Sol–gel synthesis of Au/N–TiO₂ composite for photocatalytic reduction of Cr(VI)

Abstract

Au/N–TiO₂ composites are successfully synthesized via a modified sol–gel method and their photocatalytic performance in reduction of Cr(VI) is investigated. Au/N–TiO₂ composites exhibit an enhanced photocatalytic performance in the reduction of Cr(VI) with a maximum reduction rate of 90% under visible light irradiation as compared with pure TiO₂ (34%) and N–TiO₂ (80%) due to the increase of light absorption intensity and range as well as the reduction of electron–hole pair recombination in TiO₂ with the incorporation of N and Au.

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1. Introduction

TiO₂ has attracted tremendous attention as a promising photocatalyst for widespread environmental applications owing to its intriguing optical and electric properties.^1–9 However, its universal use is restricted to ultraviolet (UV) light due to its intrinsically wide band gap (3.2 eV in the anatase phase), and the quick recombination of photo-induced electron–hole pairs has significantly decreased the photocatalytic performance of TiO₂. Recent progress has shown that the co-doping of TiO₂ by non-metal elements,^10–12 typically N or B, and metal nanomaterials with a work function lower than the conduction band of TiO₂,^13–18 such as Au, Ag or Fe, can solve these problems and enhance the photocatalytic activity of TiO₂ in the UV or visible light region. In the photocatalysis process, metal nanomaterials can act as excellent electron-acceptor/transport materials to effectively facilitate the migration of photo-induced electrons and hinder the charge recombination in electron-transfer processes due to the electronic interaction between TiO₂ and them, which enhances the photocatalytic performance,^19,20 while non-metal elements can tune the band gap and extend the photo-response of TiO₂ into the visible light region,^21–25 which is regarded as an effective approach for the utilization of solar light. Wu et al.²⁶ studied the synthesis of the Au/N–TiO₂ composite and investigated its photocatalytic activity in the degradation of methyl orange. A two-step synthesis of N–TiO₂ was carried out by the hydrolysis of titanium sulfate by ammonia followed by urea. Then, Au particles were deposited on the surface of N–TiO₂ by using the deposition-precipitation method with urea. When Au was introduced into the N–TiO₂ composite, the photocatalytic degradation rate of methyl orange increased from about 36% for N–TiO₂ to 85.4% for Au/N–TiO₂ (4 wt% Au) after reaction for 5 h under visible light irradiation. Parida et al.²⁷ studied the synthesis of Au/S,N–TiO₂ composite via a one-pot template-free homogeneous co-precipitation technique and borohydrate reduction method, and the composite achieved a higher catalytic activity in the oxidation of carbon monoxide compared to Au/TiO₂, which was attributed to the presence of oxygen vacancies and the creation of new adsorption sites for the adsorption and activation of O₂ molecules. Tian et al.¹³ carried out a simple wet-chemical method to fabricate the Au/N–TiO₂ composite which exhibited a much higher visible-light photocatalytic activity in the degradation of 2,4-dichlorophenol compared with single N-doped or Au-loaded TiO₂ due to the synergetic effects of N doping and Au loading. Iliev et al.²⁸ reported that Au/N–TiO₂ synthesized through a sol–gel and photo-reduction method improved the photocatalytic oxidation rate of oxalic acid, which was ascribed to the more efficient charge separation, the increase in the lifetime of the charge carriers and the enhancement of the efficiency of the interfacial charge transfer to adsorbed substrates. Despite the above progress to date, the exploration on Au/N–TiO₂ composites, especially on their synthesis via the sol–gel method and application in photocatalytic reduction of Cr(VI), is not nearly enough. In this work, the one-step synthesis of Au/N–TiO₂ composites is carried out via the hydrolysis of tetrabutyl titanate in HAuCl₄ and urea solution to form a transparent sol. Au/N–TiO₂ composites exhibit an enhanced photocatalytic performance in the reduction of Cr(VI) with a maximum reduction rate of 90% after reaction for 4 h under visible light irradiation as compared with pure TiO₂ (34%) and N–TiO₂ (80%).

2. Experimental

2.1 Synthesis of TiO₂–N/Au composite

4 ml tetrabutyl titanate was first dissolved in 10 ml ethanol by stirring for 30 min at room temperature to obtain solution A. A certain amount of HAuCl₄ and 429 mg urea were dissolved in 7 ml ethanol, and then 2 ml deionized water and 5 ml acetic acid were successively added to the solution with stirring for 30 min at room temperature to obtain solution B. Solution B was then added dropwise into solution A under vigorous stirring. Subsequently, the mixture solution was continuously stirred at 40 °C for the hydrolysis of tetrabutyl titanate until a transparent sol was formed. Finally, the sol was dried in air at 100 °C for 24 h, ground and heated at 500 °C for 1 h. The as-synthesized Au/N–TiO₂ samples with 0, 0.1, 0.3, 0.5 wt% Au were named as TNA-0, TNA-1, TNA-2, and TNA-3, respectively. The pure TiO₂ was also synthesized for comparison. For the electrochemical impedance spectra (EIS) testing, the as-synthesized composites with 5 wt% cellulose binder were homogenously mixed in terpineol to form a slurry. Then, the resultant slurries were coated on the FTO using a screen-printing approach. Finally, these prepared electrodes were sintered at 500 °C for 30 min.

2.2 Characterization

The surface morphology, structure and composition of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800), high-resolution transmission electron microscopy (HRTEM, JEOL-2010), X-ray diffraction spectroscopy (XRD, Holland Panalytical PRO PW3040/60) with Cu-Kα radiation (V = 30 kV, I = 25 mA), and energy dispersive X-ray spectroscopy (EDS, JEM-2100), respectively. The UV-vis absorption spectra were recorded using a Hitachi U-3900 UV-vis spectrophotometer. EIS measurement was carried out on an electrochemical workstation (AUTOLAB PGSTAT302N) under dark conditions using a three-electrode configuration with the as-prepared films as working electrode, a Pt foil as counter electrode and a standard calomel electrode as reference electrode. The electrolyte was 10 mg l⁻¹ K₂Cr₂O₇ aqueous solution. EIS were recorded in the frequency range of 0.1 Hz to 1 MHz, and the applied bias voltage and ac amplitude were set at open-circuit voltage and 10 mV AC between the counter electrode and the working electrode, respectively.

2.3 Photocatalytic experiments

The photocatalytic performance of the as-prepared samples was evaluated by photocatalytic reduction of Cr(VI) under visible light irradiation. The samples (1 g l⁻¹) were dispersed in 60 ml Cr(VI) solutions (10 mg l⁻¹) which were prepared by dissolving K₂Cr₂O₇ into deionized water. The mixed suspensions were first magnetically stirred in the dark for 0.5 h to reach the adsorption–desorption equilibrium. Under ambient conditions and stirring, the mixed suspensions were exposed to visible light irradiation produced by a 400 W metal halogen lamp with cut off filter (λ > 400 nm). At certain time intervals, 2 ml of the mixed suspensions were extracted and centrifugated to remove the photocatalyst. The filtrates were analyzed by recording UV-vis spectra of Cr(VI) using a Hitachi U-3900 UV-vis spectrophotometer.

3. Results and discussion

Fig. 1(a)–(e) show the FESEM images of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3, respectively. It is clearly seen that all the samples display spherical particles and that the average particle size decreases with the increase of Au content, showing that Au loading inhibits the growth of TiO₂ crystallites.²⁹ A decrease in particle size can lead to a larger surface area and lower volume recombination, which can facilitate the adsorption of more pollutants, and thus improve the photocatalytic performance.³⁰ The existence of N-TiO₂ in the composite is proved by the peaks of Ti, O and N in EDS spectra of TNA-3 (Fig. 1(f)). However, no diffraction peak of Au is observed, which may be due to the low amount of Au in the composite.

Fig. 1 Surface morphologies of (a) TiO₂, (b) TNA-0, (c) TNA-1, (d) TNA-2, and (e) TNA-3 by FESEM measurement; (f) EDS spectrum of TNA-3.

Fig. 2(a) and (b) show the low-magnification and high-magnification HRTEM images of TNA-3. It can be observed that Au nanoparticles have attached onto the surface of TiO₂. The large crystallite is identified as the TiO₂ particle. The lattice spacing measured for this crystalline plane is 0.352 nm, corresponding to the (101) plane of anatase TiO₂. Around the TiO₂ crystallite edge, fine crystallites are observed. The crystallites connected to the TiO₂ have lattice fringes of 0.236 nm which is ascribed to the (111) plane of Au.

Fig. 2 (a) Low-magnification and (b) high-magnification HRTEM images of TNA-3.

The XRD patterns of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3 are shown in Fig. 3. It can be observed that all the samples exhibit the diffraction peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3°, and 75.0°, indexed to (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO₂ (JCPDS 21-1272), which demonstrates that the co-doping of N and Au does not result in the development of new crystal orientations of TiO₂. Compared with pure TiO₂ and TNA-0, new peaks at 44.4°, 64.6°, and 77.6°, corresponding to the (200), (220) and (311) planes of polycrystalline Au (JCPDS 76-1802), appear in XRD pattern of TNA-2 and TNA-3, which further confirm the existence of Au in the composite. It should be noticed that the absence of the Au peak for TNA-1 should be ascribed to the low amount of Au being below the detection limit.

Fig. 3 XRD patterns of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3.

The UV-vis absorption spectra of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3 are shown in Fig. 4. It is observed that TiO₂ presents its characteristic absorption peak at 330 nm and the absorbance of Au/N–TiO₂ composites increases even in visible light region with the increase of Au content, which is similar to other reported results.^13,26,31 Such an enhancement in absorbance can increase the number of photo-generated electrons and holes to participate in the photocatalytic reaction and enhance the photocatalytic performance.³² In addition, the absorption edge of the Au/N–TiO₂ composite exhibits a red shift toward the visible light region compared with TiO₂, which is attributed to band gap narrowing by the N doping.³³ Similar results are also observed in other TiO₂-based composite materials.^34,35 Therefore, the incorporation of N and Au in TiO₂ can increase the visible light absorption intensity and range, which is beneficial to the photocatalytic performance.

Fig. 4 UV-vis absorption spectra of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3.

The photocatalytic reduction of Cr(VI) under visible light irradiation was used to evaluate the photocatalytic performance of TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3, as shown in Fig. 5. The normalized temporal concentration changes (C/C₀) of Cr(VI) during the photocatalytic process are proportional to the normalized maximum absorbance (A/A₀), which is derived from the changes in the Cr(VI) absorption profile at a given time interval. It can be observed that TNA-0 composite exhibits much higher photocatalytic reduction rate (80%) than pure TiO₂ (34%) because the doping of N extends the absorption of TiO₂ into the visible light range,^33,36 and inhibits the recombination of the charge carriers.³⁷ When Au is introduced into the composite, the removal rate is increased to 87% for TNA-1 and reaches a maximum value of 90% for TNA-2. It indicates that the incorporation of N and Au into TiO₂ play an important role in the photocatalytic performance. It is known that during photocatalysis, the adsorption of pollutants, the light absorption and the charge transportation and separation are crucial factors.^38–40 The enhancement of the photocatalytic performance should be ascribed to the increase of the adsorption of pollutants, the increase of the visible light absorption intensity and range as well as the reduction of electron–hole pair recombination in TiO₂ with the incorporation of N and Au. As shown in Fig. 6, the typical EIS of the pure TiO₂, TNA-0, and TNA-2 electrodes are presented as Nyquist plots. It is observed that the semicircle in the plot becomes shorter with the incorporation of N and Au into TiO₂, indicating a favored electron transport and the reduction of electron recombination which is related to the stepwise structure of energy levels constructed in the Au/N-TiO₂ composite, as shown in Fig. 7. The conduction band and valence band of TiO₂ are −4.2 eV and −7.4 eV (vs. vacuum).⁴¹ The work function of Au is around −5.1 eV.⁴² Such energy levels are beneficial for photo-induced electrons to transfer from the TiO₂ conduction band to Au, which could efficiently separate the photo-induced electrons and hinder the charge recombination in electron-transfer processes,^26,43 and thus enhance the photocatalytic performance. However, when the Au content is further increased above its optimum value, the photocatalytic performance deteriorates. This is ascribed to the following reasons: (i) Au can absorb some visible light^19,26,31,44 and thus there exists a light harvesting competition between N–TiO₂ and Au with the increase of Au amount, which leads to the decrease of the photocatalytic performance; (ii) the excessive Au can act as a kind of recombination center instead of providing an electron pathway and promote the recombination of electron–hole pairs in Au.^13,19,45

Fig. 5 Photocatalytic reduction of Cr(VI) by TiO₂, TNA-0, TNA-1, TNA-2, and TNA-3 under visible light irradiation. The concentrations of Cr(VI) and photocatalyst are 10 mg l⁻¹ and 1 g l⁻¹, respectively.

Fig. 6 Nyquist plots of the pure TiO₂, TNA-0, and TNA-2 electrodes. Inset is the corresponding equivalent circuit model.

Fig. 7 Schematic diagram of energy levels of N–TiO₂ and Au. CB, VB and Φ are conduction band, valence band and work function, respectively.

4. Conclusions

Au/N–TiO₂ composites are successfully synthesized via a simple sol–gel method and their photocatalytic performance is investigated. The experimental results indicate that (i) Au/N–TiO₂ composites exhibit a better photocatalytic performance than pure TiO₂ and N–TiO₂; (ii) the photocatalytic performance of the Au/N–TiO₂ composite is dependent on the proportion of Au in the composite, and the composite with 0.3 wt% Au achieves a highest reduction rate of 90%; (iii) the enhanced photocatalytic performance is mainly ascribed to the increase of visible light absorption intensity and range as well as the reduction of photoelectron–hole pair recombination in TiO₂ with the incorporation of N and Au.

Acknowledgements

Financial support from Special Project for Nanotechnology of Shanghai (No. 1052nm02700) and ECNU Reward for Excellent Doctors in Academics (No. XRZZ2011017) is gratefully acknowledged.

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