Au Nanocomposites via Sol-Gel

Synthesis of TiO2 /Au Nanocomposites via Sol-Gel Process for Photooxidation of Methanol Adel A. Ismail* and Detlef W. Bahnemann Institut für Technische Chemie,Leibniz Universität Hannover, Callinstrasse 3,D-30167 Hannover, Germany

Abstract: Well-ordered TiO2/Au nanocomposites were prepared using sol-gel process in presence of poly(ethylene oxide)106-poly(propylene oxide)70-poly(ethylene oxide)106 triblock copolymer (Pluronic F127) as the structuredirecting agents.. The content of Au into TiO2 network was amounted to be 0.3, 0.5, 1 and 2 wt%. The produced TiO2/Au gels were calcined at 500 °C for 4 hours under atmospheric conditions. The findings indicated that TEM images of 2 wt%Au/TiO2 nanocomposites correspondeds anatase TiO2 nanoparticles, which mainly consisted of nanoscale cubes and rhombohedra. These nanoparticles are homogenously distribution and have average sizes of TiO2 and Au 12 ± 2 and 20 ± 5 nm respectively. TiO2/Au nanocomposites were investigated for their photooxidation efficiencies for methanol to produce formaldehyde at room temperature. The results indicated that, the TiO2/Au nanocomposites show good performance in methanol photooxidation due to their highly ordered crystalline, one crystal phase, small particle size and strong adsorptive capacity. Additionally, the results of this study demonstrate that 1 wt%Au/TiO2 is more photoactive than pure TiO2 and P25 degussa. The photonic efficiencies of the TiO2/Au nanocomposites at varies Au content was determined. In general, a large surface area, and highly TiO2 crystalline anatase obtained contributes high adsorptive capacity and hence resulting in high photocatalytic activity.

Introduction Multicomponent metal oxides have attracted attention for their potential use in electronic, (photo)catalytic, photovoltaic, and energy storage applications. The ability to simultaneously control the nanoscale structure and composition of such materials using simple and inexpensive routes is important for that potential to be realized (1). The modification of the TiO2 nanomaterials surface with precious metals can alter the charge-transfer properties between TiO2 and the surrounding environment, thus improving the performance of TiO2 nanomaterials-based devices. The optical response of any material is largely determined by its underlying electronic structure. The electronic properties of a material are closely related to its chemical composition , its atomic arrangement, and its physical dimension for nanometer-sized materials (2). It is desirable to maintain the integrity of the crystal structure of the photocatalytic host material and to produce favorable changes in electronic structure. To increase the efficiency of charge separation involves the contact of the semiconductor particles with another semiconductor. This coupled semiconductors has led to the discovery of photoinduced vectorial electron *Corresponding author; E-mail: [email protected] ISSN 1203-8407 © 2009 Science & Technology Network, Inc.

transfer from one semiconductor to another semiconductor (3). However, the charge carrier recombination occurs within nanoseconds and hence low activity is usually observed (4). Noble metals are loaded on the semiconductor surface to solve this problem. Many investigators have demonstrated that the photocatalytic activity may be enhanced by impregnating the surface of titanium dioxide with noble metals (5, 6). The small size of the nanoparticle is beneficial for the modification of the chemical composition of TiO 2 due to the higher tolerance of the structural distortion than that of bulk materials induced by the inherent lattice strain in nanomaterials (7). Au/titania mesostructured particles are well-known to be an attractive material for a variety of optoelectronic and photonic applications while nanocrystalline anatase titania has proven to be an excellent candidate for the use in low cost photocatalysis and solar energy conversion (8). Au nanoparticles supported on TiO2 surfaces have been extensively investigated in recent years for their applications as catalysts for low-temperature CO oxidation, selective propene oxidation, and other catalytic and photocatalytic oxidation reactions (9-20). In this contribution, gold nanoparticles were incorporated into titania network through a sol-gel process. The sol-gel solution consists of CH3COOH as a complexing agent to modify the condensation kinetics J. Adv. Oxid. Technol. Vol. 12, No. 1, 2009

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A. A. Ismail and D. W. Bahnemann

of tetrabutyl orthotitanate (TBOT) and HCl partially hydrolyzes and charges the modified inorganic precursor to enhance the affinity for the hydrophilic block of the block copolymer. The methanol oxidation was chosen as the test reaction to demonstrate the photocatalytic activity of the novel TiO 2/Au nanocomposites and comparison their performance with commercial TiO2 P25 degussa. The TiO2/Au nanocomposites exhibit excellent photocatalytic oxidation of methanol in aqueous suspensions at room temperature to form formaldehyde. Additionally, the results of this study demonstrate that 1wt% Au/TiO2 is more photoactive than pure TiO2 and commercial P25 degussa.

Experimental Procedures Materails:The block copolymer surfactants EO106PO70EO106 (F-127, EO= -CH2CH2O-,PO= -CH2(CH3) CHO-), MW 12 600 g/mol) from Sigma, tetrabutyl orthotitanate (TBOT), Ti(OC(CH3)3)4, CH3COOH, HCl, CH3CH2OH, CH3OH and HAuCl4 were purchased from Aldrich. Preparation of TiO2/Au nanocomposites: 10 mmol of TBOT, 40 mmol of CH3COOH, 24 mmol of HCl , and 1.6 g of F127 were dissolved in 30 ml of ethanol(1). Separately, to prepare Au nanoparticles, a freshly prepared sodium borohydride (NaBH4, at 0.01 M) solution was added slowly, with vigorous stirring to HAuCl4 (1g dissolved in 25 ml H2O); Au nanoparticles was formed after 20 min. The calculated amount of Au nanoparticles suspension solution was added to (F127-TBOT-CH3COOH) mesophase and the mixture was stirred vigorously for 1 h. The ethanol was evaporated at 40 °C for 12 h, a transparent TiO2/Au nanocomposites were formed, and it was transferred into a 65 °C oven and aged for an additional 24 h. As made mesostructured hybrids were calcined at 500 °C in air for 4 h (ramp rate 2 °C/min) to obtain Au/TiO2 nanocomposites. Characterization: X-ray powder diffraction (XRD) data were acquired on a Bruker AXS D4 Endeavour X diffractometer using Cu Kα1/2, λα1=154,060 pm, λα2 = 154.439 radiation. FT-IR spectra were recorded with a BRUKER FRA 106 spectrometer using the standard KBr pellet method. TEM was conducted at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument equipped with a Gatan GIF 2001 energy filter and a 1k-CCD camera in order to obtain EEL spectra. SEM on a JEOL JSM-6700F field-emission instrument using a secondary electron detector (SE) at an accelerating voltage of 2 kV. The bandgap energy of the catalysts was determined using diffuse reflectance 10

J. Adv. Oxid. Technol. Vol. 12, No. 1, 2009

spectroscopy (DRS). The reflectance spectra of the samples over a range of 200–700 nm were recorded by a Varian Cary 100 Scan UV-vis system equipped with a Labsphere diffuse reflectance accessory. The spectrophotometer was equipped with an integrating sphere, and BaSO4 was used as a reference (21). A given amount of TiO2 and Au/TiO2 powder was uniformly pressed in the tablet and placed in the sample holder on integrated sphere for the reflectance measurements. Photocatalytic activity: The photoreactor consisted of a quartz reactor with an effective volume of 75 ml. UV irradiation was performed by a 450 W medium pressure xenon lamp (Osram) placed inside a quartz jacket and equipped with a cooling tube. The lamp was switched on 30 min before the beginning of the reaction to stabilize the power of its emission spectrum line > 320 nm. Photocatalytic reactions were carried out suspending 0.5 g/l of TiO2 or Au/TiO2 nanocrystallines and oxygen under atmospheric pressure was bubbled through the reaction continuously. The suspensions were sonicated in the desired concentration of methanol [30 mM] before the experiment was started. Samples were withdrawn at regular intervals from the upper part of the reactor with the catalyst being removed from the liquid phase by filtration through 0.22 µm nylon syringe filters. The photooxidation rate was determined by measuring the formaldehyde production employing the Nash method (22). This method is based on the reaction of formaldehyde with acetylacetone and ammonium acetate to form a yellow coloured product with a maximum of absorbance at 412 nm, measured after an incubation time of 15 min at 60 °C. The photonic efficiency was calculated for each experiment as the ratio of the photocatalytic degradation rate and the incident light intensity as given in the following equation (23).



r 100 I

where ξ is the photonic efficiency (%), r the photooxidation rate of methanol (mol l-1s-1), and I the Incident photon flux (4.94x10-6 Ein l-1s-1). The UV-A incident photon flow, determined by ferrioxalate actinometry (24).

Results and Discussion For synthesis TiO2/Au nanocomposites with highly order crystalline, one crystal phase, excellent photon adsorption, intrinsic highly diffusion of reactant and efficiently photocatalytic active, we carried out coassembly of TiO2/Au using F127-TBOT-CH3COOH-HCl-Au

A. A. Ismail and D. W. Bahnemann 0.030 A (101) A u(111) A: A natas e Au: G old

A (213)

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-1

q(nm ) Figure 1. SAXS patterns of 2 wt% Au/TiO2 nanocomposites calcined at 500 °C for 4 hours whereas. Wave vector q is defined as (4π/λ)sin(2θ/2), where 2θ is he scattering angle. Diffraction signal is collected in transmission geometry with respect to the planar surface of the Au/TiO2 photocatalyst. Inset, WAXRD of as made 2 wt% Au/TiO2(a) and pure TiO2 (b) and 2 wt%Au/TiO2 nanocomposites(c).

mesophase. TBOT is highly reactive toward water due to coordination expansion and rapid condensation. Exposure of ethanolic solutions of TBOT to ambient conditions causes rapid particle growths 5 orders of magnitude faster than that of the silicon alkoxide system (25). Here, nanosized titanium-oxo-acetate-Au particles were formed rapidly in the acetic acid solution and the nanoparticles are quite stable and grow slowly due to the slow introduction of water from the ambient environment and the esterification of CH3COOH(25). In this preparation system, acetic acid plays a key role in adjusting the growth of inorganic species and formation of stable nanoparticles. The TiO2/Au nanocomposite gels were calcined at 500 °C for 4 hours. The novel TiO2/Au nanocomposites prepared were thoroughly characterized using XRD, TEM, SEM, FTIR and UV-vis diffuse as follows: XRD was used to investigate the phase stability and phase transformation. Both pure TiO2 and TiO2/Au nanocomposites have anatase 101 plane peaks at about 25.4° 2 θ , yet no rutile 100 plane peaks indicating that only the anatase crystalline phase is present (Figure 1 Inset) and the crystalline phase is confirmed using TEM. Using the Scherrer equation (eq. 1), particle size (t) can be estimated based on peak width (B). Given a shape factor (k) of 0.9, λ (wavelength of the CuK α 1 x-ray source) of 0.1541 nm, B (full peak width at half maximum corrected for instrumental broadening).

t

k B  cos 

eq.1

The Scherrer equation indicates that particle sizes for TiO2 nanocomposites were ~12±2 nm for both pure TiO2 and 2%Au/TiO2. For pure anatase phase, when the Au content increased, the crystal phases of anatase, was retained and no phase transformation occurred. However, the intensity of the peaks became stronger and well-resolved, indicating that larger particles were formed. In turn, SAXS indicated that the mesostructural regularity of the TiO2/Au nanocomposites was nearly collapsed entirely (26) because of the crystal growth during calcination under high temperatures up to 500 °C for 4 hours (Figure 1). In general, a large surface area, and highly crystalline anatase obtained at high temperature contributes high adsorptive capacity, resulting in high photocatalytic activity (See Figures 6 and 7). FTIR spectra for pure TiO2 and Au/TiO2 (Figure 2) demonstrates that, there is sharp peak at 3411 cm-1 due to OH species and the absorption peak at 1634 cm-1 belonging to the Ti-O structure. The findings revealed that FT-IR spectra of Au/TiO2 nanocomposites showed no change neither in intensity nor in position for the pure TiO2 bands. This indirectly indicates that Au is dissolved into the TiO2 lattices even at high concentration. TEM profile revealed that 2 wt%Au/TiO2 nanocomposites corresponding anatase TiO2 nanoparticles J. Adv. Oxid. Technol. Vol. 12, No. 1, 2009

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Absorbance

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Figure 2. FTIR spectra for pure TiO2 and 2%Au/TiO2 after calcinations at 500 °C for 4h. Figure 4. Energy-dispersive X-ray (EDX) analysis of 2%Au/TiO2 nanocomposites. Inset, the quantitative analysis of 2%Au/TiO2 nanocomposites.

Figure 3. TEM image of 2%Au/ TiO2 nanocomposites (a), SEM image of 2Au%/TiO2 calcined at 500 °C for 4 h (b). BF-STEM and DF-STEM images of 2Au%/TiO2( c&d). 3d inset for electron diffraction of 2Au%/TiO2 .

(Figure 3a), which mainly consisted of nanoscale cubes and rhombohedra (27). These nanoparticles are homogenously distribution and have an average size of 12 ± 2 nm. The crystalline sizes estimated from the TEM images are in good agreement with that calculated from XRD pattern by using the Scherrer equation. Bright and dark field – STEM (BF and DF- STEM) (Figures 3c and 3d) were detected Au nanoparticles. The average Au particle sizes are 20±5 nm. Figure 3c inset revealed that electron diffraction of 2%Au/TiO 2 nanocomposites and evidenced highly crystalline anatase phase. Scanning electron microscopy (SEM) image (Figure 3b) shows the morphology for mesoporous 2wt%Au/ TiO2 nanocomposite. The fine particulate morphology 12


indicates that the mesoporosity is probably partly due to the intraparticle porosity (28). Energy-dispersive Xray (EDX) analysis of 2 wt%Au/TiO2 nanocomposites clearly shows the characteristic peaks of titanium and oxygen (Figure 4). Also, the quantitative results of EDX clearly indicate that the weight percents of Ti and O are 45.25 and 54.75 wt%, respectively (table 1 inset in Figure 4). Interestingly, EDX did not observed Au in the 2 wt% Au/TiO2 nanocomposites. This might be attributed to Au homogenously distribution into the TiO2 network. Figure 5 clearly shows The UV-visible DRS spectra. The reflectance data was converted to the absorption coefficient F(R1) values according to the Kubelka–Munk (eq.2) (29). The modified KubelkaMunk function was determined using the equation as follows:  (1  R) 2    h   2R 

1

2

eq.2

where R is the proportion reflected, h is Planck’s constant, and ν is the frequency of light. Plots of this parameter were used to determine bandgap energy by determining a linear model for the linear portion of the absorption transition using least squares regression and extrapolating to zero at the corresponding photon energy. The bandgap energies of catalysts were calculated according to the equation bandgap EG = hc/λ, where EG is the bandgap energy (eV), h Planck’s constant, c the light velocity (m/s), and λ the wavelength (nm) (see Figure 5). The bandgaps of pure TiO2, 0.3, 0.5. 1, 2wt %Au/TiO2 nanocomposites were

A. A. Ismail and D. W. Bahnemann 3.2

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TiO2 0.3% Au/TiO2 0.5% Au/TiO2 1% Au/TiO2 2% Au/TiO2

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Figure 5. UV–vis Kubelka–Munk transformed diffuse reflectance spectra of the TiO2 and 0.3, 0.5, 1 and 2wt%Au/TiO2 nanocomposite calcined at 500 °C for 4 h.

amounted to be 3.16, 3.15, 3.15, 3.15 and 3.14 eV respectively (Figure 5). The findings revealed that there is no significant change in the bandgap value of pure TiO2 with addition varies Au content.

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Figure 6. Photooxidation of methanol over Degussa P25 and pure TiO2 and 0.3, 0.5, 1 and 2wt%Au/TiO2 nanocrystalline and photocatalysts for HCHO production as a function of illumination time. Photocatalyst loading, 0.5 g/L; 30 mM aqueous CH3OH (O2saturated, natural pH; T = 20 °C); reaction volume, 75 mL; Io = 4.49 x 10-6 Einstein L-1 s -1 (ca. >320 nm). 9 8 7

The photocatalytic activity of pure TiO2 and TiO2/Au nanocomposites was evaluated by measuring methanol photooxidation to produce HCHO at room temperature and comparison their performance with commercial P25 degussa.When photons with energy greater than the bandgap are absorbed by Au/TiO 2 nanoparticles, electrons are promoted from the valence band to the conduction band leaving holes in the valence band. The overall reaction mechanism for methanol photooxidation can be represented as follows (24, 33): TiO2 + hν  (TiO2 (eCb + hVb+) hVb+ + OH  •OH

(1) (2)

TiO2(eCb) + Au  TiO2 –Au(e)1b TiO2 –Au(e) + O2  TiO2 – Au + O2 • CH3OH + •OH  •CH2OH + H2O

(3) (4) (5)

CH2OH + O2  HCHO + HO2•

(6)

The experiments were carried out by mixing the known concentration of methanol [30 mM] and Au/ TiO2 nanoparticles, firstly, the reactions were conducted under dark condition, no reactions were observed even after 6 h, secondly there was no reaction using UV illuminated for 60 min without photocatalysts. It is evident that both pure TiO2 and TiO2/Au nanocomposites (Figures 6 and 7) are active photocatalysts; also they exhibit higher photoactivities as compared with commercial P25 degussa TiO2 (Figures 6 and 7).

Photonic efficiency, %

Photocatalytic Activity Measurements

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6 5 4 3 2 1 0 P25

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Figure 7. Photonic efficiency of methanol over commercial degussa P25 and pure TiO2 and 0.3, 0.5, 1 and 2 wt%Au/TiO2 nanocomposites for HCHO formation. Photocatalyst loading, 0.5 g/L; 30 mM aqueous CH3OH (O2- saturated, natural pH; illumination time, 60 min. T = 20 °C); reaction volume, 75 mL; Io = 4.49 x 10-6 Einstein L-1 s -1 (ca. >320 nm). The error bars represent one standard deviation.

The observed enhancement of photocatalytic activity, relative to a commercial P25 degussa, may be related to an increase in surface area(181 m2/g) as well as with a rise in anatase mass fraction and crystallinity, (30, 31) characteristic of our pure TiO2 and TiO2/Au nanocomposites. In general, TiO2/Au nanocomposites with nanoscale and mesopores offer significant promise as heterogeneous photocatalysts and photocatalytic activity has been found to be dependent on the crystallite size due to the surface reactivity changes with the particle size (32). In this section we carried out a series of J. Adv. Oxid. Technol. Vol. 12, No. 1, 2009

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experiments to study the influence of illumination time, photonic efficiency and photocatalyts loadings on photocatalytic oxidation of methanol to produce formaldehyde as follow: Figure 6 shows the change of the HCHO derivative concentration as a function of irradiation time in the Degussa P25, pure TiO2 and TiO2/Au at different Au concentration. A linear relation of the HCHO derivative concentration with irradiation time was obtained. From this figure the growth rate of HCHO derivative was found to be 2.1x10-7 M s-1 using degussa P25, but it increased from 2.68 to 4.16 1x10-7 M s-1 with increasing Au content from 0 to 1% and then decreased to 3.76x10-7 M s-1 at 2Au%. In turn, photonic efficiency of Degussa P25 for photooxidation methanol is 4.4% that increased to 5.43% using pure titania and then photonic efficiency was increased to 8.43% with increasing Au content from 0 to 1wt% then decreased to 7.62% with increasing Au content to 2wt% (Figure 7). As a result of Au small particle size, facilitate charge transport and reduce charge recombination, the large nanoparticles may act as the centers of electron-hole recombination (20) and reduce quantum efficiency. These factors may account for the observed decreasing photocatalytic activity at a higher Au content (e.g., 1wt%). In turn, the Au particles serve as an active site for HCHO production, on which the trapped photogenerated electrons are transferred to oxygen to produce O2.- radical . It has been reported that smaller gold particles induce more negative Quasi-Fermi level shift than the bigger particles (33). The negative shift in the Quasi-Fermi level is an indication of better charge separation and more reductive power for the photocatalyst. Thus, the catalyst with smaller gold nanoparticles is more photocatalytically active than that with larger gold particle. And also Au nanoparticles possess the property of storing electrons in a quantized fashion (34-35). Generally, the performance of the Au/TiO2 photocatalyst for methanol oxidation was crucially affected by the loading amount. Herein, we conducted an experimental work at 0.1, 0.3, 0.5, 0.75 and 1 g/L to explore the influence of 1 wt%Au/TiO2 nanocomposites loadings on methanol photooxidation (Figure 8). It is important to notice that HCHO formation was increased from 0.805 to 1.57 mM with increasing Au/TiO2 photocatalyst from 0.1 to 0.5 g/l respectively. The variation of the loading in a fairly wide range (from 0.5 to 1 g/l) does not alter the HCHO formation. The findings revealed that 0.5 g/l of Au/TiO2 photocatalyst was sufficient for oxidation of methanol to produce high amount from formaldehyde (Figure 8). It is known that a large surface area, small particle-size and porosity

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Figure 8. Effect of 1wt% Au/TiO2 photocatalyst loading on photooxidation of methanol for HCHO production (30 mM CH3OH, O2-saturated, natural pH; illumination time, 60 min. T = 20 °C, Io = 4.49 x 10-6 Einstein L-1 s -1 , λ > 320 nm). The error bars represent one standard deviation.

provide more surface sites and are advantageous to allow high adsorption of reactants molecules, making the photocatalytic process more efficient (36). Additionally, the transport of methanol through the interior space can be feasible because of the mesopores structure of the resulting photocatalysts. The porous structure facilitates the harvesting of solar light due to the enlarged surface area. These can also contribute to the higher photocatalytic activity of the TiO 2/Au calcined at 500 °C.

Acknowledgment A. A. Ismail acknowledges the Alexander von Humboldt (AvH) Foundation for granting a research fellowship.

Conclusions We have described a simple methodology for the synthesis of porous TiO2/Au nanocomposite from the combination TBOT solubilized in the acetic acid solution in presence of F127 polymer.TiO 2/Au nanocomposites obtained by the acetic acid system have small particle size, a high degree of homogeneity, and thermal stability up 500 °C. The TiO2 and Au nanoparticles are homogenously distribution and have an average size of 12 ± 2 and 20±5 nm respectively. The results indicated that, the TiO2/Au nanocomposites show good performance in methanol photooxidation due to their highly ordered crystalline, one crystal phase, small particle size and strong adsorptive capacity. 1wt% Au/TiO2 is more photoactive than pure TiO2 and P25 degussa for the oxidation of methanol to formaldehyde. TiO2/Au nanocomposites can be formed as membranes, which may be easy to handle and considered as promising candidates for useful photocatalysts.


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