Towards a relationship between photoluminescence emissions and photocatalytic activity of Ag 2 SeO 4 : Combining experimental data and theoretical insights

A systematic theoretical and experimental study was carried out to find a relationship between photoluminescence emissions and photocatalytic activity of Ag 2 SeO 4 obtained by different synthesis methods (sonochemistry, ultrasonic probe, coprecipitation and microwave assisted hydrothermal synthesis). Experimental characterization techniques (XRD with Rietveld refinement, Raman, FTIR, UV-vis, XPS and photoluminescence spectroscopies) were performed to elucidate its structural order at short, medium, and long ranges. Morphological analysis evaluated by FE-SEM showed distinct morphologies due to the different methods of synthesis. Based on density functional theory (DFT) calculations it was possible to study in detail the Ag 2 SeO 4 surface properties, including its surface energy, geometry, and electronic structure for the (100),


Introduction
In the scientific community, the family of selenium-based materials stands out due to the existence of a great variety of compounds.][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Divalent metallic selenates and selenites with the general Experimental Section Synthesis Ag2SeO4 samples were synthesized by the SC, UP, CP, and MH methods.Silver nitrate (AgNO3, 99.0%) and sodium selenate (NaSeO4, BioXtra -GHS06, GHS08, GHS09) were purchased from Sigma-Aldrich.In a typical procedure, stoichiometric amounts of Ag + and SeO4 2-solutions were prepared by separately dissolving AgNO3 and Na2SeO4 in 50 mL of distilled water.Afterwards, both solutions were mixed to form a suspension.In the SC methodology, the suspension was ultrasonicated for 1 h at room temperature in a Branson (model 1510) ultrasonic cleaner, and the crystals were collected after turning off the ultrasonic equipment.In the UP methodology, an ultrasonic probe sonicator (Sonics, GEX 750) was used by inserting the probe into the suspension and maintaining it there for 1 h at room temperature.During CP, the suspension was maintained under stirring at 90 °C for 1 h, and the precipitated was collected after interrupting the stirring.In the MH method, the suspension was transferred to the MH system and maintained at 140 °C for 1 h.Subsequently, all samples were naturally cooled down to room temperature, and the precipitates were separated by centrifugation and washed with deionized water to remove any remaining ions.Finally, the crystals were collected and dried in a conventional an oven at 60 °C for 12 h.

Characterization
The crystals were structurally characterized by X-ray diffraction (XRD) using a D/Max-2000PC Rigaku (Japan) diffractometer with Cu Kα radiation (λ = 1.5406Å) in the 2θ range from 15° to 60° at a scanning speed of 2°/min in the normal routine, and from 5° to 110° at a scanning speed of 0.2°/min in the Rietveld routine.X-ray photoelectron spectroscopy (XPS) was performed using a Scienta Omicron ESCA+ spectrometer with a high-performance hemispheric analyzer (EA 125) with monochromatic Al Kα (hν = 1486.6eV) radiation as the excitation source.The operating pressure in the ultrahigh vacuum chamber (UHV) during analysis was 2x10 -9 mbar.Energy steps of 50 and 20 eV were used for the survey and high-resolution spectra, respectively.Micro-Raman spectroscopy was conducted on a Horiba Jobin-Yvon (Japan) spectrometer equipped with a charge-coupled device (CCD) detector and argon-ion laser (Melles Griot, United States) operating at 514.5 nm and a maximum power of 200 mW.Fourier-transform infrared spectroscopy (FTIR) was performed at room temperature using a Jasco FT/IR-6200 (Japan) spectrophotometer operating in diffuse reflectance mode (DRIFT) with a spectra resolution of 4 cm −1 and 32 accumulations per measurement in the range of 400−4000 cm −1 .These measurements were carried out on powder mix, which was composed of 1% by weight of each sample mixed with 99% by weight of KBr (99%, Sigma-Aldrich).The shapes and sizes of the crystals were observed on a field emission scanning electron microscope (FE-SEM) model Inspect F50 (FEI Company, Hillsboro, OR) operating at 5 kV.UV-vis diffuse reflectance spectroscopy (UV-vis DRS) was performed using a Varian (USA) spectrophotometer (model Cary 5G) in the diffuse-reflectance mode.
Photoluminescence (PL) measurements were carried out at room temperature, with the samples excited by a 355 nm laser (Cobolt/Zouk) focused on a 20-μm spot.The backscattered luminescence was dispersed by a 20-cm spectrometer and the signal was detected by a charge-coupled device detector (Andor technologies).

Theoretical methods and model systems
The theoretical calculations were performed using the CRYSTAL17 package 45 under the framework of well-defined density functional theory (DFT) approximation.

Bulk optimization
For the optimization procedure of the bulk structure, the initial parameters were obtained from the Rietveld experimental values of sample Ag2SeO4-MH.All-electron Gaussian-type function basis set Ag+_SC-doll_1998 was used for Ag atom 46 , while P_pob_TZVP_2012 was used for Se and O atoms 47 .The exchange and correlation energy was treated within the generalized gradient approximation (GGA) with B3LYP functional 48,49 .An analysis of the band gap values obtained adjusting different percentage of exact Hartree−Fock (HF) exchange for the B3LYP functional was performed and it was found that 25% provides reliable values for both experimental band gap and structural parameters of Ag2SeO4.The minimization algorithm chosen was the Broyden-Fletcher-Goldfarb-Shanno (BFGS) scheme 50 , and the convergency of energy was set to 10 -8 Hartree.The shrinking factor (Pack-Monkhorst and Gilat net) was set to 8, which provides an accurate description of the electronic structure.The vibrational modes at the Γ-point were calculated using the numerical second derivate of the total energy.The optimized geometry was obtained by considering a full optimization, atom coordinates, and cell parameters.The optimized geometry corresponds to a minimum, once all frequencies obtained are positive.

Surface optimization
The (100), (010), (001), ( 101), (011), ( 110), ( 111), (021), ( 012) and (121) surfaces of Ag2SeO4 were represented through slab models using optimized lattice parameters of the bulk.For each surface, there were different possible terminations.The surface energy (Esurf) of possible terminated plane was calculated, and the plane with the minimum surface energy was selected as the most thermodynamically stable and used in further surface calculations.The surface energy was calculated as: Eq. 1 where Eslab is the total energy of the corresponding slab, n is the number of molecular units present in the slab, Ebulk is the energy of the bulk of each polymorph, and 2A corresponds to the area of both sides of the slab.Calculating the Esurf values with the Wulff construction results in a polyhedron that depends only on the ratios between the values of the Esurf and symmetry point group. 51,52Convergence energy tests regarding the thickness for symmetrical and stoichiometric slabs were also carried out.Both parameters of 2D slab and number of layers related to the thickness are listed in Table SI-1.By tuning the values of Esurf, the available morphologies can be obtained.This methodology provides a simple relationship between Esurf and the distance of the planes and has been used in materials science to predict experimental crystal shapes. 52,53 rationalize the pathways connecting the different morphologies shapes predicted, the polyhedron energy (  ) was calculated by summing the contributions of each facet to the morphological shape and the corresponding   values, according to the expression: Eq. 2 where  (ℎ) is the percent contribution of the surface area to the total surface area of the polyhedron, and   (ℎ𝑘𝑙) is the surface energy of the corresponding surface, according to methodology proposed by our research group. 54lectrostatic potential maps were generated with the DFT methodology with the B3LYP and 6-31g(d) functional/basis set combination 55 .Gaussian 09 (https://gaussian.com/glossary/g09/) was used for the numerical data derived from atomic charges of the (001) and (111) surfaces previously optimized to plot the electrostatic potential over the electron density using a color scale RGB (Red, Green and Blue) to represent the potential values.To visualize the electrostatic surface, Jmol software was used (http://www.jmol.org/)considering the values of total charges and a distance of 1.4 Å of the atomic surface.

Photocatalytic measurements
The performance of the as-prepared photocatalysts for the photodegradation of Rhodamine B (RhB) under UV illumination (6 Philips TUV lamps, 15 W) was verified.
For this purpose, 50 mg of the photocatalyst and 50 mL of RhB (1x10 -5 mol/L) were used and placed in ultrasound bath (Ultronique Eco-Sonics, 40kHz) for 5 min.After this step, the photocatalytic solution was transferred to a double-wall sealed cup with a water circulation system maintained at 20 °C and stirred for 30 min to achieve adsorption-desorption balance in the dark.The RhB photodegradation process started by exposing the solution to UV light for 60 min and collecting aliquots at certain times, centrifuging them to remove the catalyst.The remaining solution was analyzed on an UV-vis spectrophotometer (V-660, JASCO) at the maximum RhB wavelength region (λmax = 554 nm).The RhB photodegradation mechanism was investigated through experiments using scavengers of species that may be involved in the reaction, such as pbenzoquinone (BQ 0.012 mol/L), ammonium oxalate (AO 0.012 mol/L), and tert-butyl alcohol (TBA 0.012 mol/L) as scavengers of superoxide radical (O2'), hole (h  ), and hydroxyl radical (OH*), respectively.

XRD and Rietveld refinement
The XRD patterns of the Ag2SeO4 samples obtained by different methods are presented in Fig. 1.It is possible to observe that all diffractograms contain prominent and clearly distinguishable peaks perfectly indexed to crystalline Ag2SeO4 orthorhombic phase and space group Fddd (Z = 8), according to ICSD no.41-3089. 31,41condary phases and impurities peaks cannot be identified, confirming the purity and high crystallinity of the samples, as well as their structural long-range order irrespective of the synthesis methods.The structural properties of the Ag2SeO4 samples were investigated by Rietveld refinement using the general structure analysis system (GSAS) software. 56The refined parameters were preferred orientation, lattice parameters, shift lattice constants and atomic functional positions among other instrumental and sample parameters.The background was adjusted by a Chebyshev function, while the peak profile was fitted by a convolution of Thompson-Cox-Hastings pseudo-Voigt (pV-TCH) function.The asymmetry function and the anisotropy in the half-width of the reflections were determined according to Finger et al. 57 and Stephens 58 , respectively.In addition, experimental lattice parameters, unit cell volume, statistical parameters of quality (χ 2 and RBragg) and atomic positions of the Ag2SeO4 microcrystals were also performed.

XPS
XPS analysis was carried out to provide information about the chemical composition, binding energy, atomic bonding configuration, electronic structure, and oxidation states of the constituent atoms on the surface of Ag2SeO4 samples.The survey XPS spectra of the Ag2SeO4 samples are displayed in Fig. 3, where the C, Ag, Se, and O peaks for all samples and no other elements due to impurities can be identified.2. Peaks below 300 cm -1 are common and can be ascribed to lattice modes.For this system, one peak at 98 cm -1 was identified, being also present in other related materials such as Na2SeO4. 39Peaks above 300 cm -1 can be related to internal vibrations of the tetrahedral SeO4 2-(point group Td), which split into seven Raman active modes in the orthorhombic crystal environment.Among them, the most intense peak at 812 cm -1 can be attributed to the symmetric vibration of the selenate tetrahedron. 40Some small differences observed are due to changes in the reduced masses and bonding strengths of the Ag2SeO4.These results confirm the structural short-range order of all samples and the crystallization of the materials regardless of the method of synthesis used.Intensity (arb.units)

FTIR
The FTIR spectra of the Ag2SeO4 samples are presented in Fig. 5, whereas the experimental and theoretical calculated IR bands are listed in Table 2.The band appearing around 420 cm -1 can be assigned to the O-Se-O asymmetric bonding mode.
The mode at 669 cm -1 can be attributed to the symmetric and asymmetric stretching modes of Se-O. 79,80The splitting band (820, 843 and 873 cm -1 ) with the most intense peak at 843 cm -1 is ascribed to the infrared-active Se-O stretching mode. 40,81The 900-4000 cm -1 region of the spectra typically present characteristic bands of CO2 and H2O due to room atmosphere and humidity.The modes at 1633 and 1677 cm -1 correspond to the bending vibration band of molecular H2O.2][83][84] These FTIR modes attest the structural order of all samples and are in agreement with the theoretical values listed in Table 2.
The electronic structure of Ag2SeO4 was obtained by DFT calculations, including band structure, density of states (DOS) and partial density of states (PDOS).
The band structure plot using five high symmetry lines of the Brillouin zone is shown in

FE-SEM
The FE-SEM micrographs of the Ag2SeO4 samples are shown in Fig. 7, where it is possible to observe faceted block-like particles with a high degree of heterogeneity in shape and size.Additionally, the particles present smooth surface, well-defined shapes, and are mainly aggregated with a polydisperse size distribution.ZnMoO4. 87Moreover, there are some deformed rods with hexagonal and cubic faces.The thermodynamically stable macroscopic crystal shape of Ag2SeO4 was determined according to the Esurf values of Table 3 using the VESTA software 59 .For the Ag2SeO4 crystal, four facets [(111), (010), (001) and (011)] are exposed to the vacuum, forming a truncated octahedron.Usually, the lower the energy of a surface, the more it contributes to the crystal shape.In this case, the most stable surface, i.e., (111), covers about 63.07% of the total crystal shape area, while the (010), ( 001) and (011) surfaces respectively correspond to 9.84%, 14.29% and 18.84 % of the total crystal shape area.Table SI-5 presents the contribution of each surface to the total crystal shape as well as their relative energy.
The results reveal that the octahedral microcrystals with triangular faces (morphology A in Fig. 9) obtained by the CP method is the most stable and with stability rather similar to the ideal morphology (see Fig. 10).While morphology A exposes exclusively the (111) surface and is obtained by destabilizing the Esurf of the (001) surface, the Esurf of the former surface remains unchanged.
The rhombus morphologies of different sizes observed in the synthesis by SC and MH are described mainly by the (001) and (111) surfaces.The flat rhombic shape (morphology B in Fig. 9) appears due to the increase in the Esurf of the ( 010) and (011) surfaces and the stabilization of the (001) surface, which in turn increase the Epoly.On the other hand, in the truncated rhombic shape (morphologies C and D in Fig. 9) there are smaller contributions of the (121) surface to the total morphology.The truncated rhombic shape obtained by the SC method exposes the (111) surface in minor proportion compared to the MH method.An opposite behavior is observed for the (121) surface, but with equal Epoly values for morphologies C and D. This can be understood as a competitive effect of stability between both surfaces, as indicated in Table SI-5.
It can be seen that morphology E (Fig. 9) obtained by the MH method exposes the three lowest index surfaces, that is, (100), ( 010) and (001).To obtain this morphology, it is necessary to increase the Esurf value of the (001), ( 011) and (111) surfaces to 0.30, 0.80 and 0.80 Jm -2 , respectively.The destabilization of these surfaces promotes the exposure of the (101) and (110) surfaces, thus requiring an increase in the relative energy values of these surfaces to 0.55 Jm -2 and the simultaneous stabilization of the most unstable (100) surface, reducing its Esurf value to 0.40 Jm -2 .
The synthesis by the UP method resulted in morphologies F and G (Fig. 9).
Crystal morphology F is characterized by the (001), ( 101), (011), ( 111) and (121) surfaces, with predominance of the two first.Initially, the UP method is able to promote the exposure of the (101) surface through the destabilization of the (011) and (111) surfaces by increasing the values of the Esurf to 0.61 and 0.44 Jm -2 , respectively.
Simultaneously, the Esurf values for the (001) and (121) surfaces are reduced to 0.19 and 0.40 Jm -2 , respectively.Regarding the obtention of morphology G, an increase in the Esurf of the (101) surface is needed, as shown in Fig. 9.In addition, as illustrated in Fig. 10 these morphologies have higher values of Epoly than the ideal morphology, as they are generated by the destabilization of the surfaces involved in the process.
Finally, with exception of morphology E, the most stable ( 111) and (001) surfaces are present in all obtained morphologies.Also, a rhombus morphology as a result of the SC, UP and MH methods is obtained with minimum changes in the Esurf value compared with the ideal morphology.However, when obtained by the UP method the rhombic shape exposes the (011) surface, but not when obtained by other methodologies.

Surface geometric and electronic structures
Based on the optimized structural parameters, the structural and electronic properties of the different surfaces of Ag2SeO4 were investigated.The clusters of the unsaturated Ag and Se atoms exposed on the surfaces of the samples and the corresponding neutral oxygen vacancies (    ) were described by using the Krӧger-Vink notation 88 .Because of the weak symmetry breaking in the b-c plane, there are only slightly differences between the geometric structures of the (010) and (001) surfaces of Ag2SeO4.Thus, physical properties such as surface electronic structures and surface energies are expected to be similar, which means that the discussion presented for the (010) surface is also valid for the (001).It is also concluded that except for the (100), all surfaces with index hkl < 2 are terminated with saturated Se atoms, i.e., The value of the calculated Egap for this surface is 3.12 eV, which is slightly higher than that of the bulk.The VB region is mainly described by O 2p states, while the CB presents dispersed states with small contributions of O 2p and Se 4s orbitals.shows the DOS of the surface atoms in the relaxed (011) surface slab.The value of the calculated Egap for this surface is 3.12 eV, which is slightly higher than that of the bulk.
The VB region is described by O 2p and Ag 5d states, while the CBM presents minimum contributions of O 2p states., respectively.Therefore, it can be concluded that the (012) is the most organized surface, while the (110) is the less organized one.As previously discussed, the CP method promotes a morphological control of the material with preference exposure of the (111) surface (morphology A in Fig. 9).

Photoluminescence spectroscopy
The chemical environmental of (111) surface is governed by the presence of regular

Photocatalytic activity
The photocatalytic property of the samples was evaluated by spectrophotometry at maximum absorption wavelength (λmax = 554 nm) through the photodegradation of RhB under UV light irradiation.Fig. 13 shows the absorbance spectra of samples Ag2SeO4-SC, Ag2SeO4-UP, Ag2SeO4-CP and Ag2SeO4-MH.It can be seen that the Ag2SeO4-SC is the sample that most closely matches the absorbance value 0, thus being the sample with the highest photocatalytic activity.proving the photocatalytic property of the samples synthesized in this study.Besides, the reaction kinetics of the RhB photodegradation was also calculated in order to investigate the photocatalytic performance, as shown in Fig. 14(b).
The value of the kinetic rate constant can be calculated using the pseudo-first order model (-ln(A/A0) = kt, where k is the rate constant (min -1 )).According to Fig.
14 (b), the correlation between -ln(A/A0) and irradiation time is linear, demonstrating that the photodegradation of the RhB dye follows the first order under UV light illumination.The calculated reaction rate constants k were 0.037, 0.019, 0.020, 0.015, and 0.005 min -1 for Ag2SeO4-SC, Ag2SeO4-UP, Ag2SeO4-CP, Ag2SeO4-MH, and Ag2SeO4 (without catalyst), respectively.The sample synthesized by the sonochemical method showed the highest k value, proving to be the most efficient catalyst for RhB photodegradation.Additionally, the sonochemical approach was also the best method for synthesizing Ag2SeO3 catalysts for the photodegradation of RhB under UV light. 44 is reported that PL intensity is directly correlated with the recombination of photoexcited e'─h  pairs and that a lower PL intensity means less recombination, leaving the e'─h  pairs free to act on the RhB photodegradation 89 .Sample Ag2SeO4-SC showed the lowest PL intensity, as seen in Fig. 11(a), which was also observed by Pinatti et al. 44 for the Ag2SeO3 sample synthesized by the sonochemical method.Thus, it is believed that samples synthesized by the sonochemical method tend to have a lower recombination of the e'─h  pair, which is reflected in their higher photocatalytic response.These results present similar behavior as other photocatalyst recently studied [90][91][92][93][94][95] .

Photocatalytic mechanism
To understand the photodegradation mechanism of the sample Ag2SeO4-SC, which presented the highest photocatalytic activity, experiments were carried out using p-benzoquinone (BQ), ammonium oxalate (AO) and tert-butyl alcohol as scavengers of O2 ' , h  , and OH*, respectively.According to Fig. 15, the RhB photodegradation efficiency is unaffected by the addition of BQ and TBA, indicating that O2 ' and OH* participate, to a lesser extent, in the RhB photodegradation process.When AO is added, the percentage of degradation decreases remarkably, suggesting that h  is largely responsible for the photocatalytic activity of the sample Ag2SeO4-SC.In the CB, the generated clusters have negative charge, such as [SeO4]′, acting as e' (see reaction 1).
[AgO6] • + RhB → degraded products (2)   This is in accordance with the proposed mechanism of photocatalysis, which shows that h  play a crucial role in RhB photodegradation since OH* and O2 ' do not participate in the degradation mechanism.

Figs. SI- 1 (
Figs. SI-1(a-d) show the Rietveld refinement plot of the Ag2SeO4 microcrystals obtained by different synthesis methods.Table1summarizes the experimental and

Figure 2 .
Figure 2. Unit cell representation of the Ag2SeO4 microcrystal.(a) Ball-stick model, (b) polyhedral AgO6 and SeO4 models, and (c) Ag − O and Se − O bond lengths represented by horizontal lines.Cyan, green, and red balls represent Ag, Se and O atoms, respectively.

Fig. 6 (
Fig. 6(II).It can be seen that the VB is flat and according to our assignment of the kpoints, with the valence band maximum (VBM) located along the  −  point, while the CB is more dispersive, with the conduction band minimum (CBM) located at the  point.Based on the calculated band structure, Ag2SeO4 is a semiconductor with indirect band gap and a calculated Egap value of 2.89 eV that coincides with the experimental optical Egap (Fig. SI-5).The VBM and its vicinity are mainly composed of Ag 5d orbitals with significant contribution of O 2p orbitals.The analysis of the VB region indicates the presence of Ag 5s and Ag 5d states along the evaluated energy range.The CBM, on the other hand, is predominantly formed by Ag 5s states as well as contributions of Se 4s and O 2p states (Fig. 6(III)).

Figs. 7 (
Figs.7(a, b) show the FE-SEM images of sample Ag2SeO4-SC, where rhombusshaped crystals of different sizes ranging from 1 to 6 μm can be clearly seen.Some of them have truncated face and flat morphology, while others present deformed shapes with lengths larger than widths, resulting in rod-like crystals.Crystals with a welldefined truncated hexagonal plate morphology are predominantly observed in sample Ag2SeO4-UP, as illustrated in Figs.7(c, d).It is also possible to note morphologies of various truncated octagonal rods and truncated rhombic shapes, with many of them in process of formation.Figs.7(e, f) show octahedral microcrystals with triangular faces ascribed to Ag2SeO4-CP, being these morphologies similar to the ones observed in

Figs. 7
Figs.7(g, h) show the crystal Ag2SeO4-MH, which is mainly composed of truncated cubes, in addition to rhombus crystals and rods with flat and elongated shapes.These results confirm that Ag2SeO4 can be synthesized by different methods and that various morphologies can be obtained without the use of surfactants, templates, organic solvents, or medium pH adjustment.Particularly, each synthesis method provides distinct forces of interaction (temperature, sonication, physic stirring, pressure, microwave radiation) for the [AgO6] and [SeO4] clusters, which account for the overall energy of the system, thus resulting in different morphologies observed.Also, these results reveal that morphology control can provide a great versatility for tuning and enhancing the applications of materials.

Figure 8 .
Figure 8. Geometric structure of the surface models of Ag2SeO4.Cyan, green and red balls represent Ag, Se and O atoms, respectively

Figure 9 .
Figure 9. Crystal shape computationally simulated for Ag2SeO4 synthesized by SC, UP, CP, and MH methods.The experimental shapes are included for comparison.Surface energy values are given in Jm -2

Figure 10 .
Figure 10.Polyhedron energy profile connecting the ideal and associated morphologies to the experimentally obtained data 4 ] clusters.(100) surface.The relaxed structure of the (100) surface is shown in Fig. SI-7(a).It can be seen that the (100) surface exposes three unsaturated Ag atom, being two of them fourfold coordinated and forming the equivalent undercoordinated [ 4 • 2   ] clusters, while the other one is fivefold coordinated, resulting in the undercoordinated [ 5 •    ] cluster.Both four-and fivefold undercoordinated clusters present  −  with average bond length of 2.401 Å.In addition, two undercoordinated [ 2 • 2   ] clusters are also present in this surface.The  −  surface bonds have equal length of 1.685 Å. Fig. SI-7(b) shows the DOS of the surface atoms in the relaxed (100) surface slab.The value of the calculated Egap for this surface is 0.47 eV.The sharp peaks are mainly composed of O 2p and Se 4s located in the bulk band gap region.(010) and (001) surfaces.The relaxed geometric structure of the (010)/(001) surfaces is illustrated in Fig. SI-7(c).The surfaces present two fourfold coordinated Ag atoms, indicated as undercoordinated [ 4 • 2   ] clusters.The  −  bond lengths are different in each of these clusters and separated into two groups of two, with bond length values of 2.227 and 2.796 Å in one, and 2.462 and 2.341 Å in the other.The  −  bond lengths in the [ 4 ] clusters are also separated into two groups of two, with values of 1.673 and 1.698 Å, but they are different from the bulk, which has four  −  bonds with equal length.Fig. SI-7(d) shows the DOS of the surface atoms in the relaxed (001) surface slab.It is possible to note that the value of the calculated Egap is 2.97 eV and that the surface Fermi level almost coincides with that of the bulk.The VB region is described by only one sharp peak composed of O 2p and Ag 5d states and hybridized states at approximately 0.5 eV, while the CB region presents Se 4s and O 2p hybridized states.(101) surface.The (101) surface exposes two coordinated Ag atoms separated into two group of clusters: two undercoordinated [ 5 •    ] and two [ 4 • 2   ] clusters (Fig. SI-7(e)).The two undercoordinated [ 5 •    ] clusters are equivalent to each other, as are the undercoordinated [4•    ] clusters.The five  −  bond lengths in the [ 5 •    ] clusters are different, varying from 2.331 to 2.838 Å.The tetrahedral [ 4 • 2   ] cluster are formed by four  −  bonds with lengths varying from 2.296 to 2.577 Å.The  −  average bond length is 1.70 Å, similar to the bulk value.Fig. SI-7(f) shows the DOS of the surface atoms in the relaxed (101) surface slab.

( 110 )
surface.This surface presents four unsaturated Ag atoms separated into two groups of two, as illustrated in Fig.SI-8(c).The undercoordinated [ 3 • 3   ] clusters are composed of  −  bonds with lengths of 2.227, 2.267 and 2.446 Å, whereas the [ 2 • 4   ] clusters are formed by  −  bonds with lengths of 2.158 and 2.163 Å.The [ 4 ] clusters exposed at the top surface are symmetrically equivalent and have three different  −  bonds with lengths of 1.658, 1.687 and 1.693 Å. Fig. SI-8(d) shows the DOS of the surface atoms in the relaxed (110) surface slab.The value of the calculated Egap for this surface is 3.18 eV, which is close to that of the bulk.The VB region is mainly composed of O 2p states with minor contribution of Ag 5d states.There are no states in the band gap region.(111) surface.The relaxed structure of the (111) surface is shown in Fig. SI-8(e), where it is possible to see two fourfold coordinated Ag surface atoms forming two distorted tetrahedral undercoordinated [ 4 • 2   ] clusters with  −  average bond lengths of 2.432 and 2.455 Å, which are shorter than the bulk values.The [ 4 ] surface cluster has  −  average bond length of 1.683 Å. Fig. SI-8(f) shows the DOS of the surface atoms in the relaxed (111) surface slab.In the VB region, peaks observed in the energy range of -1.5 to 0 eV are due to Ag-5d and O 2p states.The Egap of 2.97 eV obtained through the DOS plot is close to the bulk value (2.90 eV).This result is expected because the (111) surface presents the highest contribution to the total crystal area and no states occur in the band gap region of the material.(012) surface.Fig. SI-9(a) shows the relaxed geometric structure of the (012) surface, where four Ag atoms and one Se atom are observed.Of the four Ag atoms, two are fivefold and two are threefold coordinated, while the Se atom is also threefold coordinated.The undercoordinated [ 5 •    ] clusters have five different  −  bonds with lengths from 2.270 to 2.740 Å, whereas the undercoordinated [ 5 •    ] clusters have  −  bonds with lengths ranging from 2.410 to 2.426 Å.The undercoordinated [ 3 •    ] clusters have three different  −  bonds with lengths of 1.677, 1.710 and 1.788 Å, which are on average higher than that of the bulk.The calculated Egap is 2.10 eV, which is lower than the bulk value, indicating that states located in the band gap region of the bulk are expected, being them mainly composed of O 2p (Fig. SI-9(b)).(021) surface.The relaxed (021) surface illustrated in Fig. SI-9(c) exposes two fivefold and two threefold coordinated Ag atoms, while the exposed Se atom is threefold coordinated.The undercoordinated [ 5 •    ] clusters have  - bonds with lengths from 2.383 to 2.781 Å, whereas the undercoordinated [ 3 • 3   ] clusters have bond lengths of 2.383 and 2.538 Å.The undercoordinated [ 3 •    ] clusters present three equal bond lengths of 1.680 Å.The calculated Egap for this surface is 1.87 eV (Fig. SI-9(d)), which is lower than that of the bulk.The CBM is composed of O 2p and Ag 5s orbitals.(121) surface.Fig. SI-9(e) shows that the (121) surface presents four unsaturated Ag atoms, where two kinds of Ag clusters can be found: undercoordinated [ 5 •    ] and [ 4 • 2   ] clusters.The  −  bonds in the undercoordinated [ 5 •    ] clusters are separated into two groups of two, with lengths of 2.383 and 2.538 Å, plus one  −  bond with length of 2.781 Å.In the undercoordinated [ 4 • 2   ] clusters, the  −  bonds are also separated into two groups of two, with lengths of 2.0538 and 2.781 Å.Additionally, two threefold Se atoms are exposed in this surface, and the undercoordinated [ 3 •    ] clusters have bond length of 1.680 Å.According to Fig. SI-9(f), the calculated Egap for this surface is 1.23 eV.The peak located in the band gap region of the bulk is generated by O 2p and Ag 5s states as well as Se 4s states.Undercoordinated Ag clusters do not generate states in the band gap region of the bulk.However, the presence of undercoordinated Se clusters on the surface leads to the appearance of O 2p and Ag 5s states, causing the surface Egap value to be lower than that of the bulk.Surfaces terminated in Se clusters with the lowest coordination number have the lowest Egap values.From undercoordinated clusters presented at the top of each surface it is possible to obtain the number of the dangling bonds (NB), and then calculate their density (  =   /, where NB is the total number of  −  and  −  dangling bonds), which is associated with the structural order-disorder degree in the referred region.These values are listed in

Fig. 11 ( 44 Fig. 11 (
Fig.11(a) shows the PL emission spectra at room temperature of the Ag2SeO4 samples under the excitation wavelength of 355 nm.All spectra present a broadband profile covering the entire visible region, with maximum PL intensity at 665 nm.The PL band intensity of the samples follow the sequence: Ag2SeO4-CP > Ag2SeO4-UP > Ag2SeO4-MH > Ag2SeO4-SC.The lowest PL intensity is mainly associated with a lower recombination rate of electron-hole (e`─h • ) pairs.This PL behavior can be attributed to the structural and electronic arrangements of the [AgO6] and [SeO4] clusters.The electronic PL phenomenon occurs after electronic transition from the VB to the CB, forming the e`─h • pairs and subsequently emitting photon decay radiatively.The degree of order-disorder of the constituent clusters is responsible for the presence of vacancies as well as surface and bulk structural defects, which in turn play an important role in the visible emission spectra.Also, different synthesis method can change the lengths and angles of the Ag-O and Se-O bonds, modifying the electronic properties of the materials.The intense orange-red emissions observed in sample Ag2SeO4-CP can be ascribed to the deep-level defects located in the optical band gap region.In addition, we assume that CP is the method that provides more induced defects.Then, it can be implied that these crystals present medium-range structural and electronic order-

[
SeO4] and defective [ 4 • 2   ] clusters (Fig. SI-8(e)), with spatial distribution preferential for the Se and Ag cations exposed in the top of the surface, which induce the transference of the excess of electronic density to the interior of the bulk, resulting in a slightly positive surface.An analysis of the electrostatic potential of the (111) surface displayed in Fig. 12(b), confirms this is a positive surface which would increase the PL emission intensity, because a reduced local charge density on the surface decreases the probability of non-radiative transitions, indicating that the recombination rate of the photogenerated electron (e′) and hole (h•) pairs is the largest one.On the other hand, despite the   value for this surface (Fig. SI-8(f)) be very close to the   bulk and intermediate levels are not observed in the bulk region, the calculated DOS profile for (111) surface (Fig. SI-8(f)) shown a higher density of states and overlap orbital when compared with the bulk, resulting in distinct photoinduced excitations.Combining these facts, it is expected that shape-oriented Ag2SeO4 particles exhibit enhanced PL emissions.

Figure 12 .
Figure 12.Electrostatic potential maps for (a) (001) surface and (b) (111) surface.The red surface corresponds to a negative region of the electrostatic potential (-0.01 au), whereas the blue color corresponds to the region where the potential is positive (0.010 au).

Figure 14 .
Figure 14.Photocatalytic degradation (a) and kinetic fit curves for all samples under UV-light irradiation (b).
photoexcited e'─h  pairs and promoting the photocatalytic activity.

Figure 15 . 3 ECB
Figure 15.Photocatalytic degradation of RhB using Ag2SeO4-SC in the presence of different scavengers under UV-light irradiation

Figure 16 .
Figure 16.Proposed mechanism for the photodegradation of RhB under UV light irradiation using Ag2SeO4-SC

Table 1 .
Lattice parameters, unit cell volume and statistical parameters of quality

Refined formula Ag2SeO4 Lattice Parameters (Å) Cell volume (Å 3 ) RBragg (%) χ 2 (%)
Table SI-4 lists the positions of the XPS elements and the concentration of the area components for the elements Ag, Se and O of the Ag2SeO4 Raman Fig. 4 shows the Raman spectra at room temperature of Ag2SeO4 samples excited by a green laser.According to group theory analysis, the Fddd structure of Ag2SeO4 has 42 Raman and infrared active modes, as stated by the following samples.The aforementioned results prove the existence of such elements as well as the purity of the sample surface.-1,and are close to the theoretical modes predicted, as shown in Table

Table 2 .
Experimental and theoretical values of Raman and IR vibrational frequencies

Table 3 .
The calculated DB defined the