ARTICLE pubs.acs.org/JPCC
Synthesis of Cu2xSe Nanocrystals by Tuning the Reactivity of Se Yi Liu,† Qingfeng Dong,† Haotong Wei,† Yang Ning,† Haizhu Sun,†,‡ Wenjing Tian,† Hao Zhang,*,† and Bai Yang† †
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People's Republic of China ‡ College of Chemistry, Northeast Normal University, Changchun 130024, People's Republic of China
bS Supporting Information ABSTRACT: In this paper, we demonstrated a modified hotinjection method to synthesize high-quality Cu2xSe nanocrystals (NCs) in liquid paraffin without using the hazardous and unstable alkylphosphines as the ligand of Se. The key of this method is the capability for tuning the reactivity of Se at the stage of formation of Cu2xSe nuclei. The low reactivity of Se facilitated the decomposition of copper(II) acetylacetonate into Cu2O, whereas the increase of Se reactivity promoted the reaction between Cu and Se. By control of the experimental variables, such as reaction time, Se concentration, reaction temperature, and, particularly, the addition of the noncoordinating solvent of Se, high-quality Cu2xSe NCs were prepared. The resultant Cu2xSe NCs possessed an indirect band-gap absorption around 1050 nm, potentially applicable in photovoltaic investigations. As an example, the optoelectronic properties of Cu2xSe NCs were investigated, which showed a promising increase in photocurrent under AM 1.5 illumination. Because the current method was convenient and environmentally friendly, it was believed that this work would facilitate the development of a preparation technique and industrial application of copper-based photovoltaic devices.
’ INTRODUCTION With increasing global energy consumption, the fabrication of low-cost, high-efficiency photovoltaic cells has emerged as an intractable problem. Fortunately, the advances of colloidal science, for example, the synthesis of high-quality semiconductor nanocrystals (NCs), have opened the door to address this challenge. Because of the quantum confinement effect,14 colloidal semiconductor NCs have many fascinating properties, such as tunable absorption and emission spectra,5 high-efficiency interface charge separation,6,7 spatial separation,8,9 multiexcitons generation,10,11 and easy chemical modification and processing,12 which make them suitable for next-generation photovoltaic applications. Accordingly, great efforts have been devoted to synthesize various NCs with the aim to prepare high-efficiency inorganic or organicinorganic hybrid photovoltaic devices.1327 Among these NCs, IIVI and IVVI semiconductors, such as CdSe, CdTe, PbSe, and their alloys, have been studied intensively during the past decade.2835 However, despite their appealing photovoltaic properties, the intrinsic toxicity of cadmium and lead sheds a doubt on the future applicability of these NCs, particularly in view of recent environmental regulations.36 Thereby, the lesstoxic semiconductor NCs are now strongly demanded. Recently, several copper-based NCs, such as CuInS2, CuInSe2, Cu(InGa)Se2, and Cu2ZnSnS4, have been explored and regarded as the competitive candidates because of their high absorption coefficients, good photostability, low-toxicity, and high photovoltaic r 2011 American Chemical Society
efficiency.3751 For example, the vapor deposited films of Cu(InGa)Se2 with a band gap of 1.3 eV have been demonstrated to have a high power conversion efficiency of ∼20%, which is the highest among thin-film solar cell technologies.52 Because of their high potentials in solar energy conversion, many synthetic approaches have been reported over the past few years. Lu et al. had synthesized CuInS2 ternary NCs with a tunable structure and composition.40 Korgel et al. had synthesized CuInS2, CuInSe2, and Cu(InGa)Se2, respectively, and then used them as the ink to fabricate photovoltaic devices.41 Cu2ZnSnS4 had also be prepared by hot injection of an oleylamine (OLA) solution of elemental sulfur into an OLA solution containing copper(II) acetylacetonate, zinc acetylacetonate, and tin(IV) bis(acetylacetonate) dibromide at 225 °C.48 Still, there is little work about the synthesis of high-quality copper selenide NCs. Copper selenide is also widely used in solar cells,53 as optical filters and as superionic materials.54,55 Interestingly, as a kind of binary compound, copper selenide can exist in a wide range of stoichiometric compositions, for instance, Cu2Se, CuSe2, Cu3Se2, Cu5Se4, and Cu7Se5, and nonstoichiometric compositions, for instance, Cu2xSe, and can be constructed into several crystallographic forms, including monoclinic, cubic, tetragonal, and Received: January 27, 2011 Revised: April 15, 2011 Published: April 29, 2011 9909
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The Journal of Physical Chemistry C hexagonal. Among these diverse phases, the major materials are the cubic copper(I) selenide (Cu2Se, Cu2xSe), which are highly desired for the applications toward diverse photovoltaic devices. It has been reported that Cu2xSe is an extrinsic p-type semiconductor with a direct band gap of 2.2 eV and an indirect band gap of 1.4 eV.56 The indirect energy gap is well within the ideal band-gap range of 1.11.7 eV for the applications as solar energy materials. During the past years, several methods have been applied to prepare Cu2xSe NCs, including a solvothermal method,5761 sonochemical method,62,63 microwave-assisted heating method,64,65 template-assisted method,66,67 hot-injection method,6870 and so on.71,72 However, the as-prepared NCs are usually in the form of large and uncontrolled aggregates. Lately, Choi et al. reported the synthesis of the cubic-phase copper selenide nanodiscs, but a special selenium source was used, which needed to be prepared beforehand.69 Sasanka et al. also synthesized the high-quality Cu2xSe NCs by injecting a Se precursor into the mixture of OLA and octadecene contained Cu(I), but the synthesis process was so intricate.70 In addition, although the Cu2xSe NCs could be synthesized by the way mentioned above, the experimental parameters were not investigated systemically and carefully. On the basis of these works, it is necessary to develop a simple and convenient method to synthesize highquality Cu2xSe NCs. Here, we demonstrated a modified hotinjection method to synthesize high-quality Cu2xSe NCs in liquid paraffin. The key of the current synthesis was the alteration of Se reactivity, which was achievable by regulating the experimental variables, particularly the addition of the noncoordinating solvent of paraffin. The as-prepared Cu2xSe NCs possessed ideal band-gap absorption and promising photoresponsive properties, which revealed that the Cu2xSe NCs were potentially applied in photovoltaic devices.
’ EXPERIMENTAL SECTION Materials. Copper(II) acetylacetonate (Cu(acac)2, 98%) was purchased from Alfa Aesar. Selenium powder (100 mesh, 99.5%) and oleylamine (OLA, technical grade, 70%) were purchased from Aldrich. Toluene (99.5%) and methanol (99.5%) were purchased from Beijing Chemical Reagent Ltd., China. Liquid paraffin was purchased from Sinopharm Chemical Reagent Co., Ltd. All of the reagents were used as received. Synthesis. A typical synthetic procedure of Cu2xSe NCs was briefly described below. First, 0.5 mmol of Cu(acac)2 and 5 mL of OLA were mixed at room temperature and cycled between vacuum and nitrogen three times. Afterward, the mixture was kept at 60 °C under vacuum for 1 h. This solution was marked as solution A. An 8 mL portion of OLA and 0.25 mmol of Se powder were deposited in a separate flask, cycled between vacuum and nitrogen three times, vacuum pumped at 120 °C for 0.5 h, and then put under a nitrogen atmosphere for the remainder of the synthetic reaction. This solution, labeled solution B, was then heated to 220 °C and kept at this temperature for 2 h. Once the Se powder was completely dissolved, the solution B was raised to 250 °C, and 5 mL of solution A was injected into it under vigorous stirring. The solution turned immediately to dark brown. After injection, the temperature of the reaction mixture dropped to ∼200 °C, and it was allowed to recover to the preinjection temperature. The overall reaction time after injection was 4 min, after which the flask was rapidly cooled to room temperature to achieve NC products. Unless otherwise stated, the above techniques were used in all of the following synthesis.
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The NC products were purified by precipitation with methanol, followed by centrifugation at 6000 rpm for 10 min. After such a washing step, the supernatant containing unreacted precursor and byproducts was discarded. The NCs were in the precipitate. The precipitate was then redispersed in 10 mL of toluene and centrifuged at 6000 rpm for 5 min to remove poorly capped NCs and large aggregates, which settled during centrifugation. To remove excess capping ligands and remaining impurities, the product was again precipitated using methanol and centrifuged at 6000 rpm for 10 min, then redispersed in toluene or chloroform. A control experiment was also carried out by injecting the Cu(acac)2/OLA mixture into pure OLA without the addition of Se at 250 °C. The overall growth process was kept for 4 min, and the product obtained in this case was Cu2O NCs, as indicated in Figure 3. Effect of Reaction Time. To reveal the effect of reaction time, the aforementioned procedure was followed exactly with the exception of the overall reaction time changed from 4 to 60 min. Effect of Se Concentration. To investigate the effect of Se concentration on the growth of NCs, different Se concentrations, varied from 0.019 to 0.038 mmol/L were used, whereas the reaction time and temperature were kept as 4 min and 250 °C, respectively. OLA was used as the solvent cum ligand of Se. Effect of Reaction Temperature. In this section, we still used OLA as the solvent cum ligand of Se. The reaction time was 30 min, and the concentration of Se was 0.019 mmol/L. However, the reaction temperature was altered from 220 to 270 °C. Effect of Paraffin. All experiments were performed at 250 °C for 4 min, and the Cu and Se concentrations were fixed at 0.038 and 0.019 mmol/L. The solvents of Se were altered from pure OLA, to a mixture of 1/1 OLA/paraffin, to pure paraffin. Characterization. UVvisible absorption spectra were obtained using a Shimadzu 3600 UVvis-NIR spectrophotometer. Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. High-resolution TEM (HRTEM) imaging was implemented by a JEM-2100F electron microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) was investigated by using a VG ESCALAB MKII spectrometer with a Mg KR excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. Inductively coupled plasma (ICP) was performed with a PerkinElmer OPTIMA 3300DV analyzer. An X-ray powder diffraction (XRD) investigation was carried out using a Bruker-AXS D8 Discovery with a general area detector diffraction system (GADDS). A photoresponse device was fabricated from the as-prepared Cu2xSe NCs following the device configuration (indium tin oxide (ITO)/Cu2xSe/ITO) shown in the inset of Figure 7. The Cu2xSe NCs were synthesized by using paraffin as the solvent of Se at 250 °C for 4 min. The IV characteristics were recorded using a Keithley 2400 Source Meter in the dark and under simulated AM 1.5 illumination (100 mW/cm2). The scan voltage was tuned from 0 to 10 V. All the tests of the devices were processed in a glovebox.
’ RESULTS AND DISCUSSION In this study, we reported a facile and feasible hot-injection method for synthesizing Cu2xSe NCs without using hazardous alkylphosphines as a Se ligand. Also, the reactivity of Se was tunable by altering experimental variables, including reaction 9910
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Figure 1. Characterization of Cu2xSe NCs. TEM (a) and HRTEM (b) images, XRD pattern (c), UVvis absorption spectroscopy (d), and XPS Cu 2p3/2, Cu 2p1/2 (e), and Se 3d (f) spectra. The Cu2xSe NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of paraffin with 0.25 mmol of Se powder at 250 °C; the overall reaction time after injection was 4 min.
Table 1. Cu/Se Ratio of Cu2xSe NCs Synthesized with Different Solvents of Sea
Cu/Se a
OLA
OLA/paraffin = 1/1
paraffin
4.70
3.82
1.76
The results were determined by ICP measurement.
time, Se concentration, reaction temperature, and, particularly, the coordinative ability of the solvent with Se. Figure 1a shows the TEM image of the as-prepared Cu2xSe NCs. The asprepared NCs were spherical with the average diameter of 16 nm (Figure S1, Supporting Information). The HRTEM image indicated that the NCs were single crystals, with the interplanar distances of 0.34 and 0.23 nm, which were consistent with the (111) and (220) lattice planes of cubic Cu2xSe (Figure 1b). The XRD pattern firmly supported that these NCs possessed a cubic structure (Figure 1c). The diffraction peaks at 26.8, 31.0, 44.6, 53.0, 65.0, and 71.2° were consistent with the (111), (200), (220), (311), (400), and (331) planes of cubic phase Cu2xSe, respectively. Apart from these clear peaks, there was a weak diffraction peak at 36.4°, which belonged to the cubic phase Cu2O, indicating that the product contains a trace of Cu2O. The UVvis absorption spectroscopy of the NCs is shown in Figure 1d. Two obvious absorption peaks were observed. The first one appeared at 470 nm, which blue shifted with respect to the direct band gap of bulk copper selenide (2.4 eV). Besides, a broad and intense absorption peak was observed at 1050 nm, which was attributed to the transition
Figure 2. XRD patterns (a) and UVvis absorption spectra (b) of Cu2xSe NCs synthesized with different reaction times. TEM images of Cu2xSe NCs synthesized with the reaction time of 4 (c), 10 (d), 30 (e), and 60 min (f). The Cu2xSe NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of OLA with 0.25 mmol of Se powder at 250 °C.
involving the indirect band gap. The composition of the asprepared Cu2xSe NCs was characterized by ICP measurement (Table 1), which showed an average Cu/Se atomic ratio of 1.76/1. XPS was further performed to investigate the valence state of copper in the as-prepared Cu2xSe NCs. As shown in Figure 1e, the binding energy of Cu 2p3/2 and Cu 2p1/2 was 932.4 and 952.2 eV, respectively, suggesting that copper was mainly in the form of Cu(I). In addition, the Cu 2p peak had a satellite line at 940945 eV, which resulted from a little Cu(II) in Cu2xSe. The asymmetric peak at 54.2 eV was representative of the Se 3d binding energy for lattice Se2 (Figure 1f). The small peak at 5860 eV was assigned to the oxidation of a little Se in the product. Effect of Reaction Time. To give an in-depth understanding of the evolution of Cu2xSe NCs, the effect of reaction time was studied (Figure 2). Figure 2a shows the XRD patterns of the samples at different reaction stages. Comparing with the standard JCPDS card database, we found that the NCs prepared with 4 min growth were actually a mixture of cubic Cu2xSe and cubic Cu2O. Apart from the diffraction peaks of cubic Cu2xSe, there were excess diffraction peaks at 29.6, 36.4, 42.3, 61.4, and 73.6°, which were consistent with the (110), (111), (200), (220), and (311) planes of cubic Cu2O. However, as the reaction time was 10 min or longer, the diffraction peaks of cubic Cu2O disappeared and no other impurity peaks were observed. All of the diffraction peaks remained matched well to those of cubic Cu2xSe, indicating the the single-phase and high-purity Cu2xSe NCs were obtained. Note that pure Cu2O NCs were also prepared as a control experiment, and their XRD pattern is shown in Figure 3a. It further 9911
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Figure 3. XRD pattern (a), UVvis absorption spectroscopy (b), and TEM image (c) of Cu2O NCs. The Cu2O NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of OLA at 250 °C; the overall reaction time after injection was 4 min.
Table 2. Cu/Se Ratio of Cu2xSe NCs Synthesized with Different Reaction Timesa
Cu/Se a
4 min
10 min
30 min
60 min
4.70
2.03
1.91
2.01
The results were determined by ICP measurement.
confirmed that the excess peaks in Figure 2a belonged to Cu2O. CuCl2 was also used as the Cu source to synthesize Cu2xSe NCs (Figure S2, Supporting Information), but the product was not dispersible in the solution. The UVvis absorption spectra of the NCs, corresponding to the XRD characterization, are shown in Figure 2b. It was observed that the absorption spectra of the NCs prepared with 4 min growth had no visible absorption peak, which was similar to the absorption spectra of Cu2O NCs (Figure 3b). It meant that the products were not the pure Cu2xSe, but mixed with some Cu2O at the moment. However, as the reaction proceeded to about 10 min, the characteristic absorption peaks of Cu2xSe NCs appeared and became clearer during the sequential growth. After 1 h growth, absorptions of both the direct and the indirect band gaps were obvious, indicating the formation of highly crystalline Cu2xSe. Either XRD analysis or UVvis absorption spectra implied that there was a little of Cu2O that formed at the early stage of the reaction and then disappeared during the sequential growth. This consideration was further confirmed by the ICP measurement. The measured Cu/Se atomic ratios are listed in Table 2. It revealed that the evolution of NCs began as copper-rich products, whereas the amount of selenium increased relative to copper with prolonging the growth duration. Accordingly, the Cu/ Se ratio of the NCs varied from 4.70/1 to 2.03/1. After 10 min, the Cu/Se ratio was almost fixed with no regard to the time. The ICP results clearly indicated that the initially high concentration of copper relative to selenium resulted from the existence of Cu2O, whereas sequential growth promoted the formation of Cu2Se. It should be mentioned that, in this section, OLA was used as solvent cum ligand of both Cu and Se. Under this condition, the reaction system favored Cu2Se rather than Cu2xSe. The reason would be discussed latter. The size and morphology evolution of the NCs were also monitored by TEM observation (Figure 2cf). Preliminarily, the NCs were monodisperse, but highly faceted, with the average diameter of about 15 nm. After 10 min growth, they became polydisperse and the shape changed from a faceted to a spherical morphology. This variation was in accord with the character of the Ostwald ripening (OR) process.73 It was known that, if the
monomer concentration in the solution was higher than the solubility of all existing NCs, all NCs in the solution grew and the size distribution narrowed down. This was the “focusing of the size distribution”. However, if the amount of monomer in the solution was not enough for supporting all the NCs' growth, the smaller NCs would solve. This meant that the smaller NCs in the solution shrank and the bigger ones continued their growth. As a result, the size distribution broadened. This was the “defocusing of size distribution”, namely, OR. The TEM images shown in Figure 2 indicated that, as the growth duration was longer than 10 min, defocusing occurred, so the size distribution of the NCs became polydisperse. Simultaneously, the OR process smoothed the surface of the faceted NCs via monomer diffusion, making their morphology spherical. According to the aforementioned results, we could briefly speculate the growth process of the NCs as presented below. It has been reported that OLA was not only the solvent but also the ligand of Se. When the Cu precursor was injected into the system, the coordination bond between Se and OLA would decrease the reactivity of Se, thus preventing the nucleation process of NCs. Moreover, many groups had synthesized metal oxides by thermal decomposition of the corresponding metal acetylacetonate.74 It was also known that Cu(acac)2 could decompose into Cu2O when the temperature reached 284 °C. Consequently, though the current reaction temperature was 250 °C, not as high as the reported decomposition temperature, it was reasonable to consider that some Cu(acac)2 decomposed due to the low reactivity of Se. As a result, some Cu2O formed. As the reaction time was longer than 10 min, the OR of NCs became dominant. Previous literature had reported that the OR was actually a thermodynamics-favored diffusion process, in which the monomers dissociated from smaller NCs into the solution and then recrystallized. Following this mechanism, the Se monomer in the solution would diffuse into the Cu2O and convert them into Cu2xSe along with the OR process. That would be the reason why the Cu2O disappeared in the late stage of the growth process. From the discussion above, we could conclude that the reactivity of Se was the key factor affecting the nucleation and the formation of Cu2xSe NCs. Effect of Se Concentration. In this section, we explored the influence of Se concentration on the growth of the NCs by varying the Se concentration from 0.019 to 0.038 mol/L. The Cu precursor concentration was fixed at 0.038 mol/L, and the reaction time was 4 min. Figure 4a illustrates the XRD pattern of the NCs prepared with different Se concentrations. It could be seen that, when the Se concentration was 0.019 mol/L, the relative intensity of the peaks belonging to Cu2O was higher than 9912
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Figure 4. XRD patterns (a) and UVvis absorption spectra (b) of Cu2xSe NCs synthesized with different Se concentrations. TEM images of Cu2xSe NCs synthesized with the Se concentration of 0.029 (c) and 0.039 mmol/L (d). The Cu2xSe NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of OLA with different amounts of Se powder at 250 °C. The overall reaction time after injection was 4 min.
Table 3. Cu/Se Ratio of Cu2xSe NCs Synthesized with Different Concentrations of Sea
Cu/Se a
[Se] = 0.019 mol/L
[Se] = 0.029 mol/L
[Se] = 0.038 mol/L
4.70
3.65
1.96
Figure 5. XRD patterns (a) and UVvis absorption spectra (b) of Cu2xSe NCs synthesized with different reaction temperatures. TEM images of Cu2xSe NCs synthesized at 220 (c) and 270 °C (d). The Cu2xSe NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of OLA with 0.25 mmol of Se powder at different reaction temperatures. The overall reaction time after injection was 30 min.
Table 4. Cu/Se Ratio of Cu2xSe NCs Synthesized with Different Reaction Temperaturesa
Cu/Se a
The results were determined by ICP measurement.
that of the peaks of Cu2xSe. As the Se concentration was increased to 0.029 mol/L, the peaks of the Cu2xSe became dominant, while the Cu2O peaks got weak. When the Se concentration was 0.038 mol/L, the XRD pattern was in accord with cubic Cu2xSe without Cu2O peaks. The Cu/Se ratio of the NCs is listed in Table 3. With the increase of the Se precursor concentration, the Cu/Se ratio decreased from 4.70/1 to 1.96/1 due to the reduction of Cu2O. This result was well consistent with the XRD analysis. It was easy to understand that a high Se precursor concentration leads to a high chemical potential of the Se in the solution, which increased the reactivity of Se. Thus, the formation of Cu2xSe nuclei was promoted, and therefore, the formation of Cu2O was avoidable by decreasing the Cu/Se feed ratio to 1/1. Interestingly, the NCs prepared here were also favored to be Cu2Se rather than Cu2xSe. Additionally, UVvis absorption spectra indicated that the characteristic absorption peak of the NCs became distinct with the increase of Se concentration (Figure 4b). Also, the TEM images illustrated that the NCs changed from a faceted to a spherical morphology (Figures 2c and 4c,d). Following the discussion mentioned above, these results represented the transition of NCs from Cu2O to Cu2Se. It further confirmed the conclusion that the reactivity of Se was the key in determining the nucleation process of NCs. Effect of Temperature. In addition to the reaction time and Se concentration, we found the reaction temperature was another important factor in affecting the growth of Cu2xSe NCs. As shown in Figure 5a, when the reaction temperature was
220 °C
250 °C
270 °C
2.55
1.91
1.87
The results were determined by ICP measurement.
low, such as 220 °C, the product was a mixture of cubic Cu2xSe and cubic Cu2O. As the temperature was increased from 220 to 250 °C, the peaks of Cu2O disappeared and all the peaks accorded with cubic Cu2xSe. Further increasing the temperature to 270 °C led to pure cubic Cu2xSe NCs. Besides, the UVvis absorption spectra indicated that both the direct and the indirect band gaps of the products became obvious with the increase of temperature (Figure 5b), indicating the increase of Cu2xSe in the NC collective. Furthermore, the NCs prepared at 270 °C possessed a more irregular morphology in comparison with those prepared at low temperature (Figures 5c, 2c, and 5d). The ICP analysis further supported our speculation (Table 4). With the increase of reaction temperature from 220 to 250 °C, the Cu/Se ratio altered from 2.55/1 to 1.91/1, which kept constant with further increasing the temperature to 270 °C. Peng had reported that the monomer reactivity was a function of reaction temperature.75 Therefore, it was understood naturally that the reaction temperature could affect the reactivity of Se and, thereby, the nucleation process of NCs. When the temperature was low, the reactivity of Se was not high enough to support the reaction with Cu. At this condition, some Cu(acac)2 decomposed into Cu2O. When the temperature was high, the high reactivity of Se facilitated the formation of Cu2xSe nuclei, so pure Cu2xSe NCs were obtained. Effect of Paraffin. As discussed above, the reactivity of Se greatly influenced the nucleation process of the NCs, and an increase of Se reactivity promoted the formation of Cu2xSe NCs. Note that, in the above investigations, OLA was used as the solvent cum ligand of Se for synthesizing NCs. The reaction 9913
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Figure 6. XRD patterns (a) and UVvis absorption spectra (b) of Cu2xSe NCs synthesized with different solvents of Se. The Cu2xSe NCs were synthesized by injection of 5 mL of OLA containing 0.5 mmol of Cu(acac)2 into 8 mL of OLA, a mixture of 4 mL of OLA and 4 mL of paraffin, or 8 mL of paraffin at 250 °C, which contained 0.25 mmol of Se powder. The overall reaction time after injection was 4 min. (c) TEM image of Cu2xSe NCs synthesized by using the mixture of 4 mL of OLA and 4 mL of paraffin.
system favored Cu2Se rather than Cu2xSe NCs (Tables 24). This was attributed to the strong coordination between OLA and Se, keeping Se reactivity at a low level. Therefore, it was reasonable to consider the utilization of a noncoordinating solvent for further weakening the OLA/Se coordination and, therewith, increasing the reactivity of Se. In this scenario, liquid paraffin, a colorless liquid at room temperature with the boiling point above 300 °C, was selected as an alternative solvent of Se. In comparison to OLA, paraffin was cheaper and more stable in the atmosphere, and most importantly, paraffin could not coordinate with Se. It meant that, by altering the OLA/paraffin ratio, one could control the reactivity of Se. Figure 6a,b shows the XRD pattern and the absorption spectra of the NCs prepared using the solvent mixture of OLA and paraffin with different ratios. Because of the lack of the coordination between paraffin and Se, the reactivity of Se increased with the increase in the amount of paraffin, represented by the weakening of the XRD diffraction peaks indexed to Cu2O and the strengthening of Cu2xSe peaks (Figure 6a), though there was still a trace of Cu2O in the product when paraffin was used as the solvent of Se only. Also, a clear enhancement of the indirect band-gap absorption was observed with the increase of paraffin (Figure 6b). In addition, the ICP results exhibited the decreased tendency of Cu in the as-prepared NCs with increasing paraffin, which agreed with the consideration about the influence on Se reactivity. Photoresponsive Property of Cu2xSe NCs. To study the optoelectronic properties of the as-prepared Cu2xSe NCs, a photoresponse device was fabricated by sandwiching the Cu2xSe NCs between two blank ITO glasses with a device configuration of ITO/Cu2xSe/ITO (Figure 7, inset). The IV curves of the devices measured for a 10 V bias range under AM 1.5 illumination in a dark state were compared in Figure 7. Under AM 1.5 illumination, the slope of the IV curve was higher than that in the dark state. The photocurrent of Cu2xSe film increased from 2.75 105 to 4.5 105 A when the illumination was on. To improve the photoresponsive property, pyridine and butylamine were used to exchange the initial ligand of Cu2xSe NCs. However, the as-prepared Cu2xSe NCs could not be dispersed in pyridine or butylamine, even when the NCs were refluxed for a long time. This was attributed to the weak coordination ability of the pyridine and butylamine. Another way we tried was the thermal annealing to remove the ligand. Because the device was an ITO/Cu2xSe/ITO sandwich configuration, it was hard to remove the ligand by this way. Therefore, the effect of the thermal annealing was not remarkable. Further investigation was undertaken to replace or remove the ligand of the as-prepared
Figure 7. IV curves of Cu2xSe NC thin films in the dark state and under AM 1.5 illumination. Inset: experimental setup for measuring the photoresponsive properties of Cu2xSe NCs.
Cu2xSe NCs. Although the increase in the current was not as high as that of the previous report, the obvious photoresponsive behavior suggested that the as-prepared Cu2xSe NCs would be a competitive candidate in the fabrication of photovoltaic devices.
’ CONCLUSION In summary, we indicated a simple and convenient method for synthesizing high-quality Cu2xSe NCs through a modified hotinjection strategy. The as-prepared Cu2xSe NCs possessed ideal band-gap absorption and promising photoresponsive properties. Systematic studies revealed that the key in the current synthesis was the ability to tune the reactivity of Se by regulating the experimental variables, such as the reaction time, Se concentration, reaction temperature, and, particularly, the addition of the noncoordinating solvent of Se. In general, the formation of Cu2xSe was facilitated at high Se concentration, high reaction temperature, and low coordinative ability of the solvent. Contrarily, the reaction system promoted the decomposition of Cu(acac)2 into Cu2O. Because the current synthesis is convenient and environmentally friendly, it is reasonable to expect that this work will promote the development of synthetic techniques and industrial applications of copper-based NCs. ’ ASSOCIATED CONTENT
bS
Supporting Information. Size distribution of the Cu2xSe NCs and the XRD pattern of NCs using CuCl2 as the Cu source. This material is available free of charge via the Internet at http:// pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Fax: þ86 431 85193423. E-mail:
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