Directionally Dendritic Growth of Metal Chalcogenide Crystals Via Mild Template-Free Solvothermal Method

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Journal of Crystal Growth 283 (2005) 230–241 www.elsevier.com/locate/jcrysgro

Directionally dendritic growth of metal chalcogenide crystals via mild template-free solvothermal method

Ai-Miao Qina, Yue-Ping Fanga, Wen-Xia Zhaoa, Han-Qin Liua, Cheng-Yong Sua,b,Ã

aSchool of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China bThe State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

Received 10 February 2005; received in revised form10 April 2005; accepted 17 May 2005 Available online 7 July 2005 Communicated by R.M. Biefeld

Abstract

Dendritic metal chalcogenide (HgS, CdS and PbS) single crystals have been prepared via facile and mild solvothermal process in water or ethanol/water systems. SEM and TEM observations show that HgS and CdS dendrites exhibit whole or part of the two-dimensional (2D) hexagonal snowﬂake spatial pattern while PbS exhibits whole or part of the three-dimensional (3D) octahedral star-like morphology. Selected area electron diffusion (SAED) and HRTEM show that all of these dendrites are single crystals whose shape evolution is closely related to their intrinsic crystal symmetries. The factors inﬂuencing the evolution of the crystal morphology, i.e. reactant molar ratio, solvent, sulfur source, reaction time and temperature have been investigated. Two dimensional six-fold symmetrical HgS and CdS dentrites and 3D four-fold symmetrical PbS dentrite have been fabricated by a facile solvothermal method. The crystal evolution process has been investigated. r 2005 Elsevier B.V. All rights reserved.

Keywords: A1. Dentrite; A2. Solvothermal crystal growth; B1. Nanocrystals; B2. Metal chalcogenide

1. Introduction size is an important goal of modern materials chemistry although this still remains a key obstacle Construction of nano- or microscopic-scale to overcome [1]. In recent years, self-assembly of inorganic materials with well-deﬁned shape and meso-, micro-, or nanostructured functional materials with organized product arrangement has ÃCorresponding author. School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, become an active research ﬁeld in material China. Tel./fax: +86 20 84115178. synthesis and device fabrication [2], partly stimu- E-mail address: [email protected] (C.-Y. Su). lated by the success of supramolecular synthesis

A.-M. Qin et al. / Journal of Crystal Growth 283 (2005) 230–241 231 strategy which resembles many natural dynamical paper, Qian et al. reported hierarchical growth processes where formation and organization take of dentritic HgS [24] with hydrothermal treat- place simultaneously [3]. If the shape evolution ments of a Hg(II)-thiourea complex. In an effort processes are understood and predicable, it could to explore the programmable dendritic growth be possible to programthe systemto fabricate path of metal chalcogenide, we carried out a materials with desired morphology and crystal- systematic investigation to synthesize HgS, CdS linity. The subject of inorganic dendrites has been and PbS dendrites via a hydro/solvothermal drawing much attention since Nittmann and process in water or mixed ethanol/water system. Stanley [4] reported dendritic growth patterns in Hexagonal branched HgS and CdS crystals and 1986. Many models and theories have been octahedrally branched PbS crystals have been proposed to explain this ubiquitous phenomenon, obtained, whose branching modes was found to most of them focused on nonequilibrium growth exactly match the inherent crystal symmetry of the and molecular anisotropy [5]. However, the metal chalcogenide. complete understanding and controllable growth of dentrites still represent a great challenge to practitioners in many fields. 2. Experimental procedure Metal chalcogenides have attracted considerable research interest for many decades due to their 2.1. Instrumentation unique semiconducting and optical properties [6,7]. For example, mercury sulfide is a useful SEM images were obtained using a JSM-6330F material which can be widely used in ultrasonic operating at 20 kV and TEM and HRTEM transducers, catalysts, electrostatic image materi- images were obtained using a JEOL JEM-2010 als and photoelectric conversion devices [8–11]. with an accelerating voltage of 200 kV. The XRD Further more, Single HgS (cinnabar) crystal is a patterns were recorded with a D/Max-IIIA dif- famous natural chiral crystal and expected to be ( fractometer with Cu Ka radiation (l ¼ 1:54056 A) used for stereochemical reactions [12]. Much effort at a scanning rate of 0.071 sÀ1 for 2y ranging has been devoted to the synthesis of metal from10 1 to 701. A Shimadzu spectrophotometer chalcogenide of various morphologies, such as (model 2501 PC) equipped with an integrating rods [13], wires [14], tubes [15], etc., with various sphere was used to record the UV–vis diffuse techniques and methods involving microwave reflectance spectra of the sample. Raman spectro- synthesis [16], sonasynthesis [17] and electroche- scopy measurements were carried out using a mical deposition [18] and so on. The formation of Renishaw RM 3000 spectrometer with the spatial patterns of metal chalcogenide is of special 632.8 nmwavelength line froma He–Ne (25 mW) interest because subtle influencing factors may laser. result in completely different crystal shape. Qian et al. reported the synthesis of rod-based PbS dendrites with different morphologies via a sol- 2.2. Growth of HgS crystalline dentrites vothermal process [19]. Liu et al. applied surfac- tant to assemble PbS dendrites with cross-like In a typical procedure, 0.272 g (1 mmol) mercury morphology [20]. Qi et al. used thermal decom- chloride (HgCl2) was dissolved in 18 ml distilled position method to prepare the star-shaped PbS water and 0.304 g (4 mmol) thiourea (Tu) was crystals [21]. Multi-pod or dentritic CdS nanorods added by stirring, then the mixture was transferred have been obtained by means of thermal decom- into a 25 ml Teflon-lined autoclave and heated to position or evaporation-condensation methods 140 1C and maintain for 12 h. After the autoclave [22], respectively, and spindle-like morphology was cooled naturally to roomtemperature, the mediated by cauliflower- and branching-like mi- precipitate was collected and washed with distilled cropatterns of CdS was achieved by the solvother- water and absolute ethanol for several times, and mal process [23]. During the preparation of this then dried at 50 1C for 4 h. ARTICLE IN PRESS

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2.3. Growth of CdS crystalline dentrites ethanol for several times, and then dried at 60 1C for 4 h. The synthesis process is similar to that of HgS but the temperature, solvent and the molar ratio of 2.4. Growth of PbS crystalline dentrites the starting materials are different. 0.457 g

(2 mmol) cadmium chloride (CdCl2) was dissolved Lead acetate (Pb(CH3COO)2) of 1.138 g in a mixture of 24 ml EtOH/H2O (v/v ¼ 2:1), (3 mmol) was dissolved in a solvent of 33 ml and 0.457 g (6 mmol) ammonium thiocyanate EtOH/H2O (v/v ¼ 1:2 v/v), and 0.229 g (3 mmol) (NH4CSN) was added with stirring. The resulting thiourea (Tu) was added by stirring. The resulting mixture was transferred into a 30 ml Teﬂon-lined mixture was transferred into a 40 ml Teﬂon-lined autoclave and heated to 170 1C and maintain for autoclave and heated to 120 1C and maintain for 12 h. After the autoclave was cooled naturally to 12 h. After the autoclave was cooled naturally to roomtemperature, the precipitate was collected roomtemperature, the precipitate was collected and washed with distilled water and absolute and washed with distilled water and absolute

Fig. 1. Representative powder X-ray diffraction patterns of obtained samples: (a) HgS, (b) CdS and (c) PbS. ARTICLE IN PRESS

A.-M. Qin et al. / Journal of Crystal Growth 283 (2005) 230–241 233 ethanol for several times, and then dried at 60 1C compared with the standard reflection patterns of for 4 h. HgS and CdS, respectively, which is probably related to the preferential orientation of the crystals. The diffraction peaks in Fig. 1(c) are well 3. Results and discussion matched cubic phase PbS with calculated lattice ( constants a ¼ 5:935 A, (JCPDS No. 05-0592, ( 3.1. Crystal morphologies and characterization a ¼ 5:9362 A). The morphologies and structures of the samples The representative XRD patterns of obtained are examined by SEM and TEM. Fig. 2a–d shows HgS, CdS and PbS are shown in Fig. 1. All the the representative SEM images of the obtained reflections in Fig. 1(a) and (b) can be indexed to HgS, CdS and PbS samples. It can be seen that all hexagonal phase HgS and CdS, with calculated the samples have dendritic structures. The spatial ( lattice constants a ¼ 4:146, c ¼ 9:500 A, (cinnabar, patterns of HgS and CdS are quite similar, ( JCPDS No.06-0256, a ¼ 4:149, c ¼ 9:495 A) displaying whole or part of the hexagonal snow- ( and a ¼ 4:129, c ¼ 6:697 A (JCPDS No.77-2306, flake-like morphology, while PbS exhibits whole or ( a ¼ 4:136, c ¼ 6:713 A), respectively. It is worth part of the octahedral star-like pattern. For noting that the (0 0 3) reflection in Fig. 1a and the simplification, the snowflake-like HgS and CdS (0 0 2) reflection in Fig. 1b are relatively strong can be considered to possess six main trunks which

Fig. 2. SEM images of as-prepared products: (a) and (b) HgS with reactant molar ratio of HgCl2/Tu ¼ 1:4 for 12 h at 120 1C. (c) CdS with reactant molar ratio of CdCl2/NH4CNS ¼ 1:3 for 12 h at 170 1C, EtOH/H2O ¼ 75%. (d) PbS with reactant molar ratio of Pb(CH3 COO)2/Tu ¼ 1:1 for 12 h at 120 1C, EtOH/H2O ¼ 33%. ARTICLE IN PRESS

234 A.-M. Qin et al. / Journal of Crystal Growth 283 (2005) 230–241 are six-fold symmetry related. Each trunk contains secondary and tertiary branches which emerge in parallel at 601 with respect to the mother branch. Therefore, the branching mode of such pattern may be crystallographically assigned to along all ½2 1¯ 10¯ directions within a hexagonal crystal system(Bravais–Miller indices are used here to emphasize the hexagonal symmetry, otherwise, Miller indices are used in general for convenient comparison with the cited literatures). For an individual trunk in ½2 1¯ 10¯ direction, the secondary branches can be considered to grow in ½1 2¯ 10¯ and ½ 1¯ 120¯ directions, and the tertiary branches for the ½1 210¯ directed secondary branch grow in ½2 1¯ 10¯ and ½1¯ 120¯ directions (vice versa). The length of the mail trunk of HgS and CdS is about 20 and 6 mm, respectively, with the width of the secondary branch lies in the order of hundreds nanometers. By contrast, six main trunks of the PbS crystal orientated ortho- gonally in three dimensions other than in a plane as for HgS and CdS. Each trunk contains the secondary branches which are vertical with respect to the main trunk and aligned in parallel in four directions displaying four-fold symmetry. Therefore, the branching mode for such pattern may be crystallographically regarded to Fig. 3. TEM image of a single dentrite crystal of as-prepared along all ½100 directions for a cubic crystal HgS and SAED patterns taken in its different area as labeled. system. For an individual trunk in [0 0 1] direction, the secondary branches can be considered to grow in [1 0 0], ½100¯ , [0 1 0] and ½0 10¯ directions (vice versa). The size of the trunk and branch of PbS dentrite is comparable with that of HgS. Actually that the dentrite is a single crystal and grows most of the dentrites are observed in part of the parallel to (0 0 1) plane, agree with the XRD whole pattern. The possible reason will be results mentioned above. The HRTEM images discussed later, and, no matter the crystal is grown taken for different tips (circled in Fig. 4a)inan in part or whole pattern, the branching mode has individual CdS dendrite are shown in Fig. 4, which no difference. can be attributed to the [0 0 1] zone axis diffrac- To investigate the crystallinity of the products tion. All the images exhibit an interplanar distance select area electron diffraction (SAED) patterns of 0.357 nmbelonging to the (1 0 0) lattice plane of and high-resolution transmission electron micro- the hexagonal CdS phase, suggesting that the scopy (HRTEM) images have been taken. Fig. 3 dentrite grows parallel to (0 0 1) plane, in agree- shows SAED patterns for an individual HgS ment with the XRD analysis and observations for dentrite. The sharp spots are indicative of good HgS dentrite. The SAED and HRTEM results for crystallinity and consistent with the hexagonal PbS dentrite are quite similar to our previous phase indexed fromXRD reﬂections. The SAED results [20] and will not include in this contribu- patterns taken in different area of the individual tion. The preferential growth in [1 0 0] direction is dentrite (circled in Fig. 3f) are identical, suggesting obvious. ARTICLE IN PRESS

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Fig. 4. TEM image of a single dentrite crystal of CdS prepared with reactant molar ratio of CdCl2/NH4CNS ¼ 1:3 for 12 h at 170 1C, EtOH/H2O ¼ 75% with SAED pattern as insert and HRTEM taken in its different area as labeled.

3.2. Spectral properties liminary photoluminescence measurement on CdS powder with the excitation wavelength at 455 nmis Fig. 5a and b depict the optical diffuse reﬂection shown in Fig. 6. The PL spectrumshows spectra of the as-prepared HgS and CdS samples. structured emission peaks with the maximum at The optical reﬂection edge of HgS and CdS are ca. 502 nm, which is close to the band gap of CdS. 550 and 500 nm, respectively. which give the Raman spectra of HgS and PbS samples measured estimated band gap of HgS and CdS to be ca. with 632.8 nmlaser excitation are present in Fig. 7. 2.25 eV (bulk HgS 620 nm, 2.00 eV [8]) and The bands at 252, 283 and 342 cmÀ1 in Raman 2.48 eV (bulk CdS 512 nm,2.42 eV [25]). A pre- spectrumof HgS ( Fig. 7a) are assigned to Hg–S ARTICLE IN PRESS

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peak at 415 cmÀ1 ascribed to the 2LO phonon mode [28]. The mode at 205 cmÀ1 arising from ﬁrst-order longitudinal optical (LO) phonon near the G-point of Brillouin zone is absent in our experiment probably due to forbidden scattering [29].

3.3. Crystal growth process and mechanism

The general reaction routes may be represented by the following process:

MX2 þ nL !½MLnX2 (1) (M ¼ Hg, Cd, Pb; X ¼ Cl or Ac; L ¼ Tu or NH4CNS, n ¼ 1À4). decomposition ½MLnX2 À! MS: (2) Fig. 5. Reflection spectra of the as-prepared HgS and CdS According to the nonequilibriumgrowth theory, powders: (a) CdS, (b) HgS. dendritic pattern growth fromsolution depends on the applied supercooling conditions, so the external conditions may significantly affect the morphology evolution of the crystals. In order to understand the crystal growth process, various factors including the reactant molar ratio, solvent, reaction temperature and time, and sulfur source have been investigated. The experimental results indicate that the phase and morphology of HgS are remarkably influ- enced by reactant molar ratio (Rn) than those of CdS and PbS. Table 1 shows the relationship between Rn and the phase and morphology for three reactions. It was found that excess thiourea (Rn41) was needed for HgCl2 to be completely converted to HgS, otherwise, only primrose or white product was obtained which is likely to be Hg2Cl2S [9]. When Rn was kept between 1 and 2, the HgS crystals exhibit mixed morphology. When Fig. 6. Photoluminescence spectra of the as-prepared CdS. Rn42, the product displays a scarlet color and dendritic morphology with a pure cinnabar phase is formed. stretching modes, which are consistent with the Using coordination agents as templates to spectrumof the pure vermilion or cinnabar HgS control the morphologies of the crystallites [26,27]. The spectrumof PbS shows five bands have been frequently reported in literatures at 143, 270, 415, 600 and 971 cmÀ1. The peaks [13,22a,30,31], and complex structure controlling below 150 cmÀ1 are tentatively attributed to the mechanisms were proposed to explain the effect of so-called plasma lines of the excitation laser, above coordinate agents on the product morphology. 960 cmÀ1 are attributed to oxide-sulfates. The peak In the present cases, thiourea (or thiocyanate) at 270 cmÀ1 is due to two phonon process and the acts not only as a sulfur source but also as a ARTICLE IN PRESS

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Fig. 7. Raman scattering spectra of the as-prepared HgS and PbS powders: (a) HgS, (b) PbS.

Table 1 is. Therefore, the geometric structure of coordina- The relationship between reactant molar ratio (Rn) and tion precursors may play a key role to the morphologies and phase of the products formation of the morphology of the product.

Rn HgS CdS PbS It is believed that pattern growth of a crystal depends on the delicate balance between faceting 1 Mixture Dendritic Mixture 2 and branching, the former tends to make flat 1 Mixture Dendritic Dendritic surface while the later prefer to formcomplex 2 Particles, rods and dendritic Dendritic Dendritic 3 Dendritic Dendritic Dendritic structure [5]. Once the crystal growth is dominated 4 Dendritic Dendritic Dendritic by faceting, polyhedral morphology will be ob- 8 Dendritic — — tained. On the contrary, pattern-forming is dic- tated by branching process. Since the formation of MS in the present conditions is followed by decomposition of coordination precursors and 2À coordination agent. Reaction of HgCl2 with slow release of S fromthe organic compounds, thiourea can forma series of complexes with it is understandable the reaction mixture constitu- different stoichiometry (HgCl2(Tu)n, n ¼ 1 À 4) tes a nonequilibriumsystem.At the initial stage, [32] and structural geometry, serving as coordina- polyhedral nucleation is normally started, but tion precursors for crystal growth under hydro- further growth may be controlled by branching thermal conditions. When n ¼ 1 or 2, the due to ‘‘Mullins–Sekerka instability’’ of diffusion precursors exhibit 1D infinite chain structure, thus [5]. For HgS and CdS, hexagonal nucleus leads to the growth of HgS crystallites resulted in rod-like six corners which grow faster than the rest of the morphology due to faster growth along one crystal and finally give snowflake pattern with six- direction. When n ¼ 3 or 4, the geometry of the fold symmetry imparted from the inherent hex- precursors are 3D, therefore, pattern morphology agonal crystal system. As for PbS star-like pattern of the HgS crystals could be obtained. For the case may be caused by branching of octahedral nucleus of CdS and PbS, the metal complexes are normally [30] which give six apexes sticking out with formed with Cd2+ and Pb2+ ions in tetrahedral, intrinsic cubic crystal system. Since branching square or octahedral geometry, so pattern growth directions exactly follow ½2 1¯ 10¯ directions for of MS could be always possible no matter what Rn HgS (or CdS) and [1 0 0] directions for PbS, the ARTICLE IN PRESS

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ﬁnal crystal pattern will be inevitably dentritic apparently subject to subtle variations of the displaying respective symmetry. On the other hand, external conditions which may lead to different because nucleation happens simultaneously along polyhedral nucleus. Similar oriented crystal growth decomposition of the coordination precursors, has also been observed by others [33]. most of the nucleuses may stick on the surface of Solvent systemrepresents another important the precursors and prevent formation of the whole factor affecting the morphology evolution. Table 2 pattern of the dentrites. This probably accounts for shows the effect of the solvent on the phase and the reason why broken dentrites are observed morphology for three reactions. When used quantitatively. On the basis of this point, forma- EtOH/H2O (20–80%) as solvent, dentritic crystals tion of varied spatial patterns [19–24] with different can be always obtained for all three reactions. For symmetry is feasible because initial nucleation is HgS, pure phase could not be obtained when use

Table 2 The effect of solvent on the phase and morphologies of the products with Rn ¼ 3

Solvent HgS CdS PbS

Structurea Morphology Structure Morphology Structure Morphology

VEtOH/Vwater 20–80% H Dendritic H Dendritic, FCC Dendritic 100% H+FCC Mixed H Spindle-like FCC Multiarmed 0% H Dendritic H Cauliﬂower-like FCC Dendritic DMF FCC Rod-like H Spherical FCC Mixed

aH—hexagonal and FCC—face-centered cubic.

Fig. 8. (a) CdS prepared with water as solvent for 12 h at 170 1C. (b) CdS prepared with ethanol as solvent for 12 h at 170 1C. ARTICLE IN PRESS

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Fig. 9. The morphologies of PbS prepared with reactant molar ratio of Pb(CH3 COO)2/Tu ¼ 1:1 for 12 h at 120 1C in different solvent system: (a) EtOH/H2O ¼ 20%, (b) EtOH/H2O ¼ 60%, (c) EtOH/H2O ¼ 80%, (d) EtOH, (e) H2O and (f) DMF. ARTICLE IN PRESS

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Table 3 The effect source on the phase and morphologies of the products with Rn ¼ 3

S source HgS CdS PbS

Structurea Morphologyb Structure Morphology Structure Morphology

Na2S FCC P H P FCC P KCNS FCC+H P H D FCC D

CH3CSNH2 H D H C FCC D (NH2)2NHCS H D H C+HS FCC D DDTC FCC OP H HS FCC P

NH4CNS F+H P H D FCC R

aFCC—face-centered cubic and H—hexagonal. bP—particle, D—dendritic, C—cauliﬂower-like, HS—hollow spherical, OP—octahedral particle and R—rod-like.

EtOH or DMF as the mere solvent. For CdS, large (4160 1C) is needed, probably due to release of 2À cauliflower- and spindle-like crystals (assembled S anion formNH 4NCS requiring high tempera- by many small triangle flakes) can be formed when ture. The reaction time required for formation of use water or EtOH as solvent, respectively (Fig. 8a dentrites is somewhat associated to the reaction and b). Fig. 9a–f shows various morphologies of temperature. However, prolonging the reaction PbS obtained in different solvent system. The time (more than 12 h.) was found to be helpful for similar observations have also been reported in the formation of well-shaped crystals, but had no litertures [34,35], and formation of different significant influence on the morphology of the morphology in different solvent probably related crystals. to the different polarity and hydrogen-bonding ability of the solvent [23,36,37]. In order to investigate the effect of sulfur source, 4. Conclusion a series of sulfur containing compounds, such as sodiumsulfide (Na 2S), potassiumthiocyanate Dentritic HgS, CdS and PbS single crystals have KCNS, thioacetamide (CH3CSNH2), thiosemicar- been constructed by a facile hydro/solvothermal bazide (NH2NHCSNH2) and diethydithiocarba- method in water or ethanol/water solvent system mate (DDTC), were selected to replace Tu or without using any templates. The crystal growth NH4CNS. Table 3 summarizes the effect of the has been found to follow a coordination precursor sulfur source on the phase and morphology for assisted branching process, in which the intrinsic three reactions, which indicated that varied crystal symmetry represents the key-directing role. morphology, such as sphere particles, regular Other influencing factors such as reactant molar octahedron, hollow sphere microparticles, cauli- ratio, solvent, sulfur source, reaction time and flower as well as dentrite could be obtained with temperature also play important roles in crystal different sulfur sources. A similar result have been morphology evolution. The present synthetic route observed by Xie et al. too [23]. may be extended to prepare higher ordered The influencing factors of reaction temperature hierarchical organizations of other metal chalco- and time were also investigated. For HgS with Rn genide, further investigation is now underway. between 1 and 4, leaf-shaped crystals are pre- dominant at low temperature (90–120 1C). When the temperature was increased to 140 1C or higher, Acknowledgments the leafy crystals disappeared, leaving dendrites only, but the higher the temperature is, the less This work was supported by NNSF of China tertiary branches have. To obtain well-formed No. 20303027, NSF of Guangdong Province No. dentrites of CdS, higher reaction temperature 04205405 and NSFC key project No. 20131020. ARTICLE IN PRESS

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