synthesis, characterization, and optical properties of ag2mo2o7 nanowires

Phys. Status Solidi A 208, No. 8, 1937–1941 (2011) / DOI 10.1002/pssa.201026648 p s sa

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applications and materials science

Synthesis, characterization,and optical properties ofAg2Mo2O7 nanowires

Muhammad Hashim1, Chenguo Hu*,1, Yanxue Chen2, Cuiling Zhang1, Yi Xi1, and Jing Xu1

1 Department of Applied Physics, Chongqing University, Chongqing 400044, P.R. China2 School of Physics, Shandong University, Jinan 250100, P.R. China

Received 27 October 2010, revised 17 March 2011, accepted 28 March 2011

Published online 18 April 2011

Keywords hydrothermal method, photoluminescence, silver molybdate nanowires

* Corresponding author: e-mail [email protected], Phone: þ86 23 65104741, Fax: þ86 23 65105925

Ag2Mo2O7 nanowires were synthesized by facile hydrothermal

method from sodium molybdate and silver nitrate at temper-

ature of 150 8C without any surfactant or template. The

synthesized product was characterized by X-ray diffraction

(XRD), scanning electron microscopy (SEM), energy disper-

sive spectrometry (EDS), and transmission electron micro-

scopy (TEM). It was found that the product is of anorthic phase

and single crystalline nanowires with width of 200 nm and

length up to 100mm. UV–Visible reflection spectrum demon-

strated a band gap of 2.94 eV. Photoluminescence (PL)

spectrum exhibited strong violet emission centered at 412 nm

and red emission ranging from 600 to 700 nm at room

temperature. The greatly enhanced red emission was observed

at low temperature. The emission mechanism has been

discussed according to the calculated results of electron density

of states (DOSs) for Ag2Mo2O7 with consideration of oxygen

vacancies by using the Vienna ab initio simulation package

(VASP).

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Low dimensional systems representone of important frontiers in advanced material research [1].One-dimensional nanosized building blocks such as nano-rods, nanowires, nanotubes, and nanobelts are consideredsuperior when compared with conventional bulk solidsbecause their properties differ significantly from those oftheir bulk counterparts. These systems are expected todisplay the size and shape dependent optical, magnetic, andelectronic properties [2, 3]. These nanomaterials are widelyused in varied applications such as energy cells [4], sensitivesensors [5], nanoelectronic devices [6], bio-markers [7], etc.Consequently, the synthesis and fabrication of functionalmaterials with rational strategies is a key to the developmentof their applications.

Nanocrystals have been prepared by various methods,including vapor phase process [8], jet deposition [9], milling[10], sol–gel method [11], micromulsion [12], coprecipi-tation [13], template synthesis [14, 15], hydrothermalsynthesis [16, 17], and solvothermal synthesis [18].Although all these methods have their own advantages, asa simple, cost-effective, and efficient synthesis route, thehydrothermal process has been demonstrated as one of

versatile routes toward the size and morphology-controllablenanomaterials [19].

Metal molybdate is an important family of inorganicmaterials, which has wide application in various fields, suchas photoluminescence [20], microwave applications [21],optical fibers [22], scintillator materials [23], humiditysensors [24], magnetic properties [25], and catalyst [26].Some metal molybdate nanostructures, such as, Fe2(MoO4)3

nanoparticles, ZnMoO4 nanoplates, MnMoO4 nanorods,CoMoO4 nanowires have been synthesized by hydrothermalmethod [27]. Yu et al. have synthesized Ag2MoO4 particles,Ag6Mo10O33 nanowires, Ag2W2O7 nanowires and a mixturephase of Ag6Mo10O33 and Ag2Mo2O7 nanowires by thehydrothermal method [28, 29]. Nagaraju et al. have preparedsilver molybdates nanowires of the mixture phase Ag2MoO4

and Ag2Mo2O7 through the hydrothermal method [30]. Also,Bhattacharya and Ghosh synthesized Ag6Mo10O33 nano-wires, Ag2MoO4 particles and Ag2Mo2O7 nanorodsembedded in silver molybdate glassy matrix by a heat-treatment [31]. These works are all important and valuable.

In this paper, we report the synthesis of pure anorthicAg2Mo2O7 nanowires with uniform morphology through a


1938 M. Hashim et al.: Synthesis, characterization, and optical properties of Ag2Mo2O7 nanowiresp

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Figure 1 (online colour at: www.pss-a.com) XRD spectrum of thesample indicating an anorthic phase of Ag2Mo2O7.

facile hydrothermal method without any surfactant ortemplate. The as-synthesized Ag2Mo2O7 nanowires werecharacterized by X-ray diffraction (XRD), field emissionscanning electron microscopy (FE-SEM), transmissionelectron microscopy (TEM), high resolution transmissionelectron microscopy (HRTEM), and selected area electronicdiffraction pattern (SAED). The UV–Visible reflectionspectrum and photoluminescence spectrum were alsostudied. The electron density of states (DOSs) and theelectron partial density of states (PDOSs) of Ag2Mo2O7

crystal were calculated by using the Vienna ab initiosimulation package (VASP) to explain the photolumines-cence spectrum at room and low temperature.

2 Experimental section2.1 Synthesis All of the chemical reagents used in our

experiments were of the analytical grade purchased fromChongqing Chemical Company and were used withoutfurther purification. Aqueous solutions were prepared withdistilled water.

In a typical synthesis, 0.5 mmol of AgNO3 and0.25 mmol of H2MoO4 � 2H2O were dissolved in 12 mL ofdistilled water in a beaker, respectively. Then, the AgNO3

solution was added into the H2MoO4 solution slowly undermagnetic stirring to form a mixture at room temperature,which was adjusted to pH 2 using HNO3 solution. Themixture was then transferred into a Teflon-lined autoclave of25 mL capacity. Then the autoclave was sealed and put into afurnace preheated to 150 8C. After reacting 24 h the


autoclave was taken out and allowed to cool down to roomtemperature naturally. The resultant was collected andwashed several times with absolute ethanol and distilledwater to have the samples for the investigation.

2.2 Characterization An X-ray diffractometer(BDX320) equipped with graphite monochromatized CuKa radiation (l¼ 1.5418 A) was used to characterize thecrystalline phase of as-synthesized sample. A scanning rateof 48/min was applied to record the pattern in the range of

Figure 2 (online colour at: www.pss-a.com)Low (a) and high (b) magnified SEM image,EDS (c) showing the presence of Ag, Mo, and O(Si from substrate). TEM (d) and HRTEMimage (e) of the Ag2Mo2O7 nanowires synthe-sized at 150 8C for 24 h.

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Phys. Status Solidi A 208, No. 8 (2011) 1939

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Figure 3 (online colour at: www.pss-a.com) Reflection spectrumand its corresponding Kubelka–Munk function (a), and PLspectra ofthe Ag2Mo2O7 nanowires under the irradiation of the HeCd laser(325 nm) at temperature of 10, 30, 100, 200, 250, 300 K (b).

208–808. Field emission scanning electron microscope (FE-SEM: Nova 400 Nano SEM) at 15 kV, high resolutiontransmission electron microscope at 400 kV (TEM JEOL4000EX) were used to investigate the morphology and sizeof the product. The reflection spectrum was recorded usingHitachi U-4100 UV–VIS–NIR Spectrophotometer on thefilm that was prepared by casting the dispersed Ag2Mo2O7

nanowires in ethanol solution on a glass substrate. The room-temperature and low-temperature photoluminescence (PL)spectra were measured on the Ag2Mo2O7 nanowires on aglass slice under the irradiation of 30 mW HeCd laser atwavelength of 325 nm.

3 Results and discussion Figure 1 shows the XRDpattern of the synthesized sample at 150 8C, for 24 h. All thepeaks can be indexed with pure anorthic phase of Ag2Mo2O7

with lattice constants of a¼ 6.09 A, b¼ 7.50 A, andc¼ 7.68 A, which match well with the standard JCPDS cardNo.75-1505. Figures 2a and b give FESEM images of thesample in low and high magnification, which show thenanowires with width of �200 nm and length up to 100mm.To the best of our knowledge, no pure Ag2Mo2O7 nanowiresin such long length were reported. EDS pattern in Fig. 2cindicates that the elements in the sample are Ag, Mo, and Oonly (Si is from the substrate). The TEM image in Fig. 2dexhibits clean surface Ag2Mo2O7 nanowires. The HRTEMimage confirms the single crystalline structure of Ag2Mo2O7

in Fig. 2e.The optical properties of Ag2Mo2O7 nanowires were

investigated via the diffuse reflectance spectrum and PLspectrum. The energy gap (Eg) of Ag2Mo2O7 nanowires canbe estimated by extrapolating the linear part of Kubelka–Munk function, which is the ratio between the absorption andscattering factor from the optical diffuse reflectancespectrum [32]. The diffuse reflectance and Kubelka–Munkfunction of the Ag2Mo2O7 is shown in Fig. 3a. Because thesize of individual nanowires is much smaller than thethickness of the sample thin film, an ideal diffuse reflectancewith constant scattering coefficient (S) could be expected.The Kubelka–Munk function in Fig. 3a shows a clear opticalgap at about 2.94 eV, corresponding to the absorption edge,which is close to the previous report of 2.65 eV forAg2Mo2O7 particles [33]. To explore the PL property ofthe Ag2Mo2O7 nanowires, we had PL spectra under theHeCd laser irradiation (325 nm) at different temperatures, asshown in Fig. 3b. PL emission displays a violet emissionpeak around 412 nm, a red emission range from 600 to700 nm at room temperature. With the decrease in tempera-ture, we find two obvious peaks centered at 629 and 674 nm,and we find that they become much stronger at 30–10 K. Toexplain the PL spectrum, we constructed a crystal structureof Ag2Mo2O7 (Fig. 4a), and the DOSs and the PDOSs werecalculated with the VASP on the basis of density functionaltheory (DFT) by the pseudopotential plane-wave method[34], as is shown in Fig. 4b. From the PDOSs in Fig. 4b, thebottom of valence band (VB) is mainly formed byhybridization between Ag 4d and O 2p, while conduction

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band (CB) is mainly formed by hybridization between Mo 4dand O 2p. The violet emission at 412 nm is attributed toelectron transition from VB to CB corresponding to a bandedge transition. However, if Ag2Mo2O7 is a perfect crystalstructure, the luminescence in the visible region cannot beexplained from the results in Fig. 4b. Thus, the calculationwith consideration of oxygen vacancies in the crystal latticesis presented in Fig. 4c. From Fig. 4c, we can see that theoverall structure around the bottom of CB and top of VB isnot changed much, but, as the electron spin DOS near thebottom of CB splits into up and down states, two defectenergy levels within the band gap appear. According to thethermoluminescence theory [35], existence of defect energylevels might cause visible luminescence under the laserirradiation at the lower temperature. Electrons can populatethese defect levels and recombine with a VB holes moreeasily at low temperature. However, the recombinationinvolving these states is hardly observed at high temperatureas they cannot be populated by electrons due to the thermalenergy they possess. Thus, the Moþ-VOi (where i¼ 1,2.)electronic centers capture electrons from the CB, then thesecaptured electrons jump to the VB to recombine with theholes there, as shown in Fig. 4d. The red emission at 629 and674 nm from the PL spectra of the Ag2Mo2O7 nanowires


1940 M. Hashim et al.: Synthesis, characterization, and optical properties of Ag2Mo2O7 nanowiresp

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Figure 4 (online colour at: www.pss-a.com)Proposed crystal structure of Ag2Mo2O7 (a).DOSs and PDOSs for a perfect Ag2Mo2O7

crystal structure (b) and with considerationof oxygen vacancies in the crystal structure(c), a schematic diagram of the energy levelsand the corresponding three emissions (d).

might come from the recombination of these electrons andholes and they produce excellent photoluminescence at lowtemperature.

4 Conclusions Single crystalline nanowires ofAg2Mo2O7 have been synthesized by using the facilehydrothermal method at temperature of 150 8C without anysurfactant or template. XRD and SAED results confirm thatsynthesized product is of pure anorthic phase of Ag2Mo2O7

and single crystalline structure. SEM images show theAg2Mo2O7 nanowires with width of 200 nm and length up to100mm. The optical band gap of Ag2Mo2O7 nanowires is2.94 eV from the UV–Vis reflectivity spectrum. The strongviolet emission at 412 nm in the PL spectra of the Ag2Mo2O7

nanowires is a result of the recombination of CB electronswith VB holes, while strong red emission at 629 and 674 nmat low temperature is due to the population of electrons orholes in defect energy levels and recombination of theseelectrons with VB holes. More populated electrons in defectlevels result better red emission.

Acknowledgements This work has been funded by theNSFC (60976055), the Fundamental Research Funds for the CentralUniversities (CDJXS11102208) and Postgraduates’ InnovativeTraining Project (S-09109) of the 3rd-211 Project, and sharing fundof large-scale equipment of Chongqing University.

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synthesis, characterization, and optical properties of ag2mo2o7 nanowires

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