3.1 Characterization of Bi2WO6 and Pd/Bi2WO6
X-ray diffraction (XRD) patterns of Bi2WO6 and Pd/Bi2WO6 are shown in Fig. 1. The XRD pattern of pure Bi2WO6 sample exhibits diffraction peaks at 28.36°, 32.96°, 47.22°, 55.88° and 58.64o which correspond to the (131), (200), (202), (331) and (262) planes of orthorhombic Bi2WO6 phase comparing to the JCPDS No. 39–0256 [16]. The XRD patterns of Pd/Bi2WO6 nanocomposites show addition diffraction peaks at 40.11o which can be identified to the (111) plane of metallic cubic Pd phase (JCPDS No. 46-1043 [16]). There is no change in orthorhombic structure of Bi2WO6 phase in the Pd/Bi2WO6 nanocomposites. These results indicate that metallic Pd nanoparticles deposited on top of Bi2WO6 nanoplates. No other diffraction peaks were detected in both XRD patterns of Bi2WO6 and Pd/Bi2WO6. The crystallite size of Pd in Pd/Bi2WO6 nanocomposites was calculated from the Scherrer’s equation as follows.
D = Kλ/βcosθ (2)
D is the crystallite size (nm), K is the shape factor and equals 0.94 for spherical particle, λ is the wavelength of Cu Kα 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, 18]. The crystallite size of Pd nanoparticles containing in 10% Pd/Bi2WO6 nanocomposites is 8.53 nm.
SEM images of Bi2WO6 and Pd/Bi2WO6 are shown in Fig. 2. The as-prepared Bi2WO6 sample was composed of nanoplates with 100–200 nm in diameter and an average thickness of about 20 nm. The surface of Bi2WO6 nanoplates is smooth. Upon being loaded with Pd, the morphologies of 1%, 5% and 10% Pd/Bi2WO6 nanocomposites remain unchanged. SEM image of 10% Pd/Bi2WO6 nanocomposites presents clear Pd nanoparticles with particle size of < 20 nm deposited on the surface of Bi2WO6 synthesized by microwave-assisted precipitation method. The elemental constituents containing in 10% Pd/Bi2WO6 nanocomposites were analyzed by energy dispersive X-ray spectroscopy (EDS) and the elemental distribution was analyzed by EDS mapping as the results shown in Fig. 3. The EDS spectrum of 10% Pd/Bi2WO6 nanocomposites contains Pd, Bi, W and O. The EDS analysis shows that Pd in 10% Pd/Bi2WO6 nanocomposites is 8.57% by weight. The EDS mapping of 10% Pd/Bi2WO6 nanocomposites shows uniform distribution of metallic Pd nanoparticles across the whole sample.
Figure 4 shows TEM images of the as-prepared Bi2WO6 sample and Pd/Bi2WO6 nanocomposites with different contents of metallic Pd particles loaded on top. As shown in Fig. 4a, the Bi2WO6 sample exhibits nanoplates with edge of 200x100 nm. The SAED pattern of single phase of orthorhombic Bi2WO6 nanoplate (Fig. 4b) shows spots of electron diffraction pattern, certifying a single crystalline nanoplate. The pattern can be indexed to the (200), (220) and (020) planes of orthorhombic Bi2WO6 which is in good agreement with the XRD standard [19, 20]. A set of diffraction spots was specified as the [001] zone axis of orthorhombic Bi2WO6. Thus, the exposed facet of Bi2WO6 nanoplate is (001) plane [19, 20]. Figure 4c and d of 5% and 10% of Pd/Bi2WO6 samples shows Pd fine dispersive nanoparticles with size of 15–20 nm on top of Bi2WO6 nanoplates. They should be noted that a number of Pd nanoparticles were increased with the progressive increase of PdCl2 content. The average particle sizes of Pd counted for 100 particles were 9.97 ± 2.17 and 11.22 ± 2.53 nm for 5% Pd/Bi2WO6 and 10% Pd/Bi2WO6, respectively. Figure 4e shows a high-resolution TEM image of 10% Pd/Bi2WO6 nanocomposites which revealed the Schottky barriers between metallic Pd nanoparticles and Bi2WO6 nanoplates and enhanced the photocatalytic performance of heterostructure Pd/Bi2WO6 nanocomposites [1, 6, 8, 9]. Obviously, the lattice fringe spaces of 3.18 Å and 2.25 Å were well-indexed to the (131) plane of orthorhombic Bi2WO6 structure and the (111) plane of cubic Pd structure. The Fast-Fourier-Transform (FFT) diffraction pattern of a Pd nanoparticle as shown in Fig. 4f appears as clear diffraction spots with systematic alignment of the single crystalline nanoparticle.
The chemical states of Pd nanoparticles on top of Bi2WO6 nanoplates were investigated by XPS. The survey XPS spectrum of 10% Pd/Bi2WO6 (Fig. 5a) shows that the heterostructure 10% Pd/Bi2WO6 nanocomposites are composed of Pd 3d, Bi 4f, O 1s and W 4f elements corresponding to the above EDS results. The Pd 3d spectrum of 10% Pd/Bi2WO6 (Fig. 5b) contained two signals of Pd 3d5/2 and Pd 3d3/2 at 335.27 eV and 340.60 eV, respectively. The signals certified that Pd species in the catalyst mainly existed as metallic Pd nanoparticles [12, 13, 21–23]. Moreover, the peaks at higher binding energies are assigned to Pd2+ (Pd 3d5/2 at 335.96 eV and Pd 3d3/2 at 341.92 eV), Pd3+ (Pd 3d5/2 at 336.88 eV and Pd 3d3/2 at 342.49 eV) and Pd4+ (Pd 3d5/2 at 337.93 eV and Pd 3d3/2 at 343.24 eV) [13, 21–23]. Figure 5c shows two binding energy peaks of 159.42 eV and 164.76 eV in accordance with the Bi 4f7/2 and Bi 4f5/2 levels, respectively. Thus, Bi species in Bi2WO6 are attributed to the typical Bi3+ ions [1, 8, 9, 12, 19, 20]. The XPS spectrum of W 4f (Fig. 5d) shows two main binding energies at 35.67 eV for W 4f7/2 and 37.83 eV for W 4f5/2, certifying the existence of W6+ oxidation state [1, 8, 9, 19, 20]. The O 1s core level (Fig. 5e) can be de-convoluted into three peaks, which include bonds of Bi–O at 530.33 eV, W–O at 531.11 eV and O–H on top of Bi2WO6 at 532.37 eV [1, 19, 20].
The optical properties of photocatalysts were analyzed by UV-visible spectroscopy as the results shown in Fig. 6. UV-visible absorption of pure Bi2WO6 sample (Fig. 6a) shows an excellent absorption in UV-visible region due to the intrinsic energy gap of Bi2WO6 [24–26]. Comparing to Bi2WO6, 10% Pd/Bi2WO6 shows higher absorption in visible light because of the localized SPR effect of Pd nanoparticles supported on top of Bi2WO6 nanoplates [27–29]. The results indicate that heterostructure Pd/Bi2WO6 nanocomposites absorbed visible light which can lead to generate more charge carriers and to improve photocatalytic activity [26–29]. Figure 6b shows the plot of (αhν)2 versus hν of pure Bi2WO6 and 10% Pd/Bi2WO6 samples by Kubelka–Munk equation [24, 26]. The band gaps of pure Bi2WO6 and 10% Pd/Bi2WO6 samples are 2.48 eV and 2.54 eV, respectively.
The visible-light-driven photocatalytic performance of pure Bi2WO6 and Bi2WO6 doped with different contents of Pd was investigated for photodegradation of RhB. Figure 7 shows UV–visible spectra of RhB solution over 10% Pd/Bi2WO6 for different lengths of irradiation time. They can be seen that λmax of RhB at 554 nm was significantly 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 Bi2WO6 and Bi2WO6 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 photolysis of pure RhB solution was carried out under visible-light irradiation. The RhB solution was highly stable and was not degraded under visible light irradiation within 120 min. Clearly, the degradation efficiency of Bi2WO6 was improved by being loaded with Pd. The degradation efficiencies of Pd/Bi2WO6 samples were improved with increasing in Pd content from 1–10% by weight. Bi2WO6 could degrade 48.71% RhB under light irradiation within 120 min. In contrast, 10% Pd/Bi2WO6 sample could degrade almost 100% of RhB under visible light irradiation within 120 min. This sample has the highest activity for RhB degradation.
The kinetic degradation of RhB over Bi2WO6 and Pd/Bi2WO6 nanosamples was also investigated by the pseudo-first-order equation as follows.
ln(Co/Ct) = kt (3)
, where k is the first-order rate constant, Co is the initial concentration and Ct is the concentration at a time (t) [1, 3, 11, 13, 14, 17, 20]. The photodegradation of RhB by Bi2WO6 and Pd/Bi2WO6 follows the pseudo-first order kinetics (Fig. 8b). The degradation rate constant over 10 wt% Pd/Bi2WO6 nanocomposites (0.0270 min− 1) is 4.79 times that over pure Bi2WO6 nanoplates (5.64 × 10− 3 min− 1). These results were suggested that 10% Pd/Bi2WO6 nanocomposites exhibited the highest photocatalytic performance for RhB degradation. The photostability of reused 10% Pd/Bi2WO6 nanocomposites was further investigated for five recycles as the results shown in Fig. 9. There is no obvious decline in photodegradation of RhB after five reaction runs. Thus, 10% Pd/Bi2WO6 nanocomposites have excellent photostability and photocorrosion resistance under visible light irradiation.
During photodegradation of RhB solutions containing 10% Pd/Bi2WO6 nanocomposites under visible light irradiation, isopropanol (IPA), benzoquinone (BQ) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were also added for scavenging hydroxyl radical (●OH), superoxide radical (●O2−) and photogenerated hole (h+), respectively [32–35]. The photodegradation of RhB (Fig. 10) was significantly decreased to 35.78% and 25.35% for the addition of IPA and BQ. But for the addition of EDTA-2Na, the photodegradation of RhB was still quite high. According to the results, ●OH and ●O2− are the main active species for RhB degradation over 10% Pd/Bi2WO6 nanocomposites.
Based on the above results and discussion, a mechanism of the enhanced photocatalytic performance of Pd/Bi2WO6 was proposed (Fig. 11). Electrons were excited from valence band (VB) to conduction band (CB) while holes were induced in VB of Bi2WO6 under visible light irradiation [1, 9, 11, 14, 19, 20]. Subsequently, the excited electrons and photo-induced holes were transferred to the surface of Bi2WO6 photocatalyst and reacted with O2 and H2O/OH− to produce active superoxide anion radical (●O2−) and hydroxyl radical (●OH) for degradation of RhB molecules [1, 9, 11, 14, 19, 20].
Bi2WO6 + hv → e− CB + h+ VB (4)
Pd + e− CB → e− Pd (5)
e− Pd + O2 → ●O2− (6)
h+ VB + H2O/OH− → ●OH (7)
●O2−/●OH + RhB dye → Degraded products (8)
Thus, Pd nanoparticles on top of Bi2WO6 nanoplates act as electron acceptors, promote interfacial charge-transfer kinetics through the metal − semiconductor interface [1, 9, 11, 12, 14, 19, 20], improve the separation of electron − hole pairs and enhance the photocatalytic activity of Pd/Bi2WO6 nanocomposites.