Efficient Nanoclay-Based Composite Photocatalyst: The Role of

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Cite This: J. Phys. Chem. C 2018, 122, 25900−25908

Efficient Nanoclay-Based Composite Photocatalyst: The Role of Nanoclay in Photogenerated Charge Separation Denghui Jiang,† Ziran Liu,‡ Liangjie Fu,†,§ Huihua Jing,∥ and Huaming Yang*,†,§,⊥

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†

Centre for Mineral Materials, School of Minerals Processing and Bioengineering, §Hunan Key Lab of Mineral Materials and Application, and ⊥Key Lab of Clay Mineral Functional Materials in China Building Materials Industry, Central South University, Changsha 410083, China ‡ Department of Physics, Key Lab for Low-Dimensional Structures and Quantum Manipulation (Ministry of Education), Hunan Normal University, Changsha 410081, China ∥ Hunan Institute of Food Quality Supervision Inspection and Research, Changsha 410111, China S Supporting Information *

ABSTRACT: Natural clay mineral is a low-cost support for photocatalytic materials, but it exhibits insulating properties determined by intrinsic compositions of aluminosilicate, which normally makes it difficult to participate in the charge carrier separation and transfer of the photocatalytic system. Herein, natural iron-rich kaolinite clay with photoresponse is used as a multifunctional support to construct a CdS composite photocatalyst, and we find that the natural kaolinite nanosheets exhibit strong oxygen adsorption capacity by hydroxyl groups and could enhance the charge carrier separation. Density functional theory calculations show that the hydroxyl groups of kaolinite could effectively adsorb oxygen via hydrogen bonding, and the absorbed water further promotes the adsorption of oxygen. Because of these special properties, the kaolinite nanosheets not only directly improved photogenerated charge separation efficiency by a type II staggered band structure of heterostructures but also promoted the production of superoxide radicals via providing an oxygen-rich microenvironment, which resulted in the greatly enhanced photocatalytic performance of CdS nanoparticles. This work could provide a deeper understanding of the role of the iron-rich natural clay mineral as a photocatalytic support and shed light on the design of clay-based composite photocatalysts.

1. INTRODUCTION Photocatalysis represents an appealing pathway to tackle environmental and worldwide energy problems. Although many various photocatalysts have been discovered and synthesized, the low charge separation efficiency of photocatalysts still severely constrains further practical applications especially toward commercial production.1,2 Aiming to solve this scientific question, a variety of strategies have been employed to improve the charge separation and transfer efficiency of photocatalysts, such as loading cocatalysts,3−5 fabricating semiconductor heterojunctions,6−8 and introducing coating and supports.9,10 Among these methods, immobilizing the photocatalysts on a support has been demonstrated as an effective strategy to improve photocatalytic performance via enhancing the stability, adsorption capacities, and transfer efficiency of semiconductors.11 Traditional natural clay materials have been widely used as supports for photocatalytic materials because of low cost, large specific surface areas, high stability, and adsorption capacities.12−17 However, most of natural clay materials are insulators and hardly participate in the charge carrier separation and transfer of the photocatalytic system.12 Probably because of this reason, the enhanced photocatalytic abilities of clay-based © 2018 American Chemical Society

photocatalysts are generally attributed to high adsorption properties and excellent surface properties for improving the dispersity and specific surface area of semiconductors.12 Some works reported that the clay can be used as charge transfers, but these clays usually need to introduce an additional photosensitizer.18,19 Interestingly, the iron-rich clay itself such as kaolinite and montmorillonite revealed some photocatalytic activities because of existence of photoactive iron oxides which are mainly structural iron in the aluminosilicate lattice, resulting from isomorphous substitution.20−24 These clays improved the photogenerated carrier transfer efficiency of the supported photocatalyst because of its semiconducting property.25 Although a few works have reported a semiconductor−clay based photocatalyst,26−30 the essential roles of the iron-rich clay on the separation of photogenerated carriers remain unclear. On the other hand, the natural clay may provide an active microenvironment for nearby photocatalysts, thus greatly enhancing the charge carrier separation of Received: September 5, 2018 Revised: October 18, 2018 Published: October 25, 2018 25900

DOI: 10.1021/acs.jpcc.8b08663 J. Phys. Chem. C 2018, 122, 25900−25908

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The Journal of Physical Chemistry C photocatalysts.22,31 Therefore, natural iron-rich clay should be a low-cost and multifunctional support of photocatalysts. Herein, we use natural iron-rich kaolinite (Kaol) nanosheets (NSs) as a support to load CdS nanoparticles and then construct a CdS/Kaol composite photocatalyst. Natural kaolinite has only a small amount of iron oxide (0.476 wt %) but exhibited evident photoresponse like a semiconductor. In this paper, we have focused on the effects of Kaol NSs on the transfer and separation of photogenerated carriers by minimizing the effects of Kaol NSs on the dispersion, particle size, and structure of CdS. The improved photocatalytic activities of CdS/Kln composites are mainly related to the semiconducting property and oxygen adsorption capacity of Kaol NSs. On the basis of the results, the possible mechanisms of enhanced photocatalytic activity were proposed.

PL decay spectra were recorded at room temperature with an Edinburgh FLS-980 fluorescence spectrophotometer. An ultraviolet−visible light (UV−vis) spectrophotometer (PE950, PerkinElmer, USA) was used to perform the optical measurements of the product. The inductively coupled plasma mass spectrometry (ICP−MS) was performed on an Analytik Jena PlasmaQuant MS unit. Temperature programmed desorption of oxygen (O2-TPD) was carried out on a HUASI DAS-7000 dynamic adsorption apparatus. Thermal analysis [(thermogravimetric analysis (TGA)/differential scanning colorimetry (DSC)] was carried out using a MettlerToledo TGA/DSC3+/1600LF instrument at a heating rate of 10 °C/min with a gas feed (N2) of 20 mL/min. Kaol sample (0.32 mg) was added into a 1.6 mL 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) solution (75 mM). The mixture was incubated for 20 min and filtrated for electron paramagnetic resonance (EPR) spectroscopy measurements using a Bruker A300 EPR spectrometer. 2.3. Photocatalytic Activity and Stability Tests. A certain amount of CdS/Kaol sample was added to a 50 mL of 2 × 10−5 M methyl orange (MO) aqueous solution. In order to keep the same amount of CdS (17 mg) in photocatalytic tests, different quantities of samples (36.5, 77, 193, and 547 mg) were used for CdS/Kaol-3, CdS/Kaol-2, CdS/Kaol, and CdS/ Kaol-1, respectively. For comparison, CdS + Kaol-mix samples were prepared by mixing 17 mg of pure CdS nanoparticles and 176 mg of kaolinite clay. The suspended solution was magnetically stirred for 0.5 h to reach an adsorption/ desorption equilibrium in the dark. Then, the mixed solution was irradiated using a 300 W Xe lamp (CEL-HXF300, 280 mW/cm2) equipped with a 400 nm cutoff filter to remove UV light from a distance of ca. 12 cm. At a given time interval, 1 mL aliquot of the reaction suspension was taken out and centrifuged. The photocatalytic activity was evaluated according to the absorption spectra of MO, which were recorded using a UV−vis absorption spectrophotometer (Shimadzu UV2450). 2.4. Photoelectrochemical Measurements. The photocurrent was measured using a computer-controlled electrochemical work station (Gamry 5000E) in a standard threeelectrode system with the photocatalyst-coated indium tin oxide (ITO) as the working electrode, Pt foil as the counter electrode, and an Ag/AgCl as the reference electrode. The working electrodes were prepared as follows. The as-obtained samples (10 mg) were mixed with 0.5 mL of Nafion aqueous solution (1%) and ultrasonicated for 10 min to get homogeneous slurry. The slurry (0.1 mL) was dipped onto a 1 × 2 cm ITO slice and dried in air at room temperature. A 0.1 M Na2SO4 solution was used as the electrolyte. A 300 W Xe lamp equipped with a UV cutoff filter was used as the light source. Current−time curves were collected at 1.0 V versus SCE. 2.5. Computational Details. The calculations were performed within the framework of density functional theory (DFT), using the plane-wave basis set and Vanderbilt-type ultrasoft pseudopotential.33 Valence states include 3s23p2 for Si, 3s23p1 for Al, 2s22p4 for O, and 1s1 for H, respectively. The Perdew−Burke−Ernzerhof parameterization utilizing the generalized-gradient approximation scheme was adopted to deal with the exchange−correlation interactions.34 The calculations of surface adsorption were performed using the DFT semiempirical dispersion interaction correction module.35 Grimme’s method was used for DFT-D dispersion correc-

2. EXPERIMENTAL SECTION 2.1. Synthesis of CdS/Kaol Composites. Natural kaolinite clay was obtained from China Kaolin Clay Co. Ltd (Suzhou, China). The chemical composition of the sample in mass% determined by X-ray fluorescence spectroscopy was as follows: SiO2, 53.27; Al2O3, 42.46; Fe2O3, 0.476; K2O, 0.542; TiO2, 0.333; MgO, 0.155; P2O5, 0.257; and CaO, 0.09.32 Other chemical reagents were of analytical purity and purchased from the Sinopharm Chemical Reagent Company without further purification. In a typical synthesis, 3 g of kaolinite (Kaol) was added to 50 mL of 40 mM cadmium sulfate octahydrate (3CdSO4· 8H2O) aqueous solution. The dispersion was subjected to sonication for 30 min and then stirred for 24 h at room temperature. A 5 mL of 0.4 M sodium sulfide nonahydrate (Na2S·9H2O) aqueous solution was added to the above dispersion dropwise. After that, the mixture was sealed in an autoclave vessel and heated at 160 °C for 12 h. After the reaction, the yellow product was collected by centrifugation and washed with deionized water and absolute ethanol three times. Finally, the product was dried in a vacuum oven at 60 °C for 12 h, labeled as CdS/Kaol. The samples with different mass ratios of kaolinite clay (0.33, 1, and 9 g) were also obtained via the same process and labeled as CdS/Kaol-3, CdS/Kaol-2, and CdS/Kaol-1, respectively. The theoretical maximum mass ratios of CdS in CdS/Kaol-3, CdS/Kaol-2, CdS/Kaol, and CdS/Kaol-1 were 46.6, 22, 8.8, and 3.1%, respectively. For comparison, CdS + Kaol-mix contrast samples were obtained by physical mixing of pure CdS nanoparticles and kaolinite with the same mass ratio as the CdS/Kaol composites. Pure CdS nanoparticles were obtained by a similar process without the addition of kaolinite. 2.2. Characterizations. Transmission electron microscopy (TEM) images were recorded on a FEI-G20 microscope operating at 200 kV. For TEM, the dried samples were sonicated in ethanol and subsequently deposited onto a carbon grid. Scanning electron microscopy (SEM) images were obtained using a TESCAN MIRA3 field-emission scanning electron microscope. The X-ray diffraction (XRD) patterns of the products were measured using a RIGAKU D/max-2550 VB + diffractometer with Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.02 deg/s in the 2θ range from 15° to 80°. Xray photoelectron spectroscopy (XPS) measurements were obtained using an ESCALAB 250 Xi spectrometer. All of the binding energies were calibrated by the C 1s peak at 284.8 eV. The photoluminescence (PL) for the different samples was measured using a PE-LS55 fluorescence spectrophotometer. 25901

DOI: 10.1021/acs.jpcc.8b08663 J. Phys. Chem. C 2018, 122, 25900−25908

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Figure 1. (a) SEM image, (b) XRD patterns, (c,d) TEM and (e) HRTEM images of the CdS/Kaol composites. Inset in (e) is the corresponding FFT electron diffraction pattern of CdS. Scale bar = 500, 100, 20, and 2 nm for a,c,d,e, respectively.

tions.36 Considering both the calculation efficiency and accuracy, structural geometries and forces are well converged for a cutoff energy of 400 eV and 4 × 2 × 2 Monkhorst−Pack grid37 points with gamma point centered scheme. Geometry optimizations were performed to fully relax the atomic internal coordinates and the lattice parameters within the BFGS minimization algorithm till the total energy convergence of 1.0 × 10−6 eV per atom and residual force to 0.03 eV/Å. All the total energy calculations were performed in the reciprocal space using the Cambridge serial total energy package (CASTEP) code.38 Kaolinite is a 1:1 type layered aluminosilicate consisting of the silicon tetrahedral layer and an aluminum octahedral layer. The original crystal structure of kaolinite was constructed according to Wang’s data.39 The calculated lattice parameter of bulk kaolinite (a = 5.190 Å, b = 9.030 Å, c = 7.470 Å, α = 91.1°, β = 105.3°, γ = 89.7°) is in good agreement with the experimental values (a = 5.153 Å, b = 8.942 Å, c = 7.391 Å, α = 91.9°, β = 105.0°, γ = 89.8°)40 and previous calculation values (a = 5.196 Å, b = 9.007 Å, c = 7.372 Å, α = 93.0°, β = 106.0°, γ = 89.9°).41 Kaolinite surfaces are divided into (001̅) and (001) surfaces. The (001̅) surface is called Si tetrahedral surface, on which Si atoms are saturated with oxygen atoms. The (001) surface is also named Al octahedral surface composed of OH. The inner hydroxyl groups lie between the silicon tetrahedral layer and the aluminum octahedral layer. The hydroxyl groups and basal oxygen atoms of the silicon tetrahedral layer are connected by weak hydrogen bonds and van der Waals forces. Therefore, kaolinite is easily disintegrated along the (001) plane. In this study, both (001) and (001̅) surfaces of bulk kaolinite were considered. P(2 × 1)(001) and P(2 × 1)(00̅1) slabs were adopted as the substrates for oxygen adsorption. A 12 Å vacuum layer was introduced in the slabs to minimize the interactions between periodic images. The adsorption energies of the molecule on the surface were calculated as Eads = Emol/sur − Esur − Emol

where Eads is the adsorption energy, Emol/sur is the energy of the surface with the adsorbed molecule, Esur is the energy of the surface, and Emol is the energy of the molecule.

3. RESULTS AND DISCUSSION CdS/Kaol composites were synthesized by a simple wet chemical method. For comparison, CdS + Kaol-mix contrast samples were obtained by physical mixing of pure CdS nanoparticles and kaolinite with the same mass ratio as the CdS/Kaol composites. Figure 1 shows SEM and TEM images of CdS/Kaol composites. The CdS nanoparticles were evenly anchored on the surfaces and the edges of kaolinite NSs with a hexagonal lamellar structure (Figure 1a,c). The mean size of CdS nanoparticles is ca. 19 nm (Figures 1d and S1a), which is slightly smaller than that of pure CdS nanoparticles (Figure S1b). The lattice spacings of 3.58, 3.58, and 3.58 Å in the highresolution TEM (HRTEM) image are assigned to the (100), (010), and (1̅10) planes of CdS, respectively (Figure 1e). Therefore, HRTEM image and fast Fourier transform (FFT) diffraction patterns prove that the dispersive nanoparticles are CdS and crystalline. The energy-dispersive X-ray spectroscopy (EDX) data further indicate the presence of CdS (Figure S1c). XRD patterns of the obtained composites show strong CdS and kaolinite clay peaks and no other impurities (Figure 1b). In addition, there is no significant difference in the dispersity of CdS nanoparticles for CdS/Kaol and CdS + Kaol-mix composites (Figures 1a and S2b). These observations suggest that the morphology, size, dispersity, and structure of CdS in the CdS/Kaol composite show almost no noticeable changes, compared to pure CdS nanoparticles. The surface chemical composition and chemical state of the CdS/Kaol composites were further identified by XPS measurements. The peaks of Cd 3d and S 2p in the CdS/Kaol composite shifted to higher binding energies by 1 and 0.9 eV, respectively, compared to pure CdS nanoparticles (Figure S3).42 These observations indicate that there is strong interaction between CdS and kaolinite clay because of formation of Si (or Al)−O−Cd bonds.43 To investigate the light absorption property of CdS/Kaol composites, UV−vis absorption spectra were measured (Figure

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DOI: 10.1021/acs.jpcc.8b08663 J. Phys. Chem. C 2018, 122, 25900−25908

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Figure 2. (a) Photocatalytic activities of different samples for the photodegradation of MO and (b) intrinsic photodegradation rates among CdS/ Kaol, CdS + Kaol-mix, and pure CdS nanoparticles for the photodegradation of different organic contaminants.

Figure 3. (a) Photoelectric current measurements of Kaol clay and (b) ESR spectra of DMPO−•OH obtained from CdS/Kaol composites under visible-light irradiation (λ > 400 nm) and in the dark.

NSs on the photocatalytic activity of CdS for different organic contaminants. The photocatalytic stability of the CdS/Kaol composites was tested (Figure S7b,c). CdS/Kaol composites still retained nearly 80% of their original photocatalytic activity after two cycles, as compared to just one cycle of pure CdS nanoparticles. To clarify the photocorrosion rate of CdS/ Kaol and pure CdS during the photodegraded reaction, the Cd2+ concentration of the photodegraded reaction was detected by ICP analysis (Figure S7d). The Cd2+ concentration of CdS/Kaol composites was obviously lower than that of pure CdS nanoparticles. This observation suggests that the CdS/Kaol composite has lower photocorrosion rate compared to pure CdS, confirming better photocatalytic stability for the CdS/Kaol composites. Natural Fe-doped clay exhibits photoresponse and semiconducting properties because of reduced energy gap, which facilitates photogenerated carrier separation and transfer.25 To confirm the photoresponse of Kaol NSs, we conducted photocurrents and EPR measurements on spin-trapped DMPO. Under visible-light irradiation, kaolinite clay exhibited a photocurrent of approximately 0.1 μA, which was clearly higher than that of blank ITO slice, indicating obvious photoresponse of the Kaol NSs (Figure 3a). The photochemical activity of pure Kaol NSs was thereafter measured via a spin-trap DMPO to assess the production of hydroxyl radicals (•OH) in aqueous suspensions. EPR spectra of kaolinite clay irradiated for 10 min showed a strong DMPO−OH adduct signal, which has a characteristic spectrum of four strong splitting lines with a 1:2:2:1 intensity ratio (Figure 3b),16 whereas almost no noticeable DMPO− OH adduct signal was observed in the EPR spectra of kaolinite clay before irradiation. These findings confirm that Kaol NSs

S4a). The absorption bands of CdS/Kaol were shifted to the UV region compared with that of the pure CdS. According to the calculation of UV−vis spectra, the band gaps of CdS/Kaol and CdS + Kaol-mix composites were estimated to be 2.37 and 2.35 eV, respectively, which were higher than that of pure CdS nanoparticles (2.29 eV) (Figure S4b). The slight increase of band gaps of CdS/Kaol is attributed to the interaction between CdS and kaolinite clay, rather than a quantum confinement effect,43 owing to the similar band gap values of CdS/Kaol and CdS + Kaol-mix composites. The photocatalytic performance of the obtained CdS/Kaol composites was evaluated by the photodegradation of MO under visible-light irradiation. Kaol NSs showed almost no noticeable photodegradation of MO (Figure 2a). The CdS/ Kaol composites photodegraded almost all of the MO within 30 min, whereas CdS + Kaol-mix photodegraded about 90% of MO within 50 min and pure CdS nanoparticles photodegraded only 80% of MO within 60 min. The photodegradation rate of the CdS/Kaol composites was 4 times that of the pure CdS nanoparticles and 1.5 times that of CdS + Kaol-mix (Figure 2b and Table S1). In order to clarify the enhancement of Kaol on the photocatalytic activity of CdS, the apparent quantum efficiencies of different samples were calculated (Table S2). The apparent quantum efficiency of CdS/Kaol was 3.6 times that of pure CdS and 1.3 times that of CdS + Kaol-mix, which is consistent with the photodegradation rate of MO. The photocatalytic performance of CdS/Kaol composites improved with an increase of the content of Kaol NSs (Figure S5a), further proving that Kaol NSs enhance the photocatalytic activity of CdS nanoparticles. Moreover, CdS/Kaol composites also exhibited improved photocatalytic performance for the photodegradation of orange II (O II) and 4-nitrophenol (4NP) (Figure 2b), indicating the effective enhancement of Kaol 25903

DOI: 10.1021/acs.jpcc.8b08663 J. Phys. Chem. C 2018, 122, 25900−25908

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Figure 4. (a) PL spectra and (b) TRPL spectra of CdS/Kaol, CdS + Kaol-mix, and pure CdS nanoparticles at an excitation wavelength of 405 nm.

Figure 5. (a) O2-TPD profiles of raw and calcined kaolinite clay and (b) photocatalytic reaction rate comparison among CdS/Kaol, CdS + Kaolmix, CdS/Kaol-550, and CdS + Kaol-mix-550.

nanoparticles and CdS/Kaol composites. This result reveals weak interaction between CdS and Kaol NSs of the CdS + Kaol-mix composite. The transfer of photogenerated charge carriers of CdS/Kaol composites has been further supported by TRPL spectra (Figure 4b). The decay time was determined from an exponential fitting. All samples only had one decay lifetime, which were 0.89 ns (CdS/Kaol), 0.93 ns (CdS + Kaolmix), and 0.94 ns (CdS), respectively. The PL lifetime of CdS/ Kaol composites was shorter than that of pure CdS nanoparticles, indicating effective transfer of excited carriers from CdS to Kaol NSs.48,49 Surprisingly, CdS + Kaol-mix (0.93 ns) composites had the similar PL lifetime compared with pure CdS nanoparticles, suggesting that weak interface bonding hinders the separation of photogenerated charge carriers, which is also supported by the higher PL intensity of CdS + Kaol-mix than that of CdS/Kaol. However, the photocatalytic activity of CdS + Kaol-mix samples was comparable with that of CdS/Kaol composites and significantly higher than that of pure CdS. This finding implies that there is an indirect enhanced pathway in photocatalysis, besides the direct electron transport by the interface between CdS and Kaol NSs. Indirect photocatalytic enhancement of Kaol NSs may mostly be related to the reactive oxygen species (ROS) brought by kaolinite clay. In order to investigate the effects of ROS on the photocatalytic activity of CdS/Kaol composites, 1,4-benzoquinone (BQ), AgNO3, methanol (MeOH), and isopropanol (IPA) were introduced in the photodegradation of MO as •O2−, electron, hole, and •OH scavengers, respectively. The photodegradation rates of MO were significantly decreased by adding BQ, AgNO3, and MeOH (Figure S9). Meanwhile, the reaction rate was accelerated by adding O2 and IPA. These results suggest that •O2−, O2, electron, and hole contributed to the degradation of MO in the CdS/Kaol system, which is consistent with previous studies.50 On the

produce hydroxyl radicals in aqueous suspensions after visible irradiation, which is caused by the trace amount of iron oxides of clay.20−22 Hence, Kaol NSs showed weak photocatalytic ability for the photodegradation of orange II (Figure S6a) because of the trace amount of active species. In a word, ironrich Kaol NSs show the evident visible photoresponse like a semiconductor, which have the ability to transfer photogenerated carriers of nearby CdS nanoparticles. Band position alignments of heterojunction photocatalytic materials determine the charge transfer and separation of photocatalysts.44 The band gap of Kaol NSs determined from UV−vis spectra was 3.85 eV (Figure S8a), which is lower than that of ideal clay.25 The valence band maximum (VBM) of Kaol NSs was determined to be 3.58 eV by the XPS valence band spectrum (Figure S8b). The VBM value was 1.58 eV greater than that of anatase TiO2,45 indicating that Kaol NSs have a lower VBM than anatase TiO2 by about 1.58 eV relative to a normal hydrogen electrode. On the basis of the band gap and VBM of kaolinite, the electronic potentials of the Kaol NSs could be determined (Figure S8c). CdS/Kaol forms a type II staggered band structure, which suppresses the electron−hole recombination and improves the photocatalytic activity. To further investigate the charge carrier transfer of the CdS/ Kaol composites, we have characterized the samples using PL spectroscopy and time-resolved PL (TRPL) spectroscopy. The PL spectra of pure CdS nanoparticles showed three distinct emission bands (Figure 4a), with a weak band-edge emission at about 490 nm, a broad and strong trap state emission located at 540 nm, and a deep trap emission centered at 600 nm.42,46,47 For the CdS/Kaol composites, the two trap emissions were remarkably quenched, whereas band-edge emission was also reduced, indicating a fast transfer of excited carriers between CdS nanoparticles and clay.46,48 Moreover, the PL intensity of CdS + Kaol-mix was intermediate between those of pure CdS 25904

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The Journal of Physical Chemistry C basis of the obvious influence of •O2−, O2, and electron on the degradation process, we further infer that the photodegradation of MO is predominated by the electron mechanism. This is because CdS could not oxidize OH− and H2O directly by the photogenerated holes because of more positive potential of the photogenerated holes than that of • OH/OH− and •OH/H2O.51 Hence, O2 and electron play a primary role in the photodegradation of MO. The photogenerated electron is difficult to transfer from Kaol NSs to CdS because of the limitation of energy band structure (Figure S8c), but more oxygen could be provided to nearby CdS nanoparticles by the surface of Kaol NSs which have excellent adsorption properties and abundant surface hydroxyl groups. To clarify this hypothesis, we carried out O2TPD of raw and calcined kaolinite clay whose hydroxyl groups and absorbed water can be completely removed by calcination at 550 °C.52 In order to characterize oxygen absorbed on raw kaolinite clay, we directly carried out oxygen desorption without the adsorption process in the O2-TPD experiments. The two weak desorption peaks (100 and 225 °C) were ascribed to the oxygen physical adsorbed on the surface and interlayer (Figure 5a), respectively. A strong desorption peak appeared at 400 °C < T < 600 °C was attributed to the oxygen anchored by structural hydroxyl groups. The process of oxygen desorption is consistent with the dehydration and dehydroxylation of kaolinite (Figure S10). This is because hydroxyl groups or adsorbed water of kaolinite adsorbs oxygen by the hydrogen bonding.53,54 More importantly, the desorption peak of raw kaolinite was significantly stronger than that of calcined kaolinite clay, indicating that more oxygen is adsorbed on the surface of the raw kaolinite. We used calcined kaolinite clay instead of raw kaolinite clay to prepare control samples (CdS + Kaol-mix-550 and CdS/Kaol-550). As expected, the photocatalytic activities of CdS/Kaol-550 and CdS + Kaol-mix-550 were obviously poor compared with that of CdS/Kaol and CdS + Kaol-mix sample (Figures 5b and S11). These results suggest that more oxygen anchored by structural hydroxyl groups and adsorbed water of Kaol NSs obviously enhances the photocatalytic performance of nearby CdS nanoparticles. The calculations with the framework of DFT were performed to further investigate the adsorption of oxygen by hydroxyl groups and adsorbed water of kaolinite. Kaolinite (Al2Si2O5(OH)4) is a typically layer aluminosilicate mineral composed of one octahedral (Al−OH) sheet and one tetrahedral (Si−O) sheet (Figure S12). In the crystal structure of kaolinite, the Al octahedral surface composed of hydroxyl groups is the (001) surface, whereas the Si tetrahedral surface is the (001̅) surface, on which silicon atoms are saturated with oxygen atoms.39 Because of electrostatic repulsion of the oxygen atoms of the (001̅) surface, it is difficult for the (001̅) surface to further adsorb oxygen molecules. Hence, we focused on the adsorption of oxygen on the (001) surface. After the geometry optimizations of O2 at different sites on the (001) surface, the most stable adsorption structure is shown in Figure 6a, denoted as O2@(001). It can be seen that O2 adsorbed on the (001) surface is nearly parallel, leading to the formation of two O−H···O hydrogen bonds. The adsorption energy (Eads) of O2 adsorbed on the kaolinite (001) surface is 0.23 eV. Water inevitably exists on the surface of kaolinite because of rich hydroxyl groups of kaolinite. Figure 6b exhibits the most stable adsorption structure of H2O adsorbed on the (001) surface. The Eads of H2O adsorbed on the kaolinite (001) surface is much higher than that of oxygen by 0.46 eV, suggesting that

Figure 6. Different binding configurations of (a) O2, (b) H2O, and (c) O2−H2O on the kaolinite (001) surface. The purple, red, and white balls represent Al, O, and H, atoms, respectively.

adsorption of H2O molecules on the (001) surface is stronger than that of O2. To explore the adsorption of oxygen on the hydrated surface, a water molecule was added to the (001) surface of kaolinite.39,55 Figure 6c shows the most optimal configuration of O2 on the hydrated (001) surface, denoted as O2−H2O@(001). Interestingly, the Eads of O2 on the hydrated (001) surface is higher than that on the dry surface by 0.02 eV. This finding indicates that the water molecules further promote the adsorption of O2 molecules on the kaolinite (001) surface. In a word, hydroxyl groups and adsorbed water of Kaol NSs can effectively adsorb oxygen via hydrogen bonding. On the basis of the above results and analysis, a possible mechanism of the enhanced photocatalytic activity of Kaol NSs was proposed (Figure 7). In the CdS/Kaol system, there are two main pathways of improved photocatalytic performance. On the one hand, the direct photogenerated electrons transfer

Figure 7. Proposed mechanism for the enhanced photocatalytic activity of kaolinite NSs. Process I is the direct electron-transfer mechanism, and process II is the oxygen-assisted enhanced mechanism. 25905

DOI: 10.1021/acs.jpcc.8b08663 J. Phys. Chem. C 2018, 122, 25900−25908

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The Journal of Physical Chemistry C from CdS nanoparticles to Kaol NSs via the interface because of a type II staggered band structure, which suppresses the electron−hole recombination and improves the photocatalytic activity. On the other hand, Kaol NSs provide an oxygen-rich microenvironment on their surfaces by the strong oxygen adsorption capacity of hydroxyl groups. More oxygen rapidly traps photogenerated electrons of nearby CdS nanoparticles and then generates more superoxide radicals, leading to the improvement in the photocatalytic activity of CdS nanoparticles. The two mechanisms simultaneously improve the photocatalytic activity of CdS/Kaol composites. For the CdS + Kaol-mix sample, the oxygen-assisted enhanced mechanism is dominated, whereas direct electron transfer is blocked to some extent because of weak interfacial interaction between CdS and kaolinite. Hence, CdS + Kaol-mix exhibits a slightly low photocatalytic activity compared to CdS/Kaol samples. Moreover, the oxygen-assisted enhanced mechanism can well explain the phenomenon that the photocatalytic performance of CdS/Kaol composites improved with the increase of the content of Kaol NSs. In photodegradation experiments, the pHs of pure CdS and CdS/Kaol photodegradation solutions were 5.9 and 4.5, respectively, suggesting a decrease of pH induced by the Kaol NSs. To exclude the enhancement of pH on the photocatalytic performance of CdS, we compared the photocatalytic abilities of CdS nanoparticles in different pHs. The photocatalytic performances decreased as the pH decreases (Figure S13), indicating that the decrease of pH is a negative effect on the improved photocatalytic performance of CdS nanoparticles.

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4. CONCLUSIONS Natural iron-rich kaolinite NSs were used as a multifunctional support to load CdS nanoparticles. The CdS/Kaol composites had 2.6-fold higher photocatalytic activity than the pure CdS nanoparticles and retained nearly 80% of photocatalytic activity after two cycles, compared to just one cycle of pure CdS nanoparticles. The type II staggered band structure of the CdS/Kaol composites facilitates transfer of photogenerated electrons from CdS nanoparticles to Kaol NSs via the interface. On the other hand, the Kaol NSs provided an oxygen-rich microenvironment on their surfaces through the strong oxygen adsorption capacity of hydroxyl groups and then promoted the production of ROS of CdS nanoparticles, leading to the improvement of photocatalytic performance. The DFT results showed that hydroxyl groups of Kaol NSs effectively adsorbed oxygen via hydrogen bonding and that absorbed water further promoted the adsorption of oxygen on the kaolinite surface. On the basis of our study, other natural clay could also be used as multifunctional supports to improve the photocatalytic performance of photocatalysts, which is conducive to the utilization of natural clay, especially low-grade natural clay. This work could provide an underlying insight into the role of iron-rich clay as a photocatalytic support and shed new light on the design of clay-based composite photocatalysts.

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efficiencies; size distribution histograms and standard deviations of CdS nanoparticles in CdS/Kaol composites and pure CdS nanoparticles, the corresponding EDX analysis of CdS/Kaol composites, and TEM image of pure CdS nanoparticles; SEM images of Kaol and CdS + Kaol-mix; XPS spectra; UV−vis diffuse reflection spectrum and Ahν−hν curve; photodegradation rate and SEM images of CdS/Kaol prepared at different contents of Kaol; photodegradation curves, intrinsic photodegradation rates, and time-dependent absorption spectra of orange II and 4-NP; time-dependent absorption spectra of MO photodegradation solutions in the presence of CdS/Kaol samples, photocatalytic stability comparison, and cyclic photodegradation curves of CdS/Kaol, CdS + Kaol-mix, and CdS; cadmium ion concentration change of the photodegraded reaction solution using CdS/Kaol and pure CdS nanoparticles as photocatalysts, respectively; Ahν−hν curve, XPS valence band spectra, and band structure alignments of Kaol; effects of various scavengers on the photocatalytic activity of CdS/Kaol in the MO degradation; DSC, TG curves, photocatalytic activities, and intrinsic photodegradation rates of raw kaolinite and kaolinite calcined at 550 °C; crystal structure of bulk kaolinite; and photocatalytic abilities of CdS nanoparticles in different pHs (PDF)

*E-mail: [email protected]. Fax: 86-731-88710804. Phone: 86-731-88830549. ORCID

Denghui Jiang: 0000-0002-2046-0149 Liangjie Fu: 0000-0002-9761-5305 Huaming Yang: 0000-0002-3097-2850 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (41572036), the National Science Fund for distinguished Young Scholars (51225403), the Strategic Priority Research Program of Central South University (ZLXD2017005), Hunan Provincial Science and Technology Project (2016RS2004 and 2015TP1006), the National Natural Science Foundation of China (51402346), China Postdoctoral Science Foundation (2018M632984), and the Postdoctoral Science Foundation of Central South University (182043).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08663. Data fitting using a Langmuir−Hinshelwood model and reaction rate constant for the photodegradation of MO in the presence of different samples; apparent quantum 25906

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