Citation

Huanjing Liang, Peng Hua, Yufei Zhou, Zhimin Fu, Jiaoning Tang, Junfeng Niu. Fabrication of Cu/rGO/MoS₂ nanohybrid with energetic visible-light response for degradation of rhodamine B[J]. Chin. Chem. Lett., 2019, 30(12): 2245-2248  

Huanjing Liang^a,b,1, Peng Hua^a,1, Yufei Zhou^b, Zhimin Fu^c, Jiaoning Tang^a,*, Junfeng Niu^b,*     

In recent years, environmental pollution has been an increasingly serious issue due to the imperfect prevention and the lake of anticipation of the negative impact resulting from the rapid development. Water pollution caused by industrial synthetic dyes and pigments has seriously disturbed the water ecosystem, wastewater treatment technology has attracted widespread attention. Nowadays, there are many technologies to remove pollutants in wastewater, such as flocculation and sedimentation, microbial decomposition, molecular sieve adsorption, photocatalytic degradation. While among these technologies, photocatalytic degradation is known as a promising technology, which may consume only solar energy to mineralize organic pollutants in water or wastewater [1-12].

Among reported photocatalysts, the transition-metal dichalcogenides (TMDs) exhibit wide response spectra and potential in environmental contamination control [13-15]. Especially, molybdenum disulfide (MoS₂), a typical TMD, exhibits not only a narrow band gap but also two-dimensional layered graphene-like structures [16-19]. These properties result in strong absorption of visible light and smooth pathway for charge transfer. However, there are two matters of concern over MoS₂, which inhibits its catalytic performance to a certain extent. Firstly, the agglomeration of MoS₂ caused by the Van der Waals bond between MoS₂ layers decreases the number of active sites that lead to a lower catalytic efficiency. For the other thing, the poor intrinsic electrical conductivity of MoS₂ impedes charge transport and result in lower effective separation of charges [20-23]. Taking these factors into consideration, a high efficiency photocatalyst of functional MoS₂ with more active sites and higher electrical conductivity via a facile method should be designed. Reduced graphene oxide (rGO) has higher electron mobility and conductivity, which is widely used in semiconductor modification to improve the separation and migration of its carriers to achieve higher electrocatalytic or photoelectrocatalytic properties [24-26]. Moreover, graphene is a two-dimensional material with a large surface area, which is very suitable as a catalytic substrate for MoS₂ [27-29]. Cu-based materials have recently attracted extensive attention in photocatalytic applications due to its abundant natural reserves, low cost and good conductivity [30-32]. The functioned MoS₂ with combination of Cu and rGO is an effective method to improve the conductivity and inhibit the agglomeration of MoS₂.

As outlined in Scheme 1, a solvothermal method was used to synthesis a novel visible-light-driven Cu/rGO/MoS₂ ternary nanostructure. The morphology and structure of the materials were characterized by SEM/TEM images (Fig. S1 in Supporting information), Raman spectra (Fig. S2 in Supporting information) and nitrogen sorption isotherm (Fig. S3 and Table S1 in Supporting information). The crystal structure was detected by XRD (Fig. 1). For the prepared MoS₂ sample, the diffraction peaks at 13.97°, 33.09°, 39.11°, 58.82° and 68.8° correspond to (002), (100), (103), (110) and (201) facets of MoS₂ (JCPDS card No. 37-1492), respectively. Due to the weak crystallization of rGO, rGO/MoS₂ displayed similar XRD pattern to MoS₂ [33, 34]. Furthermore, the apparent diffraction peaks of Cu were observed in XRD patterns of the CRM hybrid. The diffraction peak located at 46.13° and 48.59° were assigned to (111) and (200) facets of Cu, respectively, whose intensities enhanced with the increasing Cu content.

Scheme 1

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Scheme 1. Schematic diagram of the Cu/rGO/MoS2 preparation.

Figure 1

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Fig. 1. XRD spectra of MoS2, RM and CRM catalyst.

XPS was executed to further ascertain the details of the elemental composition and chemical bonding state of CRM-30. The full XPS survey spectrum (Fig. 2a) revealed that the characteristic peaks of Mo, S, C, O and Cu as the main elemental components presented in CRM hybrid. The C 1s peak was deconvoluted into three peaks at 284.75 eV, 286.04 eV and 288.54 eV (Fig. 2b) [34-36], which regarded to the C—C, C—O and O＝C—O functional groups in the rGO, respectively [37]. Fig. 2c shows peaks at 531.03 and 532.02 eV corresponding to O 1s which can be attributed to the oxygen ingredient of the rGO. In Fig. 2d, the binding energies of 228.91 and 232.68 eV corresponding to Mo 3d_5/2 and Mo 3d_3/2 indicated that Mo was in the form of Mo⁴⁺ in MoS₂. The peaks located at 235.42 and 232.09 eV were attributed to Mo 3d_5/2 and Mo 3d_3/2 of MoO₃ or MoO₄^2-, which indicating the presence of Mo⁶⁺ in the form of MoO₃ or MoO₄^2-. For S 2p XPS spectrum (Fig. 2e), two peaks centered at 161.77 and 163.11 eV were indexed to S 2p_3/2 and S 2p_1/2 of MoS₂, respectively, indicating the presence of MoS₂ [38]. And the peaks at 168.54 and 169.97 eV were the shake-up satellite. It was difficult to distinguish the Cu⁺ and Cu⁰ because of the close proximity of their energies. In Fig. 2f, the peaks at 932.82 and 952.95 eV matched with Cu 2p_3/2 and Cu 2p_1/2 of Cu⁰, respectively. And the peaks of 942 and 962 eV corresponded to the satellite peaks. The binding energies at 935.19 and 944.28 eV corresponded to Cu 2p_3/2 and Cu 2p_1/2 of Cu+, respectively. It was probably that the excess NaBH₄ reduced Cu²⁺ to Cu⁰, and the Cu⁰ was partially oxidized to Cu⁺ and Cu²⁺ in the air [39]. In summary, the synthesized MoS₂ nanosheets were well integrated with rGO and Cu in the mixture. The Cu content of CRM-30 was 30.39 wt% by inductive coupled plasma emission spectrometer (ICP) method (Table S2 in Supporting information).

Figure 2

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Fig. 2. XPS of (a) survey spectrum, (b) C 1s, (c) O 1s, (d) Mo 3d, (e) S 2p and (f) Cu 2p of CRM-30 catalyst.

Before photocatalysis, the optical band gap, adsorption and photoluminescence spectra are shown in Figs. S4, S5 and Table S3 (Supporting information). As shown in Figs. 3a and b, in 20 min of visible-light photocatalysis process using MoS₂, the removal efficiency of RhB can reach over 90%. For the visible-light photocatalysis process using RM, RhB was removed 100% in 10 min. The modification of Cu showed obviously advantage for the photocatalytic reduction of RhB. Among CRM-10, 20, 30, 40, 50, GCM-30 displayed the best photocatalytic performance. The possible reason was high separation efficiency of photoelectronhole pairs. What's more, the amount of Cu affected the efficiency of charge transfer. Once it exceeded a certain amount, specific surface area of CRM will reduce, following an imperfect photocatalytic performance. As a representative sample, CRM-30 possesses higher photocurrent response relative than other photocatalysts, which clearly shows an efficient separation of photo-induced charge carriers [40]. The pseudo-first-order kinetic model primely described the photocatalytic degradation kinetic process of RhB. It was found that the pseudo-first order reaction kinetics (k) of CRM-30 was reckoned 0.72 min^-1, which was 3.79 times and 6.55 times higher than that of the RM (0.19 min^-1) and MoS₂ (0.11 min^-1), respectively. Fig. 3c depict that the kinetic rate constant of CRM-30 was the highest among all samples. Fig. 3d demonstrated the photocurrent-time curves of different photocatalysts. Taking RhB as a target, the photocatalytic activity of CRM was evaluated. In conclusion, the photocatalytic of ternary CRM-30 showed the superior catalytic activity and better stability (Fig. S6 in Supporting information). To identify the reactive oxygen species, p-benzoquinone (BQ), isopropanol (IPA) and EDTA-2 Na were selected as scavenger for superoxide radical anions (^·O₂^-), hydroxyl radicals (^·OH), hole (h⁺), respectively (Fig. S7 in Supporting information).

Figure 3

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Fig. 3. (a) The photocatalytic performance of MoS2, RM and CRM-30 catalyst. (b) The photocatalytic performance of CRM. (c) The pseudo-first-order degradation rate constants (k). (d) Transient photocurrent responses of different photocatalysts.

The photocatalytic reaction of CRM was illustrated in Scheme 2. The formation process of active radicals was expressed by the following Eqs. (1–6). As mentioned, the band gap of MoS₂ and CRM were 1.92 and 0.62 eV, respectively. Therefore, MoS₂ is easily excited by visible light illumination to generate electrons and holes. It is notable that the photogenerated electrons are rapidly transported from the conduction band (CB) of MoS₂ to rGO which displayed a momentous role in storing, promoting the separation, transfer and migration of charge carriers. Hence, the photocatalytic activity of RM was better than MoS₂, and in the presence of the H₂O₂, the degradation rate is obviously accelerated. Moreover, Cu can improve the conductivity and may be worked as electron trap states and contribute to the consumption of electron [41, 42]. In other words, this synergy effect may accelerate the separation of electrons and holes.

Scheme 2

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Scheme 2. Schematic drawing of the photocatalytic removal mechanism.

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Cu/rGO/MoS₂ nanocomposites were successfully fabricated via a solvothermal and chemical method. MoS₂ was responsible for optical adsorption, rGO acted as a scaffold to anchor MoS₂, and the load of Cu further contributed to separate the photogenerated charge carriers. Benefiting from the joint functions, Cu/rGO/MoS₂ exhibited superior photocatalytic performance and good stability towards degradation of organic dyes. This work will open a window for designing effective photocatalyst and promote the potential application of photocatalysis.

This study was financially supported by the National Natural Science Foundation of China (No. 51878169), the Guangdong Innovation Team Project for Colleges and Universities (No. 2016KCXTD023), and Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017).

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2019.05.046.