Photocatalytic technology is considered a potential green technology because of the following advantages: environmental protection and low energy consumption [1-4]. The conditions required for photocatalytic redox reactions are milder than the conventional catalytic conditions; moreover, the use of certain strong oxidants and harmful reducing materials can be avoided [5, 6]. Therefore, many researchers have applied photocatalysis in the field of organic synthesis [7-12]. However, in many reactions, the time required for the conversion of organics is long, and the selectivity is poor [13]. This is attributed to the low solar energy utilization efficiency and easy recombination of photogenerated electrons and holes. Recently, the photocatalytic performance has been significantly improved by using external fields such as microwave, thermal, electric, magnetic, and ultrasonic fields [14-20].

In recent years, microwave technology has been applied in environmental science, pollution control engineering, environmental monitoring and analysis, clean production, and green technology; moreover, it has been used to achieve technological innovation and breakthroughs [21]. Microwaves strongly penetrate materials and start heating directly from the inside of the materials, resulting in uniform and efficient heating, which makes microwave heating environmentally friendly [22, 23]. The essence of microwave heating is the conversion of microwave energy into thermal energy. Pyroelectric materials can convert absorbed thermal energy into electrical energy. The pyroelectric effect is a natural physical effect observed in crystals [24]. In this phenomenon, charge is released when the polarization intensity changes with temperature [25-30]. When a crystal with spontaneous polarization is heated or cooled, a spontaneous polarization change (ΔPs) occurs due to the temperature change (ΔT), resulting in the generation of surface polarization charges in a certain direction in the crystal [31]. Macroscopically, temperature changes will produce positive and negative charges (q⁺ and q^–) at both ends of the material, which are then transferred from the surface of the catalyst to the reactant molecules, promoting the reaction [32].

Tetragonal perovskite PbTiO₃ is a ferroelectric material with a high Curie temperature and a low dielectric constant [33-35]. With a change in the temperature, the spontaneous polarization of PbTiO₃ becomes more intense, resulting in a stronger pyroelectric effect [36]. In addition, PbTiO₃ exhibits another important characteristic: birefringence, making it an important material for photovoltaic applications [37]. However, as a photocatalyst, the excessively wide band gap of PbTiO₃ is not conducive to the formation of photogenerated electrons and holes [38].

In this work, we have designed a Pd/PbTiO₃ photocatalyst to promote organic synthesis under the synergistic effect of microwave field and UV light. The Pd nanoparticles (Pd NPs) promote the separation of photogenerated electrons and holes of PbTiO₃. Furthermore, we have achieved a highly active and selective Suzuki coupling reaction by exploiting the photocatalytic performance and pyroelectric effect of PbTiO₃. Combining photocatalysts with microwave radiation opens up new possibilities for interdisciplinary research.

In this study, analytical grade chemicals (Sigma Aldrich) were used without further purification. PbTiO₃ nanosheets were synthesized by the hydrothermal method. Typically, 0.4 g of Degussa P25 was dispersed in 17.5 mL of potassium hydroxide aqueous solution (7 mol L^–1), and the mixture was uniformly dispersed under vigorous stirring. Next, a certain amount of lead acetate was added to the above solution (Pb:Ti molar ratio = 1.25:1) and rapidly stirred for 2 h. Finally, 3 mL of polyethylene glycol and 14.5 mL of deionized water were added and stirred for 10 min. Then, the resulting solution was subjected to hydrothermal reaction in an oven at 200 ℃ for 12 h. The obtained product was washed several times with deionized water and dried for future use; the final product was a pale-yellow powder.

Typically, 150 mL of deionized water and 50 mL of ethanol (sacrificial agent) were placed in a 250 mL beaker at room temperature and uniformly stirred. Then, 1.0 g of PbTiO₃ powder was added to the above solution, sonicated for 10 min, and a certain amount of PdCl₂ aqueous solution was added. The resulting mixture was stirred for 30 min under irradiation with a 300 W xenon lamp (1050 mW cm^–2). The obtained solution was centrifuged, washed with deionized water for four times, and then, dried in a vacuum oven at 80 ℃. The resulting gray particles were ground and sieved to obtain a gray powder: Pd/PbTiO₃ catalyst (theoretical load: 0.5 wt%).

The crystal structure was characterized by X-ray diffraction (XRD, Rigaku D/MAX-2000) using a Cu K_α source at 40 kV and 20 mA (scan rate: 5° min^–1). The UV-Vis diffuse reflectance spectra were recorded on a spectrophotometer (Shimadzu, UV2600) with an integrating sphere attachment in the range of 200–800 nm using BaSO₄ as the reflectance standard. The morphology was characterized with a scanning electron microscope (SEM, Hitachi S4800) and a transmission electron microscope (TEM, JEOL JEM-2010, operated at 200 kV). The surface electronic states were determined by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C, Al K_α).

We selected the Suzuki carbon-carbon coupling reaction as the probe reaction. Typically, certain amounts of halobenzene (0.5 mmol), phenylboronic acid (1.2 equiv.), tetrabutylammonium bromide (TBAB, 0.5 equiv.), K₃PO₄ (3.0 equiv.), and EtOH/H₂O (1:1) were placed in a quartz reactor. Then, a certain amount of the Pd/PbTiO₃ catalyst was added and the reactor was placed in a microwave–photocatalytic reaction device. The reaction conditions were UV light (300 W mercury lamp), 600 W microwave power, 80 ℃, and 500 rpm. A Shimadzu GCMS-QP2010 SE gas chromatograph–mass spectrometer (GC–MS) was used to determine the biphenyl yield. An Rxi-5MS column with a diameter of 0.25 mm was used. The spectra were collected in the SIM mode and quantitatively analyzed using 1, 3, 5-trimethylbenzene as the internal standard.

All the peaks in the XRD patterns of the PbTiO₃ and Pd/PbTiO₃ samples (Fig. 1(a)) were assigned to the tetragonal perovskite PbTiO₃ phase (PDF #06-0452), confirming the successful synthesis of PbTiO₃. The strong and sharp diffraction peaks indicated that the prepared samples were highly crystalline [39]. The absence of Pd diffraction peaks was probably due to the small loading amount of Pd. In addition, as observed from the UV-Vis diffuse reflectance spectra, the single crystalline PbTiO₃ samples exhibited strong absorption in the UV region and weak visible absorption at 400-450 nm (Fig. 1(b)). Notably, the absorption in the visible light region was significantly enhanced after Pd NPs loading, which demonstrated the high photocatalytic reaction potential of the sample. The prepared perovskite single crystalline PbTiO₃ sample consisted of nanosheets and cubic crystals, as shown in the SEM image in Fig. 1(c). It could be clearly observed from Fig. 1(d) that the Pd NPs were supported on the surface of PbTiO₃, confirming the successful synthesis of Pd/PbTiO₃. More importantly, Pd NPs were selectively deposited on one side of single crystalline PbTiO₃, because of surface polarization charge screening. This was attributed to the fact that the metal ions coordinated to Cl^‒ were concentrated on the positive electrode surface of the single crystal and were reduced to a simple substance on the positive electrode surface, resulting in the selective deposition of the noble metal NPs.

The PbTiO₃ nanosheets had lateral dimensions of about 500 nm, as shown in the TEM image Fig. 2(a). The high-resolution TEM (HRTEM) image in Fig. 2(b) shows lattice fringes with a spacing of 0.390 nm, which was assigned to the (100) facet. Furthermore, the corresponding selected area electron diffraction (SAED) pattern (inset in Fig. 2(b)) confirmed that the PbTiO₃ nanosheet was monocrystalline and had the advantage of the (100) facet. Fig. 2(c) confirmed the presence of Pd NPs; as observed, the Pd NPs were mainly concentrated on one side of single crystalline PbTiO₃. The HRTEM image of the selected area showed a different interplanar spacing, which corresponded to the (111) facet of Pd (Fig. 2(d)).

XPS was performed to determine the chemical state and elemental composition of 0.5 wt% Pd/PbTiO₃. The XPS profile showed peaks due to Ti, O, Pd, and Pb; no impurity peak was detected, indicating the high purity of the sample (Fig. S1(a)). Furthermore, to further explore the existence of Pd and its chemical state, the high-resolution Pd 3d spectrum was recorded. Two sets of spin-orbit coupling peaks were observed (Fig. S1(b)). The Pd (3d_5/2, 3d_3/2) peaks at 334.8 and 340.3 eV were attributed to Pd²⁺ and those at 336.5 and 342.4 eV were attributed to Pd⁰ respectively, indicating that the valence and zero-valent metal Pd existed on the surface of PbTiO₃ [40].

In addition, a comparison of the spectra of the catalysts before and after the loading (Fig. 3(a-c)) showed that the Ti 2p, O 1s, and Pb 4f peak binding energies were significantly different. The Ti 2p and O 1s peaks shifted to higher binding energies after Pd deposition. This was possibly due to the interaction between the Pd NPs and PbTiO₃ substrate, indicating that the binding between supports was stronger. The change in the binding energy measured by XPS could be attributed to the bending of the electron band near the surface. When the Pd NPs were in contact with the PbTiO₃ crystal, the Fermi levels of Pd and PbTiO₃ reached equilibrium via consumption of most of the carriers near the dielectric interface, which resulted in the bending of the electron band and formation of a Schottky barrier junction [41].

To study the generation of pyroelectric charges, the thermal current responses for PbTiO₃ and Pd/PbTiO₃ with respect to the change in temperature were measured. The temperature change curve recorded during the heating process, which was controlled by a constant temperature heater, is shown in Fig. 4(a). The electrolyte temperature remained constant after reaching 60 ℃. For both PbTiO₃ and Pd/PbTiO₃, the current increased with increasing temperature at the beginning, and it decreased when the temperature was constant (Fig. 4(b)). This indicated that the change in temperature generated charges at both ends of the materials. Moreover, after PbTiO₃ was loaded with Pd NPs, the current intensity significantly increased, reaching 0.73 μA. The results showed that Pd NPs contributed to charge migration, carrier recombination inhibition, and catalyst activity improvement.

We performed the Suzuki coupling reaction using bromobenzene (Fig. 5(a)). The biphenyl yield obtained in the presence of Pd/PbTiO₃ was much higher than that obtained in the presence of PbTiO₃, under the same reaction conditions (Fig. 5(b))_. To investigate the cause of the increased reactivity, we conducted exploratory experiments. Firstly, the influence of the pyroelectric effect (induced by the microwave field on the catalyst) on the photocatalytic organic synthesis process and the possible mechanism were investigated under five different conditions (CH, MW, UV, UV + CH, and UV + MW). It was found that the reactivity under the UV and microwave co-irradiation condition was the highest (Fig. 6(a)). The yield of biphenyl reached 78.2% within 20 min, which was 14 times higher than that obtained under only the UV irradiation condition. In addition, the catalyst could promote the reaction under only heating, without UV and microwave irradiation. This was attributed to the thermal nature of the Suzuki coupling reaction and heating-induced pyroelectric effect of the catalyst, enabling the catalyst to be excited in the absence of light. Under microwave irradiation, the activity was significantly higher than that under separate heating conditions. This was due to the unique heating properties of microwave, which enabled the material to release more charges, thus promoting the reaction. The reactivity significantly decreased when PbTiO₃ was replaced with TiO₂, because TiO₂ does not exhibit any pyroelectric effect (Fig. S2).

Furthermore, the effect of microwave power on the Pd/PbTiO₃-catalyzed Suzuki coupling reaction was investigated. As shown in Fig. 6(b), as the microwave power increased, the activity significantly improved, reaching a maximum value at 600 W. To save energy, powers > 600 W were not used. Thus, in this process, microwave irradiation led to not only thermal effects but also non-thermal effects, which possible influenced the catalyst or directly the reactants. Moreover, experiments were performed to determine the stability of the catalyst. The results showed that the activity of the catalyst decreased after three reaction cycles. This was possibly due to the adsorption of organic substances on the catalyst surface, which hindered the reaction. To remove the organic matter from the surface, the catalyst was calcined at 400 ℃, thereby the catalyst activity was restored (Fig. S3). On comparing the XPS profiles of Pd/PbTiO₃ before and after the reaction under different conditions, we found that the binding energy of the Pd/PbTiO₃ catalyst did not significantly change, which reflected the excellent stability of the catalyst (Fig. S4).

To broaden the application potential of the Pd/PbTiO₃ catalyst, we performed the Suzuki coupling reaction with different substrates. High yields were obtained for different substrates in this reaction system, as shown in Table S1. Overall, microwave-assisted photocatalytic organic synthesis is an efficient and environmentally friendly method.

To determine the mechanism of the Suzuki coupling reaction catalyzed by Pd/PbTiO₃, we measured the coupling reaction activity after adding a capture agent. In the presence of ethylenediaminetetraacetic acid, the yield of biphenyl decreased from 78% to 38%, while in the presence of AgNO₃, the yield was only 28% (Fig. S5). It was found that both e^– and h⁺ influenced the reaction, with e^– playing the major role. Fig. 7 shows the schematic of a probable mechanism for the Suzuki coupling reaction using Pd/PbTiO₃ under microwave-photocatalytic conditions. Under UV light irradiation, the Pd/PbTiO₃ catalyst generated electron-hole pairs, and the generated electrons were immediately transferred between PbTiO₃ and Pd. A change in temperature resulted in a net change in the electric dipole moment of the high-temperature catalyst, which resulted in charge compensation on the surface of the thermal catalyst. The positive and negative charges (q⁺ and q^–) generated at the high temperature were transferred from the surface of the pyrolysis catalyst to the reactant molecules to participate in the reaction. At thermodynamic equilibrium (ΔT = 0), the compensated charges on the surface completely shielded the polarization charge. When PbTiO₃ was heated, the polarization density decreased (ΔPs < 0), resulting in the formation of uncompensated charges on the surface of PbTiO₃, which were then transferred to the reactant molecules to form an intermediate state. When the temperature was constant (ΔT = 0), the system returned to the initial state. When PbTiO₃ cooled down, the polarization density increased (ΔPs > 0), and PbTiO₃ absorbed the charges from the solution, thereby promoting the formation of the product. At the same time, Pd/PbTiO₃ was excited by UV light, leading to the formation of photogenerated electrons and holes, which further promoted the reaction. Under the action of the double charge, the catalyst exhibited high activity and selectivity for the Suzuki coupling reaction regardless of the substrate.

In summary, we successfully prepared Pd/PbTiO₃ nanosheets by a simple hydrothermal reaction. Because of the combination of photocatalytic and pyroelectric effects, Pd/PbTiO₃ showed excellent activity and selectivity for the Suzuki coupling reaction. Experimental results showed that positive and negative charges (q⁺ and q^–) were generated on both ends of PbTiO₃ at high temperatures, and then, were transferred from the catalyst surface to the reactant molecules to participate in the reaction. The Pd NPs further promoted charge migration and increased the activity of the catalyst. The Pd/PbTiO₃ showed great potential in organic synthesis under microwave-light irradiation, and also promotes the development of photocatalytic organic synthesis.