Facile polyol synthesis of CuS nanocrystals with a hierarchical nanoplate structure and their application for electrocatalysis and photocatalysis

Abstract

The article describes a robust method for the facile polyol synthesis of high-quality CuS nanocrystals with a controlled hierarchical nanoplate structure. The success of this method relies on manipulating the reaction kinetics with different sulfur precursors. In particular, with a medium releasing rate of S²⁻, we are able to produce CdS hierarchical spherical nanoflowers composed with multi-layered nanoplates, while a slow or fast release of S²⁻ gives the monodispersed hexagonal nanoplates or disordered nanoplate complexes, respectively. Benefitting from its large surface area and the hierarchical structure for light reflection, the CuS nanoflower with a hierarchical plate structure showed the best electrocatalytic and photocatalytic performances when benchmarking its activity with the well-shaped nanoplate and the disordered nanoplate complex.

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Introduction

Copper sulfide (CuS), as an important p-type metal sulfide semiconductor, has received special attention owing to its unique properties that enable its applications in (photo-)catalysis,^1,2 solarthermal conversion,^3,4 plasmonics,^5,6 etc. It has been well-accepted that the physicochemical properties of a CuS nanocrystal could be readily manipulated by altering its shape and architecture morphology. To this end, tremendous efforts have been devoted to the design and synthesis of CuS nanocrystals with various shapes and morphologies. Notable examples include nanoplates,^7,8 nanowires,^9,10 nanotubes,^11,12 flower-like structure,^13,14 hollow spheres,^15,16 and so on.^17–20 Particularly, in a wide variety of chemical transformations with the CuS nanocrystal as a catalyst, the {0001} facets are usually found to be more attractive to be involved,^21–23 which are similar to cadmium sulfide and anatase titanium dioxide.^24,25

Despite these successful demonstrations, there still lacks of systematic studies on the growth mechanism of such nanocrystals and their morphology-dependent properties. In general, most of the syntheses are hydrothermal/solvothermal-based which usually proceed with high temperature/pressure and thus are difficult to follow in situ.^26–29 On the other hand, the generation and nature of nuclei and seeds, and subsequent crystal growth that are governed by reaction kinetics can be significantly influenced by reaction conditions.^30–33 It is therefore hard to conduct the same synthesis under complete different growth conditions. It is also difficult to understand the growth mechanisms involved in such syntheses.

In this paper, we report the preparation of nanosized covellite CuS crystals in the form of well-shaped hexagonal nanoplate, nanoplate-based nanoflower, and nanoplate-based complex via a facile one-step polyol synthesis under atmosphere condition. Morphological transitions were completed by simply regulating the reaction kinetics using different sulfur precursors. These nanocrystals were then used to study their morphology-dependent catalytic performances for hydrogen evolution reaction and photodegradation of dye molecules. Our results indicate that CuS nanoflower composed of hierarchical nanoplates synthesized with thiourea showed the best activity, probably arising from their large specific {0001} facets as well as the hierarchical structure for enhancing light absorption.

Experimental section

Synthesis of CuS nanocrystals

CuS nanoplate-based flower was synthesized by a one-step solvothermal method. Typically, 100 mg of polyvinyl pyrrolidone (PVP, MW ≈ 55 000) and 59.47 mg of thiourea (THU) were dissolved into 6 mL diethylene glycol (DEG) and heated at 150 °C for 10 min in an oil bath under magnetic stirring. Subsequently, 2 mL DEG containing 0.028 mg of copper sulfate (CuSO₄·5H₂O) was added into the previous solution using a pipet. The reaction was then maintained for another 30 min. The product was collected through centrifugation, washed with acetone, ethanol and deionized water. The resultant CuS nanocrystal is denoted as CuS–THU. CuS hexagonal nanoplate, denoted as CuS–S, was prepared by replacing THU with the same molarity of sulfur (S); while for the synthesis of nanoplate-based CuS complex, thioacetamide (TAA) was used instead (denoted as CuS–TAA). If sodium sulfide nonahydrate (Na₂S·9H₂O) was used instead of THU, we could obtain CuS powder photocatalysts in the form of small nanoparticles (denoted as CuS–Na₂S).

Electrochemical measurements

All the electrochemical tests were performed in a three-electrode system at an electrochemical station (CHI 700D). Linear sweep voltammetry with scan rate of 10 mV s⁻¹ was conducted in 20 mL of 0.01 M KOH using Ag/AgCl (in saturated KCl solution) electrode as the reference electrode, Pt as the counter electrode, and the glassy carbon electrode covered with samples as working electrode. The measured potentials versus Ag/AgCl were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation. Before measuring, the electrolyte was purged with N₂ for 30 min to remove the oxygen in the electrolyte. The potential window was between −0.8 and 0 V vs. RHE with a scan rate of 10 mV s⁻¹.

Photocatalytic reaction for the degradation of methylene blue (MB)

Visible-light photocatalytic activities of CuS–S, CuS–TAA, CuS–THU, CuS–Na₂S and commercial P25 TiO₂ were evaluated through the decoloration of MB with an initial dye concentration of 2 × 10⁻⁵ M. Photocatalytic reactor equipped with xenon lamp and a 430 nm cut-off filter was used in this study. In a typical run, 0.03 g of photocatalyst was dispersed in 40 mL of ultrapure water using ultrasonic probe for 30 min. Predetermined amount of organic dye was subsequently added into the catalyst suspension. After that, the suspension was stirred for 30 min in the dark to ensure adsorption/desorption equilibrium before light illumination. During visible light irradiation, aliquots of the reaction suspension were collected per 20 min and centrifuged to remove photocatalyst particles. The concentration of MB substrates was then determined by measuring the light absorbance using a UV-vis spectrophotometer (Hitachi U-4100).

Instrumentations

The crystallite morphologic micrographs of all CuS samples were characterized with the aid of a JEOL JSM-7800F field-emission scanning electron microscopy (FESEM), and an FEI Tecnai F30 transmission electron microscopy (TEM). Elemental mapping over the selected region of the photocatalyst was conducted by an energy-dispersive X-ray spectrometer (EDX) attached to the FEI Tecnai F30 TEM. Chemical states of the CuS samples were obtained on an AXIS Ultra DLD X-ray photoelectron spectroscopy (XPS) developed by Shimadzu/Kratos Analytical, Japan. X-ray diffraction (XRD) measurements were performed on an X'Pert PRO diffractometer with Cu Ka irradiation (λ = 1.5438 Å). All the samples were scanned between 10° and 90° with a step size of 0.033° 2θ. Raman spectroscopy analysis is performed using a laser Raman spectrometer (Lab RAMHR 800, Horiba/Jobin Yvon, France) with a backscattering configuration using Ar⁺ laser (20 mW, 514.532 nm) as excitation source. BET surface areas were determined by a Beckman Coulter SA3100 instrument, which was applying the BET method to the N₂ absorption isotherms measured at liquid nitrogen temperature. The samples were previously outgassed at 393 K for 180 min.

Results and discussion

Characterizations of CuS nanostructures

Fig. 1a shows a representative FESEM image of the as-prepared CuS–THU sample. Clearly, it is characterized by spherical nanoflowers that are constituted by close packed hierarchical nanostructures and with a diameter of about 400 nm. A closer investigation by TEM over an individual nanoflower indicates that the hierarchical nanostructures should be nanoplates with a characteristic angle of about 120° (Fig. 1b). Significantly, the nanoplate oriented at a specific direction was, in fact, not single-crystal based. They are essentially featured in a layer-by-layer fashion with the thickness of each layer around 5 nm. This new style of CuS nanocrystals is definitely different from previous reported ones which also adopted hierarchical nanoflower structures.^7,34 We then tried to reveal the exposed crystal facets of the nanoplate. Fig. 1c presents the HRTEM image over dashed circle area of the nanoflower shown in Fig. 1b. The continuously well-resolved fringes with the same adjacent lattice distance of 3.29 Å correspond well to the (11̄00), (01̄10), and (1̄010) planes, respectively. The view direction therefore should be along the [0001] zone, indicating that the nanoplate is enclosed by {0001} facets. This result is also verified by the select-area electron diffraction (SAED) pattern as viewed from [0001] direction (Fig. 1d). Interestingly, the observed multi-spots of the SAED pattern were also found to sit at certain circles with the same center, which indicated a quasi-single-crystal feature (not simple single crystal) of the nanoplate unit, i.e., composed of layer-by-layer arranged single crystals. To confirm the elemental composition of the nanoflower, the nanocrystal was further examined by EDX mapping. As shown in Fig. 1e and f, S and Cu were homogeneously distributed throughout the nanocrystal and their mole ratio was roughly measured to be 1 : 1.

Fig. 1 (a) FESEM image of CuS–THU. (b) TEM image of a single CuS–THU nanocrystal. (c) HRTEM image of the area (indicated by the dashed circle) shown in (b). (d) The corresponding SAED pattern. (e and f) The elemental mapping images showing the distribution of Cu and S element in the CuS–THU over a single CuS–THU nanocrystal.

One benefit of the synthetic strategy is that we can track the crystal evolution by quenching the reaction at different reaction stages. Fig. 2 shows the FESEM images that details the morphological transition of the nanoflowers during the course of the reaction. Clearly, crystal growth started from the formation of round nanoplates (Fig. 2a, t = 5 min). Some tiny plate structures were also initiated for growth. After the reaction proceeded for 10 min (Fig. 2b, t = 10 min), hierarchical nanoplate structure became more notable. A further growth led to the formation of well-defined plate-based nanoflowers and increment of the crystal size (Fig. 2c, t = 15 min). It seemed that growth was completed after the reaction was proceeded for 20 min (Fig. 2d, t = 20 min) since no growth was found by prolonging the reaction time to 30 min (see Fig. 1a). Particularly, a closer investigation of the morphology and thickness of a single nanoplate structure implies that the layer-by-layer growth occurred throughout the reaction. Furthermore, the UV-vis absorption spectrum of the as-prepared CuS nanocrystal at different reaction stages was also monitored as shown in Fig. S1.† The gradually increased absorption intensity of the spectrum indicates the growth of the crystal and improvement of the hierarchical nanostructure.

Fig. 2 FESEM images of CuS–THU showing the temporal morphology evolution at the different reaction time. (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min. Insets are the corresponding photos of the reaction solution at each stage.

To explain the explicit growth mechanism of CuS nanocrystal, we also checked the impact of sulfur source by replacing THU with S powder and TAA, respectively. Fig. 3 shows the SEM images of the CuS–S and CuS–TAA. It was found that CuS–S was composed of mono-dispersed hexagonal nanoplates with a meaning size of about 180 nm (Fig. 3a and the inset), while CuS–TAA was mainly in the form of disordered nanoplate-based complex (Fig. 3b). Higher magnification image (inset in Fig. 3b) suggested that the nanoplate should possess straight edges and having a size less than 60 nm, different from the round nanoplate. Significantly, the nanoplate units in both CuS–S and CuS–TAA were featured by a single-domain crystalline lattice, without complicating presence of layer-by-layer grain boundaries. The difference by using these sulfur sources can be summarized as a variation of the releasing rate thus the steady concentration of S²⁻ in the reaction suspension. Generally, S needs to be reduced by DEG firstly before forming CuS growth monomers, whereas THU and TAA can directly release S²⁻. Since THU is more stable than TAA,¹⁷ the releasing rate of the three sulfur precursors is therefore roughly in an order of S < THU < TAA. We thus summarize the growth pathways in Fig. 4a. The plausible mechanism responsible for the formation of these CuS nanocrystals can be thus illustrated by a kinetic-controlled growth as shown in Fig. 4b and c. When keeping the steady concentration of CuS growth monomer at a low concentration by using S powder, the number of initially formed nucleus is limited. Growth is thus confined onto a specific direction, leading to the formation of CuS hexagonal nanoplate (Fig. 4b). Moreover, re-nucleation at the {0001} facets of the nascent nanoplate seed to form hierarchical structures, with plate units built together at a certain angle, also is not preferred. On the contrary, if TAA is used, the initial as well as the steady concentration of CuS monomer will be significantly increased. The sudden oversaturation of the CuS monomer leads the generation of a large amount of CuS nuclei. In this case, localized re-nucleation on a nascent plate becomes very prevalent in terms of both number and site, resulting in the formation nanoplate based complex (Fig. 4d). In addition, because of the increased concentration of the seed, the size of the nanoplate in the complex will be relatively small. A similar argument can be applied on the use of THU, which is an intermediate case. The resultant nanocrystals in such case are less complicated, but still have a flower-like structure fabricated by nanoplates (Fig. 4c). Significantly, the reason why the layer-by-layer fashion only occurred on the nanoplates in these nanoflowers is still unclear, but deserves to be resolved in the future work.

Fig. 3 FESEM images of CuS nanocrystals obtained in a standard synthesis except using (a) S, or (b) TAA instead of THU. Scale bars in the insets and images are 50 nm and 200 nm.

Fig. 4 Schematic illustration of (a) the formation of a CuS nanoplate, a CuS nanoflower, and a CuS nanoplate based complex, simply controlled by adjusting S²⁻ release rate and for the proposed growth mechanisms of (b) CuS nanoplates, (c) CuS nanoflower, and (d) CuS nanoplate based complex. Dashed circle in (b–d) represents the interface between the bulk solution and the stagnant solution. Each interface constructs a diffusion sphere. Black arrows indicate the directions of growth monomers' diffusion, while multi-arrows indicate the higher chemical potential.

To better understand the crystalline structure, the as-prepared CuS nanocrystals were analyzed by XRD, Raman, and UV-vis spectra. Fig. 5a shows the XRD patterns of the three CuS nanostructures. They are clearly characterized by identical XRD peaks that match well with a standard hexagonal CuS (reference JCPDS card: #01-078-2391). Generally, single crystals, possessing highly ordered lattice arrangements in all directions, are usually found with sharp XRD diffraction peaks, whereas polycrystals commonly have broadened ones. In our case, despite the larger size of CuS–THU, the quasi-single-crystal feature of the plate unit in its vertical direction (the layer-by-layer fashion) could lead to the reduction of diffraction peaks (peak ratio of CuS–S/CuS–TAA/CuS–THU at 47.89°: 1/0.95/0.44, for instance). The Raman spectra of the three CuS samples with an identical wavenumber at 470 cm⁻¹ should be the typical lattice vibrations of hexagonal CuS (Fig. 5b).³⁵ The peak implies the periodicity of the lattice atoms that are aligned in a certain direction and its intensity relies on the size at this direction.

Fig. 5 (a) XRD patterns, (b) Raman spectra, and (c) UV-vis absorption spectra of the as-prepared CuS nanocrystals.

The as-prepared CuS nanostructures have a good dispersity in deionized water without aggregation. Fig. 5c presents the UV-vis absorption spectra by measuring the aqueous suspensions of the three samples. They showed characteristic absorption behavior of hexagonal CuS. However, slight blue shifts were observed on CuS–S and CuS–TAA, which might be attributed to the quantum-sized thickness of them. The chemical states and compositions of the three samples were further confirmed by the XPS spectra (Fig. 6). They showed typical binding energies for Cu²⁺ (Cu 2p_3/2: 931.7 eV) and (Cu 2p_1/2: 951.5 eV) and S²⁻ (2p_3/2: 161.8 eV) and (2p_1/2: 162.8 eV), which were in good agreement with the literatures.^36–38

Fig. 6 (a) XPS survey of the as-prepared CuS nanocrystal, and the corresponding high-resolution spectra of (b) Cu 2p and (c) S 2p, respectively.

Catalytic performances

The nanoflower was firstly evaluated as catalyst for hydrogen evolution reaction (HER) by using linear sweep voltammetry (LSV) and Tafel. For a comparative study, the hexgonal nanoplates (CuS–S) and nanoplate-based complex were also investigated. The potential was normalized according to the equation: E_RHE = E_measure + 0.71 V + 0.20 V (vs. RHE), where 0.71 V was the shift caused by pH and 0.20 V was a correction due to the use of Ag/AgCl reference electrode. The measurement was conducted in 0.01 M KOH solution with a pH of 12 ± 0.01. Fig. 7a shows the polarization curve of the samples. They are found with a small overpotential (η) of ∼0.1 V for HER, beyond which the cathodic current rises rapidly under more negative potentials. Generally, CuS–THU shows the best HER activity as it presented higher current over the whole potential range and a relatively small overpotential of about 0.601 V. Fig. 7b details the corresponding Tafel plots. We then applied Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope) to fit the linear regions of the three plots. The yielding Tafel slopes of ∼113, ∼169, and ∼298 mV dec⁻¹ for CuS–THU, CuS–S, and CuS–TAA, respectively, also indicate that the highest HER rate could be obtained on the CuS–THU catalyst.

Fig. 7 (a) Polarization curves and (b) Tafel plots for the different CuS samples. The potentiodynamics runs on the photoelectrodes. Steady-state current density–voltage is operated in aqueous 0.1 mol L⁻¹ KOH solution at room temperature.

We next sought to determine whether superiority of the nanoflowers could be extended to photocatalytic MB degradation. In addition to the comparative studies conducted over CuS–S and CuS–TAA photocatalysts, commercial P25 TiO₂ and CuS powder (obtained by simple precipitation method, denoted as CuS–Na₂S) were also employed as reference photocatalysts. The measurements were performed by analyzing the concentration change in water solutions containing a certain amount of photocatalysts under visible-light irradiation (λ ≥ 430 nm) according to the reported method.^39,40 An initial MB concentration of 2 × 10⁻⁵ M was used in all tests. The characteristic UV-vis absorption peak at around 663 nm was used to monitor the degradation rate. It can be seen that negligible decomposition of MB was obtained over CuS–TAA nanoplate complexes (Fig. 8a). When the catalyst was replaced by CuS–S nanoplates, the decomposed MB increased to 23% (Fig. 8b, 120 min). Significantly, the reaction was largely activated by using CuS–THU nanoflowers. An approximate 53% of the dyes are removed from the solution after 2 h illumination (Fig. 8c). The order was in accordance with that of HER activities. This activity is even comparable to the well-known P25 TiO₂ photocatalyst, which removed almost 68% of the initial MB (Fig. 8d, 120 min). The CuS–Na₂S which were characterized by small nanoparticles with a size of about 16 nm (see Fig. S2† for the SEM images), also showed poor activity for MB degradation (about 29% after 120 min, Fig. 8e). The comparative time-course degradation performances of different photocatalysts by plotting C/C_o vs. reaction time were summarized in Fig. 8f. The superiority and pivotal role of the nanoflower structure are clearly presented.

Fig. 8 Temporal UV-vis absorption spectra during MB degradation by using (a) CuS–TAA, or (b) CuS–S, or (c) CuS–THU, or (d) TiO₂, or (e) CuS–Na₂S. (f) The corresponding time-coursed degradation performance by plotting C/C_o vs. reaction time.

In principle, catalysis has long depended on both the transportation of charge carriers and the surface redox reaction. As such, nanocrystals with large surface-to-volume ratios are attractive for use. Therefore, the BET surface areas of CuS nanocrystals prepared with different sulfur sources were carefully measured and summarized in Table 1. Clearly, CuS–THU has a similar specific surface area with that of CuS–S (30.43 vs. 30.88 m² g⁻¹). Although CuS–TAA owns the smallest plate size, the large-scale disordered aggregation to form complex has led the significant reduction of surface area (11.82 m² g⁻¹). Moreover, the hierarchical structure of nanoflowers as well as the ordered surface nanoscale pore structures would favor the mass transportation of reactants.⁴¹ On the other hand, the hierarchical structures can absorb and reflect photons at a more efficient way. These factors, taken together, should contribute to the higher (photo) catalytic activity of CuS–THU.

Table 1 The BET specific surface areas of CuS nanocrystals

Sample	CuS–S	CuS–THU	CuS–TAA
BET surface area (m^2 g^−1)	30.88	30.43	11.82

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Conclusions

In summary, CuS nanocrystals in the form of hexagonal nanoplate, nanoplate based hierarchical nanoflower, and nanoplate based complex were successfully prepared via a robust polyol synthesis. The crystal structures and growth progress were carefully analyzed. The results show that sulfur source that determines the reaction kinetics played the critical role in determining the final morphology and size of CuS nanocrystals. These CuS nanocrystals were then employed to electrocatalyze the hydrogen evolution reaction as well as visible-light-driven photodegradation of MB. The sample synthesized by THU showed best activity for both catalytic reactions compared with other CuS photocatalysts and a comparable activity of the activated P25 photocatalyst. It is suggested that the hierarchical layer-by-layer structure and highest effective BET surface area in CuS–THU could provide more active sites, leading to the improved photocatalytic activity. This method can be potentially extended to controlled synthesis of other metal sulfides.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51502240), the Natural Science Foundation of Jiangsu Province (No. BK20150378), China Postdoctoral Science Foundation (No. 2014M560769), and the China Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17221k

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