Physical Chemistry of Crystalline (K,NH4)H2PO4 in Aqueous Solution

Jul 3, 2014 - crystallization produces local distortions in structure, resulting in the difficulties in growing high-quality (K,NH4)H2PO4 single cryst...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Physical Chemistry of Crystalline (K,NH4)H2PO4 in Aqueous Solution: An in Situ Molecule Vibration Spectral Observation of the Early Formation Stage Congting Sun and Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: The competing occupancy of cation position by NH4+ and K+ during crystallization produces local distortions in structure, resulting in the difficulties in growing high-quality (K,NH4)H2PO4 single crystals with particular mixing concentration. Crystal properties such as structure, defect density, and purity always depend on the early formation stage of crystalline (K,NH4)H2PO4 molecule. Identification of the effect of K+/NH4+ mole ratios in KH2PO4−NH4H2PO4 aqueous solution on (K,NH4)H2PO4 molecule motion at the early stage of crystallization can provide the strategy to growing high-quality (K,NH4)H2PO4 crystals with particular mixing concentration. In situ molecule vibration spectroscopy was used to identify the early formation stage of crystalline (K,NH4)H2PO4 in KH2PO4−NH4H2PO4 aqueous solution with various K+/NH4+ mole ratios. (K,NH4)H2PO4 molecule motion was imaged via integrating the structural information on IR/Raman-active NH4+, H2PO4−, and hydrogen bonding infrastructures. K+/NH4+ mole ratio in KH2PO4−NH4H2PO4 aqueous solution determines the supersaturation in crystallization system as well as the competing incorporation of K+ and NH4+ into the lattice. Both lower supersaturation and stronger competition between cations hinder the crystallization of (K,NH4)H2PO4, resulting in the remarkable spectral difference before and after the formation of crystalline (K,NH4)H2PO4. Our results demonstrate the concept of competed incorporation between different cations into the anionic framework from the molecular viewpoint.

■

INTRODUCTION Crystallization is an exothermic process with the phase transformation from free ions to crystalline state via forming chemical bonds, which is the unit operation in the fabrication of products ranging from pharmaceuticals1 to nano/micromaterials with advanced performances.2−4 In order to optimize and control the formation of crystalline materials from solution, it is essential to establish an molecule-level understanding of the sequence of events involved in the crystallization process, rather than simply studying the morphological and structural properties of the bulk crystals collected at the end of the process.5 Unravelling the key steps in the crystallization process requires tools that allow the control and microscopic visualization of the early stages in nucleation and crystal growth, which determine crystal properties such as purity, size, morphology, and crystallographic structure.6 At the early stage of crystallization, there is growing evidence that different species of precursor phase exist instead of classical nucleation theory.7,8 The precursor phase can be divided into amorphous and crystalline phases, where the amorphous phase shows a specific short-range order that corresponds to the long-range order of the particular crystalline polymorph,9 and the selfassembly of nanocrystals plays a critical role in the further phase transformation to particular crystalline phase.10 Generally, experimental techniques such as X-ray, neutron, and electron © 2014 American Chemical Society

diffraction can be used to obtain the molecule structure, however, the molecule must be in crystalline state with longrange atomic positional order.11−13 It is still a challenge to determine the molecule configuration of precursor phases without long-range ordered atomic arrangement and the molecule motion in dynamic phase transformation process. Molecule vibration spectroscopy has been demonstrated as an effective tool to reflect molecule symmetry, since the number of allowed transitions is restricted by selection rules in molecule vibration spectroscopy.14,15 Both KH2PO4 and NH4H2PO4 are representatives of hydrogen bonded crystals which possess excellent electrooptic and nonlinear optical properties in addition to interesting electrical properties.16 Their identical H2PO4− framework and the similar radius of cation (K+ and NH4+) provide the possibility of the formation of (K,NH4)H2PO4 hybrid crystals.17 (K,NH4)H2PO4 crystals have interesting physical properties in a certain intermediate mixing concentration range, such as spin glass state18 and anomalous photonic conductivity,19 which act as a good model to study the chemical/physical properties− crystallographic structure relationship. However, the competing Received: May 16, 2014 Revised: July 3, 2014 Published: July 3, 2014 16043

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

Figure 1. Vibration spectroscopy selection rule for XY4 groups with tetrahedral configuration. With decreasing the symmetry of XY4 group from Td to S4 and C2v, the group factor transforms from F2 (triply degenerate) to E (doubly degenerate) and A, B (no degenerate). Since the number of vibration bands depends on the group factor, the symmetry of the group can be identified on the basis of the position and the number of vibration bands.

occupancy of cation position by NH4+ and K+ during crystallization produces local distortions in structure, resulting in the difficulties in growing high-quality (K,NH4)H2PO4 single crystals with particular mixing concentration.20 Identification of the effect of K+/NH4+ mole ratios (M(K+)/M(NH4+)) in KH2PO4−NH4H2PO4 aqueous solution on the molecule motion of (K,NH4)H2PO4 at the early stage of crystallization

can provide strategy to growing high-quality (K,NH4)H2PO4 crystals with a particular mixing concentration. The crystallization processes of both KH 2 PO 4 and NH4H2PO4 have been observed by in situ vibration spectroscopy.21−23 On the basis of spectroscopic selection rules, P−O, P−OH, N−H, and O−H···O vibration bands in the NH4H2PO4 molecule are both Raman and IR active.24,25 The molecular vibration modes of the growing crystals, including 16044

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

symmetry of the NH4+ group reduces from Td to S4, v4(NH4) splits into v4(E) at 1450 cm−1 and v4(B2+E) at 1400 cm−1. Moreover, in both crystalline KH2PO4 and NH4H2PO4, PO4 has been demonstrated with S4 symmetry, and two broad bands (2v1 at 1075 (B2+E) and 874 (A1) cm−1) appear in IR spectrum. Whereas the hydrated free H2PO4− possesses C2v symmetry, there exist four vibration modes for P−O and P− OH in the H2PO4− group, i.e., 2v1(A1) at 874 and 1075 cm−1, v3(B1) at 1150 cm−1, and v3(B2) at 940 cm−1. In the procedure of molecular configuration identification, the symmetry of constituent groups can be confirmed directly via the molecule vibration spectroscopy. In the phase transformation from amorphous to crystalline state, (K,NH4)H2PO4 molecule configuration can be determined via integrating K+, NH4+, H2PO4−, and hydrogen bonding infrastructures. Figure 2 illustrates the identification scheme of crystalline KH2PO4 and NH4H2PO4 molecules. As shown in Figure 2a, IR spectrum indicates the v1(PO4) broad vibration bands respectively at 1068 and 874 cm−1. The dramatic vibration bands at 1238, 1650, 2384, and 2730 cm−1 can be attributed to P−O−H···O−P vibration bands respectively, indicating the hydrogen bonding between H2PO4− groups. Molecule symmetry of H2PO4− group depends on the relative position of K+ and H2PO4− groups in the crystalline KH2PO4 molecule. By combing the structure symmetry of H2PO4− groups and P−O−H···O−P hydrogen bonding behaviors, it can deduce that crystalline KH2PO4 molecule possesses D2d symmetry. Likewise, the configuration of crystalline NH4H2PO4 molecule can also be deduced from the vibration bands in IR spectroscopy. Figure 2b shows the H2PO4− group with S4 symmetry and the NH4+ group with S4 symmetry. Molecule symmetry of the H2PO4− group depends on the relative position of NH4+ and H2PO4− in the crystalline NH4H2PO4 molecule. Combing the structure symmetry of H2PO4− groups and P−O−H···O−P hydrogen bonding behaviors, it can deduce that crystalline NH4H2PO4 molecule also possesses D2d symmetry. For amorphous precursor phase (K,NH4)H2PO4, IR spectra indicate the P−O, P−OH stretching vibrations (i.e., 2v1(A1) at 874 and 1075 cm−1, v3(B1) at 1150 cm−1, and v3(B2) at 940 cm−1), and N−H bending (i.e., v4(NH4+) at 1450 cm−1 (E) and 1400 cm−1 (B2+E)), which are different from both free ionic state and crystalline state. NH4+ with Td symmetry, H2PO4− with C2v symmetry, and hydrogen bonding linked (H2PO4−)n can build (K,NH4)H2PO4 molecule with C2v symmetry. With the transition of (K,NH4)H2PO4 amorphous precursor phase to crystalline phase, the intensity of N−H bending vibration at 1450 cm−1, P−OH stretching vibration at 940 cm−1, P−O stretching vibration at 1050 cm−1, and P−O−H···O−P bending vibration at 1253 cm−1 increases. Despite the changes in the intensity of these vibration bands, the type of vibration bands remains unchanged, illustrating the variations of N−H, P−O, and P−OH bond lengths (Figures S1−S11). As shown in Figure 3, the H2PO4− groups move in an anticlockwise direction surrounding the center H2PO4− group, whereas K+ ions/NH4+ groups start to move in a clockwise direction surrounding the center H 2 PO 4 − group. (K,NH 4 )H 2 PO 4 molecule configuration depends on the rotation of both NH4+ and H2PO4− groups. Combining these structural fractions, we can further confirm that the (K,NH4)H2PO4 molecule configuration undergoes the transition from C2v to D2d in the phase transition from amorphous precursor phase to crystalline phase.

internal modes of PO4 tetrahedra molecular vibrations, external modes of optical phonons and hydrogen bonding modes have been determined exactly by using micro-Raman spectroscopy.25 Likewise, these vibration characteristics for the H2PO4− group can also appear in the IR spectrum, which can provide more detailed information for resolving NH4H2PO4 molecule configuration. Attenuated total reflectance-infrared (ATR-IR) spectroscopy has been recently used to record the effects of concentrations, solution compositions, and pH value on structural dynamics of NH 4 + and H 2 PO 4 − groups in NH4H2PO4 aqueous solution.22,26 In this work we utilize in situ molecule vibration spectroscopy to monitor the formation process of crystalline (K,NH4)H2PO4 in KH2PO4−NH4H2PO4 aqueous solution with different M(K+)/M(NH4+). By combining advances in vibration spectroscopy techniques, we can identify the molecule motion of (K,NH4)H2PO4 during the crystallization process.

■

EXPERIMENTAL SECTION

The spectral studies of solution specimens were carried out at 18 °C by ATR-IR technique with ATR cell (Thermo Nexus 6700). Internal reflection element is a diamond wafer. Thermo−Nicolet−Nexus FT−IR spectrometer was utilized to record the IR spectra of (K,NH4)H2PO4 samples in different periods during the crystallization process. The absorption measurements of all aqueous solutions were conducted using a Nicolet 20DXB FT−IR spectrometer in the spectral range of 4000−525 cm−1. In particular IR measurement, 5 μL KH2PO4− NH4H2PO4 mixture solution was dropped onto the diamond wafer, forming the detectable liquid film. At room temperature (18 °C), 2 mol/L KH2PO4 and NH4H2PO4 solutions were prepared. Then, the KH2PO4−NH4H2PO4 mixture solutions were prepared under different volume ratios, i.e., 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9. Successively IR spectra of KH2PO4−NH4H2PO4 aqueous solution can be obtained with increasing time, and the time interval was selected as 40 s. Measurements were carried out after the as-prepared mixture solution was dropped on the diamond wafer. Raman scattering (Renishaw 2000) measurements were carried out at 18 °C, excited with the 512 nm line, in the spectral range from 200 to 4000 cm−1.

■

RESULTS AND DISCUSSION In molecule vibration spectroscopy, the number of allowed transitions is restricted by selection rules and thus directly reflects the symmetry of the groups, clusters, and particles.14 Figure 1 illustrates the vibration spectroscopy selection rule for the XY4 group with tetrahedral space configuration. With decreasing the symmetry from Td to S4 and C2v, group factor transforms from F 2 (triply degenerate) to E (doubly degenerate) and A, B (no degenerate). The number of vibration bands in molecule vibration spectrum depends on the group factor of XY4 group. The symmetry of the group can therefore be directly identified on the basis of the position and the number of vibration bands. When the surrounding environment of the XY4 group is uniform, the interaction between the environment and group is anisotropic. In such a case, the symmetry of the XY4 group approaches its intrinsic symmetry, which prefers to adopt a relative high symmetry to decrease its free energy. For example, the free hydrated NH4+ group has Td symmetry; correspondingly, the v4(F2) vibration band at 1450 cm−1 can appear in the IR spectrum. When the 16045

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

Figure 3. Molecule motion from (H2PO4−)n framework to amorphous precursor phase (K,NH4)H2PO4 and crystalline state (K,NH4)H2PO4. (a) The formation of (H2PO4−)n framework with C2v symmetry before nucleation. (b) The incorporation of K+ or NH4+ into the (H2PO4−)n framework. (c) Molecule configuration of (K,NH4)H2PO4 when H2PO4− groups move in an anticlockwise direction less than 45° surrounding the center H2PO4− group. (d) Molecule configuration of (K,NH4)H2PO4 when H2PO4− groups move in an anticlockwise direction about 45° surrounding the center H2PO4− group. (e) Molecule configuration of (K,NH4)H2PO4 when H2PO4− groups move in an anticlockwise direction larger than 45° surrounding the center H2PO4− group. (f) Crystalline (K,NH4)H2PO4 molecule with D2d symmetry. Phosphorus atoms are shown as orange spheres, oxygen are red, hydrogen atoms are small and gray, and K/NH4 are large and green.

not obvious for (K,NH4)H2PO4 grown from KH2PO4− NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 6:4. It can be concluded that host lattice of crystalline (K,NH4)H2PO4 depends on M(K + )/M(NH 4 + ) in KH 2 PO 4 −NH 4 H 2 PO 4 aqueous solution. For example, the KH2PO4 molecule can act as the host molecule framework in the crystallization from KH2PO4−NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 6:4 (Figure 4a). Whereas NH4H2PO4 molecule acts as the host molecule framework in the crystallization from KH2PO4− NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 4:6 (Figure 4b). Comparing IR spectra at t = 0 s in Figure 4a and b shows that a lower K+/NH4+ mole ratio corresponds to the higher intensity of v4(NH4+) at 1450 cm−1. With time prolonging, the aqueous solution condenses owing to the volatilization of H2O at 18 °C. For example, after volatilizing water from KH2PO4−NH4H2PO4 aqueous solution with K+/ NH4+ mole ratio of 6:4 for 400 s, the concentrations of KH2PO4 and NH4H2PO4 in the mixture solution become 1.69 and 1.13 mol/L, respectively (Figure S12). The nucleation of (K,NH4)H2PO4 occurs at 400 s in KH2PO4−NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 6:4 and occurs at 480 s in KH2PO4−NH4H2PO4 aqueous solution with M(K+)/ M(NH4+) = 4:6. The longer time needed for the nucleation in lower M(K+)/M(NH4+) can be attributed to the lower saturated concentration of KH2PO4 solution than that of NH4H2PO4 solution at 18 °C. That is, a higher K+/NH4+ mole ratio will result in a higher supersaturation in the aqueous crystallization system. Figure 5 shows the molecule symmetry of the (K,NH4)H2PO4 in KH2PO4−NH4H2PO4 crystallization system with different M(K+)/M(NH4+) at the nucleation stage recorded by in situ IR spectroscopy. Before the nucleation of (K,NH4)-

Figure 2. IR spectroscopy identification of molecule symmetry of crystalline KH2PO4 (a) and NH4H2PO4 (b) by combining NH4+, H2PO4− groups, and hydrogen bonding infrastructures. According to IR spectroscopy selection rule, the H2PO4− group has S4 symmetry in crystalline KH2PO4, while both NH4+ and H2PO4− group possess S4 symmetry in crystalline NH4H2PO4. Group symmetry of NH4+ and H2PO4− groups as well as the hydrogen bonding between groups demonstrate both crystalline KH2PO4 and NH4H2PO4 molecules with D2d symmetry.

Figure 4 shows the time-dependent IR spectra of the KH2PO4−NH4H2PO4 aqueous solution crystallization system. Previous spectroscopy studies illustrated that the symmetry of the NH4+ group decreased from Td to S4 during the crystallization of NH4H2PO4, which was indicated by the splitting of v4(NH4+) vibration bands at 1450 cm−1 into two bands respectively at 1400 and 1450 cm−1.21,22 In the KH2PO4−NH4H2PO4 aqueous crystallization system with M(K+)/M(NH4+) = 4:6, the symmetry of NH4+ group becomes S4 when the (K,NH4)H2PO4 molecule completely transforms into crystalline state. While this symmetric transformation is 16046

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

Figure 4. IR absorbance spectra during the crystallization process in KH2PO4−NH4H2PO4 aqueous solutions with M(K+)/M(NH4+) = 6:4 (a) and 4:6 (b). The concentration of H2PO4− in KH2PO4−NH4H2PO4 aqueous solution crystallization system is 2 mol/L. KH2PO4 acts as the host lattice when (K,NH4)H2PO4 is crystallized from the aqueous solution with M(K+)/M(NH4+) = 6:4, while NH4H2PO4 acts as the host lattice when (K,NH4)H2PO4 is crystallized from the aqueous solution with M(K+)/M(NH4+) = 4:6.

characterizes the symmetry of crystalline molecule, and the former spectra can provide structural information in the phase transformation during crystallization in KH2PO4−NH4H2PO4. Figure 6 summarizes these former IR spectra before crystalline (K,NH4)H2PO4 during the crystallization. Molecule configuration of (K,NH4)H2PO4 in the phase transformation can be identified by combining the symmetry of NH4+ and H2PO4− groups and the hydrogen bonding characteristics of N−H···O− P and P−O−H···O−P. Before transformation into crystalline state, KH2PO4 molecule that is formed in KH2PO4 aqueous solution possesses the configuration approaching to that in crystalline state (structure I in Figure 6). With slight addition of NH4+ into KH2PO4−NH4H2PO4 aqueous solution (M(K+)/ M(NH4+) = 9:1), NH4+ incorporates into (K,NH4)H2PO4 molecule, leading to the decreased symmetry of (K,NH4)H2PO4 molecule before crystalline state (structure II in Figure 6). With further decreasing M(K+)/M(NH4+) in aqueous solution (8:2, 7:3, and 6:4), the configuration of H2PO4− framework in (K,NH4)H2PO4 molecule is departure from crystalline (structure III). In the above conditions, (K,NH4)H2PO4 molecule contains less NH4+ than K+. When KH2PO4− NH4H2PO4 aqueous solution contains nearly equal K+ and NH4+ (i.e., 5:5 and 4:6), the molecular configuration of (K,NH4)H2PO4 (structure IV in Figure 6) is more close to that at the early nucleation stage. It can be attributed to the competition between K+ and NH4+ in crystallization system that inhibit the transformation of (K,NH4)H2PO4 into crystalline state. For KH2PO4−NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 3:7 and 2:8, the configuration of H2PO4− framework in (K,NH4)H2PO4 molecule becomes close to that in crystalline state (structure V). However, when M(K+)/M(NH4+) in KH2PO4−NH4H2PO4 aqueous solution approaches to 0 (1:9 and 0), the molecule configuration of H2PO4− in (K,NH4)H2PO4 molecules is close to that at the nucleation stage (structure VI). M(K+)/M(NH4+) in KH2PO4− NH4H2PO4 aqueous solution determines the superstation in crystallization system as well as the competing incorporation of

H2PO4, the appearance and increased intensity of P−O−H··· O−P vibration bands respectively at 1238 and 2384 cm−1 demonstrate the aggregation of H2PO4− groups, forming (H2PO4−)n prenucleation clusters/frameworks.22 Thereafter, K+ and NH4+ incorporate into the (H2PO4−)n framework with C2v symmetry, which can be monitored by the variations of v4(NH4+). Specially, in KH2PO4−NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 1:0 (KH2PO4 aqueous solution) or 0:1 (NH4H2PO4 aqueous solution), KH2PO4 and NH4H2PO4 molecules with C2v symmetry can form at the nucleation stage (Figure 5a and k). However, with the K+/NH4+ mole ratios approaching 5:5 in KH2PO4−NH4H2PO4 aqueous solution, the competed incorporation of K+ and NH4+ into (H2PO4−)n framework (C2v) leads to the decreased symmetry of the (K,NH4)H2PO4 molecule. When M(K+)/M(NH4+) equals to 1:0 and 9:1, KH2PO4 molecules are formed at the nucleation stage (structure I in Figure 5), while NH4H2PO4 molecules are formed at the nucleation stage when M(K+)/M(NH4+) equals to 1:9 and 0:1 (structure V in Figure 5). When M(K+)/ M(NH4+) equals 8:2, 7:3, and 6:4, (K,NH4)H2PO4 molecules that are formed at the nucleation stage contains more K+ than NH4+ (structure II in Figure 5). Likewise, when M(K+)/ M(NH 4 + ) equals to 6:4, 3:7, and 2:8, (K,NH4 )H 2 PO 4 molecules that are formed at the nucleation stage contains more NH4+ than K+ (structure IV in Figure 5). Specially, once M(K+)/M(NH4+) equals to 5:5, (K,NH4)H2PO4 with nearly equal K+ and NH4+ is formed at the nucleation stage (structure III in Figure 5). It can be deduced that K+ can be gradually substituted by NH4+ in (K,NH4)H2PO4 molecule at the nucleation stage with decreasing M(K + )/M(NH 4 + ) in KH2PO4−NH4H2PO4 aqueous crystallization system. During the crystallization of (K,NH4)H2PO4 from KH2PO4− NH4H2PO4 aqueous solution, (K,NH4)H2PO4 molecule can transform from solution phase to crystalline phase. Among time-dependent IR spectra, before and after the formation of crystalline (K,NH4)H2PO4 can be recorded by two successive spectra with the interval of 40 s (Figure 4). The later spectrum 16047

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

Figure 6. IR spectra of KH2PO4−NH4H2PO4 aqueous crystallization system before (K,NH4)H2PO4 transforms into crystalline state. The molecule motion of (K,NH4)H2PO4 occurs in KH2PO4−NH4H2PO4 crystallization system with M(K+)/M(NH4+) = 1:0 (a), 9:1 (b), 8:2 (c), 7:3 (d), 6:4 (e), 5:5 (f), 4:6 (g), 3:7 (h), 2:8 (i), 1:9 (j), and 0:1 (k), respectively. After 40 s, the (K,NH4)H2PO4 molecule will transform into crystalline state completely. Six typical molecular configurations for (K,NH4)H2PO4 (I−VI) before crystalline state have been identified by vibration bands in IR spectra.

Figure 5. IR absorbance spectra of KH2PO4−NH4H2PO4 crystallization system in the nucleation stage of (K,NH4)H2PO4. The nucleation of (K,NH 4 )H 2 PO4 occurs in KH 2 PO4 −NH 4H 2 PO4 crystallization system with M(K+)/M(NH4+) = 1:0 (a), 9:1 (b), 8:2 (c), 7:3 (d), 6:4 (e), 5:5 (f), 4:6 (g), 3:7 (h), 2:8 (i), 1:9 (j), and 0:1 (k), respectively. Five M(K+)/M(NH4+)-dependent structures of (K,NH4)H2PO4 (I−V) have been identified by vibration bands in IR spectra.

K+ and NH4+ into the lattice. With M(K+)/M(NH4+) approaching to 1:1, decreased supersaturation and strongest competitions between K+ and NH4+ result in the instantaneous phase transformation of (K,NH4)H2PO4 during crystallization process. Moreover, further decreasing M(K+)/M(NH4+) down to 0, lower supersaturation can likewise lead to the instantaneous transformation of NH4H2PO4 into crystalline state. It can produce the remarkable difference in vibration spectra before and after the formation of crystalline (K,NH4)H2PO4 that is grown in aqueous solution with M(K+)/ M(NH4+) approaching to 1 and 0. IR and Raman spectroscopy indicates the identical H2PO4− framework in crystalline-state (K,NH4)H2PO4 molecules, since all absorption bands belonging to H2PO4− appear at the relatively fixed positions independent of the M(K+)/M(NH4+). As shown in Figure 7, v3(B2+E) symmetric stretching mode at 1090 cm−1, and v1(A1) symmetric stretching mode at 875 cm−1 demonstrate H2PO4− group with S4 symmetry. Both H2PO4− symmetry and hydrogen bonding between H2PO4− groups direct the D2d (H2PO4−)n framework in crystalline (K,NH4)H2PO4, in agreement with our previous XRD studies.20 However, there are obvious differences for the vibration bands that are assigned to NH4+ group. With increasing M(K + )/M(NH 4 + ), the asymmetrical stretching mode (v3(NH4+)) at 3100 cm−1 and bending vibration mode

(v4(NH4+)) at 1450 cm−1 appear first, and the overtones and combination modes of NH4+ group occur subsequently (Figure 7a). Once M(K+)/M(NH4+) ≤ 1, v4(NH4+) splits into two bands respectively at 1450 and 1400 cm−1 in IR spectra of crystalline (K,NH4)H2PO4. We can understand these variances of spectra under different ammonium concentrations by analyzing the microscopic surroundings of ammonium ions. For crystalline (K,NH4)H2PO4 grown in KH2PO4−NH4H2PO4 aqueous solution with higher M(K+)/M(NH4+), NH4+ is isolated by K+ and H2PO4−. NH4+ group interacts with surrounding H2PO4− groups by N−H···O−P hydrogen bonding, where H2PO4− framework possesses D2d symmetry, resulting in NH4+ with Td symmetry. With the decreasing M(K+)/M(NH4+), the molecule structure of (K,NH4)H2PO4 approaches to that of crystalline NH4H2PO4, lead to the decreased symmetry of NH4+ (S4). Figure 7b shows Raman spectra of (K,NH4)H2PO4 crystallized in KH2PO4−NH4H2PO4 aqueous crystallization system with different K+/NH4+ mole ratios. All spectra display one band at approximately 915 cm−1 associated with the total symmetric breathing vibration of PO4. The cationic substitution of K+ by NH4+ can be evident by displacement of the peaks denoted as I1 and I2, respectively. The relative shift position of the peaks I1 and I2 is consistent with earlier reports by previous Raman study.27 16048

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

Figure 7. IR and Raman spectra of crystalline (K,NH4)H2PO4 at 18 °C. (a1−a11) IR spectra of (K,NH4)H2PO4 crystals crystallized from KH2PO4− NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 1:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, and 0:1, respectively. (b1−b11) Raman spectra of (K,NH4)H2PO4 crystals crystallized from KH2PO4−NH4H2PO4 aqueous solution with M(K+)/M(NH4+) = 1:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, and 0:1, respectively. The arrows indicate two selected peaks I1 and I2 of the NH4H2PO4 spectrum for analysis of K+/NH4+ substitution in (K,NH4)H2PO4.

■

■

CONCLUSION

In summary, molecular vibration spectroscopy was used to record in situ the molecule motion of (K,NH4)H2PO4 in KH2PO4−NH4H2PO4 aqueous system, paving a promising approach to image the molecule configuration at the early formation stage of the crystallization process. The effect of NH4+/K+ mole ratio in KH2PO4−NH4H2PO4 aqueous solution on the (K,NH4)H2PO4 molecule motion from ionic state to crystalline state were studied at different stages during the crystallization process. K+/NH4+ mole ratio in KH2PO4− NH4H2PO4 aqueous solution determines the supersaturation in the crystallization system as well as the competing incorporation of K+ and NH4+ into the lattice. Our present spectroscopy results show that decreased supersaturation and enhanced competitions between cations result in the instantaneous phase transformation of (K,NH4)H2PO4 during the crystallization process, which can decrease the crystal quality. Such a problem especially exists in the growth of (K,NH4)H2PO4 with M(K+)/M(NH4+) mole ratio approaching 1:1. In order to crystallize better crystals, we can increase the concentration of NH4H2PO4 in the KH2PO4−NH4H2PO4 mixed solution. Moreover, exquisite controlling the temperature during the crystallization can lead to the crystal growth with a relative low growth rate. Both of these operations will facilitate the growth of high-quality (K,NH4)H2PO4 crystals with a particular K+/NH4+ mole ratio.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86-431-85262294. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant No. 51125009), National Natural Science Foundation for Creative Research Group (Grant Nos. 20921002 and 21221061), and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged.

■

REFERENCES

(1) Foster, J. A.; Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Howard, J. A. K.; Steed, J. W. Anion-switchable supramolecular gels for controlling pharmaceutical crystal growth. Nat. Chem. 2010, 2, 1037− 1043. (2) Gardner, G. P.; Go, Y. B.; Robinson, D. M.; Smith, P. F.; Hadermann, J.; Abakumov, A.; Greenblatt, M.; Dismukes, C. Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water. Angew. Chem., Int. Ed. 2012, 51, 1616−1619. (3) Liu, F.; Song, S.; Xue, D.; Zhang, H. Folded Structured Graphene Paper for High Performance Electrode Materials. Adv. Mater. 2012, 24, 1089−1094. (4) Milanov, A. P.; Xu, K.; Laha, A.; Bugiel, E.; Ranjith, R.; Schwendt, D.; Osten, H. J.; Parala, H.; Fischer, R. A.; Devi, A. Growth of Crystalline Gd2O3 Thin Films with a High-Quality Interface on Si(100) by Low-Temperature H2O-Assisted Atomic Layer Deposition. J. Am. Chem. Soc. 2010, 132, 36−37. (5) Palmer, B. A.; Harris, K. D. M.; Guillaume, F. A Strategy for Retrospectively Mapping the Growth History of a Crystal. Angew. Chem., Int. Ed. 2010, 49, 5096−5100. (6) Ward, M. D. Snapshots of crystal growth. Science 2005, 308, 1566−1567.

ASSOCIATED CONTENT

S Supporting Information *

Additional IR spectra during the crystallization process. This material is available free of charge via the Internet at http:// pubs.acs.org. 16049

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050

The Journal of Physical Chemistry C

Article

(7) Pouget, E. M.; Bomans, P. H. H.; Goos, J. A. C. M.; Frederik, P. M.; de With, G.; Sommerdijk, N. A. J. M. The Initial Stages of Template-Controlled CaCO3 Formation Revealed by Cryo-TEM. Science 2009, 323, 1455−1458. (8) Baumgartner, J.; Dey, A.; Bomans, P. H. H.; Coadou, C. L.; Fratzl, P.; Sommerdijk, N. A. J. M.; Faivre, D. Nucleation and growth of magnetite from solution. Nat. Mater. 2013, 12, 310−314. (9) Gebauer, D.; Volkel, A.; Cölfen, H. Stable Prenucleation Calcium Carbonate Clusters. Science 2008, 322, 1819−1822. (10) Van Driessche, A. E. S.; Benning, L. G.; Rodriguez-Blanco, J. D.; Ossorio, M.; Bots, P.; Garia-Ruiz, J. M. The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation. Science 2012, 336, 69−72. (11) Brouwer, D. H.; Cadars, S.; Eckert, J.; Liu, Z.; Terasaki, O.; Chmelka, B. F. A General Protocol for Determining the Structures of Molecularly Ordered but Noncrystalline Silicate Frameworks. J. Am. Chem. Soc. 2013, 135, 5641−5655. (12) Baerlocher, C.; Xie, D.; McCusker, L. B.; Hwang, S. J.; Chan, I. Y.; Ong, K.; Burton, A. W.; Zones, S. I. Ordered silicon vacancies in the framework structure of the zeolite catalyst SSZ-74. Nat. Mater. 2008, 7, 631−635. (13) Tang, E.; Tommaso, D. D.; Deleeuw, N. H. Hydrogen transfer and micro-hydration properties of HnPO43−n (n = 0−3) in water studied by first principles molecular dynamics simulations. J. Chem. Phys. 2009, 130, 234502. (14) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Structures of neutral Au7, Au19, and Au20 clusters in the gas phase. Science 2008, 321, 674−676. (15) Pienack, N.; Bensch, W. In-Situ Monitoring of the Formation of Crystalline Solids. Angew. Chem., Int. Ed. 2011, 50, 2014−2034. (16) Ren, X.; Xu, D.; Xue, D. Crystal growth of KDP, ADP, and KADP. J. Cryst. Growth 2008, 310, 2005−2009. (17) Srinivasan, K.; Cantoni, A.; Bocelli, G. Compositional dependence of morphology and lattice parameters during growth of K1−x(NH4)xH2PO4 mixed crystals. Cryst. Res. Technol. 2010, 45, 737− 746. (18) Kwon, O. J.; Kim, J.-J. Proton-glass behavior and phase diagram of the K1−x(NH4)xH2PO4 system. Phys. Rev. B 1993, 48, 6639−6642. (19) Schmidt, V. H.; Lanceros-Mendez, S.; Meschia, S. C.; Pinto, N. J. Anomalous and normal protonic conductivity in Cs1−x(NH4)H2PO4, Cs1−x(ND4)D2PO4, and K1−x(NH4)H2PO4. Solid State Ionics 1999, 125, 147−157. (20) Xu, D.; Xue, D. Chemical bond simulation of KADP singlecrystal growth. J. Cryst. Growth 2008, 310, 1385−1390. (21) Sun, C.; Xue, D. In situ ATR-IR observation of nucleation and crystal growth of KH2PO4 in aqueous solution. CrystEngComm 2013, 15, 10445−10450. (22) Sun, C.; Xue, D. In Situ IR Spectral Observation of NH4H2PO4 Crystallization: Structural Identification of Nucleation and Crystal Growth. J. Phys. Chem. C 2013, 117, 19146−19153. (23) Sun, C.; Xu, D.; Xue, D. Direct in situ ATR-IR spectroscopy of structural dynamics of NH4H2PO4 in aqueous solution. CrystEngComm 2013, 15, 7783−7791. (24) Carten, R. L. Molecular Symmetry and Group Theory; John Wiley & Sons, Inc.: New York, 2004; pp 174−200. (25) Preston, C. M.; Adams, W. A. Laser Raman-Spectroscopic Study of Aqueous Orthophate Salts. J. Phys. Chem. 1979, 83, 814−821. (26) Sun, C.; Xue, D. Hydrogen bonding nature during ADP crystallization. J. Mol. Struct. 2014, 1059, 338−342. (27) Jurado, J. F.; Vargas-Hernandez, C.; Vargas, R. A. Raman and structural studies on the high-temperature regime of the KH2PO4NH4H2PO4 system. Rev. Mex. Fis. 2012, 58, 411−416.

16050

dx.doi.org/10.1021/jp504830u | J. Phys. Chem. C 2014, 118, 16043−16050