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Mechanochemically synthesized sub-5 nm sized CuS quantum dots with high visible-light-driven photocatalytic activity
Accepted Manuscript Title: Mechanochemically synthesized sub-5 nm sized CuS quantum dots with high visible-light-driven photocatalytic activity Author: Shun Li Zhen-Hua Ge Bo-Ping Zhang Yao Yao Huan-Chun Wang Jing Yang Yan Li Chao Gao Yuan-Hua Lin PII: DOI: Reference:
S0169-4332(16)31042-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.05.034 APSUSC 33227
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-2-2016 5-5-2016 7-5-2016
Please cite this article as: Shun Li, Zhen-Hua Ge, Bo-Ping Zhang, Yao Yao, Huan-Chun Wang, Jing Yang, Yan Li, Chao Gao, Yuan-Hua Lin, Mechanochemically synthesized sub-5nm sized CuS quantum dots with high visible-light-driven photocatalytic activity, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.05.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanochemically synthesized sub-5 nm sized CuS quantum dots with high visible-light-driven photocatalytic activity
Shun Lia‡, Zhen-Hua Gea‡, Bo-Ping Zhanga*[email protected], Yao Yaoa, Huan-Chun Wangb, Jing Yanga, Yan Lia, Chao Gaoa, Yuan-Hua Linb
a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
100083, China b
School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
*Corresponding Author.
‡
S. L. and Z. G. equally contributed to this work.
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Graphical Abstract
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Highlights • CuS quantum dots (<5 nm) were synthesized by mechanochemical ball milling. • Defects was observed in the CuS quantum dots. • They show good visible light photocatalytic activity as Fenton-like reagents.
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Abstract We report a simple mechanochemical ball milling method for synthesizing monodisperse CuS quantum dots (QDs) with sizes as small as sub-5 nm. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. The CuS QDs exhibited excellent visible-light-driven photocatalytic activity and stability for degradation of Rodanmine B aqueous solution as Fenton-like reagents. Our study opens the opportunity to low-cost and facile synthesis of QDs in large scale for future industrial applications.
Keywords: Copper sulfide; Ball milling; Nanostructures; Photocatalysis;
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1. Introduction Semiconductor nanocrystals or quantum dots (QDs) have been the subject of great interest owing to their unique size-tunable functional properties determined by quantum confinement effects, which make them remarkably promising in a wide range of applications such as biotechnology and photovoltaics [1, 2]. It is crucially important to synthesize monodisperse QDs (size variation < 10%) as their properties depend strongly on the dimensions of these nanocrystals [3, 4]. The synthesis processes for monodisperse nanoparticles that have been known until now are mostly based on solution-based chemical routes. Scientific and technological challenges coupled with environmental considerations highly urge the development of robust, green and economical mass-production methods for advanced functional nanocrystals or QDs with desirable properties. However, the synthesis of monodisperse nanocrystals or QDs with size less than 5 nm still remains a great challenge through alternative techniques. Copper chalcogenides based on earth-abundant transition metals have attracted considerable attention in recent years for diverse applications due to their unique optical and electrical properties as well as good environmental compatibility and low toxicity [5, 6]. As a well-known p-type semiconductor, CuS is one of the most intensively studied chalcogenides as promising candidates in many fields such as Li-ion batteries [7], nonlinear optics [8] and so on. Especially, the suitable band gap (~2.2 eV), together with its strong localized surface plasmon resonance (LSPR) effect, make CuS potentially ideal as low-cost light-harvesting and charge transport material for photovoltaics [9-12] and photocatalysis [13-16]. Solution synthesis of CuS nanostructures has been described in several articles including hydrothermal approach [17], in situ template-controlled method [13], and microwave synthesis [18], etc. The above methods suffer from many drawbacks such as complex reaction process and low yield. By comparison, ball-milling as a mechanochemistry method is a very simple, highly efficient and energy-saving technology for fabricating 5
various families of composite materials (e.g. oxides and metallic materials) in large scale [19, 20]. The advantages of this technique include decreased activation energy of reaction, enhanced materials activity, and improved homogeneity of obtained particles. Especially, mechanical energy of ball milling can break the chemical bonds in solid materials and produce dangling bonds, which are very active for chemical reactions [21]. Until now, a great number of important nanomaterials such as polycrystalline Si [22], ZnS [23], CdS [24], CdSe [25], ZnO [26], and N-doped carbon [21] have been fabricated through this method. Herein, we demonstrate a simple mechanochemical approach towards the synthesis of sub-5 nm sized CuS QDs with large scale (as much as 10 g could be produced in a single reaction). To the best of our knowledge, this is the first report of the formation of monodisperse QDs with good dispersibility using this technique. Our results indicate that the synthesis and mass-production for colloidal nanocrystals or QDs is far less delicate than it has been thought to be in certain material systems. In addition, the photocatalytic performance of the CuS QDs was evaluated by degradation of Rhodamine B (RhB) solution under visible light. 2. Experimental 2.1 Synthesis of CuS quantum dots The CuS powders were fabricated starting with commercially purity powders of Cu (99.9%) and S (99.9%) under 200 mesh as raw materials. The preparation procedure is schematically illustrated in Fig. 1. The mixed powders of 1 mol Cu and 1 mol S were ball-milled at 425 rpm for different time (0.5 to 2 h) in a mixture atmosphere of high-purity argon (95%) and hydrogen (5%) gases using a planetary ball mill (QM-1SP2, Nanjing University, China). Stainless steel vessels and balls were used, and the weight ratio of the ball to powder was kept at 20:1. The black product was finally obtained without any wet-milling process. 2.2 Characterizations The crystal structure of the products was examined by X-ray diffraction (XRD, Bruker D8, Germany) 6
with a Cu Kα radiation. The morphology and element composition of the samples were analyzed by field emission scanning electron microscope (FESEM, SUPRATM 55) equipped with an energy dispersive X-ray (EDX) spectroscopy. The microstructure and morphology were further characterized using transmission electron microscopy (TEM, Phililp Tecnai F20) and atomic force microscopy (AFM, Agilent 5500). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo XPS ESCALAB 250Xi instrument with an Al Kα (1486.8 eV) X-ray source. The UV-vis absorption spectra of the samples were measured by a UV-visible spectrophotometer (Hitachi U-3010) dispersed in water. Nitrogen adsorption-desorption measurements were performed on a Quantachrome AUTOSORB-1C instrument at room temperature, and the surface area was calculated using the Brunauer, Emmett and Teller (BET) method. 2.3 Photocatalytic measurements The photocatalytic activity was evaluated by degradation of RhB aqueous solution under visible-light irradiation using a 500 W Xe lamp (Beijing institute of electrical light sources, China) with a UV cutoff filter (λ > 400 nm). 0.1 g CuS powders were dispersed in 250 mL RhB solution (10 mg/L) and 2 mL H2O2 (30 wt%). Prior to irradiation, the mixed solution was stirred for 1 h under dark to establish the adsorption/desorption equilibrium between the dye and the photocatalyst. At a given time interval after irradiation, the dye concentration was monitored by measuring the maximum UV-vis absorbance at 553 nm for RhB. The degradation efficiency was evaluated by C/C0, where C0 is the initial concentration of the RhB solution, and C is the concentration at a given time. The reaction temperature was kept at room temperature with cooling water to prevent any thermal catalytic effect. 3. Results and discussion X-ray diffraction (XRD) patterns of the samples synthesized with different reaction time are shown in Fig. 2. Cu and S phases from raw powders were still residual as milling for 0.5 h besides the main CuS 7
phase because of the incomplete reaction. When prolonging reaction time to 1 h, the diffraction peaks belonging to both Cu and S phases reduced. For the sample with ball milling for 2 h, all the diffraction peaks can be perfectly indexed to hexagonal CuS (PDF#06-0464), indicating the formation of single phase CuS. The broadening of the diffraction peaks with time implies refined grain size. Moreover, for better understanding of the crystal structure, we compared the XRD patterns of CuS powders (ball milling for 2 h) with that of CuS nanoplates (NPs) synthesized by solvothermal method according to our published work [27], as shown in Fig. S1 in the supporting information. The diffraction peaks of the CuS NPs are almost at the same position with the standard PDF card (PDF#06-0464), while that of the CuS sample prepared by ball milling shifts to higher diffraction angles. The shrinking of lattice parameters is possibly due to the existence of copper vacancy that typically occur associated with CuS, which usually leads to a lot of free holes in the valance band that can support the plasmon resonance [28, 29]. According to reported works, Cu2S nanocrystals could be synthesized at a reductive atmosphere as well [30, 31]. We added its standard XRD patterns (PDF#84-1770) in Fig. 2 for comparison, and no peaks belonging to Cu2S phase was detected, which rule out the possibility of the formation of Cu2S. The morphology of the single phase CuS powders after ball milling for 2 h was further characterized by electron microscopy. The scanning electron microscope (SEM) image (Fig. S2 in the supporting information) reveals that the CuS powders are consists of ultrafine particles. The atomic ratio of Cu to S was determined to be ca. 1.03:1 by energy dispersive spectroscopy (EDS), in agreement with their stoichiometric ratio in CuS. A representative transmission electron microscopy (TEM) image (Fig. 3a) of the CuS powders clearly displays monodisperse nanocrytals with uniform size (< 5 nm) and an average diameter of 2.9 nm, indicating that CuS quantum dots (the sample is abbreviated as CuS QDs hereafter) have been successfully synthesized. From the high resolution TEM (HRTEM) image (Fig. 3b), the fringe spacing was determined to be ca. 0.19 nm (d = 0.186 nm calculated by XRD patterns), corresponding well to the (110) interplanar spacing (0.189 8
nm) of hexagonal CuS. Additionally, it can be clearly seen that the CuS QDs possess dislocations and planar defects such as stacking faults and twin boundaries. Similar types of defect were also observed in CuS nanocrystals prepared by solution-based chemical synthesis method [8]. Atomic force microscopy (AFM) was also employed to characterize the morphology of the CuS QDs dispersed on Si substrate (Fig. S3 in the supporting information), showing that the QDs are 1-5 nm in width and 0.5-3 nm in height. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical binding states of the as-prepared CuS QDs, as shown in Fig. 4. Two strong peaks at 932.3 and 952.2 eV were observed in the XPS spectrum of Cu 2p (Fig. 4a), consistent with the reported values of binding energies of Cu+ 2p3/2 and 2p1/2, respectively [32, 33]. Additionally, on the sides of Cu+ 2p3/2 and 2p1/2, two low-intensity components (935.2 and 955.1 eV) appeared, which can be assigned to the Cu2+ oxidation state [28, 34]. Meanwhile, the satellite peaks further confirms the existence of Cu2+ vacancy [13, 35], in agreement with the XRD analysis. The deconvolution of the S 2p spectrum (Fig. 4b) yields two doublets, corresponding to typical values for sulfide (S2−, 162.2 eV and 163.5 eV) and disulfide ((S2)2−, 163.6 eV and 164.7 eV) respectively, which is consistent with previous results on CuS nanodisks [28]. Although numerous micro/nano-materials have been fabricated by ball milling, the basic understanding of the formation mechanisms in nonmetallic compounds (e.g. oxides or sulfides) appears to be less advanced than it is for metallic materials, essentially because of the overall complexity of their structures, surfaces, defects and mechanical behavior. We consider that the formation of the CuS QDs within short time in the present study might be related to several possible reasons. First, the use of Cu and S powders with comparatively low hardness as raw materials allow them to be crushed with ease during the milling process. Second, thermodynamically, the energy needed for the formation of CuS is quite small since ∆G (–56.71 kJ/mol) and ∆H (–56.32 kJ/mol) are very close (373 K), thus allowing the reaction readily to occur with traditionally low energy ball milling. Moreover, the good lubricity of CuS [36], to some extent, may prevent 9
the conglutination of the nanoparticles during the ball milling process. Under the function together with above-mentioned factors, the CuS QDs were finally formed. It is worth noting that different from previous reports of mechanochemically synthesized nanoparticles, which are mostly around several tens to hundreds nanometers and are usually seriously aggregated, the as-synthesized ultrasmall CuS nanoparticles herein have uniform size and could be well dispersed in water or ethanol for several hours (for maintaining long-time stability, capping agents are still needed), showing the great advantage of these nanocrystals. Further investigations are still needed to better understand the formation mechanism of the present monodisperse CuS QDs as well as other material systems that worth to be explored. Fig. 5 shows the UV-vis absorption spectrum of the CuS QDs, together with that of CuS NPs with lateral size about a few hundred nanometers (SEM image is shown in Fig. S4 in the supporting information). Both of these two samples exhibit similar absorption feature. The photoabsorption spectrum of the CuS QDs is quite similar with that of previous reported CuS nanocrystal/quantum dot counterparts with comparable size obtained by solution method [37]. The first band transitions of these two samples occur below 500 nm (yellow dash line), arising from the 1Sh-1Se excitonic transition in semiconductor nanoparticles [8]. The absorption edge for the CuS QDs (363 nm) strongly blue-shifted compared to that of the NPs (481 nm), which may result from the quantum confinement effects that was predicted to occur in the sub-10 nm regime [17, 38]. The second broad absorption shoulder (black dash line) occurred at ca. 663 nm (CuS QDs) and 545 nm (CuS NPs), consistent with previous reports [17, 39, 40]. From the absorption edges (663 and 545 nm), the corresponding band gaps of these two samples were calculated to be 1.87 eV and 2.27 eV for the QDs and NPs, respectively. The band gap narrowing of the QDs should be owing to the presence of a large number of defects in and around the band gap of the CuS QDs, which has been also observed by Mary et al. in CuS QDs synthesized via solution based technique [8]. The absorption beyond 700 nm is characteristic of LSPR effect, which could be ascribed to free holes inter band transitions from valence states to the 10
unoccupied states [29]. As a proof of concept application of these intriguing CuS QDs, their photocatalytic activity was evaluated by degradation of RhB solution under visible light irradiation, as shown in Fig. 6. H2O2 was added before starting the photocatalytic experiment. Negligible degradation was found in the blank test with only H2O2, demonstrating that RhB is stable under illumination without photocatalyst. With CuS QDs alone, less than 60% of the RhB solution was degraded in 30 min. While the CuS QDs exhibited significantly higher photocatalytic efficiency with the assistant of H2O2; the C/C0 value decayed quickly to about 5% in 30 min, indicating an almost complete degradation of the RhB aqueous solution. We have also compared the photocatalytic performance of the CuS QDs and NPs. Obviously, the QDs sample exhibited much more superior photoactivity compared to that of the CuS NPs, which only degraded 40% under the same experimental condition. Moreover, the photocatalytic degradation rate for RhB solution of the CuS QDs is also higher than reported works such as CuS NPs [37, 41, 42], chainlike hierarchical structures [43], and hollow nanospheres [44] under similar conditions. To understand the effect of specific surface area on the photocatalytic activity, N2 adsorption-desorption isotherms of the CuS QDs and NPs samples were investigated, as shown in Fig. S5 in the supporting information. The calculated BET surface area of the QDs is as high as ca. 90.0 m2g−1, which is much higher than that of the NPs (13.1 m2g−1). Therefore, the remarkable improvement of photocatalytic activity of the CuS QDs could be attributed to the monodispersity and small crystal size that resulted in higher specific surface area with more reactive sites, shorter migration distance that reduces the recombination rate of photogenerated electron/hole pairs [45, 46]. In addition to the photocatalytic activity, the stability of photocatalysts is also one of the most concerned issues for their practical applications. Thus, we carried out cycling runs of photocatalytic experiment with the CuS QDs (Fig. 7). No apparent inactivation was observed in photodegradation activity after three consecutive cycles. The degradation rate of RhB after three runs is still above 90%, indicating good stability and recyclability of 11
the CuS QDs catalyst. Several works have been demonstrated the photocatalytic degradation of dye molecules with various CuS materials [13, 14, 47-51]. Based on our experimental results, the photocatalytic degradation mechanism of the RhB solution in the present CuS QDs could be explained as follows. When the CuS QDs is irradiated by light, electrons could be excited from the valence band (VB) to the conduction band (CB), leaving the corresponding holes in the VB. Organic dye molecules such as RhB could be decomposed to intermediates or mineralized products through an oxidation reaction by the formed hydroxyl radical (•OH) or superoxide anion (•O2 ) species, or directly by the holes accumulated in the VB. Once adding H2O2 into the aqueous -
solution, the photochemical process became complicated. H2O2 was considered to be a good electron acceptor by numerous studies, which could be converted to •OH by accepting electrons [51-53]. In addition, since the XPS results confirm the existence of cuprous ion, Cu+ in the CuS QDs may also react with H2O2 and generate more •OH species, leading to the degradation of RhB solution by the photo-Fenton reaction [52, 54]: H2O2 + Cu+ → Cu2+ + OH + •OH. Moreover, Cu+ can be regenerated by the photochemical effect and -
further producing •OH radicals continuously [55], and thus contributing to oxidize the RhB to the corresponding cationic radicals with the CuS QDs. On the other hand, it is worth mentioning that the quantum confinement effects of the CuS QDs should also play an important role on their drastically enhanced photocatalytic activity. Based on transient absorption spectroscopic characterizations, ultrafast dissociation of excitons in many semiconductor QDs/dye molecule complexes have been demonstrated such as CdS/RhB [56], Cd3P2/RhB [57], CdSe/Re-bipyridyl [58], and InP/methyl viologen [59] systems. According to these reports, quantum confinement could increase the reducing and oxidizing powers of conduction band electrons and valence band holes, respectively. On the other hand, quantum effect can also lead to strong electronic coupling between QD and dye molecules (RhB in our case) that leads to ultrafast charge transfer from QD to the dye. 12
Spectroscopic studies are needed to verify the ultrafast charge transfer processes in such systems. Further works are underway for size and chemical composition tunable copper sulfide nanocrystals by ball milling, as well as their utilization as visible-sensitizers in photovoltaic or photoelectrochemical cells. 4. Conclusion In summary, we described a facile mechanochemical route for fabricating uniform sized CuS QDs (< 5 nm), which is low-cost, and easy to scale up. Microstructural characterization revealed the existence of dislocations and planar defects such as twins and stacking faults in the CuS QDs. We further demonstrated the distinctive advantages of such QDs with high specific surface area for efficient degradation of organic pollutants as photocatalysts under visible light irradiation, indicating that they are promising to be applied in dye wastewater treatment as Fenton-like reagents.
Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 51272023 and 51472026), and the Doctoral Program of Higher Education (Grant No. 20130006110006).
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Figures and Captions
Fig. 1 Schematic illustration of the synthesis procedure of CuS QDs as photocatalysts by ball milling approach.
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Fig. 2 XRD patterns of powder samples synthesized with different reaction time.
Fig. 3 (a) TEM and (b) HRTEM images of the CuS QDs. (c) The size distribution histogram of the CuS QDs.
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Fig. 4 The XPS spectra of the CuS QDs (a) Cu 2p and (b) S 2p.
Fig. 5 UV-vis absorption spectra of the CuS QDs and NPs (inset shows the photograph of the CuS QDs dispersed in water). 22
Fig. 6 UV-vis absorption spectra of RhB solution with (a) H2O2 alone, (b) CuS QDs and (c) CuS QDs and H2O2 at different irradiation time, and (d) corresponding degradation rate of (a-c) and that of the CuS NPs sample under visible light illumination.
Fig. 7 Repeated photocatalytic test of RhB over the recycled CuS QDs.
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S0169-4332(16)31042-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.05.034 APSUSC 33227
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-2-2016 5-5-2016 7-5-2016
Please cite this article as: Shun Li, Zhen-Hua Ge, Bo-Ping Zhang, Yao Yao, Huan-Chun Wang, Jing Yang, Yan Li, Chao Gao, Yuan-Hua Lin, Mechanochemically synthesized sub-5nm sized CuS quantum dots with high visible-light-driven photocatalytic activity, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.05.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanochemically synthesized sub-5 nm sized CuS quantum dots with high visible-light-driven photocatalytic activity
Shun Lia‡, Zhen-Hua Gea‡, Bo-Ping Zhanga*[email protected], Yao Yaoa, Huan-Chun Wangb, Jing Yanga, Yan Lia, Chao Gaoa, Yuan-Hua Linb
a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
100083, China b
School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
*Corresponding Author.
‡
S. L. and Z. G. equally contributed to this work.
1
Graphical Abstract
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Highlights • CuS quantum dots (<5 nm) were synthesized by mechanochemical ball milling. • Defects was observed in the CuS quantum dots. • They show good visible light photocatalytic activity as Fenton-like reagents.
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Abstract We report a simple mechanochemical ball milling method for synthesizing monodisperse CuS quantum dots (QDs) with sizes as small as sub-5 nm. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. The CuS QDs exhibited excellent visible-light-driven photocatalytic activity and stability for degradation of Rodanmine B aqueous solution as Fenton-like reagents. Our study opens the opportunity to low-cost and facile synthesis of QDs in large scale for future industrial applications.
Keywords: Copper sulfide; Ball milling; Nanostructures; Photocatalysis;
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1. Introduction Semiconductor nanocrystals or quantum dots (QDs) have been the subject of great interest owing to their unique size-tunable functional properties determined by quantum confinement effects, which make them remarkably promising in a wide range of applications such as biotechnology and photovoltaics [1, 2]. It is crucially important to synthesize monodisperse QDs (size variation < 10%) as their properties depend strongly on the dimensions of these nanocrystals [3, 4]. The synthesis processes for monodisperse nanoparticles that have been known until now are mostly based on solution-based chemical routes. Scientific and technological challenges coupled with environmental considerations highly urge the development of robust, green and economical mass-production methods for advanced functional nanocrystals or QDs with desirable properties. However, the synthesis of monodisperse nanocrystals or QDs with size less than 5 nm still remains a great challenge through alternative techniques. Copper chalcogenides based on earth-abundant transition metals have attracted considerable attention in recent years for diverse applications due to their unique optical and electrical properties as well as good environmental compatibility and low toxicity [5, 6]. As a well-known p-type semiconductor, CuS is one of the most intensively studied chalcogenides as promising candidates in many fields such as Li-ion batteries [7], nonlinear optics [8] and so on. Especially, the suitable band gap (~2.2 eV), together with its strong localized surface plasmon resonance (LSPR) effect, make CuS potentially ideal as low-cost light-harvesting and charge transport material for photovoltaics [9-12] and photocatalysis [13-16]. Solution synthesis of CuS nanostructures has been described in several articles including hydrothermal approach [17], in situ template-controlled method [13], and microwave synthesis [18], etc. The above methods suffer from many drawbacks such as complex reaction process and low yield. By comparison, ball-milling as a mechanochemistry method is a very simple, highly efficient and energy-saving technology for fabricating 5
various families of composite materials (e.g. oxides and metallic materials) in large scale [19, 20]. The advantages of this technique include decreased activation energy of reaction, enhanced materials activity, and improved homogeneity of obtained particles. Especially, mechanical energy of ball milling can break the chemical bonds in solid materials and produce dangling bonds, which are very active for chemical reactions [21]. Until now, a great number of important nanomaterials such as polycrystalline Si [22], ZnS [23], CdS [24], CdSe [25], ZnO [26], and N-doped carbon [21] have been fabricated through this method. Herein, we demonstrate a simple mechanochemical approach towards the synthesis of sub-5 nm sized CuS QDs with large scale (as much as 10 g could be produced in a single reaction). To the best of our knowledge, this is the first report of the formation of monodisperse QDs with good dispersibility using this technique. Our results indicate that the synthesis and mass-production for colloidal nanocrystals or QDs is far less delicate than it has been thought to be in certain material systems. In addition, the photocatalytic performance of the CuS QDs was evaluated by degradation of Rhodamine B (RhB) solution under visible light. 2. Experimental 2.1 Synthesis of CuS quantum dots The CuS powders were fabricated starting with commercially purity powders of Cu (99.9%) and S (99.9%) under 200 mesh as raw materials. The preparation procedure is schematically illustrated in Fig. 1. The mixed powders of 1 mol Cu and 1 mol S were ball-milled at 425 rpm for different time (0.5 to 2 h) in a mixture atmosphere of high-purity argon (95%) and hydrogen (5%) gases using a planetary ball mill (QM-1SP2, Nanjing University, China). Stainless steel vessels and balls were used, and the weight ratio of the ball to powder was kept at 20:1. The black product was finally obtained without any wet-milling process. 2.2 Characterizations The crystal structure of the products was examined by X-ray diffraction (XRD, Bruker D8, Germany) 6
with a Cu Kα radiation. The morphology and element composition of the samples were analyzed by field emission scanning electron microscope (FESEM, SUPRATM 55) equipped with an energy dispersive X-ray (EDX) spectroscopy. The microstructure and morphology were further characterized using transmission electron microscopy (TEM, Phililp Tecnai F20) and atomic force microscopy (AFM, Agilent 5500). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo XPS ESCALAB 250Xi instrument with an Al Kα (1486.8 eV) X-ray source. The UV-vis absorption spectra of the samples were measured by a UV-visible spectrophotometer (Hitachi U-3010) dispersed in water. Nitrogen adsorption-desorption measurements were performed on a Quantachrome AUTOSORB-1C instrument at room temperature, and the surface area was calculated using the Brunauer, Emmett and Teller (BET) method. 2.3 Photocatalytic measurements The photocatalytic activity was evaluated by degradation of RhB aqueous solution under visible-light irradiation using a 500 W Xe lamp (Beijing institute of electrical light sources, China) with a UV cutoff filter (λ > 400 nm). 0.1 g CuS powders were dispersed in 250 mL RhB solution (10 mg/L) and 2 mL H2O2 (30 wt%). Prior to irradiation, the mixed solution was stirred for 1 h under dark to establish the adsorption/desorption equilibrium between the dye and the photocatalyst. At a given time interval after irradiation, the dye concentration was monitored by measuring the maximum UV-vis absorbance at 553 nm for RhB. The degradation efficiency was evaluated by C/C0, where C0 is the initial concentration of the RhB solution, and C is the concentration at a given time. The reaction temperature was kept at room temperature with cooling water to prevent any thermal catalytic effect. 3. Results and discussion X-ray diffraction (XRD) patterns of the samples synthesized with different reaction time are shown in Fig. 2. Cu and S phases from raw powders were still residual as milling for 0.5 h besides the main CuS 7
phase because of the incomplete reaction. When prolonging reaction time to 1 h, the diffraction peaks belonging to both Cu and S phases reduced. For the sample with ball milling for 2 h, all the diffraction peaks can be perfectly indexed to hexagonal CuS (PDF#06-0464), indicating the formation of single phase CuS. The broadening of the diffraction peaks with time implies refined grain size. Moreover, for better understanding of the crystal structure, we compared the XRD patterns of CuS powders (ball milling for 2 h) with that of CuS nanoplates (NPs) synthesized by solvothermal method according to our published work [27], as shown in Fig. S1 in the supporting information. The diffraction peaks of the CuS NPs are almost at the same position with the standard PDF card (PDF#06-0464), while that of the CuS sample prepared by ball milling shifts to higher diffraction angles. The shrinking of lattice parameters is possibly due to the existence of copper vacancy that typically occur associated with CuS, which usually leads to a lot of free holes in the valance band that can support the plasmon resonance [28, 29]. According to reported works, Cu2S nanocrystals could be synthesized at a reductive atmosphere as well [30, 31]. We added its standard XRD patterns (PDF#84-1770) in Fig. 2 for comparison, and no peaks belonging to Cu2S phase was detected, which rule out the possibility of the formation of Cu2S. The morphology of the single phase CuS powders after ball milling for 2 h was further characterized by electron microscopy. The scanning electron microscope (SEM) image (Fig. S2 in the supporting information) reveals that the CuS powders are consists of ultrafine particles. The atomic ratio of Cu to S was determined to be ca. 1.03:1 by energy dispersive spectroscopy (EDS), in agreement with their stoichiometric ratio in CuS. A representative transmission electron microscopy (TEM) image (Fig. 3a) of the CuS powders clearly displays monodisperse nanocrytals with uniform size (< 5 nm) and an average diameter of 2.9 nm, indicating that CuS quantum dots (the sample is abbreviated as CuS QDs hereafter) have been successfully synthesized. From the high resolution TEM (HRTEM) image (Fig. 3b), the fringe spacing was determined to be ca. 0.19 nm (d = 0.186 nm calculated by XRD patterns), corresponding well to the (110) interplanar spacing (0.189 8
nm) of hexagonal CuS. Additionally, it can be clearly seen that the CuS QDs possess dislocations and planar defects such as stacking faults and twin boundaries. Similar types of defect were also observed in CuS nanocrystals prepared by solution-based chemical synthesis method [8]. Atomic force microscopy (AFM) was also employed to characterize the morphology of the CuS QDs dispersed on Si substrate (Fig. S3 in the supporting information), showing that the QDs are 1-5 nm in width and 0.5-3 nm in height. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical binding states of the as-prepared CuS QDs, as shown in Fig. 4. Two strong peaks at 932.3 and 952.2 eV were observed in the XPS spectrum of Cu 2p (Fig. 4a), consistent with the reported values of binding energies of Cu+ 2p3/2 and 2p1/2, respectively [32, 33]. Additionally, on the sides of Cu+ 2p3/2 and 2p1/2, two low-intensity components (935.2 and 955.1 eV) appeared, which can be assigned to the Cu2+ oxidation state [28, 34]. Meanwhile, the satellite peaks further confirms the existence of Cu2+ vacancy [13, 35], in agreement with the XRD analysis. The deconvolution of the S 2p spectrum (Fig. 4b) yields two doublets, corresponding to typical values for sulfide (S2−, 162.2 eV and 163.5 eV) and disulfide ((S2)2−, 163.6 eV and 164.7 eV) respectively, which is consistent with previous results on CuS nanodisks [28]. Although numerous micro/nano-materials have been fabricated by ball milling, the basic understanding of the formation mechanisms in nonmetallic compounds (e.g. oxides or sulfides) appears to be less advanced than it is for metallic materials, essentially because of the overall complexity of their structures, surfaces, defects and mechanical behavior. We consider that the formation of the CuS QDs within short time in the present study might be related to several possible reasons. First, the use of Cu and S powders with comparatively low hardness as raw materials allow them to be crushed with ease during the milling process. Second, thermodynamically, the energy needed for the formation of CuS is quite small since ∆G (–56.71 kJ/mol) and ∆H (–56.32 kJ/mol) are very close (373 K), thus allowing the reaction readily to occur with traditionally low energy ball milling. Moreover, the good lubricity of CuS [36], to some extent, may prevent 9
the conglutination of the nanoparticles during the ball milling process. Under the function together with above-mentioned factors, the CuS QDs were finally formed. It is worth noting that different from previous reports of mechanochemically synthesized nanoparticles, which are mostly around several tens to hundreds nanometers and are usually seriously aggregated, the as-synthesized ultrasmall CuS nanoparticles herein have uniform size and could be well dispersed in water or ethanol for several hours (for maintaining long-time stability, capping agents are still needed), showing the great advantage of these nanocrystals. Further investigations are still needed to better understand the formation mechanism of the present monodisperse CuS QDs as well as other material systems that worth to be explored. Fig. 5 shows the UV-vis absorption spectrum of the CuS QDs, together with that of CuS NPs with lateral size about a few hundred nanometers (SEM image is shown in Fig. S4 in the supporting information). Both of these two samples exhibit similar absorption feature. The photoabsorption spectrum of the CuS QDs is quite similar with that of previous reported CuS nanocrystal/quantum dot counterparts with comparable size obtained by solution method [37]. The first band transitions of these two samples occur below 500 nm (yellow dash line), arising from the 1Sh-1Se excitonic transition in semiconductor nanoparticles [8]. The absorption edge for the CuS QDs (363 nm) strongly blue-shifted compared to that of the NPs (481 nm), which may result from the quantum confinement effects that was predicted to occur in the sub-10 nm regime [17, 38]. The second broad absorption shoulder (black dash line) occurred at ca. 663 nm (CuS QDs) and 545 nm (CuS NPs), consistent with previous reports [17, 39, 40]. From the absorption edges (663 and 545 nm), the corresponding band gaps of these two samples were calculated to be 1.87 eV and 2.27 eV for the QDs and NPs, respectively. The band gap narrowing of the QDs should be owing to the presence of a large number of defects in and around the band gap of the CuS QDs, which has been also observed by Mary et al. in CuS QDs synthesized via solution based technique [8]. The absorption beyond 700 nm is characteristic of LSPR effect, which could be ascribed to free holes inter band transitions from valence states to the 10
unoccupied states [29]. As a proof of concept application of these intriguing CuS QDs, their photocatalytic activity was evaluated by degradation of RhB solution under visible light irradiation, as shown in Fig. 6. H2O2 was added before starting the photocatalytic experiment. Negligible degradation was found in the blank test with only H2O2, demonstrating that RhB is stable under illumination without photocatalyst. With CuS QDs alone, less than 60% of the RhB solution was degraded in 30 min. While the CuS QDs exhibited significantly higher photocatalytic efficiency with the assistant of H2O2; the C/C0 value decayed quickly to about 5% in 30 min, indicating an almost complete degradation of the RhB aqueous solution. We have also compared the photocatalytic performance of the CuS QDs and NPs. Obviously, the QDs sample exhibited much more superior photoactivity compared to that of the CuS NPs, which only degraded 40% under the same experimental condition. Moreover, the photocatalytic degradation rate for RhB solution of the CuS QDs is also higher than reported works such as CuS NPs [37, 41, 42], chainlike hierarchical structures [43], and hollow nanospheres [44] under similar conditions. To understand the effect of specific surface area on the photocatalytic activity, N2 adsorption-desorption isotherms of the CuS QDs and NPs samples were investigated, as shown in Fig. S5 in the supporting information. The calculated BET surface area of the QDs is as high as ca. 90.0 m2g−1, which is much higher than that of the NPs (13.1 m2g−1). Therefore, the remarkable improvement of photocatalytic activity of the CuS QDs could be attributed to the monodispersity and small crystal size that resulted in higher specific surface area with more reactive sites, shorter migration distance that reduces the recombination rate of photogenerated electron/hole pairs [45, 46]. In addition to the photocatalytic activity, the stability of photocatalysts is also one of the most concerned issues for their practical applications. Thus, we carried out cycling runs of photocatalytic experiment with the CuS QDs (Fig. 7). No apparent inactivation was observed in photodegradation activity after three consecutive cycles. The degradation rate of RhB after three runs is still above 90%, indicating good stability and recyclability of 11
the CuS QDs catalyst. Several works have been demonstrated the photocatalytic degradation of dye molecules with various CuS materials [13, 14, 47-51]. Based on our experimental results, the photocatalytic degradation mechanism of the RhB solution in the present CuS QDs could be explained as follows. When the CuS QDs is irradiated by light, electrons could be excited from the valence band (VB) to the conduction band (CB), leaving the corresponding holes in the VB. Organic dye molecules such as RhB could be decomposed to intermediates or mineralized products through an oxidation reaction by the formed hydroxyl radical (•OH) or superoxide anion (•O2 ) species, or directly by the holes accumulated in the VB. Once adding H2O2 into the aqueous -
solution, the photochemical process became complicated. H2O2 was considered to be a good electron acceptor by numerous studies, which could be converted to •OH by accepting electrons [51-53]. In addition, since the XPS results confirm the existence of cuprous ion, Cu+ in the CuS QDs may also react with H2O2 and generate more •OH species, leading to the degradation of RhB solution by the photo-Fenton reaction [52, 54]: H2O2 + Cu+ → Cu2+ + OH + •OH. Moreover, Cu+ can be regenerated by the photochemical effect and -
further producing •OH radicals continuously [55], and thus contributing to oxidize the RhB to the corresponding cationic radicals with the CuS QDs. On the other hand, it is worth mentioning that the quantum confinement effects of the CuS QDs should also play an important role on their drastically enhanced photocatalytic activity. Based on transient absorption spectroscopic characterizations, ultrafast dissociation of excitons in many semiconductor QDs/dye molecule complexes have been demonstrated such as CdS/RhB [56], Cd3P2/RhB [57], CdSe/Re-bipyridyl [58], and InP/methyl viologen [59] systems. According to these reports, quantum confinement could increase the reducing and oxidizing powers of conduction band electrons and valence band holes, respectively. On the other hand, quantum effect can also lead to strong electronic coupling between QD and dye molecules (RhB in our case) that leads to ultrafast charge transfer from QD to the dye. 12
Spectroscopic studies are needed to verify the ultrafast charge transfer processes in such systems. Further works are underway for size and chemical composition tunable copper sulfide nanocrystals by ball milling, as well as their utilization as visible-sensitizers in photovoltaic or photoelectrochemical cells. 4. Conclusion In summary, we described a facile mechanochemical route for fabricating uniform sized CuS QDs (< 5 nm), which is low-cost, and easy to scale up. Microstructural characterization revealed the existence of dislocations and planar defects such as twins and stacking faults in the CuS QDs. We further demonstrated the distinctive advantages of such QDs with high specific surface area for efficient degradation of organic pollutants as photocatalysts under visible light irradiation, indicating that they are promising to be applied in dye wastewater treatment as Fenton-like reagents.
Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 51272023 and 51472026), and the Doctoral Program of Higher Education (Grant No. 20130006110006).
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Figures and Captions
Fig. 1 Schematic illustration of the synthesis procedure of CuS QDs as photocatalysts by ball milling approach.
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Fig. 2 XRD patterns of powder samples synthesized with different reaction time.
Fig. 3 (a) TEM and (b) HRTEM images of the CuS QDs. (c) The size distribution histogram of the CuS QDs.
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Fig. 4 The XPS spectra of the CuS QDs (a) Cu 2p and (b) S 2p.
Fig. 5 UV-vis absorption spectra of the CuS QDs and NPs (inset shows the photograph of the CuS QDs dispersed in water). 22
Fig. 6 UV-vis absorption spectra of RhB solution with (a) H2O2 alone, (b) CuS QDs and (c) CuS QDs and H2O2 at different irradiation time, and (d) corresponding degradation rate of (a-c) and that of the CuS NPs sample under visible light illumination.
Fig. 7 Repeated photocatalytic test of RhB over the recycled CuS QDs.
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