Plasmon-enhanced cathodic reduction for accelerating electricity generation in visible-light-assisted microbial fuel cells

Author’s Accepted Manuscript Plasmon-enhanced Cathodic Reduction for Accelerating Electricity Generation in Visible-lightassisted Microbial Fuel Cells Dan Guo, Hui-Fang Wei, Xue-Yan Yu, Qing Xia, Zixuan Chen, Jian-Rong Zhang, Rong-Bin Song, Jun-Jie Zhu www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30948-0 https://doi.org/10.1016/j.nanoen.2018.12.043 NANOEN3291

To appear in: Nano Energy Received date: 16 June 2018 Revised date: 11 December 2018 Accepted date: 13 December 2018 Cite this article as: Dan Guo, Hui-Fang Wei, Xue-Yan Yu, Qing Xia, Zixuan Chen, Jian-Rong Zhang, Rong-Bin Song and Jun-Jie Zhu, Plasmon-enhanced Cathodic Reduction for Accelerating Electricity Generation in Visible-lightassisted Microbial Fuel Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.12.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Plasmon-enhanced

Cathodic

Reduction

for

Accelerating

Electricity Generation in Visible-light-assisted Microbial Fuel Cells Dan Guoa, Hui-Fang Weia, Xue-Yan Yua, Qing Xiaa, Zixuan Chena, Jian-Rong Zhang a,b*

, Rong-Bin Song a*, Jun-Jie Zhua*

a

State Key Laboratory of Analytical Chemistry for Life Science and Collaborative

Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China b

School of Chemistry and Life Science, Nanjing University Jinling College, Nanjing

210089, China. [email protected] [email protected] [email protected] *

Corresponding authors

Abstract Visible-light-assisted microbial fuel cells (VLA-MFCs) possess great potential, in which novel photoelectric conversion process was integrated into the conventional microbial electricity generation, thus multidimensional energy conversion routes can be achieved. In this regard, developing high-efficiency photoelectric conversion strategy to obtain high performance of VLA-MFCs is highly desirable. Herein, we present a promising plasmon-induced hot-electron transfer system in which 1

three-dimensional Cu2O@plasmonic Au nanowire array was first synthesized and used as the photocathode for accelerating electricity generation of the VLA-MFC. Owning to the surface plasmon resonance excitation of Au nanocrystals, hot electrons can be generated and injected into the adjacent Cu2O semiconductor nanowire array for significantly enhancing the cathodic reduction reaction, eventually resulting in the improved performance of the VLA-MFC.

TOC Graphic：

The promising plasmon-induced hot-electron transfer system was successfully introduced into the light-assisted microbial fuel cell in which three-dimensional Cu2O@plasmonic Au nanowire array was first synthesized and used as the photocathode for accelerating electricity generation.

Keywords: Microbial fuel cells; Solar energy; Plasmon-induced hot-electron transfer 1. Introduction Energy crisis and environmental pollution are highlighted to be the major global issues in the 21st century. Development of new green and sustainable energy technology with reduced negative impact on the environment is critical to human beings. In this regard, microbial fuel cells (MFCs) possess great potential to simultaneously address both the problems by generating electrical energy from 2

microbial metabolism using organic waste as electron donor[1-5]. However, the low power output of the MFCs extremely hinders their practical application. Compared to traditional MFCs that the electricity only originates from the chemical energy in fuels, the light-assisted-MFCs have made great improvements in the power output performance due to their multidimensional energy conversion routes, which can capture electricity from both chemical energy in the fuels and light energy in the sun[6-9]. The essential feature of the light-assisted-MFCs system is the integration of the photoelectric conversion process into the conventional microbial electricity generation. In this system, suitable semiconductor materials are needed to serve as photo-absorber, simultaneously the solar energy is introduced and converted into electrical energy through the semiconductor photoelectrode. For example, a Pd nanoparticle-modified p-type Si nanowire has been introduced as a cathodic material to develop photo-MFCs and its maximum output power density were doubled due to the synergistic effect of photocathode and bioanode[10]. Moreover, a p-type cuprous oxide has also been applied as photocathode to construct light-driven microbial photoelectrochemical cell for proton reduction, leading to efficient hydrogen production and current generation[11]. Although substantial progresses have been made in the field of utilizing solar energy for high power generation of MFCs, the low photoelectric conversion efficiency of the reported semiconductor electrodes resulting from the wide bandgap and high electron-hole recombination rate is still restricting the development of the light-assisted MFCs. To overcome these drawbacks, general efforts mainly focus on strategies of sensitization with dyes or other narrow-bandgap semiconductors, modification with metal co-catalysts and so on. Especially, the strategy of the plasmon-induced hot-electron transfer has been attracting significant 3

attention due to the effect of surface plasmon resonance (SPR)[12-15]. SPR arises from the collective oscillations of the electrons near the surface of the illuminated nanostructured plasmonic metal (such as Au, Ag and Cu) and is capable of efficiently converting the energy from light to other forms of energy through plasmonic excitation[16]. During the plasmonic excitation process, energetic electrons (also referred to as “hot electrons”) can be generated and transferred to the conduction band of adjacent semiconductor by overcoming the Schottky barrier[17-20]. Meanwhile, the adjacent semiconductor can provide trapping sites for the acceptance of plasmon-excited electrons, leading to the suppression of the electron-hole recombination and promoting the photoelectrocatalytic activity of semiconductors in oxidation

or

reduction

reaction

systems

[12,

21-22].

Therefore,

the

plasmonic-metal/semiconductors system is believed to have significant potential in enhancing the electrode reaction and improving the output performance of the light-assisted-MFC,

showing

superiority over

the

conventional

co-catalyst/

semiconductor system because of its high and stable light harvesting efficiency, tunable absorption wavelength and lower electron-hole recombination rate[23]. However, up to now, this promising strategy has not been explored in any of the light-assisted-MFC systems. Herein, we present the promising plasmon-induced hot-electron transfer system for boosting electricity generation of visible-light-assisted MFC (VLA-MFC, schematic diagram shown in Figure S1 of Supporting Information). In this system, the three-dimensional Cu2O@plasmonic Au nanowire array (3D Cu2O@Au NA) on the copper foam (CF) substrate was first synthesized in situ for the photocathode of the VLA-MFC. By SPR excitation of Au nanocrystals, plasmon-induced hot electrons can be injected from Au into Cu2O in the plasmonic-metal/semiconductors system and the 4

cathodic reduction reaction of the MFC is significantly enhanced, eventually resulting in the improved performance of the whole VLA-MFC. 2. Experimental Section 2.1 Materials and chemicals Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O), sodium borohydride (NaBH4), polyvinyl pyrrolidone (PVP) were purchased from Sigma-Aldrich (Shanghai, China). Luria-Bertani (LB) broth was purchased from Nanjing Reagent Company. All reagents were used as received. Copper foam was bought from KUNSHAN JIA YI ELECTRONICS CO., LTD (1.6 mm thick). Carbon paper (CP) was bought from HeSen Electrical Co. (Shanghai, China). Ultra-pure water (18.2 MΩ resistivity, Milli-Q, Millipore) was used for all the experiments.

2.2 Apparatus X-ray diffraction (XRD) measurements were conducted on a D8 ADVANCE X-ray diffractometer (Bruker Corporation, America). Diffuse reflectance spectra were recorded on a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan) and absorbance spectra were calculated according to Kubelka−Munk theory. X-ray photoelectron spectra (XPS) were measured on a PHI 5000 VersaProbe (Ulvac-Phi, Japan). Raman spectra were measured on a Via-Reflex Laser Confocal Raman Microspectroscopy (Renishaw, England). The morphology and element mapping were characterized by field emission scanning electron microscopy (JSM-7800F, JEOL, Japan) with an energy-dispersive X−ray (EDX) spectrometer. All electrochemical measurements were performed using an electrochemical workstation (CHI660D, Chenhua, China). In the measurement of VLA-MFC, a 500 W xenon lamp coupled with an AM 1.5 global filter was utilized as the irradiation source. 5

2.3 Preparation of 3D Cu2O NA and Cu2O@Au NA electrode The copper foam (CF, 1 cm × 1 cm) was first anodized at constant current density in an alkali solution (3M NaOH) to form Cu(OH)2 nanowire array (NA) precursors on the CF. Subsequently, the Cu(OH)2 NA precursors were transformed into 3D Cu2O NA by thermal annealing at 600 ℃ at N2 atmosphere for 4h [24]. The effect of different anodized conditions on the 3D Cu2O NA photoelectrical current was examined and the optimized fabrication conditions of the 3D Cu2O NA were selected at 50 mA of the anodized current and 16 minutes of the anodized time (Figure S2, Supporting Information). The plasmonic Au was in situ deposited on the Cu2O NA electrode to fabricate the plasmonic-metal/semiconductors system of Cu2O@Au NA photocathode. In a typical synthesis, the Cu2O NA electrode was immersed in ethanol (20 mL), followed by the addition of PVP (0.1 g). After stirring for 15 min, an aqueous solution of HAuCl4·3H2O (7 mL, 1.2 mM) was added and the mixture was stirred for 5 min. Afterwards, NaBH4 (3 mL, 3 mM) was added to the mixture and stirred for another 10 min for the reduction of HAuCl4, then the primary gold nucleus on the Cu2O NA electrode was formed. After the products were washed with anhydrous ethanol, using the primary Au nucleus as the core, the second growth process of gold particles on the Cu2O NA electrode was conducted similar to that of the first time, expect that the concentration of HAuCl4·3H2O solution changed from 1.2 mM to 0.6 mM. The third growth of gold particles on the Cu2O NA electrode was exactly the same as that of the second time. Finally, the synthesized Cu2O@Au NA and Cu2O NA based on the CF substrate could be directly utilized as the photocathode without further treatment. All the experimental steps were carried out at room temperature. 2.4 Construction and operation of the VLA-MFC 6

H-shaped dual-chamber MFCs were constructed by connecting two glass bottles with a 30 mm diameter tube. The Shewanella oneidensis MR-1 cells harvested at late stationary phase were used as the model bacteria and were inoculated into the MFC anode chamber. The anode chamber contains 100 mL M9 buffer solution with 5% LB broth and 30 mM lactate as the electron donor. The catholyte was 50 mM K3[Fe(CN)6] in 100 mL phosphate buffer (PBS, pH 7.4) solution. The traditional commercial carbon paper was used as the regular cathode and the previous reported 3D nitrogen-doped graphene aerogel was used as the anode[6]. The anode and the cathode chambers were separated by a proton-exchange membrane (Nafion 211, DuPont) and connected to an external resistance of 1000 Ω with titanium wire. After the operation for two weeks, cathode was replaced by the prepared photocathode to construct the VLA-MFC. Linear sweep voltammetry was used to obtain the polarization curves in one typical cycle at a scan rate of 1 mV s-1. The current density and power output density in electrochemical measurements were normalized to the geometric surface area of the anode (9 mm diameter, 1.5 mm height) with no special instruction. To evaluate the solo photocathode performance, the response of current to potential was measured by using a three-electrode system with the photocathode as the working electrode, a Pt wire counter electrode and a saturated calomel (SCE) reference electrode. 2.5 Dark-Field Microscopy Characterization Before observation, the Au nanoparticle and Cu2O@Au were deposited on glass slide followed by successive rinsing with water and blow-drying with nitrogen gas. Dark field images and spectra measurements were carried out on the same Nikon Ti-E inverted microscope. A broadband light source (EQ-99XFC LDLS, Energetiq Technology) was used for incident illumination. True-color dark-field images are 7

captured by a color cooled digital camera (Nikon DS-RI1), and the scattering spectra of single nanoparticles were measured by a monochromator (Acton SP2300i, PI) equipped with a spectrograph CCD (PLXIS 400BR-excelon, PI) and a grating (grating density: 300 1mm-1; blazed wavelength: 500 nm).

3. Results and discussion As one kind of the typical p-type semiconductor, Cu2O possesses favorable band structures for coupling microbial electron transfer and guaranteeing efficient visible light absorption[11, 25]. Moreover, semiconductors with the nanowire morphology especially have efficient light harvesting ability along the full length of the wire, because they possess favorable ratio of the carrier diffusion length over the light absorption depth and short radial diffusion distance for the minority carriers toward the electrolyte solution[24]. Therefore, the Cu2O semiconductor with nanowire morphology has great potential to achieve high photoelectrical conversion efficiency, making it a more attractive candidate to other photocathode materials used in the VLA-MFC. In view of this, Cu2O nanowire array was selected as the semiconductor material in the plasmonic-metal/semiconductors system. Subsequently, the 3D macroporous copper foam (CF) was chosen as the base material for in situ fabricating 3D Cu2O@Au NA photocathode based on the following considerations: (1) the CF itself is a good conductive substrate electrode; (2) it not only serves as a precursor for in situ growth of Cu2O nanowire array (Cu2O NA), but also provide a binder-free support substrate for in situ growth of plasmonic Au nanocrystals; (3) its open interconnected porous structure in all directions significantly benefits mass transport in comparison to plane Cu foil/sheet. Scheme 1 presents the strategy of fabricating the binder-free 3D Cu2O@Au NA photoelectrode based on the 3D CF substrate. Briefly, a 8

pristine CF was first electrochemically anodized in 3M NaOH aqueous solution and the uniform Cu(OH)2 nanowire array (Cu(OH)2 NA) was formed on the CF surface. Then the Cu(OH)2 NA on the CF was converted to Cu2O NA through thermal treatment. Lastly, the plasmonic Au was in situ deposited onto the Cu2O NA surface carefully to fabricate the 3D Cu2O@Au NA by soaking Cu2O NA into the HAuCl4 solution and using NaBH4 as reductant (details seen in the Experimental Section).

Scheme 1. Schematic diagram of the 3D Cu2O@Au NA fabrication procedure. (a) CF, (b) Cu(OH)2 NA, (c) Cu2O NA and (d) Cu2O@Au NA.

The morphology of the as-prepared samples was characterized by scanning electron microscopy (SEM). The CF substrate shows a 3D macroporous continuous structure (Figure S3, Supporting Information), which would be beneficial for mass transfer and improving the charge transfer efficiency. Figure S4A~S4C (Supporting Information） and S5A~5C (Supporting Information) display the images of Cu(OH)2 NA and Cu2O NA samples with different scales, respectively. It is found that the prepared Cu(OH)2 sample uniformly cover every individual copper wire of the CF substrate and exhibits a nanowire array (NA) structure, with diameters of 100-300 nm and lengths of 5-10 μm. After annealing treatment, the resulting Cu2O sample inherits the NA structure 9

from the transformation of Cu(OH)2. For the image of the Cu2O@Au NA sample (Figure 1A), it can be seen that the surface become coarse compared to that of Cu2O NA and it still remains the NA structure after the in situ deposition of Au. Clearly, Figure 1B indicates that the Au nanocrystal is successfully loaded on the Cu2O nanowire surface. The transmission electron microscopy (TEM) image (Figure S6) also demonstrates the in situ formed Au nanocrystals anchored on Cu2O nanowire substrate. Moreover, the corresponding EDS element-mapping data (Figure 1C) presents a single Cu2O@Au nanowire and confirms that it is wrapped by the Cu, O and Au element. These results suggest that the 3D nanowire array structure has been successfully fabricated by the in situ preparing process.

Figure 1. SEM images of Cu2O NA (A) and Cu2O@Au NA (B). (C) Element mapping images of Cu2O@Au NA.

The crystal structures of the as-prepared Cu(OH)2 NA, Cu2O NA and Cu2O@Au NA were subsequently characterized by X-ray diffraction (XRD). As shown in Figure 2A and S7 (Supporting Information), the diffraction peaks at 43.3°, 50.4° and 74.1° in 10

the XRD patterns of these three samples all come from the CF substrate (JCPDS 04-0836). For Cu(OH)2 NA and Cu2O NA samples, the XRD patterns match well with the standard phase of orthorhombic Cu(OH)2 (JCPDS 13-0420) and cubic Cu2O (JCPDS 05-0667), respectively. The results indicates that the Cu(OH)2 was successfully converted to Cu2O with high purity after thermal treatment. Besides the peaks from Cu2O, a new diffraction peak at 38.5° that can be assigned to the (111) of Au nanocrystals (marked in red circle) was detected in the XRD pattern of Cu2O@Au NA sample (Fig. 2A). Such result suggests the successful in situ deposition of Au onto the Cu2O NA. Raman spectra (Figure S8, Supporting Information) also confirms that the Cu2O NA is well retained and no inpurity phase presents after the deposition of Au nanocrystals. To probe the optical properties, the UV-vis absorption spectra derived from the diffuse reflectance data according to the Kubelka-Munk theory were performed (Figure 2B and 2C). As shown, the Cu2O@Au NA sample can achieve enhanced light absorption especially in the visible light region compared to the Cu2O NA sample. It means a better light harvesting ability of Cu2O@Au which would benefit the photocathode performance. Moreover, the spectrum for the Cu2O@Au NA sample shows a distinct SPR longitudinal band of Au at 748 nm (donated as red circle), which is consistent with the band of Au directly deposited on the CF. The above result also demonstrates the successful depositon of Au on the Cu2O semiconductor. X-ray photoelectron spectroscopy (XPS) characterizations were conducted to reveal the existence of strong electronic interaction between Au and Cu2O. As shown in Figure 2D, the high-resolution Cu 2p XPS spectra displays that the electron binding energy of Cu 2p decreased 2.25 eV after in situ depositing Au onto the Cu2O NA. Meanwhile, the Au 4f peaks of the Cu2O@Au NA (Figure S9, Supporting Information) are positively shifted compared to those of the pure Au 11

nanoparticles[26]. These results confirm the electron transfer from Au to Cu2O NA, possibly contributing to activate the conduction band of Cu2O. Therefore, the Cu2O@Au NA has the potential to make excellent effect on the cathodic reduction reaction of the photocathode in VLA-MFC.

Figure 2. (A) XRD pattern of the Cu2O NA (a) and the Cu2O@Au NA (b). (B) Diffuse reflectance spectra and (C) Absorption spectra derived from diffuse reflectance via Kubelka-Munk theory of the Cu2O NA (a), the Cu2O@Au NA (b) and the Au nanocrystals deposited CF (c). (D) High-resolution Cu 2p XPS spectra of Cu2O NA (a) and Cu2O@Au NA (b).

To verify the as-mentioned hypothesis, the synthesized Cu2O@Au NA sample was directly used as a photocathode, and its cathodic reduction activity toward the K3[Fe(CN)6] electron acceptor was explored by a liner scan voltammetry (LSV) analysis. For comparison, the precursor Cu2O NA photoelectrode and the traditional 12

light-unresponsive electrodes, including the CF substrate electrode and the commercial carbon paper (CP) electrode, were also tested under identical conditions. As shown in Figure 3A, both the Cu2O NA electrode and the Cu2O@Au NA electrode present significant reduction activity toward the electron acceptor in the region from 0.2 V to -0.4 V (vs. SCE) (curve c-f), on the contrast, the traditional light-unresponsive electrodes (CF electrode and CP electrode, curve a and b) only exhibit little activity and low current density toward the cathodic reduction even though in the light illumination condition. The results indicate the remarkable activity of the p-type semiconductor photoelectrodes which would be beneficial to boost the cathodic reduction reaction. Moreover, for the Cu2O NA electrode and the Cu2O@Au NA electrode, it can be seen that upon visible-light illumination (curve e and f), the cathodic reduction ability of the two photocathodes is both improved compared to that under dark condition (curve c and d), this can be evidenced by the enhanced reduction current density over the entire potential region and the positively shifted onset potential of the Cu2O NA electrode (from 0.185 V to 0.234 V vs. SCE) and the Cu2O@Au NA electrode (from 0.180 V to 0.232 V vs. SCE) towards the reduction of electron acceptors. The results suggest that the visible light can be efficiently introduced into the system to promote the reduction of electron acceptors because of the utilization of p-type semiconductor of Cu2O. To further evaluate the effect of the in situ depositon of plasmonic Au on the cathodic reduction reaction, the LSV performance of the Cu2O NA electrode and the Cu2O@Au NA electrode was compared in detail under different conditions. Under no light illumination, the polarization curve of the Cu2O@Au NA electrode (curve d) along the potential scan shows slight different from that of the Cu2O NA electrode (curve c) but a little higher reduction current (7.03 mA cm-2 of curve d and 6.61 mA 13

cm-2 of curve c at –0.2 V, respectively), this slight improvement of 0.42 mA cm-2 should be attributed to the excellent electrical conductivity of Au which can be beneficial to accelerate the electron transfer and enhance the reaction current. More impressively, under the condition of the visible light illumination, the current density of the Cu2O@Au NA electrode (9.29 mA cm-2 of curve f at –0.2 V) is significantly higher than that of the Cu2O NA electrode ( 7.87 mA cm-2 of curve e at –0.2 V), this enhancement of 1.42 mA cm-2 presents a big contrast to that in the dark condition, that is to say, the cathodic reduction reaction of the electron acceptor can be meaningfully accelerated after the in situ deposition of Au onto the Cu2O NA electrode under the light illumination.

Figure 3. (A) LSV of different cathodes in a K3[Fe(CN)6] electrolyte. CF (a) and carbon paper (b) with light illumination, Cu2O NA without light illumination (c) and with light illumination (e), Cu2O@Au NA without light illumination (d) and with light illumination (f). Scan rate: 10 mV/s (B) Polarization curves and power curves of VLA-MFCs equipped with CF (a), Cu2O NA (b) and Cu2O@Au NA (c) under light illumination.

Why does the Au nanocrystals in the plasmonic-metal/semiconductor system of Cu2O@Au NA lead to the visible-light enhanced cathodic reduction reaction? To clarify the mechanism of the enhanced electricity generation between Cu2O NA and plasmon Au in light illumination condition, we further employed dark-field 14

microscopy to directly investigate the hot electron transfer process based on the SPR effect of plasmon Au. The schematic apparatus for dark-field microscopy is depicted in our previous report[27]. Figure 4A and Figure 4B demonstrate the broadband dark-field image of abundant individual Au and Cu2O@Au deposited on glass slide under the same exposure time, respectively. Red dots can be clearly observed in Au sample (Figure 4A) and the scattering spectra indicate a uniform SPR wavelength around 714 nm (Figure 4C). In contrast, for the Cu2O@Au system, the red scattering dots significantly fade (Figure 4B) and a red-shift of the SPR scattering of Au to 731 nm (Figure 4C) was detected. These results directly indicate that hot electron is exactly transferred from Au to the adjacent Cu2O owning to the SPR excitation of plasmon Au. On the basis of the above results, the underlying mechanism of the improved performance of the Cu2O@Au system under light illumination is proposed and shown in Figure 4D. Upon visible-light illumination, the Au nanocrystal essentially acts as a sensitizer for absorbing resonant photons, simultaneously generate the energetic hot electrons during the process of its SPR excitation, then the energetic hot electrons were injected into the conduction band (ECB) of the adjacent Cu2O NA. Due to the electron-injection process to the conduction band of Cu2O, the semiconductor shows an increased Fermi level (Ef) and become more inclined to combine with the electron acceptor (AOX), eventually contributing to the acceleration of the reduction reaction in cathode. On the other hand, the photoexcited holes are inclined to combine with the electrons from the bio-anode due to their positive potential, keeping the charge balance and sustaining an electric current.

15

Figure 4. Dark-field image of Au (A) and Cu2O@Au NA (B) in PBS solution. (C) SPR scattering spectra of Au in Au/ITO (a) and Cu2O@Au NA/ITO (b) in PBS solution. (D) Schematic illustration of charge carrier transfer under a visible light irradiation at Cu2O/Au interface.

Given the excellent ability to accelerate the reduction of the electron acceptor in the cathode, the Cu2O@Au NA photoelectrode is expected to be a good cathode candidate in the VLA-MFC. To confirm this hypothesis, the VLA-MFC was developed using the 3D Cu2O@Au NA as photocathode and 3D nitrogen-doped graphene self-standing sponge as anode which was reported previously in our group[6]. For comparison, other two VLA-MFCs were also constructed and measured under the same condition using the Cu2O NA electrode and the CF substrate electrode as the cathode, respectively. As shown in Figure 3B, under visible-light illumination, the maximum 16

power density (Pmax) value of the CF cathode-based MFC is only 549.1 mW m-2 (curve a), while the Cu2O NA cathode-based MFC produces a Pmax value of 2315.7 mW m-2 (curve b) due to the efficient introduce of solar light energy through the Cu2O semiconductor. More impressively, the 3D Cu2O@Au NA photocathode-based MFC produce a Pmax value as high as 2849.9 mW m-2 (curve c), which is 4.2 times and 23% higher than the two above MFCs (curve a and curve b), indicating that the synergistic effect between the Cu2O and Au nanocrystal under the light illumination condition. As summarized in Table S1 (Supporting Information), the Pmax value obtained from the Cu2O@Au nanowire array (NA) photocathode-based MFC is clearly superior than those of recently reported MFCs using the similar three-dimensional graphene-based anodes but traditional cathodes. The results fully prove that the modification of plasmon Au onto the Cu2O semiconductor cathode is a powerful strategy for promoting the performance of the VLA-MFCs derived from the SPR effect of the plasmonic-metal/semiconductors system. The reduction of internal resistance (Rint ) in the MFC can also improve the power output of the MFC. Several different measurement methods for the internal resistance of MFCs have also been suggested in Logan’s book[28], which includes polarization slope, power density peak, electrochemical impedance spectroscopy using a Nyquist plot, and current interrupt methods. Based on the estimate of the polarization slope in the linear region near the low current, the Rint of the MFCs in curve b and curve c is 143 Ω and 139 Ω, respectively, which further demonstrates that the improvement of the MFC power output indeed ascribes to the plasmon-enhanced cathodic reduction achieved from the 3D Cu2O@plasmonic Au nanowire array system, not the reduction of internal resistance in the MFC. The similar internal resistance of the two types of MFC, in which the anodic part is identical, may be because most part of the internal 17

resistance is coming from the sluggish charge transfer rate of the bio-anode. 4. Conclusion In summary, the in situ formed three-dimensional Cu2O@plasmonic Au NA was successfully synthesized and utilized as the photocathode of the VLA-MFC. Owning to the SPR effect of Au nanocrystals, the plasmon-excited hot electrons injection mechanism is found to be an efficient and promising strategy to improve the reduction ability of the photocathode, further accelerate the electricity generation of the whole VLA-MFC. The present study will provide new insight into the research of light-assisted MFCs for high power output, and more importantly, open up exciting opportunities for efficient solar energy utilization in electrochemistry energy field. Acknowledgments We gratefully appreciate financial support from the National Natural Science Foundation of China (21775067 and 21335004), the International Cooperation Foundation from Ministry of Science and Technology (2016YFE0130100), the China Postdoctoral Science Foundation (2017M621694) and the program B for Outstanding PhD candidate of Nanjing University (201801B027).

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The following are the recent personal portrait photos and biosketches of authors.

Dan Guo is currently pursuing her Ph.D. degree under the supervision of Prof. Jian-Rong Zhang and Prof. Jun-Jie Zhu at the Nanjing University, China. Her research interests include the preparation and characterization of nanomaterials and their applications in biofuel cells.

Hui-Fang Wei received her B.Sc. degree from the Henan Normal University in 2017. She is now working as a master student under the supervision of Prof. Jian-Rong Zhang and Prof. Jun-Jie Zhu in the School of Chemistry and Chemical Engineering, Nanjing University. Her research interests focus on microbial fuel cells.

Xue-Yan Yu is currently working as a master student under the supervision of 20

Prof. Li-Ping Jiang and Prof. Jun-Jie Zhu in the school of chemistry and Chemical Engineering, Nanjing university. Her interests focus on visible light assisted microbial fuel cells.

Qing Xia is currently pursuing her Ph.D. degree under the supervision of Prof. Jun-Jie Zhu in the School of Chemistry and Chemical Engineering, Nanjing University. Her research interests focus on dark-field microscopy for imaging chemical processes of single nanoparticles, single molecules and single cells.

Zixuan Chen received his Ph.D. (2015) degree from the School of Chemistry and Chemical Engineering, Nanjing University. He is currently working in Nanjing University as an associate researcher since 2016. His research interests focus on imaging techniques and biochemical analysis for single molecules, nanoparticles and cells.

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Jian-Rong Zhang was born in 1963 in Wujiang City of Jiangsu Province. He received BSc (1984), MSc (1987), and Ph.D (1990) from Nanjing University. After obtained his Ph.D, he worked at Nanjing University as a Lecturer in 1990 and a Professor in 2003. His research interests focus on biofuel cell and electroanalysis for biomolecule. He has published more than 150 papers and received a number of national and ministerial awards.

Rong-Bin Song received his BS (2010) and MS (2013) from Nanjing Tech University. After that, he started his PhD study in Nanjing University under the supervision of Prof. Jian-Rong Zhang and Prof. Jun-Jie Zhu, and received his PhD degree in 2017. During this period, he also pursued his research in Nanyang Technology University as a joint PhD student under the supervision of Prof. Qichun Zhang. Currently, he continued his post-doctoral research in Nanjing University. His research expertise involves novel functionalized nanomaterials, bacteria cell-surface coating and their applications in the microbial fuel cells.

Jun-Jie Zhu is a Professor of Chemistry at Nanjing University, China. He received his BS (1984), and PhD (1993) degrees in Chemistry from Nanjing University. After this, he started his academic career at Nanjing University in 1993. During 1998–1999, 22

he was a postdoctoral fellow in Bar-Ilan University, Israel. His current research interests are analytical chemistry and materials chemistry, which mainly focus on the study

of

nanobioanalytical

chemistry

including

nanoelectrochemistry and the fabrication of biosensors.

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bioelectrochemistry,

Highlights:

 The plasmon-induced hot-electron transfer system was found to be effective for boosting cathode performance of the light-assisted MFC and provided a promising strategy for efficient utilization of solar energy in MFCs.  The three-dimensional Cu2O@plasmonic Au nanowire array was first synthesized by in situ fabrication process and used as the photocathode for accelerating electricity generation.



A high maximum power density of 2849.9 mW m-2 is obtained in the light-assisted MFC.

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