superior photocatalytic activity, environmentally friendly, and low cost [3]. However, TiO2 with wide band gap can only absorb ultraviolet energy, which contains only about 4 % of the solar energy [4]. This phenomenon will decrease the efficiency for PEC usage due to the maximum current will increase in the range of photon absorption. Hence, the new materials with photoactive nature in visible light were developed to shift the energy band gap of TiO2. Bismuth vanadate, (BiVO4) based photoanode is an interesting candidate due to visible-light activity, good stability, and effective charge separation properties. Combining the BiVO4 that has small optical bandgap (2.4 eV) with TiO2 would improve the light absorption in the visible region [5,6]. Moreover, the formation of TiO2/BiVO4 prolongs the carrier lifetime and then promotes the separation of photoelectron and holes that exhibit an excellent performance for photocatalytic water splitting and pollutant degradation [7].
Recently, controlled synthesis of TiO2 to form a specific morphology with specific exposed crystal facet has gained much attention. Anatase TiO2 crystal phase is known to have the highest photoactivity among the others phase, rutile and brookite [8]. According to the Wuff construction, the most available facets on the anatase TiO2 crystals is dominated by (101) facets rather than (001) facets due to its less reactivity and thermodynamically more stable [9]. Therefore, (101) facets found naturally for about 90 % of the total exposed surface on the anatase TiO2 [10]. In this work, the effect of morphology and exposure (101) crystal facet of anatase TiO2 nanooctahedra on the photoelectrochemical response of TiO2 nanooctahedra/BiVO4 nanocomposite film as photoanode were investigated. Here, anatase TiO2 nanooctahedra were synthesized by a simple solvothermal method with the addition of directing agent into the reaction, followed by loading the as-prepared TiO2 nanooctahedra onto BiVO4 paste and deposition on the FTO via doctor blade technique to obtain the photoanode film.

Synthesis of TiO2 nanooctahedra
Anatase TiO2 nanooctahedra were synthesized through a solvothermal method as previously reported [8]. For the typical synthesis, 2 mmol TiF4 was added to 5 mL distilled water forming a clear solution.
Then, 35 mL of hydrazine hydrate was added to the solution with magnetic stirring. The solution mixture was transferred into a Teflon lined autoclave and heated at 200 °C for 24 h. After the reaction, the products were separated by centrifuge and rinsed with distilled water and ethanol for several times. Finally, the products were dried in an oven at 70 °C for 6 h and calcined at 450 °C for 2 h.

Fabrication of TiO2 nanooctahedra/BiVO4 nanocomposite
The photoanode was prepared via doctor blade technique according to the previous literature [11]. Bi(NO3)3.5H2O (2 mmol), NH4VO3 (2 mmol) were mixed with 2 mL of 13 M HNO3 and grinded in an agate mortar followed by the addition of as-prepared TiO2 nanooctahedra (1 mmol). Then, polyethylene glycol (0.2 mmol) was added gradually to the above mixed solution. After the solid was dissolved, one drop of Triton-X was added and grinded to form yellow-brownish paste. The resulting paste was coated on FTO and calcined at 450 °C for 3 h.

Characterizations
In order to determine the crystallinity of the as-prepared TiO2, the X-Ray diffraction (XRD) analysis was employed by using PANanalytical X'Pert Pro MPD with Cu-KȽ radiation. To determine the morphology and the particle size, transmission electron microscopy (TEM) was performed on a TECNAI G2 Spirit Twin High-Resolution. To investigate the porosity and surface area, Brunauer-Emmett-Teller (BET) surface area, Barret-Joyner-Halenda (BJH) pore volume and BET N2 adsorptiondesorption isotherm were recorded on a QUADRASOB evo (Quantachrome Instruments). To determine the optical properties, diffuse reflectance spectroscopy (DRS) was carried out using Shimadzu UV-2450 spectrophotometer and the band gap (Eg) was calculated using Kubelka-Munk method.

Photoelectrochemical measurements
The photoelectrochemical response of the photoanode film was carried out with three-electrode system by using Hokuto Dento Hz-3000 Potentiostat. TiO2 nanooctahedra/BiVO4 nanocomposite (0.9 cm × 0.9 cm) film was used as working electrode, Ag/AgCl (3 M NaCl) as reference electrode, and coiled Pt as counter electrode. An 500 W Xenon lamp was used as the visible light source with an intensity 160 mW/cm 2 measured by TENMARS TM-208 solar light meter and Na2SO4 0.5 M (pH 7) was added into the phosphate buffer 0.1 M as the electrolyte solution [12].

Fabrication and characterization of anatase TiO2 nanooctahedra
The crystalline structure of the as-prepared TiO2 was analyzed by XRD as shown in figure 1a. According to the XRD patterns, the as-prepared TiO2 was believed to show the anatase crystalline phase, which corresponds to the reference (JCPDS Card No. 21-1272) [13]. Furthermore, the morphology of the asprepared TiO2 nanooctahedra was observed by TEM analysis. From the TEM image in Figure 1b, the as-prepared TiO2 with addition of hydrazine hydrate as directing agent was found to be an octahedralshape structure with edge width in the range of 200-300 nm and length between the two-pointed ends in the range of 400-500 nm. In addition, to investigate the exposure of the (101) crystal facet, Fast Fourier Transform (FFT) analysis of the TEM images is shown in the inset in figure 1b. As shown, the result confirms that the as-prepared TiO2 nanooctahedra did expose more (101) crystal facet.

Surface and optical properties of anatase TiO2 nanooctahedra
The BET isotherms graphs in figure 2a shows a typical type (III) like with type (IV) hysteresis loops that presence at high pressure [14]. Furthermore, the pore size distribution (PSD) plot was also determined using Barrett-Joyner-Halenda (BJH) method from the desorption of the isotherm (figure 2b). According to the result, the obtained PSD plot exhibited similar features with isotherm plot, having peaks of microporous (< 2 nm) and mesoporous (2-50 nm). Additionally, the result also demonstrated that the average pore volume and BET surface area for the as-prepared TiO2 nanooctahedra were 0.026 cc/g and 16.133 m 2 /g, respectively. Furthermore, the optical properties of the samples were also investigated using UV-Vis spectroscopy. As it can be seen in figure 3, the absorbance of TiO2 nanooctahedra increases in the UV region and displays an absorption edge at 413 nm, which corresponds to a band gap of 3.0 eV.

Photoelectrochemical measurements
In order to gain the information about the film response to the light irradiation, the linear sweep voltammetry (LSV) was carried out both under dark and light exposure conditions. Figure 4a shows the obtained J-V characteristics of the as-prepared TiO2 nanooctahedra/BiVO4 photoanode. According to the result, it is obvious that there is a significant increase in the photocurrent in light irradiation, indicating the film photoactive response. Additionally, result also showed that the film produced about 0.14 mA/cm -2 of photocurrent at the thermodynamic water oxidation potential of 1.23 V. Further investigation was also carried out to evaluate the stability of the photoanode film. Here, photocurrent measurements were performed under the applied potential bias (1.4 VRHE) and light illumination. According to the result in figure 4b, the photocurrent spikes of TiO2 nanooctahedra/BiVO4 nanocomposite film reached the value of 0.26 mA/cm 2 . Nevertheless, it is also worth to note that these photocurrents were quickly decreased during the first 200 sec of light exposure before gradually reached the equilibrium. This photocurrent was found to be significantly higher than the value of bare BiVO4 photoanode film, 0.5 ɊA/cm -2 , under illumination of simulated sunlight (AM 1.5, 100 mW/cm -2 ) and  active area of photoanode 2 cm × 2 cm without applied bias [11]. It is believed that the high-energy electrons of BiVO4 that excited by the light would be energetically transferred to TiO2 and enhanced the lifetimes and separation of visible-excited charge carriers of BiVO4 [7].

Conclusion
In summary, anatase TiO2 nanooctahedra have been successfully synthesized via solvothermal method with the addition of hydrazine hydrate as a directing agent. This conclusion is confirmed by the evidence that the diffraction peaks of TiO2 powder particles are corresponded to the characteristic peak of pure anatase phase and the morphology of the anatase TiO2, found to be an octahedral shape, which preferentially exposed (101) facets. The TiO2 nanooctahedra/BiVO4 nanocomposite film has the potential to become a promising photoanode on PEC for water splitting and pollutant degradation due to its photoactive response and effective charge separation.