Nonmetallic plasmonic heterostructures with multi-synergies on boosting hot electron generation for CO2 photoreduction

Constructing multi-physical effects on semiconductors is one new horizon to develop next-generation photocatalysts. Here we use pyroelectric black phosphorus (BP) to couple with nonmetallic plasmonic tungsten oxides (WO) forming a BP/WO heterostructures as photocatalysts to convert CO 2 for CO under visible-near-infrared (Vis-NIR) light irradiation. Nonmetallic plasmonic heterostructures exhibit 26.1 µmol h − 1 g − 1 CO generation with a selectivity of 98 %, and which is 7- and 17-fold higher than those of plasmonic WO and pyroelectric BP, respectively. The interface P-O-W bonds in heterostructures are constructed to work as channels for electron transfer from BP to plasmonic WO. Moreover, the photothermal energy generated by SPR excitation on WO can make the temperature of heterostructures rapidly increasing from 24 to 86 o C in 10 min, triggering the pyroelectric BP for carriers to promote electron transfer. Multi-physical effects including plasmonic hot carriers and photothermal effect of WO, intrinsic band excitation and pyroelectric effect of BP and W-O-P bonds play synergistic roles on boosting hot electron generation for CO 2 reduction. This work provides clear proofs to demonstrate that constructing multi-physical effects on semiconductors is one useful strategy to promote NIR-harvesting for articial photosynthesis. plasmonic photothermal effect and hot carriers of WO, intrinsic band excitation and pyroelectric effect of BP and W-O-P bonds play synergistic roles on boosting hot electron generation for high-selective CO generation. This work provides one useful strategy to promote photocatalytic performance of plasmonic semiconductors by constructing synergy of multi-physical effects on heterostructures. for 10-BP/WO. The results indicate that a part of photoelectrons on BP directly transfers to WO, leading to a fast decay process of photoelectrons on BP [50] . Consequently, the photo-excited photoelectrons on BP can transfer to plasmonic WO, and the continuous electron injection can increase the electron density for enhanced SPR.


Introduction
As the free carrier density reaches 10 18 -10 21 cm − 2 , the surface plasmon resonance (SPR) generally observed on noble metals also occurs on semiconductors such as Cu 2 − x S, 1,2 MoO 3 − x , 3,4 WO 3 − x 5,6 , and Bi 2 O 3 − x 7 . Contrast to the metals 8-10 , nonmetallic plasmonic semiconductors have more complex electronic structures including intrinsic band gap, defect and SPR states, resulting in a complex excitation process and a broad light response in visible-near-infrared (Vis-NIR) region. [11][12][13][14] Currently, the most reported semiconductors with Vis-NIR SPR absorption are focused on heavily oxygen-vacancy-or cationvacancy-doped metal oxides 15,16 , and the heavy doping is a valuable strategy to increase semiconductors' carrier density, resulting in their metalloid properties on optics and catalysis [17][18][19][20][21] . Taking plasmonic WO 3 − x as an example [22][23][24] , the free-electron on heavy-oxygen-vacancy-doped WO 3 − x is excited by SPR to be hot electron for catalysis, meanwhile, the consumed electrons are supplied by the intrinsic band gap excitation or electron injection from other semiconductors for maintaining the free carrier density and SPR during photocatalysis 11,22,23 . However, the short lifetime of hot carriers 22 on plasmonic semiconductors leads to fast nonradiative decay for photothermal energy, and it restricts the hot electron generation for photocatalysis. Therefore, the synergistic utilization of photothermal effect and hot carriers is one promising strategy to break the above limitations of plasmonic semiconductors for high-e cient photocatalysis.
The pyroelectric effect is one physical phenomenon to convert thermal energy for electrical energy, and which is triggered on crystal materials with non-centrosymmetric structures and has signi cant applications on pyroelectric nanosensors, electrical generators, and pyroelectric catalysis [23][24][25][26][27][28][29][30][31] . Recently, a pyroelectric-effect-induced electrical eld was used to enhance photo-carriers separation in photocatalyst 32,33] . A ternary heterostructure of poly (vinylidene uoride-co-hexa uropropylene (PVDFHFP)/carbon nanotube (CNT)/CdS were reported with the triple functions of pyroelectric, photothermal material and photocatalyst, and in which the pyroelectric-effect-induced electrical eld was demonstrated to promote carrier migration and separation for photocatalysis 33 . Therefore, constructing pyroelectric effect on plasmonic semiconductors will be a new direction for exploring high-e cient photocatalysts. Among various pyroelectric materials, two-dimensional (2D) layered black phosphorus (BP) with broken structure by the loss-of-inversion symmetry exhibits excellent pyroelectric effect and can realize pyro-catalytic H 2 generation under cold-hot alternation 34 . Owning narrow band gap and pyroelectric effect [35][36][37] , 2D BP is one ideal material to marry with plasmonic semiconductors forming heterostructures, and in which multi-physical effects may play synergistic roles on photocatalysis.
In this work, 2D structured BP with pyroelectric effect is coupled with plasmonic tungsten oxides (WO 3 Supplementary Fig. 2), and their XRD patterns are consistent with those of BP and WO without any other peaks.
UV-Vis-NIR diffuse re ectance spectra (DRS, Fig. 1i) shows the optical properties of different samples. 2D BP has a narrow band gap and a broad light response in visible region, and WO nanowires exhibit strong SPR absorption in Vis-NIR region. 10-BP/WO heterostructure has a similar SPR band with that of WO, and the slight enhancement UV-Vis absorption is attributed to 10 wt% BP loading. Chemical structures of 10-BP/WO were analyzed by Raman spectra (Supplementary Fig. 3) and Fourier transform infrared (FTIR) spectra ( Supplementary Fig. 4). Weak signal of P-O-W bonds was detected at 1116 cm − 1 , indicating that a small amount of P-O-W bonds exist on interface to connect WO and BP 42,43 . Oxygen vacancies in 10-BP/WO heterostrutures and plasmonic WO were detected by electron paramagnetic resonance (EPR) spectra and similar concentrations were observed in Fig. 1j [44] . X-ray photoelectron spectra (XPS) show the chemical state of elements in different samples. For BP, XPS of P 2p (Fig. 1k) Fig. 6) and EPR spectra of plasmonic WO do not have changes under Vis-NIR-irradiation, con rming the electron transfer from BP to WO in BP/WO heterostructures. Moreover, the electron transfer frocess can also be observed on XPS of W 5+ 4f ( Fig. 2d) which have a pronounced increase on intensity after Vis-NIR irradiation, and the amount of W 5+ is increased from 19.2 to 34.5 mol% in heterostructures.
The kinetic process of electron transfer in different samples under light irradiation was characterized by using ultrafast transient absorption spectroscopy combined with the femtosecond laser excitation. As shown in transient absorption spectra of BP (Fig. 3a), the electron on VB of BP can be excited in a broad visible spectral region, and the highest photobleaching point is observed at 5.0 ps. Then, the excited electron has a decay resulting in a continuous decrease on spectra as time prolongs. At 2.0 ns, ∆A is near zero meaning the completed decay of photoelectrons, and BP is recovered from an excited state to the ground state. So, the total lifetime of photoelectron on BP is about 2.0 ns. While, for plasmonic WO, due to the short lifetime of plasmonic carriers 48,49 , the fast decay (< 100 ps) process and weak signals on transient absorption spectra are observed in Fig. 3b. Therefore, the transient absorption spectra ( directly transfers to WO, leading to a fast decay process of photoelectrons on BP [50] . Consequently, the photo-excited photoelectrons on BP can transfer to plasmonic WO, and the continuous electron injection can increase the electron density for enhanced SPR. Photocatalytic performance. The photocatalytic CO 2 reduction reaction (CO 2 -RR) performance is determined in a reactor using a gas-solid con guration. Under Vis-NIR (> 420 nm) light irradiation, 10-BP/WO heterostructures generate 78.3 µmol g − 1 CO in 3 hours (Fig. 4a), more than 7-and 17-fold higher than 10.9 and 4.4 µmol g − 1 of plasmonic WO and BP, respectively. CO generation rates (Supplementary  (Fig. 4b), with a selectivity of 91, 95, 95, 98, 98, and 94 %, respectively. With pyroelectric effect and narrow band gap, the increased amount of BP will generate more pyroelectric electrons and photoelectrons to transfer to plasmonic WO which can enhance hot electron generation for CO 2 -RR. However, as the amount of BP beyond 10 wt%, the plasmonic WO nanowires are covered by BP layers which hinder the SPR excitation for photocatalytic CO 2 -RR. CO generated from CO 2 is veri ed by using 13 C-labled 13 CO 2 for photocatalysis over 10-BP/WO, and 13 CO are detected by the mass spectrometry (MS) as main product (Fig. 4c).
Considering the broad light absorption of 10-BP/WO heterostructures in UV-Vis-NIR region, their photocatalytic performance under different light irradiation was investigated as shown in Fig. 4d. UV-and Vis-irradiation only generates 7.2 and 7.9 µmol g − 1 CO in 3 hours, respectively. While, NIR-irradiation promotes CO generation to be 12.5 µmol g − 1 in 3 hours. As Vis-NIR light is irradiated, CO generation is improved to 86.7 µmol g − 1 , 7-fold higher than those under NIR irradiation. While, 109.4 and 148.5 µmol g − 1 CO is generated under UV-Vis and full-spectrum (UV-Vis-NIR) light irradiation, respectively. CO generation rates (Fig. 4e) (Fig. 4g) has a similiar trend with SPR band con rming that photocatalytic CO generation is attributed to SPR excitation. More interesting is that AQE at 900-nm is higher than those in visible region, and it means NIR-irradiation can enhance CO generation. However, AQE (Fig. 6h) Fig. 5c, and 1.4, 7.9 and 13.5 µmol g − 1 CO were obtained in 3 hours, respectively. Only heating on 10-BP/WO do not improve CO 2 -RR. However, heating can greatly enhance Vis-driven CO 2 -RR on 10-BP/WO. The possible illustration is that the heating triggers the pyroelectric BP to convert thermal energy for carriers and transfer to WO promoting plasmonic CO 2 -RR. The in uence of heating on CO 2 -RR over WO and BP as catalysts were also tested in Fig. 5d, and the heating and Vis-irradiation do not play synergistic roles on CO 2 -RR. Consequently, SPR and pyrolectric effect in BP/WO heterostructures play an synergistic role on Vis snd NIR light harvesritng for continnous eletron injection and hot electron generation for high-selective CO generation.
The detailed reaction pathway during CO 2 -RR over BP/WO was monitored by in situ FTIR spectra, as shown in Fig. 6a. Before CO 2 absorption in the dark, no remarkable signals are observed on the spectra.  Fig. 6b. CO 2 is absorbed on the surface of WO 3 − x to be *CO 2 with ΔG = 0.35 eV. Then, one electron and proton are obtained by *CO 2 to be *COOH with ΔG = -0.53 eV. After reaction with the second electron and proton, *COOH becomes *CO with a ΔG = -0.45 eV. *CO reaction for next step has two different pathways, one is to desorb from surface to be free CO with a ΔG = 0.65 eV, and the other one is to reaction with one electron and proton to be *CHO with a ΔG = 0.91 eV which is generally considered as a key step for CH 4 generation. In situ FITR and DFT simulations demonstrate that CO generation is the main reaction pathway of CO 2 -RR over plasmonic WO as photocatalyst, and it is consistent with the experiment results.
Based on above results, the detailed multi-synergetic processes and the possible photocatalytic mechanism are described in schematic diagram (Fig. 6c). With abundant oxygen vacancies, a defect band (DB) formed on the bottom of the conduction band (CB) in WO nanowires. With abundent W 5+ , the DB is occupied by the electrons generating SPR on WO in Vis-NIR region, and the large energy difference between valence band (VB) and lowest unoccupied CB restricts electron excitation from VB to CB under Vis-NIR irradiation. While, the electrons on DB can be excited by SPR to be hot electron in WO. The fast decay process of hot carriers leads to short lifetime of hot electron (< 200 ps) which has been observed on transient absorption spectra (Fig. 4) Consequently, multi-physical effects including plasmonic hot carriers and photothermal effect of WO, pyrolectric effect and band excitation of BP, and electron transfer through W-O-P bonds play synergetic roles on enhancing SPR and hot electron generation, boosting high-selective CO 2 -RR for CO generation.

Discussion
In summary, a pyroelectric/plasmonic BP/WO heterostructure was constructed as photocatalysts with multi-physical effects for CO 2 reduction. 26.1 µmol h − 1 g − 1 CO was generated over the nonmetallic plasmonic heterostructures under Vis-NIR light irradiation with a selectivity of 98%, and which was 7-and 17-fold higher than those of plasmonic WO and BP, respectively. W-O-P bonds connected WO and BP were demonstrated by DFT calculations and vari ed by various spectrascopic studies as channels for electron transfer from BP to WO. The photothermal energy generated by SPR of WO can make temperature of 10-BP/WO rapidly increase from 24 to 86 o C under Vis-NIR light irradiation, triggering the pyroelectric effect of BP for carriers to promote electron transfer. Therefore, multi-synergistic effects of plasmonic hot carriers and photothermal effect on WO, band excitation and pyroelectric effects on BP, and electron transfer through W-O-P bonds promote hot electron generation for high-selective CO 2 -RR in Vis-NIR region.
Our work provids clear proofs to demonstrate that contructing multi-physical effects on semiconductors are one promising strategy to improve the Vis-NIR light haversting for arti cal photosynthesis.
All the chemical reagents were used as purchased without any further puri cation.
Sample preparation. Layered BP were obtained by using a basic NMP solvent exfoliation method. In detail, 20 mg of bulk BP was rst dispersed in 20 mL NMP, and then the dispersion was sonicated for 4 h at 100 W output power under ice-water bath. After exfoliation, the dispersion was centrifuged at 2000 rpm for 20 min with two times to remove non-exfoliated bulk BP. The supernatant was further centrifuged at 11,000 rpm for 20 min to separate BP from the supernatant. After that, the precipitations were redispersed in 10 mL NMP with ultrasonic process, resulting in BP NMP dispersion (0.5 mg mL − 1 ). WO 3 − x (WO) nanowires were obtained by using solvothermal method. In a typical procedure, 150 mg of WCl 6 powder was dissolved into 30 mL ethanol, which was vigorously stirred to obtain a yellow suspension. Subsequently, the solution was transferred to a 50 mL Te on-lined stainless steel autoclave, heated and maintained at 180 ℃ for 24 h. After naturally cooling down to room temperature, the dark blue powder was separated by centrifugation, washed with ethanol for three times and dried in a vacuum oven for overnight.
BP/WO heterostructures were obtained by a simple ultrasonic process. In brief, BP/NMP solution was centrifuged and washed with ethanol for twice to remove the NMP. Subsequently, the 10 mg WO was dispersed 10 mL ethanol with sonicated for 15 min, then the 2 mL as-prepared BP/ethanol dispersion (0.5 mg mL − 1 ) was added into above WO dispersion with sonication for 5 min. Then, the BP/WO were precipitated and obtained by centrifugation. The obtained BP/WO sample was labelled as 10-WO/BP and the different weight ratio BP for WO were prepared by adding different amounts of BP to WO.
Samples characterization. The X-ray diffraction (XRD) patterns of the sample were carried out by a Rigaku Rint-2500 diffractometer with Cu K α radiation at a scanning rate of 0.1° s − 1 . The morphologies were measured by transmission electron microscope (2100, JEOL, operated at 100 kV). X-ray photoelectron spectroscopy (XPS) measurements were performed at a Thermo Fisher Scienti c K-ALPHA + spectrometer. The binding energy was referenced to the C 1s peak at 284.6 eV of the adventitious carbon. Electron paramagnetic resonance (EPR) signal were detected by Bruker A300 spectrometer. UV-Vis-NIR diffuse re ectance spectra (UV-Vis-NIR DRS) were recorded in a UV-Vis/NIR spectrophotometer (JASO V-570). Raman spectra were obtained on a Raman microscopy (XPLORA PLUS, HORIBA) with a 532-nm laser for excitation. Fourier transform infrared (FTIR) spectra were collected by a spectrometer (THERMO) employing KBr disk method. The in situ FTIR spectra were obtained through an in situ diffuse re ectance Fourier transform infrared spectrometer (Nicolet iS50, TMO, US). The sample was degassed for 4 h at 150℃ prior to measurement. Then each sample was purged with nitrogen for one hour to blow out all the gases adsorbed on the samples. After that, a mixed gas of CO 2 and water vapor was owed into the specimen chamber for another hour to ensure sorption equilibrium before irradiation.
Photocatalytic CO 2 reduction test. 5 mg sample was mixed with 0.2 mL pure water and plastered on cover glass (4.9 cm − 2 ). The cover glass with the sample on the upside was put on the bottom of the reaction chamber (100 mL). The chamber was sealed with thick quartz cover glass and degassed with pure carbon dioxide gas for 20 min. Subsequently, Vis-NIR light supplied by a 300 W xenon lamp (Perfectlight, PLS-SXE300D) equipped with a wavelength cutoff lter (> 400 nm) irradiated the suspension. The gaseous products were analysed by gas chromatography (GC-2014A, Shimadzu) equipped with one TCD and two ame ionization detectors (FID). Other photocatalytic reaction measurements were carried following the above procedure under different light irradiation which is supplied by a 300 W xenon lamp equipped with different-wavelength cutoff lters (< 400nm, 400-780 nm, > 800 nm).  where Eads is the adsorption energy, and ΔEZPE is the difference corresponding to the zero-point energy between the adsorbed state and the gas phase, S is the entropy, and T is the temperature.

Declarations
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Supplementary Files
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