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Plasmonic Enhanced Oxygen Reduction Reaction of Silver/Graphene Electrocatalysts Fenglei Shi, Jing He, Baiyu Zhang, Jiaheng Peng, Yanling Ma, Wenlong Chen, Fan Li, Yong Qin, Yang Liu, Wen Shang, Peng Tao, Chengyi Song, Tao Deng, Xiaofeng Qian, Jian Ye, and Jianbo Wu Nano Lett., Just Accepted Manuscript • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Plasmonic Enhanced Oxygen Reduction Reaction of

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Silver/Graphene Electrocatalysts

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Fenglei Shi,†a Jing He,†b Baiyu Zhang,c Jiaheng Peng,a Yanling Ma,a Wenlong Chen,a

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Fan Li,a Yong Qin,a Yang Liu,a Wen Shang,a Peng Tao,a Chengyi Song,a Tao Deng,a,f

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Xiaofeng Qian,*c Jian Ye*b,d and Jianbo Wu*a,e,f

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a.

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Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd, Shanghai, 200240,

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People’s Republic of China.

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b.

State Key Laboratory of Metal Matrix Composites, School of Materials Science and

State Key Laboratory of Oncogenes and Related Genes, Shanghai Med-X

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Engineering Research Center, School of Biomedical Engineering, Shanghai Jiao Tong

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University, Shanghai, People’s Republic of China.

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c.

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College of Science, Texas A&M University, College Station, Texas 77843, United States

Department of Materials Science and Engineering, College of Engineering and


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d.

Shanghai Key Laboratory of Gynecologic Oncology, Ren Ji Hospital, School of

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Medicine, Shanghai Jiao Tong University, Shanghai, P. R. China

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e. Materials

Genome Initiative Center, Shanghai Jiao Tong University.

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f. Hydrogen

Science Research Center, Shanghai Jiao Tong University.

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ABSTRACT: Oxygen reduction reaction (ORR) is of paramount importance in polymer

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electrolyte membrane fuel cell (PEMFC) due to its sluggish kinetics. In this work, a

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plasmon-induced hot electrons enhancement (PIHEE) method is introduced to enhance

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ORR property of the silver (Ag)-based electrocatalysts. Three types of Ag

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nanostructures with differently localized surface plasmon resonances (LSPR) have

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been used as electrocatalysts. The thermal effect of plasmonic-enhanced ORR can be

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minimized in our work by using graphene as the support of Ag nanoparticles. By tuning

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the resonance positions and laser power, the enhancement of ORR properties of Ag

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catalysts has been optimized. Among these catalysts, Ag nanotriangles after excitation

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show the highest mass activity and reach 0.086 mA/μgAg at 0.8V, which is almost 17

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times as commercial Pt/C catalyst after the price is accounted. Our results demonstrate


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that the hot electrons generated from surface plasmon resonance can be utilized for

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electrochemical reaction, and tuning the resonance positions by light is a promising and

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viable approach to boost electrochemical reactions.

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KEYWORDS: oxygen reduction reaction, localized surface plasmon resonance, hot

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electrons, Non-Pt electrocatalyst

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Exploration of novel technologies to take advantage of alternative energy sources is of

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great importance while facing challenges from the shortage of fossil fuel and

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consequent environmental impact. During past decades, polymer electrolyte membrane

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fuel cell (PEMFC) has been one of the most attractive technologies due to its high

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efficiency and environmental friendliness.1 There is no doubt that oxygen reduction

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reaction (ORR), which occurs at cathode, is becoming a major limitation in PEMFC due

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to its extremely sluggish kinetics.2,3 Recently, many highly active ORR electrocatalysts

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have been intensively investigated.4-6 Among these catalysts, platinum (Pt) or Pt-based


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alloy catalysts with different shapes offer the highest performance as well as durability

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compared to other non-Pt metals or their alloys.7-9 Nevertheless, Pt is usually high cost

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with limited reserves, which brings primary limitations to the commercialization of these

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Pt-based catalysts, especially for mass production of vehicles.10 As for these reasons,

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recently much work has focused on other noble-metal-based electrocatalysts.11,12 There

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is a great demand to utilize the noble metal, silver (Ag), as the catalyst for ORR, since

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its cost is only about 1/68 and its global reserves is 38 fold as that of Pt, respectively.13

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However, Ag is not competitive with Pt in the following two aspects of the ORR activity:

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1) the relatively large overpotential of activated ORR on Ag surface;14 2) too weak

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oxygen adsorption for boosting ORR based on the Sabatier’s principle.15 Therefore, Ag

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was never a choice as a candidate as an ORR catalyst despite its low price and large

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reserves.

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Recently, plasmon-induced energetic hot electrons through photon absorption in

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metallic nanostructures have been applied in various fields such as chemistry, local

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heating, molecular detection, and photoelectronic devices.16-20 Typical examples of the


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plasmon-induced chemical reactions assisted by hot electrons include water splitting,21-

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phase oxidation.29,30 Among these reactions, plasmon-induced hot electrons are

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generated by optical excitation, which further contribute to the improvement in these

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afore-mentioned electrochemical reactions. Indeed, silver, a desired metal, which has a

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strong localized surface plasmon resonances (LSPR),31 is able to drastically absorb

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light and generate the hot electrons for reduction reaction.32,33 Actually, applying

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plasmon-induced hot electrons into reduction reaction has been investigated by

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combining Au, a plasmonic metal, with TiO2, a semiconductor to enhance the ORR

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properties.34 Nevertheless, how the electrons transfer between the interface and the

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mechanism of plasmon-induced hot electron enhancement still remains unclear.

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Meanwhile, the thermal contributions in the plasmonic reactions are also very difficult to

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be ruled out. How to distinguish or eliminate the thermal effect during the plasmonic

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chemical reactions is becoming more and more important. Recently, Yu et al35 used

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scanning electrochemical microscopy (SECM) to resolve the enhancement mechanism

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of plasmon-mediated photoelectrochemical reactions. Zhang et al36 and Zhou et al37

H2 production,24,25 and dissociation,26 O2 evolution27 and dissociation,28 and gas


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utilized different temperature-detectors to measure the temperature change to

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distinguish the thermal contributions in the hot electron-induced enhancement.

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Herein, we demonstrate a plasmon-induced hot electrons enhancement (PIHEE)

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method to boost ORR on Ag surface by the decay of surface plasmons on Ag

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nanostructures on graphene support, which shows a comparable activity as Pt.38

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Graphene has been applied as the support for electrocatalysis due to its high corrosion

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resistance, electronic conductivity, ionic conductivity, and chemical stability.2,39,40 Lim et

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al has demonstrated that graphene can be utilized as an excellent support because of

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its high specific surface area (theoretically 2630 m2/g), that can perfectly disperse the

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Ag nanoparticles to avoid the agglomeration of the nanoparticles and finally results in

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better ORR performance,41 especially better mass activities of Ag/graphene than that of

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the Ag/C catalyst.42 We further found that the graphene barely had plasmonic thermal

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effect on ORR compared to graphite and carbon nanotube (CNT), which was chosen as

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a carbon support to eliminate the thermal effect and investigating the plasmonic effect

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on ORR. Furthermore, this plasmon-induced ORR enhancement can be optimized by


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tuning LSPR through manipulation of Ag nanostructures to different resonance positions

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as the sources of plasmons. We synthesized three different Ag nanostructures with

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different maximum absorbance wavelengths to explore the effect of plasmon resonance

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on boosting ORR property on Ag surface. We also investigated the mechanism of the

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ORR enhancement on Ag surface from the plasmon-induced hot electron.

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Results and discussion


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Figure 1. TEM, optical characterization, and simulation of different Ag nanostructure. (a)

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TEM images, (b) experimental extinction spectra, (c) simulated extinction/absorption

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spectra, (d) schematic of simulations, and (e, f) simulated photo-excited local electric

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field (|E/E0|) distributions of a Ag (i) NW, (ii) NS, and (iii) NT. All extinction spectra are

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normalized and vertically shifted for clarity.

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polarization are noted in panel d. The electric fields in panel f are plotted in a logarithmic

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scale.

The incident light propagation and


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TEM, optical characterization, and simulation of different Ag nanostructures were

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performed to investigate their morphologies and optical properties. Figure 1a shows

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three different Ag nanostructures including nanowires (NWs) (i), nanospheres (NSs) (ii),

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and nanotriangles (NTs) (iii) (see the details of characterization in the Figure S1-2),

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which are synthesized as the LSPR generator for the generation of plasmon-induced

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hot electrons and the subsequent catalyst of the ORR activity. Because the LSPR

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wavelengths of plasmonic nanostructures are highly dependent on their size, shape,

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and compositions,43,44 we examined the experimental extinction spectra of these Ag

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nanocatalysts by the UV-Vis spectrometer. It is shown that a pronounced dipolar LSPR

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peak locates at 382 and 400 nm in the UV region for NWs and NSs, respectively (Figure

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1b). In contrast, a strong dipolar LSPR mode at 570 nm was observed in the visible

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range for NTs.

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The simulated extinction and absorption spectra (Figure 1c), schematic of simulations

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(Figure 1d) and the local electric field distributions (Figure 1e and 1f) of Ag

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nanocatalysts were obtained by performing the finite-difference-time-domain (FDTD)


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method (see the details in the section of Numerical Simulations). Generally, the

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simulated extinction spectra well duplicate the experimental extinction spectra for three

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types of Ag nanostructures in terms of peak number, peak wavelength, and overall

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spectral profile: a single LSPR peak (at 360 and 388 nm) in the UV-Vis region for NWs

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and NSs, respectively, and a strong LSPR (at 579 nm) in the visible region for NTs

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(Figure 1c). It can also be noticed that the absorption cross-section acts as the

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predominant portion in the extinction cross-section for NSs and NTs (ii and iii in Figure

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1c). While NWs have a much smaller absorption cross-section relatively and most of the

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interacted photons are damped via the scattering process (i in Figure 1c). This indicates

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that more photons will be absorbed to generate more hot electrons beyond the Fermi

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energy for NSs and NTs compared to NWs due to a relatively larger absorption cross-

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section.16 As for AgNSs and AgNTs, the latter have a higher efficiency of photon energy

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absorption, which prompts us to choose a laser whose wavelength is near the

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absorption peak of AgNTs. The experimental and simulated extinction spectra both

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reveal that the LSPR of NTs mostly approaches to the 532 nm wavelength of the

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incident laser, consequently exhibiting the largest absorption efficiency among the three


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Ag catalysts. This can be demonstrated by the calculated local electric field (|E/E0|)

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distributions of three Ag nanostructures under the irradiation of incident light in Figure

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1e, where the NT shows the largest localized electric field enhancement at 532 nm with

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a maximum value of 13.1 among these three Ag nanostructures (4.6 for NW and 4.7 for

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NS). Electric field distributions also indicate that much stronger fields are confined at the

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corners for NTs while the fields are more uniformly enhanced on the surface for NWs

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and NSs. Figure 1f further illustrates the distributions of logarithmic electric fields to

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show dipole modes of nanocatalysts. It distinctly confirms the characteristic dipolar

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feature of LSPR modes at the excitation wavelength of laser (532 nm). To simulate

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more accurately, the Ag/graphene nanocatalysts were also built to investigate whether

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the graphene affects the optical properties of Ag, consequently, little difference can be

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found by the addition of graphene support (see the details in the section of Numerical

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Simulations and Figure S3 and S4).


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Figure 2. ORR polarization curves of Ag NWs (a), NSs (b), and NTs (c) with and without

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laser excitation respectively. (d) The half-wave potential positive shift of three

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electrocatalysts. (e) ORR activity at 0.8V versus RHE normalized by the mass of Ag

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nanostructures.

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Followed the exploring the morphologies and optical properties of three different

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nanostructures of Ag, 532 nm laser source was chosen to study the plasmonic

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enhancement of the electrocatalysts and the PIHEE-induced ORR performance of three

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Ag nanocatalysts was evaluated in the three-electrode system under the irradiation. The

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ORR polarization curves of NWs, NSs, and NTs with and without laser excitation (2 W)

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are shown in Figure 2a-c, respectively. All three Ag nanocatalysts show that the limiting

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current density increases upon optical excitation and the curve shifts positively, which


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indicate that the electrocatalytic reduction of oxygen has been improved under the laser

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irradiation. After the quantitative examination of the shifting behavior of each curve, the

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half-wave shift shows 18.2 mV for NWs, 21.1 mV for NSs, and 22.6 mV for NTs,

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respectively (Figure 2d), indicating the order of the curve shift value: NWs < NSs < NTs,

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when excited by a green laser (532 nm). Further investigations indicate that the

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enhancements of mass activity at 0.8 V by PIHEE are 0.0062 mA/μgAg for AgNW

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(66.8%), 0.014 mA/μgAg for AgNS (92.9%), 0.049 mA/μgAg for AgNT (132.01%) (Figure

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2e), which results in that Ag NTs with optical excitation exhibits a mass activity of

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0.0856 mA/μgAg at 0.8 V, which is about 25% of the commercial Pt/C catalyst (0.34

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mA/μgPt at 0.8 V) (see the details in Figure S5). Considering that the diameter of the

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green laser is only 3 mm, so the area of the catalyst under illumination is only about

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0.07 cm2. Furthermore, compared to the area of RDE (0.196 cm2), the illuminated area

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of the catalyst is only 1/2.8 to the RDE. From the previous work published by Mukherjee

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et al,26 the plasmon-induced dissociation of H2 had an approximate linear relationship

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with the gold (plasmonic metal) loading. Herein, we can also fairly estimate the plasmon

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effects can be increased proportionally by the area in our experiment because the


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nanoparticles are dispersed on the graphene uniformly (Figure S1). Therefore, the ORR

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property can be further enhanced by 237.59% if the catalyst is fully illuminated. The

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theoretic value of the mass activity is shown in the Table S2. The highest mass activity

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of AgNT should be 0.1733 mA/μgAg with an enhancement of 369.6%, which is

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comparable to the ORR mass activity of Pt. Among these catalysts, AgNT after

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excitation show the highest mass activity and reach 0.086 mA/μgAg at 0.8V, which was

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25% of that of Pt/C. Hence, the mass activity of Ag can be almost 17 times as

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commercial Pt catalyst under the same price cost, considering the cost of Ag is just

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about 1/68 of platinum. Moreover, the global reserve of Pt metal is only about one over

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thirty-eight of Ag, suggesting that Ag is much easier to acquire and for mass production.

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The above experimental and theoretical extinction/absorption spectra have indicated

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that Ag NTs tend to generate more hot electrons upon the plasmon excitation. It has

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been theoretically predicted that for plasmon energies below the interband transition

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threshold16 the number of plasmon-generated hot electrons closely follows the

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absorption spectrum of Ag nanostructures (e.g., nanospheres and nanoshells).45


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Therefore, the enhancement of ORR performance of three Ag nanostructures can be

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understood in terms of hot electron generation by two aspects as follows. First, due to

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the small size of NS and NT, these two kinds of nanostructures both have relatively

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larger absorption cross-sections, which indicate that more photons will be absorbed to

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generate hot electrons. Second, the LSPR of NTs overlaps most with the excitation

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laser wavelength, which enables the most efficient excitation of plasmons. The

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associated strong electric fields and dramatically enhanced absorption cross-section

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greatly boost the hot electron production rate in NTs. These two factors play a major

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role in the fact that Ag NTs trend to generate the most amount of hot electrons.

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Figure 3. (a), ORR properties of three kinds of carbon supports with and without laser

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excitation, (b) Corresponding half-wave potential positive shift and limiting current

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density enhancement under laser irradiation. (c) Comparison of ORR properties of pure

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AgNTs, pure graphene, and the AgNT/graphene catalysts. (d) Calculated temperature

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profile of a AgNT in water illuminated by 532 nm laser with experimental a power

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density (28.3 W/cm2) used in the experiment. The dashed black triangle indicates a

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AgNT. (e) The current-time (i-t) curves of these three Ag catalysts and commercial Pt/C

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catalyst. (f) Temperature test near the electrocatalysts before and after laser irradiation.

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After exploring the ORR performance of three different Ag-based catalysts before and

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after laser excitation, the thermal effect was investigated to confirm the thermal

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contributions in the enhancement. First, three different kinds of carbon supports were

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chosen to study the plasmonic thermal effect on ORR properties with and without laser

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irradiation. The ORR properties with and without 2 W laser irradiation of graphite,


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carbon nanotube, and graphene are shown in Figure 3a, respectively. The ORR curves

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of different carbon supports show the largest limiting current density enhancement of

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0.45 mA/cm2 for graphite, 0.22 mA/cm2 for carbon nanotube, and 0.03 mA/cm2 for

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graphene, respectively (Figure 3b). Meanwhile, the ORR curves positive-shift also has

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the same trend as the limiting current density with the order of graphite > carbon

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nanotube > graphene (Figure 3b). Moreover, an improvement of 0.03 mA/cm2 in limiting

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current density and invisible positive-shift of ORR curve indicates that the thermal effect

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of graphene is barely visible and can be ignored. Hence, graphene was suitable to be

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the support for the study of plasmonic enhancement of ORR without the thermal effect.

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After the exclusion of the influence of carbon support, the thermal effect of Ag also

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should be clarified. The ORR curves of pure Ag catalyst with and without laser

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irradiation show no visible positive-shift but a little bit increasing of limiting current

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density (0.05 mA/cm2) (Figure 3c), indicating a weak thermal effect. Moreover, we

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added the ORR curves of graphene and AgNT/graphene with and without laser

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excitation into Figure 3c to have a comparison, which indicated that the ORR

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performance enhancement of AgNT had little to do with the thermal contributions of


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graphene and AgNT. Further, we took AgNT as an example to perform the calculated

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local temperature change of AgNT nanostructures in Figure 3d. The absorbed photonic

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energy can produce heat in plasmonic nanostructures, which can be reflected in the

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temperature variation.46,47 According to the law of heat transfer between Ag NT and the

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surrounding medium, the temperature distribution can be calculated numerically. With

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the experimental power density of laser, the temperature can only increase by 0.003 K

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in this theoretical model.48 This result strongly indicates that the thermal effect

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contributes quite little to the ORR enhancement process here. The curve of power

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density-temperature variation further (Figure S6) reveals that an obvious increase of

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temperature would appear only when the power density is much higher than the value in

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our experiment. To further verify that the enhancement of ORR property is originated

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from the plasmonic effect, not from the thermal effect under laser irradiation, the

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current-time (i-t) curves of these three Ag catalysts and commercial Pt/C catalyst were

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measured, respectively (Figure 3e). Before the laser irradiation, both Ag catalysts and

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Pt/C show an equilibrium current, which is consistent with the polarization curve in

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Figure 2a-c and Figure S5. Once the light is on, the currents of all three Ag catalysts


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suddenly jump to the extinction state within less than 1 second while it costs the Pt/C 55

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seconds to approach the extinction current (Figure 3e). As we know the decay of

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plasmon and the following generation of hot electrons are ultrafast process within the

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timescale of femtoseconds.16,49 In contrast, the thermal heating of nanoparticles by laser

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in a liquid environment is on the order of dozens of seconds.46,50 Moreover, the

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temperature test was performed to acquire the temperature change practically before

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and after the laser illumination. The temperature was measured close to the RDE by K-

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type thermocouples and the temperature data was acquired 10 times per second in

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Figure 3f. The temperature near the catalyst had a little bit fluctuate when the light was

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on or off. The whole temperature change is less than 0.3 oC, indicating the laser would

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not cause the visible temperature change during the reaction.


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Figure 4. The mechanism of plasmon-induced hot electrons enhancement for ORR.

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Schematics of the statuses (unconnected (a), connected (c), irradiated (e)) for the AgNT

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and graphene and the corresponding energy band diagrams (b, d, f). (g) XPS of Ag 3d

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of AgNT and AgNT/graphene. (h, i) Electric band structures of FCC Ag crystal and a

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6.5-nm thick Ag (111) slab (inset), where large p band splitting is present in the slab due

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to quantum confinement, allows a wide range of p-p intraband and interband transitions

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and p-d interband transitions.

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To investigate the mechanism of plasmon-induced hot electrons enhancement for

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ORR, we take AgNT as a typical example. Figure 4 shows the origin of the mechanism

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of plasmon-induced hot electrons enhancement for ORR. When AgNT and graphene


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were separated (Figure 4a), the corresponding Fermi level diagram is shown in the

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Figure 4b where Wm means metal’s work function (Ag) of 4.26 eV51 while Wg represents

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graphene’s work function of 4.5 eV.52 Once the AgNT contacts with graphene (Figure

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4c), the electrons above the modulated Fermi level at the Ag side prefer to transfer to

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the lower energy band at the graphene side, resulting in the redistribution of the

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electrons (Figure 4d) and the enhancement of ORR property (Figure 3c). When the

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AgNT is irradiated (Figure 4e), the localized resonant collective oscillation of the free

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electrons in Ag’s d orbitals will take place. The following decay of LSPR will lead to the

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generation of hot electrons which are excited into p orbitals. Sequentially, these hot

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electrons trend transfer to graphene, resulting in the electron loss of the AgNT (Figure

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4f). The X-ray photoelectron spectroscopy (XPS) of AgNT and AgNT/graphene was

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shown in Figure 4g. The XPS spectra of AgNT shows two peaks of Ag 3d spectra

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located at 367.3 eV and 373.4 eV, which can be assigned to Ag 3d 5/2 and 3d 3/2.

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These two peaks of AgNT both had a positive shift of 1.1 eV and 1.0 eV when mixed

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with graphene, which indicate the oxidation state of the Ag, confirming that the electrons

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indeed transfer from AgNT to graphene and the similar results had been also reported


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before.53 In addition, we performed a comparative electronic band structure calculations

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based on first-principles density functional theory54,55 for single crystalline FCC Ag and

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6.5 nm-thick Ag (111) slab. The results are presented in Figure 4h-i, which

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demonstrates that quantum confinement along the slab normal causes large splitting of

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p bands, consequently promoting a wide range of p-p intraband and interband

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excitations and Ag p-d interband transitions upon optical excitation. The p orbital-

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involved oxygen adsorption on Ag is much improved. As a result, according to the

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Sabatier’s principle, for Ag that located at the downhill of volcano plots, the enhanced

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oxygen adsorption on Ag surface leads to the improvement of ORR (Figure 2c).15,56,57


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Figure 5. ORR properties with different laser power illumination and the theory of the

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enhancement. ORR polarization curves of Ag NWs (a), NSs (b), and NTs (c) with

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different laser powers and the corresponding mass activities (insert). (d) ORR activity

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for Ag and some other metals plotted against O absorption energy.56 The data of pure

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metal in Figure 4 are from the reference,56 where the activity is the negative of activation

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energy of ORR at the equilibrium electrode potential.

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To further study the mechanism of plasmonic enhancement of Ag/graphene ORR

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properties. We performed the different laser power experiments. Figure 5a-c shows the

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ORR curves of Ag NWs, NSs, and NTs with different laser powers from 0 to 2.3 W. The


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enhancement of laser power, the improvement ORR properties increase. Meanwhile,

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the enhancements are saturated when the laser power increases to a certain degree

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(Figure 5a-c inset). Indeed, when the laser power exceeds 1.5 W, the plasmonic boost

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of ORR property of AgNW might get saturated and then fall off. The enhancement of

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mass activity in the other two nanostructures start to decay after 2 W. This interesting

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phenomenon can be explained by the Sabatier’s principle. According to the volcano plot

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for ORR activity plot against O absorption energy (Figure 5d), Ag is located at the

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downhill of the volcano plot, so the ORR activity can be increased to the peak of the

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volcano, which stands for the optimized scenario. Nevertheless, once the laser power

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exceeds the certain degree, ORR activity tends to throw over the peak to fall off. This

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phenomenon is because the O absorption energy is becoming too strong due to the

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excessive electrons transfer, sequentially result in the difficulty in OH desorption on Ag

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surface, which slows down the ORR kinetics.

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To conclude, this work demonstrates a new strategy of PIHEE to enhance ORR

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property on non-Pt noble metal exhibiting LSPR effect like Ag. We have shown that hot


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electrons that are generated from plasmon decay can subsequently participate in the

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reduction reaction of oxygen to boost the electrocatalysis. Meanwhile, the thermal effect

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of plasmon-induced hot electrons-mediated reactions can be minimized in our work by

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using graphene as the support. This environment-friendly path could be useful in fuel

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cells for generating clean energy by transferring the optical energy to chemical energy

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with low cost during utilization of hydrogen energy. Moreover, this strategy represents

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another application of plasmonics by tuning the resonance positions through

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controllable nanostructures to precisely controlling the electrocatalytic reaction. In

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general, this economical and practical plasmon-induced hot electron enhancement

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method will be beneficial to energy and environment and may open up avenues for the

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optical control of chemical reactions.

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ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the

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ACS Publications website at DOI: 10.1021/*******.


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Additional experimental methods, characterizations of Ag, numerical simulations details

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on optical spectra of Ag, calculation details on the electronic structure of Ag.

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AUTHOR INFORMATION

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Corresponding Author

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* Email: [email protected]

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*Email: [email protected]

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*Email: [email protected]

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given

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approval to the final version of the manuscript. † F. S. and J.H. contributed equally to

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this work.

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Notes

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The authors declare no competing financial interest.


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ACKNOWLEDGMENT

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This work is sponsored by the thousand talents program for distinguished young

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scholars from Chinese government (J.W. and J.Y.), National Key R&D Program of

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China (No. 2017YFB0406000), and the National Science Foundation of China

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(21875137, 51521004, 81571763, 81622026, and 51420105009), and start-up fund

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(J.W.), medical fund (YG2016MS51 and YG2017MS54) (J.Y.), and the Zhi-Yuan

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Endowed fund (T. D.) from Shanghai Jiao Tong University. B.Z. and X.Q. acknowledge

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the start-up funds from Texas A&M University and the support from Texas A&M Energy

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Institute. Portions of this research were conducted with the advanced computing

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resources provided by Texas A&M High Performance Research Computing.

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