发表文章毕业论文2014-YuanFML-Phase-controlled synthesis of nickel sulfide series via solvothermal

Phase-controlled synthesis of nickel sulfide series via solvothermal method Binxia Yuan* Shanghai Engineering Research Center of Power Generation Environment Protection Shanghai University of Electric Power, Shanghai 200090, P. R. China yuanbinxia100 Weiling Luan School of Mechanical and Power Engineering East China University of Science and Technology, Shanghai 200237, P. R. China Received 11 September 2013; Accepted 9 November 2013; Published 12 December 2013 Nickel sulfide series nanoparticles were synthesized by a simple solvothermal reduction method of nickel chloride and element sulfur in the oleylamine solvent. This method could offer potential advantages of mildness, safety, low cost, and simplified fabrication procedures. Through the adjustment of Ni/S raw material ratio, different phases of nickel sulfide, including cubic NiS2, hexagonal NiS, orthorhombic Ni7S6, and trigonal Ni3S2were obtained. In addition, the sulfur sources played important roles in the synthesis of nickel sulfide series compounds. The possible growth mechanisms had been discussed based on the influence of reaction temperatures and solvents on the phase structure and detailed composition of the final products. Finally, the obtained optical properties demonstrated that each sample had the unique absorption peak except cubic NiS2. Keywords: Nickel sulfide; phase transitions; chemical synthesis; optical properties. Sulfides of the transition metal show electronic and optical properties such as semiconductivity and photoconductivity.1,2 Among the inorganic complex, nickel sulfides are very interesting because of their promising uses as transformation toughening agents for materials applied in the semiconductor industry, catalysts for organic reactions, and coatings for photogalvanic cells.3,4According to the phase diagram,5 nickel sulfide compounds have five stable phases within room temperature range: NiS2, Ni3S4, NiS, Ni7S6and Ni3S2. Their physical andchemical properties show a strongdependenceon theircomposition,phasestructureandmorphology.Especially, nickel sulfide (NiS) has aroused increasing attention with applications in cathode materials of lithium batteries because of its high lithium activity, high theoretical capacity (590mAh?g?1), high electronic conduction and low cost,68 while nickel disulfide adopts the pyrite structure and exhibits semiconducting properties,9and Ni3S2undergoes similar volume-expansion phase transformations upon cooling and may act as a transformation toughener.10 Thus, the synthesis of the 3d transition metal sulfides had attractedgreatinterestsforseveraldecades.Traditionally,these sulfides were prepared through stoichiometric amount of the metal and sulfur heated in evacuated and sealed quartz ampoules at 5001000C.11However, the low melting point and volatilization of sulfur in some cases could make the control of composition difficult, and then several low tem- perature routes to group transition-metal sulfides were recently reported.12,13For the preparation of sulfides of nickel, several techniques, such as solid-state reaction,14chemical vapor transport,15and liquid phase technique,16,17had been devel- oped. By comparing with these methods, the solvothermal and hydrothermalmethodweremoreeffectivesynthetictechniques for preparing chalogenides with controlled phase structures and morphologies of the final products. For example, kinds of NiS morphologies, such as nanoparticle,18nanorod,19nano- bells,20hollow nanosphere,21,22etc., and NiS2dodecahe- drons23,24and dendritic25were fabricated by hydrothermal or solvothermal method. However, these methods almost re- quired multistep reaction processes, expensive additives or long reaction time. In our continuous efforts to develop a simple method, we had been interested in the preparation of nanostructures using a single-source molecular precursor method,in which an individual reactant molecule contained all theelements requiredin thefinal product.26This methodcould not only offer potential advantages of mildness, safety, low cost and simplified fabrication procedures, but also made it *Corresponding author. Functional Materials Letters Vol. 7, No. 1 (2014) 1450003 (6 pages) World Scientific Publishing Company DOI: 10.1142/S1793604714500039 1450003-1 easier to control the reaction process when compared with the useofmultiplereactionsourcesrequiringexpensiveprecursors or toxic solvents. In the paper, nickel sulfide series (cubic NiS2, hexagonal NiS, orthorhombic Ni7S6and trigonal Ni3S2) were facilely synthesized through a convenient and environmentally be- nign method for the first time. Through the adjustment of the molar ratio of raw material, the four stable phases had been obtained at room temperature, and the compositions and morphologies could be easily controlled. The formation process of nickel sulfide series was also analyzed. Finally, the absorption properties were measured in order to optimize properties of nickel sulfide series. To the best of our knowledge, this is the first report on synthesis of four kinds of nickel sulfide with the same method. Nickel chloride (NiCl2? 6H2O, analytical reagent), sulfur powder (S, 99.5%), and thiourea (CH4N2S, analytical re- agent) were purchased from Shanghai Chemical Reagent (SCR). Oleylamine (OLA, 70%), 1-octadecene (ODE, 90%, Fisher) and L-cysteine (99%) were purchased from Fluka. All chemicals and solvents were used directly without any further purification and the whole operations were carried out in the open-air. In a typical synthesis of nickel sulfide series, 1mmol NiCl2? 6H2O and a certain amount of sulfur powder were put into a three-neck flask of 50mL capacity, and 10mL OLA was added in as the solvent. After that, the mixture was heated to 260C at the rate of 25C/min and kept for 1h under magnetic stirring. Then, the flask was allowed to cool down to room temperature naturally. The sample was washed with ethanol several times in order to remove the excess solvents. Finally, the precipitate was dried, and utilized for further characterization. Through adjusting the amount of sulfur powder, the different nickel sulfide phases were obtained. For cubic NiS2, orthorhombic Ni7S6and trigonal Ni3S2, the amount of sulfur powder was strictly controlled in 10, 6/7 and 0.5mmol, respectively. The hexagonal NiS sample was obtained by controlling the amount of sulfur powder from 1 to 6mmol. X-Ray Diffraction (XRD) pattern were recorded with a Rigaku D/max 2250V diffractometer operating with Cu K radiation, and the operation voltage and current were set as 40kV and 100mA, respectively. Transmission electron mi- croscope (TEM) images were acquired using a JEOL JEM- 2100F operated at an acceleration voltage of 200kV, and the samples were prepared by dipping an amorphous carbon copper grid in dilute solution of samples dispersed in chlo- roform. Energy-dispersive spectrum (EDS) was taken on JEOL JSM-6360LV scanning electron microscope. The ab- sorption properties were acquired by Cary 50 UVVis spectrophotometer, the samples were provided by sonicating the samples in chloroform (CHCl3). In our solution-phase reaction system, a feasible and ho- mogeneous environment was provided for the nucleation and growth of the products. It was found that the original ratio between the two reactants (Ni/S) played important roles in the formation of the nickel sulfide series. Table 1 listed the reaction conditions and the yields for the synthesis of NiS2, NiS, Ni7S6and Ni3S2. The yields are calculated with respect to NiCl2? 6H2O. The phases and impurities of the prepared samples were investigated by the XRD analysis. Figure 1 showed the XRD patterns of the nickel sulfide series compound. All the dif- fraction peaks in the patterns can be identified as pure phase compounds of cubic NiS2, hexagonal NiS, orthorhombic Ni7S6and trigonal Ni3S2. The results were consistent with the value given in the standard card (cubic NiS2: JCPDS No. 11-0099; hexagonal NiS: JCPDS Card No. 02-1280; trigonal Ni3S2: JCPDS No. 44-1418). No impurity peaks (such as sulfur, Ni9S8, or Ni3S4, etc.) were observed, which indicated that the products were pure. The sharp and narrow peaks indicated the high crystallization of the products. The crystal sizes and typical morphologies of the nickel sulfide series samples were investigated by TEM images, as shown in Fig. 2. Figure 2(a) showed the TEM image of NiS2 obtained at the Ni/S molar ratio of 1:10, which demonstrated the products consisted of cubic-like and globular-like parti- cles. It could be thought that globular-like was the original state and would become the cubic-like morphologies with Table 1.The reaction conditions of nickel sulfide series synthesized from NiCl2? 6H2O. T(C)Ni/S molar ratioResidence time (h)ProductYield (%) 2601:101NiS282 2601:31NiS93 2607:61Ni7S694 2602:11Ni3S280 Fig. 1.XRD patterns of cubic NiS2, hexagonal NiS, orthorhombic Ni7S6, and trigonal Ni3S2. B. Yuan (b) and (f) NiS (Ni: S1:3); (c) and (g) Ni7S6(Ni: S7:6); (d) and (h) Ni3S2(Ni: S2:1), respectively. Phase-controlled synthesis of NiS series via solvothermal method 1450003-3 at 260C. If the reaction temperatures were over-high or over- low, the transformation process could not be observed in the XRD patterns. As the reaction energy cannot overcome the energy barrier under relatively low reaction temperature (240C), the cubic NiS2phase was not observed in the XRD patterns with the sulfur content rising. At the relatively adaptive reaction temperature (260C), the system had suffi- cient thermal energy to completely transform into cubic NiS2 with sulfur source adding. While the resultant temperatures were increased to above 260C, the rapid evaporation rate of sulfur resulted in decreasing the actual sulfur pressure and could not obtain cubic NiS2. Thus, over-high or over-low reaction temperatures would disturb the reactive system bal- ance and restrain the formation of pure cubic NiS2crystals. To investigate the morphologies variation from hexagonal NiS to cubic NiS2occurring at 260C, TEM analysis was adopted and the results were shown in Fig. 4. When the molar ratio of Ni/S was adjusted into 1:8, Fig. 4(b) showed the TEM image of the main NiS2phase with a large quantity of near-cubic particles, in agreement with the XRD patterns (Fig. 3). The chemical compositions of these samples were further investigated by using EDS (Figs. 4(d)4(f). The molar ratios of Ni and S were calculated as 1:1.123, 1:1.756, 1:1.978 for hexagonal NiS, mixed phase, cubic NiS2, re- spectively. The EDS results further indicated the evolution process. It was worthwhile to mention that addition element sulfur had strong effects on the formation of nickel sulfide series. If thiourea and L-cysteine were taken as sulfur source while keeping other conditions unchanged, the acquired samples had different morphologies and phase structures. When ele- ment sulfur was used instead of L-cysteine, the obtained sample was amorphous phase rather than nickel sulfide serie crystallization.Whenelementsulfurwas replacedbythiourea, three nickel sulfide series (hexagonal NiS, orthorhombic Ni7S6and trigonal Ni3S2) were obtained. Figure 5 showed the TEM images of the corresponding products, which was different from nickel sulfide series obtained by element sulfur. It was worthwhile to investigate the growth mechanism of nickel sulfide compounds. In the most relative reports,27,28an organic solution-phase synthetic route and its alternatives in diverse surfactants or solvents had demonstrated to be one of the most effective approaches for obtaining the samples with various shapes and sizes. Moreover, it provided a way for understanding the precipitation and growth mechanism, in which the growth of particles generally occurred through pri- marymechanisms afterfastnucleationinsolution:theOstwald ripening process and aggregation growth process.2931 Firstly, temperature-dependent experiments were per- formed in order to understand the formation process. It was (a)(b) (c)(d) (e)(f) Fig. 4.TEM images and EDS patterns of the conversion process from hexagonal NiS to cubic NiS2at the different molar ratio of Ni/S: (a) and (d) 1:3, (b) and (e) 1:8, (c) and (f) 1:10, respectively. (a)(b) (c) Fig. 5.TEM images of the nickel sulfide series: (a) NiS (Ni/thiourea1:4), (b) Ni7S6(Ni/thiourea7:6), and (c) Ni3S2(Ni/thiourea3:2) at the reac- tion temperature for 260C. B. Yuan & W. Luan 1450003-4 found that when the reaction temperature was decreased to 220C, the final products wereclear and green solution and no precipitation can be obtained. With reaction temperature in- creasing, black precipitate was observed. When the reaction temperature was increased to 240C, the XRD indicated that the samples were composed of an amorphous peak and several weak nickel sulfide series crystallization peaks. It indicated that nickel sulfide series nuclei started to appear. However, through extending the reaction time to a longer period, an amorphous peak still existed in the final product, which demonstrated that the system had no sufficient thermal energy tocompletelynucleationatrelativelylowreactiontemperature. With reaction temperature further increasing to 260C, nickel sulfide series with different morphologies were obtained. Thus, at higher reaction temperatures, the critical nuclei size as well as the energy barrier decreased, thereby developing into aggregation growth process. Secondly, if OLA solvents were fully replaced by ODE, the black precipitation was amorphous phase through XRD analysis. According to the report on CdS and CdSe NCs synthesis,3234the addition of OLAwill induce two competitive mechanisms, decelerating the growth by providing increased ligand coverage for the surface of nano- particles and accelerating the growth by activating the pre- cursor. Under the reaction temperature as 260C, the ligand apart from the surface of NCs was dominated in the quasi-gas state. Thus, a high reaction temperature would promote the enhancement of the activation capability of the OLA toward the reactivity of the precursors. Finally, to further understand the formation process, two precursor solutions were prepared. One mmol NiCl2? 6H2O and 1mmol S powder was separately added into a flask containing 5ml OLA. After that, the pre- cursorswereheatedto150Cfor30mininvigorousstirring.It was found that the clear green and yellow solutions were got, respectively. Afterwards, the two kinds of precursor were mixed together at 100C, the solution was a black suspension. Thus, it was possible that Ni2and S powder combined with OLA form relatively stable Ni-S-OLA precursor and then decomposed to form nickel sulfide series for later growth, or Ni2and S powder combined with OLA obtaining relatively stable Ni-OLA and S-OLA precursors separately and then directly integrated to obtian nickel sulfide series. To further prove it, form Ni-S-OLA or Ni-OLA as well as S-OLA pre- cursors separately. UVVis and IR analysis were used to study the reaction process of nickel sulfide series compound, as shown in Fig. 6. From the UV spectra, the absorption curve of the mixture was not the superposition of the Ni and S pre- cursors absorption curves. It could be possible that the pre- scuror was relatively stable Ni-S-OLA precursor. To further prove the conclusion, we also analyzed IR spectra. From the pattern, it revealed that the Ni-S-OLA included the character- istic peaks of NiS and OLA. The IR pattern of NiS sample had the saturated alkane (CH) peaks in 2919cm?1and 2850cm?1, which showed the samples surface connected with the organic groups, in accordance with TEM image and improving its stability. Thus, it illustrated that the formation of nickel sulfide series had a relation with temperatures and kinds of solvents. A possible schematic illustration for the formation process of nickel sulfide series was shown in Fig. 7. The optical properties of nickel sulfide series compounds dispersed in anhydrous chloroform (CHCl3) by sonication were studied, as shown in Fig. 8. The trigonal Ni3S2samples (a)(b) Fig. 6.UVVis and IR spetra of formation process of nickel sulfide. Fig. 7.Schematic diagram of the formation process. Fig. 8.UVVis absorption spectra of suspensions nickel sulfide series dispersed in CHCl3. Phase-controlled synthesis of NiS series via solvothermal method 1450003-5 exhibited a weak optical absorption locating at about 428nm. Comparing with the trigonal Ni3S2samples, the orthorhom- bic Ni7S6products possessed a well-defined, broad optical absorption peak at 484nm. A strong peak centered at around 508nm together with one weak peak at around 625nm can be observed in the hexagonal NiS samples. The absorption spectrum showed an obvious red shift compared with that of NiS hollow spheres and urchin-like structures,20,35which could be attributed to the 34nm near-spherical-like NiS with the quantum sizes effects. While for cubic NiS2products, no absorption peak was observed and it could be thought that the optical properties of NiS2crystals were strongly depended on its structure. In summary, through simple control the Ni/S molar ratio of the raw materials, the nickel sulfide series (cubic NiS2, hexagonal NiS, orthorhombic Ni7S6and trigonal Ni3S2) were selectively synthesized in flask via a one-step solvothermal- reduction process. With the sulfur content increase, hexago- nal NiS gradually transformed into cubic NiS2along with the morphologies change. Meanwhile, sulfur sources (L-cysteine and thiourea) played important roles in the phase structure and morphology of nickel sulfide series, the results indicated thiourea can obtaine three kinds of nickel sulfide series with different morphologies. Regarding the formation process of nickel sulfide series, it was considered that Ni2and S powder combined with OLA form relatively stable Ni-S- OLA precursor and then decomposed to form nickel sulfide series for later growth. In addition, the abso