Efficient visible light photocatalytic activity of p–n junction CuO/TiO₂ loaded on natural zeolite

Abstract

Highly efficient and visible-light-responsive p–n junction CuO/TiO₂-zeolite heterogeneous nanostructures had been successfully synthesized by a standard impregnation method. A detailed study of p–n junction CuO/TiO₂-zeolite impacting on the photodecoloration of a MB solution showed that the composite was highly reusable and stable for long-running photocatalytic application. The apparent rate constant of the CuO/TiO₂-zeolite was calculated to be 0.0704 min⁻¹, which is 1.4 times higher than that of TiO₂-zeolite (k = 0.048 min⁻¹) and 1.9 times higher than that of zeolite (k = 0.0368 min⁻¹). The experimental results indicated that the composites had a superior photocatalytic activity for the decoloration of dye wastewater under visible light irradiation because the p–n junction was formed between CuO and TiO₂. The assembly of p-type CuO produces a large number of p–n junction heterostructures on the surface of TiO₂, where CuO and TiO₂ form p- and n-type semiconductors, respectively. The p–n junction could efficiently suppress charge recombination, improve interfacial charge transfer, enhance visible-light adsorption and provide plentiful photocatalytic reaction active sites. This new p–n junction heteronanostructure is expected to show considerable potential application in solar-driven wastewater treatment.

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1 Introduction

Nowadays, the water shortage problem has been aggravated by dye pollutants, increasing population and industrialization.¹ Photocatalysis technology that uses ultraviolet (UV) and/or visible light as the excitation source for semiconductor catalysts to degrade dye wastewater has potentially promising applications. A variety of semiconductor photocatalysts such as TiO₂,^2,3 CdS,⁴ ZnO,⁵ CuO (ref. 6) and Al₂O₃ (ref. 7) have been studied. Compared with other semiconducting materials, TiO₂ is a promising semiconductor photocatalyst for treating dye wastewater due to its non-toxicity, low cost, no secondary pollution and chemical stability. However, further technological problems remain. First, TiO₂ is difficult to separate from the media and it is hard to adsorb organic molecules at low concentrations.⁸ Second, the wide band-gap energy and rapid recombination of photo-generated electron–hole pairs in TiO₂ results in low photocatalytic efficiency.⁹ Therefore, it is necessary to solve these problems.

Firstly, combining adsorbents such as activated carbons¹⁰ and zeolites¹¹ with a TiO₂ photocatalyst make it possible to improve adsorption and condensation properties. Zeolites have been chosen as supports since it can delocalize band gap excited electrons of TiO₂ and thereby minimize electron hole recombination.¹² In addition, nature zeolites¹³ seem to be promising supports due to its porous structure, channel sizes, good adsorption ability, low cost and abundant storage. TiO₂ loaded on zeolites integrate the photocatalytic activity of TiO₂ with the adsorption properties of the zeolite together, which induces a synergistic effect, resulting in the enhancement of photocatalytic efficiency.

On the other hand, a variety of methods such as metal and nonmetal doping,¹⁴ noble metal deposition,¹⁵ and surface modification¹⁶ dye sensitization,¹⁷ have been carried out to reduce the wide band-gap energy and utilize sunlight for TiO₂ nanoparticle. Especially, the previous research works have been demonstrated that the fabrication of a p–n junction in photocatalyst is the most effective strategy to significantly promote its photocatalytic performance because their internal electric field. For instance, it has been reported that p–n junction Cu₂O/BiVO₄ heterogeneous nanostructure shows a high decoloration rate of methylene blue under visible-light irradiation.¹⁸ CuO (1.2–1.5 eV) is an important p-type transition metal oxide which is able to work under solar irradiation. It is expected to prepare p–n CuO/TiO₂ nanostructure with the perfect photocatalytic activity under visible light illumination and highly efficient electron–hole separation. The functions of CuO in the composite material have two aspects: photogenerate electron acceptor and suppress recombination of photoexcited electron–hole pairs for improving its photocatalytic efficiency.¹⁹

This work is aimed at investigating the photocatalytic activity of p–n CuO/TiO₂ loaded on the natural zeolite by using methyl blue (MB) as a target degraded compound. In view of this, p–n CuO/TiO₂-zeolite composite material could be a more promising photocatalyst due to its superior properties such as (1) zeolite support enhanced adsorption ability and easy separation, (2) electrons and holes's effective separation enhanced light utilization rate, (3) the modified photocatalytic property makes TiO₂ not limited to UV region. The MB decoloration rate of the as-synthesized p–n junction CuO/TiO₂-zeolite composite material was significantly higher than that of reported value from TiO₂-zeolite. We have also discussed the photoinduced electronic interaction between CuO and TiO₂, in order to study the mechanism on the enhanced photocatalytic performance. What is more, the CuO/TiO₂-zeolite composite material was shown to be reused without significant deterioration of its photocatalytic property. Therefore, the p–n CuO/TiO₂-zeolite photocatalyst has a great potential for treating dye wastewater.

2 Experimental

2.1 Chemicals

The main raw material, natural zeolite was purchased from Hebei Chengde, P25 was purchased from Tianjin, China. Hydrochloric acid (36%), tetrabutylortho titanate (Ti(OC₄H₉)₄) (98.0%), diethanolamine (NH(CH₂CH₂OH)₂) (99.0%), ethanol (≥99.7%), cupric nitrate (≥99.0–102.0%), methylene blue were purchased from Tianjin, China and used as received. Double-distilled water was used throughout the experiments.

2.2 Preparation of zeolite

The purchased natural zeolite should be pretreated in order to exclude the effect of impurities of them. Typically, zeolite (Chengde, Hebei) was suspended in aqueous hydrochloric acid solution and was stirred for 4 h at 80–90 °C. Then, the zeolite was recovered by filtration, washed several times with distilled water, and then dried at 100 °C overnight. After that, the resulting materials were heated in a crucible in air at 600 °C for 6 h. Thus, obtained samples can be used directly.

2.3 Preparation of TiO₂-zeolite photocatalyst

The TiO₂-zeolite photocatalyst was prepared by a simple sol–gel method. Precursor solutions for the TiO₂ sol were prepared as follows: Ti(OC₄H₉)₄ of 3.6 mL and NH(CH₂CH₂OH)₂ of 1 mL were dissolved in ethanol of 12.2 mL. The solution was stirred vigorously for 2 h at the room temperature, followed by the addition of a mixture of distilled water (0.375 mL), ethanol (3.75 mL) and sequentially stirred for another 2 h, and then weak yellow and clear TiO₂ sol was obtained. 0.1 g of zeolite was immersed into the TiO₂ sol and the mixture was stirred for 4 h at the room temperature. The obtained samples were dried at 100 °C and, subsequently, the samples were repeated the step above. Finally, the obtained photocatalysts were calcined in air at 500 °C for 2 h and had been kept in powder form for further use.

2.4 Preparation of CuO/TiO₂-zeolite photocatalyst

The CuO/TiO₂-zeolite photocatalyst was prepared by a standard impregnation method. The TiO₂-zeolite samples were suspended in 0.5 M of Cu(NO₃)₂ solution. The solution was stirred for 4 h at room temperature, and the obtained samples were dried at 60 °C. Finally, the obtained samples were calcined at 400 °C for 4 h.

2.5 Preparation of CuO/TiO₂–indium tin oxides (CuO/TiO₂–ITO)

In order to verify whether the p–n junction CuO/TiO₂ was formed, CuO/TiO₂ was grown directly on ITO. First, TiO₂ sol was prepared as above mentioned in 2.3, and the ITO glass substrate was dipped into TiO₂ sol precursor solution at a speed of 1 mm s⁻¹ for 1 min, and then dried at 100 °C for 20 minutes. The obtained film was calcined in air at 500 °C for 2 h. Second, the TiO₂–ITO was dipped into Cu(NO₃)₂ solution at a speed of 1 mm s⁻¹ for 1 min, and then dried at 100 °C for 20 minutes. The obtained film was calcined in air at 400 °C for 4 h.

2.6 Characterization

Morphology of nanoparticles was observed using a PHILIPS XL-30 environment scanning electron microscopy (ESEM) and transmission electron microscopy (TEM, JEOL 100CX-a). X-ray diffraction (XRD) of nanoparticles was performed with a Rigaku D/max-2500 using Cu Kα radiation (k = 0.154059 nm). The UV-visible diffuse reflectance spectrum studies were performed on TU-1901 UV-visible spectrophotometer, accompanied with an integrating sphere accessory and utilizing BaSO₄ as a diffuse reflectance standard in the wavelength spanning from 250–800 nm. Photoelectrochemical characterization of the samples was performed using an electrochemical workstation (LK2005A, Tianjin, China), a three-electrode configuration, with CuO/TiO₂ composite nanoparticles as the working electrode, saturated Ag/AgCl as reference electrode, and a platinum foil as counter electrode. The potential was swept linearly at a scan rate is 50 mV s⁻¹. PEC water splitting experiments were conducted in NaOH electrolyte (1 mol L⁻¹). The stability of CuO/TiO₂ composite was measured by current–voltage (I–V) scanning from −2 to 1.50 V.

Evaluation of photocatalytic performance of CuO/TiO₂-zeolite photocatalyst was explored by the photocatalytic decoloration of MB under visible light illumination or UV lamp, activated using a fluorescent lamp which was positioned above the reactor (30 cm). The photocatalytic ability tests were accomplished in evaporating dishes. In a visible-light activated photocatalytic ability tests, 0.1 g of the resultant samples were dispersed in 20 mL of 10 mg L⁻¹ MB aqueous solution and stirred magnetically. Then MB solution was centrifuged for different times and the absorbance of the characteristic wavelength at 290 nm utilizing a calibrated UV spectrophotometer. The decoloration efficiency has been calculated as:

Decoloration% = 100 × {(A₀ − A_t)/A₀} (1)

where A₀ is the initial absorbance of dye and A_t is the absorbance value of dye after irradiation in selected time interval.

3 Results and discussion

Identification of the phase composition and structure of the photocatalysts were performed by powder XRD studies and the results are manifested in Fig. 1a. For comparison, the XRD pattern of the pure natural zeolite is similar to that reported in ref. 20. It indicates that the predominant crystalline components of zeolite are Na_1.84Al₂Si_2.88O_9.68 (pdf no. 48-0731). The intensities of diffraction peaks of zeolite decreased slightly after loading TiO₂, however, the observed diffraction peaks could be assigned to the crystalline structure of zeolite used as a support. As shown in this figure, the XRD of TiO₂-zeolite reveals TiO₂ accords with the well documented anatase phases (JCPDS 21-1272) and rutile phases (JCPDS 21-1276). However, it can be seen that the peaks of TiO₂ shifted in Fig. 1b. We thought the cause might be that certain ions in the zeolite reacted with TiO₂ during the calcination of TiO₂-zeolite. The EDS of zeolite is showed in Fig. 1c. It can be seen that there are some Fe elements in zeolite. It can be concluded that Fe doped on TiO₂ during the calcination which caused the peaks of TiO₂ shifted.^21–23 As it also can be seen from this figure the crystalline peaks of CuO phase (JCPDS 44-0766) at 2θ values 35.5°, 38.7°and 48.7° corresponding to (111), (111), (202), are shown in CuO/TiO₂-zeolite, from which anatase phases and zeolite phases can also be seen. Therefore, it can be concluded that crystalline CuO on TiO₂-zeolite has been formed.

Fig. 1 XRD patterns (a) of zeolite, TiO₂-zeolite and CuO/TiO₂-zeolite, XRD of TiO₂-zeolite (b) and EDS elemental analysis of zeolite (c).

The FESEM images in Fig. 2 confirm the morphology of the system. Planeview SEM micrographs showed that the surface of nature zeolite was smooth, with pores (Fig. 2a). It can be seen that a layer of TiO₂ nanoparticles exists on the zeolite surface (Fig. 2b), meanwhile, the TiO₂ nanoparticles have smaller size than 200 nm which contributed to TiO₂ sol was coated on the surface of zeolite primarily and then it transformed to a TiO₂ layer in the calcination process. As it is indicated in Fig. 2c, compared to the SEM images of zeolite and TiO₂-zeolite, CuO nanoparticles with size uniformity that is (50 nm × 100 nm) on average were loaded on TiO₂-zeolite. Fig. 2d shows the TEM image of CuO/TiO₂-zeolite composite. As presented in Fig. 2e, the high magnification HRTEM image clearly indicates four distinctive lattice fringes of 0.345 nm, 0.336 nm, 0.167 nm, 0.186 nm, which correspond well to the (313), (500) plane of zeolite, the (211) plane of anatase TiO₂, and the (−202) plane of CuO, respectively. The result shows that CuO was distributed on TiO₂ nanoparticle and CuO/TiO₂ was loaded on zeolite.

Fig. 2 SEM images of zeolite (a), TiO₂-zeolite (b), CuO/TiO₂-zeolite (c), and TEM image of CuO/TiO₂-zeolite (d), high magnification HRTEM image showing the corresponding crystal lattice of anatase TiO₂, CuO and zeolite.

In this context, to explore the worth of the CuO/TiO₂-zeolite composite, we have performed the photocatalytic tests using the aqueous solution of MB dye in the presence of 2 mL of H₂O₂ under visible light illumination. The decoloration performances of MB solution by zeolite without illumination, TiO₂-zeolite in the presence of 2 mL of H₂O₂ under UV light and visible light illumination were also studied. The results of adsorption and photocatalytic decoloration experiments by the prepared samples are shown in Fig. 3. It can be seen that MB solution can be decolored by 79.12% with nature zeolite, by 67.75% with TiO₂-zeolite (visible light), by 84.12% with TiO₂-zeolite (UV light), by 85.24% with CuO-zeolite (visible light) and by 89.94% with CuO/TiO₂-zeolite after 60 min. The decoloration of TiO₂-zeolite under visible light is lower than that of zeolite because the pores of zeolite are blocked, and the photocatalytic function of TiO₂ is infinitesimally small under visible light, which shows that CuO/TiO₂-zeolite composite exhibits an improvement of photocatalytic activity over that of zeolite and TiO₂-zeolite (UV light or visible light).

Fig. 3 The decoloration of MB solution by zeolite without illumination (a), TiO₂-zeolite in the presence of 2 mL of H₂O₂ under visible light (b), TiO₂-zeolite in the presence of 2 mL of H₂O₂ under UV light (c), CuO-zeolite in the presence of 2 mL of H₂O₂ under visible light (d) and CuO/TiO₂-zeolite in the presence of 2 mL of H₂O₂ under visible light (e).

Generally, the kinetic behavior of photodecoloration and adsorption reaction follows the Langmuir–Hinshelwood equation confirming the heterogeneous catalytic character. According to the Langmuir–Hinshelwood model, the rate (r) varying proportionally with the coverage (θ) as:

r = −dc/dt = −d[MB]/dt = kθ = k(KC/(1 + KC)) (2)

where r is the rate of photodegradation reaction in (mg L⁻¹ min⁻¹), k is the rate constant of photocatalysis in (mg L⁻¹ min⁻¹), K is the rate constant of adsorption in (L mg⁻¹) (Langmuir constant related to the energy of adsorption), C is the concentration of MB solution in (mg L⁻¹), c is the concentration of MB solution at any time, and t is the time in minutes.^24,25

In this system, the diluted MB solutions <1.0 mM (KC ≤ 1), and the reaction rate is fit to a pseudo-first-order reaction model. The results are nearly consistent with the linear equation:

ln(C₀/C_t) = kt (3)

where k is a first-order rate constant in (min⁻¹), C₀ is the initial concentration of diluted MB solution; C_t is the concentration of diluted MB solution at time t.^26,27 As shown in Fig. 4, the decoloration constants presented from the slope of the plots of ln(C₀/C_t) vs. t are utilized to have a more precise comparison of the photocatalytic performances of the resultant samples. The logarithmic plots imply the reaction kinetics of photodecoloration of MB accordance with the first-order reaction model. The apparent rate constant of CuO/TiO₂-zeolite is calculated to be 0.0704 min⁻¹, which is 1.4 times higher than that of TiO₂-zeolite (k = 0.048 min⁻¹), 1.9 times higher than that of zeolite (k = 0.0368 min⁻¹), 6.5 times higher than that of P25 (k = 0.0108 min⁻¹). The linear fit (R²) with more than 98% is always obtained inset of Fig. 4, disclosing an excellent agreement with the given model.

Fig. 4 The modified first order kinetics in photodecoloration of zeolite, TiO₂-zeolite and CuO/TiO₂-zeolite. The inset table shows the corresponding reaction constants, intercepts on the vertical axis and the linear fit (R²).

It is well known that TiO₂ is an n-type semiconductor with 3.2 eV, which has perfect photocatalytic activity by near ultraviolet photon, though it is not the main fraction found in the solar ray. However, it has a good photodecoloration under visible light illumination while CuO loaded on the TiO₂-zeolite. Why? To calculate the band gap energy, demonstrating their light harvesting abilities, the UV-visible diffuse reflection spectra of the prepared composites are shown in Fig. 5a. The incorporation of CuO and TiO₂ onto the zeolite framework has a significant influence on the light absorption. It can be seen that the absorption bands intensity of the TiO₂-zeolite is higher in UV domain. A remarkable increase in the absorption (∼400 nm) can be assigned to the intrinsic band gap absorption of TiO₂.²⁸ Compared to TiO₂-zeolite, the UV-vis spectra of CuO/TiO₂-zeolite shows an enhanced absorption in the visible-light region. The value of the band gap energies of the prepared composites can be evaluated by the Kubelka–Munk relation,²⁹ follows the equation:

(αhν)ⁿ = A(hν − E_g) (4)

where α is absorption coefficient, hν is light frequency, E_g is band gap, A is a constant and n is 2 or 1/2 for the allowed direct or indirect transitions respectively. A plot obtained via (αhν)² vs. hν based on the direct transition is presented in Fig. 5b, from which the rough band gap values of the samples are estimated to be 2.7 eV, and 1.48 eV corresponding to TiO₂-zeolite and CuO/TiO₂-zeolite, respectively. The result indicates a band gap narrowing of TiO₂-zeolite due to the coupling in CuO/TiO₂-zeolite. However, the estimated band gap value of TiO₂-zeolite composite is about 2.7 ev, which is smaller than the value of TiO₂ crystallite (3.2 eV). Why? With the analysis of XRD pattern and EDS of TiO₂-zeolite in Fig. 1, TiO₂ was may mollified by Fe in the zeolite.

Fig. 5 UV-visible absorption spectra of TiO₂-zeolite and CuO/TiO₂-zeolite (a) and the direct allowed optical transition of TiO₂-zeolite and CuO/TiO₂-zeolite (b).

The enhanced photocatalytic activity of the CuO/TiO₂-zeolite heterogeneous nanostructures compared to the pure TiO₂-zeolite can be ascribed to the formation of the p–n junction between p-type CuO and n-type TiO₂ semiconductors. It can be seen that before contacting of p-type CuO and n-type TiO₂, the conduction band edge and valence band edge of n-type TiO₂ are higher than that of p-type CuO, and the Fermi level of CuO is lower than that of TiO₂. After contacting of CuO and TiO₂, the Fermi level of CuO is moved up; while the Fermi level of TiO₂ is moved down until an equilibrium state is formed as shown in Fig. 6. The p–n junction was formed, while an inner electric field from n-type TiO₂ to p-type CuO is established at the equilibrium. Under visible-light irradiation, both CuO and TiO₂ can be excited to generate electron–hole pairs. As shown in Fig. 6b, according to the energy band schematic diagram, the photo-generated electrons on the conduction band of the p-type CuO, cannot transfer to that of n-type TiO₂ because the conduction band of the p-type CuO is lower than that of n-type TiO₂. However, the holes remain in the p-type CuO valence band, simultaneously photo-generated holes can migrate from the valence band of n-type TiO₂ to that of p-type CuO because the valence band of the n-type TiO₂ is higher than that of p-type CuO. The internally formed electric field promoted the migration of photo-generated carriers. Therefore, the photo-generated electrons and holes can be separated effectively and the recombination of electron–hole pairs can be reduced by the p–n junction formed between the p-type CuO and n-type TiO₂ interface. Then the separated electrons and holes are free to initiate reactions with the reactants adsorbed on the photocatalyst surface, which lead to enhance the photocatalytic activity.

Fig. 6 Schematic diagram of charge transfer between p-type CuO and n-type TiO₂.

Fig. 7 shows typical I–V characteristics of p–n junctions CuO/TiO₂–ITO under both forward and reverse bias in the dark. The current size appears the exponential rise when the forward voltage increases. However, with the negative voltage increases, the current size decreases and the leakage current is very small. Obviously I–V curve shows a typical p–n junction rectifier features of CuO/TiO₂–ITO, which indicates the formation of p–n junction, and that has a good rectification characteristic.

Fig. 7 Typical I–V characteristics of CuO/TiO₂.

The CuO/TiO₂-zeolite composite photocatalyst was also studied for its reusability to show the importance of highly reusable and stable photocatalyst for long-run photocatalytic application. Fig. 8 shows a sustainable photocatalytic performance in decoloration with CuO/TiO₂-zeolite and TiO₂-zeolite after 8 cycles. As can be seen from the figure, there was insignificant loss of photocatalytic decoloration after reusing for more than 8 times while the synthesized zeolite cannot continue to decolor MB due to its saturate adsorption, which indicates that the CuO/TiO₂-zeolite composite is stable on photocatalytic and can be reused. The opportunity to reuse the CuO/TiO₂-zeolite composite has been rendered due to its highly potential and sustainable engineering applications.

Fig. 8 Reusability experiment for decoloration efficiency of zeolite without illumination, TiO₂-zeolite under in the presence of 2 mL of H₂O₂ UV light and CuO/TiO₂-zeolite in the presence of 2 mL of H₂O₂ under visible light.

4 Conclusions

In summary, p–n junction CuO/TiO₂-zeolite heterogeneous nanostructure has been successfully prepared using a standard impregnation method which is simple and low-cost. The photocatalytic activity tests demonstrate that p–n junction CuO/TiO₂-zeolite exhibits superior photocatalytic efficient visible-light-driven photocatalytic activities towards the decoloration of MB solution. Compared to zeolite and TiO₂-zeolite, the CuO/TiO₂-zeolite exhibits a 1.6-fold enhancement in apparent rate constant (k = 0.0704 min⁻¹). The improved photocatalytic efficiency is attributed to the photo-generated electrons and holes separation enhanced by the contacting CuO nanoparticles. The considerable stability makes p–n junction CuO/TiO₂-zeolite a strong candidate for enhancing photocatalytic activities which has an important influence on the future development of highly efficient visible-light p–n junction photocatalysts for dye wastewater decoloration.

Acknowledgements

The authors gratefully acknowledge financial support from Technology Development Foundation Plan Project of Tianjin Colleges (No. 20140309).

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