Abstract
Tungsten trioxide nanoparticles anchored on titania nanotube arrays (WTiNT) fabricated by a chemical bath deposition technique in combination with a pyrolysis process. The resulting films were characterised by SEM, EDX and XRD. Results showed that the nanomaterials film have the same ordered structure as titania nanotubes. Diffuse reflectance spectra showed an increase in the visible absorption relative to bare titania nanotubes. The tungsten trioxide was successfully anchored on the titania nanotubes, which enhanced the photocatalytic activity of the WTiNT, as indicated by the efficient removal of Rhodamine B and the production of hydrogen. The resultant hybrid nanotubes array showed better photoelectrochemical activity than the bare titania nanotubes (TiNT) under Xe illumination. These WTiNT nanostructured films are promising candidates for practical photochemical reactors that drive useful reactions, such as water reduction (H2 production) and oxidation of pollutants.
Introduction
Solar energy is potentially clean, safe, and limitless, but its use at the global scale would benefit from efficient conversion of the energy into a fuel to allow storage and distribution. For solar photoconversion, specialised semiconductors are needed to absorb sunlight and use this energy to drive photochemical reactions. Metal oxides, in particular titania, have properties such as electronic band structure, high photoactivity, chemical stability, and low cost that make them good candidates for photoanodic reactions in aqueous electrolytes. 1 However, the application of titania is limited by its high band gap energy (3·2 eV), which diminishes its absorption in the visible light range of solar spectrum. One of the most studied approaches to increase the visible absorption of metal oxides is doping with anions or transition metals. Owing to its electronic bandgap (Eg = 3·2 eV, corresponding to a light wavelength λ = 380 nm), titania absorbs less than 5% of the natural solar spectrum. 1 Because W6+ has an ionic radius similar to that of Ti4+, WO3 can couple into TiO2 crystals in their co-crystallisation process during annealing, resulting in a well doped WO3/TiO2 composite. 2 Incorporating WO3 with TiO2 not only efficiently inhibits the recombination between the photogenerated holes and electrons but also reduces the band gap of TiO2.3–9 In this study, we construct tungsten trioxide nanoparticles anchored on titania nanotube arrays by depositing a tungsten trioxide nanoparticles layer on TiO2 nanotube surfaces. Normally, the approaches for producing large surface area electrodes include sol–gel deposition, sintering nanoparticles, chemical vapour deposition and electrodeposition. Among various methods for the synthesis of nanostructures, the fabrication of self-organised oxide nanotube (NT) arrays by a simple but optimized electrochemical anodisation of a metal substrate represents a most elegant and economic approach. Particularly, the formation of self-organised TiO2 nanotubular structures has received extensive attention because of the numerous functional properties of TiO2.10–16 The decoration of tungsten trioxide nanoparticles occurs homogenously and a high quality tungsten trioxide–titania films can be achieved by using chemical bath deposition (CBD) technique. This method makes it more favourable for the deposition in the tubes than at the entrance, and provides a general way for any mesoporous or microporous substrate where this type of selective deposition is desired. To the best of our knowledge, report on the preparation of tungsten trioxide–titania nanotubular composite films by anodisation and following chemical bath deposition are lacking. Figure 1 shows the two-step process developed to prepare WTiNT composite materials. In the first step, titania nanotube was prepared by anodisation of titanium foil. Then tungsten trioxide nanoparticle films deposited on the titania nanotubes by a chemical bath deposition technique. The morphology and structure were characterised by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Optical properties were investigated by UV-visible diffuse reflectance spectra (UV/vis/DRS). In addition, photocatalytic activity of this WTiNT was evaluated through the degradation of Rhodamine B (RhB) and hydrogen generation.
Schematic diagram of two-step synthetic approach to prepare WTiNT samples
Experimental
Chemicals and materials
All chemical materials were of analytical grade without further purifying before experiment and solutions were prepared with distilled water.
Fabrication of TiO2 nanotubes
A piece of titanium sheet (99·99% purity, 1 mm thick) was cut into desired dimension and the titanium electrode was first mechanically polished with different emery type abrasive papers (with the following grades: 80, 240, 800, 1200 and 2400), rinsed in a bath of distilled water, and then chemically etched by immersing in a mixture of HF and HNO3 acids for 30 s. The ratio of components HF/HNO3/H2O in the mixture was 1∶4∶5 in volume. The last step of pretreatment was rinsing with distilled water. After cleaning, anodic films were grown from titanium by anodizing of titanium foil in a DMSO electrolyte containing 2 vol-% HF at a constant voltage of 40 V for 8 h at room temperature using a platinum sheet as counter electrode.
Tungsten trioxide nanoparticles attachment
Tungsten trioxide nanoparticles were synthesized on the nanotubes by chemical bath deposition. The titanium nanotube arrays (TiNT) were soaked in a 0·1M H2SO4 solution for 90 min followed by soaking in a H2O/ethanol (4∶1) solution containing 0·05M Na2WO4 for 4 h at 70°C. The samples were rinsed with distilled water and dried in air. Then, samples were annealed by heating at 400°C for 2 h, with a ramp of 1°C min−1 to form tungsten oxide and also to obtain crystalline WTiNT samples.
Characterisation
The surface morphologies of WTiNT samples were characterised by field emission scanning electron microscopy (FE-SEM, Hitachi S-4160, Japan) and the elemental composition was estimated by energy dispersive X-ray spectroscopy (EDX). The crystalline phases were identified by XRD (Philips X'Pert). Diffraction patterns were recorded in the 2θ range from 20 to 80° at room temperature. UV-visible absorption spectra of the samples were recorded on a photospectrometer (JASCO V-570).
Photo-catalytic and photoelectrocatalytic activity tests
Photocurrent was carried out in a standard three-electrode cell containing 100 mL 1M NaOH solution (pH = 13·6) by Compactstat IviumStat (Model 2·175). The WTiNT and TiNT samples, platinum plate and an Ag/AgCl reference electrode served as the working, counter and reference electrodes respectively.
Photo-catalytic activity of samples was evaluated by degradation of the aqueous Rhodamine B (RhB) under visible light irradiation. The photo-catalytic reaction was carried in a single compartment cylindrical quartz reactor. A 200 W Xe lamp was used as a light source. The intensity of the Xenon lamp was 100 mW cm−2. A fan was used to cool down the reactor tube. Prior to illumination, the photo-catalyst sample was immersed in quartz reactor containing Rhodamine B and magnetically stirred for 2 h in the dark to ensure the establishment of an adsorption–desorption equilibrium between the photo-catalyst and Rhodamine B. Then the solution was exposed to visible light irradiation under magnetic stirring for 2 h. At each 10 min intervals, 5 mL solution was sampled and the absorbance of Rhodamine B at 550 nm was measured by a UV-vis spectrophotometer.
The photo-catalytic hydrogen production was evaluated in 50 mL aqueous solution of 1M NaOH. Hydrogen evolution was measured for 300 min and H2 gas was collected using the water displacement technique. H2 gas is produced at the counter electrode in the photo electrochemical (PEC) cell. A Pt coil spot welded to a stainless steel rod served as the cathode. The cathode was inserted into a burette where the hydrogen was collected via electrolyte displacement. The volume of hydrogen was measured by directly reading the variation of the electrolyte level in the burette for various times.
Results and discussion
The FE-SEM of the as prepared TiNT and WTiNT samples were illustrated in Fig. 2. TiNT samples (Fig. 2a and b) displayed vertically ordered nanotube arrays that the surface of them was open. WTiNT samples (Fig. 2c–e) displayed nanotube arrays with the internal diameter of the tubes is around 50–90 nm, external diameter is around 120–170 nm and wall thickness is estimated to be in the range of 40–70 nm. It can be seen that the tungsten trioxide nanoparticles are distributed on the surface of the titania nanotube arrays (Fig. 2e). Side-view SEM image of as synthesised WTiNT samples was illustrated in Fig. 2e.
XRD measurements were conducted to determine the crystal phase of the TiNT and WTiNT samples. Figure 3 shows the XRD patterns of the pure TiO2 (TiNT) and WTiNT film annealed at 400°C. It confirms the presence of anatase phase of TiO2 in the samples, and the Ti peaks were due to the titanium substrate. The WTiNT presented peaks attributed to the metal substrate of Ti, TiO2 as anatase and the formation of WO3 in the monoclinic form. The analysis of the WTiNT surface by energy dispersive X-ray spectroscopy (EDX) taking measurements on top is shown in Fig. 4. The analysis revealed that the surface present similar composition with presence of Ti as a main energy (E) = 4·5 kV, W at E = 1·7 kV and O at E = 0·5 kV confirming that WTiNT composite film was formed. The occurrence of traces of contaminants such as carbon and fluorine from precursors are also observed. The presence of C species is due to the absorption of carbon from DMSO.
a, b SEM top-view images of TiNT samples with different magnification; c–e SEM top-view images of WTiNT samples with different magnifications; f SEM side-view of WTiNT samples
XRD patterns of photocatalysts: a as prepared WTiNT; b TiNT annealed at 400°C; c as prepared WTiNT; d WTiNT annealed at 400°C
EDX spectrum of WTiNT sample
The optical properties such as reflectance spectra and optical band gap energy of TiNT and WTiNT samples were studied. Figure 5 show the optical band gap energy of TiNT film annealed at 400°C. Optical band gap energy of the WTiNT sample is shown in inset of Fig. 5. According to Tauc–Sounds equation
Tauc plot of TiNT: inset: Tauc plot of WTiNT sample
To investigate the photocurrent response of electrodes, the transient photocurrent responses of the TiNT and WTiNT samples are measured by several on-off cycles of intermittent irradiation. Figure 6 shows a comparison of I–t curves of two samples. It can be seen that the photocurrent value rapidly decreases to zero as soon as the irradiation of light turns off, and the photocurrent comes back to a constant value when the light is again on, which has a good reproducibility. This indicates that under light irradiation, most of the photogenerated electrons are transported to the walls of nanotubes, and then transferred to titanium substrate to produce photocurrent. The photocurrent value for WTiNT is ∼0·8 mA, which is higher than that of TiNT (0·65 mA), as seen in Fig. 6. The higher photocurrent indicates that more photo induced electrons can transfer efficiently from WTiNT to the counter electrode via an external circuit, thereby obtaining high photoelectrochemical activity. 17
Photocurrent density of TiNT and WTiNT photoelectrodes
Figure 7 shows the photocatalytic degradation of RhB on TiNT and WTiNT samples under Xe. The WTiNT sample exhibited a higher photocatalytic activity than the TiNT. Meanwhile, the stability of the electrode has an important role in its practical application in the degradation of organic pollutants and hydrogen production. As shown in Fig. 7 (inset), after a five-cycle experiment, TiNT and WTiNT catalysts exhibited similar catalytic performance without significant deactivation, revealing thier high stability after multiple reuses.
Photo catalytic degradation of Rhodamine B (RhB) over TiNT and WTiNT samples as (Ct/C0) versus irradiation time plot: (inset) photo catalyst stability test of TiNT and WTiNT catalysts
Figure 8 shows the amount of hydrogen generated as a function of time using TiNT and WTiNT samples under Xe light illumination. Control experiments indicated that no appreciable hydrogen production was detected in the absence of either visible light irradiation or photo catalyst, suggesting that hydrogen was produced by photo catalytic reactions on photo catalyst. By measuring the hydrogen evolved, the total amount of H2 evolved on the sample WTiNT was 17·8 mL cm−2 after 300 min, which is approximately 2·23 times higher than that on the TiNT (8·0 mL cm−2). The recyclability of WTiNT photo catalyst was tested during 8 runs of photo-catalytic reaction under visible light irradiation, as shown in Fig. 9. The amount of hydrogen evolution has no obvious decay and maintains an average value of 17 mL cm−2 after 8 continuous running (300 min per running), which indicates that these samples are a relatively stable photo catalyst and can keep the activity for a period of time.
Photo catalytic H2 production of TiNT and WTiNT samples over irradiation time
H2 evolution for WTiNT sample as function of running times
Conclusion
In summary, we demonstrate an effective approach to decorate titania nanotubes with tungsten trioxide nanoparticles uniformly (WTiNT) and employed it for water splitting and wastewater treatment. Diffuse reflectance spectra show an improvement in the visible absorption relative to bare TiO2 nanotubes. These materials were employed as photoanodes for the degradation of organic pollutants and the generation of photocurrent for hydrogen production. We showed that resultant hybrid nanotubes array (WTiNT samples) improved the photodegradation and photocurrent efficiencies, as compared to bare TiO2 nanotubes (TiNT) under Xe illumination. The results of this work suggest that the optimisation of the nanostructure and composition of these composite materials improve the photogenerated carrier transport and the absorption pathways, resulting in higher photo catalytic activity.