In,V-codoped TiO₂ nanocomposite prepared via a photochemical reduction technique as a novel high efficiency visible-light-driven nanophotocatalyst

Abstract

In the current study, a series of novel, high efficiency photocatalysts of In,V-codoped TiO₂ were developed. The TiO₂ nanoparticles were synthesized by sol–gel and hydrothermal methods and different molar percentages (0.1–1%) of vanadium (V) and Indium (In) nanoclusters were deposited over the TiO₂ nanoparticles via photochemical reduction. XRD, SEM, EDX, TEM, XPS and UV-vis DRS analyses were carried out to characterize the prepared In,V-codoped TiO₂ nanocatalysts, and methyl orange (MO) was used as the probe environmental pollutant to test the photocatalytic performance of the prepared catalysts under UV and visible light irradiation. Our study demonstrated that In and V nanoclusters were successfully deposited over TiO₂ particles via a photochemical deposition technique and the metal doping slightly suppressed TiO₂ crystal growth. The optical analysis showed a red shift in the light absorption spectrum and decrease in the band gap of In,V-codoped TiO₂ catalysts compared to that of parent TiO₂. XPS study revealed that the doped elements In and V are in oxidation state of 3 (In^III), 4 (V^IV) and 5 (V^V). The photo-oxidative decomposition of MO showed that doping of In and V can considerably improve the photocatalytic activity of TiO₂. Thus, for the first time, we demonstrated that TiO₂ codoped with binary metals of In and V can serve as a high efficiency visible-light-active photocatalyst.

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1. Introduction

In recent years, many attempts have been devoted for developing heterogeneous photocatalysts with high activity for environmental applications, including water disinfection, water purification, air purification and hazardous waste remediation.^1,2 Among the various metal oxide semiconductor photocatalysts, TiO₂ has shown to be the most suitable for environmental purposes due to its strong oxidizing power, chemical inertness, long-term stability, and cost effectiveness.^3,4 The primary event occurring on the TiO₂ surface after irradiation is the generation of electrons (e_CB⁻) and holes (h_VB⁺). In these reactions, the organic pollutants are oxidized (decomposed) by the photo-generated h_VB⁺ or by the reactive oxygen species (OH˙ and O₂⁻ radicals) formed on the illuminated TiO₂ surface. However, the practical application of TiO₂ is limited by two main factors. First, due to the wide band gap of TiO₂ (3.2 eV for anatase, 3.0 eV for rutile phase),⁵ it can only absorb the UV portion of solar light (λ < 387 nm), which is up to 5% of solar light.⁶ Therefore, to extend its practical application, many efforts have been made to design second-generation TiO₂-based photocatalysts, which would be able to induce photocatalysis under visible light irradiation. Second, the low rate of photocatalytic decomposition using TiO₂ photocatalyst is attributed to the charge recombination of photogenerated electron–hole pairs (charge carriers).

To overcome these problems, several approaches have been proposed: metal doping,^7,8 metal ion doping,^9–11 nonmetal doping,^12–14 dye-sensitizing of TiO₂ (e.g. thionine),^15,16 fabricating composites of TiO₂ with other semiconductors with a narrow band gap energy (e.g. CdS particles),¹⁷ and doping TiO₂ with an up-conversion luminescence agent.¹⁸ Among these, metal doping has been proved to be a very promising approach as it greatly extends the light absorption of TiO₂ and significantly improves the trapping efficiency of change carriers.^19–22

Raftery et al., reported that doping TiO₂ nanoparticles with vanadium cations could extend the photoresponse of TiO₂ to the visible light region (396–450 nm) and also temporarily trap the photogenerated electrons (e⁻) and holes (h⁺), thus suppressing the recombination of the charge carriers.²³ Wu and Chen found that V-doped TiO₂ exhibits photoactivity in the visible light region and shows a “red shift” in the UV-vis spectra.²⁴ Choi et al., reported that doping V⁴⁺ into TiO₂ at V/Ti ratio of 0.1–0.5 wt% can significantly increase the TiO₂ photoreactivity due to the improved interfacial charge transfer.²⁵

Wang et al., observed that by introducing In ions into the TiO₂ structure, not only new energy states emerge between the TiO₂ band gap, which results in considerable red shift in absorption, but also the ions facilitate the charge separation.²⁶ The same results were reported by another group.²⁷ They analyzed the photocatalytic activity of In-doped TiO₂ under visible light illumination. It was demonstrated that the In doping improved visible light response of the TiO₂ catalyst and enhanced the charge carrier separation, which when combined leads to significant improvement in the photocatalytic performance of TiO₂ under visible light illumination.

Modification of TiO₂ by binary doping of metal ions is a novel process for enhancing the optical efficiency of TiO₂. The synergic effect of codoping can further improve the photocatalytic activity of TiO₂. Recently, a number of literatures have reported different types of photocatalysts using binary metal-doped TiO₂. Estrellan et al. synthesized Fe,Nb-codoped TiO₂ via the sol–gel method.²⁸ Fe–Nb–TiO₂ exhibited the anatase crystalline phase with high values of crystallinity along with a red shift in light absorption. It was reported that TiO₂ co-doping with cations of Rh³⁺/Sb⁵⁺ can result in a greatly improved photocatalytic activity.²⁹ Zhang et al., prepared V,Sc-codoped TiO₂ by the sol–gel method and found that binary doping of these metals led to a considerable decline in the charge recombination and also induced a large red shift in the TiO₂ absorption spectrum.³⁰

The photochemical reduction method has recently attracted considerable attention due to its versatile advantages: (i) controlled reduction of metal ions can be carried out without using an excess of a reducing agent, (ii) the reduction reaction is uniformly performed in the solution, and (iii) light radiation is absorbed regardless of the presence of light absorbing solutes and products.³¹

The aim of the current study, which to the best of our knowledge is for the first time, is to study the synergistic effect of co-doping In and V ions on the photocatalytic activity of TiO₂ nanoparticles. The physicochemical characteristic of the prepared catalysts were investigated by XRD, SEM, EDX, TEM, XPS and UV-vis DRS. Methyl orange (MO) was used as the probe organic pollutant to monitor the photocatalytic performance of the In,V-codoped TiO₂ catalysts. Finally, it is found that the In,V-codoped TiO₂ shows a size in the nanometer range with strong light absorbance and high quantum efficiency of charge carriers and the 0.2% In–0.2% V/TiO₂ catalyst exhibited superior photocatalytic activity compared to parent TiO₂, under UV and visible light irradiation.

2. Experimental

2.1. Chemicals

Titanium(IV) tetraisopropoxide (TTIP, Ti[OCH(CH₃)₄]₄ (Merck, >98%)), glacial acetic acid (Merck, >99.8%), vanadium chloride (VCl₃, Merck), and indium chloride (InCl₃, Merck) were used as received without further purification. Deionized water was prepared by an ultra pure water system (Smart-2-Pure, TKA Co, Germany). Methyl orange (MO, M.W. = 695.58 g mol⁻¹) was provided by Alvan Co., Iran.

2.2. Synthesis of pure TiO₂ nanoparticles by hydrothermal and sol–gel methods

Pure TiO₂ nanoparticles were prepared according to previous studies by sol–gel³² and hydrothermal³³ methods. TiO₂ nanoparticles prepared by hydrothermal and sol–gel processes were calcined at 450 °C for 3 h and 500 °C for 2 h, respectively.

2.3. Preparation of In–TiO₂ and In,V-codoped TiO₂ nanoparticles via photochemical reduction

The photochemical route is a promising way to form noble metal–semiconductor nanocomposites in situ by reducing noble metal ions adsorbed on the surface of a semiconductor. It is well known that a semiconductor can be excited to generate electrons (e⁻) and holes (h⁺) in the conduction band (CB) and valence band (VB), respectively, if the energy of the photons of the incident light is larger than that of the band gap of the semiconductor.³⁴ The metal ion dopants influence the photo-efficiency of TiO₂ by acting as electron or hole trap centres within the band gap of TiO₂ and alter the e⁻/h⁺ pair recombination rate³² through the following process. The photo-reduction of the metal ions (eqn (1)) is accompanied by the elimination of photo-generated holes by water oxidation (eqn (2)):

Mⁿ⁺ + ne⁻ → M^o (1)

2H₂O + 4h⁺ → O₂ + 4H⁺ (2)

In-doped TiO₂ with varying metal content was prepared as follows:

To prepare In-doped TiO₂, first different molar percent of InCl₃ (0.1, 0.2%, 0.4%, 0.6%, 1.0% of In to Ti molar ratio) as the indium source was added to 100 ml of aqueous solution containing a certain amount of pure TiO₂ particles synthesized by sol–gel and hydrothermal processes. Then, the resulting solution was purged with a high-purity N₂ atmosphere while stirring. Furthermore, the resulting solution was transferred to a quartz reactor with its head covered and was placed under UV irradiation for 12 h, under vigorous stirring. After this stage, the precursor was filtered by centrifugation and washed with deionized water several times. The resulting powders were dried at 100 °C for 12 h.

To prepare In,V-codoped TiO₂, the same method for synthesis of In-doped TiO₂ was adopted using both InCl₃ (0.1%, 0.2%, 0.4%, 0.6%, 1.0% of In to Ti molar ratio) as the indium source and VCl₃ (0.1%, 0.2%, 0.4%, 0.6%, 1.0% of V to Ti molar ratio) as the vanadium source.

2.4. Characterization of the prepared catalysts

Crystalline phases of the prepared samples were analysed by X-ray powder diffraction (XRD, Bruker D8 Discover X-ray Diffractometer). The morphology was investigated by a transmission electron microscope (TEM, JEOL JEM3200 FS) and a scanning electron microscope (SEM, Hitachi S-4800) equipped with an energy dispersive X-ray detector (EDX). UV-vis DRS spectra of the samples were obtained by a Shimadzu 1800 spectrometer. UV-vis absorption spectra of MO degradation were obtained by UV-vis spectrophotometer (Perkin Elmer Lambda2S, Germany). XPS test was monitored by an Omicron XPS/UPS system with an Argus detector, which uses an Omicron's DAR 400 dual Mg/Al X-ray source.

2.5. Photocatalytic performance analysis

The photocatalytic activity of the prepared catalysts was analyzed by MO degradation under UV and visible light irradiation. Each time, 0.1 g photocatalyst was dispersed into 100 ml MO aqueous solution with a concentration of 10 mg l⁻¹ held in a quartz reactor (with a dimension of 12 cm × 5 cm, height and diameter, respectively). Two 400 W Osram lamps provided the visible and UV sources, located 40 cm and 25 cm away from the reactor, respectively. The reaction system was stirred in the dark for 30 min to achieve absorption equilibrium before irradiation.

3. Results and discussion

3.1. X-ray diffraction patterns

Fig. 1 shows the XRD patterns of parent TiO₂ and In,V-codoped TiO₂ catalysts. (101), (004), (200), (105), (211), (204) and (116) diffraction peaks were proof of the anatase phase for pure TiO₂.³⁵ A main peak for anatase around 2θ = 25.2° (101) has a tetragonal form.³⁵ Moreover, no rutile phase diffraction peaks were detected in the samples. Furthermore, the XRD pattern did not show any In or V phase (as in metallic or metal oxide states) and it was concluded that In and V ions were uniformly loaded onto the TiO₂ surface. There is also a relatively small shift in In,V-codoped TiO₂ compared to parent TiO₂, which shows slight distortion in the TiO₂ structure.

Fig. 1 XRD patterns of parent TiO₂ and In,V-codoped TiO₂ catalysts.

Debye–Scherrer formula was used for measuring the average crystallite size of the prepared catalysts as follows:

D = kλ/β cos θ (3)

where k is the constant taken as 0.9 here, λ is the wavelength of the X-ray radiation (λ = 0.1541 nm), β is the corrected band broadening [(FWHM) full-width at half-maximum] after subtraction of equipment broadening and θ is the Bragg angle.³⁵ By using this equation on the anatase phase (2θ = 25.2°, 48.2° and 55.2°), the average particle size was calculated for pure TiO₂ synthesized via the hydrothermal method to be about 18.75 nm. The particle size of In,V-codoped TiO₂ with 0.2 mol% metal content was estimated to be 14.3 nm. We evidently found that doping TiO₂ by In and V ions results in a decrease of the TiO₂ catalyst particle size.

Indeed, the formation of Ti–O–In or Ti–O–V inhibits the transition of the TiO₂ phase and blocks Ti–O species at the interface with TiO₂ domains, thus preventing the agglomeration of TiO₂ particles. Hence, the doping of TiO₂ by In and V minimizes the charge carrier recombination during the photocatalytic decomposition of MO, and as a result, it is expected that In,V-codoped TiO₂ shows a higher photocatalytic activity compared to pure TiO₂.

3.2. SEM-EDX analysis

Fig. 2 shows SEM micrographs of pure TiO₂ and 0.2% In–0.2% V/TiO₂ catalysts. The SEM micrographs show that the particles consist of uniform, globular and slightly agglomerated particles, and the doped metal ions had no obvious influence on the morphology of the samples. Further observation indicates that the morphology of samples is very rough, which may be beneficial to enhance the adsorption of dye due to its great surface roughness and high surface area.³⁶ Both narrow size distribution of nanoparticles and optimal dispersion are favourable for photoactivity.

Fig. 2 SEM micrographs of pure TiO₂ (A and B) and In,V-codoped TiO₂ (C and D).

The EDX measurement was carried out to verify the formation of In and V nanoclusters onto the TiO₂ surface after photochemical reduction. As it is obvious from Fig. 3, new peaks appear in the TiO₂ spectra after metal deposition, which confirms the presence of In and V in the prepared In,V/TiO₂ sample.

Fig. 3 EDX result of In,V-codoped TiO₂.

3.3. TEM analysis

TEM images of pure TiO₂ and 0.2% In–0.2%V/TiO₂ catalysts are shown in Fig. 4. Based on the images, the particle size of the 0.2% In–0.2% V/TiO₂ sample was estimated to be around 11–13 nm, which is in a good agreement with the particle size estimated from the Debye–Scherrer formula (12–15 nm). Moreover, the HRTEM image shows that the peak located at 2theta = 25° matches well with the (101) plane of anatase TiO₂ (JCPDS card no. 01-065-9124), indicating the formation of anatase TiO₂.³⁷

Fig. 4 HRTEM images of In,V-codoped TiO₂.

3.4. XPS study of the In,V-codoped TiO₂ catalyst

XPS is carried out to determine the oxidation states of In and V in the In,V-codoped TiO₂ catalyst. Fig. 5 shows the XPS spectrum for In 3d and V 2p of In,V-codoped TiO₂. As shown in Fig. 5, XPS spectrum in Ti 2p region of In,V-codoped TiO₂ shows peaks at around 456 eV (Ti 2p_3/2) and 462 eV (Ti 2p_1/2), which corresponds to Ti⁴⁺ ions in the TiO₂ lattice.³⁸ The O 1s peak of the prepared In,V-codoped TiO₂ can be seen at a binding energy of around 527 eV, which is attributed to crystal lattice oxygen (O²⁻) of In,V-codoped TiO₂ (Ti–O–Ti; Ti–O–In; Ti–O–V).³⁸

Fig. 5 XPS spectra of In,V-codoped TiO₂.

Our findings show that In in In,V-codoped TiO₂ exists in oxidation state of 3, In(III), showing peaks around 444.8 eV (In 3d_5/2) and 450.6 eV (In 3d_3/2), which is in agreement with previous reports.³⁹ Regarding the presence of V in the In,V-codoped TiO₂ catalyst, we witnessed a peak for V 2p_3/2, which consists of two peaks, one at around 516 eV, related to V⁴⁺ and around 517 eV, related to V⁵⁺.³⁸

3.5. UV-vis DRS analysis of In,V-codoped TiO₂ catalysts

It is well-known that the photocatalytic performance of a metal oxide semiconductor is closely related to its band gap structure. The UV-vis absorbance spectra of the pure and metal doped TiO₂ samples are shown in Fig. 6. By considering the absorbance spectra of pure TiO₂, the onset of the absorption appears at 380 nm, which matches well with the intrinsic band gap of anatase TiO₂ (3.2 eV). Moreover, it is obvious that there is a considerable shift in the absorption toward a higher wavelength for the In,V-codoped TiO₂ catalyst compared to pure TiO₂. The reason for that might be attributed to the appearance of the new electronic energy state in the middle of the TiO₂ band gap, which results in gap reduction between the conduction band (CB) and valence band (VB) of TiO₂, allowing TiO₂ to absorb visible light.³⁶

Fig. 6 UV-vis DRS absorption spectra of as-prepared TiO₂ and In,V-codoped TiO₂ nanoparticles with different metal content.

The band gap values of the pure TiO₂ and In,V-codoped TiO₂ catalysts were calculated⁴⁰ using the following equation:

E_g = h × c/λ (4)

where E_g = band gap energy, h = Planck's constant in eV (4.135 × 10⁻¹⁵ eV), c = velocity of light (3 × 10⁸ m s⁻¹), λ = wavelength of the band gap for corresponding catalysts. We found that the band gap values were lower for the doped catalysts (below 3 eV) compared to the pure TiO₂ catalyst (up to 3 eV).

Furthermore, the V 3d energy state and In 4d energy state play important roles in interfacial charge transfer and elimination of charge recombination. Thus, transition metal ions (V and In) would act as efficient electrons scavenger to trap the electrons of CB state of TiO₂.³⁵ Accordingly, it can be presumed that the In,V–TiO₂ photocatalyst may demonstrate higher photocatalytic activity under visible light irradiation, compared to the pure TiO₂.

3.6. Photocatalytic performance of In,V-codoped TiO₂ catalysts

Methyl orange (MO) was used as a probe environmental pollutant to study the photocatalytic activity of the pure TiO₂ and metal-doped TiO₂ catalysts. Tables 1 and 2 show the degradation results over In-doped TiO₂ with various metal content synthesized via hydrothermal-assisted photochemical reduction and sol–gel assisted photochemical reduction, respectively, and Fig. 7 and 8 demonstrate the photocatalytic performance of In,V-codoped TiO₂ with various metals content synthesized via sol–gel assisted photochemical reduction and hydrothermal-assisted photochemical reduction, respectively. Before the exposure to UV or visible light on the catalysts, the MO solution containing the catalysts was stirred in dark for half an hour. Our detection results exhibited that the MO concentration showed negligible decrease due to slight absorption on the photocatalysts surface, which showed that there was almost no MO decomposition in the absence of light irradiation.

Table 1 The photocatalytic results of MO degradation using In-doped TiO₂ nanoparticles with various In(III) content, prepared via hydrothermal assisted photochemical deposition

UV light illumination
Time (min)	15	30	60	45
Elem. (%)
0	37.32	59.77	81.07	90.80
0.05	43.91	59.78	75.71	82.56
0.1	35.42	48.58	67.82	82.68
0.2	52.41	71.72	86.51	95.09
0.5	46.28	81.99	88.66	98.47
0.8	45.21	78.47	88.12	96.17
1	38.16	60.69	81.53	92.18
2	44.67	64.37	81.23	92.87

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Visible light illumination
Time (min)	30	60	90	120	150
Elem. (%)
0	23.85	34.81	48.15	62.98	69.38
0.05	26.15	44.56	60.15	74.21	79.81
0.1	28.66	48.89	70.81	93.55	98.89
0.2	28.89	46.07	61.63	77.63	82.78
0.5	9.18	22.96	36.15	47.48	52.72
0.8	8.02	19.51	29.63	41.52	48.21
1	21.48	37.18	51.48	66.07	72.13
2	26.89	43.11	58.59	73.19	78.32

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Table 2 The photocatalytic results of MO degradation using In-doped TiO₂ nanoparticles with various In(III) content, prepared via sol–gel assisted photochemical deposition

UV light illumination
Time (min)	15	30	45	60
Elem. (%)
0	43.67	67.66	81.02	91.02
0.1	45.08	68.36	82.66	91.48
0.2	45.08	68.36	82.66	91.48
0.4	38.20	64.76	76.48	81.95
0.6	40.93	67.11	78.52	89.76
1	45.08	69.53	85.55	92.97
2	49.14	71.01	91.09	96.09

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Visible light illumination
Time (min)	30	60	90	120	150
Elem. (%)
0	20	30	42.50	52.71	61.20
0.1	12.86	25.50	41.53	48.57	53.43
0.2	30.86	52.21	69.28	83.57	92.50
0.4	27.28	47.14	52.93	70.81	80.28
0.6	21.71	45.32	60.15	64.71	74.86
1	19	27.93	41.25	50.28	57.36
2	26.36	42.07	57.07	68.43	81.92

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Fig. 7 Photocatalytic activity of pure TiO₂ and In,V-codoped TiO₂ catalysts prepared by sol–gel method, under UV and visible light irradiation.

Fig. 8 Photocatalytic activity of pure TiO₂ and In,V-codoped TiO₂ catalysts prepared by hydrothermal method, under UV and visible light irradiation.

As it is apparent from Table 1, all In-doped TiO₂ nanoparticles are visible light active. The optimal dosage of indium ion to obtain the highest photocatalytic activity for MO decomposition was found to be 0.1% (hydrothermal-assisted photodeposition) and 0.2% (sol–gel assisted photodeposition) under visible light and 0.5% (hydrothermal-assisted photodeposition) and 2% (sol–gel assisted photodeposition) under UV light illumination.

Our findings for the In,V-codoped TiO₂ catalyst prepared with the sol–gel assisted photodeposition technique (Fig. 7) illustrate no improvement in photocatalytic performance in comparison to pure TiO₂. However, in the case of the In,V-codoped TiO₂ catalyst prepared with the hydrothermal-assisted photodeposition technique (Fig. 8), we found that the TiO₂ catalyst with 0.2% metal content achieved higher rates of MO decomposition compared to the pure TiO₂ catalyst. Indeed, in the In,V-codoped TiO₂ catalyst, the metal could act as an electron trapper and thus reduce the charge recombination rate, which favours the photocatalytic activity enhancement. The improvement of pollutant degradation was initially increased with the increase of metal content, but it decreased when the metal content reached a high level.^41–43 As matter of fact, in In,V-codoped TiO₂ catalysts with metal content higher than 0.2%, the metal ions act as electron–hole recombination centres, which as a result decreases the photo-efficiency and photocatalytic activity.

To investigate the In³⁺ doping effect on the photocatalytic performance of TiO₂, it was found that loading indium ions onto TiO₂ particles prevents the particle growth and In³⁺ changes to In²⁺ as electron gets trapped by forming a low energy level between the CB and VB of TiO₂. In the absence of light irradiation, In²⁺ ions convert to In³⁺ and atmospheric O₂ traps the released electrons as electron acceptors to produce O₂⁻. The mechanism of this process is shown below:

TiO₂ + hν → TiO₂˙ + e⁻ (5)

In³⁺ + e⁻ → In²⁺ (6)

In²⁺ + oxidation → In³⁺ + e⁻ (7)

O₂ + e⁻ → O₂⁻ (8)

In fact, this metal–TiO₂ support interface is largely beneficial for the photocatalytic reactions. The close contact of metal nanoclusters with TiO₂ nanoparticles allows the photogenerated electrons (free or trapped) in TiO₂ lattices (eqn (9) and (10)) to be transferred to the metal lattices (eqn (11) and (12)) easily;

TiO₂ + hν → e_CB⁻ + h_VB⁺ (9)

Ti⁴⁺ + e_CB⁻ → Ti³⁺ (10)

M + e_CB⁻ → M⁻ (11)

Ti³⁺ + M → Ti⁴⁺ + M⁻ (12)

where hν represents the light irradiation energy, e_CB⁻ and h_VB⁺ represent the photogenerated electrons in CB and holes in the VB of TiO₂, respectively. Tiⁿ⁺ and M_m⁻ represent the titanium ions or atoms in the TiO₂ crystal and metal atoms or ions in the metallic clusters, respectively.

The number of electrons in the bulk TiO₂ is reduced, thereby the possibility of recombination (eqn (13) and (14)) declines:

e_CB⁻ + h_VB⁺ → recombination (13)

Ti³⁺ + h_VB⁺ → Ti⁴⁺ (14)

In general, most of the photogenerated electrons and holes recombine through processes in eqn (13) and (14), and only a small number remains for the photocatalytic reactions. By using IR spectroscopy, it was reported that electron transfer from the TiO₂ support to the deposited metal clusters is the bottleneck of the photocatalytic reactions.⁴⁴

Based on our findings, there is a significant potential to enhance efficiency of photocatalytic reactions through improving charge separation and charge transfer using proper metal nanoclusters loading over TiO₂ catalysts.

4. Conclusions

In the current study, a series of novel In,V-codoped TiO₂ catalysts with different In and V contents were synthesized by a photochemical reduction technique and used as photocatalysts to decompose MO as a probe pollutant in an aqueous solution. XRD and EDX analysis did not show any peaks to confirm the appearance of unwanted impurities. SEM analysis confirmed that all samples are uniform, globular and slightly agglomerated and TEM analysis confirmed the results obtained from SEM and XRD analysis. The photocatalytic activity of pure TiO₂ is greatly improved in the presence of loaded metal nanoclusters with 0.2% In and 0.2% V content. The high visible-light-driven photocatalytic activity of In and V modified TiO₂ is ascribed to the synergetic effects of (1) decreased particle size, (2) improved visible-light harvesting ability due to formation of sub-energy levels in the TiO₂ structure, and (3) increased efficiency in separation of photo-generated charge carriers. This investigation contributes to understanding the effects of the complex ion doping on TiO₂ photoactivity and thus provides a reference for improving its environmental application.

Acknowledgements

We are grateful to the Council of University of Kashan, the Iran Nanotechnology Initiative Council and the University of Texas at El Paso (UTEP) for providing financial support. We also thank Dr Peter Cooke from New Mexico State University (NMSU) and Dr Sudheer Molugu for helping us with TEM measurements, Dr Jing Wu from Texas A & M University for helping us with XPS analysis, Dr Juan Noveron from chemistry department of UTEP for providing us with the UV light reactor, and the College of Engineering at UTEP for allowing access to their SEM, EDX and XRD instruments.

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