CuO free p-Cu2O nano-surfaces prepared by oxidizing copper sheets with a slow heating rate exhibiting the highest photocurrent and the H2 evaluation rate

Abstract

Among the various metal oxide materials for solar energy applications, p-type cuprous oxide (p- Cu2O) is a promising non-toxic and low cost semiconductor with attracted attention for many decades (Mittiga et al., 2006). It was reported that p-Cu2O can be prepared by various fabrication processes such as thermal oxidation, electrochemical oxidation, chemical bath deposition and chemical vapor deposition. Low energy conversion efficiency of p-Cu2O based solar energy conversion devices is due to the prevention of the photo-generated charge carrier separation in the micron–sized Cu2O grains on the surface enhancing the recombination process. If the grains radius is reduced from micron to nano-size, the opportunities for recombination can be dramatically reduced enhancing the light absorption properties of the films. Therefore, the preparation of nano- crystalline Cu2O thin films is a key factor to improve the performance of solar application devices without destroying the crystalline properties of the Cu2O films. Hence, the present work is aim to fabricate CuO free nano-crystalline p-Cu2O by thermal oxidation of copper sheets under maintaining slow heating rate for the first time. Structural and photoelectrochemical properties are also aimed to study. Methodology

The outer layers of commercially available (99.99% purity) copper sheets 2cm×4cm were removed by sand papers and polished with Brasso Metal Polisher until obtaining a mirror like surface. Thereafter, polished copper sheets were washed with a surface detergent and distilled water several times. Well cleaned copper sheets were inserted into a Quartz Tube in a Cabolite- 301 Tube Furnace opening both ends by filling normal air during the oxidation process. Initially a heating rate 10 C/min was provided inside the furnace with copper sheets starting from the room temperature. After reaching 300 C, 400 C, 450 C and 700 C the temperature kept constant for 30min and then cooled down to room temperature. Experimentally that it was found that the 10 C/min heating rate was the most suitable to fabricate mechanically stable p-Cu2O on copper

C temperature profiles did not produce mechanically stable Cu2O films on copper sheets. Furnace temperature below 300 C was not sufficient to oxidize copper sheets to form quality Cu2O surfaces.Fig.2 shows the appearance of the Cu2O films prepared. Four different surface colors exhibited at each temperature profile expecting four different surface morphologies.

Result and Discussion Fig.01 shows the diffuse reflectance spectra for the samples prepared from 300ºC, 400ºC, 450ºC and 700ºC temperature profiles. For the samples prepared from 300ºC, 400ºC and 450ºC temperature profiles show absorption edges 630nm, 620 nm and 600nm, to the band gaps 1.98eV, 2.0eV and 2.1 eV due to band to band transitions of Cu2O. Band gap 1.4eV for an absorption edge ≈850nm can be observed for CuO crystals prepared from 700ºC temperature profile. It should be mentioned that the absorption edges for Cu2O corresponding to the 300ºC, 400 ºC and 450ºC temperature profiles cannot be observed clearly for the samples prepared at 700ºC temperature profile as shown. So that, it can be concluded the most light is absorbed by CuO regions than the Cu2O regions for the samples prepared from 700ºC temperature profile.