Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal

Journal Pre-proofs Full Length Article Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal Meijuan Chen, Xinwei Li, Yu Huang, Jie Yao, Yan Li, Shun-cheng Lee, Wingkei Ho, Tingting Huang, Kehao Chen PII: DOI: Reference:

S0169-4332(20)30531-6 https://doi.org/10.1016/j.apsusc.2020.145775 APSUSC 145775

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Applied Surface Science

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31 October 2019 8 January 2020 13 February 2020

Please cite this article as: M. Chen, X. Li, Y. Huang, J. Yao, Y. Li, S-c. Lee, W. Ho, T. Huang, K. Chen, Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145775

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Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal

Meijuan Chen1, 2, 3*, Xinwei Li2, 4, Yu Huang

2*,

Jie Yao2, Yan Li2, Shun-cheng Lee4, Wingkei Ho5,

Tingting Huang2, Kehao Chen3

1School

of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R.

China 2State

Key Laboratory of Loess and Quaternary Geology (SKLLQG), Key Laboratory of Aerosol

Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, P. R. China 3Key

Laboratory of Degraded and Unused Land Consolidation Engineering, the Ministry of Natural

Resources of China, Xi'an 710075, P. R. China 4Department

of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong

Kong, P. R. China 5Department

of Science and Environmental Studies, The Education University of Hong Kong, Hong

Kong, P. R. China

*Corresponding Author: Dr. Meijuan Chen, E-mail address: [email protected], Tel: 86-29-8339 5107 Prof. Yu Huang, E-mail address: [email protected], Tel: 86-29-6233 6261

Abstract1 Bi metal–BiPO4 (Bi-BPO) nanocomposites formed by in situ solvothermal reduction were employed as plasmonic photocatalysts for oxidative NO removal, achieving a removal efficiency of 32.8% in a continuous NO flow (400 ppb) under illumination with visible light. This high performance was ascribed to the generation of energetic hot electrons (and their subsequent surface chemical reactions) due to the surface plasmon resonance (SPR) of Bi metal, as validated by numerical simulations. The combined results of density functional theory (DFT) calculations and electrochemical analysis revealed that hot electrons are transferred from Bi metal to BPO via the Bi-BPO interface. DFT calculations further showed that enhanced O2 activation on the Bi-BPO interface facilitates the generation of both superoxide (•O2−) and hydroxyl (•OH) radicals, as confirmed by electron spin resonance, while in situ DRIFTS analysis demonstrated that NO is activated on the Bi-BPO interface and then oxidized to nitrates. Thus, this work highlights the SPR effects of Bi metal and promoted O2 and NO activation in plasmonic photocatalysis, showing that the adopted approach can be generalized to design efficient and cost-effective photocatalytic systems for the removal of other gaseous pollutants.

1BiPO

4

(BPO), Surface Plasmon Resonance (SPR), Density Functional Theory (DFT), Reactive Oxygen

Species (ROS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), Fourier Transform Infrared (FTIR), Thermogravimetric Differential Scanning Calorimeter (TG-DSC), Transmission Electron Microscopy (TEM), High-resolution TEM (HRTEM), Electron Spin Resonance (ESR), Brunauer–Emmett–Teller (BET), In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS).

Keywords: plasmonic photocatalysis, NO removal, Bi metal, O2 and NO activation

1. Introduction Photocatalytic oxidation, as an effective and green technology, has been widely employed for atmospheric removal [1-5] and is thought to rely on the surface adsorption of pollutants and the generation of reactive oxygen species (ROS) such as superoxide (•O2−) and hydroxyl (•OH) radicals [6-10]. Normally, pollutant adsorption is strongly affected by the surface electron properties of the catalyst, while ROS generation is often closely related to catalyst light adsorption properties, carrier separation efficiency, and surface O2 activation [11-14]. Hence, photocatalyst performance can be productively enhanced by rationally designing and exploiting nanomaterials to possess optimized electronic structures. Surface plasmon resonance (SPR)-induced photocatalysis takes advantage of the unique nature of plasmonic metals to significantly enhance photocatalytic reaction efficiency under the conditions of low-intensity illumination [15-20]. Typically, a plasmonic metal initially oscillates in resonance with incident photons at an applicable wavelength range to give rise to a local electromagnetic field, with subsequent non-radiative decay inducing the generation of abundant hot electrons [21-23] that activate surface O2 to generate ROS for pollutant oxidation. Although plasmonic photocatalysts are usually limited to noble metals such as Au and Pt [24-26], Dong et al. showed that earth-abundant Bi metal also exhibits SPR effect and demonstrated the direct photocatalytic activity for ppb-level NO removal [27]. Since then, Bi metal–based plasmonic photocatalysts have been widely applied for disinfection and pollutant removal [28-32]. Nevertheless, hot carriers produced by plasmonic metals are easily devitalized by radiative scattering and thus usually fail to initiate surface reactions [33].

The use of supports capable of accepting hot electrons to hinder their inactivation can be an effective method of enhancing the catalytic activity of Bi metal. Ideally, the support should not only facilitate hot electron transfer but also be chemically stable so as not to sacrifice performance. In situ partial reduction of Bi metal in Bi-based nanomaterials allows Bi metal surface deposition and affords nanocomposites with an interface for internal charge transfer [34-37]. Bismuth phosphate (BiPO4, BPO) has been widely employed in Bi-based nanomaterials intended for photocatalytic applications because of its high stability, resistance to deactivation, low cost, and superior activity [38]. However, in contrast to visible light–responsive semiconductors, bulk BPO has a band gap of 3.5–4.1 eV and therefore cannot directly harvest visible light to initiate photocatalytic reactions [39]. This drawback highlights the potential of in situ Bi-BPO nanocomposite synthesis to profit from the SPR effect of Bi metal and generate an interface capable of efficient charge transfer for O2 and NO activation. Herein, an in situ generated Bi metal-BPO (Bi-BPO) nanocomposite was used for the removal of 400 ppb gaseous NO under illumination with visible light, and the photocatalytic performance of this material and the NO removal mechanism were probed by numerical simulations, electrochemical analysis, and in situ DRIFTS. The results showed that a local electric field is generated around Bi metal upon illumination at 420 nm, further revealing that the constructed interface efficiently promotes hot electron transfer from Bi metal to BPO and the activation of O2 to generate superoxide (•O2−) and hydroxyl (•OH) radicals. A mechanism of plasmonic photocatalytic NO oxidation on Bi-BPO was proposed for the first time, highlighting the Bi metal SPR effect in photocatalysis and the pivotal role of O2 and NO activation.

2. Experimental 2.1. Catalyst synthesis All solvents and reagents were of analytical grade and were used without further purification. Typically, Bi(NO3)3·5H2O, NaH2PO4·2H2O, and glucose (1 mmol each) were dispersed in ethylene glycol (15 mL) under 3-h vigorous stirring to form a suspension, which was then transferred into a 20-mL Teflon-lined stainless steel autoclave and heated at 160 °C for different times (Fig. S1). The obtained samples were collected by centrifugation, washed three times each with absolute ethanol and deionized water, and oven-dried at 70 °C. Products prepared at heating times of 72, 96, and 120 h were denoted as Bi-BPO-1, Bi-BPO-2, and Bi-BPO-3, respectively. Bulk BPO was synthesized in the absence of glucose and heating. Pure Bi metal was prepared using NaBH4 as reducing agents. Typically, 1 mmol of Bi (NO3)3·5H2O was dispersed in ethylene glycol (15 mL) firstly. At the same time, 5 mL NaBH4 solution (50 mmol/L) was added dropwise into the above suspension, followed by stirring and then ageing for 1 h, respectively. The resulting precipitates were filtered and rinsed with ethanol, deionized water for four times, and dried at 40 °C in air. The as prepared black power is pure Bi metal. The as prepared black power is pure Bi metal.

2.2. Characterization Sample crystal structure was analyzed by powder X-ray diffraction (pXRD; PANalytical X’pert Pro, PANalytical Corp., the Netherlands) at a scan rate of 0.017° min−1 for 2θ = 20–80°. X-ray photoelectron spectroscopy (XPS; Escalab250Xi, ThermoFisher Scientific) analysis was performed using monochromatic Al Kα radiation (hν = 1486.71 eV). All binding energies were calibrated by C 1s peak at 284.8 eV as a reference. Surface etching process taken out by Ar ion gun under high

vacuum circumstance. Scanning electron microscopy (SEM; MAIA3, TESCAN, Czech) was used to characterize the morphology, structure and element mapping of the obtained products. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging were carried out on an FEI Tecnai G2 F20 microscope (FEI Corp., Japan) at an acceleration voltage of 200 kV. The thermogravimetric-differential scanning calorimetry analysis (TG-DSC: NETZSCHSTA 409 PC/PG), 20 mg dry sample was sealed in an Al2O3 crucible with a lid and scanned at a rate of 20 oC/min. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet iS50FT-IR spectrometer (Thermo, USA)

on samples embedded in KBr pellets. A Varian Cary 100 Scan UV-visible

spectrometer equipped with a labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of catalysts in the range of 200–800 nm. Electron spin resonance (ESR) spectra (ER200-SRC, Bruker, Germany) were recorded for solution-phase mixtures of as-prepared photocatalysts with different trapping agents (aqueous dispersion for DMPO-•OH, methanol dispersion for DMPO-•O2−) upon illumination at 420 nm. The photoelectrochemical properties of bare BPO and Bi-BPO-2 were evaluated using a Parstat 4000 electrochemical workstation (USA) in a conventional three-electrode cell, with a Pt plate and a Ag/AgCl electrode employed as counter and reference electrodes, respectively. Typically, a 50-mg sample was dispersed in ethanolic Nafion (5 mL, 1 wt%) through sonication to obtain a homogeneous suspension. A fluorine-doped tin oxide conducting glass was dip-coated with the above suspension and dried at room temperature to afford the working electrode. Current-time curves were measured at 0 V vs. Ag/AgCl in 0.1 M Na2SO3 solution upon illumination with a 300-W Xe lamp (MICROSOLAR 300, Perfect Light, China). Electrochemical impedance spectra were recorded in the frequency range of 0.1 Hz to 100 kHz at open-circuit voltage (voltage amplitude = 5 mV) in 1

mM K3Fe(CN)6/K4Fe(CN)6 solution. In situ DRIFTS measurements were conducted using a Nicolet iS50FT-IR spectrometer (Thermo, USA) equipped with an in situ diffuse reflectance cell (Harrick) and a high-temperature reaction chamber featuring three gas ports and two coolant ports. High-purity He, high-purity O2, and 100 ppm NO in He were used as feeds, and a three-way ball valve was used to switch between target (NO) and purge (He) gases. The total gas flow rate equaled 30 mL min−1, and NO concentration was set to 50 ppm by dilution of O2. The chamber was enclosed by a dome with three windows, two for IR light entrance and detection (made of ZnSe), and one for photocatalyst illumination (made of UV quartz). A 300-W Xe lamp (420 nm cut-off) was used as a visible light source.

2.3. Density functional theory (DFT) calculations Herein, p(3 × 3) supercell slabs with a 25-Å vacuum space were utilized for BPO (120) crystal facets. A Bi8 cluster that is part of the unit cell was added above to represent partly reduced Bi metal. All spin-polarized DFT calculations were performed using the Cambridge Sequential Total Energy Package in Material Studio 8.0[40, 41], with electron-ion interactions expressed by the projector-augmented wave method[42]. The Perdew-Burke-Ernzerhof generalized gradient approximation was used to describe exchange-correlation energies and potential[43]. For geometry optimization and electron density difference calculations, a large convergence of the plane-wave expansion was obtained with an energy cut-off of 400 eV, and the Brillouin zone was sampled in a 2 × 2 × 1 Monkhorst-Pack set[44]. The geometries were optimized when the energy, force, and maximal displacement converged to 2.0 × 10−5 eV atom−1, 0.05 eV Å−1, and 2 × 10−3 Å, respectively. NO and O2 adsorption on BPO was also modeled, with adsorption energy (Eads) defined as (1)

Eads = (E(slab) + E(adsorbate)) − E(adsorbate/slab) (1) where E(adsorbate/slab), E(slab), and E(adsorbate) are the total energies of the slab with the adsorbate in the equilibrium state, the slab surface, and the free adsorbate, respectively. According to this definition, more positive values reflect a stronger interaction between the adsorbed species and the BPO (120) surface.

2.4. Photocatalytic activity test Photocatalytic activities were evaluated by NO removal under illumination with visible light and simulated solar light provided by a 300-W commercial Xe lamp. The photocatalyst (0.1 g) was homogeneously dispersed in a glass Petri dish (diameter = 90 mm), which was then placed in a 0.40-L dark stainless steel chamber covered with quartz glass. The concentration of NO in the feed gas was ~400 ppb, and the feed flow rate was controlled at 1.2 L min−1. After the establishment of an adsorption-desorption equilibrium between gases and photocatalysts, the Xe lamp was turned on, and NO concentration was continuously monitored by a chemiluminescence NOx analyzer (Thermo Environmental Instruments, Inc., Model 42c). NO removal efficiency (R) was calculated as Where C0 is the initial concentration of NO before illumination, and Ct is the NO co (2) R = 100% × (C0 − Ct)/C0 (2) ncentration measured on-line during illumination. 3. Results and discussion 3.1. Catalyst structure and morphology The XRD pattern of BPO (Fig. 1a) matched that of monoclinic BiPO4 (JCPDS No. 80-209) [45]

and contained no other peaks, which was indicative of high product purity. The characteristic peaks at 38.1° and 39.7° observed for Bi-BPO-2 and Bi-BPO-3 were attributed to Bi metal (JCPDS No. 05-519). The XRD spectrum of pure Bi metal was showed in Fig. S2. Elemental composition was probed by XPS (Fig. S3a), which revealed the presence of Bi, O, and P elements. P 2p spectra featured a peak at 132.7 eV (Fig. S3b) corresponding to P(V) [46], while the O 1s spectra (Fig. S3c) could be deconvoluted into peaks at 532.5 and 530.8 eV, ascribed to surface–adsorbed O species and lattice O, respectively [47, 48]. Peaks at 164.9 and 159.7 eV in the Bi 4f spectrum of BPO (Fig. 2b) were attributed to Bi 4f5/2 and Bi 4f7/2 transitions of Bi3+ in BiPO4 [49, 50]. In the case of Bi-BPO-2, the above peaks shifted to 165.3 and 159.9 eV, respectively, which was ascribed to the partial reduction of Bi3+ to Bi0 and confirmed by the observation of Bi0 peaks at 162.4 and 157.1 eV after etching [37]. This behavior was ascribed to the tendency of Bi metal to be easily oxidized by atmospheric O2 to form a thin surface layer of amorphous bismuth oxide [51]. After 180-s surface etching, the atomic% of Bi slightly increased (Fig. 1c), which indicated that more Bi metal was reduced on the BPO surface. In SEM images, BPO comprised stacks of well-dispersed nanoparticles (Fig. S4a, b). Slight differently, Bi-BPO-1 sample is comprised of nanoparticles partly aggregated together (Fig. S4c, d). Distinctively, Bi-BPO-2 and Bi-BPO-3 sample presented solid nanospheres surrounded by nanoparticle microstructures (Fig. S5). The elemental mapping images of Bi-BPO-2 sample indicates that the solid nanosphere is mainly composed of Bi element (Fig. S6). Moreover, TEM imaging (Figs. 1d, S7a, and b) showed that the size of nanoparticles in BPO is about 20–30-nm. Fig. 1e, S7c, d and S8a, b show that Bi-BPO-2 contained solid nanospheres with sizes of 200–250 nm. Further HRTEM examination (Fig. 1f, S8c, d) revealed that the nanospheres and nanoparticles featured ordered lattice

fringes with spacings of 3.28 and 3.02 Å, respectively, which well matched those of the (012) planes of Bi metal and the (120) planes of BPO, respectively [52]. A clear interface was observed between Bi (012) and BPO (120). The physisorption isotherms of BPO and Bi-BPO were classified as type-V according to IUPAC (Fig. S9a) [53]. The Brunauer–Emmett–Teller (BET) surface areas of BPO, Bi-BPO-1,

Bi-BPO-2,

and

Bi-BPO-3

were

calculated

from

the

corresponding

N2

adsorption–desorption isotherms as 19.43, 16.87, 16.52, and 16.15 m2 g−1, respectively. The corresponding Barrett–Joyner–Halenda pore size distributions are given in Fig. S9b, and sample BET surface areas, total pore volumes, and pore diameters are listed in Table S1. XRD, XPS, TEM, and BET results combined to indicate that progressive in situ Bi metal formation with increasing time. Further, FTIR and TG-DSC results in Fig. S10 proved that both glucose and ethylene glycol in this system will be decomposed almost completely during solvothermal process and leave few carbonaceous

materials

on

the

catalyst.

Fig. 1. (a) XRD patterns of BPO and Bi-BPO samples. (b) Bi 4f spectra of BPO, Bi-BPO-2, and Bi-BPO-2 etched to a depth of 20 nm. (c) Elemental composition of Bi-BPO-2 as a function of

etching time. TEM images of (d) BPO and (e) Bi-BPO-2. (f) HRTEM image of Bi-BPO-2.

3.2. Electronic, magnetic, and optical properties of Bi metal on BPO DFT calculations (Fig. 2a) were performed assuming the phase structure of Bi-BPO-2 for 3 × 3 supercell slabs of the BPO crystal featuring exposed (120) facets with an eight-atom Bi cluster. The obtained charge difference distribution (Fig. 2b) revealed that 0.32 e electrons would migrate from the top Bi metal to BPO, suggesting that the presence of Bi metal on BPO would enhance internal electron transfer at the interface. The SPR effect of Bi metal was simulated using a rigorous Maxwell solver, with the numerical simulation conducted for two Bi spheres with reference to the morphology in Fig. S7c (detailed methods are described in the Supporting Information). An obvious local electric field was observed under illumination with 420-nm light to verify the SPR effect of Bi metal, with subsequent non-radiative decay affording energetic hot electrons for chemical reactions [21-23]. More importantly, the intensity of the local electromagnetic field was enhanced in the region between two Bi metal spheres, which implied that the adjacent local plasmon oscillations could be accumulated to furnish higher-energy hot carriers for photocatalysis. The SPR properties of Bi metal were characterized by UV-vis absorption spectroscopy (Fig. 2d), which showed that Bi metal deposition on BPO enhanced absorption in the visible-light region. Carrier separation efficiency was probed by photocurrent density measurements (Fig. 2e) and electrochemical impedance spectroscopy (Fig. S11) of BPO and Bi-BPO-2 under illumination with visible light. The increased photocurrent and decreased impedance radius of Bi-BPO-2 implied an enhanced transfer of photogenerated carriers across the Bi-BPO interface [54].

Fig. 2. (a) Electronic morphology of the Bi-BiPO4 surface and (b) the corresponding charge difference distribution. Electron accumulation and depletion regions are shown in green and yellow, respectively. (c) Simulation of SPR-induced local electromagnetic field in Bi metal (Bi metal modeling was performed with reference to Fig. S7c) under illumination with 420-nm light. (d) UV-vis diffuse reflectance spectra of BPO and Bi-BPO. (e) Photocurrents of BPO and Bi-BPO-2 in response to light (420 nm) on/off switching every 20 s for 180 s.

3.3. Adsorbed O2 activation and ROS generation O2 activation by hot electrons is an essential step of plasmonic photocatalysis, affording active free radicals that directly react with contaminants. Therefore, DFT calculations were used to model O2 activation on three different adsorption sites (Fig. 3a), namely on the BPO top, Bi top, and the Bi-BPO interface (relative top, side, front view were given in Fig. S12). The O–O bond length increased to 1.44 Å for O2 adsorbed on the Bi-BPO interface, which exceeded values observed for the Bi top (1.28 Å) and the BPO top (1.24 Å). Moreover, the energy of O2 molecule adsorption energy (Eads) in the three positions was in the order of Bi-BPO interface (-2.40 eV) < Bi top (-0.39 eV) < BPO top (-0.08 eV). Thus, the Bi-BPO interface was concluded to be the most beneficial position for O2 activation, featuring the most negative Eads and the longest O–O bond.

Consequently, the ESR signals of DMPO-•O2− (Fig. 3b) observed for Bi-BPO-2 were more intense than those observed for pristine BPO under illumination with visible light, as energetic hot electrons were largely injected into O2 to produce significant amounts of •O2−. The thus produced superoxide radicals could be further converted to hydroxyl radicals (•O2− → H2O2 → •OH), which were also detected by ESR (Fig. 3c) [55]. The band gap of bare BPO was estimated from the corresponding (αhν)1/2 vs. photon energy (hν) plot as 3.75 eV (Fig. S13) and implied that BPO is unable to respond to visible light to generate reactive radicals. However, Bi metal SPR in Bi-BPO-2 allowed for hot electron generation and significantly promoted surface O2 activation to facilitate the generation of ROS for NO oxidation.

Fig. 3. (a) O2 activation pattern on Bi-BPO, O2 on the BPO top, Bi top, and the Bi-BPO interface, respectively (front view). Eads is the adsorption energy of O2 molecules, with negative values denoting exothermicity. DMPO (5, 5’-dimethyl-1-pirroline-N-oxide) spin-trapping ESR spectra of BPO and Bi-BPO-2 recorded (b) in methanolic dispersion for DMPO-•O2− and (c) in aqueous dispersion for DMPO-•OH.

3.4. NO photocatalytic oxidation and corresponding mechanism Sample photocatalytic activity was evaluated in terms of NO removal efficiency under illumination with visible light (Fig. 4a), as NO cannot be photodegraded in the absence of a catalyst [56]. Bare BPO showed negligible activity due to its wide band gap (>3.0 eV). In contrast, Bi-BiPO nanocomposites were photocatalytically active, with the NO removal efficiency decreasing in the order of Bi-BPO-2 (32.8%) > Bi-BPO-3 (29.7%) > Bi-BPO-1 (26.2%). However, pure Bi metal shows negligible photocatalytic activity (Fig. S14). Thus, although Bi metal presence increased photocatalytic performance, excess Bi had the opposite effect. Five-fold cycling of Bi-BPO-2 (Fig. 4b) revealed that this nanocomposite was photochemically stable and underwent little deactivation. To gain insights into NO activation on Bi-BPO-2, NO adsorption on the BPO top, Bi top, and the Bi-BPO interface were modeled by DFT calculations (Fig. S15). Similar to the case of O2 adsorption, the Bi-BPO interface was the most favorable position for NO activation, as indicated by the most negative adsorption energy Bi-BPO interface (-0.94 eV) < Bi top (-0.74 eV) < BPO top (0.07 eV) and the longest N–O bond Bi-BPO interface (1.23 Å) < Bi top (1.20 Å) < BPO top (1.20 Å). Moreover, Fig. 4c shows the local density of states (LDOS) for the N atom, revealing that NO on the Bi-BPO interface contributed to the Fermi level stronger than NO on the Bi top or the BPO top. This finding confirms that NO molecules are more likely to be activated on the Bi-BPO interface and then take part in the following reactions [57].

Fig. 4. (a) NO removal efficiencies of BPO and Bi-BPO under illumination with visible light, (b) cycling test results for Bi-BPO-2, (c) LDOS of the N atom for NO adsorption on the BPO top, Bi top, and the Bi-BPO interface. (d) In situ DRIFTS during photocatalytic NO removal over Bi-BPO-2. In situ DRIFTS (Fig. 4d) was used for the direct monitoring of NO and O2 adsorption and reactions on the catalyst surface to elucidate the mechanism of photocatalytic NO oxidation on Bi-BPO-2. Several adsorbed NOx species were detected, with their adsorption bands listed in Table 1. The corresponding background spectrum was recorded before NO injection into the reaction chamber. The band at 879 cm−1 was attributed to pyramidal Bi–OO– units in Bi-BPO-2 [58, 59], while the band at 1748 cm−1 was ascribed to adsorbed NO [60].The negative broad band at 3400–3200 cm−1 was ascribed to surface OH− ions [61]. Bands at 1193−1142 cm−1 attributable to NO−/NOH [62] implied that NO reacted with surface OH− to form NOH and NO2−: 2NO + OH− → NOH + NO2− [63]. The obvious depletion of surface H2O after the light switched on was mainly due

to the generation of •OH. Other notable absorption bands were assigned to the stretching vibrations of bidentate NO3− (1577, 1263, and 1047 cm−1) [62, 64] and nitro species (1330 cm−1) [58, 64, 65]. Those intensities detected after illumination are largely promoted due to the continuous photocatalytic conversion process on Bi-BPO-2 sample with enhanced generated ROS. Table 1. Assignments of IR bands observed during photocatalytic NO oxidation. Peak position (cm−1)

Assignment

Ref.

3400–3200 (broad)

surface H2O

[61]

1748

NO

[60]

1577,1263, 1047

Bidentate NO3−

[62, 64]

1330

Nitro species

[58, 64, 65]

1167

NO−/NOH

[62]

879

Surface peroxo, v(Bi–OO– )

[58, 59]

Upon illumination of Bi-BPO-2 with visible light (Fig. 5), the SPR effect of Bi metal resulted in the generation of hot carriers that were transferred to BPO via the Bi-BPO interface. At the same time, O2 was activated at the above interface to afford •O2− and •OH that reacted with NO (pre-activated at the interface) to ultimately convert to final oxidative products nitrates.

Fig. 5. Proposed mechanism of photocatalytic NO oxidation over Bi-BPO-2.

4. Conclusion Bi-BPO nanocomposites synthesized by in situ solvothermal reduction were shown to be effective and durable photocatalysts for ppb-level NO removal from a continuous air flow under ambient conditions. Numerical simulations suggested that the SPR effect of in situ generated Bi metal promoted the generation of hot electrons that participated in subsequent surface reactions. DFT calculations and electrochemical analysis revealed that such hot electrons were transferred from Bi metal to BPO via the corresponding interface, which was the most favorable position for O2 activation to generate •O2− and •OH. NO was also activated on the Bi-BPO interface and took part in subsequent reactions, as traced by in situ DRIFTS analysis. Thus, this work highlights the SPR effects of Bi metal on O2 and NO activation in plasmonic photocatalysis, demonstrating that the adopted strategy can be generalized to the design of efficient and cost-effective photocatalytic systems for the removal of other gaseous pollutants.

Acknowledgements This research was financially supported by the National Science Foundation of China (Nos. 41877481), the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS (No. SKLLQG1729), the Fundamental Research Funds for the Central Universities (No. xjj2018249), the Opening Fund of Key Laboratory of Degraded and Unused Land Consolidation Engineering, the Ministry of Natural Resources (No. SZDJ2019-15), and the China Postdoctoral Science Foundation (grant number 2018M643669). It was also partially supported by the National Key Research and Development Program of China (2016YFA0203000).

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Graphical abstract Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal Meijuan Chen1, 2, 3*, Xinwei Li2, 4, Yu Huang Tingting Huang2, Kehao Chen3

2*,

Jie Yao2, Yan Li2, Shun-cheng Lee4, Wingkei Ho5,

Highlights • Bi-BiPO4 nanocomposites were prepared as plasmonic photocatalysts for NO removal • High NO removal efficiency (32.8%) was achieved in a continuous NO flow (400 ppb) • Surface plasmon resonance of Bi metal promoted hot electron generation • Hot electrons were transferred from Bi metal to BiPO4 via the Bi-BiPO4 interface • Our approach can be extended to photocatalysts for the removal of other pollutants

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Credit author statement Meijuan Chen: Conceptualization, Investigation, Writing- Original draft preparation, Methodology. Xinwei Li: Data curation, Writing- Original draft preparation. Yu Huang: Project administration, Supervision, validation, Writing - Review & Editing. Jie Yao: Investigation. Yan Li: Data curation. Shun-cheng Lee: Project administration. Wingkei Ho: Writing - Review & Editing. Tingting Huang: Software. Kehao Chen: Writing - Review & Editing.