Improved photocatalytic activity and mechanism of Cu₂O/N–TiO₂ prepared by a two-step method

Abstract

Cu₂O/N–TiO₂ was achieved through a two-step route by deposition of CuO on Degussa P25 TiO₂ using a wet impregnation method and then treatment with N₂/Ar plasma. The morphology, composition, and structure of the prepared catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). XRD spectra indicated that neither the low-content copper oxides nor the plasma treatment changed the crystalline phase of TiO₂. XPS results indicated the transformation of Cu species from CuO to Cu₂O and the doping of N after plasma treatment. EPR spectra confirmed the formation of Ti³⁺ that was also induced by plasma treatment. TEM analysis revealed that the well crystallized Cu₂O nanoparticles, existed with sizes of ∼3 nm, dispersed uniformly on the surface of TiO₂. As for the optical absorption curves obtained by UV-vis absorption spectroscopy, an extended absorption edge of Cu₂O/N–TiO₂ was confirmed compared with that of TiO₂ and CuO/TiO₂. In addition, the photodegradation efficiency of methyl orange (MO) solutions over Cu₂O/N–TiO₂ were markedly enhanced under both the visible and full-spectrum light irradiation in comparison with TiO₂ and CuO/TiO₂. The mechanism of photocatalytic reaction over Cu₂O/N–TiO₂ corresponding to UV and visible light irradiation was discussed.

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1. Introduction

Titanium dioxide has been extensively studied and used in a vast variety of applications ascribed to the exceptional physical and chemical properties in photocatalysis,^1,2 however, its photocatalytic efficiency is restricted by the large band gap (3.2 eV for anatase) and the rapid recombination rate of photogenerated charge carriers. Generally, TiO₂ can only be excited by UV light under 387 nm that just accounts for 3–5% of solar spectrum, of which less than 10% of the energy can be utilized due to the high recombination rate.^3,4

So far, numerous modification methods have been developed to enhance the photocatalytic activity of TiO₂ by shifting the optical response to the visible light region and decrease the electron–hole recombination rate, such as non-mental doping,^5,6 transition metal loading,⁷ self-modification,⁸ and so on. Among these methods, nitrogen doping is regarded as one of the most effective approaches to extend the absorption edge from the UV to the visible light region owing to the induced N 2p state,^4,9–11 but the interband states always cause the decrease of oxidation/reduction potential of photogenerated charge carries. Cu₂O is one of the p-type direct band gap semiconductors with a narrow band gap of ∼2 eV, Cu₂O nanoparticle-loaded TiO₂ was proved to be an efficient photocatalytic material by effective separation of charge carriers and reducing of the recombination rate of electron–hole pairs in nano–nano heterostructure, and the absorption of Cu₂O to visible light greatly extends the absorption range of Cu₂O/TiO₂ heterostructure.^12–17

Recently, there has been growing interests about the multiple modification methods, such as the codoping of TiO₂ with N and Cu. Morikawa et al. prepared Cu loaded TiO_2−xN_x using a wet impregnation method and revealed that the photocatalytic activity of metal oxide loaded TiO_2−xN_x was twice than that of TiO_2−xN_x.¹⁸ Co-modifications of TiO₂ with Cu and N were also achieved by other methods, such as using N₂ gas plasma in the presence of Cu foil¹⁹ or coprecipitation.²⁰ It is worth noting that plasma treatment was taken as an effective way to induce surface modification. All the modified photocatalysts resulted in photocatalytic activity enhancement and absorption edge expansion, but the mechanism of the superior performance was rarely explored, especially the relationship among the dopants and the effect of each other.²¹

In this paper, Cu₂O/N–TiO₂ was prepared through a two-step route by deposition of CuO nanoparticles using a wet impregnation method and then treatment with N₂/Ar dielectric barrier discharge (DBD) plasma. The morphology, structure, and composition of the photocatalysts were systematically characterized by X-ray diffraction (XRD), UV-vis absorption spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). And the photocatalytic activity was evaluated by monitoring the photodecoloration of methyl orange (MO) solution under irradiation of 300 W xenon light with 400 nm cut-off or not. The mechanisms of the charge carrier redistribution and photocatalytic activity enhancement under irradiation of light in different wavelength were also proposed.

2. Experimental

2.1 Preparation of Cu₂O/N–TiO₂ samples

CuO nanoparticles (Cu : Ti = 0–5 at%) were deposited on Degussa P25 TiO₂ using a minor modified simple impregnation method described by Liu et al.²² Briefly, 1 g of TiO₂ was dispersed in 100 mL of 0.25 M NaOH aqueous solution, and then a certain volume of 0.05 M Cu(NO₃)₂·3H₂O solution was added dropwise under vigorous stirring. After further stirring for 6 h, the precipitates were filtrated and washed with deionized water until pH to 7, then dried at 80 °C for 12 h and subsequently calcined at 400 °C for 2 h in air. The final powders were in blue to black with the increasing Cu : Ti ratios. At last, the resultant CuO/TiO₂ was then treated with N₂/Ar plasma in a DBD plasma reactor.

The experimental set-up arrangement for the plasma treatment is presented in Fig. 1. Molybdenum wire was served as the outer electrode and winded around a 60 mm outer diameter corundum tube compactly, and a stainless steel rod (40 mm in diameter) was settled in the center of the corundum tube and acted as the inner electrode, which was connected to the high voltage. The discharge gap between the corundum tube and inner electrode was 7 mm, where a quartz boat with the samples was placed. During the experiments, a high-voltage pulse power supply was used to generate dielectric barrier discharge plasma between the steel electrode and the outer electrode that attached to the ground.

Fig. 1 Sketch of the experimental set-up arrangement for the plasma treatment.

N₂ (99.999%) and Ar (99.999%) were introduced into the reactor, the pressure and flow rates of N₂ and Ar were controlled with the mass flow controllers. The experiments were carried out with the changes in the power delivered to the plasma, plasma treatment time, reaction pressure, and gas composition.

2.2 Structural characterization

XRD analysis was carried out to verify the crystal structures of the photocatalysts in a diffractometer (Rigaku, Smartlab3) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA. The patterns were recorded in a range from 20° to 80° (2θ) with a step size of 0.02° s⁻¹.

UV-vis absorption spectra was collected over the wavelength range 300–800 nm on a PerkinElmer Lambda 950 scanning spectrophotometer fitted with a 150 mm integrating sphere attachment. Polytetrafluoroethene was used as the reference.

The lattice structure was visualized by TEM using a JEOL JEM 2010F electron microscope instrument, equipped with a field emission gun (FEG) electron source using an acceleration voltage of 200 kV.

XPS analysis was performed on a PHI 5000 VersaProbe system to measure the surface properties of the photocatalysts. The general C 1s, O 1s, N 1s, Ti 2p, and Cu 2p core level spectrum scan was performed. All binding energies were referenced to the C 1s peak of the adventitious carbon on the surface at 284.6 eV.

EPR spectra were recorded at 77 K on an EMX 10/12 spectrometer working in the X-band with 10 mg of the sample introduced into an EPR quartz probe cell.

2.3 Photocatalytic evaluation

The photocatalytic activity of each photocatalyst was evaluated by monitoring the decomposition of MO in a self-assembled apparatus. Before irradiation, an adsorption–desorption equilibrium was attained by stirring using a magnetic stirrer for 30 minutes.

A solution of 100 mL of MO (10 mg L⁻¹) and 0.10 g of catalyst were placed into a water-cooled jacket beaker, which was open to air. The sample was illuminated using a xenon light source of 300 W (CEL-HXF 300) at a distance of 5 cm from the reactor, and the photocatalytic reactions were carried out at neutral pH conditions. For visible-light irradiation, UV-cut filter of 400 nm was used to eliminate UV light irradiation. The temperature of the suspension was kept at about 20 °C by an external cooling jacket with recycled water. The suspension was irradiated for different time intervals (20 or 120 minutes for full-spectrum and visible light, respectively), and a 5 mL aliquot was sampled and filtrated through a 0.45 μm syringe filter to remove the photocatalyst particles, then the dye photodegradation was monitored by measuring the absorbance of MO concentration before (C₀) and after (C) irradiation using a UV-vis spectrometer (Spectrumlab 752s) at 464 nm, and the decomposition efficiency (%) was used to represent the photocatalytic activity.

3. Results and discussion

3.1 Characterization of TiO₂, CuO/TiO₂, and Cu₂O/N–TiO₂

XRD patterns of TiO₂, CuO/TiO₂, and Cu₂O/N–TiO₂ are shown in Fig. 2. The P25 TiO₂ powder is composed of about 20% rutile and 80% anatase, and when P25 was deposited with Cu nanoparticles, the changes on peak position or intensity were rarely observed on the X-ray powder patterns of titania polymorphs. For all the photocatalysts, no significant characteristic diffraction peaks responding to Cu or Cu_xO were detected even when the Cu : Ti ratio was increased up to 5%, it perhaps attributed to the extremely small clusters and/or the lower copper content in these samples,^23,24 which could be confirmed by TEM. Furthermore, the N₂/Ar plasma treatment also didn't change XRD patterns, which means it has no effect on the crystal structure of TiO₂.

Fig. 2 XRD patterns of TiO₂, CuO/TiO₂, and Cu₂O/N–TiO₂ samples.

Fig. 3 shows a comparison of the UV-vis absorption spectra of the photocatalysts. According to these spectra, the loaded copper and plasma treatment didn't induce the change of band gap, and the absorption at the wavelength shorter than 400 nm was attributed to the intrinsic band gap absorption of TiO₂.¹² Meanwhile, it's obviously that the light absorption by CuO/TiO₂ and plasma-treated Cu₂O/N–TiO₂ was strongly affected compared with the spectra of TiO₂.

Fig. 3 Optical absorption curves of TiO₂, CuO/TiO₂, and plasma treated Cu₂O/N–TiO₂.

For CuO/TiO₂, a great increase in the absorption at wavelength longer than 400 nm was observed. Since CuO is a semiconductor with a narrow band gap, it can be excited by visible light and obtain absorption at 400–600 nm centered at 500 nm.²⁵ While the increase of the absorption can also be observed in the 700–800 nm wavelength region because of d–d transition of Cu²⁺ ions. This kind of absorption was considered to have no contribution to the increase of photogenerated carries but just extend their lifetime.¹⁸ Electrons transfer from the valence band (VB) of TiO₂ to the metal ions attached to the surface of TiO₂, which is known as interfacial charge transfer (IFCT), may also result in absorption around 450 nm.²⁶

Moreover, a red shift of absorption edges is referring to the sample after plasma treatment. It has been reported that oxygen vacancies (Ov) could be generated on TiO₂ during N₂ plasma process and produced TiO_2−xN_x.^27–30 The plasma treatment leads to the configuration of Ov and N 2p state located at ∼1.18 and ∼2.48 eV below the conduction band (CB),^31–33 which also causes visible light absorption.

TEM images are shown in Fig. 4. The HR-TEM images show the clear lattice fringes of TiO₂ nanoparticles and confirm the crystallinity of the TiO₂, but Cu₂O rarely exists in 1% Cu₂O/N–TiO₂ due to the low content (Fig. 4b), which is consistent with the former research.²² In Fig. 4c and d, the nanoparticles, in the size of ∼3 nm, are well dispersed on the surface of TiO₂ in 5% Cu₂O/N–TiO₂. Since the charge transfer rate across the interface of Cu₂O and TiO₂ relies on the size of Cu₂O particles, the diffusion of charge carries across the interface can be more favorable in this work.^24,34,35 As shown in Fig. 4c and d, the fringes with a distance of 0.208 nm match well with {111} planes of cubic Cu₂O.³⁶ A lattice fringe of 0.240 nm is assigned to {001} plane of rutile TiO₂.

Fig. 4 TEM images of (a) TiO₂, (b) 1% Cu₂O/N–TiO₂, (c and d) 5% Cu₂O/N–TiO₂.

The XPS analysis was carried out to determine the surface composition of samples and to identify the valence state of various species. As illustrated in Fig. 5a, before the plasma treatment, the Cu 2p spectra of CuO/TiO₂ samples reveal two peaks (Cu 2p_3/2 at 933.1 eV, and Cu 2p_1/2 at 952.9 eV), but the satellite peak (at ∼942.2 eV), which can be characterized as Cu²⁺,²² is not obvious due to fact that the concentration of Cu is low (1% in this research) in comparison with other researches.²³ Interestingly, the peak related to the Cu²⁺ state shifts to lower binding energy compared with usual 933.2–934.6 eV,³⁷ because highly dispersed or calcined Cu²⁺ species have a lower binding energy than bulk Cu²⁺ in CuO.^38,39 While after plasma treatment, the binding energy of Cu 2p_3/2 shifted to a low binding energy of 931.7 eV, suggesting that Cu, Cu₂O or Cu hydroxides are present on the catalyst surface. The Cu²⁺ was reduced to Cu⁺ by high energetic electrons and active species during plasma treatment.¹⁹ Combined with previous TEM images, it's believed that the Cu species incorporated on the surface of TiO₂ mainly exist as Cu₂O.²³

Fig. 5 XPS spectra of (a) Cu 2p, (b) N 1s, (c) Ti 2p, and (d) O 1s before and after treatment, respectively.

N 1s spectrum of plasma-treated sample was shown in Fig. 5b, and the peaks at 399.4 and 396.3 eV were found. Many researchers thought the N 1s peaks around 399–400 eV were corresponding to chemisorbed γ-N₂ molecules incorporated into TiO₂ lattice, while other researchers assigned the peaks in the range of 399–400 to the presence of NO and/or interstitial nitrogen species⁴⁰ for the reason that molecular N₂ is impossible to be chemisorbed on metal oxides at room temperature. N 1s peak around 396 eV has been assigned to β-N atoms in Ti–N bonds and shows the substitution of O by N in the TiO₂ lattice, which is responsible for the visible light absorption.^4,41

As shown in Fig. 5c, the peaks of Ti 2p_1/2 and Ti 2p_3/2 for the undoped TiO₂ sample were observed at 465.0 and 459.5 eV, respectively, which indicated that the Ti of TiO₂ is in the form of Ti⁴⁺ ions. When N₂/Ar plasma was applied to change CuO/TiO₂ into Cu₂O/N–TiO₂, the peaks of Ti 2p_1/2 and Ti 2p_3/2 were shifted to 464.0 eV and 458.5 eV, respectively. This clearly showed a decrease in the binding energy of Ti 2p, which indicated that nitrogen was successfully incorporated into the TiO₂ lattice.^20,42 Ti³⁺ may be formed due to the chemical modification induced by the sputtering of plasma.⁴³ Fig. 5d presents the O 1s spectra for the samples. Peaks of the O 1s region for the TiO₂ sample were observed at 528.1 and 529.2 eV. The peak at 528.1 eV could be attributed to the oxygen in the TiO₂ lattice, while the signal at 529.2 eV could be ascribed to the metal oxide and metal–OH. The peaks of the O 1s region moved to significantly lower binding-energy levels for the N-doped TiO₂ sample, indicating the incorporation of nitrogen into the TiO₂.²⁰

Furthermore, EPR spectra (Fig. 6) of photocatalysts recorded at 77 K was obtained to identify the presence of Ti³⁺ before and after plasma treatment. It was reported that paramagnetic Ti³⁺ has a g-value of 1.94–1.99.^30,44 The unmodified TiO₂ and plasma-treated TiO₂ all reveal the signal of Ti³⁺. The signal of pristine P25 at g = 1.975 indicates the existence of intrinsic defect, and the stronger EPR signal of plasma-treated P25 attributed to the plasma treatment. Meanwhile, the surface Ti³⁺ defect sites would adsorb and react with oxygen molecules to form O⁻ and O²⁻ species, which have signals around g = 2.00.⁴⁵ But they may overlap with the broad Ti³⁺ signal and thus be invisible in our spectrum.⁴⁶

Fig. 6 EPR spectra of the samples before and after plasma treatment recorded at 77 K.

3.2 Photocatalytic activity for the photodegradation of MO

To evaluate and compare the photocatalytic activity of TiO₂, CuO/TiO₂, and Cu₂O/N–TiO₂, the degradation of MO under visible light irradiation for 120 min and full-spectrum light irradiation for 20 min was monitored, and the values obtained by the repetition (three times) are shown in Fig. 7(a). From the figure, the photodecoloration efficiency of MO follows the trend of Cu₂O/N–TiO₂ > CuO/TiO₂ > TiO₂, which revealed the promotion effects of Cu species and plasma treatment no matter under visible or solar irradiation.⁴⁷ Photocatalytic activity of CuO/TiO₂ and Cu₂O/N–TiO₂ nanoparticles was considerably increased with respect to that of pure TiO₂. The repeated experiments for the photodegradation of MO were performed by Cu₂O/N–TiO₂ to investigate the photochemical stability of the catalyst, and the results are shown in Fig. 7(b). The photocatalytic activity of reused Cu₂O/N–TiO₂ was slightly decreased even after being repeatedly used for 5 cycles, which may due to the aggregation of photocatalyst during separation and dry process, and the photochemical stability of the Cu₂O/N–TiO₂ under the irradiation of both the full-spectrum light and the visible light was considerable in general. According to Dong et al., metal ions deposited on the surface of N doped TiO₂ could improve the stability of the doping nitrogen, thus enhancing the photochemical stability of as-prepared photocatalyst.⁴⁸

Fig. 7 (a) Degradation of MO under visible light irradiation for 120 min (blue) and full-spectrum light irradiation for 20 min (red) by TiO₂, CuO/TiO₂, and Cu₂O/N–TiO₂ (three repetition experiments), respectively. (b) Photochemical experiments of Cu₂O/N–TiO₂ under visible light irradiation (blue) and full-spectrum light irradiation (red) after being repeatedly used for 5 cycles.

For CuO/TiO₂, the photoactivity was augmented gradually with the increase of Cu : TiO₂ ratio (Fig. 8). However, it was decreased with the further increase of copper above 1.0%, so we choose the Cu/TiO₂ in the ratio of 1% for the subsequent plasma treatment. This may be due to the fact that the excess copper species on TiO₂ surface act as charge recombination center, which is detrimental to the photocatalytic activity of the catalyst.⁴⁹ It's also possible that the CuO and Cu₂O particles loaded onto the TiO₂ photocatalyst absorb the light, resulting in a decrease of light reaching to the catalyst.¹⁸ Additionally, the densely covered nanoparticles reduce the opportunity of interaction between catalysts and substances.

Fig. 8 Degradation of MO under visible light irradiation for 120 min (black) and full-spectrum light irradiation for 20 min (red) by CuO/TiO₂ in varies Cu/TiO₂ ratio (three repetition experiments), respectively.

Meanwhile, it seems that plasma treatment is an effective way to induce the promotion of photodegradation efficiency. The power delivered to the plasma, treatment time, reaction pressure, and gas composition all have effects on the treated samples.^42,50,51 In this study, the Cu₂O/N–TiO₂ was obtained under the optimal plasma conditions of N₂ : Ar = 4 : 1, 20 min of time, and current at 1.5 A (380 volts). Long-time plasma treatment would lead to the formation of excess Ov, which acts as recombination site for electrons and holes and leads to poor performance.⁵² The concentration of N species and ratio of β-N to γ-N are deeply affected by the gas composition, and the low proportion of N₂ will lead to the decrease of doped N species. Additionally, different N precursors form different groups in TiO₂, which may have diametrically opposite effects on catalytic activity.⁴² The power delivered to the plasma treatment is also a controllable element for the character of catalyst.¹⁹

3.3 Mechanism of photocatalytic reaction over Cu₂O/N–TiO₂

Both the absorption edge and photocatalytic activity are improved by Cu oxides deposition and band structure modification. Cu₂O is a relatively narrow band gap p-type semiconductor, in which both the conduction and the valence bands lie above those of TiO₂. Generally, it's known that the combination of Cu₂O and TiO₂ forms type-II heterojunction, in which electrons transfer from Cu₂O to TiO₂, whereas holes transfer in the opposite direction to achieve e–h separation.^17,43,53 According to the band structure, when the photocatalytic test is carried out under visible light that only Cu₂O can be excited due to its narrow band gap, electrons can be excited in p-type Cu₂O and move to TiO₂. Caused by the charge flow, the efficiency of the visible light utilization can be significantly improved, leading to a higher photocatalytic activity. Moreover, the photoabsorption of Cu₂O-loaded TiO₂ extends into the visible light region.

Besides, it's believed that the induced N exists as N 2p localized states shallow above the VB of TiO₂ other than mixed with O 2p,^9–11 which composes the VB. The N 2p intraband was thought to be the origin of visible light response, and photoexcitation of N 2p to Ti 3d (Ti 3d ← N 2p transition) provides visible light-induced photocatalysis over N–TiO₂.²¹

Apart from the substitutional N in the lattice, other defects also play a vital role in visible light response, such as interstitial N and Ov. For example, the grafted Cu species promote the formation of one-electron defect sites (Ov).⁵⁴ Especially, Ti³⁺ species were inevitably induced by collision between active species and the surface of catalyst during the plasma treatment.^28,30,55,56 The Ti³⁺ would be formed as isolated states in the forbidden gap. This isolated band has various electric levels from 0.3 and 0.8 eV below the CB minimum. The formed Ti³⁺ states in its forbidden gap would cause visible light absorption, although this kind of effect was negligible.^32,57 The active species generated during the plasma treatment could partially reduce TiO₂ and induce a shift of the absorption edge. Our results are in agreement with previous studies that Ar, H₂, and N₂ plasma were used.^{28,29,47,55,58} Meanwhile, the Ti³⁺ species on the TiO₂ support may also induce efficient catalytic activity by a strong electron interaction between Cu²⁺ in highly dispersed CuO and Ti³⁺ on rutile TiO₂ (Cu²⁺ + Ti³⁺ → Cu⁺ + Ti⁴⁺), which leads to the high activity of the catalysts.³⁹

Combined with the previous reports, we postulated that there exist two specific mechanisms with regard to photogenerated charge carrier transferring in conjunction with visible and UV light irradiation, respectively.

On the one hand, when irradiated by UV light that can activate TiO₂ even N 2p state, the photoexcited electrons are activated from VB to the CB of N–TiO₂, then transfer to the loaded Cu species, which inhibits the recombination of electrons and holes, and leads to the promotion of catalytic efficiency (Fig. 9A).^21,59 At the same time, Cu₂O can be excited due to the narrower band gap compared with TiO₂. The generated electrons transfer to CB and are ready to react with absorbents.

Fig. 9 The schematic diagram of the mechanism for charge carrier redistribution in Cu₂O/N–TiO₂ irradiated by (A) full-spectrum light and (B) visible light.

On the other hand, when the energy of photons is not strong enough to stimulate electrons in VB of TiO₂ (Fig. 9B), the excited electrons from N 2p states will be trapped by deep defect levels and can't transfer to Cu species unless extra energy was applied.²¹ But the electrons stimulated from Cu₂O can migrate to CB of TiO₂ because of the disequilibrium between TiO₂ and Cu₂O.⁵⁹ Meanwhile, the IFCT mechanism may be also suitable for the Cu₂O/N–TiO₂ system. The interband structure of Cu₂O/N–TiO₂ is more favorable to induce IFCT than CuO/TiO₂.^26,60,61 Just like Cu/WO₃, the electrons generated by interband excitation after visible light irradiation can also transfer to Cu species through IFCT.²⁶ Then the electrons and holes react with substances on the interface, respectively. Irie et al.^26,60 proposed that the visible light sensitivity was caused by the photo-induced IFCT from the VB of oxides to Cu ions, and the reduced Cu species act as a multi-electron oxygen reduction catalyst. Visible light can be absorbed by the IFCT between the VB of TiO₂ and surface-modified Cu ions. In addition, the strong oxidation power of holes produced in the VB of TiO₂ is utilized. This speculation was experimentally confirmed using electron spin resonance and chemiluminescence.^61,62

Overall, the generated holes and electrons react with water and oxygen, respectively, to form O-centered radical intermediates such as ˙OH, ˙OOH, and ROO˙.⁶³ The intermediate active species will react with substances that absorbed on the surface of catalyst.

There are other opinions think that the Cu species enhanced photocatalytic activity under visible light by a strong interaction with intermediates. For example, Li et al.⁶⁴ attributed the photocatalytic improvements to the surface/interfacial Cu²⁺ sites in the 0.1% CuO/TiO₂ nanocomposite that function as unique adsorption sites for reactant molecules and facilitate the subsequent photocatalytic degradation. But the information of active intermediates has not been obtained yet.

Though the effect of N₂/Ar plasma treatment on Cu₂O can't be characterized in our study, it should still be taken into consideration. The N doping of Cu₂O would lead to the optical band gap widening and shift of VB and CB. The interband would be formed also, resulting in enhancement of absorption ability for solar light.^65,66

4. Conclusion

Cu₂O/N–TiO₂ was achieved by loading with optimal content of CuO on TiO₂ and followed by N₂/Ar plasma treatment, and efficient photocatalytic ability for MO degradation under both visible and full-spectrum light irradiation was obtained. After Cu deposition and plasma treatment, the band gap of TiO₂ is still preserved. The highly dispersed Cu species transfer to well-crystal Cu₂O after N₂/Ar plasma treatment. Meanwhile, substitutional N and other defects (like Ov and interstitial N) are induced at the same time. Due to the formation of p–n heterojunction and intraband deep localized states, the trapping of electrons and the charge carrier lifetime enhancement of Cu₂O/N–TiO₂ are more effective than that of TiO₂ and CuO/TiO₂.

We speculated the mechanism for enhancement of the photocatalytic activity irradiated by light at different wavelength. Mechanisms for enhancement of the photocatalytic activity differ depending on the excitation wavelength. When irradiated by full-spectrum light that can activate electrons on VB of TiO₂, the generated charge carriers transfer to the loaded Cu species and lead to the decrease of recombination of electron and hole, which would enhance the photocatalytic activity. While irradiated by visible light that can only stimulate the electrons on VB of Cu₂O and interband states of N–TiO₂, the charge carriers can be separated well via the migration from Cu₂O to TiO₂.

Acknowledgements

The authors acknowledge the project supported by the Natural Science Foundation of Jiangsu province, China (no. BK2011808) and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (no. 10KJB61005).

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