One-pot synthesis of CuInS2 nanocrystals using different anions to engineer their morphology and crystal phase.

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DOI: 10.1039/C5DT01111F

One-pot Synthesis of CuInS2 Nanocrystals Using Different Anions to Engineer Morphology and Crystal Phase

a

Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing 100044, P. R.

China E-mail: [email protected]; Tel: +86-10-51683627 b

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing

JiaoTong University, Beijing 100044, P. R. China Email: [email protected]

Abstract A simple one-pot colloidal method has been described to engineer ternary CuInS2 nanocrystals with different crystal phases and morphologies, in which dodecanethiol is chosen as the sulfur source and the capping ligands. By careful choice of the anions in the metal precursors and manipulation the reaction conditions including the reactant mole ratios and reaction temperature, CuInS2 nanocrystals with chalcopyrite, zincblende and wurtzite phases have been successfully synthesized. The types of the anions in the metal precursors have been found to be essential for determining the crystal phase and morphology of the as-obtained CuInS2 nanocrystals. Especially, the presence of Cl- ions plays an important role in the formation of CuInS2 nanoplates with a wurtzite–zincblende polytypism structure. In addition, the mole ratios of Cu to In precursors have also a significant effect on the crystal phase and morphology, and the intermediate Cu2S-CuInS2 heteronanostructures are formed which are critical for the anisotropic growth of CuInS2 nanocrystals. Furthermore, the optical absorption results of the as-obtained CuInS2 nanocrystals exhibit a strong dependence on the crystal phase and size.

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Published on 15 April 2015. Downloaded by East Carolina University on 23/04/2015 11:21:33.

Aiwei Tang,*a,b Zunlan Hu, a Zhe Yin, a Haihang Ye,a Chunhe Yang,a and Feng Teng*b

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DOI: 10.1039/C5DT01111F

1. Introduction In the past several decades, copper-based semiconductor nanocrystals (NCs) have attracted much attention due to their high absorption coefficient, low toxicity and their potential applications in


the most promising alternatives to II-VI or IV-VI semiconductor NCs that contain toxic elements, such as Cd, Pb, Hg, etc, which restricts their wide applications in optoelectronic devices and biological fields.8 Among different types of copper-based NCs, ternary I-III-VI2 semiconductors have become the research focus due to their appropriate band-gap energies as well as excellent electrical and optical properties.9-11 As one of the most important ternary I-III-VI2-type semiconductors, copper indium disulfide (CuInS2) is a direct semiconductor with the band gap of ～1.5 eV which is well-matched with the solar spectrum, and also has high optical absorption coefficient (～10-5 cm-1) and good stability against radiation. Moreover, the CuInS2 nanocrystals exhibit size-dependent optical properties, and the photoluminescence can be tuned from ultraviolet to the near infrared region. 8, 10-16 Previous studies have proved that there are three crystal phases for bulk CuInS2: chalcopyrite, zincblende and wurtzite.17-19 In most cases, bulk CuInS2 is often present in the form of tegragonal chalcopyrite phase, in which Cu and In atoms are ordered within the unit cell. With the development of synthesis of colloidal NCs, the nanocrystalline CuInS2 have been synthesized in the form of zincblende and wurtzite structures at room temperature. As compared to tetragonal chalcopyrite phase, the metal ions become disordered and are randomly distributed over the cation sites in zincblende and wurtzite structures, which are in close associated with the stacking patterns of sulfur anions.18, 20 Over the past few years, most of the published reports mainly focused on the synthesis of tetragonal chalcopyrite CuInS2 NCs and their applications in thin film solar cells by sputtering or evaporation techniques, but the crystallinity was not good and the size distribution was broad.21-23 Since the pioneering synthesis of zincblende and wurtzite CuInS2 NCs was reported by Pan et al in 2008, different colloidal synthetic methods, such as solvo/hydrothermal methods, hot-injection techniques, single-source pyrolysis and microwave radiation methods, have been developed to prepare zincblende and wurtzite CuInS2 NCs with different morphologies by using different metal salts and sulfur sources.24 However, one of the challenges in synthesis of CuInS2 NCs may derive from the different chemical properties of two cations used in the reaction, which often leads to the formation of hetero-nanostructures composed of Cu2S-In2S3.5,

8

In

addition, another challenge is that the anions of different Cu and In precursors have an important effect on the crystal phase and morphology of the resultant CuInS2 NCs based on previous reports.8 So far, there have been a few reports on the controllable synthesis of wurtzite and zincblende CuInS2 NCs by varying the Cu/In precursor ratios, the sulfur sources and the organic surfactant as well as the reaction temperature.13, 18-20, 24-28 However, the effect of metal precursors on the phase control of CuInS2 NCs was rarely discussed in the previous reports except recent 2


low-cost thin film solar cells.1-8 As a result, copper-based semiconductor NCs have become one of

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work reported by Wang and his co-workers.28 In this work, the author adopted a solvothermal approach to prepare chalcopyrite and wurtzite CuInS2 NCs via varying the In precursors in the assistance of some surfactants. Last but not least, it is still challenging to develop a convenient and low-cost method with large-scale potential for synthesis of different-phase CuInS2 NCs. Herein,


and morphologies, which involves the direct reaction of different Cu and In precursors with DDT in a non-coordinating solvent ODE at a relatively high temperature. Chalcopyrite, wurtzite and zincblende CuInS2 phases can be selectively synthesized by carefully manipulating the anions in the Cu or In sources and the metal precursors ratios. With the variation of crystal phase of the as-obtained CuInS2 NCs, the size and morphology can also be changed. Moreover, the presence of Cl- ions in the reaction system has a vital effect on the formation of CuInS2 nanoplates with wurtzite–zincblende polytypism. The CuInS2 NCs exhibit different optical absorption properties, which are dependent on the crystal phase and morphology.

2. Experimental section 2.1 Materials All the following chemicals are used as received without any purification. Copper (II) acetylacetonate (Cu(acac)2), copper (II) nitrate hydrate (Cu(NO3)2•3H2O), indium (III) nitrate hydrate (In(NO3)3•4.5H2O), indium (III) chloride (InCl3) were purchased from Aladdin Reagent Company. Indium (III) acetylacetonate (In(acac)3), copper (I) chloride (CuCl), 1-Octadecene (ODE, 90%) were purchased from Alfa Aesar; n-dodecanethiol (DDT), ethanol and chloroform were purchased from Sinopharm Chemical Reagent. 2.2 Synthesis of different CuInS2 NCs For a typical synthesis of CuInS2 nanoplates (Sample B), 2.5 mmol of Cu(acac)2 and 2.5 mmol of InCl3 were mixed with 5 mL of DDT and 25 mL of ODE in a 50 mL three-neck flask, and then the mixture was degassed using N2 flow under magnetic stirring for about 20 min. Afterwards, the mixture was heated slowly to 240 oC and kept for 120 min. After the reaction was finished, the mixture was naturally cooled to room temperature by removing the heating mantle. The mixture was precipitated by adding ethanol and centrifugated at 6000 rpm for 10 min, and the supernatant was descarded. The as-obtained product was purified by three repeated actions of dissolving the precipitate in chloroform and then re-precipitating the Samples out by adding excess ethanol. Finally, the products were dispersed in chloroform or dried in vacuum for further characterization. Similarly, the irregular nanodisks (Sample A) and very small nanoparticles (Sample C) could be obtained by using 2.5 mmol of In(NO3)3 and 2.5 mmol In(acac)3 instead of 2.5 mmol of InCl3, respectively. The reaction was conducted under the same conditions mentioned above. To study the Cu/In precursor ratios and reaction temperature on the crystal phase and morphology of CuInS2 NCs, Cu(acac)2 and In(acac)3 were selected as Cu and In precursors, and other reaction condition was kept the same except the Cu/In ratio was changed to 1.7/1 and the reaction 3


we present a facile one-pot colloidal route for engineering CuInS2 NCs with different structures

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temperature was increased to 250 oC. In addition, other samples have also synthesized by using CuCl and Cu(NO3)2 as Cu sources, in which three In sources, such as In(NO3)3, InCl3 and In(acac)3, have been used while other reaction conditions are kept the same as Sample A. The detailed reaction conditions and the corresponding results of various products are summarized in


2.3 Characterization The crystal phase of the as-obtained products was characterized by using powder X-ray diffraction (XRD) patterns on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.54056 Å). The transmission electron microscopy (TEM) images of the resultant products were taken with a JEM-1400 transmission electron microscope operating at an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) images were performed on a JEM-2010 with an acceleration voltage of 200 kV. The UV-Vis absorption spectra were performed on a Shimadzu-UV 3101 spectrophotometer. All the characterizations were carried out at room temperature.

3. Results and discussion Previous study has proved that the surfactant and the amount of sulfur sources have a significant effect on the phase control of CuInS2 NCs.26, 27 To exclude the effects of the surfactant and the amount of sulfur sources on the formation of CuInS2 NCs, in our case, a non-coordinating agent (ODE) is selected as the reaction media and DDT is used as sulfur source and capping ligand. A series of experiments have been carried out through using different Cu and In precursors. First of all, the effect of In precursors with different anions on the morphology and crystal phase of CuInS2 NCs has been studied, in which Cu(acac)2 is selected as Cu source, and In(NO3)3, InCl3, In(acac)3 are chosen as In sources while other reaction conditions including the dosage of DDT, the reaction temperature and the amount of metal sources are kept the same. Fig.1 shows the XRD patterns and TEM images of Sample A-C obtained at 60 min. As shown in Fig.1a, the XRD pattern of Sample A exhibits a typical hexagonal characteristic, which could be indexed as a wurtzite phase according to previous report.10 The corresponding TEM image of Sample A shown in Fig.1b exhibits a disk-shape with an average diameter of 8.5±0.8 nm. As a matter of fact, some nanodisks deposit flat with their faces on the substrate, and others are perpendicular to the substrate. The HRTEM shown in the inset demonstrates the single crystalline nature, and clear lattice fringes with an interplanar spacing of 0.32 nm can be observed, which can be ascribed to (002) plane of wurtzite CuInS2. To study the growth mechanism of Sample A, the TEM images and XRD pattern collected at different reaction time have been measured and shown in Fig.S1 and Fig.S2†, respectively. As shown in Fig.S1a and b†, the products obtained at and 10 min are spherical shape with an average diameter of 6.1±0.7 nm and 9.1±0.5 nm. Prolonging the reaction time to 30 and 60 min, the morphology of the products is disk-like and some

4


Table.1.

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heterostructured nanocrystals can be observed in the Fig.S1d, which are labeled by the red arrows. The HRTEM image shown in Fig.S1e confirms the formation of Cu1.94S-CuInS2 heterostructured NCs, which indicates that the formation of the intermediate Cu1.94S-CuInS2 NCs is essential in the growth of CuInS2 NCs. The formation mechanism of wurtzite CuInS2 NCs will be discussed in the


of a mixed phase of monoclinic Cu1.94S and wurtzite CuInS2 (Fig S2†). Furthermore, the diffraction peaks at 2θ=37.4o and 2θ=46.3o shift to 2θ=38.8o and 2θ=46.7o when the reaction time reaches 60 min, simultaneously the diffraction peak at 2θ=48.6o disappears and the peak at 2θ=50.7o becomes narrow and stronger, which suggests that the monoclinic Cu1.94S is first formed and acts as a foundation for further growth of CuInS2 NCs. Based on the Hard and Soft Acid and Base theory (HSAB), in our experimental system, Cu+ and In3+ ions act as soft and hard acids, and DDT is a soft base, thus the affinity of Cu+ ions to DDT is higher than that of In3+ ions due to the soft-soft compatibility.10,

25

According to the reports of Li’s group and Cui’s group, the formation

mechanism of the cation-diffusion process could be explained as follow:

10, 12

the intrinsic

+

characteristic that the Cu ions have high mobility at relatively high temperature allows the Cu+ ions to easily flow out of the lattice, and then the In3+ ions diffuse into the Cu1.94S lattice, and CuInS2 would be formed, which brings about the phase transformation. The transformation process of CuInS2 NCs from the intermediate Cu1.94S-CuInS2 heterostructured NCs will be discussed in the following part. When the In precursor is changed to InCl3, a distinct change in the crystal phase and morphology can be observed. The XRD pattern shown in Fig.1a indicates that Sample B is composed of wurtzite and zincblende CuInS2 phase. In consideration that the (002) plane of wutzite CuInS2 has the same diffraction distance with the (111) plane of zincblende CuInS2, the diffraction peak at 2θ=28.5o of Sample B is much stronger than that of Sample A, which is in consistent with the previous result.26, 27 As shown in Fig. 1c, all the nanocrystals are plate-like with their faces parallel to the substrates due to their large diameter. Obviously, the “face-to-face” aggregation of some nanoplates arises from the high surface energy due to the ultrathin CuInS2 nanoplates. The interplanar spacing of Sample B shown in the inset of Fig.1c is measured to be about 0.33 nm, which corresponds to (002) plane of wurtzite CuInS2 and (111) plane of zincblende CuInS2. As similar to the formation process of Sample A, the monoclinic Cu1.94S first nucleates and directs the further growth of wurtzite CuInS2 through the cation-diffusion process, which can be confirmed by the XRD results shown in the Fig.S3a†. Interestingly, the products obtained at 0 and 10 min exhibit a plate shape (Fig.S3b and c†), which demonstrates that the Cl- ions play a significant role in the phase and morphology change. It has been stated previously in Tang’s report that the Cl- ions are preferentially adsorbed on the (110) facets, thus the growth along the direction is more favorable.28 Due to the consistency of the (002) facets of wurtzite CuInS2 and the (111) facets of zincblende phase, the zincblende structure could grow on the face of the wurtzite CuInS2 nanoplates, which facilitates the formation of wurtzite–zincblende polytypism of CuInS2 nanoplates.26 To further study when and how the anions affect the morphologies of the CuInS2, a control experiment has been carried out, in which 5


following part. Accordingly, the XRD result of Sample A obtained at 0 min indicates the existence

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the InCl3 in ODE is injected quickly into the reaction system after the formation of Cu1.94S, and the corresponding XRD and TEM images are given in Fig.S4†. After injection of InCl3, plate-like CuInS2 nanocrystals are formed with some heterostructures (Fig.S4c†), which is consistent with the results of Sample A. When In(acac)3 is used as In source, the crystal phase and morphology


shown in the XRD pattern of Sample C suggests the resulting sample can be indexed as tetragonal chalcopyrite CuInS2 phase. Three obvious diffraction peaks correspond to (112), (204)/(220) and (312)/(116) planes of tetragonal chalcopyrite phase. It should be noted that the difference between the chalcopyrite and zincblende phases is very small, and it cannot often be distinguished from one another. Herein, however, a typical diffraction peak assigned to (101) plane of chalcopyrite phase is observed in the region of 15-20o, which is not present in the cubic zincblende CuInS2 phase. This result confirms the formation of tetragonal CuInS2 phase. The broadening of the diffraction peak suggests the nature of small size of Sample C. To further study the formation mechanism of Sample C, the XRD pattern and TEM image of Sample C obtained at 0 min indicate that the tetragonal chalcopyrite CuInS2 nanoparticles with very small size are formed at the initial stage of the reaction (Fig. S5†). This result suggests that the formation mechanism is very different from that of Sample A and B. A typical TEM image of Sample C obtained at 60 min indicates the resultant products are very small with a quasi-spherical shape (Fig. 1d). Further increasing the reaction time, the morphology of Sample C still remains irregular shape and the size is still small, and some aggregation appears in the sample obtained after 120 min (Fig.S6a-e†). The temporal evolution of UV-Vis absorption spectra of Sample C (Fig.S6h†) indicates that the absorption band shifts to longer wavelength with the reaction time increasing, and the absorption band obtained at 180 min is located at about 660 nm, which shows an obvious blue-shift as compared to the energy band gap of 1.07 eV for zincblende CuInS2. This blue-shift can be attributed to the quantum confinement effect due to their size less than Bhor radius.10 By comparison the difference in morphology and crystal phase of the samples synthesized using three different In sources, it can be seen that the anions in the In sources have a critical role in the formation of the structure and morphology of the as-obtained CuInS2 NCs. Considering that the formation process of Sample C is different from that of Sample A and B, two control experiments were carried out by varying the reaction temperature and the Cu/In precursor ratios based on the synthetic route of Sample C. When the reaction temperature is increased to 250 oC while the amount of Cu and In sources (Cu/In ratio is 1:1) is kept the same as that of Sample C, the XRD and TEM image of Sample D are depicted in Fig.2a and b respectively. As shown in Fig.2a, the as-obtained product may be indexed as zincblende CuInS2. It is noted that the diffraction peak in the range of 15-20o assigned to tetragonal chalcopyrite CuInS2 can be neglected as compared to that of Sample C (Fig.S7c in the supporting information). Irregular nanoparticles with average size less than 5 nm can be observed in Fig.2b. Even the reaction time is prolonged to 120 min, the size of the products is still small and some aggregation appears in the products (see Fig.S7a and b†). The absorption spectra of Sample D shown in Fig.S7d† indicate 6


are very different from those of Sample A and B. As depicted in Fig. 1a, all the diffraction peaks

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that the absorption band shifts to longer wavelength with the reaction time increasing, which is similar to that of Sample C. By comparison Sample C and D, there is little change in the size and morphology, but the crystal phase is changed from tetragonal chalcopyrite phase to cubic zincblende phase, which may arise from the disordered arrangement of metal ions at relatively


growth of CuInS2 NCs, in which metal nitrate salts are used as preucrsors.10 In our case, if the Cu/In precursor ratio is changed to 1.6:1 while other reaction conditions were kept the same as Sample C, the as-obtained products (Sample E) are composed of pure wurtzite CuInS2 phase, and no other obvious impurity is detected in the XRD pattern shown in Fig. 2a. Moreover, as shown in Fig.2c, Sample E exhibits a bullet shape with an average length of 10.1±1.3 nm and an average bullet caliber of 8.2±0.7 nm. It should be noted that the bottom edge of most nanobullets is much darker than the other part, which can be regarded as the formation of heterostructured Cu2S-CuInS2 NCs due to the excess Cu sources used in the synthesis of Sample E. As a matter of fact, the formation of Cu2S-CuInS2 heterostructures is an essential step during the growth of CuInS2 NCs, which has been discussed in detail by both the Cui’s and Kolny-Olesiak’s group, respectively.1, 12 In Cui’s work, the formation of the CuInS2 NCs can be regarded as three steps: the separate nuceation of Cu2S and CuInS NCs, the epitaxial growth of CuInS2 onto Cu2S phase to form Cu2S-CuInS2 heterostructured NCs due to little lattice distoration and the conversion from heterostructured NCs to CuInS2 NCs through the inter-diffusion of Cu+ and In3+ ions due to relatively high mobility of Cu ions.12 Further analysis of single particle can be confirmed by the HRTEM image shown in Fig.2d, and the as-measured lattice spacing of 0.32 nm corresponds to (002) plane of the wurtzite CuInS2 phase, indicating that Sample E grows along the direction of axis. The morphology evolution of Sample E with the reaction time is depicted in Fig.S6a-d†, and the nanorods and nanospheres as well as some heterostructured NCs coexist in the Sample obtained at 120 min, indicating that different nucleation and growth process take place with the exhaustion of Cu2S seeds, which has been observed by Kolny-Olesiak et al.1 As shown in Fig.S6e†, the diffraction peak indexed as (200) plane of zincblende CuInS2 is observed in the XRD pattern of the products obtained at 120 min, suggesting that zincblende CuInS2 phase appears in the later growth process of Sample E. The absorption spectrum of the aliquot obtained after 30 min (Fig. S6f †) shows the absorption edge beyond 750 nm, which could be ascribed to the defect-based absorbance.12 Based on the aforementioned experimental results, it can be concluded that the formation of Cu2S nuclei is critical for the formation of wurtzite CuInS2. Otherwise, the CuInS2 nuclei can be directly formed which leads to the formation of zincblende and chalcopyrite CuInS2 phase. It has been previously reported that the CuInS2 NCs with different crystal phases have different optical band gaps.10 Herein, optical absorption spectra have been employed to study the optical properties of the CuInS2 NCs with different crystal phase. Fig.3 displays the absorption spectra of Sample A-E dispersed in chloroform. As stated aforementioned that Sample C has a tetragonal chalcopyrite phase while Sample D is composed of cubic zincblende phase, and both of 7


higher temperature. Previous report has pointed out that the Cu/In precursor ratio is critical for the

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the two samples exhibit an irregular morphology with an average size less than 5 nm. It can be seen that Sample C and D exhibit a more or less pronounced absorption shoulder, and an obvious red-shift can be observed in the absorption spectrum of Sample D as compared to that of Sample C, which can be attributed to the quantum confinement effect. In contrast, Samples A and E with the


absorption shoulder is located at about 750 nm, which is in good agreement with the previous result for wurtzite CuInS2 NCs.10 Similarly, there is no well-defined excitonic absorption peak but a broad absorption with a tail up to 900 nm can be observed in Sample B with a wurtzite-zincblende polytypism. As a matter of fact, the feature of the absorption spectra of CuInS2 NCs is attributed to the intraband states or inhomogeneity of elemental distribution.8, 12 To further study the effect of the anions in the metal sources on the crystal phase and morphology of the as-obtained products, we chose a covalent compound CuCl to replace Cu(acac)2 as Cu sources to react with different In sources while other reaction conditions remained the same

as Sample A-C. Moreover, the synthesis of CuInS2 NCs has also been performed by

using Cu(NO3)2 and In(acac)3 as Cu and In sources, respectively. The XRD patterns and TEM images of Sample F-I are depicted in Fig.4 and Fig.5, respectively. As shown in Fig.4, Sample F possesses a pure zincblende phase of CuInS2 without any other impurity, but the size is small with an undefined morphology (Fig.5a and b). The morphology evolution of Sample F with the reaction time shown in Fig.S8a-f† indicates that very small spherical nanoparticles are formed at the initial reaction stage and then the nanoparticles grow into the large undefined nanoparticles with the increasing reaction time. The XRD patterns suggest that the as-obtained products are kept in zincblende CuInS2 phase for different reaction time (Fig.S9g†), in which no diffraction peaks are observed in the region from 15 to 20o, and the tetragonal chalcopyrite phase is ruled out. Similar to Sample B, the wurtzite–zincblende polytypism is formed when the InCl3 is used as In sources (Fig.4), and the hexagonal nanoplates are dominant in the morphology of Sample G (shown in Fig.5 c and d). To contrast with Sample A, Sample H consists of mixture of wurtzite and zincblende CuInS2 phase (Fig.4), which is synthesized using In(NO3)3 In source. Furthermore, the morphology is changed from nanodisks (Sample A) to nanoplates (Sample H) with the Cu precursor varying from Cu(acac)2 to CuCl (Fig.5e and f). This indicates that the Cl ions play an important role in the phase and morphology change. As a matter of fact, the morphology of Sample H is evolved from quasi-nanoflowers to nanoplates with the reaction time prolonging (Fig.S10a-c†). The XRD patterns shown in Fig.S8d† indicate that the wurtzite CuInS2 phase is dominant in the products obtained at early stage of the reaction, in which some inneglectable diffraction peaks assigned to monoclinic Cu1.94S phase can be observed. As the reaction time is increased, the diffraction peak indexed as (002) plane of zincblende CuInS2 phase appears in the products, which indicates the zincblende phase is present in Sample H obtained at longer reaction time. This can be attributed to the presence of Cl- ions in the Sample H, which makes the growth of the sample along the direction, and thus the nanoplates is formed at the later reaction stage. However, if we use Cu(NO3)2 and In(acac)3 as Cu and In sources while other reaction 8


wurtzite structure show an absorption over the wavelength range from 400 nm to 900 nm, and the

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conditions remain the same as Sample F-H, wurtzite CuInS2 phase is observed without any other impurity (Fig.4). As shown in Fig.5 g and h, mixture of nanodisks and small nanoparticles are obtained, in which the nanodisks can self-assemble into face-to-face chains (Fig.5g). The TEM images and XRD results shown in Fig.S11† confirm that the morphology is evolved from uniform


is unchanged when the reaction time is increased from 60 min to 180 min. Based on the XRD results of the products (Sample C, F and I) obtained by using different Cu salts and neutral In(acac)3 as In sources, three different structures including chalcopyrite, zincblende and wurtzite phases can be obtained, which indicates that the Cu sources have a vital effect on the crystal phase the CuInS2 NCs. The aforementioned results also confirm that the anions of metal sources play acritical effecton the crystal phase and morphology of the CuInS2 NCs. In previous study, oleic acid and oleylamine were often used to control over the crystal phase since they were hard bases, which could collaborate with the soft base DDT to offer suitable acidic or basic environment on the basis the hard-soft acid-base model.18, 29 However, only quasi-neutral DDT and neutral ODE were used in our case except the metal sources, thus it is difficult to interpret the effect of metal sources on the crystal phase and morphology merely in terms of the HSAB model. Moreover, Lei et al suggested that the acidity-basicity of the reaction system was essential for the formation zincblende CuInS2 phase, and the acidic or basic system was favorable for the formation of zincblende phase.18 However, herein, zincblende CuInS2 phase could be obtained in the neutral reaction system when the ratio of the Cu to In sources is kept at 1:1. We take Sample F as an example, CuCl is a covalent compound and In(acac)3 is an organometallic complex, and they should be neutral, but Sample F is a zincblende phase. If excess Cu sources are used in the neutral reaction system, wurtzite CuInS2 phase would be formed, such as Sample E. Therefore, it is difficult to explain the formation of zincblende CuInS2 phase merely by the acidity-basicity of the reaction system. As demonstrated by Pan et al that the Cu2S formed at the initial stage was a critical factor for the formation of wurtzite CuInS2 NCs.24 This deduction can be testified by Sample E. Moreover, in our case, wurtzite CuInS2 phase is also formed in the presence of NO3-, which is beneficial to the formation of Cu1.94S nuclei and the diffusion of In3+ into the Cu1.94S lattice. If the Cu(NO3)2 or In(NO3)3 is used as Cu or In source, a relatively acetic reaction system may be formed due to the hydrolysis of Cu2+ and In3+, which may enhance the reactant reactivity and thus facilitate the formation of CuInS2 NCs. However, CuInS2 nanoplates with wurtzite–zincblende polytypism can be formed in the presence of Cl- ions due to their adsorption on the specific facet. Therefore, the crystal phase and morphology of the CuInS2 NCs can be tailored effectively by fine selecting the anions in the sources, which opens a new stratery towards control over the morphology and structure of the ternary semiconductor nanocrystals.

4. Conclusions In summary, a simple one-pot colloidal approach has been adopted to prepare ternary CuInS2 9


nanospheres to the mixture of irregular nanodisks and small nanoparticles, and the wurtzite phase

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NCs with different structures and morphologies, in which DDT is used as sulfur source and neutral ODE serves as reaction media. It has been found that the crystal phase, size and morphology can be modulated effectively by manipulating the anions in the metal sources and the mole ratios of the reactants in the reaction. Relatively smaller CuInS2 nanoparticles with pure chalcopyrite and


nanocrystals with pure wurtzite phase have been successfully synthesized which is in close associated with the formation of Cu1.94S nuclei, which is favorable of further anisotropic growth of CuInS2 NCs. If the Cl- ions are present in the reaction system, CuInS2 nanoplates with wurtzite–zincblende polytypism would be formed due to the effect of Cl- ions on the growth process by specific adsorption. The as-obtained CuInS2 NCs with variable structures and morphologies exhibit different optical absorption properties. This study opens the door towards the manipulation of the crystal phase and morphology of inorganic nanocrystals via a simple chemical route in terms of varying anions in the metal precursors.

Acknowledgements This work is partly supported by the Fundamental Research Funds for the Central Universities (2014JBZ010), National Natural Science Foundation of China (61108063), and National Science Foundation for Distinguished Young Scholars of China (61125505).

†Electronic Supplementary Information (ESI) available: TEM images and XRD patterns of Sample A-I for different reaction times and the HRTEM image of Sample A, and the absorption spectra of Sample C-E, and the XRD patterns and TEM images of the product synthesized using hot-injection method. See DOI:10.1039/c4dtxxxx

Notes and references 1.

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zincblende phases can be formed in the neutral reaction system while relatively larger CuInS2

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7192-7198.

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Table and Figure Captions

Table.1 Summary of detailed reaction conditions, crystal phase and morphology of different

Fig.1 (a) XRD patterns of the Sample A-C using different In precursors at 240 oC for 60 min, and the bottom lines are the standard chalcopyrite (JCPDS 85-1575) and the simulated zincblende and wurtzite CuInS2; TEM images of the corresponding samples: (b) Sample A, (c) Sample B and (d) Sample C, and the insets of figure (b) and (c) show the corresponding HRTEM image. Fig.2 (a) XRD patterns of the Sample D and Sample E, and the bottom lines are the simulated reference zincblende and wurtzite CuInS2; TEM images of (b) Sample D and (c) Sample E; (d) HRTEM image of Sample E. Fig.3 Absorption spectra of the samples synthesized under different reaction conditions: (a) Sample C; (b) Sample D; (c) Sample E; (d) Sample A and (e) Sample B. All the samples were collected at 60 min and measured in chloroform. Fig.4 XRD patterns of Sample F-I, and the bottom lines are the simulated reference zincblende and wurtzite CuInS2. Fig.5 TEM images of (a, b) Sample F, (c, d) Sample G, (e, f) Sample H and (g, h) Sample I obtained at 60 min (top panel) and 120 min (bottom panel).

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CuInS2 NCs




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The effects of anions on the phases and morphologies of ternary CuInS2 nanocrystals have been described.

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TOC Figure

Phase-selective synthesis of Cu2ZnSnS4 nanocrystals using different sulfur precursors.

Complete colloidal synthesis of Cu₂SnSe₃ nanocrystals with crystal phase and shape control.

ZnS nanocrystals through hybrid flow reactor.

Atomic imaging of phase transitions and morphology transformations in nanocrystals.

Synthesis of CuInS2 nanocrystals from a molecular complex - characterization of the orthorhombic domain structure.

Phase transfer-based synthesis of HgS nanocrystals.

Complex perovskite oxide nanocrystals: low-temperature synthesis and crystal structure.

Facile Morphology-Controlled Synthesis of Organolead Iodide Perovskite Nanocrystals Using Binary Capping Agents.

Solution-phase synthesis of titanium dioxide nanoparticles and nanocrystals.

The phase transformation of CuInS2 from chalcopyrite to wurtzite.

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Synthesis of 1,4-bis(phenylethynyl)benzenes and their application as blue phase liquid crystal composition.

Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents.

Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol.

The evaluation of the impact of titania nanotube covers morphology and crystal phase on their biological properties.

Amelogenin Affects Brushite Crystal Morphology and Promotes Its Phase Transformation to Monetite.

The Photoluminescent Properties of New Cationic Iridium(III) Complexes Using Different Anions and Their Applications in White Light-Emitting Diodes.

Surfactant-controlled composition and crystal structure of manganese(II) sulfide nanocrystals prepared by solvothermal synthesis.

Facile Synthesis of Monodisperse Gold Nanocrystals Using Virola oleifera.

Using argument notation to engineer biological simulations with increased confidence.

Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals.

Facile synthesis of pentacle gold-copper alloy nanocrystals and their plasmonic and catalytic properties.

Using nature's platform to engineer bio-artificial lungs.

Corrigendum: Sustainable, Rapid Synthesis of Bright-Luminescent CuInS2-ZnS Alloyed Nanocrystals: Multistage Nano-xenotoxicity Assessment and Intravital Fluorescence Bioimaging in Zebrafish-Embryos.