ChemComm
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 7128 Received 10th March 2014, Accepted 12th May 2014
View Article Online View Journal | View Issue
Colloidal synthesis and photocatalytic properties of orthorhombic AgGaS2 nanocrystals† Cong-Min Fan,‡ab Michelle D. Regulacio,‡a Chen Ye,ac Suo Hon Lim,a Yuangang Zheng,a Qing-Hua Xu,c An-Wu Xu*b and Ming-Yong Han*a
DOI: 10.1039/c4cc01778a www.rsc.org/chemcomm
AgGaS2 (AGS) nanocrystals that exist in the orthorhombic phase were successfully prepared for the first time through a one-pot colloidal synthetic strategy using suitable coordinating solvents. These orthorhombic AGS nanocrystals were found to display great potential in visible-light-driven photocatalysis.
Ternary I–III–VI2 semiconductors (where I = Cu, Ag; III = Ga, In; VI = S, Se) are among the most widely investigated inorganic semiconductors in recent years owing to their band structures, which are well-suited for light-harvesting and light-emitting applications.1 Their environmentally benign metal components make them particularly attractive as alternative materials to the technologically useful, but toxic, Cd- and Pb-containing semiconductors. A great deal of attention has been given to the Cu-based systems (e.g. CuInS2, CuGaSe2) and their alloys, which display properties that can be exploited for use in the areas of optoelectronics, photovoltaics, thermoelectrics and photocatalysis.1,2 Although the Ag-based systems also exhibit technologically applicable properties, they are not as well explored as their Cu-based counterparts mainly because they are much more challenging to prepare, especially in the nanoscale. Much of the studies on the Ag-based systems have been limited to AgInS2 (AIS), a direct-gap semiconductor that has shown great promise in photoelectrics, and in biological imaging and labeling applications.3 AIS is known to exist in two types of cation-ordered structures: (1) the thermodynamically stable tetragonal structure (known as chalcopyrite structure) and (2) the metastable orthorhombic structure.4 These two AIS phases have
a
Institute of Materials Research and Engineering, A*STAR, Singapore 117602. E-mail: [email protected] b Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected] c Department of Chemistry, National University of Singapore, Singapore 117543 † Electronic supplementary information (ESI) available: Experimental details, refined diffraction patterns, crystal structure, EDX and XPS spectra, TEM images, XRD patterns, absorption spectra, Tauc plots and degradation efficiency. See DOI: 10.1039/c4cc01778a ‡ These authors contributed equally to this work.
7128 | Chem. Commun., 2014, 50, 7128--7131
been successfully prepared in the bulk form as well as in the nanoscale. A notable yet relatively less-investigated member of the I–III–VI2 family is AgGaS2 (AGS), an attractive material for optoelectronics and photocatalytic applications. Similar to AIS, the thermodynamically stable phase of AGS has the cation-ordered tetragonal structure. Tetragonal AGS can be readily synthesized in the bulk form by the conventional solid-state method, while its nanoscopic form can be obtained via solvothermal and hydrothermal routes and through the colloidal chemical approach.5 However, unlike in the case of AIS, the orthorhombic phase has never been previously observed for AGS. Aside from AIS, the only other I–III–VI2 semiconductor that has been experimentally shown to exist in the metastable cation-ordered orthorhombic structure is its selenium analogue, AgInSe2 (AISe). Orthorhombic AISe was first synthesized by Vittal and co-workers in nanocrystalline form through the colloidal synthetic method.6 In this communication, we present the first report on the colloidal nanocrystal synthesis of the orthorhombic polymorph of AGS using suitable coordinating solvents. In addition, we demonstrate the potential of these orthorhombic AGS nanocrystals as visible-lightresponsive photocatalysts in dye degradation. In a typical synthesis, 0.1 mmol of each of the Ag(I) and Ga(III) dithiocarbamate complexes were mixed with 10 mL of coordinating solvent at room temperature, and the resulting mixture was degassed under vacuum at 80–100 1C for 20 min. The reaction mixture was then rapidly heated up to 280 1C under an inert atmosphere and held at this temperature for 2 h (see ESI† for experimental details). Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the as-synthesized AGS nanocrystals using the following coordinating solvents: dodecanethiol (DDT), hexadecanethiol (HDT), oleylamine (OM) and hexadecylamine (HDA). The patterns did not match well with the pattern reported in the JCPDS database for tetragonal AGS (#27-0615). The similarity of the patterns to that of orthorhombic AIS strongly hinted that the phase exhibited by our nanocrystalline AGS is isostructural with the orthorhombic polymorph of its indium analogue. Using the crystallographic data for orthorhombic AIS as reference,7 we have carried out Rietveld analysis of our diffraction data (Fig. S1a in ESI†) and determined the lattice parameters for
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
Communication
ChemComm
Fig. 1 XRD pattern of the AgGaS2 (AGS) nanocrystals prepared using (a) DDT, (b) HDT, (c) OM and (d) HDA. Shown at the bottom is the simulated pattern for orthorhombic AGS.
Fig. 2 TEM images of the AGS nanocrystals prepared using (a) DDT, (b) HDT, (c) OM and (d) HDA. HRTEM images are shown in the insets.
orthorhombic AGS. The refined lattice parameters are a = 6.577(4) Å, b = 8.066(5) Å and c = 6.451(4) Å. When compared to the lattice parameters reported in the literature for orthorhombic AIS (JCPDS #25-1328: a = 7.001 Å, b = 8.278 Å and c = 6.698 Å), the obtained values for orthorhombic AGS are smaller. This is due to the smaller ionic radius of Ga3+ relative to that of In3+. With the obtained lattice parameters, we have simulated the diffraction pattern for orthorhombic AGS using the Diamond 3.2 software and this is shown as the red pattern in Fig. 1. All the diffraction peaks in the experimental pattern can be indexed according to the simulated pattern, confirming that our AGS nanocrystals exist in the orthorhombic phase. The crystal structure of orthorhombic AGS is depicted in Fig. S1b (ESI†). In contrast to the known tetragonal polymorph of AGS (space group I4% 2d), which is a zinc-blende-related phase, orthorhombic AGS (space group Pna21) has a wurtzite-derived crystal structure. In this structure type, the sulfur anions form a hexagonal close-packed array, with the Ag+ and Ga3+ cations occupying half of the tetrahedral holes. If the cations are randomly distributed in the tetrahedral sites, the structure would have been similar to wurtzite ZnS. However, the ordering of Ag+ and Ga3+ in the cation sublattice reduces the hexagonal symmetry of the basic wurtzite cell to orthorhombic. The transmission electron microscopy (TEM) images of the orthorhombic AGS nanocrystals are displayed in Fig. 2. The nanocrystals are well separated and are fairly monodisperse in size and shape. The nanocrystals prepared using alkanethiols are pyramidal in shape with an average size of 16.3 1.1 nm (DDT) and 17.4 1.8 nm (HDT). Meanwhile, the use of primary alkylamines has resulted in polyhedral nanocrystals measuring 18.4 1.2 nm (OM) and 19.1 1.3 nm (HDA). The difference in morphology is due to the selective binding affinity of the coordinating solvents to certain crystal facets. High-resolution TEM images reveal continuous lattice fringes, indicating that the nanocrystals are single-crystalline. The lattice fringes with interplanar spacings of 3.21 and 3.06 Å correspond to the (002) and (210) lattice planes, respectively. The presence
of Ag, Ga and S is evidenced by the energy dispersive X-ray (EDX) spectrum shown in Fig. S2 (ESI†). Peak analysis gives an average Ag/Ga/S composition ratio that is in accordance with the expected ratio of 1 : 1 : 2. X-ray photoelectron spectroscopy (XPS) confirms the presence of the three constituent elements in their expected oxidation states (Fig. S3, ESI†). The two peaks centered at 368.1 and 374.1 eV, with a separation of 6.0 eV, is indicative of monovalent Ag.8 The peaks observed at 1118.1 and 1145.0 eV, with a separation of 26.9 eV, are attributable to trivalent Ga.9 The peaks seen at 162 and 163 eV are in good agreement with those reported for S in metal sulfides.9,10 In colloidal nanocrystal synthesis of metal sulfides, metal dithiocarbamate complexes have been some of the most widely used precursors because they can easily decompose into metal sulfides upon heat treatment.11 It has been reported that the solution-phase thermolysis of metal dithiocarbamates can be influenced by the nature of coordinating solvents.10,12 We have used different types of coordinating solvents in our synthesis and the results of our experiments are summarized in Table S1 in ESI.† Phase-pure orthorhombic AGS was obtained with the use of long-chain alkanethiols, such as DDT and HDT. This is consistent with previous reports that the presence of alkanethiols during metal dithiocarbamate thermolysis can induce the formation of the wurtzite-related phase of multinary sulfides.9,10,12c It is believed that the ability of thiols to strongly coordinate to the metal ions and serve as an additional sulfur source can affect the growth kinetics of multinary sulfide nanocrystals in a manner that is favorable to the stabilization of the metastable wurtzite-derived phases.1a Interestingly, we have found that the use of primary alkylamines, such as OM and HDA, also produces orthorhombic AGS, whereas employing a tertiary alkylamine, such as trioctylamine (TOA), yields a mixture of tetragonal AGS and Ag9GaS6 (Fig. S4, ESI†). Previous studies have shown that alkylamines, being Lewis bases, can promote the decomposition of metal dithiocarbamates, and the decomposition rate varies
This journal is © The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7128--7131 | 7129
View Article Online
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
ChemComm
with the type of alkylamine.12a,b Primary alkylamines can decompose metal dithiocarbamates at a much faster rate than the more sterically hindered tertiary alkylamines. The generation of orthorhombic AGS in the presence of primary alkylamines indicates that a rapid precursor decomposition rate is necessary to achieve growth conditions that are conducive to the formation of this metastable AGS phase. Meanwhile, employing oleic acid (OA) as solvent did not generate AGS but produced a mixture of Ag and Ag2S (Fig. S5a, ESI†). The use of trioctylphosphine (TOP), on the other hand, yielded Ag as the sole product (Fig. S5b, ESI†). In these last two cases, the Ga(III) precursor did not readily decompose in solution under the conditions employed. The room-temperature absorption spectra of the orthorhombic AGS nanocrystals prepared in different solvents are shown in Fig. S6a (ESI†). A broad shoulder at around 420 nm can be observed in all spectra. The optical band gap, Eg, is determined using a Tauc plot (Fig. S6b, ESI†), which is the plot of (ahn)2 versus hn (where a = absorption coefficient; hn = photon energy). By extrapolating the linear portion of the plot, the band gap is estimated to be around 2.69–2.71 eV. The size-dependent band gap of I–III–VI2 semiconductor nanocrystals has been theoretically investigated by Omata et al.13 On the basis of their calculations, size-induced widening of the band gap (i.e., quantum confinement effect) would be significantly observed for AGS nanocrystals with size smaller than 5 nm. Because the dimensions of our AGS nanocrystals are much larger than this, they are not expected to exhibit the quantum confinement effect. Thus, the band gap observed for our AGS nanocrystals can be assumed as the bulk band gap of orthorhombic AGS. Its close proximity to the reported band gap of tetragonal AGS (Eg = 2.687– 2.71 eV)14 tells us that the tetragonal and orthorhombic phases of AGS exhibit very similar band gap energies. We have evaluated the photocatalytic potential of the orthorhombic AGS nanocrystals for dye degradation under visible-light illumination. Rhodamine B (RhB), a widely utilized nonbiodegradable dye, was used in the study. Fig. 3a shows that under visible-light illumination, the presence of AGS nanocrystals in an aqueous RhB solution causes a considerable decrease in the intensity of the RhB absorption peaks, which is indicative of a dramatic drop in RhB concentration due to degradation. By contrast, there is no substantial change in RhB concentration when a similar mixture was kept in the dark (Fig. S7a, ESI†), implying that the degradation of RhB over AGS is induced by visible light. To show that RhB does not undergo photolysis (i.e., degradation by light alone), the experiment was also done in the absence of the orthorhombic AGS nanocrystals under visible-light illumination. Without the nanocrystals, the RhB concentration remained unchanged as seen in Fig. S7b (ESI†). This confirms that the decrease in RhB concentration observed in Fig. 3a is due to the photocatalytic activity of orthorhombic AGS. The degradation efficiency, Eff, was calculated for the nanocrystal samples prepared using different coordinating solvents. As shown in Fig. 3b, all four samples exhibit promising photocatalytic behavior under visible-light illumination, with Eff reaching 86–93% after an illumination time of 90 min. The diffraction patterns of our material before and after the photocatalytic experiment are essentially identical (Fig. S8, ESI†), indicating that the orthorhombic
7130 | Chem. Commun., 2014, 50, 7128--7131
Communication
Fig. 3 (a) Temporal evolution of the absorption spectra of an aqueous RhB solution in the presence of orthorhombic AGS nanocrystals under visible-light illumination. (b) Degradation efficiency (Eff) as a function of time for nanocrystal samples prepared using different solvents.
AGS nanocrystals did not decompose or change in phase under visible-light illumination. In summary, this work has demonstrated the first successful synthesis of colloidal AGS nanocrystals that have the wurtzitederived orthorhombic crystal structure. A facile injection-free synthetic strategy was employed wherein Ag(I) and Ga(III) dithiocarbamates are co-thermally decomposed in solution. It was found that the use of long-chain alkanethiols (DDT and HDT) and primary alkylamines (OM and HDA) as coordinating solvents can lead to the formation of the orthorhombic polymorph of AGS. The band gap of orthorhombic AGS is determined to be B2.7 eV, which lies in the visible spectrum. Under visible-light illumination, the orthorhombic AGS nanocrystals effectively served as photocatalysts for the degradation of RhB.
Notes and references 1 (a) F. J. Fan, L. Wu and S. H. Yu, Energy Environ. Sci., 2014, 7, 190; (b) D. Aldakov, A. Lefrancois and P. Reiss, J. Mater. Chem. C, 2013, 1, 3756. 2 (a) J. Zhang, R. Xie and W. Yang, Chem. Mater., 2011, 23, 3357; (b) M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770; (c) C. Ye, M. D. Regulacio, S. H. Lim, Q. H. Xu and M. Y. Han, Chem. – Eur. J., 2012, 18, 11258. 3 (a) M. Deng, S. Shen, X. Wang, Y. Zhang, H. Xu, T. Zhang and Q. Wang, CrystEngComm, 2013, 15, 6443; (b) M. D. Regulacio, K. Y. Win, S. L. Lo, S. Y. Zhang, X. Zhang, S. Wang, M. Y. Han and Y. Zheng, Nanoscale, 2013, 5, 2322; (c) W. W. Xiong, G. H. Yang, X. C. Wu and J. J. Zhu, J. Mater. Chem. B, 2013, 1, 4160. 4 R. S. Roth, H. S. Parker and W. S. Brower, Mater. Res. Bull., 1973, 8, 333.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Communication
11 (a) T. Trindade, P. O’Brien and X. Zhang, Chem. Mater., 1997, 9, 523; (b) M. D. Regulacio, S. Kar, E. Zuniga, G. Wang, N. R. Dollahon, G. T. Yee and S. L. Stoll, Chem. Mater., 2008, 20, 3368. 12 (a) N. Pradhan, B. Katz and S. Efrima, J. Phys. Chem. B, 2003, 107, 13843; (b) Y. K. Jung, J. I. Kim and J. K. Lee, J. Am. Chem. Soc., 2010, 132, 178; (c) D. Pan, L. An, Z. Sun, W. Hou, Y. Yang, Z. Yang and Y. Lu, J. Am. Chem. Soc., 2008, 130, 5620. 13 T. Omata, K. Nose and S. Otsuka-Yao-Matsuo, J. Appl. Phys., 2009, 105, 073106. 14 (a) J. P. Aicardi and J. P. Leyris, J. Phys. Chem. Solids, 1982, 43, 1023; (b) J. S. Jang, P. H. Borse, J. S. Lee, S. H. Choi and H. G. Kim, J. Chem. Phys., 2008, 128, 154717; (c) J. Sun, G. Chen, G. Xiong, J. Pei and H. Dong, Int. J. Hydrogen Energy, 2013, 38, 10731.
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
5 (a) J. S. Jang, S. H. Choi, N. Shin, C. Yu and J. S. Lee, J. Solid State Chem., 2007, 180, 1110; (b) J. Hu, Q. Lu, K. Tang, Y. Qian, G. Zhoub and X. Liu, Chem. Commun., 1999, 1093; (c) J. Hu, B. Deng, K. Tang, Q. Lu, R. Jiang and Y. Qian, Solid State Sci., 2001, 3, 275; (d) F. Huang, J. Zhou, J. Xu and Y. Wang, Nanoscale, 2014, 6, 2340. 6 M. T. Ng, C. B. Boothroyd and J. J. Vittal, J. Am. Chem. Soc., 2006, 128, 7118. 7 G. Delgado, A. J. Mora, C. Pineda and T. Tinoco, Mater. Res. Bull., 2001, 36, 2507. 8 K. Li, B. Chai, T. Peng, J. Mao and L. Zan, RSC Adv., 2013, 3, 253. 9 M. D. Regulacio, C. Ye, S. H. Lim, Y. Zheng, Q. H. Xu and M. Y. Han, CrystEngComm, 2013, 15, 5214. 10 M. D. Regulacio, C. Ye, S. H. Lim, M. Bosman, E. Ye, S. Chen, Q. H. Xu and M. Y. Han, Chem. – Eur. J., 2012, 18, 3127.
ChemComm
This journal is © The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7128--7131 | 7131
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 7128 Received 10th March 2014, Accepted 12th May 2014
View Article Online View Journal | View Issue
Colloidal synthesis and photocatalytic properties of orthorhombic AgGaS2 nanocrystals† Cong-Min Fan,‡ab Michelle D. Regulacio,‡a Chen Ye,ac Suo Hon Lim,a Yuangang Zheng,a Qing-Hua Xu,c An-Wu Xu*b and Ming-Yong Han*a
DOI: 10.1039/c4cc01778a www.rsc.org/chemcomm
AgGaS2 (AGS) nanocrystals that exist in the orthorhombic phase were successfully prepared for the first time through a one-pot colloidal synthetic strategy using suitable coordinating solvents. These orthorhombic AGS nanocrystals were found to display great potential in visible-light-driven photocatalysis.
Ternary I–III–VI2 semiconductors (where I = Cu, Ag; III = Ga, In; VI = S, Se) are among the most widely investigated inorganic semiconductors in recent years owing to their band structures, which are well-suited for light-harvesting and light-emitting applications.1 Their environmentally benign metal components make them particularly attractive as alternative materials to the technologically useful, but toxic, Cd- and Pb-containing semiconductors. A great deal of attention has been given to the Cu-based systems (e.g. CuInS2, CuGaSe2) and their alloys, which display properties that can be exploited for use in the areas of optoelectronics, photovoltaics, thermoelectrics and photocatalysis.1,2 Although the Ag-based systems also exhibit technologically applicable properties, they are not as well explored as their Cu-based counterparts mainly because they are much more challenging to prepare, especially in the nanoscale. Much of the studies on the Ag-based systems have been limited to AgInS2 (AIS), a direct-gap semiconductor that has shown great promise in photoelectrics, and in biological imaging and labeling applications.3 AIS is known to exist in two types of cation-ordered structures: (1) the thermodynamically stable tetragonal structure (known as chalcopyrite structure) and (2) the metastable orthorhombic structure.4 These two AIS phases have
a
Institute of Materials Research and Engineering, A*STAR, Singapore 117602. E-mail: [email protected] b Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected] c Department of Chemistry, National University of Singapore, Singapore 117543 † Electronic supplementary information (ESI) available: Experimental details, refined diffraction patterns, crystal structure, EDX and XPS spectra, TEM images, XRD patterns, absorption spectra, Tauc plots and degradation efficiency. See DOI: 10.1039/c4cc01778a ‡ These authors contributed equally to this work.
7128 | Chem. Commun., 2014, 50, 7128--7131
been successfully prepared in the bulk form as well as in the nanoscale. A notable yet relatively less-investigated member of the I–III–VI2 family is AgGaS2 (AGS), an attractive material for optoelectronics and photocatalytic applications. Similar to AIS, the thermodynamically stable phase of AGS has the cation-ordered tetragonal structure. Tetragonal AGS can be readily synthesized in the bulk form by the conventional solid-state method, while its nanoscopic form can be obtained via solvothermal and hydrothermal routes and through the colloidal chemical approach.5 However, unlike in the case of AIS, the orthorhombic phase has never been previously observed for AGS. Aside from AIS, the only other I–III–VI2 semiconductor that has been experimentally shown to exist in the metastable cation-ordered orthorhombic structure is its selenium analogue, AgInSe2 (AISe). Orthorhombic AISe was first synthesized by Vittal and co-workers in nanocrystalline form through the colloidal synthetic method.6 In this communication, we present the first report on the colloidal nanocrystal synthesis of the orthorhombic polymorph of AGS using suitable coordinating solvents. In addition, we demonstrate the potential of these orthorhombic AGS nanocrystals as visible-lightresponsive photocatalysts in dye degradation. In a typical synthesis, 0.1 mmol of each of the Ag(I) and Ga(III) dithiocarbamate complexes were mixed with 10 mL of coordinating solvent at room temperature, and the resulting mixture was degassed under vacuum at 80–100 1C for 20 min. The reaction mixture was then rapidly heated up to 280 1C under an inert atmosphere and held at this temperature for 2 h (see ESI† for experimental details). Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the as-synthesized AGS nanocrystals using the following coordinating solvents: dodecanethiol (DDT), hexadecanethiol (HDT), oleylamine (OM) and hexadecylamine (HDA). The patterns did not match well with the pattern reported in the JCPDS database for tetragonal AGS (#27-0615). The similarity of the patterns to that of orthorhombic AIS strongly hinted that the phase exhibited by our nanocrystalline AGS is isostructural with the orthorhombic polymorph of its indium analogue. Using the crystallographic data for orthorhombic AIS as reference,7 we have carried out Rietveld analysis of our diffraction data (Fig. S1a in ESI†) and determined the lattice parameters for
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
Communication
ChemComm
Fig. 1 XRD pattern of the AgGaS2 (AGS) nanocrystals prepared using (a) DDT, (b) HDT, (c) OM and (d) HDA. Shown at the bottom is the simulated pattern for orthorhombic AGS.
Fig. 2 TEM images of the AGS nanocrystals prepared using (a) DDT, (b) HDT, (c) OM and (d) HDA. HRTEM images are shown in the insets.
orthorhombic AGS. The refined lattice parameters are a = 6.577(4) Å, b = 8.066(5) Å and c = 6.451(4) Å. When compared to the lattice parameters reported in the literature for orthorhombic AIS (JCPDS #25-1328: a = 7.001 Å, b = 8.278 Å and c = 6.698 Å), the obtained values for orthorhombic AGS are smaller. This is due to the smaller ionic radius of Ga3+ relative to that of In3+. With the obtained lattice parameters, we have simulated the diffraction pattern for orthorhombic AGS using the Diamond 3.2 software and this is shown as the red pattern in Fig. 1. All the diffraction peaks in the experimental pattern can be indexed according to the simulated pattern, confirming that our AGS nanocrystals exist in the orthorhombic phase. The crystal structure of orthorhombic AGS is depicted in Fig. S1b (ESI†). In contrast to the known tetragonal polymorph of AGS (space group I4% 2d), which is a zinc-blende-related phase, orthorhombic AGS (space group Pna21) has a wurtzite-derived crystal structure. In this structure type, the sulfur anions form a hexagonal close-packed array, with the Ag+ and Ga3+ cations occupying half of the tetrahedral holes. If the cations are randomly distributed in the tetrahedral sites, the structure would have been similar to wurtzite ZnS. However, the ordering of Ag+ and Ga3+ in the cation sublattice reduces the hexagonal symmetry of the basic wurtzite cell to orthorhombic. The transmission electron microscopy (TEM) images of the orthorhombic AGS nanocrystals are displayed in Fig. 2. The nanocrystals are well separated and are fairly monodisperse in size and shape. The nanocrystals prepared using alkanethiols are pyramidal in shape with an average size of 16.3 1.1 nm (DDT) and 17.4 1.8 nm (HDT). Meanwhile, the use of primary alkylamines has resulted in polyhedral nanocrystals measuring 18.4 1.2 nm (OM) and 19.1 1.3 nm (HDA). The difference in morphology is due to the selective binding affinity of the coordinating solvents to certain crystal facets. High-resolution TEM images reveal continuous lattice fringes, indicating that the nanocrystals are single-crystalline. The lattice fringes with interplanar spacings of 3.21 and 3.06 Å correspond to the (002) and (210) lattice planes, respectively. The presence
of Ag, Ga and S is evidenced by the energy dispersive X-ray (EDX) spectrum shown in Fig. S2 (ESI†). Peak analysis gives an average Ag/Ga/S composition ratio that is in accordance with the expected ratio of 1 : 1 : 2. X-ray photoelectron spectroscopy (XPS) confirms the presence of the three constituent elements in their expected oxidation states (Fig. S3, ESI†). The two peaks centered at 368.1 and 374.1 eV, with a separation of 6.0 eV, is indicative of monovalent Ag.8 The peaks observed at 1118.1 and 1145.0 eV, with a separation of 26.9 eV, are attributable to trivalent Ga.9 The peaks seen at 162 and 163 eV are in good agreement with those reported for S in metal sulfides.9,10 In colloidal nanocrystal synthesis of metal sulfides, metal dithiocarbamate complexes have been some of the most widely used precursors because they can easily decompose into metal sulfides upon heat treatment.11 It has been reported that the solution-phase thermolysis of metal dithiocarbamates can be influenced by the nature of coordinating solvents.10,12 We have used different types of coordinating solvents in our synthesis and the results of our experiments are summarized in Table S1 in ESI.† Phase-pure orthorhombic AGS was obtained with the use of long-chain alkanethiols, such as DDT and HDT. This is consistent with previous reports that the presence of alkanethiols during metal dithiocarbamate thermolysis can induce the formation of the wurtzite-related phase of multinary sulfides.9,10,12c It is believed that the ability of thiols to strongly coordinate to the metal ions and serve as an additional sulfur source can affect the growth kinetics of multinary sulfide nanocrystals in a manner that is favorable to the stabilization of the metastable wurtzite-derived phases.1a Interestingly, we have found that the use of primary alkylamines, such as OM and HDA, also produces orthorhombic AGS, whereas employing a tertiary alkylamine, such as trioctylamine (TOA), yields a mixture of tetragonal AGS and Ag9GaS6 (Fig. S4, ESI†). Previous studies have shown that alkylamines, being Lewis bases, can promote the decomposition of metal dithiocarbamates, and the decomposition rate varies
This journal is © The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7128--7131 | 7129
View Article Online
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
ChemComm
with the type of alkylamine.12a,b Primary alkylamines can decompose metal dithiocarbamates at a much faster rate than the more sterically hindered tertiary alkylamines. The generation of orthorhombic AGS in the presence of primary alkylamines indicates that a rapid precursor decomposition rate is necessary to achieve growth conditions that are conducive to the formation of this metastable AGS phase. Meanwhile, employing oleic acid (OA) as solvent did not generate AGS but produced a mixture of Ag and Ag2S (Fig. S5a, ESI†). The use of trioctylphosphine (TOP), on the other hand, yielded Ag as the sole product (Fig. S5b, ESI†). In these last two cases, the Ga(III) precursor did not readily decompose in solution under the conditions employed. The room-temperature absorption spectra of the orthorhombic AGS nanocrystals prepared in different solvents are shown in Fig. S6a (ESI†). A broad shoulder at around 420 nm can be observed in all spectra. The optical band gap, Eg, is determined using a Tauc plot (Fig. S6b, ESI†), which is the plot of (ahn)2 versus hn (where a = absorption coefficient; hn = photon energy). By extrapolating the linear portion of the plot, the band gap is estimated to be around 2.69–2.71 eV. The size-dependent band gap of I–III–VI2 semiconductor nanocrystals has been theoretically investigated by Omata et al.13 On the basis of their calculations, size-induced widening of the band gap (i.e., quantum confinement effect) would be significantly observed for AGS nanocrystals with size smaller than 5 nm. Because the dimensions of our AGS nanocrystals are much larger than this, they are not expected to exhibit the quantum confinement effect. Thus, the band gap observed for our AGS nanocrystals can be assumed as the bulk band gap of orthorhombic AGS. Its close proximity to the reported band gap of tetragonal AGS (Eg = 2.687– 2.71 eV)14 tells us that the tetragonal and orthorhombic phases of AGS exhibit very similar band gap energies. We have evaluated the photocatalytic potential of the orthorhombic AGS nanocrystals for dye degradation under visible-light illumination. Rhodamine B (RhB), a widely utilized nonbiodegradable dye, was used in the study. Fig. 3a shows that under visible-light illumination, the presence of AGS nanocrystals in an aqueous RhB solution causes a considerable decrease in the intensity of the RhB absorption peaks, which is indicative of a dramatic drop in RhB concentration due to degradation. By contrast, there is no substantial change in RhB concentration when a similar mixture was kept in the dark (Fig. S7a, ESI†), implying that the degradation of RhB over AGS is induced by visible light. To show that RhB does not undergo photolysis (i.e., degradation by light alone), the experiment was also done in the absence of the orthorhombic AGS nanocrystals under visible-light illumination. Without the nanocrystals, the RhB concentration remained unchanged as seen in Fig. S7b (ESI†). This confirms that the decrease in RhB concentration observed in Fig. 3a is due to the photocatalytic activity of orthorhombic AGS. The degradation efficiency, Eff, was calculated for the nanocrystal samples prepared using different coordinating solvents. As shown in Fig. 3b, all four samples exhibit promising photocatalytic behavior under visible-light illumination, with Eff reaching 86–93% after an illumination time of 90 min. The diffraction patterns of our material before and after the photocatalytic experiment are essentially identical (Fig. S8, ESI†), indicating that the orthorhombic
7130 | Chem. Commun., 2014, 50, 7128--7131
Communication
Fig. 3 (a) Temporal evolution of the absorption spectra of an aqueous RhB solution in the presence of orthorhombic AGS nanocrystals under visible-light illumination. (b) Degradation efficiency (Eff) as a function of time for nanocrystal samples prepared using different solvents.
AGS nanocrystals did not decompose or change in phase under visible-light illumination. In summary, this work has demonstrated the first successful synthesis of colloidal AGS nanocrystals that have the wurtzitederived orthorhombic crystal structure. A facile injection-free synthetic strategy was employed wherein Ag(I) and Ga(III) dithiocarbamates are co-thermally decomposed in solution. It was found that the use of long-chain alkanethiols (DDT and HDT) and primary alkylamines (OM and HDA) as coordinating solvents can lead to the formation of the orthorhombic polymorph of AGS. The band gap of orthorhombic AGS is determined to be B2.7 eV, which lies in the visible spectrum. Under visible-light illumination, the orthorhombic AGS nanocrystals effectively served as photocatalysts for the degradation of RhB.
Notes and references 1 (a) F. J. Fan, L. Wu and S. H. Yu, Energy Environ. Sci., 2014, 7, 190; (b) D. Aldakov, A. Lefrancois and P. Reiss, J. Mater. Chem. C, 2013, 1, 3756. 2 (a) J. Zhang, R. Xie and W. Yang, Chem. Mater., 2011, 23, 3357; (b) M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770; (c) C. Ye, M. D. Regulacio, S. H. Lim, Q. H. Xu and M. Y. Han, Chem. – Eur. J., 2012, 18, 11258. 3 (a) M. Deng, S. Shen, X. Wang, Y. Zhang, H. Xu, T. Zhang and Q. Wang, CrystEngComm, 2013, 15, 6443; (b) M. D. Regulacio, K. Y. Win, S. L. Lo, S. Y. Zhang, X. Zhang, S. Wang, M. Y. Han and Y. Zheng, Nanoscale, 2013, 5, 2322; (c) W. W. Xiong, G. H. Yang, X. C. Wu and J. J. Zhu, J. Mater. Chem. B, 2013, 1, 4160. 4 R. S. Roth, H. S. Parker and W. S. Brower, Mater. Res. Bull., 1973, 8, 333.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Communication
11 (a) T. Trindade, P. O’Brien and X. Zhang, Chem. Mater., 1997, 9, 523; (b) M. D. Regulacio, S. Kar, E. Zuniga, G. Wang, N. R. Dollahon, G. T. Yee and S. L. Stoll, Chem. Mater., 2008, 20, 3368. 12 (a) N. Pradhan, B. Katz and S. Efrima, J. Phys. Chem. B, 2003, 107, 13843; (b) Y. K. Jung, J. I. Kim and J. K. Lee, J. Am. Chem. Soc., 2010, 132, 178; (c) D. Pan, L. An, Z. Sun, W. Hou, Y. Yang, Z. Yang and Y. Lu, J. Am. Chem. Soc., 2008, 130, 5620. 13 T. Omata, K. Nose and S. Otsuka-Yao-Matsuo, J. Appl. Phys., 2009, 105, 073106. 14 (a) J. P. Aicardi and J. P. Leyris, J. Phys. Chem. Solids, 1982, 43, 1023; (b) J. S. Jang, P. H. Borse, J. S. Lee, S. H. Choi and H. G. Kim, J. Chem. Phys., 2008, 128, 154717; (c) J. Sun, G. Chen, G. Xiong, J. Pei and H. Dong, Int. J. Hydrogen Energy, 2013, 38, 10731.
Published on 12 May 2014. Downloaded by Brown University on 22/10/2014 18:04:17.
5 (a) J. S. Jang, S. H. Choi, N. Shin, C. Yu and J. S. Lee, J. Solid State Chem., 2007, 180, 1110; (b) J. Hu, Q. Lu, K. Tang, Y. Qian, G. Zhoub and X. Liu, Chem. Commun., 1999, 1093; (c) J. Hu, B. Deng, K. Tang, Q. Lu, R. Jiang and Y. Qian, Solid State Sci., 2001, 3, 275; (d) F. Huang, J. Zhou, J. Xu and Y. Wang, Nanoscale, 2014, 6, 2340. 6 M. T. Ng, C. B. Boothroyd and J. J. Vittal, J. Am. Chem. Soc., 2006, 128, 7118. 7 G. Delgado, A. J. Mora, C. Pineda and T. Tinoco, Mater. Res. Bull., 2001, 36, 2507. 8 K. Li, B. Chai, T. Peng, J. Mao and L. Zan, RSC Adv., 2013, 3, 253. 9 M. D. Regulacio, C. Ye, S. H. Lim, Y. Zheng, Q. H. Xu and M. Y. Han, CrystEngComm, 2013, 15, 5214. 10 M. D. Regulacio, C. Ye, S. H. Lim, M. Bosman, E. Ye, S. Chen, Q. H. Xu and M. Y. Han, Chem. – Eur. J., 2012, 18, 3127.
ChemComm
This journal is © The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7128--7131 | 7131
