Full-Spectrum Photocatalytic Activity of ZnO/CuO/ZnFe₂O₄ Nanocomposite as a PhotoFenton-Like Catalyst

Abstract

Deriving photocatalysts by the calcination of hydrotalcite-like compounds has attracted growing interest for extending their photocatalytic activity to the visible and even near-infrared (NIR) light regions. Herein, we describe the acquisition of a ZnO/CuO/ZnFe₂O₄ nanocomposite with good photoFenton-like catalytic activity under UV, visible and near-infrared (NIR) light irradiation by optimizing the calcination temperature of the coprecipitation product of Zn²⁺, Cu²⁺ and Fe³⁺. The ZnO/CuO/ZnFe₂O₄ nanocomposite is composed of symbiotic crystals of ZnO, CuO and ZnFe₂O₄, which enable the nanocomposite to show absorption in the UV, visible and NIR light regions and to produce a transient photocurrent in the presence of H₂O₂ under NIR irradiation. The full-spectrum photoFenton-like catalyst shows improved performance for the degradation of methyl orange with an increasing amount of H₂O₂ and is very stable in the recycling process. We believe that the ZnO/CuO/ZnFe₂O₄ nanocomposite is a promising full-spectrum photoFenton-like catalyst for the degradation of organic pollutants.

1. Introduction

Hydrotalcite-like compounds (HLCs), or layered double hydroxides (LDHs), are a class of anionic clays with brucite-like sheets consisting of adjustable mixed metal hydroxides and interlayer anions [1,2]. Many divalent cations, such as Mg²⁺, Zn²⁺, Co²⁺, Ni²⁺, Mn²⁺, Fe²⁺ and Cu²⁺, and trivalent cations, such as Al³⁺, Fe³⁺, Cr³⁺, Mn³⁺, Ga³⁺ and In³⁺, have been reported to be involved in the preparation of HLCs [1,2,3,4,5]. By calcinating the HLCs at high temperatures, mixed metal oxides (MMOs) with heterojunctions at metal oxide interfaces are formed due to dehydration, dehydroxylation and anion decomposition [6]. When photoactive metal oxides with suitable band gap structures are involved, MMOs derived from HLC calcination generally show superior photocatalytic performance compared to single metal oxides as semiconductor photocatalysts due to the extended absorption spectrum of the MMOs and reduced recombination rates for photogenerated carriers [7,8,9]. Deriving HLC-based photocatalysts by calcination has, therefore, attracted growing interest recently from the viewpoint of designing photocatalysts to either improve their photocatalytic performance [10,11,12,13] or extend their photocatalytic activity to the visible light region [14,15,16].

It is well-known that near-infrared (NIR) light accounts for 44% of the harvestable solar energy. Developing full-spectrum photocatalysts active in the NIR region is necessary for the better use of solar energy in photocatalytic processes [17,18,19,20,21,22]. Many strategies have been developed for fabricating NIR-active photocatalysts involving combination with upconversion materials [23], sensitization with NIR-responsive dyes [24], manipulation of defect bands, vacancies, and other photosensitive sites in semiconductors [17,25,26], and the incorporation of narrow-band-gap semiconductors [26]. However, HLC-based photocatalysts with near-infrared (NIR) activity have rarely been reported [27,28,29]. Of the few reported NIR light active photocatalysts derived from HLC calcination, Er³⁺-doped ZnO/ZnAl₂O₄ multiphase oxide (MPO) was prepared by the calcination of Zn/Al/Er-HLC and acquired its the NIR light activity from the doping of Er³⁺ as an upconversion species in ZnO/ZnAl₂O₄ MPO [27]. Er³⁺-doped ZnO-CuO-ZnAl₂O₄-MPO was produced through calcination of Zn/Cu/Al/Er-HLC. It absorbs NIR light because of the incorporation of narrow-band-gap CuO and the doping of Er³⁺. The excellent NIR photocatalytic performance of Er³⁺-doped ZnO-CuO-ZnAl₂O₄-MPO was ascribed to the formation of n–p–n heterojunctions among Er³⁺-doped n-ZnAl₂O₄, Er³⁺-doped p-CuO and Er³⁺-doped n-ZnO, which greatly suppresses the recombination of photogenerated electron–hole pairs and extends the life-times of the charge carriers [28]. The ZnO/ZnFe₂O₄ nanocomposite derived from the calcination of Zn/Fe-LDH also absorbs NIR light due to the presence of ZnFe₂O₄. However, the separation of photogenerated electron–hole pairs under NIR light irradiation depends on the presence of H₂O₂. Consequently, the ZnO/ZnFe₂O₄ nanocomposite catalyzes the degradation of organic pollutants under NIR light irradiation as a photoFenton-like catalyst [29].

Coprecipitation is one of the most popular methods for preparing HLCs [1]. In this work, a new full-spectrum photoFenton-like catalyst ZnO/CuO/ZnFe₂O₄ nanocomposite was developed by calcinating the coprecipitation product of Zn²⁺, Cu²⁺ and Fe³⁺. The obtained ZnO/CuO/ZnFe₂O₄ nanocomposite was found to show good optical absorption from the UV to NIR light regions due to the coexistence of CuO and ZnFe₂O₄ and found to possess excellent photocatalytic activities in the presence of H₂O₂ under UV, visible and NIR light. The ZnO/CuO/ZnFe₂O₄ nanocomposite has also very stable catalytic activities under all tested light irradiation. This work provides a new idea for developing full-spectrum photoFenton-like catalysts.

2. Results and Discussion

2.1. Preparation and Optimization

The ZnO/CuO/ZnFe₂O₄ nanocomposite was prepared by coprecipitation of Zn(NO₃)₂, Cu(NO₃)₂ and Fe(NO₃)₃ with mixed NaOH and Na₂CO₃ followed by calcination of the coprecipitation product. The molar ratios of Zn²⁺ to Cu²⁺ and (Zn²⁺ + Cu²⁺) to Fe³⁺ were both controlled to be 3:1, enabling the incorporation of suitable amounts of copper oxide and ferrite after calcination. Meanwhile, the formation of oxide and spinel (ferrite) phases depends on the calcination temperature, which consequently affects the photoFenton catalytic activity of the as-prepared catalyst. The crystal phases for the calcination products derived from various temperatures were first analyzed using the X-ray diffraction (XRD) technique. The photoFenton-like catalytic activity was analyzed by comparing the removal rate of methyl orange (MO) in the presence of H₂O₂ under light irradiation with that due to adsorption in the dark without H₂O₂.

Figure 1 shows the XRD patterns and photoFenton-like catalytic activities for the calcination products. As shown in Figure 1, the XRD pattern of the calcination product derived from 200 °C shows only a very weak distinguishable peak assigned to the (003) plane of an HLC, indicating that no oxide phase is formed at 200 °C. However, this calcination product removes a significant fraction of MO in the dark, with a MO removal rate under either UV, visible or NIR light irradiation higher than that in the dark, indicative of good absorption of MO and considerable full-spectrum photoFenton-like catalytic activity. By further increasing the calcination temperature to 400 °C, the hexagonal phase ZnO (JCPDS card no. 36–1451) is observed to appear, while the absorption capacity and photoFenton-like catalytic activity of the calcination product do not change significantly. As the calcination temperature increases to 600 °C, hexagonal phase ZnO and monoclinic phase CuO (JCPDS card no. 45–0937) are found to appear in the calcination product. The MO absorption capacity is significantly reduced while the photoFenton-like catalytic activity is significantly improved especially under irradiation by UV and NIR light. This observation demonstrates that the occurrence of CuO favors the development of NIR photoFenton-like catalytic activity for the calcination product.

When the calcination temperature reaches 800 °C, the spinel cubic phase ZnFe₂O₄ (JCPDS card no. 22–1012) is formed in the calcination product, with ZnO and CuO crystal size significantly increased, judging from the XRD patterns. However, no spinel phase assigned to CuFe₂O₄ was found in the corresponding calcination product. Meanwhile, the adsorption capacity of the calcination product is further remarkably reduced while its photoFenton-like catalytic activity is further improved probably due to the formation of ZnFe₂O₄. The crystal phases of the calcination product do not change significantly after further increasing the temperature to 1000 °C. Nevertheless, the adsorption capacity of the calcination product is slightly decreased, and its photoFenton-like catalytic activity is greatly lowered, suggesting the importance of adsorption for the photoFenton catalytic degradation of MO. Therefore, for the production of the ZnO/CuO/ZnFe₂O₄ nanocomposite, the calcination temperature was chosen to be 800 °C.

2.2. Characterization

The morphology and microstructure of the ZnO/CuO/ZnFe₂O₄ nanocomposite derived from calcination at 800 °C were analyzed using field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM), respectively. Figure 2 shows the FE-SEM images of the ZnO/CuO/ZnFe₂O₄ nanocomposite and its precursor, as well as the TEM and HR-TEM images of the ZnO/CuO/ZnFe₂O₄ nanocomposite. The insets in Figure 2d show the magnified images and Fourier transform electron diffraction (FTED) patterns for the corresponding components.

The FE-SEM images shown in Figure 2a,b reveal that the precursor of the ZnO/CuO/ZnFe₂O₄ nanocomposite, i.e., the coprecipitation product of Zn²⁺, Cu²⁺ and Fe³⁺, consists of irregular nanoparticles with particle sizes ranging from 10 to 30 nm. The ZnO/CuO/ZnFe₂O₄ nanocomposite derived from calcination of the precursor at 800 °C also consists of irregular particles. The particle size falls in the range of 100–200 nm, which is much greater than that the size of the precursor due to the intergrowth of various crystal phases.

The TEM and HR-TEM images as well as the FTED patterns shown in Figure 2c,d indicate that the irregular particles in the ZnO/CuO/ZnFe₂O₄ nanocomposite are composed of well-crystallized monoclinic CuO, cubic ZnFe₂O₄ and hexagonal ZnO, corroborating the results from the XRD analysis. The CuO, ZnO and ZnFe₂O₄ crystals grow together. Transitions and distortions of lattice fringes are observed among these crystals, yet no clear interface is formed. This observation indicates that the ZnO/CuO/ZnFe₂O₄ nanocomposite is a symbiotic crystal of ZnO, CuO and ZnFe₂O₄, which formed the crystal simultaneously from the coprecipitation product of Zn²⁺, Cu²⁺ and Fe³⁺ without being well-separated.

The chemical states of the ZnO/CuOZnFe₂O₄ nanocomposite were analyzed using X-ray photoelectron spectroscopy (XPS). Figure 3 shows the core-level XPS spectra measured for Zn 2p, Cu 2p, Fe 2p and O 1s for the ZnO/CuOZnFe₂O₄ nanocomposite. As shown in Figure 3a, the Zn 2p core-level XPS spectrum shows two peaks at binding energies of 1045.0 eV and 1021.9 eV, which are assigned to Zn 2p_1/2 and Zn 2p_3/2 of tetrahedral Zn²⁺, respectively [29], confirming that the oxidation state of Zn is +2 in the ZnO/CuOZnFe₂O₄ nanocomposite.

In the Cu 2p spectrum for the ZnO/CuOZnFe₂O₄ nanocomposite, as shown in Figure 3b, two main peaks are observed assigned to Cu 2p_1/2 and Cu 2p_3/2, located at 953.7 eV and 933.8 eV, respectively. In addition, two satellite peaks due Cu 2p_3/2 are found, corroborating the element Cu occurring as CuO in the ZnO/CuOZnFe₂O₄ nanocomposite [30]. In the Fe 2p spectrum ranging from 730 to 705 eV (Figure 3c), two main peaks belonging to Fe 2p_1/2 and Fe 2p_3/2 appear at 725.3 eV and 711.6 eV, while the satellite peak of Fe 2p_3/2 is located at 719.3 eV, indicative of the presence of Fe as Fe³⁺ [29,31]. The Fe 2p_2/3 binding energy value of 711.6 eV matches well with that for Fe 2p_3/2 in ZnFe₂O₄ [32]. Meanwhile, the Fe 2p_3/2 peak can be deconvoluted into two peaks located at 712.5 eV and 711.2 eV, which are assigned to octahedral and tetrahedral Fe³⁺, respectively [31]. This observation indicates that the spinel ZnFe₂O₄ in the ZnO/CuOZnFe₂O₄ nanocomposite has a partially inverse spinel structure [29].

The O 1s spectrum shown in Figure 3d displays an asymmetric peak, which can be deconvoluted into three peaks: a low bonding energy peak at 529.8 eV, a middle binding energy peak at 530.7 eV and a high binding energy peak at 531.7 eV. The low binding energy peak is assigned to lattice oxygen O²⁻ from the Zn–O, Cu–O and Fe–O linkages [28,29]. It is much stronger than the other two binding energy peaks, indicating that the majority of oxygen occurs as lattice oxygen O²⁻ in the ZnO/CuO/ZnFe₂O₄ nanocomposite [33]. The middle binding energy peak is associated with O²⁻ in the oxygen-deficient regions, indicative of the presence of oxygen vacancies in the ZnO/CuO/ZnFe₂O₄ nanocomposite [29]. The high binding energy peak is attributed to the absorbed oxygen species such as O₂, H₂O and CO₂ [33]; this peak is weaker than the other two binding energy peaks due to the high crystallinity of the ZnO/CuO/ZnFe₂O₄ nanocomposite.

2.3. Photo-Fenton-like Catalytic Activity

The photoFention-like catalytic activities for the ZnO/CuO/ZnFe₂O₄ nanocomposite under UV, visible and NIR light irradiation were further investigated by varying the amount of H₂O₂. To determine if the catalytic degradation of MO in the presence of the ZnO/CuO/ZnFe₂O₄ nanocomposite and H₂O₂ is initiated by light, H₂O₂ or their simultaneous action, the removal rates for MO were also analyzed in the presence of the ZnO/CuO/ZnFe₂O₄ nanocomposite in the dark, in the presence of the ZnO/CuO/ZnFe₂O₄ nanocomposite and H₂O₂ in the dark as well in the presence of the ZnO/CuO/ZnFe₂O₄ nanocomposite under light irradiation. The stability of the photoFenton-like catalyst reported in this work, ZnO/CuO/ZnFe₂O₄, was evaluated by performing cycle analyses, with the results shown in Figure 4.

As shown in Figure 4a,c,e, the combination of the ZnO/CuO/ZnFe₂O₄ nanocomposite with H₂O₂ (1.2 mL) without light irradiation cannot significantly improve the MO removal rate, compared to the adsorption of the ZnO/CuO/ZnFe₂O₄ nanocomposite in the dark, indicating that the ZnO/CuO/ZnFe₂O₄ nanocomposite is not a competent Fenton catalyst. Exposure of the ZnO/CuO/ZnFe₂O₄ nanocomposite to UV light only slightly improves the MO removal rate, while exposure to visible or NIR light does not grant any significant improvement for the removal of MO, implying that the ZnO/CuO/ZnFe₂O₄ nanocomposite is not a broad-spectrum photocatalyst. However, the introduction of H₂O₂ does not only significantly improve the UV photocatalytic activity of the ZnO/CuO/ZnFe₂O₄ nanocomposite but also produces visible and NIR photodegradation effects on MO, confirming the full-spectrum photocatalytic activity of ZnO/CuO/ZnFe₂O₄ nanocomposite as a photoFenton-like catalyst. Meanwhile, the photocatalytic activities of the ZnO/CuO/ZnFe₂O₄ nanocomposite under UV, visible and NIR light are further improved by increasing the amount of H₂O_2. When the amount of H₂O₂ is increased from 0.3 to 1.2 mL, the MO removal rate can be further increased from 60.1% to 93.1% under UV light irradiation, from 32.3% to 67.8 under visible light irradiation, and from 64.3% to 93.4% under NIR light irradiation, respectively.

As shown in Figure 4b,d,f, the ZnO/CuO/ZnFe₂O₄ nanocomposite is very stable under UV, visible and NIR light irradiation as a photoFenton-like catalyst. The MO removal rates are only reduced by 3.6%, 5.3% and 1.8% under UV, visible and NIR light irradiation, respectively, after the ZnO/CuO/ZnFe₂O₄ nanocomposite is repeatedly used three times. The slight reduction of the photocatalytic efficiency after repeated use is probably due to contamination and loss of the catalyst during separation.

2.4. Optical Properties and Photocurrent Response

To explain the origin for the full-spectrum photoFenton-like catalytic activity of ZnO/CuO/ZnFe₂O₄ nanocomposite, especially in the NIR light region, its optical absorption properties were analyzed using UV-vis-NIR diffuse reflectance spectroscopy, with its transient photocurrent response in the presence of H₂O₂ under NIR light irradiation analyzed using an electrochemical workstation. Figure 5a shows the UV-vis-NIR diffuse reflectance spectrum (DRS) for the ZnO/CuO/ZnFe₂O₄ nanocomposite compared to that for ZnO and the ZnO/ZnFe₂O₄ nanocomposite. ZnO only shows absorption in the UV region with an absorption edge of 386 nm. The ZnO/ZnFe₂O₄ nanocomposite extends the optical absorption to the visible light region with an absorption edge of 788 nm due to the incorporation of ZnFe₂O₄. The ZnO/CuO/ZnFe₂O₄ nanocomposite further extends the optical absorption to the NIR light region with an absorption edge of 968 nm due to the introduction of CuO. This full-spectrum light absorption for the ZnO/CuO/ZnFe₂O₄ nanocomposite provides the prerequisite for the ZnO/CuO/ZnFe₂O₄ nanocomposite to produce full-spectrum photoFenton-like catalytic activity.

From the equation E_g = 1240/λ_g and the absorption edges (λ_g) acquired from DRS, the band gaps (E_g) for ZnO, ZnO/ZnFe₂O₄, and the ZnO/CuO/ZnFe₂O₄ nanocomposite can be estimated to be 3.21 eV, 1.57 eV and 1.28 eV, respectively. The E_g values for ZnO/ZnFe₂O₄ and the ZnO/CuO/ZnFe₂O₄ nanocomposite are very similar to that of ZnFe₂O₄ [29] and CuO [28]. Therefore, the energy bands for CuO and ZnFe₂O₄ in the ZnO/CuO/ZnFe₂O₄ nanocomposite must determine the same in the ZnO/CuO/ZnFe₂O₄ nanocomposite. The two bands can be estimated to be 1.57 eV and 1.28 eV using the absorption edge for ZnO/CuO/ZnFe₂O₄ and the ZnO/ZnFe₂O₄ nanocomposite, respectively.

In a photoFenton-like process, H₂O₂ acts as an electron acceptor for reacting with the photogenerated electrons, providing hydroxyl radicals (•OH) for the degradation of organic pollutant [29,34]. To produce •OH in the presence of H₂O₂, the photogenerated electrons in the ZnO/CuO/ZnFe₂O₄ nanocomposite should have a potential lower than the redox potential of H₂O₂/•OH (+0.38 eV), i.e., the conduction band (CB) potential of either ZnO, CuO or ZnFe₂O₄ should be lower than +0.38 eV. The CB potentials for ZnO, ZnFe₂O₄ and CuO were calculated to be −0.32 eV, +0.58 eV and +0.67 eV, respectively, according to their band gap energies and the concept of Mulliken’s electronegativity [28]. The ZnO/CuO/ZnFe₂O₄ nanocomposite cannot provide •OH in the presence of H₂O₂ under visible light or NIR light irradiation if the ZnO, ZnFe₂O₄ and CuO components occur as a physical mixture, since neither ZnFe₂O₄ nor CuO can generate electrons acceptable to H₂O₂.

Figure 5b shows the transient photocurrent response of the ZnO/CuO/ZnFe₂O₄ nanocomposite under NIR light irradiation in the presence of H₂O₂ with on–off switch times of 100 s. As shown in Figure 5b, the ZnO/CuO/ZnFe₂O₄ nanocomposite produces a strong transient photocurrent in the presence of H₂O₂ under NIR irradiation, while the fluorine-doped tin oxide (FTO) glass hardly shows any photocurrent response under the same conditions. This observation implies that the ZnO/CuO/ZnFe₂O₄ nanocomposite possesses high transfer efficiency for photogenerated electrons and good separation of photogenerated electron–hole pairs in the presence of H₂O₂ under NIR light irradiation [17,29].

2.5. Suggested Photocatalytic Mechanism

Based on the above investigations, the excitation and transfer processes for the charge carriers under light irradiation, as well as the production of •OH, which are the active species in the photoFenton-like process, are shown in Figure 6.

The formation of p-n-p heterojunctions shown in Figure 6 is reasonably expected, as is evident from the transitions and distortions of the lattice fringes observed among the n-ZnO, p-CuO and p-ZnFe₂O₄ nanocrystals (Figure 2d), and the diffusion of electrons (e⁻) from n-ZnO to p-CuO and p-ZnFe₂O₄ as well as holes (h⁺) from p-CuO and p-ZnFe₂O₄ to n-ZnO in the ZnO/CuO/ZnFe₂O₄ nanocomposite. The formation of the p-n-p junction causes the energy bands in n-ZnO to bend downwards and enables the energy bands in p-CuO and p-ZnFe₂O₄ to bend upwards to form a unified Fermi level among the ZnO, CuO and ZnFe₂O₄ nanocrystals. Consequently, the CB potentials for CuO and ZnFe₂O₄ become even lower than that for ZnO. The photogenerated electrons in the CuO and ZnFe₂O₄ components under visible and NIR light irradiation can, therefore, either be captured by H₂O₂ to form •OH, which can oxidize organic pollutants such as MO, or transfer to the CB of the ZnO component, leading to the separation of photogenerated electrons and holes.

3. Experimental

3.1. Preparation of the ZnO/CuO/ZnFe₂O₄ Nanocomposite

The precursor for the ZnO/CuO/ZnFe₂O₄ nanocomposite was synthesized using a coprecipitation method [35]. A mixed salt aqueous solution containing 0.045 M Zn(NO₃)₂·6H₂O, 0.015 M Cu(NO₃)₂∙3H₂O and 0.02 M Fe(NO₃)₃·9H₂O and a mixed alkali aqueous solution containing 0.35 M NaOH and 0.05 M Na₂CO₃ were simultaneously added dropwise to vigorously stirred distilled water at a rate where the pH remained at approximately 10.5. After this addition, the resulting slurry was mixed for 30 min and aged at 65 °C for 3 h. This slurry was then filtered, thoroughly washed with deionized water, dried at 50 °C for 24 h, and milled to obtain the precursor. The precursor was heated to 800 °C in an electric tubular furnace at a rate of 2 °C min⁻¹ before being calcined for 3 h. After being cooled naturally to room temperature, the product was milled to obtain the ZnO/CuO/ZnFe₂O₄ nanocomposite.

ZnO nanoparticles and the ZnO/ZnFe₂O₄ nanocomposite were also prepared using similar coprecipitation and calcination procedures, except that the Cu(NO₃)₂∙3H₂O and Fe(NO₃)₃·9H₂O were replaced by the same mole of Zn(NO₃)₂·6H₂O, and Cu(NO₃)₂∙3H₂O was replaced by the same mole of Zn(NO₃)₂·6H₂O.

3.2. Characterization

The XRD patterns for the various calcination products were recorded using a Bruke D8 Advance powder X-ray diffractometer (Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15406 nm). The FE-SEM and HR-TEM images of the samples were obtained with a Hitachi S-4800 field emission scanning electron microscope (Hitachi, Tokyo, Japan) and a JOEL JEM-2100 high-resolution transmission electron microscope (Akishima, Tokyo, Japan), respectively. The XPS spectra were collected by an ESCALAB 250 spectrometer (VG Scientific Ltd., United Kingdom) equipped with a monochromatized Al Kα X-ray source. All of the binding energies were calibrated C1s peak at a binding energy of 284.6 eV. The UV–Vis-NIR DRS were recorded on a spectrophotometer (Cary 5000, Varian, Inc., Palo Alto, CA, USA) with an integrating sphere attachment in the wavelength range of 200–1200 nm. The transient photocurrents were measured using a CHI660E electrochemical workstation (Chenhua Ins. Inc., Shanghai, China) with a standard three-electrode assembly, where Ag/AgCl was used as the reference electrode, a Pt wire was used as the counter electrode, and ZnO/CuO/ZnFe₂O₄ nanocomposite-coated FTO conductive glass (Zhuhai Kaivo Optoelectronic Technology Co., Ltd., Zhuhai, China) was used as the working electrode. The electrolyte was a 0.5 M Na₂SO₄ aqueous solution [29].

3.3. Photocatalytic Activity Test

Photocatalytic experiments under UV and visible light irradiation were carried out in a photocatalytic reactor at 25 °C. A 500 W mercury lamp with a primary wavelength of 365 nm and a 350 W Xe arc lamp equipped with an UV optical filter with a cutoff wavelength of 380 nm were used as UV light and visible light sources, respectively. Experiments under NIR light irradiation were performed at a temperature below 30 °C in a self-assembled experimental device, as reported previously [17,29]. A 200 W infrared lamp with a cutoff filter for cutting off the light below 800 nm was employed as the near-infrared source. 20 mg of the nanocomposite sample was suspended in 20 mL of 20 mg/L MO aqueous solution followed by sonication for several seconds; then, a predetermined amount of H₂O₂ (10 wt. %) was added. The above suspension either remained in the dark or was irradiated under UV, visible or NIR light. At given time intervals, 3 mL of suspension was collected and immediately centrifuged to remove the solid catalyst at 10000 rpm for 5 min. The residual concentration of MO solution was determined by measuring its absorbance at 465 nm with a UV-Vis spectrophotometer (Hitachi UV-300, Tokyo, Japan).

4. Conclusions

By calcinating the coprecipitation product of Zn²⁺, Cu²⁺ and Fe³⁺ at different temperatures, a series of nanocomposites with UV, visible and NIR photocatalytic activities in the presence of H₂O₂ were prepared. Among the various nanocomposites, the ZnO/CuO/ZnFe₂O₄ nanocomposite derived from calcination at 800 °C composed of ZnO, CuO and ZnFe₂O₄ nanocrystal heterojunctions shows the highest photoFenton-like activity. The ZnFe₂O₄ and CuO components extend the optical absorption of the ZnO/CuO/ZnFe₂O₄ nanocomposite to the visible and NIR regions, respectively. The presence of H₂O₂ promotes the separation of photogenerated electrons and holes and confers the ZnO/CuO/ZnFe₂O₄ nanocomposite with full-spectrum photocatalytic activity. The photocatalytic activity of the ZnO/CuO/ZnFe₂O₄ nanocomposite is improved by increasing the amount of H₂O₂ and remains mostly unchanged after undergoing three cycles of repeated use. The ZnO/CuO/ZnFe₂O₄ nanocomposite is a promising full-spectrum photoFenton-like catalyst for the degradation of organic pollutants.