Surfactant-Mediated Morphology and Photocatalytic Activity of α-Ag2WO4 Material

In the present work, the morphology (hexagonal rod-like vs cuboid-like) of an α-Ag2WO4 solid-state material is manipulated by a simple controlled-precipitation method, with and without the presence of the anionic surfactant sodium dodecyl sulfate (SDS), respectively, over short reaction times. Characterization techniques, such as X-ray diffraction analysis, Rietveld refinement analysis, Fourier-transform (FT) infrared spectroscopy, FT Raman spectroscopy, UV−vis spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction, energy-dispersive X-ray spectroscopy, field emission-scanning electron microscopy (FE-SEM), and photoluminescence emission, are employed to disclose the structural and electronic properties of the α-Ag2WO4 material. First-principles calculations were performed to (i) obtain the relative stability of the six low-index surfaces of α-Ag2WO4; (ii) rationalize the crystal morphologies observed in FE-SEM images (using the Wulff construction); and (iii) determine the energy profiles associated with the transformation process between both morphologies induced by the presence of SDS. Finally, we demonstrate a relationship between morphology and photocatalytic activity, evaluated by photodegradation of Rhodamine B dye under UV light, based on the different numbers of unsaturated superficial Ag and W cations (local coordination, i.e., clusters) of each surface.


INTRODUCTION
Organic chemicals are still largely employed by the textile industries and play a critical role in the pollution of river water, causing chemical and biological changes in aquatic systems, which can kill animals and plants. 1 This problem makes the improvement and elucidation of processes like the gas sensing 2 capabilities and photocatalytic degradation of these dyes a priority. 3 Ag 2 WO 4 appears to be a good alternative to the extensively researched materials currently used as photocatalysts (e.g., TiO 2 4 , ZnO, 5 WO 3 6 ) and should thus be further studied and understood. Recent research reported the photocatalytic activity of α-7 and β-8 Ag 2 WO 4 under UV light. 9 Orthorhombic α-Ag 2 WO 4 , 10 which also presents good luminescent, 11 antimicrobial, 12 and gas-sensing 13 properties, offers new opportunities to further optimize the facetdependent photocatalytic performance of semiconductors.
The structure of α-Ag 2 WO 4 was resolved by our research group. 14 15 The properties of this semiconductor change according to the crystallographic phase, size, and morphology, and these variables are all dependent on the synthesis procedure. Each surface of a crystal can possess a particular surface energy, atomic distribution, local coordination (clusters) of exposed metals, vacancies, etc. These results tend to confirm the hypothesis that the properties observed for this material result from a compromise between the presence of local defects (short-range disorder) and crystal lattice ordering (long-range order). Therefore, finding a synthesis method to control these characteristics is crucial to determine the properties and subsequent technological applications of this material.
α-Ag 2 WO 4 materials, at nano-and microscale, can be obtained by traditional synthesis methods, 16 or, more recently, by conventional hydrothermal (CH) 17 and microwave hydrothermal (MH) 18 methods. However, there are limitations related to these methods, such as the difficulties of large-scale reproduction, the necessity of specific equipment, the small quantities of product obtained, and safety concerns. Therefore, achieving precise control over the synthesis, morphology, and properties of α-Ag 2 WO 4 has become a hot research subject. In this context, controlled-precipitation (CP) 19 is a simple and facile method that is widely employed in the production of α-Ag 2 WO 4 , which has good bactericide properties and photocatalytic activity. By varying some synthetic parameters of the CP method, such as temperature, stirring time, solvents, or use of capping agents, 20 one can obtain significant changes in the morphology and size of the final samples and, consequently, in some highly surface-dependent properties like photocatalytic activity. 7 The properties of nanoscale materials and the changes in these properties along with size or shape is a hot research field. 21 Unraveling the relationship between the atomic structure and the functional properties can be a crucial step, especially in complex materials, where short-range interactions, rather than the average structure, can define the actual properties and behavior of the material. One of the main factors determining functionality is the crystal morphology, because the number of active sites is clearly surface-dependent. The variation of reaction conditions can produce different morphologies and, consequently, modify their properties dramatically. However, the control of a crystal morphology is a complex process depending on several factors. Surfactants are particularly important due to their influence on the (nano) particle structure and other physical and chemical properties, having the remarkable ability to control crystal growth and direct it in a morphology-and size-controlled manner. 22 Achieving a detailed understanding of mechanisms by which surfactants can control the morphology of as-synthesized material is of paramount importance, given the effects on the photocatalytic activity of the material.
In this study, we seek to fulfill a threefold objective. The first is to report the novel formation of α-Ag 2 WO 4 by a simple CP method, with short reaction time, with and without the presence of an anionic surfactant (sodium dodecyl sulfate, SDS), to evaluate the influence of this surfactant on crystal formation and the resultant structure and morphology. Second, the photocatalytic degradation of Rhodamine B (RhB) will be investigated as a function of the obtained morphology. The third aim is to determine the relationship between morphology and photocatalytic activity. This paper is organized as follows: Section 2 presents the experimental procedures and computational methods; Section 3 contains the results and discussion; and, finally, Section 4 provides a summary of conclusions.

METHODS
2.1. Synthesis. The α-Ag 2 WO 4 crystals were prepared by a simple CP method at 90°C in the presence of the anionic SDS (C 12 H 25 SO 4 Na; 90% purity, Synth) as follows: 1 × 10 −3 mol of dihydrate sodium tungstate (Na 2 WO 4 ·2H 2 O; 99% purity, Sigma-Aldrich;) and 2 × 10 −3 mol of silver nitrate (AgNO 3 ; 98.80% purity, Cennabras) were dissolved separately in 50 mL of deionized water each. SDS (1 g) was added to the sodium tungstate solution, and the solutions were heated under continuous stirring. After the two solutions reached 90°C, the silver nitrate solution was poured into the sodium tungstate solution and allowed to stir for 5 min. The precipitate was then decanted and washed several times with distilled water. Washing of the precipitate was necessary to eliminate the Na + , NO 3− , or SO 4 2− ions and residual organic compounds. Finally, the precipitate was dried with acetone at room temperature and collected after an overnight period. The reaction was also performed under the same conditions but without the presence of SDS. The precipitates obtained from the reaction without and with SDS are light beige and light brown, respectively.
2.2. Characterization. The α-Ag 2 WO 4 crystals were structurally characterized by X-ray diffraction (XRD) using a D/Max-2500PC diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 5°to 75°, with a scanning velocity of 2°min −1 , in the normal routine, and from 10°to 110°with a scanning velocity of 1°/min in the Rietveld routine. The shapes and sizes of the products obtained were observed with a field-emission scanning electron microscope (FE-SEM), operated at 5−10 kV (Supra 35-VP, Carl Zeiss, Germany). Transmission electron microscopy (TEM) analysis was performed to verify additional morphology details inside a Jeol JEM 2100F with a field emission gun (FEG) operating at 200 kV. Some crystallographic aspects of the samples were also verified with selected area electron diffraction (SAED) analysis and high-resolution transmission electron microscopy (HR-TEM) in the same equipment. Additionally, energy dispersive X-ray spectroscopy (EDS) was also employed to obtain a local elementary composition with EDAX equipment. To verify the complete elimination of the surfactant, thermogravimetric analysis (TGA) was performed with a NETZSCH TG 209  F1 instrument, within the temperature range from 22 to 900°C , and with a rate of 10k/min, under an atmosphere of synthetic air. Fourier-transform infrared (FT-IR) spectroscopy was performed in a Bomem-Michelson spectrophotometer in transmittance mode (model MB102), in the range from 250 to 1000 cm −1 , using KBr pellets as a reference. Raman spectra were recorded using the iHR550 spectrometer Horiba Jobin-Yvon coupled to a CCD detector and an argon-ion laser (Melles Griot) operating at 514.5 nm with a maximum power of 200 mW and a fiber microscope. UV−Vis diffuse reflectance measurements were obtained using a Varian Cary spectrometer model 5G on the diffuse reflectance mode, with a wavelength range from 200 to 800 nm, and a scan speed of 600 nm min −1 . Photoluminescence (PL) measurements were obtained at room temperature with a monochromator (Thermal Jarrel-Ash Monospect-27) linked to a photomultiplier model R446 (Hamatsu Photonics) and a double monochromator (Jobin-Yvon-U1000) directly linked to a system of photon counting. A Krypton laser (Coherent Innova 90k) with a wavelength of 350 nm was used as the excitation source, with a maximum exit power rate of 500 mW. After the passage of the laser beam through the optical chopper, the power was reduced and maintained at 40 mW on the samples. In addition, the surface areas of the samples were obtained by physiosorption of N 2 at 77 K in a Micromeritics equipment (ASAP 2420). Before the analysis the samples were heated to 120°C for 10 h under vacuum to eliminate water and physically adsorbed. The surface area was calculated with the Brunauer−Emmet−Teller (BET) equation. 23 2.3. Photocatalysis Assays. To perform the photocatalysis experiments, 50 mL of RhB (95%, Mallinckrodt) solution (1 × 10 −5 mol L −1 ) with a pH of 4.0 was mixed with 50 mg of α-Ag 2 WO 4 . The mixture, consisting of the dye and photocatalyst, was irradiated in a dark box using six UV lamps (Phillips TL-D, 15 W). Before UV illumination, the suspensions were sonicated for 10 min in an ultrasonic bath (42 kHz, model 1510) to allow saturated absorption of RhB onto the catalyst. Subsequently, the dispersion containing the catalyst and the dye was transferred to a vessel inside the photoreactor, with the temperature maintained at 20°C via a thermostatic bath and vigorous stirring. At 0, 5, 10, 20, 30, 50, 70, 90, and 120 min, a 2 mL aliquot of the suspension was removed from the photocatalytic system and placed in a plastic tube, which was centrifuged at 13 000 rpm for 10 min to separate the solid catalyst from the liquid phase. Finally, the kinetics of the dye photodegradation process were monitored in a commercial cuvette via UV−vis spectroscopy (JASCO V-660) at 554 nm. This procedure was subsequently performed for each synthesized α-Ag 2 WO 4 sample.
2.4. Computational Details. From the calculated surface energy (E surf ) values for the six low-index crystals facets reported by Andreś et al., 24 it is possible to determine the equilibrium shape and ideal morphology, by using the Wulff construction. Furthermore, by changing the relative surface energy values of each facet, we found a map of available morphologies for α-Ag 2 WO 4 , as well as the pathways linking the ideal and given morphologies, including both hexagonal rod-like and cuboid-like morphologies. 24 This methodology provides a simple relationship between E surf and the distance of the planes and has been used in materials science to predict experimental crystal shapes. 25 In this study, we go further by calculating the energy profiles of the paths connecting different morphologies from the values of the polyhedron energy (E polyhedron ) by a function of the form where C i is the percentage contribution of the surface area to the total surface area of the polyhedron, C i = A i /A polyhedron , and E surf i is the surface energy of the corresponding surface. The energy profiles were calculated by decreasing and/or increasing the E surf values of a given surface of the polyhedron.

RESULTS AND DISCUSSION
3.1. X-ray Diffraction Analysis. Figure 2 shows the XRD patterns of the synthesized α-Ag 2 WO 4 crystals. According to the Inorganic Crystal Structure Database, ICSD card 34−61, these crystals have an orthorhombic structure, without any deleterious phases, with the space group Pn2n and point group symmetry (C 2v 10 ). 26 Therefore, the samples synthesized by the CP method with and without anionic surfactant SDS present a pure α-phase structure. Both crystals have sharp and welldefined diffraction peaks, which indicate a good degree of longrange structural order in the lattice, and they are all in good agreement with the respective databases. There are also no significant variations in peak intensity or position between both samples.
3.2. Rietveld Refinement Analysis. The structural refinement using the Rietveld method 27 was performed using GSAS program, version 1. The refinement plots for the α- The Journal of Physical Chemistry C Article Ag 2 WO 4 crystals synthesized by the CP method at 90°C without and with the presence of SDS, respectively, were depicted in Figure 3A,B. The Rietveld refinement confirms the orthorhombic structure, with only a small variation between the values obtained in the refinement and the literature. 14 The structural results (a, b, c cell parameters and volume) with the statistical reliability parameters (R wp and GOF) are shown in Table 1. The statistical reliability of the parameters indicates a high agreement between the calculated and observed X-ray patterns for both samples.

Morphological Analysis. FE-SEM analysis is a very
powerful tool to provide morphological information, such as the arrangement of atoms in the structure and the degree of structural and electronic order/disorder. Figure 4 shows the FE-SEM images of the experimentally obtained products. Figure 4A is an image of α-Ag 2 WO 4 crystals obtained from the short period stirring synthesis without surfactant under 90°C. The observed morphology is hexagonal rod-like, in accordance with previous studies found in literature, 7 and is depicted in Figure 4B. The hexagonal rod-like α-Ag 2 WO 4 crystals usually have a length of nearly 2 μm. In this case, probably due to the short time of stirring (5 min), the crystals are smaller (see Figure 5). Figure 4C displays the morphology of the product obtained when the anionic surfactant SDS is included in the CP method, maintaining the other conditions constant. It is possible to observe several cuboid-like crystals, depicted in Figure 4D. Although there are some reports in the literature of rectangular nanorod crystals of Ag 2 WO 4 combined with hexagonal rods, 28 this is the first report in which the predominance of cuboid-like morphology is observed. The samples synthesized without and with anionic surfactant SDS are designated, according to their morphology, as α-Ag 2 WO 4 -R (α-Ag 2 WO 4 hexagonal rod-like crystals) and α-Ag 2 WO 4 -C (α-Ag 2 WO 4 cuboid-like crystals), respectively.
The FE-SEM image ( Figure 4D) also shows two crystals, one possessing a cubic shape, α-Ag 2 WO 4 -C, and the other being exactly twice the length of the first one. An analysis of the histogram in Figure 5 renders a length distribution ranging from 100 to 300 nm, in the case of the cuboid crystals, with an average of nearly 200 nm. These results seem to indicate that the basic cubic blocks link to each other by coalescence in such a way that the theoretical cubes become elongated cuboid-like particles.
As studied before by our work group, 15,29 the irradiation of α-Ag 2 WO 4 by an electron beam generates the formation of metallic Ag nanoparticles on their surface. 30 Figure 6A,B displays the TEM images of the Ag 2 WO 4 -R and α-Ag 2 WO 4 -C samples, respectively. Figure 6C,D displays the SAED analysis by HR-TEM. The patterns are diffuse halo rings typical from amorphous materials. The presence of these diffuse rings indicates the changes in interatomic spacing induced by the deformation process. 29 The formation of Ag nanoparticles at some regions on the surface α-Ag 2 WO 4 are confirmed by HR-TEM analysis as it is shown in Figure 6E,G, as the interplanar distances between (111) planes, 2.35 Å, of the Ag cubic structure, according to ICSD No. 064706. The local elementary analysis EDS shown in Figure 6F,H also confirms the presence    Figure 7B presents the Raman scattering spectra and the vibrational modes in the range of 50−1200 cm −1 at room temperature. Although there are 21 known vibrational modes  The calculated interplanar distance corresponding to cubic Ag metal at the α-Ag 2 WO 4 -R and α-Ag 2 WO 4 -C samples, respectively.
The Journal of Physical Chemistry C Article for α-Ag 2 WO 4 (6 A 1g , 5 A 2g , 5B 1g , and 5B 2g ) 32 it is possible to detect 12 of them for both samples. According to Turkovic et al., 32 there are two active A 1g external modes within the region of smaller wavenumbers, at 44 and 60 cm −1 , that are due to the translational movements of Ag cations in a rigid molecular unit. In our spectra, it is possible to observe, for both materials, an intense band at ∼58 cm −1 . This peak is identical in intensity and definition for both samples. The active modes between 500 to 100 cm −1 are related to external vibrational modes of [AgO n ] (n = 7, 6, 4, 2). 28 Although this region presents lower-intensity bands, it seems to indicate some slight differences between the spectra. There is an intense band at 878 cm −1 , attributed to the symmetric stretching of the W−O bond in [WO 6 ] octahedral clusters. The active modes, between 500 and 1000 cm −1 , can be assigned to the vibrations in [WO 6 ] clusters. This result confirms that the Raman spectra are the result of the presence of short-range order at the [AgO n ] clusters within the crystal lattice.
3.5. UV−Vis Optical Spectroscopy Analysis. The optical band gap energy (E gap ) values for both α-Ag 2 WO 4 samples were determined by UV−vis spectroscopy. With the utilization of the Wood and Tauc method 33 we could estimate the energy gaps of the materials. The spectra obtained were converted to the Kubelka−Munk function, by the method proposed by Kubelka and Munk-Aussig, 34 which is a reliable method to estimate E gap values with good accuracy within the limits of assumptions in three dimensions. According to Kim et al. 35 and Tang et al., 36 Ag 2 WO 4 crystals exhibit an optical absorption spectrum governed by direct electronic transitions between the valence band (VB) and conduction band (CB). On the basis of this information, the E gap values of our α-Ag 2 WO 4 crystals were calculated using n = 0.5.
UV−Vis spectroscopy provides structural information in terms of the structure of electronic bands and the level of organization at both short and medium ranges. A characteristic behavior of UV−vis absorbance curves is the formation of an exponential decay within the regions of lesser energies known as Urbach Tail. 33  Both values fall within the range of values previously reported. Although the differences in value might be small, it could be related to the individual band gaps of each surface given that the E gap value is the result of bulk and surface contributions. Moreover, one might consider the influence of some possible defects, such as structural distortions, associated with changes in bond lengths and angles, atomic dislocation, local coordination changes, oxygen vacancies, etc. 37 The presence of such defects is very dependent on the synthesis parameters, such as solvent, time, temperature, pressure, surfactants, or contaminants. From an electronic point of view, these defects generate intermediate energy states in the forbidden region between the VB and CB, while an increase of the E gap values can be attributed to a reduction in  α-Ag 2 WO 4 has PL emission at room temperature with a broadband character. 38 This is typical of materials where the relaxation step occurs in several paths, with the participation of several energy states within the band gap, according to the broadband model. This broadband is generally composed of blue and green light components, which indicates a high level of mainly shallow defects. This emission is attributed to intrinsic transitions of the [WO 4 ] 2− clusters, as well as charge-transfer processes between distorted [WO 6 ] to undistorted [WO 6 ] clusters, 39 which generates shallow defects adjacent to the bands. Figure 9 presents the PL spectra at room temperature. Both samples show PL emissions from 380 to 800 nm. We can observe that the PL behavior of α-Ag 2 WO 4 -R presents a broad band with maximum emission at 452 nm, corresponding to the blue region of the electromagnetic spectrum. Though the α-Ag 2 WO 4 -C also presents emission in the blue region (456 nm), this band is much less intense, and the curve is much more shifted into the whole spectrum, with emission in the red region (640 nm) with almost the same intensity of the blue emission peak. This behavior can be associated with the decreasing number of distorted [WO 6 ] clusters and increasing presence of distorted [AgO n ] (n = 7, 6, 4, and 2) clusters, which can be attributed to oxygen vacancies, which induce more disorder to the material and more deep defects in the forbidden region.
3.7. Morphology Control. Surface energy does not only dictate the surface structure and stability, but it is also a fingerprint to measure the catalytic activity and selectivity, as well as to follow the degradation process and the crystallization pathway. The surface energy value provides insight into the resistance of a given material to sintering, ripening, and in dissolution processes. Surface energy is not a constant material property and may depend on the environment that surrounds the surface, such as the adsorption of a reactant, surfactant, or any species present in the solution, which can promote the stability of a particular surface, structure, or facet. Therefore, the control of the crystal morphology is a complex process depending on several factors related to crystal internal structures and external agents. Hence, the synthesis process can have a strong influence on the final product. In our study, when the anionic surfactant SDS is included in the synthesis of α-Ag 2 WO 4 , the result indicates that SDS works as a morphology-controlling agent.
It is well-established that, during crystal growth, the shape evolution is controlled by the surface energy of each face. Under equilibrium conditions, faces with high surface energies tend to grow rapidly, so that they usually disappear to minimize the total surface energy. 40 In a given environment, driven by the experimental conditions, the surface energies of different surfaces can be effectively decreased by the selective adsorption of appropriate inorganic or organic agents. 41 Bakshi 22 showed that the adsorption of a surfactant in aqueous media takes place through bilayer formation. The polar heads of surfactant molecules adhere to the charged nanoparticle surfaces, exposing the organic surfactant tails to the bulk solution, where they are insoluble. A second surfactant layer adsorbs to the first one, making a bilayer. This kind of adsorption is usually selective to a specific crystal plane, which leads to an ordered shape evolution. Hydrophobicity is a very important characteristic in surfactant action and has a strong correlation with the final morphology. The cubic shape is usually generated in the presence of a highly hydrophobic medium.
In this way, the surface plays an important role in the material growth process, and it is fundamental to study the atomic arrangement on each surface. Table 2 presents the surface energy values for each surface. 24 By employing the Wulff construction we can obtain the ideal morphology of α-Ag 2 WO 4 , as well as the experimentally observed hexagonal rodlike α-Ag 2 WO 4 -R (see Figure 4B) and the cuboid-like α-Ag 2 WO 4 -C (see Figure 4D) morphologies. The E surf values show the following stability order: (010) < (100) < (001) < (110) < (101) < (011). Figure 10 43 A detailed analysis comparing the two different surfaces in the α-Ag 2 WO 4 -R and α-Ag 2 WO 4 -C morphologies, namely, the (101) and (100) exposed facets (see Figure 10), indicates that both surfaces have Ag1 and Ag2 cations at the top of each surface, but the (101) surface presents more under-coordinated [WO 6 ] clusters near the top of the surface with respect to the (100) surface.
The natural starting point for predicting morphology type is to establish the relative stabilities of all surfaces, where the lowest surface energy will be thermodynamically preferred. While it is well-established that the kinetics required to reach different morphologies from a specific precursor state can often be manipulated by the presence of a surfactant to result in the synthesis of another morphology, the relative stability of kinetically accessible morphologies then falls within a fairly narrow range of energies. The relative stabilities of each morphology can be directly modified if they are grown in the presence of a surfactant. The surface energies of the different facets can be modified by interaction with the surfactant, which causes reordering in their relative stabilities.
Wulff's crystal representation of the optimized α-Ag 2 WO 4 is depicted in the central part of Figure 11, and the different morphologies, hexagonal rod-like α-Ag 2 WO 4 -R and cuboid-like α-Ag 2 WO 4 -C, can be obtained by assuming different values for the surface energies of the different facets. This interpretation has the advantage that all faces grow from the initial α-Ag 2 WO 4 crystal (α-Ag 2 WO 4 −ideal), depending on their surface energy value.
From the calculated values of polyhedron energies reported in Table 2, we can obtain the energy profile that links the ideal morphology to the experimentally observed hexagonal rod-like, α-Ag 2 WO 4 -R (see a Reaction Path (A) on Figure 11) and cuboid-like α-Ag 2 WO 4 -C (see Reaction Path (B) on Figure 11) morphologies. Figure 11 illustrates the good agreement between the experimental and theoretical morphologies. The principal idea of this representation is that the α-Ag 2 WO 4 -R and α-Ag 2 WO 4 -C, associated with stable morphologies, can be energetically related to the ideal morphology along two alternative pathways via intermediates A−G.
The pathway depicted in Figure 11A involves five steps: the first one is achieved by decreasing the value of E surf of the (101) surface with appearance of the A morphology, followed by two stages related to an increase and decrease in the values of E surf for the (100) and the (001) surfaces, with formation of the B and C morphologies, respectively. The C morphology resembles the experimental α-Ag 2 WO 4 -R (see Figure 4A); however, it is not elongated, as seen in Figure 4B. From C and by increasing the values of E surf for the (010) surface, an intermediate D morphology is reached, and by increasing the value of E surf for the (010) surface, the E morphology is obtained, corresponding to a maximum value of E polyhedron . Finally, by increasing the value of E surf for the (110) surface, the α-Ag 2 WO 4 -R morphology is achieved (see Figure 4B). Along this pathway, there is an energy barrier in the fourth step.
The pathway depicted in Figure 11B corresponds to the process linking the α-Ag 2 WO 4 -C morphology and the ideal morphology. This path involves a decreasing in E surf values for the (100) and (001) surfaces and is a barrierless process, via E and F morphologies. This is the energetically favorable path and allows us to explain the effect of the anionic surfactant SDS in the synthesis process. As observed in the FE-SEM images in Figure 4C,D, the Ag 2 WO 4 -C morphology can be cubic and elongated-cubic. In this way, the four morphologies (ideal from theoretical calculations, F, G, and Ag 2 WO 4 -C morphologies from Figure 11B) involved in the pathway can be associated with the crystal obtained by the CP method at 90°C with anionic surfactant SDS, since that Ag 2 WO 4 -C morphology is formed by the combination of (010), (100), and (001) surfaces.
The order of stability of α-Ag 2 WO 4 morphologies is α-Ag 2 WO 4 -C < α-Ag 2 WO 4 ideal < α-Ag 2 WO 4 -R, as shown in Figure 11. The presence of the anionic surfactant SDS in the synthesis of the α-Ag 2 WO 4 crystal prevents the formation of α-Ag 2 WO 4 -R, resulting in the disappearance of (101) surface and the appearance of the (100) surface to render the α-Ag 2 WO 4 -C morphology. Analysis of Figure 11B indicates that the anionic surfactant SDS interacts with and stabilizes the (100) and (001) surfaces, mainly on the Ag cations of the under-coordinated clusters. SDS also prevents increase of E surf of the (010) surface to form the elongated hexagonal rod-like morphology α-Ag 2 WO 4 -R.
SDS has proven to be an appropriate morphology-directing agent in the CP synthesis of α-Ag 2 WO 4 . This is primarily related to the surface adsorption of the anionic part of the surfactant on the different crystal surfaces, thus controlling their The Journal of Physical Chemistry C Article overall morphology. This kind of adsorption is usually selective to a specific crystal plane, because the active centers of the surface are mainly associated with under-coordinated Ag cations, that is, [AgO n ] clusters, and consequently, such adsorption leads to an ordered morphology evolution.
3.8. Photocatalytic Degradation. The structural analysis of both synthesized samples of α-Ag 2 WO 4 shows that the main difference between the samples is related to surface effects. There are no significant differences in the results of XRD, FT-IR, and FT-Raman spectroscopy between both morphologies. PL analysis, however, showed significant differences in the types of defects. These experimental results, combined with the theoretical simulation of the surfaces and clusters, indicates that the behavior observed in photocatalytic reactions is due, exclusively, to morphological aspects, and a particular crystal plane enhances both the reduction/oxidation sites and the photocatalysis process. 44 Generally, the photocatalytic process involves the following three steps: (i) the absorption of photons with energy larger than the band gap of the photocatalyst; (ii) the generation, separation, migration, or recombination of photogenerated electron−hole pairs, and (iii) the redox reactions on the photocatalyst surface. The degradation process of RhB is based on an oxidative attack by active oxygen species on an N-ethyl group. The loss of an ethyl group (N,N,N′-triethylated) at RhB has λ max = 539 nm, the loss of two ethyl groups, that results in N,N′-diethylated Rhodamine has a λ max = 522 nm, the N′ethylated Rhodamine (λ max = 510 nm), and the de-ethylated form present at λ max = 498 nm. 45 We monitored the degradation process by analyzing the maximum absorption bands of the RhB spectra as a function of time for both α-Ag 2 WO 4 -R and α-Ag 2 WO 4 -C samples. The photocatalytic activities of the α-Ag 2 WO 4 -R and α-Ag 2 WO 4 -C samples are given in Figure 12A,B, respectively. The degradation of RhB UV radiation without photocatalyst or in the dark in the presence of photocatalyst is performed to understand the influence of the light or photocatalyst. The photocatalytic degradation of RhB follows pseudo-first-order kinetics, exhibiting a linear relationship between log(C 0 /C t ) and the reaction time. Analysis of Figure 12A indicates a significant reduction (almost 80%) at the absorption maximum of the RhB solution after 120 min of reaction in the presence of α-Ag 2 WO 4 -R as catalyst. This result correlates well with the results found in the literature for the same catalyst and conditions. 7 However, the control experiment showed that, without a photocatalyst and with α-Ag 2 WO 4 -C, RhB hardly decomposed during photolysis over a period of 120 min. Analysis of Figure 12A,B indicates that RhB exhibits one band with maximum absorption at 554 nm. In addition, Figure 12A also illustrates a significant decrease in the height of the absorption maximum of RhB during the photodegradation process, while Figure 12B shows that, after 120 min, a small percentage of the dye was degraded (only 37%). This value is quite close to that of the photolysis reaction in the absence of a catalyst. Figure 12C,D displays the linear plots of the standard kinetic data curves obtained for the RhB photodegradation process. The discoloration reaction follows first-order kinetics and can be described by the relationship d[C]/dt = k[C], where [C] is the RhB concentration, and k indicates the overall photodegradation rate constant and activity. By plotting log(C/C 0 ) as a function of time through linear regression, where C 0 is the initial concentration of RhB and C is the concentration at time t, we obtained the constant k (min −1 ) for the photocatalysis The Journal of Physical Chemistry C Article under UV−vis irradiation of each sample, from the gradients of the simulated straight lines. The corresponding values of k for both Ag 2 WO 4 -R and Ag 2 WO 4 -C are 4.3 × 10 −3 min −1 and 1.8 × 10 −3 min −1 , respectively, while the corresponding value without a catalyst is 2.1 × 10 −3 min −1 . These results point out that only the α-Ag 2 WO 4 -R is working as a catalyst, whereas the cuboid-like morphology of α-Ag 2 WO 4 negatively affects the photocatalytic activity of α-Ag 2 WO 4 . For comparison purposes, the value reported for commercial TiO 2 anatase (Aldrich, 99.7%), under similar experimental conditions, is k = 2.48 (min −1 ); this is a widely applied standard for several authors. 46 Several factors can influence reaction rate, namely, the structural defects ascribed to ordered−disordered clusters, crystallographic preferred orientation, intermediate electronic levels, high surface energy, roughness, high active surface area, facets, and so on. 47 The presence of residual anionic surfactant can also contribute to a lower photocatalytic efficiency, acting as hole scavengers. 48 However, we ensured the complete removal of surfactant, as verified by TGA (Supporting Information). According to BET data, α-Ag 2 WO 4 possesses a surface area of 3.72 m 2 /g for the α-Ag 2 WO 4 -R and only 1.22 m 2 /g for the α-Ag 2 WO 4 -C. The larger surface area for the α-Ag 2 WO 4 -R in relation to α-Ag 2 WO 4 -C is in agreement with the superior performance as photocatalyst of α-Ag 2 WO 4 -R with respect to α-Ag 2 WO 4 -C.
Each surface defect can act as an adsorption site for oxygen and water. In the presence of air or oxygen, irradiated semiconductors can destroy many organic contaminants. The activation of Ag 2 WO 4 by light (hν) produces electron−hole pairs, which are powerful oxidizing and reducing agents, respectively, and the main determinant factor of catalyst efficiency is the low recombination rate between photogenerated electrons and holes on the semiconductor surface.
In the degradation of organic compounds (such as RhB), the hydroxyl radical OH*, which comes from the oxidation of adsorbed water, is the primary oxidant; and the presence of oxygen can prevent the recombination of hole−electron pairs.
For the generation of all the reactive oxygen species, strong adsorption of oxygen molecule O 2 with concomitant electron transfer process from the oxygen vacancies 50 is necessary. In our case, both distorted [WO 6 ] and [AgO n ] clusters possess different electron density and oxygen vacancies with the capability of producing electron−hole pairs. However, thanks to the different nature of the Ag−O and W−O bonds within these clusters, the lability and the stability of these vacancies change according to the Ag/W under-coordinated clusters. The predominant presence of the surface (101) is responsible for the hexagonal rod-like morphology, α-Ag 2 WO 4 -R, in which present under-coordinated Ag and W cations to be active for the RhB photocatalytic degradation.
Although the hexagonal rod-like morphology, α-Ag 2 WO 4 -R, imparts a photocatalytic effect, its activity is much lower than other common catalysts, like TiO 2 . Roca and co-workers 7 synthesized other more energetic facets of α-Ag 2 WO 4 by means of the microwave-assisted hydrothermal method, which is a very efficient method, but with several limitations mentioned previously. We show here that the simple use of a surfactant in a short time reaction can efficiently promote changes in morphology that directly influence the final properties of α-Ag 2 WO 4 .

CONCLUSIONS
Good crystal facet engineering is crucial for the successful synthesis of semiconductor materials with functional applications. To develop functional materials, it is important to control their morphology and structure. The fundamental issue is a lack of in-depth knowledge about nanoparticle formation and crystallization mechanisms and about their surface chemistry. It is important to point out that the chemical environment strongly influences the growth and morphology of the crystals. Altering environmental parameters, for instance, by including surfactants, may result in modification of the surface energies of crystal faces, and consequently, the crystals obtained can present different morphologies.
The main conclusions of the present study can be summarized as follows: (i) We have experimentally obtained the cuboid-like morphology of α-Ag 2 WO 4 -C using a simple controlled-precipitation method with the anionic surfactant SDS dissolved in a sodium tungstate solution. (ii) Although the Wulff construction, obtained by first-principles calculations, renders that the ideal morphology of α-Ag 2 WO 4 is the cube, this morphology is not spontaneously reached by the simple combination of those precursors in the aqueous medium. (iii) α-Ag 2 WO 4 -C morphology is composed of the (001), (010), and (100) surfaces, and the adsorption of the SDS anionic head on the (100) facet provokes the stabilization of the (100) surface and prevents the growth process along the (101) surface. (iv) By calculating the energy profiles connecting these morphologies, a deeper insight, at atomic level, on the role of SDS surfactant is achieved, and hence, SDS has been successfully used as a morphology-controlling agent for the synthesis of α-Ag 2 WO 4 -C. (v) The calculated energy profile, based on the Wulff construction, points out that the anionic surfactant SDS interacts with and stabilizes the (100) 6 ] clusters, with a minor contribution of under-coordinated [AgO 7 ] clusters. (vii) Correlation between the exposed surfaces and photocatalytic activity was revealed, and an explanation of this behavior, arising from different morphologies and structural data, was provided. (viii) Theoretical calculations confirm the rationality of the experimental scheme and elucidate the underlying reason for the photodegradation process of Rhodamine B.
These results highlight the importance of considering the chemical environment and the adsorption of surfactant along with the synthetic route when determining the stability and activity of a catalyst with a defined morphology. Our model also reveals some details about the mechanism involved and gives some important information about how to improve photocatalytic activity by designing the morphology of the patterned surface or improving the synthesis conditions or restructuring/ shape control techniques.
Rather than individual numbers, the most important outcomes of these simulations are the general, chemical trends they have revealed. There, and here, predictive atomistic simulations are likely to play an increasingly important role. The rationally controlled chemical synthesis of well-defined morphologies may open a valuable synthetic route, and, in terms of future perspectives, this morphology control strategy can be extended to explore other Ag-based materials, where shape is the most important characteristic for application.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01898.