Synthesis and Characterization of Heterostructure Pd/Bi2WO6 Nanocomposites with Enhanced Properties of Visible-Light-Driven Photocatalyst

Heterostructure Pd/Bi 2 WO 6 nanocomposites were successful synthesized in ethylene glycol by microwave-assisted deposition method at 300 W for 10 min. Effect of the loaded Pd on phase, composition, morphology and visible-light-driven photocatalytic properties of Bi 2 WO 6 was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fast-Fourier-Transform (FFT) diffraction, UV-visible absorption and X-ray photoelectron spectroscopy (XPS). In this research, good distribution of cubic phase of spherical Pd nanoparticles with particle size of 15–20 nm supported on orthorhombic Bi 2 WO 6 thin nanoplates. The 10% Pd/Bi 2 WO 6 nanocomposites reveal major metallic Pd 0 species containing in Bi 2 WO 6 sample. Microwave can be used to synthesize metallic Pd nanoparticles supporting on top of Bi 2 WO 6 nanoplates. Photocatalytic activities of Bi 2 WO 6 loaded with different weight contents of Pd were monitored through photodegradation of cationic rhodamine B (RhB) dye under visible light irradiation of a xenon lamp. The 10% Pd/Bi 2 WO 6 nanocomposites have the highest photocatalytic activity because Pd nanoparticles as electron acceptors promote interfacial charge-transfer through Pd/Bi 2 WO 6 heterojunction.


Introduction
In recent years, semiconductor based photocatalysis as green method is a candidate degradation of harmful organic pollutants in water and air to CO 2 , H 2 O and other small molecules at room temperature [1][2][3][4][5]. However, the practical applications of visible-light-driven semiconductor based photocatalysts are still restricted by fast rate of photo-induced charge recombination [1-3, 6, 7]. Bi 2 WO 6 as a member of the Aurivillius oxide family is composed of perovskite [WO 4 ] 2− layers sandwiched between [Bi 2 O 2 ] 2+ layers to form internal electric eld between the slabs which can lead to enhance the e cient separation of charge carrier and photocatalytic activity [3,[7][8][9]. Bi 2 WO 6 with narrow band gap of 2.7 eV has been studied as an e cient visible-light-driven photocatalyst due to its high physical and chemical stability and high photostability [1,8,10]. Moreover, the photocatalytic activity of bare Bi 2 WO 6 is limited by fast recombination of photo-induced charge carriers and low e ciency of photo-induced charge transfer [1,9,10]. Thus, improving the photocatalytic performance of Bi 2 WO 6 as photocatalyst is worth to be investigated.
Noble metals have been used as electronic accepter from conduction band of semiconductor to enhance the separation of photo-induced electrons and holes and to promote photocatalytic activity of the semiconductor [1,[11][12][13]. There are reports of Pd nanoparticles loaded on semiconductor with enhanced visible-light-driven photocatalytic activity by surface plasmon resonance (SPR) effect, Mott-Schottky interface and increase of valence band edge [12][13][14][15] nanocomposites by microwave-assisted deposition method.
To prepare Pd/Bi 2 WO 6 nanocomposites by microwave-assisted deposition method, each of 1%, 5% and 10% PdCl 2 by weight was dissolved in 100 ml ethylene glycol as a reducing reagent. Then, 2.50 g asprepared Bi 2 WO 6 nanoplates were added with continued stirring for 30 min. The whole system was transferred in a microwave oven and heated at 300 W for 10 min. In the end, heterostructure 1%, 5% and 10% Pd/Bi 2 WO 6 nanocomposites were separated, washed by water and ethanol several times and dried for further characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM) connected with energy dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fast-Fourier-Transform (FFT) diffraction and UV-visible absorption spectroscopy.
Photocatalytic activities of heterostructure Pd/Bi 2 WO 6 nanocomposites with different weight contents of Pd were investigated by photodegradation of RhB under visible light irradiation (λ ≥ 420 nm) from a xenon lamp with a 420 nm cutoff lter. 0.2 g heterostructure 0-10% Pd/Bi 2 WO 6 nanocomposites were suspended in 200 ml of 1x10 − 5 M RhB solutions (pH = 5.2) with being stirred in the dark for 30 min.
During visible light illumination, 5 ml RhB solution was sampled at a given time interval and spun around to separate heterostructure Pd/Bi 2 WO 6 nanocomposites by forcing the heavier solid to the outer edge.
The residual concentration of RhB was measured at an absorption peak of 554 nm by UV-visible spectroscopy. The decolorization e ciency has been calculated by the following. D is the crystallite size (nm), K is the shape factor and equals 0.94 for spherical particle, λ is the wavelength of Cu K α line (λ = 0.154056 nm), β is the full width at half maximum (FWHM) in radian and θ is the Bragg's angle [3, 17,  shows uniform distribution of metallic Pd nanoparticles across the whole sample. Figure 4 shows TEM images of the as-prepared Bi 2 WO 6 sample and Pd/Bi 2 WO 6 nanocomposites with different contents of metallic Pd particles loaded on top. As shown in Fig. 4a, the Bi 2 WO 6 sample exhibits nanoplates with edge of 200x100 nm. The SAED pattern of single phase of orthorhombic Bi 2 WO 6 nanoplate ( Fig. 4b) shows spots of electron diffraction pattern, certifying a single crystalline nanoplate.
The optical properties of photocatalysts were analyzed by UV-visible spectroscopy as the results shown in Fig. 6. UV-visible absorption of pure Bi 2 WO 6 sample (Fig. 6a) shows an excellent absorption in UVvisible region due to the intrinsic energy gap of Bi 2 WO 6 [24][25][26]. Comparing to Bi 2 WO 6 , 10% Pd/Bi 2 WO 6 shows higher absorption in visible light because of the localized SPR effect of Pd nanoparticles supported on top of Bi 2 WO 6 nanoplates [27][28][29]. The results indicate that heterostructure Pd/Bi 2 WO 6 nanocomposites absorbed visible light which can lead to generate more charge carriers and to improve photocatalytic activity [26][27][28][29]. Figure 6b shows the plot of (αhν) 2 versus hν of pure Bi 2 WO 6 and 10% Pd/Bi 2 WO 6 samples by Kubelka-Munk equation [24,26]. The band gaps of pure Bi 2 WO 6 and 10% Pd/Bi 2 WO 6 samples are 2.48 eV and 2.54 eV, respectively.
The visible-light-driven photocatalytic performance of pure Bi 2 WO 6 and Bi 2 WO 6 doped with different contents of Pd was investigated for photodegradation of RhB. Figure 7 shows UV-visible spectra of RhB solution over 10% Pd/Bi 2 WO 6 for different lengths of irradiation time. They can be seen that λ max of RhB at 554 nm was signi cantly decreased with increasing in irradiation time and was slightly blue shifted because of deethylation of ethyl group and decomposition of RhB [20,30,31].
The photocatalytic performance of pure Bi 2 WO 6 and Bi 2 WO 6 doped with different contents of Pd under visible light irradiation was estimated through the change of RhB concentration as a function of irradiation time (Fig. 8a) The kinetic degradation of RhB over Bi 2 WO 6 and Pd/Bi 2 WO 6 nanosamples was also investigated by the pseudo-rst-order equation as follows.
, where k is the rst-order rate constant, C o is the initial concentration and C t is the concentration at a time (t) [1,3,11,13,14,17,20]. The photodegradation of RhB by Bi 2 WO 6 and Pd/Bi 2 WO 6 follows the pseudorst order kinetics (Fig. 8b) Based on the above results and discussion, a mechanism of the enhanced photocatalytic performance of Pd/Bi 2 WO 6 was proposed (Fig. 11). Electrons were excited from valence band (VB) to conduction band (CB) while holes were induced in VB of Bi 2 WO 6 under visible light irradiation [1,9,11,14,19,20].
Subsequently, the excited electrons and photo-induced holes were transferred to the surface of Bi 2 WO 6 photocatalyst and reacted with O 2 and H 2 O/OH − to produce active superoxide anion radical ( • O 2 − ) and hydroxyl radical ( • OH) for degradation of RhB molecules [1,9,11,14,19,20]. nanocomposites are considered as a promising photocatalyst for wastewater treatment.
Declarations Figure 10 Photodegradation of RhB solutions containing different active scavengers comparing with that without a scavenger over 10% Pd/Bi2WO6 nanocomposites.

Figure 11
Schematic diagram for photocatalytic mechanism of Pd/Bi2WO6 nanocomposites.