Synthesis and characterization of nanosized inorganic–organic hybrid between phosphotungstic acid and rhodamine 6G

Yanbin Rena, Lin Chenb, and Hongcheng Panc

College of Chemistry and Bioengineering, Guilin University of Technology,

Guilin 541004, P. R. China

aemail: 792299196@qq.com, bemail: a414388720@qq.com,

cemail: panhongcheng@glut.edu.cn

Keywords: synthesis, nanoparticles, hybrid, phosphotungstic acid, rhodamine 6G.

Abstract. inorganic–organic hybrid nanoparticles between phosphotungstic acid (PTA) and

rhodamine 6G (R6G) were synthesized by mixing a solution of 0.1 mmol R6G in ethanol (40 mL)

and 40 mL of aqueous solution of PTA (0.2 g). The as-prepared PTA-R6G nanoparticles were

characterized by scanning electron microscopy (SEM), UV-vis spectra, and Fourier transform-

infrared (FT-IR) spectra. The SEM studies show that the PTA-R6G nanoparticles have an average

diameter of about 111 nm and the atomic ratio of W to P is 12.7:1, near to the chemical

stoichiometry of PTA. UV-vis spectra confirm that the R6G aggregates formed in PTA-R6G

nanoparticles are J-type. The Keggin structure of PTA is maintained in the PTA-R6G hybrid

nanoparticles, as demonstrated by FT-IR.

Introduction

Organic-inorganic hybrid materials have gained much attention because of their potential

applications in optics, electronics, and catalysis[1-4]. With the large number of chemical and

structural modifications available, it is possible to design specific properties and produce novel

hybrid materials with both inorganic and organic characteristics[5].

Polyoxometalate anions have recently been employed as inorganic building blocks for the

synthesis of the functional hybrid materials[5-7]. The combination of the inorganic building blocks

with appropriate organic molecules has been shown to form a variety of polyoxometalate-based

hybrid materials[5]. For instance, Guo et al. synthesized four salts that combining the

triarylmethane dye cations (pararosaniline and crystal violet) with the hexametalates [M6O19]2-

(M =

Mo, W)[7].

Rhodamine 6G (R6G) is a laser dye with strong absorption in the visible and has a high

fluorescence yield. R6G is also used in sensor[8, 9], spectroscopic probes[10, 11], and fluorescence

resonance-energy-transfer (FRET)[12]. Considerable interest has been shown in recent years to

prepare the hybrid material of R6G with inorganic materials. H-type aggregates of R6G were

capped on SiO2 and SnO2 nanocrystallites[13]. These dye aggregates extends the photoresponse of

nanocrystalline SnO2 film. A maximum incident photon-to-photocurrent efficiency of ~1% was

observed for the photosensitized current generation.

Herein, we synthesized organic–inorganic hybrid nanomaterials by combining phosphotungstic

acid (PTA) as inorganic building blocks and R6G as the organic ligand. Scanning electron

microscopy (SEM), UV-vis spectra, and Fourier transform-infrared (FT-IR) spectra were carried

out to investigate the PTA-R6G nanoparticles.

Experimental Section

Materials. R6G was supplied by Hualan chemical (Shanghai, China). PTA was purchased from

Qiangshun chemical (Shanghai, China). Anhydrous ethanol were from Xilong chemical factory

(Shantou, China). Other reagents were of analytical grade. Ultrapure water (resistivity > 18 MΩ cm)

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was obtained from a WP-UP-IV-30 purification system (Sichuan, China) and used in all

experiments.

Synthesis of PTA-R6G nanoparticles. To a solution of 0.1 mmol R6G in ethanol (40 mL) was

added 40 mL of aqueous solution of PTA (H3PW12O40, 0.2 g). The mixture was stirred, heated at

40˚C for 2 h, and cooled to room temperature. The solution was centrifuged at 4000 revolutions per

minute (rpm) for 10 min. The resulting purplish red precipitates were washed five times with water,

centrifuging 10 min at 4000 rpm with each wash. The precipitates were air-dried at 40˚C.

SEM, UV-vis, and IR spectral measurements. The surface morphology and composition of the

PTA-R6G nanoparticles were observed with a Hitachi S-4800 scanning electron microscope (SEM)

equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. UV-vis absorption spectra

were recorded on a CARY 50 spectrophotometer (Varian, USA). FT-IR spectra were measured by a

Nicolet iS10 spectrometer (Thermo Nicolet, USA) using KBr pallets.

Results and discussion

SEM and EDS characterization. Detailed morphological and structural investigations were

carried out by using SEM analysis. Fig. 1a shows the SEM image of the PTA-R6G nanoparticles. A

histogram of particle diameters constructed in Fig. 1b indicates an average diameter of 111 nm with

a standard deviation of 32 nm. EDS analysis confirms the presence of tungsten and phosphorus in

the nanoparticles, as shown in Fig. 1c. The atomic ratio of W to P is 12.7:1, near to the chemical

stoichiometry of PTA (H3PW12O40).

Fig.1. (a) SEM image, (b) the size distribution histogram, and (c) EDS of the PTA-R6G

nanoparticles.

UV-vis spectra. The R6G monomer has a characteristic absorption maximum at 526 nm in aqueous

solution (Fig.2). In PTA-R6G suspension, a red shift in the absorption maximum is seen with a

maximum at 555 nm. According to molecular exciton model, the excitonic singlet state of the dye

aggregate splits into two levels, one being lower and the other being higher in energy than the

corresponding monomer singlet excited state[13, 14]. The transition to a higher energy level is

allowed for H-type aggregates, while transition to a lower energy level is allowed for J-aggregates.

The red shift in the absorption maximum and the broadness of the absorption band observed in Fig.

2 confirm that the R6G aggregates formed in PTA-R6G nanoparticles are J-type.

178 Advanced Materials and Processing Technologies: IFMPT 2014

Fig. 2. UV-vis spectra of R6G and PTA-R6G nanoparticles in aqueous solution.

IR spectra. Fig. 3 presents the IR spectra of PTA, R6G, and PTA-R6G. The IR spectrum of PTA

(Keggin-type) in Fig. 3 show four characteristic bands for P–Oa stretching vibration (1080 cm-1

),

W–Od stretching (984 cm-1

), W–Ob–W bridges ("inter" bridges between corner-sharing octahedra,

891 cm-1

),and W–Oc–W bridges ("intra" bridges between edge-sharing octahedra, 808 cm-1

)[15].

The IR spectrum of the PTA-R6G hybrid material also indicated the similar characteristic peaks to

the Keggin structure of PTA[15-17]. Slight frequency shifts in W–Od, W–Ob–W, and W–Oc–W

bands of PTA-R6G were observed, in comparison with PTA. Such behavior indicates that the

Keggin unit of PTA interacts (hydrogen bonding) with amino groups of R6G.

Fig. 3. IR spectra of PTA, R6G, and PTA-R6G nanoparticles.

Advanced Materials Research Vol. 900 179

Conclusions

In this study, the PTA-R6G hybrid nanoparticles were synthesized via a simple solution method.

EDS analysis of the PTA-R6G nanoparticles gives an atomic ratio of W/P = 12.7:1, which is very

close to the chemical stoichiometry of PTA. The red shift in the absorption maximum and the

broadness of the absorption band of the PTA-R6G nanoparticles confirm that the R6G aggregates

formed in PTA-R6G nanoparticles are J-type. Four characteristic IR absorption bands of PTA were

also observed in the PTA-R6G nanoparticles, which demonstrate that the Keggin structure of PTA

is maintained in the PTA-R6G nanoparticles.

Acknowledgements

This work is supported by National Natural Science Foundation of China (20905016, 21265005)

and Guangxi Natural Science Foundation (2013GXNSFBB053009).

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