Chemically-Synthesised, Atomically-Precise Gold Clusters Deposited and Activated on Titania - Term Paper Example

Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles Name: Institution: Date: INTRODUCTION 1.0 Background of the Project A considerable attention has been drawn towards clusters of noble metal supported on metal oxides due to their wide applications in catalysis. Illumination of semiconductor particles in a solution of metal salt in aqueous phase leads to deposition of nanoparticles of metal oxide on the surface of the semi-conductor material. This phenomenon is what is described as photodeposition. When gold is supported as ultrafine particles and dispersed on TiO2, it can catalyze significant industrial reactions, for instance, the reaction involving partial oxidation of propylene and CO (Heiz & Landman, 2007). The catalysis and selectivity of Au clusters are largely influenced by the size, shape and geometrical distribution of Au particles on the support. This means that the methods used in the preparation of the materials is fundamental to the properties of the Au particles. Gold is chemically inert in its bulk form. However, when deposited as dispersed particles on a support of a metal oxide of a transition metal, its activity is improved (Primo, et al., 2011). Clusters reveal unique chemical and physical properties that are not scalable from the bulk material as they have nanoscale dimensions. Thus, clusters form the building blocks of nanocatalytic materials. Au nanoparticles find a wide range of potential applications – from bio-labeling, catalysis and electron transfer processes. This project investigates the photodeposition of Au on TiO2. TiO2 Semi-conductor Material Many semi-conductor materials are candidates for photocatalysis. An ideal photocatalytic material needs to meet the requirements listed below: A small band-gap Capable of providing enough active sites A suitable location of the edges of the valence band and conduction compared to the oxidation and reduction potentials that are related to photocatalytic reactions. Efficient electron/hole separation, leading to a lower rate of charge carrier recombination. Low production costs. Stability Non-toxicity Durability under irradiation High photo-corrossion resistance TiO2 meets most of these requirements, which explains why it is the most studied photocatalyst. However, it has a relatively large bandgap under UV light, making photocatalysis possible to a wavelength of about 413 nm (Tada, et al., 2011). The wide band gap of the material depends on the size and crystal phase of TiO2. When the material absorbs UV light at a wavelength greater than that of the band gap, electrons and holes are photo-generated and get trapped at the material surface and take part in redox reactions. Figure 1: Bandgap of TiO2 The wide bandgap limits the fraction of solar spectrum that can be harvested using TiO2. There are two ways to solve this; doping the TiO2 or using an alternative material, such as tungsten trioxide. The balance of the above requirements has lead to extensive efforts to use TiO2 while increasing the bandgap of the semi-conductor through doping with C, F,S or N. It is a very robust material with excellent photochemical stability (Wenderich, 2016). It is for this reason and its availability, lack of toxicity and affordability that TiO2 is by far the most convinient photocatalytic material. Of the three different crystalline phases in which this semiconductor material exists, the most active phase is anatase, while the rutile and amorphous phases being less active. Brookite is another phase of TiO2 that is photocatalytically inactive. TiO2 can be typically synthesized by sol-gel to amorphous phase which through calcination, it is transformed into anatase at moderate temperatures (Wenderich & Mul, 2016). At temperatures of about 500 oC a transformation from anatase to rutile phase is initiated. This can be transformed to the brookite phase at much elevated temperatures. Due to titanium oxide’s optical and electrical properties, it can be utilized in a number of fields; from solar cells to photocatalysts, biosensors and self-cleaning. Photodeposition of Au nanoparticles on TiO2 TiO2 has the highest photocatalytic activity among the semi-conductor materials, but it requires irridiation of shorter wavelength than the onset of the absorption band – which is approximately 350 nm. The photocatalytic activity of titenium oxide in its active anatase phase is evident with UV light. Therefore, there is need to expand the photoresponse of the semi-conductor to wider wavelengths because solar light corresponds to most visible region. Au/TiO2 material is the modification of TiO2 surface in the active anatase phase by depositing Au nanoparticles (Anderson, et al., 2013). The metal modification does not become part of the solid framework, but is rather in a distinct phase in interfacial contact between the metal and the semiconductor. Doping TiO2 with Au nanoparticles supported on the semiconductor is an approach that has been widely utilized to increase the photocatalytic response of TiO2 in the visible light region. Au is suitably used because of its property of chemical inertness towards heterogeneous catalytic photo-oxidation (Bagheri, et al., 2014). Au cluster as a cocatalyst Nanosized Au particles work as an efficient co-catalyst for photocatalytic water splitting by enhancing the evolution of H2. The activity of photocatalytic metal oxides can be greatly improved when co-catalytic nanoparticles are deposited onto the photocatalyst. This is attributed to the fact that the co-catalyst can act as a beacon for charge carrier, and it can also reduce the activation energy required for photocatalytic reactions (Chusuei, et al., 2001). Initially as the amount of co-catalyst deposited increases the photocatalytic activity increases, up to an optimum, and further loading decreases the photocatalytic activity. Au Nano-clusters for Solar Energy Production A more interesting pathway of solar energy production is the conversion of solar power to hydrocarbon fuels. Advantages of hydrocarbon fuels include high energy density, existing global infrastructure, and stability. This process involves capturing H2O and CO2, and using solar energy to dissociate these compounds into CO and H2 and finally recombine them into a hydrogen fuel, such as methanol. Another pathway that has been extensively investigated for solar fuel is the electrochemical water splitting which has shown possibility of efficiencies of up to 12% in hydrogen-production set-up (Bagheri, et al., 2014). Semi-conductor-based photocatalysis has become the most promising pathway for large scale photocatalysis. TiO2 semi-conductor has shown the capability to split water into O2 and H2. Under UV irradiation. This is possible due to the semi-conductor nature of the material that causes incident UV light to excite electrons from valence band with higher energy states to the conduction band across the band gap. This leads to charge migration and separation which interacts with the molecules adsorbed on reaching the surface (Anderson, et al., 2013). 1.2 Motivation Nanoparticles of metal oxide deposited on the surface of a semiconductor material by photodeposition find a lot of applications in photocatalytic synthesis of solar fuel, air purification and treatment of waste water. Metal nanoparticles in these applications can significantly enhance the performance of semi-conductor materials in reactions that are light-stimulated, including improving their stability. In the case of Au, it provides stimulated light absorption through the effects of plasmonic field (Tada, et al., 2011). Today, the world is faced with several challenges to continue providing high level human wealth. For instance, the demands for energy are always increasing as the world reserves of fossil fuels are running out. This requires an alternative energy source to sustain the energy requirements. Clean energy options such as wind and hydraulic energy are on the rise. However, the energy produced from these sources cannot fully cover the energy demand. Solar energy has the potential to become a reliable source of energy in the world. A major challenge of solar energy is its storage for later use. Currently, solar energy has to be used directly or stored indirectly in capacitors and batteries. Direct storage of the energy is a promising development to overcome drawbacks from photo-voltaic (Wenderich & Mul, 2016). Hence, photocatalysis provides an alternative outcome, where the energy generated by solar is stored by synthesis of a fuel. In photocatalysis, solar energy can be stored directly in hydrogen through splitting of water molecules, or as hydrocarbons when CO2 is reduced. Photocatalysis is also the basis of degradation of compounds that are harmful in the gas phase. Photodeposition of Au over TiO2 is used as a means of improving the photocatalytic activity of the material. Heterogeneous photocatalysis by titanium oxide is an effective way of degrading a number of organic pollutants found in air and waste water. Photocatalytic oxidation can be used to degrade volatile organic compounds (VOCs) found in indoor areas that would otherwise cause symptoms associated with the sick building syndrome, such as headaches. Photocatalysis can also provide a solution for clean drinking water, where photocatalytic materials are utilized in degradation of harmful substances such as bacteria, metal ions and dyes in water (Tada, et al., 2011). This provides a promising technology for cleaning the environment. Another advantage of photocatalysis is the ability to obtain high selectivities via a green process. 1.3 Expected Results The procedure used to prepare the Au-TiO2 determines the catalytic activity of the material. There are several methods that can be used to prepare TiO2 supported Au nanoparticles. It is expected that the photodeposition method will produce nanoparticles with excellent photocatalytic activity. This efficient photocatalytic activity is required for generation of hydrogen, decomposition of phenol, carboxylic acid degradation, dye decolonization, among other processes. Parameters such as particle size, Au loading, spatial structuring, surface area, and other factors are expected to play a significant role in the activity of the photocatalyst. References Read More

Figure 1: Bandgap of TiO2 The wide bandgap limits the fraction of solar spectrum that can be harvested using TiO2. There are two ways to solve this; doping the TiO2 or using an alternative material, such as tungsten trioxide. The balance of the above requirements has lead to extensive efforts to use TiO2 while increasing the bandgap of the semi-conductor through doping with C, F,S or N. It is a very robust material with excellent photochemical stability (Wenderich, 2016). It is for this reason and its availability, lack of toxicity and affordability that TiO2 is by far the most convinient photocatalytic material.

Of the three different crystalline phases in which this semiconductor material exists, the most active phase is anatase, while the rutile and amorphous phases being less active. Brookite is another phase of TiO2 that is photocatalytically inactive. TiO2 can be typically synthesized by sol-gel to amorphous phase which through calcination, it is transformed into anatase at moderate temperatures (Wenderich & Mul, 2016). At temperatures of about 500 oC a transformation from anatase to rutile phase is initiated.

This can be transformed to the brookite phase at much elevated temperatures. Due to titanium oxide’s optical and electrical properties, it can be utilized in a number of fields; from solar cells to photocatalysts, biosensors and self-cleaning. Photodeposition of Au nanoparticles on TiO2 TiO2 has the highest photocatalytic activity among the semi-conductor materials, but it requires irridiation of shorter wavelength than the onset of the absorption band – which is approximately 350 nm. The photocatalytic activity of titenium oxide in its active anatase phase is evident with UV light.

Therefore, there is need to expand the photoresponse of the semi-conductor to wider wavelengths because solar light corresponds to most visible region. Au/TiO2 material is the modification of TiO2 surface in the active anatase phase by depositing Au nanoparticles (Anderson, et al., 2013). The metal modification does not become part of the solid framework, but is rather in a distinct phase in interfacial contact between the metal and the semiconductor. Doping TiO2 with Au nanoparticles supported on the semiconductor is an approach that has been widely utilized to increase the photocatalytic response of TiO2 in the visible light region.

Au is suitably used because of its property of chemical inertness towards heterogeneous catalytic photo-oxidation (Bagheri, et al., 2014). Au cluster as a cocatalyst Nanosized Au particles work as an efficient co-catalyst for photocatalytic water splitting by enhancing the evolution of H2. The activity of photocatalytic metal oxides can be greatly improved when co-catalytic nanoparticles are deposited onto the photocatalyst. This is attributed to the fact that the co-catalyst can act as a beacon for charge carrier, and it can also reduce the activation energy required for photocatalytic reactions (Chusuei, et al., 2001). Initially as the amount of co-catalyst deposited increases the photocatalytic activity increases, up to an optimum, and further loading decreases the photocatalytic activity.

Au Nano-clusters for Solar Energy Production A more interesting pathway of solar energy production is the conversion of solar power to hydrocarbon fuels. Advantages of hydrocarbon fuels include high energy density, existing global infrastructure, and stability. This process involves capturing H2O and CO2, and using solar energy to dissociate these compounds into CO and H2 and finally recombine them into a hydrogen fuel, such as methanol. Another pathway that has been extensively investigated for solar fuel is the electrochemical water splitting which has shown possibility of efficiencies of up to 12% in hydrogen-production set-up (Bagheri, et al., 2014). Semi-conductor-based photocatalysis has become the most promising pathway for large scale photocatalysis.

TiO2 semi-conductor has shown the capability to split water into O2 and H2.

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