Experimental procedure

Preparation of pure and Pd doped WO ₃ Np

Chemicals and reagents

The employed chemicals, such as H₂WO₄ (99.97%) and PdCl₂.2H₂O (99.97%) were purchased from Sigma-Aldrich. Ethanol, NaOH, HCl, MB and RhB dyes (99.99%) were purchased from Merck. All the above chemicals were of analytical grade, and were used without any further purification.

H₂WO₄ and PdCl₂ were used as W and Pd sources, respectively. The starting solution was prepared by adding H₂WO₄ to 10 mL NaOH, which resulted in a yellow color solution, due to proton exchange process. A proper amount of PdCl₂.2H₂O in 0, 5 and 10 wt% deionized water was added to the above solution. The solution pH was adjusted to 2, with the addition of 2 mL HCl. The dropping rate had to be well controlled for achieving chemical homogeneity. Then, 5 mL distilled water were added, to obtain a MWI quick response with the starting solution. The resulting precipitate was transferred into a teflon lined household microwave oven (2.45 GHz), under optimum power of 360 W, and irradiated for 5 min, in air. After MWI process, the resulting substance was dried in convection mode, at 100 ºC, for 5 min. Pure WO₃ Np were prepared in a similar manner, without Pd. The final prepared products were pale yellow and light grey, for pure and Pd-doped WO₃, respectively. Fig. 1 shows the flow chart representation of Pd-WO₃ Np experimental procedure synthesis.

XRD analysis

Non-destructive XRD technique was employed for categorizing Pd-doped WO₃ Np structural analysis and phase crystallinity, as shown in Fig. 2.

From the X- ray analysis, characteristic peaks were seen in all diffraction patterns, which were indexed to the standard card (JCPDS 83-0950). As shown in Fig. 2, all WO₃ samples have a monoclinic structure, and Pd doping did not change their crystal assemblies. No extra peaks, except for monoclinic WO₃, were seen, which can be explained by Pd species small amount and high dispersion in the samples. Moreover, the diffraction peaks of the Pd doped WO₃ samples have slightly shifted towards a smaller diffraction angle than that of the pure one. Pure WO₃ lattice parameters were estimated at: a = 7.2456Å; b = 7.4324Å; c = 7.5821Å; and β = 91.001. After Pd doping, the lattice parameters decreased with higher Ct of Pd (Table 1). The difference in lattice parameters was due to Pd²⁺ ion (0.72 Å) smaller atomic radius than that of the W⁶⁺ ion (0.78Å).

The average crystalline size of pure and Pd doped WO₃ Np was estimated using Debye Scherrer’s formula, as in equation (1).

where d is the mean crystallite size and K is the shape factor of 0.89.

The average crystalline size of pure and 10 wt% Pd doped WO₃ was calculated as 41 and 28 nm, respectively. This result suggests that the grain growth was concealed due to Pd doping of W-site.

TEM analysis

Fig. 3 shows TEM of pure and Pd (5 and 10 wt%) doped WO₃ Np. Both pure and Pd-doped WO₃ samples clearly show mono-dispersed spherical forms with an average diameter from 50 to 30 nm, which agrees with the crystallite size estimated by XRD.

UV-vis TSA

In order to study the optical properties and band E_g of Pd-doped WO₃ samples, they were characterized by UV-vis TSA, as shown in Fig. 4a. In all the samples, optical transparency was from 70 to 85%, but it decreased with higher Ct of Pd. Also, a significant red shift, with the increase in Ct of Pd from 0 to 10 wt%, was seen in the absorption edges, which indicates the decrease in WO₃ band E_g on doping. The absorption coefficient was calculated from TSA, using eq. (2).

where T is optical transmission and t is the samples thickness. The direct band E_g of pure and Pd doped WO₃ samples was calculated from Fig. 4b), using eq. (3).

where α indicates the absorption coefficient, h denotes Planck’s constant, ν is incident radiation frequency and m is the factor ruling the direct or indirect movement of electrons from the valence to the conduction band.

E_g was calculated as 3.02, 2.84 and 2.73 eV, for pure, 5 and 10 wt % Pd doped WO₃ samples, respectively (Fig. 4b). The observed decrease in E_g can be explained by the formation of intermediate energy level of pure WO₃, between valence and conduction bands, due to Pd doping.

Pl spectra analysis

In conformity, Pl emission spectra arose from the recombination of free carriers, for all synthesized Np. Pl emission spectra of both pure and Pd doped WO₃ Np are shown in Fig. 5. They were in the range from 300 to 600 nm, using a 325 nm He-Cd laser source.

The bands at 371 and 408 nm were attributed to near UV emission, due to O vacancies. These bands were caused by the localized state of O vacancies in the WO₃-x needle-like structure, as reported by ³⁵. A weak shoulder peak, at 458 nm, corresponds to the bluish green emission caused by defects in the samples. The green emission peak, at 515 nm, indicates V⁺O vacancies. This emission was due to different Pl centers, namely, defect energy levels created by W interstitials and O vacancies. Pl intensity of the Pd-doped sample was significantly higher than that of pure WO₃, because of the recombination of photo-excited electrons and holes. Then, a lower Pl intensity (pure sample) might indicate a low recombination rate of those electrons and holes under VL irradiation.

FTIR

FTIR spectroscopy is an interesting characterization technique for analyzing the crystalline quality structure disorder and defects in Pd doped WO₃ Sc_. FTIR spectra of pure and Pd-doped WO₃ Np are shown in Fig. 6. All the spectra display similar IR active modes. The bands at 615.12 cm^-1 were due to W-O bond stretching vibrations. The region from 3000 to 3800 cm^-1 corresponds to OH stretching vibration of the absorbed surface or internally bonded water molecules ³⁶. The bending mode of H-O-H was located at about 1601.23 cm^-1. In addition to the above bands, the small peak positioned at 131.29 cm^-1 was associated with carbonate pieces absorption by the sample, during the annealing process ³⁷. No peaks were detected for Pd. This result confirmed that Pd ions substituted WO₃ site regular lattice.

Pc activity set up

Pc activities of pure and Pd-doped WO₃ samples were evaluated by MB and RhB degradation, under VL irradiation. For the Pc process, the dyes solutions were prepared at a specific Ct (e.g. 200 mL, Ct₀ = 10 mg L^-1), and with a particular amount of synthesized material. A 125 W high pressure Hg lamp was used as a light source. The dyes solutions were irradiated in the horizontal direction, and the distance between the lamp and the samples was kept within 25 cm. Then, the solutions were kept in a dark room and well mixed with the magnetic stirrer, for more than 30 min, to attain equilibrium. Ct of MB and RhB in each sample were analyzed using an UV-vis spectrophotometer, at a wavelength of 664 nm. Pc efficiency was calculated from the expression given by eq. (4).

where C₀ and C are Ct of MB and RhB dyes before and after a certain irradiation time, respectively.

Pc activity measurements

Pc activities of pure and Pd-doped WO₃ samples were evaluated based on their ability to degrade MB and RhB under VL. The findings revealed that irradiation time and Ct of Pc induced the reactive dyes degradation. Figs. 7 and 8 show the catalyst effect Ct on MB and RhB degradation profiles under VL irradiation. In both cases, the degradation was linearly increased with higher Ct of Pd. Particularly, the significant improvement in degradation was observed in the Pd-doped WO₃ catalyst. After 120 min, MB degradation was 56, 73 and 84%, for pure and Pd (5 and 10 wt%) doped WO₃ samples, respectively.

Similarly, pure, 5 and 10 wt% Pd doped WO₃ samples degradation efficiency of RhB was about 59, 78 and 92%, respectively. The significant improvement in the Pc performance was due to an increase in SA, as well as to O vacancies created by Pd doping. In addition, lower E_g was another crucial factor that increased Pc efficiency. Sequentially, the catalyst was subject to multiple cycles under the same procedure, for confirming that it was stable and reusable.

Fig. 9 shows the seven runs of RhB photo degradation by using Pd (10 wt%) doped WO₃ Pc. The result shows the slight variation in the decolorization/degeneration, due to incomplete recollection and loss during the washing process.

Fig. 10 shows the schematic representation of the Pc mechanism for RhB discoloration. One possible reason for the high SA and O vacancies of the Pd-WO₃ Pc may be the active ^•OH generation. The active •OH radicals were produced on the Pc surface by the reaction of photo generated h⁺vb holes and adsorbed OH^-. Other reason may be that the smaller E_g of Pd-WO₃ catalyst (2.56 eV) allowed OH^- oxidization to active OH ³⁸, which is the primary oxidizing species in Pc processes. Therefore, higher Ct of OH^- and, thus, increased Pc generation of active ^•OH, from the Pd-WO₃ surface, enhanced MB and RhB discoloration.

Abbreviations

E_g: band gap energy

FTIR: Fourier transform infrared

H₂WO₄: tungstic acid

h⁺vb: valence band holes

JCPDS: Joint Committee on Powder Diffraction Standards

MB: methylene blue

MWI: microwave irradiation

NaOH: sodium hydroxide

Np: nanoparticles

^• OH: hydroxyl radicals

OH^-: hydroxyl anions

Pc: photocatalytic/photocatalyst

PdCl₂.2H₂O: palladium chloride dihydrate

Pl: photoluminescence

RhB: rhodamine B

SA: surface area

Sc: semiconductor

TEM: transmission electron microscopy

TSA: transmission spectra analysis

UV-vis: visible ultraviolet

VL: visible light

WO₃: tungsten trioxide

WO₃.H₂O: tungstite

XRD: X-ray diffraction

Symbols definition

(: full width half maximum

(: order of diffraction

θ: Bragg’s reflection angle in degree

(: wavelength of the incident beam

ν: frequency of incident light