Synergetic effect of functionalized carbon nanotubes on ZnCr–mixed metal oxides for enhanced solar light-driven photocatalytic performance

Abstract

In this work, ZnCr–mixed metal oxides (MMO) hybridized with functionalized carbon nanotubes (A-CNTs) have been successfully fabricated via vacuum calcination of the layered double hydroxide precursors. The structural and morphological characterization of the samples was investigated by multiple techniques such as XRD, SEM, TEM, XPS, BET, Raman and UV-vis DRS. The results showed that A-CNTs were well incorporated into the MMO nanoparticles to form a nanohybrid structure dependent upon intimate interfacial contact. As compared to pristine MMO, the obtained MMO–CNTs nanohybrids exhibited significantly enhanced photocatalytic performance for bisphenol A (BPA) degradation under simulative solar light irradiation, providing powerful evidence for the superiority of the hybridization with A-CNTs. The key role of A-CNTs played in enhancing the photocatalytic activity of the nanohybrids was probably ascribed to the larger surface area, higher visible light absorption and the more efficient restriction of charge carriers recombination.

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1. Introduction

Over the past decades, increasing endeavors have been focused on semiconductor based heterogeneous photocatalysis for environmental remediation and photon energy conversion.^1,2 Particularly, searching for visible-light-induced semiconductors with excellent photocatalytic efficiency and physicochemical stability is critical to the practical application of photocatalysis. To date, heterogeneous photocatalysis assisted by common metal oxides such as ZnO, TiO₂ and their derivatives has proven to be an efficient and acceptable method for the purification of organic pollutants in aqueous phase.^3–8 However, the wide band gap (E_g > 3.0 eV) and large hole binding energy have limited the practical applications due to the low efficiency in utilizing solar energy and high rate of electron–hole recombination.^9,10 For the sake of driving the metal oxide semiconductors to be sensitive toward visible light irradiation with high electron–hole pair separation efficiency, considerable attentions have been paid to the construction of heterostructures or hybrids by assembling the semiconductors with designed materials, such as metals, metal oxides and carbon nanomaterials.^11–14 As for ZnO, although metal doping has been proved to be able to improve the photocatalytic activity, the transition efficiency is still limited due to its inherent light absorption property.¹⁵ Constructing heterojunctions by coupling ZnO with other metal oxides may present an alternative method to enhance the photocatalytic efficiency, due to the feasibility of charge transfer and separation through the junctions at interfaces and the interfacial defect sites that act as catalytic “hot spots”.^16,17 However, it remains a challenge to construct highly effective semiconductor heterostructures with a fine homogeneity of the composites.

Interestingly, layered double hydroxides (LDHs), which are known as anionic clays with brucite-like layered structures that can be represented by the general formula [M^II_1−xM^III_x(OH)₂]^x+ (Aⁿ⁻)_x/n·mH₂O, have been deemed to be one of the most promising precursors to prepare crystallized mixed metal oxides (MMO) through calcination treatment.^18–21 The pre-arrangement of metal cations in the LDH structure results in the uniform distribution of the metal oxides in the calcined compounds, which can bring forth an unexpected and highly improved catalytic performance of the composites.¹⁸ The derivative mixed metal oxides have been employed as catalysts in varied redox reactions to transform organic pollutants.^20,21 Recently, Zn-containing LDHs have been extensively explored as catalyst precursors on account of the advanced catalytic activity of the derived mixed metal oxides.^18,22 In our laboratory,²³ we have found that ZnAlTi–mixed metal oxides deriving from the LDH precursor can be used as effective photocatalysts in the degradation of dye under solar light irradiation.

Carbon nanotubes (CNTs), a member of carbon nanomaterials with a cylindrical sp²-hybridized graphitic framework, have been recognized as ideal catalyst supports to disperse and immobilize active materials, because of their exceptional properties.^24,25 For instance, the large specific surface area of the network structure can provide more exposure of active sites for facilitating the dispersion of metal oxides.²⁶ Besides, the extraordinary electrical conductivity of the continuous network can inevitably lead to highly improved charge transfer efficiency, associated with the accelerated catalytic reactions.^27,28 As documented previously, the nanohybrids by coupling carbon nanomaterials with active materials presented a synergetic combination of the distinct properties of each component in the heterogeneous catalysis systems.^29,30 Recent studies in our laboratory have demonstrated the superiorities of using carbon nanomaterials as supports to anchor metal oxides for catalytic degradation of organic pollutants.^27,31 The results revealed that the sp²-hybridized and zigzag carbons in the graphene layers played vital roles in facilitating the catalytic redox reactions in aqueous solutions. Moreover, it is due to the inert surface of nanocarbon materials that allows limited binding sites to immobilize guest materials and thus generally leads to poor dispersion and severe agglomeration of the active components, many attempts have been carried out with particular emphasis upon the surface functionalization of such materials.^29,32 It was reported that the functionalization process could inevitably introduce defects into the graphene lattice that resulted in increased number of oxygen species adsorbed on the active sites and thus presumably improved the catalytic performance.³³

In this study, we first focused on combining the superiorities of functionalized CNTs with ZnCr–LDHs to fabricate LDH–CNTs nanocomposites via a facile co-precipitation method. The nanohybrids of CNTs and mixed metal oxides (MMO–CNTs) were derived from the composite precursors of LDH–CNTs upon vacuum calcination. The photocatalytic behaviors of MMO–CNTs nanohybrids were investigated for the degradation of bisphenol A (BPA) under solar light irradiation. Particular attention was paid to the physicochemical and optical properties of the obtained nanohybrids. Further investigation was underway to probe into the role of CNTs in the nanohybrids and the photocatalytic mechanism, which to our knowledge has not been published in the previous works.

2. Experimental methods

2.1 Preparation of photocatalysts

Pristine multi-walled CNTs with an outer diameter of <8 nm, inner diameter of 2–5 nm and a length of 0.5–2 mm, were acid-treated with a mixture of concentrated HNO₃ and H₂SO₄ (v : v = 1 : 3) by refluxing for 4 h according to the previous literature.²⁷ The product was recovered by filtration, washing and vacuum drying to obtain the functionalized CNTs, namely A-CNTs. The nanocomposite precursors LDH–CNTs were synthesized through a homogeneous co-precipitation method. 20 mg of above A-CNTs were suspended in deionized water by ultrasonication for 4 h, then certain amounts of Zn(NO₃)₂·6H₂O and Cr(NO₃)₃·9H₂O with Zn/Cr ratio of 2 were dissolved in the above suspension to obtain a mixed solution, which was then titrated dropwise by a mixed alkali solution containing NaOH and Na₂CO₃ until the solution pH reached 6.5–7.0. After that, the slurry was aged in a water bath at 65 °C for 24 h. The resulting slurry was then filtered, washed with distilled water and absolute ethanol alternately and dried at 60 °C, yielding the LDH–CNTs-x (x = 0.02, 0.05, 0.10, 0.20) precursors, where x signified the weight ratio of CNTs to LDH. The as-synthesized LDH–CNTs-x were calcined at 700 °C for 3 h in vacuum to obtain the final products, which were denoted as MMO–CNTs-x. The synthesis of pristine MMO was carried out under identical conditions apart from the coupling of CNTs.

2.2 Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a D/MAX-III A X-ray diffractometer (Rigaku Ltd., Japan) by using a nickel-filtered Cu Kα radiation source. The morphologies were observed using a ZEISS Merlin (SEM, Carl Zeiss, Germany) field-emission scanning electron microscope and a JEM-3010 transmission electron microscope (TEM, JEOL Ltd., Japan). The determination of specific surface areas and pore size distributions were obtained by N₂ adsorption/desorption isotherm analysis on a ASAP-2010 Chemi-sorption Surface Area Analyzer (Micromeritics, America). Raman spectra were conducted on a InVia Raman spectrometer (Renishaw). The X-ray photoelectron spectra (XPS) were measured by a AES430S X-ray photoelectron spectrometer (ANELVA, Japan) and the binding energy was referred to the C 1s peak, located at 284.6 eV. Solid-state UV-vis diffuse reflection spectra (UV-vis DRS) were obtained using a UV-2440 spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere attachment, and BaSO₄ was used as the reference. Photoluminescence spectra (PL) measurements were performed on a F-7000 fluorescence spectrophotometer (Hitachi, Japan). For the sake of evaluating the stability of the samples, the leaching amounts of zinc and chromium after reactions were determined by a Zeeman atomic absorption spectrometer (Hitachi, Z-2000).

2.3 Photocatalytic tests

The photocatalytic activities of the samples were evaluated by degrading 10 mg L⁻¹ BPA solution under irradiation utilizing a 300 W Xe arc lamp (GXH-GXZ, 100 V, 3 A). Typically, 0.125 g of photocatalyst was dispersed in 250 mL of BPA solution (10 mg L⁻¹). The reaction mixture was vigorously agitated for 30 min in dark to obtain the adsorption/desorption equilibrium. Then the suspension was exposed to light irradiation and the concentration of BPA at given time intervals was monitored by a L-2000 high performance liquid chromatography (Hitachi, Japan) equipped with a Polar-RP 80 column (Synergi 4u, 250 × 4.6 mm) and a HP UV detector. The detection wavelength was selected as 276 nm. The mobile phase was composed of water and acetonitrile (v/v, 50/50) at a flow rate of 1.0 mL min⁻¹. To determine the radical species formed during the photocatalytic degradation process, three sets of quenching experiments were carried out by the addition of p-benzoquinone (BZQ), tert-butanol (TBA) and disodium ethylenediaminetetraacetate (Na₂-EDTA) into the BPA solution to obtain a fixed concentration.

2.4 Electrochemical measurements

The electrochemical properties of the samples were investigated on a CHI 600D workstation (Chenhua Instruments, Co., Shanghai) with a three-electrode system, in which an Ag/AgCl and a platinum foil were used as the reference and counter electrode, respectively. The as-prepared samples were fixed to the glassy carbon electrode to serve as the working electrode by the following steps: 2 mg of samples was ultrasonically dispersed in a mixed solution containing 1 mL of ethanol and 10 μL of Nafion emulsion, 10 μL of which was then coated on the glassy carbon disk and air-dried at ambient temperature. The electrochemical impedance spectroscopy (EIS) investigations were carried out in 0.1 M KCl electrolyte solution by using a 5.0 mM redox probe composed of K₃[Fe(CN)₆] and K₄[Fe(CN)₆]. The EIS data were obtained at AC amplitude of 5 mV over the frequency range from 1 MHz to 0.01 Hz at 0.2 V. For the photocurrent measurement (I–t), 0.2 M Na₂SO₄ solution was used as the electrolyte and the initial voltage was set as −0.6 V.

3. Results and discussion

3.1 Enhanced photocatalytic activity

The photocatalytic activities of the MMO–CNTs nanohybrids were evaluated by the degradation of BPA aqueous solution under simulative solar light irradiation. Note that the photocatalytic decomposition of BPA could not be stimulated in the absence of light illumination or any catalyst. The photocatalytic activity of the as-prepared nanohybrids shows a dependence on the loading amount of A-CNTs. As shown in Fig. 1a, a remarkable increase of degradation efficiency of BPA can be clearly observed for the MMO–CNTs nanohybrids, compared with pristine MMO. The photocatalytic activity of the MMO–CNTs nanohybrids increases gradually as the loading amount of A-CNTs increases, demonstrating the superiority of MMO–CNTs hybrid nanostructures. Particularly, the photocatalytic degradation efficiency of BPA after 200 min on the sample hybridized with 10 wt% of CNTs (MMO–CNTs-0.10) is as high as ∼100%. However, the decreased photocatalytic performance for MMO–CNTs-0.20 may be ascribed to the excessive amount of opaque CNTs that lead to increased absorbance and scattering of photons.¹ In addition, the photodegradation process of BPA was kinetically fitted to the pseudo-first-order reaction, and the much larger apparent reaction rate constants (k_a) for the MMO–CNTs nanohybrids can be clearly seen in Fig. 1b. The evaluated apparent rate constant reaches the highest value of 0.0265 min⁻¹ for MMO–CNTs-0.10 sample, which exceeds that of pristine MMO by ∼2 times (k_a = 0.0086 min⁻¹). The increased k_a indicates that the photocatalytic activity of MMO nanoparticles can be significantly improved by the hybridization of CNTs, and the optimal loading amount of CNTs in the nanohybrids is suggested to be 10 wt%.

Fig. 1 Effect of CNTs loading in MMO–CNTs nanohybrids on photocatalytic degradation of BPA aqueous solution at ambient temperature (a), the pseudo-first-order kinetic for the photodegradation of BPA (b). Reaction conditions: [BPA]₀ = 10 mg L⁻¹, catalyst dosage = 0.5 g L⁻¹, initial pH = 7.14.

3.2 Structural and morphological characterizations

The XRD patterns of the LDH–CNTs samples and the derivative MMO–CNTs samples are shown in Fig. 2. It can be seen that all the LDH-containing samples display the characteristic reflections corresponding to two-dimensional hydrotalcite-like structure with a hexagonal lattice and R3̄m symmetry.^34,35 Two reflections at 25.7° and 43.2° observed for A-CNTs can be indexed to the intense (002) plane and (101) plane of graphite layers, respectively (JCPDS no. 41-1487).²⁷ However, no typical reflections of A-CNTs can be distinguished within the curves of LDH–CNTs samples, proving that A-CNTs are well incorporated into the hydrotalcite network.^32,36 Comparing with that of pristine LDH, the broadening reflections as the amount of hybridized CNTs increases obviously indicate a reduction of particle size and crystallinity for LDH phase in the LDH–CNTs hybrids (Fig. S1†), which may be ascribed to the orientation/confinement impact of A-CNTs on the growth of LDH nuclei.^32,36,37 More specifically, structural defects may be introduced into LDH nanosheets with the coupling of A-CNTs, because of the modified nucleation circumstances or possibly induced curvature.^27,38 The structural data of the samples are given in Table 1. It can be found that the basal spacing (d₀₀₃) of LDH–CNTs characterized as 0.779 nm shows a slight increase in comparison to that of pristine LDH (d₀₀₃ = 0.755 nm), which may be attributed to the intimate interaction between the brucite-like nanosheets and A-CNTs that gives rise to the weakened electrostatic interaction between the positively charged laminates and the charge-balancing interlayer species (CO₃²⁻).^32,37 Correspondingly, the average crystallite sizes of LDH phase in the nanocomposites, as calculated by Scherrer's equation,³⁷ are in the nanoscale range of 12–16 nm which are smaller than that of pristine LDH. After calcination, the hydrotalcite-like structure was destroyed to form well crystallized mixed metal oxides, as all the newly appearing reflections are found to be perfectly indexed as the hexagonal wurtzite ZnO (JCPDS card # 36-1451) and cubic ZnCr₂O₄ spinel (JCPDS card # 22-1107). Moreover, the weaker reflection intensities of MMO–CNTs than that of pristine MMO indicate the smaller particle sizes and lower crystallinity of the MMO phase.

Fig. 2 XRD patterns of LDH, LDH–CNTs and A-CNTs (a), MMO and MMO–CNTs (b).

Table 1 The basal spacing (d₀₀₃), average crystallite size (D_e), BET surface area (S_BET), total pore volume (V_t) and average pore size (D_p) of samples

Sample	d003 nm	De nm	Sample	SBET m^2 g^−1	Vt cm^3 g^−1	Dp nm
LDH	0.754	14.10	MMO	13.98	0.15	42.18
LDH–CNTs-0.02	0.755	15.46	MMO–CNTs-0.02	15.97	0.08	20.28
LDH–CNTs-0.05	0.761	13.92	MMO–CNTs-0.05	31.29	0.15	20.93
LDH–CNTs-0.10	0.776	12.33	MMO–CNTs-0.10	35.15	0.16	16.79
LDH–CNTs-0.20	0.779	12.55	MMO–CNTs-0.20	67.85	0.25	14.69

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The morphology and microstructure of the samples were investigated by SEM and TEM. Fig. 3 comparatively exemplifies the SEM images of LDH, MMO, LDH–CNTs and MMO–CNTs. It can be found that pristine LDH shows a typical platelet-like morphology, while the LDH platelets in the LDH–CNTs hybrids are morphologically disordered and dimensionally reduced, signifying a breakage or fracture of the platelets during the self-assembly process of exfoliated nanosheets.³⁸ Here, the self-assembly of LDH nuclei deposited onto A-CNTs surface to form a nanohybrid structure is mainly attributed to the interaction between the positively-charged LDH platelets and negatively-charged CNTs.^32,38 Moreover, it can be observed that the introduced A-CNTs are well incorporated into the sheet-shaped LDH networks. The insufficient number of defect sites, especially for the nanohybrids with low loadings of A-CNTs, endows A-CNTs to be wrapped up in the LDH crystallites. The pristine MMO presents the morphology of uniform-sized nanoparticles due to the dehydration and decomposition of carbonate ions that result in the collapse of the layered structure. For MMO–CNTs sample, it shows that A-CNTs can be homogenously dispersed on a large number of nanoparticles, which can be further confirmed by the typical TEM micrographs (Fig. 4). As is shown, MMO–CNTs hybrids reveal the uniform sizes of about 3–5 nm, which are much smaller than that of MMO (∼15 nm). The intact crystalline structure and interfacial feature of ZnO and ZnCr₂O₄ for MMO can be clearly seen in the HRTEM image. For comparison, the elementary composition of LDH and LDH–CNTs was particularly investigated by EDX spectra, and the corresponding data is shown in Table 2. The relative content of C and O exhibits an increase with the introduction of A-CNTs, while the molar ratio of elemental Zn to Cr decreases slightly, revealing that the nucleation of LDH was influenced by A-CNTs.

Fig. 3 SEM images of LDH (a), MMO (b), LDH–CNTs (c) and MMO–CNTs (d).

Fig. 4 TEM images of LDH–CNTs (a), MMO–CNTs (b), HRTEM image of MMO (c) and EDX spectra of LDH–CNTs (d).

Table 2 Chemical composition of the samples

Element	LDH		LDH–CNTs-0.10
	wt%	Atomic%	wt%	Atomic%
C	9.79	24.47	21.89	33.11
O	19.21	37.29	44.15	52.45
Zn	51.78	25.89	24.59	9.22
Cr	19.22	12.45	9.41	5.22

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Fig. 5 shows the Raman spectra of A-CNTs, pristine MMO, LDHs–CNTs and MMO–CNTs nanohybrids for comparison. All the CNTs-containing samples present two intense peaks at around 1322 and 1574 cm⁻¹, which can be indexed to the D and G band of carbon materials, respectively, demonstrating that A-CNTs have been well incorporated in both LDHs–CNTs and MMO–CNTs hybrids. It is well known that the D band is characteristic of the degree of conjugation disruption in the hexagonal graphite lattice and the G band is associated with the presence of sp²-bonded carbon atoms in the tubular structure.³⁹ The value of I_D/I_G, which is commonly used for evaluating the graphitization degree and the average size of sp² domains in carbon materials, is calculated as 0.43 for A-CNTs, indicative of a high graphitization. An intense increase of I_D/I_G for MMO–CNTs-0.10 (I_D/I_G, 1.31) and LDH–CNTs-0.10 (I_D/I_G, 1.22) reveals a revocation of sp² domains, demonstrating the successful consumption of oxygen-containing groups on the outer shells of CNTs due to in situ decoration of LDH nuclei or mixed metal oxides.⁴⁰ Upon calcination, lower crystalline order of graphite phase occurred due to the slightly higher I_D/I_G value of MMO–CNTs than that of LDH–CNTs. Additionally, four other peaks observed in the spectrum of MMO–CNTs can be assigned to the Raman-active modes of wurtzite ZnO (325 and 428 cm⁻¹)⁴¹ and ZnCr₂O₄ spinel (509 and 673 cm⁻¹),^42,43 implying the successful integration of metal oxides upon A-CNTs.

Fig. 5 Raman spectra of A-CNTs, MMO, LDH–CNTs and MMO–CNTs.

To get further insight into the chemical composition and bonds configuration of the synthesized samples, XPS analysis was carried out and the results are presented in Fig. 6. Core spectra of Zn 2p, Cr 2p, C 1s and O 1s can be clearly identified from the XPS survey (Fig. 6a). As for A-CNTs, five peaks arose from the deconvolution of the C 1s core spectrum can be assigned to the sp² graphitized carbon (284.2 eV), C–O group (285.1 eV), C–OH/C=O group (286.1 eV), C–O–C/C=O group (287.1 eV) and the O=C–O group (289.0 eV).⁴⁴ The intensity of the peaks that are assigned to C–O group and C–OH/C=O group suggests a considerable oxidation degree in the graphite lattice of A-CNTs. The typical signal of sp² graphitized carbon (284.8 eV) in the C 1s spectrum of MMO–CNTs-0.10 nanohybrid further demonstrates the integration of MMO and A-CNTs, which is consistent with the above results. Besides, a dramatic change for the high-resolution XPS spectra of Zn 2p can also be observed for MMO before and after their hybridization with A-CNTs, implying the formation of chemical bonds at the interface of A-CNTs and MMO. For pure MMO, the asymmetric peaks of Zn 2p_3/2 and Zn 2p_1/2 are located at 1020.6 and 1043.9 eV, respectively. However, the corresponding peaks are centered at 1021.8 and 1044.8 eV for the MMO–CNTs-0.10 nanohybrids. A similar case is observed for Cr 2p XPS spectra, only the positive BE shift is relatively slight for the MMO–CNTs-0.10 nanohybrids comparing with MMO. Noticeably, the chemical valence, chemical circumstances and distribution status of elements can be connected with BE shift and the decrease of electron density can lead to the increase of BE value.⁴⁵ As a result, the charge transfer from MMO to A-CNTs surface at the interface of the hybridized nanostructure should be responsible for the positive BE shift of Zn 2p and Cr 2p. This phenomenon has also been found in other CNTs-based composites reported in the previous literature.³⁷ It is suggested that the intimate interaction facilitates the charge transfer of the nanohybrid structure.

Fig. 6 XPS survey spectra (a) of MMO–CNTs, high resolution core level spectra of C 1s (b), Zn 2p (c), Cr 2p (d) for MMO and MMO–CNTs.

The textural properties of the samples were investigated by N₂ adsorption/desorption measurements (Fig. 7) and the corresponding data are summarized in Table 1. It can be observed that all the samples present type IV isotherm with a thin hysteresis loop of H3 type appearing at relatively high pressures, which is associated with slit-shape capillaries due to the aggregates of metal oxides particles.^15,20 This behavior is frequently characteristic of mesoporous materials with three-dimensional interconnected pore geometry that can be confirmed by the corresponding pore size distribution curves. The much more apparent hysteresis loop on the isotherms of the samples with increased A-CNTs contents is indicative of increased porosity of the materials. With the introduction of A-CNTs into the nanohybrids, the BET specific surface area of the samples increases sharply from 13.98 m² g⁻¹ to 67.85 m² g⁻¹ and the pore volume increases from 0.15 cm³ g⁻¹ to 0.25 cm³ g⁻¹, as depicted in Table 1. Note that the decreased pore volume for sample MMO–CNTs-0.02 with low loading amount of A-CNTs is possibly due to the stacked structure of irregular particles in the hybrids. In addition, the MMO–CNTs samples exhibit a narrower pore size distribution centered at 2–40 nm than pristine MMO, which can be reasonably explained by the introduced inner pores of A-CNTs.²⁷ This finding clearly demonstrates that the loaded A-CNTs in the nanohybrids makes possible the inhibited aggregation of the particles and thus give rise to more exposure of active sites in the present system.

Fig. 7 Nitrogen adsorption–desorption isotherms and pore size distribution curves of the obtained MMO–CNTs samples.

The optical properties of MMO–CNTs with different loading amount of A-CNTs were investigated by UV-vis diffuse reflectance spectra, as depicted in Fig. 8. It is well known that blank ZnO can only absorb in the UV region because of its large band gap energy. In present work, the absorption in the visible light region for pristine MMO is attributed to the existence of ZnCr₂O₄ spinel, which absorbs over the whole solar spectrum.^46,47 For the case of ZnCr₂O₄, the transfer occurs predominantly because of the photoexcitation of O 2p to Cr-3dt_2g (∼420 nm) in an octahedral environment and Cr-3dt_2g to Cr-3d_eg (∼570 nm) corresponding to the d–d transition of Cr³⁺.^47,48 With the hybridization of A-CNTs, the MMO–CNTs samples exhibit the same absorption edge with that of pristine MMO. The band gap energy of the MMO–CNTs samples barely shows no change due to the fact that CNTs in the hybrid structure can be characterized as free graphitic carbon. More interestingly, the MMO–CNTs nanohybrids display broad absorption bands with highly enhanced absorbance in the visible light region, which shows a positive correlation with the increasing content of A-CNTs in the hybrid nanostructures. It is, accordingly, suggested that introduced A-CNTs may lead to an increase of surface charge on metal oxides in the nanohybrids, further affecting the fundamental generation of electron–hole pairs under irradiation. As a result, the modifications of charge transfer presumably contribute to the higher photocatalytic efficiency of MMO–CNTs nanohybrids.

Fig. 8 UV-vis diffuse reflectance spectra of the MMO–CNTs samples.

3.3 Role of CNTs in enhancing charge transfer efficiency

It has been well-established that the interfacial charge transfer, which is closely relevant to the probability of recombination of electron–hole pairs, is the key factor that affecting the photocatalytic activities of photocatalysts.⁴⁹ To this end, EIS, transient photocurrent, and PL measurements were carried out to figure out the influence of CNTs on the interfacial charge transfer efficiency of the MMO–CNTs nanohybrids.

The EIS measurements were employed to investigate the interfacial charge transfer behaviors of the prepared samples. As can be seen in the EIS Nyquist plots (Fig. 9a), the semicircle radius of MMO–CNTs-0.10 is much smaller that of pristine MMO. It is noteworthy that the smaller semicircle radius corresponds to a smaller charge transfer resistance, which can provide evidence for the higher separation efficiency of photogenerated charge carriers and faster interfacial charge transfer from MMO to A-CNTs.³⁶ In this regard, the presence of A-CNTs with superior electrical conductivity is significant to maintain high capacity during the repeated conversion reactions.

Fig. 9 Electrochemical impedance spectroscopy (EIS) Nyquist plot (a), transient photocurrent responses (b) and PL spectra (c) of MMO and MMO–CNTs-0.10.

Fig. 9b illustrates the transient photocurrent tests for the photoactive materials over repeated on/off cycles. The prompt and reproducible photocurrent response originated from intermittent visible light irradiation implies the rapid charge separation and recombination within the photocatalyst. It is well known that the photocurrent response is closely related to the separation efficiency of photogenerated charge carriers within the materials.⁴⁹ As for MMO–CNTs-0.10, the much higher photocurrent density than that of MMO testifies the higher charge separation efficiency and longer lifetime of photogenerated electrons and holes,¹¹ which can be attributed to the intimate contact at the interface between A-CNTs and MMO nanoparticles.

The recombination process of photogenerated charge carriers dependent upon the charge separation efficiency can be further demonstrated by PL emission spectra. It is well known that the lower recombination rate of photogenerated electron–hole pairs corresponds to the lower PL intensity.⁵⁰ PL signals for MMO and MMO–CNTs-0.10 under excitation at around 300 nm are depicted in Fig. 9c. The significantly diminished PL intensity for MMO–CNTs-0.10 nanohybrid as compared to pristine MMO indicates a reduced recombination rate of photogenerated electron–hole pairs, which further confirms the benefit of A-CNTs in promoting charge transfer within the obtained hybrids.

3.4 Photocatalytic mechanism investigation

At photocatalyst interface, free electrons and active holes can react with adsorbed O₂ and H₂O to produce O₂⁻˙ and ˙OH radicals, respectively, both of which are possibly responsible for the photocatalytic degradation of BPA. To reveal the photocatalytic mechanism in the present system, the electron spin resonance (ESR) spin-trap technique was employed and the signals of trapping the predominant reactive oxygen species are depicted in Fig. 10. Both the signals of DMPO–˙OH and DMPO–O₂⁻˙ can be clearly detected when pristine MMO and MMO–CNTs-0.10 dispersions are under irradiation. In Fig. 10a, four characteristic peaks of DMPO–˙OH emerge in MMO and MMO–CNTs-0.10 aqueous dispersions under visible light irradiation, respectively. Interestingly, the signal intensities of ˙OH radicals for MMO–CNTs-0.10 nanohybrid are slightly weaker than those for MMO. However, the characteristic peaks corresponding to O₂⁻˙ radicals (Fig. 10b) detected for MMO–CNTs-0.10 nanohybrid are much stronger than that of MMO. The ESR results demonstrate that both ˙OH and O₂⁻˙ radicals are generated on the surface of MMO–CNTs-0.10 nanohybrid in the present system, with O₂⁻˙ radicals presumably as the predominant reactive species. Furthermore, the quenching experiment for BPA degradation with the introduction of reactive species scavengers was performed, as shown in Fig. 11. The photocatalytic degradation efficiency of BPA over MMO–CNTs-0.10 nanohybrid was slightly reduced with the presence of tert-butanol (TBA, ˙OH scavenger). However, the addition of 5 mM benzoquinone (BZQ, O₂⁻˙ radical scavenger) caused an obvious decrease from almost 100% to 29% in 200 min. Notably, the presence of Na₂-EDTA as holes scavenger in the same system led to a fast deactivation of the MMO–CNTs-0.10 photocatalyst. It is, accordingly, speculated that O₂⁻˙ oxidation combined with direct hole oxidation dominate the photocatalytic degradation of BPA, while ˙OH oxidation also contribute to the photocatalytic process to some degree.

Fig. 10 ESR spectra of radical adducts trapped by DMPO in MMO and MMO–CNTs-0.10 aqueous dispersions (a) and methanol dispersions (b).

Fig. 11 Photocatalytic degradation of BPA with the addition of hole and radical scavengers. Reaction conditions: [BPA]₀ = 10 mg L⁻¹, catalyst dosage = 0.5 g L⁻¹, TBA and Na₂-EDTA dosage = 2 mM.

Based on the results described above, the key role of A-CNTs played in enhancing the photocatalytic activity of MMO–CNTs nanohybrids can be attributed to three main respects: firstly, the nanohybrids with higher specific surface area provide more active sites for adsorption of organics and photocatalytic reactions; secondly, the extremely extended absorbance in visible light region caused by A-CNTs introduction has an influence on the fundamental generation of electron–hole pairs under irradiation; last, the intimate interaction between MMO and conductive A-CNTs can lead to a elimination of potential barrier at the grain boundaries between MMO and A-CNTs, resulting in highly efficient separation of electron–hole pairs and thus more reactive oxygen species participating in the degradation of BPA. More specifically, the specific π-conjugated structure and outstanding electrical conductivity endow A-CNTs with rapid charge transportation, through which the photoexcited electrons can be transported from MMO to A-CNTs. Note that the huge π–π network of CNTs can serve as a sink for the photoexcited electrons because charge transfer from the ZnO conduction band (CB) (E_CB = −0.5 V vs. NHE) to the CNTs conduction band (E_CB ≥ −0.3 V vs. NHE) is energetically feasible.^51,52 In addition, within the heterostructure of ZnO (E_g = 3.37 eV) and ZnCr₂O₄ (E_g ∼ 1.5 eV), the potentials of CB and VB of ZnO are more negatively charged than that of ZnCr₂O₄.^11,46 Because of the wide band gap, ZnO cannot be excited by visible light irradiation. However, ZnO in contact with ZnCr₂O₄ can act as electron acceptor favoring the electron–hole separation, because of the relatively higher CB bottom of ZnCr₂O₄ than that of ZnO.²⁹ The schematic illustration of charge transfer process in the MMO–CNTs nanohybrid structure can be exemplified in Scheme 1.

Scheme 1 Schematic illustration of the enhanced photocatalytic degradation of BPA by MMO–CNTs nanohybrids.

3.5 Photocatalyst stability

Since MMO originated from hydrotalcite-like precursor generally suffers from memory effect and possible photocorrosion that inhibit the photocatalytic activity for practical applications,¹¹ it is of great significance to probe the photostability of the MMO–CNTs nanohybrids. As shown in Fig. 12, it can be found that MMO–CNTs is photochemically stable under irradiation in the present system in view of the negligible change of photocatalytic performance, even though the photocatalyst has gone through four successive recycles. The slight decrease of BPA degradation efficiency may be attributed to trace leaching of metals from the catalysts during the degradation process.²¹ The leaching amount of zinc and chromium was detected with respective as <0.01 mg L⁻¹ and 0.571 mg L⁻¹ at neutral pH. Furthermore, the outstanding photostability of the hybrid nanostructure can also be testified by XRD patterns (Fig. S2†) of the used and fresh samples, taking the intact crystalline phase into account.¹¹ It is speculated that trace metal leaching has little effect on the crystalline structure of the nanohybrid.

Fig. 12 Cycling runs over MMO–CNTs-0.10 photocatalyst for degradation of BPA aqueous solution at ambient temperature. Reaction conditions: [BPA]₀ = 10 mg L⁻¹, catalyst dosage = 0.5 g L⁻¹, initial pH = 7.14.

4. Conclusions

In conclusion, the MMO–CNTs nanohybrids have been successfully synthesized via a facile precursor route. The resulting MMO–CNTs nanohybrids exhibit highly enhanced photocatalytic performance and cycling stability, as compared to pristine MMO. This finding can be attributable to the heterostructure of ZnO and ZnCr₂O₄ spinel that provides a synergistic effect of multi-component metal oxides for such nanohybrids and the introduction of A-CNTs that facilitates the effective separation of photogenerated electron–hole pairs. The as-prepared MMO–CNTs nanohybrids can serve as a promising candidate for the photocatalyst materials for further applications in wastewater treatment utilizing solar light.

Acknowledgements

The authors are grateful for financial support from the National Science Foundation of China (Grant No. 41472038, 41273122, 41073058), the Science and Technology Plan of Guangdong Province, China (Grant No. 2014A020216002) and the Fundamental Research Funds for the Central Universities, SCUT (Grant No. 2015ZP007).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05291f

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