Author Correction: Pyroelectric nanoplates for reduction of CO2 to methanol driven by temperature-variation

Carbon dioxide (CO2) is a problematic greenhouse gas, although its conversion to alternative fuels represents a promising approach to limit its long-term effects. Here, pyroelectric nanostructured materials are shown to utilize temperature-variations and to reduce CO2 for methanol. Layered perovskite bismuth tungstate nanoplates harvest heat energy from temperature-variation, driving pyroelectric catalytic CO2 reduction for methanol at temperatures between 15 °C and 70 °C. The methanol yield can be as high as 55.0 μmol⋅g−1 after experiencing 20 cycles of temperature-variation. This efficient, cost-effective, and environmental-friendly pyroelectric catalytic CO2 reduction route provides an avenue towards utilizing natural diurnal temperature-variation for future methanol economy. CO2 is a problematic greenhouse gas, although its conversion to alternative fuels represents a promising approach to limit its long-term effects. Here, the authors demonstrate that CO2 can be reduced to methanol through pyroelectric catalysis under temperature variation near room temperature.

F or hundreds of years, fossil fuels have been the main energy source for human activities and industrial manufacture. With the development of human society, the decrease in fossil energy and the increase of CO 2 concentration have aroused great attention. For instance, energy crisis, greenhouse effect and ocean acidification are some of the main problems facing humanity [1][2][3] . Converting CO 2 into hydrocarbon fuels is considered as one of the ideal solutions, which can solve not only the environmental problems but also the high requirements of energy consumption. Various methods have been explored to convert CO 2 to organic fuels, such as photocatalytic reduction, electrocatalytic reduction, biological transformation, hydrogenation, and dry reforming [4][5][6][7] . Nevertheless, hydrogenation of CO 2 to form CH 3 OH process requires high operating temperatures (200-250°C) and high pressures (5-10 MPa), which limit the yield of methanol 8 . As a matter of fact, photocatalytic reduction of CO 2 can be carried out at mild temperature and pressure, but it does not work in dark 9 .
Temperature variation is a recurring phenomenon in our daily life 10 . It would be meaningful to harvest such abundant energy source during temperature variation. Such a motive is reasonable because pyroelectric materials can convert heat energy into electric energy via repeating cooling or heating process [11][12][13] . Pyroelectric materials can produce positive and negative electric charges during temperature variation. The free charges generated through pyroelectric process can be applied to catalytic processes such as dye decomposition [14][15][16] and water splitting 17,18 . Theoretical calculation shows that a pyroelectric engine in an ideal condition can reach an energy conversion efficiency as high as 84-92%, which is much higher than the photovoltaic energy conversion efficiency typically in the range of 20% 19,20 . Theoretically, Kakekhani et al. 21 have proved the feasibility of pyroelectric catalytic water splitting. However, to our best knowledge, there is no report about collecting the energy using pyroelectric materials from temperature variation for CO 2 reduction.
The catalytic performance of ferroelectrics has been studied for 70 years. For example, the internal fields from the polarization of the ferroelectrics can separate electrons and holes, thus enhancing the catalytic efficiency 17 . Ferroelectric polarization can affect molecular adsorption and desorption from the surface of the materials 21 . It is well known that all ferroelectric materials are pyroelectric materials. As the simplest member of bismuth layerstructured Aurivillius phase, bismuth tungstate (Bi 2 WO 6 ) exhibits good ferroelectric and pyroelectric properties. Meanwhile, Bi 2 WO 6 has some other interesting properties such as high ion conductivity, large spontaneous polarization (P ≅ 50 µC cm −2 ), high Curie temperature (T C = 950°C), and photocatalytic property 22,23 . As Bi 2 WO 6 is constructed by alternating (Bi 2 O 2 ) 2+ and (WO 4 ) 2− layers, such a layered structure enables high thermal and chemical stabilities 24 . More importantly, the suitable energy band structure and surface properties of Bi 2 WO 6 allow it for CO 2 reduction into renewable hydrocarbon fuel 25,26 . In this work, through pyroelectric catalysis, CO 2 is reduced to CH 3 OH at temperature variation below 100°C. The efficiency has reached as high as 55.0 µmol g −1 after experiencing 20 cycles between 15°C and 70°C. Our experimental work provides a new route to CO 2 reduction for methanol through a pyroelectric catalytic process, which can be carried out near room temperature.

Results and discussion
Characterization of Bi 2 WO 6 . Previous study shows that Bi 2 WO 6 is ferroelectric with an orthorhombic structure 27 . Bi 2 WO 6 nanoplates were synthesized by hydrothermal process in this work (see the experimental details in the section of Methods). In order to identify the phase of the synthesized Bi 2 WO 6 , X-ray diffraction (XRD) analysis was performed at room temperature. As shown in Fig. 1a, all the diffraction peaks can be assigned to Bi 2 WO 6 according to the standard JCPDS card No. 79-2381 (space group: Pca2 1 ; point group: mm2; orthorhombic crystal system).
It can be seen from Fig. 1b that the synthesized Bi 2 WO 6 has a plate-like morphology with an average size of 250 nm. Figure 1c presents the image of Bi 2 WO 6 from transmission electron microscopy (TEM), where the nanoplate has the similar feature size as the ones of scanning electron microscope (SEM) images in Fig. 1b. The high-resolution transmission electron microscopy (HRTEM) image of Bi 2 WO 6 is shown in Fig. 1d. It clearly shows the single-crystalline nature of Bi 2 WO 6 nanoplate with a lattice plane intervals of 0.27 nm, corresponding to the (002)/(200) plane, respectively. The aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of Bi 2 WO 6 sample is presented in Fig. 1e. The light/dark gray contrast spots correspond to Bi (Z = 83, where Z is the atomic number) and W (Z = 74) atom columns, respectively. The inset image denotes the fast Fourier transformation (FFT) of the STEM image. The FFT image shows the zone axis of the STEM image is [103], which is perpendicular to b direction. Therefore, the STEM image reflects the layered structure of Bi 2 WO 6 , which is sandwiched by alternating perovskite-like (WO 4 ) 2− and fluorite-like (Bi 2 O 2 ) 2+ blocks. A comparison between the STEM image and the structure model is schematically illustrated in Fig. 1f. The left picture in Fig. 1f is the magnified image of the area marked in a red rectangle in Fig. 1e. The inset in Fig. 1e shows the simulated diffraction pattern in the [103] projection direction. Complete structure model is shown on the right side of Fig. 1f.
Bi 2 WO 6 is paraelectric with a high-symmetry body-centered tetragonal structure (space group symmetry I4/mmm) at high temperature. When the temperature drops, symmetry of the crystal structure will be broken, and the distortion of the symmetry tetragonal structure makes Bi 2 WO 6 generate ferroelectric properties. This mainly includes two aspects. First, the ions displace along the [110] axis of the tetragonal structure. Secondly, the WO 6 octahedra rotates around the a and c axes 28 . In order to characterize the ferroelectric properties of the assynthesized Bi 2 WO 6 nanoplates, ferroelectric domains of Bi 2 WO 6 nanoplates are observed using a piezoelectric force microscope (PFM) at a slow scanning frequency of 1 Hz with an area of 0.8 × 0.8 μm 2 . The nanoplates' morphologies of Bi 2 WO 6 in Fig. 2a are consistent with the results of TEM and SEM. Figure 2b, c show the vertical piezoresponse amplitude and phase image, respectively. The distinct contrast in the images illustrates the different polarization in the Bi 2 WO 6 nanoplates. Figure 2d, e display the local piezoelectric hysteresis loops of the Bi 2 WO 6 , including both "off" state (piezoelectric displacement contribution only) and "on" state (both the piezoelectric contribution and the displacement resulted from electrostatic interaction). The phase angles at "off" and "on" states change about 150°under 60 V DC bias field, confirming the occurrence of a local polarization switching under an electric field. The butterfly-shaped hysteresis loop further verifies the local ferro-/piezoelectric response of Bi 2 WO 6 nanoplate.
It is noted that Bi 2 WO 6 also shows pyroelectric properties, where imbalanced polarization charges can generate electric field when the material undergoes temperature variation. The voltage produced by pyroelectric effect can be driving force for electrochemical reactions. Figure S1 shows the pyro-potential distribution across a Bi 2 WO 6 nanoplate fitted by COMSOL finite element simulation, in which different colors represent different potentials. It can be seen that potential difference occurs on the surfaces of the Bi 2 WO 6 nanoplates. In general, ferroelectric materials have greater pyroelectric and piezoelectric coefficients than non-ferroelectrics 28 . In order to demonstrate Bi 2 WO 6 generate free charges through temperature variation, pyrocurrent response of Bi 2 WO 6 nanoplates is measured. Once the temperature of the pyroelectric material changes, the pyroelectric charges can be generated quickly due to the pyroelectric effect. The pyroelectric current can be expressed by Eq. (1), where I and p are the pyroelectric current and the pyroelectric coefficient of Bi 2 WO 6 , respectively. A is the area of electrode. dT/dt is the rate of temperature fluctuation. Therefore, the pyroelectric current is proportional to dT/dt, any temperature change of the pyroelectric material will cause it to generate free charges. Figure 2f, g show the current change generated by Bi 2 WO 6 nanoplates with the infrared signal. Under infrared radiation, a sharp increase of current density is induced by the pyroelectric effect due to the rapid increase of temperature within Bi 2 WO 6 nanoplates. The current density decays slowly due to the decrease of temperature change rate, and maintains at a steady value under the equilibrium condition. When the infrared radiation is turned off, due to the instantaneous temperature decrease (dT/dt < 0), the redistribution of the pyroelectric charges will produce a reverse current. The output current returns to zero while there is no temperature change and infrared radiation. To clarify the temperature effect, we further use xenon lamp (UV light) instead of the infrared radiation to illuminate the sample. There is no pyro-current signal generate, which further confirms that Bi 2 WO 6 generates free charge under temperature variation(see Fig. S2).
Pyroelectric catalytic activity of Bi 2 WO 6 . In order to evaluate the pyroelectric catalytic activity of Bi 2 WO 6 , CO 2 reduction test was carried out under the condition of temperature variation. As shown in Fig. 3a, the methanol yield increases with the increasing thermal cycles. The total methanol yield reaches 20.5 µmol g −1 without adding any sacrificial agent after 20 thermal cycles. 1 H NMR in Fig. S3 also demonstrates that no other products can be detected in the liquid phase. Meanwhile, the analyses of gaseous products in Fig. S4 show only a small amount of CH 4 and CO (0.11 µmol g −1 and 0.20 µmol g −1 , respectively), indicating high selectivity of Bi 2 WO 6 pyroelectric catalytic CO 2 reduction to CH 3 OH. The oxygen production detection is shown in Fig. S5, the amount of O 2 is roughly 1.5 times the amount of methanol. It has been similarly reported that the photocatalytic process works due to the recombination of electrons and holes. Such a process will significantly affect the catalytic efficiency 29,30 . To reduce the occurrence of electrons recombination with holes, sacrificial agents are usually added to the reaction system. Figure 3b shows that more methanol can be generated by using Na 2 SO 3 as negative charge sacrificial agent. The methanol yield can be as high as 55.0 µmol g −1 after 20 temperature-variation cycles, which is 2.5 times more than that without Na 2 SO 3 . The range of temperature variation can also affect the pyroelectric catalytic CO 2 reduction. Figure S6 shows the yield of methanol after 10 thermal cycles in different temperature ranges (15-40°C, 15-50°C, 15-70°C, 15-85°C). In Fig. S6, the methanol yield increases as the temperature range increases. The pyro-induced charges (dQ) can be expressed in Eq. (2) A larger temperature range can generate more pyro-charges, leading to better pyroelectric catalytic results. It is also noted that the Bi 2 WO 6 nanoplates maintain their crystal structure and morphology after pyroelectric catalytic reduction as confirmed by the XRD analysis and SEM characterization (see Fig. S7). To further prove that CO 2 reduction comes from pyroelectric catalysis, experiment without Bi 2 WO 6 nanoplates is performed, the result in Fig. S8a shows that no methanol or other products can be detected under temperature variation without Bi 2 WO 6 nanoplates. Furthermore, no methanol or other products can be detected when the test is carried out with the presence of Bi 2 WO 6 nanoplates for 10 h at temperatures of 15°C, 45°C and 70°C, respectively (Fig. S8b). CO 2 is a linear molecule, which is one of the most thermodynamically stable carbon compounds, it's hard to break the bonding of C=O 8,31 .
Pyroelectric charges can react with O 2 and OH − in water to form O ÁÀ 2 and ÁOH. Such reactions can be expressed as shown in Eqs. (3)-(5).
To have a better understanding of the pyroelectric catalysis, O ÁÀ 2 and ÁOH detections are performed. Experimentally, the ÁOH can be detected by fluorescence spectrometry using terephthalic acid as a photoluminescent ÁOH trapping agent. The O ÁÀ 2 can be detected by UV-Vis spectrophotometer since O ÁÀ 2 can react with nitro-blue tetrazolium (BNT) to produce diformazan and monoformazan. As shown in Fig. 3c, significant fluorescence emission at~425 nm associated with 2-hydroxyterephthalic acid is observed upon the temperature-variation cycles. The gradual increase of luminescence intensity with temperature-variation cycles indicates the formation of ÁOH. Figure 3d presents the absorption spectra of diformazan and monoformazan, which are produced by BNT reacted with O ÁÀ 2 . The increase of peak absorption at~630 nm and 720 nm with the temperaturevariation cycles indicates the formation of O ÁÀ 2 32 . The electron spin resonance characterization in Fig. S9 can further prove O ÁÀ 2 and ÁOH generated through temperature variation. O ÁÀ 2 and ÁOH are considered to be the main active species in dye decomposition 33 . Except for the pyroelectric catalysis of CO 2 reduction, we further performed RhB pyroelectric catalytic decomposition experiment to fully demonstrate the pyroelectric catalytic activities of Bi 2 WO 6 . In fact, RhB pyroelectric catalytic decomposition is a visual evidence to prove the redox ability of pyroelectric charges. Accordingly, Rhodamine B (RhB) solution (5 mg L −1 ) is used to demonstrate the pyroelectric catalytic dye decomposition of Bi 2 WO 6 in Fig. S10a, b. In order to prove that methanol is the product of CO 2 reduction, we manage the isotopic labeling experiment using 13 CO 2 as feedstock. To avoid 12 CO 2 , we have done the additional experimental using NaOH instead of NaHCO 3 . The 1 H NMR spectrum of the reaction solution (Fig. 3e) clearly shows the formation of methanol (δ = 3.34 ppm) when the unlabeled CO 2 is used as feedstock. While Fig. 3 Catalytic activities of Bi 2 WO 6 . a Methanol yield through pyroelectric catalytic CO 2 conversion without Na 2 SO 3 , and b with Na 2 SO 3 as sacrificial agent. c The fluorescence spectra of 2-hydroxyterephthalic acid, d the absorption spectra of diformazan and monoformazan. e 1H NMR spectra of the pyroelectric catalytic reaction solution with unlabeled CO 2 and f 1H NMR spectra of the pyroelectric catalytic reaction solution with labeled 13 CO 2 . NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20517-1 ARTICLE NATURE COMMUNICATIONS | (2021) 12:318 | https://doi.org/10.1038/s41467-020-20517-1 | www.nature.com/naturecommunications using 13 CO 2 instead of CO 2 , the 1 H NMR spectrum of the reaction solution in Fig. 3f shows doublet peaks between 3.7 and 3.0 ppm, which is attributed to the proton coupled with the 13 C of 13 CH 3 OH 34 . The results directly indicate that CO 2 is the carbon source for the pyroelectric catalytic CO 2 reduction into CH 3 OH. The time course change of the intensity is shown in Fig. S11.
Theoretical calculation of CO 2 reduction reaction path. To better illustrate the reaction mechanism for the CO 2 reduction, we employ first-principles calculations with SIESTA package, which is based on density functional theory (DFT) 35 . The pseudopotentials are constructed by the Troullier-Martins scheme 36 . The Ceperley-Alder exchange-correlation functional as parameterized by Perdew and Zunger is employed for the local density approximation (LDA) 37,38 . In all calculations, the double-ζ plus polarization basis sets are chosen for all atoms. The atomic structures are fully relaxed using the conjugated gradient method until the Hellman-Feynman force on each atom is smaller than 0.02 eV Å −1 . Since Bi 2 WO 6 consists of alternative (WO 4 ) 2− and (Bi 2 O 2 ) 2+ layers as discussed above, a slab model is constructed for the Bi 2 WO 6 (001) surface. The top of the slab is terminated by the WO square network, and the bottom of the slab is saturated by H atoms, as shown in Fig. S12. It is known that oxygen vacancies commonly exist in oxide semiconductors 39 . However, it is found that the formation energy of an oxygen vacancy in Bi 2 WO 6 (001) is as large as 3.2 eV, which indicates that the density of oxygen vacancies in Bi 2 WO 6 (001) is ignorable. To explore the possible process of the CO 2 reduction, it is necessary to figure out the ground-state adsorption configuration of the CO 2 molecule on Bi 2 WO 6 (001). Figure 4a shows five different adsorption configurations for CO 2 . The lowest adsorption energy is −3.6 eV, which implies that the CO 2 molecule is strongly bound to Bi 2 WO 6 (001). In this case, the CO 2 molecule is bent with one C-W bond (2.00 Å) and two O-W bonds (2.09 and 2.26 Å) as shown in Fig. 4b (step "0"), which is different from previous report 9 . The C-O bond lengths are elongated by 0.1 Å because of the interaction between CO 2 and Bi 2 WO 6 (001). The CO 2 reduction starts when the hydrogen ions in the solvent interact with the CO 2 molecule. Note that DFT calculations, the hydrogen atom as proton (H + ) and electron (e − ) cannot be separated directly. To model the reaction between H and radical on Bi 2 WO 6 (001), a H atom is placed beside a certain site of the radical and carried out DFT calculations to optimize the interaction between them. Electron charge transfer happens between the H atom and radical, usually from H to the radical so that the H atom finally becomes H + , according to the chemical bonding between H and the radical (e.g., CO 2 molecule in this work). In other words, charge separation can be reached after selfconsistent-field iterations. In addition, the gas phase of H is assumed because the solvent does not involve in the reaction of the CO 2 reduction. The process of the CO 2 reduction is divided into a series of steps and the reaction energies are calculated step by step. All possible structural configurations along with addition of one H ion are considered for each step simulation. For instance, from step "0" to step "1", the H ion may bind to the CO 2 molecule through C or O atom, or to the Bi 2 WO 6 (001) surface through W or O atoms, which results in different products. To determine the most possible reaction, the reaction energy of each product is estimated as: ΔE ¼ E n ð Þ À E n À 1 ð ÞÀμ H , where E n ð Þ is the total energy of a certain product at the n th step and μ H is the chemical potential of H. After optimizing all structural configurations, the case with the lowest reaction energy at each step is plotted in Fig. 4b. Obviously, the structural configurations in Fig. 4b are the most possible products for each step. Further, Fig. 4c shows the reaction energy of the most possible product at each step. Here, the first three H ions at the first three steps prefer to bind to the C atom, and these reactions are exothermic due to the large negative reaction energies as seen in Fig. 4c. As a consequence, one C-O bond is broken, then a CH 3 O * radical and a separate O ion are produced at step "3". Then the subsequent H ions will be attracted by the separated O ion until a H 2 O molecule forms (step "5"). However, the H 2 O molecule is not released from Bi 2 WO 6 (001), because it requires a large activation energy of about 1.7 eV. Finally, a methanol (CH 3 OH) molecule is produced after one more H ion attaches to the CH 3 O * radical (step "6"). As shown in Fig. 4c, the first four reaction steps are exothermic while the last three are endothermic. In particular, an activation energy Fig. 4 Adsorption configurations and reaction path of CO 2 into CH 3 OH on Bi 2 WO 6 . a Five different adsorption configurations for CO 2 on Bi 2 WO 6 . The red, gray, purple, and brown spheres stands for O, W, Bi, and C atoms. b Structures and c reaction energies for the CO 2 reduction. Eight reaction steps are considered. The red, cyan, light gray, and dark gray spheres stand for O, W, H, and C atoms. Only the atoms around CO 2 are highlighted. The reaction energies for the side product (methanol) at steps "4" and "5" are indicated by short dashed lines. The short red (dashed) lines are estimated from the free energies. of 1.6 eV (equal to the reaction energy) is needed for the CH 3 OH molecule to be detached from the Bi 2 WO 6 (001) surface, i.e., from step "6" to step "7". Note that CH 3 OH may also be produced at steps "4" and "5" (short dashed lines in Fig. 4c), but the corresponding activation energies are 1.77 and 1.87 eV, respectively, even larger than that for step "7". Therefore, the possibility to produce CH 3 OH at steps "4" and "5" is very small, because the major reactions at the first two steps are exothermic. Nevertheless, the overall process of the CO 2 reduction is still exothermic as shown in Fig. 4c, that is, the activation energies in the last three reaction steps can be compensated by the energy released in the first four steps. In principle, the CO 2 reduction happens spontaneously. However, energy supply might be required while the energy loss in a solvent environment. This is the reason that the temperature is not high in our experiments. It is worth pointing out that, when the CH 3 OH molecule is detached from the Bi 2 WO 6 (001) surface, the zero-point energy and enthalpy contribute to the free energy significantly 40 . Therefore, the reaction energies are also estimated through the free energies for CH 3 OH production indicated by the red lines in Fig. 4c. Interestingly, the activation energy of step "7" decreases to 0.95 eV, which implies that the reaction may happen at relatively high temperature, as found in our experiments.
On the basis of the above analysis, the mechanism of pyroelectric catalytic CO 2 reduction induced by temperature variation is proposed as shown in Fig. 5. When the temperature of Bi 2 WO 6 remains stable, the internal spontaneous polarization is balanced with the external bound charges (Fig. 5a). It has been reported that the spontaneous polarization intensity of pyroelectric materials decreases as its temperature increases 41 . That is to say, the increase in temperature will reduce the polarization of the pyroelectric catalyst, thereby breaking the balance and generating free charges. The free negative charges react with adsorbed CO 2 to form methanol and the free positive charges would be captured by Na 2 SO 3 to form Na 2 SO 4 (Fig. 5b). As a result, balance is established again between polarization and bonding charges (Fig. 5c). On the other hand, the decrease of temperature causes the increase of spontaneous polarization, and the equilibrium will be broken again, thus leading to opposite charges transfer and CO 2 reduction process (Fig. 5d). Then the catalyst temperature returns to its original value and at the same time it returns to its original equilibrium. Therefore, the continuous thermal cycles will cause continuous CO 2 reduction reaction.
In summary, this work introduces the use of pyroelectric materials to reduce CO 2 to methanol driven by temperature variation. Experimental results show that the yield of methanol generation from CO 2 can be as high as 55.0 µmol g −1 after 20 cycles of temperature variation. This efficient and environmentally friendly process based on the pyroelectric nanomaterial Bi 2 WO 6 provides great potential for CO 2 reduction in utilizing environmental heat energy near room temperature.
Preparation of Bi 2 WO 6 nanoplates. Bi 2 WO 6 nanoplates were synthesized through hydrothermal process. In a typical process, 485 mg of Bi(NO 3 ) 3 ·5H 2 O (1 mmol) and 165 mg of Na 2 WO 4 ·2H 2 O (0.5 mmol) were added into the mixed solution. White precipitate appeared immediately in the solution. After being washed for several times, the collected precipitate was added into a 50 mL Teflonlined autoclave and filled with deionized water up to 80% of the total volume. Then the autoclave was sealed into a stainless steel tank and kept at 433 K for 20 h. After reactions, the white as-prepared sample was centrifugated at 2400 × g and washed three times with deionized water. Finally, the collected products were dried in vacuum at 333 K for 12 h for further use.
Characterization. The crystal structure was test by an X-ray diffractomer (Philips PW3040/60, the Netherlands) with monochromatic Cu Kα radiation (λ = 1.5406 Å, 2θ = 20°−80°). The morphologies of the Bi 2 WO 6 sample was characterized by a transmission electron microscope (TEM, Hitachi H-7650, Japan) and a field emission transmission electron microscopy (FETEM, Scios, USA) with an accelerated voltage of 200 kV. The high-resolution transmission electron microscopy (HRTEM) image was acquired through a field emission transmission electron microscopy (FEI Tecnai G2 F20 S-TWIN, USA) with the accelerated voltage of 200 kV. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy image was obtained on a 300 kV aberration-corrected JEM-ARM300F. The piezoelectric property of the Bi 2 WO 6 sample was characterized with piezoresponse force microscopy (PFM, MFP-3D, USA). Photoluminescence (PL) measurements were carried out with a Horiba spectrofluorometer (Fluoromax-4, France) in air. Pyro-current response was measured on a CHI 660E electrochemical workstation using a three-electrode cell. The UV-visible absorption spectra are recorded on a UV2501PC (Shimadzu, Japan).
Pyroelectric catalytic CO 2 reduction activity test. In the pyroelectric catalytic CO 2 conversion process, Bi 2 WO 6 powder (40 mg) was suspended in 5 mL 0.2 M NaHCO 3 solution in a 50 mL flask with the addition of 0.3 M Na 2 SO 3 as sacrificial donor. High purity CO 2 gas was bubbled into the flask for 10 min. Then the flask was immediately sealed with a rubber stopper. Then the flask was immediately sealed with a rubber stopper. The sample was suspended in the solution under magnetic stirring, being applied alternating temperature between 15°C and 70°C in water bath. The entire catalytic process is performed in dark. The detailed Fig. 5 The mechanism of pyro-catalytic CO 2 reduction induced by pyroelectric Bi 2 WO 6 nanoplate. a The temperature of catalyst remains constant, its spontaneous polarization in equilibrium with the bound charges. b The rapid rise in temperature broke the balance, and thus induce CO 2 reduction reaction. c A new balance is established after the CO 2 reduction reaction. d When the temperature drops, the opposite charges transfer, leading to a new CO 2 reduction process. NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20517-1 ARTICLE temperature profile and schematic diagram of the process can be found from Figs. S13 and S14, respectively. To detect the formation of methanol, 1 mL solution was fetched out and analyzed by using a gas chromatograph (Persee G5) equipped with a KB-5 column connected to a flame ionization detector. For the nuclear magnetic resonance (NMR) test, 800 μL reaction solution, 100 μL D 2 O and 50 μL DMSO (0.1% vol aqueous solution) were taken into nuclear magnetic tubes, and detected with an NMR spectrometer with superconducting magnet (AVANCE NEO 400 MHz, Switzerland).

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.