Single-Crystal Mesoporous ZnO Thin Films Composed of Nanowalls

This paper presents a controlled, large scale fabrication of mesoporous ZnO thin films. The entire ZnO mesoporous film is one piece of a single crystal, while high porosity made of nanowalls is present. The growth mechanism was proposed in comparison with the growth of ZnO nanowires. The ZnO mesoporous film was successfully applied as a gas sensor. The fabrication and growth analysis of the mesoporous ZnO thin film give general guidance for the controlled growth of nanostructures. It also provides a unique structure with a superhigh surface-to-volume ratio for surface-related applications. and nanohelices. 3-7 Among them, aligned ZnO NWs with controlled size, growth direction, and surfaces have shown great potential for building nanodevices with superior mechanical, electrical, optical, and piezoelectric properties. It has been discovered that the aligned ZnO NWs sometimes came with a continuous sheet-like structure connecting the bottom of the NWs. Once the sheet became dominant, a new morphology called nanowalls appeared. 8,9 As a quasi-two-dimensional nano- structure with a high porosity, the vertically aligned ZnO nanowalls exhibit great promise for sensors, fuel cells, solar cells, and catalysts. 10 Although formation mechanisms have been proposed, lack of rational control makes the nanowall often be accompanied by aligned ZnO NWs that stand directly above, and receiving a single-phased nanowall structure becomes more or less unpredictable In this paper, we report a controlled growth of single crystal mesoporous ZnO thin films composed of vertically aligned ZnO walls. After analyzing the growth condition and structure of the mesoporous films, we present a general understanding on the vapor concentration-induced growth orientation selection. Guided by this discovery, formation of NWs with the nanowall structure can be intentionally turned on and off so that a single- phased nanowall structure was observed repetitively. A potential application of the film as a gas sensor has also been demonstrated. 2. Experimental Section The mesoporous ZnO film was fabricated in a double tube system. In this system, an alumina tube with 2-in. inner diameter (ID) was place inside a tube furnace with both ends sealed by a metal cap. Then a 95% Ar/5% O2 mixture was introduced into the tube as carrying and reactive gases at a total flow rate of 50 sccm. The source materials (a mixture of 0.3 g of ZnO powder and 0.3 g of graphite power) were loaded at the center of a 20 cm long quartz tube with 0.5-in. inner diameter. The substrate used for the growth is a GaN epitaxial film grown on single crystal alumina substrate. The substrate was covered by a layer of 0.4 nm thick Au as catalyst and was placed at one end of the small tube. The small quartz tube was then positioned inside the alumina tube with both tube centers aligned together and the substrate pointing downstream of the gas flow. At a tube vacuum of 20 torr, the tube furnace was heated up to 950 °C at a rate of 50 °C /min and the substrate was located in the ∼700 °C temperature zone. The temperature and vacuum were held steady for 2 h for ZnO deposition. Then the furnace was turned off, and the tube was cooled to room temperature with the vacuum unchanged.