Heterogeneous Deposition of Cu 2 O Nanoparticles on TiO 2 Nanotube Array Films in Organic Solvent

A novel method for decoration of anodic TiO 2 nanotube array films (NAFs) with Cu 2 O nanoparticles has been reported. The method is based on the reduction of Cu(II) in a mixture of ethylene glycol and N,N-dimethylformamide at 120C for 16 h, where the resulting Cu 2 O can heterogeneously nucleate and grow on TiO 2 NAFs. The nanosized Cu 2 O is found to be well dispersed on the wall of TiO 2 nanotubes without blocking the nanotube, a commonly observed phenomenon in the case of deposition of Cu 2 O via electrochemical method. The amount of Cu 2 O deposited on the TiO 2 NAFs can be varied by adjusting the concentration of Cu(II) in the organic solution. UV-vis spectra measurement indicates that the decoration of TiO 2 NAFs with Cu 2 O nanoparticles greatly improves their ability to respond to visible light. By examining the photocurrent and photodegradation of methyl orange under simulated sunlight, it is found that these Cu 2 O-decorated TiO 2 NAFs show much more photoactive in comparison with the as-prepared TiO 2 NAFs.


Introduction
Because of their large aspect ratio and high specific surface area, materials with one-dimensional (1D) structures, for example, nanotubes and nanowires, often exhibit different performances compared to the bulk counterparts.The unique properties of these 1D nanostructures have shown potential applications in many fields, such as electronics, catalysis, data storage, optics, and sensors [1][2][3].1D TiO 2 nanostructures are of great scientific and technical interest because they exhibit excellent photocatalytic activities [4][5][6][7].Over the past decade, great attention has been paid into the synthesis and application of TiO 2 nanotube arrays prepared by anodic oxidation of Ti metal in F − -containing solutions [7][8][9][10][11].The highly ordered nanotube arrays not only possess high surface area, but also provide an efficient transport channel for photogenerated electrons [12].Furthermore, unlike the powder-typed TiO 2 photocatalysts which often need be immobilized onto solid substrates for practical application [13,14], the TiO 2 nanotube arrays are grown on Ti substrates and thus the formed TiO 2 nanotube array films (NAFs) can be directly used as photoanodes for photoinduced redox reactions such as water splitting [15] and decomposition of harmful compounds [7].
However, TiO 2 nanotube arrays possess a wide band gap (∼3.2 eV) and thus only respond well to ultraviolet light, which is a great hindrance to their use under sunlight.To extend their light-response scope from ultraviolet to visible light region, a common approach is postdecoration of TiO 2 NAFs with narrow band gap semiconductors, for example, Cu 2 O [16][17][18][19][20], Fe 2 O 3 [21] and CdS [22].When TiO 2 NAFs are decorated with Cu 2 O, a p-type semiconductor with a direct band gap of ∼2.2 eV, electrons excited under visible light may transfer from the conduction band of Cu 2 O to that of TiO 2 since the conduction band (CB) edge for Cu 2 O is much higher than that of TiO 2 [23].As a result, the recombination probability of the photoexcited electrons and holes will be reduced, leading to a great improvement in photocatalytic activity.So far, the methods for the decoration of TiO 2 nanotube arrays with Cu 2 O mainly include electrodeposition [16][17][18], sonoelectrochemical deposition [19], and photocatalytic reduction [20].In the present work, we report a new method for loading of Cu 2 O nanoparticles onto TiO 2 NAFs.The method is based on the reduction of Cu (II) in a mixture of ethylene glycol (EG) and N,Ndimethylformamide (DMF), where the resulting Cu 2 O can heterogeneously nucleate and grow on TiO 2 NAFs.In comparison with commonly used electrodeposition (including sonoelectrochemical deposition), the size of Cu 2 O is small, and the nanosized Cu 2 O is well dispersed on the wall of TiO 2 nanotubes without blocking the nanotube.

Experimental Details
2.1.Synthesis of the Films.Ti foils were cut into pieces (7.2 cm × 1.7 cm × 0.4 mm), polished with abrasive paper, and then washed with deionized water.The polished Ti pieces were degreased in a mixed solution of NaOH and Na 2 CO 3 (the ratio of NaOH : Na 2 CO 3 : H 2 O by weight is 5 : 2 : 100, resp.) at 85 ∘ C for 1.5 h, and then washed with deionized water.Before anodic oxidization, one side of the pretreated Ti piece was sealed with epoxy resin, and then etched in a 10 wt% HF aqueous solution at room temperature for about 20 s, followed by washing with deionized water.The anodic oxidization of the Ti piece was conducted in an EG solution containing KF (0.7 wt%) and H 2 O (1.8 wt%) at ∼25 ∘ C, where a Cu plate was used as cathode and a constant voltage of 50 V was applied between two electrodes.The anodic oxide layer was formed by a three-step method.Firstly, the Ti piece was anodized for 2 h, and then the grown oxide layer was removed by an adhesive tape.Secondly, the above procedure was repeated.Finally, the Ti piece was reanodized under the same conditions for 1 h.After anodization, the sample was washed thoroughly with deionized water and then dried in the oven at 40 ∘ C for about 12 h.
Deposition of Cu 2 O on TiO 2 NAFs was conducted in a 50 mL Teflon-lined autoclave.The autoclave was filled with a solution containing 30 mL of EG and 10 mL of DMF, where a certain amount of CuSO 4 (ranging from 0.005 to 0.05 g) was previously dissolved.The as-prepared TiO 2 NAFs were immersed in the organic solution, and then the sealed autoclave was kept in an oven at 120 ∘ C for 16 h.After the autoclave was cooled down to room temperature naturally, the resulting samples were removed from the organic solution, washed several times with deionized water, and subsequently dried in an oven at 40 ∘ C for about 12 h.

Characterization and Photocatalytic Activity Evaluation of the Films.
The surface morphology of the films was examined using a scanning electron microscope (SEM, Hitachi S-4700) operating at 15 kV.X-ray diffraction (XRD) analysis was performed on a Thermo ARL XTRA X-ray diffractometer using Cu K X-ray source.The chemical composition of the as-prepared film was characterized by an energy-dispersive X-ray spectrometer (EDS) attached to SEM operating at 15 kV.The ultraviolet-visible (UV-vis) diffuse reflectance spectra were recorded on a UV-2550 (SHIMADSU) spectrophotometer with BaSO 4 as the reference.The photoelectrochemical property of the film electrodes was evaluated in a threeelectrode cell using a Pt wire as counter electrode and a saturated calomel electrode (SCE) as reference electrodes.If needed, the working electrode could be irradiated from the front side by a sunlight-simulation lamp (Osram Ultra Vitalux 300W).The current with or without irradiation was measured in a 0.25 M Na 2 SO 4 aqueous solution using a potentiostat (CHI 620B, CHI Co.).Photocatalytic activities of the samples were evaluated by the photodegradation of methyl orange (MO) solution with an initial concentration of 5.0 mg/L under simulated sunlight.The photodegradation experiments were conducted in a quartz reactor.In each test, one piece of the sample was hung in the liquid.Prior to irradiation, the suspension was kept in the dark for 60 min to achieve the adsorption-desorption equilibrium between the photocatalyst and methyl orange.Then, the solution was exposed to the light irradiation, and samples were taken at given time interval to analyze the concentration of MO by measuring the absorbance with the spectrophotometer.

Results and Discussion
To decorate TiO 2 NAFs with nanosized Cu 2 O, a suitable condition for reduction of Cu 2+ in organic solvent should be chosen.Figure 1  TiO 2 nanotubes, and treatment in CuSO 4 -containing organic solution cannot remove fluorine from the TiO 2 nanotube layer.The observation of fluorine in the nanotube layer should result from the fact that F − will migrate towards the Ti anode during the anodizing process [24].From Figure 3(b), we can also find that the amount of Cu in the film shows an increasing trend as the concentration of CuSO 4 in the organic solution increases.The increasing trend can be roughly fitted by a logarithm equation of  = 6.991 + 0.8747lnx.The surface morphology of treated TiO 2 NAFs can be found in Figure 4.As can be seen from Figures 4(a the film surface.Compared with the corresponding powder materials, nanosized materials dispersed on the film surface is often more difficult to be detected by conventional XRD technique when their crystal size or amount is not large enough.In addition, since the small-sized Cu 2 O particles tend to aggregate due to high surface energy (see Figure 1(b)), the particles of large size observed in Figure 4 might also be composed of several small-sized particles, which further make it difficult to detect Cu 2 O phase by XRD.
To determine whether the treatment of TiO 2 NAFs in CuSO 4 -containing organic solution can extend their lightresponse scope from ultraviolet to visible light region, we have measured the UV-vis diffuse reflectance spectrum of the TiO 2 NAFs treated in organic solution containing relatively high amount of CuSO 4 (viz.0.01 g and 0.05 g). Figure 5(a) shows the UV-vis diffuse reflectance spectrum of these two treated TiO 2 NAFs.For comparison, the UV-vis diffuse reflectance spectrum of the untreated film is also shown.Compared with the untreated film, both two treated TiO 2 NAFs exhibit significant increases in photoadsorption at the wavelength larger than 400 nm, suggesting that they can respond well to visible light.The absorption coefficient  follows the equation ℎ] = (ℎ] −   )  , where ℎ, ], ,   , and  are, respectively, plank constant, light frequency, proportionality coefficient that depends on the properties of the material, band gap, and a constant that can take different values depending on the type of electronic transition [25].For a permitted direct transition,  = 0.5.Figure 5(b) shows the plot of (ℎ]) 2 against ℎ] for three films, where the value of   is obtained by extrapolating the linear part of the graphics to the axis of the abscissa (see dashed red lines).The band gaps estimated from the plots of (ℎ]) The photoelectrochemical property of the as-prepared or Cu 2 O-decorated TiO 2 film electrodes is investigated by measuring the anodic photocurrent in a 0.25 M Na 2 SO 4 aqueous solution.As can be seen from Figure 6(a), without light irradiation (from 0-30 s), the dark currents for all films are almost equal to zero.When the light is on (from 30 to 60 s), the photocurrent increases sharply till reaching a certain value.If the light is off (from 60 to 90 s), the photocurrent declines rapidly to about zero.A similar phenomenon can also be observed in the range of 90-150 s.The observed photocurrent represents the anodic oxidation of water to oxygen by the photogenerated holes at the film electrode under light irradiation.For the untreated TiO 2 film, under light irradiation the electrons are excited from the valence band to conduction band of TiO 2 to form photogenerated electron-hole pairs.The photogenerated electrons and holes are separated under the external potential bias, and most electrons are transferred to titanium substrate to produce photocurrent with the hole oxidizing water to oxygen on the surface of the anode.It is clear from Figure 6(a) that the decoration of Cu 2 O can lead to a great rise in photocurrent density, where the photocurrent density for the treated TiO 2 NAF in organic solution containing 0.05 g CuSO 4 is about 3 times higher than that for the untreated TiO 2 NAF.It is also interesting to note that the photocurrent density almost increases linearly with the amount of Cu in the film (see Figure 6(b)).These results suggest that the decoration of Cu 2 O can greatly improve the water splitting performance of the TiO 2 NAFs under sunlight.The comparison of photocatalytic activity between untreated and treated TiO 2 NAFs in organic solution containing 0.05 g CuSO 4 is also evaluated by the photodegradation of MO.The MO aqueous solution shows an intense absorption band centered at ∼464 nm and the peak intensity is proportional to its concentration.Figure 7 shows a comparison of the temporal evolution of the adsorption spectra of MO solution degraded by two films.It is obvious that the Cu 2 O-decorated TiO 2 NAF exhibits better photocatalytic activity than the undecorated one.The degradation efficiency of MO for the Cu 2 O-decorated film reaches ∼54.7% in 3 h, while that for the undecorated film is ∼31.2%.
The enhanced activity of the Cu 2 O-decorated NAFs observed in our experiments can be attributed to the combined effect of several factors.Firstly, under simulated sunlight, TiO 2 can be excited by UV light, and Cu 2 O can be excited by visible light, which will generate more electrons and holes for photocatalytic reactions as compared to undecorated TiO 2 NAF.Secondly, the combination of TiO 2 with Cu 2 O will lead to a reduced recombination of the photoexcited electrons and holes due to the difference between the band edges of Cu 2 O and TiO 2 semiconductors.As shown in Figure 8, the electron excited under visible light may transfer from the conduction band of Cu 2 O to that of TiO 2 since the conduction band edge for Cu 2 O is higher than that of TiO 2 [23].As a result, the recombination probability of the photoexcited electrons and holes will be reduced, leading to an improvement in photocatalytic activity.

Conclusions
In summary, we have presented a novel method for modification of anodic TiO 2 nanotube array films with Cu 2 O nanoparticles.The method is based on the theory of heterogeneous nucleation and growth in an organic solvent (ethylene glycol and N,N-dimethylformamide) containing CuSO 4 .The Cu 2 O nanoparticles are found to be well dispersed on the wall of TiO 2 nanotubes without blocking the nanotube, and the amount of Cu 2 O deposited on the TiO 2 nanotube array films shows an increasing trend as the concentration of CuSO 4 increases.The decorated nanotube array films can respond well to both ultraviolet and visible light and show much better photocatalytic activity than the undecorated film.
(a) shows the XRD patterns of the products obtained by reduction of Cu 2+ (2 mmol) in a mixture of EG and DMF at different temperatures.At a temperature of 120 ∘ C, all the diffraction peaks appearing in the XRD pattern of the product (indicated by the solid circles) can be indexed to cubic Cu 2 O phase (JCPDS number 65-3288), at which the peaks at 2 values of 29.6 ∘ , 36.5 ∘ , 42.4 ∘ , and 61.5 ∘ correspond to 110, 111, 200, and 220 lattice planes of Cu 2 O, respectively.The broaden peaks indicate that the size of Cu 2 O is very small.The average crystal size calculated by Scherrer's equation for (111) reflections of Cu 2 O is about 10 nm.The SEM image shown in Figure 1(b) reveals that these smallsized Cu 2 O nanocrystals are severely aggregated as a result of reduction in surface energy.When the reduction is conducted at 140 ∘ C, we can observe two new peaks at 2 of 43.2 ∘ and 50.4 ∘ (indicated by open circles), which can be, respectively, assigned to the diffraction of (111) and (200) planes of cubic Cu (JCPDS number 04-0836).The result indicates the formation of Cu at 140 ∘ C. As the temperature is raised to 160 ∘ C, all the diffraction peaks of Cu 2 O disappear.Moreover, the peaks assigned to Cu become very sharp, suggesting that the growth of Cu crystals of large size occurs.This is confirmed by the SEM image of this sample (see Figure 1(c)), where Cu microcrystals can be observed.Therefore, we chose a temperature of 120 ∘ C to deposit Cu 2 O on TiO 2 nanotube arrays, where the concentration of Cu 2+ in the mixture of EG and DMF is changed to control the amount of Cu 2 O deposited on the films.

Figure 2 (Figure 1 :Figure 2 :Figure 3 :
Figure 1: (a) XRD patterns of the products obtained by reduction of CuSO 4 (2 mmol) in a mixture of EG and DMF at different temperatures.(b) and (c) are the corresponding SEM images of the products obtained at 120 ∘ C and 160 ∘ C, respectively.

Figure 7 :
Figure 7: Comparison of the temporal evolution of the adsorption spectra of MO solution degraded by two TiO 2 NAFs: (a) untreated and (b) after treatment in the mixture of EG and DMF containing 0.05 g CuSO 4 at 120 ∘ C for 16 h.
2verses photon energy (ℎ]) are about 3.27, 2.20, and 2.16 eV for untreated TiO 2 NAF and two treated TiO 2 NAFs, respectively, (see Figure5(b)). Figure 8: Schematic diagram for describing the band gap and electron transfer for the Cu 2 O/TiO 2 system.CB, VB, and NHE are the abbreviations of conduction band, valance band, and normal hydrogen electrode, respectively.The observed decrease in the band gap after treatment in CuSO 4 -containing organic solutions is in line with the UVvis adsorption spectra with a red shift.Since the band gap of Cu 2 O is about 2.2 eV, the value of 2.20 and 2.16 eV obtained for two treated NAFs confirms that Cu 2 O is deposited on TiO 2 NAF.