Decorating (001) dominant anatase TiO2 nanoflakes array with uniform WO3 clusters for enhanced photoelectrochemical water decontamination

Accepted Manuscript Title: Decorating (001) Dominant Anatase TiO2 Nanoflakes Array with Uniform WO3 Clusters for Enhanced Photoelectrochemical Water Decontamination Authors: Guanda Zhou, Ting Zhao, Ruifeng Qian, Xin Xia, Songyuan Dai, Ahmed Alsaedi, Tasawar Hayat, Jia Hong Pan PII: DOI: Reference:

S0920-5861(18)30925-8 https://doi.org/10.1016/j.cattod.2018.12.004 CATTOD 11821

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

27 September 2018 23 November 2018 3 December 2018

Please cite this article as: Zhou G, Zhao T, Qian R, Xia X, Dai S, Alsaedi A, Hayat T, Hong Pan J, Decorating (001) Dominant Anatase TiO2 Nanoflakes Array with Uniform WO3 Clusters for Enhanced Photoelectrochemical Water Decontamination, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Decorating (001) Dominant Anatase TiO2 Nanoflakes Array with Uniform WO3 Clusters for Enhanced Photoelectrochemical Water Decontamination

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Guanda Zhou,a Ting Zhao,a Ruifeng Qian,a Xin Xia,a,* Songyuan Dai,a Ahmed

Beijing Key Laboratory of Energy Safety and Clean Utilization, School of

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a

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Alsaedi,b Tasawar Hayat,b,c Jia Hong Pana,*

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Renewable Energy, North China Electric Power University, Beijing, 102206, China b

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NAAM Research Group, Department of Mathematics, Faculty of Science, King

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

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c

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Abdulaziz University, Jeddah 21589, Saudi Arabia

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* To whom all correspondence should be addressed E-mail: [email protected]; [email protected];

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Tel: +86 10 6177 2407; Fax: +86 10 6177 2079.

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Graphic Abstract

Coating of (NH4)2WO4

TiO2 Array

TTIP+AcAc +HCl+NH4F

(NH4)2WO4/TiO2

FTO/TiO2

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WO3/TiO2

Highlights

WO3 decorated TiO2 arrays were synthesized through a mild hydrothermal

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

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process;

Chelating agent AcAc were used to control crystal morphology;

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3.Efficient degradation of MB molecules were observed when using WO3/TiO2

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ABSTRACT

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as photocatalyst.

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A facile two-step chemical bath deposition (CBD) method has been developed for the

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preparation of uniformly crystalline anatase WO3/TiO2 array on FTO substrate. The synthesis starts from the hydrothermal growth of TiO2 array in a homogenous HCl

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aqueous solution containing stabilized titanium isopropoxide, NH4F and acetylacetone (AcAc). Electron microscopy and the XRD analysis suggest the addition of AcAc chelating agent can facilitate the preferential growth of anatase (001) facets that are interconnected and vertically aligned. While (101) dominant TiO2 bipyramid array may form without AcAc. The subsequent decoration of WO3 clusters on TiO2 array is 2

achieved by post-depositing TiO2 array in (NH4)2WO4 aqueous solution followed with a calcination at 450 oC, the resultant WO3/TiO2 array shows a significantly elevated photocurrent performances owing to the high separation efficiency of charge carriers. The enhanced photoelectrocatalytic properties are explained by the efficient charge

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carrier separation at the heterojunction between WO3 and TiO2. A faster photoelectrochemical degradation of methylene under simulated sunlight further

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demonstrates the potential usefulness of WO3/TiO2 arrayed photoelectrocatalyst in

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solar-driven environmental purification and solar fuel synthesis.

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Keywords: TiO2, WO3, hydrothermal process, semiconductor photoelectrocatalysis;

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water treatment

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1. Introduction

The effective utilization of semiconducting metal oxides for photoelectrochemical

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(PEC) environmental remediation and water splitting has been attracted numerous research interest since the discovery of Honda-Fujishima effect in early 1970s [1].

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Continuous efforts have been devoted to developing efficient light-driven photocatalysts for solar energy harvesting and water purification[2, 3]. Metal oxide

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semiconductors, such as TiO2 [4-6], ZnO [3, 7, 8], BiVO4 [9-12], and WO3 [13-18] have been extensively studied as photocatalysts to decompose organic pollutants through photo-oxidation reactions. An ideal photocatalytic material requires strong redox capability and stability, a suitable band gap with a broad light absorption spectrum, and excellent stabilities against the photocorrosion in water at different pH values [1, 19]. 3

Unfortunately, traditional photocatalysts, represented by TiO2, give a relatively low photoelectric conversion efficiency resulting from the facile recombination of photogenerated electrons and holes. Moreover, the application of single TiO2 semiconductor as photocatalyst has also been restricted to ultraviolet environment owing to its wide band gap (Eg = 3.2 eV, corresponding to a light wavelength λ = 380 nm) [3, 20, 21].

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Various dopants and duo metal oxides holding comparable band positions have been

utilized to boost the PEC performances by sufficiently improving the light absorption

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and charge carrier separation [22-25].

WO3 has a slightly narrower band gap (Eg = 2.6 eV, corresponding to a light

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wavelength λ = 477nm) than TiO2 [6, 13, 14, 16, 26-31]. The nanostructured WO3 films

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have been successfully prepared by several techniques, such as CVD, spin-coating,

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electrodeposition and sol-gel precipitation. [32-35]

Yagi et al. [36] reported the

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synthesis of nanostructured WO3 platelets which produced significant photoanodic

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current on illumination of UV and visible light below 470nm. Since W6+ has a similar ionic radius with Ti4+, WO3 can easily couple into TiO2 crystals through a co-

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crystallization process. Several previous studies have demonstrated the band gap of

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WO3-TiO2 can be progressively reduced through WO3 doping, e.g. for highly ordered TiO2 and WO3/TiO2 nanotubes, the band gap decreased from 3.23 eV for TiO2 to 2.78

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eV for WO3/TiO2 [32]. Emerging attempts have also made to synthesize coupled WO3/TiO2 nanostructures with high surface area and favorable morphology for the potential applications in environmental remediation. Puddu et al.[37] reported the enhanced

photocatalytic

activity

of

hydrothermally

synthesized

WO3/TiO2

nanocomposites for trichloroethylene degradation. Ramos-Delgado et al. [16] 4

synthesized 2wt% WO3/TiO2 via a sol-gel process for efficient photocatalytic degradation of malathion pesticide. Very recently, Gao et al. [27] hydrothermally deposited heterostructured WO3/TiO2 nanofilms on the substrate of wood fibers with significantly enhanced photocurrent densities.

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Aiming to growing well crystallized WO3/TiO2 array via a simple route, we have

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developed a two-step chemical bath deposition (CBD) method to prepare uniform crystallized WO3 decorated TiO2 arrays. The morphology, optical and electronic

properties and photocatalytic activity of obtained WO3/TiO2 films were characterized

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through several experimental techniques. The bicomponent WO3/TiO2 photocatalysts

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was found to extend the range of visible light adsorption as well as increase the

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efficiency of photo-current generation and electron-hole separation, giving a significantly improved PEC performances in degradation of methylene blue (MB)

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under the irradiation of simulated sunlight. These promising features of WO3/TiO2

purification.

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films offers potential utilization of this material in solar water splitting and water

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2. Experiment

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2.1 Synthesis of WO3/TiO2 Array All the chemicals were purchased from Sigma-Aldrich without further purification. Figure 1 illustrated the two-step CBD route to WO3 decorated anatase TiO2 nanoflakes array. The nutrient solution for anatase TiO2 nanoflakes array was prepared by adding 0.8 ml of titanium isopropoxide (TIP, >97 wt%) dropwise into a 30 ml of HCl aqueous 5

solution HCl (5.0 mol/L) (36 wt% HCl) containing small amount of acetylacetone (AcAc, >99%) and NH4F (0.10 g) [34, 38] [39]. The high concentration of HCl ensures the stabilization of TIP by complexing [40, 41]. After stirring for 15 min, the clear, yellowish, homogeneous nutrient solution was transferred to a 50-ml Teflon-lined

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hydrothermal synthesis autoclave reactor. A pre-cleaned fluorine doped tin oxide (FTO) coated glass slide (Pilkington TEC 7; sheet resistance: ~7 Ω/sq; thickness: 2.2 mm)

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with a size of size of 2×4 cm2 was then immersed into the solution and leant face down and against the Teflon wall, before the autoclave was tightly covered and kept in an

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electronic oven at 180°C for 4.5 h. The system was cooled down naturally after

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finishing the hydrothermal reaction. The obtained white array grown on FTO substrate

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was recovered and sufficiently rinsed with DI water repeatedly, and the calcined at

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350°C for 1 h to completely remove any residuals and organics on the surface of TiO 2

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array. The obtained array was used as substrate for the second-step CBD process or calcined at 450 oC for 1 h to obtain final TiO2 nanoflakes array with dominant anatase

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(001) facets. The sample was denoted as TA.

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The second-step CBD was conducted by soaking the above TiO2 array into a 1.0 wt% (NH4)2WO4 aqueous solution for 2h at room temperature. (NH4)2WO4 solution

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was made by dissolving tungstic acid in a diluted ammonia solution. The array was then recovered and rinsed with DI water repeatedly, followed by calcination at 450 °C for 1 h to obtain WO3-decorated TiO2 nanoflakes array (denoted as WT) . For comparison, anatase (101) dominant TiO2 bipyramid array, denoted as BA, was synthesized directly through a similar hydrothermal reaction without adding AcAc 6

followed with a similar thermal treatment. 2.2 Materials characterization and PEC measurement The morphologies and microstructures of the as-prepared samples were observed

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using a field emission scanning electron microscope (SEM Hitachi S4800) at 5 kV and a Tecnai F20 high-resolution field emission transmission electron microscope (TEM).

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X-ray diffraction (XRD) patterns were characterized on a Bruker D8 Advance

diffractometer with CuK radiation (λ = 0.15418 nm) in the 2 range of 20−60o. The optical transmission spectra were measured using a Metash UV8000A UV-Visible

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Spectrophotometer. PEC measurement was performed on a CHI 760E workstation with

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a Na2SO4 electrolyte (1.0 mol/L) as the electrolyte. A Xe arc lamp (Perfectlight FX300,

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light intensity: 100 mW/cm2) was used as the light source. A three-electrode configuration consisting of TiO2-based array on FTO as working electrode, a Pt counter

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electrode and a saturated Ag/AgCl as reference electrode was employed as reference

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standard. PEC degradation of MB solution (0.20 mg/L) was conducted in the same system. Before measurement, the arrays were immersed in the MB solution for 1h

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before subjected to light irradiation. The transient concentrations of MB at different irradiation time intervals were measured by detecting the optical absorbance at 663 nm

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on the Metash UV8000A UV-Visible Spectrophotometer.

Results and discussion We first discuss the structure and the formation of TiO2 array. SEM micrographs 7

shown in Figures 2a-e reveal the hydrothermal- time-dependent morphological evolution during CBD process. The nanoflakes gradually grow and tend to interconnect with the elapse of the time. At a reaction time more than 3 h, the growth significantly slow down and no obvious change in structure. Interestingly, the optimal PEC

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performances are found in the TiO2 array reacted for 4.5 h, which may be due to the improved crystallinity and reduced crystal defects upon a prolonged growth time.

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Figure 2 shows the representative SEM images of WT, TA and BA. Without adding AcAc, BA presents bipyramidal morphology in which the prominent facet is anatase

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(101) as suggested in various previous studies [42]. When using AcAc as a chelating

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agent, well-defined TiO2 alignment comprised of two-dimensional (2-D) nanoflakes

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has successfully grown on FTO substrate after 4.5 h hydrothermal reaction, as shown

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in Figure 2 c and d. The nanoflakes, acting as building blocks, present a uniform size

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and shape distribution of interdigitated decahedrons which indicated anatase (101) facets exposed to the surface, while the dominant anatase (001) facets are

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interconnected and almost vertical to the substrate. The orientation of the single-crystal

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anatase can be further demonstrated by the XRD analysis. In Figure 3, the XRD pattern recorded directly on the films shows that all the prominent reflection peaks are well match with to a tetragonal (I41/amd) anatase TiO2 phase with lattice parameters of a =

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3.784 Å and c = 9.508 Å (Powder Diffraction File (PDF) No. 21–1272, International Centre for Diffraction Data (ICDD)). An extraordinarily strong reflection peak at 2θ of 25.4o can be assigned to the anatase (101) plane. Interestingly, the characteristic (004) diffraction peak at 2θ of 37.2o cannot be found, although its intensity is enhanced in 8

powdered counterpart with dominant (001) facets. These results evidence the vertical alignment of (001) facets.

AcAc is found to be critically important to the final structure of array. It is a well-

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known chelating agent and able to complex with TIP and thus hinder the attack of water molecules to Ti. Based on the stabilized TIP, preferential growth along <001> direction

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guided by the adsorption of fluorine ion can be induced due to slow kinetics of

hydrolysis and condensation, thereby facilitating the formation of anatase single crystals with dominant (001) facets with a higher energy (~0.90 J/m2) vertically

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oriented on FTO substrate. On the contrary, in the absence of AcAc, (001) facets are

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prone to elimination, and energetical growth along <001> direction is triggered,

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resulting in a bipyramidal morphology with dominant low-energy (101) facets (~0.44

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J/m2).

A representative square nanoflake of TiO2 recorded at a low magnification is

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shown in Figure 4a. The corresponding selected area electron diffraction (SAED)

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pattern along <001> zone axis shown in Figure 4b can be indexed as the 200 and 020 Bragg reflections from the rectangular facets [43]. High-resolution TEM images in Figures 4c and d show the presence of crystallized (001) facets of WO3/TiO2. The

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coexistence of (001) atomic planes with a lattice spacing of 4.7 Å and tiny WO3 clusters in a uniform size of less than 10nm decorated on TiO2 surfaces can be clearly observed.

The UV-Vis absorbance spectra in Figure 5 shows remarkably visible light absorption effect of the tested TiO2 samples (BA, TA and WT) comparing to the bare 9

FTO substrate. The transmittance of visible light gives a red shift between BA and TA, which implies a band gap shrinking accompanied with the modification of TiO2 crystal morphology. By adding WO3 on TiO2 arrays, the adsorption wavelength for WT was further extended, although that the effect is not significant due to the small amount of

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WO3. The enhanced light adsorption behavior of WT could be attributed to both crystal

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morphology change and WO3 doping.

The current density-voltage plots, electrochemical impedance spectroscopy and photocurrent potential curves have been employed to investigate the photoelectric Current density-voltage

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properties of prepared samples as presented in Figure 6.

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curves were measured with an applied bias voltage in the range of -1.0 to 1.0V. The J-

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V plots in Figure 6 (a) suggest the TA and WT arrays prepared with AcAc chelating agent give higher photocurrents than BA. The elevated current density indicated

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elevated electron-hole separation in the WO3 decorated TiO2 films (WT). In Figure 6

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(b), electrochemical impedance spectroscopy was applied to illustrate the inter-facial charge transfer properties of the prepared samples. The photocurrents measurements

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have been performed at a voltage bias of 1.0 V by scanning within the frequency range of 1-105. The charge transfer resistance of a photocatalyst can be evaluated through the

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semicircular diameter of Nyquist curve. A larger semicircular diameter indicates a stronger charge transfer resistance. When semi-infinite diffusion was the ratedominating step, the Nyquist curve gives a linear relation with a slope of 45 o. While charge transfer reaction occurred, the Nyquist curve is semicircular. [44, 45]. According to Figure 6 (b), the TA and WT electrodes present much smaller semicircles than the 10

BA electrode, which indicated an effective separation of photogenerated electron-hole pairs and fast electron transfer occurred. Photocurrent potential curves of decorated TiO2 films have been recorded under voltage bias of 1.0 V with 1.0 mol/L Na2SO4 solution as the electrolyte. As shown in

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Figure 6 (c), when exposing the samples (BA, TA and WT) to simulated sunlight (λ

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=300-800 nm, 100 mW/cm2), positive photocurrents were produced instantaneously,

which suggest good photoelectric responses for all the tested samples. The positive values of photocurrents demonstrated n-type semiconductors, i.e. photo induced

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electrons transfer from FTO substrate to TiO2 films. However, due to the recombination

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of photo-generated electron-hole pairs, the photocurrent observed for sample BA is

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very small (about 0.1mA/cm2). In contrast, the WT electrode can produce a photocurrent density 5 times larger (0.5 mA/cm2). This remarkably rise of photocurrent

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density suggests an efficient electron generation and charge-separation effect of combining WO3 and TiO2 photocatalysts. Without decoration of WO3 clusters, the TA

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electrode produced a photocurrent of 0.4 mA/cm2 which is slightly lower than WT, but

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still much higher than using BA electrode. This result implies that the orientation of TiO2 arrays also has a significant influence on the photocurrent generation, i.e. exposing

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the lower coordinated (101) facet of anatase TiO2 to visible light could significantly accelerate electron generation. The PEC activities of WO3 decorated TiO2 arrays and undoped TiO2 films were evaluated using MB as a probe molecule. The prepared photoelectrodes were immersed in MB solution (0.20 mg/L) for 1h before subjected to UV irradiation. As presented in 11

Figure 7(a), decreases of MB concentration with time were clearly observed when using different photocatalysts of BA, TA and WT. WT gives the most prominent influence on MB degradation. Half amount of MB molecules has been eliminated in 60 min through photoelectron oxidation. When using BA photocatalyst, the concentration

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of MB only reduced 30% in 60 min. Langmuir–Hinshelwood model has been applied here to describe the photodegradation reaction with the first order kinetics. The apparent

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rate constant (kapp) was employed as the basic kinetic parameter to evaluate the

performance of different photocatalysts, i.e. ln[C0/C] = kappt [44]. C0 denotes the initial

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concentration of MB and C is concentration of MB remaining in the solution at

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irradiation time (t). Figure 7(b) plots the variation in ln[C0/C] as a function of reaction

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time. The calculated apparent rate constant (kapp) values for BA, TA and TW are

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0.00655，0.00916，and 0.00999 min-1, respectively. TA photoanode shows a superior

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photocatalytic activity, WT exhibits an even higher photocatalytic activity, suggested

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by a kapp constant rising of over 50%, comparing to BA. A mechanism can be proposed to explain the improved photocatalytic performance

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of TA and WT. With AcAc, the resulting TA presents a highly ordered orientation with exposed (100) and (101) facets. The vertically aligned array is assumed to possess a

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larger and more accessible surface area to the aqueous medium for photoelectrocatalytic degradation of MB. With the additional decoration with WO3 clusters that have an energy level below the conduction band of TiO2 (~0.5 eV lower as presented in Figure 8), WO3 can serves as an electron sink to trap the photogenerated electrons from TiO2. In opposite, the photogenerated holes would be accumulated within the valence band 12

of TiO2, which are able to to trap OH- and form radical hydroxyl species (OH), a powerful oxidizing agent to degrade MB[46]. As a result, the photocatalytic behavior would be further accelerated for WT.

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3. Conclusions

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Single crystalline TiO2 arrays have been successfully synthesized through a mild

hydrothermal process. The chelating agent, AcAc is found to facilitate the reproducible growth of TiO2 single crystals with dominant (001) facets, which in turn improves their

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interconnection. The resulting mechanically stable TiO2 arrays with exposed (101)

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facets on surface show an enhanced visible-light absorption. We further show that the

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decoration of WO3 clusters can deliver significant elevated photocurrent performances. Especially, much smaller electrochemical impedance and five times larger photocurrent

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comparing to the pure TiO2. WO3 incorporated into TiO2 could promote electron

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generation and charge separation during the photochemical reactions. Furthermore, a promising PEC activity of the prepared TA-WO3 photocatalyst is evidenced by the

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efficient degradation of MB molecules.

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Acknowledgements This work is supported by the National Nature Science Foundation of China (No. 51772094), Fundamental Research Funds for the Central University (No. JB2016004).

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[38] J. Zhu, S. Wang, Z. Bian, S. Xie, C. Cai, J. Wang, H. Yang, H. Li, Solvothermally controllable synthesis of anatase TiO2 nanocrystals with dominant {001} facets and enhanced photocatalytic activity, CrystEngComm, 12 (2010) 2219-2224. [39] Y. Wang, H. Zhang, P. Liu, T. Sun, Y. Li, H. Yang, X. Yao, H. Zhao, Nature of visible-light responsive fluorinated titanium dioxides, Journal of Materials Chemistry A, 1 (2013) 12948-12953. [40] Z. Xiong, H. Dou, J. Pan, J. Ma, C. Xu, X.S. Zhao, Synthesis of mesoporous anatase TiO 2 with a combined template method and photocatalysis, CrystEngComm, 12 (2010) 3455-3457. [41] J.H. Pan, X.S. Zhao, W.I. Lee, Block copolymer-templated synthesis of highly organized mesoporous TiO2-based films and their photoelectrochemical applications, Chemical Engineering Journal, 170 (2011)

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363-380.

[42] H.G. Yang, Anatase TiO2, single crystals with a large percentage of reactive facets, Nature, 453 (2008) 638.

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[43] M. Dozzi, E. Selli, Specific Facets-Dominated Anatase TiO2: Fluorine-Mediated Synthesis and Photoactivity, 2013.

[44] S.A.K. Leghari, S. Sajjad, F. Chen, J. Zhang, WO3/TiO2 composite with morphology change via hydrothermal template-free route as an efficient visible light photocatalyst, Chemical Engineering Journal, 166 (2011) 906-915.

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[45] J. Lin, R. Zong, M. Zhou, Y. Zhu, Photoelectric catalytic degradation of methylene blue by C60modified TiO2 nanotube array, Applied Catalysis B: Environmental, 89 (2009) 425-431.

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[46] A. Mills, J. Wang, Photobleaching of methylene blue sensitised by TiO 2: an ambiguous system,

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Journal of Photochemistry and Photobiology A: Chemistry, 127 (1999) 123-134.

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Coating of (NH4)2WO4

TiO2 Array

TTIP+AcAc +HCl+NH4F

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Figures 1. Schematic of WO3 decorated TiO2 (WT) by a two-step chemical bath

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deposition (CBD) method.

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(a)

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(c) 2 mm

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Figure 2. Morphologies of AcAc modified (TA) nanoflake TiO2 arrays and bipyramid (BA) without AcAc. SEM images from top view of TA samples after hydrothermal treatment at 180°C for (a)1h, (b) 2h, and (c, d) 4.5h; (e) cross-sectional view of TA after hydrothermal treatment at 180°C for 4.5h; (f) top view of BA.

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(103) (004) (112)

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ICDD #21-1272

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Figure 3. XRD pattern of WO3 decorated (WT) and bipyramid (BA) TiO2 arrays calcinated at 450°C for 1h.

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Figure 4. TEM micrograph of TA nanoflake (a) and its corresponding SAED pattern (b); high-resolution TEM images (c, d) of WT nanoflakes decorated with welldispersed WO3 clusters.

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Transmittance (%)

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Figure 5. UV-VIS spectra of bipyramid TiO2 (BA), AcAc modified TiO2 (TA), WO3 decorated TiO2 (WT) and bare FTO glass. 20

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Figure 6. Linear sweep voltammetry (LSV) curves (a), photocurrent-potential curves (b), and electrochemical impedance spectroscopy (EIS) Nyquist plots (c) of BA, TA and WT.

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Figure 7. Time-course photoelectrocatalytic degradation of methylene blue (a) and the corresponding apparent rate constant (b) under sunlight irradiation over BA, TA and WT photoanodes.

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Figure 8. Band edge positions of TiO2 and WO3 and their charge carrier separation when forming heterojunction and upon photoexcitation.

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