Observation of Combined Effect of Temperature and Pressure on

Jul 25, 2014 - Cubic to Hexagonal Phase Transformation in ZnS at the Nanoscale ..... powders in a recently in-house designed ball mill (KC0)33−39...
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Communication pubs.acs.org/crystal

Observation of Combined Effect of Temperature and Pressure on Cubic to Hexagonal Phase Transformation in ZnS at the Nanoscale C. S. Tiwary,*,† S. Saha,‡ P. Kumbhakar,‡ and K. Chattopadhyay† †

Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Nanoscience Laboratory, Department of Physics, National Institute of Technology, Durgapur 713209, India

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ABSTRACT: The temperature of allotropic phase transformation in ZnS (cubic to wurtzite) changes with pressure and particle size. In this paper we have explored the interrelation among these through a detailed study of ZnS powders obtained by a temperature-controlled high energy milling process. By employing the combined effect of temperature and pressure in an indigenously built cryomill, we have demonstrated a large-scale, low-temperature synthesis of wurtzite ZnS nanoparticles. The synthesized products have been characterized for their phase and microstructure by the use of X-ray diffraction and transmission electron microscopic techniques. Further, it has been demonstrated that the synthesized materials exhibit photoluminescence emissions in the UV−visible region with an unusual doublet pattern due to the presence of both cubic and hexagonal wurtzite domains in the same particles. By further fine-tuning the processing conditions, it may be possible to achieve controlled defect related photoluminescence emissions from the ZnS nanoparticles.

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he last two decades have seen extensive research to understand the relation among the structural and optical, electrical and chemical properties of ZnS in the context of its projected applications in different photonics devices as well as for harvesting solar energy.1−7 Generally, ZnS crystallizes in two polymorphic forms: the wurtzite (hexagonal (P63mc)) and the sphalerite (cubic (F43m)), although growth of a phase with BCT structure of ZnS has also been reported earlier.8−11 In the normal condition, the cubic sphalerite phase is more stable than the wurtzite (hexagonal) phase due to a free energy difference of ∼10 kJ/mol. The phase transformation from cubic to hexagonal phase takes place at 1020 °C under a pressure of 1 bar.8 From a point of view of application, the wurtzite phase exhibits better photoluminescence properties than sphalerite.12 Hence, several attempts have been made in recent times to prepare the wurtzite phase at a lower temperature by controlling the size with the help of chemical precipitation, chemical vapor deposition, thermal deposition, and thin film deposition methods.13−16 The phase stability of these nonequilibrium processed samples is not well-established, and this lack of knowledge hinders their direct application. Recently by employing high-temperature annealing of the cubic phase, stable nanocrystals of wurtzite phase have been obtained.12,14,15 The reduction of particle size led to a decrease in transformation temperature, and hence, the nanometer-sized particles of the sphalerite phase of ZnS undergo transformation to the wurtzite phase at a much lower annealing temperature of 400 °C.17 Huang et al. have reported that the transformation temperature can be as low as 225 °C for 7 nm sized particles.18 In our previous work, we have also reported the transformation temperature of 300 °C for ZnS nanoparticles having a particle size of ∼5 nm.19 © 2014 American Chemical Society

There are other reports dealing with the effect of pressure on the synthesis of hexagonal phase of ZnS at normal temperature. The cubic to hexagonal phase transformation of ZnS has been reported at a high pressure of 30 GPa at room temperature.11,20,21 At the same time, the hexagonal wurtzite can be transformed back to cubic zinc blend structure at a pressure of 20.4−25.5 GPa. In recent experiments, strong correlations between pressure (15.6, 19.0, and 20.5 GPa) and particle size (10 μm, 36 and 11 nm) have been reported.20,23 In order to understand the effect of temperature and pressure on transformation, we like to draw attention to a general and detailed thermodynamical approach by Li et al.24 In this work, transformation temperature Tc has been correlated with the particle size (D) and the applied pressure (P). The relation is determined through the consideration of Gibbs free energy difference ΔG between two phases (cubic and wurtzite). The ΔG(T, P, D) function combines the effects of particle size, pressure, and temperature and can be given as ΔG(T , P , D) = ΔGv (T , 0, ∞) + ΔGs(D) + ΔGe(P , D) (1)

Here ΔGv(T, 0, ∞) is the temperature-dependent Gibbs free energy difference of bulk crystal, ΔGs(D) is size-dependent surface free energy difference, and ΔGe(P,D) is size- and pressure-dependent elastic energy. Assuming ΔG(T, P, D) = 0 at T = Tc(D) one obtains a empirical relation between transformation temperature and pressure as a function of size as Received: May 5, 2014 Revised: June 27, 2014 Published: July 25, 2014 4240

dx.doi.org/10.1021/cg500657e | Cryst. Growth Des. 2014, 14, 4240−4246

Crystal Growth & Design

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Figure 1. (a) Variation of transformation temperature (K) as a function of particle size (nm) at different pressures. The transformation temperature in the absence of pressure for different sizes from the literature is shown in square box, and current investigation results are shown in circle. (b) The root-mean-square (RMS) strain and WZ fraction at different ball to powder ratios (BPRs). Inset shows the variation of crystallite size at different BPRs. The inset shows the XRD plot for different BPRs. (c) Fraction of WZ for different milling durations for RT and cryo+RT. Inset shows XRD for both conditions. (d) Variation of crystallite size as a function of milling time for RT and cryo+RT. Inset showing RMS. strain for the same.

However, no experimental observation is available in the literature to confirm this trend. The “top to down” approach of reducing particle size through mechanical impact using a high energy ball mill can be an ideal technique to study the pressure and temperature induced phase transformation. The preparation of nanoparticles by ball milling is also scalable and hence can be available for synthesizing nanoparticles in large quantity. In the recent past, there are several reports of producing ZnS nanoparticles using a ball mill.27−29 All the above experiments yield cubic ZnS with a particle size of 4.4 nm for ZnS), will have a relatively smaller surface to volume ratio and can exhibit stronger intensity of PL emissions due to the reduction of surface states and hence nonradiative recombination paths.

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CONCLUSION We have shown that ball milling with a high ball to power ratio yields high pressure along with a localized temperature rise that influences and promotes cubic to wurtzite phase transformation in ZnS. The current paper experimentally establishes the interrelation between pressure, temperature, and size using temperature-controlled milling. We have shown that reduction of size reduces the transformation temperature, and application of pressure further reduces the transformation temperature. It has been shown that the combination of cryo and room temperature milling can give rise to very fine stable wurtzite nanoparticles. The effect of processing condition gives rise to defects in the structure. Synthesized samples show defectrelated PL emission in the UV−visible region with the unusual doublet pattern due to the presence of both cubic and hexagonal wurtzite domains in the sample particles. Hence, tuning the processing condition can give rise to controlled PL emission from ZnS nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*E mail: [email protected], chandrasekhar@platinum. materials.iisc.ernet.in. Tel.: +919449156997. Fax: +91 22932262. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the electron microscopy facilities available at the Advanced Facility for Microscopy and Microanalysis, Indian Institute of Science (IISc), Bangalore, India. The authors are grateful to UGCNRCM, IISc, Bangalore, for the financial support. Authors are also thankful to Mr. A. K. Kole, Nanoscience Lab, Department of Physics, NIT Durgapur, for his help in analyzing some results.

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REFERENCES

(1) Chattopadhyay, M.; Kumbhakar, P.; Tiwary, C. S.; Sarkar, R.; Mitra, A. K.; Chatterjee, U. J. Appl. Phys. 2009, 105, 105. (2) Kole, A. K.; Kumbhakar, P. Results Phys. 2012, 2, 150. (3) Zhang, H.; Gilbert, B.; Huangs, F.; Banfield, J. F. Nature 2003, 424, 1025. (4) Fang, X.; Zhai, T.; Gautam, U.; Li, l.; Wu, L.; Bando, Y.; Golberg, D. Prog. Mater. Sci. 2011, 56, 175. (5) Fang, X.; Ye, C.; Zhang, L.; Wang, Y.; Wu, Y. Adv. Funct. Mater. 2005, 15, 63. (6) Liu, H.; Hu, L.; Watanabe, K.; Hu, X.; Dierres, B.; Kim, B.; Sekiguchi, T.; Fang, X. Adv. Funct. Mater. 2013, 23, 3701. (7) Murugadose, G.; Ramasany, V.; Rajeshkumar, M. Appl. Nanosci. 2014, 4, 449. (8) Zhang, H.; Huangs, F.; Gilbert, B.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 13051. (9) Yan, J.; Fang, X.; Zhang, L.; Bando, Y.; Gautam, U.; Dierres, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 8, 2794. 4246

dx.doi.org/10.1021/cg500657e | Cryst. Growth Des. 2014, 14, 4240−4246