Electromagnetic induction derived micro-electric potential in metal-semiconductor core-shell hybrid nanostructure enhancing charge separation for high performance photocatalysis

Journal Pre-proof Electromagnetic induction derived micro-electric potential in metal-semiconductor core-shell hybrid nanostructure enhancing charge separation for high performance photocatalysis Wenqiang Gao, Qilu Liu, Shan Zhang, Yuying Yang, Xiaofei Zhang, Hang Zhao, Wei Qin, Weijia Zhou, Xiaoning Wang, Hong Liu, Yuanhua Sang PII:

S2211-2855(20)30181-6

DOI:

https://doi.org/10.1016/j.nanoen.2020.104624

Reference:

NANOEN 104624

To appear in:

Nano Energy

Received Date: 21 December 2019 Revised Date:

9 February 2020

Accepted Date: 14 February 2020

Please cite this article as: W. Gao, Q. Liu, S. Zhang, Y. Yang, X. Zhang, H. Zhao, W. Qin, W. Zhou, X. Wang, H. Liu, Y. Sang, Electromagnetic induction derived micro-electric potential in metalsemiconductor core-shell hybrid nanostructure enhancing charge separation for high performance photocatalysis, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104624. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

Wenqiang Gao: Conceptualization, Methodology, Software, Writing- Original draft preparation. Investigation. Qilu Liu: Software. Shan Zhang: Writing-Review & Editing. Yuying Yang: Formal analysis. Xiaofei Zhang: Writing-Review & Editing. Hang Zhao: Visualization. Wei Qin: Formal analysis, Writing-Review & Editing. Weijia Zhou: Writing-Review& Editing, Supervision. Xiaoning Wang: Writing-Review & Editing. Hong Liu: Writing-Review & Editing, Supervision, Project administration, Funding acquisition. Yuanhua Sang: WritingReviewing and Editing, Supervision, Project administration, Funding acquisition.

Electromagnetic induction derived micro-electric potential in metal-semiconductor core-shell hybrid nanostructure enhancing charge separation for high performance photocatalysis a

a

a

a

a

a

Wenqiang Gao , Qilu Liu , Shan Zhang , Yuying Yang , Xiaofei Zhang , Hang Zhao , Wei a

Qin , Weijia Zhoub, Xiaoning Wangc, Hong Liu a

a,b,*

, Yuanhua Sanga,*

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,

China. b

Institute for Advanced Interdisciplinary Research (iAIR), University of Jinan, Jinan

250022, China. c

School of Transportation and Civil engineering, Shandong Jiaotong University, Jinan

250357, China.

Abstract The application of an external electric field is one of the most efficient approaches for photo-induced charge separation. However, the application of the electric-field- enhanced charge separation is limited in powder catalysts. In this study, the

electromagnetic

induction

derived

micro-electric

potential

in

metal-semiconductor core-shell hybrid nanostructure was used to enhance charge separation in the shell semiconductor photocatalysts. The efficiency of photocatalytic hydrogen production can be improved around 110% by utilizing a core-shell nanostructure with a permanent magnet that moves under the normal photocatalytic reactor device. This electromagnetic induction derived electric field via the metal-semiconductor core-shell structure shows efficient conversions from relative 1

motion to electric potential that provide a new opportunity to enhance photocatalytic performance with non-contacted interaction.

1. Introduction Semiconductor photocatalysis is a sustainable approach that has excellent potential in realizing effective conversion from solar energy to chemical energy [1-3]. Efficient photocatalytic performance is still a topic of significant interest in the study of photocatalysis [4-5]. Apart from the improvement in light absorption, the suppression of photo-induced charge recombination is also a challenge. Typically, the construction of a built-in electric field in a photocatalyst is the key strategy for suppressing charge recombination and promoting charge transfer [6-7]. One of the main approaches to achieving these objectives is the construction of semiconductor heterostructures based on Schottky junctions [8-10], p-n junctions [11-13], and band structure alignment [14-17], which can generate the built-in electric field at the interface of the heterostructure due to their different energy band properties that is required to achieve charge separation. Spontaneous charge separation via a built-in potential is analogous to the flow of a river based on the underlying topography. However, it is determined by the intrinsic properties of materials with relatively low efficiency. To facilitate manual charge separation, an external field is considered to realize controllable and efficient charge separation. The external electric field of an electrode is acceptable, which means that the electric field is applied through the external circuit. It provides a significant improvement in charge separation [18]. For 2

example, Fe2O3, with a low charge mobility and high electron-hole recombination rate, was improved via the application of an external electric field [19]. It must be noted that electrodes are required for the application of an external electric field. The requirement of good conductivity and semiconductor property of the photocatalyst limit the thickness of the catalyst layer on the electrodes. An external circuit is also required, which results in a complex system. These requirements constrain the loading amount of the photocatalyst in the system. Although the photoelectrochemical efficiency is always significantly improved, the total photocatalytic rate is insufficient. In contrast, for powder photocatalysts with high photocatalytic activity, the separation of charges cannot be facilitated via the connection to an external circuit. For this reason, most studies focus on the heterostructure construction to generate the built-in electric field at the interfaces. External-field-stimulated charge separation is rarely applied to the powder photocatalysts. Wireless electric field application has been a significant challenge until now. Fortunately, there exist specific nanomaterials that possess or generate electric potential under appropriate conditions [20-21]. During the combination of these nanomaterials with photocatalysts, the as-provided potential serves as the wireless electric potential supply by dispersing along with the photocatalysts in a suspension. Therefore, it can generate the electric field required to promote charge separation in a photocatalyst. For example, the spontaneous polarization of the piezoelectric materials forms a surface potential along the polarization. By introducing the photocatalyst to the surface of the piezoelectric 3

material, the photo-induced charge in the photocatalyst can be effectively separated. BaTiO3@Ag2O composite has been proposed by our group for this purpose [22]. Stimulation via external ultrasound results in surface free-charge saturation of BaTiO3 nanocubes. Thus, it shows an obvious charge separation to eliminate the self-reduction of Ag2O with improved photocatalytic performance. This is considered to be an effective strategy to improve photocatalytic performance by promoting the separation of photocatalytic charges under the action of a micro-electric potential [23-25]. Magnetic field-induced electricity is widely used in our daily lives. According to the principle of electromagnetic induction discovered by Faraday [26], metal-based nanomaterials that move perpendicular to the direction of a magnetic field generate a motion-induced electromotive force. The microscopic description is that the electron distribution in the metal is polarized due to the Lorentz force that occurs during cutting the magnetic induction lines. The Lorentz force in magnetic field mainly means the force on a charge induced by the relative motion between the charge and the magnetic field. Considering piezoelectric materials based on composite photocatalysts for which spontaneous polarization-induced potential functions as an electric field to promote charge separation, it is believed that the magnetic field derived micro-electric potential in the metal nanomaterial would also promote the separation of photoinduced charges in the photocatalyst. Therefore, we proposed a core-shell hybrid nanostructure photocatalyst that generates the micro-electric potential based on the electromagnetic induction in the core structure and promotes 4

the separation of photo-generated charges in the shell structure. Photocatalytic hydrogen (H2) production was investigated to evaluate the variation of the photocatalytic property. Briefly, gold nanorods (Au NR) were used as a metal core to realize the magnetic field derived micro-electric potential. CdS nanoparticles (NP) were loaded on Au NR to form the core-shell nanostructure. Enhancement of photocatalytic H2 production was achieved by utilizing a rotating magnet under the photocatalytic reactor. The mechanism of the improvement is also discussed. This work further extends the strategy of using an external field to facilitate the separation of photo-induced charges, which can be applied to a wide range of nanocomposite of semiconductor photocatalysts.

2. Results and discussion

Figure 1. (a) XRD pattern of the Au nanorod (NR)-CdS nanoparticles (NP). (b) High-resolution Cd 3d XPS spectra of pure CdS and Au NR-CdS NP and (c) Au 4f XPS spectra of Au NR and Au

5

NR-CdS NP. (d) UV-vis absorption spectra of pure CdS, Au NR and Au NR-CdS NP. (e-f) TEM images of the Au NR. (g-h) TEM image, High-resolution TEM image and FFT image of the Au NR-CdS NP. (i) EDS elemental mapping of Au, Cd, S, and Mix, respectively.

The hybrid nanostructure and morphology of Au NR-CdS NP were investigated based on XRD pattern, XPS spectrum, UV-vis absorption spectra, HRTEM images, FFT image and EDS elemental mapping (Figure 1). The XRD pattern presented in Figure 1(a) shows the diffraction peaks corresponding to the hexagonal greenockite CdS phase (JCPDS No .41-1049) and the face-centered cubic Au phase (JCPDS No. 65-2870). The XPS survey spectra of the Au NR-CdS NP core-shell nanocomposite confirm the presence of Cd, S, and Au (Figure S1). Analysis of the high-resolution Cd 3d and Au 4f XPS spectra illustrates that only the characteristic peaks attributable to Cd2+ and Au0 are detected (Figure 1 (b,c)). This indicates that no obvious defect is detected in the as-obtained CdS NP, as well as the stable metallic character of Au. A new phase/structure of Au NR-CdS NP core-shell nanocomposite was not formed during the synthesis, which guarantees the individual properties of the Au NR and CdS NP. Moreover, the electronic state of Cd is essentially unchanged for the pure CdS and Au NR-CdS NP as shown in Figure 1(b). However, the CdS NP has an obvious effect on the electronic state of Au NR, showing as a higher FWHM of the XPS spectra for Au NR as shown in Figure 1(c). It indicates that the CdS coating on the surface of the Au NR causes the surface deformation of the Au NR. On the other hand, it also proves the successful synthesis of Au NR-CdS NP nanocomposite. Different morphologies of Au nanoparticles also correspond to different light 6

absorption ranges. The light absorption band of the as-synthesized Au NR lies at approximately 790 nm (Figure 1d), whereas that of the Au NR-CdS NP core-shell nanocomposite lies at approximately 1050 nm. This redshift can be attributed to the increase of the refractive index of the medium around the Au NR after the formation of the Au NR-CdS NP nanocomposite [27]. It further demonstrates the successful loading of CdS on the surface of the Au NR. In addition, the Au NR-CdS NP core-shell nanostructure exhibits the characteristic absorption of CdS at 560 nm, which is consistent with the absorption characteristics of pure CdS. It is notable that the light absorption at approximately 1050 nm is transformed into thermal energy inside the photo-induced hot electrons, and has little effect on water splitting [28-31]. Therefore, it is preferable that the photo-induced charge carriers generated in CdS play a dominant role in the photocatalytic H2 production of the Au NR-CdS nanocomposite. The Au NR is uniform with a length of 60-80 nm and up to tens of nanometers in diameter, and the surface of Au NR is relatively pristine (Figure 1 (e,f)). Figure 1 (g,h) confirms that the uniform Au NR-CdS NP core-shell nanostructures are successfully synthesized. The length of the composite does not change much and remains between 70 nm and 90 nm, however, the diameter increases to 40 nm. Furthermore, due to the surface area is much higher than the tip for the Au NR, which is more easily to deposit CdS by cadmium thiobenzoates at a certain temperature. Hence, CdS layer is thicker around the sides of the Au nanorodes compared to their tip. The FFT images of the as-obtained CdS NP confirm the polycrystalline cubic structure which correspond to the (100) and (101) planes, respectively. The good 7

crystalline structure of the CdS that coat the Au NR is further confirmed by the HRTEM images, which would assist in the reduction of the photo-induced charge carrier recombination during transport. The energy-dispersive X-ray (EDX) elemental mappings intuitively show that the core is composed of Au and the shell is composed of Cd and S (Figure 1i and Figure S2). Apart from the Au metalcore in the CdS based core-shell structure, we also investigated the use of the non-metal nanorod as the core. The material characterization of the hydroxyapatite (HAP)-CdS NP is shown in detail in the Supporting Information (Figure S3 and S4).

Figure 2. (a) A schematic diagram of the experimental setup showing the magnetic field-assisted photocatalytic H2 production system. (b) Photocatalytic H2 production with no magnetic field (NMF) or with a magnetic field (MF) in the presence of CdS NP, HAP-CdS NP and Au NR-CdS NP under 160 mW cm-2 Xe lamp illumination. (c) Photocatalytic H2 production

8

for different magnetic induction intensities in the presence of Au NR-CdS NP under 160 mW cm-2 300 W Xe lamp illumination. (d) Photocatalytic H2 production under different light intensities of 300 W Xe lamp illumination in the presence of Au NR-CdS NP with NMF and MF.

The photocatalytic activity is limited by the charge recombination in the catalyst; however, performing direct analysis of the fate of the photo-induced charges is difficult. Fortunately, photocatalytic H2 production with the Au NR-CdS NP core shell can be performed with and without a magnetic field (MF and NMF, respectively) to indirectly evaluate the effects of the magnetic field on the process of photocatalysis (Figure 2). To realize the cutting of the magnetic induction line in the external magnetic field of the photocatalysts dispersed in the photocatalytic hydrogen production reactor, a magnetic field-photocatalytic reaction setup was designed as illustrated in Figure 2(a). The intensity of the magnetic field was measured using a Gauss meter; intensities of 810 and 510 Gauss were defined as large and small magnetic fields, respectively, as shown in Figure S5-6. Figure 2b shows the photocatalytic performance of the different nanocomposites such as the CdS NP, HAP-CdS NP, and Au NR-CdS NP core-shell nanostructures in the large magnetic field and non-magnetic field using the magnetic photocatalysis setup. The photocatalytic H2 production rates of the CdS NP and HAP-CdS NP are 70.6 and 75.6 µmol g-1 h-1, respectively, at an NMF under 160 mW cm-2 of the Xe lamp illumination. These rates exhibited little change as 72.5 and 82 µmol g-1 h-1 for large MF (810 Gauss) in the case of the CdS NP and HAp NR-CdS NP (Figure S7). For the Au NR-CdS NP core-shell nanostructure as photocatalysts for photocatalytic H2 9

production, the H2 production rate is 105 µmol g-1 h-1, which is higher than that of the individual CdS NP (60 µmol g-1 h-1). This is likely due to the heterostructure that improves the photo-generated charge separation. Surprisingly, photocatalytic H2 production of Au NR-CdS NP yielded an improvement of 110% for a large MF (222.8 µmol g-1 h-1) compared to the NMF (105 µmol g-1 h-1) for the same reaction conditions. In terms of the material structure, the main difference between the Au NR-CdS NP and the HAP-CdS NP is the metallic conductor. Therefore, it is predicted that the different response of the Au NR and HAP NR in a magnetic field that cutting the magnetic induction line is likely the key reason for the significant improvement. Electrons moving in a magnetic field sustain the Lorenz force. The free electrons in the metallic conductor form a polarized distribution driven by the Lorenz force that generates a motion-induced electromotive force. Without free electron, the HAP NR cannot generate an electromotive force. This force is likely a key reason for the improvement of the photocatalytic property of CdS. To critically evaluate the suggested reasons for the improved photocatalytic performance under MF conditions, the photocatalytic activities of the Au NR-CdS NP core-shell nanostructure were assessed at different MF intensities (Figure 2(c)). At the same stirring speed (350 rpm) and light intensity (160 mW cm-2), the photocatalytic H2 production rates for the Au NR-CdS NP core-shell nanostructure are 105, 176, and 223 µmol g-1 h-1 for the conditions of NMF, small MF, and large MF, respectively. The significant improvement in the photocatalytic property is highly correlated to the ⇀

increase of the magnetic induction intensity (B). 10

Notably, higher light intensities can stimulate more photoinduced charge carriers in semiconductor photocatalysts. The effects of MF on additional charge carriers would be more significant and easily identifiable. Hence, the photo-induced charge carriers influenced in MF was further investigated for various light intensities as exhibited in Figure 2(d). Photocatalytic H2 production with Au NR-CdS NP core-shell nanostructure with NMF and a large MF for illumination by a Xe lamp were obtained using various light intensities of 80, 120, and 160 mW cm-2. The H2 production rates are 61, 87 and 105 µmol g-1 h-1 with NMF for 80, 120 and 160 mW cm-2 light illumination. This result shows that the photocatalytic performance is improved by ~43% and ~72% when the light intensity increases by 50% and 100%, respectively. It is generally consistent with the photocatalytic mechanism that more photo-induced charges with higher light intensity would result in higher catalytic performance [32-33]. By introducing the large MF condition, the photocatalytic H2 production rates are 80, 142 and 223 µmol g-1 h-1 under 80, 120 and 160 mW cm-2 light illumination, respectively. There were improvements of 31%, 63% and 112% for the large MF condition compared to the NMF condition for 80, 120 and 160 mW cm-2 light illumination, respectively. It is noteworthy that with the increase of the light intensity, the improvement of the photocatalytic H2 production is more evident. This non-linear improvement indicates two basic aspects. First, at higher photo-induced charge carrier generation, the charge utilization rate is lower due to higher charge recombination rate. Second, the presence of MF helps in charge separation and results in more charges being utilized instead of recombination. 11

Figure 3. (a) The current density of Au NR at large MF without illumination, and (b) 180 s and 200 s. (c) Time dependence of open circuit potential of Au NR at large MF without illumination, and (d) 180 s and 200 s. (e) Induced electromotive force of Au, a schematic diagram of the relative motion between the magnetic field and the Au NR, and COMSOL simulated distribution of surface-induced electromotive force of Au NR at large MF.

Based on a previous design, the metal conductor (Au NR) generated a motion-based electromotive force during movement in the magnetic field. This property plays one of the key roles in the enhancement of charge separation. Therefore, the current density or potential of Au NR in the electrochemical test during 12

cutting of the magnetic induction line at MF condition was studied (Figure 3). The experimental setup is designed as shown in Figure S9 (a). A magnetic field with a controlled rotation speed is placed at the side of the electrochemical testing device. The current density and open circuit potential (OCP) with on-off rotation of the large MF are recorded using the magnetic electrochemical setup (Figure 3(a-d)). A peak-to-peak current density of 2 µA cm-2 is recorded and the current density of two cycles is consistent for the Au NR loaded electrode. However, there is a negligible response to the blank electrode (Figure 3 (a-b)). The current density gradually increases when the speed of rotation of the magnet increases from 0 to 350 rpm, and finally approaches a plateau. A current density is not generated in a motionless magnetic field, which indicates that there is a relationship between the electromagnetic induction current and the rotation speed. Simultaneously, the current density with on-off rotation of the small MF shows the same pattern change with that of the large MF, but with a decreased current density (Figure S8). Similarly, the OCP shows a variation up to 3 mV from 0 to 350 rpm in the magnetic field (Figure 3(c, d)). The change of the OCP can be assigned to the motion-induced electromotive force. ⇀

This force increases with the increase of the magnetic induction intensity (B) and the ⇀

rotation speed (V) of the magnetic field, in addition to the length of the Au NR ⇀

⇀

perpendicular to the plane of B×V. The distribution of the induced electromotive force generated under the MF condition was simulated using COMOSOL, and a schematic diagram illustrates the effect of the magnetic field on the moving Au NR in Figure 3e. It indicates that the 13

induced electromotive force is the highest in the middle of the Au NR and that it decreases gradually for both sides. There is almost a parallel and homogeneous electric field generated in the Au NR based on the electromagnetic induction. However, due to the accumulation of charges at both ends of the Au NR, part of the micro-potential is counteracted. The surface of the Au NR must show the electric field inside the Au NR near the surface. This surface potential works as the companion potential supply system to drive charge separation in the surface loaded CdS NP. It is notable that the electric field is determined only by the and

( =

× ), which

is the same for the Lorenz force applied to electrons moving in a magnetic field. It indicates that the morphology is not related to the electromagnetic induction derived micro-electric potential. The direction can be determined using the right-hand rule (Figure S10).

14

Figure 4. (a) Schematic diagram of the magnetic field Mott-Schotty and photocurrent density of Au NR-CdS NP core-shell nanostructure set. (b) Schematic diagram of the effect of the magnetic field on the moving Au NR-CdS NP core-shell nanostructure under illumination. (c) Mott-Schottky plots of Au NR-CdS NP core-shell nanostructure under NMF and MF conditions for non-illumination and illuminated using a 300 W Xe lamp. The electrolyte is 0.1 M Na2SO4. (e) I-t curves for the Au NR-CdS NP core-shell nanostructure and blank electrode for on-off switching of the illumination under NMF and MF (350 rpm magnet rotation) conditions.

The effect of the magnetic field on the photo-generated charge carriers in the Au NR-CdS NP core-shell nanostructure was investigated using Mott-Schottky plots, and the I-t curves for on-off switching of the illumination (Figure 4). A device similar to the electrochemical test unit (Figure 3(a)) with the Au NR-CdS NP core-shell nanostructure loaded on the F-doped tin oxide coated glass (FTO) was used to 15

investigate the PEC property as shown in Figure 4(a). Based on the experimental results in Figure 3, it is proposed that the induced electromotive force generated via Au NR at the MF can interact with the photo-induced charges of CdS via mutual attraction of positive and negative charges to improve charge separation. This will improve charge separation. A schematic diagram illustrates the effect of the magnetic field on the moving Au NR-CdS NP core-shell nanostructure under illumination in Figure 4(b). In addition, the influence of the electromotive force on the photo-induced charge carrier of CdS is further explored via photoelectrochemical tests, and the results are shown in Figure 4 (c,d). The Mott-Schottky plots describe the reciprocal relationship for the capacitance squared and the potential difference between a semiconductor and an electrolyte. The charge carrier concentration could also be analyzed using the Mott-Schottky plot [34-35]. In this regard, Mott-Schottky plots of the Au NR-CdS NP core-shell nanostructure were recorded sequentially with and without light illumination for the MF and NMF conditions (Figure 4(c)). The negative slope indicates that photo-induced electrons are the dominant charge carriers. The analyzed flat-band potentials of the Au NR-CdS NP core-shell nanostructure is similar and independent of the light or/and magnetic field conditions. According to the function, slope = 2(εε0A2eND)-1, where ε and ε0 are the dielectric constants, A represents the electrode area, and ND represents the dopant density in the semiconductor, which can be assigned to the charge carrier density. A smaller slope indicates a higher carrier density. The slopes change little under the dark with NMF and MF conditions. This indicates that the electromagnetic induced surface potential 16

of the Au NR does not influence the intrinsic charge carrier density. With light illumination, the slope is smaller compared to that of the dark in the NMF condition. This is consistent with the generation of a photo-induced charge, which results in a higher charge density. When illuminated under the MF condition, the slope decreases further. The calculated ND for the NMF condition with light illumination is 1.54×1017 cm-3, whereas the value increases to 1.91×1017 cm-3 when the MF condition is utilized. The increase in the ratio is approximately 25%, which is not consistent with improvement in excess of 100% for the rate of photocatalytic H2 generation. Given that the Mott-Schottky measurement was performed with a biased application, the photo-induced charge separation of CdS has been clearly improved due to the external electric field of the electrode. Similarly, the current density is also related to the photo-generated charge carrier density. Therefore, the I-t curves of the Au NR-CdS NP core-shell nanostructure and the blank sample (FTO) were recorded under NMF and MF conditions to analyze the variation of the charge carrier density (Figure 4d). The photocurrent densities of the blank sample are the same at 1.5 µA cm-2 at a bias of 0.2 V for both NMF and MF (350 rpm) conditions. This confirms that the FTO conductive layer could not be induced to generate extra electric field under MF conditions [36-37]. However, the photocurrent densities of the Au NR-CdS NP core-shell nanostructure produced a fluctuation of 2.1-2.8 µA cm-2 at a bias of 0.2 V under MF (350 rpm) conditions. This is higher than the value of 2 µA cm-2 under NMF conditions. The Au NRs loaded electrode generates an additional bias and current (Figure 3). The dark currents of the Au NR-CdS NP core-shell (Figure 4d) are 17

the same as the pure Au NR (Figure 3a) at both the NMF and MF conditions. This implies that the photo-current improvement is related to the electromagnetic induction current. Therefore, the improvement is likely associated with the enhanced charge separation of the CdS NP due to the external electric field of Au NR by magnetic field driven. It is noted that the minimum photocurrent for the CdS at MF conditions is similar to that of the NMF condition. This is consistent with the periodic behavior of the motion-induced electromotive force of Au NR under the MF condition (Figure 3b). It also implies that the directivity of the electromotive force of Au NR under the MF condition does not influence the improvement of the photo-induced charge separation in the CdS NP. In addition to the generation of the motion-induced electromotive force of Au NRs cores, it could improve the photo-induced charge separation of CdS shell.

18

Figure 5. (a) Schematic diagram of CdS structure. Electron density difference maps before and after the application of an external electric field in different directions and the calculated bond distance of CdS (b) without an external electric field. (c) Application of an external electric field (0.05 V m-1) in the direction of an axis of CdS. (d) Application of an external electric field (0.05 V m-1) in the direction of the c axis of CdS. (e) The separation of electrons and holes in CdS nanoparticles attached to the two opposite surfaces of an Au NR with opposite polarization charges due to the electromagnetic induction effect (f) Schematic diagram of the enhancement of photocatalysis with the metal-semiconductor core-shell structure in a magnetic field.

19

To gain insight into the charge transfer in the CdS NP based on the Au NR-CdS NP core-shell nanostructure at MF, the electronic structure of the CdS was analyzed with and without the application of an electric field, instead of magnetic field influence. The initial structure of the hexagonal greenockite CdS is shown in Figure 5(a). The motion-induced electromotive force on the Au NR induced under the MF condition was applied to the CdS NP. An electric field of 0.05 V m-1 was used to study the effect of the electromotive force on the electronic structure of CdS. As shown in Figure 5(b), initially, there are gaps along the c-axis and a-axis. This indicates that the transport path of the charge carrier is blocked in some directions. With the electric field applied along the a-axis, the isolated charge density merges at the a-axis (Figure 5(c)). The formation of the path allows the charge transfer to be free of the direction limitation. Similarly, with the electric field applied along the c-axis, the isolated charge density merges with the other part at the c-axis (Figure 5(d)). The average bond distances of the CdS are 1.37 Å and 1.35 Å with an external electric field along the a-axis and c-axis, respectively. They are all larger than that of the initial value (1.33 Å). Larger bond distance implies a weaker locality of the valance electrons, which would improve its transport. The effect of the motion-induced electromotive force on the separation of the photo-induced charge carrier is illustrated in Figure 5(e). For the Au NR-CdS NP core-shell nanostructure, the Lorentz force is produced inside the Au NR through the metal (Au NR) and cuts the magnetic induction line at the rotating magnetic field. Then the electrons inside the Au NR are polarized with a distribution along the 20

direction of the Lorentz force. The polarized electron distribution results in a charge concentration at the surface. Thus, there is a space electric field caused by the charge concentration at the surface. This could drive the photo-induced electrons and holes to move towards opposite directions, realizing enhanced charge separation. The separation of charge by motional electromotive force is the main aspect of the enhancement of the photocatalytic performance, which is illustrated in Figure 5(f). The Au NR core functions as the non-contacted relative motion and electric potential converter. In addition, it plays a role as the companion micro-electric potential, and generates an external electric field to drive charge separation. By combining the metal-semiconductor photocatalyst core-shell nanostructure with the external magnetic field, a companion micro-electric potential can be introduced in the photocatalytic material, which results in the efficient separation of the electron-hole pairs in the photoelectric conversion process of metal-semiconductor core-shell hybrid materials. In particular, for a core-shell companion micro-electric potential system, it is not only possible for Au NR to generate a polarized electric field by cutting the magnetic induction lines at the magnetic field, but it also has broad applicability for Au nanoparticles of a certain size, and is not limited to a special morphology. This is because the perspective of electron-polarized distribution induced by a Lorenz force, for a metal conductor with any morphology less than a specific size, there would be a concentration of surface charge with the generation of a space electric field. Considering the motion-induced electromotive force, all the metal cores can be equivalent to the metal rod at each cross-section to generate a polarized electric field 21

by cutting the magnetic induction lines in the magnetic field (details in Figure S11 of the Supporting Information). Moreover, it is proposed that the electric field is determined only by the and

(

=

×

), which indicates the similar

improvement with different morphology of core structure. Therefore, Au NP-CdS NP core-shell nanostructures were also prepared to evaluate the photocatalytic H2 production under the MF condition (Figures S12-S14 in the Supporting Information), and the results confirm that the photocatalytic performance is also improved under this condition. Moreover, other metals could also work well with this construction, such as Ag and Mo, etc. 3. Conclusion In summary, the significant effect of an external magnetic field with respect to improving

photocatalytic

H2

production

was

demonstrated

with

the

metal-semiconductor core-shell hybrid nanostructure. With the Au NR-CdS NP core shell hybrid nanostructure, a 110% enhancement of photocatalytic H2 production was achieved by applying a moving magnetic field under the photocatalytic hydrogen production reactor. The enhancement should be due to the electromagnetic induction derived micro-electric potential of the Au NR core, which works as the external electric field driving the photo-induced charges in the CdS shell moving in the opposite directions. A metal-semiconductor core-shell structure is proposed as the composite photocatalyst with a micro-electric potential supply inside during the relative motion in the magnetic field. This hybrid nanostructure provides an efficient route to apply external electric potential on powder photocatalyst wirelessly, thereby improving its performance. Therefore, for specific metal-semiconductor core-shell structures, many kinds of photocatalysts can enhance the photocatalytic properties 22

due to the influence of a magnetic field. 4. Experimental procedures 4.1. Materials All the chemicals used in this work were analytic grade and commercially available. Gold trichloride (HAuCl4·3H2O), sodium borohydride (NaBH4), hexadecyl trimethyl ammonium bromide (CTAB), silver nitrate (AgNO3), L-ascorbic acid (AA), hydrochloric

acid

(HCl),

thiobenzoic

acid

(PhCOSH),

sodium

carbonate

(Na2CO3·10H2O), cadmium chloride (CdCl2·2.5H2O), nickel chloride (NiCl2·4H2O), sodium sulfite (Na2SO3, 99.5%), sodium sulfide (Na2S, 99.5%), were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals and Alfa Aesar were used without further purification. 4.2. Preparation Preparation of Au nanorod and metal thiobenzoates The initial Au NRs were prepared using a seed-growth method and Metal thiobenzoates were synthesized via the reaction of sodium thiobenzoate with metal nitrates in an aqueous solution to produce precipitates that correspond to the previously reported [38-39]. (See Supporting Information). Preparation of Au NR-CdS core-shell nanostructures The gold nanorod (AuNR) dispersion (28 mL) was mixed with sodium hydroxide (1 M, 560 µL). The resulting solution was washed via centrifugation at 10000 rpm for 15 min and then dissolved in CTAB solution (0.1 M, 20 mL). (PhCOS)2Cd (20 mg) was then added while magnetic stirring was performed at room temperature until (PhCOS)2Cd was completely dissolved. AgNO3 (0.01 M 200 µl) solution was then added to the solution and stirred for 20 min. The solution was placed in a Teflon-lined, 50-mL autoclave and heated at 140 °C for 3 h. The final product was centrifuged and 23

washed prior to further use. For the preparation of CdS NPs, Au NRs and AgNO3 do not need to be added in the aforementioned synthesis steps [27]. In this reaction system, CTAB can guarantee the dispersion of Au NR, and AgNO3 ensures that metal thiobenzoates can be adhered to the surface of Au NRs. Moreover, the thickness of the shell can be adjusted by changing the amount of metal thiobenzoates. Preparation of hydroxyapatite (HAP)-CdS nanoparticles nanocomposites HAP nanobelts were synthesized using the hydrothermal method and loaded with CdS nanoparticles using the chemical precipitation method. See Supporting Information for detailed steps of the synthesis process [40]. 4.3. Characterization X-ray diffraction (XRD) data were collected using a powder X-ray diffractometer (Cu Kα, λ=0.15406 nm, Bruker D8 Advance, Germany) to verify the phase composition of the material. A high-resolution transmission electron microscopy (HRTEM) system equipped with an Oxford EDX analysis instrument was used to study the morphology of the different samples, including the lattice image and composition. To analyze the defects and valence state of the samples, X-ray photoelectron spectroscopy (XPS) was used (ESCALAB 250). The absorption spectra of the samples were acquired based on UV-vis spectroscopic measurements (Hitachi UV-3100). Photoluminescent spectra of the Au NR-CdS core-shell nanostructure were recorded with and without the influence of a magnetic field. The equipment was built in the lab and was based on a fiber spectrograph (NOVA-EX, 320-1113 nm, Shanghai Ideaoptics, China). The excitation wavelength was 360 nm. 4.4. Photocatalytic activity evaluation The photocatalytic activity of the different samples was tested on the basis of the photocatalytic H2 production. Aqueous suspensions of Au NR-CdS core-shell 24

nanostructure were centrifuged at 10 000 rpm for 15 min to remove excess CTAB, washed with Milli-Q ultrapure water three times, and finally redispersed in Milli-Q ultrapure water. The photocatalyst suspension was mixed with 50 ml of an aqueous solution containing 0.25 M Na2SO3, 0.35 M Na2S, and 1 mM NiCl2 as sacrificial reagents under a 300 W Xenon lamp as the light source. The temperature of the suspension system in the process of the photocatalytic H2 production was maintained at 15 °C using an external circulating water cooling system. The amount of hydrogen gas was automatically detected using a gas chromatography system (Shimadzu GC-9A) equipped with a thermal conductivity detector (TCD). The error was analyzed based on three repeated experiments. 4.5. Photoelectrochemical tests The photoelectrochemical properties of the samples were analyzed using an electrochemical workstation (CHI660C Instruments) and a standard three-electrode cell in a 1 M NaOH aqueous solution. An Ag/AgCl electrode (saturated with KCl) and a carbon electrode were applied to the reference electrode and counter electrode, respectively. On one hand, the electrochemical data obtained due to the interaction between a metal conductor (Au NR) and the magnetic field was examined. Suspensions of the Au NR in a mixture of ethanol (4 mL) and Nafion (30 µL) were well dispersed at a concentration of 4 mg mL-1. The suspensions (10 µL) were drop-casted onto the glassy carbon electrode with a working area of 0.19 cm2. The I-t curves were recorded with and without a rotating magnetic field (0-350 rpm) under the illumination of a 300 Xe lamp (100 mW cm-2) with light ON-OFF intervals of 50 s and an applied bias of 0.2 V. Open circuit voltages were also recorded for rotating magnet ON-OFF intervals of 50 s. On the other hand, the photoelectrochemical of Au NR-CdS core-shell nanostructure was detected by preparing suspensions of the 25

sample on FTO glass as the work electrodes. The interaction between the static sample and the rotating magnet was realized by designing a new apparatus (see the Supporting Information Figure S9 (b)). I-t curves were recorded with or without a rotating magnetic field (0–350 rpm) under the illumination of a 300 Xe lamp (100 mW cm-2) with light ON-OFF intervals of 50 s and an applied bias of 0.2 V. Mott-Schottky plots of Au NR-CdS core-shell nanostructures for NMF or rotating MF (350 rpm) were recorded at 1000 Hz for illumination by a 300 Xe lamp (100 mW cm-2). The electrolyte was 0.1 M Na2SO4. The carrier densities in the Au NR-CdS core-shell nanostructure were calculated under various conditions. 4.6. Theoretical calculations Based on density functional calculations using the CASTEP package [41-43], the first-principles method was utilized to calculate the electronic structures of CdS under various electronic fields. The functional developed by Perdew, Burke, Ernzerhof, (PBE) [44] for a generalized gradient approximation (GGA) [45] was adopted to describe the exchange-correlation energy. An optimized ultra-soft pseudopotential [46] was used to model the effective interaction between the atomic core and valence electrons, in which Cd 4d105s2 and S 3s23p4 electrons were treated as valence electrons. The Brillouin zones were set as 4 × 4 × 2 with a separation of Monkhorst-Pack [47] k-point sampling of 0.07 Å -1. To achieve energy convergence, the plane-wave cut-off energy was 300 eV. The induction current density of Au NR simulations is obtained using two-dimensional Maxwell calculations based on a finite element method solver (COMSOL). One unit cell consists of a plane with length and width of 60 nm and 10 nm, respectively, and it was modeled using periodic boundary conditions. Acknowledgement 26

The authors would thank for the calculation supporting from Prof. Mingwen Zhao of Shandong University. And the authors wish to acknowledge the funding of National Key Research and Development Program of China (2017YFE0102700), National Natural Science Foundation of China (Grant Nos. 51732007), Major Innovation Projects in Shandong Province (2018YFJH0503), the Science Fund for Distinguished Young Scholars of Shandong Province （ ZR2019JQ16 ） and the Fundamental Research Funds of Shandong University (2018WLJH64). Thanks for the supporting from Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong.

Appendix A. Supplementary data References [1] Z. G. Zou, J. H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625-627. [2] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253-278. [3] X. C. Wang, K. Maeda, A. Thomas, K. Domen, M. Antonietti, Nat. mater. 8 (2009) 76-80. [4] L. Liao, Q. H. Zhang, Z. H. Su, S. Baldelli, J. M. Bao, Nat. Nanotechnol. 9 (2014) 69-73. [5] J. Liu, Y. Liu, N. Y. Liu, J. Zhong, Z. H. Kang, Science 347 (2015) 970-974. [6] C. Ye, J. X. Li, Z. J. Li, C. H. Tung, L. Z. Wu, ACS Catal. 5 (2015) 6973-6979. [7] J. X. Low, J. G. Yu, Adv. Mater. 29 (2017)1601694. [8] S. Bai, X. Y. Li, Q. Kong, Y. J. Xiong, Adv. Mater. 27 (2015) 3444-3452. [9] J. D. Xiao, L. L. Han, H. L. Jiang, Angew. Chem. Int. Edit. 57 (2018) 1103-1107. [10] Y. Tian, T. Tatsuma, J. Am. Chem. Soc.127 (2005) 7632-7637. [11] M. Yan, Y. Q. Hua, F. F. Zhu, H. Q. Shen, Appl. Catal. B 202 (2017) 518-527. 27

[12] F. K. Meng, J. T. Li, S. K. Cushing, M. J. Zhi, N. Q. Wu, J. Am. Chem. Soc. 135 (2013) 10286-10289. [13] S. Ida, A. Takashiba, S. Koga, H. Hagiwara, T. Ishihara, J. Am. Chem. Soc. 136 (2014) 1872-1878. [14] H. J. Li, W. G. Tu, Y. Zhou, Z. G. Zou, Adv. Sci. 3 (2016) 1500389. [15] Z. Y. Zhan, J. D. Huang, Y. R. Fang, M. Y. Zhang, K. C. Liu, B. Dong, Adv. Mater. 29 (2017) 1606688. [16] P. Zhou, J. G. Yu, M. Jaroniec, Adv. Mater. 26 (2014) 4920-4935. [17] Q. Wang, T. Hisatomi, Q. X. Jia, H. Tokudome, M. Zhong, C. Z. Wang, Z. H. Pan, T. Takata, K. Domen, Nat. Mater. 15 (2016) 611-615. [18] J. Li, L. J. Cai, J. Shang, Y. Yu, L. Z. Zhang, Adv. Mater. 28 (2016) 4059-4064. [19] X. N. Wang, W. Q. Gao, Z. H. Zhao, Y. H. Sang, H. Liu, Appl. Catal. B 248 (2019) 388-393. [20] Z. L. Wang, J. H. Song, Science 312 (2006) 242-246. [21] C. Zhang, W. Tang, C. B. Han, F. R. Fan, Z. L. Wang, Adv. Mater. 26(2014) 3580-3591. [22] H. D. Li, Y. H. Sang, H. Liu, Z. L. Wang, Nano lett. 15 (2015) 2372-2379. [23] H. W. Huang, S. C. Tu, C. Zeng, T. R. Zhang, A. H. Reshak, Y. H. Zhang, Angew. Chem. Int. Edit. 56 (2017) 11860-11864. [24] C. F. Tan, W. L. Ong, G. W. Ho, ACS Nano 9 (2015) 7661-7670. [25] X. Y. Xue, W. l. Zang, P. Deng, Q. Wang, Z. L. Wang, Nano Energy 13 (2015) 414-422. [26] I. Galili, D. Kaplan, Y. Lehavi, Am. J. phys. 74 (2006) 337-343. [27] H. H. Wang, Z. H. Sun, Q.H. Lu, F. W. Zeng, D. S. Su, Small 8 (2012) 1167-1172. [28] M. Li, X. F. Yu, S. Liang, X. N. Peng, Z. J. Yang, Y. L. Wang, Q. Q. Wang, Adv. Funct. Mater. 28

21 (2011) 1788-1794. [29] M. L. Brongersma, N. J. Halas, P. Nordlander, Nat. Nanotechnol. 10 (2015) 25-34. [30] Y. Shi, J. Wang, C. Wang, T. T. Zhai, W. J. Bao, J. J. Xu, X. H. Xia, H. Y. Chen, J. Am. Chem. Soc. 137 (2015) 7365-7370. [31] K. F. Wu, W. E. R. Córdoba, Y. Yang, T. Q. Lian, Nano lett. 13 (2013) 5255-5263. [32] Z. F. Jiang, H. l. Sun, T. Q. Wang, T. C. An, H. J. Zhao, J. G. Yu, P. K. Wong, Energ. Environ. Sci. 11(2018) 2382-2389. [33] W. X. Yang, R. Godin, H. K. B. Moss, Y. F. Dong, S. A. J. Hillman, L. Steier, E. Reisner, J. R. Durrant, J. Am. Chem. Soc. 141 (2019) 11219−11229. [34] G. Horowitz, J. Electroanal. Chem. 159 (1983) 421-436. [35] K. Gelderman, L. Lee, S. W. Donne, J. Chem. Educ. 84 (2007) 685-688. [36] J. E. Hirsch, Phys. Rev. Lett. 83 (1999) 1834-1837. [37] K. S. Novoselov, Z. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan, G. S. Boebinger, P. Kim, A. K. Geim, Science 315 (2007) 1379-1379. [38] Z. H. Sun, Z. Yang, J. H. Zhou, M. H. Yeung, W. H. Ni, H. K. Wu, J. F. Wang, Angew. Chem. Int. Ed. 48 (2009) 2881-2885. [39] W. H. Ni, X. S. Kou, Z. Yang, J. F. Wang, ACS Nano 2 (2008) 677-686. [40] B. J. Ma, S. Zhang, J. C. Qiu, H. Liu, Nanoscale 8 (2016) 11580-11587. [41] J. Clark et al., Zeitschrift Fur Kristallographie 220 (2005) 567-570. [42] W. Kohn, L. J. Sham, Phys. Rev. 140 (1965) 1133-&. [43] M. C. Payne, M. P. Teter, D. C. Allan, T. A. Arias, J. D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045-1097. 29

[44] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865-3868. [45] J. P. Perdew, Y. Wang, Phys. Rev. B 46 (1992) 12947-12954. [46] D. Sanchezportal, E. Artacho, J. M. Soler, Solid State Commun. 95 (1995) 685-690. [47] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 13 (1976) 5188-5192.

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1. A magnetic-field-induced electromotive force was used to promote photo-induced charge separation for powder photocatalysts. 2. The efficiency of photocatalytic hydrogen production can be improved around 110% via a metal-semiconductor core-shell nanostructure in an external magnetic field. 3. Provide a new strategy to enhance photocatalytic characteristics based on non-contacted interaction.

Wenqiang Gao obtained his B.E. degree from University of Jinan in 2016. Currently, he studies as a doctor student in Prof. Hong Liu’s group in State Key Laboratory of Crystal Materials, Shandong University, China since September 2016. His research interests are synthesis of nanomaterials and their application in energy area, including photocatalysis and solar electricity generation.

Qilu Liu is a master student in Prof. Hong Liu's group in State Key Laboratory of Crystal Materials, Shandong University, China since September 2018. His research interests are structure and properties of lithium niobate crystals, ferroelectric and piezoelectric materials.

Shan Zhang obtained her B.E. degree from Shandong University in 2015. Currently, she studies as a doctor student in Prof. Hong Liu’s group in State Key Laboratory of Crystal Materials,

Shandong University, China since September 2015. Her research interests are synthesis of nanomaterials and their application in tissue engineering cell imaging, and drug delivery. .

Yuying Yang studies as a doctor student in Prof. Wei Qin’s group in school of physics, Shandong University, China. Her current research is mainly focused on spin-related properties of organic charge transfer crystals.

Xiaofei Zhang obtained his B.E. degree from Shandong University in 2015. Currently, he studies as a doctoral student in Prof. Hong Liu’s group in State Key Laboratory of Crystal Materials, Shandong University, China since September 2015. His research interests are synthesis of nanomaterials and their application for solar energy conversion, including photocatalysis, solar thermal and solar thermoelectricity generation.

Hang Zhao studies as a master student in Prof. Hong Liu’s team in state Key laboratory of Crystal Materials, Shandong University, China. His current research is mainly focused on tissue engineering.

Prof. Dr. Wei Qin, is working at School of Physics as a professor in Shandong University since 2017. In 2013, he receives his PhD degree from Shandong University. Then, he joined the University of Kansas (United States) as a postdoctoral researcher, and 2015~2016, he moved to the University of Tennessee Knoxville (United States) as a research associate. Now, he mainly works on the topics of organic spintronics.

Prof. Dr. Weijia Zhou completed his Ph.D. at Shandong University in 2012. Now, Dr. Zhou is a

professor in the Institute for Advanced Interdisciplinary Research (IAIR), University of Jinan (UJN), Shandong. His research interests are related to the design and synthesis of functional materials and devices for new energy conversion and storage, including photo and electro-catalytic water splitting, CO2 reduction and supercapacitor.

Dr. Xiaoning Wang obtained her B.S. degree in Physics at Shandong University in China in 2007, and received her Ph.D degree in Materials Physics and Chemistry at Shandong University in 2019. She is now working as a lecturer in School of Transportation and Civil engineering, Shandong Jiaotong University, China. Her research focuses on nanomaterials for solar light conversion, especially for photocatalysts and their industrial application.

Prof. Dr. Hong Liu is a professor in State Key Laboratory of Crystal Materials, Shandong University. He received his Ph.D. degree in 2001 from Shandong University (China). His current research is mainly focused on chemical processing of nanomaterials for energy related

applications including photocatalysis, tissue engineering, especially the interaction between stem cell and nanostructure of biomaterials as well as the nonlinear optical crystals.

Prof. Dr. Yuanhua Sang obtained his B.S. degree at Shandong University in China in 2007 and received his Ph.D degree at Shandong University in July 2012. Now, he works as an associate professor in State Key Laboratory of Crystal Materials, Shandong University, China. His research interests are functional crystal materials for nonlinear optics, biomaterials for tissue engineering and stem cells differentiation, and nanomaterials for solar light conversion, especially for photocatalysis and photothermal application.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: