Effect of Morphology and Doping on the Photoelectrochemical Performance of Zinc Oxide
As a clean and inexhaustible energy source, solar energy is the most important alternative to 11011-renewable fossil fuels to solve the problems of an energy shortage, global warming, and environmental pollution. The key issue is how to efficiently harvest and utilize solar energy. Photoelectrochemical (PEC) solar energy conversion is regarded as one of the most promising technologies for solar energy conversion and application. However, there are some challenges of employing this material for PEC water splitting application: (a) conduction band (CB) potential is not high enough to drive water reduction without bias, (b) a relatively low absorption coefficient because of indirect bandgap and (c) poor majority earner conductivity and short hole diffusion length (2-4 mn). Metal oxides carved into hierarchical nanostructures are thought to be promising for improving photo-electrochemical performance by enhancing charge separation and transport. ZnO is studied as one of the most relevant optoelectronic materials, due to its unique optical (like large direct bandgap of 3.37 eV) and electrical properties (high exciton room temperature binding energy 60 meV, high electron mobility, etc.). However, the efficiency of the ZnO based photoanode has limited success because of its high recombination rate of eir (electron-hole) pairs and poor catalytic activity. Over the years, several research efforts have been devoted to explore the effective methods to address those drawbacks. Such attempts have been employed through various approaches, such as doping, hetero-structure, dye sensitization, quantum dot sensitization, and co-catalysts modification. Out of these, nowadays, impurity-doped ZnO has been given more and more attention because the impurity element directly and simply enhances conductivity, earner concentration, and optoelectronic performance of ZnO.
The quest of achieving sustainable, clean, green energy has always led mankind to think off about alternate sources of energy. Several attempts are also done so far to achieve the same by scientists across the globe. Apart from all the available renewable resources; solar energy is considered to be the best among all with regard to its ability in generating energy at a significant level (Tee et al., 2017; Tsao, Lewis, and Crabtree, 2006). Several solar energy-driven applications are being tried nowadays to harvest sunlight at its fiillest. Extraction of hydrogen front water is one of the possible solutions to meet the world's energy demands. When this evolved gas is further consumed it generates water as the only by-product, which proves its credibility as a clean green energy solution (Figure 8.1). In order to realize this, photoelectrochemical (PEC) water splitting has emerged as a broad area of research. Basically in this process hydrogen and oxygen are collected by splitting water in presence of photoelectrodes (semiconductors). But the lowered efficiency in these types of systems put hurdles in further commercialization. On one hand, we have stable oxide semiconductors but with considerably reduced efficiency while on the other hand, highly efficient (as compared with oxide semiconductors) semiconductors suffer with short life span. Henceforth to come up with highly efficient stable systems several approaches has been made.
After the first experiment by Fujishima and Honda (1972) to generate H, and O, from ТЮ, by means of water splitting several other semiconductors have been taken under trail for the same. In order to utilize hydrogen fuel, several research groups have developed many metal oxide nanostructures (Chen et al., 2017) such as ZnO (Djurisic et al., 2012; Kolodziejczak-Radzimska and Jesionowski, 2014), Cu,0 (Pan et al., 2018; Yang et al., 2016), TiO, (Chen et al., 2015; Guo et al., 2016; Schneider et al., 2014), SnO, (Outemzabet et al., 2015), Fe,03 (Cha et al., 2011), W03 (Vidyarthi et al., 2011; Xu et al., 2015), BiV04 (Luo et al., 2008; Sun et al.,
FIGURE 8.1 Various energy-driven water splitting routes by using thermal, electrical, biochemical, and photonic energy or then- combinations.
Source: Reprinted with permission from Tee et al. (2017). https://creativeconnnons.org,'' licenses by/4.0/
2014), CuW04 (Salimi et al., 2019; Valenti et al., 2015), Ag3P04 (Lin et al., 2012; Martin et al., 2015), V,05 (Andrews et al., 2018), etc., through their potential application in PEC water splitting. Among all, the ZnO has been acknowledged as a promising photoanodic material due to its suitable band edges, high electron mobility and lower electrical resistance as well as its environmentally benign nature. As far as solar energy-driven water splitting application is concerned, the suitable band positions of ZnO, i.e., conduction band minimum (CBM) and valence band (VB) maximum (VBM) are -0.31 V and +2.89 V respectively (Here both of the values mentioned are in normal hydrogen electrode (NHE) scale). However, the large bandgap (3.37 eV) of ZnO nanomaterials substantially limits its photoconversion efficiency. Being a UV active material it always lacks with regard to its limited ability in further use of solar spectrum. This emphasizes more focus on increasing the visible region absorption ability of Z11O through various processes like sensitization with dyes or quantum dots, and doping with heteroatoms. Basically doping with heteroatoms or Impurity doping is widely investigated as an effective tool to increase or enhance electrical conductivity and narrowing optical band gap of semiconductors (Baruah and Dutta, 2009; Bharathi et al., 2014; Ilican, 2013; Panigrahy et ah, 2010). Till now, various types of dopants have been introduced into ZnO in order to improve the photoconversion efficiency.
Another important aspect for ZnO is the facile tunable properties along with a wide range of morphologies. The morphology has also been known to have a substantial impact on the photoelectrochemical performance (Govatsi et ah, 2018). Therefore, ZnO of different morphology like nanosheets (Hsu et ah, 2011), nanorods (NRs) (Sharma et ah, 2018a, b), nanowires (NWs) (Wang et ah, 2019), nanopencils (Wang et ah, 2015), nanotree (Ren et ah, 2016), nano triangles (Chandrasekaran et ah, 2016), nanotertrapods (Qiu et ah, 2012), nanocorals (Aim et ah, 2008; Shet, 2011), etc., were developed. The usually occurring fast surface recombination rate of photoinduced electron-hole pairs are well managed by ID nanostructure. For a better photoactive semiconductor, a material should be capable of charge collection along with effective separation and transfer of the same to the surrounding medium. NRs possess large surface-area-to-volume ratio which provides large surface area to the surrounding medium; thus helping the minority carriers to get diffused easily at the semiconductor electrolyte interface, leading to an enhanced charge separation. Furthermore, the facile synthesis of ID nanostructure along with tuneable diameter suffices with the carrier diffusion length leading to easy removal of the carriers from the surface, hi view of these overall features, these NRs have emerged as a potential candidate for realizing efficient photoelectrochemical devices. Not only this but also ZnO has shown its immense fidelity in several other applications like transistors (Li et al., 2008), ultraviolet (UY) photodetectors (Boruah et al., 2017), UV nanolaser, Field emitter, Photovoltaics (Djurisic et al., 2014), Sensors (Wei et al., 2011), Gas sensors (Singh et al., 2012), etc. Materials which offer photoelectrochemical water splitting possess unique characteristics. Lin and group have reported the detailed physical and chemical processes involved in water splitting (Lin et al., 2011). The reaction involves the following steps:
- 1. Light Absorption and separation of charge earners;
- 2. Charge transfer;
- 3. Charge transport;
- 4. Surface chemical reactions.
The material should possess a suitable bandgap (>1.23 eV) to permit light absorption. Also, the material should have proper band edge positions to split H,0 molecules. Since the number of incident photons received per unit area is fixed under certain standard condition (i.e., AM 1.5 with intensity of 100 mW/cm2); so to match the kinetics material should be catalytically active for both oxidation and reduction of H,0. Furthermore, the semiconductor needs to be resistant to photo corrosion and should survive reactions in water under intense illumination (Lin et al., 2011).
In the case of defect-free ZnO, photogenerated carriers are vulnerable for surface-bulk recombination resulting in low photonic efficiency. The presence of defects such as oxygen vacancy, zinc interstitial, and oxygen interstitial can temporarily inhibit the charge carrier recombination process and magnificently improve the reaction rates (Kayaci et al., 2014; Pei et al., 2013). Impurity doping at the cationic/anionic sites is a promising approach to alter the structure-electronic properties of any semiconductors. The significant improvement in the photonic efficiency with all these dopants is successfully achieved. The following observations are largely encountered upon impurity doping: (1) change in the defect chemistry—substituting at lattice sites in the Zn-O-Zn (cationic doping) and Zn-O-O (anionic doping) framework leads to the formation of Zn-O-M. The change in the local atomic configuration modifies the electronic environment and simultaneously introduces structural defects, lattice disorder, and lattice stress/strain. The segregation of these dopants along the grain boundary region inhibits the growth of crystallite size; (2) mismatch in the ionic radius between the dopant and host ion is the basis for changes in the defect chemistry (Sushma and Girish Kumar, 2017).
In this chapter, we present a discussion regarding the effect of various morphology of ZnO on the photoelectrochemical performance of ZnO. We also tried to put an insight on the effect of various dopants on the water splitting performance of ZnO NRs (Figure 8.2). Also, emphasis has been given on the basics of photoelectrochemical water splitting and different synthesis methods of ZnO nanostructures.
FIGURE 8.2 Periodic table highlighted with the elements doped in ZuO nanorods discussed in this chapter.