Structural evolution from CuS nanoflowers to Cu₉S₅ nanosheets and their applications in environmental pollution removal and photothermal conversion

Abstract

Three-dimensional (3D) CuS nanoflowers and 2D single crystalline Cu₉S₅ nanosheets with irregular hexagonal holes have been successfully synthesized by easily hydrothermally treating a solution of copper dichloride dihydrates and thiourea at 180 °C for several hours without any surfactants and morphology-controlling agents. Irregular hexagonal holes were widely distributed over the Cu₉S₅ nanosheets, which could efficiently enlarge the surface area. The UV-vis-NIR absorption spectroscopy showed an obvious increase of the near-IR absorbance. The influence of the reaction time, morphology, and crystal phase on the photocatalysis and photothermal conversion was discussed. The results revealed that CuS nanoflowers displayed the maximum removal value of rhodamine B of 67%. The Cu₉S₅ nanosheets obtained with the increase reaction time displayed an adsorption and photo-degradation decrease for RhB. The exposure of the aqueous dispersion of the Cu₉S₅ nanosheets prepared with 6 h (2.0 g L⁻¹) under the irradiation of a 980 nm laser can increase its temperature by 25.1 °C in 600 s, exhibiting a good photothermal conversion performance. The photothermal efficacy increases with the samples obtained from 1 h to 18 h.

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Introduction

Colloidal nonstoichiometric copper chalcogenide Cu_2−xY (Y = S, Se, Te and x = 0–1) nanocrystals absorbing radiation in the near-infrared (NIR) region have received increasing attention due to their localized surface plasmon resonances (LSPRs),^1–7 which is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material stimulated by incident light. These plasmonic nanomaterials with the LSPR property show intense light absorption and scattering with potential uses in many fields, ranging from photovoltaics⁸ to bio-imaging.⁹ It is worth noting that those nanoparticles exhibit significant absorption abilities in the NIR window, which make it possible to transform NIR irradiation into thermal energy as a photosensitizer, imparting benign optical energy into tumors for thermal ablation of tumor cells, namely photothermal ablation (PTA) therapy.^10–13 NIR laser-induced PTA has received much attention because of the less absorption of NIR laser by biological tissues, meaning that more NIR laser penetrate deeply into biological tissues. Some materials with unique optical properties, primarily noble metal nanostructures such as Au nanoshells,^14,15 nanorods,^16,17 nanocages,^18,19 hollow nanospheres,^20–22 and Pd nanosheets,²³ but also carbon nanotubes,^24,25 graphene oxide(GO),²⁶ reduced graphene oxide(RGO),²⁷ as well as some organic compounds, including indocyanine green²⁸ and polyaniline nanoparticles,¹⁰ and other oxides nanostructures²⁹ have been investigated as photothermal coupling agents to enhance the efficacy of PTA therapy.¹¹ As a type of important plasmonic semiconductor, copper chalcogenide provides an alternative platform for PTA removal of tumor cells in cancer therapy because of their controllable absorption in the NIR spectrum, low-cost fabrication, and acceptable cytotoxicity.

Another advantage of the plasmonic semiconductor copper chalcogenide (Cu_2−xY) nanocrystals is the tunable optical properties by varying value of x in a wide range between 0 and 1, from the Cu-rich Cu₂S (chalcocite) to CuS (covellite). For example, several polymorphs of nonstoichiometric Cu_2−xS compounds including Cu₂S (chalcocite), Cu_1.94S (djurleite), Cu_1.8S (digenite), Cu_1.75S (anilite) and CuS (covellite) are p-type semiconductors and exhibit a stoichiometry-dependent band gap^30,31 size-, geometry-, and phase-, dependence of LSPR related absorption properties in Cu_2−xY nanocrystals have also been extensively investigated besides composition.^32–37 For example, Burda and his co-workers found that the blue-shift of the absorbance spectra in NIR region occurred with the increase of free carrier concentration in different nonstoichiometric Cu_2−xS crystal phrases.³¹ Zhu group could tune the LSPR frequency of the Cu_1.94S (djurleite) nanocrystals by varying their morphologies and sizes.³⁸ Effect of exposure in air and surface ligand groups on the plasmonic band of the djurleite nanocrystals has also been investigated in Zhu's work.

Semiconductor nanocrystals are widely studied as photocatalysts for hydrogen generation and environmental pollution removal.³⁹ Their catalytic activity depends on the their optical absorption properties, crystallinity, defects and surface area.⁴⁰ Larger optical absorption efficiency, higher crystallinity, smaller photocatalysts are beneficial to the photocatalytic activity due to the increase of photoelectrons and holes, charge separation, and more catalytic sites.⁴¹ Defects always give rise to the chance of charge recombination, which decrease the photocatalytic activity. Photothermal conversion efficiency of plasmonic semiconductors is also influenced by the plasmon resonance wavelength, composite, morphology, surface coating, and crystalline structure.⁴² Thus, it is important to investigate the relationship between properties and structures of materials.

Due to the attractive applications in chemiluminescence,⁴³ PTA therapy, bioimaging and photovoltaics, copper chalcogenide nanocrystals with different shapes and sizes have been prepared via variety of synthetic methods, especially nanostructured copper sulfides,³¹ including nanorods,⁴⁴ nanoribbons,⁴⁵ nanotubes,⁴⁶ nanoflakes,⁴⁷ nanowalls,⁴⁸ nanocages,⁴⁹ nanoflowers,⁵⁰ hollow spheres,⁵¹ and quadrates.⁵² The LSPR behaviors of the plasmonic semiconductor copper chalcogenide nanocrystals have been investigated experimentally and theoretically.^53,54 However, it is still a challenge to understand of structure-to-property relations and control the nanocrystal morphologies, especially the nanostructure with secondary structures. For example, mesoporous anatase–silica nanocomposites with secondary mesopores exhibited the superhigh photocatalytic degradation of rhodamine B, acid red, microcystin-LR compared to counterpart without secondary mesopores.⁵⁵

Herein, we report the fabrication of 2D single crystalline Cu₉S₅ nanosheets with irregular hexagonal secondary holes by a facile hydrothermal route. Chemical compositions and morphologies of the as-obtained nanoparticles have been investigated at different reaction time. Their NIR laser photothermal behavior and photodegradation of organic pollutions under radiation of simulated sunlight are studied. It is interesting founding that nanoflowers of CuS were found at initial stage and then converted into the Cu₉S₅ nanosheets with holes, which exhibit different adsorption and photodecomposition capabilities for rhodamine B (RhB). The reaction time plays a key role in controlling their morphology and crystalline phase. And due to the samples have strong absorption in NIR region, the Cu₉S₅ nanosheets has good photothermal conversion performance as potential photothermal agents.

Results and discussion

3D flower-like covellite CuS nanostructure and rhombohedral phase of Cu₉S₅ digenite nanosheets were obtained via hydrothermal method using copper dichloride dihydrates and thiourea as copper and sulfur resources at 180 °C with the reaction time several hours. The XRD patterns of the as-prepared samples synthesized at various reaction times are shown in Fig. 1A. The typical diffraction peaks of the flower-like samples obtained at 1 hour between 20° and 80° accurately matched those of the standard hexagonal phase of CuS covellite on the JCPDS card (no. 03-1090) with lattice constants of a = b = 3.802 Å and c = 16.43 Å. The additional peak at 2θ = 26.6° labeled by asterisk shows the [100] peak of the standard hexagonal phase of CuCl on the JCPDS card (no. 09-0017). Except the most intense peak of CuCl, no other diffractive peaks of impurities containing copper were found, meaning that a tiny amount CuCl was present as impurity. And all the diffraction peaks of other samples with sheet-like nanostructures at 2θ values of 26.243°, 27.769°, 29.247°, 32.160°, 35.773°, 37.136°, 41.483°, 46.173°, 51.754°, 54.704°, 57.404°, 67.328° and 71.394° in this pattern are identical to (1 0 1) (0 0 15) (1 0 7) (1 0 10) (1 0 13) (0 1 14) (0 1 17) (1 1 0) (1 1 12) (1 1 15) (0 0 30) (2 0 20) and (1 1 27) crystal planes of the standard rhombohedral phase of Cu₉S₅ digenite (JCPDS no. 47-1748) with lattice constants of a = 3.93 Å and c = 48.14 Å. No additional reflections of any impurity were observed. The intense peaks confirmed that all the nanosheets have well crystallized nature.

Fig. 1 (A) XRD patterns of the CuS nanoflowers synthesized at: 1 h and the Cu₉S₅ nanosheets samples synthesized at 3 h, 6 h, 12 h, 18 h. The XRD patterns of hexagonal phase of CuS and rhombohedral phase of Cu₉S₅ are also shown, respectively. Impurity of tiny amount CuCl is labeled by asterisk. (B and C) The diffraction peaks of (0 0 15) crystal planes of the samples obtained at 3 h, 6 h, 12 h, 18 h. And their corresponding Lorentzian curves. (D) N₂ adsorption/desorption isotherm curve and pore size distribution (inset) of the sample synthesized at 3 h.

The instrumental Bragg peak broadening contribution was used for estimation of average size of primary crystalline domains or crystals by Scherrer's equation: τ = (kλ)/(β cos θ), where τ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size, k is the shape factor with a typical value of 0.89, λ is the X-ray wavelength (the value of 1.54 in this work), β is the line broadening full width at half maximum (FWHM) peak height, after subtracting the instrumental line broadening, in radians, and θ is the Bragg angle.^56,57 Fig. 1B shows the diffraction peaks of the (0 0 15) crystal plane of the samples obtained at 3 h, 6 h, 12 h and 18 h, respectively. Rietveld refinement technique is applied to fit a number of well resolved individual peaks of (0 0 15). The peak shapes shown in Fig. 1C are simply approximated to the convolution of Lorentzian functions. The variation of FWHM is clearly observed with the react time prolonged. As shown by an arrow, the values of the FWHM decrease from 0.006044 rad of 3 h, 0.005313 rad of 6 h, 0.005120 rad of 12 h to 0.004259 rad of 18 h, respectively. After the estimation with Scherrer's equation, the average sizes in the [0 0 15] direction are calculated about 22.7 nm, 25.8 nm, 26.8 nm, and 32.2 nm, respectively. Fig. 1D shows the N₂ adsorption/desorption isotherm curve, which could be classified as a type IV isotherm,^58–60 revealing their mesoporous structure characteristics. The Brunauer–Emmett–Teller surface area and total pore volume are 12.03 m² g⁻¹ and 0.07 cm³ g⁻¹, respectively. However, the mesoporous size from the pore size distribution curve are from 3 nm to 165 nm (inset of Fig. 1D), showing their wide pore contribution.

Fig. 2A shows SEM images of the as-prepared CuS nanoflowers synthesized after 1 h, low-magnification scanning electron microscopy images shows that CuS nanoflowers were formed in high yield. Few of nanowires can be obvious among the products of as-obtained flower-like nanostructures. High-magnification SEM image (Fig. 2B) was further investigated, showing that the thickness of nanosheets forming the CuS nanoflowers was about 25 nm. The nanosheets of the flowers-like structure disorderedly grow in different directions, forming rough sphere structure. The nanosheets substituted for nanoflowers were obtained when reaction time increased (Fig. 2C–F). Combined with the analysis results of XRD, the crystal phase conversion was accompanied by the morphological changes. The formed nanosheets were rhombohedral Cu₉S₅ while nanoflowers were hexagonal CuS. It is interesting that the nanosheets were polyporous with many heterogeneous hexagonal holes. Fig. 2C shows the nanosheets obtained after 3 h. The thickness of the nanosheets was around 22 nm. The nanosheets were very likely to be Cu₉S₅. After 6 h, nanosheets were formed as well (Fig. 2D), many holes in the nanosheets were obviously seen, the thickness of the Cu₉S₅ nanosheets can be measured to be about 25 nm. After 12 h, Cu₉S₅ nanosheets with larger thickness about 26 nm emerged (Fig. 2E), a few irregular particles were mixed as impurity in the samples in SEM images. When the reaction time was further elongated to 18 h, pure Cu₉S₅ nanosheets were obtained (Fig. 2F). Many irregular hexagonal holes lie multi-dispersely in the nanosheets with wide contribution, which is in agreement with the BET findings. The thickness of the nanosheets at 18 h can be measured to be about 32 nm. Meanwhile, the size of the holes in nanosheets slightly increases from 3 h to 18 h.

Fig. 2 SEM images of the as-prepared samples synthesized at: (A) 1.0 h, (B) 1.0 h, (C) 3.0 h, (D) 6.0 h, (E) 12.0 h and (F) 18.0 h.

High-magnification scanning electron microscopy images (Fig. 3A and C) of Cu₉S₅ synthesized at 12 h and 18 h and the corresponding transmission electron microscopy images (Fig. 3B and D) were further investigated to estimate the variation of the thickness and pore size of the nanosheets. Despite wide pore contribution, the average pore size of the nanosheets apparently increase with prolonging the reaction time. A similar trend is also found for the thickness of the nanosheets.

Fig. 3 High-magnification SEM images and their corresponding TEM images of the as-prepared Cu₉S₅ samples synthesized at (A and B) 12.0 h and (C and D) 18.0 h.

Fig. 4A shows the representative transmission electron microscopy (TEM) images of the CuS nanostructures obtained after 1 h. Flower-like shapes are obviously found. Apparent contrast of the edges of the nanoflowers indicate that the super-thin flakes compose the nanoflowers. Further structural investigations of the as-obtained CuS nanoflowers were elaborated by high-resolution TEM (HRTEM) image. Fig. 4B shows a lattice plane distance measured to be 3.21 Å, in close proximity to the lattice space of (101) planes of a cubic phase CuS at 3.22 Å. Fig. 4C and D also confirmed the as-obtained Cu₉S₅ nanosheet-like structure at the reaction time of 6 h, in agreement with XRD findings. The HRTEM image of Fig. 4D shows well defined two-dimensional lattice planes with an interplanar spacing about 1.97 Å, which can be assigned to the (110) plane of rhombohedral Cu₉S₅ 1.96 Å. The corresponding SAED image (inset of Fig. 4D) indicating that the nanosheets is a single-crystalline and can be indexed to the [20 1] zone axis of the Cu₉S₅ sheet-like nanostructures.⁶¹

Fig. 4 TEM images of the as-prepared CuS samples synthesized at (A) 1.0 h, (B) HRTEM image of an individual CuS nanoflowers, (C) TEM images of the as-prepared Cu₉S₅ samples synthesized at 6.0 h, (D) HRTEM image of an individual Cu₉S₅ nanosheet, and SAED pattern (inset).

The reactions processes for synthesis of Cu₉S₅ nanosheets in this experiment can be described as follows:

NH₂CSNH₂ + 3H₂O → H₂S + 2NH₄⁺ + CO₃²⁻

H₂S + Cu²⁺ → CuS + 2H⁺

2CuCl₂ + NH₂CSNH₂ + H₂O → 2CuCl + S + 2HCl + NH₂CONH₂

CuS + 8CuCl + 4H₂S → Cu₉S₅ + 8HCl

Hydrogen sulfide was from the decomposition of thiourea at an early stage of reaction, which was combined with the aquated cupric ions to form CuS. Then cupric chlorides could be reduced to cuprous chlorides precipitate by thiourea in acidic solution, which also be determined by XRD patterns in the samples at the reaction early stage of 1 h. So the superthin nanosheets of CuS were initially formed by the aggregation of CuS nanoclusters and trace of CuCl, then assembling into flower-like structures. Finally, Cu₉S₅ nanoparticles were obtained by the determination of value of solubility product constant (K_sp) between the chlorides and sulfides, where chloride ions were replaced by sulfur ions. Cu₉S₅ nanosheets were obtained by solid-state reaction, in which many holes occurred and grew, which probably resulted from the disappearance of CuCl.

As shown in Fig. 5A, the removal performance for RhB over irradiation time shows apparently different adsorption and photodecomposition activities for RhB of all the samples. The amount of RhB adsorbed were figured out by sonicating the solution for 10 min and stirring for 30 min before light on. The photodecompositions for RhB in the course of next 60 min were also shown in Fig. 5A. For clarity, the amount of adsorbed and photocomposed RhB of all the samples were significantly compared by generating histogram statistics in Fig. 5B.

Fig. 5 (A) UV-vis absorbance changes of RhB over irradiation time under simulated sunlight, the photocatalysts was synthesized at controlled reaction time. (B) Comparisons of the adsorption and photo-decomposition of RhB by the photocatalysts: 1 h, 3.0 h, 6.0 h, 12.0 h and 18.0 h.

The amount of adsorbed RhB on the samples synthesized with controlled reaction time from 1 h to 18 h followed the order 18 h < 12 h < 6 h < 1 h, 3 h, indicating that the reaction time and the particle phase influenced the adsorption behaviors for RhB. Among the samples, the maximum adsorption value for the RhB is up to the 50.2% by the nanosheets obtained at 3 h. The Cu₉S₅ nanosheets synthesized at 3 h with inhomogeneous holes revealed better RhB adsorption capability than other reported shapes of copper sulfides,⁶² possibly due to their unique polyporous structure. The amount of photodecomposed RhB on these nanomaterials followed the order 18 h < 12 h < 6 h < 3 h, 1 h. The total amounts of the RhB removed are from 35.7% up to 67% after adsorption and photo-decomposition treatments with the as-obtained samples. The total removal amounts for RhB of the nanoflowers are better than that of nanosheets. The decrease of RhB removal capability of the samples with prolonging the react time is likely due to the increase of the pore size and thickness of nanosheets, which was confirmed by XRD and SEM findings.

Fig. 6A represents the UV-vis-NIR absorption spectrum of as prepared samples in the range of 400–1000 nm, revealing the absorption edge at ca. 640 nm. Specifically, all of the as-obtained samples exhibit an obvious increased absorption in the near-IR widow with the increase of wavelength, similar to the observations of the reported copper sulfides.⁶³ The enhancement of NIR absorbance of the samples is mainly attributed to the localized surface plasmon resonances (LSPRs) because of a relatively high carrier (holes) concentration and a strong free carrier absorption of the p-type semiconductor, which is similar to the previous works of Luther¹ and Zhao.³¹

Fig. 6 (A) UV-vis absorption spectra of the as-prepared samples obtained at: 1.0 h, 3.0 h, 6.0 h, 12.0 h and 18.0 h; (B) the temperature elevation of the aqueous dispersion of Cu₉S₅ with different concentrations (i.e., 0.125, 0.25, 0.5, 1.0 and 2.0 g L⁻¹) as a function of time (0–600 s) under the irradiation of 980 nm laser, and water was used as a control; (C) plot of temperature change over a period of 600 s versus the aqueous dispersion of the Cu₉S₅; (D) the temperature elevation of the aqueous dispersion containing the as-prepared samples obtained at: 1.0 h, 3.0 h, 6.0 h, 12.0 h and 18.0 h, respectively, with the concentration of 1.0 g L⁻¹ as a function of irradiation time of 980 nm laser; (E) temperature variations of the sample synthesized at 6 h under repeated 980 nm laser irradiation for ten cycles.

The samples at reaction time from 6 h to 18 h display similar strong absorption in NIR region of the samples. So the Cu₉S₅ nanosheets prepared with 6 h were chosen as a model photothermal agent to make an investigation of the photothermal conversion performance under irradiation of 980 nm laser in detail. And the light-to-heat conversion efficiencies of all samples were discussed. The as-synthesized Cu₉S₅ nanosheets dispersed into aqueous solution. Compared with pure water as a negative control, the photothermal conversion performance of the Cu₉S₅ nanosheets was examined by detecting the extent of temperature increase of the solution. Fig. 6B shows the temperature increase of the aqueous dispersion of the Cu₉S₅ nanosheets over the irradiation time, and the temperature rose more rapidly with the increase of the concentration of Cu₉S₅ nanosheets in solution. The temperature of Cu₉S₅ aqueous dispersion (e.g. 2 g L⁻¹) could be elevated by 25.1 °C with the irritation of 980 nm laser (11.7 w cm⁻²) for 600 s. However, pure water was slightly increased by less than 2.3 °C under the same condition. The above findings confirmed the Cu₉S₅ can rapidly and efficiently convert the NIR-laser energy into thermal energy, elevating the temperature of whole solution. Moreover, with the increase of the concentration (i.e., from 0.125 to 0.25, 0.5, 1.0 and 2.0 g L⁻¹), the temperature of the Cu₉S₅ aqueous dispersion could be increased by 4.1, 6, 11.2, 16 and 25.1 °C, respectively (Fig. 6C). Fig. 6D shows the temperature elevation of the aqueous dispersion on these nanomaterials synthesized with controlled reaction time from 1 h to 18 h followed the order 1 h < 3 h < 6 h < 12 h, 18 h, indicating that the reaction time, and the particle phase influenced the photothermal conversion performance. In consideration of the shape change of the nanosheets with the reaction time, the increase of pore size and thickness of samples positively influence on photothermal conversion performance. Besides, whether the repeated irradiation could affect the efficiency of the photothermal agent or not should be assessed.^{13,60,64–66} The sample at 6 h with the concentration of 1 g L⁻¹ were chosen to test their stability and repeatability. In each of ten heating cycles, NIR light irradiation was applied for 10 min followed by a 20 min cooling period to reach room temperature naturally. Fig. 6E reveals that the temperature could raise to 16 °C with slightly fluctuate in ten cycles of repeating laser on–off, indicating that the influence of the repeated irradiation on the photothermal efficiency of the Cu₉S₅ nanosheets is little significant, indicating an excellent photothermal stability of Cu₉S₅ nanosheets. As is known to all, hyperthermic therapy revolving intracellular heat stress in the temperature range of 41–44 °C damages cancer cells.^29,67 Assuming that the in vivo temperature of a healthy human body is 37 °C, tumor region can easily be heated up to 44 °C within 600 s by irradiation with NIR wavenumber laser when it was moderately injected with an aqueous Cu₉S₅ solution, which could efficiently induce cancer cells death.

Experimental

Materials

All chemicals such as copper chloride dihydrate (CuCl₂·2H₂O), thiourea were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used without further purification. Deionized water was purified with a Milli Q system (Millipore).

Synthesis

0.10 g of CuCl₂·2H₂O and 0.02 g of thiourea were mixed in a Teflon-lined 25 mL capacity autoclave. The autoclave was kept in oven at 180 °C for several hours. The resulting suspension was collected by centrifugation at 4500 rpm for 30 min and washed with deionized water and alcohol several times, respectively, then dried in vacuum.

Characterization

The XRD was performed using a X'Pert Pro diffractometer operated at 40 kV voltage and 40 mA current with Cu Kα irradiation (λ = 1.5418 Å). The SEM images were obtained on a JSM-6700F scanning electron microscope. The samples dispersed in water were dropped onto an aluminium sample stage and naturally dried. The TEM, HRTEM and SAED were recorded by a JEOL JEM-2100F transmission electron microscope. A small amount of sample dispersed in ethanol was dropped onto a thin holey carbon film, and dried for 30 min before TEM measurement. The UV-vis-NIR absorption spectra were measured on Lambda 35 spectrophotometer within the range from 1100 nm to 400 nm. The specific surface area and pore volume of the sample was measured using a Micromeritics ASAP 2020 M apparatus.

Photocatalytic pollution removal

The photocatalytic activities of the as-obtained samples were determined by measuring the photocatalytic decomposition processes of rhodamine B (RhB), which is usually used as model organic pollution. Decomposition process for RhB: 4.0 mg of photocatalysts was dispersed into 10.0 mL rhodamine B solution with optical density of 1.0 (O.D. = 1.0). In order to compare the difference between adsorption and photodecomposition, the mixture was sonicated for 10 min and stirred for 20 min before lighting on to estimate the contribution of the RhB adsorption on the samples' surface. A 150 watt Xe lamp (81 094, Newport) with an AM 1.5 air mass filter as a solar simulator was used to decompose the model pollution.

Measurement of photothermal performance

For measuring temperature change due to the photothermal conversion of as-synthesized samples, 980 nm NIR laser light was delivered through a 2 mm short pathlength quartz cuvette containing aqueous dispersion (0.5 mL) with different samples (i.e., 1, 3, 6, 12 and 18 h) and the samples of 6 h with various concentrations (i.e., 0.125, 0.25, 0.5, 1 and 2 g L⁻¹). 980 nm diode laser with an external adjustable power (0–4 W) and 5 mm diameter laser module (Xi'an Tours Radium Hirsh Laser Technology Co., Ltd China) was used as light source. The output power density was independently calibrated to be 11.7 W cm⁻² using a hand-held optical power meter (VLP-2000, Ranbond Technology (HK) Co., Ltd China). A thermocouple with an accuracy of ±0.1 °C was inserted perpendicularly to the laser beam into the aqueous dispersion of samples. The temperature was recorded one time in the first minute per 10 s, the second minute per 20 s, then in the next 8 min per 30 s.

Conclusions

In summary, we have elucidated a facile hydrothermal route to synthesize Cu₉S₅ nanosheets. At first stage, we synthesized three-dimensional (3D) CuS nanoflowers, and with the time prolonged, we synthesized 2D single crystalline Cu₉S₅ nanosheets with secondary irregular hexagonal holes at 180 °C for several hours. The holes in the as-obtained Cu₉S₅ nanosheets had wide distribution. The UV-vis spectrum showed that the increased absorbance in NIR region due to LSPR. The CuS nanoflowers displayed the maximum removal value of the RhB by 67%. The Cu₉S₅ nanosheets at the reaction time of 6 h displayed good photothermal conversion performance, with their temperature increment by 25.1 °C in 600 s with its concentration up to 2.0 g L⁻¹ under the radiation of 980 nm laser with power density of 11.7 W cm⁻². That is, the results showed that reaction time, morphology and crystal phase intensely affected their performance. The initial products with higher specific surface showed the better removal for organic pollution. The samples with prolonging the react time displayed good light-to-heat conversion.

Acknowledgements

Financial supports from State Key Development Program for Basic Research of China (2014CB643306), National Natural Science Foundation of China (21071096, 21141007), and NSF of Shanghai (15ZR1420500) are gratefully acknowledged. Y. Z. thanks Key National Research Project (2016YFB0300700) for its financial support.

Notes and references

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