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Evidence of reaction rate influencing cubic and hexagonal phase formation process in CdS nanocrystals
Chemical Physics Letters 652 (2016) 11–15
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Evidence of reaction rate influencing cubic and hexagonal phase formation process in CdS nanocrystals Kuldeep Deka, M.P.C. Kalita ⇑ Department of Physics, Gauhati University, Guwahati 781014, Assam, India
a r t i c l e
i n f o
Article history: Received 11 March 2016 In final form 9 April 2016 Available online 12 April 2016
a b s t r a c t CdS nanocrystals are synthesized by co-precipitation method using 2-mercaptoethanol (ME) as capping agent. Cubic, hexagonal and their mixture are obtained by varying the ME concentration. Lower (higher) ME concentration results in cubic (hexagonal) phase. The crystallite sizes are in the range 3–7 nm. Increase in ME concentration lead to lower reaction rate between Cd2+ and S2 of the precursors, and slower reaction rate is found to favor hexagonal phase formation over the cubic one in CdS nanocrystals. Role of reaction rate in the phase formation process provides a way to synthesize CdS nanocrystals in desired crystal phase. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Physical properties such as optical, thermal, mechanical and magnetic of crystalline materials strongly depend on their crystal structure. Microcrystalline materials of a particular composition may exist in different crystal structures at different pressures and temperatures. In nanocrystalline materials, increased fraction of surface atoms, nature of bonding at the surface, etc. may further influence their structural phase [1]. CdS is a group II–VI semiconductor with a direct band gap of 2.42 eV. CdS is widely used in optoelectronic devices such as thin film solar cells [2], light emitting diodes [3], photo detectors [4], gas sensors [5] and electrically driven lasers [6]. Microcrystalline CdS generally occurs in hexagonal (wurtzite) phase at atmospheric pressure up to a temperature of 1200 °C [7]. However, the internal energy difference between the stable hexagonal (wurtzite) and the cubic (zinc blende) structures is very small (1.1 meV/atom) and this leads to the formation of cubic phase at atmospheric pressure along with the hexagonal one [8]. Transformation from hexagonal to cubic as well as cubic to hexagonal can take place upon application of suitable temperature and pressure [9]. Although, there is only subtle difference in the arrangement of atoms in the wurtzite and zinc blende structures, their properties are significantly different [8]. In nanocrystalline CdS, the relative stability of hexagonal and cubic phases changes, and in recent years, considerable amount of research work is reported in the literature on phase controlled synthesis of CdS nanocrytals, and on cubic to hexagonal as well as hexagonal to cubic phase transformations [10–14]. Singh ⇑ Corresponding author. E-mail address: [email protected] (M.P.C. Kalita). http://dx.doi.org/10.1016/j.cplett.2016.04.023 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
et al. reported that increased amount of the surfactant cetyl trimethyl ammonium bromide (CTAB) leads to hexagonal to cubic phase transformation [10]. Zou et al. observed cubic to hexagonal phase transformation with increase of the surfactant cetyl trimethyl ammonium chloride (CTAC) [11]. Banerjee et al. reported on thiophenol capped CdS and observed phase transformation from hexagonal to cubic phase with increase in capping agent concentration [12]. In these works, cubic phase is reported to be obtained for smaller sizes of nano-crystallites. It is also inferred in some reports that cubic can be the stable structural phase for CdS nanocrytals [11,12]. However, Vogel et al. reported that hexagonal phase is formed in smaller crystallites in 2-mercaptoethanol capped CdS nanocrystals [14]. Therefore, it is not clear whether structural phase of CdS is size dependent or not. Further, these reports show that it is not always possible to have a control over structural phase simply by varying the sizes of the nanocrystals. Therefore, it is imperative to understand the factors which influence the phase formation process to have a control in the synthesis of CdS nanocrystals in cubic and hexagonal phases. In the present work, we report formation of CdS nanocrystals with different crystal structures viz. cubic, hexagonal and their mixture depending upon the concentration of 2-mercaptoethanol (ME) concentration which act as capping agent. Increase in ME concentration increases the bonding probability between Cd2+ ion of the precursor and S2 ion of ME and thereby, lowers the reaction rate between Cd2+ and S2 ions of the precursors, and it is observed that slower reaction rates favor hexagonal phase formation over the cubic one in CdS nanocrystals. A similar phase formation observed with changing S2 ion of the precursor (hexagonal for lower and cubic for higher) at constant Cd2+ ion and ME concentration further shows that reaction rate plays a role in
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controlling the crystal phase in CdS nanocrystals. UV–Vis absorption spectroscopic measurements show presence of confinement effects with widening of band gaps of the nanocrystals as compared to their bulk values while fourier transform infrared spectroscopic studies show capping of CdS by ME. 2. Experimental details For the synthesis of CdS nanocrystals, required amount of cadmium chloride (CdCl2) and sodium sulfide (Na2S) are dissolved together with required amount of ME. The solution is kept undisturbed for 48 h and then the precipitates are filtered and washed in triple distilled water. The precipitates are allowed to dry for a few days. The samples are obtained as powders and are characterized by X-ray diffractometer (Rigaku, TTRAX-III/Philips X-pert Pro PW 1830) with Cu Ka radiation, transmission electron microscope (TEM) (Jeol, JEM-2100), UV–Vis absorption spectrophotometer (Varian, Carry 300) and fourier transform infrared (FT-IR) spectrometer (Shimadzu, FTIR 3000). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the CdS nanocrystals with Cd:S molar ratio 1:1 and ME concentrations 50, 100, 150 and 200 mM. The XRD patterns (a) and (b) exhibit three prominent peaks at 2h values at 26.48°, 30.50°, 43.61° and 51.70° corresponding to the diffraction from (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively of bulk cubic zinc blende phase of CdS (JCPDS card no.: 89-0440). However, there is a peak of very low intensity at 24.95° in the XRD pattern (b), which is indexed as (1 0 0) peak of hexagonal (wurtzite) CdS. This peak becomes more prominent in (c) and (d). The XRD patterns (c) and (d) show peaks at 25.25°, 26.76°, 43.70° and 51.83° which could be indexed to the (1 0 0), (0 0 2), (1 1 0) and (1 1 2) planes of bulk hexagonal (wurtzite) phase of CdS (JCPDS card no.: 77-2306). The diffraction peaks get broader with increase in the ME concentration indicating decrease in crystallite size. The crystallite sizes are calculated using the Scherrer formula [15] and are presented in Table 1. The microstructures of the CdS nanocrystals are further investigated by TEM. The TEM results of CdS nanocrystals synthesized with Cd:S molarity ratio 1:1 and ME concentrations 50 mM and
Fig. 1. XRD patterns of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d). X-ray diffractometer used: Rigaku, TTRAX-III.
Table 1 Summary of the structural phase and crystallite size of the CdS nanocrystals synthesized under different conditions. Cd:S ratio
ME (mM)
Phase
Miller plane
Crystallite size (nm)
1:1 1:1 1:1 1:1 1:0.75 1:0.50 1:0.25
50 100 150 200 50 50 50
Cubic Cubic + Hexagonal Hexagonal Hexagonal Cubic Hexagonal Hexagonal
(2 2 0) (2 2 0) (1 1 0) (1 1 0) (2 2 0) (2 2 0) (2 2 0)
6.3 6.1 3.6 3.3 5.3 3.4 3.0
200 mM are presented in Fig. 2a and b respectively. From the figures it is clear that the shapes of the nanocrystals are spherical. The average crystallite size of the CdS nanocrystals with 50 mM ME is 7 nm (Fig. 2a(i)) while with 200 mM ME is 3 nm (Fig. 2b (i)). The selected area electron diffraction (SAED) pattern of the CdS nanocrystals with 50 mM ME can be indexed to cubic (zinc blende) crystal phase (Fig. 2a(iii)) while with 200 mM ME can be indexed to hexagonal (wurtzite) crystal phase (Fig. 2b(iii)). Therefore, TEM results give supportive information about average crystallite size and nature of the crystal phase as obtained from XRD analysis. TEM results show the crystallites to be spherical in shape which could not be inferred from the XRD analysis. The UV–Vis absorption spectroscopy is employed to obtain the band gaps of the CdS nanocrystals. Fig. 3 shows the corresponding UV–Vis absorption spectra of the CdS nanocrystals. The band gaps of the nanocrystals are measured using the Tauc relation [16] given by Eq. (1) and are presented in the insets of Fig. 3.
ðahmÞ ¼ Kðhm Eg Þ 2
ð1Þ
where a is the absorption co-efficient, hm is the photon energy, K is a constant and Eg is the band gap energy. The band gaps of the CdS nanocrystals with ME concentrations 50 mM, 100 mM, 150 mM and 200 mM are found to be 2.47 eV, 2.48 eV, 2.63 eV and 2.65 eV respectively. Thus, the band gaps are blue-shifted from the bulk value which is 2.42 eV. Further, the band gaps increase with decrease in crystallite sizes. The widening of band gaps show presence of confinement effects in the nanocrystals. The bonding nature of ME with the CdS nanocrystals is investigated using FT-IR spectroscopy. The FT-IR spectra of CdS nanocrystals are shown in Fig. 4. A broadband around 3420 cm1 are due to the O–H stretching vibration while the doublet centered at 2930 cm1 and 2850 cm1 is due to the symmetric and asymmetric stretching vibration of the CH2 groups respectively. The absorption bands at 1620 cm1 and 1415 are attributed to bending vibrations of H2O and H–C–H while absorption band at 1110 cm1 is attributed to C–O stretching vibration. The absorption band for S–H bond (2500 cm1) is found to be absent in CdS nanocrystals which occurs in free ME [17]. The absence of the S–H vibration in the FT-IR spectra is the result of covalent bonding between the thiols and Cd atom on the nanocrystal surface. Thus, FT-IR studies show capping of the CdS nanocrystals by ME. From the XRD and TEM studies, it is clear that CdS nanocrystals synthesized with 50 mM ME is cubic (zinc blende) while for high ME concentrations hexagonal (wurtzite) phase is found to be produced. FT-IR studies show capping of the CdS nanocrystals by ME. The average crystallite sizes of the CdS nanocrytals with cubic phase is 7 nm while with hexagonal phase is 3 nm. Thus, smaller crystallites are found to have hexagonal phase while larger ones take cubic phase. This result is in contrary to some of the earlier observations by Singh et al. [10], Zou et al. [11] and Banerjee et al. [12] that smaller crystallites of CdS favor cubic phase, but
K. Deka, M.P.C. Kalita / Chemical Physics Letters 652 (2016) 11–15
13
Fig. 2. TEM micrographs of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a) and 200 mM (b). Insets: (i) crystallite size distribution (ii) high resolution TEM micrograph and (iii) SAED pattern.
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Fig. 3. UV–Vis absorption spectra of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d). Inset: Tauc plots of the respective absorption spectrum.
Fig. 5. XRD patterns of CdS nanocrystals with 50 mM ME concentration and Cd:S molarity ratios 1:0.75 (a), 1:0.50 (b) and 1:0.25 (c). X-ray diffractometer used: Philips X-pert Pro PW 1830.
precursors, hexagonal (cubic) phase is preferred in case of CdS nanocrystals. 4. Conclusions
Fig. 4. FT-IR spectra of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d).
In summary, ME capped CdS nanocrystals with cubic and hexagonal phases are successfully synthesized at room temperature. Cubic, hexagonal and their mixture can be produced by changing the ME concentration. Increase in ME concentration lead to decrease in reaction rate between Cd2+ and S2 ions of the precursors and lower reaction rate favor hexagonal phase. The role of reaction rate in phase formation process is further evidenced by the observation of favoring hexagonal phase formation at lower S2 ion at constant Cd2+ ion and ME concentration. The present work shows the crystal phase of CdS nanocrystals can be controlled by varying the reaction rate of the Cd2+ and S2 ions of the precursors and thereby provides a way to obtain CdS nanocrytsals in desired structural phase. Acknowledgements
agrees well with the observation of hexagonal phase for smaller crystallites of CdS by Vogel et al. [14]. We make the proposition that the phase formation process in CdS nanocrystals can rather be related to the reaction rate between the Cd2+ and S2 ions. Increase in ME concentration increases the bonding probability between Cd2+ ion of the precursor and S2 ion of ME and thereby, lowers the reaction rate between Cd2+ and S2 ions of the precursors, and slower reaction rates between Cd2+ and S2 ions of the precursors favor the stable hexagonal phase. In order to validate the proposed mechanism of role reaction rate on phase formation process in CdS nanocrystals, we prepared a series of CdS samples with constant ME concentration of 50 mM and varying the Cd:S ratio as 1:0.75, 1:0.50 and 1:0.25. The XRD patterns of these samples are presented in Fig. 5. For higher concentration of S2 ion, cubic CdS is formed but for lower concentrations of S2 gradually hexagonal phase of CdS is found to evolve. Hence, we infer that for slower (faster) reaction between Cd2+ and S2 ions of the
The work is supported by DST, India through project no. SB/FTP/ PS-008/2013. We thank Department of Physics, IIT Guwahati and SAIF, Gauhati University for providing the XRD facilities; SAIF, NEHU for providing the TEM facility and Department of Chemistry, Gauhati University for providing the FT-IR facility. References [1] S.A. Acharya, N. Maheshwari, L. Tatikondewar, A. Kshirsagar, S.K. Kulkarni, Cryst. Growth Des. 13 (2013) 1369. [2] N. Novkovski, A. Tanusevski, D. Gracin, J. Phys. D: Appl. Phys. 48 (2015) 395105. [3] N. Kouklin, L. Menon, A.Z. Wong, D.W. Thompson, J.A. Woollam, P.F. Williams, S. Bandyopadhyay, Appl. Phys. Lett. 79 (2001) 4423. [4] K. Deng, L. Li, Adv. Mater. 26 (2014) 2619. [5] A. Giberti, D. Casotti, G. Cruciani, B. Fabbri, A. Gaiardo, V. Guidi, C. Malagu, G. Zonta, S. Gherardi, Sensors Actuat. B: Chem. 207 (2015) 504. [6] X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241. [7] J. Xiao, B. Wen, R. Melnik, Y. Kawazoe, X. Zhang, Phys. Chem. Chem. Phys. 16 (2014) 14899.
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Y. Yeh, Z.W. Lu, S. Froyen, A. Zunger, Phys. Rev. B 46 (1992) 10086. O. Zelaya-Angel, R. Lozada-Morales, Phys. Rev. B 62 (2000) 13064. V. Singh, P.K. Sharma, P. Chauhan, J. Phys. Chem. Solids 121 (2010) 202. Y. Zou, L. Dongsheng, D. Yang, Nanoscale Res. Lett. 6 (2011) 374. R. Banerjee, R. Jayakrishnan, P. Ayyub, J. Phys. Condens. Matter 12 (2000) 10647.
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[13] M. Pal, N.R. Mathews, P. Santiago, X. Mathew, J. Nanopart. Res. 14 (2012) 916. [14] W. Vogel, J. Urban, M. Kundu, S.K. Kulkarni, Langmuir 13 (1997) 827. [15] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, third ed., Prentice Hall, New Jersey, 2001. [16] J. Tauc, Mater. Res. Bull. 5 (1970) 721. [17] H. Fujii, K. Inata, M. Ohtaki, K. Eguchi, H. Arai, J. Mater. Sci. 36 (2001) 527.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Evidence of reaction rate influencing cubic and hexagonal phase formation process in CdS nanocrystals Kuldeep Deka, M.P.C. Kalita ⇑ Department of Physics, Gauhati University, Guwahati 781014, Assam, India
a r t i c l e
i n f o
Article history: Received 11 March 2016 In final form 9 April 2016 Available online 12 April 2016
a b s t r a c t CdS nanocrystals are synthesized by co-precipitation method using 2-mercaptoethanol (ME) as capping agent. Cubic, hexagonal and their mixture are obtained by varying the ME concentration. Lower (higher) ME concentration results in cubic (hexagonal) phase. The crystallite sizes are in the range 3–7 nm. Increase in ME concentration lead to lower reaction rate between Cd2+ and S2 of the precursors, and slower reaction rate is found to favor hexagonal phase formation over the cubic one in CdS nanocrystals. Role of reaction rate in the phase formation process provides a way to synthesize CdS nanocrystals in desired crystal phase. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Physical properties such as optical, thermal, mechanical and magnetic of crystalline materials strongly depend on their crystal structure. Microcrystalline materials of a particular composition may exist in different crystal structures at different pressures and temperatures. In nanocrystalline materials, increased fraction of surface atoms, nature of bonding at the surface, etc. may further influence their structural phase [1]. CdS is a group II–VI semiconductor with a direct band gap of 2.42 eV. CdS is widely used in optoelectronic devices such as thin film solar cells [2], light emitting diodes [3], photo detectors [4], gas sensors [5] and electrically driven lasers [6]. Microcrystalline CdS generally occurs in hexagonal (wurtzite) phase at atmospheric pressure up to a temperature of 1200 °C [7]. However, the internal energy difference between the stable hexagonal (wurtzite) and the cubic (zinc blende) structures is very small (1.1 meV/atom) and this leads to the formation of cubic phase at atmospheric pressure along with the hexagonal one [8]. Transformation from hexagonal to cubic as well as cubic to hexagonal can take place upon application of suitable temperature and pressure [9]. Although, there is only subtle difference in the arrangement of atoms in the wurtzite and zinc blende structures, their properties are significantly different [8]. In nanocrystalline CdS, the relative stability of hexagonal and cubic phases changes, and in recent years, considerable amount of research work is reported in the literature on phase controlled synthesis of CdS nanocrytals, and on cubic to hexagonal as well as hexagonal to cubic phase transformations [10–14]. Singh ⇑ Corresponding author. E-mail address: [email protected] (M.P.C. Kalita). http://dx.doi.org/10.1016/j.cplett.2016.04.023 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
et al. reported that increased amount of the surfactant cetyl trimethyl ammonium bromide (CTAB) leads to hexagonal to cubic phase transformation [10]. Zou et al. observed cubic to hexagonal phase transformation with increase of the surfactant cetyl trimethyl ammonium chloride (CTAC) [11]. Banerjee et al. reported on thiophenol capped CdS and observed phase transformation from hexagonal to cubic phase with increase in capping agent concentration [12]. In these works, cubic phase is reported to be obtained for smaller sizes of nano-crystallites. It is also inferred in some reports that cubic can be the stable structural phase for CdS nanocrytals [11,12]. However, Vogel et al. reported that hexagonal phase is formed in smaller crystallites in 2-mercaptoethanol capped CdS nanocrystals [14]. Therefore, it is not clear whether structural phase of CdS is size dependent or not. Further, these reports show that it is not always possible to have a control over structural phase simply by varying the sizes of the nanocrystals. Therefore, it is imperative to understand the factors which influence the phase formation process to have a control in the synthesis of CdS nanocrystals in cubic and hexagonal phases. In the present work, we report formation of CdS nanocrystals with different crystal structures viz. cubic, hexagonal and their mixture depending upon the concentration of 2-mercaptoethanol (ME) concentration which act as capping agent. Increase in ME concentration increases the bonding probability between Cd2+ ion of the precursor and S2 ion of ME and thereby, lowers the reaction rate between Cd2+ and S2 ions of the precursors, and it is observed that slower reaction rates favor hexagonal phase formation over the cubic one in CdS nanocrystals. A similar phase formation observed with changing S2 ion of the precursor (hexagonal for lower and cubic for higher) at constant Cd2+ ion and ME concentration further shows that reaction rate plays a role in
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K. Deka, M.P.C. Kalita / Chemical Physics Letters 652 (2016) 11–15
controlling the crystal phase in CdS nanocrystals. UV–Vis absorption spectroscopic measurements show presence of confinement effects with widening of band gaps of the nanocrystals as compared to their bulk values while fourier transform infrared spectroscopic studies show capping of CdS by ME. 2. Experimental details For the synthesis of CdS nanocrystals, required amount of cadmium chloride (CdCl2) and sodium sulfide (Na2S) are dissolved together with required amount of ME. The solution is kept undisturbed for 48 h and then the precipitates are filtered and washed in triple distilled water. The precipitates are allowed to dry for a few days. The samples are obtained as powders and are characterized by X-ray diffractometer (Rigaku, TTRAX-III/Philips X-pert Pro PW 1830) with Cu Ka radiation, transmission electron microscope (TEM) (Jeol, JEM-2100), UV–Vis absorption spectrophotometer (Varian, Carry 300) and fourier transform infrared (FT-IR) spectrometer (Shimadzu, FTIR 3000). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the CdS nanocrystals with Cd:S molar ratio 1:1 and ME concentrations 50, 100, 150 and 200 mM. The XRD patterns (a) and (b) exhibit three prominent peaks at 2h values at 26.48°, 30.50°, 43.61° and 51.70° corresponding to the diffraction from (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively of bulk cubic zinc blende phase of CdS (JCPDS card no.: 89-0440). However, there is a peak of very low intensity at 24.95° in the XRD pattern (b), which is indexed as (1 0 0) peak of hexagonal (wurtzite) CdS. This peak becomes more prominent in (c) and (d). The XRD patterns (c) and (d) show peaks at 25.25°, 26.76°, 43.70° and 51.83° which could be indexed to the (1 0 0), (0 0 2), (1 1 0) and (1 1 2) planes of bulk hexagonal (wurtzite) phase of CdS (JCPDS card no.: 77-2306). The diffraction peaks get broader with increase in the ME concentration indicating decrease in crystallite size. The crystallite sizes are calculated using the Scherrer formula [15] and are presented in Table 1. The microstructures of the CdS nanocrystals are further investigated by TEM. The TEM results of CdS nanocrystals synthesized with Cd:S molarity ratio 1:1 and ME concentrations 50 mM and
Fig. 1. XRD patterns of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d). X-ray diffractometer used: Rigaku, TTRAX-III.
Table 1 Summary of the structural phase and crystallite size of the CdS nanocrystals synthesized under different conditions. Cd:S ratio
ME (mM)
Phase
Miller plane
Crystallite size (nm)
1:1 1:1 1:1 1:1 1:0.75 1:0.50 1:0.25
50 100 150 200 50 50 50
Cubic Cubic + Hexagonal Hexagonal Hexagonal Cubic Hexagonal Hexagonal
(2 2 0) (2 2 0) (1 1 0) (1 1 0) (2 2 0) (2 2 0) (2 2 0)
6.3 6.1 3.6 3.3 5.3 3.4 3.0
200 mM are presented in Fig. 2a and b respectively. From the figures it is clear that the shapes of the nanocrystals are spherical. The average crystallite size of the CdS nanocrystals with 50 mM ME is 7 nm (Fig. 2a(i)) while with 200 mM ME is 3 nm (Fig. 2b (i)). The selected area electron diffraction (SAED) pattern of the CdS nanocrystals with 50 mM ME can be indexed to cubic (zinc blende) crystal phase (Fig. 2a(iii)) while with 200 mM ME can be indexed to hexagonal (wurtzite) crystal phase (Fig. 2b(iii)). Therefore, TEM results give supportive information about average crystallite size and nature of the crystal phase as obtained from XRD analysis. TEM results show the crystallites to be spherical in shape which could not be inferred from the XRD analysis. The UV–Vis absorption spectroscopy is employed to obtain the band gaps of the CdS nanocrystals. Fig. 3 shows the corresponding UV–Vis absorption spectra of the CdS nanocrystals. The band gaps of the nanocrystals are measured using the Tauc relation [16] given by Eq. (1) and are presented in the insets of Fig. 3.
ðahmÞ ¼ Kðhm Eg Þ 2
ð1Þ
where a is the absorption co-efficient, hm is the photon energy, K is a constant and Eg is the band gap energy. The band gaps of the CdS nanocrystals with ME concentrations 50 mM, 100 mM, 150 mM and 200 mM are found to be 2.47 eV, 2.48 eV, 2.63 eV and 2.65 eV respectively. Thus, the band gaps are blue-shifted from the bulk value which is 2.42 eV. Further, the band gaps increase with decrease in crystallite sizes. The widening of band gaps show presence of confinement effects in the nanocrystals. The bonding nature of ME with the CdS nanocrystals is investigated using FT-IR spectroscopy. The FT-IR spectra of CdS nanocrystals are shown in Fig. 4. A broadband around 3420 cm1 are due to the O–H stretching vibration while the doublet centered at 2930 cm1 and 2850 cm1 is due to the symmetric and asymmetric stretching vibration of the CH2 groups respectively. The absorption bands at 1620 cm1 and 1415 are attributed to bending vibrations of H2O and H–C–H while absorption band at 1110 cm1 is attributed to C–O stretching vibration. The absorption band for S–H bond (2500 cm1) is found to be absent in CdS nanocrystals which occurs in free ME [17]. The absence of the S–H vibration in the FT-IR spectra is the result of covalent bonding between the thiols and Cd atom on the nanocrystal surface. Thus, FT-IR studies show capping of the CdS nanocrystals by ME. From the XRD and TEM studies, it is clear that CdS nanocrystals synthesized with 50 mM ME is cubic (zinc blende) while for high ME concentrations hexagonal (wurtzite) phase is found to be produced. FT-IR studies show capping of the CdS nanocrystals by ME. The average crystallite sizes of the CdS nanocrytals with cubic phase is 7 nm while with hexagonal phase is 3 nm. Thus, smaller crystallites are found to have hexagonal phase while larger ones take cubic phase. This result is in contrary to some of the earlier observations by Singh et al. [10], Zou et al. [11] and Banerjee et al. [12] that smaller crystallites of CdS favor cubic phase, but
K. Deka, M.P.C. Kalita / Chemical Physics Letters 652 (2016) 11–15
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Fig. 2. TEM micrographs of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a) and 200 mM (b). Insets: (i) crystallite size distribution (ii) high resolution TEM micrograph and (iii) SAED pattern.
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Fig. 3. UV–Vis absorption spectra of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d). Inset: Tauc plots of the respective absorption spectrum.
Fig. 5. XRD patterns of CdS nanocrystals with 50 mM ME concentration and Cd:S molarity ratios 1:0.75 (a), 1:0.50 (b) and 1:0.25 (c). X-ray diffractometer used: Philips X-pert Pro PW 1830.
precursors, hexagonal (cubic) phase is preferred in case of CdS nanocrystals. 4. Conclusions
Fig. 4. FT-IR spectra of CdS nanocrystals with Cd:S molarity ratio 1:1 and ME concentrations 50 mM (a), 100 mM (b), 150 mM (c) and 200 mM (d).
In summary, ME capped CdS nanocrystals with cubic and hexagonal phases are successfully synthesized at room temperature. Cubic, hexagonal and their mixture can be produced by changing the ME concentration. Increase in ME concentration lead to decrease in reaction rate between Cd2+ and S2 ions of the precursors and lower reaction rate favor hexagonal phase. The role of reaction rate in phase formation process is further evidenced by the observation of favoring hexagonal phase formation at lower S2 ion at constant Cd2+ ion and ME concentration. The present work shows the crystal phase of CdS nanocrystals can be controlled by varying the reaction rate of the Cd2+ and S2 ions of the precursors and thereby provides a way to obtain CdS nanocrytsals in desired structural phase. Acknowledgements
agrees well with the observation of hexagonal phase for smaller crystallites of CdS by Vogel et al. [14]. We make the proposition that the phase formation process in CdS nanocrystals can rather be related to the reaction rate between the Cd2+ and S2 ions. Increase in ME concentration increases the bonding probability between Cd2+ ion of the precursor and S2 ion of ME and thereby, lowers the reaction rate between Cd2+ and S2 ions of the precursors, and slower reaction rates between Cd2+ and S2 ions of the precursors favor the stable hexagonal phase. In order to validate the proposed mechanism of role reaction rate on phase formation process in CdS nanocrystals, we prepared a series of CdS samples with constant ME concentration of 50 mM and varying the Cd:S ratio as 1:0.75, 1:0.50 and 1:0.25. The XRD patterns of these samples are presented in Fig. 5. For higher concentration of S2 ion, cubic CdS is formed but for lower concentrations of S2 gradually hexagonal phase of CdS is found to evolve. Hence, we infer that for slower (faster) reaction between Cd2+ and S2 ions of the
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