Investigation on photocatalytic potential of Au–Ta₂O₅ semiconductor nanoparticle by degrading Methyl Orange in aqueous solution by illuminating with visible light

Abstract

A semiconductor photocatalytic process has shown great potential as a low cost, environmentally friendly and sustainable treatment technology for the treatment of wastewater. Hence, a wide band gap Ta₂O₅ semiconductor nanoparticle was prepared by the hydrothermal method and considerable efforts have been taken to narrow the band gap, i.e., the surface modification has been done with a noble metal (Au⁰) by a deposition precipitation method. As synthesized, Au–Ta₂O₅ semiconductor was well characterized by X-ray diffraction (XRD), X-ray photo electron spectroscopy (XPS), transmission electron microscopy (TEM) and diffused reflectance UV-vis spectroscopy (DRS). The photocatalytic potential of Au–Ta₂O₅ semiconductor was investigated by degrading Methyl Orange in the presence and absence of electron acceptors by illuminating with visible light of intensity 80 600 ± 10 Lux.

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Introduction

Advanced oxidation processes (AOPs) are the most effective technology for treatment of wastewaters containing organic compounds, and among these AOPs photocatalysis in particular is most promising.^1–3 In this perspective, a semiconductor photocatalyst for purification of water should be chemically and biologically inert, photocatalytically active, easy to produce and use and activated by sunlight. In the semiconductor photocatalyst, TiO₂ became one of the most widely used materials for use in solar cells, pollutant degradation, photolysis of water, gas sensor and bio applications due to its unique and favorable physiochemical properties.^4–7 However, recently numerous efforts have been put forward for the fabrication and application of efficient semiconductor manufacture, not only TiO₂ but also other oxide semiconductors.

Among the oxide semiconductors, tantalum oxide (Ta₂O₅) has many outstanding properties such as a wide band gap (∼3.9 eV), photoactivity in near-UV areas, good thermal and chemical stability, good conductivity, good catalyst for a variety of chemical reactions, etc.^8–11 Therefore it is being widely studied and used for a wide variety of applications, e.g. as anti-reflection coating for solar cells, wave guides for surface acoustic devices and capacitor material for dynamic random access memories (DRAM).^11–18 Hence, we planned to study visible light assisted photocatalytic degradation of organic pollutant (Methyl Orange, MeOr) in aqueous solution using tantalum oxide (Ta₂O₅) semiconductor. Ta₂O₅ semiconductor cannot efficiently utilize the visible light of the solar energy because of its comparably large band gap. To overcome the above problem, considerable efforts have been taken to narrow the band gap, i.e., surface modification with a noble metal (Au⁰) have been explored in an effort to increase visible light absorption or suppress recombination of photogenerated carriers owing to the unique plasmon absorbance features.^19–26 To the best of our knowledge, investigations on the preparation and characterization of gold embedded tantalum oxide (Au–Ta₂O₅) for photocatalytic degradation of Methyl Orange have never been reported. The activity of Au–Ta₂O₅ nanoparticle is enhanced further by adding external electron acceptors such as PMS (peroxomonosulphate), PDS (peroxodisulphate) and H₂O₂ (hydrogen peroxide) which also suppresses the recombination of photogenerated carriers.

Experimental details

Materials

Tantalum(V) chloride, Methyl Orange (MeOr; dye content 85%) and chloroauric acid trihydrate (HAuCl₄·3H₂O) were purchased from Sigma-Aldrich and used as such. Potassium peroxomonosulphate, a triple salt with composition 2KHSO₅·KHSO₄·K₂SO₄ (commercially known as Oxone) from Janssen Chimica (Belgium) was used as received. Peroxodisulphate (PDS) and hydrogen peroxide (H₂O₂) was an analytical grade reagent purchased from E−Merck. Unless otherwise specified, all reagents used were of analytical grade and all solutions were prepared by using Millipore water.

Preparation of Ta₂O₅ semiconductor nanoparticle

Ta₂O₅ was prepared by a hydrothermal method as follows. First, 100 ml of NaOH (0.01 M) was added rapidly into 300 mL of TaCl₅ (0.05 M) containing diethanolamine (0.1 mL; which acts as a stabilizer). After mixing, the solution was allowed to stir for 1 h at room temperature and then was transferred into a Teflon autoclave for crystallization by heating up to 80 °C for 48 h. After cooling to room temperature, the powder was washed several times with distilled water (until the filtrate does not show any precipitate with AgNO₃) and ethanol (thrice) to remove impurities, then dried in vacuum oven at room temperature. The dried powder was calcinated at 700 °C for 3 h in a temperature programmed muffle furnace, which yields Ta₂O₅ nanoparticles.

Preparation of Au–Ta₂O₅ semiconductor nanoparticle

The preparation of Au–Ta₂O₅ nanocatalyst was carried out by deposition-precipitation processes^27,28 with NaOH as follows: 100 mL of aqueous solution of HAuCl₄·3H₂O (1.14 × 10⁻³ M) was taken and heated to 80 °C. The pH of the solution was adjusted to 7 by dropwise addition of NaOH (1 M), then 1 g of Ta₂O₅ (previously calcinated at 700 °C for 3 h) was dispersed into the solution and the pH of the resultant solution was readjusted to 7 by dropwise addition of NaOH (1 M). The suspension was then kept in a thermostat at 80 °C with vigorous stirring for 2 h and the solids were collected by centrifugation (12 000 rpm for 10 min) and washed with 100 ml of Millipore water under stirring for 10 min at 50 °C. The washing procedure was repeated several times and the samples were dried in vacuum at 100 °C for 2 h in order to get pure Au–Ta₂O₅ nanocatalyst. The final yield of gold loading was ∼8 atomic weight % on Ta₂O₅, based on the assumption that all Au ions were loaded on Ta₂O₅ surface during the doping procedure. Finally for characterization purposes, the prepared catalyst was calcined at 250 °C with a heating rate 10 °C min⁻¹ for 4 h.

Characterization techniques

Material phase analysis of the prepared nanostructures was done by powder X-ray diffraction, XRD (measured using Rigakudiffractometer, Cu-Kα radiation, Japan), X-ray photoelectron spectroscopy, XPS (measured using Physical Electronics PHI 5600 XPS spectrophotometer with monochromatic Al-Kα (1486.6 eV) excitation source, and the morphology and particle size of the samples were analyzed through high resolution transmission electron microscopy (HRTEM; recorded using JEOL JEM2010 model) respectively. The diffuse reflectance spectra of the samples were recorded in the wavelength range 230–800 nm using a UV–visible spectrophotometer (T90+, PG instruments, UK) equipped with an integrating sphere accessory. BaSO₄ was used as a reference. The mineralization of the dye was monitored by measuring the total organic carbon (TOC) content with a TOC-V_CPH analyzer (Shimadzu Company, Japan). Prior to analysis, the instrument was calibrated with potassium hydrogen phthalate for TC analysis, and sodium carbonate/bicarbonate of different concentrations was used as standards to get reproducible results for TIC. TOC₀ is the TOC measured after the equilibrium adsorption of the dye on the Ta₂O₅ and Au–Ta₂O₅ surface and TOC obtained at various irradiation times is denoted as TOC_t.

Evaluation of photocatalytic efficiency

The photocatalytic experiments were conducted under ambient atmospheric conditions and at natural pH (6.0) using 150 W tungsten halogen lamp (λ ≥ 400 nm; the intensity of incident radiation is 80 600 ± 10 Lux measured using Extec, USA) as the light source. In order to attain adsorption/desorption equilibrium between the dye molecules and the catalyst surface, the solution was stirred for about 45 min in the dark, prior to irradiation. The apparent kinetics of disappearance of the substrate, MeOr, was determined by following the concentration of the substrate (λ_max = 486 nm) using UV-VIS spectrophotometer (T90 + model purchased from PG Instruments, UK) after a certain period of irradiation of the photocatalyst suspension and then filtered with a 0.45 μm polyvinylidene fluoride (PVDF) filter. No dye degradation was observed when the dye solution was illuminated in the absence of catalysts and electron acceptors. The electron acceptors were added only after stirring for 45 min in the dark.

Results and discussion

Fig. 1 shows the XRD patterns of as-synthesised Ta₂O₅ and Au–Ta₂O₅ nanoparticles. In both Ta₂O₅ and Au–Ta₂O₅ nanoparticles, the orthorhombic β-phase of Ta₂O₅ diffraction patterns can be clearly seen,²⁹ which indicates that the crystal structure of Ta₂O₅ was not affected [ICSD-43498] but crystallinity was affected (strong intensity of the main peaks in Au–Ta₂O₅) due to the doping of gold nanoparticles on the Ta₂O₅. That is, 2θ value corresponding to 23.06 (001), 28.62 (100), 37.02 (101), 46.94 (002), 50.54 (110), 55.72 (102), 58.84 (200), 64.18 (201), 71.30 (112), 73.16 (003) and 78.38 (202) revealed the Ta₂O₅ phase to have an orthorhombic structure (JCPDS Card No. 25-0922).^30,31 The diffraction peaks of Au were not observed which reveals good dispersion of Au in the Ta₂O₅ crystal structure. Furthermore, the peak broadening was noticed in the XRD, indicating that there is grain refinement occurring due to gold doping. The calculation of the crystallite size from the line broadening of (001) diffraction peak using the Scherrer formula before and after gold doping shows that the crystallite size progressively decreased from 32 to 21 (±5) nm, which also supports the observed XRD peak broadening.

Fig. 1 XRD spectra of Ta₂O₅ and Au–Ta₂O₅ nanoparticle.

The chemical composition of as-prepared Ta₂O₅ and Au–Ta₂O₅ nanoparticles was obtained by XPS measurements. No peak of other elements except C, O, Ta and Au indicates the observed product was highly pure (Fig. 2A). In the XPS spectra of the as-prepared Ta₂O₅ and Au–Ta₂O₅ nanoparticles, Ta (4f_7/2) and Ta (4f_5/2) peaks lie at 23.5 and 24.9 eV (Fig. 2B), the C (1s) peak lies at 284 eV (Fig. 2C), the broad peak that lies at 528 eV is assigned to O (1s) (Fig. 2D), and finally Au (4f_7/2) peaks lies at 81.2 eV and Au (4f_5/2) peak lies at 85.2 eV (Fig. 2E). These energy values correspond to Ta⁵⁺ in Ta₂O₅ and Au⁽⁰⁾ in Au–Ta₂O₅ as reported in the literature.^17,32–34

Fig. 2 XPS spectra of Au–Ta₂O₅ nanoparticle. (A) Survey spectrum, (B) Ta (4f) line, (C) C(1s) line, (D) O (1s) line and (E) Au (4f) line.

TEM and HRTEM images of Ta₂O₅ and Au–Ta₂O₅ are presented in Fig. 3. As-prepared Ta₂O₅ exists as sphere like morphology with a uniform grain size in the range 25–30 nm. HRTEM analysis provides further insight into the nanostructure i.e., the observed lattice fringes may belong to the individual nanocrystal of the Ta₂O₅ sphere, which is consistent with the (200) plane as reported earlier by Agrawal et al.^35,36 In addition, the fast Fourier transform (FFT) image (Fig. 3E) of the selected area of HRTEM micrograph, shows a polycrystalline ring diffraction pattern, which may correspond to L-Ta₂O₅ crystalline phase.³¹ Upon deposition of Au on the Ta₂O₅ crystalline phase, the dimensional uniform (sphere morphology) of Ta₂O₅ is lost, while in addition the particle size increases to 100 nm and above (Fig. 3C). Also the observed HRTEM (Fig. 3D) and the fast Fourier transform (FFT) image (Fig. 3F) are different from the earlier, i.e., the selected area of the HRTEM micrograph shows both polycrystalline diffraction rings and single crystal diffraction dots which correspond to both Ta₂O₅ and Au nanoparticles. The composition of the prepared material (Au–Ta₂O₅) was determined by the EDX technique (Fig. 3G), which supports the presence of 8 atomic weight percentage of Au in Ta₂O₅. The observed TEM image looks big in size after gold doping, which may be due to Ta₂O₅ particles joined together to form nanosheet-like morphology without any change in the orthorhombic β-phase of Ta₂O₅.

Fig. 3 TEM and HRTEM image of Ta₂O₅ (A, B) and Au–Ta₂O₅ (C, D) nanoparticle. FFT image of Ta₂O₅ (E) and Au–Ta₂O₅ (F). EDX pattern (G) and elemental composition of Au–Ta₂O₅ nanoparticle are provided.

The UV-vis diffused absorbance spectrum of Ta₂O₅ and Au–Ta₂O₅ are shown in Fig. 4. Pure Ta₂O₅ nanoparticles show an absorption onset band in the UV region at approximately 340 nm (band gap calculated is ∼3.55 eV from Tauc plot, see inset of Fig. 4), which may be indexed to the presence of Ta species as Ta⁵⁺ due to the electronic excitation of the valence band O (2P) electron to the conduction band Ta (5d) level.^17,37,38 However in the case of prepared Au–Ta₂O₅ nanoparticles, a new absorption band (around 580 nm) is seen in the visible region and may be due to the surface plasmon resonance of Au⁰ nanoparticles impregnated on the surface of Ta₂O₅ nanoparticles. The band gap calculated for Au–Ta₂O₅ nanoparticles is ∼3.09 eV from the Tauc plot (see inset of Fig. 4).

Fig. 4 Solid state diffuse reflectance UV-vis spectra of Ta₂O₅ and Au–Ta₂O₅ nanoparticle. Inset shows the corresponding Tauc plots.

Under visible light irradiation, the photocatalytic degradation of Methyl Orange (MeOr) was performed using prepared Au–Ta₂O₅ nanoparticles. After confirming that there is no appreciable degradation of MeOr with Au–Ta₂O₅/visible light irradiation alone, we studied photocatalytic degradation of MeOr (5 × 10⁻⁵ M) in the presence of both Au–Ta₂O₅ nanoparticles (0.2 to 1.8 g L⁻¹) and visible light (80 600 ± 10 Lux) by monitoring the decrease in the absorption maxima of MeOr (λ_max 493 nm) as a function of irradiation time in 1 h using UV-vis absorption spectroscopy. First order rate constants were evaluated from the slopes of the -ln[C/C_o] vs. time plots (Fig. 5). The optimum concentration of Au–Ta₂O₅ nanocatalysts provides maximum photocatalytic efficiency at 1 g L⁻¹ (1.2 × 10⁻⁴ s⁻¹; see inset of Fig. 5) which is comparatively higher than the rate obtained for bare Ta₂O₅ nanocatalyst (0.3 × 10⁻⁴ s⁻¹) with the same concentration. Generally, the photocatalytic activity depends on the optimal amount of the catalyst loading and the total number of incident photons available in the reaction medium. This can be rationalized in terms of availability of active sites on Au–Ta₂O₅ surface and the light penetration into the suspension. The availability of the active sites increases with an increase in the catalyst loading up to 1 g L⁻¹ of Au–Ta₂O₅ nanoparticles. However, increasing the catalyst loading beyond 1 g L⁻¹, the light scattering effect induced by the Au–Ta₂O₅ catalyst also increased, which reduced the number of photons available for photocatalytic reactions and as a consequence, the rate of photocatalytic activity got reduced (inset of Fig. 5). Therefore, 1 g L⁻¹ of Au–Ta₂O₅ catalyst loading was considered as the optimum quantity for the photocatalytic degradation of Methyl Orange. Furthermore, the photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles by shifting the Fermi level to more negative potentials. The negative shift in the Fermi level is an indication of better electron and hole charge separation and more reductive power towards the degradation of pollutants. This may be due to the surface plasmon resonance of Au⁰ nanoparticles impregnated on the surface of Ta₂O₅ nanoparticles which absorbs a higher number of photons at that concentration. That is, gold present in the Au–Ta₂O₅ can enhance the photocatalytic activity by creating a local electric field (oxygen vacancy and crystal defects) and in addition the optical vibration of the gold surface plasmon can make a reasonable alternation in the Fermi level equilibration, which leads to easier electronic transition from the valence band O (2P) electron to the conduction band Ta (5d) level.³⁸

Fig. 5 Plot of variation of Au–Ta₂O₅ nanoparticle ((■) 0.2 g L⁻¹, (●) 0.6 g L⁻¹, (▲) 1.0 g L⁻¹, (▼) 1.4 g L⁻¹, (◆) 1.8 g L⁻¹) upon irradiation time in the presence of fixed initial concentration of MeOr (5 × 10⁻⁵ M). Inset shows a plot of the photodegradation rate of various concentrations of Au–Ta₂O₅ nanoparticles.

To enhance the photocatalytic activity of Au–Ta₂O₅ nanocatalysts further, experiments were carried out with external electron acceptors such as PMS, PDS and H₂O₂, and the observed rate constants (Table 1) are in the following order, i.e., PMS (10.09 × 10⁻⁴ s⁻¹) > PDS (3.89 × 10⁻⁴ s⁻¹) > H₂O₂ (3.21 × 10⁻⁴ s⁻¹) (experimental conditions are as follows: MeOr (5 × 10⁻⁵ M); Au–Ta₂O₅ (1 g L⁻¹) and PMS = PDS = H₂O₂ (0.25 mM) (Fig. 6). The use of these electron acceptors alone in the absence of Au–Ta₂O₅ nanocatalysts did not degrade the MeOr dye. Such an enhanced rate constant may be due to the (i) generation of surface active radicals by a reaction between electron acceptors and Au–Ta₂O₅ nanocatalysts^23,34,38,39 and (ii) immediate trapping of the photogenerated electrons by the electron acceptors.^23,39 Finally, the extent of photocatalyzed mineralization of MeOr was measured in the presence of Au–Ta₂O₅ and electron acceptors after 8 h illumination of visible light (Fig. 7). With Au–Ta₂O₅ and electron acceptor (PMS) about 57% of the TOC was removed after discoloration of MeOr. However with bare Au–Ta₂O₅ the mineralization of MeOr took place at a much lower rate and TOC was reduced to only 28%, while the rate of mineralization is 42 and 37% in the case of PDS and H₂O₂. Thus, the addition of electron acceptors to the Au–Ta₂O₅ nanocatalyst accelerates the mineralization process, which may be due to the generation of surface active radicals by a reaction between electron acceptors and Au–Ta₂O₅. Hence in the available literature,^23,34,38 it is considered that the embedded Au on Ta₂O₅ upon illumination by visible light can promote the transfer of electrons (e⁻) in the Ta₂O₅ conduction band to outer oxygen, which is dissolved in water and meanwhile it degrades the MeOr. However the electrons would be promptly transported in the case of added electron acceptors, which avoid recombination of e⁻/h⁺ pairs. Thus, these studies clearly indicate that excitation of surface plasmons may play a major role for the visible light assisted photocatalytic activity. Linic et al.⁴⁰ explained the role of surface plasmon mediated chemical transformations by predicting three different possible mechanisms, however many different factors affect the overall performance. Therefore, further research is ongoing in our lab to predict the perfect mechanism.

Table 1 Comparison of photocatalytic degradation rate of MeOr in the presence of Ta₂O₅, Au-Ta₂O₅ nanoparticle with and without electron transfer agents. Concentrations are maintained as follows: MeOr (5 × 10⁻⁵ M), PMS = PDS = H₂O₂ (0.25 mM), Ta₂O₅ = Au-Ta₂O₅ = 1 g L⁻¹

S. No.	Name of catalyst	Name of oxidant	Presence of light	Absence of light	Rate constant × 10^−4 (s^−1)
1	—	—	Yes	—	0.000012
2	Ta2O5	—	Yes	—	0.31
3	Ta2O5	PMS	Yes	—	0.71
4	Ta2O5	PDS	Yes	—	0.45
5	Ta2O5	H2O2	Yes	—	0.42
6	Au-Ta2O5	—	—	Yes	0.0000667
			Yes	—	1.2
7	Au-Ta2O5	PMS	—	Yes	0.000765
			Yes	—	10.09
8	Au-Ta2O5	PDS	—	Yes	0.0000987
			Yes	—	3.89
9	Au-Ta2O5	H2O2	—	Yes	0.000087
			Yes	—	3.21

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Fig. 6 Comparison of the photocatalytic degradation rate of MeOr in the presence of Ta₂O₅, Au–Ta₂O₅ nanoparticles with and without electron transfer agents. Concentrations are maintained as follows: MeOr (5 × 10⁻⁵ M), PMS = PDS = H₂O₂ (0.25 mM), Ta₂O₅ = Au–Ta₂O₅ = 1 g L⁻¹. Bar diagram notations A1, B1, C1 and D1 are Au–Ta₂O₅/PMS, Au–Ta₂O₅/PDS, Au–Ta₂O₅/H₂O₂ and Au–Ta₂O₅ alone respectively under illumination of visible light whereas under dark conditions are indicated as notations A2, B2, C2 and D2. Illumination experiments with bare Ta₂O₅ alone, Ta₂O₅/PMS, Ta₂O₅/PDS and Ta₂O₅/H₂O₂ are indicated as E, F, G and H respectively. Notations I is a control experiment in the presence of visible light only without any catalyst and electron transfer agents.

Fig. 7 Comparison of the photomineralization rate of MeOr in the presence of Au–Ta₂O₅ nanoparticle without any electron transfer agents (◆), Au–Ta₂O₅with PMS (■), PDS (▲) and H₂O₂ (×). Concentrations are maintained as follows: MeOr (5 × 10⁻⁵ M), PMS=PDS = H₂O₂ (0.25 mM), Au–Ta₂O₅ = 1 g L⁻¹.

Conclusions

Highly efficient Au–Ta₂O₅ nanoparticle was prepared for photocatalytic degradation of MeOr. An 8 atomic weight percentage gold nanoparticles were embedded on the Ta₂O₅ surface, extending the absorption of Ta₂O₅ towards the visible region. TEM photographs shows that there is fine dispersion of gold nanoparticles on the Ta₂O₅ semiconductor surface and EDX confirmed the presence of 8 atomic weight percentage of gold on the Ta₂O₅ semiconductor surface. Photocatalytic activity was evaluated in the presence and absence of electron acceptors and the maximum degradation rate (10.09 × 10⁻⁴ s⁻¹) was achieved for Au–Ta₂O₅ nanoparticles with PMS under visible light illumination. Based on the available results, it may be concluded that wide band gap semiconductor nanoparticles may activate into an efficient visible light photocatalyst.

Acknowledgements

The research described herein was financially supported by the National Science Council (NSC), Taiwan, under contract number NSC-98-2221-E-35-12-MY3. S.A. thanks Feng Chia University, Taiwan, for the Visiting Professor appointment. Also, S.A. thanks CSIR, India for the sanction of a major research project (CSIR reference No. 02(0021)/11/EMR-II).

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