Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles - Research Paper Example

Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles Results and Discussions In this research, we used two types of gold; (i) Chloroauric acid [HAuCl4], and (ii) Au clusters [Au9 (PPh3)8)] (NO3)3. We used UV light and solar simulator as the sources of illumination to irradiate the samples. The HAuCl4 was pre-irradiated for 6 minutes before putting the sample into the solution. There were two techniques that were used to irradiate the samples. The first technique was the “pulsed” technique – 4 x 5 seconds exposure to light with a 30-secont gap between pulses. Therefore, with this technique, the overall exposure time was 20 seconds. The second technique was the “continuous” technique in which the samples were exposed to light for 20 seconds. We also varied the duration of illumination and the concentration of samples to study the effect of these variables on the photo-deposition of gold on titanium dioxide. The results of the research are presented and discussed in the following sections. i) XPS Results for Chloroauric acid [HAuCl4] a) UV as a source of irradiation Figure 1: XPS Results of Chloroauric acid [HAuCl4] under UV source Effect of Pre-irradiation of HAuCl4 Solution Using UV as a source of illumination Photolysis of AuCl4- produces AuCl3- and a radical chlorine species that is highly reactive. The AuCl3- disproportionate rapidly to regenerate AuCl4- and produce AuCl2-, which is a metastable intermediate that can further undergo slow disproportionation to give Au0. The AuCl2- can readily be reduced at the surface of TiO2, thus activating the HAuCl4 salt solution before photodeposition [18]. Furthermore, the free Cl radical can abstract hydrogen from alcohols or organics to form a very powerful reducing species that can generate Au NPs. Preirradiation of the HAuCl4 solution for 6 minutes increases the amount of Au that is deposited onto the TiO2 for both continuous and pulsed techniques. The non preirradiated solution generated nanoclusters of Au at a binging energy of 85.0 eV. Preirradiation increases the amount of Au deposited on TiO2 because it leads to increase in the rate of Au photodeposition onto the surface of TiO2 as the Au salt is activated prior to deposition on TiO2. Effect of Irradiation Technique The irradiation techniques of light exposure also seemed to play a significant role in determining the final formation of the Au-TiO2 hybrid product. When continuous technique is compared to the pulsed technique, the rate at which Au NPs are deposited on titanium dioxide in the pulsed technique is higher than in the continuous technique. The likely reason for this is that during the continuous irradiation, there is accumulation of photoexcited electrons, which generate Au clusters [18]. On the other hand, during a pulsed irradiation, there is discharge of electrons between the pulses which generates Au NPs [20]. b) Solar simulator as a source of illumination Figure 2: XPS Results of Chloroauric acid [HAuCl4] under solar simulator The effect of pre-irradiation From figure 2, irradiating pre-irradiated HAuCl4 solution increases the amount of Au deposited on the TiO2 in both continuous and pulsed techniques. The non preirradiation generate Au NPs at a binding energy of about 85.0 eV. Pre-irradiation increases the amount of Au NPs deposited on TiO2 because the Au salt is activated before heterogenous deposition onto TiO2. The effect of irradiation technique Comparing between the two irradiation techniques, there was almost equal amount of Au NPs deposited in both continuous and pulsed techniques. In the non preirradiation, the amount of Au nanoclusters is a slightly higher in the pulsed technique than in the continuous technique. Comparing between solar simulator result and UV light. In both solar simulator and UV, Au NPs are generated at binding energy of 85.0 eV. However, the UV photodeposition process occurred much faster than in the solar simulator. This is because the absorption spectrum of TiO2 is largely found in the UV region. A solar simulator produces a near natural light of the sun with a typical spectrum which contains 4% of UV [19]. Using UV light in the photo-excitation process provides a specific wavelength of 370nm which activates the metal oxide faster to form Au NPs [18]. c) Varying Duration of UV illumination In this part, we used UV light for non-preirradiated sample and continuous irradiation technique with varying durations of UV exposure. Figure 3: XPS Results of Chloroauric acid [HAuCl4] under varying duration of UV illumination from 10-50 s From figure 3, it is observed that increasing the time of exposure increases the rate of Au deposition on TiO2. The relative intensity of HBP decreases for the non-irradiated sample within the first 20seconds, while LBP is observed to increase within the same duration. As the duration of irradiation increases, the Au clusters start to grow and form larger particles [17]. Longer exposure times generate Au nanoclusters on TiO2 faster because the conditions lead to accumulation of electrons at the deposition sites of Au NPs[18]. d) Varying concentrations of HAuCl4 salt solution In this part, we used UV light with non-preirradiated solution and continuous technique for a duration of 10 seconds, with varying concentrations of HAuCl4. Figure 4: XPS Results of Chloroauric acid [HAuCl4] under UV illumination in varying concentrations From figure 4, the relative intensity of HBP seems to change slightly with varying concentrations. For the LBP, it is observed that the relative intensity is increasing with increasing concentration. This is attributed to the fact that at higher concentrations, more high-energy sites on which Au NPs photodeposit become available, than in lower concentrations [18]. ii) XPS results for Au9 For this type of gold sample, we used the UV light only with non-preirradiation and continuous illumination technique. a) Different concentrations of Au9 with different irradiation time Figure 5: XPS for different concentrations of Au9 under different durations of illumination From figure 5, it can be observed that at a concentration of 0.1mM and 0.01mM, the relative intensity of Au9 decreases as the duration of irradiation increases. Clusters begin to grow to form larger particles (agglomeration) as irradiation time increases. The binding energy also shifts from 85 eV to 84 eV. At a concentration level of 0.001mM, the relative intensity of Au9 increases as the duration of irradiation increases. The binding energy remains relatively stable at about 85 eV and the Au NPs form a cluster on the surface. b) Annealing Annealing of theAu9 samples was carried out by heating the samples at 200oC for 10 minutes to remove the phosphine (PPh3) ligands. The results before and after heating are presented in figures 6 to 8. After annealing the samples, the PPh3 ligands which protect the Au9 clusters are lost and two things may occur: either the Au9 cluster cores maintain their identity; or the cluster cores merge to form larger particles (see figure 9). [16] observes that heating of clusters at 200oC produces two effects. The first effect is the formation of Au particles that are slightly larger in size from the aggregation of ultra-small clusters. The second effect is that the Au clusters exhibit Au-O bonds (the oxygen is likely to come from TiO2). The XPS measurements indicate that there is partial agglomeration leading to formation of larger particles [21]. As the ligands are removed, the core of the Au9 nanoclusters become in a closer contact with the TiO2 substrate. The binding energy as found by XPS is dependent on the initial state (chemical shift) and the final state effects which relate to the size of the Au cluster [22]. [A] 0.1mM Au9 10s and 20s before and after heat treatment Figure: 6(a): 0.1mM Au9 before heat treatment Figure: 6(b): 0.1mM Au9 after heat treatment Before heating, there is high LBP relative intensity of Au clusters for the first 10 seconds (see figure 6(b)). The LBP position is around 85 eV before heating, and after heating, the LBP position is around 84 eV. Thus, heat treatment results in a decrease in HBP binding energy. The downward shift in the low binding energy peaks is attributed to the fact that Au becomes partially agglomerated after heating to form larger aggregates. Agglomeration of Au clusters affects its signal intensity, and thus, its peak position [23]. [B] 0.01mM Au9 10s and 20s before and after heat treatment The samples were heated at 200oC for 10 minutes as in [A] above. The results are shown in figure 7(a) and 7(b). Figure 7(a): 0.01mM Au9 before heat treatment Figure 7(b): 0.01mM Au9 after heat treatment Before heating, Au clusters are obtained at binding energy of 85.27 eV within the first 10 seconds. After heating from the LBP, Au clusters partially agglomerate at binding energy of 84.61eV after 10 seconds. However, the agglomeration is less compared to the 0.1mM concentration. [C] 0.001mM Au9 10s and 20s before and after heat treatment The samples were heated as earlier described to remove the ligands. Figure 8(a): 0.001mM Au9 before heat treatment Figure 8(b): 0.001mM Au9 after heat treatment Before heating, the Au clusters are obtained at the LBP of 85.66 eV after 10 seconds. After heat treatment, the LBP is at 85.14eV and the Au clusters are non-agglomerated. Figure 9: A schematic diagram of Au nanoclusters deposited on TiO2 before and after annealing (Source: [21]). It is observed that the higher the concentration of Au9, the higher the degree of particle agglomeration. At lower concentrations, the degree of agglomeration reduces. Heating of the Au9 samples at 200oC leads to partial agglomeration of gold nanoclusters which cause the formation of larger particles of Au. Binding peaks with binding energies in the range of 84-84.5 eV can be attributed to Au clusters with small aggregates. The agglomeration of the Au clusters also plays a role on the intensity of the signal obtained. Because there is limited electron mean free path, electrons that originate from larger clusters tend to be attenuated more compared to those emitted from smaller size clusters [23]. Conclusions The photodeposition of Au on TiO2 substrate was investigated using the XPS technique to observe process conditions, Relative intensity of Au Nps and the binding energy of the particles. We investigated the effect of source of illumination, preirradiation, the technique of illumination, the duration of UV illumination, the concentration and the effect due to heat treatment of the samples. Preirriadation of the samples increases the rate of photodeposition of Au NPS on the surface of titenia. Using the pulsed technique, the rate of Au NPs photodeposition on the titenia is higher than in the continuous technique. It was also observed that increasing the time of exposure to the UV light and the concentration of Au solution increases the rate of Au deposition on TiO2. For Au9 with different concentrations, the relative intensity of Au9 decreases with increase in concentration and the low binding-energy peak shifts from 85 eV to 84 eV. After heat treatment, part of the Au9 NPs undergo partial agglomeration. References 16. Anderson, D. P. et al., 2013. Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II. Phys. Chem. Chem. Phys., Volume 15, pp. 14806--14813. 17. Chong, S. & Yang, T. C.-K., 2017. Illumination wavelength and time dependent nano gold photo-deposition and CO oxidation. Results in Physics, Volume 7, p. 1167–1174. 18. Fernando, J. F. S. et al., 2016. Controlling Au Photodeposition on Large ZnO Nanoparticles. Applied Materials and Interfaces, Volume 8, p. 14271−14283. 19. Nguyen, B. H. & Nguyen, V. H., 2015. Recent advances in research on plasmonic enhancement of photocatalysis. Advances in Natural Sciences: Nanoscience and Nanotechnology, Volume 6, pp. 1-17. 20. Primo, A., Cormaa, A. & Garcíaa, H., 2011. Titania supported gold nanoparticles as photocatalyst. Phys. Chem. Chem. Phys., Volume 13, pp. 886-910. 21. Qahtani, H. S. A. et al., 2016. Grouping and aggregation of ligand protected Au9 clusters on TiO2 nanosheets. RSC Adv, Volume 6, p. 110765–110774. 22. Qahtani, H. S. A. et al., 2017. Aggregation Behavior of Ligand-Protected Au9 Clusters on Sputtered Atomic Layer Deposition TiO2. Journal of Physical Chemistry, pp. A-I. 23. Ruzicka, J.-Y.et al., 2015. Toward Control of Gold Cluster Aggregation on TiO2 via Surface Treatments. Journal of Physical Chemistry, Volume 119, p. 24465−24474. Read More

The likely reason for this is that during the continuous irradiation, there is accumulation of photoexcited electrons, which generate Au clusters [18]. On the other hand, during a pulsed irradiation, there is discharge of electrons between the pulses which generates Au NPs [20]. b) Solar simulator as a source of illumination Figure 2: XPS Results of Chloroauric acid [HAuCl4] under solar simulator The effect of pre-irradiation From figure 2, irradiating pre-irradiated HAuCl4 solution increases the amount of Au deposited on the TiO2 in both continuous and pulsed techniques.

The non preirradiation generate Au NPs at a binding energy of about 85.0 eV. Pre-irradiation increases the amount of Au NPs deposited on TiO2 because the Au salt is activated before heterogenous deposition onto TiO2. The effect of irradiation technique Comparing between the two irradiation techniques, there was almost equal amount of Au NPs deposited in both continuous and pulsed techniques. In the non preirradiation, the amount of Au nanoclusters is a slightly higher in the pulsed technique than in the continuous technique.

Comparing between solar simulator result and UV light. In both solar simulator and UV, Au NPs are generated at binding energy of 85.0 eV. However, the UV photodeposition process occurred much faster than in the solar simulator. This is because the absorption spectrum of TiO2 is largely found in the UV region. A solar simulator produces a near natural light of the sun with a typical spectrum which contains 4% of UV [19]. Using UV light in the photo-excitation process provides a specific wavelength of 370nm which activates the metal oxide faster to form Au NPs [18]. c) Varying Duration of UV illumination In this part, we used UV light for non-preirradiated sample and continuous irradiation technique with varying durations of UV exposure.

Figure 3: XPS Results of Chloroauric acid [HAuCl4] under varying duration of UV illumination from 10-50 s From figure 3, it is observed that increasing the time of exposure increases the rate of Au deposition on TiO2. The relative intensity of HBP decreases for the non-irradiated sample within the first 20seconds, while LBP is observed to increase within the same duration. As the duration of irradiation increases, the Au clusters start to grow and form larger particles [17]. Longer exposure times generate Au nanoclusters on TiO2 faster because the conditions lead to accumulation of electrons at the deposition sites of Au NPs[18]. d) Varying concentrations of HAuCl4 salt solution In this part, we used UV light with non-preirradiated solution and continuous technique for a duration of 10 seconds, with varying concentrations of HAuCl4.

Figure 4: XPS Results of Chloroauric acid [HAuCl4] under UV illumination in varying concentrations From figure 4, the relative intensity of HBP seems to change slightly with varying concentrations. For the LBP, it is observed that the relative intensity is increasing with increasing concentration. This is attributed to the fact that at higher concentrations, more high-energy sites on which Au NPs photodeposit become available, than in lower concentrations [18]. ii) XPS results for Au9 For this type of gold sample, we used the UV light only with non-preirradiation and continuous illumination technique. a) Different concentrations of Au9 with different irradiation time Figure 5: XPS for different concentrations of Au9 under different durations of illumination From figure 5, it can be observed that at a concentration of 0.1mM and 0.01mM, the relative intensity of Au9 decreases as the duration of irradiation increases.

Clusters begin to grow to form larger particles (agglomeration) as irradiation time increases. The binding energy also shifts from 85 eV to 84 eV. At a concentration level of 0.001mM, the relative intensity of Au9 increases as the duration of irradiation increases. The binding energy remains relatively stable at about 85 eV and the Au NPs form a cluster on the surface.

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