# {0 0 1}-Facet-Exposed Ag4V2O7 Nanoplates: Additive-Free Hydrothermal Synthesis and Enhanced Photocatalytic Activity

Authors: Nguyen Duc Van; Ngo Thi Hong Le
Journal: Journal of Nanomaterials
 (2018)

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## Abstract

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.


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## Body

## 1. Introduction

Novel ternary oxide-based photocatalysts including silver vanadates as alternatives to traditional TiO2 have been recently attracted a lot of attention in both basic research and applications [1–4]. 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 [5–9]. The reduction in the average grain size of a photocatalyst from micro- to 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 random-oriented ones [10–12]. 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 ZnSnO3 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]+[BF4]−), an ionic liquid, as a morphology-controlling agent, the anatase TiO2 nanocrystals enclosed by exposed {0 0 1}-facets were obtained via the release of F− ions from [BF4]− 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 Ta5+ ions into rutile TiO2 crystal lattice enabled tuning of the surface energy of lattice planes and caused induced microstrains that would retard the growth of rutile TiO2 crystals along the c-axis direction. As a result, the Ta5+-doped rutile TiO2 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 additive-free 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 101¯1 and flat 0001¯ 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 Nb2O5·H2O instead of frequently used, commercialized Nb2O5 as a precursor, the c-axis-oriented and {0 0 1}-facet-exposed rhombohedral NaNbO3 plate-like microcrystals with enhanced photocatalytic activity were resulted [19]. The formation of the faceted rhombohedral NaNbO3 plate-like microcrystals synthesized by a template-free hydrothermal method was suggested to be attributed to the difference in chemical reactivity between Nb2O5·H2O and other polymorphisms of Nb2O5. However, the facet engineering research topic has not been yet carried out for silver vanadates in general and silver pyrovanadate, Ag4V2O7, in particular [20–23]. In details, most of these works studied the enhancing photocatalytic activities of Ag4V2O7-based photocatalysts via controlling their microstructural and morphological properties by a hydrothermal synthesis method. Without using any surfactant, only random-oriented, single crystalline Ag4V2O7 microcrystals were obtained [20]. The similar situation was also observed for Ag4V2O7-based photocatalytic composites such as Ag2O/Ag3VO4/Ag4V2O7 or Ag3VO4/Ag4V2O7 synthesized by surfactant-free hydrothermal route when only Ag4V2O7 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 Ag4V2O7 decorated by Ag nanoparticles [26]. To date, to the best of our knowledge, the synthesis and photocatalytic activity of faceted Ag4V2O7 in general and {0 0 1}-faceted Ag4V2O7 nanosized powders in particular have not yet been reported.In this work, the additive-free hydrothermal synthesis of Ag4V2O7 nanoplates with exposed {0 0 1}-facets from NH4VO3, AgNO3, and ammonia was performed for the first time. The photodegradation of rhodamine B (RhB) over these samples under solar light irradiation was also investigated.


## 2. Materials and Methods

### 2.1. Synthesis of Ag4V2O7

For synthesis of Ag4V2O7, 0.468 g NH4VO3 (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 AgNO3 (Sigma-Aldrich, 99.5%) and 4 mL of NH3 (Sigma-Aldrich, p.a.) with different concentrations during stirring. The obtained mixture with the Ag+/V5+ 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.


### 2.2. Characterization Methods

The crystalline structural characterization of the synthesized samples was carried out by X-ray powder diffraction (Bruker D8 Advance diffractometer, CuKα radiation (λ = 1.5406 Å)). 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 Ag4V2O7 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 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.


## 2.1. Synthesis of Ag4V2O7

For synthesis of Ag4V2O7, 0.468 g NH4VO3 (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 AgNO3 (Sigma-Aldrich, 99.5%) and 4 mL of NH3 (Sigma-Aldrich, p.a.) with different concentrations during stirring. The obtained mixture with the Ag+/V5+ 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.


## 2.2. Characterization Methods

The crystalline structural characterization of the synthesized samples was carried out by X-ray powder diffraction (Bruker D8 Advance diffractometer, CuKα radiation (λ = 1.5406 Å)). 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 Ag4V2O7 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 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.


## 3. Results and Discussion

The crystal phase evolution of samples with different hydrothermal temperatures was performed with results as shown in Figure1. For samples synthesized at hydrothermal temperatures ranging from 100 to 140°C for 4 h, the pure crystalline orthorhombic phase of Ag4V2O7 (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 I004/I620 ratio reached the highest value of 2.5 with the sample synthesized at 140°C in comparison to that of 0.28 documented from the standard pattern of Ag4V2O7. However, for the sample autoclaved at 160°C, the Ag4V2O7 phase underwent a rapid phase transformation into β-AgVO3 with no trace of Ag4V2O7 was detected but the pure β-AgVO3 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.Figure 1
XRD diagrams of the samples synthesized at hydrothermal temperature of (a) 100, (b) 120, (c) 140, (d) 160, and (e) 220°C for 4 h.From SEM images of random- andc-axis-oriented Ag4V2O7 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 Ag4V2O7 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 Ag4V2O7, respectively (Figure 3). The exposed {0 0 1}-facets of the obtained Ag4V2O7 nanoplates were then derived. Thus, in combination with aforementioned XRD results, these observations evidenced that the Ag4V2O7 nanoplates favoured to grow along [0 0 1] direction. Based on all above characterization results, it is clear that the single-phase Ag4V2O7 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+/V5+ molar ratio of 1 : 1 with respect to published hydrothermal procedures that always produced Ag4V2O7 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-AgVO3 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 Ag4V2O7 phase can be suggested to differ from that reported in literature and can be described as follows [27, 28]:
(1)VO3−+OH–→VO43−+H+(2)2VO43−+2H+→V2O74−+H2O(3)Ag++2NH3→AgNH32+(4)4AgNH32++V2O74−→Ag4V2O7+8NH3Figure 2
FE-SEM images of Ag4V2O7 samples synthesized at (a) 100 and (b) 140°C for 4 h.
(a)
(b)Figure 3
HRTEM image and FFT pattern (inset) of the Ag4V2O7 sample synthesized at 140°C for 4 h.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(NH3)2]+ complexes (3). Therefore, the phase formation rate of Ag4V2O7 became slower than the case of aqueous Ag+ ions for the reason that it is necessary for Ag+ ions to be released from [Ag(NH3)2]+ (4), similar to the previous works [29, 30]. This probably facilitates Ag4V2O7 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 Ag4V2O7 plates in nanoscale although no additives were used with respect to the published work [26]. When temperature was increased over 140°C, the β-AgVO3 phase occurred uniquely. It is worthy to note that to obtain the single-phase Ag4V2O7 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 Ag4V2O7 [19, 20].Figure 4
(a) DR-UV-Vis spectra and (b) Tauc’s plots of the random- andc-axis-oriented Ag4V2O7 samples.
(a)
(b)The evolution of the RhB photodegradation rate as a function of irradiation time over the random- andc-axis-oriented Ag4V2O7 samples autoclaved at 100 and 140°C, respectively, was indicated in Figure 5.Figure 5
Rhodamine B photodegradation efficiency under solar light irradiation over (A)c-axis and (B) random-oriented Ag4V2O7 samples.After being irradiated for 90 minutes under a solar source, the rhodamine B photodegradation efficiency of 92% was found for thec-axis-oriented Ag4V2O7 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 Ag4V2O7 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 Ag4V2O7 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 Ag4V2O7 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 Ag4V2O7 nanoplates was not only significantly higher than that reported for silver pyrovanadate [20, 24] but also for other vanadate-based nanophotocatalysts such as Ag3VO4, FeVO4, FeVO4/V2O5, BiVO4, and BiVO4/TiO2 [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 TiO2 for photodegradation of dyes. This originated from the fact that their photocatalytic activity is distinctive superior to BiFeO3 and BiFeO3-Bi2WO6 and comparable to, for example, Ag2O/Ag3VO4/Ag4V2O7 and Ag3PO4/SBA-15 [24, 34–36]. 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 Ag3PO4 [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].Figure6 demonstrated the recyclability testing results of RhB over the random- and c-axis-oriented Ag4V2O7 samples for three recycling photocatalytic runs under solar light irradiation. The results revealed that, for c-axis-oriented Ag4V2O7 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 Ag4V2O7 samples are reusable with high stability after three RhB photodegradation cycles.Figure 6
Photodegradation recyclability test of rhodamine B over (a)c-axis and (b) random-oriented Ag4V2O7 samples under solar light irradiation.


## 4. Conclusions

In summary, single-phase silver pyrovanadate, Ag4V2O7, nanoplates were synthesized readily by an additive-free hydrothermal method at temperatures from 100 to 140°C by using NH4VO3, AgNO3, 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}-facet-exposed 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 Ag4V2O7 samples under solar light irradiation was 71 and 92%, respectively. The enhanced photocatalytic activity of the c-axis-oriented Ag4V2O7 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(NH3)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 silver-containing ternary oxides without using any additives.



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