Three-Dimensional Zn0.5Cd0.5S/Reduced Graphene Oxide Hybrid Aerogel: Facile Synthesis and the Visible-Light-Driven Photocatalytic Property for Reduction of Cr(VI) in Water

Abstract

A series of three-dimensional Z [subscript] n x [/subscript] C [subscript] d 1 - x [/subscript] S/reduced graphene oxide (Z [subscript] n x [/subscript] C [subscript] d 1 - x [/subscript] S/RGO) hybrid aerogels was successfully synthesized based on a one-pot hydrothermal approach, which were subsequently used as visible-light-driven photocatalysts for photoreduction of Cr(VI) in water. Over 95% of Cr(VI) was photoreduced by Zn0.5Cd0.5S/RGO aerogel material within 140 min, and such photocatalytic performance was superior to that of other Z [subscript] n x [/subscript] C [subscript] d 1 - x [/subscript] S/RGO aerogel materials ( x ≠ 0.5 ) and bare Zn0.5Cd0.5S. It was assumed that the enhanced photocatalytic activity of Zn0.5Cd0.5S/RGO aerogel was attributed to its high specific surface area and the preferable synergetic catalytic effect between Zn0.5Cd0.5S and RGO. Besides, Zn0.5Cd0.5S/RGO aerogel materials were robust and durable enough so that they could be reused several times with merely limited loss of photocatalytic activity. The chemical composition, phase, structure, and morphology of Zn0.5Cd0.5S/RGO aerogel material were carefully examined by a number of techniques like XRD, SEM, TEM, BET, Raman characterizations, and so on. It was found that Zn0.5Cd0.5S/RGO aerogel possessed hierarchically porous architecture with the specific surface area as high as 260.8 m2 g-1. The Zn0.5Cd0.5S component incorporated in Zn0.5Cd0.5S/RGO aerogel existed in the form of solid solution nanoparticles, which were uniformly distributed in the RGO matrix.

Academic Editor:Takuya Tsuzuki

Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Yongchuan, Chongqing 402160, China

Received 21 April 2016; Revised 4 August 2016; Accepted 11 August 2016

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Water pollution is becoming more and more serious all over the word and threatening the survival and development of human beings. Among the water pollutants, heavy metal ions especially hexavalent chromium Cr(VI) are acutely hazardous because of the extreme toxicity and high mobility in aqueous environment [1, 2]. Several techniques such as membrane separation, adsorption, ion exchange, and photocatalytic reduction have been utilized to treat Cr(VI) contamination [1-5]. Compared with other methods, heterogeneous photocatalytic reduction using nanostructured semiconductor photocatalysts has been considered as a promising way to remove Cr(VI), particularly when it is present at low concentrations [2, 6]. However, most of the traditional photocatalysts like TiO₂ and ZnO are only active in UV light region, which contributes to about 4% of the solar energy, thus severely limiting their practical applications [2, 6, 7]. Considering the large proportion (around 43%) of the solar energy from visible light, it is highly desirable to explore novel visible-light-driven photocatalysts to achieve high-performance photocatalytic reactions for environmental remediation and protection.

Recently, metal sulfides have attracted tremendous attention due to their prominent physicochemical properties. Among them, CdS is regarded as an excellent visible-light-responsive photocatalyst candidate on account of the narrow band ( _{E g} = 2.4 eV) as well as its appropriate conduction band edge (more negative than the H₂ O/H₂ redox potential) [8-10]. However, CdS often suffers from photocorrosion and instability in photocatalytic systems [9, 10]. To overcome the disadvantages, lots of strategies have been proposed including the preparation of multicomponent chalcogenide solid solutions [9-16]. For example, by introducing ZnS into CdS, various semiconductor Z _{n x} C _{d 1 - x} S solid solutions with different structures and morphologies were produced, which not only possessed improved photocorrosion resistance but also exhibited efficient visible-light-responsive photocatalytic activities for degradation of organic dyes [9-13, 15, 16], whereas direct use of nanosized Z _{n x} C _{d 1 - x} S matters in photocatalytic reactions readily intrigues aggregation behavior owing to the small size and brings about fast recombination of photogenerated electrons and holes [9-11]. Both of the two negative factors could severely lower the photocatalytic abilities. Therefore, immobilization, dispersion, or hybridization of Z _{n x} C _{d 1 - x} S solid solutions with diverse solid substrates like semiconductor metal oxide, graphitic carbon nitride, carbon nanotubes, and other carbonaceous species has been explored [9, 12, 17-19].

Graphene has fascinated the materials research community since its discovery due to the unique properties. In particular, its huge specific surface area makes it an ideal support for depositing or growing guest substances, and, meanwhile, its exceptional electrical conductivity makes it an excellent electron acceptor and transporter [20-25]. Consequently, a large variety of semiconductor photocatalysts were hybridized with reduced graphene oxide (RGO) for photocatalytic applications, leading to the formation of the corresponding RGO-containing composites [8, 9, 13, 20, 21, 23-25]. It has been proven that RGO incorporated in the hybrid photocatalysts facilitated the charge transfer and suppressed the recombination of charge carries during photocatalytic reactions thus giving rise to their enhanced photocatalytic performances [8, 20, 21, 23, 24]. However, even so, some drawbacks for the synthesis of RGO-based hybrid photocatalysts still exist. For instance, most of the reported works were focused on the fabrication of two-dimensional (2D) RGO-based composites; that is, photocatalytic active species were just anchored on the surface of RGO sheets. In such circumstances, serious restacking or agglomeration of 2D RGO sheets would inevitably take place during drying and usage as a result of the van der Waals force, which dramatically reduced the accessible surface area of the hybrid photocatalysts and limited the transport of mass and photoexcited electrons to a large extent [26]. To address these issues, self-assembly of 2D RGO sheets into macroscopic three-dimensional (3D) architecture seems to be a feasible choice since it is capable of effectively circumventing the restacking of RGO sheets, providing hierarchically porous structure with large specific surface area, and contributing to the fast electron transport [26]. Accordingly, some functional nanoparticles have been successfully introduced, embedded, or captured in the 3D RGO networks, yielding novel RGO-based aerogel materials [8, 20, 26-41]. Previously, SnO₂ /RGO and TiO₂ /RGO hydrogels were generated via hydrothermal treatment of aqueous dispersions of graphene oxide (GO) in the presence of SnO₂ and TiO₂ nanoparticles, respectively [26, 27]. Besides, by mixing aqueous suspension of GO with the solution of suitable metal salt and reducing agent, a homogeneous mixture was produced. Then, it was subjected to hydrothermal treatment, resulting in the formation of metal nanoparticle (including Au, Ag, Pt, and Ni)/RGO hydrogels [28-33]. Other metal oxide (including Fe₃ O₄ , Fe₂ O₃ , MnO₂ , CeO₂ , Co₃ O₄ , and VO₂ )/RGO and metal sulfide (including MoS₂ , CoS₂ , ZnS, and NiS)/RGO hydrogels were similarly hydrothermally developed as well [20, 34-41]. These hydrogels were easily converted into the corresponding aerogels after a vacuum freeze-drying process, which displayed extraordinary performances in plenty of challenging fields like catalysis, sensing, adsorption, supercapacitors, lithium storage, and so on [8, 20, 26-41]. Motivated by the remarkable properties of RGO-based aerogel materials and their facile synthetic strategy, herein, we report a series of 3D Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels, which were fabricated based on the one-pot hydrothermal method employing Zn(OAc)₂ and Cd(OAc)₂ as metal precursors and thiourea as sulfur source. In contrast with other Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels ( x ≠ 0.5 ), RGO aerogel, and bare Zn_0.5 Cd_0.5 S solid solution, Zn_0.5 Cd_0.5 S/RGO hybrid aerogel gave the best performance toward the photocatalytic reduction of Cr(VI) in water under visible light and such hybrid photocatalyst was robust, durable, and photocorrosion resistant enough so that it was able to be reused many times with limited loss of photocatalytic activity, exhibiting outstanding practical application potential.

2. Materials and Methods

2.1. Materials and Reagents

GO sheets were provided by Nanjing JCNANO Technology Co., Ltd. (Nanjing, China). Zn(OAc)₂ ·2H₂ O, Cd(OAc)₂ ·2H₂ O, and thiourea CS(NH₂ )₂ were bought by Sinopharm Chemical Reagent Co., Ltd. K₂ Cr₂ O₇ was purchased from Sigma-Aldrich Co., Ltd., and employed as the Cr(VI) source for the photocatalytic experiments. All chemicals were guaranteed reagents and used as received. All aqueous solutions and dispersions were prepared with Milli-Q water (resistivity, 18.2 MΩ cm).

2.2. Synthesis of 3D Zn _x Cd_1-x S/RGO Hybrid Aerogels

3D Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels were synthesized based on a one-pot hydrothermal process. In a typical procedure, Zn(OAc)₂ ·2H₂ O and Cd(OAc)₂ ·2H₂ O with fixed total amount of 2 mmol but different molar ratios were dissolved in 1 mL water. This 1 mL of solution and another 1 mL of aqueous solution containing 8 mmol of thiourea were sequentially added to 18 mL of aqueous GO suspension containing 40 mg GO sheets under vigorous stirring. The resulting homogeneous mixture was then transferred into a stainless-steel autoclave with the capacity of 50 mL and sealed to heat at 180°C for 12 h. After the hydrothermal reaction, the autoclave was naturally cooled to room temperature and a cylindrical Z _{n x} C _{d 1 - x} S/RGO hydrogel was located at its bottom, which was then taken out, washed with water, and immersed in abundant fresh water for 3 days to remove the impurities. Subsequently, the hydrogel was freeze-dried under vacuum to obtain the corresponding Z _{n x} C _{d 1 - x} S/RGO hybrid aerogel. It is noted that the x value is the molar ratio of Zn precursor to the two kinds of metal precursors, and when it equals 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1, the corresponding hybrid aerogels are labeled as CdS/RGO, Zn_0.2 Cd_0.8 S/RGO, Zn_0.4 Cd_0.6 S/RGO, Zn_0.5 Cd_0.5 S/RGO, Zn_0.6 Cd_0.6 S/RGO, Zn_0.8 Cd_0.2 S/RGO, and ZnS/RGO, respectively. For comparison, RGO aerogel was likewise fabricated in the absence of any metal precursor and Zn_0.5 Cd_0.5 S solid solution was also prepared according to the synthetic conditions of Zn_0.5 Cd_0.5 S/RGO hydrogel but without addition of GO sheets.

2.3. Photocatalytic Reduction of Cr(VI) under Visible Light

Photocatalytic reduction of Cr(VI) in water using the currently developed Zn_0.5 Cd_0.5 S/RGO hybrid aerogel as visible-light-driven photocatalyst was tested by employing a photoreactor (JOYN-GHX-A, Shanghai JOYN Electronic Co., Ltd.), which was equipped with a xenon lamp (500 W) for visible light irradiation. Typically, 50 mg of Zn_0.5 Cd_0.5 S/RGO aerogel material was dispersed into 50 mL of aqueous solution of K₂ Cr₂ O₇ (20 mg L^-1 ). The resulting suspension was put into the photocatalytic setup and exposed to visible light illumination under continuous stirring with a constant horizontal distance (10 cm) between the xenon lamp and reaction mixture. At 20 min intervals, 3 mL of aliquots was extracted from the reaction system and centrifuged to get rid of the photocatalyst, while the upper clear solutions were analyzed on an UV-vis spectrophotometer. The variations of the concentrations of Cr(VI) in reaction mixture with irradiation time were monitored by recording the absorption band changes of the upper clear solutions located at 370 nm. For comparison, the photoreduction of Cr(VI) catalyzed by other Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels materials (x ≠ 0.5), Zn_0.5 Cd_0.5 S solid solution, and RGO aerogel material as well as the photoreduction of Cr(VI) without any photocatalyst was conducted under the identical experimental conditions. Moreover, repetitive photoreduction of Cr(VI) under visible light using Zn_0.5 Cd_0.5 S/RGO aerogel material as photocatalyst was evaluated as well. After the first run of photocatalytic reaction, the suspension was centrifuged to separate the photocatalyst, which was once again dispersed in a fresh solution of K₂ Cr₂ O₇ to form a new reaction mixture for the next round of photoreduction.

2.4. Characterizations

UV-vis spectra were measured on a Hitachi U-3900 spectrophotometer in the transmission mode with pure water as reference standard. Raman spectra were collected on a Horiba Scientific Raman spectrometer using 532 nm laser line as the excitation source. X-ray photoelectron spectra (XPS) were obtained from a VG ESCALAB MARK II instrument with a monochromatic and focused Mg K α radiation source ( h [upsilon] = 1253.6 eV) operating at an energy of 15 kV/300 W under a residual pressure of ~2 × 10^-9 mbar. Atomic adsorption spectroscopy was analyzed on a Shimadzu AA-680 atomic absorption spectrophotometer fixed with deuterium background corrector using air-acetylene flame. Field emission scanning electron microscopy (FE-SEM) observations were carried out by using a Hitachi S4800 instrument working at an acceleration voltage of 3 kV, and all of the specimens were directly observed owing to their excellent conductivity. Transmission electron microscopy (TEM) inspections were performed on a Tecnai G2 F20 instrument operating at an acceleration voltage of 200 kV. Energy dispersive X-ray spectra (EDS) and two-dimensional element mapping of materials were detected on Oxford Energy Dispersive X-ray equipment working at an acceleration of 20 kV. Nitrogen adsorption-desorption isotherm was made at 77 K employing a Micromeritics ASAP 2020 equipment; the specific surface area was estimated through the Brunauer-Emmett-Teller (BET) model; and the pore size distribution was analyzed by the Barrett-Joyner-Halenda (BJH) method. Thermogravimetric analysis (TGA) was conducted using a Netzsch TG 209F1 apparatus at a heating rate of 10°C min^-1 in air flow. X-ray diffraction (XRD) patterns were acquired on a Bruker D8 Advance diffractometer with a CuKα ( λ = 0.15418 nm) radiation source.

3. Results and Discussion

3.1. Composition, Structure, and Morphology of Zn_0.5 Cd_0.5 S/RGO Hybrid Aerogel

To understand the crystal phase and structural information of the Z _{n x} C _{d 1 - x} S/RGO hybrid aerogel materials, XRD technique was applied. As presented in Figure 1, three main diffraction peaks of CdS/RGO aerogel centered at 2 θ = 26.6°, 43.9°, and 52.0° are well indexed to (111), (220), and (311) crystal planes of cubic CdS (JCPDS number 65-2887), respectively. Likewise, the XRD patterns of the other Z _{n x} C _{d 1 - x} S/RGO aerogels also show three diffraction peaks analogous to those for CdS/RGO aerogel, which are indicative of cubic zinc blende structure of all the Z _{n x} C _{d 1 - x} S components incorporated in these samples [9, 19]. Notably, as the x value goes up, namely, as the Zn content increases, gradual shift of the diffraction peaks to larger angles takes place, suggesting that the produced Z _{n x} C _{d 1 - x} S components comprise of the corresponding solid solutions rather than a simple mixture of ZnS with CdS [9, 19, 42]. The generation of Z _{n x} C _{d 1 - x} S solid solutions means that Zn²⁺ has incorporated into the lattice of CdS or entered its interstitial sites by virtue of the smaller ionic radius of Zn²⁺ (0.74 Å) than that of Cd²⁺ (0.97 Å) [9, 19, 42]. Nevertheless, the characteristic broad peak of RGO ( 2 θ = ^{25 °} , shown in Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2016/6201546) does not appear in the XRD patterns of all Z _{n x} C _{d 1 - x} S/RGO aerogels, whose absence is likely ascribed to the weak diffraction intensity [8, 9, 43].

Figure 1: XRD patterns of the currently developed Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels with different x values.

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Figure 2 is the Raman spectra of GO sheets and Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material. Obviously, both curves show two prominent bands at 1350 cm^-1 (D band) and 1590 cm^-1 (G band), demonstrating the existence of carbon species [8, 21, 31]. D band is commonly related to the structural disruption and defects of the symmetrical hexagonal graphitic lattice, while G band is attributed to the first-order scattering of _{E 2 g} phonons within sp² -bonded carbon materials [8, 20, 21]. The peak intensity ratio of D to G band, _{I D} / _{I G} , can be used to estimate the disorder level and defect degree of the sp² -bonded carbon domains of graphite materials [20, 21]. As can be directly calculated from the peak intensity displayed in Figure 2, the values of _{I D} / _{I G} for GO sheets and Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material are 0.85 and 1.05, respectively. The former is much lower than the latter, manifesting that the GO sheets underwent a significant removal of oxygen-containing groups and were indeed reduced to RGO during the hydrothermal process [20, 31, 40].

Figure 2: Raman spectra of GO sheets (blue curve) and Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material (black curve).

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XPS spectroscopy is a useful tool for discovering the surface state of composite materials. Figure 3(a) is the high-resolution XPS spectrum of GO sheets for C 1s region. Figures 3(b)-3(e) present a set of high-resolution XPS spectra of Zn_0.5 Cd_0.5 S/RGO aerogel for C 1s, Zn 2p, Cd 3d, and S 2p regions, respectively, and the detected signals verify the presence of the four elements in this product. Both of the C 1s spectra can be resolved into four Gaussian peaks. The dominant peak at 284.6 eV is assigned to the aromatic sp² C=C bonding in graphitic structure, while the other three peaks centered around 286.6 eV, 287.5 eV, and 289.3 eV result from the oxygen-containing functional groups like C-OH, C-O-C, and HO-C=O, respectively [8, 9, 22]. Most of the carbon atoms in GO sheets and Zn_0.5 Cd_0.5 S/RGO aerogel are bonded in the form of sp² . However, in comparison with the C 1s spectrum of GO sheets, the peak intensities for the oxygen-containing functional groups in the C 1s spectrum of Zn_0.5 Cd_0.5 S/RGO aerogel remarkably decrease, implying the abundant loss of these oxygen-containing functional groups during the hydrothermal reaction [8, 9, 20, 22]. Namely, GO sheets have been sufficiently reduced to RGO component in such process [8, 9, 20, 22]. Moreover, XPS Zn 2p and Cd 3d spectra disclose representative curves for Zn 2p and Cd 3d doublets with the binding energies (BE) of 1046.7 eV for Zn 2p_1/2 , 1022.8 eV for Zn 2p_3/2 , 411.7 eV for Cd 3d_3/2 , and 405.0 eV for Cd 3d_5/2 , respectively, which are in good agreement with the previously reported values for Z _{n x} C _{d 1 - x} S solid solution [9, 16, 17]. The BE splitting of 23.9 eV for Zn 2p doublets and 6.7 eV for Cd 3d doublets verifies that the chemical states of elements Zn and Cd within the Zn_0.5 Cd_0.5 S/RGO hybrid aerogel are both +2 valence [9, 17]. That is, they exist in the form of Zn²⁺ and Cd²⁺ , respectively [9, 17]. XPS S 2p spectrum gives a single peak located at 162.1 eV, demonstrating the existence of element S in the form S^2- , which should arise from Zn-S and Cd-S bonds [9, 15-17]. The content of RGO incorporated in Zn_0.5 Cd_0.5 S/RGO hybrid aerogel was determined by its TGA measurement from room temperature to 700°C. As exhibited in Figure 4, the TGA profile shows a slight weight loss of 2.5% at below 200°C, which should be owing to the physically and chemically adsorbed water molecules [44, 45]. Generally, a further increase of temperature from 200 to 600°C leads to the decomposition and complete burning of RGO within the sample to CO₂ [44, 45]. Therefore, according to the weight loss profile of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material, the RGO content within it is calculated to be 46.9%.

Figure 3: (a) High-resolution XPS spectrum of C 1s region of GO sheets. (b-e) High-resolution XPS spectra of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material for C 1s, Zn 2p, Cd 3d, and S 2p regions, respectively.

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Figure 4: TGA profile of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material.

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The as-prepared bulk Zn_0.5 Cd_0.5 S/RGO aerogel is monolithic and free-standing as shown in the inset of Figure 5(a). Its microscopic morphology and structure are carefully examined by FE-SEM and TEM observations. As can be clearly seen in the low-magnified FE-SEM image (Figure 5(a)), the Zn_0.5 Cd_0.5 S/RGO aerogel features a well-defined and interconnected porous RGO network with lots of continuous macropores on micrometer scale. The ultrathin, flexible, and crumpled RGO sheets can be identified by closer inspections, which meanwhile reveal considerable narrow mesopores and voids with irregular shape created by the their accumulation (Figures 5(b) and 5(c)), demonstrating the hierarchically porous architecture. Besides, the high-magnified FE-SEM images (Figures 5(b) and 5(c)) show that a great number of nanoparticles with the size of several tens of nanometers are evenly embedded in the 3D RGO framework, which should be the Zn_0.5 Cd_0.5 S solid solution. In addition to the XPS analysis, the chemical composition of Zn_0.5 Cd_0.5 S/RGO aerogel is further validated by its EDS spectrum where elements C, Zn, Cd, and S are available and the atomic ratio of Zn : Cd : S is quite close to 1 : 1 : 2 (Figures 5(d) and 5(e)), commendably matching with the stoichiometric ratio of Zn_0.5 Cd_0.5 S solid solution. Figures 5(f)-5(i) are EDS mapping images of elements C, Zn, Cd, and S within Zn_0.5 Cd_0.5 S/RGO aerogel, respectively, which visualize their spatial distribution. Evidently, the uniform signals are full of the same detection zone, manifesting the homogeneity of all the four elements in this specimen. Figure 5(j) is a TEM image of Zn_0.5 Cd_0.5 S/RGO aerogel with low magnification, displaying that Zn_0.5 Cd_0.5 S solid solution nanoparticles are uniformly and densely dispersed on the surface of RGO sheets. Such TEM observation result agrees well with the morphological characteristics revealed by FE-SEM examinations (Figures 5(b) and 5(c)). Interestingly, the Zn_0.5 Cd_0.5 S solid solution nanoparticles observed in the FE-SEM images actually consist of smaller nanocrystals, which are merely several nanometers in diameter as revealed by the high-magnified TEM image (Figure 5(k)). Figure 5(l) presents a high-resolution TEM (HRTEM) image of an individual Zn_0.5 Cd_0.5 S nanocrystal within Zn_0.5 Cd_0.5 S/RGO aerogel, where the lattice fringes are clearly visible, indicating the highly crystalline nature. The interplanar distance is calculated to be 0.31 nm, which corresponds to the (111) plane of cubic zinc blende phase of Zn_0.5 Cd_0.5 S solid solution [9, 16, 43-45]. From all these above data and results, it is inferred that the reduction of GO to RGO, the formation, growth, and crystallization of Zn_0.5 Cd_0.5 S/RGO solid solution nanoparticles, their deposition on RGO substrate, and the self-assembly of 2D RGO sheets to 3D porous structure through π - π stacking interactions were almost synchronously accomplished, resulting in the hybridization of Zn_0.5 Cd_0.5 S solid solution nanoparticles with RGO to form the 3D Zn_0.5 Cd_0.5 S/RGO hybrid hydrogel [26]. Therefore, after it was freeze-dried under vacuum, the macroscopic 3D cylindrical shape was retained and the microscopic hierarchically porous architecture was generated, readily yielding the 3D Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material with unique structure and morphology.

Figure 5: (a-c) SEM images of Zn_0.5 Cd_0.5 S/RGO aerogel at different magnifications; the inset in (a) is the digital photograph of a monolithic Zn_0.5 Cd_0.5 S/RGO aerogel. (d) EDS spectrum of Zn_0.5 Cd_0.5 S/RGO aerogel. (e) The result of elemental contents of C, Zn, Cd, and S contained in Zn_0.5 Cd_0.5 S/RGO aerogel, which was obtained from the EDS spectrum shown in (d). (f-i) EDS elemental mapping images of C, Zn, Cd, and S within Zn_0.5 Cd_0.5 S/RGO aerogel, suggesting their uniform distribution in the sample. (j and k) TEM images of Zn_0.5 Cd_0.5 S/RGO aerogel at low and high magnifications, respectively. (l) HRTEM image of an individual Zn_0.5 Cd_0.5 S solid solution nanocrystal trapped in the matrix of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel, showing its (111) lattice plane.

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The porous property of Zn_0.5 Cd_0.5 S/RGO aerogel was surveyed by BET measurement. Figure 6(a) depicts its typical nitrogen adsorption-desorption isotherm. According to the IUPAC nomenclature, the isotherm belongs to type IV with a H4 hysteresis loop at the relative pressure ( P / _{P 0} ) between 0.3 and 0.95, which is associated with the capillary condensation occurring in mesopores, indicating the presence of narrow slit-shaped mesopores in the hybrid aerogel material [46-48]. The corresponding pore size distribution data obtained from the adsorption branch of isotherm curve by the BJH method are shown in Figure 6(b), illustrating that the pore size mainly ranges from several to dozens of nanometers. These BET results are quite consistent with the FE-SEM examinations (Figures 5(b) and 5(c)). Moreover, the specific surface area of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel reaches up to 260.8 m² g^-1 , which is much larger than that of RGO aerogel (147.1 m² g^-1 ) and Zn_0.5 Cd_0.5 S solid solution (66.4 m² g^-1 ), implying that the generation and decoration of Zn_0.5 Cd_0.5 S solid solution nanoparticles within 3D RGO matrix not only functionalize the final product (i.e., the Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material) but also serve as the spacer to partially alleviate the agglomeration of RGO sheets. As such, the hierarchically porous architecture of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material with higher specific surface area can enlarge reaction space, offer numerous reactive sites, favor mass transport, and, hence, ensure its splendid photocatalytic property.

Figure 6: (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel.

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(b) [figure omitted; refer to PDF]

3.2. Photocatalytic Property of Zn_0.5 Cd_0.5 S/RGO Hybrid Aerogel Material

The photocatalytic activity of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material was evaluated by conducting the photoreduction of Cr(VI) in aqueous media under visible light irradiation. As control, the photocatalytic performances of Z _{n x} C _{d 1 - x} S hybrid aerogel materials ( x ≠ 0.5 ), Zn_0.5 Cd_0.5 S solid solution, and RGO aerogel material were tested under the same conditions as well. The results are shown in Figure 6, where _{C 0} and C are the initial concentration of Cr(VI) and its measured concentration at irradiation time t , respectively. Apparently, without any photocatalyst, the self-reduction of Cr(VI) could be neglected. For the currently developed Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material, it took 140 min to realize the photoreduction of Cr(VI) over 95%. Such photocatalytic performance is much superior to that of other photocatalysts since only 29.2%, 54.8%, 84.1%, 76.8%, 36.3%, 9.7%, 5.1%, and 41.3% of Cr(VI) were photoreduced by CdS/RGO, Zn_0.2 Cd_0.8 S/RGO, Zn_0.4 Cd_0.6 S/RGO, Zn_0.6 Cd_0.4 S/RGO, Zn_0.8 Cd_0.2 S/RGO, ZnS/RGO, RGO aerogel, and bare Zn_0.5 Cd_0.5 S solid solution after identical illumination time, respectively (Figure 7(a)). The kinetics of photocatalytic reduction reaction of Cr(VI) were also surveyed. As plotted in Figure 7(b), linear correlation is fitted in each case, suggesting that the photoreduction process obeys a pseudo-first-order kinetic model. Therefore, the photoreduction rate constant ( k ) can be directly determined in terms of the slopes of the fitted lines. Impressively, the photocatalytic rate for Zn_0.5 Cd_0.5 S/RGO aerogel ( k = 0.098 min^-1 ) is the fastest among all the photocatalysts synthesized in this work, and their photocatalytic activities follow the increasing order of ZnS/RGO aerogel < CdS/RGO aerogel < Zn_0.8 Cd_0.2 S/RGO aerogel < bare Zn_0.5 Cd_0.5 S solid solution < Zn_0.2 Cd_0.8 S/RGO aerogel < Zn_0.6 Cd_0.4 S/RGO aerogel < Zn_0.4 Cd_0.6 S/RGO aerogel < Zn_0.5 Cd_0.5 S/RGO aerogel. During the photoreduction process, the valence electrons of Z _{n x} C _{d 1 - x} S solid solution were excited to the conduction band by absorbing visible light and electron-hole pairs were generated [49, 50]. As a consequence, Cr(VI) could be reduced to Cr(III) by capturing the photoexcited electrons; in parallel, the holes on the valence band were also reactive enough so that they could oxidize water molecules to oxygen [49, 50]. When RGO is composited with Z _{n x} C _{d 1 - x} S solid solution, due to rather more negative conduction band potential of Z _{n x} C _{d 1 - x} S solid solution compared with RGO as well as the exceptional conductivity and π -conjugation structure of RGO, the photoexcited electrons could be injected into RGO sheets, effectively promoting the electron-hole pair separation [9, 50, 51]. In this manner, RGO actually functioned as an electron collector and transporter and significantly prolonged the lifetime of charge carriers, thus speeding up the photoreduction reaction of Cr(VI) [9, 22, 50]. Therefore, in contrast with Zn_0.5 Cd_0.5 S solid solution, the catalytic synergistic effect between Zn_0.5 Cd_0.5 S and RGO as well as the unique hierarchically porous structure with enhanced specific surface area should be responsible for the stronger photocatalytic performance of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material. In previous reports, Z _{n x} C _{d 1 - x} S solid solution was hybridized with carbon nanotubes (CNTs), TiO₂ , and CuS to form Z _{n x} C _{d 1 - x} S/CNTs, Z _{n x} C _{d 1 - x} S/TiO₂ , and Z _{n x} C _{d 1 - x} S/CuS composites, respectively, which were utilized as visible-light-driven photocatalysts for photocatalytic applications [17, 18, 52]. Their photocatalytic activities varied as the x value changed as well and depended on the band energy, the adsorption ability for visible light, the rate of interfacial charge transfer, and the separation efficiency of electron-hole pairs [17, 18, 52]. Accordingly, it is assumed that when x value is equal to 0.5, Zn_0.5 Cd_0.5 S/RGO aerogel could optimally balance these factors, resulting in the best photocatalytic performance among all the Z _{n x} C _{d 1 - x} S/RGO aerogel materials.

Figure 7: (a) Performances of photoreduction of Cr(VI) under visible light catalyzed by Z _{n x} C _{d 1 - x} S/RGO hybrid aerogels, RGO aerogel, and bare Zn_0.5 Cd_0.5 S solid solution as well as the performance of photoreduction of Cr(VI) under visible light in the absence of any photocatalyst. (b) Kinetic curves of the visible-light-driven photocatalytic reduction reactions of Cr(VI) catalyzed by the currently developed photocatalysts. (c) Recycling experiments of photocatalytic reduction of Cr(VI) over Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

To obtain the information about the stability and recyclability of Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material, repetitive photoreduction of Cr(VI) under visible light illumination was carried out, and the results were presented in Figure 7(c). The photocatalytic activity is appreciably retained after successively using this photocatalyst for several times, because the percentage of photoreduced Cr(VI) is still as high as 88.4% for the fifth visible-light-driven photocatalytic reaction. Notably, the phase, structure, and morphology of recycled Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material remain intact and it maintains the porous architecture with Zn_0.5 Cd_0.5 S solid solution firmly trapped in the RGO backbone as before (Figures S2 and S3), indicating its excellent durability and robustness. Furthermore, after consecutive photoreduction, we examined the aqueous solution of reaction mixture via atomic adsorption spectroscopy to detect the potential release of toxic Cd ions from photocatalyst. However, no noticeable signal of Cd ions is available, implying that Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material is stable and environmentally friendly enough and hardly causes the secondary water pollution.

4. Conclusions

In summary, a group of 3D Z _{n x} C _{d 1 - x} S/RGO hydrogels with different x values were prepared through a facile one-pot hydrothermal process, which was converted into self-supporting monolithic aerogels after freeze-drying. It was found that Zn_0.5 Cd_0.5 S/RGO hybrid aerogel had hierarchically porous network with Zn_0.5 Cd_0.5 S solid solution nanoparticles well distributed in the RGO matrix. Thanks to the large specific surface area and the preferable synergetic catalytic effect between Zn_0.5 Cd_0.5 S solid solution and RGO, Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material exhibited the strongest visible-light-driven photocatalytic activity toward the photoreduction of Cr(VI) as compared with other Z _{n x} C _{d 1 - x} S/RGO aerogel materials ( x ≠ 0.5 ) and bare Zn_0.5 Cd_0.5 S solid solution. Besides, Zn_0.5 Cd_0.5 S/RGO hybrid aerogel material featured satisfactory photocatalytic stability and recyclability. Relying on the convenience and versatility of the present methodology, it should be facile to fulfill the incorporation of other active or functional species with different constituents and shapes into RGO framework to construct more RGO-based aerogel materials with distinct microscopic morphologies, high specific surface area, and reinforced properties, which would find applications in lots of important fields such as catalysis, energy storage, and environmental protection.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21401015, 21403020, and 21101136), the Basic and Frontier Research Program of Chongqing Municipality (cstc2014jcyjA50012, cstc2015jcyjA90020, and cstc2016jcyjA0137), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501104, KJ1501101, and KJ1501126), the Natural Science Foundation of Yongchuan (Ycstc, 2014nc4001), and the Foundation of Chongqing University of Arts and Sciences (R2013CJ04 and R2012CJ15).

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