5 V, which is in good agreement with the observation confirmed by XRD spectra shown in Figure 1. Figure 2d,e shows the SEM micrographs of films deposited at −0.7 and −0.9 V vs. the reference electrode, respectively. These films exhibit a granular spherical morphology, and the average diameter of the grains tends to be approximately 50 nm. Optical properties Figure 3 illustrates the optical absorption spectra for all the samples of cuprous oxide thin films deposited on Ti sheets at different applied potentials. As can be seen, there is an absorption edge in the range of 500 to 620 nm. Comparing these curves, it can be found RGFP966 that the absorption edges show redshift then blueshift with increasing the applied potential.

Figure 3 UV–vis absorption spectra of Cu 2 O thin films. The photoabsorption in the visible light range for Cu2O film at −0.1 V vs. the reference electrode with cubic structure was more than 50% stronger than that for Cu2O film with pyramid shaped structure, which can be seen from Figure 2a,b. It can originate from the reason that the cubic structure film has more surfaces to adsorb light, leading to stronger photoabsorption [27]. Cu2O film deposited at −0.5 V vs. the reference electrode with the strongest absorption FK506 mw is due to the resonance absorption of metal copper particles, which can be

also confirmed by XRD spectra of Figure 1. The decrease of the absorption coefficient of Cu2O films deposited at −0.7 and −0.9 V may be due to too much nucleation covering the entire Ti sheets. It decreases gaps, and defects of the films then reduce the scattering of light. The cuprous oxide is a typical direct band gap semiconductor. The absorption coefficient satisfies the equation (ahv)2 = A(hv − E g )

for a direct band gap material [28]. Here, a is the absorption coefficient, A is a constant, hv is the discrete photon energy, and E g is the band gap energy. The band gap E g is obtained by extrapolation of the plot next of (ahv)2 vs. hv, and the estimated direct band gaps of Cu2O films are listed in Table 1. Based on the data of Figure 4 and Table 1, it can be found that the band gap of Cu2O films first decreases and then increases with the applied potential which becomes more cathodic. The intercepts to the (ahv)2 vs. hv plot for the samples S1 and S2 give the value of band gap as 1.90 and 1.83 eV, respectively. Due to the presence of metal Cu particles, the absorption edge of the sample S3 is 1.69 eV. Figure 4 shows (ahv)2 vs. hv plot for the samples S4 and S5, and the obtained band gap values are 2.00 and 2.03 eV, respectively. This is also consistent with previous XRD results and coincides with Grez’s observation [29]. Table 1 The estimated direct band gaps of Cu 2 O films Applied potential (V) −0.1 −0.3 −0.5 −0.7 −0.9 Band gap (eV) 1.90 1.83 1.69 2.00 2.03 Figure 4 Square of the absorption energy as a function of photon energy of Cu 2 O films.