Uncovering Metastable α-Ag2MoO4 Phase Under Ambient Conditions

May 4, 2015 - Formation of Ag nanoparticles on metastable β-Ag 2 WO 4 microcrystals induced by electron irradiation. Roman Alvarez Roca , Pablo S. Le...
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Uncovering Metastable α‑Ag2MoO4 Phase Under Ambient Conditions. Overcoming High Pressures by 2,3-Bis(2-pyridyl)pyrazine Doping Choon Hwee Bernard Ng and Wai Yip Fan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 S Supporting Information *

ABSTRACT: The study of metastable phases has been elusive due to their propensity to convert to the stable phase. In this work, 3-bis(2-pyridyl)pyrazine (dpp)-doping was successfully used to relieve the high pressures required for the formation of metastable α-Ag2MoO4. α-Ag2MoO4 spheres were prepared via solution-phase precipitation under ambient conditions with the addition of dpp. While the properties of β-Ag2MoO4 have been well-studied, the difficulty in preparing the α-phase has limited its investigations to theoretical calculations. The overdue synthesis of α-Ag2MoO4 allowed empirical snapshots of its electronic and optical properties. The spheres show absorption in the visible/nearIR regions, and the band gap was determined to be 1.26 eV. Unlike β-Ag2MoO4, which is known to decay into Ag filaments on electron irradiation, the α-Ag2MoO4 spheres maintain their structural and electronic integrity. The inclusion of dpp into the crystal lattice is used to explain the experimental observations.

1. INTRODUCTION There has been a considerable library of research on silver molybdate (Ag2MoO4) due to its diverse applications in hightemperature lubrication,1,2 gas-sensing,3 and as ion-conducting glasses4 and surface-enhanced Raman scattering substrates.5 Early methods of preparing Ag2MoO4 such as solid-state reaction of the oxide mixture,6,7 melt-quenching,8 and Czochralski growth9 were plagued by high temperature requirements, long reaction times, sophisticated equipment, and polydispersity in shapes and sizes of the products. In recent years, Ag2MoO4 nano- and microstructures with an assortment of morphologies have been prepared using more facile and efficient approaches such as solution phase precipitation,10,11 conventional hydrothermal12,13 and microwave-assisted hydrothermal methods.14,15 Ag2MoO4 exists in two forms: α-Ag2MoO4 with a tetragonal K2NiF4-type structure and β-Ag2MoO4with a spinel-type cubic structure.16−19 β-Ag2MoO4 is the thermodynamically stable phase, while α-Ag2MoO4 has been reported to exist only at high pressures.17,19 Not surprisingly, the plethora of syntheses reported above yield exclusively the cubic β-phase. While the properties of β-Ag2MoO4 have been well-studied, the inability to prepare the metastable α-phase has limited its investigations to theoretical calculations.19 Metastable phases of materials have been reported to exhibit different physical and chemical properties from their thermodynamically stable counterparts.20,21 The prospect of novel properties and unique applications has fueled research in the preparation of materials of metastable phases. For instance, metastable calcium carbonate (vaterite and aragonite) has been studied for its applications as functional materials.22−24 Because © XXXX American Chemical Society

of the propensity of the metastable phase to transform into its more stable phases, the preparation of metastable materials remains a challenge.25,26 In this work, the addition of 2,3-bis(2-pyridyl)pyrazine (dpp) in the solution-phase precipitation of Ag2MoO4 from AgNO3 and Na2MoO4·2H2O under ambient conditions resulted in the unexpected quantitative formation of metastable tetragonal αAg2MoO4 (see Figure S1, Supporting Information for the structure of dpp). To the best of our knowledge, this is the first time metastable α-Ag2MoO4 has been prepared under ambient conditions. Characterization of the sub-micrometer spheres revealed unique optical and electronic properties. The influence of dpp on room temperature stabilization of the metastable phase was also investigated.

2. EXPERIMENTAL SECTION 2.1. Synthesis of α-Ag2MoO4 Microspheres. The reagent-grade chemicals were obtained from Sigma-Aldrich and used without further purification. α-Ag2MoO4 microspheres were synthesized by a solution phase method. In a typical synthesis, 4 mL of aqueous silver nitrate (AgNO3, 0.04 mmol) was mixed with 4 mL of ethanolic 2,3-bis(2pyridyl)pyrazine (dpp, 0.03 mmol) under stirring (300 rpm) in a 20 mL sample vial. After 5 min of stirring, 4 mL of aqueous sodium molybdate dehydrate (Na2MoO4·2H2O, 0.02 mmol) was added, and the mixture was allowed to stir for an additional 60 min to ensure complete reaction. The yellow precipitate was obtained via centrifugation (5000 rpm, 30 min) and washed three times with Received: April 2, 2015 Revised: April 30, 2015

A

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Figure 1. XRD spectra of Ag2MoO4 prepared with different concentrations of dpp where R = [dpp]/[Na2MoO4·2H2O].

obtained as the exclusive product. For R ≥ 2.0, precipitation of Ag2MoO4 does not occur. The sizes and morphologies of the products were characterized by scanning electron microscopy (SEM). As shown in Figure 2A,B, the products obtained in the absence of dpp exhibit an irregular faceted morphology with sizes of 2−4 μm. With the introduction of dpp in the reaction, the products show greater monodispersity for its size and morphology. For R = 0.15, the particles exhibit a bud-like morphology with dimensions of 500−550 nm (Figure 2C,D). The products obtained for R = 0.75 consist of two main types: the bud-like particles as well spherical aggregates. The sizes of the particles range from 550−650 nm (Figure 2E,F). As shown in Figure 2G,H, uniform spheres with diameters of 900−980 nm were obtained on increasing R to 1.5. Under high magnifications, it can be observed that these spheres have a hierarchical structure assembled from ∼10 nm particles (Figure S2, Supporting Information). The α-Ag2MoO4 spheres (R = 1.5) were further characterized by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). As shown in Figure 3A, the particles exhibit a spherical morphology with a porous structure. The selected area electron diffraction (SAED) pattern obtained for a single particle consists of discontinuous rings which suggest a polycrystalline microstructure (Figure 3B). The polycrystalline nature of the spheres can be established from HRTEM images where numerous domains with differently oriented lattice fringes can be observed. Sets of fringes with interplanar spacings of 3.87, 3.02, and 2.71 Å can be determined, which correspond to the {210}, {004} and {310} planes of α-Ag2MoO4 (JCPDS 00-021-1340) (Figure 3C,D). Energy dispersive X-ray (EDX) measurements of the spheres confirm the presence of Ag, Mo, and O and shows a Ag:Mo atomic ratio of about 2:1 (Figure S3, Supporting Information). During TEM imaging, the β-Ag2MoO4 particles were observed to interact with the electron beam, causing degradation of the structure and the appearance of Ag filaments (Figure S4, Supporting Information). Such beam effects have been reported for β-Ag2MoO427 as well as other Ag containing

water. The purified product was then redispersed in 10 mL of deionized water for characterization. 2.2. Controlled Experiments. Experiments investigating the effect of the concentration of dpp on the products were conducted as above with a change in the concentration of the dpp to the stated amounts (i.e., R = [dpp]/[Na2MoO4·2H2O]). Experiments investigating the effect of the organic dopant on the products were conducted as above with dpp being replaced by 0.03 mmol of pyrazine, 2,2′bipyridyl or poly(vinylpyrrolidone) (PVP, Mw ≈ 55000). 2.3. Instrumentation. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the JEOL 3010 transmission electron microscope operating at 300 kV. Samples for TEM were prepared by drop-casting 5 μL of the aqueous dispersion onto Formvar-coated copper grids (150 mesh), followed by drying in air. Scanning electron microscopy (SEM) was performed on the JEOL JSM-6701F scanning electron microscope operating at 5 kV. Samples for SEM were prepared by drop-casting 5 μL of the aqueous dispersion onto silica substrates, followed by drying in air. Powder Xray diffraction (XRD) studies were performed using the Bruker D5005 diffractometer (Cu Kα λ = 0.15418 nm). Concentrated aqueous dispersions of the samples were dropcast onto glass slides and dried in air. Ultraviolet−visible spectroscopy was performed on the Shimadzu UV1660 spectrometer, using aqueous samples in 1 cm width quartz cuvettes. Fourier transform infrared (FTIR) spectra were recorded on a Varian Excalibur 3100 spectrometer with KBr pellets.

3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) spectra of the products formed with different concentrations of dpp where R = [dpp]/[Na2MoO4·2H2O]. In the absence of dpp (R = 0), the product obtained gave reflections that can be indexed to the thermodynamically stable β-Ag2MoO4 (JCPDS 00-008-0473). The intense and well-defined diffraction peaks indicate a good degree of structural order at long-range. With the addition of dpp, the peaks arising from β-Ag2MoO4 decreased in intensity, and new peaks appeared that can be matched to the reflections of α-Ag2MoO4 (JCPDS 00-008-0473). The general decrease in peak intensities indicates loss of crystallinity which suggests the formation of defects on the introduction of dpp. Increasing the amount of dpp led to an increase in intensities of the αAg2MoO4 reflections and a corresponding diminishing of the βAg2MoO4 reflections. At R = 1.5, the metastable α-Ag2MoO4 is B

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Figure 3. (A) TEM image of α-Ag2MoO4 spheres. (B) SAED pattern obtained from a single sphere. (C, D) HRTEM images of a single sphere showing the polycrystalline microstructure.

Figure 2. SEM image of as-prepared products for (A, B) R = 0, (C, D) R = 0.15, (E, F) R = 0.75, and (G, H) R = 1.5 where R = [dpp]/ [Na2MoO4·2H2O].

compounds such as α-Ag2WO428 and β-AgI.29 Interestingly, the α-Ag2MoO4 spheres were observed to show immunity against the electron beam. The structural and chemical integrity of the spheres were maintained even on high magnifications which permit the imaging of the α-Ag2MoO4 lattice fringes. The optical properties of the washed products were studied using UV−visible spectroscopy (Figure 4). For the β-Ag2MoO4 particles (R = 0), absorption occurs only in the UV region. The peak at 208 nm and shoulder around 232 nm have been reported for MoO42− ions.30 When dpp was introduced into the synthesis (R = 0.15−1.5), absorption bands appear in the visible and near-IR regions. The absorption maxima undergo a red shift from 500 to ∼620 nm with an increase in the concentration of dpp used. At R = 1.5, a broad absorption that extends into the near-IR region is observed. Also, for Ag2MoO4 particles prepared with dpp, their UV spectra showed additional features in the region of ∼300 nm which could be assigned to the π → π* transitions for dpp.31 The band structures of the products were further explored using the classical Tauc approach.32 The optical band gap and absorption coefficient of semiconductor oxides can be calculated by the following equation:

Figure 4. UV spectra of the products prepared at R = 0, 0.15, 0.75, and 1.5 where R = [dpp]/[Na2MoO4·2H2O].

where α is the absorption coefficient, hν is the photon energy, C is a constant, Eg is the optical band gap, and n is a constant associated with different types of electronic transitions (n = 1/2 for direct allowed, n = 2 for indirect allowed, n = 1.5 for direct forbidden, and n = 3 for indirect forbidden). On the basis of theoretical calculations, both α- and β-Ag2MoO4 crystals exhibit optical absorptions governed by indirect and allowed electronic transitions.15,19 Hence, the indirect band gap energies of the products can be estimated from a plot of (αhν)1/2 against hν, where Eg is the intercept of the tangent of the linear region to the abscissa (Figure 5). The band gap of the β-Ag2MoO4 particles (R = 0) is determined to be 3.89 eV. The value is marginally lower than the theoretical calculated band gap for bulk β-Ag2MoO4 (4.19

αhν = C(hν − Eg )n C

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dpp absorption features are observed for products obtained at R > 0 (Figure 4). The result was corroborated by Fourier transform infrared (FTIR) spectroscopic analysis (Figure 6).

Figure 5. Tauc plots for the determination of optical band gap for the products prepared at R = 0, 0.15, 0.75, and 1.5 where R = [dpp]/ [Na2MoO4·2H2O].

eV).19 The band gap energy decreased to 3.44 eV on the addition of dpp, and an increase in the amount of dpp used led to a further reduction to 3.12 eV for R = 0.75. When R = 1.5, a distinct linear region appears which corresponds to a much reduced band gap of 1.26 eV. The series of experimental observations appear to be in good agreement with the inclusion of dpp into the Ag2MoO4 lattice during precipitation. When AgNO3 is initially mixed with dpp, Ag+ can form a complex with dpp through the formation of coordinate bonds from the lone pairs on the N atoms to the electron deficient Ag center (1). When a sufficiently high concentration of dpp is used, the precipitation of Ag2MoO4 is inhibited due to the low concentration of free Ag+ as observed in our experiments. In the presence of intermediate concentrations of dpp, precipitation occurs (2). The removal of free Ag+ in solution promotes the dissociation of the Ag+dpp complex and drives the precipitation reaction. Ag + + x dpp ⇌ [Ag(dpp)x ]+

(1)

2Ag + + MoO4 2 −Ag 2MoO4 ↓

(2)

Figure 6. FTIR spectra of dpp and the washed products prepared at R = 0 and 1.5 where R = [dpp]/[Na2MoO4·2H2O].

The purified R = 1.5 sample showed weak absorptions at 1384.4 and 1033.7 cm−1 corresponding to that of dpp, which are not observed for the R = 0 sample. Both UV−visible and FTIR characterization indicate the presence of dpp in the products which support the possibility of dpp being trapped within the Ag2MoO4 lattice. The formation of Ag filaments on β-Ag2MoO4 on electron beam irradiation has been investigated by Andrés et al.27 On the basis of theoretical results, it is suggested that the electrons imposed on the material is transferred between [AgO6] and [MoO4] clusters in the spinel structure through the lattice network, and Ag nucleation occurs primarily from the reduction of the Ag centers in the [AgO6] clusters (Figure 7A). In a similar study conducted by the group, it was found that for the [AgOx] (x = 2, 4, 6, 7) clusters in α-Ag2WO4, the [AgO2] and [AgO4] are preferentially reduced to form the Ag filaments.28 In our case, α-Ag2MoO4 crystallizes in a tetragonal K2NiF4type structure which consists of octahedrally coordinated Mo clusters and Ag centers in 9-fold coordination with O, i.e., [AgO9] (Figure 7B).19 Hence, the structural and electronic immunity of α-Ag2MoO4 spheres against the electron beam could be due to the greater shrouding of the Ag centers by negatively charged O atoms, which affords an electrostatic shield preventing the incident electrons from interacting with

We attempted to monitor the growth of the α-Ag2MoO4 spheres by sampling the reaction mixture for R = 1.5 at different times. The aliquots were dropcast onto silica substrates and dried rapidly under reduced pressure. As shown in Figure S5, Supporting Information, the spheres were formed in less than 30 s, which indicates that the precipitation had occurred rapidly. Under the influence of the rapid precipitation process as well as electrostatic interactions with Ag+, it is possible for dpp to be trapped within the Ag2MoO4 lattice during crystal formation. The inclusion of dpp in the lattice gives rise to defects which disrupt the long-range order resulting in the loss of crystallinity as suggested by the weak reflections in the XRD spectra (Figure 1). The presence of residual dpp in the thoroughly washed dppdoped samples is suggested from the UV−visible spectra where D

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Figure 7. Representation of unit cells: (A) cubic spinel (β-Ag2MoO4) and (B) K2NiF4-type structure (α-Ag2MoO4). Orange lines depict the coordination of Ag centers to O in the lattice.

Figure 8. XRD and SEM images of the as-prepared products for controlled experiments with (A) pyrazine, (B) 2,2′-bipyridyl, and (C) polyvinylpyrrolidone (PVP) as the dopant. XRD reflections are matched to β-Ag2MoO4 (JCPDS 00-008-0473).

the Ag centers. Another possible contribution arises from the disruption of the electron transfer through the lattice by dpp inclusion. The decrease in the band gap energies of the products on addition of dpp as well as on increasing the amount of dpp used can be attributed to the inclusion of dpp into the Ag2MoO4 lattice. The disruption of the lattice gives rise to defect states which leads to the appearance of energy tails in the valence band maxima and the conduction band minima, and hence a narrowing of the band gap, i.e., Urbach tail.33 This narrowing of the band gap results in the appearance of absorptions in the region of 500−650 nm observed for the Ag2MoO4 particles prepared with dpp. Increasing the concentration of dpp results in the formation of more defect states which leads to increased tailing and hence smaller band gaps, i.e., 3.89 eV for R = 0 to 3.10 eV for R = 1.5. At R = 1.5, the appearance of a distinct linear region in the Tauc plot corresponding to a much reduced energy gap of 1.26 eV is likely to be due to the quantitative formation of αAg2MoO4. This is consistent with theoretical studies on the electronic structure of Ag2MoO4 polymorphs which established a significantly smaller band gap for α-Ag2MoO4 (0.62 eV) compared to β-Ag2MoO4 (4.19 eV).19

The incorporation of dopants into the lattice of a crystal has been reported to yield metastable phases for several compounds.34,35 The manifestation of the metastable phase is attributed to the dopant effectively disrupting the transformation of the metastable phase to the stable phase. In our case, the inclusion of dpp at interstices or grain boundaries could have restricted atomic arrangement involved in phase transformation which allows for the metastable α-Ag2MoO4 to be formed under ambient conditions. The stark difference between the morphologies and microstructures of α-Ag2MoO4 (hierarchical and polycrystalline) and β-Ag2MoO4 (faceted and single-crystalline) suggests another possible stabilization mechanism. Previous studies have revealed that in the nanometer size regime, an excess of energy associated with increased surface area can cause surface reconstruction to occur leading to the appearance of metastable phases (surface energy theory).18 In our case, the addition of dpp could hinder the atomic growth of Ag2MoO4 crystallites through surface passivation. Particle growth then takes place via aggregation of small crystallites into hierarchical structures. As shown in Figure 3C,D, the α-Ag2MoO4 spheres are composed of numerous domains of less than 10 nm. Because of the excess energy associated with the polycrystalline microstructure as well E

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as the porous morphology, the metastable α-Ag2MoO4 could have been preferred. Preliminary experiments were carried out to investigate the influence of dpp on the manifestation of the metastable phase, in which dpp was replaced with other organic dopants: pyrazine, 2,2′-bipyridyl and polyvinylpyrrolidone (PVP) (Figure S1). The dopants used are chemically similar to dpp and hence are expected to have similar effectiveness in passivating the Ag2MoO4 crystallites. However, from XRD analyses, the products formed with all three dopants yielded the stable cubic β-Ag2MoO4 (Figure 8). SEM images of the products also show faceted morphologies similar to that formed in the absence of dopants. These observations appear to favor the mechanism in which the metastable phase is stabilized by the inclusion of dpp into the lattice. The results also suggest that the inclusion of the organic dopant into the Ag2MoO4 lattice depends critically on the size of the dopant. In the controlled syntheses, the dopants were either too big or too small to be embedded into the lattice. More detailed experiments are expected to shed light on the exact mechanism for the stabilization of the metastable phase by dpp.

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4. CONCLUSION Metastable α-Ag2MoO4 spheres were prepared for the first time under ambient conditions using dpp as a dopant. Unlike βAg2MoO4 which has a large band gap (∼4 eV) permitting the absorption of only a small fraction (5%) of solar energy, the asprepared α-Ag2MoO4 spheres boasts a band gap of 1.26 eV, showing good promise as photocatalysts. The availability of a facile method to prepare α-Ag2MoO4 is expected to catalyze further empirical studies on the elusive and little-studied αphase.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

High magnification SEM image and EDS spectrum of αAg2MoO4 spheres, TEM images showing the effect of electron irradiation on β-Ag2MoO4 microcrystals, SEM image of the product for R = 1.5 after 30 s. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00455.

Corresponding Author

*E-mail: [email protected]. Fax: 6567791691. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by a National University of Singapore research grant (Grant No. 143-000-553-112). C.H.B.N. thanks NUS for a PGF research scholarship.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00455 Cryst. Growth Des. XXXX, XXX, XXX−XXX