INTRODUCTION

The heterogeneous photocatalysis of organic pollutants on semiconductor surfaces has attracted much attention as a green technique. Therefore many semiconductor catalysts such as TiO2, ZrO2, CeO2, and CuO2..etc, have received much attention due to their wide applications. Semiconductors are highly effective for the degradation of multiple types of organic pollutants, such as detergents, dyes, pesticides, and volatile organic compounds[1-4]. However, the fast recombination rate of photo-generated electron/hole pairs hinders the commercialization of this technology [5]. Thus, it is of great interest to improve the photocatalytic activity of semiconductors for the degradation process. Zirconium oxide (n-type) semiconductors are widely used in a variety of technological fields such as fuel cell electrolyte, engineering ceramic, catalyst, catalyst support, oxygen sensor, and gate dielectric [6–7]. However, as a wide band-gap (5.0 eV) semiconductor, ZrO2 requires UV-light (

CeO2 shows promising photocatalytic activity for the degradation of various organic dye pollutants such as Methylene Blue (MB), acid orange 7 (AO7), but the broad bandgap energy and the electronic potential position in the conductance and valence bands of this material seriously limit its further application as a photocatalyst utilizing solar energy [11-13]. A combination of two semiconductors with different and gap level energies of type n-n junction showed exhibited better photocatalytic properties than single ones. However, when p-type semiconductors and n-type junction semiconductors will be formed; the inner electric field will also be produced in the interface. Once optical excitation occurs, a free electron (e-) and an electronic vacancy (h+) are formed, separated, and migrated effectively in a semiconductor being partially localized structural defective centers of its crystalline lattice, hence improving the electrical properties of the semiconductor system [10]. The photocatalytic efficiency of a p-CuO/ n-CeO2-ZrO2 system was studied, in which CeO2-ZrO2 of (n-type semiconductors) were associated with different concentrations of CuO (p-type semiconductor) to form p-n heterojunction photocatalysts. Due to its narrow bandgap energy of 1.2 eV and its catalytic, electronic, electrical, and thermal properties, CuO was selected as a sensitizer semiconductor Whereas Physical properties such as high-temperature superconductivity were exhibited [14]. The objective of this study is to assess the effect of p-CuO loading on the n-CeO2-ZrO2 photocatalytic properties prepared by the process of solution combustion.

MATERIALS AND METHODS

Materials

Copper (II) Nitrate Trihydrate (Cu(NO3)2.3H2O), Cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O), Zirconyl nitrate (ZrO(NO3)2), glycine. All the chemicals were used without further purification. Throughout the experiment, all solutions were prepared with deionized water of resistivity not less than 18.2MΩ/cm.

Preparation of the catalyst

The P-n CuO/CeO2ZrO2 heterojunction photocatalysts were synthesized by the auto solution combustion method using glycine as fuel at temperature 400ºC for 4h (Fig.1). The stoichiometric composition of the solution mixtures (Oxidizer and fuel) were calculated in the percentage synthesis 0.00, 0.02, 0.04, 0.06, and 0.08M respectively of Copper (II) Nitrate Trihydrate Cu(NO3)2.3H2O, 0.3M Cerium (III) nitrate hexahydrate Ce(NO3)3.6H2O and 0.3M Zirconyl nitrate ZrO(NO3)2.H2O was dissolved in 10 ml of distilled water for each one. The solutions were mixed thoroughly by stirring well at room temperature for 1h to reach the adsorption equilibrium. Then, 0.1M glycine (NH2CH2COOH) was dissolved in 20ml of distilled water separately and added to the mixture drop by drop under constant stirring at 80°C. A small amount of ammonia was used to adjust pH to 7. After 40 minutes viscous blue color of the gel was formed. The temperature of the gel was raised to 200ºC and an auto-ignition process started and the final product was obtained as ash precipitate. Then, the product was thoroughly washed with ethanol followed by distilled water several times with centrifuged at 6,000 rpm for 5 minutes to dispose of free Cu2+ ions. The obtained product was dried under a vacuum desiccator oven at 60ºC for 120 minutes. Finally, the sample was grind into pestle mortar and heated in a crucible at 400 °C for 4h in a muffle furnace [15-16].

Characterization techniques

The crystal structures of the powders were characterized by X‐ray diffraction (XRD, Proto, Canada) studies were carried out at room temperature with Cu Ka radiation. The intensity data were collected at 25 ◦C over a 2θ range of 20–70◦. The energy dispersive X-ray spectrum (EDS) technique was used to characterize the elemental composition using JSM-IT300 (JEOL, Japan). The morphologies of the photocatalyst were inspected with SEM German LEO- 1530VP electron microscopy operating at 10 kV. The Brunauer–Emmett–Teller (BET) surface area of the catalyst was determined nitrogen adsorption measurement at 77K (QuantachromeAutosorb-1model). X‐ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD. V. UV–vis absorption spectra were recorded on a SHIMADZU UV-3600 spectrometer.

Evaluation of photocatalytic activity

To evaluate the photocatalytic activity of the degradation MB by p-n heterojunction photocatalysts p-CuO/n-CeO2-ZrO2 under visible light irradiation. In the present study, the prepared photocatalyst in a varied concentration was dispersed separately on an aqueous solution (100 ml) of M.B (10 mg/L-1). The suspension was well stirred for 30 minutes in darkness to reach the adsorption equilibrium. The mixture was magnetically stirred during visible light irradiation. At regular intervals, the samples of 10ml were collected from the suspension and centrifuged at 5000rpm for 10min. The concentration of aqueous M.B was determined by using a UV-vis spectrophotometer (Shanghai UV-722)at 660nm. Shows the photocatalytic degradation rated (C/C0) of the MB on the p-n junction CuO/CeO2ZrO2 and n- CeO2ZrO2 photocatalysts, where C0 and Ct referred to the equilibrated and the original dye concentrations.

RESULTS AND DISCUSSION

The crustal structure of p-CuO/n-CeO2-ZrO2 heterojunction photocatalysts prepared with different concentrations of Cu(NO3)2.3H2O was investigated by X-ray diffraction (XRD). Fig.2 shownall the diffraction peaks for CeO2ZrO2 on the 2θ scale at 28.6°(111), 33.2°(200), 47.5°(220), 56.4°(311), and 59.2°(203) planes, clearly exhibit the tetragonal, monoclinic phase of ZrO2 together with fluorite phase of CeO2. Interestingly, additional phases such as Cu, CuO, Cu2O cannot be detected in all the samples, which attributed to Cupric oxide particles were so small to be detected by XRD(less than 3nm) or well dispersed on the n-CeO2ZrO2 photocatalysts [17,18]. As well, We noticed the intensity of the diffraction peaks of photocatalysts increased with the amount of Cu(NO3)2.3H2O insertion. All diffraction peaks can be identified and assigned according to the JCPDS data (#43-1002 for CeO2 and #81-1544 for t-ZrO2 and 81-1314 for m-ZrO2). The average crystallite size of materials calculated from the full width at half maximum intensity (FWHM) of the mean reflection (111) was in the range 6.9-8.6 nm (Table 1).

Since the CuOcannot be identified by XRD, EDS was used to verify the presence and value of CuO in the p-CuO/n-CeO2ZrO2 composites. (Fig.3a) and Table.1 shows the EDS data of p-CuO/nCeO2ZrO2, the elements of Ce, Zr, Cu, O, and C can be observed and the molar ratios of p-CuO to n-CeO2ZrO2 in the composites prepared with different concentrations of Cu(NO3)2.3H2O (0.02, 0.04, 0.06 and 0.08 M) are calculated to be 0.014, 0.018,0.021 and 0.025, which increases with the concentration of Cu(NO3)2.3H2O. The result also reveals the content of p-CuO dispersed on n-CeO2ZrO2 particles is so small that it is hard to highly crystallize.

The morphology of CuO/CeO2ZrO2 catalyst prepared with 0.06 M Cu (NO3)2.3H2O was investigated by SEM micrographs. (Fig.3b) shows the image of small particles with a uniform distribution is adhere to the CeO2ZrO2 nanoparticles. The small particles are considered to be CuO.

XPS was employed to confirm that p-CuO on the surface of n-CeO2ZrO2 particles. Fig.4 indicates the surface elemental composition and valance state of the prepared catalyst in 0.06M Cu(NO3)2.3H2O solution. The peaks of Cu 2p, Ce 3d, O 1s, C 1s, and Zr 3d verify the existence of Cu, Ce, O, C, and Zr from the survey spectrum (Fig. 4a). Ambient contamination and adsorbed impurities lead to the appearance of the small C 1s peak. From (Fig. 4b), the peaks of Cu 2p1/2 and Cu 2p3/2 appear in the binding energies of 952.9 and 932.9 eV, respectively, which also imply the existence of CuO because of which also imply the existence of CuO is the Cu 2p1/2 and Cu 2p3/2 shake up intensively at binding energies of 961.9 and 942.0eV, respectively, which are the characteristic peaks of CuO [19,20].

Textural properties

All synthesized oxides (n-CeO2ZrO2 and p-CuO/n-CeO2ZrO2) calcined at 400 °C presents the same nitrogen adsorption-desorption isotherms. The nitrogen isotherm and the textural properties of the photocatalysts are respectively given in Fig. 5 and Table 1. The only difference in the isotherms of these photocatalysts is in the absorbed volumes, which show different specific surface area results. The BET specific surface area of the p-CuO/n-CeO2ZrO2 photocatalysts decreased with the increase of CuO loading, revealing a difference in the specific surface areas. This is attributed to the formation of agglomerates during the synthesis.

PHOTOCATALYTIC ACTIVITY

Fig.6 The influence of the Cu precursor amount on the photocatalytic activities of p-n CuO/CeO2-ZrO2 photocatalysts has been investigated through MB degradation under visible-light irradiation(λ> 420 nm), all the samples show much higher photocatalytic activity than n-CeO2ZrO2. It can also be found that the photocatalytic activity of p-n CuO/CeO2ZrO2 heterojunction photocatalysts increases with the increasing amount of Cu (NO3)2.3H2O and reaches the best for the sample prepared in 0.06 M Cu (NO3)2.3H2O solution. About 98% of MB is degraded after 3h irradiation, while only 78.6% of MB can be decomposed using n-type CeO2-ZrO2 as photocatalyst. The degradation percentage is not further increased when the concentration of Cu (NO3)2.3H2O solution increases from 0.06 to 0.08 M. That is to say, the results indicate that the amount of CuO in CuO/CeO2-ZrO2 can play an important role in the photocatalytic performance because an insufficient CuO amount cannot effectively separate electrons and holes photogenerated from photocatalyst, resulting in low photocatalytic activity. On the other hand, an excess CuO amount can also decrease the photocatalytic activity of the photocatalyst as shown in the sample prepared with 0.08 M Cu(NO3)2.3H2O.

Because adsorption also plays an important role in most photocatalytic reactions, the adsorption capacity of all catalysts during the whole decolorization process was investigated (Fig.7). The percentage of MB decolorization was calculated according to Eq. (1):

Where Ao is the initial absorbance of MB, and A is the absorbance of MB after “t” minutes. n-CeO2ZrO2 has the superior adsorption capacity to MB. From this pattern, almost 70% of MB molecules adsorb on the n-CeO2-ZrO2 surface on the dark adsorption stage and only 35% of MB are decomposed by photocatalysis (Fig.7a). Hereby, the high decolorization efficiency of n- CeO2ZrO2 can be mainly attributed to high adsorption capability to MB molecules.

After loaded with CuO, however, the dark adsorption capacity becomes much lower even with the least loading amount of CuO (Fig.7b), which may be attributed to the reduced specific surface area as seen from Table.1 Actually, the photocatalytic activity usually improves with adsorption capacity during the photocatalysis process. But it is observed in (Fig.7d) that the sample prepared in 0.06M Cu (NO3)2.3H2O solution with the best photocatalytic activity only adsorbs about 40% of MB during the dark adsorption process. From these data, it is apparent that the high decolorization percentage of p-CuO/n-CeO2ZrO2photocatalyst is not mainly attributed to the adsorption capability anymore.

UV-vis diffuse reflectance spectra (DRS) of n-type CeO2ZrO2 and p-type CuO loaded n- CeO2ZrO2 (prepared in 0.06 M of Cu (NO3)2.3H2O solution) are shown in Fig.8. It shows that the p-n CuO/ CeO2ZrO2 heterojunction photocatalysts have a redshift and increased absorption in the visible range. The optical band gap energy (Eg) was calculated based on the absorbance spectrum of the powders according to the equation of Eg=1240/λAbsorp.Edge. n-CeO2ZrO2 and p-n heterojunction CuO/CeO2ZrO2 show the bandgap absorption onset at 460 and 472 nm, which correspond to bandgap energies of 2.70 and 2.62eV, respectively. Consequently, the optical absorption edge of the p-n CuO/CeO2ZrO2 composite shifts to the lower energy region compared to the n-CeO2ZrO2, and its absorption is stronger in the wavelength range of 400–600 nm. That is to say, p-n CuO/CeO2ZrO2 composite photocatalyst has high optical absorption capability than n-type CeO2-ZrO2, and more photo-introduced electrons/holes pairs can be generated under visible light irradiation.

Mechanism of Degradation

The p-CuO/n-CeO2ZrO2 photocatalysis mechanism (based on the CB & VB) is graphically presented in scheme1. we noticed an increased photocatalytic activity of the p-n heterojunction photocatalyst p-CuO/n-CeO2ZrO2 can be ascribed to its improved photogenerated charge separation. The bandgap of p-Cu/n-CeO2ZrO2 was 2.62eV which could be excited by visible light irradiation. So, when P-CuO and nCeO2ZrO2 were connected, p-n heterostructure was formed between p-CuO/n-CeO2ZrO2, and the inner electric field was produced at the same time in the interface, following the model described by Chen et al [21,22].

With the effect of the inner electric field, the holes flow into the negative field while the electrons move to the positive field. Therefore, the photogenerated electron-hole pairs were separated more effectively, the photogenerated electrons took place from the p-CuO transfer to CB of n-CeO2ZrO2; whereas the photogenerated holes from the n-CeO2ZrO2 transformed to the VB of p-CuO. Thus, due to the transfer of electrons and holes in the p-n CuO/CeO2ZrO2 heterojunction a significant increase in the life-time of the photogenerated electron-hole pair was observed. These electron/hole pairs are free to initiate a series of chemical reactions that eventually mineralize the MB. According to the above observations, the p–n junction formation model and the schematic diagram of the photoexcited electron-hole separation process are illustrated in Fig.9.

O2•− and OH•, as highly oxidative radical species, were generated with the electrons captured by the adsorbed oxygen molecules and h+ trapped by the surface hydroxyl, respectively.

Generally, the degradation of MB by p-n CuO/CeO2ZrO2 in aqueous solution results from reactive oxygen species (O2•−, H2O2, and OH•) and hole (h+) generation under visible light irradiation. The relevant reactions can be described as follows:

p-n CuO/CeO2ZrO2 + hν→ p-n CuO/CeO2ZrO2 (hVB+) + p-n CuO/CeO2ZrO2 (eCB-) (1)

To investigate the role of reactive oxidative species(ROSs) in MB photocatalytic degradation, the scavengers catalase (CAT), benzoquinone (BQ), t-butanol (t-OH), and ammonium oxalate (AO) were used to trap hydroxyl radical (OH•), H2O2, superoxide radicals (O2•− )and photo-generated holes (h+), respectively. Fig. 10 displays the relationship of different scavengers and the photodegradation of MB solution in the presence of an S4 photocatalyst. It can found the introduction of CAT doesn’t cause significant deactivation of S4 photocatalyst. But, the photocatalytic activity of S4 significantly decreases when the t-OH, OA, and BQ were added. These results suggest showed the contribution of OH•, h+, O2•− in the photocatalytic degradation process. Whereas the H2O2 did not play an important role in MB degradation by p-n CuO/CeO2ZrO2.

CONCLUSION

The p–n junction photocatalyst CuO/CeO2ZrO2was prepared by the auto solution combustion method using glycine as the fuel at temperature 400C for 4h. The structural and optical properties of the p–n junction photocatalyst p-CuO/n-CeO2ZrO2were characterized by XRD, EDS, UV-vis DRS. The photocatalyst prepared in 0.06M Cu (NO3)2.3H2O solution has the best photocatalytic activity. The adsorption capability of p-n CuO/CeO2ZrO2 photocatalyst to MB was much lower than n-type CeZrO4, but the photocatalytic activity was much higher under visible light irradiation. The enhancement of photocatalytic degradation could be explained by the improved visible light absorption and the formation of p–n junction.

CONFLICTS OF INTEREST

The authors declare that there are no conflicts of interest.