Enhanced photoelectrochemical and photocatalytic activity by Cu₂O/SrTiO₃ p–n heterojunction via a facile deposition–precipitation technique

Abstract

In our study, a new visible-light-driven photocatalyst Cu₂O/SrTiO₃ (C/S) heterojunction was firstly prepared by a simple, facile and effective deposition–precipitation technique. The particle size of the Cu₂O nanoparticle is only about 5 nm and the SrTiO₃ (STO) nanocube is about 50 nm when modified by Cu₂O nanoparticles. The samples are used as photocatalysts for photodegrading tetracycline (TC) under visible light irradiation. The 9-Cu₂O/SrTiO₃ (9-C/S) sample heterojunction shows the highest TC degradation ratio (77.65%), which is caused by the photogenerated electrons of the Cu₂O nanoparticles moving from the conduction band of Cu₂O to that of SrTiO₃, resulting in the separation of electrons and holes. This study not only shows a possibility for substituting noble metals with low-cost Cu₂O nanoparticles in photocatalytic degradation but also exhibits a facile deposition–precipitation technique for synthesizing narrow/wide band gap photocatalysts.

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1. Introduction

Owing to the ability of decomposing organic pollutants completely and splitting water into oxygen and hydrogen under light irradiation, photocatalysis has been widely used in green energy and environmental water treatment.^1–3 Currently, photocatalytically active semiconductors with good prospects are TiO₂, SrTiO₃ and ZnO due to their outstanding properties, including chemical stability, strong oxidizing activity, corrosion resistance, and nontoxicity.^4–6 However, the wide band gap (3.2 eV) of SrTiO₃ corresponds to its low absorption of solar light. Therefore, for practical applications, many methods have been developed for SrTiO₃ modification: noble metal deposition, metal or nonmetal ion doping, sensitization with organic polymers, and coupling with the other semiconductors.^7–9

Many studies, including experimental results and theoretical calculations, have demonstrated that the heterojunction formed by TiO₂ and modified metal oxides contributes to the efficient separation of photo-generated electron–hole pairs, which prolong the lifetime of excited electrons and holes.^10–12 Among various photocatalysts, p-type Cu₂O represents an important type of metal oxide. It has many advantageous characteristics, such as low cost, nontoxicity, unique optical and electrical properties and narrow band gap of 2.0 eV. Cu₂O is used in hydrogen production, sensors, superconductors, solar cells, and photocatalysis.^13–16 The narrow band gap contributes to its effective utilization of solar energy, whereas its strong adsorption of molecular oxygen could scavenge photoelectrons, minimizing the electron–hole pair recombination on its surface. It had been reported that the valence and conduction bands of Cu₂O are both higher than those of TiO₂, which thermodynamically favors the movement of excited electrons and holes between them and could subsequently enhance the separation of charge carriers to decrease their recombination.^17–20 Zhang et al. loaded polyhedral Cu₂O particles on TiO₂ nanotube arrays through electrodeposition and found significant improvement in the visible-light activity as compared to pure TiO₂ nanotubes.²¹ In addition, the Cu₂O/TiO₂ heterojunction could store multi-electrons, which can be utilized for follow-up dark reactions.²²

Herein, owing to the similar valence and conduction band position between SrTiO₃ and TiO₂, we introduce the C/S heterojunction. Dipika Sharma et al. have synthesized a C/S photoelectrode, which was used for hydrogen generation.²³ However, the morphology of Cu₂O is anomalistic, and the synthesis is much more complicated than our deposition–precipitation technique. To the best of our knowledge, there is no report in the literature about the preparation of C/S heterojunction by a simple, facile and effective method. In this study, for the first time, we successfully loaded Cu₂O nanoparticles (NPs) onto the surface of STO nanocubes (NCs) through a facile deposition–precipitation technique. This method has the following advantages: (1) the Cu₂O NPs are uniformly dispersed on STO NCs and a (2) heterojunction can form between STO NCs and Cu₂O NPs, which induces visible-light absorption and efficient separation of photo-generated electrons and holes. The heterojunctions exhibit much better efficiency for the degradation of tetracycline (TC) under visible light irradiation compared with pure STO NCs and Cu₂O NPs, which can be ascribed to the p–n junctions between STO NCs and Cu₂O NPs. This study not only shows a possibility for substituting noble metals with low-cost Cu₂O NPs in photocatalytic degradation but also exhibits a facile deposition–precipitation technique for synthesizing narrow/wide band gap photocatalysts.

2. Experimental

2.1. Materials

Titania TiO₂ (P25) was purchased from Degussa (Germany). Sr(OH)₂·8H₂O, CuSO₄·5H₂O, KOH, NaOH, L-ascorbic acid solution and ethanol were purchased from Aladdin (Shanghai, China). All the reagents are of analytical grade and used without further purification. Deionized water is used in the study.

2.2. Catalysts synthesis

2.2.1 Synthesis of STO NCs. The STO NCs were prepared by a simple hydrothermal method: 3 mmol Sr(OH)₂·8H₂O and 3 mmol TiO₂ were mixed in 33.3 mL deionized water with 2.1 g KOH under vigorous stirring, then the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally treated in an air-flow electric oven at 150 °C for 72 h. After natural cooling, the white STO NCs were collected by centrifugation and washed with deionizer water and ethanol several times, and then dried at 60 °C in air for 12 h.

2.2.2 Synthesis of C/S heterojunction. 0.1 g STO NCs and a certain amount of CuSO₄·5H₂O were dissolved in 20 mL of 0.2 M NaOH solution under magnetic stirring. Then, 1.0 mL of 0.1 M L-ascorbic acid solution was added. The C/S heterojunction was harvested by centrifugation and washed with distilled water and ethanol several times. The pure Cu₂O NPs were prepared similarly without adding STO NCs.

2.3. Photocatalytic degradation of tetracycline

The photocatalytic degradation of tetracycline (TC) was performed as reported in previous research works. 0.1 g of photocatalysts was added to 100 mL of tetracycline solution (10 mg L⁻¹). In order to clear up the impact of adsorption, prior to light illumination, the suspension was endlessly stirred in the dark for 30 min. A 150 W Xe lamp equipped with a filter to cut off light of wavelength <420 nm was used as the light source. At a given time interval, 10 mL of the suspension was sampled and measured by a UV-vis spectrometer at the maximum absorbance (357 nm for TC).A₀ is the initial absorbance of TC at absorption equilibrium, while A_i is the absorbance after the sampling analysis.

3. Results and discussion

3.1. Morphology structure of STO NCs decorated with Cu₂O NPs

As shown in Fig. 1a, the size of STO is about 50 nm and the samples have aggregated, so we cannot observe the morphological structure of 9-C/S heterojunction from the scanning electron microscopy (SEM) image. However, the transmission electron microscopy (TEM) image provides insights into the structure of the 9-C/S heterojunction. As shown in Fig. 1b, the Cu₂O NPs are uniformly dispersed on the surface of STO NCs (about 50 nm). The morphologies and structures of 9-C/S heterojunction are further studied by HRTEM (Fig. 1c). The lattice fringes in the pure STO image have an interplanar spacing d = 0.225 nm, which perfectly corresponds to the (111) plane of STO, and the interplanar spacing d = 0.213 nm corresponds to the (200) plane of Cu₂O. From the HRTEM of 9-C/S heterojunction, the particle size of Cu₂O is about 5 nm.

Fig. 1 SEM image of 9-C/S heterojunction (a); TEM image of 9-C/S heterojunction (b); HRTEM image of 9-C/S heterojunction (c).

3.2. Crystal structure and chemical composition of STO NCs decorated with Cu₂O NPs

In order to investigative the crystal phase composition, purity, and valency of the as-prepared samples, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements were carried out. The crystal structures of the samples were analyzed by XRD on a D/MAX-2500 X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα (λ = 1.54178 Å) radiation at a scan rate of 5° min⁻¹. As shown in Fig. 2a, all the diffraction peaks observed at values of 22.8°, 32.2°, 40°, 46.5°, 58°, 68° and 77.2° of STO NCs (I) match that of the pure SrTiO₃ (JCPDS: 35-0734),⁷ and no other impurity peaks are detected. The pattern is also suitable for Cu₂O (II) (JCPDS: 05-0667),¹³ which implies high purity of the as-prepared samples by our experimental strategies. EDX images were also collected on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.) using a 200 kV accelerating voltage. As displayed in Fig. 2b, the result of EDX gives the signals of Sr, Ti, O and Cu elements. XPS analysis was carried out by a Thermo ESCALAB 250X (America) electron spectrometer using 150 W Al Kλ radiations. In Fig. 2c, XPS survey spectrum is carried out to clearly demonstrate the existence of Ti, O, and Cu in the sample. The emergence of C element can be attributed to the presence of carbon in the environment. Fig. 2d shows the high resolution XPS scans over the Cu 2p peak. The binding energies of Cu 2p3/2 and 2p1/2 are 932.5 and 952.8 eV, respectively, indicating that the sample contains Cu⁺ rather than Cu²⁺ (the characteristic peaks of Cu 2p3/2 for Cu(0), Cu(I), and Cu(II) are at 932 eV, 932.7 eV, and 933.6 eV, respectively).²⁴ The weak peaks shake up at about 942 eV in Fig. 2d is the shake-up peaks of Cu(II). This is because Cu₂O is chemically unstable and partial Cu₂O was oxidized to CuO. This can be ascribed to the relatively small amount and the amorphous nature of CuO due to the surface oxidization of Cu₂O.²⁵

Fig. 2 (a) X-ray diffraction patterns of as-synthesized SrTiO₃ NCs (I), Cu₂O NPs (II) and Cu₂O/STO heterojunction (III); (b) energy dispersive X-ray spectroscopy spectra of the Cu₂O/STO heterojunction; (c) X-ray photoelectron spectroscopy survey spectrum of Cu₂O/STO heterojunction; (d) the high resolution XPS scans over Cu 2p peak.

3.3. Optical properties of STO NCs decorated with Cu₂O NPs

The optical properties of these samples were investigated using UV-vis diffuse reflectance spectroscopy (UV-vis), electrochemical impedance spectroscopy (EIS) and photoresponse density. UV-vis spectrum of the as-prepared samples was obtained from a UV2550 UV-vis spectrophotometer (Shimadzu, Japan) using BaSO₄ as a reference. Fig. 3a shows the UV-vis/DR spectra of the STO NCs, Cu₂O NPs and Cu₂O/STO heterojunction. Clearly, the absorption edge of pure STO is at approximately 400 nm, which agreed well with the band gap energy of STO NCs (E_g = 3.2 eV). After coupling the Cu₂O NPs, the Cu₂O/STO heterojunction shows a strong absorption both in UV and visible light, and the absorption is even extended to larger than 500 nm, indicating that this method can overcome the lack of visible light response of STO NCs.

Fig. 3 (a) UV-vis diffuse reflectance spectra of STO NCs, Cu₂O NPs and 9-Cu₂O/STO heterojunction; (b) electrochemical impedance spectroscopy spectra of pure SrTiO₃ NCs and 9-Cu₂O/STO heterojunction; (c) photoresponse density of STO NCs and Cu₂O/STO heterojunction.

In addition, another electrochemical analysis, EIS, has also been done. As we all know, the impedance spectrum represents the degree of charge transfer and the relative higher separation of degree of the photogenerated electron/hole pair with smaller size of radius of semicircle.²⁶ As shown in Fig. 3b, the radius of 9-C/S heterojunction is much smaller than that of the pure STO NCs, which means a more effective separation of the photogenerated electron/hole pairs between the Cu₂O NPs and the STO NCs surface.

Another interesting phenomenon occurs in the transient photocurrent of STO NCs and 9-C/S heterojunction under a visible light pulse of 30 s. As shown in Fig. 3c, firstly, the photocurrent was almost measured to be zero in the dark; secondly, the photocurrent emerged without delay along with the irradiation. Lastly, when the irradiation was suspended, the current space fell to zero. It means that the sample is sensitive to light. The photocurrent density of the 9-C/S heterojunction is roughly 4 times that of pure STO NCs, suggesting the excellent ability of electronic transmission and separation of photogenerated electron/hole pairs of 9-C/S heterojunction.²⁷

3.4. Photocatalytic degradation of tetracycline under visible light illumination

The photocatalytic abilities of the as-prepared catalysts were confirmed by their photodegradation on a typical antibiotic pollutant, tetracycline, under visible light irradiation. As shown in Fig. 4a, the degradation of tetracycline can be neglected with the photocatalyst of STO NCs, owing to its wide band gap (3.2 eV); moreover, the pure Cu₂O NPs exhibit a degradation rate of only 21.42% under visible light irradiation. When combined with Cu₂O NPs, the C/S heterojunction show significant increase in the photodegradation of TC compared to pure STO NCs and Cu₂O NPs. The most predominant degradation ability could be obtained using a 9 wt% Cu₂O NPs loading. This experimental result agrees with that of UV-vis diffuse reflectance, and further confirms that this method can enhance the photocatalytic activity of narrow/wide band gap photocatalysts.

Fig. 4 (a) Photocatalytic degradation ratios of TC with different samples under visible light irradiation, (b) the pictorial diagram of the photocatalytic degradation ratios of TC with different samples, (c) the first-kinetic of the photocatalytic degradation of TC, (d) apparent rate constant values (pink color) and the intercept (purple color) for the photodegradation of the TC solution over different photocatalysts in 140 min under visible light irradiation, (e) TOC removal curves of 9-Cu₂O/STO heterojunction under visible light irradiation, (f) degradation curve trend contrasts tetracycline and TOC in 100 min.

In Fig. 4c, the linear relationship of ln(C₀/C) as a function of time implied that the photodegradation of TC followed an apparent first order kinetics, which can be calculated by ln(C₀/C) = kKt ≈ K_appt, where C₀ and C are the initial and reaction concentrations of TC, respectively. K_app represents the degradation rate constant and an index of photocatalytic ability, and its value is proportional to the photocatalytic ability.⁷ Therefore, as shown in Fig. 4d, the K_app of the photocatalytic degradation of TC by pure STO NCs and C/S heterojunction prepared by deposition of various amounts of Cu₂O NPs were 0.00079, 0.0035, 0.0052, 0.0069, 0.013 and 0.010 min⁻¹, respectively. This value is consistent with the photocatalytic ability.

In order to determine that the achievement is due to the photocatalysis instead of physical adsorption, total organic carbon (TOC) analyses were also conducted. TOC is a relevant parameter for the overall determination of the organic pollution of effluent and wastewaters. TOC analyses were conducted on a multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer. As shown in Fig. 4e, the decomposition of TC with the photocatalyst of 9-C/S heterojunction under visible light reached 47.6%, which is much lower than that of the photocatalysis. In Fig. 4f, similar trends of TOC removal and degradation curves indicate that our photocatalysts have enormous potential in photodegrading antibiotics. In addition, a lot of other substances were also discovered. In the test of optical properties and photocatalytic ability, we can draw the conclusion that this heterojunction can separate photogenerated electron/hole pairs and partly overcome the recombination of electrons and holes, thus excellently improving the photocatalytic ability.

3.5. Mechanism of the enhancement of photocatalytic activity of STO NCs decorated with Cu₂O NPs

As is well known, all types of active species are relevant to photocatalysis, so in order to investigate the active species in our experiments, we conducted a series of active species trapping experiments: triethanolamine (TEA) is for H⁺,^28–31 iso-propanol (IPA) is for ˙OH (ref. 31) benzoquinone (BQ) is for ˙O₂⁻ (ref. 30) and AgNO₃ is for e⁻.^28–31 As shown in Fig. 5a and b, when IPA is added into the reaction system, the degradation ratio becomes 21.20%, suggesting that ˙OH made a difference. A similar and obvious suppression phenomenon also occurs, and the addition of AgNO₃ and BQ leads to 90% and 80% decrease in the photocatalytic degradation rate of TC under the 9-C/S heterojunction, respectively. O₂ can be reduced by one electron into ˙O₂⁻, so the effect of addition of AgNO₃ is similar to that of BQ. In contrast, the addition of TEA seems to have a little effect on the photocatalytic activity. This result fully confirmed that the three active species that promote the photodegradation are H⁺, ˙OH and ˙O₂⁻. In addition, ˙OH and ˙O₂⁻ play main roles in the photocatalytic degradation system.

Fig. 5 (a) Photocatalytic degradation ratios of TC using different radical scavengers over 9-Cu₂O/STO heterojunction under visible light irradiation for 100 min; (b) the pictorial diagram of photocatalytic degradation ratios of TC using different radical scavengers over 9-Cu₂O/STO heterojunction under visible light irradiation for 100 min; (c) DMPO spin-trapping ESR spectra of TC solutions after visible light irradiation by 9-Cu₂O/STO–H₂O–DMPO; (d) DMPO spin-trapping ESR spectra of TC solutions after visible light irradiation by 9-Cu₂O/STO–CH₃OH–DMPO.

The generation of reactive species is further confirmed by an ESR technique with DMPO as a spin-trapping reagent in the visible-light-irradiation condition. Before the experiment, 10 mg photocatalysts was dissolved into 1 mL H₂O (CH₃OH) and 40 μL DMPO to form solution A: 9-Cu₂O/STO–H₂O–DMPO for detecting ˙OH (solution B: 9-Cu₂O/STO–CH₃OH–DMPO for detecting ˙O₂⁻). As shown in Fig. 5c, there are weak characteristic peaks assigned to DMPO–˙OH adducts, which suggested that there are ˙OH reactive species generated. Moreover, the strong characteristic peaks with the peak area ratio 1 : 1 : 1 : 1 : 1 : 1 are assigned to the DMPO–˙O₂⁻ adducts shown in Fig. 5d, suggesting the existence of ˙O₂⁻.³² This is because the photogenerated holes in the VB of Cu₂O NPs, and the photogenerated electrons in the CB of Cu₂O NPs will migrate to the CB of STO NCs. As a result, the photogenerated electrons and holes are accumulated in the CB of STO NCs and the VB of Cu₂O NPs, respectively. Owing to the positive E_VB of Cu₂O NPs (+0.46 V vs. NHE at pH = 0), the photogenerated H⁺ in the VB of Cu₂O NPs cannot reduce OH⁻ into ˙OH with the redox potential of +2.7 V vs. NHE at pH = 0, but the positive E_BCB (−0.3 V vs. NHE at pH = 0) of STO NCs and the photogenerated electrons in the CB of STO NCs can reduce O₂ into ˙O₂⁻ with the redox potential of −0.046 V vs. NHE at pH = 0.³³ However, ˙O₂⁻ can generate ˙OH with H₂O. Therefore, combining the experimental results of active species trapping and ESR technique, we can conclude that ˙OH and ˙O₂⁻ play very important roles in this exploration. As shown in Fig. 6, under visible irradiation, owing to the band gap, only the Cu₂O NPs can generate electron/hole pairs (reaction (2)); moreover, the electrons can move from VB to CB of Cu₂O NPs, and further to the CB of STO NCs (reaction (3)). As a result, the photogenerated electrons and holes are accumulated in the CB of STO NCs and the VB of Cu₂O NPs, respectively. The holes left in the VB of Cu₂O NPs are going to decompose TC directly. The electrons will react with O₂ to produce ˙O₂⁻ (reaction (4)), portion of which will afford ˙OH (reaction (5) and (6)). The major electron transfer steps in the abovementioned photocatalytic mechanism under visible light irradiation are summarized by the following equations:

Cu₂O + hν → Cu₂O(h_VB⁺) + Cu₂O(e_CB⁻) (2)

Cu₂O(e_CB⁻) + SrTiO₃ → Cu₂O + SrTiO₃(e_CB⁻) (3)

SrTiO₃(e_CB⁻) + O₂ → SrTiO₃ + ˙O₂⁻ (4)

˙O₂⁻ + H₂O → ˙HO₂ + OH⁻ (5)

HO₂˙ + H₂O → ˙OH + H₂O₂ (6)

Fig. 6 Mechanistic pathway of electrons and holes under visible light illumination over 9-Cu₂O/STO heterojunction.

4. Conclusion

In summary, C/S heterojunction is synthesized for the first time via a facile deposition–precipitation technique. Compared to STO NCs and Cu₂O NPs, the prepared C/S heterojunction shows perfect photodegradation of TC. The perfect Cu₂O NPs decorating content is about 9 wt% and 9-C/S heterojunction had the best photocatalytic activity reaching 77.65%, this is because of the efficient separation of electrons and holes between Cu₂O NPs and STO NCs. This study not only shows a possibility for substituting noble metals by low-cost Cu₂O nanoparticles in the photocatalytic degradation but also exhibits a facile deposition–precipitation technique for synthesizing narrow band gap/wide band gap photocatalysts.

Acknowledgements

The authors would like to acknowledge the National Natural Science Foundation of China (21276116, 21477050, 21301076, 21303074 and 21201085), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011,BK2012701), the Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014010), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).

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