Scalable synthesis of Cu2S double-superlattice nanoparticle systems with enhanced UV/visible-light-driven photocatalytic activity
Graphical abstract
Introduction
Cuprous sulfide (Cu2S) nanoparticles, as important p-type semiconductor nanomaterials, are holding great potential for many applications like cold cathodes [1], [2] and nanoscale switches [3], [4]. And attributed to its exceptional combination of a bulk band gap of 1.2 eV, an absorption coefficient of >104 cm−1, the elemental abundance as well as low toxicity [5], Cu2S is especially considered as an ideal light absorbing material for photothermal [6], optoelectronic [7], [8], photovoltaic [9], [10], [11], [12], [13] and photocatalytic [2], [8], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] applications. Till now, Cu2S nanoparticles with various morphologies such us nanorods [24], nanowires [25] and nanovesicles [26] have been synthesized. And further, diversiform noble metal-Cu2S [23], [27], [28], semiconductor-Cu2S [17], [18], [19], [20], [21], [22] as well as carbon nanomaterial-Cu2S [7], [19] heterostructures including alloyed particles [22], heterodimers [21], [27], core-shell structures [23], p-n heterojunctions [17], [18], [19] and many other forms of hybrid nanocomposites have been designed and obtained successfully.
It was since the first discovery of photocatalytic water splitting on titania in 1972 [29] that tremendous efforts have been devoted to the development of highly active photocatalysts. Particularly, for the development of Cu2S-based efficient photocatalysts toward degradation of organic dyes recently, to construct three-dimensionally (3D) self-assembled nanostructures has become one of the research focuses. Typically in some cases, open porous hierarchical Cu2S microsponges derived from ultrathin nanosheets with 99% exposed (1 1 1) facets were reported by Liu et al. [14] and demonstrated >90 times of photoreactivity for the degradation of phenol. Also, Cu2S microrings as well as hollow spheres constructed from self-assembled pristine nanoplates were reported by Zhao et al. [2] and Jiang et al. [15], respectively and both showed excellent photocatalytic performances by testing the degradation efficiency of methylene blue (MB). Besides, Peng et al. [16] reported the controllable synthesis of flower-like Cu2S nanostructures through a template-free polyol process. The self-assembled nanoflowers finally demonstrated higher photocatalytic activity under visible light irradiation than several other forms of Cu2S assemblies like the nanorod arrays and nanowires.
On the other hand, superlattices containing long-range periodic compositional and structural features, typically on the nanometer scale, have attracted scientific attention for over 60 years and still remained an active area of research [30]. Generally speaking, there are two classes of nanomaterials we usually dub them as “superlattices”, one of which is the atomic-level superlattice nanowires, nanobelts or multilayer thin films [31], [32], [33], [34], [35], [36], [37], and the other is the particle-level 3D ordered nanoparticle assemblies [38], [39], [40], [41], [42], [43], [44]. Since these superlattices possess unique properties which are different from either the monodispersed nanoparticles or bulk materials of each component, for example, the narrowed band gaps [45], they can offer intriguing possibility for many applications including developing novel photocatalytic systems [45], [46], [47], [48], [49], [50]. Nevertheless, the controllable preparation of superlattices still remains a challenge in chemistry and materials science, and currently for obtaining atomic-level superlattices, chemical vapor deposition (CVD) and the more expensive physical vapor deposition (PVD) processes are the most commonly used methods, yet neither of them can achieve scalable preparation which is always a formidable task [51], [52], [53]. What's more, the integration of both the atomic- and particle-level superlattices into a single system and the subsequently correlated studies have been rarely done.
Here in this work, by utilizing a one-pot solvothermal method, we have synthesized Cu2S nanoparticles with both atomic- and particle-level superlattice structures, which we named them double-superlattices (DSLs). The size and shape of pristine Cu2S nanoparticles were highly tunable by controlling the reaction time or added amount of the reactants. These Cu2S nanoparticles were firstly identified to contain inherent atomic-level superlattices, and consequently, accompanied by addition of the polar solvent into non-polar colloidal dispersions during the post-treatment process, 3D assembled nanoparticle arrays, that is the DSLs were eventually obtained. Amazingly, these DSLs demonstrated enhanced activities during photocatalytic degradation tests toward MB solutions although the photocatalytic activities were influenced mutually by their size, shape, crystallinity, self-assembling behavior and many other factors according to the experimental results. Besides, it is also mentionable that the synthesis finally achieved gram-scale owing to the relatively high yields and little loss of the one-pot solvothermal process. It is to the best of our knowledge that the DSL structures discussed in this work were referred to for the first time, and it is also believable that not only the novel structures and scalable synthesis of the DSLs, but also their highly enhanced photocatalytic activities would make much sense to the further applications of Cu2S as efficient photocatalysts and other functional materials.
Section snippets
Materials and chemicals
All reagents, including the copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, AR), 1-dodecanethiol (DDT, AR), ammonia solution (NH4OH, AR), n-hexane (AR), tetrachloroethylene (AR), anhydrous ethanol (AR) and methylene blue (MB, BS) were purchased from Sinopharm Chemical Reagent Company and used as received. And in all experiments, highly pure water (Millipore) with the resistivity greater than 18.0 MΩ cm was used.
Synthetic procedures
For each reaction, the detailed adding amount of reactants and reaction time of the
Results and discussion
Typically for the identification of the phase structure and purity of as-synthesized Cu2S nanoparticles, XRD pattern and Raman spectrum were recorded. The XRD pattern, as is shown in Fig. 1(a), demonstrates the pure Cu2S component of the synthesized nanoparticles. Nevertheless, two different crystalline phases of Cu2S, that is the chalcocite phase with space group of hexagonal P63/mmc (PDF#26-1116) and chalcocite-M phase with space group of monoclinic P21/c (PDF#33-0490), were coexisted.
Conclusions
In summary, for the first time we have developed a facile one-pot solvothermal process for nearly gram-scale synthesis of size- and shape-tunable Cu2S nanoparticles. XRD characterization has revealed the unique coexistence of two different phases of Cu2S, hexagonal chalcocite and monoclinic chalcocite-M phase. It is intriguing that distinct atomic-level superlattice structures were identified to intensively exist in these Cu2S nanoparticles. And it was after the post-treatment process of
Acknowledgements
This work was financially supported by the National Natural Foundation of China (Nos. 11274066, 51172047, 50872145 and 51102050) and the Ministry of Science and Technology of China (973 Project Nos. 2013CB932901 and 2009CB930803). Besides, the authors are grateful to the “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (09SG01).
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