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Fabrication of MgFe2O4/MoS2 Heterostructure Nanowires for Photoelectrochemical Catalysis Weiqiang Fan, Meng Li, Hong-Ye Bai, Dongbo Xu, Chao Chen, Chunfa Li, Yilin Ge, and Weidong Shi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03887 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Fabrication of MgFe2O4/MoS2 Heterostructure Nanowires for Photoelectrochemical Catalysis Weiqiang Fan, Meng Li, Hongye Bai, Dongbo Xu, Chao Chen, Chunfa Li, Yilin Ge, Weidong Shi*

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China. *Corresponding author: Tel.: +86 511 8879 0187 Fax: +86 511 8879 1108 E-mail: [email protected]

Abstract A novel one-dimensional MgFe2O4/MoS2 heterostructure has been successfully designed and fabricated. The bare MgFe2O4 was obtained as uniform nanowires through electrospinning, and MoS2 thin film appeared on the surface of MgFe2O4 after further chemical vapor deposition. The structure of MgFe2O4/MoS2 heterostructure was systematic investigated by XRD, HTEM, XPS and Raman spectra. According to electrochemical

impedance

spectroscopy

(EIS)

results,

MgFe2O4/MoS2

heterostructure showed the lower charge-transfer resistance compared with bare MgFe2O4, which indicated that the MoS2 played an important role in the enhancement of electron/holes mobility. MgFe2O4/MoS2 heterostructure can efficiently degrade tetracycline (TC), since the superoxide free-radical can be produced by sample under illumination due to the active species trapping and electron spin resonance (ESR) measurement, and the optimal photoelectrochemical degradation rate of TC can achieved up to 92% (radiation intensity: 47 mW/cm2, 2 h). Taking account of its unique semiconductor band gap structure, MgFe2O4/MoS2 can also be used as an

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photoelectrochemical anode for hydrogen production by water splitting, and the hydrogen production rate of MgFe2O4/MoS2 was 5.8 mmol/h•m2 (radiation intensity: 47 mW/cm2), which is about 1.7 times than MgFe2O4.

Keywords: MgFe2O4/MoS2; Heterostructure; Photoelectrochemical degradation; Tetracycline; Hydrogen production


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1. Introduction Photoelectrochemistry (PEC), new emerging environment-energy technology, could effectively utilize solar energy for dye degradation and hydrogen production.[1] In PEC cell, the electrode produced the photo-generated charge and carried out the oxidation-reduction.[2] The popular matrix of photoelectrode is semiconductor, because semiconductor possesses rich band gap structure and physical and chemical stability.[3,4] In addition to the nature of semiconductor itself, the morphology of the semiconductor also plays a key role in PEC performance of photoelectrode. Since nanoscience and nanotechnology came into being, one-dimensional (1D) nanostructures have made great advances in the field of physics, chemistry, energy and biochemistry, as well as the development of novel functional materials, because 1D nanostructure takes a nature of special chemical and physical properties, which was different from the conventional materials.[5,6] Several methods have been applied to synthesize 1D functional materials, such as hydrothermal method, template method and electrospinning method.[7-11] By contrast, electrospinning was an uncomplicated and convenient method, which could be used to prepare the various anticipative 1D nanostructures (including nanowires, nanorods and nanotubes).[12] Commonly, most 1D nanostructures fabricated by electrospinning are assembled with small particles (size was from 1nm to 100 nm), and that further makes them possess higher strength, diffusivity and plasticity. Thus, 1D nanostructure with above properties has an important application in the preparation of photoelectronchemical functional materials. MgFe2O4 belongs to face-centered cubic close-packed spinel structure and n-type semiconductor.[13,14] MgFe2O4 has been investigated in-depth in the field of energy storage and conversion, due to outstanding magnetic and photoelectric property.[15]


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More importantly, the band gap of MgFe2O4 is around 1.9 eV, so MgFe2O4 can respond to both ultraviolet and visible light, which makes it became a kind of potential matrixes for preparing photocatalysis.[16] Unfortunately, the disadvantage resulting from the fast recombination of photo-generated charges cause the non ideal photocatalytic

activities

of

bare

MgFe2O4

for

degradation

or

hydrogen

production.[17,18] One of effective routes to overcome the limitation of bare MgFe2O4 is to fabricate 1D heterostruture, since the 1D heterostruture can not only accelerate directional transmission of electrons and holes, but also suppress charge recombination.[19] As previous reports, MoS2 with excellent electronic conductivity is a suitable unit for the construction of heterostructure, because MoS2 usually crystallized with two-dimensional layer structure for the benefit of close coating onside heterogeneous semiconductor.[20-23] Moreover, the narrow band gap of MoS2 can also extend the absorption range and improve the utilization efficiency of light, so various heterostructure built from MoS2 has been widely studied for photocatalysis.[24,25] Pure MgFe2O4 or MoS2 nanomaterials had been received more attentions, but the 1D heterostructure based on the two matrixes rarely researched, so the 1D composite of MoS2 and MgFe2O4 might be endowed with enhanced photoelectrochemical properties [26,27] In this work, novel co MoS2/MgFe2O4 nanowires with p-n type heterojunction had been successfully synthesized, and the obtained samples demonstrated significantly enhanced photoelectrochemical degradation and hydrogen production performance. The photoelectrochemical degradation property of MoS2/MgFe2O4 nanowires was investigated over tetracycline (TC) under irradiation. Compared with bare MgFe2O4, MgFe2O4/MoS2 (ω=10%) nanowires exhibited the best performance of


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photoelectrochemical degradation and hydrogen production. This enhancement of photoelectrochemical properties can be attributed to the formation of MgFe2O4/MoS2 heterostructure, which effectively improved the utilization ratio of solar and repressed the recombination of photo-generated charge.

2. Experimental 2.1 Materials Polyvinylpyrrolidone

(PVP,

K-90),

Magnesium

nitrate

hexahydrate

(Mg(NO3)2•6H2O, SCR), iron (III) nitrate nonahydrate (Fe(NO3)3•9H2O, SCR), hexaammonium heptamolybdate tetxahydrate [(NH4)6Mo7O24•4H2O, SCR] and thiourea (H2NCSNH2, SCR) were used as received without further purification. 2.2 Preparation of MgFe2O4 nanowires 1 mmol Mg(NO3)2•6H2O, 3 mmol Fe(NO3)3•9H2O and 1g PVP were dissolved in 2 mL deionized ethanol/water (1:3) mixed solvent. Subsequently, the obtained sol was stirred for 2 h. The precursor nanowires were synthesized through electrospinning (25 kV). Finally, the MgFe2O4 nanowires were prepared by calcinations at 500 oC for 1 h (The heating rate was at 1 oC/min for 25–400 oC, then 5oC/min for 400–500 oC) 2.3 Preparation of MgFe2O4/MoS2 A chemical vapor deposition method was used to synthesize MgFe2O4/MoS2 through. 1 mg (NH4)6Mo7O24•4H2O and 9.5 mg H2NCSNH2 as Mo-S source were mixed evenly. Then, 10 mg MgFe2O4 nanowires and the Mo-S source were placed at the ends of the quartz boat respectively, and then quartz boat was covered and calcined in a pipe furnace under high purity nitrogen atmosphere for 4 h (2 oC/min, 500 oC). After above steps, the sample as MgFe2O4/MoS2 (ω=10%) heterostructure nanowires was successfully obtained. By altering the proportion between MgFe2O4


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nanowires and the Mo-S source, a series of samples (MoS2:MgFe2O4=2%, 5%, 20% and 30%) were also prepared with the same method. Finally, the samples were spread on foamed nickel sheets (1.5 cm×3 cm), which were further applied as PEC photoelectrodes. 2.4 Photoelectrochemical degradation of tetracycline (TC) The photoelectrochemical degradation of TC was performed in a three-electrode cell (MgFe2O4/MoS2 as the working electrode, Pt foil as the counter electrode and Ag/AgCl electrode as a reference electrode), while the 50 mL TC aqueous solution (5%) was used as electrolyte. The illumination intensity is 47 mW•cm-2 (Newport/Oriel, QE-PV-SI).

In order to study the detail mechanism of

photoelectrochemical degradation, the active species trapping experiments were investigated, where 0.5 mL trapping agent such as ethylenediaminetetraacetic acid (EDTA), isopropanol (IPA) and p-benzoquinone (BQ). Furhermore, the electron spin resonance (ESR) analysis was used to confirm the existence of hydroxyl radicals (•OH) and superoxide radical (•O2−) under illumination. The photoelectrochemical degradation rate was calculated with the following formula. DR=1-C/C0 C0 was initial concentration of TC aqueous solution, while the C was the TC concentration after degradation for 2 h. 2.5 Photoelectrochemical H2 production Photoelectrochemical water splitting experiment was carried out in three electrode isolated system (working photoelectrode and calomel electrode in 120 mL deionized water/methanol solution (6:1), Pt electrode in Na2SO4 dilute aqueous


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solution) with 0.5 V applied bias. After illumination (47 mW•cm-2) for 5 h, the hydrogen evolution rate was then measured and calculated by gas chromatography. 2.6 Electrochemical impedance spectroscopy (EIS) The electrochemical impedance spectroscopy was measured by CHI660 B electrochemical analyzer (CH Instruments, and the frequency spectrum ranged from 0.01 Hz to100 kHz under the open circuit voltage for -0.35 V. The diameter of the semicircle corresponded to charge transfer resistance from the Nyquist plot. 2.7 Characterization The MgFe2O4/MoS2 nanowires were measured by X-ray diffraction (XRD, D8 ADVANCE), scanning electron microscope (SEM, JSM6480), solid UV diffuse reflectance (UV-2450), transmission electron microscope (TEM, JEM-2100), Raman (DXR, USA), energy dispersive X-ray detector

(EDX, F20 S-TWIN electron

microscope), X-ray photoelectron spectrometry (XPS, Thermo ESCALAB 250Xi, USA) and electron spin resonance (ESR, A300-10/12, BRUKER, Germany). Total organic carbons (TOC) were characterized by multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer.

3. Results and Discussions We present a possible synthesis process (Scheme 1) for synthesizing 1D heterostructure with MgFe2O4 and MoS2. The sol of Mo-S source was first prepared and employed as precursor for electrospinning. Then, the bare MgFe2O4 nanowires were synthesized after removing the organic ingredient by calcination. The successful introduction of MoS2 has been finally realized through chemical vapor deposition. We point out that, experimentally, the above synthesis process is easy and controllable to fabricate 1D MgFe2O4/MoS2 nanowires.


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Crystalline structure was characterized by XRD patterns (Fig.1), and the samples were measured in the form of pressed sheet where MgFe2O4/MoS2 was loaded on nickel foam. In addition to diffraction peak of nickel metal, the diffraction peaks of MgFe2O4 sample (Fig.1a) at 30.1o, 35.4o, 57.0o and 62.5ocan be corresponded to the (220), (311), (511) and (440) diffraction planes of MgFe2O4 (PDF:71-1232), which revealed that the pure MgFe2O4 has been successfully synthezied by electrospinning and calcination. However, the characteristic peaks of MoS2 cannot be distinguish from MgFe2O4/MoS2 (2%–30%, Fig.1b–1f), which may be attributed to low content of MoS2. Moreover, the MoS2 commonly forms with two-dimensional structure, so its degree of crystallization will be limited, which may be another reason for the disappearance of the MoS2 XRD peaks. Other efforts to confirm the existence of MoS2, and EDX and Raman spectra were further studied as shown in Fig.2 and Fig.3, respectively. According to the EDX spectra (Fig.2), the peaks of all elements (O, S, Mo, Fe and Mg) could be clearly distinguished, but the elements S and Mo were overlap together, since the EDX position of the two elements were very close. Furthermore, Raman spectra of MgFe2O4 and MgFe2O4/MoS2 were recorded, respectively. A single peak resulting from MgFe2O4 appeared at 380 cm-1, although its intensity was not high.[28,29] According to the Raman spectra of MgFe2O4/MoS2, two prominent peaks located at 383 cm-1 and 407 cm-1 can be correspond to the first-order Raman active modes with E12g and A1g symmetries of MoS2. The E2g pattern involved the formation of the metal atoms lattice within the metal surface for chalcogen, while A1g element atoms were against the upper and lower lattice plane vibrations.[30-33] Therefore, the EDX and Raman spectra offered a preliminary proof for the successfully synthesis of MgFe2O4/MoS2.


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All the involved chemical elements on the superficies of MgFe2O4/MoS2 were further identified by XPS spectra, as shown in Fig.4. The full survey XPS spectrum revealed that the sample composed of Mo, S, Fe, Mg and O elements. The peak of C 1s could also be observed in the XPS spectrum, due to the residual carbon during precursor decomposition at high temperature. The high-resolution spectra of Mo 3d and S 2p exhibited peaks at 225.8 eV (Mo 3d5/2) and 161.8 eV (S 2p3/2), which suggested that Mo and S in the MoS2 existed as tetravalent and divalent, respectively.[34,35] The peaks at 710.2 eV (Fe 2p3/2), 530.0 (O 1s) and 1303.1 eV (Mg 1s) corresponded to three different atoms of MgFe2O4.[36-38] As a surface of the XPS characterization technique, the composition ratio within a very shallow depth to the surface could be determined. XPS spectra further confirmed that the MoS2 has been successfully obtained and introduced into MgFe2O4/MoS2 heterostructure. SEM images of samples were observed in Fig. 5. According to Fig. 5a, MgFe2O4 nanowires with 1D nanostructure has been successfully synthesized through electrospinning, and the diameter of MgFe2O4 nanowires was c.a. 500 nm. After further loading MoS2, uniform thin sheet has appeared on the surface of MgFe2O4 nanowires, and thin sheet can be attributed to the formation of MoS2. With increasing the loading quantity (ω=2%–30%) of MoS2, the number of MoS2 thin sheet also became more, this phenomenon indicated that the MoS2 on the surface of MgFe2O4 could be well controlled through alter the experiment condition. Therefore, SEM image is a facile route to distinguish the component of materials, since the MoS2 exhibited as thin film morphology, while the MgFe2O4 was fabricated as nanowire morphology. Moreover, experimental controllability also offered the opportunity to optimize photoelectrochemical activity of MgFe2O4/MoS2 composted system. According this, the MgFe2O4/MoS2 may be had a wide application.


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The SEM mapping technology was also carried out in order to analyze the element distribution of MgFe2O4/MoS2 (Fig. S1 and S2). Based on Fig. S2, we could distinctly see O, S, Fe, Mg, Mo and Au elements, and the entire elements were emerged and uniform distributed in the MgFe2O4/MoS2 sample. Taking account of sample being broken by ultrasonic, we could not clearly observe the nanowire and nanosheet morphology in the SEM mapping images. The detail nanostructure of samples was investigated in-dpeth by TEM and HRTEM images (Fig. 6). Bare MgFe2O4 displayed as regular nanowires (Fig. 6a), and the high magnified TEM images (Fig. 6b and 6c) showed that the MgFe2O4 nanowires formed with many micropores, which might be caused by decomposition of organic components in electrospinning precursor. The MoS2 nanosheet has grown on the surface of MgFe2O4 nanowires, which can be well observed in the TEM images of MgFe2O4/MoS2 (Fig. 6d) and HRTEM images (Fig.6e). The connection between MgFe2O4 and MoS2 was very tight, and the unique structure would be advantageous to reduce the defects of interface and increase the separation of the photogenerated charges. The MoS2 nanosheet has been further identified through HRTEM (Fig. 6f), where the lattice fringes were 0.27 nm corresponded to MoS2 (101) crystal face. [39-42] EIS data of MgFe2O4 and MgFe2O4/MoS2 (Fig. 7) were measured to investigate the charge transfer process between the semiconductor and electrolyte interface, and the EIS curves can be fitted with equivalent circuit in insert image. The high frequency capacitive loop was usually connected with the chemical corrosion and charge transfer, while the low frequency inductive loop might be attributed to the inhibition or adsorption. It could be observed that both the EIS curves represented obvious semicircles at high frequency. The Nyquist plots at 298 K was in the absence


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and presence of the samples, which could be ascribed to the corrosion controlled by the charge transfer in alkaline medium or coupling action.[43] Compared with MgFe2O4, the semicircle diameter of MgFe2O4/MoS2 significantly decreased, due to the significantly diminished impedance of heterostrucutre. The phenomenon indicated that lower charge transfer resistance and faster separation of photogenerated charge would endow MgFe2O4/MoS2 heterostrucutre with enhanced photoelectrochemical activity.[44,45] However, the phase angle was decreased drastically to be an imperfect semicircle, there are two reasons for this problem: on one hand, it might be caused by the inconsistent or adsorption of the electrode surface; on the other hand, it may relate to the poor conductivity or solution resistance.[46,47] Therefore, EIS results gave an important proof that the MgFe2O4/MoS2 heterostructure has effectively optimized the photoelectrochemical activities of bared MgFe2O4. Today's society, pollution has gradually become a serious problem, particularly for water pollution, which is manly caused by harmful chemical substances such as antibiotics. Tetracycline (TC) as a kind of wide-spectrum antibiotics can slowly cause physiological system disorders of human body, so how to effectively remove environmental antibiotics becomes a hot research topic.[48-50] Herein, we applied a three-electrode cell to degrade TC, and the photoelectrochemical degradation rates of various samples were shown in Fig.8. The photoelectrochemical degradation rates of MgFe2O4/MoS2 (2%–30%) were better than bare MgFe2O4, which revealed that MgFe2O4/MoS2 heretostructure could accelerate electric charge separation and inhibited the recombination of photogenerated electrons and holes. Photoelectrochemical degradation rates of MgFe2O4/MoS2 gradually increased with increasing the quantity of MoS2, but excess MoS2 contrarily reduced degradation activities, because too much MoS2 will depress the contact of MgFe2O4


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with TC solution, so the optimal composited proportion of MgFe2O4/MoS2 was 10%, of which the photoelectrochemical degradation rate of TC can achieved up to 92%. Moreover, the responding ability to light is one of most important factors for evaluating the activity of sample, so we studied the photocurrent curve of TC degradation by MgFe2O4/MoS2 as shown in Fig. S3, and it can be observed that the photocurrent intensity was decrease sharply when the light was off, which clearly indicated that the MgFe2O4/MoS2 possessed high responding ability. A total organic carbon (TOC) was used to analyse the mineralization index of the TC degradation. Figure S4 showed the TOC analysis date with MgFe2O4/MoS2 (ω=10%) photoelectrochemical

on

the

degradation

of

TC

under

light.

After

photoelectrochemical catalytic reaction for 120 min, 41.1% TOC was removed, and it was lower than photoelectrocatalytic degradation ratio, which may be attributed the residual of small organic fragment. Moreover, the trend of TOC was also proved the degradation efficiency of photoelectrocatalytic. Besides, the UV-vis absorption spectra of samples were also characterized as shown in Fig. 9, the introduction of MoS2 effectively expanded the spectral absorption range of MgFe2O4, and this is another factor for the higher photoelectrochemical degradation of MgFe2O4/MoS2. The active species trapping experiments for TC degradation by MgFe2O4/MoS2 were measure in-detail to understand the photoelectrochemical degradation mechanism. According to Fig. 10, the degradation performance was mostly unchanged, when IPA as capture agent involved in the reaction process, which can be attributed to the inexistence of •OH or the •OH took no influence on TC degradation.[51] However, the degradation activity of MgFe2O4/MoS2 has been significantly depressed after EDTA or BQ taking the place of IPA. The depressed effect of BQ was the most significant, and the final degradation rate was down to 29%,


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which revealed that a large number of •O2- was produced during the reaction and played a key role in the TC degradation.[52] While, the EDTA also reduced the degradation rate, but just down to 70%. The reason for the lower degradation rate over EDTA can be explained that the partial holes in the solution also made a positive contribution to the photoelectrochemical degradation.[53] Furthermore, ESR technique was applied to further check whether superoxide anion (•O2-) has formed during photoelectrochemical degradation under irradiation. As shown in Fig. 11, the six characteristic peaks of DMPO-•O2- indicated that •O2radicals were produced in MgFe2O4/MoS2 reaction systems under light, since the substantial O2 and electrons could recombine to form •O2-, then these as-produced •O2- caused oxidation reaction in the process of TC degradation.[54] Therefore, the ESR spectra confirmed that •O2- was the primary active species responsible for the photoelectrochemical degradation of TC, and this result well agreed with the capture agent experiments. According to above characterization, a proposed mechanism for TC degradation over MgFe2O4/MoS2 was deduced. As shown in Fig. 12, the photoelectrons would transfer from conduction band of MgFe2O4 to MoS2, while the photogenerated holes contrarily migrated to valence band of MgFe2O4. Thus, the electrons and holes effectively separated, which greatly inhibited their recombination. The photogenerated electrons and oxygen recombined to form •O2-, while the photogenerated holes directly oxidized TC. However, it should be noted that the generation of •O2- took the dominant role for TC degradation,[55,56] and the holes also participated in the oxidation reaction.[57-59] •O2- and holes as strong oxidants could degrade TC molecular to organic debris or inorganic compound. Hence, it is confirmed in theory that MgFe2O4/MoS2 structure held higher charge separation efficiency and more


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powerful oxidation ability, which was in accordance with the experimental results. Due to the suitable band gap structure of both MgFe2O4 and MoS2, the obtained sample also has ability of photoelectrochemical water splitting for hydrogen production. The hydrogen production analysis was investigated at 0.5 V vs calomel electrode under illumination, and the schematic diagram of especial instrument was shown in Fig. 13. Hydrogen production rate (Fig. 14) was measured and calculated through gas chromatography. The hydrogen production rate of MgFe2O4/MoS2 (ω=10%) was 5.8 mmol/h•m2, which was about 1.7 times than that of bare MgFe2O4 (3.5 mmol/h•m2). The I-V curves of MgFe2O4/MoS2 (ω=10%) (Figure S5) showed that the photocurrent transient achieved to 0.066 mA/cm2 with the potential at 0.5 V vs Ag/AgCl, while the photocurrent density of the electrode was obtained by subtraction of the respective dark current, which well indicated that the sample possessed the responsibility to light. In order to better study its photoelectric effect, the incident-photon-to-current-conversion efficiency (IPCE) measurement was also measured showing on Figure S6, and the result was gained by the following formula: IPCE = (1240×I)/(λ×Jlight). While the I (mA/cm2) was the current density, λ(nm) was the wavelength of incident light and Jlight (mW/cm2) was power density. The MgFe2O4/MoS2 (ω=10%) had a value at about 16.3% at 300 nm, which indicated that the MgFe2O4/MoS2 (ω=10%) can effectively convert photo energy into electronic energy. The result revealed that the introduction of MoS2 can not only increase the TC degradation activity, but also enhance the performance of photoelectrochemical water splitting. Proposed process of hydrogen production is that: CH3OH as sacrificial agent can consume holes and form HCHO and H+, then H+ passed through nafion membrance


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into another reaction vessel.[60] Both conduction band potentials of MgFe2O4 and MoS2 was more negative than reduction potential of water, so the photogenerated electrons of MgFe2O4/MoS2 can effectively reduced H+ to produce hydrogen.[61-63] Moreover, the CH3OH can reactive with holes, this process also protect the MoS2 from oxidation, which improved the stability of MgFe2O4/MoS2 sample.

4. Conclusions MgFe2O4/MoS2 has been successfully synthesized through a relatively simple method. Compared with bare MgFe2O4, fabrication of MgFe2O4/MoS2 heterostructure can not only extend the absorption spectra range, but also accelerate the separation of photogenerated electrons and holes. According to unique band gap structure, as-prepared MgFe2O4/MoS2 exhibited significantly photoelectrochemical activity for the TC degradation and hydrogen production. The TC degradation over MgFe2O4/MoS2 mainly resulted from the formation of •O2- and holes, and the rate of hydrogen production was 5.8 mmol/h•m2, when the MgFe2O4/MoS2 (10%) used as the photoanode.

Acknowledgements The authors are grateful for National Natural Science Foundation of China (21401082, and 21201085), Open Project of State Key Laboratory of Rare Earth Resource

Utilizations

(RERU2015004),

Youth

Backbone

Teacher

Training

Engineering of Jiangsu University and Chinese-German Cooperation Research Project (GZ1091).


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Scheme. 1. Schematic illustration for the growth of MgFe2O4/MoS2.


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Fig.1. XRD patterns of MgFe2O4/MoS2 on the nickel foam. (a) MgFe2O4; (b) MgFe2O4/MoS2 (ω=2%); (c) MgFe2O4/MoS2 (ω=5%); (d) MgFe2O4/MoS2 (ω=10%); (e) MgFe2O4/MoS2 (ω=20%) and (f) MgFe2O4/MoS2 (ω=30%).


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Fig. 2. EDX spectrum of MgFe2O4/MoS2


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Fig. 3. Raman spectra of MgFe2O4, MoS2 and MgFe2O4/MoS2


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Fig. 4. XPS spectra of MgFe2O4/MoS2. (a) wide spectrum; (b) Mo 3d; (c) S 2p; (d) Fe 2p; (e) O 1s and (f) Mg 1s.


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Fig. 5. SEM images of (a) MgFe2O4; (b) MgFe2O4/MoS2 (ω=2%); (c) MgFe2O4/MoS2 (ω=5%); (d) MgFe2O4/MoS2 (ω=10%); (e) MgFe2O4/MoS2 (ω=20%) and (f) MgFe2O4/MoS2 (ω=30%).


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Fig. 6. TEM images of MgFe2O4 (a, b, c) and HRTEM image of MgFe2O4/MoS2 (d, e, f)


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Fig. 7. EIS of (a) MgFe2O4 and (b) MgFe2O4/MoS2 (ω=10%).


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Fig. 8. Photoelectrochemical degradation rates of MgFe2O4 and MgFe2O4/MoS2.


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Fig. 9. Solid UV diffuse reflectance spectra of MgFe2O4 and MgFe2O4/MoS2 (ω=10%).


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Fig. 10. Active species trapping experiments of MgFe2O4/MoS2


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Fig. 11. ESR spectra of MgFe2O4/MoS2 (ω=10%) in methanol.


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Fig. 12. Photoelectrocatalysis mechanisms for MgFe2O4/MoS2 (ω=10%)


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Fig. 13. Schematic diagram of hydrogen production apparatus


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Fig. 14. H2 production rates of (a) MgFe2O4/MoS2 (ω=10%) and (b) MgFe2O4 for 1h.

、


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Fig. 15. Water-splitting mechanism of MgFe2O4/MoS2 (ω=10%)


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Table of contents graphic 80x47mm (300 x 300 DPI)


MoS2 Heterostructure ... - ACS Publications

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