Direct Observation of Curved Surface Enhanced Disordering in Ag2S

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C: Physical Processes in Nanomaterials and Nanostructures

Direct Observation of Curved Surface Enhanced Disordering in AgS Nanoparticles 2

Jun Liu, Lu Chen, Hangsheng Yang, Ze Zhang, and Yong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10346 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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The Journal of Physical Chemistry

Direct Observation of Curved Surface Enhanced Disordering in Ag2S Nanoparticles Jun Liu, Lu Chen, Hangsheng Yang, Ze Zhang, Yong Wang* State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China

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ABSTRACT: Surface induced order-disorder phase transition has been widely studied on flat bulk surfaces, while such a transition is poorly understood on curved surfaces in nanoscale. Here, we report a direct observation of the dynamic behaviors of surface-initiated disordering in Ag2S nanoparticles using atomic resolution in-situ transmission electron microscopy. It was found that the disordering behavior is different from the traditional model that the disordered layer follows a logarithmical thickness dependence with temperature. In particular, the disordering is largely enhanced at higher temperature when the radius of the residual order phase is getting smaller. Moreover, the correlation length of disordered phase was found to be several times larger than the typical value for bulk surfaces. This significantly enhanced disordering in nanoparticles could be attributed to the extra driving force provided by the decreasing of order-disorder interface area.


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1.

INTRODUCTION

Being one of the most fundamental physicochemical phenomena in nature, first order solid-solid phase transition have been widely observed and extensively studied1-8, yet a detailed understanding of its microscopic mechanism is still evolving8-14. For a homogeneous system, once the nucleation barrier is crossed, the system will eventually evolve to a daughter phase that corresponds to the global minimization of the free energy15. However, for a system with free surfaces, the first order phase transition becomes complicated and the surface contribution must be seriously considered. In 1982, based on Landau theory of phase transitions, Lipowsky and coworkers16-17 predicted that the transition starts with the appearance of a disordered (high temperature) phase in the surface region of the crystal, while the bulk remains in its ordered state (such behavior is referred as pretransition18 in solid-solid phase transition). Specifically, the thickness ( l ) of the disordered layer grows logarithmically with temperature19: l  l0 ln T0 / (Tc  T )

(1)

where l0 is the correlation length within the disordered phase. The constant T0 specifies the temperature difference of Tc  Ts , Ts is the temperature at which the buildup of a disordered layer starts. Tc is the bulk phase transition temperature and temperature T is between Tc and Ts 19. The first experimental evaluation of this theory in solid-solid phase transition was reported in 198820 on the (100) surface of bulk Cu3Au by Dosch and coworkers using XRD. They demonstrated that the thickness of the disordered surface layer grows logarithmically as Tc is approached. This result confirmed that the model proposed by Lipowsky and coworkers works well in bulk materials with a flat surface. When the size of material decreased to nanoscale, the


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increased surface to volume ratio and surface curvature have a significant impact on the dynamic behaviors of phase transition of nanoparticles3-4, 21-24. However, limited studies25 have been done to explore such an effect in first order solid-solid transition at the atomic scale due to the lack of advanced in-situ techniques. As a consequence, whether the equation (1) is still valid or not, and how the disordering will proceed in nanoparticles remains unknown. In this study, the order-disorder ( α - β ) phase transition of individual Ag2S nanoparticles was investigated by taking advantage of recent advances in in-situ transmission electron microscopy (TEM) and microelectromechanical system (MEMS) technology, which ensured accurate temperature control and atomic resolution imaging at high temperature26-30. Our in-situ TEM results revealed that phase transition of Ag2S nanoparticles did start with the appearance of a surface disordered layer. However the thickness of the surface disorder layer was deviated from the traditional logarithmical dependence with temperature. Moreover, the estimated correlation length of Ag2S nanoparticle was much larger than that for bulk surfaces. 2.

METHODS

2.1. Preparation of Ag2S nanoparticles Ag2S nanoparticles were synthesized through a “soft” route as previously reported31. In a typical synthesis, 0.1 g AgNO3 (AR, Sinopharm Chemical Reagent Co., Ltd) was dissolved in 10 mL octadecylamine (90%, Aladdin) at 190°C. 0.1 g sulfur powder (CP, Sinopharm Chemical Reagent Co., Ltd) was then added to the solution. After magnetic stirring (10 min), the mixing solution was maintained at 190°C for 22 h (stirring and degassing are not necessary). The final products were collected and washed with ethanol for several times. The nanoparticles show the α phase at room


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temperature and have an average diameter of 32 nm as shown in Supporting Information Figure S1. 2.2. TEM characterizations A suspension liquid with Ag2S nanoparticles dispersed in ethanol was dropped onto a microelectromechanical system (MEMS) based heating chip functionalized with electron-transparent windows and heating spiral, which exhibits a negligible thermal drift. The chip was then mounted onto a double-tilt heating holder (Wildfire D6 for FEI, DENSsolutions, the temperature-control accuracy can be found in Supporting Information Figure S2 and S3). Before TEM characterization, argon plasma cleaning (Fischione Model 1020 plasma cleaner) was conducted on the samples to remove the chemical agents and adsorbates. In-situ observations were carried out by conventional TEM (Tecnai G2 F20, FEI, Operated at 200 kV) and Cs-corrected TEM (Titan G2 60-300 with an image aberration corrector, FEI, Operated at 300 kV). 3.

RESULTS AND DISCUSSION

Bulk Ag2S exhibits an order-disorder transition32-33 between α phase and β phase at 177°C. The α -Ag2S is monoclinic with a space group of P21/n, in which ordered Ag atoms partially occupy

the interstices of a distorted body-centered cubic (BCC) sulfur lattice frame7, 34. When α -Ag2S transforms into β -Ag2S (a space group of Im-3m), the BCC sulfur sublattice remains rigid, but disordered Ag atoms randomly distributed over the interstices of sulfur lattice35-36. Figure 1a and 1b are high resolution transmission electron microscopy (HRTEM) images of the same Ag2S nanoparticle before ( α ) and after ( β ) phase transition, and the insets are corresponding fast Fourier transformation (FFT) patterns. The zone axis of Figure 1a can be indexed as [2-10]α


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(detailed information can be found in the Supporting Information Figure S4.). Based on the orientation relationship of α -Ag2S and β -Ag2S, the zone axis of Figure 1b is indexed as [20-1]β . All diffraction spots in the FFT pattern of FCC β - Ag2S can be observed in the FFT pattern of α Ag2S, which indicates that the S sublattice framework in α -Ag2S has nearly the same lattice parameters as those in the BCC β -Ag2S. This is consistent with the structures of Ag2S determined by Frueh34. The disappearance of (001)α in FFT of β -Ag2S, which resulted in a large interplanar spacing, makes it intuitive to distinguish β -Ag2S from α -Ag2S25.


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Figure 1. In situ HRTEM images of the α - β phase transition process of the same Ag2S nanoparticle. (a-b) HRTEM images of α -Ag2S and β -Ag2S, corresponding FFT patterns are shown in the right panels. (c-f) Procedure of phase transition from α to β , β -Ag2S appeared at the surface of the nanoparticle and the thickness of β -Ag2S increased with the increase of temperature.


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The phase transition process of Ag2S nanoparticle was studied by in-situ HRTEM. At 151°C the Ag2S nanoparticle showed pure α phase, as shown in Figure 1c. It should be noted that all the temperatures in this study are calibrated, which have considered the heating effect of electron beam (detailed information can be found in the Supporting Information Figure S5 and S6). With temperature increased, β -Ag2S started to appear at the surface of the nanoparticle, which is consistent with the results in FeCoPd nanoparticles reported by Kovács et al.25 in 2009. As shown in Figure 1d and 1e, the thickness increased with temperature. Note that in our experiment, HRTEM images were acquired at least 30 s after the change of temperature to ensure the nanoparticles have reached the thermodynamic equilibrium state (refer to Supporting Information Figure S7 and S8). When the sample was heated to 163°C, the nanoparticle transformed to β phase completely as shown in Figure 1f. It should be noted that the as-measured phase transition temperature of Ag2S nanoparticle is 14°C lower than the bulk phase transition temperature of 177°C, which could be attributed to the size effect24-25.


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Figure 2. (a-c) Sequential Cs-corrected HRTEM images showing the thickness of β phase of region A, B and C in Figure 1c at different temperatures. (d-f) Relation between ln T0 / (Tc  T ) and the thickness of β -Ag2S surface layer. The initial slope of the diagram l0, which is the smallest one, is measured according to the red lines for comparison. To explore the relationship between the disorder layer ( β phase) thickness and temperature in detail, the layer thickness ( l ) of three typical regions of the nanoparticle at different temperatures was measured as shown in Figure 2a-c. From Figure 2a, one can see that the thickness of the β


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layer on region A increased with temperature. The l ~ ln T0 / (Tc  T ) plot of region A is shown in Figure 2d. According to equation (1), l should be proportional to ln T0 / (Tc  T ) as indicated by the red line. It fits well below 156°C, however, the measured result deviated from the red line evidently between 156°C and 162°C. Similar phenomena were also observed in Figure 2e and 2f measured from region B and region C. It should be noted that Lipowsky’s theory is derived from a semi-infinite system, where the area of interface kept unchanged during phase transition. However, in the case of Ag2S nanoparticles, the α - β interface is continuously decreasing during phase transition, which provided extra driving force for the phase transition. Moreover, the extra driving force will increase with temperature, since the reduction of the interface area caused by transition of the same amount of Ag2S is negatively correlated to the radius of the residual α -Ag2S (refer to Supporting Information Figure S9.). As a result, the surface induced phase transition is further enhanced, with the l ~ ln T0 / (Tc  T ) plot being upturned. According to equation (1), the slope of l ~ ln T0 / (Tc  T ) diagram ( l0 ) is the correlation length within the disordered phase. As mentioned above, if the disordering is enhanced, the slope l0 of Ag2S nanoparticle should be distinctly larger than bulk system. As the slope of l ~ ln T0 / (Tc  T ) diagram of Ag2S nanoparticle is no longer a constant, we measured the initial slope l0 _ initial (also the smallest l0 ) of l ~ ln T0 / (Tc  T ) diagram. The l0 _ initial of region A, B and C is measured to be 3.26 nm, 11.95 nm and 3.20 nm, respectively, as shown in Figure 2. For a first-order transition, l0 should be in the order of the lattice constant ( 3a0 for the (100) surface of Cu3Au and 2.5a0 for the (110) surface of Pb).20, 37 However, the smallest l0 measured in the present study ranged from 6.6a to 25.6a ( a =0.486 nm)38, which is approximately one order of magnitude higher than the


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lattice constant of β -Ag2S. This result confirmed that the surface-initiated disordering is enhanced by the extra driving force provided by the reduction of the interface area in nanosystems.

Figure 3. Heating and cooling stages of the sample during phase transition. (a) At 150°C, no β Ag2S is observed. (b) At 161°C, about 5 nm of β -Ag2S is observed. (c) At 156°C, the phase boundary moved backward with the layer thickness decreased to 1.5 nm. (d) When the nanoparticle was cooled to 150°C, it transformed to pure α -Ag2S completely. (e) FFT pattern of the framed area in the yellow square in (b). (f) FFT pattern of the framed area in the white square in (b). To affirm the thickness of β phase is a state function of temperature, the temperature induced reversible movement of the phase interface was also studied. Figure 3a is a HRTEM image of another particle at 150°C, showing a pure α phase. When the nanoparticle was heated to 161°C, which is higher than the onset temperature of pretransition (150°C), a surface β -Ag2S layer with a thickness of about 5 nm is observed (Figure 3b). When the particle was cooled from 161°C to 156°C, the phase boundary moved backward accordingly with a reduced β layer thickness of 1.5 nm as shown in Figure 3c. In particular, when the temperature was cooled to 150°C, no β phase is observed (Figure 3d). This indicated that the thickness of β phase layer is determined by


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temperature and the movement of the phase interface is reversible, which demonstrated that the phase transition is not restricted by dynamic factor but thermodynamic one16. This maneuverability might offer a promising prospect in regulating performance of nanoscale devices. In contrast to the surface-induced order to disorder transition, during β to α transition the nanoparticle kept pure β phase even it was cooled to 143°C (Figure 4a) from 163°C (Figure 1f). Then it rapidly transformed to pure α phase at 142°C (Figure 4b), which is far below the transformation point (162°C, order to disorder) of the nanoparticle, indicating that the disorder to order transition of Ag2S nanoparticles still follows a classic nucleation and growth pathway. This is consistent with Lipowsky’s theory that surface-induced ordering and surface-induced disordering is exclusive in the same system39.

Figure 4. Phase transition from β -Ag2S back to α -Ag2S. (a) At 143°C, the nanoparticle is still pure β phase. (b) When cooled to 142°C, the nanoparticle transformed to pure α phase. Insets are magnification of the framed area.


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4.

CONCLUSIONS

In summary, the dynamic behaviors of surface-initiated disordering in single Ag2S nanoparticles were studied for the first time. Our results demonstrated that the disordering of Ag2S nanoparticles behaved differently compared to the traditional theory for bulk materials, and particularly at high temperature. Moreover, the measured correlation length was found to be several times larger than that expected for bulk surfaces. The extra driving force provided by the decreasing area of the order-disorder interface in nanoparticles is responsible for this enhanced disordering. Our results provide a complete picture of surface-induced phase transition, which offers new insight into the fundamental microscopic mechanism of phase transitions in nanoscale.

ASSOCIATED CONTENT Supporting Information Figure S1: TEM image of Ag2S nanoparticles. Figure S2-3: Temperature control accuracy of Wildfire D6. Figure S4: Zone axis index of Ag2S nanoparticle in Figure 1(in the main text). Figure S5-6: Temperature calibration. Figure S7-8: Time needed to reach equilibrium state. Figure S9: Detailed information about Model about interface area reduction induced by transformation of the same amount of α -Ag2S to β -Ag2S. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y.W.)


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Author Contributions Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the support of National Natural Science Foundation of China (Nos. 51390474, 11327901). REFERENCES (1) Zhang, H.; Huang, F.; Gilbert, B.; Banfield, J. F., Molecular Dynamics Simulations, Thermodynamic Analysis, and Experimental Study of Phase Stability of Zinc Sulfide Nanoparticles. J. Phys. Chem. B 2003, 107, 13051-13060. (2) Davydov, V. A.; Rakhmanina, A. V.; Rols, S.; Agafonov, V.; Pulikkathara, M. X.; Vander Wal, R. L.; Khabashesku, V. N., Size-Dependent Phase Transition of Diamond to Graphite at High Pressures. J. Phys. Chem. C 2007, 111, 12918-12925. (3) Li, Y.; Qi, W.; Li, Y.; Janssens, E.; Huang, B., Modeling the Size-Dependent Solid–Solid Phase Transition Temperature of Cu2S Nanosolids. J. Phys. Chem. C 2012, 116, 9800-9804. (4) Rivest, J. B.; Fong, L.-K.; Jain, P. K.; Toney, M. F.; Alivisatos, A. P., Size Dependence of a Temperature-Induced Solid–Solid Phase Transition in Copper(I) Sulfide. J. Phys. Chem. Lett 2011, 2, 2402-2406. (5) Chen, B.; Ten Brink, G. H.; Palasantzas, G.; Kooi, B. J., Crystallization Kinetics of Gesbte Phase-Change Nanoparticles Resolved by Ultrafast Calorimetry. J. Phys. Chem. C 2017, 121, 8569-8578. (6) Huang, F.; Banfield, J. F., Size-Dependent Phase Transformation Kinetics in Nanocrystalline Zns. J. Am. Chem. Soc. 2005, 127, 4523-4529. (7) Sadovnikov, S. I.; Gusev, A. I.; Chukin, A. V.; Rempel, A. A., High-Temperature X-Ray Diffraction and Thermal Expansion of Nanocrystalline and Coarse-Crystalline Acanthite AlphaAg2S and Argentite Beta-Ag2S. Phys. Chem. Chem. Phys. 2016, 18, 4617-4626. (8) Sadovnikov, S. I.; Gusev, A. I.; Rempel, A. A., An in Situ High-Temperature Scanning Electron Microscopy Study of Acanthite-Argentite Phase Transformation in Nanocrystalline Silver Sulfide Powder. Phys. Chem. Chem. Phys. 2015, 17, 20495-20501. (9) Tong, H. C.; Wayman, C. M., Direct Evidence of Pretransformation Lattice Instabilities. Phys. Rev. Lett. 1974, 32, 1185-1188. (10) Dosch, H.; Mailänder, L.; Reichert, H.; Peisl, J.; Johnson, R. L., Long-Range Order near The Cu3Au(001) Surface by Evanescent X-Ray Scattering. Phys. Rev. B 1991, 43, 13172-13186. (11) Zheng, H.; Rivest, J. B.; Miller, T. A.; Sadtler, B.; Lindenberg, A.; Toney, M. F.; Wang, L. W.; Kisielowski, C.; Alivisatos, A. P., Observation of Transient Structural-Transformation Dynamics in a Cu2S Nanorod. Science 2011, 333, 206-209.


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TOC Graphic


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TOC Graphic 82x44mm (300 x 300 DPI)


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Figure 1



Figure 2


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Figure 4


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Figure S1 in Supporting Information 121x39mm (300 x 300 DPI)





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Figure S4 in Supporting Information


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Figure S9 in Supporting Information


Direct Observation of Curved Surface Enhanced Disordering in Ag2S

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