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Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features
Accepted Manuscript Title: Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features Authors: Buvaneswari Gopal, Aishwarya Muralidharan, Rangarajan Bakthavatsalam, Subramanian Nellaiappan, Annamalai Senthil Kumar PII: DOI: Reference:
S0025-5408(16)30556-6 http://dx.doi.org/doi:10.1016/j.materresbull.2017.03.020 MRB 9212
To appear in:
MRB
Received date: Revised date: Accepted date:
12-8-2016 13-2-2017 11-3-2017
Please cite this article as: Buvaneswari Gopal, Aishwarya Muralidharan, Rangarajan Bakthavatsalam, Subramanian Nellaiappan, Annamalai Senthil Kumar, Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features Buvaneswari Gopal*, Aishwarya Muralidharan, Rangarajan Bakthavatsalam, Subramanian Nellaiappan and Annamalai Senthil Kumar Department of Chemistry, School of Advanced Sciences VIT University, Vellore-632 014, Tamil Nadu, India
*Corresponding author Department of Chemistry, School of Advanced Sciences VIT University, Vellore-632 014, Tamil Nadu, India Ph: 91-0416—2202338/2202393; Fax:91-0416-2243092; Email:[email protected]
Graphical abstract
V5+ ion substitution results in an unusual reduction in the optical band gap of BPO4 from ~8.9 eV to the value of ~2.2 eV (BP1-xVxO4, x = 0.05 and 0.1). The newly developed phosphovanadates (BP0.95V0.05O4 and BP0.9V0.1O4) show absorbance edge in the visible region, possess stable yellow color and exhibit tunable redox features. Highlights
(i)
Partial substitution of V5+ ion for P5+ in BPO4 hugely reduces the band gap of BPO4 from 8.9 eV to the order of 2.2 eV.
(ii)
The phosphovanadates colorimetrically detect hydrazine and hydrogen peroxide.
1
Abstract This article documents the unexpected optical property observed in chemically modified BPO4 which is carried out by partial substitution of P5+ ion by V5+ ion and redox behaviour of the resultant new phases. Structurally, the extent of bigger V5+ ion substitution for smaller P5+ ion in the phosphate lattice is found to be restricted. Powder X-Ray diffraction and FT-IR analysis confirmed BP0.95V0.05O4 and BP0.9V0.1O4 as optimized compositions and were further characterized by UV-VIS-NIR and SEM-EDS techniques. Nevertheless, such small replacement of PO4 moiety by VO4 moiety in BPO4 lattice resulted in greatly enhanced optical absorption and the absorption edge values were found to be around 569 and 591 nm in the case of BP0.95V0.05O4 and BP0.9V0.1O4 respectively against the absence of such absorption in the parent BPO4. In addition, the new phosphovanadates exhibited redox behaviour on interacting with hydrazine and hydrogen peroxide.
Keywords: A. ceramics, A. oxides, B. optical properties, C. infrared spectroscopy, C. X-ray diffraction.
1. Introduction
Designing materials by chemical modification of a host material either by introducing or extracting a species into or out of a matrix has led to discovery of new properties. Current work focuses on one such system of the formula BPO4 and explores the change in its optical property by introducing V5+ ion.
The boron phosphate crystallizing in tetragonal symmetry (I-4) at room temperature is visualized as a three dimensional network of interconnected BO4 and PO4 tetrahedra as noted
2
in SiO4 interlinked framework of cristobalite [1]. The phosphate has been used as a raw material in the synthesis of other metal phosphates [2], as a catalyst [3, 4] and explored for nonlinear optical property [5], anisotropic thermal expansion behavior [6] and humidity sensing property [7]. Recently, BPO4 in its nanoform has been studied as an inorganic material for boron neutron capture therapy (BNCT) application [8]. Dogan et al studied the impact of the addition of BPO4 on the thermal behavior of polypropylene and polyamide-6 fiber [9]. The phosphate has also been investigated for battery applications [10,11].
To the best of our knowledge very few studies are available on the aliovalent cation doped BPO4. This could be due to the matrix’s crystal chemical restrictions towards such ionic substitution. However, two significant substitutions resulted in interesting products. For example, Li doped BPO4 has been studied for electrolyte application [10,11]. Ba2+ doped BPO4 has been found to be a bluish white luminescent material [12]. Current work investigates the extent of replacement of P5+ by V5+ and its impact on the structure and optical absorbance of BPO4. It is observed that the BPO4 network accommodates V5+ partially and its optical band gap is drastically reduced. Interestingly, the resultant V5+ substituted phase is found to respond to hydrazine and the treated phase in reverse to hydrogen peroxide.
Exposure to a medium containing hydrazine and hydrogen peroxide beyond the risk level leads to health problems [13,14]. It is convenient to detect such species by colorimetric sensing in which the presence of the species could be identified by the color change and the method is simple, inexpensive and safe [15]. Intensity of the color could be correlated to the concentration of the species and thus quantification becomes possible. Current work
3
demonstrates a rapid colorimetric response of the synthesized phosphovanadate for both hydrazine and hydrogen peroxide.
2.
Experimental
2.1
Synthesis of BPxV1-xO4
2.2
Materials
Boric acid (S.D.Fine chem. Ltd, 99.5%), Diammonium hydrogen orthophosphate (S.D.Fine chem. Ltd, 98-100%) and ammonium metavanadate (S.D.Fine chem. Ltd, 99.5%) have been utilized.
2.3
Solution method
Accurately weighed stoichiometric amounts of B, P and V sources were made into solution using double distilled water and ammoniacal water respectively. The solutions were mixed together and heated to dryness. The dried powder was ground well and heated at 800 C for 15 h.
2.4
Sol-gel method
The solutions of B, P and V sources were mixed together and to the mixture 10% starch solution was added. The sol obtained was condensed by slow heating on a hot plate and the gel was heated to dryness. The dried mass was ground well and heated at 800 C for 15 h .
2.5
Combustion method
The solutions of the starting materials were mixed together and to the solution calculated quantity of urea was added. The resulting slurry was introduced into a preheated furnace (500
4
C) till the decomposition process aided by combustion reaction was completed. The fluffy mass collected was cooled, ground well and heated at 800 C for 15 h.
2.6
Characterization
The parent and vanadium substituted phases have been analyzed by powder XRD, FT-IR, SEM-EDS analysis. Powder XRD patterns were obtained using Bruker D8 Advanced powder diffractometer (Cu Kα ). The unit cell parameters were calculated by least square refinement method. FT-IR spectra were collected on a JASCO FT-IR 4100 using KBr pellet technique. Surface morphology and elemental analysis have been performed by SEM-EDX using JEOL Model JSM - 6390LV. UV-Vis Diffuse reflectance spectra were recorded using Jasco spectrophotometer (model V-560) equipped with 150 mm integrating sphere. The color characteristics of the materials were assessed through L*a*b* coordinates according to CIE (Commission Internationale de l’Eclairage).
2.7
Interaction with hydrazine and hydrogen peroxide
Yellow powder of BP0.9V0.1O4 [BP91, (0.025g)] was spread over a glass plate and was interacted with 0.04 ml of hydrazine of the concentration range 100-1 mM The treated phosphate has been made in contact with hydrogen peroxide (concentration range 100-1 Mm) which makes the material to regain its original yellow color. Both the testing were performed at room temperature.
3.
Results and discussion
3.1 Phase formation and Optical absorption The V5+ substituted BPO4 compounds have been synthesized by solution method (SM), combustion method (CM) and sol-gel method (SG). The extent of substitution of V5+ and the
5
pure phase formation are found to vary with the method of synthesis. Indexing the powder XRD patterns given in Fig. 1(i) based on the reported data (JCPDS 34-0132) of the parent phase BPO4 reveals that the sol-gel method leads to the formation of V5+ substituted phase with maximum of x = 0.1 (BP0.9V0.1O4). The solution method limits the introduction of V5+ in BPO4 up to x = 0.05 (Fig.S1) whereas, impurity peak is noticed in the product obtained by combustion method with x = 0.05 (Fig. S2). Compared to solution and combustion methods, the step in between the initial mixing and final heating in sol-gel method involves the formation of metal ion incorporated polymeric precursor and thus provides space for more vanadium ions to enter into the lattice. Further heating decomposes the organic moiety and stabilizes the phosphovanadate phase. Having studied the impact of method of synthesis on the extent of replacement of P5+ by V5+, it is important to find the extent of replacement of P5+ by V5+ crystallographically in BPO4 lattice. Hence, the phases with ‘x’ values 0.15, 0.5 and 1.0 have been tried by sol-gel method. It is noted that with x = 0.15, V2O5 starts forming, proceeding to x = 0.5 leads to the formation of mixture of B2O3 and V2O5 and x = 1.0 results in the formation of brownish glassy material. Powder X-Ray diffraction analysis designates that the framework of BPO4 does not get disturbed up to the replacement of 10% of crystallographic P5+ by V5+. The contributory factors towards this could be the difference in size of the ions and the characteristic BO4 and PO4 interconnection in BPO4.
The unit cell parameters (Table 1) obtained for the parent phase BPO4 agree well with the reported values [6]. The values of the V5+ substituted phases depict that the chemical substitution does not bring significant change in the unit cell dimensions regardless of the size difference between P5+ and V5+ ions (0.17 Å and 0.36 Å for four fold coordination respectively) [16]. This could probably be due to less amount of replacement of P5+ by V5+ and open framework structure of BPO4. The open space possessed by the 3-dimensional
6
network of BPO4 could accommodate such small changes when P5+ is randomly replaced by V5+ and thus the resultant change in unit cell dimensions is noted to be feeble. It is worth to note that studies on the doping of ions such as Li+ and Ba2+ in the BPO4 lattice have reported negligible change in the lattice parameters [10,12].
Infrared spectral analysis also confirms the incorporation of V5+ into BPO4 lattice. Figure 2 displays the characteristic PO4 and BO4 vibrational modes of BPO4 network. The two groups of bands appear in the range 571- 550 cm-1 and 633-607 cm-1 in all the three spectra represent the bending vibrational modes of PO4 group. The asymmetric stretching modes of PO4 tetrahedra are represented by the band in the range 1070-1190 cm-1. The band denoted by the wave number ~938 cm-1 is due to the asymmetric stretching vibrational mode of B-O of BO4 tetrahedra [12, 17]. The prominent peaks at 434, 420 and 406 cm-1 in Fig. 2(ii) are attributed to the V-O stretching vibration [18]. The peaks are less intense in BP0.95V0.05O4 compared to BP0.9V0.1O4 due to the lesser percentage of V5+ substitution. SEM images (Fig. 1) show the influence of V5+ substitution on the surface morphology of the particles. The parent phase exhibits agglomerated small size particles which get diffused and grow into larger size particles without acquiring any specific shape in phosphovanadate phases. The results obtained by EDS analysis (Table S1) of the compositions confirm the presence of vanadium whose wt% increases from 6.2 to 11.2 with x = 0.05 and 0.1.
The parent compound BPO4 is white in color and as reported it is found to be non-absorbing in the region 200-2200 nm [19]. The replacement of P5+ by V5+ changes the color of the sample to yellow and orange yellow (Fig. 3) with the content of V5+, x = 0.05 and 0.1 respectively. Diffuse reflectance measurements on the V5+ substituted phases (Fig. 3) show an enormous increase in the optical absorbance in the wavelength range 200-500 nm against
7
the parent BPO4. The absorbance edge values for the compounds BP0.95V0.05O4 and BP0.9V0.1O4 are found to be 569.4 nm and 591.3 nm respectively. Thus, the introduction of V5+ into the B-P-O network drastically reduces the optical band gap value (8.9 eV) [12] of BPO4 to 2.18 and 2.10 eV in the case of BP0.95V0.05O4 and BP0.9V0.1O4 respectively. The strong absorption in the region 200-500 nm is comparable to that of reported for 6 mol% Ba doped BPO4 [12], where Ba - doping makes the sample to absorb in the region 300-500 nm. In addition, Ba-doped BPO4 exhibits photoluminescence due to the formation of paramagnetic defects. Whereas, V5+ substitution in BPO4 makes the phosphate to respond colorimetrically to the presence of reducing atmosphere such as hydrazine (discussed in detail below).
The color parameters are compiled in Table 1. The white to yellow color change is indicated by the higher positive b* values in the case of both x = 0.05 and x = 0.1 samples. In the latter case, the slightly higher positive a* value specifies orange tinge in the color due to the mixing of red with yellow. Increase in V5+ content is found to decrease the brightness as revealed by the L* values. The test by interacting the phosphovandates with acid and base (0.05 M HNO3 and NaOH, KOH) confirms that the compounds are highly acid resistive.
V5+ ion substitution in BPO4 lattice (1) Reduces the optical band gap of BPO4 (2) brings color change of BPO4 from white to yellow. Introduction of V5+ ion converts BPO4 into multi cationic complex oxide which influences its electronic structure and hence its optical absorbance. Change in the optical absorption is expected based on the facts that (i) the substitution of less electronegative V for P may decrease the covalency of the framework bonding made up of P-O in pure BPO4 and influence the electronic structure and thus the
8
band gap [20] (ii) the O to M charge transfer excitation energy is noted to decrease in the order (PO4)3- > (VO4)3- [21].
In the case of BPO4 formed by interconnected discrete BO4 and PO4 tetrahedra, large separation between occupied O 2p orbitals and unoccupied B and P orbitals resulted in wide band gap [20]. The replacement of P5+ ion by V5+ ion inserts the electronic component of V3d in conduction band which is otherwise formed by B-O and P-O hybridization orbitals [22]. As a result the band gap energy of the phosphate is significantly reduced.
The observation of white color of the parent compound, BPO4 and the yellow color of the V5+ ion substituted compounds is in agreement with the absence of absorbance in the former and the appearance of absorption edge (~ 569 - 590 nm) in the latter. The absorption edge in the visible region and the red shifted absorption edge with increase in V5+ content suggest the obvious contribution of electronic component of V5+ in the band gap reduction and thus the color of the material. The broad absorption in the visible region could be due to the contribution from O-V CT band [23].
3.2 Impact of V5+ ion substitution on redox behaviour Interestingly, the resultant phosphovanadate is found to respond rapidly on interacting with hydrazine by changing its yellow color to green. The property has been explored by carrying out the experiments using BP0.9V0.1O4 (BP91). Once 0.04 ml of hydrazine taken in a syringe is interacted with the powder sample (0.025 g), the color of the sample turns green (Fig. 4). The tested concentrations of hydrazine are 100 mM, 10 mM, 1mM and 500 mM. The color change is noted to be spontaneous. Slight excess of hydrazine is needed to obtain the color change in the case of low concentrations (1000 and 500 mM). Treatment with further lower
9
concentration of hydrazine (250 mM) required longer duration (overnight). The shades of different green colors observed depending on the concentration of hydrazine are shown in Fig.S3. The observed color changes depicted a clear evidence and visual contrast before and after the interaction. Powder X-Ray diffraction analysis of the sample treated with hydrazine (BP91G) shows that the phase does not undergo degradation during its interaction with hydrazine (Fig. S4). The rapid response of the oxide by changing its color may be used to provide visual indication of the presence of hydrazine.
The effect of interfering ions is studied. The solutions (100 mM) containing (i) K+, Na+, Cl-, SO42-, Ca2+, CO32-, F-, HPO42-, Mg2+, Zn2+, Fe3+ and Cu2+ ions (solution A) and (ii) all these ions and hydrazine (solution B) are tested. The sample is interacted with solution A and B separately and the response of the sample is shown in Fig. 4(ii). No color change is observed in the case of solution A and the color change of the sample to green in the case of solution B confirming the selectivity.
The green phosphovanadate responds to hydrogen peroxide by changing its color. Figure. 4(i) shows how the original yellow color of the sample resumed after making the hydrazine treated BP0.9V0.1O4 to interact with hydrogen peroxide (0.04 ml). The green sample (BP91G) is treated with hydrogen peroxide of varied concentration in the range 100- 1 mM. The response is found to be spontaneous with 100 mM H2O2 and takes ~1 min with 10mM. But the response is noted to be feeble with 1 mM H2O2. Thus, the derived phosphovanadate exhibits novel property of sensing both hydrazine and hydrogen peroxide. It is presumed that the presence of V5+ makes this redox behaviour feasible.
10
In order to understand the redox feature, V5+ doped BPO4 compound (BP0.9V0.1O4) [BP91] was subjected to electrochemical studies (Fig. 5), wherein a suspension prepared using 3mg of BP0.9V0.1O4 with 500 L of 5% Nafion® (electro-inactive proton conducting polymer) coated (5 L) on a cleaned glassy carbon electrode (0.0707 cm2) was used as a working electrode along with Ag/AgCl as reference and 3 mm platinum disc as a counter electrodes. Exposure of the working electrode in 0.05 M KNO3-HNO3 (0.1 N, pH~3) solution under open-circuit condition resulted to a potential value 0.61 V vs Ag/AgCl, which is equal to 1.0 V vs reversible hydrogen electrode (RHE) [24]. This value is closer to the Eo value of V5+/V4+ redox system (Eo = 1 V vs RHE) which supports the presence of V5+ in the matrix. In further the modified electrode is subjected to hydrazine and hydrogen peroxide electrochemical reactions by studying with cyclic voltammetry in discrete conditions showing welldefined oxidation and reduction current signals at starting potentials, +0.60 ±0.02 and 0.05±0.02 V vs Ag/AgCl which correspond to the standard redox potentials of V5+/V4+ and V4+/V3+ (Eo = 0.34 V vs RHE or =-0.14 V vs Ag/AgCl) respectively. It was observed that H2O2 involves in a dual function like oxidation at V5+/V4+ potential and reduction at V4+/V3+ potential parallely. These observations likely indicate the reversible mediation of vanadium redox states on interaction with hydrazine and hydrogen peroxide which in turn reveals the change in the color of the materials observed in this work.
As control experiments BPO4 (BP) without V5+ was also subjected to the electrochemical studies under similar conditions but no such redox specific observations were noticed and thus supports the speculation. Overall, this work demonstrates a reversible redox active system which responds to both hydrazine and hydrogen peroxide. The advantages of the current system are: simple, stable, fast response and cost effectiveness. This new system can
11
be used as a qualitative sensor for screening of hydrazine and hydrogen peroxide in real samples.
4.
Conclusions
In conclusion, current investigation leads to the following observations: (i) chemical modification of BPO4 by V5+ ion substitution hugely reduces the optical band gap of BPO4 from 8.9 eV to the order of 2.2 eV. (ii) the resultant yellow colored phosphovandate exhibits a redox feature which is shown by the drastic stable color change from yellow to green upon interacting with hydrazine and the treated phase is found to respond reversibly to hydrogen peroxide. The results suggest for a potential application of the simple phosphate system in the fields such as photocatalytic, chemical sensor and pigments.
Acknowledgements The authors thank VIT University for providing all required facilities to carry out the experiments.
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Behrooz Zargar , Amir Hatamie, A Simple and Fast Colorimetric Method for Detection of Hydrazine in Water Samples Based on Formation of Gold Nanoparticles as a Colorimetric Probe, Sens. Actuators. B, 182 (2013) 706.
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Figure captions
Fig. 1. Powder X-Ray diffraction patterns and SEM images of BP1-xVxO4, x = 0 (a), 0.05 (b), 0.1(c), 0.15(d) (sol-gel method) [*- impurity peaks] Fig. 2. FT-IR spectra of BP1-xVxO4, x = 0 (a), 0.05 (b) and 0.1(c) Fig. 3. UV-VIS-NIR absorption spectra of BP1-xO4, x = 0(a), 0.05(b), 0.1(c) Fig. 4. Response of BP0.9V0.1O4 to (i) hydrazine (HY) and hydrogen peroxide (HO) (ii) to solution containing (a) interfering ions (b) interfering ions along with hydrazine Fig. 5. Cyclic voltammogram of GCE/BP91-Nf modified electrode in 500M H2O2 (A), 5 mM hydrazine (B) and open circuit potential data in 500 M H2O2 (C) in 0.05 M KNO3HNO3 buffer solution at a scan rate of 10 mv/s. Control sample GCE/BP-Nf in 500uM H2O2 (D), 5 mM hydrazine (E) [condition: CME: GCE/BP91-Nf (5% Nf) – 5L ; Buffer : 0.05 M KNO3-HNO3] [*BP91- BP0.9V0.1O4, BP –BPO4]
15
a
10 20 30 40 50
2
16 (220) (213)
*
(103) (211) (202)
(112)
c
(002) (101) (110)
Intensity (a.u)
Figure 1
d
* *
b
60 70
Figure 2
(i)
BPSG2 BP95SG1 BP91SG1
a
% Transmittance
b c
946 938
4000
3500
3000
2500 2000 1500 -1 Wavenumber (cm )
1000
500
% Transmittance
(ii)
c 406
b 434
420
a
500
-1
Wavenumber (cm )
17
400
BPSG2 BP95SG1 BP91SG1
Figure 3
1.0
BP B95 BP91
c
c
Eg, BP95SG = 1 BP91SG, = 2.10
b Absorbance
0.8 b
0.6 a
0.4
0.2 200
a 300
569.4 400
500
591.3 600
Wavelength (nm)
18
700
800
Figure 4
19
Figure 5
20
Table 1. Lattice parameters and CIE L*a* b* colorimetric data for the composition BP1-xVxO4
Composition
Lattice parameters (Å)
Color coordinates
a
c
L*
a*
b*
BPO4
4.343(1)
6.638(4)
78.07
0.22
1.71
BP0.95V0.05O4
4.338(1)
6.642(4)
73.05
3.74
40.72
BP0.9V0.1O4
4.344(1)
6.643(4)
68.11
11.55
42.34
T
21
S0025-5408(16)30556-6 http://dx.doi.org/doi:10.1016/j.materresbull.2017.03.020 MRB 9212
To appear in:
MRB
Received date: Revised date: Accepted date:
12-8-2016 13-2-2017 11-3-2017
Please cite this article as: Buvaneswari Gopal, Aishwarya Muralidharan, Rangarajan Bakthavatsalam, Subramanian Nellaiappan, Annamalai Senthil Kumar, Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Unusual observation of optical property of V5+ substituted BPO4 and its tunable redox features Buvaneswari Gopal*, Aishwarya Muralidharan, Rangarajan Bakthavatsalam, Subramanian Nellaiappan and Annamalai Senthil Kumar Department of Chemistry, School of Advanced Sciences VIT University, Vellore-632 014, Tamil Nadu, India
*Corresponding author Department of Chemistry, School of Advanced Sciences VIT University, Vellore-632 014, Tamil Nadu, India Ph: 91-0416—2202338/2202393; Fax:91-0416-2243092; Email:[email protected]
Graphical abstract
V5+ ion substitution results in an unusual reduction in the optical band gap of BPO4 from ~8.9 eV to the value of ~2.2 eV (BP1-xVxO4, x = 0.05 and 0.1). The newly developed phosphovanadates (BP0.95V0.05O4 and BP0.9V0.1O4) show absorbance edge in the visible region, possess stable yellow color and exhibit tunable redox features. Highlights
(i)
Partial substitution of V5+ ion for P5+ in BPO4 hugely reduces the band gap of BPO4 from 8.9 eV to the order of 2.2 eV.
(ii)
The phosphovanadates colorimetrically detect hydrazine and hydrogen peroxide.
1
Abstract This article documents the unexpected optical property observed in chemically modified BPO4 which is carried out by partial substitution of P5+ ion by V5+ ion and redox behaviour of the resultant new phases. Structurally, the extent of bigger V5+ ion substitution for smaller P5+ ion in the phosphate lattice is found to be restricted. Powder X-Ray diffraction and FT-IR analysis confirmed BP0.95V0.05O4 and BP0.9V0.1O4 as optimized compositions and were further characterized by UV-VIS-NIR and SEM-EDS techniques. Nevertheless, such small replacement of PO4 moiety by VO4 moiety in BPO4 lattice resulted in greatly enhanced optical absorption and the absorption edge values were found to be around 569 and 591 nm in the case of BP0.95V0.05O4 and BP0.9V0.1O4 respectively against the absence of such absorption in the parent BPO4. In addition, the new phosphovanadates exhibited redox behaviour on interacting with hydrazine and hydrogen peroxide.
Keywords: A. ceramics, A. oxides, B. optical properties, C. infrared spectroscopy, C. X-ray diffraction.
1. Introduction
Designing materials by chemical modification of a host material either by introducing or extracting a species into or out of a matrix has led to discovery of new properties. Current work focuses on one such system of the formula BPO4 and explores the change in its optical property by introducing V5+ ion.
The boron phosphate crystallizing in tetragonal symmetry (I-4) at room temperature is visualized as a three dimensional network of interconnected BO4 and PO4 tetrahedra as noted
2
in SiO4 interlinked framework of cristobalite [1]. The phosphate has been used as a raw material in the synthesis of other metal phosphates [2], as a catalyst [3, 4] and explored for nonlinear optical property [5], anisotropic thermal expansion behavior [6] and humidity sensing property [7]. Recently, BPO4 in its nanoform has been studied as an inorganic material for boron neutron capture therapy (BNCT) application [8]. Dogan et al studied the impact of the addition of BPO4 on the thermal behavior of polypropylene and polyamide-6 fiber [9]. The phosphate has also been investigated for battery applications [10,11].
To the best of our knowledge very few studies are available on the aliovalent cation doped BPO4. This could be due to the matrix’s crystal chemical restrictions towards such ionic substitution. However, two significant substitutions resulted in interesting products. For example, Li doped BPO4 has been studied for electrolyte application [10,11]. Ba2+ doped BPO4 has been found to be a bluish white luminescent material [12]. Current work investigates the extent of replacement of P5+ by V5+ and its impact on the structure and optical absorbance of BPO4. It is observed that the BPO4 network accommodates V5+ partially and its optical band gap is drastically reduced. Interestingly, the resultant V5+ substituted phase is found to respond to hydrazine and the treated phase in reverse to hydrogen peroxide.
Exposure to a medium containing hydrazine and hydrogen peroxide beyond the risk level leads to health problems [13,14]. It is convenient to detect such species by colorimetric sensing in which the presence of the species could be identified by the color change and the method is simple, inexpensive and safe [15]. Intensity of the color could be correlated to the concentration of the species and thus quantification becomes possible. Current work
3
demonstrates a rapid colorimetric response of the synthesized phosphovanadate for both hydrazine and hydrogen peroxide.
2.
Experimental
2.1
Synthesis of BPxV1-xO4
2.2
Materials
Boric acid (S.D.Fine chem. Ltd, 99.5%), Diammonium hydrogen orthophosphate (S.D.Fine chem. Ltd, 98-100%) and ammonium metavanadate (S.D.Fine chem. Ltd, 99.5%) have been utilized.
2.3
Solution method
Accurately weighed stoichiometric amounts of B, P and V sources were made into solution using double distilled water and ammoniacal water respectively. The solutions were mixed together and heated to dryness. The dried powder was ground well and heated at 800 C for 15 h.
2.4
Sol-gel method
The solutions of B, P and V sources were mixed together and to the mixture 10% starch solution was added. The sol obtained was condensed by slow heating on a hot plate and the gel was heated to dryness. The dried mass was ground well and heated at 800 C for 15 h .
2.5
Combustion method
The solutions of the starting materials were mixed together and to the solution calculated quantity of urea was added. The resulting slurry was introduced into a preheated furnace (500
4
C) till the decomposition process aided by combustion reaction was completed. The fluffy mass collected was cooled, ground well and heated at 800 C for 15 h.
2.6
Characterization
The parent and vanadium substituted phases have been analyzed by powder XRD, FT-IR, SEM-EDS analysis. Powder XRD patterns were obtained using Bruker D8 Advanced powder diffractometer (Cu Kα ). The unit cell parameters were calculated by least square refinement method. FT-IR spectra were collected on a JASCO FT-IR 4100 using KBr pellet technique. Surface morphology and elemental analysis have been performed by SEM-EDX using JEOL Model JSM - 6390LV. UV-Vis Diffuse reflectance spectra were recorded using Jasco spectrophotometer (model V-560) equipped with 150 mm integrating sphere. The color characteristics of the materials were assessed through L*a*b* coordinates according to CIE (Commission Internationale de l’Eclairage).
2.7
Interaction with hydrazine and hydrogen peroxide
Yellow powder of BP0.9V0.1O4 [BP91, (0.025g)] was spread over a glass plate and was interacted with 0.04 ml of hydrazine of the concentration range 100-1 mM The treated phosphate has been made in contact with hydrogen peroxide (concentration range 100-1 Mm) which makes the material to regain its original yellow color. Both the testing were performed at room temperature.
3.
Results and discussion
3.1 Phase formation and Optical absorption The V5+ substituted BPO4 compounds have been synthesized by solution method (SM), combustion method (CM) and sol-gel method (SG). The extent of substitution of V5+ and the
5
pure phase formation are found to vary with the method of synthesis. Indexing the powder XRD patterns given in Fig. 1(i) based on the reported data (JCPDS 34-0132) of the parent phase BPO4 reveals that the sol-gel method leads to the formation of V5+ substituted phase with maximum of x = 0.1 (BP0.9V0.1O4). The solution method limits the introduction of V5+ in BPO4 up to x = 0.05 (Fig.S1) whereas, impurity peak is noticed in the product obtained by combustion method with x = 0.05 (Fig. S2). Compared to solution and combustion methods, the step in between the initial mixing and final heating in sol-gel method involves the formation of metal ion incorporated polymeric precursor and thus provides space for more vanadium ions to enter into the lattice. Further heating decomposes the organic moiety and stabilizes the phosphovanadate phase. Having studied the impact of method of synthesis on the extent of replacement of P5+ by V5+, it is important to find the extent of replacement of P5+ by V5+ crystallographically in BPO4 lattice. Hence, the phases with ‘x’ values 0.15, 0.5 and 1.0 have been tried by sol-gel method. It is noted that with x = 0.15, V2O5 starts forming, proceeding to x = 0.5 leads to the formation of mixture of B2O3 and V2O5 and x = 1.0 results in the formation of brownish glassy material. Powder X-Ray diffraction analysis designates that the framework of BPO4 does not get disturbed up to the replacement of 10% of crystallographic P5+ by V5+. The contributory factors towards this could be the difference in size of the ions and the characteristic BO4 and PO4 interconnection in BPO4.
The unit cell parameters (Table 1) obtained for the parent phase BPO4 agree well with the reported values [6]. The values of the V5+ substituted phases depict that the chemical substitution does not bring significant change in the unit cell dimensions regardless of the size difference between P5+ and V5+ ions (0.17 Å and 0.36 Å for four fold coordination respectively) [16]. This could probably be due to less amount of replacement of P5+ by V5+ and open framework structure of BPO4. The open space possessed by the 3-dimensional
6
network of BPO4 could accommodate such small changes when P5+ is randomly replaced by V5+ and thus the resultant change in unit cell dimensions is noted to be feeble. It is worth to note that studies on the doping of ions such as Li+ and Ba2+ in the BPO4 lattice have reported negligible change in the lattice parameters [10,12].
Infrared spectral analysis also confirms the incorporation of V5+ into BPO4 lattice. Figure 2 displays the characteristic PO4 and BO4 vibrational modes of BPO4 network. The two groups of bands appear in the range 571- 550 cm-1 and 633-607 cm-1 in all the three spectra represent the bending vibrational modes of PO4 group. The asymmetric stretching modes of PO4 tetrahedra are represented by the band in the range 1070-1190 cm-1. The band denoted by the wave number ~938 cm-1 is due to the asymmetric stretching vibrational mode of B-O of BO4 tetrahedra [12, 17]. The prominent peaks at 434, 420 and 406 cm-1 in Fig. 2(ii) are attributed to the V-O stretching vibration [18]. The peaks are less intense in BP0.95V0.05O4 compared to BP0.9V0.1O4 due to the lesser percentage of V5+ substitution. SEM images (Fig. 1) show the influence of V5+ substitution on the surface morphology of the particles. The parent phase exhibits agglomerated small size particles which get diffused and grow into larger size particles without acquiring any specific shape in phosphovanadate phases. The results obtained by EDS analysis (Table S1) of the compositions confirm the presence of vanadium whose wt% increases from 6.2 to 11.2 with x = 0.05 and 0.1.
The parent compound BPO4 is white in color and as reported it is found to be non-absorbing in the region 200-2200 nm [19]. The replacement of P5+ by V5+ changes the color of the sample to yellow and orange yellow (Fig. 3) with the content of V5+, x = 0.05 and 0.1 respectively. Diffuse reflectance measurements on the V5+ substituted phases (Fig. 3) show an enormous increase in the optical absorbance in the wavelength range 200-500 nm against
7
the parent BPO4. The absorbance edge values for the compounds BP0.95V0.05O4 and BP0.9V0.1O4 are found to be 569.4 nm and 591.3 nm respectively. Thus, the introduction of V5+ into the B-P-O network drastically reduces the optical band gap value (8.9 eV) [12] of BPO4 to 2.18 and 2.10 eV in the case of BP0.95V0.05O4 and BP0.9V0.1O4 respectively. The strong absorption in the region 200-500 nm is comparable to that of reported for 6 mol% Ba doped BPO4 [12], where Ba - doping makes the sample to absorb in the region 300-500 nm. In addition, Ba-doped BPO4 exhibits photoluminescence due to the formation of paramagnetic defects. Whereas, V5+ substitution in BPO4 makes the phosphate to respond colorimetrically to the presence of reducing atmosphere such as hydrazine (discussed in detail below).
The color parameters are compiled in Table 1. The white to yellow color change is indicated by the higher positive b* values in the case of both x = 0.05 and x = 0.1 samples. In the latter case, the slightly higher positive a* value specifies orange tinge in the color due to the mixing of red with yellow. Increase in V5+ content is found to decrease the brightness as revealed by the L* values. The test by interacting the phosphovandates with acid and base (0.05 M HNO3 and NaOH, KOH) confirms that the compounds are highly acid resistive.
V5+ ion substitution in BPO4 lattice (1) Reduces the optical band gap of BPO4 (2) brings color change of BPO4 from white to yellow. Introduction of V5+ ion converts BPO4 into multi cationic complex oxide which influences its electronic structure and hence its optical absorbance. Change in the optical absorption is expected based on the facts that (i) the substitution of less electronegative V for P may decrease the covalency of the framework bonding made up of P-O in pure BPO4 and influence the electronic structure and thus the
8
band gap [20] (ii) the O to M charge transfer excitation energy is noted to decrease in the order (PO4)3- > (VO4)3- [21].
In the case of BPO4 formed by interconnected discrete BO4 and PO4 tetrahedra, large separation between occupied O 2p orbitals and unoccupied B and P orbitals resulted in wide band gap [20]. The replacement of P5+ ion by V5+ ion inserts the electronic component of V3d in conduction band which is otherwise formed by B-O and P-O hybridization orbitals [22]. As a result the band gap energy of the phosphate is significantly reduced.
The observation of white color of the parent compound, BPO4 and the yellow color of the V5+ ion substituted compounds is in agreement with the absence of absorbance in the former and the appearance of absorption edge (~ 569 - 590 nm) in the latter. The absorption edge in the visible region and the red shifted absorption edge with increase in V5+ content suggest the obvious contribution of electronic component of V5+ in the band gap reduction and thus the color of the material. The broad absorption in the visible region could be due to the contribution from O-V CT band [23].
3.2 Impact of V5+ ion substitution on redox behaviour Interestingly, the resultant phosphovanadate is found to respond rapidly on interacting with hydrazine by changing its yellow color to green. The property has been explored by carrying out the experiments using BP0.9V0.1O4 (BP91). Once 0.04 ml of hydrazine taken in a syringe is interacted with the powder sample (0.025 g), the color of the sample turns green (Fig. 4). The tested concentrations of hydrazine are 100 mM, 10 mM, 1mM and 500 mM. The color change is noted to be spontaneous. Slight excess of hydrazine is needed to obtain the color change in the case of low concentrations (1000 and 500 mM). Treatment with further lower
9
concentration of hydrazine (250 mM) required longer duration (overnight). The shades of different green colors observed depending on the concentration of hydrazine are shown in Fig.S3. The observed color changes depicted a clear evidence and visual contrast before and after the interaction. Powder X-Ray diffraction analysis of the sample treated with hydrazine (BP91G) shows that the phase does not undergo degradation during its interaction with hydrazine (Fig. S4). The rapid response of the oxide by changing its color may be used to provide visual indication of the presence of hydrazine.
The effect of interfering ions is studied. The solutions (100 mM) containing (i) K+, Na+, Cl-, SO42-, Ca2+, CO32-, F-, HPO42-, Mg2+, Zn2+, Fe3+ and Cu2+ ions (solution A) and (ii) all these ions and hydrazine (solution B) are tested. The sample is interacted with solution A and B separately and the response of the sample is shown in Fig. 4(ii). No color change is observed in the case of solution A and the color change of the sample to green in the case of solution B confirming the selectivity.
The green phosphovanadate responds to hydrogen peroxide by changing its color. Figure. 4(i) shows how the original yellow color of the sample resumed after making the hydrazine treated BP0.9V0.1O4 to interact with hydrogen peroxide (0.04 ml). The green sample (BP91G) is treated with hydrogen peroxide of varied concentration in the range 100- 1 mM. The response is found to be spontaneous with 100 mM H2O2 and takes ~1 min with 10mM. But the response is noted to be feeble with 1 mM H2O2. Thus, the derived phosphovanadate exhibits novel property of sensing both hydrazine and hydrogen peroxide. It is presumed that the presence of V5+ makes this redox behaviour feasible.
10
In order to understand the redox feature, V5+ doped BPO4 compound (BP0.9V0.1O4) [BP91] was subjected to electrochemical studies (Fig. 5), wherein a suspension prepared using 3mg of BP0.9V0.1O4 with 500 L of 5% Nafion® (electro-inactive proton conducting polymer) coated (5 L) on a cleaned glassy carbon electrode (0.0707 cm2) was used as a working electrode along with Ag/AgCl as reference and 3 mm platinum disc as a counter electrodes. Exposure of the working electrode in 0.05 M KNO3-HNO3 (0.1 N, pH~3) solution under open-circuit condition resulted to a potential value 0.61 V vs Ag/AgCl, which is equal to 1.0 V vs reversible hydrogen electrode (RHE) [24]. This value is closer to the Eo value of V5+/V4+ redox system (Eo = 1 V vs RHE) which supports the presence of V5+ in the matrix. In further the modified electrode is subjected to hydrazine and hydrogen peroxide electrochemical reactions by studying with cyclic voltammetry in discrete conditions showing welldefined oxidation and reduction current signals at starting potentials, +0.60 ±0.02 and 0.05±0.02 V vs Ag/AgCl which correspond to the standard redox potentials of V5+/V4+ and V4+/V3+ (Eo = 0.34 V vs RHE or =-0.14 V vs Ag/AgCl) respectively. It was observed that H2O2 involves in a dual function like oxidation at V5+/V4+ potential and reduction at V4+/V3+ potential parallely. These observations likely indicate the reversible mediation of vanadium redox states on interaction with hydrazine and hydrogen peroxide which in turn reveals the change in the color of the materials observed in this work.
As control experiments BPO4 (BP) without V5+ was also subjected to the electrochemical studies under similar conditions but no such redox specific observations were noticed and thus supports the speculation. Overall, this work demonstrates a reversible redox active system which responds to both hydrazine and hydrogen peroxide. The advantages of the current system are: simple, stable, fast response and cost effectiveness. This new system can
11
be used as a qualitative sensor for screening of hydrazine and hydrogen peroxide in real samples.
4.
Conclusions
In conclusion, current investigation leads to the following observations: (i) chemical modification of BPO4 by V5+ ion substitution hugely reduces the optical band gap of BPO4 from 8.9 eV to the order of 2.2 eV. (ii) the resultant yellow colored phosphovandate exhibits a redox feature which is shown by the drastic stable color change from yellow to green upon interacting with hydrazine and the treated phase is found to respond reversibly to hydrogen peroxide. The results suggest for a potential application of the simple phosphate system in the fields such as photocatalytic, chemical sensor and pigments.
Acknowledgements The authors thank VIT University for providing all required facilities to carry out the experiments.
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Figure captions
Fig. 1. Powder X-Ray diffraction patterns and SEM images of BP1-xVxO4, x = 0 (a), 0.05 (b), 0.1(c), 0.15(d) (sol-gel method) [*- impurity peaks] Fig. 2. FT-IR spectra of BP1-xVxO4, x = 0 (a), 0.05 (b) and 0.1(c) Fig. 3. UV-VIS-NIR absorption spectra of BP1-xO4, x = 0(a), 0.05(b), 0.1(c) Fig. 4. Response of BP0.9V0.1O4 to (i) hydrazine (HY) and hydrogen peroxide (HO) (ii) to solution containing (a) interfering ions (b) interfering ions along with hydrazine Fig. 5. Cyclic voltammogram of GCE/BP91-Nf modified electrode in 500M H2O2 (A), 5 mM hydrazine (B) and open circuit potential data in 500 M H2O2 (C) in 0.05 M KNO3HNO3 buffer solution at a scan rate of 10 mv/s. Control sample GCE/BP-Nf in 500uM H2O2 (D), 5 mM hydrazine (E) [condition: CME: GCE/BP91-Nf (5% Nf) – 5L ; Buffer : 0.05 M KNO3-HNO3] [*BP91- BP0.9V0.1O4, BP –BPO4]
15
a
10 20 30 40 50
2
16 (220) (213)
*
(103) (211) (202)
(112)
c
(002) (101) (110)
Intensity (a.u)
Figure 1
d
* *
b
60 70
Figure 2
(i)
BPSG2 BP95SG1 BP91SG1
a
% Transmittance
b c
946 938
4000
3500
3000
2500 2000 1500 -1 Wavenumber (cm )
1000
500
% Transmittance
(ii)
c 406
b 434
420
a
500
-1
Wavenumber (cm )
17
400
BPSG2 BP95SG1 BP91SG1
Figure 3
1.0
BP B95 BP91
c
c
Eg, BP95SG = 1 BP91SG, = 2.10
b Absorbance
0.8 b
0.6 a
0.4
0.2 200
a 300
569.4 400
500
591.3 600
Wavelength (nm)
18
700
800
Figure 4
19
Figure 5
20
Table 1. Lattice parameters and CIE L*a* b* colorimetric data for the composition BP1-xVxO4
Composition
Lattice parameters (Å)
Color coordinates
a
c
L*
a*
b*
BPO4
4.343(1)
6.638(4)
78.07
0.22
1.71
BP0.95V0.05O4
4.338(1)
6.642(4)
73.05
3.74
40.72
BP0.9V0.1O4
4.344(1)
6.643(4)
68.11
11.55
42.34
T
21