Fabrication and Enhanced Photoelectrochemical Performance of

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Fabrication and Enhanced Photoelectrochemical Performance of MoS2/S-Doped g-C3N4 Heterojunction Film Lijuan Ye, Dan Wang, and Shijian Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11326 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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ACS Applied Materials & Interfaces

Fabrication and Enhanced Photoelectrochemical Performance of MoS2/S-Doped g-C3N4 Heterojunction Film Lijuan Yea, Dan Wangb, Shijian Chena, * a

School of Physics, Chongqing University, Shapingba, Chongqing 401331, People's Republic of China.

b

School of media and mathematics & Physics, Jilin Engineering Normal University, Changchun 130052, People's Republic of China.

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ABSTRACT

We report on a novel MoS2/S-doped g-C3N4 heterojunction film with high visible-light photoelectrochemical (PEC) performance. The heterojunction films are prepared by CVD growth of S-doped g-C3N4 film on indium-tin oxide (ITO) glass substrates, with subsequent deposition of a low bandgap, 1.69 eV, visible-light response MoS2 layer by hydrothermal synthesis. Adding thiourea into melamine as the co-precursor not only facilitates the growth of g-C3N4 films but also introduces S dopants into the films, which significantly improves the PEC performance. The fabricated MoS2/S-doped g-C3N4 heterojunction film offers an enhanced anodic photocurrent of as high as ~1.2×10-4 A/cm2 at an applied potential of +0.5 V vs. Ag/AgCl under the visible light irradiation. The enhanced PEC performance of MoS2/S-doped g-C3N4 film is believed due to the improved light absorption and the efficient charge separation of the photogenerated charge at the MoS2/S-doped g-C3N4 interface. The convenient preparation of carbon nitride based heterojunction films in this work can be widely used to design new heterojunction photoelectrodes or photocatalysts with high performance for H2 evolution.

Keywords: photoelectrochemical, film, S-doped g-C3N4, MoS2, p-n heterojunction

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1. INTRODUCTION Since Fujishima and Honda discovered the photo-assisted electrochemical water oxidation

of

n-type

TiO2

single-crystal

electrode

in

the

early

1970s,

photoelectrochemical (PEC) water-splitting of semiconductor materials, as an environment-friend, sustainable and renewable approach for oxygen and hydrogen evolution, has attracted considerable attention. As is well known, the hydrogen and oxygen evolution are the results of redox reactions of photogenerated electrons and holes, respectively. To improve the efficiency of water-splitting, it is essential to suppress the spontaneous recombination of photogenerated electron and hole pairs. Recent reports have demonstrated that constructing heterojunction structures with binary or ternary materials could greatly facilitate the transfer and separation of photogenerated charge carriers (i.e. electrons and holes).1,

2

For instance, the

TiO2/CdS, WO3/BiVO4, MoS2/CdS and WO3/C3N4/CoOx systems could significantly inhibit the recombination of photogenerated charge carriers and show more outstanding performance than those single materials.3-6

Recently, layered g-C3N4 has become a very valuable material in photocatalysis-driven field due to the appropriate electronic structure (bang gap ~2.7 eV), metal-free, peculiar thermal and chemical stability and as well as low-cost preparation.7 Various modified routes, such as: template method for increasing the surface area 8, loading of co-catalyst 9, non-metal-doping(S 10, P 11, B 12, C 13 and O 14 etc.), coupling with other function materials (graphene

15

, TiO2 16 and MoS2

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17

etc.)


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and so on, usually could be used to improve the photocatalytic performance of g-C3N4. However, the challenge in preparation of g-C3N4 film with high quality hindered the application in PEC field. Although the fabrication of g-C3N4 film has been reported in recent years, the substrates were chosen as those semiconductor electrode materials such as ZnO

18

, TiO2 18, CuInS2 19, and CuGaSe2

20

, and the

primary focuses were on the influences of g-C3N4 on the properties of electrode materials or the heterostructures. The g-C3N4 films were also obtained through electrophoretic deposition, dip-coating or spin-coating the powders on the conduction substrates such as fluorine doped tin oxide (FTO) or indium tin oxide (ITO), but the measured PEC performance always showed very poor. 21-23 Therefore, we try to seek a simple method to grow g-C3N4 film with high PEC performance on a common conduction substrate and then, the construction of heterojunction shall be further considered.

Usually, the two-dimensional (2D) layered structure is beneficial to shorten the charge transport time and distance.24 As a result, it is possible for the 2D heterojunction to show more efficient charge separation and transfer. Among numerous heteromaterials, MoS2, 2D layered transition metal chalcogenide, exhibits large specific surface areas, good conductivity, relatively high mobility, and narrow band gap (1.2 eV -1.9 eV).25 The latest reports have suggested that nanostructured MoS2 could exhibit p-type conductivity characteristic and be used as an efficient photocathode for photoelectrochemical hydrogen production.26, 4 / 39


27

Also, since the

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analogous layered structures have minimized lattice mismatch, the planar growth of MoS2 over the g-C3N4 surface could be realized.

24

For example, MoS2 has been

loaded on the mesoporous g-C3N4 by via an impregnation-sulfidation approach to form MoS2/g-C3N4 layered nanojunctions, accompanying with improved H2 production performance.28 The possibility of the directional migration of photogenerated electrons between MoS2 and g-C3N4 has been confirmed by the theory calculations on their intrinsic band structures.29 In addition, the MoS2/g-C3N4 heterostructures have been reported to be used as high efficiency photocatalyst for pollutant degradation and also to exhibit superior electrochemical performance for lithium-ion batteries.30, 31

Therefore, in this paper, we first prepared carbon nitride films with high PEC performance via a conventional CVD method. Then molybdenum sulfide films were hydrothermally grown on carbon nitride films to construct the heterojunction structures. The fabricated MoS2/S-doped g-C3N4 heterojunction films exhibits much higher PEC performance compared with S-doped g-C3N4 and pure g-C3N4 films at the same conditions. Finally, the possible charge transfer mechanism in heterojunction films was discussed and mainly attributed to the improved light absorption and the construction of p-n heterojunction.

2. EXPERIMENT SECTION

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Figure 1. The schematic diagram of preparation of film samples. The reagents in this work were purchased from Adamas and used without further purification.

2.1. Preparation of CN thin film on substrate (ITO) The preparation process of CN thin film is schematically shown in Figure 1. Briefly, 0.5 g thiourea was added into 2 g melamine and after grinding and mixing, the mixture was placed at the bottom of a crucible. The indium−tin oxide (ITO) substrate (1.5 cm × 2.0 cm) was put above the mixture. Then the capped crucible was transferred into a muffle furnace and heated at 550 °C for 4h under air atmosphere with a heating rate of 2 °C per minute. After cooling naturally, the obtained transparent light yellow color smooth CN film (labeled as mt-CN) was cleaned by a strong N2 airflow. Without addition of thiourea, the yellow CN film (labeled as m-CN) was also prepared by the same procedure.

2.2. Preparation of heterojunction film

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The procedure for preparing heterojunction films are as following: firstly, the precursor solution of MoS2 was prepared by dissolving sodium molybdate and thiourea (1:5 molar ratio) in 50 ml deionized (DI) water (resistivity ~ 18.25 MΩ/cm). Then, the prepared mt-CN film sample was immersed in a 25 ml Teflon-lined autoclave, which contained 3 ml above precursor solution. After maintaining at 180 °C for 24 hours, the cooled film sample was washed with DI water. Finally, the obtained dark color sample was dried with N2 airflow and named as mt-CN/MoS2 for convenience.

2.3. Material Characterization techniques

X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert Powder diffractometer with Cu Kα radiation (λ=1.54 Å). Diffraction angle (2-theta) ranged from 10 ° to 70 ° and the scanning step was 0.02 °. The Raman spectra were acquired on LabRAM HR Evolution spectrometer (HORIBA Jobin Yvon) with an excitation wavelength of 532 nm. The photoluminescence (PL) spectra were excited at room temperature using He-Cd laser (325 nm). The surface and cross-section morphology images of films were obtained by a XL-30 field emission scanning electron microscope (XL-30 FESEM, Philips) with an energy dispersive X-ray spectrometer (EDXS). The UV-vis absorption spectra were performed on a UV4100 spectrophotometer (Hitachi, Japan) equipped with an integrating sphere. X-ray photoelectron spectra (XPS) were carried on a K-Alpha X-ray photoelectron spectrometer (Thermo Scientific). The electrical conductivity types of films were 7 / 39



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determined by the Hall Effect measurements using Ecopia HMS-3000 system at room temperature.

2.4. Photoelectrochemical (PEC) measurements All the obtained film samples with a geometric area of 1 cm2 could be directly used as working electrodes for photoelectrochemical (PEC) tests. The PEC performance of film electrodes was studied in a photoelectrochemical cell with a standard three-electrode setup using PARSTAT 2273 advanced electrochemical system (Princeton). The platinum plate (1×1 cm2) and Ag/AgCl was used as the counter electrode and reference electrode, respectively. 0.1 M Na2SO4 aqueous solution was used as the electrolyte. The Hayashi LA-410UV-3 with a 150 W Xe lamp (XE4030) equipped with an ultraviolet cut-filter (λ > 420 nm) was used as the visible light source (~ 100 mW/cm2). In detail, the steady state linear sweep voltammetry (LSV) measurements were carried out with anodic sweeping (from -1.0 V to +1.0 V) at a scan rate of 10 mV/s before and after light irradiation. The periodically illuminated LSV measurements with on-off light of 10 s were also recorded. The chronoamperometry was used to detect the photocurrent and its stability under an applied voltage of + 0.5 V vs. Ag/AgCl. In addition, the electrochemical impedance spectroscopies (EIS) of each system were measured with a frequency range from 100 kHz to 100 mHz. The Mott−Schottky plots were obtained at 10 KHz frequency. Finally, the transient open circuit potentials (OCP) were also tested. 8 / 39


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3. RESULTS AND DISCUSSIONS 3.1. Structure and Composition of Films

Figure 2 (a) shows the XRD patterns of the prepared m-CN, mt-CN and mt-CN/MoS2 film samples, including that of ITO substrate. Due to the strong background signal from ITO, the XRD patterns of films are weak. As melamine is used as the only precursor to grow CN thin film (i.e. m-CN sample) on ITO, a weak peak is observed at 27.67o, which is indexed to the (002) plane of g-C3N4 (JCPDS No. 87-1526), indicating the graphite-like stacking of the conjugated aromatic units of carbon nitride

21, 32

. As the mixture of thiourea and melamine was used as precursor,

the (002) peak of the obtained film (i.e. mt-CN sample) becomes much stronger, which implies the presence of larger crystalline domains in mt-CN sample. Obviously, the addition of thiourea improves the crystal quality of CN film, which is attributed to that the sulfur species (-SH) in thiourea as leaving groups could facilitate the polymerization/condensation process to form polymer CN network with modified texture.33, 34 Comparing the XRD patterns of CN films with those of CN powders also reveals that the prepared CN thin films in such closed system are a tri-s-triazine based polymer, i.e. g-C3N4 (see Figure S2 in Electronic Supporting Information). After MoS2 is grown on the mt-CN film sample, the typical XRD diffraction peaks of g-C3N4 and MoS2 cannot be observed clearly. On the one hand, this is mainly because the MoS2 layer is too thin to be detected and the XRD peak intensity of MoS2 synthesized by hydrothermal method is usually weak.35, 36 On the other hand, to some 9 / 39



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extent, the MoS2 layer screens the signal of mt-CN film. However, the characteristic peaks of MoS2 and g-C3N4 can be detected by Raman spectra.

Figure 2. (a) X-ray diffraction patterns of film samples. The inset is the magnified view of (002) diffraction peak of CN film samples. (b) Raman spectra of mt-CN/MoS2 film sample and ITO substrate. The inset is the typical Raman peaks of MoS2.

Raman spectroscopy, as an available, rapid and nondestructive surface characterization technique, is widely used to probe the vibrational properties of bonding of MoS2.37 The energy of 532 nm excitation wavelength could resonate with π-π* electronic transition in ring structures, which makes it suitable to the study of aromatic clustering processes in sp2-dominated solid.38 Figure 2 (b) shows the Raman spectra of mt-CN/MoS2 film sample and ITO substrate excited by 532 nm laser. Except the peaks of ITO substrate, two obvious peaks in mt-CN/MoS2 film sample are observed at 377 cm-1 and 402 cm-1 and ascribed to the E12g and A1g Raman active vibration modes of MoS2, respectively, which correspond to the in-plane and out-of-plane vibration of S-Mo-S layer.39 In addition, the peak (labeled as VII) 10 / 39


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centered around 1580 cm-1 in mt-CN/MoS2 sample could be also observed from the Raman spectra of mt-CN film sample, mt-CN and m-CN powder samples, as shown as the Figure S4 (a), which is attributed to the G (for graphite) band due to the in-plane bond stretching vibrations of g-C3N4.40, 41 The G mode is a single resonance and is related to the bond stretching of all pairs of C sp2 atoms in aromatic clusters or in the chain structures.38, 42 Thus, the Raman spectra results indicate the coexistence of MoS2 and g-C3N4 in mt-CN/MoS2 film sample.

Figure 3. The high-resolve XPS spectra of (a) C 1s, (b) N 1s, (c) S 2p and (d) Mo 3d for mt-CN/MoS2 film sample. The gray, red, green and rose color lines represent the originally measured XPS spectra, the fitted survey spectra, the fitted components and baselines, respectively.

To precisely ensure the components of film samples, the X-ray photoelectron spectroscopy (XPS) was conducted to detect the elements and analyze the chemical 11 / 39



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state. All binding energies were calibrated by the C 1s peak (284.5 eV) arising from adventitious carbon. The XPS survey spectra of all the samples and the high-resolution XPS spectra of C, N and S elements from m-CN and mt-CN film samples are shown in Figure S5. The high-resolution XPS spectra of C, N, S and Mo elements from mt-CN/MoS2 film sample are displayed in Figure 3. Due to the asymmetric peak shape, these spectra could be deconvoluted into several Gaussian-Lorenzian peaks.

Firstly, three fitting peaks are obtained in the C 1s core level spectrum (Figure 3 (a)). The strong peak at 284.50 eV could be attributed to the adventitious carbon-containing contaminations, i.e. C–C, C–H, and/or C=C bonds.43 The weak peak at 286.08 eV could be assigned to the C–N–C group of g-C3N4, which is the characteristic of C-N groups and usually observed from carbon nitrides.43 The other weak peak at high binding energy of 288.33 eV is typically ascribed to the tertiary carbon C-(N)3 in the g-C3N4 lattice, which means this peak is identified as the sp2-bonded carbon in N-containing aromatic skeleton rings (N2-C=N).43 However, this peak shifts towards higher bind energy compared with m-CN (287.89 eV) and mt-CN (287.69 eV) film samples (see the Figure S5 (b)). It is worth noting that in all the film samples, the peaks represented the C combined with O in the form of O=CO or C-O bonds are not be observed, implying that the C element in CN materials didn’t suffer oxidation.44 Secondly, two obvious peaks centered at 395.53 eV and 399.46 eV from high-resolution XPS spectrum of N 1s (Figure 3(b)) are usually ascribed to the 12 / 39


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N-C bonds and tertiary nitrogen N–(C)3 groups, respectively.44 The latter peak shifts to higher bind energy (~ 400 eV) in m-CN and mt-CN film samples (see the Figure S5 (c)). The obtained strong peak at 161,44 eV for the high-resolution XPS spectrum of S 2p in Figure 3(c) is usually attributed to the S-Mo bonds in the MoS2, while the weak peak at163.27 eV is reasonably ascribed to the C-S bonds. Since such peak has also been observed in S doped g-C3N4 system from other reports, where the sulfur atoms substituted for lattice nitrogen sites.10 Also, it has been reported that the binding energy of S 2p3/2 in CS2 material is located at 163.7 eV or 163.9 eV.10, 45 Importantly, the peaks from the S-N bonds are not been observed, indicating that the S atoms are just incorporated into the sites of nitrogen.46 This is consistent with the theoretical research finding, which has demonstrated that the S atom preferentially substitutes for the edge N site of g-C3N4 lattice.47 Similarly, for the etched mt-CN film sample, the C-S bond is also observed, as shown in Figure S5 (d), which just shifts towards the lower binding energy of 162.43 eV. This also demonstrates that the S impurities have been implanted homogeneously into CN materials by substituting the nitrogen sites after mixing thiourea into the precursor of melamine, and in theory, it probably leads the mt-CN film sample to exhibit n-type conductivity. Besides, the non-metal S element doping has been reported to enhance the photocatalysis activity of g-C3N4,10, 34

therefore, it is reasonable to believe that the S dopant would play an important role

in improving the PEC properties of CN film in our studies. Lastly, from the Mo 3d XPS spectrum in Figure 3(d), the obtained peaks 228.40 eV and 232.24 eV are known to be attributed to Mo 3d5/2 and Mo 3d3/2, respectively, which are the typical 13 / 39



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values for Mo4+ species in pure MoS2. The peak centered at 225.75 eV is attributed to S 2s. The peak located 235.60 eV implies the presence of Mo6+, revealing the trace of molybdenum oxide (MoO3) in mt-CN/MoS2 film sample due to the oxidation on the surface of MoS2 film. The above XRD, Raman and XPS analysis results indicate that the obtained m-CN film sample is graphitic like layered carbon nitride (g-C3N4) material and the mt-CN film sample is in-situ S doped g-C3N4. The mt-CN/MoS2 sample is a heterojunction film consisted of MoS2 and S doped g-C3N4. Moreover, compared with mt-CN sample, the binding energies of C 1s and N 1s shift for mt-CN/MoS2 sample. Usually, the binding energy shift has been used to indicate the existence of strong electronic coupling, revealing the chemical bonding interaction between MoS2 and S-doped g-C3N4 layer, which is beneficial for the charge transport between the adjacent components during the photocatalytic process. 48

3.2. Morphologies of Films Figure 4 displays the SEM images of surface and cross-section morphology, as well as the EDX spectra of ITO, m-CN, mt-CN and mt-CN/MoS2 film samples, respectively. It can be seen that ITO layer is about 200 nm thick and composes of particles tightly. For the m-CN film sample (i.e. g-C3N4), just a very thin and uneven thick (< 100 nm) film is formed on the top of ITO layer, and most surface area of the film is covered by many macroscopic agglomerated m-CN powders, as shown in Figure S6 (a and b). For the mt-CN film sample (i.e. S-doped g-C3N4), it is clear that 14 / 39


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the film shows transparent light yellow color and smooth surface without any agglomerations on it (see Figure 6S (a and c)). Besides, the surface morphology shows unique lotus leaf-like structure with some wrinkles. More importantly, the film appears more compact and thick (>200 nm), comparing with that of m-CN film sample. It is obvious that the thiourea facilitates the deposition of film and introduces the S impurity into g-C3N4 (see the corresponding EDX spectrum), which is consistent with the results of XRD and XPS. For the mt-CN/MoS2 sample, the film becomes dark color (see Figure S6 a) and shows a coral-like structure, which is beneficial to capture more light energy for photocatalysis. It can be seen that the film on ITO substrate consists of two layers: MoS2 layer with average thickness of 100 nm and mt-CN layer of about 50 nm thick. Also, the C, N, S and Mo elements are all detected by EDXS and XPS, which confirmed the formation of MoS2/S-doped g-C3N4 heterojunction film. Herein, the fabrication, specific structure and composition of film samples in this work can be presented as the schematic diagram shown in Figure S7.

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Figure 4. (a1-a4) the low-resolution surface SEM, (b1-b4) high-resolution surface SEM, (c1-c4) cross-section morphologies and (d1-d4) the EDX spectra of ITO, m-CN, mt-CN and mt-CN/MoS2 film samples, respectively.

3.3. Optical properties of Films

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Figure 5. (a) Absorption spectra of m-CN, mt-CN and mt-CN/MoS2 film samples. (b), (c) and (d) are the corresponding tauc plots.

As shown in Figure 5 (a), the prepared m-CN film sample shows an absorption edge around 470 nm. By fitting the optical transition at the absorption edge using the Tauc/David−Mott model 49, the band gap is evaluated to 2.79 eV, as shown in Figure 5(b). As is well known, the visible light response of g-C3N4 originates from the electron transition from the valence band (VB) derived from N2p orbitals to the conduction band (CB) formed by C2p orbitals. Several reports have suggested that the sulfur-mediated precursor could alter the packing styles of π-conjugated aromatic species to modify the optical property and band gap of g-C3N4 polymers.10, 46 Here, for the mt-CN sample, it can be observed that the film shows much stronger absorption with an abrupt edge at 430 nm, where the edge is equivalent to band gap of 2.92 eV (see Figure 5(c)). Apparently, the thiourea, as a sulfur-mediator, affects the band gap of g-C3N4. Compared with m-CN film sample, the band gap of mt-CN film sample is much wider. This phenomenon has also been reported in other S doped g-C3N4 system, where the homogeneous doping of S atoms and quantum size effect are used for interpreting the widening of band gap of g-C3N4.10 While MoS2 layer is deposited on the mt-CN film sample, the obtained mt-CN/MoS2 sample exhibits much stronger and broader absorption in UV-visible light region and the color of film changes from light yellow to grey black. This phenomenon has always been observed in g-C3N4/graphene 15, MoS2/g-C3N4 17, C/g-C3N4 50, Fe3O4/g-C3N4 51 systems and so on. 17 / 39



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Consequently, under the same light irradiation condition, the mt-CN/MoS2 sample could absorb more photons to produce more photogenerated electron-hole pairs, promoting the PEC and photocatalytic activity. In addition, the small absorption peak around 570 nm gives rise to a gradual edge, according to which the optical transition band gap of mt-CN/MoS2 sample is estimated about 1.69 eV, as shown in Figure 5(d). Obviously, the observed dark film should be ascribed to the MoS2 consisted of multilayer stacked S−Mo−S structures.36

3.4. PEC Performance

PEC properties can be used to analyze the photoresponse of materials to evaluate their PEC or photocatalysis water-splitting potential. Photocurrent (I), as a main indicator for photoresponse, is formed by the charge carrier flow, which reflects the generation of electrons and holes excited by light irradiation.

Figure 6. (a) The periodically illuminated photocurrent-potential (I−V) curves of m-CN, mt-CN and mt-CN/MoS2 film samples. The labeled 1.6×10-4 A/cm2 presents the instantaneous photocurrent of mt-CN/MoS2 sample at +0.5 V vs. Ag/AgCl. (b) The photocurrent-time (I-t) 18 / 39


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curves of mt-CN and mt-CN/MoS2 film samples under chopped visible light irradiation with an applied bias of +0.5 V vs. Ag/AgCl. The inset is the amplified view of the instantaneous photocurrent of mt-CN thin film sample at the same condition. The V-axes (photocurrent=0) are indicated by the gray dotted lines. The marked On and Off words represent turning the light on or off.

Figure 6 (a) shows the photocurrent-potential (I-V) curves of m-CN, mt-CN and mt-CN/MoS2 film samples under periodically illumination of 10 s. It can be seen that m-CN film sample barely shows photocurrent response (The obtained photocurrent is extremely weak: ~10-7 A/cm2, as shown in Figure S8). On the contrary, the mt-CN film sample shows apparent photocurrent response (~10-5 A/cm2), which is increased by two orders of magnitude compared with that of m-CN film sample. On the one hand, this is due to that the crystal quality and thickness of CN film have been improved by adding thiourea into precursor. On the other hand, this is mainly attributed to the introduction of S dopant, which could promote the photo-reduction and photo-oxidation activity of CN film, benefiting the enhancement of photoelectrochemical or photocatalytic performance. After growing MoS2 layer on mt-CN film sample, the obtained mt-CN/MoS2 heterojunction film sample shows larger photocurrent response at same polarization potential, and the photocurrent reaches ~ 10-4 A/cm2. The further increase in photocurrent is mainly attributed to the enhanced light absorption and the formation of heterojunction between mt-CN and

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MoS2, which facilitate the generation, transfer and separation of photoinduced electrons and holes.

To study the photoresponse of mt-CN and mt-CN/MoS2 film samples at specific polarization potential, the instantaneous photocurrent was measured by light chopping at an applied bias of +0.5 V vs. Ag/AgCl, as shown in the Figure 6 (b). With intermittent light irradiation of 10 s, mt-CN thin film sample can reach relative stable anodic photocurrent of 7 ×10-5 A/cm2 after undergoing 12 light on-offs (see the inset), which is much higher than that of CN films spin-coated or dip-coated on the conductive substrates (such as FTO and ITO) using CN powders.30, 31 However, an apparent decay phenomenon happens to the peak current, indicating a fraction of photogenerated holes accumulate at the surface and recombine with electrons from the CB. Until the transfer and generation of photogenerated electron−hole pairs reach equilibration, a stable photocurrent is formed. Comparing the photocurrent responses of mt-CN and mt-CN/MoS2 film samples, it is obvious that the deposited MoS2 layer greatly enhances the instantaneous anodic photocurrent of mt-CN film sample, stably reaching ~1.2×10-4 A/cm2. This is in agreement with the result in visible-light-driven electrochemical impedance spectra (EIS) presented by Nyquist plots in Figure S9, where the radius of incomplete semicircles from the mt-CN/MoS2 film sample is distinctly smaller than that from mt-CN sample, implying that the charge transfer resistance is reduced after depositing MoS2 layer. As a result, about doubly enhanced photocurrent response is observed in mt-CN/MoS2 film sample. Although under the 20 / 39


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applied potential of +0.5 V vs. Ag/AgCl the polarization current of mt-CN/MoS2 film sample in the dark shows a small decrease with time, the photo-generated current remains relative stable.

3.5. PL spectra

Figure 7. The PL spectra of m-CN, mt-CN and mt-CN/MoS2 film samples under excitation at 325 nm laser.

It is well known that the PEC or photocatalytic activity are directly related to the amount of photogenerated carriers, so monitoring the concentration of carriers can be used to interpret the PEC or photocatalytic potential of materials. Photoluminescence (PL) spectra is just originated from radiative recombination of charge carriers, thus, it can be used to detect the change of carrier concentration in a semiconductor under light illumination. The PL spectra of m-CN, mt-CN and mt-CN/MoS2 film samples were measured at room temperature and shown in Figure 7. A strong luminescence peak at 430 nm is detected in the m-CN film sample, indicating the strong 21 / 39



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recombination of photogenerated charge carriers under the 325 nm laser excitation. On the contrary, the prepared mt-CN film sample exhibits much weak PL signal under the same excitation conditions. When the thin MoS2 layer is deposited on the mt-CN film sample, the obtained mt-CN/MoS2 film sample shows negligible luminescence intensity. Lower luminescence intensity indicates a lower recombination efficiency of photogenerated charge carriers, which means more photogenerated electrons and holes could be used for the redox reaction, achieving a higher photocurrent. Therefore, the mt-CN/MoS2 film sample could exhibit much higher photocurrent than mt-CN film sample, and far higher than m-CN film sample, as above measured I-V and I-t results.

3.6. Possible Charge Transfer Mechanism

To shed light on the injection direction of photogenerated electrons, the open circuit potential (OCP) transient tests of the prepared mt-CN and mt-CN/MoS2 film samples were carried out and the results were shown in Figure 8 (a). It can be seen that mt-CN film sample shows a negative increase in voltage under light irradiation, which indicates the photogenerated electrons are injected from the mt-CN film into the ITO substrate,52,

53

resulting in the formation of anodic photocurrent in I-V and I-t

measurements. In PEC tests, the anodic current (positive current value) is defined by electrons in semiconductors as the working electrode transferring to the counter electrode through the external circuit (see the schematic diagram in Figure 9 (a)); otherwise, the cathodic current (negative current value) is formed by the electrons 22 / 39


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directly transferring to the electrolyte side. Usually, n-type semiconductor is used as photoanode to obtain anodic photocurrent, while p-type semiconductor is used as photocathode to produce cathodic photocurrent. Therefore, it can be considered that the prepared mt-CN film acts an n-type semiconductor material. Similarly, for the mt-CN/MoS2 film sample, the measured OCP also shows a negatively increasing voltage under light irradiation, which suggests the photogenerated electrons are transferred to ITO from MoS2 layer by passing through mt-CN film layer and continued to the counter electrode, also implying the photoanode role of mt-CN/MoS2 sample. It is noted that the generated photovoltage (i.e. the difference between the voltages in dark and under irradiation) of mt-CN/MoS2 sample (1.197 V vs. Ag/AgCl) is larger than that of mt-CN film sample (1.121 V vs. Ag/AgCl) as the light is on, indicating that the prepared mt-CN/MoS2 sample shows more remarkable photoelectric conversion ability.

Figure 8. (a) Transient OCP of mt-CN and mt-CN/MoS2 film samples in the dark and under light illumination. (b) Mott–Schottky curves of mt-CN and mt-CN/MoS2 film samples.

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Furthermore, Mott–Schottky (M-S) plot based on capacitance that is derived from the electrochemical impedance can be used to qualitatively analyze the conductivity types and the carrier concentration of semiconductors.5 The positive slope in the linear segment of M-S plot reflects the n-type nature of a semiconductor, while the negative slope indicates the p-type nature. According to the M-S equation 5, the concentration of carriers (N) is derived from the function of the reciprocal of slope in M-S linear plot (i.e. N ~ slope-1), therefore, the value of such slope can be used to estimate the change of carrier concentration. The drop of value in slope indicates the increase in the concentration of charge carrier. Figure 8(b) shows the M-S plots of mt-CN and mt-CN/MoS2 film samples at a frequency of 10 kHz. As shown, the mt-CN film sample shows positive slope, indicating the n-type characteristic. This is consistent with the above XPS and OCP analyses. Notably, the mt-CN/MoS2 film sample also shows a positive slope, but the value of slope is smaller than that of mt-CN sample, suggesting an increase of carrier densities and the enhancement of conductivity due to the growth of MoS2 layer.54 Additionally, the V axis intercept of linear region in M-S plot could be used for the rough estimation of flat band potential (Efb) of semiconductor. As is well known, the CB potential of n-type semiconductor lies close to the Efb and thus, the CB edge potential of mt-CN film sample is about -1.38 V vs. Ag/AgCl, which is equivalent to -1.18 V vs. NHE. Considering the band gap value from the absorption spectra, it is evaluated that the VB edge potential of mt-CN sample is about 1.54 V vs. Ag/AgCl, 24 / 39


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corresponding to 1.74 V vs. NHE, which is in agreement with the previous reports 7. According to reports, the CB and VB edge potentials of p-type MoS2 are at 0.4 and 2.27 V vs. NHE, respectively 5, and those of n-type MoS2 are around 0.45 and 2.15 V vs. NHE,55 both of which are doubtlessly more positive than the band edge potentials of mt-CN film sample. Hence, the schematic diagram of band energies of mt-CN sample and MoS2 layer before contact can be determined cursorily, as shown in Figure 9 (b). Since the conduction type of undoped MoS2 depends on the experimental preparation methods and in our previous work, it has reported the MoS2 grown by hydrothermal method could show p-type conductivity.36 Here, the electrical conduction types of samples were further characterized by Hall Effect measurements. The results in Table S1 suggest that the prepared m-CN and mt-CN samples exhibit n-type conductivity, while the mt-CN/MoS2 sample shows p-type conductivity. Thus, it is confirmed that the MoS2 layer in mt-CN/MoS2 sample works as a p-type semiconductor material.

Consequently, a p-n heterojunction is formed between the p-type MoS2 layer and the n type mt-CN material. Under equilibrium condition the Fermi level in the mt-CN and MoS2 materials is equal and constant, accompanying with the energy band bending. The energy bands of MoS2 will shift upward to the negative potential, while the bands of mt-CN will shift downward to the positive potential, as shown as Figure 9 (c). This is also demonstrated by the M-S plot of mt-CN/MoS2 sample in Figure 8 (b), where the Efb is about -1.17 V vs. Ag/AgCl (i.e. -0.95 V vs. NHE) and obviously 25 / 39



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shifts towards positive potential compared with that of bare mt-CN film sample. Naturally, under light irradiation, the photoinduced electrons in CB of MoS2 will transfer to the CB of mt-CN material and continue migrating to the ITO conductive layer and entering the external circuit to the counter electrode for producing hydrogen. As a result, anodic photocurrent is obtained, which is consistent with the measured results. It is noteworthy that an anodic shift appears in both photocurrent onset potential in Figure S10 and Efb in Figure 8(b) after MoS2 layer is deposited on the mt-CN film material. Also, the photocurrent onset potentials of both mt-CN and mt-CN/MoS2 film samples shift toward anodic direction from the corresponding Efb, which is commonly observed for the n-type semiconductors and is usually caused by slow kinetics and/or surface recombination.56 Due to the slow kinetics of water oxidation, the peroxo species become kinetically competitive with O2 evolution and may be formed at the electrolyte side of photoanode (i.e. mt-CN/MoS2 film sample), even though the production of O2 is thermodynamically more feasible than formation of peroxo species.56 Thus, it is reasonably to believe that at the photoanode, the water is preferentially oxidized into H2O2 by the photogenerated holes, as shown in Figure 9(c). The feasibility has been confirmed by the reported sunlight-driven H2O2 production from water by g-C3N4.57 As is well-known, MoS2 is usually used as co-catalysts to activate the photocatalytic functions of a given photocatalytic system, and more importantly, MoS2 itself is a very high efficient electrocatalyst for H2 evolution. MoS2 has previously been used a co-catalyst in the MoS2/g-C3N4 composite powders for H2 evolution and the heterojunction formed between MoS2 26 / 39


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and g-C3N4 is found to benefit the charge transfer.29, 58 Here, due to the formation of p-n heterojunction, the photogenerated carriers are separated and transferred more efficiently. Combining with the improved light utilization as shown in Figure 5(a), the MoS2 layer promotes mt-CN/MoS2 film sample to show an enhanced PEC performance. Therefore, compared with the bare mt-CN film sample, the measured higher anodic photocurrent of mt-CN/MoS2 heterojunction film is benefited from the effective separation and transfer of photogenerated charge carriers due to the formation of p-n heterojunction between MoS2 layer and mt-CN material, as well as the improved light absorption.

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Figure 9. The schematic diagrams of (a) formation of anodic photocurrent, (b) band energy position of MoS2 and mt-CN material before contact, and (c) the charge transfer mechanism of mt-CN/ MoS2 sample in the case of p-n type contact.

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CONCLUSIONS In summary, S doped graphitic carbon nitride (g-C3N4) film was successfully deposited on the indium−tin oxide (ITO) glass substrate by pyrocondensation polymerization of the mixture of melamine and thiourea. XRD, Raman, SEM and XPS characterizations indicate that the crystalline quality of g-C3N4 film is improved by the mediation of thiourea and S dopants are also introduced into the film. Consequently, the obtained S-doped g-C3N4 film shows much higher anodic photocurrent than that of pure g-C3N4 film. Furthermore, a thin MoS2 layer is fabricated on the S-doped g-C3N4 film by a hydrothermal route and the obtained MoS2/S-doped g-C3N4 heterojunction film exhibits more excellent PEC performance. Under the visible light irradiation, the anodic photocurrent generated by heterojunction film stably reaches ~1.2×10-4 mA/cm2 at an applied potential of +0.5 V vs. Ag/AgCl, which is nearly twice that of S-doped g-C3N4 film. The enhanced PEC performance is attributed to the synergetic effect from the increasing photogenarated carriers due to the improved light absorption and the efficient charge separation due to the formation of p-n heterojunction structure between MoS2 and S-doped g-C3N4. Both the fabricated S-doped g-C3N4 film and MoS2/S-doped g-C3N4 heterojunction film can be the promising candidates as photoanodes for PEC cells. The convenient preparation method of film in this work can be used to design new heterojunction film materials based on carbon nitrides.

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AUTHOR INFORMATION Corresponding Author: Shijian Chen

*E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank Professor Xintong Zhang from Northeast Normal University for helpful discussions and constructive suggestions. This work is supported by the National Natural Science Foundation of China (NSFC) (grants 11304406, 61307035 and 51502111).

SUPPORTING INFORMATION XRD and Raman of precursors, CN films and powders form; XPS of CN films; optical photographs of samples; schematic diagram of the fabrication, structure and composition of film; I-t & I-V of pure CN film; steady state I-V and EIS Nyquist plots of S-doped CN and heterojunction films under on-off light; Hall data. This material is available free of charge via the Internet at http://pubs.acs.org.

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ABSTRACT GRAPHIC

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Fabrication and Enhanced Photoelectrochemical Performance of

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