Detection of Photodeposition of Gold on Titanium Dioxide Nanoparticles - Term Paper Example

Investigation of Photodeposition of Gold on Titanium Dioxide Nanoparticles Student name/number: Institution: Date: 1.0 INTRODUCTION Background to the project In the recent years, nanoparticles and nanoclusters of noble metals supported on metal oxides have drawn a lot more interest due to their wide applications in heterogeneous catalysis [5]. According to Chusuei, et al. (2001), Gold (Au) in particular nanoclusters supported on Titenium dioxide (TiO2) has the capability to catalyze some industrial reactions, such as partial oxidation of propylene to form propylene oxide and oxidation of carbon monoxide (CO). Au is chemically inert in tis bulk form, but when deposited as nanoclusters on an oxide of a transition metal, its catalytic activity is greatly improved. Thus, nanoclusters reveal unique chemical and physical properties when they are dimensionally reduced to the nanoscale. These unique properties are not scalable from the bulk material [7]. Catalytic activity is strongly dependent on the size of the nanoclusters on the support [5]. Metal nanoparticles supported on a semiconductor surface of a metal oxide find applications in industrial photocatalysis processes, photocatalytic synthesis of solar fuel, purification, medicine, surface technology and waste water treatment [14]. According to Wenderich & Mul (2016), nanoparticles can significantly enhance the performance and stability of semiconductors in reactions that are stimulated by absorption of light energy. With sutable loading, photocatalysts can function as (i) charge-carriers upon photoexcitation, implicating electron/hole recombination; (ii) active sties for reactions involving transfer of charges; and (iii) stimulated light absorption, particularly for gold and silvey. In this project, we will investigate the photodeposition of Au on TiO2 nanoparticles to make an attempt to observe process condtions, particle size distribution, photocatalytic activities, oxidation states of the metals formed, among other observations. 1.1 Photodeposition Photodeposition is a method used in the preparation of photocatalysts and metal-supported catalysts. It is a phenomena where illumination of a slurry of particles of a semiconductor in an aqueous phase solution leads to deposition of well-defined nanoparticles of metal oxide on the surface of the semiconductor [14]. Generalized equations of reductive and oxidative photodeposition for a metal M is as follows: Oxidative photodeposition: M n+ (aq) + ne- M(s) Reductive Photodeposition: M n+ (aq) + nh++ nH2O MOn(s) + 2nH+ [14]. Thus, photodeposition is based on light-induced electro-chemistry and involves oxidative and reductive photodeposition as shown in figure 1 below. Figure 1: Schematic illustration of (a) reductive and (b) oxidative photodeposition [14]. CB – conduction band; D – electron donor, M – metal; VB – valence band; A – electron acceptor; VB – valence band; – number of electron-holes. The irradiation of metal oxide powder with UV light causes reduction of metal cations with appropriate redox potential from the photo-excited electrons. This creates metal particles on the surface of the metal oxide. The way in which adsorption takes place and the efficiency of charge separation in turn influence the photocatalytic activity of the material [11]. 1.2 Photocatalysis Photocatalysis is the process in which light energy is converted into chemical energy. For this process to occur, a material that can provide a temporary state decay to form some chemical species when irradiated with light is required. Semiconductor metal oxides, such as ZnO and TiO2 are typically used as photocatalysts [12]. Upon light absorption, charge separation occurs with the formation of an electron in the conduction band and an electron hole in the valence band [12]. The pair of electron-hole created may collapse or separate to form independent pairs of electron and holes and allow the transfer of charge between the electron-hole pairs and the reactants on the surface of the TiO2 semiconductor [2]. At this stage, photo-excitaion takes place . In TiO2, electrons diffuse faster in the conduction band while hole-migration occurs by charge-jumping from neighbouring sites. In heterogenous photocatalysis, photo-induced reactions occur at the surface of the catalyst [9]. Titenium dioxide has been widely used as a photocatalyst in a series of oxidative and reductive reactions on the surface of the semiconductor. This is largely contributed by the existence of lone electron in its outer orbital [4]. Electron and holes that fail to be annihilated move to the particle surface where they can react with other chemical species [12]. UV light is commonly used in the photo-excitation process to reduce the metal onto the surface of the metal oxide, activating the metal oxide. UV light excites valence band electrons to the conduction band of the semiconductor, creating holes in the valence band. Figure 2 shows a schamatic diagram of this process and the reactions involved. Figure 2: A schamatic diagram of the photo-excitation process of TiO2 as a catalyst [15]. O – oxidation; R-reduction. The process of TiO2 activation under UV light can be represented by the equation: TiO2 + hv h+ + e- In this reaction, h+ is is a powerful oxidizing agent, while e- is a powerful reducing agent [6]. The oxidative and reactiuve reactions are as follows: OH- + h+ OH (Oxidative reaction) O2ads + e- (Reductive reaction) 1.3 Solar Energy In the last few decades, more attention has been focused on the photocatalytic activity of TiO2 semiconductor with a strong redox power under photoinduction. The current challenge in photocatalysis is to develop a photocatalyst that enables efficient utilization of sunlight as a source of energy[6]. A study done by Fujishima and Honda attempted to convert solar energy to chemical energy on the surface of TiO2 semiconductors. During the process of photocatalytic water splitting, water dissociates upon illumination by solar energy, and the flat band potential of TiO2 semiconductor shifts upwards. In this simulation, the active species are the holes which are able to oxidize OH- to hydrogen peroxide (H2O2) and oxygen, while hydrogen is produced on the counterelectrode. However, only 10% of solar energy is absorbed, making it impractical to use TiO2 in commercial solar energy converters [6]. In addition to this, TiO2 has a low capability to reduce hydrogen, and it requires a small voltage to help the photolytic process. Due to the poor efficiency of TiO2, studies are underway to obtain materials which have a smaller band gap e.g. InP catalyzed by Pt, GaAs catalyzed with Ru etc. A schematic diagram of a photochemical photocell is shown in figure 3. Figure 3: Principle of an electrochemical photocell. 1: n-type TiO2 electrode; 2: platinum counter electrode; 3: ionically conducting separator; 4: gas buret; 5: load resistance; 6: Voltmeter [6]. 1.4 Catalytic Activity of Gold Gold in its bulk nature is an inert material, but Au nanoparticles in the size range of 3-10 nm in diameter have been found to be catalatically active for a number of chemical reactions [10]. Several explanationations have been proposed for the catalytic activeness of Au nanosize particles, including the oxidation state of Au, quantum size effects, charge transfer or support-induced strain, and the presence of very low-cordinated atoms of Au in nanosize particles [3]. There is a likelihood that the above effects occur simultenously. Au9 clusters supported on titania surface have centered polyhedral geometries which allow Au-O bonds, more likely on the oxygen of titenia [1]. Gold Nanoparticles as co-catalytic  Au nanoparticles supported on a metal oxide of a transition metal provide a catalytically active material. The use of nano size particles provides a laerger contact area between the active catalyst and the reacting materials. This ensures that the catalyst is effectively and efficiently used to facilitate a reaction process [3]. Gold Nanoclusters as co-catalytic In recent findings, researchers have shown that the reactivity of nanosize particles less than 1 nm is even more dramatic. Nanoparticles of this size are considered as nanoclusters and is established for nanoclusters prepared both through ultra-high vacuum (UHV) conditions and by chemical synthesis [1]. One atom less or more in a cluster of nanoparticles can completely alter the catalytic activity of nano-cluster material [8]. According to Anderson, et al. (2013), the smaller nanoclusters of Au tend to exhibit higher binding energy shifts, and the binding energy decreases with increasing cluster size before eventually converging to the binding energy value of the bulk material. Thus, the size of nanoclusters on the support strongly influence the catalytic activity as well as selectivity of Au clusters [5]. Figure 3: An overview of the crystal structures of Au8, Au9, and Au11 arranged from the left column to the right column respectively [1]. 1.5 TiO2 as Photocatalyst material Titenium dioxide is widely applied as a photocatalyst due to its excellent properties: (i) it has high catalytic efficiency, very stable, durable, safe and inexpensive; (ii) does not require chemical additives; (iii) promotes oxidation of pollutants at ambient temperature; and (iv) can completely degrade a wide range of pollutants at certain operation conditions [15]. Durability and stability under irradiation is one of the prerequisutes of a photocatalyst. For this reason, chalcogenides and metal suphide are not used as photocatalysts even thought these materials exhibit a high catalytic activity [12]. They also tend to exhibit photocorrosion and dissolve under irradiation in aquous phase [12]. Some reserachers have demonstrated that TiO2 film has a higher catalytic activity compared to the most active TiO2 powder [15]. Furthermore, TiO2 is very robust with high photochemical activity, available and affordable, and does not have toxicity, thus making it the most used photocatalytic material. Titenium dioxide has two crystal phases: rutile and anatase. Anatase is more superior to rutile for photocatalytic application [15]. This is because the location of the conduction band of anatase is more suitable for activating conjugate reactions that involve electrons. Furthermore, during a photo-oxidation reaction, the surface peroxide groups formed at anatase are very stable as opposed to those formed at the surface of rutile [15]. 1.6 The motivation to perform the research work The motivation behind undertaking this research work is the fact that nanoclusters have emerged and labeled as superior catalysts that are utilized in a many applications. However, there has been little understanding about fabrication and activation of these catalysts by photodeposition, particularly Au clusters supported on the surface of TiO2 which has found a wide range of applications than other inert supports. 1.7 Expected Achievement It is expected that by undertaking this research study, the researchers and other people who may be interested will have a better understanding of fabrication and activation of Au clusters supported on titanium dioxide. The overall achievement will be gain of understanding of the interesting field of nanotechnology. References Read More

1.2 Photocatalysis Photocatalysis is the process in which light energy is converted into chemical energy. For this process to occur, a material that can provide a temporary state decay to form some chemical species when irradiated with light is required. Semiconductor metal oxides, such as ZnO and TiO2 are typically used as photocatalysts [12]. Upon light absorption, charge separation occurs with the formation of an electron in the conduction band and an electron hole in the valence band [12].

The pair of electron-hole created may collapse or separate to form independent pairs of electron and holes and allow the transfer of charge between the electron-hole pairs and the reactants on the surface of the TiO2 semiconductor [2]. At this stage, photo-excitaion takes place . In TiO2, electrons diffuse faster in the conduction band while hole-migration occurs by charge-jumping from neighbouring sites. In heterogenous photocatalysis, photo-induced reactions occur at the surface of the catalyst [9].

Titenium dioxide has been widely used as a photocatalyst in a series of oxidative and reductive reactions on the surface of the semiconductor. This is largely contributed by the existence of lone electron in its outer orbital [4]. Electron and holes that fail to be annihilated move to the particle surface where they can react with other chemical species [12]. UV light is commonly used in the photo-excitation process to reduce the metal onto the surface of the metal oxide, activating the metal oxide.

UV light excites valence band electrons to the conduction band of the semiconductor, creating holes in the valence band. Figure 2 shows a schamatic diagram of this process and the reactions involved. Figure 2: A schamatic diagram of the photo-excitation process of TiO2 as a catalyst [15]. O – oxidation; R-reduction. The process of TiO2 activation under UV light can be represented by the equation: TiO2 + hv h+ + e- In this reaction, h+ is is a powerful oxidizing agent, while e- is a powerful reducing agent [6].

The oxidative and reactiuve reactions are as follows: OH- + h+ OH (Oxidative reaction) O2ads + e- (Reductive reaction) 1.3 Solar Energy In the last few decades, more attention has been focused on the photocatalytic activity of TiO2 semiconductor with a strong redox power under photoinduction. The current challenge in photocatalysis is to develop a photocatalyst that enables efficient utilization of sunlight as a source of energy[6]. A study done by Fujishima and Honda attempted to convert solar energy to chemical energy on the surface of TiO2 semiconductors.

During the process of photocatalytic water splitting, water dissociates upon illumination by solar energy, and the flat band potential of TiO2 semiconductor shifts upwards. In this simulation, the active species are the holes which are able to oxidize OH- to hydrogen peroxide (H2O2) and oxygen, while hydrogen is produced on the counterelectrode. However, only 10% of solar energy is absorbed, making it impractical to use TiO2 in commercial solar energy converters [6]. In addition to this, TiO2 has a low capability to reduce hydrogen, and it requires a small voltage to help the photolytic process.

Due to the poor efficiency of TiO2, studies are underway to obtain materials which have a smaller band gap e.g. InP catalyzed by Pt, GaAs catalyzed with Ru etc. A schematic diagram of a photochemical photocell is shown in figure 3. Figure 3: Principle of an electrochemical photocell. 1: n-type TiO2 electrode; 2: platinum counter electrode; 3: ionically conducting separator; 4: gas buret; 5: load resistance; 6: Voltmeter [6]. 1.4 Catalytic Activity of Gold Gold in its bulk nature is an inert material, but Au nanoparticles in the size range of 3-10 nm in diameter have been found to be catalatically active for a number of chemical reactions [10].

Several explanationations have been proposed for the catalytic activeness of Au nanosize particles, including the oxidation state of Au, quantum size effects, charge transfer or support-induced strain, and the presence of very low-cordinated atoms of Au in nanosize particles [3].

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