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. 2023 Jan 16;3(2):468–475. doi: 10.1021/jacsau.2c00596

VSe2–xOx@Pd Sensor for Operando Self-Monitoring of Palladium-Catalyzed Reactions

Mingze Li 1, Yunjia Wei 1, Xingce Fan 1, Guoqun Li 1, Xiao Tang 1, Weiqiao Xia 1, Qi Hao 1,*, Teng Qiu 1,*
PMCID: PMC9975834  PMID: 36873688

Abstract

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Operando monitoring of catalytic reaction kinetics plays a key role in investigating the reaction pathways and revealing the reaction mechanisms. Surface-enhanced Raman scattering (SERS) has been demonstrated as an innovative tool in tracking molecular dynamics in heterogeneous reactions. However, the SERS performance of most catalytic metals is inadequate. In this work, we propose hybridized VSe2–xOx@Pd sensors to track the molecular dynamics in Pd-catalyzed reactions. Benefiting from metal–support interactions (MSI), the VSe2–xOx@Pd realizes strong charge transfer and enriched density of states near the Fermi level, thereby strongly intensifying the photoinduced charge transfer (PICT) to the adsorbed molecules and consequently enhancing the SERS signals. The excellent SERS performance of the VSe2–xOx@Pd offers the possibility for self-monitoring the Pd-catalyzed reaction. Taking the Suzuki–Miyaura coupling reaction as an example, operando investigations of Pd-catalyzed reactions were demonstrated on the VSe2–xOx@Pd, and the contributions from PICT resonance were illustrated by wavelength-dependent studies. Our work demonstrates the feasibility of improved SERS performance of catalytic metals by modulating the MSI and offers a valid means to investigate the mechanisms of Pd-catalyzed reactions based on VSe2–xOx@Pd sensors.

Keywords: two-dimensional materials, vanadium selenide, metal−support interaction, palladium catalysts, Suzuki−Miyaura coupling reaction, photoinduced charge transfer, surface-enhanced Raman spectroscopy

Introduction

Palladium (Pd) has become a cornerstone of synthetic chemistry by virtue of its excellent stability and superior surface catalytic activity.1 The Pd-catalyzed coupling reactions have been widely applied in the fabrication of organic molecular frameworks, including the synthesis of aniline, substituted alkene, and functional group-containing olefins as well as styrene and biphenyl.2−4 Nevertheless, the dynamics of theses complicated reactions are not always clear because of a lack of experimental techniques in clarifying the molecular evolutions at the palladium interface.

Surface-enhanced Raman scattering (SERS), which features single-molecule sensitivity and chemically specific identification, has been demonstrated as an innovative tool in tracking the molecular dynamics in heterogeneous reactions.5 It generally employs metallic plasmonic structures to generate localized surface plasmon resonance (LSPR) to boost the Raman scattering of the target molecule with an enhancement factor up to 108–1010.6,7 This strategy, which is known as the electromagnetic (EM) enhancement, has been successfully applied in investigating palladium-catalyzed reactions by constructing plasmonic core–shell or core–satellite composites.8,9 However, the LSPR-based SERS strategy is accompanied by the plasmon-induced hot carriers and thermal effects.10 To be specific, the plasmon would decay into energetic hot carriers within the metal via Landau damping and then transfer the energy to the surrounding environment and cause significant thermal effects near the metal surface.11,12 These additional factors may cast a shadow on the validity of the findings of the Pd-catalyzed coupling reactions.

Considering the difficulties in addressing the above intrinsic limitations of EM, it is of great importance to circumvent these obstacles to realize self-monitoring of Pd-catalyzed reactions.13 Photoinduced charge transfer (PICT) processes between the Pd catalyst and molecules could magnify the Raman scattering cross section of the molecule–Pd catalyst system (also called the chemical mechanism, CM), which is the key for plasmon-free SERS enhancement.14−17 However, the enhancement factor of the PICT-based strategy is originally weak. Although heterojunction engineering,18−22 defect engineering,23−26 and many other surface techniques have been proposed to improve the sensitivity of CM-based strategy,27−29 it is hard to improve the PICT between the Pd catalyst and molecules. As reported in the literature, metal–support interactions (MSI), including the orbital rehybridizations and charge transfer across the metal–support interface, would profoundly affect the properties of the catalysts. Modulating MSI by selecting suitable support has been considered as an effective approach to modulate the electronic structure of supported metal catalysts.30,31 We suggest that rational structural design of catalysts based on MSI may improve the PICT between Pd catalyst and molecules, enabling the Pd-support system to track the SERS dynamics in Pd-catalyzed reactions.

In this work, we propose two-dimensional Pd hybridized defective VSe2 (VSe2–xOx@Pd) sensors to realize self-monitoring of Pd-catalyzed reactions based on operando SERS strategy. The significantly improved SERS performance of VSe2–xOx@Pd is attributed to the electronic structure modulation of atomic-level Pd clusters due to MSI. For one thing, the surface defects of selenium provide favorable sites for the doping of oxygen and assembly of Pd clusters, authorizing energetic charge exchanges between the Pd and VSe2–xOx nanosheets. For another, the combination of the Pd clusters and metallic phase VSe2–xOx nanosheets further enriches the density of states (DOS) at the Fermi level of the hybrid system, and improves the probability of charge transfer between the hybrid system and adsorbed molecules.32,33 These advantages endow VSe2–xOx@Pd sensors the ability to visualize the Pd-catalyzed coupling reactions in a valid manner.

Results and Discussion

Efficient assembly of the Pd catalyst on VSe2 nanosheets is the prerequisite to fabricate the VSe2–xOx@Pd sensor. Layered VSe2 is bound by a van der Waals force, and its crystal structure is depicted in Figure 1a. Manufacturing Se defects on exfoliated VSe2 is conducive to the selectively assembly of Pd clusters (detailed experimental methods are shown in Figure S1), and the schematic illustration is shown in Figure 1b. As for the pristine VSe2 nanosheet, it was characterized by high-resolution transmission electron microscopy (TEM) in Figure 1c. The fast Fourier transform (FFT) pattern in the inset suggests a 6-fold rotational symmetric diffraction, which is consistent with the lattice structure of the hexagonal VSe2 crystal in Figure 1a.34 The flat surface of exfoliated VSe2 nanosheets is displayed in Figure 1d by scanning electron microscopy (SEM) using Si substrate as a contrast, and the thickness of the thinnest exfoliated VSe2 nanosheet was estimated to be ∼3.2 nm by atomic force microscopy (AFM) in Figure 1e.

Figure 1.

Figure 1

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Preparation and characterization of the samples. (a) Top and side views of the atomic structure of the layered VSe2 crystal. (b) Schematic illustration of the fabrication mechanism of the VSe2–xOx@Pd sensor. (c) High-resolution TEM image of ultrathin VSe2 nanosheets. Inset: corresponding FFT image. (d) SEM, (e) AFM topography image and the corresponding height profile of the ultrathin VSe2 nanosheets. (f) High-resolution XPS of Se 3d, Pd 3d and V 2p spectra of the VSe2 nanosheets and VSe2–xOx@Pd, respectively. (g) XRD of the VSe2 nanosheets and VSe2–xOx@Pd sensor.

Furthermore, more experimental evidence was provided to demonstrate the assembly of Pd clusters on VSe2. First, X-ray photoelectron spectroscopy (XPS) was conducted to verify the chemical state and composition of the VSe2–xOx@Pd sample. As Figure 1f shows, the appearance of Pd 3d peaks proved the existence of the Pd element on VSe2–xOx@Pd, and the binding energy of Pd 3d5/2 of VSe2–xOx@Pd is about 336 eV, which is between the metallic Pd (335 eV) and Pd–O (336.5 eV), suggesting that the Pd clusters were positively charged overall.35,36 Meanwhile, the broadened Se 3d peak at 54.6 eV of VSe2–xOx@Pd arises from the superposition of the peaks of Pd 4p and Se–O bond.37 The broadened V 2p3/2 and V 2p1/2 peaks at ∼513.6 and ∼521.2 eV of VSe2–xOx@Pd originate from the V5+ component induced by oxygen doping.32,38 Raman characterizations also demonstrate that the oxygen doping had been introduced to the sample (Figures S2). In addition, the X-ray diffraction (XRD) of the pristine VSe2 and VSe2–xOx@Pd sensors are compared in Figure 1g. The XRD shows that the peaks of the VSe2 were unchanged after Pd loading, indicating that the lattice of VSe2–xOx is not severely damaged during the treatment.39 Specifically, the diffraction peaks corresponding to the metallic Pd crystal are not observed, indicating that the Pd clusters were dispersed on VSe2–xOx and no bulk formed.40

To further reveal the microtopography of the VSe2–xOx@Pd surface, the energy-dispersive spectroscopy (EDS) elemental analyses combined with TEM and high angle annular dark-field (HAADF) were conducted to exhibit the elemental distribution on VSe2–xOx@Pd in Figure 2a–c. As for the prepared VSe2–xOx@Pd sensor, the Pd element was densely distributed on the surface. The weights of different elements of the entire sample are supplemented in Figure S3. Moreover, the high-resolution TEM in Figure 2d reveals that the atomic-level Pd clusters, marked by red boxes, were loaded at the oxygen-doping selenium defects on the VSe2 surface (large area TEM image see Figure S4), resulting in the rougher surface of VSe2–xOx@Pd (Figure S5). The typical nanostructure of VSe2–xOx@Pd was emphasized in Figure 2d and selected to be simulated for subsequent calculations.

Figure 2.

Figure 2

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(a) TEM image, (b) HAADF image, and (c) corresponding elemental mappings of the as-prepared VSe2–xOx@Pd nanosheets. (d) From left to right: High-resolution TEM images of the VSe2–xOx@Pd sensor with different magnifications, schematic diagrams of the zoomed TEM image and corresponding simulated atomic arrangements of VSe2–xOx@Pd, respectively. The dispersed bright dots in the red boxes are the Pd clusters loaded on VSe2–xOx.

The selectively assembly of Pd cluster on oxygen-doping selenium defects is important to the formation of VSe2–xOx@Pd. The role of oxygen-doping selenium defects can be demonstrated by density-functional theory (DFT) calculations, as shown in Figure 3a. The DFT results show that the energy barrier (Eb) for the dechlorination of [PdCl4]2– ion on the VSe2–xOx surface is only 13.1 kcal/mol, whereas Eb reaches 21.1 kcal/mol on the VSe2 surface. The results show that the VSe2–xOx provides more favorable sites for the reduction of [PdCl4]2– at room temperature. Meanwhile, as shown in Figure 3b, after the assembly of Pd clusters on the pristine VSe2 (VSe2@Pd) or VSe2–xOx surface, the charge exchange between the Pd cluster and VSe2–xOx of VSe2–xOx@Pd (0.139 e/molecule) is an order of magnitude higher than that of VSe2@Pd (0.014 e/molecule). The strong charge exchange in VSe2–xOx@Pd leads to positively charged Pd clusters, which is consistent with the XPS data in Figure 1f. All results demonstrate that the presence of oxygen-doping selenium defects facilitates the formation of Pd clusters and charge exchange between Pd clusters and VSe2–xOx, which may favor molecule–VSe2–xOx@Pd interactions.

Figure 3.

Figure 3

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Formation of Pd cluster on VSe2 and VSe2–xOx: the oxygen-doping Se defects matter. (a) The minimum energy pathways, including the initial state (IS), transition state (TS) and final state (FS), for dichlorination of the [PdCl4]2– ion on VSe2 and VSe2–xOx surface, respectively. (b) The charge exchange in typical nanostructure of VSe2@Pd and VSe2–xOx@Pd. The red and green colors represent the region of the increased electron density and decreased electron density, respectively.

To investigate the SERS performance of the VSe2–xOx@Pd sensor, 4-bromothiophenol (BTP) and rhodamine 6G (R6G) were employed as Raman probes for SERS studies under 532 and 633 nm lasers, respectively. Additionally, to further clarify the commendable SERS performance of VSe2–xOx@Pd sensor, VSe2@Pd, pristine VSe2, and graphene (typical metallic two-dimensional material) were taken as comparisons. As shown in Figure 4a, the fingerprint signals of BTP (C–Br vibrational mode at 1065 cm–1 and C–C vibrational mode of benzene ring at 1560 cm–1) are clearly discerned from the VSe2–xOx@Pd sensor, whereas they are fairly weak from the other substrates.41,42 Like BTP, the strongest SERS signals of R6G were observed from the VSe2–xOx@Pd sensor in controlled experiments (Figure 4b). The SERS signals from the VSe2–xOx@Pd samples of different batches are highly reproducible with a relative standard deviation of ∼9.11% (Figure S6). The enhancement factor of VSe2–xOx@Pd for 10–3 M BTP is calculated to be about 1.04× 103 (Figure S7). Considering that the plasmonic effects of the planar VSe2–xOx@Pd are negligible under our experimental conditions because the plasmonic adsorption of VSe2 is located at the infrared band and the plasmonic resonance effect of highly dispersed Pd clusters is also negligible,43 we can deduce that the CM enhancement is the dominating factor for SERS. As shown in Figure 4c, the SERS signals of molecules on the VSe2–xOx@Pd showed a strong dependence on the laser energy. Strong SERS signals of R6G were observed at 532 nm and no signal was discerned at 633 nm, whereas stronger SERS was obtained at 633 nm for BTP and a weakened signal turned up at 532 nm. This selective enhancement, which differs in CM enhancement compared the conventional EM enhancement, is determined by the coupling between the laser wavelength and molecular energy bands and can be considered as strong evidence to demonstrate that CM is the dominating mechanism for the VSe2–xOx@Pd SERS sensor.44

Figure 4.

Figure 4

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Performance and mechanisms of the VSe2–xOx@Pd sensor. SERS spectra of (a) 10–6 M R6G and (b) 10–3 M BTP from VSe2–xOx@Pd sensor, VSe2@Pd, pristine VSe2 nanosheets, and graphene. (c) Integrated Raman intensity of 10–3 M BTP and 10–6 M R6G on different substrates under 532 or 633 nm laser. (d) DOS near the Fermi energy of Pd cluster, VSe2, VSe2@Pd, and VSe2–xOx@Pd. (e) Schematic diagram of the PICT between the VSe2–xOx@Pd sensor and R6G or BTP under excitation. The black arrows indicate the inhibited charge transfer pathways. (f) Calculated binding energy and corresponding side views of the electron density difference isosurface (0.002 electron/bohr3) of different adsorption sites of BTP on VSe2–xOx@Pd and VSe2@Pd, respectively. The red and green colors represent the region of the increased electron density and decreased electron density, respectively.

To reveal the underlying SERS mechanism of the VSe2–xOx@Pd sensor, corresponding first-principles calculations based on density functional theory were conducted. According to the CM mechanisms, the PICT in analytes-VSe2–xOx@Pd system would be significantly related to the band structure of VSe2–xOx@Pd. The hybrid band structure of VSe2–xOx@Pd originated from the combined contribution of the Pd cluster and VSe2 (Figure S8). Compared with the band structure of pristine VSe2 and Pd cluster, the density of state (DOS) near the Fermi level of VSe2–xOx@Pd is more abundant (Figure 4d). According to Fermi’s golden rule, the large number of allowed energy states would be beneficial to the charge transition.45−47 As illustrated in Figure 4e, the higher DOS at the Fermi level promotes the PICT between the VSe2–xOx@Pd and molecular energy levels under excitation, resulting in a significant enhancement for Raman scattering.

Besides, the positively charged Pd offers stronger adsorption sites for the negatively charged groups, and thus improved PICT can be expected. Taking the BTP molecule as an example, we studied the adsorption of BTP on VSe2–xOx@Pd and VSe2@Pd surfaces with two main adsorption sites (top-site and bridge-site, see Figure S9). The results in Figure 4f show that the binding energies of BTP-VSe2–xOx@Pd and BTP-VSe2@Pd were 1.86 and 1.16 eV, respectively, for the bridge-site and 1.28 and 1.18 eV, respectively, for the top-site. The binding energy of the bridge-adsorbed BTP-VSe2–xOx@Pd was ∼70% higher than that of the BTP-VSe2@Pd, suggesting a significantly stronger coupling between the BTP and VSe2–xOx@Pd. Moreover, the electron density difference isosurfaces (0.002 electron/bohr3) of the above complexes exhibit the electron perturbation that arises from the charge transfer from BTP to the substrates, suggesting the formation of an interface dipole. Benefiting from the strong charge exchange between Pd clusters and VSe2–xOx, the strongest electron transfer (0.254 e/molecule) occurred between BTP and VSe2–xOx@Pd at the bridge-site, indicating a more pronounced ground-state charge transfer (GSCT). Both of these effects strengthen the molecular adsorption on Pd clusters, which is beneficial for the CM-based SERS enhancement.

The excellent SERS performance of the VSe2–xOx@Pd sensor provides the possibility to realize self-monitoring of Pd-catalyzed reaction. The Suzuki–Miyaura coupling reaction (SM reaction) is a category of palladium-catalyzed cross-coupling reactions between aryl (or alkenyl) boronic acid (or boronic acid esters) and halogenated aromatics (or alkenes).48−50 In this part, we adopt a Pd-catalyzed coupling between BTP and phenylboronic acid (BPA) to form 4-mercaptobiphenyl (MBP) in an alkaline liquid-phase environment as a model SM reaction for investigation (Figure 5a). The reaction processes of the SM reaction on VSe2–xOx@Pd are divided into three steps. On the one hand, the C–X (X = Cl, Br, I) bond of the halogenated arene adsorbed on Pd clusters in the VSe2–xOx@Pd system would break under photoexcitation. This step is the rate-determining step, in which the electrons are transferred from Pd clusters to C and Br atoms to form C–Pd and Br–Pd bonds. On the other hand, phenylboronic acid would be converted to borate anion with the assistance of CO32– and subsequently combines with the hole in VSe2–xOx@Pd to activate the C–B bond. The organic species in the above two processes would couple to form the final biphenyl product.51Figure 5b presents the typical SERS spectra of the SM reaction on the VSe2–xOx@Pd under 633 nm laser irradiation for 16 min. The distinctive new modes at 1600 cm–1 were observed during the reactions. The SERS spectra at the wavenumber ranging from 1500 to 1670 cm–1 were extracted and replotted in Figure 5c. It can be found that the peak at 1560 cm–1 (BTP) weakened as the reaction proceeded while the peaks at 1600 cm–1 (MBP) intensified. There is no peak of byproduct 4,4′-biphenyldithiol observed (details available in section IV of the Supporting Information, Figures S10–S17).52−55 For comparison, the typical SERS spectra and contour plots employing the VSe2–xOx@Pd sensor under 532 nm laser are displayed in Figure 5d,e. According to the PICT resonance theory, the charge transfer between the VSe2–xOx@Pd and BTP would be efficiently promoted when the laser energy (E633 = 1.96 eV) is closed to the energy gap between the Fermi level of the Pd cluster and the LUMO level of BTP (∼1.86 eV) (details available in section V of the Supporting Information, Figures S18 and S19). The results show that the SM reaction can still proceed but the breakage rate of the C–Br bond at 1065 cm–1 is obviously slower under a nonresonant 532 nm laser (Figure 5f). The C–Br bond breaking reaction follows the second-order kinetics under both 633 and 532 nm lasers (Figure S20), and the reaction constant is k633 ∼ 0.058 and k532 ∼ 0.021, respectively. The time-dependent conversion rates of BTP under different lasers were depicted in Figure 5g. By comparing the conversion rates of BTP on VSe2–xOx@Pd under different lasers, the contribution of the PICT resonance effect in the catalytic process can be clearly evaluated, which is difficult to study using conventional LSPR-based sensors.

Figure 5.

Figure 5

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SERS investigations of the SM reaction on VSe2–xOx@Pd. (a) Schematic diagram of the SM reaction. (b–e) Typical time-dependent SERS spectra of the SM reaction on the VSe2–xOx@Pd sensor and corresponding Raman contour plots under 633 nm (b, c) and 532 nm laser (d, e), respectively. (f) Normalized breakage rates of the C–Br bond fitted with second order kinetics and (g) time-dependent conversion rates of BTP on VSe2–xOx@Pd under different lasers.

Conclusion

In summary, we performed operando SERS monitoring of the Pd-catalyzed SM reaction with the VSe2–xOx@Pd sensors. The oxygen-doped selenium defects provide efficient adsorption sites for the Pd clusters and facilitate the charge exchanges between the Pd clusters and VSe2–xOx. Meanwhile, the combination of Pd and VSe2–xOx significantly increases DOS near the Fermi level of VSe2–xOx@Pd. These MSI between Pd and VSe2–xOx significantly promote the PICT between the Pd clusters and adsorbed molecules, leading to intense CM enhancement which is in favor of the operando SERS monitoring of the SM reactions on Pd clusters. We observed the wavelength dependence of the SM reactions on the VSe2–xOx@Pd sensor and explored the contributions from PICT resonance to the catalytic conversion rate. This work demonstrates the feasibility of the VSe2–xOx@Pd for self-monitoring of Pd-catalyzed reactions and offers a valid MSI-based approach to investigate the reaction mechanisms.

Experimental Methods

Sample Preparation

Single-crystal VSe2 was prepared by the chemical vapor transport method using 5% excess selenium as the transporting agent. High-purity V and Se powder elements were mixed and sealed in a silica ampule. The VSe2 nanosheets were obtained by a mechanical exfoliation method. The exfoliated VSe2 nanosheets were annealed in vacuum at 350 °C for 30 min and subsequently immersed in 10 mL of Na2PdCl4 aqueous solution (10–3 mol/L) for 20 min to fabricate the VSe2–xOx@Pd sensor. The VSe2@Pd samples were produced by reducing and assembling Pd clusters onto the pristine VSe2 nanosheets and naturally drying.

The Au NPs were fabricated by employing ultrathin anodic aluminum oxide membranes as templates in thermal evaporation. The prepared Au NPs were transferred in the reaction vessel where 4 mL of deionized water, 0.3 mL of Na2PdCl4 solution (2.4 mM), 2 μL of HCl solution (6 M), and 1 mL of ascorbic acid solution (100 mM) were mixed under vigorous stirring at 80 °C for 3 h to fabricated the Au@Pd NPs.

Raman Measurements

R6G (99%, Sigma-Aldrich) and BTP (97%, Sigma-Aldrich) were employed as SERS probes. The samples were immersed in 10 mL of solution containing SERS probes for 10 min for molecule adsorption and naturally dried in the air. The Raman measurements were performed with a HORIBA Jobin Yvon Lab RAM HR 800 micro-Raman spectrometer system with a laser spot of ∼1 μm2. The excitation wavelength was 532 or 633 nm, and the laser power was ∼2 mW unless specified. The Raman spectra were obtained with 10 s acquisition time unless specified. For the SERS measurements at the liquid state, we collected the signals from substrates in a self-designed reaction cell filled with reaction solution. The spectra for comparison were obtained under identical measurement conditions and normalized by an internal standard method.

Density Functional Theory (DFT) Calculations

First-principles calculations were performed by using the plane-wave technique as implemented in the Vienna Ab Initio simulation package (VASP). The ion–electron interaction was described by the projector augmented wave (PAW) method, and the generalized gradient approximation (GGA) was expressed by the functional of Perdew, Burke, and Ernzerhof (PBE). The Brillouin zone integration was sampled by the gamma only method. The systems were simulated with a periodic boundary condition by placing a 4-atom Pd cluster upon the 75-atom intact monolayer VSe2 (V: 25 and S: 50) for the VSe2@Pd and a 4-atom Pd cluster upon the 75-atom O-doped monolayer VSe2 (V: 25, O: 3, and S: 47) for VSe2–xOx@Pd. The PAW method was used to describe the wave function of the core region, and the valence wave function was extended to a linear combination of plane waves with 500 eV cutoff energy. The geometry was fully relaxed without any constraint until the force on each atom was less than 0.02 eV/Å. Grimme’s DFT-D3 correction was employed for dispersion interaction.

Acknowledgments

T.Q. acknowledges the National Natural Science Foundation of China (Grant No. 11874108). Q.H. acknowledges the National Natural Science Foundation of China (Grant No. 22004016).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.2c00596.

  • Supplementary characterizations of the samples, detailed SERS studies of VSe2–xOx@Pd sensor, details of DFT calculations, controlled experiments between the plasmonic Au@Pd NPs and VSe2–xOx@Pd sensor, calculation of the degree of charge-transfer (ρCT), kinetic analysis of the SM reaction (PDF)

Author Contributions

‡ M.L. and Y.W. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CRediT: Mingze Li conceptualization, investigation, methodology, writing-original draft; Yunjia Wei conceptualization, investigation, methodology; Xingce Fan writing-review & editing; Guoqun Li investigation; Xiao Tang investigation; Weiqiao Xia writing-review & editing; Qi Hao funding acquisition, supervision, writing-review & editing; Teng Qiu funding acquisition, supervision, writing-review & editing.

The authors declare no competing financial interest.

Supplementary Material

au2c00596_si_001.pdf (1.6MB, pdf)

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