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Preparation of TiO2/Graphic Carbon Nitride Core-Shell Array as Photoanode for Efficient Photoelectrochemical Water Splitting Xiaoli Fan, Tao Wang, Bin Gao, Hao Gong, Hairong Xue, Hu Guo, Li Song, Wei Xia, Xianli Huang, and Jianping He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03107 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Preparation of TiO2/Graphic Carbon Nitride Core-Shell Array as Photoanode for Efficient Photoelectrochemical Water Splitting Xiaoli Fan, Tao Wang*, Bin Gao, Hao Gong, Hairong Xue, Hu Guo, Li Song, Wei Xia, Xianli Huang, Jianping He*

College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, 210016 Nanjing, PR China. E-mail address: [email protected], [email protected]; Tel: +86 25 52112906; Fax: +86 25 52112626.

ABSTRACT: Photoelectrochemical (PEC) oxygen

evolution reaction over

photoanode is a promising process for renewable energy. The fascinating properties of graphic carbon nitride (g-CN) in water splitting make the photoelectrode engineering of it for PEC usage quite meaningful. In this work, we report the fabrication of the core-shell structured TiO2/g-CN composite film by hydrothermal growth for TiO2 nanorod arrays and solvothermal growth for the g-CN layer. Herein, TiO2 is used as an effective electron transfer layer and g-CN as visible light absorption layer. Different reaction conditions were investigated in order to obtain the uniform TiO2/g-CN

nanorod

core-shell

structure.

Outstanding

photoelectrochemical

performances of the optimized composites were obtained than that of pristine TiO2 or g-CN since the heterojunction of high quality between g-CN and TiO2 turned out to effectively reduce the recombination of charge carriers and improve the photoelectric conversion ability. Thus, the photocurrent density under visible light of TiO2/g-CN reached to 80.9 µA cm-2, 21 times of g-CN under 0.6 V (vs. SCE). Finally, a systematical photoelectrocatalytic mechanism of charge carrier migration and recombination path in TiO2/g-CN nanorod core-shell heterojunction was proposed, which can be considered as probable explanation of efficient PEC performance. 1

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INTRODUCTION As energy crisis and environment issues have become more and more intensified, exploring methods for effectively utilizing the inexhaustible solar energy is of utmost desirable.1 Solar-driven water splitting provides one of the most practical candidates for conversion of solar energy directly into chemical energies, in form of sustainable hydrogen.2-4 Since Fujishima and Honda first reported the splitting of water on TiO2 based semiconductor electrode in 1972,5 photoelectrochemical (PEC) procedure has emerged as an inspiring alternative to utilize the solar energy, which combines photocatalysis as well as electrocatalysis and is beneficial for gas collection at separate electrodes.6 In fabricating a PEC system, design and development of effective semiconductor-based photoanodes has become critical importance for researches. Titanium dioxide has been considered as the hot candidate for years and investigated sufficiently due to its attracting features like outstanding electric properties, nontoxicity, excellent stability and cost effective.7-9 However, TiO2 with a large band gap of 3.2 eV, can only response to the ultraviolet radiation, accounting for less than 5% of the whole solar spectrum.10,11 As a result, developing photoanodes with visible light response semiconductors has drawn considerable research attentions. For instance, WO3 (2.7 eV),12-14 Fe2O3 (2.2 eV),15,16 BiVO4 (2.4 eV)17,18 and Ag3PO4 (2.4 eV)19,20 appear to be more valuable for practical application. Nonetheless, intrinsic drawbacks still restrict their practical use, which consists of inadequate absorption of light, short holes diffusion length, unsatisfactory electronic mobility or low carrier lifetime.21-23 To search for novel semiconductor as a photoelectrode remains meaningful and vital so as to better utilize the solar energy. In recent years, the metal-free graphic carbon nitride (g-CN) with unique layered structure and electronic properties has attracted intensive interest as a new class of visible light driven organic polymer semiconductor photocatalyst,24,25 due to the attracting advantages of moderate bandgap energy of 2.7 eV for visible light absorption,26 proper band position for enough oxidizing ability, low cost with 2


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earth-abundant elements,27 thermal and chemical stability as well as environmental sustainability.28 However, the quick recombination rate of photo-generated electrons and holes still prejudices the performance of g-CN. Thus, in the area of PEC application, further efforts are still needed in order to get the satisfying film electrode.29 In photocatalysis, in addition to the choice of semiconductor with outstanding optical absorption properties and proper band positions, heterojunction construction is usually a prospective approach to get superior properties. Heterojunctions can be formed between two semiconductors with matched band edge, which allows efficient separation and transfer of photogenerated charge carriers at the interfaces.30-32 For g-CN, TiO2/g-CN,33,34 g-CN/ZnO,35 g-CN/Ag3PO4,36 WO3/g-CN37 composites with heterojunctions have exhibited superior catalytic property compared with bare g-CN. Furthermore, as for PEC application, nanostructure engineering of photoanodes with high surface area and favorable morphology is considered as effective strategy so as to get superior performance.38,39 Thus, compared with films obtained via bulk powders stacked by binders, which shows deleterious grain boundary effects and bad adhension with substrate,40 it is preferential to fabricate photoanodes in form of semiconductor film directly grown on the substrate. However, strategies of fabricating nanostructured g-CN films with uniform distribution and large area on substrates still need efforts to realize. We have prepared the g-CN film possessing intimate contact with the FTO substrate via a solvothermal synthesis with melamine and cyanuric chlorideas the precursors as well as acetonitrile being solvent.41 However, the morphology of the film appears likely to be a thick layer with large particles consisting of smaller nanoscale particles, which is somewhat not beneficial enough for effective separation of charge carriers and further optimization is needed. Thus, it is necessary to develop a film photoanode of high quality. One dimensional structure with vertically aligned arrays on substrates has received great attentions in PEC area, which can offer large surface area and direct charge transport paths at the same time.42,43 TiO2 nanoarrays have been accepted as outstanding substrates for further investigation of the fabrication and mechanism of 3


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highly efficient photoanodes. 44,45 Based on these consideration, we have chosen TiO2 nanorod (NR) arrays as substrates for further growth of g-CN and fabricated the TiO2/g-CN heterojunction with nanorod array structure for effective PEC water splitting application in this work. The TiO2 nanorod (NR) arrays are prepared directly onto FTO substrate as electron conductive material. A following solvent thermal process is adopted to construct a g-CN layer with melamine and cyanuric chloride as the precursors. The influence of both hydrothermal time of TiO2 and precursor concentrations of g-CN on the morphological structure and photoelectrochemical properties of the TiO2/g-CN composite are investigated. The optimized TiO2/g-CN exhibits desirable core-shell structure with thin g-CN layer uniformly covering the TiO2 nanorod arrays and superior PEC water oxidation performance is observed.

EXPERIMENTAL SECTION Hydrothermal preparation of TiO2 nanorod arrays TiO2 nanorod arrays are fabricated via a hydrothermal progress. In detail, 0.5 mL tetrabutyl titanate was added dropwise into the hydrochloric acid solution accompanied by intense stirring, which was prepared with volume ratio of concentrated hydrochloric acid to water being 1:1. After stirred for 30 min, the precursor solution was transferred to the Teflon-lined autoclave with a FTO substrate leaning against the autoclave wall and the conductive side facing down. In advance, the FTO substrates were cleaned sufficiently by sonication in acetone, ethanol and water in sequence. The hydrothermal reaction was carried out at 150 °C with different durations and the TiO2 nanorod arrays on FTO substrate were at last obtained after a heat treatment at 500 °C for 1 h, denoted as TiO2-t, where t represents for hydrothermal duration (2 h, 4 h and 6 h). Solvothermal preparation of TiO2/g-CN composites To further prepare g-CN films, the solvothermal reaction was implemented using 25 mL acetonitrile as solvent, variable concentrations of cyanuric chloride and melamine with fixed molar ratio of 2:1 as precursors, and the obtained TiO2 nanorod arrays on FTO as substrates. The solvothermal precursor solution was stirred for 6 h without air 4


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at room temperature and further transferred into Teflon-lined autoclave and maintained at 180 °C for 24 h. The resulted films were rinsed with acetonitrile, water and ethanol sequentially and calcined at 520 °C for 1 h under N2 atmosphere. The samples were denoted as TiO2/g-CN-a/b/c/d (additive amount of cyanuric chloride and melamine being: 2 mmol, 1 mmol; b: 1.5 mmol, 0.75 mmol; c: 1 mmol, 0.5 mmol; d: 0.5 mmol, 0.25 mmol, respectively). Characterization and photoelectrochemical measurements A D8 advance diffractometer (Germany Bruker) was applied to record the X-ray diffraction patterns, using Cu Kα radiation (λ = 0.154056 nm). The top view and cross-sectional morphologies of the prepared films were observed on scanning electron microscope (SEM) using Hitachi FE-SEM S4800, Japan. Transmission electron microscope (FEI Tecnai G2) was employed to characterize the structural images via transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The energy dispersive X-ray spectroscopy (EDX) is implemented with the STEM mode. UV-vis diffuse reflectance spectra (UV-vis DRS) was carried out on a Shimadzu UV-3600 spectrophotometer with BaSO4 as standard background reference to characterize the optical properties of the samples. Photoluminescence (PL) spectra was collected on a Cary Eclipse fluorospectro photometer, using an excitation wavelength of 320 nm. All electrochemical and photoelectrochemical measurements were carried out on a three-electrode configuration with the synthesized films with an exposed area of 1 cm2 as working electrodes, a Pt foil as counter electrodes and the saturated calomel electrode (SCE) as reference electrodes, respectively. The electrolyte is 0.2 mol L-1 Na2SO4 solution and is purged with N2 for 30 min before measurement. Light resource is a 200 W xenon lamp (Newport, Oriel Instruments U.S.A.) equipped with a 420 nm cut off filter (20 mW cm-2). Linear sweep voltammograms (LSV) at a scan rate of 10 mV s−1 and amperometric i-t curves under chopped light were recorded on CHI660A electrochemical workstation. Electrochemical impedance spectra (EIS) were performed on Zahner IM6 in the frequency from 100 kHz to 0.01 Hz at 0.60 V (vs. SCE) with alternating current (AC) voltage of 10 mV. 5


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RESULTS AND DISCUSSION Effects of hydrothermal durations of TiO2 nanoarray

Figure 1. (a) Schematic illustration of the formation process of TiO2/g-CN films on the FTO substrate. (b) Wide angel X-ray diffraction (XRD) patterns of the TiO2 and TiO2/g-CN films with different hydrothermal duration.

Figure

1a

illustrates

the

engineering

strategy

of

the

TiO2/g-CN

films.

Hydrothermal process was taken to synthesize the well aligned TiO2 nanorod arrays. Then, the g-CN layers are further fabricated onto the TiO2 nanorod films via the solvothermal growth strategy. The effects of different hydrothermal time of TiO2 as well as precursor concentrations of the g-CN layer are investigated to optimize the heterojunction structure. In order to examine the crystalline phase of the synthesized TiO2 and TiO2/g-CN composite films, wide angel X-ray diffraction (XRD) patterns are presented in Figure 1b. Taking TiO2-4h as a sample, the TiO2 film shows diffraction peaks at 36.3°and 62.9°, which are respectively indexed to the (101) and (002) crystal planes of the well crystalized rutile phase (JCPDS No.21-1276).46 Disappearance of crystal planes such as (110), (111) and (211) indicates the selectivity of crystal orientation growth of TiO2 on the FTO substrates. For all TiO2/g-CN films with various hydrothermal time of TiO2, existence of rutile phase TiO2 can be verified. Specially, taking the diffraction peaks at 37.9° of FTO as a reference, the intensity of (101) and (002) grow synchronously when hydrothermal duration prolonged from 2h to 4h, indicating the nucleation and growth at early stage both along the two directions. Further extending of hydrothermal reaction time to 6h, the peak strength of the (002) plane significantly increased, which manifests the rapid growth of TiO2 in the direction of the [001] in accompany with the increase of thickness. As for the 6


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g-CN in composite films, no obvious diffraction peak appears. The tiny amount and relatively bad crystallinity of g-CN might attribute to the unobservable diffraction peaks. However, the collected powders crapped off the FTO substrates obtained after hydrothermal reaction reveal the characteristic diffraction peaks of g-CN at 13.1° and 27.5° (Figure S1), corresponding to the (100) and (002) crystal planes, respectively.47 This indicates the formation of g-CN and no impurity phase exists.

Figure 2. SEM images of (a) TiO2-2h, (b) TiO2-2h/g-CN, (c) TiO2-4h, (d) TiO2-4h/g-CN, (e) TiO2-6h and (f) TiO2-6h/g-CN.

The morphology characteristic is illustrated intuitively by SEM images. As Figure S2 shown, the epitaxial grown g-CN films through the solvothermal reaction progress appear to be stacked onto the FTO substrates with the micro-spheres consisting of smaller nanoparticles. This thick and compact film is not in favor of separation and migration of the short lifetime charge carriers though a continuous and complete film 7


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has formed on FTO. Figure 2 is the SEM images of TiO2 films with different reaction time and the corresponding TiO2/g-CN films. TiO2 nanorod arrays with uniform distribution provide large specific surface area and plentiful active sites for the nucleation and growth of g-CN film. As shown in Figure 2a, TiO2-2h exhibits inclined and interlaced nanorods with small diameter and space. As a result, the TiO2-2h/g-CN show incomplete coverage of g-CN with large bulk stacking on the TiO2 arrays (Figure 2b). From Figure 2c and 2e, TiO2-4h and TiO2-6h possess nanorod arrays almost perpendicular to the substrates with well ordering and larger diameters and spaces. After the solvent thermal progress, g-CN fill into the intervals of TiO2 arrays and the core-shell structured TiO2/g-CN with uniform thin g-CN layer is observed, which may contribute to interfacial reaction and separation of charge carriers. Under the same solvent thermal conditions, excess g-CN in TiO2-4h/g-CN load on the surface of TiO2 and form a partial thin g-CN coverage (Figure 2d). While for the TiO2-6h/g-CN, a thick and compact g-CN layer is observed (Figure 2f). The above analysis indicates that morphology of TiO2 arrays has great influence on the solvothermal growth of the g-CN layer and morphology of large bulk agglomeration can been effectively confined under optimal experiment conditions.

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Figure 3. (a) Linear sweep voltammograms of TiO2, g-CN and TiO2/g-CN NRs under illumination (λ > 420 nm) and dark states. (b) Corresponding photoconversion efficiency as a function of the applied potential. (c) Linear sweep voltammograms with chopped light illumination at a scan rate of 10 mV s−1. (d) Amperometric i-t curves with chopped light illumination at a potential of 0.6 V (vs. SCE) in 0.2 M Na2SO4.

To investigate the photoelectrochemical properties of the TiO2, g-CN and TiO2/g-CN composite films, linear sweep voltammetry (LSV) measurements and amperometric i-t curves were carried out. In Figure 3a, the solid lines represent the i-V curves under light illumination, while the dash lines stand for the curves performed under dark environment. As illustrated, the TiO2 photoanode exhibits little photocurrent for the wide band gap energy can hardly absorb visible light. As for the g-CN film synthesized via the solvothermal method, photocurrent is small as well, of a few microamps, due to the high recombination rate of photo-generated electrons and holes and the bulk morphology restricting the charge carriers transmission. However, when taking TiO2 as an electron transfer layer, all the TiO2/g-CN composites with core-shell structure exhibit superior PEC performance. Among them, 9


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TiO2-4h/g-CN shows the highest current density. The corresponding photoconversion efficiency plots are presented in Figure 3b to evaluate the conversion efficiency from light energy to chemical energy. The pursuant equation is introduced in detail in the Supporting Information. A maximum photoconversion efficiency is measured to be 0.068% at 0.17 V vs. SCE for the TiO2-4h/g-CN, while the efficiency for pristine TiO2 or g-CN are quite tiny. These results demonstrate the enhanced PEC performance for TiO2/g-CN composite due to the heterojunction formation. The i-V curves under chopped light illumination (Figure 3c) show the same tendency of as Figure 3a, in which prompt photocurrent response is observed. As shown in Figure 3d, the transient photocurrent curves under chopped light illumination demonstrate prompt and reproducible generation of the photocurrent, in which photocurrent densities at 0.6 V (vs. SCE) of TiO2, g-CN, TiO2-2h/g-CN, TiO2-4h/g-CN and TiO2-6h/g-CN being 2.2 µA cm-2, 3.7 µA cm-2, 20.9 µA cm-2, 43.3 µA cm-2, 30.5 µA cm-2, respectively, indicating markedly improved photocurrent after coupled TiO2 and g-CN. The current density of TiO2-4h/g-CN is 11 times higher than that of pure g-CN film. For the TiO2-2h/g-CN, the smallest photocurrent density among the TiO2/g-CN composites probably due to the morphology of large bulk g-CN stacked on the surface of TiO2, in which large surface area electrode failed to be formed, as can be seen in the SEM images (Figure 2b). As for the TiO2-6h/g-CN, a dense g-CN layer on the top of TiO2 somewhat prevent the contact with the electrolyte and draw back the superior PEC performance at interface. Thus, the TiO2/g-CN core-shell structure of high quality is preferred as proper space can provide active area and sufficient contact with electrolyte in order to obtain the enhanced PEC performance.

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Figure 4. (a) UV-vis diffuse reflectance spectrum and (b) Tauc plots of TiO2, g-CN and TiO2/g-CN. Mott-Schottky curves of (c) g-CN and (d) TiO2-4h. (e) Schematic diagram of PEC water oxidation mechanism on the TiO2/g-CN arrays under visible light.

The light absorbance properties of the TiO2, g-CN and TiO2/g-CN films were investigated via the UV-vis diffuse reflectance spectrum (Figure 4a) and the corresponding Tauc plots are illustrated in Figure 4b. As shown, the rutile phase TiO2 shows an absorption edge at around 410 nm, consistent with the band gap energy of 3.02 eV, which is in accordance with other reports.48 The g-CN synthesized via solvent thermal process possesses visible light response with the band gap energy of 2.05 eV. Compared with the g-CN prepared via the traditional calcination polymerization, absorption edge of g-CN from solvothermal method shows obvious red-shift. The resulting TiO2/g-CN composite shows an obvious visible light absorption in the range of wavelength from 400 nm to 600 nm. Mott-Schottky curves of g-CN and TiO2 is presented in order to further investigate the charge transport mechanism of semiconductor. As shown in Figure 4c and 4d, the positive slopes for both g-CN and TiO2 demonstrate the characteristic of n-type semiconductor. According to the Mott-Schotty equation (Supporting Information), the flat band for g-CN and TiO2 is speculated by the x-intercept to be -1.0 V and -0.81 V, respectively. As a result, the PEC schematic diagram on TiO2/g-CN is presented in Figure 4e to explore the behavior of charge carriers inside the electrode as well as at the interface 11


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with electrolyte. Under visible light illumination, electrons in g-CN can be generated to the conductive band with holes left in the valence band. As aforementioned, the flat band potential of g-CN is testified to be at -1.0 V, which is more negative than that of TiO2 (-0.81 V). Since the conduction band (ECB) potential is very close to the EFB in the case of n-type semiconductor, we approximately consider the ECB is around the EFB. The well matched junction enables photo-generated electrons in g-CN to migrate to the conduction band of TiO2 layer and then further migrate to the external circuit and generate photocurrent. Meanwhile, holes migrate to the electrode-electrolyte interface to realize PEC water oxidation. What counts is that the heterojunction between TiO2 and g-CN make a significant contribution to better separation and migration of charge carriers and inhibition of the high recombination rate between photo-produced electrons and holes.

Figure 5. (a) Fluorescence spectra of g-CN and TiO2/g-CN composites. (b) Electrochemical impedance spectra under visible light of TiO2, g-CN and TiO2/g-CN NR arrays at a potential of 0.6 V (vs. SCE) in 0.2 M Na2SO4 solution. Inset is the equivalent circuit.

In order to characterize the separation and recombination rate of the photo-generated charge carriers within the TiO2/g-CN composites, photoluminescence measurement (Figure 5a) is implemented. In the fluorescence spectra, decreased emission peak intensity indicates that the fluorescence-associated recombination rate between photo-excited electrons and holes is efficiently restrained. Under the same excitation wavelength at 320 nm, the pristine g-CN exhibit a emission peak with the strongest intensity at about 476 nm, which is ascribe to the recombination of leaped π 12


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electrons. After coupled with TiO2, the PL intensity appears to decrease, suggesting that the recombination rate between electrons and holes is efficiently restrained. In other words, formation of interfacial junctions in the composites can effectively promote the separation and migration of the charge carriers and then accelerate the PEC water splitting performance. As the increase of the TiO2 hydrothermal reaction time, the intensities gradually weaken since formation of heterojunction with larger area makes for photo-induced charge separation. Combined with SEM images (Figure 2), area of the formed heterojunction between TiO2 and g-CN shares the same tendency with the PL spectra. Electrochemical impedance spectra performed in the frequency range from 100 kHz to 0.01 Hz at 0.6 V (vs. SCE) are displayed in Figure 5b to further explore the charge transfer process. The inset image in Figure 5b is the corresponding equivalent circuit, where RS, Rct, Wo and CPE stands for the solution resistance, charge transfer resistance, mass transport component (Warburg impedance) and double layer capacitance, respectively. According to the spectra, the TiO2/g-CN composites demonstrate much smaller diameter of the Nyquist circle than that of bare g-CN or TiO2, declaring the lower charge transfer resistance (Rct) and enhanced charge transfer. Fitting parameters from the EIS spectra via Zview-impedance analysis software 2.80 are presented in Table S1. As shown, Rs of all composites are around the same value. The fitted Rct of g-CN, TiO2-4h, TiO2-2h/g-CN, TiO2-4h/g-CN and TiO2-6h/g-CN are calculated to be 333320, 273460, 104320, 23905, 65340 Ω, respectively. Among them, TiO2-4h/g-CN exhibits the smallest resistance, which is on account of formation of high quality heterojunction. Fabrication of uniform g-CN layer can effectively facilitate the charge transfer at the semiconductor and electrolyte interface. The TiO2-2h/g-CN with bulk stacked g-CN decreases the contact and heterojunction area between TiO2 and g-CN, resulting in relatively high recombination rate of charge carriers and large resistance. The dense and integral g-CN overlayer on TiO2-6h/g-CN hinders the effective charge transfer and reaction with the electrolyte and decreases the contact area, leading to the large diffusion impedance (Table S1) and defective utilization of charge carriers. As a result, TiO2-4h/g-CN with heterojunction of high 13


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quality emerges to be beneficial for the superior PEC property and the results are in accordance with the photoelectric measurements. Effects of solvothermal precursor concentrations of g-CN layer

Figure 6. SEM images of TiO2/g-CN composites with different precursor concentrations: (a) TiO2/g-CN-a, (b) TiO2/g-CN-b, (c) TiO2/g-CN-c and (d) TiO2/g-CN-d.

In order to further prepare the optimized heterojunction structure, quantitative test through controlling the precursor concentrations to change the loading amount of the g-CN is carried out. Based on the achievements above, the TiO2 hydrothermal reactive time of 4 h is adopted as substrate. As observed in Figure 6a, the TiO2/g-CN composites with high g-CN precursor concentration exhibit large bulks accumulated on the top of the TiO2/g-CN core-shell structures. By decreasing the concentration, a core-shell structure is formed with the disappearance of the thick g-CN bulk layer in Figure 6b and 6c. Specially, under concentration of c (Figure 6c), a more uniform core-shell structure, with thinner g-CN layers homogeneously dispersed right on the surface of TiO2 nanorods as well as providing plenty of pores, than b (Figure 6b) can be observed. However, when further decreasing the concentration to d (Figure 6d), the core-shell failed to come into being with only partial linkage of TiO2 NRs by tiny g-CN plates due to the reduced reaction probabilities under low concentration.

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Figure 7. Cross-section SEM images of TiO2/g-CN composites: (a) TiO2/g-CN-c, (b) and (c) TiO2/g-CN-a. Corresponding mappings of (c) TiO2/g-CN-a: (d) O element, (e) Si element, (f) Ti element, (g) N element and (h) C element.

Figure 7a illustrates the cross-section SEM image of TiO2/g-CN-c. As shown, the TiO2 arrays exhibit a height of about 2 µm. A core-shell structure is confirmed with thin g-CN layer uniformly covering on the TiO2 arrays, which is in accordance with the image in Figure 6c. As a comparison, the cross-section SEM images of TiO2/g-CN-a composite with high g-CN precursor concentrations are presented in Figure 7b and 7c. The TiO2/g-CN-a composite results in a thicker g-CN layer on the surface of the TiO2 nanorods, with bulk accumulated. Furthermore, we choose TiO2/g-CN-a to demonstrate element distributions. The images of N (Figure 7g) and C (Figure 7h) mappings accord with the g-CN layer and verify that the packed layer in the case of TiO2/g-CN composite with high g-CN precursor concentrations is g-CN. The mapping graphs of O (Figure 7d) and Ti (Figure 7f) match well with the TiO2 15


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nanorods in SEM images of TiO2/g-CN-a. Moreover, Si element together with O element (Figure 7e) shows distribution in accordance with the substrates.

Figure 8. (a) TEM and (b, c) HRTEM images of TiO2/g-CN-c composite.

TEM and HRTEM measurements are employed to identify the structure of TiO2/g-CN composite and presented in Figure 8. In Figure 8a, the core-shell structure is intuitively confirmed in the TEM image of TiO2/g-CN-c. As shown, the nanorods are wrapped with a thin silk-like layer, labeled with red arrows. HRTEM images of the interfacial region in Figure 8b and 8c (amplification of the green region in Figure 8b) verify that the interface is made up with the crystalline (101) plane of rutile and the amorphous thin layer. Combined with the EDS in Figure S3, the crystalline region mainly consists of Ti and O elements. While the amorphous area displays obvious peaks of C and N elements. The results further convince the successful fabrication of core-shell structure with amorphous g-CN covered on TiO2 nanorods.

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Figure 9. (a) Linear sweep voltammograms of TiO2/g-CN NRs under illumination (λ > 420 nm) and dark states. (b) Corresponding photoconversion efficiency. (c) Linear sweep voltammograms with chopped light illumination at a scan rate of 10 mV s−1. (d) Amperometric i-t curves with chopped light illumination at a potential of 0.6 V (vs. SCE) in 0.2 M Na2SO4.

Linear sweep voltammograms and amperometric i-t curves were further measured to demonstrate the PEC performance of TiO2/g-CN with different laden amount of g-CN (Figure 9). In Figure 9a and 9c, as the precursor concentrations decrease, the enhanced PEC performance is observed first, with TiO2/g-CN-c displaying the largest photocurrent. However, the extreme low concentration, in the case of TiO2/g-CN-d, results in reduced photoelectric properties. Figure 9b verifies that the photoconversion efficiency further improve to 0.135% at 0.16 V vs. SCE for the TiO2/g-CN-c. Combined with the structural analyst in Figure 6, the PEC performance is in accordance with the area and quality of the TiO2/g-CN heterojunction. At the potential of 0.6 V (vs. SCE) (Figure 9d), the photoelectric response decreases in the order of TiO2/g-CN-c > TiO2/g-CN-b > TiO2/g-CN-a > TiO2/g-CN-d. Specifically, the photocurrent density of TiO2/g-CN-c reaches to as high as 80.9 µA cm-2, which is 17


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more than 20 times higher than that of the pure g-CN film. The incident photon-to-electron conversion efficiency (IPCE) is measured according to the Supporting Information. The curves in Figure S4 reveal that pristine g-CN exhibits weak photoresponse. Meanwhile, TiO2 possesses higher IPCE only under light of short wavelength with little photoresponse detected at visible light region. The IPCE of TiO2/g-CN-c is higher than that of TiO2 or g-CN at the whole spectrum and demonstrates the well visible light response. The significant enhancement of IPCE for TiO2/g-CN composite indicates the promoted separation and migration of charge carriers due to the formation of TiO2/g-CN heterojunction, which also coincides with the UV-vis diffuse reflectance spectroscopy (Figure 4a). Besides, the i-V curves under AM 1.5G simulated solar source irradiation (Figure S5a) and the corresponding photoconversion efficiency (Figure S5b) share the same conclusion that TiO2/g-CN-c exhibits superior PEC performances than pristine TiO2 or g-CN. Due to the existence of ultraviolet light in AM 1.5G simulated solar source, the performance of TiO2 electrode is enlarged than that under visible light. However, after coupling with g-CN, the performance of TiO2/g-CN-c composite is further enhanced, indicating that the construction of heterojunction structure has greatly promoted the PEC properties. Based on the photoconversion efficiency, time courses of oxygen evolution for TiO2/g-CN-c under AM 1.5G simulated sunlight has been monitored at 0.16 V vs. SCE presented in Figure S6. The photocurrent in Figure S6a verify the outstanding stability of TiO2/g-CN-c composite. The time courses of gas evolution (Figure S6b) demonstrates that the detected amount of oxygen evolution are almost in accordance with theoretical value.

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Figure 10. (a) Fluorescence spectra of TiO2/g-CN composites. (b) Electrochemical impedance spectra under visible light of TiO2/g-CN nanorod arrays at a potential of 0.6 V (vs. SCE) in 0.2 M Na2SO4 solution. Schematic illustration of charge carrier migration and recombination path on (c) bare g-CN films and (d) TiO2/g-CN array composites.

The PL spectra in Figure 10a verify that TiO2/g-CN-a with high g-CN precursor concentrations exhibits the highest PL peak intensity. As precursor concentrations decrease, peak emission intensities gradually reduce, in which TiO2/g-CN-c with superior

junction

between

TiO2

and

g-CN

exhibits

the

very

weak

fluorescence strength, indicating the reduced recombination rate. As illustrated in the structural analyst (SEM) in Figure 6, heterojunction with high area in all the composites of TiO2/g-CN-a, TiO2/g-CN-b and TiO2/g-CN-c is formed. Therefore, overgrowth of g-CN results in revere recombination within g-CN in the case of TiO2/g-CN-a and TiO2/g-CN-b. The lowest intensity of TiO2/g-CN-d might be on account of the extreme small amount of g-CN, which contributes to the low recombination probabilities of photo-generated carrier. The electrochemical impedance spectra (Figure 10b) is further carried out to characterize interface charge 19


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transfer ability. According to the fitting parameters in Table S2, the charge transfer resistance (Rct) of TiO2/g-CN-a, TiO2/g-CN-b, TiO2/g-CN-c and TiO2/g-CN-d are 91012, 19923, 10045 and 189610 Ω, respectively. The charge transfer resistance shows the tendency of TiO2/g-CN-c < TiO2/g-CN-b < TiO2/g-CN-a < TiO2/g-CN-d, in which TiO2/g-CN-c possesses the lowest Nyquist circle radius. The reason may be explained by the unique core-shell structure in TiO2/g-CN-c guarantees the formation of heterojunction between the TiO2 and g-CN, which is in agreement with the photoelectric measurement. To vividly illustrate the separation, migration as well as recombination of charge carriers on the TiO2/g-CN composite, schematic are provided in Figure 10c and 10d. The g-CN film prepared directly on the FTO substrates through solvent thermal reaction (Figure 10c) is consists of stacked smaller particles. The abundant interfaces between the bulk particles bring about extensive recombination centers for photo-generated electrons and holes, which significantly lowers the corresponding PEC performance. Nevertheless, as seen in Figure 10d, the core-shell structures of TiO2/g-CN come up with great advantages. Specifically, the well aligned TiO2 NR arrays provides growth centers for g-CN film during the solvent thermal progress and can effectively restrict the overgrowth of it, resulting in g-CN layer as shell uniformly covering on the TiO2 core. This formation of TiO2/g-CN core-shell structure offers extensive surface areas for reactive centers. Meanwhile, heterojunction between TiO2 and g-CN, with TiO2 as electron transport material at the same time, effectually facilitates the separation and migration of photo-generated charge carriers and brings about high photoelectric efficiency. Moreover, the fact that the direction of holes transporting is perpendicular to that of electrons as well as the lighting direction allows the film electrodes to be thick enough for adequate light absorption. This can effectively promote the PEC performance since the holes diffusion length is usually shorter than the optical penetration depth. In the case of the TiO2/g-CN composites, the thick is almost as the length of TiO2 nanorods, which is beneficial for light absorption, while the holes diffusion length is as the thick of g-CN layer, which offers more possibility for holes to reach the interfacial with electrolyte to take part in 20


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oxidation reaction there other than recombination. The unique system guarantees the superior photoelectrochemical properties and provides exciting prospect for fabrication of PEC electrodes.

CONCLUSIONS In this work, we report the fabrication of TiO2/g-CN heterojunction of high quality with core-shell nanorod arrays structure as PEC water oxidation electrodes. The TiO2/g-CN composite possesses superior performance since the heterojunction can effectively facilitate separation and transfer of photo-generated electrons and holes and the core-shell structure provides plenty of contact area with the electrolyte, making for the prompt usage of holes. Via controlling the reactive duration of TiO2, film with optimized TiO2 NR arrays is obtained, which further helps to get a uniform g-CN thin layer under proper precursor concentration of g-CN. This structure efficiently suppresses the bulk effect with high orientation and the space between nanorods. As a result, the TiO2/g-CN heterojunction film exhibit photocurrent density of 80.9 µA cm-2, 21 times as high as the bare g-CN film.

ASSOCIATED CONTENT Supporting Information Detailed descriptions of Mott-Schotty equation, photoconversion efficiency, incident photon-to-electron conversion efficiency and oxygen evolution measurements. XRD of TiO2/g-CN powders; SEM of g-CN film; EDX spectroscopy of TiO2/g-CN-c composite; IPCE spectra; Linear sweep voltammograms and photoconversion efficiency under AM 1.5G simulated solar source; time courses of photocurrent density and oxygen evolution; Tables of parameters from the EIS spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Fax: +86 25 52112626. Phone: +86 25 52112906. 21


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Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The authors express their appreciations for the financial support from the Natural Science Foundation of Jiangsu Province (BK20160795), the National Natural Science Foundation of China (51602153, 51372115 and 11575084), the Research Start-up Fund of NUAA (90YAH16008) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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The TiO2/g-CN heterojunction nanoarray with core-shell structure were synthesized on FTO substrates as efficient photoanodes for water oxidation, showing superior photoelectrochemical activity.

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