Enhanced photoelectrochemical and photocatalytic activity of WO3-surface modified TiO2 thin film
© Qamar et al.; licensee Springer. 2015
Received: 16 November 2014
Accepted: 10 January 2015
Published: 6 February 2015
Development of nanostructured photocatalysts for harnessing solar energy in energy-efficient and environmentally benign way remains an important area of research. Pure and WO3-surface modified thin films of TiO2 were prepared by magnetron sputtering on indium tin oxide glass, and photoelectrochemical and photocatalytic activities of these films were studied. TiO2 particles were <50 nm, while deposited WO3 particles were <20 nm in size. An enhancement in the photocurrent was observed when the TiO2 surface was modified WO3 nanoparticles. Effect of potential, WO3 amount, and radiations of different wavelengths on the photoelectrochemical activity of TiO2 electrodes was investigated. Photocatalytic activity of TiO2 and WO3-modified TiO2 for the decolorization of methyl orange was tested.
KeywordsPhotocatalysis WO3/TiO2 thin film Photoelectrochemistry Sputtering
Semiconductor-mediated photocatalytic process (advanced oxidation process (AOT)) has emerged as one of the most promising chemical oxidation processes, anticipated to play a crucial role in water treatment as standalone processes or in combination with conventional technologies [1-4]. It has now been well established that metal oxide-mediated photocatalysis is an attractive and promising technology to be applied in environmental clean up, clean energy production (H2 production from water splitting), self-cleaning surface, CO2 reduction under solar light or illuminated light source, and green synthetic organic chemistry (some selective photocatalytic oxidation reactions) [5-12]. The fundamentals of heterogeneous photocatalytic oxidation processes have been well documented in the literature [13-15]. Briefly, by shining light of energy equal to or greater than the band gap of semiconductor, an electron may be promoted from the valence band to the conduction band (e−cb) leaving behind an electron vacancy or ‘hole’ in the valence band (h+vb). If charge separation is maintained, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with absorbed species. Specially, h+vb may react with surface-bound H2O or OH− to produce hydroxyl radical (OH•), and e−cb is picked up by oxygen to generate superoxide radical anion (O2 −•). These reactive species are primarily responsible for the photodegradation of organic pollutants. TiO2 is currently the best known and most widely used photocatalytic material because it is photostable, nontoxic, and relatively inexpensive [3,4]. One practical problem associated with semiconductors is the undesired electron/hole recombination process which in the absence of a proper electron acceptor or donor is extremely efficient and hence represents the major energy-wasting step, thus limiting the quantum yield (more than 90% charge carriers recombine). Several scientific strategies, such as doping with transition metal ions , deposition of noble metals , dye sensitization , and coupling with other low band gap semiconductors [19-21], have been put forward to prevent the electron-hole pair recombination in the semiconductor and improve the photocatalytic activity. Among the oxide semiconductors, coupling TiO2 with WO3 has been the subject of intensive investigations owing to a small band gap between 2.4 and 2.8 eV, a deeper valence band (+3.1 eV), effective absorption of the solar spectrum, unique physicochemical properties, and resilience to photocorrosion [22-25].
Semiconductor-based thin films are the subject of great interest not only for their excellent properties such as high chemical inertness, high thermal stability, and corrosion resistance but also for their excellent mechanical, optical, electrical, electronic, and catalytic properties. To the best of our knowledge, no attempt has been made to study the photoelectrochemical and photochemical property of WO3/TiO2 bilayers prepared by sputtering method. In the study presented here, we demonstrated the preparation of pure and WO3-surface modified TiO2 thin films using plasma-assisted sputtering method and studied their photoelectrochemical and photocatalytic properties.
Titanium (99.999%) and tungsten (99.99%) targets were obtained from Semiconductor Wafer, Inc. (Hsinchu, Taiwan), while indium tin oxide (ITO)-coated glass slide, sodium sulfate (>99.0%), and methyl orange (dye content ca 85%) were obtained from Sigma-Aldrich (St. Louis, MO, USA).
Preparation of thin films
Thin films were fabricated by automatic sputter coater (NSC-4000) onto ITO substrates using high-purity titanium and tungsten targets. Before sputtering, the substrates were cleaned for 15 min in methanol by ultrasonication. Furthermore, surfaces of titanium and tungsten targets were cleaned before each experiment by a pre-sputtering process for 1 min. The base pressure in the chamber was less than 2 × 10−6 torr, and the working pressure was set to 7 mtorr by adjusting the O2 gas flow at 70 sccm. The distance between the target and the substrate was fixed at 10 cm. The depositions of thin films were done at two different steps and conditions: TiO2 thin film was deposited first using 120 W for 40 min time deposition using radio frequency (rf) in pure oxygen. In the second step, WO3 flashed using a DC reactive sputtering with 70 W for 1, 2.5, 5, and 10 min on TiO2 thin film in high-purity oxygen environment. The substrate temperature was 300 K, while the sputtering rates of titanium and tungsten were 0.7 and 1.5 Å/s, respectively.
Morphological characterization of the films was carried out by employing field emission scanning electron microscope (FESEM) while the elemental analysis was performed by energy-dispersive X-ray spectroscopy (EDS). Optical property was studied using a UV-vis spectrophotometer.
Evaluation of photoelectrochemical and photocatalytic activity
Photoelectrochemical behavior was studied using a three-electrode photoelectrochemical cell and a potentiostat. Saturated calomel electrode (SCE) and coiled platinum electrodes were used as a reference and counter electrodes, respectively. Irradiation of films was carried out under light with different wavelengths generated from 300-W xenon lamp. In all cases, the coated side (consisting of TiO2 or WO3/TiO2) of the films was irradiated. Different modules for UV and UV-vis along with cut off filters were used to get the radiations of desired wavelengths.
The photocatalytic tests were performed in a small photocell equipped with a quartz window and a magnetic stirring bar. For irradiation experiments, 100 mL of methyl orange solution with desired concentration was taken into the photocell and thin film slide was immersed into the dye solution. Irradiation was carried out using the abovementioned xenon light source. Samples (approximately 5 mL) were taken at regular time intervals from the cell, and concentration was determined by UV-visible spectrophotometer. Decomposition (decrease in absorption intensity vs. irradiation time) of the dye was monitored by measuring the change in absorbance.
Results and discussion
Figure 1D shows the UV-visible absorption spectra of pure TiO2 as well as TiO2 films modified with WO3. The spectra corresponded to a typical absorption behavior of TiO2, as obtained by other researchers as well. The absorption spectra of WO3-modified titania films were almost similar to that of bare titania, and surface modification by a low band gap semiconductor (WO3, approximately 2.8 eV) could not contribute in any notable visible band gap narrowing. The band gaps of bare TiO2 film and WO3/TiO2 film, prepared after 5 min, were calculated to be 3.3 and 3.2 eV, respectively.
The surface of TiO2 could be effectively functionalized by depositing nanoparticulates of WO3 by magnetron sputtering method. Microscopic analyses showed that TiO2 nanoparticles were nonspherical, irregular in shape, and <50 nm in size while WO3 nanoparticles were well distributed throughout TiO2 surface and were smaller (<20 nm) in size as compared to TiO2. Improvement in photocurrent and photocatalytic activity of TiO2 thin film was observed, approximately 50% and approximately 40%, respectively, after surface modification with WO3 nanoparticles. Both the films, TiO2 and WO3/TiO2, showed the highest photocurrent under 320-nm radiation, which continuously decreased with decreasing photonic energy. This study has presented a representative example by investigating the photoelectrochemical and the photochemical behavior of WO3-modified TiO2 thin film; this simple and effective methodology could be applied to develop other mixed-metal oxides for solar energy conversion and environmental decontamination.
The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science and Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project no. 10-NAN1387-04 as part of the National Science, Technology and Innovation Plan. The support of the Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals is gratefully acknowledged.
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