Photocatalytic degradation of 4-chlorophenol over Ag/MFe₂O₄ (M = Co, Zn, Cu, and Ni) prepared by a modified chemical co-precipitation method: a comparative study

Abstract

In this study, MFe₂O₄ (M = Zn, Cu, Co and Ni) nanoparticles were prepared by a modified chemical co-precipitation method followed by annealing. Then, Ag/MFe₂O₄ nanoparticles were synthesised via a wetness impregnation method. The structural properties of the samples were systematically investigated by XRD, SEM, TEM, EDX, DRS, BET, SPV and FT-IR techniques. The photocatalytic performances of the prepared MFe₂O₄ and Ag/MFe₂O₄ samples were comparatively studied by the photodegradation of 4-chlorophenol under xenon lamp illumination in a FTIR reaction cell. The photodegradation rate of 4-chlorophenol was 90% for the Ag/CoFe₂O₄ sample. The cheap cost and easy synthesis of the photocatalysts indicate the potential for a better thermal disposal of chlorophenol pollutants at lower temperatures.

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1. Introduction

Over the past decades, chlorophenols have been an important organic pollutant of water pollutants.¹ They are listed as one of 129 priority pollutants by the U.S. EPA.² In the 1950s, 4-chlorophenol (4-CP) was a typical pollutant among these chlorophenols, and was used in disinfectants, antiseptics and herbicides, and was also applied in the large-scale disinfection of drinking water.^2–6 It can result in negative effects such as carcinogenic, teratogenic, mutagenic and acute toxicity. It is difficult to dispose of these pollutants by usual treatment methods. Compared to the conventional multi-treatment processes, photocatalysis is an effective method which transforms chlorophenol organic compounds into small inorganic molecules.

TiO₂ is known as an efficient photocatalyst because of its nontoxicity, low-cost, and photostability, which has led to many promising applications in photovoltaics,⁷ photocatalysis,⁸ photo-electrochromics and sensors.⁹ J. Theurich et al. reported that TiO₂-based materials showed preeminent photocatalytic activities in the photodegradation of 4-CP under UV-irradiation.¹⁰ P. Rangsunvigit et al. reported that the addition of Pt in TiO₂ can improve the catalytic activity and the highest activity was obtained with 1% Pt/TiO₂. For TiO₂–SiO₂, the highest activity was achieved with 10% SiO₂–TiO₂ which has the highest adsorption capacity.¹¹ However, TiO₂ nanomaterials are normally transparent in the visible light region. By developing new visible light-induced catalysts, it is possible to satisfy sustainable energy demands.

Recently, some composite oxides such as spinel AB₂O₄ have shown better selectivity and sensitivity to certain gases than single-metal oxides. Nanoparticles of spinel ferrites MFe₂O₄ (M = Zn, Co, Ni, Cu etc.) are an emerging new type of catalyst which people have shown great interest in by addressing the relationship between their physical chemistry properties and their crystal structure.¹² Some synthetic methods to prepare MFe₂O₄ ferrites such as sol–gel,¹³ micro-emulsion,^14,15 citrate gel,¹⁶ precipitation,¹⁷ precursor techniques^18–20 host template,²¹ and mechanical alloying²² have been developed. A chemical co-precipitation method is more suitable to prepare nanostructured oxides because of its simplicity and good control of grain size. B. Veronica et al. have reported superparamagnetism and interparticle interactions in ZnFe₂O₄ nanocrystals.²³ The method could also be extended to synthesize CoFe₂O₄, CuFe₂O₄ and NiFe₂O₄ nanocrystals. J. Jiang et al.²⁴ reported nanostructured NiFe₂O₄ via a refluxing route which exhibited typical superparamagnetic behavior at room temperature, with a present finite coercivity of 353 Oe at 5 K. S. Saadi et al.²⁵ and H. Yang et al.²⁶ studied photoassisted hydrogen evolution over CuFe₂O₄, and they found that CuFe₂O₄ exhibited promising photocatalytic activity. S. Li et al.²⁷ have studied the fabrication and characterization of TiO₂/BaAl₂O₄. J. Okal et al.²⁸ have reported the catalytic combustion of methane over Ru/ZnAl₂O₄ nanoparticles. C. Ragupathi et al.²⁹ have studied the photocatalytic activities of ZnAl₂O₄ which was prepared by a microwave method.

Supported noble metal catalysts have better activities and handling properties. Moreover, the synthesis conditions are mild. Noble metal/semiconductor oxide composites are very attractive because of their enhanced efficiency of photocatalytic activity. It was reported that the addition of the noble metals Pt, Au, Rh to TiO₂ can enhance the photocatalytic efficiency.³⁰ An efficient support, including activated carbon,³¹ silica,³² alumina³³ and zeolites,³⁴ could improve the activity, recycling, and selectivity of Ag catalyst systems. Recently, Liu Yang et al.³⁵ reported that Ag nanoparticles in mesoporous TiO₂ were not only an antimicrobial but also an intensifier of photocatalysis. Jia Ren et al.³⁶ reported Ag-loaded Bi₂WO₆ photocatalysts that exhibited a significant increase in the photocatalytic activity of inactivating E. coli, a Gram-negative bacterium, and S. epidermidis, a Gram-positive bacterium under visible light irradiation (λ > 420 nm). Recently, spinel type oxide catalysts have been used as supports for Ru and Pd and a distinct metal–support interaction has been found.^37,38 However, the photocatalytic properties and the activities of these spinel type oxides for the degradation of 4-CP have not been reported.

In this work, MFe₂O₄ (M = Zn, Co, Ni and Cu) nanoparticles with a brick-like spinel superstructure were prepared by a modified chemical co-precipitation method and post calcinations at 673 K, 773 K and 873 K, and then 1% wt noble metal Ag loading on the MFe₂O₄ (M = Zn, Co, Ni and Cu) (673 K) by a wetness impregnation method, which produced the Ag/MFe₂O₄ nanomaterials. These MFe₂O₄ and Ag/MFe₂O₄ catalysts were studied for their photocatalytic degradation of 4-CP under xenon lamp irradiation. The photocatalytic mechanism is discussed, thereby showing the method of environmental purification and the need for new active catalysts.

2. Experimental section

2.1 Materials and methods

The MFe₂O₄ (M = Zn, Co, Ni and Cu) catalysts with brick-like superstructures were prepared by a co-precipitation method. The starting materials were FeSO₄·7H₂O and MSO₄·nH₂O (M = Zn, Co, Ni and Cu). A 0.1 M (100 mL) solution of FeSO₄·7H₂O and 0.2 M (100 mL) MSO₄·nH₂O was mixed in deionized water. A 0.3 M (25 mL) solution of Na₂C₂O₄ was prepared and added to the salt solution. The solution was continuously stirred at 353 K and allowed to cool slowly to room temperature. A ferrioxalate precipitate appeared. Then the precipitate was washed twice with distilled water and ethanol, respectively. The separated precipitate was dried overnight at 373 K. The precursor was divided into three portions and calcined at 673 K, 773 K and 873 K for 2 h, respectively. Consequently, MFe₂O₄ (M = Zn, Co, Ni and Cu) samples were produced. The samples are named in Table 1.

Table 1 The crystallite size, calcination temperature and photodegradation ratio for 4-CP of the synthesized samples

Abbreviated name of synthesized samples	Prepared samples	Calcination temperature (K)	Average crystallite size (nm)	Photocatalytic degradation ratio of 4-CP with the sample (%)
Ag/Zn	Ag/ZnFe2O4	673	13	88
Zn1	ZnFe2O4	673	14	81
Zn2	ZnFe2O4	773	15	77
Zn3	ZnFe2O4	873	18	65
Ag/Co	Ag/CoFe2O4	673	11	90
Co1	CoFe2O4	673	12	79
Co2	CoFe2O4	773	14	70
Co3	CoFe2O4	873	16	60
Ag/Ni	Ag/NiFe2O4	673	8	86
Ni1	NiFe2O4	673	8	78
Ni2	NiFe2O4	773	10	68
Ni3	NiFe2O4	873	14	55
Ag/Cu	Ag/CuFe2O4	673	25	81
Cu1	CuFe2O4	673	26	62
Cu2	CuFe2O4	773	22	58
Cu3	CuFe2O4	873	21	52
Without catalyst				7.5

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The catalysts of MFe₂O₄ (M = Zn, Co, Ni and Cu) (calcined at 673 K) with Ag supports were prepared by an impregnation strategy.³⁹ AgNO₃ solution was used as the precursor. The silver loading was 1 wt%. After evaporation, the catalyst was then dried at 100 °C for 12 h and calcined at 673 K in air for 6 h. Finally, the catalyst was slowly cooled to room temperature. The Ag/MFe₂O₄ (M = Zn, Co, Ni and Cu) samples are named in Table 1.

All chemicals in this work were analytical grade reagents and used as starting materials without further purification.

2.2 Characterization of the catalysts

The crystal structures of the MFe₂O₄ and Ag/MFe₂O₄ (M = Zn, Co, Ni and Cu) samples were examined by X-ray diffraction (XRD, Rigaku D/max) with Cu Kα radiation (λ = 0.15418 nm) over the 2θ range of 20–80°. The surface area was measured by Brunauer–Emmett–Teller (BET), and N₂ gas adsorption–desorption isotherms were determined using a Micromeritics ASAP-2000. Pore volume was calculated by the BJH model. The light absorption properties were measured using a UV-Vis diffuse reflectance spectrophotometer (JASCO, UV-550) with a wavelength range of 200–800 nm. The morphology of these prepared samples was characterized by scanning electronic microscopy (SEM) with a JSM-6700 LV and by transmission electron microscopy (TEM, Hitachi 800 system at 200 kV). Energy dispersive X-ray analysis (EDX, Horiba 7593 H) was performed to determine the elemental concentration distribution of the samples. Fourier transform infrared (FTIR, BRUKER VERTEX 70) spectra were recorded in the range 4000–450 cm⁻¹ with a 4 cm⁻¹ resolution. XPS data were obtained using a Perkin-Elmer PHI 5600 electron spectrometer which used achromatic Al Kα radiation (1486.6 eV) with Ar⁺ sputtering to remove the surface layer of the sample.

The characteristics of photogenerated charge carriers in the visible light spectrum were examined by a lock-in-based surface photovoltage (SPV) measurement system. The system consists of monochromatic light from a monochromator (Omni-λ3005) and a xenon lamp (500 W), a photovoltaic sample cell and a lock-in amplifier (SR830-DSP) with an optical chopper (model SR540). The AC photovoltage signal of the sample was obtained using a capacitor structure. The highest intensity monochromatic light in the spectrum was less than 80 μW cm⁻². The effective overlapping area of the two electrodes was about 1 cm² for testing. The corresponding phase spectra were taken with the SPV spectra by a computer. SPV measurements were carried out in an ambient environment and the raw data were used directly.

2.3 Measurement of photocatalytic activities

The photodegradation of 4-chlorophenol was performed under a xenon lamp (XQ-500 W) with an intensity of 40 mW cm⁻². The reaction system was a self-made photochemical reactor, demonstrated in ref. 40. In the test, the initial concentration of the 4-CP (C₆H₄ClO) solution was 20 mg L⁻¹. Reaction suspensions were prepared by adding 0.01 g catalyst powder into the 4-CP solution (100 mL). The reaction suspension was stirred in a magnetic stirrer in the dark for 30 minutes to reach an adsorption/desorption equilibrium. Then the test was performed under the xenon lamp irradiation. After a given irradiation time, 4.0 mL samples were withdrawn, and the collected samples are centrifuged at 6000 rpm for 5 minutes in order to separate the photocatalyst from the 4-CP solution. The absorption spectra of 4-CP were measured by a UV-Vis spectrophotometer.

3. Results and discussion

3.1 XRD analysis

From Fig. 1a, the undoped NiFe₂O₄ samples Ni1, Ni2, Ni3 and the Ag-doped NiFe₂O₄ sample Ag/Ni can be assigned to the inverse spinel structure, according to the standard XRD patterns (JCPDS patterns no. 21-1272). The peaks with 2θ values of 30.6°, 36.0°, 44.0°, 57.8° and 63.2° correspond to the crystal planes (220), (311), (400), (511), and (440) of crystalline NiFe₂O₄, respectively.

Fig. 1 (a) The XRD patterns of the NiFe₂O₄ (Ni1, Ni2 and Ni3) samples and the Ag/NiFe₂O₄ (Ag/Ni) sample, (b) the CoFe₂O₄ (Co1, Co2 and Co3) samples and the Ag/CoFe₂O₄ (Ag/Co) sample, (c) the CuFe₂O₄ (Cu1, Cu2 and Cu3) samples and the Ag/CuFe₂O₄ (Ag/Cu) sample, and (d) the as-synthesized precursor, the ZnFe₂O₄ (Zn1, Zn2 and Zn3) samples and the Ag/ZnFe₂O₄ (Ag/Zn) sample.

The crystallite size was calculated by the Scherrer equation:

D = 0.89λ/β cos θ

where D is the average crystal size, λ is the wavelength of Cu Kα (0.15406 nm), β is the full width at half maximum, and θ is the diffraction angle.⁴¹

The average crystallite size is calculated from the (311) peak of the crystal plane using Scherrer's equation. In this case the average crystallite sizes were 8 nm for Ni1, 10 nm for Ni2, 14 nm for Ni3 and 8 nm for Ag/Ni (see Table 1).

Fig. 1b shows the XRD patterns of the undoped CoFe₂O₄ samples Co1, Co2, Co3 and the Ag-doped CoFe₂O₄ sample Ag/Co. The results indicate that the final product is CoFe₂O₄ with the expected inverse spinel structure. The peaks with 2θ values of 30.7° (with index number of 220), 36.3° (311), 43.6° (400), 57.9° (511), and 63.2° (440) reveal the formation of CoFe₂O₄. The average crystallite sizes of these samples are about 12 nm (Co1), 14 nm (Co2), 16 nm (Co3), and 11 nm (Ag/Co).

The XRD patterns of the undoped CuFe₂O₄ samples Cu1, Cu2, Cu3 and the Ag-doped CuFe₂O₄ sample Ag/Cu are shown in Fig. 1c. In the XRD pattern of Cu1 and Ag/Cu, a phase of copper ferrite appears, and the diffraction angle and intensity of the characteristic peaks are consistent with that of the standard JCPDS card no. 34-0425. Copper ferrite with a spinel crystal structure phase can be directly formed after calcination (673 K), and it does not require other synthetic techniques.⁴² There exists a small amount of CuO impurity phase in the as-obtained CuFe₂O₄ when the calcination temperature is increased (773 K and 873 K). From the XRD data in Fig. 1c, the average sizes of these samples are about 26 nm (Cu1), 22 nm (Cu2), 21 nm (Cu3), and 21 nm (Ag/Cu).

Fig. 1d shows the XRD patterns of the as-synthesized precursor for ZnFe₂O₄ and the Zn1, Zn2, Zn3 and Ag/Zn samples. It was noted that the diffraction peaks of the as-synthesized precursor did not match the phase of zinc ferrite. The observed diffraction peaks from the (220), (311), (400), (422), (511) and (440) crystal planes are well matched with the cubic spinel structure of the ZnFe₂O₄ phase (JCPDS card no. 22-1012). From the XRD data of Fig. 1d, the average sizes of these samples are about 14 nm (Zn1), 15 nm (Zn2), 18 nm (Zn3), and 13 nm (Ag/Zn). The average crystallite size of the Ag/MFe₂O₄ sample seems smaller than the undoped MFe₂O₄. This phenomenon is probably due to the reaction between MFe₂O₄ and HNO₃ where some of the MFe₂O₄ particles were destroyed into smaller ones.⁴³

The crystallinity will increase as the calcining temperature rises. Despite this, a little of the CuO and Fe₂O₃ phases existed in the powders which were calcined below 773 K.⁴⁴ The augmentation in the calcination temperature of the CuFe₂O₄ sample led to a decrease in the degree of ordering and crystallite size of both the CuO and Fe₂O₃ phases. This decrease may be due to the solid-state reaction between CuO and Fe₂O₃ solids yielding a nanocrystalline CuFe₂O₄ catalyst.⁴⁵

From Fig. 1, the peaks due to Ag species cannot be observed due to the low doping content of Ag in the samples. As the calcination temperature increases, the peak intensity of the MFe₂O₄ samples also increases. This indicates that the calcination temperature plays a role in the formation of the spinel crystal structure and morphology. The average crystallite size of Ag/MFe₂O₄ samples seems smaller than undoped MFe₂O₄ samples. The results are consistent with the description reported by X. Li et al.⁴⁶

The specific surface areas of the MFe₂O₄ and Ag/MFe₂O₄ samples are shown in Table 2. The surface areas of Ag/Co, Ag/Zn, Ag/Ni, Ag/Cu are 187, 166, 153.5 and 142.3 m² g⁻¹, respectively. The mesoporous Ag/CoFe₂O₄ has the highest specific surface area. The pore volumes of Ag/Co, Ag/Zn, Ag/Ni, Ag/Cu are 0.12, 0.162, 0.181 and 0.20 cm³ g⁻¹, respectively. The results show that the specific surface areas of the samples with silver loading are greater than the other samples. With the increase in the specific surface area, the pore volume will be reduced.

Table 2 The specific surface area and pore volume of the MFe₂O₄ (M = Zn, Cu, Co and Ni) and Ag/MFe₂O₄ samples

Abbreviation name	Prepared sample	BET surface area/m^2 g^−1	Pore volume/cm^3 g^−1
Ag/Zn	Ag/ZnFe2O4	166	0.162
Zn1	ZnFe2O4	138	0.22
Ag/Co	Ag/CoFe2O4	187	0.12
Co1	CoFe2O4	136	0.257
Ag/Ni	Ag/NiFe2O4	153.5	0.181
Ni1	NiFe2O4	129	0.29
Ag/Cu	Ag/CuFe2O4	142.3	0.20
Cu1	CuFe2O4	108	0.34

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3.2 SEM and TEM analysis

The SEM images in Fig. 2 show the morphology of Co1 (Fig. 2a), Co2 (Fig. 2c), Co3 (Fig. 2b), and Ag/Co (Fig. 2d). The SEM images in Fig. 3 show the morphology of Ni1 (Fig. 3a), Ni2 (Fig. 3c), Ni3 (Fig. 3b) and Ag/Ni (Fig. 3d). The CoFe₂O₄ and NiFe₂O₄ powders calcined at different temperatures are completely composed of “timber”. Fig. 2e shows the TEM micrograph of Ag/Co. Fig. 3e and f show the TEM micrographs of Ag/Ni. Both the porous blocks (Ag/Ni and Ag/Co) were formed through the agglomeration of numerous Ag/Ni and Ag/Co nanoparticles, respectively.

Fig. 2 The SEM images of the calcined CoFe₂O₄ samples at 673 K (Co1) (a), 773 K (Co2) (c), 873 K (Co3) (b) and the Ag/CoFe₂O₄ (Ag/Co) sample at 673 K (d), and (e) the TEM image of the calcined Ag/CoFe₂O₄ sample (Ag/Co) at 673 K.

Fig. 3 The SEM images of the calcined NiFe₂O₄ samples at 673 K (Ni1) (a), 773 K (Ni2) (c), 873 K (Ni3) (b) and the Ag/NiFe₂O₄ (Ag/Ni) sample at 673 K (d), and (e and f) the TEM images of the calcined Ag/NiFe₂O₄ sample (Ag/Ni) at 673 K.

Fig. 4 presents the TEM images of the Cu1 and Ag/Cu samples. It is found that blocks (Fig. 4a) are formed through the agglomeration of numerous nanocrystalline CuFe₂O₄ particles. Fig. 4b shows the TEM micrograph of the Ag/Cu sample. The nanoparticles distinctly exhibit a narrow size distribution with sizes ranging from 20 to 30 nm. Fig. 5 presents the TEM images of the Zn1 and Ag/Zn samples. It is found that blocks (Fig. 5a) are formed through the agglomeration of numerous nanocrystalline ZnFe₂O₄ particles. Fig. 5b shows the TEM micrograph of the Ag/Zn sample. The nanoparticles distinctly exhibit a narrow size distribution with sizes of 20 nm, approximately. These results are in good agreement with the XRD analysis.

Fig. 4 The TEM images of the CuFe₂O₄ (Cu1) (a) and the Ag/CuFe₂O₄ (Ag/Cu) samples (b), and the EDS pattern of the CuFe₂O₄ (Cu1) sample (c).

Fig. 5 The TEM images of the ZnFe₂O₄ (Zn1) (a) and the Ag/ZnFe₂O₄ (Ag/Zn) samples (b), and the EDS pattern of the Ag/ZnFe₂O₄ (Ag/Zn) sample (c).

From the above images, we cannot distinguish the Ag species in the samples. So the EDS patterns of the prepared Cu1 and Ag/Zn samples were measured. The EDS spectrum in Fig. 4c exhibits Cu, Fe and O peaks and also reflects the atomic ratio of Cu, Fe and O in the prepared Cu1 sample (Table 3). The peaks due to metallic silver are identified in the EDS pattern (Fig. 5c) of the Ag/Zn sample. Table 4 exhibits the atomic ratio of the elements in the prepared Ag/Zn sample.

Table 3 The atomic ratio of the elements in the CuFe₂O₄ (Cu1) sample

Element	Weight ratio (%)	Atomic ratio (%)
C	29.34	44.34
O	40.99	46.46
Fe	19.03	6.16
Cu	10.64	3.04
Total	100.00	100.00

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Table 4 The atomic ratio of the elements in the Ag/ZnFe₂O₄ (Ag/Zn) sample

Element	Weight ratio (%)	Atomic ratio (%)
C	31.25	47.59
O	37.46	42.79
Fe	19.44	6.34
Zn	11.61	3.24
Ag	0.24	0.04
Total	100.00	100.00

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3.3 DRS analysis

The UV-Vis spectra of the MFe₂O₄ and Ag/MFe₂O₄ (M = Zn, Co, Ni and Cu) samples are shown in Fig. 6. The MFe₂O₄ and Ag/MFe₂O₄ samples have strong absorption in the range of the whole spectrum (200–800 nm). As shown in Fig. 6, we clearly observed the characteristic bands of absorption at 500–700 nm. Wider and stronger absorption spectra are found for Ag/MFe₂O₄ samples.

Fig. 6 (a) The UV-Vis absorption spectra of the NiFe₂O₄ (Ni1, Ni2 and Ni3) samples and the Ag/NiFe₂O₄ (Ag/Ni) sample, (b) the CoFe₂O₄ (Co1, Co2 and Co3) samples and the Ag/CoFe₂O₄ (Ag/Co) sample, (c) the CuFe₂O₄ (Cu1, Cu2 and Cu3) samples and the Ag/CuFe₂O₄ (Ag/Cu) sample, and (d) the ZnFe₂O₄ (Zn1, Zn2 and Zn3) samples and the Ag/ZnFe₂O₄ (Ag/Zn) sample.

Due to the mesoporous structure, the photo-generated electron–hole pairs in the Ag/MFe₂O₄ nanocrystal are separated mainly via diffusion. With the improved separation efficiency of these photo-induced electron–hole pairs into charge carriers, the SPV response and photocatalytic activity of Ag/MFe₂O₄ are higher than the MFe₂O₄ samples. Moreover, because the Fermi level of Ag (ca. 4.3 eV)⁴⁷ is much lower in electronic energy than the conduction band edge of the MFe₂O₄ catalysts,⁴⁸ the Ag species probably adsorbed onto the surface of MFe₂O₄ plays a key role in trapping and transporting the photo-generated electrons. Therefore, the Ag/MFe₂O₄ sample exhibits a higher photocatalytic activity than MFe₂O₄.

For the Ag/CoFe₂O₄ sample, the absorption intensity is visibly stronger than the other samples. So the Ag/CoFe₂O₄ sample may have a better absorption capability to visible light.

3.4 FTIR analysis

The FTIR spectra of the MFe₂O₄ and Ag/MFe₂O₄ (M = Zn, Co, Ni and Cu) samples are shown in Fig. 7. The main absorption bands γ1 and γ2 around 600 and 400 cm⁻¹ are attributed to the stretching vibrations of the tetrahedral and octahedral sites, respectively, which are responsible for the formation of MFe₂O₄.⁴⁹ The band at 580 cm⁻¹ is attributed to the Fe–O bond vibration of the samples.^50,51

Fig. 7 (a) The FTIR spectra of the NiFe₂O₄ (Ni1, Ni2 and Ni3) samples and the Ag/NiFe₂O₄ (Ag/Ni) sample, (b) the CoFe₂O₄ (Co1, Co2 and Co3) samples and the Ag/CoFe₂O₄ (Ag/Co) sample, (c) the CuFe₂O₄ (Cu1, Cu2 and Cu3) samples and the Ag/CuFe₂O₄ (Ag/Cu) sample, and (d) the ZnFe₂O₄ (Zn1, Zn2 and Zn3) samples and the Ag/ZnFe₂O₄ (Ag/Zn) sample.

The peaks at 3455 cm⁻¹, 1340 cm⁻¹ and 1610 cm⁻¹ can be assigned to the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups.⁵² The peak at 1185 cm⁻¹ is attributed to the C–O stretching vibration mode.⁵³ The peaks around 1000 cm⁻¹ (930 and 1030 cm⁻¹) in Fig. 7c and d correspond to the C–H stretching vibrations in C=C bonds.^54,55

3.5 XPS analysis

The XPS analysis of the MFe₂O₄ (M = Zn, Cu, Co and Ni) and Ag/MFe₂O₄ samples are shown in Fig. 8–11. Fig. 8 shows the high resolution spectra of the ZnFe₂O₄ and Ag/ZnFe₂O₄ samples. Fig. 8a shows the Zn peaks of Zn 2p3/2 and Zn 2p1/2 at 1023.4 eV and 1047.9 eV, respectively. Fe peaks of Fe 2p1/2 and Fe 2p3/2 at 728.9 eV and 715 eV, respectively, and the O 1s peak at 534.1 eV are shown in Fig. 8b and c, which indicate the existence of ZnFe₂O₄ in the sample. Fig. 8d shows the spectra of the Ag peak with Ag 3d3/2 at 372.1 eV. No contaminating species were observed within the sensitivity of this technique.

Fig. 8 The XPS spectra of the ZnFe₂O₄ and Ag/ZnFe₂O₄ samples, the Zn peaks (a), the Fe peaks (b), the O peak (c) and the Ag peak (d).

Fig. 9 shows the high resolution XPS spectra of Co 2p, Fe 2p, O 1s and Ag 3d. In Fig. 9a, two peaks appear at 780.31 eV and 788.2 eV, which are attributable to Co 2p3/2. From Fig. 9b, the peaks of Fe 2p3/2 and Fe 2p1/2 are located at 710.98 eV and 724.25 eV, respectively. Fig. 9c shows the XPS spectra of the C 1s peak at 530.5 eV. These peaks indicate the existence of CoFe₂O₄ in the sample. Fig. 9d shows the XPS spectra of the Ag peak with Ag 3d5/2 at 368.8 eV.

Fig. 9 The XPS spectra of the CoFe₂O₄ and Ag/CoFe₂O₄ samples, the Co peaks (a), the Fe peaks (b), the O peak (c) and the Ag peak (d).

Fig. 10 shows the XPS spectra of the NiFe₂O₄ and Ag/NiFe₂O₄ samples. The peak located at 860.2 eV is attributed to the Ni 2p3/2 as shown in Fig. 10a. From Fig. 10b, the peaks of Fe 2p3/2 and Fe 2p1/2 are located at 710.9 eV and 725.2 eV, respectively. Fig. 10c represents the XPS spectra of O 1s with a single peak at 530 eV. The spectrum shows that the Ag 3d3/2 is approximately at 372.6 eV in Fig. 10d.

Fig. 10 The XPS spectra of the NiFe₂O₄ and Ag/NiFe₂O₄ samples, the Ni peaks (a), the Fe peaks (b), the O peak (c) and the Ag peak (d).

Fig. 11 shows the XPS spectra of the CuFe₂O₄ and Ag/CuFe₂O₄ samples. Fig. 11a shows the peaks of Cu with Cu 2p3/2 at 936.7 eV. The Fe 2p shows two peaks at binding energies of 728.9 and 715.7 eV which are attributable to Fe 2p1/2 and Fe 2p3/2, respectively. Fig. 11c shows the O peak of O 1s at 534.5 eV. Fig. 11d shows the XPS spectra of the Ag peak with Ag 3d3/2 at 372.8 eV.

Fig. 11 The XPS spectra of the CuFe₂O₄ and Ag/CuFe₂O₄ samples, the Cu peaks (a), the Fe peaks (b), the O peak (c) and the Ag peak (d).

3.6 SPV analysis

Fig. 12 shows the SPV amplitudes and phase spectra of the MFe₂O₄ and Ag/MFe₂O₄ (M = Zn, Co, Ni and Cu) samples. The Ag/MFe₂O₄ samples exhibit a larger SPV response than the MFe₂O₄ samples. The higher SPV signal may imply the higher separation rate of the photo-generated charge carriers.⁵⁶ From Fig. 12b, d, f and h, the SPV phase values for the Ag/MFe₂O₄ samples below 600 nm are about 90°. This indicates that the photo-generated electrons accumulate on the surface of the samples.⁵⁷ It could be observed by comparison that the MFe₂O₄ samples have a smaller phase retardation. The electronic band gaps of the dispersed nanocrystals in the “timber-like” aggregation of Ag/MFe₂O₄ would improve the redox activity of their photo-generated electrons and holes. In Fig. 12g, the SPV spectrum of the Ag/ZnFe₂O₄ sample has a 50 nm blue shift with respect to ZnFe₂O₄. Moreover, the SPV signal of the Ag/CoFe₂O₄ sample is significantly larger than CoFe₂O₄ and the other samples (Fig. 12a). For Ag/CoFe₂O₄, the efficiency of charge separation is higher than the other samples, so that Ag/CoFe₂O₄ has higher photocatalytic activity.

Fig. 12 The specific surface photovoltage spectra (a and c) and corresponding phase spectra (b and d) of the MFe₂O₄ (M = Co1, Co2, Co3 and Ni1, Ni2, Ni3) samples and the Ag/MFe₂O₄ (M = Co, Ni) samples, and the specific surface photovoltage spectra (e and g) and corresponding phase spectra (f and h) of the MFe₂O₄ (M = Cu1 and Zn1) samples and the Ag/MFe₂O₄ (M = Cu and Zn) samples.

3.7 Comparison of the photocatalytic activities of the MFe₂O₄ and Ag/MFe₂O₄ samples

Fig. 13 shows the comparison of the photocatalytic activities of the MFe₂O₄ and Ag/MFe₂O₄ (M = Co, Zn, Ni and Cu) samples, studied by the photodegradation of a solution of 4-CP under xenon lamp illumination. After 120 minutes, the 4-CP solution cannot be photodegraded under xenon lamp irradiation without any catalyst, while the 4-CP solution can be photodegraded by the MFe₂O₄ and Ag/MFe₂O₄ samples. The degradation ratios of the 4-CP solution with the MFe₂O₄ and Ag/MFe₂O₄ samples are shown in Table 1 and Fig. 13a. The results show that the photodegradation ratio of the 4-CP solution with the Ag/CoFe₂O₄ sample is up to 90% and that the photodegradation ratios with the Ag/ZnFe₂O₄, Ag/NiFe₂O₄ and Ag/CuFe₂O₄ samples are 88%, 86% and 81%, respectively. This implies that the Ag/CoFe₂O₄ sample has a higher photocatalytic activity than the other samples.

Fig. 13 (a) The photocatalytic degradation ratio of 4-CP with the MFe₂O₄ and the Ag/MFe₂O₄ (M = Co, Ni, Cu and Zn) samples, and (b) the concentration change of 4-CP over 120 min under xenon light illumination.

There are many influential factors which have an impact on the decomposition rate, such as the following: (1) MFe₂O₄ nanoparticles absorb photon energy greater than the band gap energy and generate photo-generated electron–hole pairs in the bulk; (2) photoexcited carriers separate and move to the surface without recombination; (3) water molecules (or H⁺) are reduced and oxidized by the photo-generated electrons and holes to produce H₂ and O₂, respectively;⁵⁸ (4) redox reactions with adsorbed reactants.⁵⁹ Metal properties have some influence, but the above factors are clearly more important than the nature of the metal on the degradation efficiency.

According to XRD studies, the crystallite size of Ag/MFe₂O₄ samples is smaller than that of the MFe₂O₄ samples. The Ag/MFe₂O₄ samples have more hydrogen-bonded, surface water molecules and hydroxyl groups adsorbed at the surface due to the smaller crystallite size. Radical groups have been shown to be capable of contributing to the oxidation process of organic substances. With the decrease in particle size, the catalytic activity of the Ag/CoFe₂O₄ sample is enhanced because of the widened band gap. Moreover, from UV-vis-DRS analysis of the MFe₂O₄ and Ag/MFe₂O₄ (M = Co, Zn, Ni and Cu) samples, a Ag dopant enhances the absorbance of visible light. Furthermore, small particles of the semiconductor became almost totally depleted upon immersion in the electrolyte (allowing for large photovoltages).⁶⁰

We learned by reading the literature that there are many loading quantities, such as 0.05 wt%,⁶¹ 0.98 wt%, 1.3 wt%,⁶² 1.5 wt%,⁶³ 3.2 wt%⁶⁴ and 12 wt%.⁶⁵ It was concluded through experiments that the sample with 1 wt% Ag loading has the strongest adsorption capacity. Samples with Ag loading are not only antimicrobial but also enhance photocatalysis. Three factors are ascribed to the excellent antibacterial abilities of the mesoporous Ag/MFe₂O₄ samples. First, their high specific surface area and small particle size, which would result in better activity. Second, Ag nanoparticles are an excellent antimicrobial material, even though only a tiny amount of Ag is doped in the pores of Ag/MFe₂O₄ (Ag/M < 0.01, mole ratio). Furthermore, the Ag nanoparticles could act as trapping centers for photo-generated electrons in the conduction band of MFe₂O₄, leaving holes in the valence band of MFe₂O₄ and inhibiting the recombination of electrons and holes. These factors will lead to an improved quantum efficiency for photochemical reactions.^66,67 This implies that the Ag/CoFe₂O₄ sample calcined at 673 K shows a more enhanced photocatalytic activity than the other samples.

3.8 Stability and reusability of the MFe₂O₄ and Ag/MFe₂O₄ (M = Co, Zn, Cu, and Ni) catalysts

We also conducted stability experiments with the MFe₂O₄ and Ag/MFe₂O₄ samples over five reaction cycles under the same xenon lamp illumination test conditions. The photodegradation ratio of 4-CP over Ag/CoFe₂O₄ is shown in Fig. 14a and reaches 75.3% after the fifth repetition, which is slightly lower than that (90%) of the fresh Ag/CoFe₂O₄ catalyst. The photodegradation ratio of the Ag/ZnFe₂O₄, Ag/NiFe₂O₄ and Ag/CuFe₂O₄ samples after the fifth repetition are 71%, 68.1% and 65.2%, respectively. The results show that MFe₂O₄ and Ag/MFe₂O₄ have good stability and reusability as catalysts.

Fig. 14 The photocatalytic degradation ratio of 4-CP over the Ag/CoFe₂O sample (a), Ag/ZnFe₂O₄ sample (b), Ag/NiFe₂O₄ sample (c), and Ag/CuFe₂O₄ sample (d) over five repetitions.

3.9 Intermediate analysis of the photocatalytic degradation of 4-CP

The main intermediates of the photocatalytic oxidation of 4-CP were hydroquinone (HQ), benzoquinone (BQ), and hydroxyhydroquinone (HHQ) as measured by HPLC.

Under dark conditions, the adsorption/desorption processes were equilibrated. 4-CP quickly disappeared during the first 30 min due to its adsorption on the sample surface.⁶⁸ Intermediates produced by the photo-oxidation of 4-CP interact with the catalyst on the surface so that they can be released to the gas phase. Hydroxyl groups on the surface of the catalyst are able to react with the intermediates, which are then retained on the catalyst surface. Subsequently they can be converted into other products, like CO₂ and H₂O.⁶⁹

4-CP was almost fully degraded, but 44% of the total organic carbon was still present in the solution after 120 minutes. We can learn from this phenomenon that the degradation efficiency was greater than the mineralization efficiency. Therefore, there are transient organic intermediates present in the photocatalytic system. The intermediate products were subsequently oxidized rapidly by the heterojunction anode.⁷⁰ We can come to the conclusion that we should prolong the illumination time for complete mineralization.⁷¹

4. Conclusions

In summary, MFe₂O₄ (M = Zn, Cu, Co and Ni) nanocrystals were synthesized by a modified chemical co-precipitation method using different calcining temperatures (673 K, 773 K and 873 K). Then, based on the MFe₂O₄ (673 K) sample, we prepared the Ag/MFe₂O₄ (M = Zn, Cu, Co and Ni) catalysts through the traditional wetness impregnation strategy. Within the visible region of the spectra, these Ag/MFe₂O₄ nanocrystals possess attractive photovoltage responses, enhanced photocatalytic activities and good stabilities and reusabilities for the degradation of 4-CP. The Ag/CoFe₂O₄ catalyst with a spinel crystal structure annealed at 673 K promotes the adsorbates onto the active sites and results in a higher photo-induced reactivity. So the Ag/CoFe₂O₄ nanoparticles investigated here have an expected and remarkable photocatalytic activity and could be potentially applied in environmental purification in the future.

Acknowledgements

The project was supported by Open Research Fund of Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of Ministry of Education (no. 2014G1502033), Engineering Research Center of Groundwater and Eco-Environment of Shaanxi Province, National Nature Science Foundation of China (no. 20877013 and NSFC-RGC 21061160495), the National High Technology Research and Development Program of China (863 Program) (no. 2010AA064902), the Major State Basic Research Development Program of China (973 Program) (no. 2011CB936002), the Excellent Talents Program of Liaoning Provincial University (LR2010090) and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

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