INTRODUCTION

ZnO is known as an II-VI semiconductor with a wurtzite crystal structure and an energy band gap of 3.37 eV at room temperature[1]. ZnO nanostructures have already been extensively investigated for photocatalytic degradation of organic pollutants because of their high activity, biocompatibility and good chemical and mechanical stability[2]. In this process, ZnO is illuminated by incident photons with energy level higher than its band gap energy resulting in excitation of the electrons (e^-) from the valence band to the conduction band with subsequent holes (h⁺) production in the valence band. The photo-generated holes react with either water (H₂O) or hydroxyl ions (OH^•) adsorbed on the catalyst surface to produce hydroxyl radicals (OH^-)
and the photogenerated electrons react with oxygen (O₂) to form superoxide ions (O₂^•-) (). As a result, the decomposition of the organic materials can be achieved by their reaction with these radicals[3]. High energy band gap of ZnO restricts its photocatalytic activity in the UV region. Various techniques have already been employed for band gap tuning of ZnO to make it active within the visible region [4]. In recent works, CdS[5], CNT[6], and carbon[7] has been introduced to the ZnO matrix to inducephotocatalytic activity under the visible light. Furthermore, the coupling of ZnO with narrow band gap semiconductors has been employed as a helpful technique to shift the photocatalytic activity of ZnO toward the visible region. Cadmium oxide (CdO) is a n-type semiconductor with a direct band gap of 2.5 eV; therefore, the band gap of ZnO could be tuned by CdO alloying. Various methods including solvothermal[8], electrospinning[9], electrochemical deposition[10], and sol-gel[11] have already been reported for the preparation of ZnO-CdO alloy. The present work discusses the synthesis of Cd_xZn_1-xO nanostructures by sequential anodic dissolution method. The prepared samples are characterized by different techniques and are used for the photodegradation of methyl orange under visible light. In this study, we compared the photocatalytic activity ofCd_xZn_1-xO (x= 0.10,0.20,0.30,0.40,0.50) nanoparticles by using methyl orange (MO) dye as a model organic pollutant under visible light. Through these experiments, the influence of the amount of Cd on the photocatalytic performance of the nanoparticles was investigated. The fabricated nanoparticles were characterized using a variety of analytical methods, including field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and UV-Visible spectroscopy.

EXPERIMENTAL

Chemicals and reagents

Me₄NCl was purchased from Merck (Darmstat, Germany) and was used without further purification. Two cylindrical electrodes of zinc metal and cadmium metal (purity of 99%) with a diameter of 1 cm and a height of 10 cm were purchased from Merck. All solutions were prepared with deionized water prepared through an Absaz water purification system (Tehran, Iran). The specifications of the prepared water are presented in Table 1.

Electrosynthesis

Two cylindrical electrodes of zinc metal and cadmium metal were used as sacrificial anodes with a diameter of 1 cm and a height of 10 cm. The sacrificial anodes were placed on both sides, 1cm apart from a steel 316L planar electrode working as the cathode. Initially, the electrodes had been wiped out of possible contaminations by sonication in ethanol and deionized water. An aqueous solution of 0.1M Me₄NCl salt was utilized as both supporting electrolyte and stabilizer under stirring and thermostatic conditions (at 60˚C). The electrosynthesis was carried out by applying a constant potential of 15V for 30 min. An electronic circuitry was used to switch the electric potential between the interchangeable anodes. Five samples with different cadmium contents were prepared by adjusting the duty cycle of the anode switching. The anode switching was carried out by means of a particular electronic circuit comprehensively described in our previous works [12-15].

A few minutes after the application of voltage, the solution turned milky due to co-precipitation of anodically dissolved Zn/Cd ions. After completion of the reaction, the obtained suspension was washed three times with water and ethanol. This precipitate was separated from the solution by centrifugation and washed with distilled water three times. The resultant precipitate was dried in an oven at 60 Cº for 6 hr and further calcinated at 450 Cº for 4 hrs. Fig. 1 shows the schematic diagram of the experimental setup.

Characterization

The morphology of the obtained nanostructures was studied by means of a TESCAN VEGA3-SB scanning electron microscope. The phase and crystallinity were characterized using STOE XD-3A X-ray diffractometer with Cu Kα radiation at λ=1.54056 A˚. The photodegradation kinetics was spectrophotometrically determined by means of a Shimadzu UV-2100 spectrometer. DRS of the samples were recorded by means of an Ava Spec-2048TEC spectrometer.

Photocatalytic activity measurements

The photocatalytic activity of Cd_xZn_1-xO nanostructures was examined by measuring the rate of MO degradation in aqueous solution under visible light irradiation. The experiments were carried out in 50 mL beaker was filled with 15 mL of 10^-5 M MO and 6 mg of Cd_xZn_1-xO nanoparticles. Prior to the irradiation, the solution was kept in a dark place for 60 min to obtain adsorption-desorption equilibrium and the concentration of the solution was determined as initial concentration (C₀) of the dye solution. The solution was illuminated by a light source with a power of 80 lux and source-sample distance of 18 cm. The concentrations of MO in the solution were monitored and analyzed by applying the Beer-Lambert equation and measuring the absorbance at 664 nm using a UV-vis spectrophotometer.

RESULTS AND DISCUSSION

Electrosynthesis mechanism

The electrosynthesis of nanomaterials by anodic dissolution has been described in the literature [16, 17]. In this method, a metallic electrode undergoes anodic electrooxidation at sufficiently positive potential as the following equation:

M_(s)→ Mⁿ⁺+ne^- (1)

The electrogenerated metallic cation reacts with the hydroxyl ions initially present in the solution to obtain a metal hydroxide precursor:

Mⁿ⁺+nOH^-→M(OH)_n (2)

The quaternary ammonium salt acts as a mild source of hydroxyl ions and the capping agent which prevents aggregation of the produced metal hydroxide nuclei. The latter action is crucial to obtain fine nanoparticles.

In this work, the anodic potential is sequentially applied to Zn and Cd electrodes. During t_Cd, when the potential is applied to the Cd electrode, the latter is anodically oxidized to Cd²⁺:

Cd_(s) → Cd²⁺+ 2e^-t_Cd (3)

Zn(s) → Zn²⁺ + 2e^- t_Zn (4)

Similarly, the Zn electrode dissolves to Zn²⁺during t_Zn:

The dissolved Cd²⁺ and Zn²⁺cations react with hydroxyl ions to obtain a mixed hydroxide precursor that loss water to make the final product.

(1-x)Zn²⁺ + xCd²⁺ + yOH^- → Cd_xZn_1-x(OH)_y (5)

Crystal structure

XRD patterns of Cd_xZn_1-xO composites are shown in Fig. 2. The X-ray diffraction peaks of ZnO can be attributed to the hexagonal wurtzite phase of (ICDD card No.: 79-0208) and also the lattice constant values of a = b = 0.326 nm and c = 0.521 nm. However, in XRD patterns of the Cd_xZn_1-xO composites, two phases are observed with the significant broadening of wurtzite ZnO peaks. One is the hexagonal structure of ZnO and another is a cubic structure of CdO with peaks (111), (200), (211) and (220) (Card No. 73-2245) and lattice constant values a = b = c = 0.470 nm. The average crystallite size could be evaluated from the base peak according to Debye-Scherrer equation (D = 0.891λ/βcosα); where D is average grain size,λ the X-ray wavelength (0.1540 nm) and α and β are the diffraction angles and full width at half-maximum, respectively). The average particle size values were found to be 16.15, 28.85, 31.70, 45.12 and 57.21nm for various Cd_xZn_1-xO nanostructures with x=0.1, 0.2.0.3, 0.4 and 0.5, respectively.

Diffuse reflectance spectroscopy

The UV-visible diffuse reflectance spectra of theCd_xZn_1-xO composites with different compositions in the wavelength range of 300–700 nm are shown in Fig. 3. All the prepared samples show absorption in the visible region. It can also be observed that there is a slight shift in the absorption edge as the x (the content of CdO) in the composite increases. Compared with the optical absorption of bare ZnO (∼403 nm), Cd_xZn_1-xO composite has two absorption edges [18]; the main edge due to ZnO and the second one due to CdO. The absorption edges for the Cd_xZn_1-xO composite indicates that the photocatalytic activity of ZnO can be drawn toward the visible region by composite formation with CdO. Samples with a high amount of Cd (x=0.4 and 0.5) showed peak offsets between 400-600nm probably due to the formation of impurities. A possible impurity in the chloride-rich alkaline medium is CdOHCl [19].

Calculation of the band gap from diffuse reflectance

Taking into consideration the UV–visible absorption spectra and utilizing Tauc model (eq. 6), the optical band gap energy of the nanoparticle of Cd_xZn_1-xO was calculated from Eq. 6[20]:

(αhν)² = K(hν-E_g) (6)

Where α is the absorption coefficient, hν the photon energy, K an invariant and E_g the band gap energy. Fig. 4 shows the plot of (αhν)²vs.hν while using the absorption data in which the extrapolated intercept provided the related E_g. To examine the consequence of CdO focus on a property of ZnO, we obtain different band gap energy by varying x which are summarized inTable 2. In accordance with our data analysis, band gap narrowing was observed by increasing the amount of Cd. It is well known that CdO is actually a low bandgap energy semiconductor, unlike ZnO, hence, it is expected that the band gap energy nanoparticle of the Cdo/ZnO must be of these types of materials (pure ZnO and CdO). Other researchers also reported band gap energy decline in the mixed ZnO and CdO composition in comparison with pure ZnO [21, 22].

Morphology

Fig. 5(a,b, c, d and e) show the surface morphology of Cd_xZn_1-xO nanostructures with x= 0.1, 0.2, 0.3, 0.4 and 0.5, respectively. As can be seen, the morphology of the sample with x=0.1 (Fig. 5 (a)) comprises spherical nanoparticle with an average particle size of 23.4nm. The morphology of the nanostructure changed to interlocked nanosheets for the sample with x=0.2 (Fig. 5 (b)). The average diameter of the nanosheets was found to be 25.5 nm. The greater values of x resulted in the formation of a combination of nanosheets and nanoparticles morphology as demonstrated for x=0.3, 0.4 and 0.5 as Fig. 5 (c, d, e), respectively.

FT-IR

The as-synthesized Cd_xZn_1-xO composites were further characterized by FT-IR spectrometry analysis. Fig .6displays the FT-IR spectra of ZnO and CdO–ZnO composites. The peak which is observed from 400 cm^-1 to 600 cm^-1 is attributed to the Zn-O bond vibrations of the particles [23]. The broad absorption band at ∼3400–3500 cm⁻¹ corresponds to the O-H stretching vibrations of water present in ZnO and CdO–ZnO[24] composites and other absorption bands at ∼2916 cm⁻¹ are assigned to the residual organic component [25]. Absorption at 1485 cm⁻¹ showed characteristic absorption of the Cd-O bond.

Photocatalytic activity

Methyl orange (MO) has often been used as a model organic pollutant for photocatalytic degradation study of semiconductors [26, 27]. The photocatalytic activity of the Cd_xZn_1-xO nanoparticle was evaluated by measuring the rate of the degradation of MO dye under the visible light. The irradiation was performed for 120 min. It is well known that the photocatalytic degradation of MO is attributed to first order kinetics explained by the following equation [28]:

ln (C₀/C) = kt (7)

Where C₀ and C are the initial and time-dependent concentrations of MO, respectively. The ln(C₀/C) quantity was plotted as a function of irradiation time for various photocatalysts in Fig. 7. The linear behavior shows that the photocatalysis reaction follows the first order kinetics as described by Eq. 6. The reaction rate constants (k) from the slope of the linear fit were calculated (Table 1). Therefore, the MO degradation is principally assigned to the photocatalytic activity of the Cd_xZn_1-xO nanoparticle. The photocatalytic activity enhancement was observed for x=0.2in comparison to other nanoparticles.

Further investigation of photocatalytic activity under visible radiation has also been performed for comparison. As discussed within the diffuse reflectance spectroscopy pure ZnO has no absorption in the visible region. Therefore, it has no photocatalytic activity under the visible radiation. The incorporation of Cd in the nanostructures improves the photodegradation kinetics This enhancement within the photocatalytic activity of Cd_xZn_1-xO nanoparticles by CdO addition (x≠0), relates to the band gap reduction that caused the absorption of visible radiation by the Cd_xZn_1-xO nanoparticle. In order to achieve high photocatalytic activity under visible light, it is necessary to reduce the band gap of a semiconductor. On the other hand, lower energy band gap may increase the rate of recombination resulting in loss of the photocatalytic activity.

CONCLUSIONS

Photocatalysts are potential candidates in nanotechnology because of their promise in the conversion of solar power into chemical energy as well as in the degradation of numerous organic pollutants. The high energy band gap of common metal oxide semiconductors restricts their use as photocatalytic agents. The lowering of the energy band gap of semiconductors allows for greater efficiencies towards photocatalytic reactions. This is because the separation of photogenerated electron-hole pairs is increased by a hetero-junction between two semiconductors with matching electronic band structures. This heterojunction is important for the improved photocatalytic activity of the photocatalyst. The incorporation of CdO into the ZnO matrix led to band gap narrowing resulting in significant improvement of the photocatalytic activity under visible light. The highest visible light photocatalytic activity was observed for Cd_xZn_1-xO with x=0.2. The decline in size and doping along with other suitable catalytic composites enhance the rate of the photocatalytic degradation of complex organic molecules. Sequential anodic dissolution is a facile route for the preparation of Cd_xZn_1-xO nanostructures with a wide range of energy band gap and various morphologies. The produced nanoparticle of Cd_x/Zn_1-xO can be utilized as an effective photocatalyst for organic materials degradation under visible light. In summary, the introduced CdO/ZnO may have good industrial applications in the field of wastewater treatment because of the less cost from the required activation energy (solar radiation), the fast and effective degradation of the organic materials as well as the negligible polluting the environment effect from the degradation reactions products (CO₂ and H₂O).

CONFLICTS OF INTEREST

The authors declare that there are no conflicts of interest.