Enhanced photoelectrochemical properties of nanocrystalline TiO2 electrode by surface sensitization with CuxO quantum dots

Nanoporous anatase TiO2 films were fabricated by a screen-printing method, and CuxO quantum dots (QDs) were deposited on the TiO2 films through successive ionic layer adsorption and reaction (SILAR). The amount of CuxO QDs on the TiO2 films are controlled by changing the number of SILAR cycles. The morphology, microstructure, optical, and photoelectrochemical properties of different CuxO sensitized TiO2 films (CuxO/TiO2) were investigated in detail. The nanoporous TiO2 film offers a large surface area for anchoring QDs. QD deposited samples exhibited a significant improvement in photoelectrochemical performance than the bare of TiO2. CuxO/TiO2, prepared with 7 SILAR cycles, showed the best photoelectrochemical properties, where the photocurrent density was enhanced to 500.01 μA/cm2 compared with 168.88 μA/cm2 of bare TiO2 under visible light. These results indicate that the designed CuxO/TiO2 structure possesses superior charge separation efficiency and photoelectrochemical properties.

reaction (SILAR) method 38 . SILAR is a simple fabrication methodology that combines successive layer adsorption with chemical redox reaction. The typical process involves successively immersing the TiO 2 materials in + Cu(NH ) 3 4 2 and H 2 O 2 aqueous solutions successively for as many cycles as desired to achieve a uniform deposition of Cu x O QDs. Not only is the overall process environmentally friendly, cost-effective, and can be carried out in normal atmospheric pressure and room temperature, but the density of QDs can also be easily controlled by merely varying the number of deposition cycles.
In this study, we report the preparation of nanoporous anatase TiO 2 films on transparent conductive fluorine-doped tin oxide (FTO) substrates by screen-printing and the subsequent deposition of Cu x O QDs on the TiO 2 films via SILAR. The number of deposition cycles, and thus QD loading density, was varied to investigate the effect on the photoelectrochemical properties of nanocrystalline TiO 2 electrode and derive the optimal density of Cu x O QDs that enhanced the photoelectrochemical activity. The sample was also recycled 7 times to demonstrate its improved absorbance in the visible range and enhanced photoelectrochemical properties.

Experimental
Preparation of TiO 2 films. All the chemical reagents were used as received. The colloidal were prepared by hydrolysis of titanium tetraisopropoxide as described by elsewhere 39 . The TiO 2 films were synthesized directly on transparent fluorine-doped tin oxide (FTO, TEC-8, LOF) conducting glass substrates by screen-printing, followed by calcining the samples at 500 °C for 30 min in air.  Photoelectrochemical measurements. Photoelectrochemical measurements were carried out in a three-electrode configuration with the as-prepared sample as the working electrode, Pt foil as the counter electrode, and saturated Ag/AgCl as the reference electrode 41 . A 0.1 M Na 2 SO 3 aqueous solution was used as the electrolyte. Photocurrent measurements were taken as a function of voltage by an electrochemistry workstation (CHI 660D, Shanghai Chenhua instrument). The working electrode was illuminated by a 300 W Xe lamp. An ultraviolet cutoff filter was inserted in between the light source and the quartz cell to exclude UV light with wavelength below 420 nm. Photoresponses of the different samples were determined by using a light on-off cycle of 60 s at a bias of 0 V versus the Ag/AgCl electrode.

Results and Discussion
XRD patterns. Figure 1 shows XRD patterns of S0 and S9. The XRD pattern of the substrate (FTO) was used as a reference. After the TiO 2 products were formed on the FTO substrates, all FTO diffraction peaks became weaker. Both S0 and S9 samples have similar patterns, in that they display three peaks at 2θ = 36.02°, 62.56°, and 69.01°. These peaks can be attributed to the (101), (002), and (301) diffraction peaks of anatase TiO 2 , respectively, as they are in good agreement with the standard pattern of anatase TiO 2 (PDF#65-1119). In addition, the diffraction peaks of Cu x O QDs are not observed in S9, implying the low content and small size of QDs. Therefore, we can conclude that the formation of Cu x O QDs does not influence the crystalline structure of the TiO 2 electrodes.
Morphological analysis. Figure 2 presents SEM images of TiO 2 films coated with CuxO QDs. The magnification and high magnification SEM images ( Fig. 2a and b) show that the TiO 2 film is porous and uniform.

XPS analysis.
In order to clarify the elemental composition and valence state of Cu x O QDs, XPS characterizations was conducted. As shown in Fig. 4a, the general survey spectrum for Cu x O QDs modified TiO 2 electrodes contains Cu, O, Ti, and C elements. The small amount of carbon could have resulted from adventitious hydrocarbons from the XPS instrument itself and can be taken as the standard signal for the correction of other peaks 42 . From the Ti 2p spectrum (Fig. 4b), two main peaks at bonding energies of 458.6 and 461.4 eV were assigned to Ti 2p 3/2 and Ti 2p 1/2 , respectively 43,44 . Figure 4c shows a representative Cu 2p core level XPS spectrum with two peaks at 933.2 and 953.0 eV for atmospheric conditions at room temperature, and the oxidation products include Cu 2 O and CuO 45,46 . Furthermore, two fitted peaks (Fig. 4d) from the O 1 s spectrum are observed around 529.7 and 531.9 eV, which can be assigned to the lattice oxygen and surface hydroxyl oxygen of TiO 2 47 , respectively. Figure 5 shows the absorption spectra of TiO 2 electrodes sensitized with different SILAR cycles of Cu x O QDs. The average absorbance can be calculated and the results are listed in Table 1. It can be seen that with an increase in the number of SILAR cycles, the absorbance increases at wavelengths 400 to 700 nm. This can be attributed to the SPR of Cu QDs and narrow band gap of CuO and Cu 2 O 48 . In addition, the red-shift of the absorption edge of Cu x O/TiO 2 is due to the broadening of size distribution of Cu x O QDs 49 . Based on the UV-Visible absorption spectra, a plot of (αhv) 2 versus energy (hv) is shown in Fig. 6, and the Eg values of different Cu x O samples are shown in Table 1. It can be seen that the absorption bands of Cu x O/TiO 2 show large variation, which change from 2.90 to 2.50 eV. The band gap of Cu x O/TiO 2 (S9) was 2.50 eV, which was smaller than that of TiO 2 (2.90 eV). These results suggest that the formation of the Cu x O/TiO 2 nanostructures decreased the recombination of photogenerated electrons and holes and improved the photoelectrochemical ability of the TiO 2 electrodes. PL spectra. To investigate charge transfer between photogenerated electctrons and hole pairs, photoluminescence (PL) emission spectroscopy was used to measure the recombination of free charge carriers. The emission peaks at 420 and 475 nm are assigned to exciton-casued PL resulting from band edge free excitons and defects of TiO 2 50 . PL peak intensity correlates directly with the defect densities in materials. The higher PL intensity typically indicates a higher recombination rate of the photo-generated electrons and holes 51 . As shown in Fig. 7, the PL intensity of Cu x O/TiO 2 reveals a significant decreases with increasing Cu x O QDs. This is due to a decrease of radiative recombination processes 52 . When the Cu x O QDs are deposited on TiO 2 electrodes, TiO 2 can easily bond with the Cu x O QDs to form the Cu x O/TiO 2 composites. The photo-induced electrons can be trapped in the Cu 2p energy level below the conduction band in the Cu x O/TiO 2 composites, which inhibit the recombination of electron-hole pairs. The intensity of the peaks at 420 and 475 nm are the lowest for S7, which exhibit high quantum efficiency and higher photoelectrochemical properties.

UV-Visible absorption spectra.
Photoelectrochemical studies. Figure 8 shows the time-dependent photocurrent curves of Cu x O/TiO 2 under visible light illumination. It is generally known that transient photocurrent always reflect the transfer and separation of photoinduced charge carriers under intermittent illumination. As the light is turned on, the photocurrent values increase while the photocurrent values decrease rapidly as the light is turned off. This suggests that all samples have good reproducibility. In addition, photocurrents increase with more SILAR cycles, which indicate that the photocurrent of Cu x O/TiO 2 have a significant enhancement compared to bare TiO 2 . Moreover, after 7 SILAR cycles, the sample shows the highest photocurrent value of ca. 138 μA/cm 2 , which is about 13 times higher than that for bare TiO 2 . Nevertheless, when the SILAR cycles increase to 9, the photocurrent value decreases to ca. 120 μA/cm 2 . The increase in photocurrent may be attributed to the stronger SPR effect of Cu x O QDs, which improves the light absorption of TiO 2 . With more than 9 SILAR cycles, the aggregation of Cu x O QDs lead to a large particle size of Cu x O QDs, which could block the surface active sites of TiO 2 and act as potential barrier of charge transfer, resulting in a decrease of the photoelectrochemical properties [53][54][55][56] . Figure 9 shows the LSV curves of the different samples in the dark and under light irradiation. Photocurrent values are shown in Table 1. The photocurrents of the Cu x O/TiO 2 samples improved compared with that of bare TiO 2 , suggesting that the Cu x O/TiO 2 exhibit a stronger ability to separate photo-generated electron-hole pairs. The Cu x O/TiO 2 prepared with a different number of SILAR cycles (0, 3, 5, 7 and 9 times) exhibited photocurrent values of 168.88, 248.95, 390.98, 500.01, and 424.15 μA/cm 2 at 1.0 V (vs Ag/AgCl) under visible light irradiation, respectively. Clearly, photocurrent densities of the Cu x O/TiO 2 first increase then decrease with increasing the SILAR cycles. The S7 showed the strongest photocurrent value, and exhibits the best photoelectrochemical property, which is consistent with the PL results. When 9 SILAR cycles were used, the CuxO QDs aggregated to form a compact granular morphology, resulting in a lower surface area and reduced photocurrent 57 .

Conclusions
In summary, Cu x O QDs are deposited on the nanoporous anatase TiO 2 films by a screen-printing method, followed by successive ionic adsorption and reaction (SILAR). The microstructure, morphology, and loading amounts of the Cu x O QDs on the TiO 2 films are controlled by changing the number of SILAR cycles. The Cu x O/TiO 2 absorbs more light and exhibits enhanced photoelectrochemical properties compared to bare TiO 2 . Moreover, under visible light illumination, the TiO 2 sensitized with 7 SILAR cycles of Cu x O QDs shows the best photoelectrochemical properties, where the photocurrent density is increased to 500.01 μA/cm 2 , 2.96 times higher than the bare TiO 2 electrode with 168.88 μA/cm 2 . The superior photoelectrochemical properties of the Cu x O/TiO 2 nanostructures could be ascribed to the large surface area of nanoporous TiO 2 electrode and the SPR effect of Cu x O QDs. The electrode design of Cu x O/TiO 2 will be beneficial for application of solar energy conversion and wastewater degradation.