Growth and spectroscopy studies of ADP single crystals with l -proline and l -arginine amino acids

Materials Chemistry and Physics 130 (2011) 24–32

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Growth and spectroscopy studies of ADP single crystals with l-proline and l-arginine amino acids F. Ben Brahim a,∗ , A. Bulou b a b

Laboratoire des Sciences des Matériaux et de l’Environnement (MES-Lab), Faculté des Sciences de Sfax, Route de Soukra km 3.5, BP 1171, 3000 Sfax, Tunisia Laboratoire de Physique de l’Etat Condensé (LPEC), UMR CNRS no 6087, Faculté des Sciences et Techniques, Université du Maine, 72085 Le Mans Cedex 09, France

a r t i c l e

i n f o

Article history: Received 26 July 2010 Received in revised form 13 February 2011 Accepted 23 February 2011 Keywords: D. Crystal symmetry ADP crystals Slow evaporation IR Raman and NMR spectroscopy

a b s t r a c t Ammonium dihydrogen orthophosphate (ADP) single crystals were grown with l-proline and l-arginine amino acids as additives at 4% molar concentrations. X-ray diffractograms, vibrational FT-IR, and Raman and NMR MAS of 31 P and 1 H spectra of pure and treated compounds were recorded at room temperature and interpreted. A shift of the wavenumbers of some internal modes of both PO4 and NH4 tetrahedrons species was observed. At the same time, some characteristic vibrational modes of N–H and C–H bonds of the organic molecules appeared in IR spectra. However, NMR results show a little shift of the resonance of the phosphorus and hydrogen nucleus, associated with some shoulders for this latter. These observations indicate that the host-additives interactions were achieved. © 2011 Published by Elsevier B.V.

1. Introduction The isomorphous ammonium dihydrogen orthophosphate (ADP) and potassium dihydrogen orthophosphate (KDP) are the oldest crystals grown in large size for various applications [1–3]. However, ADP crystal is widely used due to its piezo-electric property [4]. It is also used as the second, third and forth harmonic generator for Nd:YAG and Nd:YLF lasers and for electro-optical applications such as Q-switches for Nd:YAG, Nd:YLF, Ti:Sapphire and Alexandrite lasers, as well as well as for acousto-optical applications. Studies on ADP crystals are gaining more interest because of their unique nonlinear optical, dielectric and antiferroelectric properties [5,6]. Over the last few years, water-soluble crystals containing complex organic molecules have raised considerable interest [7]. These crystals can be successfully used for simulation of crystal growth mechanism [8,9] and investigation of phase transition in the crystals [10]. Several other studies have investigated the crystal habit modification by metallic cations [11]. It was observed that these latter influence the crystal growth rate, leading to different geometrical shapes of crystals [12–14]. In the case of KDP, the selective adsorption of some metallic ions on the specific growth surfaces like the prismatic section (1 0 0) or pyramidal section (1 0 1), suppress the growth of those faces [15,16]. It was reported by Guzman et al. [17], that when impurity segregates into the crystals, the distribu-

∗ Corresponding author. Tel.: +216 74 27 43 90; fax: +216 74 27 44 37. E-mail address: [email protected] (F. Ben Brahim). 0254-0584/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2011.02.070

tion of impurity between aqueous solution and crystal occurs. It was found that Pb2+ , Al3+ and Fe3+ ions were concentrated in KDP crystals and their concentrations are different in the prismatic and pyramidal sections of KDP crystals. Besides, some theoretical studies have tried to explain the impurity effect on the crystal growth [18–20]. The growth mechanism is usually understood with respect to the kinetic parameters such as the growth rate of crystal, step velocity, and the stability of crystal surfaces [21–23]. However, these parameters are most of the time obtained under specific growth conditions without considering the significant influence of the intrinsic characteristics (crystal structure and chemical bond) of crystal, and their applications are consequently limited [24]. Microscopically, chemical bonding theory of single crystal growth [15,25] shows that the crystal growth includes crystal morphology, crystal defects, and growth rate, which are all related to the constituent growth units and their chemical bonding process [26–29]. Therefore, it is significant to understand the crystal growth from the viewpoint of the intrinsic characteristics of crystal. Previous studies show that hydrogen bonds play a dramatic role in governing the property and growth of KDP and ADP single crystals [13,30–33]. Due to quite a number of hydrogen bonds in ADP, the present work proposes to observe their microscopic characteristics and particularly the various spectroscopic behaviors, using the NMR and the FT-IR and Raman methods. Improvement in the quality of the KDP and ADP crystals and the performance of their based devices can be realized with suitable dopants. To analyze the influence of the dopants on the non-linear optical properties of such materials, several studies have been carried out [7,34–48].

25

(323)

(303)

(204)

(312) (301)

(202)

Intensity (a. u.)

(200)

(112)

(101)

F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

(c)

(b)

Fig. 1. As grown ADP crystals: from the pure mother liquor (a), from the mother liquor with 4 mol% l-proline (b), from the mother liquor with 4 mol% l-arginine (c).

(a) 10

20

30

40

50

60

Diffraction angle, 2 teta (degree) This paper deals with the synthesis and spectroscopic study of l-proline and l-arginine amino acids grafted on ADP crystals.

Fig. 2. Powder X-ray diffraction patterns of ADP crystallites. Pure ADP (a), grown with 4 mol% l-proline (b), grown with 4 mol% l-arginine (c).

2. Experimental 2.1. Crystal growth Pure ADP crystals were grown from aqueous solution of ammonium monohydrogen orthophosphate and phosphoric acid (85 wt%) by slow evaporation. The same method was followed for grafted amino acids on ADP crystals (4 mol% l-proline or l-arginine). The mother solution was saturated at the initial pH values around 4, and then amino acid solution, l-proline or l-arginine, was added and stirred. The growth was carried out for few days at room temperature. The grown crystals are shown in Fig. 1. In the figure, (a) is pure ADP, (b) is l-proline grafted on ADP and (c) is l-arginine grafted on ADP. 2.2. Characterization An X-ray diffractometer (X’PertPro PANalytical) was used to characterize the ˚ The patterns are compared as-grown crystals with Cu K␣ radiation ( = 0.15406 A). with the standard diffraction pattern (DIFFRAC plus 2009 Evaluation) in order to identify the crystal phase and the crystal structure. The vibrational measurements were carried out at room temperature. Fourier transform infrared spectra were obtained, with random orientations, from potassium bromide pellets on a Perkin-Elmer FT-IR Spectrum BX spectrometer in the region 4000–400 cm−1 . Spectral resolution is better than 4 cm−1 . Raman spectra were registered on oriented single crystal from 100 to 3800 cm−1 using a T64000 Jobin-Yvon multi-channel spectrometer (in triple subtractive configuration, 1800 tr. mm−1 grating) with cooled CCD as a detector. An argon–krypton laser (coherent spectrum) of about 20 mW power (on the sample) was used for the excitation (514.5 nm). All measurements were carried out at room temperature under an X50LF objective microscope (in the backscattering geometry). The spectral steps typically are 0.7 cm−1 . All NMR experiments were carried out on a Brucker 300WB spectrometer, equipped with chemagnetic triple-resonance 5 mm probe, with resonance frequencies of 121.49 and 300.13 MHz for 31 P and 1 H, respectively. The /2 pulse lengths for 31 P and 1 H were typically 5 and 7 ␮s and the recycle delays were 60 and 1 s, respectively. The number of scans acquired for the 31 P spectra was 64, while that for 1 H ones was 1024 and the MAS spinning speed was 8 kHz. The chemical shifts were referenced, at 0 ppm, to 85 wt% H3 PO4 solution for 31 P and tetra methyl silane (TMS) for 1 H.

3. Results and discussion 3.1. X-ray diffraction studies Powder X-ray diffraction (XRD) is useful to determine the crystallinity and the purity. Grown crystals were ground in an agate mortar and pestle in order to determine the crystal phases by X-ray diffraction. Fig. 2 shows X-ray powder diffraction patterns of pure ADP and those of the materials grown with amino-acid solutions at 4 mol% of l-proline and l-arginine. As it is seen in this figure, no additional peaks are present in the XRD spectra of crystals obtained in presence of additives, showing the absence of any additional phase due to the amino-acids. Thus, the structure of ADP is not distorted when 4 mol% l-proline or l-arginine

is added with ADP, showing that the additives had not entered into the lattice sites of ADP. Similar results were observed by Dhanaraj et al. [42] and Rajesh et al. [36]. The former authors have used the l-arginine monohydrochloride (lAHCl) and l-alanine as additives in 5% molar concentrations. Whereas, the latter authors have performed their materials growth in the presence of l-lysine monohydrochloride dihydrate (l-LMHCl dihydrate) at 2 mol%. Rajesh et al. [36] have observed only a slight variation in the intensity and noted that there was no shift in the angle (2). For the pure ADP from the single crystal XRD, they found the following unit cell ˚ c = 7.564 A, ˚ ˛ = ˇ =  = 90◦ , belonging to parameters a = b = 7.510 A, tetragonal system, and for crystals grown with the additive, the ˚ c = 7.558 A. ˚ However, Dhaunit cell parameters are a = b = 7.504 A, naraj et al. [42] have remarked, that both single and powder crystal XRD studies show that their dopant has not entered into the lattice sites of ADP. They also have noted that their observed values are in good agreement with those reported by Xu and Xue [26,49]. Using the commercially available ADP as a started material, these two groups of authors have experimentally observed the (1 0 1), (2 0 0), (1 1 2), (2 0 2), (3 0 1) and (3 1 2) prominent peaks of pure and doped ADP crystals. However, using the ammonium monohydrogen orthophosphate (AMP) as a starting material in this work, we have obtained materials with improved crystallinity, more pronounced resolution and intensity of peaks. They are the (1 0 1), (2 0 0), (1 1 2), (2 0 2), (3 0 1), (3 1 2), (3 0 3), (2 0 4) and (3 2 3) reticular plants. 3.2. Group theoretical analysis 3.2.1. Factor group analysis The tetragonal crystal ADP belongs to point group D2d , form¯ ing the space group I 42d at room temperature. The cell dimensions are a = b = 7.473(1) Å and c = 7.542(1) Å and the primitive cell contains four formular units [50]. Table 1, reported in [51], gives the decomposition of the zone-center lattice modes into irreducible representations in the paraelectric phase. This leads to determine the symmetries of the vibrations and to predict the IR and Raman active modes. 61 normal modes are predicted, excluding the acoustic ones. These are distributed as follows: optic = 7A1 (R) + 9B1 (R) + 9B2 (R, IR) + 18E(R, IR) The B2 and E modes are Raman (R) and infrared (IR) active optical phonon modes, while A1 and B1 are only Raman active. It is clear that the theoretically observable Raman peaks and infrared reflection bands do not exceeded 61 and 27, respectively.

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F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

Table 1 ¯ space group). g denotes the degeneracy of the representation, Reduction into irreducible representations of the 72 zone-center modes of ADP in the paraelectric phase (I 42d Ntot the total number of modes, NAc , NT and NH the number of acoustical, translational external, and proton modes, respectively. NL and Nint are the number of libration and internal modes for both tetrahedra species, PO4 and NH4 [51].

A1 A2 B1 B2 E

g

Ntot

NAc

NT

NL (PO4 )

NL (NH4 )

NH

Nint (PO4 )

Nint (NH4 )

Basis functions. Raman and IR activities

1 1 1 1 2

7 8 9 10 19

0 0 0 1 1

0 0 2 1 3

1 1 0 0 2

1 1 0 0 2

1 2 1 2 3

11 + 12 11 + 12 12 + 13 + 14 12 + 13 + 14 23 + 24

11 + 12 11 + 12 12 + 13 + 14 12 + 13 + 14 23 + 24

xx + yy, zz – xx − yy xy, Pz yz, Px xz, Py

The selection rules for the Raman-scattering tensor of the D2d point group are [52,53]:

 A1 =

a 0 0 0 a 0 0 0 b



 ,

B1 =

a 0 0 a 0 0

0 0 b



 B2 (z) =

,

0 d d 0 0 0

0 0 0



are represented as follows: vib = A1 (R) + E(R) + 2F2 (R, IR)

 ,

E(x) =

The symmetry species B2 (z), E(x) and E(y) show dipole moments oriented along the z-, x- and y-directions, respectively. The significant scattering configurations are x(zz)¯x, x(yz)¯x, z(yx)¯z and x(yy)¯x, corresponding to the symmetry species A1 , E, B2 and (A1 + B1 ), respectively, due to the equivalence of the axes x and y for the measurement of the Raman spectra of uniaxial crystals with the point group D2d [54]. 3.2.2. Vibrations of the PO4 and NH4 groups The normal vibrational modes of a free PO4 3− and NH4 + under perfect Td symmetry are expected to be around 936, 420, 1004 and 573 cm−1 [55] for PO4 3− , and 3033 (3061), 1685 (1661), 3134 (3131) and 1397 (1416) cm−1 for NH4 + [56]. In the case of the NH4 + ion, frequencies values in parenthesis were reported by Jordanov and Zellner [57], as observed by means of optical levitation and Raman spectroscopy. They correspond to 1 (A1 ), 2 (E), 3 (F2 ) and 4 (F2 ) modes, respectively. The 1 and 3 are stretching vibration modes, and 2 and 4 are bending vibration modes. The some modes are Raman active, whereas only 3 (asymmetrical stretching) and 4 (out-of-plane bending) are infrared active. The 3 and 4 modes are triply degenerate, 2 is doubly degenerate, and 1 is non-degenerate. According to the group theoretical,these modes

0 0 0 0 0 e

0 e 0

ν1

Site group S4 symmetry

A1

E

F2

E ν4

• 3 stretching modes: 33 (B2 , 2E) • 4 deformation modes: 2 (B2 ); 34 (B2 , 2E) 3.2.3. Analysis of IR spectra Infrared spectra of the pure ADP and of the grafted on l-proline and l-arginine amino acids are given in Fig. 3. The corresponding assignments and their symmetry species are given in Table 3. The interaction effect between the ADP host and the grafted amino acid

Factor group D2d symmetry A1

(ν1+ν2)

(R)

A2

(ν1+ν2)

(I)

B1

(ν2+ν3+ν4)

(R)

B2

(ν2+ν3+ν4) (R, IR)

E

(ν3+ν4)

F2

R: Raman active, IR: infrared active, and I: inactive.



Infrared

B ν3

and E(y) =

0 0 e 0 0 0 e 0 0

• 5 stretching modes: 1 (A1 ); 43 (B1 , B2 , 2E) • 7 deformation modes: 32 (A1 , B1 , B2 ); 44 (B1 , B2 , 2E)

A ν2



The symmetry of both PO4 3− and NH4 + ions is reduced from Td to S4 in the ADP crystal. Table 2 shows the correlation diagram between the free PO4 3− and NH4 + groups’ vibrations in Td symmetry, and their internal vibrations in D2d factor group symmetry, through the S4 site group in the crystal. The correlation shows that 12 Raman and 7 IR lines are predicted as follows: Raman

Table 2 Correlation scheme for the internal vibrations of both the PO4 and the NH4 groups [58].

Free ion Td symmetry



(R, IR)

F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

40

Transmittance (%)

(c) 30

(b)

20

(a)

10

0 500

1000

1500

2000

2500

3000

3500

4000

4500

-1

Wavenumber (cm ) Fig. 3. IR-spectra of ADP crystals: pure ADP (a); ADP crystal grown from the solution with 4 mol% l-proline (b); ADP crystal grown from the solution with 4 mol% l-arginine (c).

materials can be observed. Certain IR-active bands of the ion fundamental vibrations, derived from the 2 , 3 or 4 vibrations of the free ions, have shifted or splitted. Factor group analysis predicts fourteen IR active modes for both PO4 3− and NH4 + group ion (B2 + E symmetries). Concerning the PO4 3− group, the line observed at 454 cm−1 is assigned to the symmetric bending mode 2 (B2 symmetry). The result is in accordance with the prediction of one band for this mode by factor group analysis. However, for the asymmetric bending mode 4 , only one band is observed at 547 cm−1 , while three are predicted (B2 + 2E). These two bending vibrations 2 and 4 modes are easily assigned as they do not overlap with other vibrations. Our previous work [59], dealing with a synthesized doped compound deriving from the KDP isotype, indicated that there was a paradox. According to factor group analysis (Tables 1 and 2), the symmetric stretching mode 1 (PO4 ) is IR inactive. Several observed spectra showing this vibrational mode [43,46,59,60], deal with Table 3 Wavenumber (in cm−1 ) of absorption peaks in FT-IR spectra and their assignments for pure ADP and l-proline and l-arginine grafted on crystals. Pure ADP

ADP-Pro

ADP-Arg

Symmetry species

Assignment

454s 547s 913s 1107s 1295s 1402s 1460sh ≈1640b

454s 547s 913s 1100s 1295s 1402s 1452m ≈1640b

454s 547s 920s 1100s 1287s 1402s 1452m ≈1640b

B2 B2 + E A1 B2 + E B2 + E B2 + E E B2

2337w 2366m 2488sh – 3006sh 3128b – 3487sh

2337w 2366m 2473sh 2862sh 3013sh 3113sh 3258 –

2337w 2366m 2466sh 2869sh 3020sh 3121sh 3250 –

2 (PO4 ) 4 (PO4 ) 1 (P−OH)a 3 (P O);  O–H ˇO–H b 4 (NH4 ) 4 (NH4 ); ı(CHx ); s (COO− ) 2 (NH4 ) ı(N+ –H· · ·O) as (COO− ) O–H c O–H O–H s ,as (CHx ) (N+ –H· · ·O) 3 (NH4 ) 3 (NH4 ) 3 (NH4 ) (O–H)Free

B2 E E

Abbreviations: : stretching; ı: bending; ˇ: in-plane bending; : out-of plane bending; s: symmetric; as: asymmetric; s: strong; m: medium; w: weak; b: broad; sh: shoulder. a Not predicted by the factor group theory. b Observed by Liu et al. [63] at 1300 cm−1 . c Observed by Liu et al. [63] at 2428 and 2745 cm−1 .

27

paradox studies including the present work. Our recent structural study explains this paradox [59]. The structure is locally constituted by H2 PO4 entities in which these four oxygen atoms are not equivalent. So, the S4 sites symmetry correspond to average positions of the PO4 tetrahedra in the paraelectric phase. This leads to a supplementary reduction in the symmetry of the anion. Thus, the non polar and IR inactive 1 mode (A1 symmetry) appears at 913 cm−1 for the pure ADP. It is slightly shifted up in the case of the compound grown with l-arginine amino acid. 1 mode corresponds to the 1 (P–OH) and 1 (P O) symmetric stretching modes. As reported in Tominaga’s [61] spectroscopic study on KDP which showed that the local and instantaneous site symmetry of PO4 tetrahedra above Tc is C2 . This symmetry is as the one below Tc. However, the S4 sites correspond to average positions of the PO4 tedrahedra in the paraelectric phase. Thus, the local symmetry of phosphate ion tetrahedra in KDP and isomorphous crystals is dynamically broken even at room temperature, as indicated by the appearance of lines forbidden by the relevant spectroscopic selection rules. This explains the appearance of the vibrational symmetric stretching mode 1 (PO4 ), which is non polar, in the IR spectra. In the same context, dealing with a Zn(NH4 )2 (SO4 )2 ·6H2 O crystal study, Madhurambal et al. [56] have noted that the presence of an IR inactive mode as a broad band in FT-IR is probably due to strain induced by the crystal fields. The strong band located at 1107 cm−1 , for the pure material, is attributed to the asymmetric stretching mode 3 of P O bond, while three are predicted, one of B2 symmetry and two of a degenerated E one. This band is down shift by 7 cm−1 for the two grafted on amino acids compounds. This shifting confirms the interaction between ADP and the organic material, which originates from the possibility of hydrogen bonding, primarily between the nucleophilic O− of the phosphate unit of ADP and the amino group of the amino acids. Therefore, this group is protoned in the acidic reactional media (pH ≈ 4). A relatively strong band appears at 1402 cm− 1, considering the NH4 + ion IR active modes, and another weak shoulder located at 1460 cm−1 for the pure material. These are assigned to the asymmetric bending mode 4 , while three lines are predicted (B2 + 2E). The band at 1460 shifts by 8 cm−1 to lower wavenumbers (1452 cm−1 ) and becomes more pronounced, for the materials grown in presence of the two amino acids, l-proline and l-arginine. This fact can be considered as a removal of degeneracy of the asymmetric bending mode 4 (B2 + 2E) of the cation. The removal of degeneracy together with the appreciable shift indicate that the angular distortion of NH4 + ion is greater than the linear distortion. Madhurambal et al. [56] reported a similar result, dealing with Zn(NH4 )2 (SO4 )2 ·6H2 O crystal study. The distortion in the NH4 tetrahedron, which also exists in pure ADP is enhanced, due to the interaction between ADP and the organic material through the ammonium ion. In addition, the modification observed in this frequency region is probably due to an interference phenomenon with the extra vibrational lines of CHx asymmetric bending and COO− symmetric stretching modes of the grafted organic molecules. The ADP host and the amino acid additives interactions are also clearly noticeable in the frequency region of the 2 (NH4 + ) symmetric bending band (B2 symmetry). This band appears as a broad one at around 1640 cm−1 for all the samples. It becomes broader in the high wavenumber sides for the materials grown in amino acid solutions, of l-proline and l-arginine. This result could be related to two phenomena; the first one is the hydrogen bonding interaction between the H2 PO4 − group of the host and the NH4 + one of the additives. The second is the overlapping of the internal mode 2 (NH4 + ) band of the host, with the extra vibrational lines of N+ –H· · ·O bending as well as COO− asymmetric stretching modes of the organic molecules. Besides, it was reported by Rajesh et al. [62], that the broadness of the IR vibration lines is probably due to the hydrogen bonding interaction. They have also noted that changes in the

F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

3.2.4. Raman scattering investigation The Raman tensors, reported in Table 1, show that the A1 , B2 and E modes can be observed alone in the zz, xy and xz geometries, respectively. While in the xx and yy polarization, both the A1 and B1 modes are expected. The polarized Raman spectra recorded are shown in Figs. 4–6, and the frequencies of the observed bands and their vibrational assignments are given in Table 4. The Raman spectra of the crystals under study were recorded in a wide frequency region (from 100 to 3800 cm−1 ) with a spectral resolution of ∼0.7 cm−1 . The spontaneous Raman spectra recorded in the x(zz)¯x polarization scheme (A1 symmetry) for ADP and grafted on l-proline and l-arginine crystals show four lines (Fig. 4). They correspond to the two internal modes, 2 and 1 , of both PO4 3− and NH4 + group ions. The frequencies’ values are 346, 929 and

100000

Intensity (A. U.)

80000

60000

40000

(c) (b)

20000

(a)

0 0

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 4. Spontaneous Raman spectra, of pure ADP (a) and those of grafted on l-proline (b) and l-arginine crystals (c), at the x(zz)¯x polarization geometry in a wide spectral range, for the totally symmetric vibration, 1 and 2 , of the phosphate group (PO4 ) and the ammonium group (NH4 ).

20000

15000

Intensity (A. U.)

hydrogen bonds lead to some variations in the stretching vibrations and in the peak positions. In the case of the pure material, two bands appear at 3006, as a shoulder, and a broad one at 3128 cm−1 . These are attributed to the 3 (NH4 + ) asymmetric stretching mode (B2 + E). The behavior of 3 band at 3128 cm−1 is similar to 4 (NH4 + ) when additives were used. It separates into two bands, 3113 and 3258 cm−1 for the grafted on l-proline material and 3121 and 3250 cm−1 for that obtained with l-arginine. This is also probably due to a symmetry reduction of NH4 + group. Factor group analysis also predicts three bands for the asymmetric stretching 3 mode (B2 + 2E) as in the case of the 4 mode. Fig. 3 reveals the disappearance of the (OH)free band, observed at 3487 cm−1 in the case of the pure material, when the ADP crystal was grown in presence of the two amino acids. Thus, we may conclude that the hydrogen bonding occurs between the nucleophilic O− of the phosphate anion of ADP and the amino group of the grafted molecules. We note that the amino group is already protoned in the acidic media at a pH value around 4 units. Moreover, a supplementary band appears as a shoulder at around 2860 cm−1 for the treated materials. This band is due to the C–H stretching of CH2 and CH which confirms that amino acids grafting was successfully achieved, although their content in the crystals was less than that in the solutions. Similar results were observed by Rajesh et al. [62] in their work dealing with the growth of ADP crystals with ammonium malate as an additive. Although Rajesh et al. [62] have concluded that there is distinct evidence for the presence of the additive in the lattice of ADP, we think that they have also dealt with a grafting phenomenon and the additive has not entered into the structure. In fact, the organic molecules are too big to permit their intercalations into crystallographic sites. In spite of this, it is clear that the interaction between ADP and the organic material is mostly ensured by hydrogen bonding. The mechanism of such interaction is not well defined. However, all the observed spectral changes are completely in accordance with previous works [37,42,36,62]. Different organic molecules were used in the growth media of ADP, such as amino acids [42,36], EDTA [37] and ammonium malate [62]. As a first conclusion, activation of infrared-inactive modes (1 PO4 3− ) and removal of degeneracies (4 NH4 + ) occur, because of the symmetry reduction from Td to S4 site group. Moreover, the frequency shift of some modes, together with the activation of infrared inactive modes, and the lifting of degeneracy may give the following information: the internal modes of the phosphate and ammonium groups are somehow modified in the crystal environment by the effect of the crystal field on these two anion groups. These phenomena are accentuated by the interaction with organic additives, such as amino acids. This indicates that both PO4 3− and NH4 + ions have a distorted structure in pure ADP, and more in both grafted on l-arginine and l-proline crystals.

10000

(c) (b)

5000

(a) 0 0

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 5. Spontaneous Raman spectra, of pure ADP (a) and those of grafted on l-proline (b) and l-arginine crystals (c), at the x(yy)¯x polarization geometry in a wide spectral range.

15000

Intensity (A. U.)

28

10000

(c) 5000

(b)

(a) 0 0

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 6. Spontaneous Raman spectra of pure ADP (a) and those of grafted on l-proline (b) and l-arginine crystals (c) at the x(yz)¯x polarization geometry in a wide spectral range.

F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

29

Table 4 Experimental wavenumbers (in cm−1 ) of scattering peaks in FT-Raman spectra and their assignments for pure ADP and l-proline and l-arginine grafted on crystals for various geometries. x(zz)¯x A1

x(yy)¯xA1 + B1

x(yz)¯x E

Assignment

ADP

ADP-Pro

ADP-Arg

ADP

ADP-Pro

ADP-Arg

ADP

ADP-Pro

ADP-Arg

– – – 346s – – – – 929vs – – – – – 1657s – – – – 3171s – – –

– – – 354s – – – – 935vs – – – – – 1665s – – – – 3178s – – –

130vw 182vw – 354s – – – – 935vs – – – – – 1665s – – – – 3178s – – –

130w 182m – 346m 405vw – 480m 570w 927vs 1016sh 1129 1307vw 1441w – 1657m 1739sh 2358sh 2649sh 2872sh 3115s – – 3416vw

– 182m – 339m – – 480m 555vw 927vs 1039sh – 1300vw 1434vw – 1657m 1739sh 2380sh 2671sh 2872sh 3133s – – 3424vw

– 182m – 339m – – 480m 555w 927vs 1024sh – 1300vw 1441w – 1657m 1725sh 2380sh 2679sh 2879sh 3133s – – 3431vw

130s 182s 286m – – – 480vw 577m 927w – 1129w 1314w 1434w – – – – – 2865sh – 3096sh 3245s –

130s 182s 286m – – – – 577m 927w – 1129w 1314w – 1472vw 1657vw – – – 2850sh – 3088sh 3230s –

130s 182s 286m – – 472w – 577m 927w – 1129w 1307vw 1434vw 1472vw 1657w – – – 2865sh – 3104sh 3237s –

T(NH4 ) L(PO4 ) L(NH4 ) 2 (PO4 ) 2 (PO4 ) 2 (PO4 ) 4 (PO4 ) 1 (PO4 ) 3 (PO4 ) 3 (PO4 ) ␤O−H a 4 (NH4 ) 4 (NH4 ) 2 (NH4 ) 2 (NH4 ) O−H O−H O−H 1 (NH4 ) 3 (NH4 ) 3 (NH4 ) 3 (NH4 )

Abbreviations: : stretching; ˇ: in-plane bending; vs: very strong; s: strong; m: medium; w: weak; vw: very weak; b: broad; sh: shoulder; T: translational; N: librational. a Observed by Liu et al. [63], in B1 and B2 symmetries at 1307 and at 1328 cm−1 in E one.

1657, 3171 cm−1 , respectively, for the pure material. These were observed, for pure ADP, by Gorelik et al. [53] at 342, 924 and at 1661.9 and 3158 cm−1 , respectively. Group theory analysis also predicts the same number of internal modes for this geometry (Table 1). It is worth noting that the wavenumber of these bands are systematically slightly shifted with respect to those in the “free” PO4 and NH4 groups. These are 420, 936, 1685 and 3033 cm−1 , respectively, indicating a distortion in the tetrahedrons groups related to the deviation from the Td symmetry. These latter bands shift to 354, 935, 1665 and 3178 cm−1 , respectively, as it can be seen in Table 4 for the grafted on amino acids l-proline and l-arginine ADP crystals. This also indicates an accentuation of the structural distortion by the grafting organic molecules, showing the effect of the crystal field on the phosphate and the ammonium groups, related to the crystal environment modification. Meanwhile, two low-wavenumber weak peaks appear at 130 and 182 cm−1 in the case of the l-arginine grafted on compound, belonging to the lattice vibration modes of ADP crystal. The former corresponds to the translational mode of NH4 + ion, observed at the same frequency by Madhurambal et al. [56]. However, the line at 182 cm−1 is assigned to the librational mode of PO4 3− ion, as observed by Kim and Sherman [64] at 174 cm−1 . Although the librational mode is predicted by factor group analysis, the translational one consists in a contamination by a line in another geometry. This contamination may be due to an increase in the structural disorder. Thus, six of the seven A1 species Raman-active optical modes have been recorded. Fig. 5 shows the Raman spectra at room temperature, with the scattering geometry x(yy)¯x. The frequency assignments of the lattice vibration modes of the pure ADP and those of the grafted on amino-acids, are also given in Table 4. Sixteen Raman components (7A1 + 9B1 ) are predicted in this polarization geometry. Almost all these bands are observed for the non polar symmetry species A1 and B1 . In the case of the former, the corresponding lines for the pure ADP material are located at 182, 346, 927, 1657 and 3115 cm−1 . However, the B1 species bands are observed at 130,480, 570, 1016, 1307, 1441, 1739, 2358, 2649, 2872 and 3416 cm−1 .

The two strongest lines at 927 and 3115 cm−1 correspond to the symmetric stretching vibration modes 1 of PO4 3− and NH4 + ions, respectively. Only the second molecular vibration is affected by grafting, it shifts to 3133 cm−1 when l-proline and l-arginine amino-acids are added, because of the small perturbation of the surrounding crystalline field. Two bands are predicted and observed for each ion, dealing with the degenerated symmetric bending mode 2 . For the pure ADP, they are located at 346 and 480 cm−1 for PO4 3− and at 1657 and 1739 cm−1 for NH4 + . They are also slightly shifted by the surrounding crystalline field, when the additives are used. On the other hand, four bands located at 570, 1016, 1441, and 3416 cm−1 are observed for pure ADP crystal, as expected by the group theory analysis. The two former are attributed to 4 and 3 modes of PO4 3− and the two latter to 4 and 3 ones of NH4 + . Results in Table 4, clearly show that the asymmetric vibrational modes of PO4 3− are more affected by grafting on organic molecules. The peak positions of 3 (PO4 3− ) band red-shifts to 555 cm−1 , whereas that of the 3 band shows a blue-shift to 1039 for the l-proline grafted on material, and to 1024 for the l-arginine grafted on compound. At the same time, the 3 (NH4 + ) up-shifts to 3424 for the l-proline grafted on material, and to 3431 cm−1 when l-arginine was added. Meanwhile, two lines, among the six external modes predicted by group theory (3A1 + 3B1 ), are observed at 130 and 182 cm−1 . The former is due to the translation of NH4 (B1 species), whereas the second one corresponds to the libration of PO4 (A1 species). It is worth mentioning that the NH4 translational mode weak line located at 130 cm−1 disappears when the amino-acids were added. Kim and Sherman [64] have studied the Raman scattering on potassium-ammonium dihydrogen phosphate systems [(KDP)1−x (ADP)x ] and mentioned a previous work by Keeling and Pepinsky [65]. It was reported that in the ADP spectra many lattice bands are much weaker than the corresponding bands of KDP. Courtens and Vogt [51] have also noted that the phenomenon is usually considered to be the result of the hydrogen-bonding interaction between PO4 groups and NH4 groups in ADP crystals. In fact, each of the two ion groups is considered to be a structure maker and could form hydrogen bond with molecules surrounding

30

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its tetrahedral structure. Two amino-acids were used as grafting materials in the present work, l-proline and l-arginine. However, Simon [66] reported that in the case of KDP a lowest-frequency highly damped mode was observed. Accordingly, this latter mode, which is the strongest polar one (IR active), is symmetry-forbidden ¯ in the space group I 42d. The latter stands for the average position of protons between their two sites, leading to a supplementary degree of freedom related to the proton motion. Thus, the observed low-frequency extra mode is due to a supplementary degree of freedom. However, in this work the disappearance of the external mode located at 130 cm−1 (A1 species), may be related to a decrease in freedom degrees, following the establishment of hydrogen bonds with the grafted l-proline and l-arginine amino acids on ADP material. Similar spectroscopic features were also observed from the Fourier transform infrared transmittance, given in Fig. 3, mainly in the frequencies vibrational regions of free O–H. Group theory analysis predicts eighteen bands concerning the E symmetry polarization (Table 1). Experimentally, 12 lines are observed. Fig. 6 shows the spontaneous Raman spectra obtained in the x(yz)¯x -polarization scheme for ADP and grafted on l-arginine and l-proline amino acids, where a number of overlapping Raman lines are observed. A similar spectrum was obtained for the pure ADP by Gorelik et al. [53] in this geometry. It was already reported by Broberg et al. [67], that the broad line near 290 cm−1 in the x(yz)¯x is a feature existing in ADP attributed to the E librational (torsional) mode of the NH4 + ion. In the present work, this mode is observed with a medium intensity at 286 cm−1 for all the three spectra in Fig. 6, while two bands are predicted by factor group analysis (Table 1). Kim and Sherman [64] have also observed this mode as a very weak shoulder band at around 280 cm−1 and another broad one at 174 cm−1 . They have assigned this latter to the PO4 3− librational mode, which appears in this work as a strong peak at 182 cm−1 . Two bands are also predicted in this geometry for the libration of PO4 3− ion (Table 1). The two degenerated modes 4 and 3 are predicted by factor group analysis in this geometry, dealing with the internal normal modes. In the case of the PO4 3− ions, the corresponding lines are observed at 577 cm−1 and at 1129 cm−1 , respectively. Although these two modes are not affected by the grafting materials for the phosphate anion, those corresponding to NH4 + ions seem to be altered, particularly the 3 stretching mode. The two observed, and predicted, bands for this latter mode shifted from the values of the pure material when amino-acids were grafted. This suggests that the grafted molecules linear distorted the NH4 + ions in this geometry. Besides, it was reported by Madhurambal et al. [56], dealing with the Zn(NH4 )2 (SO4 )2 ·6H2 O structural study’s crystal, that shift in bands and intensity variations are due to the variation in strength of the different bonds. In our case, it seems that the hydrogen bonding between ADP material and the grafted on lproline and l-arginine amino-acids explains the observed shifting phenomenon. It is worth noting that the non polar symmetric stretching mode 1 (PO4 3− ), observed in the IR spectra (Fig. 3), appears in this geometry (E symmetry) despite the fact that it was not predicted by the factor group theory for the x(yz)¯x polarization. The band observed at 927 cm−1 could be a contamination by the very intense line of that 1 mode observed in the A1 symmetry. Moreover, it was reported by Dakhlaoui et al. [68] that a structural disorder may give rise to the emergence of lines expected in a different geometry. However, in the present work, this disorder seems to be accentuated by the grafting organic molecules. The observed weak signal at 1657 cm−1 , for the treated ADP materials of the symmetric bending mode 2 (NH4 + ) (E symmetry forbidden), is also in agreement with such a conclusion. Moreover, the degeneracy of the asymmetric bending mode 4 (NH4 + ) seems to be removed. A second very weak line appears at 1472 cm−1 when the amino acids were introduced.

(b) (c) (a)

-15

-10

-5

0

5

10

15

ppm Fig. 7. 31 P MAS NMR spectra of the pure (a) and grafted on l-proline (b) and larginine (c) ADP compounds with spinning frequency of 8 kHz, repetition time of 60 s and 64 scans.

The splitting of 4 mode in both FT-IR (Table 3, Fig. 3) and Raman (Table 4, Fig. 6) spectra is also related to the angular distortion of the NH4 + ion, when grafting amino-acids were added. This distortion is also attributed to the eventual hydrogen bonding between the ADP host material and the grafted on amino-acids. It is worth noting that the Raman scattering in the x(yx)¯x geometry (B2 symmetry) was not carried out in the present work. The corresponding polar modes lines were deduced from the IR spectra and those of Raman scattering x(yz)¯x configuration (E species). 3.3. NMR analysis We have performed the 31 P and 1 H MAS NMR measurements on the studied compounds at room temperature. The 31 P spectra, given in Fig. 7, show only one isotropic line at 0.917, 1.045 and 1.173 ppm for the pure material and those obtained with l-proline and l-arginine, respectively. This explains that (PO4 3− ) anions have the same local symmetry. Thus, all the phosphorus atoms are crystallographically equivalent in the tetragonal NH4 H2 PO4 structure. However, it was reported by Chen et al. [69] that the P chemical shift should move to low-field with the increase of the associated H atoms. Consequently, the little down-field shift of the resonance of the phosphorus atom is probably related to hydrogen bonds established with the amino acid molecules. The mechanism may consist in a deshielding related to a slight lengthening of the covalent P–O moiety of the P–O· · ·H–X hydrogen bond, where X can be a nitrogen or an oxygen atom provided by the amino and the carboxylic groups of the amino acid molecules. As a consequence, attenuation of the major shielding provided by the sigma bonding electron pair of P–O bond is observed. The 1 H MAS NMR spectra of the same samples used for the 31 P NMR measurements are given in Fig. 8. Two main so broad isotropic peaks at around 14 and 6 ppm were observed. The former is assigned to H atoms of NH4 groups and the latter to that of H2 PO4 . These resonances are located at around 14.201 and 6.701 ppm for the pure compound (Fig. 8a), at 14.097 and 6.375 ppm for the lproline grafted on material (Fig. 8b) and at 15.795 and 6.945 ppm for the l-arginine grafted on product (Fig. 8c). However, the two latter spectra underwent little variations. They were associated with some shoulders on the high-field side at 5.415 and 2.373 ppm for the (b) spectrum and at 5.334; 3.512 and 2.015 ppm for the (c) one.

F. Ben Brahim, A. Bulou / Materials Chemistry and Physics 130 (2011) 24–32

31

Acknowledgments

Sh ou ld er s

The authors thank Mrs. A.-M. Mercier (CNRS Engineer) and Mr. C. Galven (CNRS Assistant Engineer), Laboratoire des Oxydes et Fluorures, Faculté des Sciences et Techniques, Université du Maine, Le Mans, France. Tanks are due to Pr. T. LABASSI from University of Tunis for reading the manuscript.

(c) (a) (b)

-5.0

-2.5

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

ppm Fig. 8. H MAS NMR spectra of the of the pure (a) and grafted on l-proline (b) and l-arginine (c) ADP compounds with spinning frequency of 8 kHz, repetition time of 1 s and 1024 scans. 1

They probably correspond to the adsorbed organic molecules of the amino acid additives. The presence of these molecules, grafted on the ammonium dihydrogen orthophosphate host material, causes the little shifts of the resonance peaks of the host intrinsic protons of NH4 and H2 PO4 groups. Chen et al. [69] have assigned the peaks observed in the high-field range of 1.0–1.6 ppm of the 1 H proton spectrum of their synthesized compound, to the residue organic solvent n-BuOH they have used. 4. Conclusion In this work, we have observed in the spectral changes the interaction between the ADP host material and the grafted on l-proline and l-arginine amino acids additives. NMR and FT-IR and Raman spectroscopic studies were carried out. IR and Raman results show that some fundamental vibrational bands, derived from the PO4 3− and NH4 + free ions, shift by a few cm−1 from the corresponding values of the pure material. Other bands separate and give rise to two lines. This finding concerns the asymmetric bending mode 4 and the asymmetric stretching one 3 of the NH4 + ion. The band at around 1640 cm−1 becomes broader in presence of the additive, showing the hydrogen bonding establishment between the H2 PO4 − group of the host and the NH3 + of the additives as well as the overlap between the symmetric bending band 2 of NH4 + and (C–H), ı(N+ –H· · ·O), and (C O) of the additives. The 31 P MAS-NMR spectra contain a single sharp peak at around 1 ppm, showing no additional peaks assigned to the influence of amino-acid additives. However, spectra are distinguishable by a little down-field shift for the two compounds obtained in presence of the organic molecules, l-proline and l-arginine. In addition, the main two peaks at around 6 and 14 ppm in the 1 H MAS-NMR spectra of the amino-acids grafted on ADP crystals, show shifts associated to some shoulders at the up-field region. They are probably corresponding to the protons of the adsorbed organic molecules. Our results clearly show that the combination of the IR, Raman and 31 P and 1 H MAS-NMR spectroscopic techniques highlights an interaction between ADP crystals and amino-acids organic molecules. The mechanism of such interaction is not well defined. Work is now in progress in order to investigate the 15 N and 13 C CPMAS NMR in these compounds as well as their thermal behaviors. A supplementary study dealing with the physical properties such as non linear optic (NLO) is also underway.

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