Fabrication of an Ag/Fe₂O₃/ZnO ternary composite with enhanced photocatalytic performance

Abstract

A novel Ag/Fe₂O₃/ZnO ternary composite was fabricated using the chemical deposition and photochemical deposition methods. Its structure and optical properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet-visible spectrophotometry. The Ag/Fe₂O₃/ZnO ternary composite exhibited a greater improvement in photocatalytic activity compared with the Fe₂O₃/ZnO or Ag/ZnO binary composite. The degradation rate of Ag/Fe₂O₃/ZnO towards methyl orange (MO) and iodoform (CHI₃) within 120 min was 80% and 94%, respectively. More importantly, the Ag/Fe₂O₃/ZnO composite had a much higher degradation efficiency towards iodoform, which indicates its efficient and selective degradation characteristics. The results show that the Ag/Fe₂O₃/ZnO ternary composite holds great potential in the removal of carcinogenic iodoform from sewage.

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1. Introduction

Environment and human health are now threatened by the large amount of pollutants in wastewater. This threat is still great despite the fact that there is a low amount of halogenated hydrocarbon in wastewater, for example, iodoform, which can cause cancer.^1–3 Thus, the removal of such toxic organic pollutants in wastewater is a highly essential issue. Semiconductor photocatalysts, such as TiO₂ and ZnO, can be a green and efficient technology in the field of sewage treatment, because they can effectively dispose many organic pollutants in the sewage compared with other approaches. In addition, most photocatalytic materials are non-toxic, stable and low-cost.^4,5

Although ZnO has numerous advantages, such as high quantum efficiency, non-toxicity and strong ultraviolet absorption,⁶ its catalytic efficiency is still not ideal. The main reasons for this include the following reason: (1) the band gap of ZnO is too wide, only the ultraviolet part of sunlight can be used; (2) ZnO is an amphoteric oxide semiconductor, it easily dissolves in strong acid and alkali solution; (3) the electron–hole pairs in ZnO are easy to recombine, which reduces its photocatalytic efficiency. In this regard, combining ZnO with other metal compounds, such as CuO and Fe₂O₃, has been proven to be effective in improving the photocatalytic performance of ZnO. However, ZnO binary composite materials also have some defects. They cannot selectively photodegrade some hazard pollutants in wastewater, especially halogenated hydrocarbons. Furthermore, their degradation efficiency decreases obviously when there is a variety of pollutants in wastewater. Recent studies have found that ZnO-based ternary composites have greater superiority than binary composites.^7,8 Moreover, they have the function of selective degradation of some pollutants.⁹ In addition, it has been found that Ag has strong adsorption of highly carcinogenic halogenated hydrocarbon.¹⁰

In this study, an Ag/Fe₂O₃/ZnO ternary composite was synthesized via chemical deposition and photochemical deposition processes. The photocatalytic activity of Ag/Fe₂O₃/ZnO was investigated in comparison with Fe₂O₃/ZnO or Ag/ZnO. Furthermore, the effects of structure and interfacial electronic interactions of ZnO, Fe₂O₃ and Ag on their photocatalytic activity were also investigated. The Ag/Fe₂O₃/ZnO ternary composite had effective photodegradation towards MO and iodoform under the irradiation of simulated daylight, and it had a higher degradation efficiency towards iodoform. The results indicate the potential applications of Ag/Fe₂O₃/ZnO in sewage treatment.

2. Experiments

2.1 Preparation of the Ag/Fe₂O₃/ZnO ternary composite

First, a ZnO seed layer was prepared via a sol–gel method. Briefly, 40 mL anhydrous ethanol, 0.5 M zinc acetate-2-hydrate, and 0.5 M ethanol amine were mixed and stirred at 70 °C for 3 h. The resultant sol was transparent. The solution was then spin-coated on a glass substrate at 500 rpm for 10 s and 3000 rpm for 30 s, respectively. After that, the glass substrate was heated at 300 °C for 2 h to remove the solvent. Subsequently, the growth of ZnO nanorods (NR) was carried out by suspending the glass substrate in a 40 mL beaker that was filled with an equimolar aqueous solution of 0.1 M zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and 0.1 M methenamine (C₆H₁₂N₄, HMT) at 75 °C for 3 h.^11,12

Fe₂O₃ nanoparticles were then deposited on the ZnO nanorods via photochemical deposition. The ZnO nanorod-coated substrate was immersed in a mixture of 4 mM FeCl₃·6H₂O and NaNO₃·6H₂O aqueous solution, and then irradiated with UV light for 1 h (wavelength 365 nm, power 56 W and working distance 0.3 m).^13,14

Ag nanoparticles were further prepared via photochemical deposition. The Fe₂O₃/ZnO nanorod-coated substrate was immersed in a 1 mM AgNO₃ and NaNO₃·6H₂O aqueous solution, and then irradiated with UV light (wavelength 365 nm, power 56 W and working distance 0.3 m) at room temperature for 40 min. The prepared sample was post-annealed in ambient air at 400 °C for 2 h to obtain a crystal structure.

2.2 Characterization

The crystal structure of the products was characterized via X-ray diffraction (XRD, Siemens D5005). Scanning electron microscopy (SEM, S4800) was employed to investigate their morphology. Transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM) and selected area electron diffraction (SAED) were conducted on a Tecnai G2 F20 S-TWIN. UV-vis absorption spectra were obtained using a UV-2401PC spectrometer.

2.3 Photocatalytic studies

Iodoform (halogenated hydrocarbon) and MO (organic dye) are two common pollutants in wastewater. Iodoform and MO were chosen to investigate the photo-degradation characteristic of the Ag/Fe₂O₃/ZnO ternary composite and Fe₂O₃/ZnO and Ag/ZnO binary composite. Samples were placed in 30 mL of iodoform solution (1.65 mg L⁻¹) and MO solution (6.67 mg L⁻¹) under the irradiation of simulated sunlight (xenon lamp, wavelength 320–780 mm, and light intensity at the sample is approximately 50 mW cm⁻²). Aliquots of iodoform and MO were taken every 30 min to analyze their ultraviolet absorbance.

To investigate the reusability of the Ag/Fe₂O₃/ZnO ternary composite, the Ag/Fe₂O₃/ZnO modified glass substrates were placed in 30 mL of iodoform solution (1.65 mg L⁻¹) and MO solution (6.67 mg L⁻¹) under the irradiation of simulated sunlight for 5 cycles. In each cycle, the iodoform and MO solutions were degraded by Ag/Fe₂O₃/ZnO for 120 min and then their maximum absorbance was measured. In the next cycle, the solutions of iodoform and MO were replaced, and the same Ag/Fe₂O₃/ZnO modified glass substrates were reused to degrade iodoform and MO for 120 min and then their maximum absorbance was measured.

3. Result and discussion

3.1 Characterization of materials

Fig. 1 shows the XRD pattern of the as-prepared sample. The typical diffraction peaks (0 0 2), (1 0 2), (1 1 0), (1 0 3), (2 0 1) of hexagonal wurtzite ZnO are observed, which are in good agreement with the standard patterns (JSPDS no. 36-1451). In addition, three diffraction peaks (2 0 6), (2 1 12), (5 2 1) (JCPDS no. 13-0458) of Fe₂O₃ and four diffraction peaks (2 2 0), (3 1 1), (3 3 1), (4 2 0) (JSPDS no. 01-1164) of Ag are observed, which reveal that the obtained Ag/Fe₂O₃/ZnO ternary composite has a good crystalline structure.

Fig. 1 XRD patterns of the Ag/Fe₂O₃/ZnO ternary composite.

The as-prepared samples were characterized via SEM. As shown in Fig. 2a, it is obvious that the wurtzite ZnO rods have a hexagonal structure, and their surface is relatively smooth (Fig. 2a inset). Under the irradiation of UV light, the Fe₂O₃ nanoparticles are coated on the surface of ZnO to form a Fe₂O₃/ZnO binary composite (Fig. 2b), which results in a rough layer on the nanorods (Fig. 2b inset). After the decoration of Ag nanoparticles, granular materials are observed on the surface of Fe₂O₃/ZnO (Fig. 2c inset), which indicates the formation of the Ag/Fe₂O₃/ZnO ternary composite.

Fig. 2 SEM images of (a) ZnO nanorod arrays, (b) Fe₂O₃/ZnO binary composite and (c) Ag/Fe₂O₃/ZnO ternary composite.

To further clarify the structure and morphology of the Ag/Fe₂O₃/ZnO ternary composite, HRTEM was performed. As shown in Fig. 3a, it is noted that the product has hierarchical structures. The high-magnification image of Fig. 3a (indicated by the white rectangle) suggests that the lattice spacing is about 0.248 nm, 0.155 nm and 0.144 nm (Fig. 3b), which correspond to the (1 0 1) plane of ZnO, (5 2 1) plane of Fe₂O₃, and (2 2 0) plane of Ag. The data indicate the existence of ZnO, Fe₂O₃ and Ag in the composite. The corresponding selected area electron diffraction (SAED) patterns further confirm the successful formation of the Ag/Fe₂O₃/ZnO ternary composite (Fig. 3c).

Fig. 3 (a) TEM image of the Ag/Fe₂O₃/ZnO ternary composite, (b) HR-TEM image of the area indicated by the white rectangle in (a), and (c) the SAED pattern of (a).

3.2 Growth mechanism

The proposed growth mechanism of the Ag/Fe₂O₃/ZnO ternary composite is shown in Fig. 4. First, with the addition of HMT and Zn(OH)₄²⁻, the ZnO rod arrays were coated on the glass substrate with the ZnO seed layer under hydrothermal treatment at 75 °C (ref. 15) (Fig. 4a). Second, under UV irradiation, ZnO could form electronic–hole pairs. As a result, a great deal of OH⁻ and Fe³⁺ were deposited on the surface of the ZnO nanorods to form [Fe(OH)₄]⁻ ions, and then they removed OH⁻ and H₂O to form Fe₂O₃ (ref. 16) (Fig. 4b). Likewise, with continued UV irradiation, more and more Fe₂O₃ particles were obtained, which led to a thick layer covered on the surface of ZnO (Fig. 4c). Finally, in the aqueous solution of AgNO₃ and NaNO₃·6H₂O, Ag⁺ and OH⁻ were deposited on the surface of Fe₂O₃/ZnO, thus generating nanosized silver particles¹⁷ (Fig. 4d).

Fig. 4 The proposed growth model of the Ag/Fe₂O₃/ZnO ternary composite, (a) ZnO nanorods were grown on a glass substrate, (b) Fe₂O₃ nanoparticles were attached to the surface of ZnO, (c) the Fe₂O₃ layer was coated on the surface of ZnO and (d) Ag nanoparticles were uniformly deposited on the surface of Fe₂O₃/ZnO to form the Ag/Fe₂O₃/ZnO ternary composite.

3.3 Photocatalytic activity

Fig. 5 shows the UV-vis absorption spectra of Ag/ZnO, Fe₂O₃/ZnO and Ag/Fe₂O₃/ZnO. Remarkably, the absorption wavelength of the Ag/Fe₂O₃/ZnO ternary composite is up to 650 nm, which is much wider than that of Ag/ZnO or Fe₂O₃/ZnO. This result indicates that Ag/Fe₂O₃/ZnO can improve the absorption intensity in the visible region. Therefore, the Ag/Fe₂O₃/ZnO ternary composite has great potential in terms of light degradation.

Fig. 5 UV-vis absorption spectra of Ag/ZnO, Fe₂O₃/ZnO and Ag/Fe₂O₃/ZnO.

The photodegradation efficiency of Fe₂O₃/ZnO, Ag/ZnO, and Ag/Fe₂O₃/ZnO under illumination and without illumination (Fig. S1†) was investigated. To calculate the concentration of the iodoform and MO solutions, aliquots of the solutions were collected every 30 min and the absorption intensity at their maximum absorbance wavelength was measured. Fig. 6 shows the UV-vis absorption spectra of iodoform and MO degraded by Fe₂O₃/ZnO, Ag/ZnO, and Ag/Fe₂O₃/ZnO. When Ag/ZnO was used as the photocatalyst material, the characteristic absorption peaks of iodoform and MO dropped to about 44% and 50% of their initial absorption intensity after 120 min (Fig. 6a), respectively. As for Fe₂O₃/ZnO, the absorption peaks of iodoform and MO reduced to about 40% of their initial absorption intensity after 120 min (Fig. 6b). Interestingly, when the Ag/Fe₂O₃/ZnO composite was used, the absorption peaks of iodoform and MO rapidly decreased to about 5% and 22% of their initial intensity after 120 min, respectively (Fig. 6c). Obviously, the Ag/Fe₂O₃/ZnO ternary composite had a much higher degradation efficiency towards iodoform and MO compared to Ag/ZnO or Fe₂O₃/ZnO.

Fig. 6 Time-dependent absorption spectra of iodoform and MO degraded by (a) Ag/ZnO, (b) Fe₂O₃/ZnO and (c) Ag/Fe₂O₃/ZnO.

By considering the peak intensity of MO and iodoform, we plotted the corresponding degradation rates of Fe₂O₃/ZnO, Ag/ZnO and Ag/Fe₂O₃/ZnO, as shown in Fig. 7. Degradation efficiency (y-axis), is defined as (1 − C/C₀) × 100%, and the X-axis is reaction time in minutes. C₀ is the initial concentration after the equilibrium adsorption, and C is the remaining concentration of iodoform and MO.^18,19 Under the irradiation of simulated sunlight, the degradation efficiency of Ag/Fe₂O₃/ZnO for iodoform and MO was 94% and 80% (curve 1 and 2) after 120 min, respectively, which are much higher than that of Fe₂O₃/ZnO (58% and 60%, curve 3 and 4, respectively) or Ag/ZnO (56% and 50%, curve 5 and 6, respectively). In addition, in the mixture of three organics, the degradation efficiency of Ag/Fe₂O₃/ZnO for iodoform, MO and RhB was 89%, 45% and 40% (Fig. S2†), respectively. The data indicate that Ag/Fe₂O₃/ZnO exhibits a greater improvement in photocatalytic activity compared to Fe₂O₃/ZnO or Ag/ZnO. More importantly, the results also suggest that the Ag/Fe₂O₃/ZnO ternary composite has efficient and selective degradation towards iodoform.

Fig. 7 Photo-degradation efficiency of iodoform and MO under illumination. Degradation curve of Ag/Fe₂O₃/ZnO for iodoform (curve 1), and for MO (curve 2), degradation curve of Fe₂O₃/ZnO for MO (curve 3), and for iodoform (curve 4), degradation curve of Ag/ZnO for iodoform (curve 5), and for MO (curve 6).

3.4 Reusability of the Ag/Fe₂O₃/ZnO composite

To examine the reusability of the Ag/Fe₂O₃/ZnO ternary composite, it was used as the photocatalyst material for 5 cycles. The iodoform and MO solutions were replaced every 120 min and their maximum absorbance was measured. After every cycle, the corresponding degradation rates of Ag/Fe₂O₃/ZnO for iodoform and MO were about 92%, 89%, 86%, 85% and 82%, and 75%, 73%, 71%, 70% and 68% (Fig. 8b), respectively. The degradation efficiency of the Ag/Fe₂O₃/ZnO ternary composite for iodoform and MO solutions dropped to about 10% and 7% after 5 cycles (Fig. 8a), respectively. The reason for this may be that the Ag/Fe₂O₃/ZnO ternary composite participates in the process of degradation, and thus there is some consumption of Ag/Fe₂O₃/ZnO after each cycle.

Fig. 8 (a) Cycle time-dependent absorption spectra of iodoform and MO solutions degraded by the Ag/Fe₂O₃/ZnO ternary composite, and (b) cycle degradation curves of Ag/Fe₂O₃/ZnO for iodoform (curve 1) and MO (curve 2).

3.5 Proposed mechanism for the enhancement of photocatalytic activity by Ag/Fe₂O₃/ZnO

The different components in the Ag/Fe₂O₃/ZnO ternary composite, Fe₂O₃/ZnO and Ag/ZnO binary composite result in the difference in photocatalytic activities. The proposed photocatalytic degradation mechanism of iodoform and MO by Ag/Fe₂O₃/ZnO is illustrated in Fig. 9. The process can be described as follows: (1) under illumination, the electrons of ZnO in Ag/Fe₂O₃/ZnO are excited to generate electron–hole pairs, (2) the electrons in the valence band (VB) of ZnO are transferred to the (CB) conduction band of Fe₂O₃, (3) and then they are transferred to the CB of Ag. The high separation rate of electron–hole pairs makes it easier to form O²⁻ active ions and OH˙ radicals on the surface of Ag/Fe₂O₃/ZnO to degrade iodoform and MO and to enhance photo-degradation activity. This process can be written as follows:^20–25

O₂ + e⁻ → O₂⁻ (1)

O₂⁻ + H⁺ → HO₂˙ (2)

2HO₂˙ → H₂O₂ + O₂ (3)

H₂O₂ + O₂⁻ → OH˙ + O₂ + OH⁻ (4)

OH˙ + pollutants → CO₂ + H₂O (5)

Fig. 9 Schematic of iodoform and MO degradation by the Ag/Fe₂O₃/ZnO ternary composite.

At the same time, under the action of H⁺ and e⁻, iodoform can be decomposed by Ag due to the following reactions:^26–28

Ag⁺ → Ag²⁺ + e⁻ (6)

Ag²⁺ + H₂O → Ag⁺ + H⁺ + OH˙ (7)

CHI₃ + 3H⁺ + 3e⁻ → CH₄ + 3I⁻ (8)

CH₄ + OH˙ → H₂O + CO₂ (9)

I⁻ + Ag⁺ → AgI (10)

This provides another way for the Ag/Fe₂O₃/ZnO to degrade iodoform, which can improve the degradation efficiency of iodoform. Taken together, the abovementioned reactions account for the selective property of the Ag/Fe₂O₃/ZnO ternary composite towards iodoform degradation.²⁹

4. Conclusions

In summary, an Ag/Fe₂O₃/ZnO ternary composite has been successfully synthesized via chemical deposition and photochemical deposition in an aqueous solution. The Ag/Fe₂O₃/ZnO ternary composite shows a much higher photocatalytic performance compared with the Fe₂O₃/ZnO or Ag/ZnO binary composite. Moreover, the Ag/Fe₂O₃/ZnO ternary composite displays efficient and selective degradation of carcinogenic iodoform in sewage. This provides a promising strategy for the efficient degradation of halogenated hydrocarbons.

Acknowledgements

We acknowledge the financial support from the Ministry of Education of China (IRT1148), the Priority Academic Program Development of Jiangsu Higher Education Institutions (YX03001), the Natural Science Foundation of Jiangsu Province (BK20131376, BY2014015), the Opening Project of State Key Laboratory of New Ceramic and Fine Processing (KF201310) and the Synergistic Innovation Center for Organic Electronics and Information Displays.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20403h

‡ These authors contributed equally to this work.

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