Disclosing the Electronic Structure and Optical Properties of Ag4V2O7 crystals: Experimental and Theoretical Insights

Please do not adjust margins a. INCTMN-UFSCar, Universidade Federal de São Carlos, P.O. Box 676, 13565–905, São Carlos, SP, Brazil. b. Departament de Química Física i Analítica, Universitat Jaume I, 12071, Castelló de la Plana, Spain. c. , Brazil d. PPGQ-Universidade Estadual do Piauí, Rua João Cabral, 2231, P.O. Box 381, 64002–150, Teresina, PI, Brazil. e. INCTMN-UNESP, Universidade Estadual Paulista, P.O. Box 355, 14801–907 Araraquara, SP, Brazil *laeciosc@gmail.com; †Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x Received 18th May 2016, Accepted 00th January 20xx

Among the reported silver vanadates, silver pyrovanadate (Ag 4 V 2 O 7 ) has drawn extensive interest in the field of photocatalysis 6,7,10 . Various techniques, such as mechanochemical reactions 22 , oxide mixture or solid-state reaction 2 , precipitation with calcination at long times 2 , and molten metal fluxes 23 have been reported. However, these methods require high temperatures, long processing times, and sophisticated equipment with high maintenance costs. In addition, the final product is obtained with some deleterious phases, such as V 2 O 5 , amorphous AgVO 3 , Ag 3 VO 4 , and Ag 2 O, with inhomogeneous sizes and shapes. To avoid these drawbacks, several synthetic routes have recently been developed and employed in the preparation of Ag 4 V 2 O 7 micro and nanocrystals 10,24 , and pure Ag 4 V 2 O 7 crystals have been obtained through several wet chemistry-based techniques, such as conventional and surfactant-assisted hydrothermal syntheses 5,7,9,25 or microemulsions 26 . These methods circumvent the problems encountered in earlier synthetic methods and facilitate the synthesis of single phase crystals with homogeneous sizes and shapes. In particular, our lab has successfully achieved the preparation of various complex ternary metal oxides, including -Ag 2 WO 4 27 and -Ag 2 WO 4 28 , using facile and readily scalable techniques in environmentally friendly solvents (water) at low processing temperatures.
To the best of our knowledge, the geometric and electronic structures of Ag 4 V 2 O 7 crystals have not been investigated either theoretically or experimentally. This encouraged us to investigate the geometry, cluster coordination, and electronic structure of Ag 4 V 2 O 7 microcrystals. Five years ago, our research groups initiated a major experimental and theoretical collaboration to elucidate the structure, chemical bonding, electronic, and optical properties of numerous ternary metal oxide photocatalysts, bactericides, sensors, photoluminescent complexes, etc., such as -Ag 2  . These studies have made significant advances in the formal understanding of the properties and application of these compounds at a qualitative or semiquantitative level, by combining experimental data and first-principles calculations.
Here, we report for the first time the synthesis of Ag 4 V 2 O 7 microcrystals by a simple precipitation method at low temperature. The synthesized Ag 4 V 2 O 7 crystals were characterized by X-ray diffraction (XRD) and Rietveld refinement, Raman spectroscopy, field emission scanning electron microscopy (FESEM), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and photoluminescence (PL) measurements.
First-principles quantum-mechanical calculations using density functional theory (DFT) were carried out in order to correlate the results of XRD, Raman spectroscopy, FESEM, and the PL measurements forAg 4 There is a need to link the length and complexity scales between the levels of theory used in calculations and the variable space in which experiments take place; the present work can thus be considered an attempt to bridge this gap.

Characterization
X-ray diffraction using a Rigaku-DMax/2500PC (Japan) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 10° to 80° with a scanning rate of 0.02°/min. Micro-Raman spectroscopy was carried out using an T64000 spectrometer (Horiba obin-Yvon, Japan) coupled to a CCD Synapse detector and an argonion laser, operating at 514 nm with maximum power of 7 mW. The spectra were measured in the range from 100 cm -1 to 1100 cm -1 . UV-vis spectra were obtained in a Varian spectrophotometer model Cary 5G (USA) in diffuse reflection mode. The morphologies were investigated with a field emission scanning electron microscopy (FE-SEM) Supra 35-VP Carl Zeiss (Germany) operated in 15 KV. The PL measurements were performed with a Monospec 27 monochromator Thermal Jarrel Ash (USA) coupled to a R446 photomultiplier Hamamatsu Photonics (Japan). A krypton ion laser Coherent Innova 90 K (USA) (λ = 350 nm) was used as excitation source, keeping its maximum output power at 500 mW. All experiments measurements were performed at room temperature.

Theoretical Calculations
Calculations on the periodic Ag 4 V 2 O 7 structure were performed with the CRYSTAL14 sofware package 36 . Tungsten was described by a large-core ECP, derived by Hay and Wadt, and modified by Cora et al. 37 . Silver and oxygen centers were described using HAYWSC-311d31G and O (6-31d1G) basis sets, respectively, which were taken from the Crystal web site 38 . A Range-separated hybrid functional, the screened-Coulomb HSE06 was used in order to give the accurate band gaps for the computed structures. The diagonalization of the Fock matrix was performed at adequate k-points grids in the reciprocal space. The thresholds controlling the accuracy of the calculation of the Coulomb and exchange integrals were set to10 -8 and 10 -14 , and the percent of Fock/Kohn-Sham matrices mixing was set to 40 (IPMIX keyword) 36 . The empirical correction scheme to energy that considers the long-range dispersion contributions proposed by Grimme 39 and implemented by Bucko et al. 40 for periodic systems was used.
In the relaxed configuration, the forces on the atoms are less than 0.0001 hartree/bohr = 0.005 eV/Å, and deviations of the stress tensor from a diagonal hydrostatic form are less than 0.1 GPa. The band structure and the density of states (DOS) projected on atoms and orbitals of bulk Ag 4 V 2 O 7 was constructed along the appropriate high-symmetry directions of the corresponding irreducible Brillouin zone. The vibrationalfrequencies calculation in CRYSTAL is performed at the -point within the harmonic approximation, and the dynamic matrix is computed by the numerical evaluation of the first derivative of analytical atomic gradients. First-principles total-energy calculations on the periodic Ag 4 V 2 O 7 structure were performed within the density functional theory (DFT) along with projector augmented wave (PAW) potentials implemented in the VASP program 41 . The Kohn-Sham equations were solved using the screened hybrid functional proposed by Heyd, Scuseria, and Ernzerhof (HSE) 42 , in which a percentage of exact nonlocal Fock exchange was added to the Perdew, Purke, and Ernzerhof functional (25%), with a screening of 0.2 Bohr −1 applied to the partition of the Coulomb potential into long-range and short-range terms. The plane-wave expansion was truncated at cut-off energy of 400 eV and the Brillouin zones were sampled through Monkhorst-Pack special k-points grids to ensure geometrical and energetic convergence. Conjugate gradient algorithms were used for Please do not adjust margins Please do not adjust margins unit-cell relaxations and atomic positions, until the residual forces and stress in the equilibrium geometry were of the order of 0.005 eV Å -1 and 0.01 GPa, respectively. The band structure and the density of states (DOS) projected on the atoms and orbitals of bulk Ag 4 V 2 O 7 were constructed along the appropriate high-symmetry directions of the corresponding irreducible Brillouin zone. Vibrational-frequency calculations were performed at the -point within the harmonic approximation, and the dynamic matrix was computed by a numerical evaluation of the first derivative of the analytical atomic gradients.  23 . According to the literature 23 , pure Ag 4 V 2 O 7 microcrystals present a space group (Pbca),a pointgroup symmetry ( ), and sixteen molecular formula units per unit cell (Z = 16). In order to confirm this result, a structural refinement by means of the Rietveld method 43 , based on the construction of diffraction patterns calculated according to a structural model 44 , was performed using the general structure analysis (GSAS) program 45 .

XRD pattern and Rietveld refinement analysis
The calculated patterns were adjusted to fit the observed patterns and thus provide the structural parameters of the material and the diffraction profile. In this work, the Rietveld method was applied to adjust the atomic positions, lattice parameters, and unit cell volume. Figure 1(b) shows that the structural refinement results are mostly consistent with the ICSD No. 38065 reported by Masse et al. 23 . However, the lowangle region, where the most intense peaks are located, reveals a major difference related to narrow peaks and high intensities in the pattern. The quality of a structural refinement is generally examined using R-values (R wp , R Bragg , R p , χ 2 , and S).These values were determined for our crystals and found to be consistent with an orthorhombic structure. However, the experimentally observed XRD patterns and theoretically calculated data display small differences near zero on the intensity scale, as illustrated by the line Y Obs -Y Calc . More details regarding the Rietveld refinement results are displayed in Table S1. The Rietveld refinement plot for the Ag 4 V 2 O 7 microcrystals is shown in Fig. 1(b).

Unit cell representation and symmetry, geometry, and coordination of the clusters in Ag 4 V 2 O 7 crystals
Figures 2(a) and 2(b) show a schematic representation of the orthorhombic Ag 4 V 2 O 7 unit cell in which different clusters, i.e. the local coordination of V and Ag atoms, are depicted. The symmetry, geometry, and coordination data for each cluster, as well as the lattice parameters and atomic positions calculated by geometry optimization, are listed, while those obtained from the Rietveld refinement data are presented in Table S1.  Fig. 2(a) The lattice parameters, unit-cell volume, and internal atomic coordinates for Ag 4 V 2 O 7 from the Rietveld refinement and DFT calculations are reported in Table S1  In order to interpret these data, we presume that these disagreements in the atomic positions of O atoms are attributed to the nature of the starting sample (polycrystalline powder or single crystal). Although single-crystal data are essential for the refinement of crystal-structure models, we believe that the formation of different distortions in the A better understanding of the properties of this material requires more detailed information on local structures and cell parameters. In other words, if there is partial order in the distribution of Ag and V atoms, crystal cells observed at the local level should be different from those determined by XRD measurements. Raman spectroscopy probes the full vibrational spectrum of interest since it is sensitive to shortrange structural order, i.e., the local coordination at both Ag and V centers.

Micro-Raman spectroscopy analysis
Raman spectroscopy can be employed as a probe to investigate the degree of structural order-disorder in materials at short-range; in our case, the local coordination of both Ag and V atoms associated to different clusters, [AgO y ] (y = 5 and 6) and [VO z ] (z = 4 and 5), as the building blocks of Ag 4 V 2 O 7 crystals. To the best of our knowledge, the Raman characteristics of Ag 4 V 2 O 7 have not been previously reported, either theoretically or experimentally. Consequently, we extended our systematic structural investigation toward Raman spectroscopy and the theoretical analysis of the vibrational modes of Ag 4 V 2 O 7 .The micro-Raman spectrum of Ag 4 V 2 O 7 microcrystals is shown in Fig. 3. is notable that the Raman spectrum of the synthesized crystals exhibits broad vibrational modes, indicating short-range structural disorder. This characteristic can be related to very rapid kinetics under the synthetic conditions and to intrinsic structural disorder in the lattice, as demonstrated by the Rietveld refinement and first-principles calculations.
Therefore, A g , and B g are Raman-active modes, and A u , B 1u , B 2u and B 3u are active vibrational modes in the infrared spectrum. The A and B modes are nondegenerate; subscripts '' g " and " u '' indicate the parity under inversion in centrosymmetric Ag 4 V 2 O 7 crystals.
Analysis of the micro-Raman spectrum presented in Fig.3  A comparison of the experimental Raman active modes with the closest theoretically calculated ones is presented in Table S3.
A good agreement between the experimental and calculated Raman-active modes is observed, and these results allow us to confirm the 3D hexagon-like structure of the Ag 4 V 2 O 7 microcrystals obtained in this work.
FE-SEM images of the microcrystals are depicted in Fig.  4(a-c). Fig. 4(a) shows a large quantity of small Ag 4 V 2 O 7 microcrystals with a well-defined 3D hexagon-like morphology. Moreover, some of these hexagons have surface defects and are formed by small nanocrystals through a self-assembly process due to environment during the synthetic process. The microcrystals have an average size of approximately 2.7 μm width with a thickness of about 1 μm, and are formed through the aggregation of several nanocrystals and plates with an average size of approximately 375 nm, as can be seen in Fig.  S2(a-d)(SI). These microcrystals have only 8 faces in the first few minutes of the reaction; however, after 10 min of the reaction at 30 °C, fast growth occurs to afford 3D hexagon-like Ag 4 V 2 O 7 microcrystals with 14 faces, as shown in Fig. S2(e) (SI). In Fig. 4(b), a series of Ag 4 V 2 O 7 microcrystals with a more defined shape can be observed due to thermodynamic processes and the chemical synthesis method employed. We also note that these crystals display a very regular shape and size, as is evident from Fig. 4(c) and the computationally simulated crystal shape shown in the inset.  Fig. 4(d)).
When the relative stability of the facets changes (increases or decreases), more than one facet type appears in the resulting morphology, resulting in morphology variations. A3D hexagon-like Ag 4 V 2 O 7 morphology with 14 faces is obtained if the surface energies of (110) and (010) increase to 0.60 and 0.68 J/m 2 , respectively, and the surface energy of (100) decreases to 0.30 J/m 2 (see Fig. 4(e)). However, a morphology having only 8 faces is produced when the surface energy of (100) decreases to 0.05 J/m 2 and that of (011) and (001)   As seen in Fig. 5(a), an E gap value of 2.45 eV was obtained for our 3D hexagon-like Ag 4 V 2 O 7 microcrystals, as calculated by extrapolating the linear portion of the UV-vis curve and Fig.  5(c) In principle, we believe that this behavior is related to presence of intermediary energy levels between the valence band (VB) and the conduction band (CB), since the exponential optical absorption edge and E gap are controlled by the degree of structural order-disorder in the lattice. Calculations yield a direct band gap value of 2.86 eV from Γ to the Γ points in the Brillouin zone and, for a simplified description, this difference can be mainly attributed to the distortions of both tetrahedral/trigonal bipyramidal [VO z ] (z = 4 and 5) clusters and trigonal bipyramidal/octahedral [AgO y ] (y= 6 and 5) clusters at short-and medium-range, and is present in Fig 5  (b).
At this point, it is important to note that determination of the structural order-disorder in a crystalline solid plays a crucial role in the understanding of the relation between its physical properties and its electronic structure, and advanced methodologies allow nowadays for the precise control of the composition and properties of nanomaterials 55,56 . For a given material, structural disorder can present useful properties, such as ferroelectricity, piezoelectricity, and nonlinear optical behavior 57-60 . Therefore, it is important to create disorder in order to obtain new materials with unique physical properties that would be otherwise inaccessible in well-ordered crystal structures 61 . In this context, PL properties are environmentsensitive and significantly affected by the structural orderdisorder degree that accompany changes in crystal size and morphology during the synthetic process.
The PL spectrum at room temperature of Ag 4 V 2 O 7 microcrystals is shown in Fig. 6. The PL spectrum exhibits a typical broad band profile, which can be associated with multiphonon or multilevel processes, i.e., a solid system where relaxation occurs by several pathways that involve the participation of numerous energy states within the band gap. The PL spectrum covers a broad range of wavelengths, from 355 to 600 nm, centered at Please do not adjust margins Please do not adjust margins 450 nm in the blue region of the visible spectra for the 3D hexagon-like Ag 4 V 2 O 7 microcrystals (see Fig. 6).
The photoluminescence emission of vanadate-based compounds has been associated with charge transfer transitions from the oxygen ligands O 2- 6 ]...) clusters (see Fig. 2(b)). Therefore, we can assume that the distorted tetrahedral [VO 4 ] clusters and distorted trigonal bipyramidal [VO 5 ] clusters are the main responsible through electronic transitions between the VB and CB.
As there is an interconnection between the distorted trigonal bipyramidal [AgO 5 ] clusters and the distorted octahedral [AgO 6 ] clusters in the orthorhombic lattice, it is possible to conclude that any distortion caused on the [VO 4 ] and [VO 5 ] clusters also promotes a slight deformation of the O-Ag-O bonds that form part of the chain. The association of different vanadium and silver clusters yields a wide range of polarization charges with the formation of electron-hole pairs between clusters. In instances involving electronic conduction properties, these clusters are able to present cluster-to-cluster charge transfer (CCCT) from vanadium to silver (or vice-versa) by means of excitations involving electronic transitions. This CCCT mechanism induces the formation of different energy levels within the forbidden band gap (structural orderdisorder effect). Particularly, this phenomenon has its origin during the crystal formation and organization stages, which are directly dependent on interactions between tetrahedral/trigonal bipyramidal [VO z ] (z= 4 and 5) clusters and trigonal bipyramidal/octahedral [AgO y ] (y= 6 and 5) clusters. Therefore, these structural defects promote a symmetry break, causing polarization of the structure by electronic charge transfer from ordered (o) to disordered (d) clusters (formation of electron-hole pairs). The corresponding equations are presented in Support Information, S4.
The PL bands arise from photogenerated electron-hole pair processes and the electronic transition between the VB (2p levels of O atoms and 4d levels of Ag atoms) and the CB (3d levels of V atoms). Breaking symmetry processes in these clusters with distortions and tilts create a huge number of different structures and subsequently different material properties related to local (short), intermediate, and longrange structural order-disorder.

Conclusions
In summary, new research on the synthesis of Ag 4 V 2 O 7 crystals with novel properties has attracted great attention because of the variety of their potential applications. This study not only provides new information on the geometry, cluster coordination, and electronic structure of Ag 4 V 2 O 7 microcrystals, but also illustrates the potential of combining experimental techniques and first-principles DFT calculations.
This combination led us to develop a systematic procedure to study the structure and electronic DOS of Ag 4 V 2 O 7 . We found that it exhibits an orthorhombic structure, formed by two types of clusters of V atoms, [VO 4 ] and [VO 5 ], and two types of clusters of Ag atoms, [AgO 5 ] and [ AgO 6 ]. These clusters act as building blocks for the Ag 4 V 2 O 7 structure. Features in the Raman spectra were identified through comparison with calculated vibrational frequencies, and this confirmed the predicted structure of this material. The UV-vis spectrum indicated that Ag 4 V 2 O 7 microcrystals have a direct band gap with a value of E gap = 2.45 eV. The present work provides a new direction toward the design of this crystalline material and the search for practical applications in, for example, biology, catalysis, and photoluminescent materials. This knowledge may help in developing effective processing routines to enhance the performance of bulk heterojunction solar cells. With the combined insight provided by multiscale simulations and experiments, it may be possible to develop effective tempering routes to fine-tune the electronic structure of organic semiconductor materials. This would allow developing nanostructure arrays with great potential in technological applications such as optical sensors and photoelectronic materials.