TiO₂ nanocrystal incorporated with CuO and its optical properties

TiO₂ is of the particular interest because of its special properties, such as being a wide forbidden energy band gap semiconductor, friendliness to living organisms, stability in water, and having strong photocatalytic properties when its crystal grain size is reduced to tens of nanometers. Having large energy band gaps (3.0 and 3.2 eV for rutile and anatase phase of TiO₂, respectively) TiO₂ has been considered as a photocatalyst in ultraviolet light. Ultraviolet light contributes only about 3–5% of total solar power, so the photovoltaic or photocatalytic activity of TiO₂ materials under sunlight is very low. In order to increase the photovoltaic and photocatalytic activity of TiO₂ materials under sunlight, TiO₂ was modified by doping metal ions such as Fe, La, Mo...for Ti [1, 2] as well as non-metal ions such as N, C, F [3, 4]. It was shown that the substitution of Ti by metal or non-metal ions is effective in enhancing the photocatalytic activity of TiO₂ [3]. However, many studies have demonstrated that the monodoping elements can act as recombination centers where photo-induced charges recombine, which reduces charge quantity and photovoltaic, photocatalytic activity of TiO₂ [5]. This is a reason for fewer practical applications of this material till now. Fortunately, recent studies found that the defect bands can be passive and will not be active as carrier recombination centers when co-doped with two different elements in oxide semiconductors [6]. In recent years, many studies on the co-doping effect of transition metal and non-metal elements have been carried out and have given considerable enhancements on visible light absorption and photocatalytic properties of TiO₂ [7]. Gai et al [6] simulated the co-doping process of C–Cr and C–Mo. The results showed that the energy band gap of the material decreased depending on the metal and non-metal ion pairs. In the case of co-doping of C–Mo and C–Cr pairs the TiO₂ band gap can be reduced by 0.9 and 1.2 eV, respectively. Many experimental studies on the modified TiO₂ by co-doping C, Mo and Bo have also been carried out and reported [7, 8]. The experimental results show that doping of TiO₂ material is a suitable method for manufacturing modified TiO₂ active in the region of visible light. It was found also that CuO is an oxide co-catalyst suitably incorporated with TiO₂. Recently, groups of authors reported results concerning the incorporation of TiO₂ with CuO and Cu₂O [9, 10]. Xu et al [10] reported an efficient photocatalyst for hydrogen generation from water using Cu incorporated TiO₂ samples fabricated by the sol–gel process. Liau and Chang modified TiO₂ electrodes by incorporation of TiO₂ with Cu₂O and reported that the Cu₂O addition reduced the electrode resistance and consequently enhanced the photocatalytic efficiency in water splitting [11]. Recently, Nguyen et al have discussed the synthesis of TiO₂ nanostructures, their physical as well as chemical properties and application possibilities, and then achieved an essential review focused on the visible light-responsive titania-based nanostructures for applications in photocatalytic, photovoltaic and photoelectrochemical electrodes [12]. The review article opened the idea for production of TiO₂ nanostructures as well as the realization of making TiO₂ photoanodes of high efficiency in hydrogen generation and/or in other photovoltaic effects. Looking at the suitable photocatalytic material working in visible light, our research achieved both demands. The first is to produce TiO₂ photocatalytic material by doping Cu into TiO₂ structure, and the second is simultaneously to incorporate TiO₂ with CuO in the aim to get a co-catalytic system to be used for hydrogen generation from water by sunlight.

In this article we present recent results concerned with Cu-doped TiO₂ material in the search for the photocatalyt that works in the visible light region.

We synthesized TiO₂ doped with Cu by using wet chemical processing. Titanium isopropoxide (Ti(i-OC₃H₇)₄) (TPOT), axetalacetone C₅H₈O₂ (ACT) and Cu(NO₃)_2·3H₂O from Aldrich company were used as starting materials in synthesis of TiO₂ samples doped with Cu. The process is as follows: first, a mixture of TPOT and ACT weighted in a mole ratio of 1 : 1 was mixed and stirred for 1 h with temperature remaining at 80 °C. After that, the temperature was reduced to 60 °C and Cu(NO₃)₂·3H₂O 1 M was slowly dripped into it with continuous stirring for 4 h. In order to prepare the TiO₂ nanoparticles we simultaneously dripped in H₂O and stirred the mixture until the precipitation was seen in solution. To finish the wet processing a continued stir has carried out for 12 h at room temperature. Finally, using a centrifugal machine we separated TiO₂ powder from the solution. The separated powder was dried at 100 °C for 1 h and then heated at 500 °C for 5 °h. The crystalline structure of all the samples was identified by using an x-ray diffractometer Siemens D5000, and the morphology of the samples was evaluated by using a Hitachi scanning electron microscope (SEM) 4800S. The optical absorption and luminescence of all the samples at room temperature were also carried out by using a UV–Vis Cary 5000 and a high-resolution luminescence spectrometer, respectively.

Figure 1 shows x-ray diffraction patterns of the Cu-doped TiO₂ samples. The results show that the sample with Cu concentrations smaller than 8 at% is of pure TiO₂ anatase phase (curves a and b in figure 1).

Zoom In Zoom Out Reset image size

In the given x-ray pattern only the characteristic lines of TiO₂ anatase phase with a large spectral half-width were observed. In the x-ray patterns we did not observe any lines of other unexpected phases. This implies that Cu partially substituted for Ti in TiO₂ structure. However, copper may exist in the sample with a quantity so low that the x-ray equipment could not detect it. The half-width of the x-ray line proved that the crystalline grain size of TiO₂ particles is considerably small. The crystal grain size of the sample estimated by using Scherer formula was about 5–6 nm. This value is smaller than that evaluated from the SEM images as shown in figure 2. This might be due to the congregation of small particles. According to Mackenzie [13], the substitution of Ti⁴⁺ by metal ions of lower valence values increased oxygen deficiency, leading to the electrical neutralization that reduced crystal grain size of TiO₂ particles. This effect has been observed in the TiO₂ sample doped with Co [15]. It was particularly enhanced when Ti⁴⁺ was substituted by a metal of smaller valence value. In the x-ray diffraction patterns of samples with higher Cu concentrations (13 and 18%) characteristic lines for the tenorite CuO at 2θ of 35.5 appeared (see curves c and d in figure 1). To evaluate photocatalytic activity of the photo-anode working in the visible light region, we first investigate its optical properties such as its absorption in the visible light and also its luminescence spectra. The inset in figure 3 presents a red shift of the absorption edge of the TiO₂ samples in dependence of the doping Cu concentration.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

It is evident to suggest the overlapping of the absorption bands of TiO₂ and Cu_xO, where x can change in a range of 1.0–2.0. In dependence on Cu concentration we observed a shoulder around 800 nm in the case of the sample doped by Cu with a concentration of 8 at% that attributed to the absorption concerned with the transitions between states created by doping Cu in TiO₂. We know that the bulk TiO₂ crystal is an indirect semiconductor and its absorption index α(hν) should follow the (hν – Eg)^3/2 dependence. Normally nanomaterials exhibit an absorption spectrum such as that of the direct band gap semiconductor, and its absorption coefficient α(hν) follows the (hν–Eg)^1/2 law. Based on the absorption data presented in the inset in figure 3 we plot the curves of (αhν)² in dependence on (hν–Eg) in figure 3 (red, deep blue and moss green). The red curve presents that for the sample doped by Cu with concentration of 8 at%.

This curve consists of three linear lines such that their prolongations cut the x-axis at 2.49, 1.94 and 1.10 eV. This result confirmed an incorporation of TiO₂ with Cu_xO and the overlapping of their energy bands. The energy value of 2.49 eV is attributed to the band gap of the TiO₂ doped by Cu. The experimental evidence proved again that Cu has substituted Ti in TiO₂ structure. The value 1.10 eV coincided with the band gap of CuO whose existence is confirmed by x-ray diffraction in figure 1. It was known that CuO has a band gap of around 1.1 eV and Cu₂O has a band gap of 2.1 eV, then the obtained value of 1.94 eV is supposed to be the band gap of Cu_xO, where x is smaller than 2. The absorption of samples increased with increasing Cu concentration and its absorption edge shifted significantly to the red, as seen in the inset of figure 3. For the samples doped with 13 and 18 at% of Cu (the deep blue and moss green in figure 3) we have seen only two linear lines that cut the x-axis at 1.10 and 1.58 eV. We consider these values as the band gaps of CuO and Cu_xO, respectively, that co-existed in the samples. In these two samples the absorption edge of TiO₂ was not observed due to the existence of Cu_xO with a stronger absorption that covered the absorption spectra of TiO₂. In summary, we concluded that Cu has substituted Ti in TiO₂ structure and/or incorporated in the form of Cu_xO with TiO₂ that have a red shift in absorption spectra.

In order to understand the transformation of the energy of the light absorbed in the Cu-doped TiO₂ nanocrystals we have searched for radiative transitions in them. We carried out luminescence measurements with the excitation light of 325 and 442 nm (figure 4). No emission was observed for all the samples excited by a light of 325 nm wavelength. On the contrary, strong wide emissions in a range of 450–900 nm were observed when excited by a light of 442 nm wavelength. This recorded experimental luminescence result is an abnormal phenomenon. How to explain it? In principle, due to the excitation by a light (325 nm wavelength) having energy greater than the band gap of TiO₂, electrons must be excited directly from valence band to conduction band and consequently yield free charge pairs of electrons and holes in conduction and valence bands, respectively. These photo-excited electrons have two competitive ways to come back to the initial states. They can relax to the exciton traps in band gap and subsequently relax again to valence band by the radiative transitions via trapped exciton recombination and/or the non radiative transitions. Based on the report of [14] the nature of these relaxation processes happening in TiO₂ nanocrystal can be illustrated as in figure 5.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

In the case of the crystal having a size larger than the free diffusion length of charge in the valence and conduction bands, the photo-excited electrons have a long enough lifetime that they effectively relax to exciton traps and subsequently emit luminescence via the trapped exciton recombination. As is known, the probability to trap excited electrons to the surface states (normally they play as redox centers) depends on the number of surface states, which significantly increases with reducing the size of TiO₂ nanocrystals. Therefore, the probability of trapping the excited electrons to the surface states dominates over the probability of relaxation to the exciton traps when the size of TiO₂ nanocrystals is smaller than the free diffusion length of electrons. If the particle size is much smaller than the diffusion length of electrons, then the photo-excited electrons will probably be trapped into the surface states rather than relaxed to the exciton traps in the band gap. This implies that the non-existence of the emission of Cu-doped TiO₂ nanocrystals excited by a light of 325 nm is related to the absence of relaxation process of the excited electrons to the exciton traps in the band gap. This effect effectively reduces the number of electrons in the exciton traps that contribute an important role in luminescence behavior of Cu-doped TiO₂ nanocrystals. Consequently, the TiO₂ nanocrystals with size smaller than diffusion length are not luminescent. However, the nonexistence of emission depends on various reasons. According to Umebayashi et al [5], metal ions substituted for Ti⁴⁺ in TiO₂ act as recombination centers and this depresses luminescence of material. As shown above, the absence of luminescent emission in our case is not due to the recombination centers created by the doping elements. Conversely, it depends on the size of nanocrystals. For an example, the Co and/or C-doped TiO₂ powder with the crystal grain size of about 20 nm has strong luminescence excited by a light of 325 nm [15]. This means that the photo-excited electrons have effectively relaxed to exciton traps and then they are not able to contribute to photocatalytic processes. This supports that the TiO₂ crystals with large grain size are less responsive in photocatalyst.

In order to prove that the nonexistence of emission of the Cu-doped TiO₂ nanocrystals in our case is not a result of any other non-radiative recombination, we took a test experiment to measure photoluminescence on the same samples used in the previous experiment with an exciting light of 442 nm in wavelength. We have obtained interesting results. In contrast to the case of 325 nm excitation, all the samples emitted a wide luminescence band from 450 to 900 nm when they were excited by the light of 442 nm wavelength. For understanding and explaining this obtained result, we use a modified energy configuration as presented in figure 6.

Zoom In Zoom Out Reset image size

It is evident that energy of the exciting light is smaller than the band gap, therefore electrons from valence band are first excited to certain impurity states in the band gap (created by the doping elements and/or oxygen deficiency) and subsequently excited from these impurity states to the conduction band. It can be called a two-step excitation model. In the second step, a part of the excited electrons was excited again from the impurity states to the band gap, the remaining part kept in impurity states and/or transferred into the exciton traps, which have a contribution to luminescence emission. This obtained luminescence result proved that the impurities do not act as the non-radiative recombination centers. So it confirms again that the nonexistence of emission is a consequence of the absence of the relaxation of the photo-excited electrons from the conduction band to the exciton traps. The part of the excited electrons that were continuously excited to the conduction band in the second step have been trapped into the surface states, acting as the redox centers being active in photocatalysis. In principle, the stronger the luminescence is, the weaker is the photocatalytic material. So it is reasonable to suppose that the Cu-doped TiO₂ nanocrystals with grain size of about 5 nm are an expected photocatalytic material working in visible light. The wide luminescence band is a combination of some other radiative transitions such as the exciton trap in TiO₂ and the radiative transitions between energy states of Cu impurity. Using minimum square method we analyzed the recorded luminescence spectra. The best fitting results show that the luminescence spectra fitted well by using a formula consisting of three single peaks distributed in Gauss function as shown in figure 7. According to Liau and Chang [11], the band with maximum at energy of about 2.3 eV is attributed to the contribution of the trapped exciton in TiO₂. Two other bands with maxima at energies of 1.8 and 1.4 eV are supposed to be related with transitions between the states of the Cu²⁺ ion in crystals of TiO₂. These Cu related peaks shift a little bit to the lower energy as Cu concentration is increased. Moreover, the obtained result shows that a strong energy transfer between the energy levels of Cu and exciton traps occurred in TiO₂ material.

Zoom In Zoom Out Reset image size

We have successfully synthesized the anatase TiO₂ doped with Cu by means of wet chemical processing. Cu²⁺ has substituted Ti⁴⁺ in TiO₂. The substitution of Cu²⁺ for Ti⁴⁺ has prevented the development of crystalline TiO₂ particles, which effectively helped to manufacture TiO₂/Cu materials with small crystal grain size of about 5 nm. The absorption edge strongly shifted to the infrared region of about 900 nm wavelengths. Absorption of visible light and absence of luminescence when the sample was excited by light of 325 nm suggest that the TiO₂/Cu of size 5 nm would be a photocatalytic material responsive in the visible light region. The luminescence of material when excited by 442 nm light is the experimental evidence of an effective energy transfer between exciton states and Cu²⁺ ion and especially the confirmation of the abilities of TiO₂ nanocrystals in photocatalytic applications using sunlight.

This work was supported in part by the Project 'Opening the New Direction in Materials Science Research' under the guidance of Academician Nguyen Van Hieu. The author Le Van Hong would like to express his gratitude to Ms Tran Thanh Thuy and Mr The Anh for taking the luminescence and absorption measurements.