Figure 7.4a shows the I-T plots and the photocurrent onsets generated by the excitation of UV irradiation in air at room temperature with a bias voltage of 1 V. After a longer N2 treatment (900 s) for ZnO NWs and NTs, the ON–OFF cycles were exponential in generating a photocurrent with a reproducible response. It was noted that the N2 plasma treatment for the NTs not only induced an increase in the dark current, but also enhanced the absolute magnitude of the photocurrent. The photoresponse enhancement for the NTs shows a remarkable increase by one order of magnitude over that of the NWs. It was assumed that the generation of an oxygen defect normally releases two electrons according to O�× → 1/2O�+ V��� + 2e�, which contributes to
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the intrinsic n-type nature of the ZnO nanostructure. This implies that oxygen defect vacancy (V��) sites in the depletion layer are produced and become so unstable that they tend to be stabilized by combining with ambient negative charges, such as OH− from physisorbed water molecules in the surrounding atmosphere (air), to form V��–OH− bonds, which result in a decrease in free electrons. Therefore, under UV excitation, V��–OH− pairs are broken, and electrons can be released from the detached hydroxyl OH− groups on the surface of the ZnO nanostructures; this results in an apparent enhancement in photoresponse due to the larger surface/volume ratio of ZnO nanotubes without position dependent measurements of the UV illuminated lamp.
Figure 7.4 (a) Time traces of the current with the chopped light for samples measured after a 900 s N2
plasma exposure under an applied voltage of +1 eV (electron conduction). The inset shows the schematic diagram of the corresponding measurement method. (b) Energy-band diagram for 300 ◦C annealed and N2 plasma-treated ZnO under UV illumination.
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On the contrary, the slow decay in the NTs at light off suggests a better stability and a more gradual electron–hole recombination than that found in NWs, which might be related to the defect sites, which capture or release the bypassing electrons and slow the reduction of the mobility of the photoexcited carriers in the ZnO NTs.
Figure 7.4b shows a schematic diagram to illustrate the electronic band behavior for ZnO NTs. A sufficient photon energy can generate electron–hole pairs, then excited electrons in the conduction band and unpaired holes neutralize oxygen ions at the surface [155, 156, 162, 163].
Due to the higher surface-to-volume ratio of the lower-dimensional NTs, the abundance of incident photons at the N2-treated surface can readily break down the V��–OH− bonds, and the electrons transferred can subsequently overcome the large band gap. Evidently, the conductivity is governed by the carrier concentration, which is associated with the surface morphology of the ZnO NTs. The large direct band gap of the NTs (3.208 eV) demonstrates the enhanced carrier concentration, which leads to a decrease in the band gap and the Schottky barrier (ɸB) at the grain boundaries.
7.5 Summary
In summary, a chemically based approach was used to develop ZnO nanotube (NT) and nanowire (NW) structures. The PL results show that a longer nitrogen plasma exposure can significantly improve the UV emission of ZnO nanotubes. Furthermore, the current–voltage characteristics show that the photocurrent (4.82×10−7 A) of NTs can be remarkably enhanced compared to that (0.571×10−7 A) of ZnO NWs after exposure to N2 plasma for 900 s. This treatment also enhances the photoresponse behavior of the plasma-treated NTs (by a factor of 20 over that of NWs). Such a promising NT structure using a nitrogen plasma treatment may offer a
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method of producing high optical quality ZnO nanostructures and could potentially be useful in the designs of 1D ZnO-based solar cells and optoelectronic devices.
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Chapter 8
Selective Oxygen-Plasma-Etching Technique for the Formation of ZnO-FTO Heterostructure Nanotubes and Their Rectified
Photocatalytic Properties
8.1 Introduction
Low-dimensional, nanostructured materials have attracted much attention for advanced electronic, optoelectronic, and photocatalytic applications because of their high sensitivity level and surface-volume characteristics. Of particular interest are coaxial nanotubes, in which core-shell hybrids are composed of oxide semiconductors, for use as three-dimensional (3D) transparent conductive (TCO) electrodes in photoelectrochemical cells.
In dye-sensitized solar cells (DSSCs), typically a TCO electrode has been used as the back contact for the nanostructured TiO2 film. In particular, heterostructured, onedimensional (1D) electrodes have received great attention because of their chemically oxidative and hybrid properties, which were proposed to improve the conversion efficiency as an alternative approach to DSSCs [99, 164-166]. Recently, Wang et al. demonstrated that the photoelectric conversion efficiency of the DSSCs made from an indium-tin oxide (ITO) nanowire array embedded with the TiO2 photoelectrode was higher than that of a pristine TiO2 film or arrays of ITO/TiO2 core-shell nanowires [99]. Liao and co-workers reported hybrid CdS/P3HT photovoltaic devices using FTO-coated ZnO nanorod (NR) arrays as 3D electrodes. The FTO-coated ZnO NR length and thickness of the FTO layer were interpreted to enhance the photovoltaic performance [164].
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Generally, the unique properties of ZnO are utilized not only in various aligned nanostructures for charge-carrier transportation and light emission but also as a free-standing NR, which is suited for uniformly nanosized templates and can be functionalized via interfacial solid-state diffusion with a surrounding shell layer. As a shell layer, the transparent conducting FTOis thought to be an ideal candidate electrode for solar cells. In addition, it was well-known that a high aspect ratio of 1D FTO nanostructures can provide far greater surface area than thin films, offering the additional challenge of obtaining effective charge-carrier collection and transport. Therefore, if the ZnO-embedded FTO could put forward the idea of extension into nanotubes with a completely hollow core, the large interfacial area between FTO and ZnO will play a critical role in improving the conduction path through nanoscale effects [61]. However, there have not been any reports involving the fabrication of a ZnO/FTO hollow nanotube and related optoelectronic properties. Therefore, when ZnO as the active light-absorbing component is combined with oriented FTO nanotube arrays, this composite nanostructure possesses excellent charge-transport characteristics, which will be a benefit for photovoltaic devices.
To the present, there have been a number of reports on template-assisted oxide nanotube formation by various synthesis methods. Selective etching is one important technique to produce controllable structures, including wet-chemical etching [62, 92, 167-169] and hydrothermal treatment [91, 170-173]. Zeng et al. developed wet-etched Zn-ZnO core-shell nanoparticles (NPs), where H+ ions incorporated from a weak acid solution diffused along lattice defects and grain boundaries in the ZnO shell layer to eliminate Zn core materials via a redox-precipitation process at the interface [92]. Fan et al. developed ZnAl2O4 nanotube structures using hydrothermal calcination. The formation mechanism includes the occurrence of defects and voids along the core-shell interface, and the Kirkendall effect with surface diffusion produces
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hollow interiors [91]. Yao et al. proposed a two-step etching process on ZnO NRs, which entails both chemical and plasma etching at low temperature [173]. A wet acid solution provides a rough isotropic etch of hexagonal edges, and then argon plasma is employed to fabricate controllable nanotips for enhanced field-emission devices. However, the wet-etching process leads to a morphology with a high degree of disorder, causing a lower photocatalytic efficiency due to photocorrosion. Therefore, it is very important to develop a dry-etching synthesis process for the metal oxide semiconductor in solar cell applications. To date, there has been little research on dry-etching mechanisms, in which a controllable etching rate could improve the optical properties of the materials. The use of oxygen plasma to eliminate the inner core of metal oxide compounds has only been studied in a few cases, but this novel method could introduce the benefits of hollow nanoscale materials.
In this paper, we propose a fabrication method to synthesize ZnO-FTO composite nanotubes using water-based spray pyrolysis for depositing FTO NPs on arrayed ZnO NRs and oxygen plasma for the etching process. During the process, various ions and radicals generated by the oxygen plasma, including O+, O2+, and O*, can readily diffuse into oxygen vacancies [174-176] to create negative charges on the ZnO NR surface. This results in an accelerated etching rate, producing a large number of voids in ZnO and forming a hollow ZnO-FTO heterostructure. The mechanism of etching evolution of the ZnO-FTO nanotubes via oxygen plasma was also investigated in this study. In addition, the corresponding enhanced photoresponse of the heterostructures will also be discussed.
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