g-C3N4 Hybrid Photocatalysts with Enhanced

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Fabrication of CoTiO3/g-C3N4 hybrid photocatalysts with enhanced H2 evolution: Z-scheme photocatalytic mechanism insight Rongqin Ye, Hua-Bin Fang, Yan-Zhen Zheng, Nan Li, Yuan Wang, and Xia Tao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01850 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Fabrication of CoTiO3/g-C3N4 hybrid photocatalysts with enhanced H2 evolution: Z-scheme photocatalytic mechanism insight RongQin Ye,† HuaBin Fang, † Yan-Zhen Zheng,*, †, ‡ Nan Li, † Yuan Wang† and Xia Tao*, † † State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. ‡ Research Centre of the Ministry of Education for High Gravity Engineering & Technology, Beijing University of Chemical Technology, Beijing 100029, China.

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ABSTRACT: A novel direct Z-scheme CoTiO3/g-C3N4 (CT-U) photocatalytic system with different weight percentage of CoTiO3 was synthesized using a facile in-situ growth method for H2 evolution from water splitting. The as-prepared CT-U composites composed of 1D CoTiO3 microrod and 2D g-C3N4 nanosheet were characterized by various techniques including XRD, SEM, TEM, XPS, FTIR and UV-vis. Results demonstrate that the CT-U composite photocatalysts were successfully fabricated, with intimate interfacial contact and heterojunction interaction between g-C3N4 and CoTiO3 which can significantly boost the photocatalytic activity compared with prinstine g-C3N4 and CoTiO3. The most enhanced H2 evolution rate of 858 μmol·h-1·g-1 and high quantum efficiency (38.4% at 365 nm, 3.23% at 420 ± 20 nm) are achieved at an optimal 0.15% CT-U. Meanwhile, the 0.15% CT-U sample exhibits good photocatalytic stability in recycling H2 evolution. Accordingly, direct Z-scheme mechanism capable of leading efficient charge carrier separation and strong reduction ability for enhanced H2 production was proposed, and further evidenced by PL, Photoelectrochemical analysis and ESR assay.

KEYWORDS: g-C3N4, CoTiO3, H2 production, quantum efficiency, Z-scheme mechanism, photocatayst.

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1. INTRODUCTION Hydrogen (H2) has gained much attention as a sustainable and carbon-neutral fuel carrier.1 Since Fujishima and Honda reported H2 evolution from water splitting by using TiO2 electrode under light irradiation in 1972,2 photocatalytic water splitting has become one of the most fascinating and promising technologies. Among numerous photocatalysts, graphitic carbon nitride (g-C3N4) has been rapidly developed due to its non-toxic, proper energy band level for H2 production (2.7 eV), ability of absorbing visible light, good physicochemical stability and facile fabrication via one-step polymerization.3-5 Unfortunately, the pure g-C3N4 seriously suffers from poor photocatalytic efficiency because of a high recombination rate of photo-generated charge carriers.6-7 To settle this problem, g-C3N4-based heterojunctions, such as TiO2/g-C3N4,8 BiOCl/g-C3N4,9 WS2/g-C3N4,10 and AgBr/g-C3N411 were prepared. However, the redox ability of photoexcited electrons and holes would be declined as the typical heterojunction forms.

Very recently, the construction of Z-scheme photocatalytic system has become an effective approach to not only allow interfacial charge migration upon excitation and boost the separation efficiency of photogenerated electron-hole pairs, but also preserve excellent redox ability.12-14 The charge carrier transfer path of Z-scheme heterojunction is depicted in Fig.1.15 Regretfully, an ideal g-C3N4-based Z-scheme photocatalyst with sufficient efficiency meeting the practical application is yet to be demonstrated so far.16 Moreover, in-depth investigation on charge separation and electron transport

mechanism

towards

high-performance

g-C3N4-based

Z-scheme

heterojunction

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photocatalysts under sunlight still is rare. Thus exploitation of new highly efficient g-C3N4-based Z-scheme photocatalysts and intense research about the photocatalytic reaction mechanism are desirable.

CoTiO3, a classical ABO3 perovskite oxide, is a narrow band gap semiconductor (Eg = 2.25 eV) with suitable electronic band structure, large absorption coefficient, high carrier mobility as well as stability and has been widely applied as gas sensor, magnetic recorder, catalyst and high-k dielectric ceramic on account of its unique photochemical and physical properties.17 Very recently, CoTiO3 has been explored for photocatalytic oxidation evolution reaction with no co-catalyst.18 Furthermore, CoTiO3 has suitable band edges (ECB ~ 0.1 eV, EVB ~ 2.35eV), which can match well with g-C3N4 (ECB ~ -1.3 eV, EVB ~ 1.4 eV) to probably form a direct Z-scheme photocataytic system.13,

19

However, to date, no report concerning the preparation of CoTiO3/g-C3N4 hybrid

composite and its photocatalytic performance appears.

In the present work, a direct Z-scheme photocatalyst composed of one-dimensional CoTiO3 rods and two-dimensional g-C3N4 sheets has been successfully fabricated through calcining the mixture of preformed CoTiO3 and urea (a g-C3N4 precursor). Herein, g-C3N4 was directly grown on CoTiO3 rod surface to form an intimate two phase contacting layer, which is in favor of rapid electron transfer. The CoTiO3/g-C3N4 hybrid photocatalysts (denoted as CT-U) were characterized in detail. The photocatalytic activities of hybrid CT-U samples were investigated through H2 production from water splitting with Pt as co-catalyst and environment-friendly ethanol as sacrifice agent under artificial sunlight irradiation. Ultimately, the 0.15% CT-U sample exhibits the optimum 4


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photocatalytic activity for H2 production as well as possesses satisfactory photostability after four photocatalytic cycling runs. In addition, the electron transfer mechanism for the significant improvement in the photocatalytic performance of the hybrid photocatalyst was also proposed and proved by photoluminescence (PL) technique, photoelectrochemical assay (PEC) and electron spin resonance (ESR) analysis further.

2. EXPERIMENTAL SECTION 2.1 Materials

All of the chemicals used in present work were analytically pure and without further purification. Ethanol was supplied by Sinopharm Chemical Reagent Beijing Co. (Beijing, China). Titanium butoxide (99.0%), ethyene glycol (EG, 99.0%), Cobalt (II) acetate tetrahydrate (99.9% metal basis) were supplied by Aladdin. Urea (99.0%) was purchased by Beijing Chemical Co. (China). Deionized (DI) water was used throughout the experiments.

2.2 Photocatalyst preparation

The CoTiO3 powder was prepared by a simple precipitation process according to the previous report.18 Typically, 2.49 g Co(CH3COO)2·4H2O powder and 3.4 ml Ti(OC4H9)4 were dissolved in 60 ml ethylene glycol (EG) under stirring. The reaction was kept for about 5 h. The light pink precipitate was obtained by alternatively centrifuging and rinsing with ethanol to ensure the removal of residual ions and impurities, and consequently drying at 60 °C under vacuum. The final

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products were then collected by calcining in air at 700 °C for 4 h with 2 °C/min for further use.

The direct solid-state Z-scheme CoTiO3/g-C3N4 photocatalysts were synthesized by an in-situ growth strategy. The synthesis procedure is schematically shown in Fig. 2. An appropriate amount of CoTiO3 powder was mixed with 10 g of urea into an agate mortar and grounded together. The resultant powder was then transferred to a covered alumina crucible followed by annealing at a 500 °C for 2 h and 520 °C for 2 h. The resulting product was rinsed with water/ethanol, dried in vacuum at 60 °C overnight. The CoTiO3/g-C3N4 composites prepared with different weight radios of CoTiO3 and urea in 0.1, 0.15, 0.2, 0.3, 0.5, 5 wt.% are denominated as 0.1% CT-U, 0.15% CT-U, 0.2% CT-U, 0.3% CT-U, and 0.5% CT-U, 5% CT-U respectively. For comparison, pure g-C3N4 was also prepared similar to the method mentioned above by directly thermal treatment of urea but without CoTiO3. The mixture of 15 mg CoTiO3 and pure g-C3N4 (made from 10 g of urea), named 0.15% CT-CN, was also prepared by directly milling.

2.3 Characterization

The crystalline structure of samples was characterized by powder X-ray diffraction (XRD) using Cu Ka radiation (λ=0.15406 nm) on a Rigaku D/max-2500 VB2+/PC diffractometer in the 2θ range of 10°~80° at a scanning rate of 10°/min. Fourier transform infrared (FTIR) spectra were measured on a Bruker Vertex 70v spectrometer. The morphology of as-prepared samples was characterized by scanning electron microscopy (SEM) (JEOL JSM-6701F) and high-resolution transmission electron microscopy (HR-TEM) (JEOL JEM-3010). An energy-dispersive

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spectroscopy (EDS) measurement was performed with an X-ray energy dispersive spectrometer installed on a JEOL-6701F microscope. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB250 X-ray photoelectron spectrometer using Al Ka as an X-ray source. The optical property of samples was determined by using the absorption spectra recorded on a UV-vis spectrophotometer (Rerkin Elmer Lambda 950 UV/VIS). Photoluminescence (PL) spectra were obtained on a Hitachi F7000 with an excitation wavelength of 340 nm. Photoelectrochemical measurements were performed in a standard three-electrode quartz cell with a 0.5 M Na2SO4 electrolyte solution. A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The sample films, as the working electrodes, were fabricated on indium-tin oxide glass (ITO) as substrates that were ultrasonically cleaned in deionized water, isopropanol, ethanol for 15 min in sequence and then dried. Specifically, 10 mg of sample was first dispersed in DMF (1 mL) to obtain a suspension solution. The suspension was then spin-coated onto the pretreated ITO substrate, followed by drying in vacuum at 120 °C for 2 h for close adhesion. The on–off light photoresponse and electrochemical impedance spectroscopy (EIS) measurements were performed at the open-circuit potential. The frequency range was fixed from 1 to 105 Hz. The photocurrent density-voltage (I-V) curve was measured in the bias sweep range of −1 ~ 1 V. Electron spin resonance (ESR) signals of spin-trapped paramagnetic species with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were conducted with JEOL JES-FA200 X-band spectrometer.

2.4 Photocatalytic experiment for water splitting

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The photocatalytic H2-production experiments were performed in an outer irradiation type photoreactor (50 ml quartz glass). 20 mg of catalyst sample was suspended in an ethanol solution (10 vol.%). Note that ethanol is used as a sacrificial electron donor. 3 wt.% Pt was photodeposited on the surface of catalyst using H2PtCl6 as a precursor. Before irradiation, the suspension is thoroughly degassed to remove air by bubbling N2 for 20 min. During the water splitting experiment, the suspension was stirred and irradiated under simulated solar light using a 300 W Xeon lamp (Perfect Light, Beijing) at a working current of 17.5 A for 4 h. The irradiation intensity on the flask was ca. 190 mW/cm2 for simulated solar light and ca. 152 mW/cm2 for visible light with a 420 nm cutoff filter. The reaction temperature was adjusted to room temperature by a flow of cooling recycling water. The evolved gas was analyzed by a gas chromatograph (GC-TP2080, BFTP, China, TCD, nitrogen as carrier gas and TDX-01 carbon molecular sieve column). The quantum efficiency (QE) of photocatalyst was measured using the same experiment condition, except for irradiation light with a series of monochromatic light with wavelength 365 nm (9 mW/cm2), 420 ± 20 nm (72 mW/cm2), and 630 nm (114 mW/cm2). The QE values were calculated using the following equations:

QE (%) 

number of reacted electrons 100 number of incident photons

2  number of evolved H 2 molecules  100 number of incident photons

(1)

3. RESULTS AND DISCUSSION 3.1 Characterization of as-prepared samples 8


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XRD patterns of g-C3N4, CoTiO3 and a series of CT-U photocatalysts are shown in Fig. 3. The diffraction peaks of pristine g-C3N4 appear at 13.1° and 27.4°, assigned to the interplanar structural packing motif (100) of tri-s-triazine units and the interlayer stacking peak (002) of aromatic systems (JCPDS 87-1526), respectively. The two diffraction peaks are in accordance with those of g-C3N4 in the literature.20-22 For the bare CoTiO3, the characteristic peaks with 2θ values of 23.9°, 32.8°, 35.4°, 40.5°, 49.0°, 53.5°, 61.9°, 63.5° can be indexed to the (012), (104), (110), (113), (024), (116), (214) and (300) diffraction planes of CoTiO3 (JCPDS 15-0866), respectively. From the XRD patterns of composite CT-U photocatalysts, both of the characteristic peaks of g-C3N4 and CoTiO3 can be visualized without other impurity phases. This indicates that the well-crystallized CoTiO3 has been successfully loaded on the g-C3N4 to form two-phase CT-U hybrid photocatalysts. Besides, with the increasing of CoTiO3 content in CT-U composite, the diffraction peak intensity of CoTiO3 becomes stronger, whereas the peak intensity of g-C3N4 weakens gradually. It is deduced that the presence of CoTiO3 crystal may lead to restriction of crystallization of g-C3N4.20 To further verify this phenomenon, 5% CT-U is also prepared and characterized by XRD technique. As a result, no characteristic peaks related to g-C3N4 can be observed (Fig. S1).

To investigate the morphology of as-prepared samples and the heterojunction interface between CoTiO3 and g-C3N4, we preformed SEM and HRTEM measurements. Fig. 4a and b show the representative SEM images of g-C3N4 and CoTiO3, respectively. As can be seen, the pure g-C3N4 is composed of irregular nanostructured crystals stacking layers with curly nanosheets. CoTiO3 particles appear as micro-scale rods with average diameter of ~ 1μm and length of ~ 3 μm.

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Each CoTiO3 microrod consists of packed nanoparticles with the diameter of ~ 160 nm. Fig. 4c and d display the SEM and TEM images of CT-U hybrid composite. It can be observed that g-C3N4 successfully twines along the overall surface of CoTiO3 microrods, leading to the formation of a heterostructure interface. The HRTEM image, as displayed in Fig. 4e, provides more detailed information about the interfacial structure of the hybrid materials. Clearly, a well-crystallized CoTiO3 is covered with the 2D g-C3N4 layer. Though no lattice fringe of g-C3N4 is observed, its honeycomb porous structure is obviously discernable. Moreover, the crystal lattice spacing of 0.27 nm alongside the interface is in accordance with the (104) facet of high-crystalline CoTiO3 (JCPDS 15-0866), suggesting that intimate junctions are formed between the (104) plane of CoTiO3 and g-C3N4.23 To further verify the combination of CoTiO3 and g-C3N4 in the composites, EDS and elemental mapping measurements were also performed. The EDS spectrum of 0.15% CT-U composite (Fig. 4f) confirms the existence of C, N, Co, Ti and O elements in the composite. The overall elemental maps of the CT-U are also given in Fig. 5, which exhibits the dispersion of CoTiO3 rods within the g-C3N4, implying that mircrorod-structure CoTiO3 is completely and closely surrounded by g-C3N4.

The surface chemical states of CT-U were investigated by XPS technique as depicted in Fig. 6a. The photoelectron peaks of C, N, Co, Ti and O elements can be observed in the survey spectrum of 0.15% CT-U composite. The high resolution XPS profiles show peak positions at binding energies of 780.6 eV and 458.3 eV, corresponding to elements in the format of the oxidized states of Co 2p and Ti 2p for CoTiO3, respectively.24 The C 1s spectrum (Fig. 6b) exhibits two obvious

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peaks at 288.2 eV and 284.9 eV for g-C3N4 and 0.15% CT-U. The strong C peak at 288.2 eV is identified as sp2-bonded carbon (N–C=N), and the weak one at 284.9 eV corresponds to graphitic carbon.22 Note that the existence of C element peaks at 284.9 eV for pure CoTiO 3 may be considered to originate from some graphitic carbon residual on the surface of CoTiO3 in the process of calcining the glycolate precursor i.e. titanium-cobalt-EG (Ti-Co-EG). Accordingly, in comparison with the pristine g-C3N4, the intensity of peak at 284.9 eV for 0.15% CT-U strengthens significantly arising from the superposition of the graphitic carbon peak within pure CoTiO3. This demonstrates that CoTiO3 is successfully combined with g-C3N4.

The N 1s wide peak for g-C3N4 (Fig. 6c) could be fitted into three peaks at 401.1, 398.7 and 400.0 eV (Fig. S2), attributed to the binding energy of C–N–H, C=N–C and N–(C)3 functional groups, respectively.25 Compared with N 1s peaks at 398.7 eV and 400.0 eV for g-C3N4 (Fig. S2a), the N 1s peaks for 0.15% CT-U (Fig. S2b) shift to 398.6 eV and 399.6 eV, suggesting that the chemical microenvironment of N atoms in C=N-C and N–(C)3 groups has changed since g-C3N4 hybridizes with CoTiO3.25 Such a binding energy change resulting from the change of local bonding environment of species, can be also observed from the XPS spectra of O 1s (Fig. 6d), Co 2p (Fig. 6e) and Ti 2p (Fig. 6f) peaks. The oxygen element has two chemical states in CoTiO3, namely the crystal lattice oxygen at ca. 529.9 eV and the adsorbed oxygen at ca. 532.1 eV. The O1s, Co 2p and Ti 2p peaks of CoTiO3 shift to a higher binding energy, whereas the N 1s of g-C3N4 binding energy decreases accordingly. Such opposite tendency illustrates that the total electron density from CoTiO3 phase immigrates to g-C3N4 phase to some degree, facilitating the formation

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of the Co-O-N or Ti-O-N bond in the interface between CoTiO3 and g-C3N4.26-28 The changes of photoelectron peak intensity and binding energy demonstrate the existence of heterojunction interaction in the CoTiO3/g-C3N4 hybrid photocatalyst.

Fig. 7a exhibits FTIR spectra of g-C3N4, CoTiO3, and C-U hybrid photocatalysts. As for pure CoTiO3, the bands at 3309~3650 cm-1 and 1631 cm-1 are attributable to the stretching vibrations of -OH and δ-OH groups from the physically adsorbed water.29 The bands between 500 and 600 cm-1 can be assigned to the vibrations of metal ions from OR groups linked to Co or Ti. 30 As for the g-C3N4, the typical peaks located at 2820-3645, 1200-1650, and 810 cm-1, correspond to the vibrational absorption of N-H and O-H, aromatic C-N heterocyclic unites and the triazine unit, respectively.31 After introducing CoTiO3 within g-C3N4, the main characteristic peaks of OR groups for CoTiO3 and C-N heterocycycles with triazine units for g-C3N4 are discernable in all CT-U hybrid photocatalysts. However, it is worth noting that for the CT-U composites, the characteristic peak at the 2820-3645 cm-1 region is observed to extend to 2688-3645 cm-1, which is thought to be resulting from the generation of a certain chemical bond to link the CoTiO3 crystal and g-C3N4 between the O atom from CoTiO3 and N-H from g-C3N4.23, 32 Integrated with the XPS analytical results, the molecular interaction between perovskite CoTiO3 and tri-s-triazine g-C3N4 through some chemical bonds such as Co-O-N or Ti-O-N is schematically shown in Fig. 7b. The FTIR result further verifies the existence of heterojunction interaction in the CT-U composites, which is consistent with that of the XPS analysis.

The optical property of as-prepared photocatalysts is investigated using UV-vis 12


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photo-absorption spectra ranging from ultraviolet to visible region, as shown in Fig. 8a. The pure g-C3N4 exhibits an absorption edge at ~450 nm, in keeping with the value reported previously.11 The pure CoTiO3 exhibits a strong absorption over a wide spectra region from UV to visible light. The sharp region below 483 nm is due to O2-→Ti4+ charge-transfer interaction.18, 24 Moreover, the two absorption bands observed at ca. 537 and 610 nm, namely Co2+→Ti4+ charge-transfer bands, are most likely to originate from the crystal field splitting of CoTiO3.18 In the absorption spectra of CT-U composites, it can be clearly found the absorption peak at 610 nm caused by d-d transition of Co2+ assigned to CoTiO3. Furthermore, the more content of CoTiO3 corresponds to the stronger intensity of the characteristic peak, suggesting heterojunction interaction between g-C3N4 and CoTiO3.

The band gap energy of as-synthesized samples can be estimated via the Tauc plot calculated from the equation as follows:

(ahv)r = A(hv - Eg)

(2)

where a, h, v, A and Eg are the absorption coefficient, Planck constant, light frequency, proportionality and energy band gap, respectively. The band gap values of g-C3N4 and CoTiO3 can be obtained from the intercept of linear regime in plots of (ahv)1/2 vs. hv (r = 2 for direct transition, r = 1/2 for indirect transition).21 As shown in Fig. 8b, the band gaps of g-C3N4 and CoTiO3 are estimated to be around 2.74 eV and 2.25 eV, respectively. Additionally, as for 0.15% CT-U, the energy band gap is calculated to be approximately 2.78 eV, which is a bit higher than that of pure

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g-C3N4 (~2.74 eV). A higher energy band gap for the composite could be beneficial for improving the redox potential of photocatalytic reaction, which is important for water splitting.22

3.2 Photocatalytic H2-production activity

Photocatalytic hydrogen evolution over as-prepared samples loaded with 3 wt.% Pt as co-catalyst was evaluated under artificial sunlight irradiation using ethanol as hole scavenger. Fig. 9a shows the photocatalytic activities for H2 production as function of the composited amount of CoTiO3 for various CT-U photocatalysts. The H2 evolution rate (HER) of pure g-C3N4 is approximate 422 μmolh-1g-1, consistent to the previous report on g-C3N4 prepared from urea.33 It is worth noting that no hydrogen generation can be traced when only pure CoTiO3 is used as photocatalyst. Except 0.5% CT-U, the H2 evolution generated by all other CT-U composites is superior to that of pure g-C3N4. It is found that the 0.15% CT-U photocatalyst possesses the maximum HER of ca. 858 μmolh-1g-1 among a series of CT-U, which is ~ 2 times higher than that of the pure g-C3N4 under artificial sunlight illumination. At the same reaction condition, the HER of mixture 0.15% CT-CN is determined to be 578 μmolh-1g-1, far less than that of 0.15% CT-U (see Fig. S3). It confirms the formation of heterojunctions in the CT-U photocatalytic system. In contrast, the HER by other CoTiO3 loaded CT-U photocatalysts are 644 μmolh-1g-1 for 0.1% CT-U, 541 μmolh-1g-1 for 0.2% CT-U, 488 μmolh-1g-1 for 0.3% CT-U, and 263 μmolh-1g-1 for 0.5% CT-U respectively. The impairing H2 evolution for the CT-U loaded with the content of CoTiO3 higher than 0.15%, is probably resulting from the excessive restriction of g-C3N4 crystallization with the existence of too much CoTiO3 crystal (see the XRD analysis).34 After changing to use visible light 14


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irradiation (λ > 420 nm), the 0.15% CT-U still exhibits a higher photocatalytic HER (22.2 μmolh-1g-1) than pristine g-C3N4 (16.6μmolh-1g-1) as shown in Fig. S4. Thus, 0.15% CT-U, according to its exceedingly optimal H2-evolution performance, is chosen to further study the detailed photocatalysis in quantum yield, photochemical stability, photoelectrochemical activity and ESR analysis hereinafter.

As displayed in Fig. 9b, the quantum efficiencies of 0.15% CT-U exposed in different region of incident light wavelength are measured to be 38.4% at 365 nm and 3.23% at 420 ± 20 nm, which are both higher than that of pure g-C3N4 (16.0% at 365 nm; 0.83% at 420 ± 20 nm). However, no H2 can be detected for 0.15% CT-U under the wavelength light of 630 nm possibly due to its poor photoabsorption in this region (see UV-vis spectra in Fig. 8a). It indicates that the H2 evolution reaction proceeds via the photoabsorption of photocatalyst.

Apart from the photocatalytic H2-evolution performance, four runs of recycling H2 evolution reaction (4h / per time) for the 0.15% CT-U were also performed under artificial sunlight irradiation to investigate the stability (Fig. 9c). One can observe that the hydrogen evolution rate (HER) is slowly declined, maintaining a high value of 755 μmolh-1g-1 after four successive cycling hydrogen evolution experiments, which reveals that the as prepared 0.15% CT-U composite possesses high stability for its practical application. Furthermore, a comparison of the XRD patterns of 0.15% CT-U composite before and after the four cycling photocatalytic hydrogen evolution reaction is shown in Fig. 9d. No apparent change is found before and after cycling experiments, suggesting that the 0.15% CT-U composite possesses a good photostability without 15



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structural degradation during the photocatalytic reaction process.

3.3 Possible photocatalytic mechanism of CoTiO3/g-C3N4

Based on the above experimental results, the enhancement of the photocatalytic activity of CoTiO3/g-C3N4 primarily may originate from the synergistic effect of CoTiO3 and g-C3N4 associated with the formation of heterojunctions on the solid-solid contact interface. In terms of the band gap structures of CoTiO3 and g-C3N4, there are two kinds of possible heterojunction mechanisms for photogenerated charge carrier separation in the photocatalytic systems: i) traditional heterojunction-type and ii) direct Z-scheme type.26 If the photocatalytic reaction mechanism of CoTiO3/g-C3N4 photocatalyst is in keeping with the first kind of mechanism illustrated in Fig. 10a, the transfer of the photoexcited electrons will inevitably occur from the CB of g-C3N4 with negative potential to the CB of CoTiO3 with positive potential, hence leading to the decrease in electron reducibility. As such, the accumulated electrons from the CB of CoTiO3 cannot reduce O2 to form ·O2- as well as H+ into H2 without Pt co-catalyst. Simultaneously, the photo-generated holes will transfer from the VB of CoTiO3 to the VB of g-C3N4, resulting in the decrease in hole oxidizability. Hence, when the photo-generated electron-hole transfer is in accordance with the conventional type, it is unfavorable for the CoTiO3/g-C3N4 composite to form the active species, and results in lower photocatalytic H2-evolution performance. It conflicts with the foregoing results of photocatalytic H2 production under our experimental condition (see Fig. 9a). Therefore, based on the above results and data analysis, the photocatalytic mechanism of CoTiO3/g-C3N4 for enhanced H2 evolution could be proposed in accordance with direct Z-scheme 16


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and schematically represented in Fig. 10b. In the photocatalytic system, both of CoTiO3 and g-C3N4 act as photosensitizers under irradiation due to their narrow band gap structures. The absorption of photon by CoTiO3 and g-C3N4 leads to excitation of electrons from their respective VB to CB, leaving holes in their VB. Meanwhile, a great quantity of defects could be readily accumulated at the solid-solid hetero-interface, which possesses some similar properties to conductor such as analogous energy levels and low electric resistance. These features prompt the interface to be apt to serve as a recombination center of e--h+ pairs.15 Thus, the electrons generated from CB of CoTiO3 could immigrate to VB of g-C3N4 via solid-solid hetero-interface, and then recombine with the local holes, consequently accelerating the separation of photo-generated electrons from CB of g-C3N4. Subsequently, the separated electrons are transferred to Pt nanoparticles, the electron aggregates, and participate in the surface reduction for H2-evolution. Meanwhile, the photogenerated holes on the surface of CoTiO3 could be quickly immigrated and consumed by the sacrificial ethanol, which would facilitate the process of charge separation and consequently improve the photcatalytic activity.

3.4 Evidence of the mechanism

3.4.1 Photoluminescence emission spectra

To verify the above proposed Z-scheme mechanism for CoTiO3/g-C3N4 hybrid photocatalysts, PL analysis as an effective and commonly used method was employed to explore the electron transfer and recombination behavior of photocatalytic materials.27 Generally speaking, a higher PL

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intensity indicates a higher recombination rate of photoexcited electron-hole pairs.35 Fig. 11a displays the PL spectra of the pure g-C3N4 and CoTiO3/g-C3N4 hybrid photocatalysts excited by 340nm and the emission peak is displayed at around 455nm. Obviously, the emission intensities of all composite photocatalysts are higher than that of pristine g-C3N4 except 0.5% CT-U, and the 0.15% CT-U photocatalyst shows the strongest emission. This phenomenon is originated from more photoexcited charge carriers generated in CT-U photocatalytic system than pristine g-C3N4 and faster recombination rate via the interface between photoinduced electrons from CB of CoTiO3 and photoinduced holes from VB of g-C3N4, facilitating the active charge separation between CB of g-C3N4 and VB of CoTiO3. Hence, 0.15% CT-U photocatalyst occupies the highest recombination rate of interfacial charge, namely the most efficient surface charge separation. However, when the amount of CoTiO3 reaches 0.5%, excess CoTiO3 species inhibit the growth of active sites on the surface of g-C3N4 and act as recombination center itself, thereby, reducing the efficiency of charge separation.30 These results are in accordance with the photocatalytic activity of H2 production for CT-U photocatalysts, which confirm that the CoTiO3/g-C3N4 photocatalytic system is a typical Z-scheme photocatalyst.36-37

3.4.2 Photoelectrochemical analysis (PEC)

The photoelectrochemical performance is widely considered as the efficient evidence for demonstrating the electron-hole pair separation and transfer in the composite photocatalysts.38-40 Accordingly, the transient photocurrent responses (I-t), electrochemical inpedance spectroscopy (EIS) and photocurrent density-voltage (I-V) are employed to further explain the photocatalytic 18


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enhancement mechanism for H2 evolution. I-t curves for the pure g-C3N4 and 0.15% CT-U under several on/off visible irradiation cycles are displayed in Fig. 11b. The photocurrent intensity keeps almost unchanged upon the light on and decreases to zero once the light is switched off, indicating a rapid photocurrent response to the on-off intermittent irradiation. Moreover, the photocurrent of 0.15% CT-U is superior to that of pure g-C3N4, indicating that the hybrid photocatalyst could achieve a higher separation and transfer efficiency of photogenerated electron-hole, and hence enhance the photocatalytic activity.41

The EIS Nyquist plot is associated with the electrolyte solution and the interfacial charge-transfer resistances.40 The arc at high frequency region characterizes the charge transfer process and the smaller diameter of the arc represents the lower interfacial charge-transfer resistance. As displayed in Fig. 11c, the arc for 0.15% CT-U film is much smaller than that of g-C3N4 under illumination, suggesting that the heterojunction between CoTiO3 and g-C3N4 accelerates the photogenerated e--h+ pair separation and interfacial carrier migration across electrode/electrolyte.42 Furthermore, an obvious enhancement of photocurrent can be observed by 0.15% CT-U film in the I-V curves (Fig. 11d), demonstrating more favorable HER kinetics at the 0.15% CT-U film/electrolyte interface.43-44 Both EIS and I-V analyses demonstrate that efficient charge separation and transfer facilitate the enhancement of photocatalytic activity of H2 evolution for 0.15% CT-U.

3.4.3 ESR analysis

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In order to confirm the presented reaction mechanism of electron-hole separation process in-depth, the active superoxide (·O2-) species in parallel with hydrogen evolution and hydroxyl (·OH) radicals in the pure g-C3N4, CoTiO3 and 0.15% CT-U photocatalytic reaction systems were traced by ESR spin-trapping technique with DMPO under UV and visible light illumination. As displayed in Fig. 12, no ESR signal can be observed for the 0.15% CT-U photocatalyst in the dark condition. For the ESR signals of pure g-C3N4 and 0.15% CT-U photocatalyst, the characteristic peaks of DMPO-·O2- adducts (Fig. 12a-b) and DMPO-·OH adducts (Fig. 12c-d) are observed under both UV and visible irradiation. The ·O2- and ·OH signal intensities of 0.15% CT-U composite are obviously stronger than that of pure g-C3N4, suggesting that the amount of ·O2- and ·OH radicals generated on the 0.15% CT-U surface is more than that of pure g-C3N4. It further verifies that more efficient electron-hole separation occurs on the surface of CT-U composite and concludes simultaneously that the CT-U system follows the Z-scheme mechanism with high photocatalytic reduction and oxidation performance. In addition, note that no obvious characteristic peaks of DMPO-·O2- adducts or DMPO-·OH adducts can be found in single CoTiO3 under light irradiation, implying few ·O2- and ·OH radicals are generated on the surface of CoTiO3. This is also in accordance with its lack of H2-evolution photocatalytic activity.

To conclude, it can be confirmed that the proposed reaction mechanism of direct Z-scheme is reasonable for CoTiO3/g-C3N4 composite photocatalyst. That is, the CoTiO3/g-C3N4 composite boosts the efficient separation of photo-generated electron-hole pairs by accelerating the interfacial charge carrier recombination, resulting in the strong redox ability for enhanced H2 evolution from

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water splitting.

4. CONCLUSIONS In conclusion, we presented a novel Z-scheme heterostructure photocatalyst for hydrogen evolution with effectively intimate CoTiO3 (104)/g-C3N4 interfaces linked through the Co-O-N or Ti-O-N bond, fabricated by a facile solid-state in-situ synthesis strategy. Under an optimal experimental condition, the 0.15% CT-U composite photocatalyst exhibits the optimal photocatalytic performance for H2 production owing to the formation of close interface contact in the heterojunction between CoTiO3 and g-C3N4. The direct Z-scheme mechanism was inferred and further evidenced by PL, PEC and ESR analyses. The as-prepared composite photocatalysts are highly efficient, environment-friendly and recyclable. This study on the direct solid-state Z-scheme CoTiO3/g-C3N4 may not only provide useful information for further development of novel heterojunction photocatalytic systems based on perovskite, polymer and composite photocatalyst for photocatalytic H2 production, but also be significant in meeting the demands of environmental domains.

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Figures

Fig. 1 Schematic illustration of the charge carrier transfer path of direct solid-state Z-scheme heterojunction.

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Fig. 2 Schematic illustration of procedure for preparing CT-U hybrid photocatalysts.

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Fig. 3 XRD patterns of CoTiO3, g-C3N4 and CT-U photocatalysts.

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Fig. 4 SEM images of g-C3N4 (a) and CoTiO3 (b), CT-U (c) as well as TEM (d), HRTEM (e) images and EDS spectrum (f) of 0.15% CT-U.

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Fig. 5 SEM image (a) and Element mapping of CT-U (b), N (c), Co (d), Ti (e) and C (f).

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Fig. 6 XPS spectra of g-C3N4, CoTiO3 and 0.15% CT-U samples, (a) survey spectrum of the 0.15% CT-U sample; (b) C 1s; (c) N 1s; (d) O 1s; (e) Co 2p; (f) Ti 2p.

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Fig. 7 (a) FTIR spectra of CoTiO3, g-C3N4 and CT-U photocatalysts; (b) Schematic illustration of molecular interaction between perovskite CoTiO3 and tri-s-triazine g-C3N4 through the formation of chemical bonds.

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Fig. 8 (a) UV-vis absorption spectra of CoTiO3, g-C3N4 and CT-U photocatalysts (the insert pictures show the color change of the samples); (b) Plots of the (ahv)0.5 vs. Photon energy (hv) for g-C3N4, CoTiO3 and 0.15% CT-U.

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Fig. 9 (a) Photocatalytic activities for H2 production with different ratios of CT-U photocatalysts; (b) Wavelength-dependent QE and hydrogen evolution from water by 0.15% CT-U and g-C3N4; (c) Recyclability of the 0.15% CT-U photocatalyst for H2 evolution from water splitting under simulated sunlight irradiation; (d) The XRD patterns of 0.15% CT-U photocatalyst before and after water splitting experiment.

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Fig. 10 Schematic illustration of traditional heterojunction-type (a) and direct Z-scheme (b) mechanisms for charge carrier separation.

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Fig. 11 (a) PL emission spectra of g-C3N4 and CT-U photocatalysts; (b) Transient photocurrent responses, (c) EIS and (d) I-V curves of as-prepared g-C3N4 and 0.15% CT-U photocatalyst under light irradiation.

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Fig. 12 ESR signals of DMPO-·O2- in methanol dispersion with UV (a) and visible light (b) irradiation; ESR signals of DMPO-·OH in aqueous dispersion with UV (c) and visible light (d) irradiation.

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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications websites. XRD patterns for CoTiO3, g-C3N4 and 5% CT-U photocatalyst; The N 1s XPS spectra of g-C3N4 and 0.15% CT-U hybrid material; Photocatalytic H2 production activities for g-C3N4, 0.15% CT-U and 0.15% CT-CN mixture under simulated solar light; Photocatalytic H2 production activities for g-C3N4 and 0.15% CT-U under visible light (λ > 420 nm).

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected] (Xia Tao), [email protected] (Yan-Zhen Zheng); Tel: +86-10-64453680, Fax: +86-10-64434784. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work has been supported by the National Natural Science Foundation of China (Nos. 21176019, 21377011, 21476019), and Beijing Higher Education Young Elite Teacher Project (YETP0487). 34


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g-C3N4 Hybrid Photocatalysts with Enhanced

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