Synthesis, Characterization, and Proton Conduction Behavior of

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Article Cite This: ACS Omega 2019, 4, 3716−3725

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Synthesis, Characterization, and Proton Conduction Behavior of Thorium and Cerium Phosphonates Tarun F. Parangi*,†,‡ and Uma V. Chudasama‡ †

Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India Applied Chemistry Department, The M.S. University of Baroda, Vadodara 390 001, Gujarat, India

‡

ACS Omega 2019.4:3716-3725. Downloaded from pubs.acs.org by 46.161.59.56 on 02/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Thorium (Th4+) and cerium (Ce4+) phosphonates have been synthesized by the sol−gel method using various phosphonic acids and analyzed using elemental and CHN analysis, spectral analysis (Fourier transform infrared spectroscopy and X-ray diffraction), thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The proton transport study was performed by measuring proton conductance at different temperatures using an impedance analyzer. The Grotthuss mechanism of the proton conduction has been discussed thoroughly on the basis of results obtained from the impedance measurement; the specific proton conduction (σ) and activation energy (Ea) have been evaluated and compared with the reported proton-conducting system having similar phosphonate moieties. used as proton conductors. Demadis et al.9 have prepared crystalline phosphonocarboxylates of Li+, Na+, K+, and Cs+ metal ions using hydroxyphosphonoacetate. As a result, all solid materials show comparable good proton conductivity, which could be due to the structural role of water molecules and the hydrogen-bonding networks.9 Cabeza et al.10 also have reported the crystalline structure of Ca2+ -phosphonate using 5(dihydroxyphosphoryl)isophthalic acid, which has −COOH as protogenic groups exhibiting good proton conductivity in range of magnitude of 10−3 S·cm−1. The authors11 also have reported the solvothermal synthesis of Mg 2+ -phosphonate using hexamethylenediamine-tetra(methylenephosphonic acid), which shows remarkable proton conductivity (in the range of ∼10−5 S·cm−1) because of having P−OH and −NH+ groups as proton sources. Ploehn et al.12 have prepared polymer composite materials using a combination of mixed metals and phenylphosphonic acid (M = Mg, Ca, Sr, and Pb) and explored its dielectric permittivity, which was found to be higher than its parent composite material. Klapper et al.13 have synthesized Al3+-phosphonate by using hexakis(p-phosphonatophenyl)benzene, which exhibits notable proton conductivity (∼10−3 S·cm−1) close to that for Nafion. The enhanced conductivity may be due to the regular pore size distribution, which could make the lattice H2O molecules or other guest ions/molecules easy to be trapped.13 Among the transition-metal ions, phosphonates of tetravalent metal ions (Zr14,15 and Ti16−19) have been extensively studied because of their very good thermal and chemical stabilities along

1. INTRODUCTION Metal phosphonates are a novel type of metal−organic hybrid porous material that exhibit high water, chemical, and thermal stabilities.1,2 The tetrahedral phosphonic acids (O3P−OR or O3P−R) are obtained when −H or −OH of the phosphoric acid (O3P−OH) is replaced by alkyl/aryl (−R) groups, which are reacted with metal ions such as Zr4+, Ti4+, Sn4+, Ce4+, Th4+, and so forth to synthesize M+4-phosphonates.3−5 The flexible coordination sites and strong coordination attachment of the phosphonate ligand (phosphonic acid) can form a variety of metal phosphonates possessing significant structures and promising characteristics. This could depend on the structural type of the phosphonic acid and the synthetic route used, where there are two- or three-dimensional inorgano-organic hybrid polymeric structures in which organic species are bridged via phosphorous atoms to an inorganic two-dimensional matrix.1 The proton exchange/transport can be enhanced through the surface or interlayer region of metal phosphonate compared to that of metal phosphate because of the presence of such organic functional groups acting as the proton source.6 Boilot et al. have reported the synthesis of Zr4+-phosphate and -phosphonate using organic covalent grafting with phosphoric acid and phosphonic acid, respectively. The study shows that Zr4+phosphonate has higher/enhanced proton conductivity compared to Zr4+-phosphate, which is due to the grafting of the P− OH groups.7 Hogarth et al. have also demonstrated that the modification or variation of a number of experimental parameters of the synthetic route, such as the sol−gel method, could be favorable for improving hydration and thus proton conduction.8 From the literature survey, there are many reports on the synthesis and characterization, in which metal phosphonates are © 2019 American Chemical Society

Received: December 11, 2018 Accepted: February 5, 2019 Published: February 19, 2019 3716

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with good proton conduction behavior. Vivani et al.14 have prepared Zr4+-phosphate and Zr4+-phosphonate based on glyphosine using the Moedritzer−Irani synthesis route in the mild condition. The proton conduction behavior was studied as a function of temperature and relative humidity, which shows that the conductivity of ∼10−3 S·cm−1 is essentially because of the surface proton transport.14 Costantino et al.20 have studied proton-transport properties of Zr4+-phosphonate, where the proton conductivity values are found in the range of 10−3 S·cm−1 at different temperatures and humidities.20 Bhaumik et al.5 have synthesized porous Sn4+-phosphonate and explored its utility as a solid acid catalyst for esterification because of the high surface area and crystalline structure of the material. Kitagawa et al.21 have investigated homochiral nanotubular metal phosphonates of Ni2+ and Co2+ using (1-phenylethylamino)methylphosphonate and studied the proton conduction behavior where proton conductivity is observed with a magnitude of ∼10−6 S·cm−1. The authors also have reported proton conductivity in the range of 10−12 S·cm−1 for Co2+phosphonate using (benzylazanediyl)bis(methylene)diphosphonic acid.22 Schröder et al.23 have studied a model “free diffusion inside a sphere” for the proton conduction mechanism of the phosphonate-based metal−organic framework and confirmed proton conduction in Ni2+/Co2+-benzene1,3,5-p-phenylphosphonate. Shimizu et al.24 have the synthesized phosphonate metal−organic framework of inorganicorganic solid Zn2+-1,3,5-benzenetriphosphonate and explored its proton-transport properties through observed conductivity in the region of ∼10−6 S·cm−1. Among lanthanide metal phosphonate, Krautscheid et al.25 have reported the hydrothermal synthesis of La3+-triazolylphosphonate-based hybrid metal−organic framework materials. The results show that the proton conductivity was significantly enhanced with increasing temperature and relative humidity, which could be due to the availability of the charge-carrier concentration and maintaining high charge density, respectively. Shimizu et al.2 also have prepared highly acidic porous La3+phosphonate using 1,2,4,5-tetrakisphosphonomethylbenzene, which shows good water stability and very good proton conductivity (∼10−3 S·cm−1). Cabeza et al.26 have reported the proton conduction behavior of crystalline La3+-tetraphosphonates. Zang et al.27 have prepared Eu3+-phosphonate, which shows good anhydrous proton conductivity (∼10−3 S·cm−1). Demadis et al.28 have synthesized phosphonates of lanthanide metals such as La, Pr, Sm, Eu, Gd, Tb, Dy, Ho, and so forth. The materials exhibit comparable proton conduction values of ∼10−3 S·cm−1, which remain stable for the work conditions used. Paz et al.29 have reported microwave-assisted synthesis of porous Y3+phosphonate. As a result, the material possesses very good thermal stability and excellent proton conduction value (∼10−2 S·cm−1) under ambient conditions. Yuan et al.30 have prepared mesoporous cerium phosphonate nanostructured hybrid material by incorporating the cationic surfactant with cerium and organo-phosphonic functionalities by using the template method, thereby showing very good efficiency to be used as an optical sensor. The discussed reports expose the significance of the metal phosphonate, and the materials could be used as effective solidstate proton conductors by varying the synthesis route as well as various phosphonic acids. The reports also demonstrate the way to investigate novel proton-conducting materials and to understand the conduction mechanism is now a prior demand, the promising use of these materials being in catalysis, fuel cells,

sensors, and so forth. Earlier, we prepared amorphous M4+phosphates and -phosphonates (M = Zr, Sn, and Ti) and studied their proton conduction behavior.6,31−34 Literature survey shows that not much research has focused toward phosphonates of thorium (Th4+) and cerium (Ce4+). In our earlier report,35 we synthesized thorium and cerium phosphates and measured proton conductivity, where fairly good conductivity was observed in the range of ∼10−7 S·cm−1. Therefore, we aim to study thorium and cerium phosphonates containing more protonic sites (P−OH groups) in the organo-phosphonate species, and hence enhanced proton conductivity was expected. HEDP (1-hydroxyethylidene-1,1-diphosphonic acid), 6,34 ATMP (amino tris(methylenephosphonic acid)),6 EDTMP (ethylenediamine tetra(methylenephosphonic acid)), and DETPMP (diethylenetriamine penta(methylenephosphonic acid))6 are phosphonic acids containing structural hydroxyl (−OH) groups of five, six, eight, and ten, respectively (Figure S1). The current study deals with the synthesis, characterization, and proton-transport performance of thorium phosphonates (ThHEDP, ThATMP, ThDETPMP, and ThEDTMP) and cerium phosphonates (CeHEDP, CeATMP, CeDETPMP, and CeEDTMP). On the basis of the obtained results, the specific proton conduction (σ) and activation energy (Ea) have been evaluated and compared among the cerium and thorium phosphonates and with that of other proton-conducting systems of metal-organophosphonates.

2. RESULTS AND DISCUSSION 2.1. Material Characterization. Thorium(IV) phosphonates and cerium(IV) phosphonates were observed as hard granules with off-white and pale yellowish appearance, respectively. As discussed earlier in the text for physicochemical and ion-exchange characteristics, the results for all materials have been presented in Supporting Information Tables S1−S8. Inductively coupled plasma atomic emission spectroscopy (ICP−AES) results for thorium-phosphonate are as follows: for ThHEDP, % Th = 47.18 and % P = 12.40, for ThATMP, % Th = 40.20 and % P = 16.50, for ThDETPMP, % Th = 21.45 and % P = 14.52, and for ThEDTMP, Th = 27.10 and P = 15.36. For CeHEDP, % Ce = 47.18 and % P = 12.40, for CeATMP, % Ce = 40.20 and % P = 16.50, for CeDETPMP, % Ce = 21.45 and % P = 14.52, and for CeEDTMP, Ce = 27.10 and P = 15.36. Energydispersive X-ray spectroscopy (EDX) graphs (Figures S2−S9) are in correlation with the found elemental analysis. From the CHN analysis data, %C = 5.10, % H = 3.20 in ThHEDP, % C = 6.0, % H = 3.56, % N = 2.09 in ThATMP, % C = 10.44, % H = 3.88, % N = 4.06 in ThDETPMP, and % C = 8.48, % H = 2.74, % N = 3.64 in ThEDTMP. For ceriumphosphonates, % C = 7.45, % H = 3.16 in CeHEDP, % C = 7.38, % H = 3.05, % N = 2.48 in CeATMP, % C = 12.91, % H = 4.33, % N = 4.91 in CeDETPMP, and % C = 11.09, % H = 3.59, % N = 4.14 in CeEDTMP. The observed CHN analysis data can be correlated with those of theoretical values. Thermogravimetric analysis (TGA) of thorium phosphonates (Figure 1) and cerium phosphonates (Figure 2) shows two major weight loss regions, where loss of moisture/hydrated water occurs up to ∼120 °C and in the temperature range of 120−180 °C. The first weight loss can be assigned to more than one water molecule. The second weight loss occurred in two steps (except in the case of ThHEDP and ThATMP), namely, in the range 180−350 and 350−500 °C and is due to the condensation of structural hydroxyl (−OH) groups, decom3717

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Figure 1. TGA of thorium phosphonates.

Figure 3. XRD of thorium phosphonates.

Figure 2. TGA of cerium phosphonates.

position of organic moieties, and transformation of the dehydrated compound into metal pyrophosphate because of the collapse of the structure at a high temperature.32 From the ICP−AES, CHN, and TGA data, ThHEDP, ThATMP, ThDETPMP, and ThEDTMP were determined to have the formula as Th(C2H8P2O7)·4H2O, Th(C3H12P3O9N)· 3H2O, Th(C9H28P5O15N3)·8H2O, and Th(C6H20P4O12N2)· 10H2O, respectively. Similarly, CeHEDP, CeATMP, CeDETPMP, and CeEDTMP have the formula as Ce(C2H8P2O7)·5H2O, Ce(C3H12P3O9N)·5H2O, Ce(C9H28P5O15N3)·6H2O, and Ce(C6H20P4O12N2)·3H2O, respectively, where the Alberti and Torracca (1968) formula36 has been used to calculate the H2O molecules. X-ray diffraction (XRD) patterns of thorium and cerium phosphonates (Figures 3 and 4) indicate that the materials are amorphous in nature because of the absence of sharp peaks. The observed peak at ∼28 2θ scale in Figures 3 and 4 shows the presence of a large amount of amorphous carbon material.37 Exceptionally, in the case of CeHEDP, sharp peaks were observed indicating the material to be of crystalline nature. Scanning electron microscopy (SEM) images of the materials (Figures S10−S17) show an irregular morphology. A broad band observed in the region ∼3400 cm−1 for in the Fourier transform infrared spectroscopy (FTIR) spectra of thorium phosphonates (Figure 5) and cerium phosphonates (Figure 6) due to symmetric and asymmetric O−H stretching vibrations of structural −OH groups represent exchangeable/ protonic sites (H+ of the −OH groups), which are referred to as defective P−OH groups.38 The broad band at ∼1040−1100 cm−1 and a sharp medium band at ∼1630−1640 cm−1 are due to

Figure 4. XRD of cerium phosphonates.

aliphatic PO stretching and aquo H−O−H bending, respectively.39 The band at ∼1450−1475 cm−1 is due to overlapped C−H bending of −CH2 groups, P−C stretching vibrations, and the presence of a secondary amine for ATMP and a tertiary amine for DETPMP and EDTMP.40 The protonating ability [Na+-cation exchange capacity (CEC)] has been determined as 3.25, 2.68, 1.74, 1.97, 2.58, 2.60, 3.41, and 2.32 mequiv·g−1 for ThHEDP, ThATMP, ThDETPMP, ThEDTMP, CeHEDP, CeATMP, CeDETPMP, and CeEDTMP, respectively, at room temperature. The calcination study (effect of heating up to 500 °C in the interval of 50 °C) shows that CEC values gradually reduce with heating because of removal of hydrated water and condensation of −OH groups representing the presence of protonic sites (Tables S1− S8).32 The FTIR spectra (Figure S18) of the calcined samples are also in favor of the above observations, which could be highlighted as disappearance or decreasing the intensities of the peaks at ∼3400 and ∼1638 cm−1 responsible for the −OH group.32,39 3718

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Figure 5. FTIR spectra of ThHEDP (a), ThATMP (b), ThDETPMP (c), and ThEDTMP (d).

The order of specific conductance at 30 °C (Table 1) is found to be ThHEDP (5.39 × 10−6 S·cm−1) > ThEDTMP (1.06 × 10−6 S·cm−1) > ThATMP (6.73 × 10−7 S·cm−1) > ThDETPMP (5.86 × 10−7 S·cm−1). Similarly (Table 2), the order of specific conductance at 30 °C for cerium phosphonate is observed to be as follows: CeHEDP (8.30 × 10−7 S·cm−1) > CeEDTMP (4.17 × 10−7 S·cm−1) > CeATMP (3.95 × 10−7 S·cm−1) > CeDETPMP (3.83 × 10−7 S·cm−1). The observations resulting from such effective factors are ease with which a proton can jump to another sites, availability of conducting protonic sites, and complexity in structure.45,46 The proton conductivity of the solid material is also affected by the high density of charge carriers, crystallinity of the material, surface layer of particles of a sample, a number of vacant-accessible sites, coordinated metal ion size, temperature, hydrophobicity, activation energy (Ea), and so forth.45−47 The conductivity variation observed along with a change in temperature allows calculating the activation energy (Ea) from the Arrhenius plots of log(σT) versus 1000/T. The obtained Ea values result from the contribution of many characteristics such as the concentration of mobile ions (protons), a number of vacant sites, good connectivity among the sites, the structure of used phosphonic acid, and the charge-to-size ratio of the coordinated metal ions.48 In the temperature range of 90−120 °C, the Arrhenius fit to the data shows the activation energy (Tables 1 and 2), and linearity is observed in Arrhenius plots in the range of 90−120 °C (Figures 9 and 10). The Grotthuss mechanism has been used to understand the proton transport in solid compounds. The lower values of Ea (0.06−0.20 kcal·mol−1 for cerium phosphonate and 0.3−1.9 kcal·mol−1 for thorium

2.2. Impedance Analysis. The Nyquist plot (imaginary Z″ vs the real Z′ components of the impedance vector Z) consists in a single depressed semicircle, representing the frequency response of the pellet.40−42 In each experiment, the pellet specific conductivity (σ) is derived from the sample impedance (R) (which is getting through extrapolation of the frequency curve crossing to the Z′ axis) and the thickness (d) and surface area (A) of the pellet. During the pellet preparation of the samples, no structural changes occur, suggesting that no irreversible transformations occur during conductivity measurement at different temperatures. Figures 7 and 8 show the complex impedance plots (at 30 °C) for thorium phosphonate and cerium phosphonate, respectively. As a result (Tables 1 and 2), the tendency of specific conductance values (σ) is parallel as observed earlier,6,35 where σ decreases with increasing temperature in each case. These results are well supported by the calcination study and thus the trend of the loss of water of hydration as well as condensation of structural hydroxyl (−OH) groups. The observations indicate that proton conduction occurs through a Grotthuss-type mechanism43 where the conductivity is facilitated by the hydrated water located on the surface and hence protons are able to diffuse through the interlayer space, which makes materials to be proton conductors.44 This is well understood by the calcination studies where loss of protonic sites observed because of condensation of −OH groups which have an effect on proton conductance as conductivity decreases with increasing temperature also favoring to be the conduction is protonic. 3719

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Figure 6. FTIR spectra of CeHEDP (a), CeATMP (b), CeDETPMP (c), and CeEDTMP (d).

Figure 7. Complex impedance plot at 30 °C for (a) ThHEDP, (b) ThATMP, (c) ThDETPMP, and (d) ThEDTMP

phosphonate) show easy proton transportation following the Grotthuss type mechanism, where lower activation energy (Ea)

is observed, which totally corresponds to participation of water (H2O) molecules on the solid surface.44 The result regarding the 3720

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Figure 8. Complex impedance plot at 30 °C for (a) CeHEDP, (b) CeATMP, (c) CeDETPMP, and (d) CeEDTMP

Table 1. Specific Conductance σ (S·cm−1) of ThATMP, ThDETPMP, ThHEDP, and ThEDTMP at Different Temperatures temperature (°C)

ThATMP

ThDETPMP

ThHEDP

ThEDTMP

30 40 50 60 70 80 90 100 110 120 130 Ea (kcal·mol−1)

6.73 × 10−7 6.17 × 10−7 6.08 × 10−7 5.23 × 10−7 5.00 × 10−7 4.20 × 10−7 4.17 × 10−7 4.09 × 10−7 4.05 × 10−7 3.90 × 10−7 3.62 × 10−7 1.90

5.86 × 10−7 4.89 × 10−7 4.62 × 10−7 4.60 × 10−7 4.39 × 10−7 4.30 × 10−7 4.22 × 10−7 4.18 × 10−7 3.94 × 10−7 3.44 × 10−7 1.64 × 10−7 0.30

5.39 × 10−6 5.10 × 10−6 5.03 × 10−6 4.92 × 10−6 3.83 × 10−6 3.05 × 10−6 1.60 × 10−6 1.55 × 10−6 1.22 × 10−6 1.16 × 10−6 1.08 × 10−6 0.68

1.06 × 10−6 5.35 × 10−7 5.35 × 10−7 5.00 × 10−7 4.40 × 10−7 4.23 × 10−7 4.23 × 10−7 3.24 × 10−7 2.82 × 10−7 2.30 × 10−7 1.92 × 10−7 1.21

Table 2. . Specific Conductance σ (S·cm−1) of CeHEDP, CeATMP, CeDETPMP, and CeEDTMP at Different Temperatures temperature (°C)

CeHEDP

CeATMP

CeDETPMP

CeEDTMP

30 40 50 60 70 80 90 100 110 120 130 Ea (kcal·mol−1)

8.30 × 10−7 8.15 × 10−7 7.64 × 10−7 6.44 × 10−7 6.25 × 10−7 4.74 × 10−7 4.55 × 10−7 3.65 × 10−7 3.50 × 10−7 3.48 × 10−7 3.16 × 10−7 0.14

3.95 × 10−7 3.95 × 10−7 3.93 × 10−7 3.92 × 10−7 3.87 × 10−7 3.84 × 10−7 3.77 × 10−7 3.64 × 10−7 3.62 × 10−7 3.61 × 10−7 3.59 × 10−7 0.20

3.83 × 10−7 3.75 × 10−7 3.70 × 10−7 3.61 × 10−7 3.58 × 10−7 3.43 × 10−7 3.37 × 10−7 3.33 × 10−7 3.30 × 10−7 3.30 × 10−7 3.28 × 10−7 0.20

4.17 × 10−7 4.15 × 10−7 4.10 × 10−7 3.90 × 10−7 3.80 × 10−7 3.54 × 10−7 3.45 × 10−7 3.43 × 10−7 3.36 × 10−7 3.30 × 10−7 3.30 × 10−7 0.16

On comparing various phosphonate moieties, the HEDP containing well-spaced five −OH (structural hydroxyl) groups shows low Ea values and high values of proton conductivity. In

low Ea values were also reported earlier by Clearfield and Yaroslavtsev.46,47 3721

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Figure 9. Arrhenius plots (temperature range 90−120 °C) for (a) ThHEDP, (b) ThATMP, (c) ThDETPMP, and (d) ThEDTMP

Figure 10. Arrhenius plots (temperature range 90−120 °C) for (a) CeHEDP, (b) CeATMP, (c) CeDETPMP, and (d) CeEDTMP

the case of ATMP species, it contains six −OH groups compared to HEDP. In the case of DETPMP and EDTMP, both contain more number of −OH groups (10 and 12, respectively), because of which more number of protonic sites (H+ of the −OH) are expected to have higher conductivity values. On the other hand, higher values of Ea in the case of ATMP and EDTMP are

attributed to the steric effect of structures of both the phosphonic acids.6 Comparison was done among the coordinated metal ions (Th4+ and Ce4+) where thorium phosphonates exhibit higher proton conductivity compared to cerium phosphonates. The results could be attributed to higher CEC values giving more exchangeable protons of −OH groups and 3722

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Chemie Pvt. Ltd. Mumbai, Maharashtra, India. HEDP, 50% (∼4.25 M) [density (D) = 1.75 g/cm3 and molecular weight (MW) = 206.02 g (CAS no. 2809-21-4)], ATMP, 50% (∼1.66 M) [D = 1.30 g/cm3 and MW = 299.05 g (CAS no. 6419-19-8)], EDTMP, 24.46% (∼0.72 M) [D = 1.28 g/cm3 and MW = 436 g (CAS no. 1429-50-1)], and DETPMP, 50% (∼1.18 M) [D = 1.35 g/cm3 and MW = 573.2 g (CAS no. 15827-60-8)] were purchased from Hydro Chem India Pvt. Ltd. Vadodara, Gujarat, India. Deionized (DI) water has been used for each experiments. 4.2. Material Synthesis. In the context of material preparation, the synthetic methods have always been proven as effective factors that tune the material characteristics and hence the performance. The sol−gel synthesis method is currently recognized as the most important chemical method because of several advantages such as employment of a low temperature, high homogeneity, stability and flexibility of processing allowing varying reaction parameters.52 To have a material with very good CEC (also called protonating ability) was the prior endeavour where the number of experimental parameters has been optimized through sol−gel synthesis route (the material synthesis given in Supporting Information Tables S9−S16 describes optimization of experimental conditions for materials preparation). 4.3. Characterization. Physicochemical characteristics for the synthesized materials were studied according to literature methods.53,54 The protonating ability/CEC was measured as Na+-CEC in mequiv·g−1 using the earlier reported column method.55 Elemental analysis for thorium, cerium, and phosphorus content was performed by using ICP−AES (Thermo Electron IRIS Intrepid II XSP). The percentage of elements C, H, and N was analyzed using a CHN analyzer (Elementar Vario EL III). Spectral analysis was performed, and FTIR spectra (Shimadzu, model 8400S, using KBr pellet) and XRD (2θ = 10°−80°) patterns (Bruker, model AXS D8, with Cu Kα radiation with a nickel filter) were obtained. TGA was performed on a Shimadzu thermal analyzer (model TGA 50) at a heating rate of 10 °C·min−1. SEM and EDX were obtained on a JEOL scanning electron microscope (model JSM-5610-SLV). 4.4. Proton Conductivity Studies. Impedance data were recorded by performing impedance measurements as described in our earlier published reports6,32,35 (Supporting Information Section S2).

thus more available conducting protons. Higher conductivity values were observed for ThHEDP than CeHEDP followed by CEC values, and a similar trend was observed in the case of ThATMP and CeATMP. The observed conductivity order could be correlated with the acidic character of the metal ions, which is related to ionic radii, charge, and coordinate site (phosphonic acids) of coordinated metal ions (0.97 Å for Ce4+ and 1.05 Å for Th4+). In addition, randomness is observed for ThDETPMP and ThEDTMP and CeDETPMP and CeEDTMP where the proton conductivity data do not follow CEC values. For this explanation, the physicochemical characteristics are effective factors, where cerium phosphonates, CeDETPMP and CeEDTMP, were obtained at a very low yield in a powder form, which may affect pellet formation while thorium phosphonates were obtained with good yields, and hard glassy material was found to be easy to make a pellet. We also have performed chemical and thermal stability studies for each case where cerium phosphonates (CeDETPMP and CeEDTMP) were comparatively not stable among all synthesized materials. Materials are observed to be resistant to acidic medium and the organic solvents studied but not suitable with base medium, and maximum tolerable limits have been presented in the Supporting InformationTables S1−S8. The comparison made with the reported proton conducting system possessing similar organo-phosphonates. The present synthesized materials are less conductive than reported materials (such as zirconium (Zr4+)-6 and titanium (Ti4+)-33phosphonates), may be due to acidic nature of the metal ions employed to obtain the metal4+-phosphonates. The acidic character of the coordinated metal ions is related to ionic radii, charge, and coordinate site (phosphonic acids). Zr4+ (0.86 Å) and Ti4+ (0.74 Å) with smaller ionic radii and hence a high charge density shows higher proton conduction compared to cerium- and thorium-phosphonates (the bigger the cation, the higher the proton conductivity).49,50 The observed specific conductivity trend is ZrHEDP6 (4.13 × 10−4) > TiHEDP33 (1.34 × 10−4) and ThHEDP (5.39 × 10−6) > CeHEDP (8.30 × 10−7). Further, the obtained Ea values for ThHEDP (0.68 kcal· mol−1) and CeHEDP (0.14 kcal·mol−1) are a result of the contribution of such factors as discussed earlier in the text and low compared to ZrHEDP (2.36 kcal·mol−1)6 and TiHEDP (2.87 kcal·mol−1).33 Stenina et al. have reported that high Ea values are because of strong H-bonds existing between the structural −OH groups in the metal-phosphonates, and hence more energy is required for proton transfer from one site to another.51

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03468.

3. CONCLUSIONS The aim was to investigate the proton-transport performance of thorium (Th4+)- and cerium (Ce4+)-phosphonates. In conclusion, considering the comparable proton conductivity and good thermal and chemical stabilities, the synthesized materials exhibit promising characteristics to be used in several applications in the field of materials science. Further, many studies based on the preparation methods of metal phosphonates and alternative synthetic routes would help tune the surface characteristics possessing more exchangeable sites with the unexpected features and hence better proton conduction.

Optimization of experimental parameters for material synthesis, impedance measurement for proton conduction study, physicochemical and ion-exchange properties of the synthesized materials, and instrumental characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tarun F. Parangi: 0000-0002-2367-5372

4. EXPERIMENTAL SECTION 4.1. Chemicals. Thorium nitrate (Th(NO3)4·5H2O) and ceric sulfate (Ce(SO4)2·6H2O) were procured from Loba

Notes

The authors declare no competing financial interest. 3723

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ACKNOWLEDGMENTS The authors acknowledge DAE-BRNS (BARC-Mumbai) for the financial help and research fellowship.

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