{ 0 0 1 }-Facet-Exposed Ag 4 V 2 O 7 Nanoplates : Additive-Free Hydrothermal Synthesis and Enhanced Photocatalytic Activity

The synthesis of silver pyrovanadate, Ag4V2O7, nanoplates with exposed {0 0 1}-facets by a facile, additive-free hydrothermal method was described in this paper. The photocatalytic activity of rhodamine B over Ag4V2O7 samples under solar light irradiation was also evaluated. By using an equimolar mixture of NH4VO3 and AgNO3 with the presence of a suitable amount of ammonia, Ag4V2O7 nanoplates were obtained readily and purely at temperatures from 100 to 140°C for 4 h. In particular, the c-axis orientation growth of Ag4V2O7 nanoplates occurred and increased monotonously with temperatures in the range of over 100 up to 140°C. Further increase in hydrothermal temperature up to 220°C, the Ag4V2O7 phase no longer existed and the β-AgVO3 phase was formed instead. The photocatalytic activity of the optimized Ag4V2O7 sample comprising {0 0 1}-facet-exposed nanoplates with the highest degree of orientation was significantly higher than that of the random-oriented sample. The effects of using ammonia as a complexing agent on the structure, microstructure, texture, exposed facet, and photocatalytic activity of Ag4V2O7 samples were also investigated for the first time.


Introduction
Novel ternary oxide-based photocatalysts including silver vanadates as alternatives to traditional TiO 2 have been recently attracted a lot of attention in both basic research and applications [1][2][3][4].For these photocatalytic systems, current research activities focus on enhancing their photocatalytic performances via morphological controlling, crystal facet engineering, and reducing band gap by elemental doping separately or simultaneously [5][6][7][8][9].The reduction in the average grain size of a photocatalyst from microto nanoscale, for instance, was confirmed to enhance the photocatalytic activity due to the increase in its surface area and light absorption [5].By applying crystal facet engineering, in another approach, the surface energy of textured grains of obtained photocatalytic samples with certain exposed facet is higher than that of randomoriented ones [10][11][12].This originated from the fact that the photocatalytic reactivity and efficiency of a photocatalyst are strongly affected by their surface energy and chemisorption properties.Comparing to the case of randomly oriented samples, the photocatalytic process is more accelerated with these textured grains because the adsorption-desorption rates of available species existed on the surface become faster.However, due to their thermodynamic instability with higher surface energy, these highly reactive facets tend to be eliminated during the crystal growth process [13].To solve this problem, additives were usually employed to tune the growth rates of desired crystal planes [14].Particularly, for the crystal facet engineering of a powdered photocatalyst prepared by solution chemistry, various chemicals like surfactants [15], ionic liquids [16], or dopants [17] can be used as additives.By using sodium dodecylbenzenesulfonate (SDBS) and cetyltrimethylammonium bromide (CTAB) as anionic and cationic surfactants, respectively, the ratio of the growth rate along the [1 0 0] direction to that of along [1 1 1] direction of ZnSnO 3 can be varied controllably [15].Recently, ionic liquids were reported to be used as green alternatives to cytotoxic surfactants such as CTAB to control the facet engineering process.By using 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim] + [BF 4 ] − ), an ionic liquid, as a morphology-controlling agent, the anatase TiO 2 nanocrystals enclosed by exposed {0 0 1}-facets were obtained via the release of F − ions from [BF 4 ] − anions that stabilized the {0 0 1}-facets, leading to the preferred growth of these facets [16].In another approach, Yang et al. indicated that the doping of Ta 5+ ions into rutile TiO 2 crystal lattice enabled tuning of the surface energy of lattice planes and caused induced microstrains that would retard the growth of rutile TiO 2 crystals along the c-axis direction.As a result, the Ta 5 + -doped rutile TiO 2 arrays were grown preferably along [1 1 0] direction, instead of along c-axis observed for undoped samples [17].There are only a limited number of additivefree chemical synthesis procedures of faceted photocatalysts, in which other preparative parameters than additives were exploited for facet engineering so far [18,19].Liang et al. synthesized nanoparticle-aggregated cone-like ZnO mesocrystals enclosed by curved 1 0 1 1 and flat 0 0 0 1 surfaces by a water-induced precursor-hydrolysis process in a water/ organic solvent system [18].The results showed that the volume ratio between organic solvent and water played a key role for tailoring the final crystal shape of ZnO with controllable facets.In our recent work, by using freshly produced Nb 2 O 5 •H 2 O instead of frequently used, commercialized Nb 2 O 5 as a precursor, the c-axis-oriented and {0 0 1}-facetexposed rhombohedral NaNbO 3 plate-like microcrystals with enhanced photocatalytic activity were resulted [19].The formation of the faceted rhombohedral NaNbO 3 platelike microcrystals synthesized by a template-free hydrothermal method was suggested to be attributed to the difference in chemical reactivity between Nb 2 O 5 •H 2 O and other polymorphisms of Nb 2 O 5 .However, the facet engineering research topic has not been yet carried out for silver vanadates in general and silver pyrovanadate, Ag 4 V 2 O 7 , in particular [20][21][22][23].In details, most of these works studied the enhancing photocatalytic activities of Ag 4 V 2 O 7 -based photocatalysts via controlling their microstructural and morphological properties by a hydrothermal synthesis method.Without using any surfactant, only random-oriented, single crystalline Ag 4 V 2 O 7 microcrystals were obtained [20].The similar situation was also observed for Ag 4 V 2 O 7 -based photocatalytic composites such as Ag 2 O/Ag 3 VO 4 /Ag 4 V 2 O 7 or Ag 3 VO 4 /Ag 4 V 2 O 7 synthesized by surfactant-free hydrothermal route when only Ag 4 V 2 O 7 powders with microscale sizes were obtained [24,25].It is surveyed that there was only one work reported on the sodium dodecyl sulfate-assisted hydrothermal synthesis of the nanosized Ag 4 V 2 O 7 decorated by Ag nanoparticles [26].To date, to the best of our knowledge, the synthesis and photocatalytic activity of faceted Ag 4 V 2 O 7 in general and {0 0 1}-faceted Ag 4 V 2 O 7 nanosized powders in particular have not yet been reported.
In this work, the additive-free hydrothermal synthesis of Ag 4 V 2 O 7 nanoplates with exposed {0 0 1}-facets from NH 4 VO 3 , AgNO 3 , and ammonia was performed for the first time.The photodegradation of rhodamine B (RhB) over these samples under solar light irradiation was also investigated.

Synthesis of Ag
For synthesis of Ag 4 V 2 O 7 , 0.468 g NH 4 VO 3 (Sigma-Aldrich) was firstly dissolved in 50 mL deionized water at 80 °C to obtain a yellow solution.This solution was added into a mixture containing 0.68 g AgNO 3 (Sigma-Aldrich, 99.5%) and 4 mL of NH 3 (Sigma-Aldrich, p.a.) with different concentrations during stirring.The obtained mixture with the Ag + /V 5+ molar ratio of 1 : 1 was transferred into a 125 mL Teflon-lined stainless steel autoclave.The autoclave was filled by deionized water until 80% of its inner volume was occupied, sealed, and heated at a certain temperature for 4 h under autogenous pressure.The samples were filtered, washed several times with deionized water until a neutral pH value was reached, and dried at 50 °C for 12 h.

Characterization Methods.
The crystalline structural characterization of the synthesized samples was carried out by X-ray powder diffraction (Bruker D8 Advance diffractometer, CuK α radiation (λ = 1.5406Å)).Their morphological, microstructural, and optical properties were analyzed by using high-resolution transmission electron microscopy (JEOL 2100), field-emission scanning electron microscopy (FE-SEM) (Hitachi S-4800), and UV-Vis diffuse reflectance spectrometry (DR-UV-Vis) (JASCO V670).The photodegradation of rhodamine B over synthesized Ag 4 V 2 O 7 samples was evaluated under solar light irradiation using a solar simulator (Model: 69907, Newport) with the applied light intensity of 100 mW•cm −2 .In details, 0.05 g of the photocatalytic sample was added to 50 mL of the 1.10 −6 ppm RhB solution.Before being subjected to photocatalytic test, the solution was stirred magnetically for 60 min in the dark to allow the adsorption-desorption  Journal of Nanomaterials equilibrium to be reached.After every 30 minutes, 3 mL of tested solution was taken and centrifuged to remove the suspended photocatalyst for RhB concentration determination with an aid of a Shimadzu UV-1800 spectrophotometer.

Results and Discussion
The crystal phase evolution of samples with different hydrothermal temperatures was performed with results as shown in Figure 1.For samples synthesized at hydrothermal temperatures ranging from 100 to 140 °C for 4 h, the pure crystalline orthorhombic phase of Ag 4 V 2 O 7 (ICDD card # 77-0097) was obtained.The most interesting feature is that when temperatures valued at 120 and 140 °C, the diffraction intensities of (0 0 4) peak located at 2θ angle of 25.57°became significantly dominated to that of (6 2 0) peak, indicating a c-axis crystallographic orientation.The I 0 0 4 /I From SEM images of random-and c-axis-oriented Ag 4 V 2 O 7 powders synthesized hydrothermally at 100 and 140 °C for 4 h, respectively, the nanoplates were observed in both samples (Figure 2).One can recognized that when the hydrothermal temperature increased from 100 to 140 °C, the average diameter and thickness of the plate-like grains increased slightly from 370 to 400 nm and from 54 to 60 nm, respectively.From the HRTEM image and corresponding FFT pattern taken on the surface of Ag 4 V 2 O 7 nanoplates of the sample synthesized hydrothermally at 140 °C for 4 h, two orthogonal lattice fringes with the interplanar spacings of 0.31 and 0.27 nm were well indexed to (6 0 0) and (0 4 0) lattice planes of Ag 4 V 2 O 7 , respectively (Figure 3).The exposed {0 0 1}-facets of the obtained Ag 4 V 2 O 7 nanoplates were then derived.Thus, in combination with aforementioned XRD results, these observations evidenced that the Ag 4 V 2 O 7 nanoplates favoured to grow along [0 0 1] direction.Based on all above characterization results, it is clear that the single-phase Ag 4 V 2 O 7 nanopowders having relative high homogeneity and well-defined morphology were given at hydrothermal temperatures ranging from 100 to 140 °C.This might benefit from using a suitable amount of ammonia and the Ag + /V 5+ molar ratio of 1 : 1 with respect to published hydrothermal procedures that always produced Ag 4 V 2 O 7 microcrystals, with or without additives [20,23].The autoclaving of an ammonia-free reaction mixture at 140 °C for 4 h was also performed with the single-phase powders of β-AgVO 3 as as-synthesized product (figure not shown).With the presence of ammonia, in mild basic medium of the hydrothermal mixture, the formation of the desired Ag 4 V 2 O 7 phase can be suggested to differ from that reported in literature and can be described as follows [27,28]:

Journal of Nanomaterials
The pH of our reaction mixture was adjusted to over 7 by ammonia solution while this value was remained lower in previous procedures [22,25].This enabled a part of ammonia amount to react with Ag + ions to form [Ag(NH 3 ) 2 ] + complexes (3).Therefore, the phase formation rate of Ag 4 V 2 O 7 became slower than the case of aqueous Ag + ions for the reason that it is necessary for Ag + ions to be released from [Ag(NH 3 ) 2 ] + (4), similar to the previous works [29,30].This probably facilitates Ag 4 V 2 O 7 crystals of samples synthesized at temperatures ranging from 100 to 140 °C to nucleate and grow anisotropically.In addition, the slow release of Ag + ions might be also a reason of obtaining the average size of Ag 4 V 2 O 7 plates in nanoscale although no additives were used with respect to the published work [26].When temperature was increased over 140 °C, the β-AgVO 3 phase occurred uniquely.It is worthy to note that to obtain the single-phase Ag 4 V 2 O 7 crystals with the highest c-axis orientation degree and yield of about 40%, the optimized amount and concentration of ammonia of 4 mL and 2.5 M, respectively, providing the pH of 9.5 for reaction mixture are required strictly.As illustrated in Figure 4, the band gap energy values of the random-and c-axis-oriented samples of 2.44 and 2.47 eV, respectively, were almost the same and fell in the range found for Ag 4 V 2 O 7 [19,20].
The evolution of the RhB photodegradation rate as a function of irradiation time over the random-and c-axis-oriented Ag 4 V 2 O 7 samples autoclaved at 100 and 140 °C, respectively, was indicated in Figure 5.
After being irradiated for 90 minutes under a solar source, the rhodamine B photodegradation efficiency of 92% was found for the c-axis-oriented Ag 4 V 2 O 7 sample while that of the random-oriented sample valued at 71%.From all investigated results, it can be proposed that the grain size and the reactivity of exposed {0 0 1}-facets are two main factors contributing to the enhanced photocatalytic activity of the synthesized samples.However, compared to the case of the random-oriented sample, the higher overall photocatalytic activity that the c-axis-oriented Ag 4 V 2 O 7 sample exhibited was probably because their dominated exposed {0 0 1}-facets accelerated the photocatalytic processes on the surface.This can be explained by the fact that although the random-oriented sample possessed a higher size-induced photocatalytic activity than that of the c-axis-oriented Ag 4 V 2 O 7 sample comprising larger nanoplates, the facet-induced photocatalytic enhancement dominated over this size-induced one in our study.Thus, for the {0 0 1}-facet-exposed Ag 4 V 2 O 7 sample, both main photocatalytic activity-enhancing solutions for synthesizing nanoscale photocatalysts and crystal facet engineering were combined by simply exploiting the excellent complexing ability of Ag + ions with ammonia.For the photodegradation of RhB under visible light irradiation, the photocatalytic efficiency of this faceted Ag 4 V 2 O 7 nanoplates was not only significantly higher than that reported for silver pyrovanadate [20,24] [24,[34][35][36].From these results, it is expected to apply the additive-free facet engineering approach to silver vanadate-based nanophotocatalysts via forming complexes of Ag + ions as an intermediate, similar to the case of Ag 3 PO 4 [37].It should be noted that commonly used crystal facet engineering techniques like using template, ionic liquids, or surfactant as structure-directing agents, to a certain extent, are limited for silver-containing photocatalysts due to the ease of reducing and forming undesired insoluble compounds of Ag + ions during the synthesis process [9,38].Figure 6 demonstrated the recyclability testing results of RhB over the random-and c-axis-oriented Ag 4 V 2 O 7 samples for three recycling photocatalytic runs under solar light irradiation.The results revealed that, for c-axis-oriented Ag 4 V 2 O 7 sample, the photodegradation efficiency decreased to 89.7% and 86.5% for the second and third cycles, respectively.The similar trend was also observed for the case of the random-oriented sample with photodegradation efficiency of 66.6% and 64.3% for the second and the third cycles, respectively, lower than that of the first cycle (71%).These slight decreases in photodegradation efficiency indicated that both random-and c-axis-oriented Ag 4 V 2 O 7 samples are reusable with high stability after three RhB photodegradation cycles.

Conclusions
In summary, single-phase silver pyrovanadate, Ag 4 V 2 O 7 , nanoplates were synthesized readily by an additive-free hydrothermal method at temperatures from 100 to 140 °C by using NH 4 VO 3 , AgNO 3 , and ammonia as starting materials.The increase in the hydrothermal temperature facilitates silver pyrovanadate to grow preferably along [0 0 1] crystallographic direction and provided {0 0 1}-facetexposed nanoplates with the highest c-axis orientation degree at 140 °C for the first time.The photodegradation efficiency of rhodamine B over random-and c-axis-oriented Ag 4 V 2 O 7 samples under solar light irradiation was 71 and 92%, respectively.The enhanced photocatalytic activity of the c-axis-oriented Ag 4 V 2 O 7 sample is attributed to their exposed {0 0 1}-facets.This work is expected to provide the possibility to use ammonia as both a pH adjusting agent and a complexing agent to form [Ag(NH 3 ) 2 ] + complexes with Ag + ions, allowing photocatalytic crystals to nucleate and grow anisotropically with a slow rate to obtain faceted nanophotocatalysts of silver vanadate and other silvercontaining ternary oxides without using any additives.

Figure 3 :
Figure 3: HRTEM image and FFT pattern (inset) of the Ag 4 V 2 O 7 sample synthesized at 140 °C for 4 h.

Figure 6 :
Figure 6: Photodegradation recyclability test of rhodamine B over (a) c-axis and (b) random-oriented Ag 4 V 2 O 7 samples under solar light irradiation.
62 0 ratio reached the highest value of 2.5 with the sample synthesized at 140 °Cin comparison to that of 0.28 documented from the standard pattern of Ag 4 V 2 O 7 .However, for the sample autoclaved at 160 °C, the Ag 4 V 2 O 7 phase underwent a rapid phase transformation into β-AgVO 3 with no trace of Ag 4 V 2 O 7 was detected but the pure β-AgVO 3 phase with monoclinic structure (ICDD card # 29-1154) existed instead.This silver metavanadate phase remained uniquely with further increase in reaction temperature up to 220 °C.
but also for other vanadate-based nanophotocatalysts such as Ag 3 VO 4 , FeVO 4 , FeVO 4 /V 2 O 5 , BiVO 4 , and BiVO 4 /TiO 2[24, 31- 33].Furthermore, in comparison to other novel visible light active ternary oxide-based photocatalysts, it is quite perspective to use our synthesized materials as competitive alternatives to TiO 2 for photodegradation of dyes.This originated from the fact that their photocatalytic activity is distinctive superior to BiFeO 3 and BiFeO 3 -Bi 2 WO 6 and comparable to, for example, Ag 2 O/Ag 3 VO 4 /Ag 4 V 2 O 7 and Ag 3 PO 4 /SBA-15