Deposition ofZn Islands

A newer technique to modify the surface of a-Fe2O3 with Zn dots was attempted by the authors' group in 2007 [89]. These samples exhibited much better PEC responses, compared to doping or layering of the same metal. We modified the surface of 1.5 at% Zn-doped iron oxide (prepared by spray pyrolysis) by depositing thermally evaporated Zn through a mesh of pore diameter ^0.7 mm, using a vacuum-coating unit. The heights of the Zn dots were varied in the range 100 to 260 A by varying the processing time of thermal evaporation. The schematic diagram explaining the method of preparation of sample is shown in Figure 13.14.

All dotted samples exhibited n-type behavior. The resistivity in Zn-doped samples was found to increase by one order of magnitude [41], whereas the presence of Zn-dotted islands on the surface of Zn-doped a-Fe2O3 thin films gave a decrease in resistivity, which could be due to a difference in the work functions of the two materials. A decrease in the resistivity from 2.6 x 107 to 5.7 x 106 O cm was observed on increasing the height of the Zn island to 230 Aon the a-Fe2O3 surface. However, a slight increase in resistivity up to 9.2 x 106 O cm was observed after loading with 260 A thick Zn-dotted islands. This increase in resistivity at a height of 260 A was due to interdiffusion of the Zn islands in the iron oxide near the interface, which resulted in the loss of the charge carriers.

Figure 13.14 Schematic diagram of Zn dotted islands on an a Fe2O3 thin film. (Reproduced from S. Kumari, A.P. Singh, C. Tripathi et al., Enhanced photoelectrochemical response of Zn dotted hematite, International Journal of Photoenergy, 87467, 1 6, 2007, Hindawi Publishing Corporation.)

Doping of 1.5 at% Zn in a-Fe2O3 thin films prepared by the spray-pyrolysis method enhanced the photocurrent density from 0.061 to 0.321 mA cm-2 at 0.7 V versus SCE. Deposition of Zn dots on 1.5 at% Zn-doped a-Fe2O3 thin films with increasing height of dots resulted in continuous enhancement in the photocurrent, as shown in Figure 13.15. Structures with —230 A thick Zn islands exhibited the maximum photocurrent density of — 1.282 mA cm 2, at an applied potential of 0.7 V versus SCE. The increase in photocurrent density in this study was due to conversion of the Zn dots on the surface of a-Fe2O3 into wide-bandgap ZnO, at the time of annealing, which acted as an efficient catalyst for the swift migration of the photogenerated charge carriers. However, a decrease in photocurrent density with a-Fe2O3 deposited with thicker dots (260 A) was correlated with an increase in resistivity and a decrease in the a-Fe2O3 surface area, due to interdiffusion of Zn. A minor change in donor density and flatband potential was observed on increasing the height of the dots. Observed values of photocurrent density on the surface of a-Fe2O3 and onset potential, along with resistivity, for various modified/unmodified thin films of a-Fe2O3 are given in Table 13.2.

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