Article pubs.acs.org/crystal
In Situ Observation of the Humidity Controlled Polymorphic Phase Transformation in Glycine Microcrystals Dmitry Isakov,*,†,‡ Daria Petukhova,† Semen Vasilev,† Alla Nuraeva,† Timur Khazamov,† Ensieh Seyedhosseini,§ Pavel Zelenovskiy,† Vladimir Ya. Shur,† and Andrei L. Kholkin†,§ †
Institute of Natural Sciences, Ural Federal University, Lenin Ave. 51, Ekaterinburg, 620000, Russia Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom § Department of Materials and Ceramic Engineering & CICECO, University of Aveiro, 3810-193 Aveiro, Portugal ‡
S Supporting Information *
ABSTRACT: Glycine is a model crystal exhibiting three polymorphic phases and important functional properties such as piezoelectricity and ferroelectricity. We report here in situ observation of the irreversible transformation of the solutiongrown glycine crystals from a β phase into a γ phase. The slow transformation process was monitored by piezoresponse force microscopy at room temperature. The process of β to γ conversion was entirely controlled by the variation of relative humidity in the sample chamber. The results show that the rate of phase transformation in glycine is humidity dependent with a threshold of about 25% RH. It is demonstrated that the phase boundary is highly rugged and the transformation front propagates inhomogeneously along the polar axis of the β phase. The mechanism of the phase transformation is discussed.
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INTRODUCTION Crystals of the amino acid glycine possess multiple possible polymorphic states under ambient conditions, thus serving as a model material for the fast growing field of physics and chemistry of polymorphism.1 Polymorph control is especially important in pharmaceutical manufacturing in which the products are mostly prepared in the crystalline form.2 Besides, glycine is the simplest and smallest amino acid, and it serves as a biomimetic structural prototype for proteins and other amino acids.3 Furthermore, recently glycine has attracted increased attention as a promising (multi)functional molecular material because of its outstanding piezoelectric,4,5 ferroelectric,6 and nonlinear optical properties.7,8 Glycine crystals consist of molecules in the form of zwitterions (NH3+−CH2−COO−) linked through NH···O hydrogen bonds.9 Under ambient conditions, there are three possible polymorphic phases, α, β, and γ, that can be observed. In addition, under some extreme conditions other metastable phases were also obtained.10,11 The α form of glycine (P21/n space group, Z = 4) represents sheets of zwitterions connected by hydrogen bonds, whereas the sheets are organized through van der Waals forces in centrosymmetric double layers. The molecular arrangement in the crystal lattice of β glycine (P21 space group, Z = 2) is quite similar. However, in contrast to αform, molecular sheets form here noncentrosymmetric single layers. The monoclinic b-axis in both β and α polymorphs corresponds to the stacking direction of molecular layers. The crystal structure of γ-glycine (P31 space group, Z = 3) is essentially different. The zwitterions are organized in triple helices around the 31 screw-axis and are linked into a 3D © 2014 American Chemical Society
hydrogen bonded noncentrosymmetric network. The arrangement of glycine molecules in the crystal structure dictates the difference in the properties of polymorphs. The stability of glycine polymorphs under ambient conditions is ordered in a sequence β < α < γ, that is, the β phase is the least stable.12,13 However, metastable α and β polymorphs can be preserved in a dry atmosphere for a long time because they have large enough kinetic barriers for transformation. Under humid NH3 vapor, α-phase typically converts into the γ-polymorph.14 The β-polymorph is also metastable, and in the presence of humid air, under increasing temperature, in the presence of NH3, or under grinding, it irreversibly transforms into α or γ forms.9,15−17 The useful polar β phase with the highest nonlinear optical properties and ferroelectric behavior can be stabilized on a platinum substrate7 or by crystallization under nanoscale confinement.5,18,19 It is evident that the mechanism of the polymorphic phase transitions must involve cooperative reorganization of the entire hydrogen bonded network in the crystal. This mechanism, being kinetically controlled, requires detailed study at the mesoscopic scale where the motion of the phase boundary can be visualized using local optical or other nondestructive methods. The aim of the present work was the in situ investigation of polymorphic transformation of metastable β-crystals of glycine into γ-form by piezoresponse force microscopy (PFM). PFM is a powerful technique for high Received: May 23, 2014 Revised: June 20, 2014 Published: July 23, 2014 4138
dx.doi.org/10.1021/cg500747x | Cryst. Growth Des. 2014, 14, 4138−4142
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Article
resolution imaging and manipulations of piezoelectric and ferroelectric materials, providing qualitative information on domain morphology and its evolution.20
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EXPERIMENTAL SECTION
Microcrystals of β-glycine were grown by the evaporation of a 20 μL drop of 0.13 M aqueous solution on the Pt(111) substrate (Si/SiO2/ Ti/Pt, Nanotec, Korea) under ambient conditions at relative humidity (RH) about 22−24%. The evaporation results in predominately crystallized β-form of glycine.7 Crystallization of the needle-shaped βglycine for the used concentration is facilitated according to Ostwald’s rule of stages, which states that crystallization from a solution should start from the nucleation of the least stable polymorph.21,22 The resulting microcrystals exhibited two types of morphology such as dendrites that mainly located at the edge of the drop and transparent flat single microcrystals of average size of about 50 × 300 × 20 μm3 with the longest size parallel to b-axis. The in situ observation of phase transformation of glycine was conveniently visualized by the PFM method. PFM technique uses standard deflection-based feedback of the electromechanical response of the sample to an applied electric voltage due to the converse piezoelectric effect.23,24 When a sinusoidal V cos(ωt) bias is applied, the first harmonic component of the bias-induced tip deflection, d1ωcos(ωt + ϕ) will determine a local induced electromechanical deformation of the sample, where d1ω and ϕ are piezoresponse amplitude and phase, correspondingly. In this way, both the contact topography and the vertical and lateral piezoresponse signals can be visualized. In our study, a commercial atomic force microscope (Ntegra Aura, NT-MDT, Russia) with a Ti/Pt coated tip was used. PFM imaging was performed in both in-plane and out-of-plane modes under application of ac-voltage (15 V peak-to-peak, 17.4 kHz). The experiment was carried out at room temperature when the Pt substrate with the as-grown sample of glycine on its surface was held before measurements in a custom chamber with dispensed vapor and humidity probe sensor. The polymorphous form and single crystallinity of the glycine crystal were verified by confocal Raman microscope Alpha300AR (WITec GmbH, Germany). A 488 nm solid state laser (power 27 mW) was used for the excitation of Raman scattering collected by 100× objective in 180° backscattering geometry. The diffraction grating with 600 lines per millimeter provides the spectral resolution of about 3 cm−1.
Figure 1. (a) Optical micrograph of as-grown β-glycine crystal, (b) lateral PFM image of diphasic glycine crystal at the beginning of phase transformation, and (c) corresponding PFM image for terminated transformation. The dark area in the PFM images corresponds to βphase; the bright one belongs to γ-phase.
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RESULTS AND DISCUSSION For experimental study, transparent and flat crystals have been chosen (see Figure 1a). Before measurements, the sample was kept in a dry atmosphere (RH about 24%), and no phase changes were observed. The polymorphic transformation into the γ-phase was triggered by a suddenly increased humidity inside the chamber to RH 30% without any direct effect on the crystal surface morphology. This transformation was visualized by PFM cyclically performed every 20 min under slowly decreasing RH (ΔH/Δt = −0.022% RH/min). Slight temperature rise (ΔT/Δt = 5 × 10−3 K/min) caused by instrument heating was observed during the experiment. Figure 1b,c presents the PFM images of the multiphase glycine in the beginning of phase transformation and in the final stage of controllable termination of the transformation process, when the RH reached 25%. The kinetics of the transformation visualized by PFM is shown in Figure 2 as a sequence of images showing position of the phase front at different times. To simplify the visualization of the phase transformation, the PFM image was converted to black-and-white where the black area represents γ-phase while white area corresponds to the original β-phase. Each image (50 × 50 μm2) was recorded for about 18 min with scanning
Figure 2. Time variation of the phases in diphase glycine crystal during β to γ phase transformation.
direction from top-to-bottom (along c-axis of the β-glycine crystal) and left-to-right (along b-axis). As seen from the image sequence, the front of the γ-phase moves predominantly along the polar b-axis of the initial crystal (see axes determined in Figure 1a). The dynamics of the front motion is very irregular and suggests the well-known phenomenon of stick−slip motion in friction.25,26 This is probably due to the pinning of the moving phase boundary on the surface or bulk defects, as well as the effect of inhomogeneous mechanical stress distribution. The predominant propagation of the new phase along the 2fold axis of β-glycine can be caused by the process being more kinetically efficient if the phase front is normal to the cleavage 4139
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plane.27 It worth nothing that after the transformation occurred, the resulting γ glycine phase had lost its optical transparency. Figure 3b shows the time-dependent variation of relative surface area of the γ-phase which represents the phase
Figure 3. Time dependences of (a) humidity and (b) corresponding γphase content; (c) dependence of ΔSγ/Δt on humidity.
transformation kinetics. The time dependence of relative humidity is shown in Figure 3a. It is seen that the step-like increase of relative humidity from 24% to 30% causes a quick increase of the relative surface area of the γ phase up to 50%. The rate of phase transformation, ΔSγ/Δt, as a function of instant humidity is shown in Figure 3c. Three characteristic areas can be highlighted: (i) the high humidity range of about 28−30% RH is characterized by the fast transformation rate (about 9 μm2/min); (ii) a low (2−3 μm2/min) transformation rate occurs at 26−28% RH; (iii) there is a threshold value at 25% RH below which the transformation rate is zero. It is evident that the phase transformation from β- to γ-glycine is fully controlled by the humidity, and the phase boundary can be easily positioned using humidity just above threshold. After the experiment, the diphasic glycine crystal was held in a dry air (
