We examined the interfacial charge transfer effect on photocatalysts using a patterned CuO thin film deposited on a rutile TiO2 (110) substrate. Photocatalytic activity was visualized based on the formation of metal Ag particles resulting from the photoreduction of Ag+ ions under visible-light illumination. Ag particles were selectively deposited near the edge of CuO film for several nanometer thick CuO film, indicating that interfacial excitation from the valence band maximum of TiO2 to the conduction band minimum of CuO plays a key role in efficient photocatalytic activity of CuO nanocluster-grafted TiO2 systems with visible-light sensitivity.

Titanium dioxide (TiO2) has attracted considerable attention as an efficient photocatalyst for water splitting reactions and environmental purification.1 However, TiO2 is a wide bandgap semiconductor and can only be activated under ultraviolet (UV) light irradiation, thereby limiting its practical application. To extend the light absorption into the visible light range and allow the utilization of solar and indoor light sources, TiO2 has been doped with various transition metal cations,2 such as Cr, Fe, V, Ni, Mn, Nb, and Cu, and with anions,3 including N, C, and S. Despite such modification, most doped TiO2 systems remain unsuitable for practical use because their quantum efficiencies under visible light are too low to drive efficient photocatalytic reactions, as excited charge carriers at dopant sites have low oxidation and/or reduction power and the mobilities of electrons and holes are much lower than those of non-doped TiO2.4,5 In addition, because the doped sites act as recombination centers, the photocatalytic activity of these systems deteriorates even under UV irradiation.5 

In contrast to the metal-ion doping of TiO2, Irie et al.6,7 recently reported a novel approach for developing an efficient visible-light-sensitive photocatalyst by the facile grafting of copper oxide nanoclusters onto pristine TiO2 particles. When nano-sized (<3 nm) CuO nanoclusters are grafted onto the surface of rutile TiO2, electrons in the valence band of TiO2 can be excited to the conduction band of the CuO clusters through an interfacial charge transfer (IFCT) process under visible light irradiation.8 Hush et al. theoretically predicted the IFCT process between metal ions on a solid electrode in 1968,9 and Sutin et al. experimentally demonstrated IFCT at a semiconductor interface.10 When nano-sized CuO particles are grafted on the surface of TiO2, the Cu-derived wave functions of CuO are coupled with the Ti-derived wave functions of TiO2, making interfacial excitation possible between the two semiconductors. Electron spin resonance (ESR)11 and X-ray absorption fine structure (XAFS)7 analyses have revealed that excited electrons are trapped at CuO sites to form Cu(I) species, and that excited holes are generated at the valence band of TiO2. Trapped electrons in CuO sites are highly reactive towards the reduction of oxygen molecules in air or silver ions in aqueous media, whereas the excited holes in the TiO2 valence band have strong oxidative power to decompose organic molecules, such as aldehydes and alcohols. By optimizing the structure of CuO clusters, the quantum efficiency of this system under visible-light irradiation has exceeded 90%.12 The technique of nanocluster grafting has also been successfully applied to other wide-gap oxide semiconductor light harvesters, including ZnO13 and SrTiO3.14 

The IFCT process between CuO and TiO2 has been comprehensively studied spectroscopically;7,11 however, most of these studies were conducted using powder or particulate systems. Herein, we have focused on well-defined thin film structures with clear single interface to examine the effect of the interface between CuO and TiO2 on the efficiency of IFCT. CuO thin films with different thicknesses were grown on rutile single TiO2 crystal substrates by pulsed laser deposition (PLD) and patterned using a metal mask or photo-lithography method. To investigate the photo-excitation process between CuO and TiO2, we performed a probe photo-deposition reaction, which consisted of the photocatalytic reduction of silver ions (Ag+) to metal silver (Ag), under visible-light irradiation. The potential of excited electrons in the conduction band minimum (CBM) of CuO is more negative than that of the redox potential of Ag/Ag+ (+0.8 V vs normal hydrogen electrode (NHE)), leading to the reduction reaction of Ag+ ions. We therefore monitored the deposited Ag nanoparticles by atomic force microscope (AFM) to determine the reduction site and to discuss the catalytic process mediated by this heterostructure.

CuO thin films were grown using PLD at room temperature with an O2 partial pressure of 10 Pa on single crystalline rutile TiO2 with the (110) surface, which is the most thermodynamically stable face of TiO2. The substrate surface was atomically flat with a step-terrace structure. CuO pellet was used as a target and a KrF excimer laser with a wavelength of 248 nm was used for the CuO ablation. The thickness of the CuO film was calibrated based on X-ray reflection measurements (see supplementary material S1).15 Patterned CuO films were fabricated by photo-lithography and a lift-off technique or by using a metal mask.

Figure 1 shows X-ray diffraction (XRD) patterns of the 30-nm thick film and the bare TiO2 substrate. A CuO peak was observed for the 30 nm thick film in addition to the rutile TiO2 110 peaks. We also measured XRD patterns for thinner films (supplementary material S2),15 where no peaks other than those corresponding to rutile TiO2 were present when the film thickness was 9 nm or smaller. These results imply that film on TiO2(110) is not well crystallized or the crystallite size is so small when its thickness is smaller than 9 nm. On the basis of our XRD patterns, the main crystal phase of deposited films is CuO but the possibility of defect formation with lower copper valence number would not be excluded. The formation of CuO has been confirmed also by the visible light absorption spectra (supplementary material S3):15 the absorption starts around 900 nm, which agrees with the bandgap of bulk CuO of 1.3–1.4 eV.16,17

FIG. 1.

XRD patterns for CuO/TiO2 thin films with various CuO thicknesses. The Cu-Kα line was used for measurement. Peaks at 24.8° and 50.8° are due to Cu-Kβ line, while that at 54.0° originates from the tungsten filament of X-ray source (W Lα1 line).

FIG. 1.

XRD patterns for CuO/TiO2 thin films with various CuO thicknesses. The Cu-Kα line was used for measurement. Peaks at 24.8° and 50.8° are due to Cu-Kβ line, while that at 54.0° originates from the tungsten filament of X-ray source (W Lα1 line).

Close modal

Next, we investigated the photocatalytic reaction sites in the patterned CuO/TiO2 films by the probe photo-reduction of silver ions (Ag+) into metallic Ag particles. Patterned CuO/TiO2 samples were immersed in 1.0 mM aqueous solution of silver nitrate (AgNO3) and the CuO film was then irradiated with blue light emitting diode (LED) light for 1 h. The photodeposition of Ag particles on the CuO films with thicknesses of 100 nm (Fig. 2(a)) and 6 nm (Fig. 2(b)) was examined by AFM. Triangular Ag particles were deposited on the CuO films, as observed in a previous report,18 while no particles were observed before light irradiation (FIG. S4 in our supplementary material).15 Notably, the mode of Ag particle deposition differed between the thick and thin CuO films. We have confirmed that the deposited particles are metallic silver by X-ray photoelectron spectroscopy (XPS) measurements (FIG. S5 in our supplementary material).15 In contrast to the thick CuO film, Ag particles were selectively deposited on the edges of the thin CuO film, near the surface of the TiO2 substrate. Further, the density of Ag particles at the edges of the thin CuO film was higher than that observed on the thick CuO film, indicating that the reduction of Ag ions was very efficient at the edge regions of the ultrathin CuO films grown on a rutile TiO2 substrate. These trends were reproduced at different positions of patterned CuO films in the AFM images with lower magnification (FIG. S7).15 

FIG. 2.

AFM images after photodeposition of Ag particles on patterned CuO films with thicknesses of (a) 100 nm and (b) 6 nm. The edges of the CuO film were formed using photo-lithography (round shape) and a metal mask (square shape) for the 100-nm and 6-nm thick films, respectively. AFM line profiles along AB (a) and CD (b) are shown in (c) and (d).

FIG. 2.

AFM images after photodeposition of Ag particles on patterned CuO films with thicknesses of (a) 100 nm and (b) 6 nm. The edges of the CuO film were formed using photo-lithography (round shape) and a metal mask (square shape) for the 100-nm and 6-nm thick films, respectively. AFM line profiles along AB (a) and CD (b) are shown in (c) and (d).

Close modal

The observed film thickness dependence of deposition mode would be understood by assuming that the internal excitation in CuO is dominant for thick films while the IFCT is dominant for thin films. As shown in the simplified energy diagrams in Fig. 3, the CBM of TiO2 is located at approximately 0 V versus NHE,19 whereas the CBM of CuO is formed at +0.2 V vs. NHE.6 At a CuO film thickness of 100 nm, the bandgap excitation of CuO itself would be dominant under visible light irradiation (process (i) in Fig. 3(a)). In this scheme, a portion of the photogenerated electrons would reduce Ag+ to Ag, whereas the remaining photogenerated electrons would recombine with photogenerated holes (process (ii)). The remaining holes would oxidize water molecules, as the potential of the excited holes is more positive than that required for water oxidation (+1.23 V vs. NHE). In contrast, the contribution of IFCT (process (iii)) to Ag deposition would be small for thick (100 nm) CuO films, because the photogenerated electrons would recombine with holes as they diffuse 100 nm to the surface of the film (process (iv)). In the ultrathin CuO film, in contrast, IFCT absorption (process (vii)) would be dominant in any region where CuO is deposited. Near the edges of the CuO films, photogenerated holes in TiO2 can laterally diffuse to the bare substrate region, where they would react with water molecules. However, photogenerated holes in the TiO2 located far from the film edges would not access the aqueous medium and would therefore predominantly recombine with photogenerated electrons (process (viii)). As the electric state and crystallinity of ultrathin CuO films are different from those of bulk CuO, the contribution of excitation process that occurs in CuO itself (processes (v) and (vi)) is not clear at present.

FIG. 3.

Schematic energy diagrams and proposed photocatalytic processes (i-viii) for CuO/TiO2 heterostructures with (a) thick and (b) thin CuO films.

FIG. 3.

Schematic energy diagrams and proposed photocatalytic processes (i-viii) for CuO/TiO2 heterostructures with (a) thick and (b) thin CuO films.

Close modal

In summary, we observed a unique photocatalytic reaction in patterned CuO films grown on single crystals of rutile TiO2. The reaction sites in the CuO/TiO2 heterostructure strongly depend on the CuO film thickness. Ag particles were selectively deposited near the edges of the ultrathin CuO film, whereas those were distributed over the surface of the thick CuO film. The selectivity of the photocatalytic reaction can be understood by assuming an interfacial excitation process between TiO2 and CuO when the thickness of CuO is several nanometers. A systematic study using the present model system would be highly valuable to optimize the key parameters of CuO-TiO2 catalysts, including CuO thickness, CuO/TiO2 exposed surface ratio, film edge density, and edge-to-edge distance. It is expected that the present findings concerning the factors influencing the photocatalytic efficiency of TiO2 can be extended to other fabrication processes, such as sputtering, chemical vapor deposition, and sol-gel coating. Further, the present methodology is expected to aid the development of efficient photocatalyst films with self-cleaning, deodorization, and anti-bacterial properties, which are particularly suited for indoor light sources, such as white fluorescent bulbs or white light emitting diode.

This work was supported by a grant from JST, PRESTO, and from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Elements Science and Technology Project and Elements Strategy Initiative to Form Core Research Center), and MEXT KAKENHI, Grant No. 26410234. We also thank Mr. Greg Newton for the critical reading of the manuscript.

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Supplementary Material