The Role of Nanoclay in Photogenerated Charge Separation

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C: Energy Conversion and Storage; Energy and Charge Transport

An Efficient Nanoclay-based Composite Photocatalyst: The Role of Nanoclay in Photogenerated Charge Separation Denghui Jiang, Ziran Liu, Liangjie Fu, Huihua Jing, and Huaming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08663 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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The Journal of Physical Chemistry

An Efficient Nanoclay-based Composite Photocatalyst: The Role of Nanoclay in Photogenerated Charge Separation

Denghui Jiang,a Ziran Liu,b Liangjie Fu,a,c Huihua Jing,d Huaming Yang*,a,c,e

a

Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

b

Department of Physics, Key Lab for Low-Dimensional Structures and Quantum Manipulation (Ministry of Education), Hunan Normal University, Changsha 410081, China

c

Hunan Key Lab of Mineral Materials and Application, Central South University, Changsha 410083, China d Hunan e

Institute of Food Quality Supervision Inspection and Research, Changsha 410111, China

Key Lab of Clay Mineral Functional Materials in China Building Materials Industry, Central South University, Changsha 410083, China

*

Corresponding author, Email: [email protected], Fax: 86-731-88710804, Tel.: 86-731-88830549

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 photocatalytic system. Herein, natural iron-rich kaolinite clay with photoresponse is used as a multifunctional support to construct 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 (DFT) 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 radical 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 role of iron-rich natural clay mineral as a photocatalytic support and shed light on the design of clay-based composite photocatalysts.


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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 application especially toward commercial production.1-2 Aiming to solve this scientific question, a variety of strategies have been employed to improve charge separation and transfer efficiency of photocatalysts, such as loading cocatalysts,

3-5

fabricating semiconductor heterojunctions6-8

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 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 photocatalytic system.12 Probably due to this reason, the enhanced photocatalytic abilities of clay-based photocatalysts are generally attributed to high adsorption properties and excellent surface properties for improving dispersity and specific surface area of semiconductor.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, due to existence of photoactive iron oxides which are mainly structural iron in the aluminosilicate lattice, resulting from isomorphous substitution.20-24 These clay improved photogenerated carriers transfer efficiency of supported photocatalyst, due to its semiconducting property.25 Although a few works have reported semiconductor-clay based photocatalyst,26-30 the essential roles of iron-rich clay on separation of photogenerated carriers



remain unclear. On the other hand, the natural clay may provide an active microenvironment for nearby photocatalysts, leading to greatly enhance the charge carrier separation of photocatalysts.22, 31 Therefore, natural iron-rich clay should be a low-cost and multifunctional supports of photocatalysts. Herein, we use natural iron-rich kaolinite (Kaol) nanosheets (NSs) as a support to load CdS nanoparticles and then construct CdS/Kaol composite photocatalyst. Natural kaolinite has only a small amount of iron oxide (0.476 wt%), but exhibited evident photoresponse like semiconductor. In this paper, we have focused on the effects of Kaol NSs on the transfer and separation of photogenerated carriers by minimizing 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. Based on the results, the possible mechanisms of enhanced photocatalytic activity were proposed.

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 (XRF) was as follows: SiO2, 53.27; Al2O3, 42.46; Fe2O3, 0.476; K2O, 0.542; TiO2, 0.333; MgO, 0.155; P2O5, 0.257 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 kaolinite (Kaol) were 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. 5 mL of 0.4 M sodium sulfide nonahydrate (Na2S·9H2O) aqueous solution was added to the above dispersion dropwise. After that, the


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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 DI water and absolute ethanol for three times. Finally, the product was dried in a vacuum oven at 60 °C for 12 h, labelled as CdS/Kaol. The samples with different mass ratio of kaolinite clay (0.33 g, 1 g and 9 g) were also obtained via the same process and labelled as CdS/Kaol-3, CdS/Kaol-2 and CdS/Kaol-1, respectively. Theoretical maximum mass ratio of CdS in CdS/Kaol -3, CdS/Kaol-2, CdS/Kaol and CdS/Kaol1 were 46.6%, 22%, 8.8% and 3.1%, respectively. For comparison, CdS+Kaol-mix contrast samples were obtained by physical mixing 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 kaolinlite. 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 SEM. 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), and at a scanning rate of 0.02 deg/s in the 2θ range from 15° to 80°. X-ray 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 of samples were measured using a PE-LS55 fluorescence spectrophotometer. Photoluminescence decay spectra were recorded at room temperature with an Edinburgh FLS-980 fluorescence spectrophotometer. An ultraviolet–visible light (UV–vis) spectrophotometer (PE-950, Perkin-Elmer, USA) was used to perform the optical measurements of the product. The inductively coupled plasma mass spectrometry (ICP-MS) was performed on Analytikjena PlasmaQuant ® MS unit. Temperature



programmed desorption of oxygen (O2-TPD) was carried out on a HUASI DAS-7000 dynamic adsorption apparatus. Thermal analysis (TGA/DSC) was carried out using a Mettler-Toledo TGA/DSC3+/1600LF instrument at a heating rate of 10 °C/min with a gas feed (N2) of 20 mL/min. 0.32 mg Kaol sample was added into a 1.6 mL DMPO solution (75 mM). The mixture was incubated for 20 min, filtrated for electron paramagnetic resonance (EPR) spectroscopy measurements using a Bruker A300 electron paramagnetic resonance 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, the different quantity of samples (36.5 mg, 77 mg, 193 mg and 547 mg) were used for CdS/Kaol-3, CdS/Kaol-2, CdS/Kaol and CdS/Kaol-1. 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. And then, the mixed solution was irradiated using a 300 W Xe lamp (CEL-HXF300, 280 mw/cm2) equipped with a 400 nm cut-off 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. Photocatalytic activity was evaluated according to the absorption spectra of MO, recorded using a UV-vis absorption spectrophotometer (Shimadzu UV2450). 2.4 Photoelectrochemical Measurements. Photocurrent was measured using a computercontrolled electrochemical work station (Gamry 5000E) in a standard three electrode system with the photocatalyst-coated ITO as the working electrode, Pt foil as the counter electrode, and an Ag/AgCl as a reference electrode. The working electrodes was prepared as follows. The asobtained 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 × 2cm ITO slice and dried in air at room temperature. A 0.1 M Na2SO4 solution was used as the


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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 vs 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 the 3s23p2 for Si, 3s23p1 for Al, 2s22p4 for O and 1s1 for H, respectively. The Perdew-Burke-Ernzerhof (PBE) parameterization utilizing generalizedgradient approximation (GGA) scheme was adopted to deal with the exchange-correlation interactions

34

. The claculations of surface adsorption were performed using the DFT semi-

empirical dispersion interaction correction (DFT-SEDC) module.35 Grimme’s method was used for DFT-D dispersion corrections 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 grid

37

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) code38. 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 of 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 (00 1 ) and (001) surfaces. The (00 1 ) 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



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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 (00 1 ) surface of bulk kaolinite were considered. P(2×1)(001) and P(2×1)(00-1) 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 molecule on surface was calculated as: Eads=Emol/sur-Esur-Emol

(1)

Where Eads is the adsorption energy, Emol/sur is the energy of the surface with 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 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 nanosheets with a hexagonal lamellar structure (Figure 1a and 1c). The mean size of CdS nanoparticles is ca. 19 nm (Figure 1d and S1a) and slightly smaller than that of pure CdS nanoparticles (Figure S1b). The lattice spacing of 3.58 Å, 3.58 Å and 3.58 Å in -

HRTEM image are assigned to the (100), (010) and ( 1 10) planes of CdS, respectively (Figure 1e). Therefore, HRTEM image and FFT (fast Fourier transform) diffraction patterns prove the dispersive nanoparticles are CdS and crystalline. The EDS data further indicate the presence of CdS (Figure S1c). X-ray diffraction (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


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difference in the dispersity of CdS nanoparticle for CdS/Kaol and CdS+Kaol-mix composites (Figure 1a and S2b). These observations suggest that the morphology, size, dispersity and structure of CdS in CdS/Kaol composite show almost no noticeable changes, compared to pure CdS nanoparticles.

b

a

●

●

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CdS+Kaol-mix CdS

Kaol 10

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20

d

30

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 (deg)

e 3.58 Å

60

70

80

3.58 Å d(010)

d(110)

CdS

CdS

3.58 Å d(100)

Kaol CdS

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 and e, respectively.

The surface chemical composition and chemical state of the CdS/Kaol composites were further identified by X-ray photoelectron spectroscopy (XPS) measurements.The peaks of Cd 3d and S 2p in CdS/Kaol composite shifted to higher binding energies by 1 eV and 0.9 eV compared to pure CdS nanoparticles (Figure S3), respectively.

42

These observations indicate that there is

strong interaction between CdS and kaolinite clay, due to formation of Si (or Al)-O-Cd bonds 43.



To investigate light absorption property of CdS/Kaol composites, UV−vis absorption spectra were measured (Figure S4a). The absorption bands of CdS/Kaol were shifted to the ultraviolet 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 eV and 2.35 eV, respectively, which were higher 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. Photocatalytic performance of the obtained CdS/Kaol composites was evaluated by photodegradation of methyl orange (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, while CdS+Kaol-mix about 90% within 50 min and pure CdS nanoparticles only 80% 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 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 as increase of 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 (4-NP) (Figure 2b), indicating that the effective enhancement of Kaol NSs on photocatalytic activity of CdS for different organic contaminant.


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Photocatalytic stability of the CdS/Kaol composites was tested (Figure S7b and S7c). CdS/Kaol composites still retained nearly 80% of its original photocatalytic activity after two cycles, as compared to just one cycles of pure CdS nanoparticles. To clarify the photocorrosion rate of the CdS/Kaol and pure CdS during photodegraded reaction, the Cd2+ concentration of 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 the pure CdS, confirming better photocatalytic stability for the CdS/Kaol composites.

a 1.0

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light on

light off

-1

Reaction rate (min )

0.8 0.6

C/C0

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0.4 0.2 0.0 -40

MO Kaol CdS CdS+Kaol-mix CdS/Kaol -20

0

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60

CdS CdS+Kln-mix CdS/Kln

0.12

0.09

0.06

0.03

0.00

Time (min)

MO

O II

4-NP

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

Natural Fe doped clay exhibits photoresponse and semiconducting properties due to reduced energy gap, which facilitates photogenerated carrier separation and transfer

25

. To confirm

photoresponse of Kaol NSs, we conducted photocurrents and EPR measurements on spintrapped 5, 5-dimethyl-1-pyrroline-N-oxide (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 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. While, almost no noticeable DMPO–OH adduct signal was observed in the EPR spectra of kaolinite clay before irradiating. These findings confirm that Kaol NSs produces hydroxyl radicals in aqueous suspensions after visible irradiating, which is caused by trace amount iron oxides of clay 20-22. Hence, Kaol NSs showed weak photocatalytic ability for the photodegradation of Orange II (Figure S6a), due to the trace amount of active species. In a word, iron-rich Kaol NSs show the evident visible photoresponse like semiconductor, which have the ability to transfer photogenerated carrier of nearby CdS nanoparticles.

a

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K aol

Irradiating for 10 min

Intensity (a.u.)

0.1

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on off

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Before irradiating

B lank 260

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300

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Time (S)

3480

3500

3520

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Magnetic Field (G)

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 dark.

Band position alignments of heterojunction photocatalytic materials determine charge transfer and separation of photocatalysts44. The band gap of Kaol NSs determined from UV–vis spectra was 3.85 eV (Figure S8a), which is lower than that of ideal clay25. The valence band maximum (VBM) of Kaol NSs was determined to be 3.58 eV by the XPS valence band spectrum (Figure


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S8b). The VBM value was 1.58 eV greater than that of anatase TiO2 45, indicating that Kaol NSs has a lower VBM than anatase TiO2 by about 1.58 eV relative to a normal hydrogen electrode (NHE). Based on the band gap and VBM of kaolinite, the electronic potentials of the Kaol NSs could be determined (Figure S8c). The CdS/Kaol forms a type II staggered band structure, which suppresses the electron–hole recombination and improves the photocatalytic activity. To further investigate charge carrier transfer of the CdS/Kaol composites, we have characterized the samples using photoluminescence (PL) spectroscopy and time-resolved photoluminescence (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 centred at 600 nm.

42, 46-47

. For the CdS/Kaol composites, the two trap emissions were remarkably quenched,

while 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 nanoparticles and CdS/Kaol composites. This result reveals weak interaction between CdS and Kaol NSs of 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 were determined from an exponentials fitting. All samples only had one decay lifetime, which were 0.89 ns (CdS/Kaol), 0.93 ns (CdS+Kaol-mix) 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 separation of photogenerated charge carriers, which is also supported by higher PL intensity of CdS+Kaolmix 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 are an indirect enhanced pathway in photocatalysis process, besides the direct electron transport by the interface between CdS and Kaol NSs.

a

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CdS

CdS/Kaol

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τ= 0.89 ns τ= 0.93 ns τ= 0.94 ns

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CdS/Kaol CdS+Kaol-mix CdS

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

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 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 holes contributed to the degradation of MO in the CdS/Kaol system, which is consistent with previous studies.50 Based on the obvious influence of •O2-, O2 and electron on the degradation process, we further infer that photodegradation of MO is predominated by the electron mechanism. This is because that CdS could not oxidize OH− and H2O directly by the photogenerated holes, due to


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more positive potential of the photogenerated holes than that of the •OH/OH− and •OH/H2O.51 Hence, O2 and electron play a primary role in the photodegradation of MO. Photogenerated electron is difficult to transfer from Kaol NSs to CdS, due to 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 temperatureprogrammed desorption of oxygen (O2-TPD) 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 adsorption process in the O2-TPD experiments. The two weak desorption peaks (100 °C 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 that hydroxyl groups or adsorbed water of kaolinite adsorb oxygen by the hydrogen bonding 5354

. 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 prepared 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 (Figure 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.



a

b

0.12

CdS/Kaol

-1

Reaction rate (min )

R aw

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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550 ℃ 100

200

300

400

500

600

Temperature(C)

700

800

0.08

CdS+Kaol-mix CdS/Kaol-550

0.04

CdS+Kaol-mix-550 CdS

0.00

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

The calculations with the framework of density functional theory (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 crystal structure of kaolinite, the Al octahedral surface composed of hydroxyl groups is (001) surface, while Si -

tetrahedral surface is (00 1 ) surface, on which silicon atoms are saturated with oxygen atoms39. -

-

Due to electrostatic repulsion of the oxygen atoms of the (00 1 ) surface, it is difficult for (00 1 ) surface to further adsorb oxygen molecules. Hence, we focused on 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 is adsorbed on the (001) surface in nearly parallel, leading to the formation of two O–H•••O Hydrogen-bonds. The adsorption energy (Eads) of O2 adsorbed on kaolinite (001) surface is 0.23 eV. Water inevitably exists on the surface of kaolinite, due to 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 kaolinite (001) surface is much higher than that of oxygen by 0.46


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eV, suggesting that adsorption of H2O molecules on (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 hydrated (001) surface, denoted as O2-H2O@(001). Interestingly, the Eads of O2 on hydrated (001) surface is higher than that on dry surface by 0.02 eV. This finding indicates that the water molecules further promote adsorption of O2 molecules on 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 from CdS nanoparticles to Kaol NSs via the interface due to 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 its 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 improve 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 while direct electron transfer is blocked to some extent, due to 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 pH 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 photocatalytic performance of CdS, we compared the photocatalytic abilities of CdS nanoparticles in different pH. The photocatalytic performances decreased as the pH decrease (Figure S13), indicating that the decrease of pH is a negative effect on the improved photocatalytic performance of CdS nanoparticles.

a 2.206 2.198

E ads = 0.23 eV

T op view

S ide view

b 2.105 2.012 1.701

E ads = 0.69 eV

T op view

S ide view

c

2.608

T op view

2.224

E ads = 0.25 eV

S ide view

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


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MO

CO2 ●-

O H ...O

O2 2

Ⅱ

Ⅰ

O H ...O

H2O

2

Figure 7. Proposed mechanism for the enhanced photocatalytic activity of kaolinite nanosheets. Process Ⅰ is the direct electrons transfer mechanism, process Ⅱ is the oxygen-assisted enhanced mechanism.

4. CONCLUSIONS Natural iron-rich kaolinite nanosheets 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, as compared to just one cycles 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 its surfaces through the strong oxygen adsorption capacity of hydroxyl groups, and then promoted 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 absorbed water further promoted



adsorption of oxygen on kaolinite surface. Based on our study, other natural clay could also be used as multifunctional supports to improve 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 of role of iron-rich clay as a photocatalytic support and sheds new light on the design of clay-based composite photocatalysts.

ASSOCIATED CONTENT Supporting Information Data fitting using a Langmuir-Hinshelwood model and reaction rate constant for photodegradation of MO in the presence of different samples (Table S1), apparent quantum efficiencies (Table S2), 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, TEM image of pure CdS nanoparticles (Figure S1), SEM images of Kaol and CdS+Kaol-mix. (Figure S2), XPS spectra (Figure S3), UV–vis diffuse reflection spectrum and Ahν–hν curve (Figure S4), photodegradation rate and SEM images of CdS/Kaol prepared at different content of Kaol (Figure S5), photodegradation curves, intrinsic photodegradation rates and time-dependent absorption spectra of Orange II and 4-NP (Figure S6), Time-dependent absorption spectra of MO photodegradation solutions in present of CdS/Kaol samples, photocatalytic stability comparison and cyclic photodegradation curves of CdS/Kaol, CdS+Kaol-mix and CdS. The cadmium ion concentration change of photodegraded reaction solution using CdS/Kaol and pure CdS nanoparticles as photocatalysts respectively (Figure S7), Ahν–hν curve, XPS valence band spectra and band structure alignments of Kaol (Figure S8), effects of various scavengers on the photocatalytic activity of CdS/Kaol in the MO degradation (Figure S9), DSC, TG curves, Photocatalytic activities and intrinsic photodegradation rates of raw kaolinite and kaolinite calcined at 550 °C (Figure S10 and S11)


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and crystal structure of bulk kaolinite (Figure S12). Photocatalytic abilities of CdS nanoparticles in different pH (Figure S13).

AUTHOR INFORMATION Corresponding Author Email: [email protected], Fax: 86-731-88710804, Tel.: 86-731-88830549 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 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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TOC Graphic

MO ●-

O H ...O

O2 2

Ⅱ

Ⅰ

Kaol

CdS O H ...O

2


CO2 H2O


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TOC Graphic

MO ●-

OH ... O2

O2

Ⅰ

Kaol

Ⅱ

CdS OH ... O2

ACS Paragon Plus Environment

CO2 H2O

The Role of Nanoclay in Photogenerated Charge Separation

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