Serendipitous Synthesis of Ag1.92Mo3O10·H2O Nanowires from

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Serendipitous Synthesis of Ag1.92Mo3O10·H2O Nanowires from AgNO3‑Assisted Etching of Ammonium Phosphomolybdate: A Material with High Adsorption Capacity Chanchal Mondal,† Jaya Pal,† Kuntal Kumar Pal,† Anup Kumar Sasmal,† Mainak Ganguly,† Anindita Roy,† P. K. Manna,‡ and Tarasankar Pal*,† †

Indian Institute of Technology, Kharagpur, Kharagpur 721302, West Bengal, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

‡

S Supporting Information *

ABSTRACT: Ultralong Ag1.92Mo3O10·H2O nanowires have been serendipitously obtained due to selective etching of ammonium phosphomolybdate (APM) only by Ag+ ions in water under stirring conditions. The spherical yellow APM particle upon etching by Ag+ ions generates a hollow sphere, and PO43− ions are expelled as a consequence of etching. The etching and hollowing disrupt the APM structure. Concentration of the etching agent and reaction time are crucial for the formation of Ag1.92Mo3O10·H2O nanowire. The growth of nanowires occurs probably due to etching followed by Ostwald ripening, oriented attachment, and splitting process. Finally, the as-synthesized nanowire exhibits a high capacity to adsorb cationic dyes on its surface. It shows superb adsorption properties, with maximum adsorption capacity of 110 mg g−1, 175 mg g−1, 160 mg g−1 for Methylene Blue, Methyl Green, Crystal Violet, respectively. Moreover, the adsorption process of Methylene Blue on the nanomaterial was investigated taking it as a representative adsorbate. The selective adsorption capability of the nanomaterial toward cationic dye molecules makes it a competent candidate for water purification.

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of silver molybdenum oxides adopting magnetron sputtering,12 chemical vapor deposition,13 hydrothermal treatment,14 and laser annealing.15 Although these experiments have led to outstanding outcomes, the experimental conditions involve high temperatures, and most of them are time-consuming and traditionally synthesized by harsh reactions involving a MoO3/ Ag2O system. The reports of electrical,16 optical,17 and photoswitching properties18 of the 1D nanostructures are noteworthy. However, there is no report for the synthesis of silver molybdate nanowire exploiting a solution-based etching route involving APM and AgNO3 as the straightforward precursor material at room temperature under stirring conditions. Selective etching of robust APM by AgNO3 in water engenders the reported nanowires through oriented attachment of ammonium molybdate nuclei followed by splitting. The removal of dyes from wastewater is a serious concern nowadays because they cause severe damage to the environment as these compounds are extremely lethal and not biodegradable. It is difficult to degrade dye materials because they are very stable against light and oxidation reactions.19 In

INTRODUCTION Polyoxometalates (POM) are extremely large transition metal anionic clusters made of octahedra-linked shared oxygen atoms.1 These compounds have gained momentum not only because of their structural diversity but also for their innumerable applications in various fields, for instance, as a catalyst, sensors, and smart windows, and researchers have developed many methods for synthesis of these nanomaterials in recent years.2−5 For example, Liu et al. reported a general strategy for fabricating polyoxometalate nanostructures, the electrochemistry-assisted laser ablation in liquid, which is a green, simple, and catalyst-free approach under the ambient environment.6 The synthesis of one-dimensional (1D) architectures (rods, wires, tubes, and belts) of molybdates and vanadate-based oxides as building blocks, and their selfassembly into an exotic structure, have fetched noteworthy interest because of their novel physical and chemical properties and potential utilities in a variety of fields, e.g., energy, electronic, optical device fabrication, scintillator materials, sensors, and photocatalysis.7−10 Among them, silver molybdates have received significant attention as a vital family of conducting glass and ammonia-sensing materials as their morphologies can easily be manipulated by solution-based synthesis.11 However, like most metal oxides, during the past decades various methods have been employed for the synthesis © XXXX American Chemical Society

Received: April 28, 2014 Revised: August 12, 2014

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the process of removing dye materials from contaminated water, numerous methods including adsorption, membrane processes, flocculation or sedimentation, photocatalytic degradation, and biological treatment have been extensively explored.20 Among them, adsorption technology is preferred due to their high efficiency, economic viability, and simplicity in design and execution. Thus, adsorbents with a high capacity play a crucial role in the wastewater treatment through the adsorption of dyes. To the best of our knowledge, silver molybdate has not been employed as an adsorbent for the adsorption of dye molecules. Herein, for the first time, we have demonstrated a simple, cost-effective methodology for the synthesis of silver molybdate exploiting AgNO3 and APM as the starting material and used the product as an adsorbent with a high adsorptive capacity for environmental remediation.

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Figure 1. XRD patterns of (a) (NH4)3[PMo12O40], (b) Ag(MoO2)PO4, (c) Ag1.92Mo3O10·H2O.

EXPERIMENTAL SECTION 9.7900(8) Å. These values are slightly higher than those reported for isostructural Ag2Mo3O10·2H2O nanowires (a = 13.2182(3) Å, b = 7.5968(2) Å, c = 9.7610(2) Å).11 FESEM images show that only nanowire morphology with an average diameter of 200 nm is formed (Figure 2). These

Materials and Analytical Instrument. The relative information are portrayed in Supporting Information. Synthesis of Ag1.92Mo3O10·H2O Nanowire. In a typical preparation of silver molybdate nanowire, 0.1 g of APM was added to 45 mL of water in a beaker. The resultant mixture was sonicated for 15 min. After proper sonication, 5 mL of 0.1 M AgNO3 was added into the suspension, and the mixture was stirred for 6 h at room temperature and pressure. After completion of the reaction, a yellow color precipitate was found at the bottom of the beaker. The precipitate was collected and washed with water and ethanol. The product was dried under a vacuum for characterization. Adsorption of Dye Molecules. The adsorption performance of the as-prepared nanowire was examined via the adsorption of different dyes in aqueous solution. Adsorption experiments were carried out under stirring conditions at room temperature (25 °C) in the dark. In a typical procedure, 20 mg of adsorbent was added to 500 mL of 2 × 10−5 M methylene blue (MB) solution, followed by stirring at 200 rpm. After certain time intervals, the aliquots were taken out from the suspension and centrifuged to separate the adsorbents from the suspension. The concentration of residual MB in the solution was monitored using a UV−visible spectrophotometer (Shimadzu UV2450, Japan). Adsorption capacity was calculated using the following equation, adsorption capacity = (C0 − Cf )VMM(II)/Ws

(1)

where C0 and Cf are initial and final concentration of aqueous dye solution in M, V (L) is total volume, MM(II) (mg) is atomic weight of dye molecule, and Ws (g) is weight of Nanomaterial. The Cf value is evaluated from a standard graph. In a similar fashion, the adsorption property of the as-synthesized material toward various dye molecules was studied.

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RESULTS AND DISCUSSION The phase purity of the as-prepared nanowires was evaluated by X-ray diffraction (XRD) analysis. Figure 1c depicts the XRD pattern of the as-synthesized nanowires, and the diffraction peaks can be indexed as Ag1.92Mo3O10·H2O phase with orthorhombic crystal structure (JCPDS File No. 82-1954). Figure 1, panels a and b show the XRD pattern of APM and Ag(MoO2)PO4 respectively. The as-prepared nanomaterial exhibits excellent stability under ambient conditions. Rietveld Analysis. We have carried out the Rietveld refinement of the room-temperature X-ray diffraction pattern (Figure S1, Supporting Information) of the Ag1.92Mo3O10·H2O nanowires using the FULLPROF program.21 The nanowires are found to crystallize in an orthorhombic crystal structure under the space group Pnma. The lattice parameters are determined as a = 13.2367(11) Å, b = 7.6187(7) Å, and c =

Figure 2. FESEM images of Ag1.92Mo3O10·H2O nanowire (a, b) medium magnification, (c) low magnification, (d) high magnification.

nanowires have lengths of about several tens of micrometers, and they are automatically interlaced with each other. Close inspection reveals the smooth surface of the nanowire. This geometry is completely different from that of the ammonium phosphomolybdate particles (Figure 3). Diffuse reflection spectrum (DRS) analysis (Figure S2a, Supporting Information) of the as-prepared nanowire shows a strong absorption in the spectral region of 200−500 nm, with a maximum absorbance at 310 nm. A Tauc approach was employed to calculate the band gap energy (Eg) of the as-synthesized nanowire. The relationB

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Figure 3. FESEM images of the products obtained at different hydrothermal reaction times: (a) 0 min, (b) 60 min, (c) 120 min, (d) 180 min, (e) 240 min, (f) 300 min.

respectively, which are in very good agreement with that reported data.25 In the Ag 3d spectrum, the peaks (Figure S5a) at 368.01 and 374.12 eV are assigned to Ag+ for silver molybdate system. In Figure S5c a peak at 530 eV corresponds to the O 1s spectrum. The P 2p signal peaks (Figure S5d) at 133.6 eV is due to binding energy for PO43−. This shows that phosphate ion is present on the surface of the composite which is also a reason for the nanomaterial to become a cationic dye adsorbent. Some key structural information was obtained from the FTIR analysis, which is depicted in Figure S6, Supporting Information. The bands (one sharp band, two weak bands) located between 817 and 966 cm−1 are ascribed to symmetric vibration bands of the MoO4 of the molybdate ion. The sharp signatures at 780 and 670 cm−1 are assigned to the O−Mo−O stretching frequency in the silver molybdate sample. Many absorption bands of samples are present; generally, the infrared bands at 3324 and 1650 cm−1 are attributed to the OH stretching vibration and bending vibration of water molecules. A relatively weak signature at 1065 cm−1 is due to the presence of PO43− ion on the surface.5 The N2 adsorption−desorption isotherm (Figure S7, Supporting Information) of silver molybdate wire morphology shows that the specific surface area and pore volume of the material are 58.027 m2/g and 0.298 cm3/g, respectively. Pores presumably arise from the spacing generated due to interlacing of the smooth wirelike morphology. The results indicate that the as-synthesized interlaced sample is mesoporous in nature. The high specific surface area and pore volume provoked us to use them as a novel adsorbent nanomaterial to assist adsorption performance. To realize the mechanism of Ag1.92Mo3O10.H2O nanowire growth in water, we looked into the time-dependent product formation. XRD analysis at different reaction time intervals

ship of absorption coefficient (α) on the photon energy is given in the following equation: αEp = K (Ep − Eg )1/2

where K is a constant, α is the absorption coefficient, Ep stands for the discrete photoenergy, and Eg denotes the band gap energy. Figure S2b displays the plot of (αEp)2 vs Ep which is based on the direct transition, and the extrapolated value of Ep at α = 0 provides absorption edge energy correspondence to band gap of as-synthesized nanowire (Eg) = 2.81 eV.7 Interestingly, we noticed a very weak signature appeared at in the spectral region between 620 and 780 nm. Figure S2c displays the magnified spectra in the spectral region between 620 and 780 nm. This is presumably due to the aggregation of few isolated Ag+ ions into Ag(0) clusters.22−24 EDX analysis as depicted in Figure S3, Supporting Information reveals the presence of Ag, Mo, O as elements present in the fibrous morphology. HRTEM experiments were carried out in order to obtain more insight into the crystal structure and sample purity of the as-synthesized nanowires (Figure S4a,b, Supporting Information). It coincides well with the FESEM images. Figure S4c shows the SAED pattern of the as-synthesized nanowire. By calculation, the diffraction point can be assigned to (122), (113), and (404) facets of Ag1.92Mo3O10·H2O nanowire. Similarly, fringe spacing measurement (Figure S4d) clearly shows that the interplanar spacing is 2.91 Å, which coincides well with the (122) crystallographic plane of the rhombohedral Ag1.92Mo3O10·H2O. XPS analysis was performed on Ag1.92Mo3O10·H2O pellets to understand the chemical state of the element on the surface of the nanomaterial. Figure S5b, Supporting Information shows binding energies at 232.5 and 235.6 eV, which are attributed to the Mo 3d5/2 and 3d3/2 lines for Ag1.92Mo3O10.H2O nanowires, C

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and the nanobundles start to form, signifying that the longer microrods grow at the expense of smaller particulates as there is energy difference between large particles and smaller particles. The nanowires of smaller diameter develop through a side-byside attachment to form nanobundles. With the progress of reaction, a splitting process resulted in nanowires. Crystal splitting is governed by fast crystal growth which in turn depends on solution oversaturation. Punin suggested that splitting only occurs when oversaturation exceeds beyond a certain critical value, which is specific for materials and given conditions. Depending on the level of supersaturation, different degrees of splitting of the material occurs, which leads to a number of subforms of split crystals. At lower temperatures, when few nuclei formation occurs, fast growth may result in a metastable situation, where the average size of the crystals exceeds a certain limit. Then thermodynamically favorable crystal splitting occurs, and in the present case presumably this is the reason for the nanobundles to split into a single crystal of Ag1.92Mo3O10·H2O nanowire (Scheme 1).29 In APM ammo-

revealed that a pale yellow product was formed after 60 min of stirring of the suspended APM (s) in aqueous solution of AgNO3. All the diffraction peaks of the intermediate (Figure 1b) can be indexed to Ag(MoO2)PO4 (JCPDS File No. 741395). However, some incorporated impurities may be due to APM and other silver phosphomolybdate present in the sample. The diffraction peaks (Figure 1c) were changed after 5 h of stirring, suggesting the formation of a new compound. All the diffraction peaks then indexed as Ag1.92Mo3O10·H2O nanomaterial. The presence of expelled NH4+ ion in aqueous medium was tested via the formation of a brown precipitate with Nessler reagent taking the supernatant into consideration. Again, confirmation of PO43− in the supernatant was carried out by the canary yellow precipitate, (NH4)3PMo12O40 formation in dil. HNO3 medium to confirm the expulsion of PO43− from the APM matrix. This test and XRD analysis of the final product cojointly indicate that all the PO43− ions are quantitatively removed from the APM skeleton by Ag+ ion due to etching. However, with prolonged reaction time no change occurs in the nanowire morphology, and hence the diffraction pattern of the product remains unaltered indicating the completion of reaction. FESEM images indicate that the starting material APM are spherical in shape having an average diameter of 1.5 μm (Figure 3a). Distinctive hollow spheres (Figure 3b) resulted by stirring an water insoluble APM suspension in AgNO3 solution for 60 min. After 2 h of stirring, the hollow spheres become coexistent with nanobundles (Figure 3c). These nanobundles were generated by the coalescence of several tiny nanowires. The number of the nanobundles increased at the expense of hollow spheres (Figure 3d) with further increase of reaction time. After a specified reaction time of 5 h, numerous large nanowires were seen (Figure 3e), and the number of hollow spheres as well as nanobundles disappeared considerably indicating that the longer wires grew at the cost of smaller wires akin to the proposition of oriented attachment. Fascinatingly, it is observed that hollow spheres disappeared completely, and the bundles began to split into nanowires. After completion of the reaction, i.e., after the elapse of ∼6 h, the nanowires were generated depicting a single unique morphology (Figure 3f). In the present synthetic route, neither surfactants nor templates were employed. Above experimental results inferred that the formation of inorganic Ag1.92Mo3O10.H2O nanowires were presumably due to etching followed by Ostwald ripening, splitting, and oriented attachment process.26−28 From the above results, one can conclude that etching of spherical APM particles occurs, and gradual consumption of the etching reagents, AgNO3 by molybdate is understandable. Actually, the concentration of etching agent and reaction time are crucial to synthesize the desired 1D morphology. Excess AgNO3 promptly etches APM, generating the observed hollow spheres. Conversely, low Ag+ concentrations take a long time to etch and generate hollow spheres. The etched product is insoluble silver molybdate which nucleates slowly in the reaction mixture containing expelled NH4+ and PO43− ions. The slow etching relates to slow nucleation of insoluble silver molybdate. Then Ostwald ripening, oriented attachment and splitting processes conjointly evolve nanowires. After that well-known “Ostwald-ripening” helps to understand the growth of nanowires with a smooth surface structure. Ostwald-ripening process basically states the formation of nuclei from a supersaturated medium followed by subsequent crystal growth. With the increase of reaction time, the irregular and ill-defined smaller particles/wires disappear,

Scheme 1. Schematic Illustration of the Growth Mechanism in the Synthesis of Ag1.92Mo3O10·H2O Nanowires

nium ion is exchangeable. Hence NH4+ is exchanged with the incoming Ag+ ion and silver phosphomolybdate is formed instantaneously. In the second step, hollowing of the solid particles occur due to the expulsion of PO43− from the water insoluble APM backbone. This process of etching is selective and reported for the first time. The whole process is viewed as the etching induced disruption of the hollow spheres followed by the precipitation of Ag1.92Mo3O10·H2O. Thus, insolubility matter drives the reaction for nanowire formation through crystal growth. We have employed other reagents, e.g., Ni(NO3)2, Co(NO3)2, Fe(NO3)2, BaSO4, NaNO3, KNO3, Zn(NO3)2, NH4NO3, NiSO4, CoSO4, MnSO4 instead of AgNO3 to examine the role of Ag+. However, no etching was observed in other cases. To explore the effect of anion other silver salts were employed, e.g., Ag2SO4, Ag(OAc). They are capable of forming silver molybdate at room temperature under stirring conditions. From the above observation, it can be concluded that only silver is competent to etch APM to generate Ag1.92Mo3O10·H2O nanowire. To further confirm the Ag+ induced APM etching, we have also carried out a reaction of AgNO3, silver molybdate, and ammonium dihydrogen phosphate, but we have not observed any such hierarchical nanowire-like morphology. So here we can conclude that only etching of APM by AgNO3 brings such wirelike morphology. D

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dominated by negative charges (OH−) and (PO43−) confirmed from FTIR analysis (Figure S10a, Supporting Information), the nanomaterial shows good selectivity toward positive organic groups due to the electrostatic attraction. DRS spectrum of MB adsorbed nanomaterial as depicted in Figure S10b shows that the corresponding peak of MB and the nanomaterial at 664 nm, 320 nm significantly shifted to lower wavelength of 598 and 302 nm, respectively. This suggests a significant electrostatic attraction between MB and the nanomaterial. This infers that the nanomaterial is not capable of adsorbing Methyl Orange, Eosin, and Rose Bengal as the dyes bear a negative charge. The high degree of adsorption of cationic dye is attributed to a higher surface area of the nanomaterial. Entrenchment of the dye molecules into the interspaces between the interlaced nanowires is crucial for effective adsorption.31,32 Here, from the FESEM images, it is clear that the nanomaterial possesses many vacant sites which is responsible for its capability of dye adsorption. The adsorption capacity of the nanomaterial was calculated for various dye molecules, and they are summarized in Table 1. The adsorption capacity shows that the as-

Adsorption of Dye Molecules. The adsorption activity of the as-synthesized nanomaterial was demonstrated using various dye molecules. Among them MB was chosen as a representative dye. MB was removed from the solution by its adsorption on the nanomaterial, as shown in Figure 4. The

Figure 4. Absorption spectra of MB solution at different adsorptive time intervals in the presence of Ag1.92Mo3O10·H2O nanowire under dark conditions.

Table 1. Adsorption Capacity of Ag1.92Mo3O10·H2O Nanowire for Various Dyes

characteristic absorption peak of MB at 662 nm decreased dramatically in the first 10 min. Digital images of Figure 5 indicate that the color of MB fades away with an increase of adsorptive time and the yellow color of the adsorbent turned deep blue. This was due to the strong electrostatic attraction that operates between positively charged MB and negatively charged nanomaterial.30 Concentration of MB decreased with extension of the adsorptive time, demonstrating the increase of adsorption of dye molecules with time. However, in the later stage adsorption was much slower than that in the beginning stage. It can be stated that an increased reaction time decreased vacant surface sites and hence, repulsion occurred between adsorbed MB molecules and incoming MB molecules. Moreover, other organic dyes with different charges, e.g., Crystal Violet, Methyl Green, Malachite Green, Methyl Violet, Methyl Orange, Eosin, Rose Bengal were employed to evaluate the adsorption selectivity of the nanomaterial. The result reports that the cationic dyes are removed by adsorption (Figure S8, Supporting Information), while very little decrease in concentration occurs for Methyl Orange, Eosin, and Rose Bengal (Figure S9, Supporting Information). As the surface is

dye

Methylene Blue

Methyl Green

Crystal Violet

adsorption capacity (mg/g)

110

175

160

synthesized nanomaterial is highly competent to remove cationic dyes through adsorption, indicating its possible utilization in wastewater treatment.33−35 The regeneration performance of the adsorbent is a crucial factor for its practical application. For this purpose, after each cycle of operation the adsorbent was carefully separated through centrifugation, then washed with ethanol to diminish the amount of adsorbed dye on its surface and dried for reutilization as an adsorbent. The results divulge that the adsorption capability of the nanomaterial up to fourth cycle of operation remains almost the same (93% retention of adsorption capacity) and decreased slightly after fourth cycles of operation which is shown in Figure S11, Supporting Information. The XRD pattern of the nanomaterial was carried out after the fourth cycle of operation, which shows that no change occurs in the XRD pattern after the adsorption process (Figure S12, Supporting Information). The results indicate that

Figure 5. Digital images of MB solutions with increasing adsorptive time (a) 0 min, (b) 10 min, (c) 30 min, (d) 90 min, (e) 150 min, (f) color of the nanomaterial after adsorbing MB. E

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Table 2. Adsorption Kinetic Parameters of MB Adsorption pseudo-first-order model material silver molybdate

K1 (min−1) 0.01395

qe,cal (mg/g) 93.69

pseudo-second-order model R2

K2(g/mg·min) 2.91 × 10

0.99094

−4

qe,cal (mg/g)

R2

qe,exp (mg/g)

100.50

0.99904

108.63

Figure 6. (a) Pseudo-first-order kinetic plot, (b) pseudo-second-order kinetic plots for the adsorption of MB onto Ag1.92Mo3O10·H2O nanowire, (c) Langmuir, and (d) Freundlich isotherms for the adsorption of MB onto Ag1.92Mo3O10·H2O nanowire.

time t (min), respectively. k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and pseudo-second-order adsorption rate constants, respectively. The kinetic parameters and the correlation coefficients (R2) for pseudo-first-order and pseudo-second-order adsorption are given in Table 2. Figure 6 depicts that all the experimental data can fit well to the pseudo-second-order model with a very high correlation coefficients (R2 > 0.999). The values of qe,cal obtained from pseudo-second-order model were also very close to the values of qe,exp, which suggests that the adsorption process of MB follows pseudo-second-order model. The adsorption data also agree well to the pseudo-first-order model. However, the value of qe does not match the value of qt (Figure 6, Table 2). This suggests that the present adsorption kinetics does not follow a pseudo-second-order model. These data infer that the control of adsorption occurs by chemisorptions not by mass transfer. The effect of the initial concentration and interaction between the adsorbent and adsorbate were crucial for adsorption experiment. It can be shown in the adsorption isotherm by adopting the Langmuir (eq 3) and Freundlich (eq 4) isotherms.38,39

the as-synthesized nanomaterials have high stability and do not get corroded in the adsorption−desorption process, which signifies that it can act as a potential candidate in wastewater treatment as an adsorbent. The adsorption performance of silver molybdate nanowires was also carried out by annealing the sample at 450 °C. It was observed that the as-prepared sample shows much better adsorption capacity than the annealed sample (Figure S13, Supporting Information). This may attributed to mitigation of the porous moiety due to heat treatment as well as removal of the negative ions present on the surface of the as-synthesized Ag1.92Mo3O10·H2O nanowire. Kinetics of Dye Adsorption. The kinetics of MB removal by the as-synthesized Ag1.92Mo3O10·H2O nanowire was investigated employing pseudo-first-order (eq 1) and pseudosecond-order kinetic models (eq 2).36,37 The pseudo first order equation is expressed as ln(qe − qt) = ln qe − k1t

(1)

The pseudo second order equation is expressed as

t /qt = 1/k 2qe 2 + t /qe

(2) −1

where qe and qt (mg g ) are the amount of adsorbate adsorbed, i.e., adsorption capacities at equilibrium and at any

1/qe = 1/qmax + 1/Ceqmax b F

(3)

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Table 3. Comparison of Data of Two Isotherm Models for MB Adsorption Langmuir isotherm

Freundlich isotherm

material

b (L/mg)

qm (mg/g)

R2

KF ((mg1−n Ln)/g)

n

R2

silver molybdate

0.0972

145.13

0.99956

10.59

0.42

0.9912

ln qe = ln KF + (1/n) ln Ce

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(4)

Corresponding Author

where qmax (mg/g) is maximum adsorption capacity corresponding to complete monolayer coverage, Ce is the equilibrium concentration, qe is the amount adsorbed at equilibrium (mg/g), b is the equilibrium constant (L·mg−1) and it is related to binding strength, KF is a rough indicator of the adsorption capacity, and n is the adsorption intensity. The observed R2 values of Langmuir and Freundlich isotherm are 0.99956 and 0.9912 respectively for MB adsorption onto silver molybdate nanomaterial (Table 3). This indicates that the experimental data have a good agreement with Langmuir isotherm model (Figure 6c), while for Freundlich model these data do not fit well (Figure 6d). Thus, it can be concluded that the adsorption process occurs through the Langmuir adsorption model, i.e., monolayer adsorption. Effect of pH. The solution pH is a crucial parameter that affects the adsorption of dye molecules as it can affect the surface charge of the adsorbent, the degree of ionization of dyes, as well as the structure of the dye molecules. To understand the effect of the initial pH of the solution on the dyes adsorption onto the as-prepared nanowire experiment was carried out at different pH values ranging from 1 to 10 (Figure S14, Supporting Information). With increasing pH (2−7), the surface of the catalyst might become more negatively charged and the electrostatic attraction force between the adsorbent and MB molecules enhanced to a greater extent which causes an increase of adsorption capacity for the MB dyes. The adsorption of MB decreases dramatically at a higher pH (above 7) presumably due to repulsion of catalyst and MB. It may be ascribed to hydrolysis of the adsorbent surfaces, which creates positively charged sorption sites.40,41

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are thankful to IIT Kharagpur, BRNS, CSIR, New Delhi, for financial assistance.

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REFERENCES

(1) Hill, C. L.; White, G. C. Introduction: PolyoxometalatesMulticomponent Molecular Vehicles to Probe Fundamental Issues and Practical Problems. Chem. Rev. 1998, 98, 1−2. (2) Basu, M.; Sarkar, S.; Pande, S.; Jana, S.; Sinha, A. K.; Sarkar, S.; Pradham, M.; Pal, A.; Pal, T. Hydroxylation of Benzophenone with Ammonium Phosphomolybdate in the Solid State via UV Photoactivation. Chem. Commun. 2009, 46, 7191−7193. (3) Sonoyama, N.; Suganuma, Y.; Kume, T.; Quan, Z. Lithium Intercalation Reaction into the Keggin Type Polyoxomolybdates. J. Power Sources 2011, 196, 6822−6827. (4) Liang, Y.; Zhu, L. F.; Liu, P.; Li, H. B.; Xiao, J.; Ji, X. W.; Yang, G. W. Ag2V4O11 Nanostructures for Highly Ethanol Sensitive Performance. CrystEngComm 2013, 15, 6131−6135. (5) Liang, Y.; Liu, P.; Li, H. B.; Yang, G. W. ZnMoO4 Micro- and Nanostructures Synthesized by Electrochemistry-Assisted Laser Ablation in Liquids and Their Optical Properties. Cryst. Growth Des. 2012, 12, 4487−4493. (6) Liu, P.; Liang, Y.; Lin, X.; Wang, C.; Yang, G. A General Strategy to Fabricate Simple Polyoxometalate Nanostructures: Electrochemistry-Assisted Laser Ablation in Liquid. ACS Nano 2011, 5, 4748−4755. (7) Feng, M.; Zhang, M.; Song, J.-M.; Li, X.-G.; Yu, S.-H. Ultra- long Silver Trimolybdate Nanowires: Synthesis, Phase Transformation, Stability, and Their Photocatalytic, Optical, and Electrical Properties. ACS Nano 2011, 5, 6726−6735. (8) Mondal, C.; Ganguly, M.; Pal, J.; Sahoo, R.; Sinha, A. K.; Pal, T. Pure Inorganic Gel: A New Host with Tremendous Sorption Capability. Chem. Commun. 2013, 49, 9428−9430. (9) Qu, W. M.; Wlodarski, W.; Mayer, J. U. Comparative Study on Micromorphology and Humidity Sensitive Properties of Thin-Film and Thick-Film Humidity Sensors Based on Semiconducting MnWO4. Sens. Actuators, B 2000, 64, 76−82. (10) Zhang, K.; Heo, N.; Shi, X.; Park, J. H. Chemically Modified Graphene Oxide-wrapped Quasi-micro Ag Decorated Silver Trimolybdate Nanowires for Photocatalytic Applications. J. Phys. Chem. C 2013, 117, 24023−24032. (11) Hakouk, K.; Deniard, P.; Lagaunie, L.; Guillot-Deudon, C.; Harel, S.; Wang, Z.; Huang, B.; Koo, H.-J; Whangbo, M.-H; Jobic, S.; Dessapt, R. Novel Soft-Chemistry Route of Ag2Mo3O10·2H2O Nanowires and In Situ Photogeneration of a Ag@Ag2Mo3O10·2H2O Plasmonic Heterostructure. Inorg. Chem. 2013, 52, 6440−6449. (12) Ferroni, M.; Guidi, V.; Mertinelli, G.; Nelli, P.; Sacerdoti, M.; Sberveglieri, G. Characterization of a Molybdenum Oxide Sputtered Thin Film as a Gas Sensor. Thin Solid Films 1997, 307, 148−151. (13) Hussain, O. M.; Rao, K. S.; Madhuri, K. V.; Ramana, C. V.; Naidu, B. S.; Pai, S.; John, J.; Pinto, R. Growth and Characteristics of Reactive Pulsed Laser Deposited Molybdenum Trioxide Thin Films. Appl. Phys. A: Mater. Sci. Process 2002, 75, 417−422. (14) Sing, D. P.; Sirota, B.; Talpatra, S.; Kohli, P.; Rebholz, C.; Aouadi, S. M. Broom-like and Flower-like Heterostructures of Silver

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CONCLUSION In summary, an easy and environmentally friendly procedure for preparing Ag1.92Mo3O10·H2O nanowire was reported exploiting Ag+ induced etching of APM. A growth mechanism was proposed for the generation of this unusual product. The synthesized Ag1.92Mo3O10·H2O nanowire exhibited excellent removal capability toward cationic dyes, suggesting its potential application in wastewater treatment. The present study provides a prospective in fabricating further functional hierarchical architectures because it allows a high degree of control over their structure and morphology. The ultralong nanowire of Ag1.92Mo3O10·H2O is expected to be a potential candidate for applications in environmental remediation because of its simplistic, cost-effective, and scaled-up preparation methodology and superb dye adsorption capacity.

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S Supporting Information *

Experimental details, FTIR, EDX spectra, digital images, FESEM images, XRD pattern, Rietveld-refined X-ray diffraction patterns, DRS spectrum, XPS spectrum, BET analysis, adsorption spectra all are available free of charge via the Internet at http://pubs.acs.org. G

dx.doi.org/10.1021/cg500600p | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Molybdate through pH Controlled Self Assembly. J. Nanopart. Res. 2012, 14, 781(1−11). (15) Brechignac, C.; Cahuzac, Ph; Kebaili, N.; Lando, A.; Masson, A.; Schmidt, M. Synthesis of Silver Molybdate Clusters Driven by Laserannealing. J. Chem. Phys. 2004, 121, 9617(1−6). (16) Cui, X. J.; Yu, S. H.; Li, L. L.; Liu, B.; Li, H. B.; Mo, M. S.; Liu, X. M. Selective Synthesis and Characterization of Single-crystal Silver Molybdate/tungstate Nanowires by a Hydrothermal Process. Chem. Eur. J. 2004, 10, 218−223. (17) Nagaraju, G.; Chandrappa, G. T.; Livage, J. Synthesis and Characterization of Silver Molybdate Nanowires, Nanorods and Multipods. Bull. Mater. Sci. 2008, 31, 367−371. (18) Chen, L.; Shao, Q.; Shao, M. W.; Wei, X. W.; Wu, Z. C. Photoswitches of One-dimensional Ag2MO4 (M = Cr, Mo, and W). J. Phys. Chem. C 2009, 113, 1764−1768. (19) Hameed, B. H.; Rahman, A. A. Removal of Phenol from Aqueous Solutions by Adsorption onto Activated Carbon Prepared from Biomass Material. J. Hazard. Mater. 2008, 160, 576−581. (20) Wu, N.; Wei, H.; Zhang, L. Efficient Removal of Heavy Metal Ions with Biopolymer Template Synthesized Mesoporous Titania Beads of Hundreds of Micrometers Size. Environ. Sci. Technol. 2012, 46, 419−425. (21) Rodríguez-Carvajal, J. Recent Developments of the Program FULLPROF. Commission on Powder Diffraction (IUCr) Newsletter 2001, 26, 1219. (22) Agostini, G.; Usseglio, S.; Groppo, E.; Uddin, M. J.; Prestipino, C.; Bordiga, S.; Zecchina, A.; Solari, P. L.; Lamberti, C. From Isolated Ag+ Ions to Aggregated Ag0 Nanoclusters in Silver- Exchanged Engelhard Titanosilicate (ETS-10) Molecular Sieve: Reversible Behavior. Chem. Mater. 2009, 21, 1343−1353. (23) Zou, X. Q.; Dong, S. J. Surface-Enhanced Raman Scattering Studies on Aggregated Silver Nanoplates in Aqueous Solution. J. Phys. Chem. B 2006, 110, 21545−21550. (24) Zeng, J. B.; Jia, H. Y.; An, J.; Han, X. X.; Xu, W. Q.; Zhao, B.; Ozaki, Y. Preparation and SERS Study of Triangular Silver Nanoparticle Self-Assembled Films. J. Raman Spectrosc. 2008, 39, 1673−1678. (25) Cui, X.; Yu, S.-H.; Li, L.; Biao, L.; Li, H.; Mo, M.; Liu, X.-M. Selective Synthesis and Characterization of Single-Crystal Silver Molybdate/Tungstate Nanowires by a Hydrothermal Process. Chem.Eur. J. 2004, 10, 218−223. (26) Peng, Z. A.; Peng, X. Mechanisms of the Shape Evolution of CdSe Nanocrystals. J. Am. Chem. Soc. 2001, 123, 1389−1395. (27) Li, R. F.; Luo, Z. T.; Papadimitrakopoulos, F. Redox-assisted Asymmetric Ostwald Ripening of CdSe Dots to Rods. J. Am. Chem. Soc. 2006, 128, 6280−6281. (28) Wang, Q.; Wen, Z.; Li, J. Solvent-Controlled Synthesis and Electrochemical Lithium Storage of One-Dimensional TiO2 Nanostructures. Inorg. Chem. 2006, 45, 6944−6949. (29) Punin, Y. O. Cleavage of Crystals [minerals]. Zap. Vses. Mineral. Ova. 1981, 110, 666−686. (30) Lim, Y. W. L.; Tang, Y. X.; Cheng, Y. H.; Chen, Z. Morphology, Crystal Structure and Adsorption Performance of Hydrothermally Synthesized Titania and Titanate Nanostructures. Nanoscale 2010, 2, 2751−2757. (31) Feng, M.; You, W.; Wu, Z.; Chen, Q.; Zhan, H. Mildly Alkaline Preparation and Methylene Blue Adsorption Capacity of Hierarchical Flower-like Sodium Titanate. ACS Appl. Mater. Interfaces 2013, 5, 12654−1266. (32) Cao, C.-Y.; Qu, J.; Wei, F.; Liu, H.; Song, W.-G. Superb Adsorption Capacity and Mechanism of Flowerlike Magnesium Oxide Nanostructures for Lead and Cadmium Ions. ACS Appl. Mater. Interfaces 2012, 4, 4283−4287. (33) Wang, Y.; Chen, J.; Lu, L.; Lin, Z. Reversible Switch between Bulk MgCO3·3H2 O and Mg(OH) 2 Micro/Nanorods Induces Continuous Selective Preconcentration of Anionic Dyes. ACS Appl. Mater. Interfaces 2013, 5, 7698−7703.

(34) Saha, B.; Das, S.; Saikia, J.; Das, G. Preferential and Enhanced Adsorption of Different Dyes on Iron Oxide Nanoparticles: A Comparative Study. J. Phys. Chem. C 2011, 115, 8024−8033. (35) Wang, Y. J.; Chen, D. G.; Wang, Y. D.; Huang, F.; Hu, Q. C.; Lin, Z. Tunable Surface Charge of ZnS: Cu Nano-adsorbent Induced the Selective Preconcentration of Cationic Dyes from Wastewater. Nanoscale 2012, 4, 3665−3668. (36) Lagergren, S.; Svenska, K. About the Theory of So Called Adsorption of Soluble Substances. K. Sven. Vetenskapsad. Handl. 1898, 24, 1−9. (37) Ho, Y. S.; McKay, G. Pseudo-second Order Model for Sorption Processes. Process Biochem. 1999, 34, 451−465. (38) Langmuir, I. Constitution and Fundamental Properties of Solids and Liquids. I. Solids. J. Am. Chem. Soc. 1916, 38, 2221−2295. (39) Freundlich, H. M. F. Adsorption in Solids. Z. Phys. Chem. 1906, 57A, 385−470. (40) Saha, B.; Das, S.; Saikia, J.; Das, G. Preferential and Enhanced Adsorption of Different Dyes on Iron Oxide Nanoparticles: A Comparative Study. J. Phys. Chem. C 2011, 115, 8024−8033. (41) Yu, X. Y.; Luo, T.; Jia, Y.; Zhang, Y. X.; Liu, J. H.; Huang, X. J. Porous Hierarchically Micro-/Nanostructured MgO: Morphology Control and Their Excellent Performance in As(III) and As(V) Removal. J. Phys. Chem. C 2011, 115, 22242−22250.

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