Photocatalysis is an environmentally friendly technology, with promising applications in environmental purification and in converting solar energy to chemical energy ^{[1, 2, 3, 4, 5, 6]}. Under suitable light irradiation, most permanent organic pollutants in wastewater can be rapidly degraded by an appropriate photocatalyst. The main advantage of such process is relatively simple, and secondary pollution can be avoided. Many semiconductor photocatalysts have been developed, including large band gap semiconductors (e.g. TiO₂ ^[7], ZnO ^[8]), narrow band gap semiconductors (e.g. CdS ^[9], BiOBr(I) ^{[10, 11]}, Ag₂CO₃ ^{[12, 13]}), plasmonic noble metal (e.g. Au, Ag) nanoparticles (NPs) ^[14], Au₂₅(SR)₁₈ nanoclusters ^[15], and metal free photocatalysts (e.g. C₃N₄) ^[16].

Lead semiconductors are an important solid material. Lead tungstate (PbWO₄) has received much recent interest, because it is widely used in inorganic scintillators in high-energy physics. PbWO₄ has favorable properties as a scintillator, including a short decay time (10 ns), high energy density (8.28 cm³), low light yield (300 photons/MeV), short radiation length (0.9 cm), and high irradiation damage resistance (107 rad) ^[17]. The excitonic luminescence, thermoluminescence, and other optical properties of PbWO₄ crystals strongly depend on their morphology and nanostructure ^[18]. PbWO₄ nanocrystals with morphologies of tetragonal-bipyramidal microcrystals ^[17], microspheres ^[19], nanorods ^[19], nanospindles ^[20], nanobelts ^[21], and nanodendrites ^[22] have been reported. PbXO₄ (X = W, Mo) materials have attracted much recent attention as potential photocatalysts ^{[23, 24, 25, 26, 27, 28, 29]}. We previously demonstrated that the photocatalytic activity of PbWO₄ was determined by its microstructure and morphology ^[29]. Hierarchical PbWO₄ microspheres exhibited the highest activity among the morphologies of 14-faceted polyhedrons, hierarchical microspheres and nanoparticles. The PbWO₄ microspheres were also easily separated and recycled. Designing and fabricating high quality PbWO₄ crystals with desired microstructure or morphology is of fundamental and technological interest. Noble metal/composite structures are another strategy for improving photocatalytic performance ^{[30, 31]}. In noble metal/semiconductor composites, the recombination of photo-generated electrons (e^‒) and holes (h⁺) is minimized because of efficient charge separation, in which e^‒ accumulate on the metal and h⁺ remain on the photocatalyst surface .

In the current study, we prepared rod-shaped Pt/PbWO₄ microcrystals (1‒3 μm) by a hydrothermal route and subsequent calcination. Pt NPs of different contents (0.5‒2 wt%) were then photo-deposited on the PbWO₄ microcrystals. The photocatalytic performances of the resulting catalysts in the degradation of aqueous acid orange II dye were investigated. To the best of our knowledge, such rod-shaped Pt/PbWO₄ microcrystals have not been previously reported.

Rod-shaped PbWO₄ microcrystals were fabricated by a hydrothermal route with subsequent calcination. Na₂WO₄·2H₂O (1.658 g) was dissolved in 100 mL of deionized (DI) water under stirring, to obtain solution A. 1.056 g of Pb(NO₃)₂ and 0.8 g of CTAB were dissolved in 40 mL of DI water, and the resulting solution was added dropwise to solution A. After stirring for 30 min, the mixture was placed in a polytetrafluorethylene-lined stainless steel autoclave and maintained at 160 °C for 18 h. The resulting white powder was collected by centrifugation, washed twice with DI water, and dried in an oven at 100 °C. The sample was then calcined in a furnace for 2 h at 400 °C.

The Pt/PbWO₄ composite microcrystals were prepared by the photo-deposition method. Approximately 1 g of PbWO₄, 4 mL of methanol and the appropriate amount of H₂PtCl₆·6H₂O were added to 40 mL of DI water. The suspension was stirred in a 100 mL Pyrex round bottom flask for 30 min while purging with a stream of N₂, and then irradiated with an ultraviolet (UV) lamp (λ_max = 365 nm) for 4 h. After irradiation, the powder was separated by centrifugation, washed with DI water, and dried under vacuum at 120 °C for 10 h. The catalyst loaded with x wt% Pt was denoted x%Pt/PbWO₄. The final Pt contents of the Pt/PbWO₄ composites were determined by X-ray fluorescence analysis (Magix 601) to be ~0.5 wt%, 1 wt% or 2 wt%.

Powder X-ray diffraction (XRD) patterns were recorded at a scanning rate of 0.05°/s, using a Bruker D8 advance X-ray diffractometer at 40 kV and 40 mA with a Cu K_α radiation source (λ = 0.15418 nm). Scanning electron microscopy (SEM) images were collected using a SU8000 scanning electron microscope, and were used to investigate sample morphologies. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected using a Tecnai 20 FEG microscope. Surface compositions were determined by X-ray photoelectron spectroscopy (XPS), using a PHI Quantum 2000 XPS system with a monochromatic Al K_α source and a charge neutralizer. All binding energies were referenced to the C 1s peak of surface adventitious carbon at 284.8 eV. Photoluminescence (PL) emission spectra were recorded using a fluorescence spectrometer (Hitachi F-4500, Japan). Fourier-transform infrared (FT-IR) spectra were recorded using a Nicolet 470 FT-IR spectrometer (USA). Samples were pressed by a KBr disk preparation apparatus. Ultraviolet-visible (UV-Vis) diffuse reflectance spectra (DRS) were recorded using a UV-Vis spectrophotometer (UV-2550, Shimadzu).

The photocatalytic activities of the bare PbWO₄ and Pt/PbWO₄ samples were determined by the degradation of aqueous acid orange II dye. 50 mg of photocatalyst was suspended in 100 mL acid orange II (20 mg/L). A 500 W Xe lamp (CHF-XM-500W) was used as the light source. Before irradiation, the suspension was stirred by magnetic force in the dark for 40 min to ensure sorption equilibrium between the dye and photocatalyst. During the degradation process, the reaction temperature was maintained at 20 °C by the circulation of water. After fixed irradiation intervals, ~2 mL aliquots of suspension were removed, and the solids removed by centrifugation. The acid orange II concentration of the clear upper layer was determined by UV-Vis spectrophotometry, from its absorbance at λ = 484 nm.

The phase composition and crystal properties of the samples were analyzed by XRD. Fig. 1 shows the XRD patterns of the bare PbWO₄ and Pt/PbWO₄ samples. Similar diffraction peaks at 2θ = 27.43°, 29.66°, 32.75°, 44.72°, 47.07°, 51.27°, 55.34° and 56.58° were observed in the XRD patterns of the four samples. These peaks corresponded to the (400), (211), (410), (212), (511), (402), (013), and (‒603) planes, respectively, which were indexed to the monoclinic stolzite phase of PbWO₄, with unit cell constants of a = 13.525 Å, b = 4.968 Å, and c = 5.546 Å (PDF 16-0156). The space group belongs to P21/a(14). The strong and sharp diffraction peaks indicated a high degree of crystallinity. No peaks of any other phases were detected in any of these patterns, because of the small crystallite size and low Pt concentration.

Fig. 1. XRD patterns of PbWO4 and Pt/PbWO4 containing varying Pt concentrations.

The average crystallite size of the bare PbWO₄ and Pt/PbWO₄ samples was estimated using the Scherrer equation. The strongest diffraction plane of PbWO₄ was the (400) plane, which was used in this calculation. Table 1 shows the average crystallite sizes and surface areas of the bare PbWO₄ and Pt/PbWO₄ composites. The average crystallite size of PbWO₄ was 65‒70 nm. While small variations in crystallite size were observed, the Pt loading appeared to have little effect on the crystal properties of PbWO₄. Table 1 indicates that the bare PbWO₄ and Pt/PbWO₄ composites had very small specific surface areas (1.5‒1.9 m²/g). The Pt NP loading resulted in a small increase in surface area.

Table 1 Crystallite sizes and specific surface areas of the samples. Sample Average crystallite size (nm) Specific surface area (m2/g) PbWO4 67.9 1.6 0.5%Pt/PbWO4 69.7 1.5 1%Pt/PbWO4 65.2 1.8 2%Pt/PbWO4 65.7 1.9

Table 1 Crystallite sizes and specific surface areas of the samples.

Fig. 2 shows a representative SEM image of the bare PbWO₄, showing well-dispersed rod-shaped particles. The rods were typically ~1‒1.5 μm long, consistent with a morphology of rod-shaped PbWO₄ microcrystals.

Fig. 2. SEM image of the PbWO4 sample.

The morphology and crystallinity of the 1%Pt/PbWO₄ sample were further investigated by TEM. Fig. 3(a) shows a TEM image of a well-formed PbWO₄ microrod, with a smooth surface, diameter of 100 nm, and length of >1 μm. Some small spherical particles of diameter 1‒5 nm were observed on the smooth microrod surface, consistent with Pt NPs deposited on the surface of PbWO₄ microcrystals. Fig. 3(b) shows a high magnification TEM image of a single Pt particle, indicating a particle size of ~6 nm. A close-contact Pt-PbWO₄ interface was produced across the entire Pt/PbWO₄ composite. In this Pt/PbWO₄ system, the recombination of photo-generated e^‒ and h⁺ could be suppressed, because e^‒ accumulated on Pt NPs which were separation from h⁺.

Fig. 3. TEM images of the Pt(1%)/PbWO4 sample Low (a) and high (b) magnification TEM images; (c) HRTEM image; (d) SAED pattern of the PbWO4 microrod in (a).

Fig. 3(c) shows a HRTEM image of a PbWO₄ microrod, in which the crystal face was structurally uniform. The interplanar spacing of ~0.55 nm corresponded to the ^[001] lattice spacing of monoclinic PbWO₄. The vertical direction of the ^[001] plane corresponded to the (001) crystal axis (red line). The XRD analysis showed that the PbWO₄ was of monoclinic phase with α = 90°, β = 107.7°, and γ = 90° (PDF card No. 16-0156), with an intersection angle of 27.63° between the (001) crystal direction and crystal growth direction. Thus, the growth direction of the PbWO₄ microrods was along the (‒102) crystal direction. The selective area electron diffraction (SAED) pattern in Fig. 3(d) recorded from the body of the PbWO₄ microrod showed that it was of high crystallinity.

XPS was used to investigate thesurface composition and elemental states of 1%Pt/PbWO₄. Fig. 4(a) shows the XPS survey spectrum of 1%Pt/PbWO₄, showing that the sample mainly consisted of Pb, W, O, and Pt. The mass fractions of Pt, Pb, W, and O on the 1%Pt/PbWO₄ surface were estimated to be 0.96%, 45.0%, 43.0%, and 14.0%, respectively. High-resolution XPS spectra of the O 1s, W 4f, Pb 4f, and Pt 4f regions are shown in Fig. 4(b), (c), (d) and (e), respectively. The O 1s region (Fig. 4(b)) contained two peaks at 530.3 and 532.1 eV, which were attributed to O²⁻ in PbWO₄ and surface hydroxyl groups, respectively. The W 4f region (Fig. 4(c)) contained two peaks at 35.7 and 37.6 eV, which were attributed to the binding energies of W 4f_7/2 and W 4f_5/2, and were similar to literature values for W^{6+ [32]}. The high-resolution XPS spectrum of the core levels of the Pt 4f region is shown in Fig. 4(e). The binding energies of 74.76 and 71.46 eV corresponded to the Pt 4f_5/2 and Pt 4f_7/2 states, respectively, which were consistent with Pt(0) ^[33].

Fig. 4. XPS spectra of the 1%Pt/PbWO4 sample. (a) Survey spectrum; (b) O 1s; (c) W 4f; (d) Pb 4f; (e) Pt 4f.

UV-Vis DRS was used to determine the absorption of the samples at different wavelengths (200‒800 nm). Fig. 5 shows the UV-Vis DRS spectra of the bare PbWO₄ and Pt/PbWO₄ samples. Bare PbWO₄ absorbed at ~330 nm, which was outside the visible region. Visible light (350-600 nm) absorption was observed for the Pt/PbWO₄ samples. Pt NPs deposited on semiconductors can reportedly induce surface plasma absorption in the visible range ^[30]. The visible light absorption of Pt/PbWO₄ originated from surface plasma absorption, and was consistent with the presence of Pt NPs.

Fig. 5. UV-Vis DRS spectra of the bare PbWO4 and Pt/PbWO4 samples.

The catalyst surface properties were further analyzed by FT-IR spectroscopy. Fig. 6 shows the FT-IR spectra of the samples. According to the Ref. ^{[34, 35]}, the characteristic absorption band at ~700‒900 cm^‒1 was ascribed to the W‒O stretching vibration of WO₄ tetrahedra. Bands at ~1625 and ~3447 cm^‒1 were assigned to the OH bending and stretching modes, respectively. A relatively strong intensity of the absorption peak of ‒OH groups was observed for the Pt/PbWO₄ samples. During photocatalysis, ‒OH groups can capture photogenerated h⁺ and transform to ^•OH radicals, which are the main reactive species for the decomposition of organic dyes. Therefore, an increase in the number of surface ‒OH groups potentially improves photocatalytic activity.

Fig. 6. FT-IR spectra of the bare PbWO4 and Pt/PbWO4 samples.

Fig. 7 shows the room temperature PL emission spectra of the bare PbWO₄ and Pt/PbWO₄ samples. All samples exhibited an emission peak at ~476 nm. The PL emission of PbWO₄ microcrystals typically consists of two bands, named green luminescence and blue luminescence ^[36]. The peaks at ~470 and ~500 nm corresponded to blue and green luminescence, respectively. The emission peak at ~476 nm could be attributed to the radiative recombination of self-trapped excitons localized on regular WO₄ complex anions. The intensity of the PL peaks of Pt/PbWO₄ was much weaker than that of PbWO₄. The PL spectrum of a semiconductor largely results from the radiative recombination of self-trapped excitons. The weak intensity suggested a low recombination rate of e⁻/h⁺ pairs, and high photon efficiency for catalysis.

Fig. 7. PL spectra of the bare PbWO4 and Pt/PbWO4 samples.

Acid orange II was chosen as the photodegradation target under Xe lamp illumination. Fig. 8(a) shows that during irradiation, the photolysis of acid orange II did not occur in the absence of a photocatalyst. Introducing bare PbWO₄ microcrystals resulted in the rapid degradation of acid orange II, suggesting the PbWO₄ possessed high photocatalytic activity. The high crystallinity and UV absorption of the bare PbWO₄ contributed to its high activity. Loading Pt NPs into the PbWO₄ resulted in a large increase in activity, which was closely related to the Pt NP content. Increasing the Pt NP content from 0.5% to 1% resulted in an obvious increase in photocatalytic activity. Further increasing the Pt NP content to 2% resulted in only a small increase in activity. Acid orange II was completely decomposed within 50 min under irradiation in the presence of Pt/PbWO₄. To evaluate the photocatalytic performance of the Pt/PbWO₄ microcrystals, the activity of the standard photocatalyst P25 (TiO₂, ~80% anatase 20% rutile, Degussa Co., Dossenheim, Germany) was compared under the same conditions. The Pt/PbWO₄ microcrystals exhibited higher activity than P25.

Fig. 8. Decomposition of acid orange II under Xe lamp irradiation. (a) Change in acid orange II solution concentration with reaction time; (b) Kinetics of acid orange II decomposition.

The pseudo first order kinetics model expressed by ln(C₀/C) = kt was used to describe the kinetics of the acid orange II degradation. Fig. 8(b) shows the kinetics of acid orange II degradation in solution, based on the data in Fig. 8(a). The rate constants were obtained from the regression lines in Fig. 8(b). A good correlation to the pseudo first order model (R² > 0.99) was observed for the four samples. The rate constants over PbWO₄, 0.5%Pt/PbWO₄, 1%Pt/PbWO₄, and 2%Pt/PbWO₄ were calculated to be 0.028, 0.036, 0.051, and 0.054 min^‒1, respectively. Based on the above surface area data, the apparent rate constants for the degradation per unit surface area of the four catalysts were ~0.0175, 0.024, 0.0283 and 0.0284 min^-1m^-2, respectively.

One advantage of the rod-shaped Pt/PbWO₄ microcrystals was their ease of separation and recycling, which resulted from their high density. The stability of the 1%Pt/PbWO₄ catalyst was therefore tested over consecutive reactions, as shown in Fig. 9. The activity of the photocatalyst slightly decreased with increasing recycling number. The degradation rate of acid orange II decreased from 93% in the first reaction to 88% in the fourth reaction. This indicated that the photocatalyst had high photocatalytic stability. The rod-shaped Pt/PbWO₄ microcrystals are therefore promising for practical application.

Fig. 9. Stability of the 1%Pt/PbWO4 microcrystals over consecutive reactions.

Based on the characterization and photocatalysis results, a diagram accounting for the high photocatalytic activity of the rod-shaped 1%Pt/PbWO₄ microcrystals was proposed, as shown schematically in Fig. 10.

Fig. 10. Proposed mechanism for the photocatalytic reaction in the rod-shaped 1%Pt/PbWO4 microcrystals.

For the bare PbWO₄ microcrystals under Xe lamp irradiation, UV light excited electrons in the PbWO₄ valence band (VB) to the conduction band (CB), resulting in holes in the VB. The e^‒ and h⁺ could convert to reactive radicals like O₂^‒ and ^•OH, which decomposed the dye. The fraction of UV light emitted by the source was low, but the high crystallinity and rod-shaped structure of PbWO₄ lead to a high quantum efficiency in the photocatalytic reaction. Two explanations are given for the enhanced photocatalytic performance of the 1%Pt/PbWO₄ microcrystals and the effect of Pt NPs. Pt NPs deposited on the PbWO₄ microcrystals could act as electron traps, promoting e^‒/h⁺ separation, as suggested in Fig. 7. The trapped electrons could react with surface-adsorbed O₂, to generate O₂^‒. On the other hand, holes accumulated in the VB of PbWO₄ could react with H₂O to give form ^•OH radicals. O₂^‒ and ^•OH radicals were the main active species responsible for the decomposition of the dye. Quenching and radical production experiments were carried out, to confirm the identity of the radicals involved in dye degradation in the presence of Pt/PbWO₄. p-Benzoquinone (BZQ) is an O₂^•− radical scavenger ^[6]. Fig. 11 shows that adding 2 m mol BZQ rapidly deactivated the Pt/PbWO₄. BZQ could have captured O₂^•−, thus decreasing the amount of O₂^•− available for photocatalytic reaction. ^•OH radicals could also have been involved in dye degradation. Coumarin readily reacts with ^•OH to form the highly fluorescent product 7-hydroxycoumarin. Under irradiation, ^•OH radicals generated in Pt/PbWO₄ solution in the presence of Fe³⁺ were detected by a fluorescence technique, using coumarin as an ^•OH trap. Fig. 12 shows PL spectra of coumarin-^•OH adducts (7-hydroxycoumarin), generated in the irradiated 1%Pt/PbWO₄ suspension. The intensity of the PL peak increased with increasing time, indicating an increase in the amount of generated ^•OH radicals.

Fig. 11. Effect of BZQ on the photocatalytic degradation of acid orange II by 1%Pt/PbWO4.

Fig. 12. PL spectra of aqueous Pt/PbWO4-coumarin (1 mmol/L) under irradiation.

Photocatalysis could also have been mediated by the surface plasmon resonance (SPR) of the Pt NPs ^[30]. Fig. 5 shows that the presence of the Pt NPs increased the visible light (350‒600 nm) absorption of the composite. SPR-mediated photocatalysis could have involved the SPR-induced excitation of noble metal NPs under visible light, and electrons could have transferred from the Pt NPs to the CB of PbWO₄. These electrons could then be trapped by O₂, to form reactive oxygen species like O₂^‒. The resulting positively charged Pt⁺ could also oxidize organic molecules. The BET analysis showed that loading with Pt NPs did not change the surface area of the catalyst. Therefore, the Pt NP content could have influenced the efficiency of e^‒/h⁺ separation. A suitable Pt NP content could provide good particle dispersion on the PbWO₄ surface. Excess Pt NPs would result in large overlapping particles, leading to new recombination centers, and adversely effecting charge separation.

Rod-shaped PbWO₄ microcrystals of high crystallinity were synthesized via a hydrothermal route with subsequent calcination. Pt NPs were subsequently deposited on the PbWO₄ microcrystals by a photo-deposition method, producing rod-shaped Pt/PbWO₄ composite microcrystals. Pt/PbWO₄ exhibited high activity and stability in the photodegradation of acid orange II dye. The favorable crystal structure and high crystallinity resulted in the good photocatalytic activity of PbWO₄. The well-dispersed Pt NPs produced a SPR effect that promoted visible light harvesting. The Pt NPs also acted as electron traps to promote electron-hole separation, which further promoted the photocatalytic reaction. The rod-shaped Pt/PbWO₄ microcrystals have potential in the degradation of organic pollutants and other environmental purification applications.