Efficient and Stable Photoelectrochemical Seawater Splitting with TiO2

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Efficient and Stable Photoelectrochemical Seawater Splitting with TiO@g-CN Nanorod Arrays Decorated by Co-Pi 2

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Yuangang Li, Rongrong Wang, Huajing Li, Xiaoliang Wei, Juan Feng, Kaiqiang Liu, Yong-Qiang Dang, and Anning Zhou J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 11, 2015

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The Journal of Physical Chemistry

Efficient and Stable Photoelectrochemical Seawater Splitting

with

TiO2@g-C3N4

Nanorod

Arrays

Decorated by Co-Pi Yuangang Li*1, Rongrong Wang1, Huajing Li1, Xiaoliang Wei1, Juan Feng1, Kaiqiang Liu2, Yongqiang Dang1 and Anning Zhou1

1 College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China. 2 Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China Corresponding author, E-mail: [email protected] (Yuangang Li), Tel. /Fax: 0086 29 85583183;

Abstract: Co-Pi decorated TiO2@graphitic carbon nitrides (g-C3N4) nanorod arrays (denoted as CCNRs) with different mass ratios of g-C3N4 have been constructed on the FTO substrate through three processes, hydrothermal growth, chemical bath deposition and electrodeposition. Firstly, TiO2 nanorod arrays were grown onto a FTO substrate by a hydrothermal method. Secondly, g-C3N4 was coated onto the TiO2 nanorod arrays by immersing the above substrate with TiO2 nanorod arrays into a solution of urea and then heated at higher temperature. In this procedure, the amount of the g-C3N4 on the TiO2 nanorod arrays can be controlled by tuning the concentration of the urea solution. At last, Co-Pi were decorated on the surface of the TiO2@g-C3N4 by electrodeposition. The as-prepared CCNRs were characterized by XRD, FESEM, TEM, XPS, UV-vis and FTIR, respectively, which illustrated that Co-Pi were successfully decorated on the hybrid TiO2@g-C3N4 nanorod arrays. Photoelectrochemical (PEC) measurements have demonstrated that the prepared CCNRs serve as an efficient

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and stable photoanode for PEC seawater splitting. The photocurrent density reaches 1.6 mA/cm2 under 100 mW/cm2 (AM1.5G) light illumination at 1.23 V (RHE). More significantly, the CCNRs photoanode is quite stable during seawater splitting and the performance remain undiminished even after ten hours continuous illumination. Finally, a systematical photocatalytic mechanism of the Co-Pi decorated TiO2@g-C3N4 was proposed and it can be considered as potential explanation of enhanced PEC performance. Key words: Photoelectrochemical water splitting; Seawater; Nanorod arrays; Semiconductors; Photo catalysis

1. Introduction Hydrogen evolution from solar-driven seawater splitting is an ideal way to solve the energy crisis 1-4, since solar energy is one of inexhaustible energy sources and seawater is the most abundant and geographically balanced natural resource available on the earth 5-7. Well-known, semiconductor-based photoelectrodes 8, 9 have been extensively explored in several aspects (Cu2O

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, Fe2O3 11, CdS 12, 13, ZnO 14, BiVO4 15 and TiO2 16), because they are proved to be potential devices for

converting solar energy into fuels within photoelectrochemical (PEC) cells. In the case of material selection, titanium dioxide is one of the most commonly employed materials as photocatalyst because of its nontoxicity, low-cost and brilliant photochemical and chemical stability 17-19. Nevertheless, owing to its large band gap of about 3.0 eV, pristine TiO2 does not respond to visible region of solar spectrum and can be excited only by UV irradiation that is about 4% of solar power spectrum. Hence, great efforts have been making to expand its absorption range to visible region 20, such as element doping 21, 22

, dye- sensitization 23, 24, precious metal decoration25 or coupling with other semiconductors to form heterojunction 26-28.

Furthermore, in the case of electrode architectures, photoelectrodes derived from nanorod arrays 29 have addressed current materials limitations by decoupling length scales of light absorption and charge diffusion and offering sufficient surface area for the photo-generated electrons or holes to diffuse into electrolyte

30-32

. In this way the recombination rate of

electron-hole pairs will be reduced, and the performance of the devices will be improved correspondingly. Recently, metal-free polymeric semiconductor, graphite-like carbon nitride (g-C3N4) 33, 34 with moderate band gap of


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2.7 eV 35, has attracted more and more attentions for its inherent chemical and thermal stability 36, 37. Unlike transitional metal oxides and sulfide semiconductor photocatalysts, g-C3N4 behaves very stable performance in acid or alkaline electrolytes ascribing to strong covalent bonds between carbon and nitride atoms in its structure 38. However, applications of pure g-C3N4 are limited largely because of its low quantum efficiency and high electron-hole recombination rate

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.

Therefore, it lays a large space to explore and construct novel composite materials to remedy these deficiencies of pure g-C3N4. Herein, we combined TiO2 and g-C3N4 to fabricate TiO2@g-C3N4 nanorod arrays

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with different mass ratios of

g-C3N4 via hydrothermal growth and chemical bath deposition. Moreover, the composite nanorod arrays were decorated with Co-Pi particles 41-43 to offset the inadequacy of g-C3N4 44. To the best of our knowledge, this is the first report upon semiconductor nanorod arrays modified with tunable g-C3N4 mass ratio. As reported, Co-Pi decorated TiO2@g-C3N4 nanorod arrays (CCNRs) used for photoelectrode within PEC cells are still unexplored. More importantly, the results in this work have proved that the composited nanorod arrays as photoelectrodes exhibit very efficient and stable performances for PEC seawater splitting.

Experimental section 2. Sample preparation Materials: All the reagents employed in this work were analytical grade and used without further purification. Urea (CO(NH2)2), tetrabutyl titanate (C16H36O4Ti) and methanol (CH3OH) were purchased from Aladdin Chemical Reagent Co., Ltd. Additionally, hydrochloric acid (HCl), acetone (C3H6O) and absolute ethanol (C2H5OH) were bought from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water was used throughout the experiment. Fluorine-doped tin oxide (FTO) (14Ω/square) was obtained from Huanan Xiangcheng Technology Co., Ltd. The CCNRs photoelectrode was fabricated on FTO substrate via three simple processes, including hydrothermal growth, chemical bath deposition (CBD) and electro-deposition, as illustrated in Scheme 1.



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Scheme 1. Schematic illustration for the preparation of CCNRs

a. Synthesis of pristineTiO2 nanorod arrays Pristine TiO2 NRs were prepared using a simple hydrothermal growth as reported previously

45

. In detail, 12 mL of

ultrapure water was mixed with equal amount of concentrated hydrochloric acid (mass fraction 36.5-38%). The mixture was stirred for 5 min under ambient conditions, and then 0.4 mL of tetrabutyl titanate was added. Subsequently, the above mixture was transferred to 50 mL of Teflon-lined stainless steel autoclave. And a piece of cleaned FTO substrate（ 1×6 cm2） was placed into it, with an appropriate angle against the wall of the Teflon-liner and conductive side facing down. The autoclave was kept in an oven at 150 oC for 18 h. After cooling down to room temperature, the FTO substrate with TiO2 nanorod arrays was taken out, rinsed with ultrapure water for several times, and then dried in an oven at 60 oC.

b. Fabrication of TiO2@g-C3N4 nanorod arrays In a typical synthesis of TiO2@g-C3N4 NRs, different mass of urea powder were disovled in 10 mL of methanol with constant stirring speed for about 10 min. And then a piece of FTO substrate with TiO2 nanorod arrays was immersed into methanol solution and standing for 30 min. After being taken out, the FTO substrate was placed in a crucible with a lid, a


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semi-closed environment. After being maintained at 80 oC for 30 min in oven, the crucible was transfer to a muffle furnace and maintained at 550 oC for 3 h. Another FTO substrate with TiO2 nanorod arrays can be used as a reference through the same thermal procedure without being immersed in the urea solution.

c. Fabrication of CCNRs Co-Pi particles were deposited with a three electrode system at 1.1 V (vs. Ag/AgCl) in electrolyte containing 0.15 mM cobalt nitrate and 0.1 M potassium phosphate (pH=7) with platinum wire as a counter electrode46. After the electrodeposition for 20 min, Co-Pi was successfully covered onto the surface of TiO2@g-C3N4 NRs.

Characterization The crystalline structure of samples were identified using a Shimadzu 7000S X-ray diffract meter (XRD) with Cu-Kɑ radiation. The morphologies and particle sizes of the as-prepared samples and high-resolution images were studied with a field-emission Tecnai G2 F20 transmission electron microscopy (TEM) with an acceleration voltage of 200 kV. EDX mapping analyse is carried out with the same TEM with STEM mode. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 field-emission scanning electron microscopy. The UV-vis reflectance spectroscopy was measured by a Thermo Scientific Evolution 220 ultraviolet-visible spectrophotometer equipped with an integrating sphere using barium sulfate as the reference. The electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV) and open circuit voltage decay (OCVD) measurements were conducted with an accurate electrochemical workstation (CHI 660E) in a three-electrode system using platinum wire as counter electrode and Ag/AgCl as reference electrode. X-ray photoelectron spectroscopy (XPS) data were obtain from a Kratos Axis Ultra DLD X-ray photoelectron spectrometer, with which a monochromatic Al Kα (hν = 1486.69 eV) source was equipped. The incident photon-to-current conversion efficiency (IPCE) was measured by the same electrochemical workstation with a Xe lamp (300 W) coupled with a monochromator. The Fourier transform infrared spectra (FTIR) were conducted on a Nicolet Avatar 360E Fourier transform infrared (FTIR) spectrophotometer under attenuated total reflection (ATR) pattern. Inductively coupled plasma



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(ICP) measurement was conducted with Varian 715-ES.

PEC measurements The PEC properties of as-prepared samples were evaluated by a three-electrode electrochemical workstation (CHI 660 E, China). For the three-electrode system, we used the as-prepared nanorod arrays as the working electrode, Ag/AgCl in 3 M KCl as the reference electrode and platinum wire as the counter electrode. An aqueous solution containing 0.10 M Na2SO4 (pH=6.8) was used as electrolyte under bubbling nitrogen for 30 min prior to all the PEC measurements. When we conducted experiments, a 300 W xenon lamp with light intensity of 100 mW/cm2 (AM 1.5G) acted as light source. LSV was carried out at a scan rate of 10 mV/s. During photocatalytic process, the potential of the working electrodes (vs. Ag/AgCl) can be converted to the reversible hydrogen electrode (RHE) by the Nernst equation: VRHE = VAg/AgCl + 0.059 pH + 0.197.42. Natural seawater used in the experiment was obtained from Huanghai (36o 03' 07.4" N, 120o 19' 27.3" E), Shandong province, China. The pH of natural seawater was 6.4 (after boil and filtration)

3 Results and discussion 3.1 Characterization of composition and structure The structures and morphologies of the as-prepared samples were observed by SEM. As exhibited in Fig. 1A, TiO2 nanorods are equably grown on the surface of FTO. Moreover, as seen from Fig. 1B, the shape of pristine TiO2 nanorods is tetragonal and the average diameter is about 200 nm. Furthermore, from the cross-section SEM image (Fig. 1A inset) of TiO2 NRs, it is seen that the length of the nanorod is about 2.2 µm. After decoration with g-C3N4, the diameter of the nanorods slightly increased (Fig. 1C). However, Co-Pi particles were not found from SEM image of CCNRs shown in Fig.

1D, which probably is due to the shape of Co-Pi particles similar to the topmost morphologies of TiO2 nanorods. On the other hand, it is very difficult to distinguish Co-Pi from g-C3N4 by SEM, but the existence of Co-Pi will be further confirmed by TEM and XPS analysis.


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Fig. 1 Typical top-view SEM images of: (A) pristine TiO2 NRs at low magnification (the inset is its cross-section image) and (B) high magnification; (C) SEM images of TiO2@g-C3N4 NRs and (D) CCNRs; (E) TEM images of pristine TiO2 NRs and (F)

TiO2@g-C3N4 NRs. (The mass ratio of N to Ti is 0.12 in hybrid samples) As a result, it can be seen from Fig. 1E that the diameter of the pristine TiO2 nanorod is about 200 nm, which is in agreement with the results of SEM measurements. The selected area electron diffraction (SAED) pattern of pristine TiO2 (Fig. 1E inset) indicates that the pristine TiO2 nanorod is single-crystalline. Corresponding to SAED results, HRTEM image shown in Fig. S1 clearly illustrates that the adjacent lattice fringes with interplanar spacing d110 = 3.1 nm and d001 = 2.9 nm and the growth direction of well-crystallized TiO2 nanorods is (110) crystal plane

47

. In addition, Fig. 1F

demonstrates that the entire surface of TiO2 was evenly covered with g-C3N4 and formed good interfacial contact. The EDX mapping has also been employed to illustrate the element distribution in the TiO2@g-C3N4 NRs (Fig. S2). It shows in the



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mapping that the elements Ti, O, C and N are evenly distributing in the whole sample. This result does further demonstrate the g-C3N4 uniformly distributing on the surface of TiO2 nanorods. Meanwhile, the existence of Co-Pi can also be confirmed by TEM examination shown in Fig. S3. X-ray diffraction (XRD) measurement was conducted to determine the composition and crystal-phase properties of pure g-C3N4, TiO2NRs, TiO2@g-C3N4 NRs and CCNRs. As for the pure g-C3N4 sample, the two distinct diffraction peaks at 27.5 and 13.1o corresponds to the (002) and (100) crystal planes. The main diffraction peaks of pristine TiO2 located at 2θ values of 36.3, 62.2, and 65.8o can be well indexed to (101), (002) and (112) crystal planes, respectively 48. All the peaks of TiO2 NRs compliance with rutile phase (JCPDS 21-1276). No other impurity phase is detected. The enhancement of (002) peak in the pattern showed that the nanorods grow along (001) direction 49, which is in accordance with the results of HRTEM. Compared to the XRD patterns of TiO2, no different diffraction peaks for g-C3N4 can be observed in the TiO2@g-C3N4 arrays and CCNRs. This might be ascribing to the fact that the amount of g-C3N4 was very less and well dispersed onto the surface of TiO2. Notably, it was also no diffraction peaks for Co can be observed in the nanocomposites of CCNRs, which might be the fact of amorphous Co-Pi 50. To determine the existence of g-C3N4 in the composite photoelectrode, FTIR technique was applied. As shown in Fig. 2B, the FTIR spectrum of pure g-C3N4 is very similar to CCNRs as previously reported 51. Several sharp adsorption peaks located at 1256, 1329, 1420, 1575, and 1635 cm−1 are related to the typical stretching modes of aromatic C-N 51. The peak higher than 3000 cm-1 is usually considered as absorbed H2O molecules on the surface of the materials 52. In addition, the peak at 808 cm-1 was commonly regard as the out-of-plane bending modes of thiazine unites. The broad range from 400 to 1200 cm-1 was denoted as the stretching vibration of Ti-O-Ti in TiO2 53. The absorption peaks of TiO2 are not clear in the CCNRs, which could be due to the fully coverage of g-C3N4 over TiO2. On the basis of FTIR spectra, the existence of g-C3N4 in CCNRs was confirmed.


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Fig. 2 (A) XRD patterns of pristine TiO2 NRs, TiO2@g-C3N4 NRs, and CCNRs; (B) FTIR spectra of TiO2 NRs, TiO2@g-C3N4 NRs and CCNRs; (C) XPS survey spectrum of CCNRs; High resolution XPS spectra of Ti 2p (D), and C1s (E); N 1s (F). (The mass ratio of N to Ti is 0.12 in hybrid samples).

To further confirm the existence of g-C3N4 in CCNRs and study the surface chemical state of CCNRs, X-ray photoelectron spectroscopy (XPS) was employed. Obvious photoelectron peaks of Ti, O, C, N, and Co elements are observed in the XPS survey spectra from the surface of CCNRs (Fig. 2C). The high resolution XPS spectra of Ti 2p (Fig. 2D) show two different peaks Ti 2p1/2 and Ti 2p3/2 with binding energies of 464.2 eV and 458.5 eV, respectively

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.

Combined with the results of XRD analysis, these peaks were derived from Ti4+ in TiO2. The high-resolution C 1s XPS spectrum (Fig. 2E) reveals two evident signals centered at 284.6 eV and 288.14 eV, indicating that carbon possess two diverse chemical states 55. The peak at 284.88 eV is the signal of graphite-like sp2 C-C, which can be ascribed to carbon species adsorbed on the surface of g-C3N4. Whereas, the second peak located at 288.18 eV indicate the formation of N-sp3 C



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(N-C=N) 56. The curves of N1s (Fig. 2F) region can be divided into three peaks located at 398.5 eV, 399.7eV, and 401.1 eV. The 398.5 eV peak is usually assigned to sp2-hybridized nitrogen (C-N=C) dominated in the g-C3N4. The other two weak peaks correspond to sp3 C-N bonds and amino groups with a hydrogen atom (C-N-H), respectively 57. Therefore, the high resolution N 1s and C 1s XPS spectra further confirmed the formation of g-C3N4 in the composite. The Co 2p1/2 and Co 2p3/2 spin orbit 58 (Fig. S4) correspond to Co3+ and Co2+. These results matched well with previously reported species data. To sum up, due to this explicit surface property of CCNRs, g-C3N4 and Co-Pi particles indeed covered onto the surface of TiO2 through CBD and electro deposition.

3.2 Optical absorption properties UV-vis diffuse reflectance spectroscopy was harnessed to identify the optical absorption properties of the as-prepared samples. The absorption edge for pure g-C3N4 is about 460 nm, which represented a band gap of ≈ 2.7 eV. Meanwhile, the band gap of pristine TiO2 is found to be about 3.0 eV, which was in agreement with the previous report. It is clearly seen from Fig. 3A that the TiO2@g-C3N4 nanorod arrays show slightly red-shifted

59

compared with pristine TiO2. It can be

attributed to interaction between TiO2 and g-C3N4. The curves based on the Kubelka Munk function versus the energy of light are shown in Fig. 3A inset. The estimated band gap values of the as-prepared samples are about 2.71, 2.85, 2.86 and 3.01 eV approximately, corresponding to pure g-C3N4, TiO2@g-C3N4, CCNRs and pristine TiO2 NRs, respectively. The band gap narrowing phenomenon can be attributed to the chemical bonding between g-C3N4 and TiO2. After the modification of Co-Pi, the light absorption also slightly changed 50.

3.3 Photoelectrochemical performance To investigate PEC properties of CCNRs, Linear Sweep Voltammetry (LSV) measurement was performed. As can be shown in Fig. 3B, transient photocurrent responses of pristine TiO2 is 0.42 mA/cm2 under 100 mW/cm2 (AM 1.5G) intermittent illumination in 0.1 M Na2SO4 aqueous solution (pH=6.8) at 1.23 V (Vs RHE). After coating with g-C3N4, the photocurrent density of TiO2@g-C3N4 composite nanorod arrays shows remarkable increase over several on-off cycles. In


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order to determine the relationship of laden g-C3N4 and PEC performance of TiO2@g-C3N4, we further made quantitative test through controlling the loading amount of g-C3N4 by the concentration of the precursor (urea). It can be seen from Fig. 3C that the amount of g-C3N4 has a significant effect on the photocurrent density of the as-prepared TiO2@g-C3N4. The photocurrent density increase with the mass percentage of g-C3N4 (determined by the ICP measurement) at the low g-C3N4 content and decrease at higher g-C3N4 content. A maximum photocurrent density of 1.15 mA/cm2 appeared at the g-C3N4 mass percentage of about 10%. This current density is 2.6 times as that of pure TiO2. For the formation of heterojunction, with the increase of g-C3N4 content charge carrier recombination becomes to decrease and the absorption range broadened too, which lead to the increase of current density with the content of g-C3N4. Once the content of g-C3N4 reaches a certain threshold, further increase of g-C3N4 will lead to the increase of the resistance as well as light absorption effect will be diminished thus the photocurrent density will decrease with the g-C3N4 percentage. After the electrodeposition of Co-Pi, the photocurrent density of TiO2@g-C3N4 NRs further increased and reached a maximum value of about 1.6 mA/cm2 at VRHE =1.23 V, which is 3.8 times as that of pure TiO2 and 1.6 times as that of the optimized TiO2@g-C3N4. It was caused by the quickly transfer of photo-generated electrons. It has been well established that the further enhancement of photocurrent is caused by the efficient separation and transport of photo-excited electron-hole pairs. This conclusion is also consistent with the result of EIS. The EIS is a useful tool to evaluate the kinetics of charge transfer at the interface of electrode/electrolyte. Fig. S7 shows EIS spectra for pristine TiO2 NRs, TiO2@g-C3N4 NRs and CCNRs in a solution containing 0.5 M K3Fe(CN)6 and 0.5 M K4Fe(CN)6. As to the EIS spectra, the radius of arc represents the value of resistance. The largest arc of TiO2 represents a large charge transfer resistance existing in the charge-transfer process of TiO2. After coating with g-C3N4, the arc radius of TiO2@g-C3N4 NRs greatly decreased. This phenomenon demonstrates that the separation efficiency of electron-hole can be effectively improved by structuring heterojunction. The minimum radius of CCNRs represents the facile transport of electrons was occurred in the presence of Co-Pi. In other words, the CCNRs composite has good



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conductivity and this result could contribute to explain the improvement photoelectrochemical performance of CCNRs.

Fig. 3 (A) UV-vis diffuses reflectance spectra; (B) Photocurrent density versus potential curves for pristine TiO2 NRs,

TiO2@g-C3N4 NRs and CCNRs; (C) Photocurrent density over g-C3N4 Modified TiO2 with different mass ratios; (D) IPCE of pristineTiO2 NRs, TiO2@g-C3N4 NRs, and CCNRs. (The mass ratio of N to Ti is 0.12 in hybrid samples).

3.4 IPCE The incident photon-to-current conversion efficiency (IPCE) of the as-prepared samples was measured at 0.63 V (VRHE=1.23V) with a 300 W Xe lamp coupled with a monochromator. The IPCE values were calculated with the following equation 60:

IPCE =

(1240 × J ph ) (λ × I light )

× 100%

Where JPh is the photocurrent density (mA/cm2) produced by excited electrons, λ the incident light wavelength (nm) and Ilight the incident light power density (mW/cm2) for each wavelength. Fig. 3D shows that the outline of all three plots are consistent well with the UV-vis absorption spectrum of each sample, respectively. The IPCE of TiO2@g-C3N4 is higher than that of pure TiO2 NRs at whole spectrum, which indicates that photogenerated electron-hole pairs are efficiently separated in the TiO2@g-C3N4 NRs heterostructure. After modification with Co-Pi the IPCE value was further increased and the maximum IPCE of 41 % at 380 nm was obtained. Compared with pristine TiO2, the significant IPCE increase of


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TiO2@g-C3N4 and CCNRs in the range of 430 to 500 nm is well matched with the result of UV-vis diffuse reflectance spectroscopy, which means that the absorption at longer wavelength after coupled with g-C3N4 is really contributing to the performance. There are three factors determine the IPCE: the light-harvesting efficiency, the charge injection efficiency, and the charge collection efficiency. As to our composite electrode, the vertical TiO2 nanorod structure provides a large surface area, leading to the enhanced light harvesting efficiency. The formation of heterojunction between g-C3N4 and TiO2, leads to the directional flow of the charge. The outermost Co-Pi layer offers a direct pathway for excited holes. In this way, the charge injection efficiency and collection efficiency were improved. Therefore, the IPCE of CCNRs nanorod arrays can reach to such a high level.

3.5 Mott–Schottky To further investigate the enhanced photoelectrochemical performance and the fast charge transport and collection of CCNRs, we also implemented Mott-Schottky measurement. From Mott-Schottky analysis, we can obtain flat band potential (Efb) and the charge carrier density of the interface between semiconductor electrode and electrolyte. The capacitance of the semiconductor can be calculated by the Mott-Schottky equation 61: 2 1/CSC = [2/e0 ε 0εNd A 2 ][(E - Efb ) - (k b T/e0 )]

According to the equation, three curves can be drawn using E against 1/C2, and Efb of the corresponding electrode is ascertained by the x-intercept (Fig. 4). The positive slope of TiO2@g-C3N4 and CCNRs plot suggests the expected n-type semiconductor of TiO2 in the nanocomposites. For n-type semiconductor flat band potential is consistent with the bottom of the conduction band. The flat band voltage for TiO2, TiO2@g-C3N4 and CCNRs were calculated to be -0.34, -0.42 and -0.43, respectively. The low Efb value indirect represents high Nd value 62. So, the charge will migrate from higher Fermi level to lower, which will result in the formation of space charge layer. That is to say, the bias potential applied to TiO2@g-C3N4 and CCNRs reduced, compared to pristine TiO2. The Efb of TiO2@g-C3N4 and CCNRs shows almost no difference illustrate that the modification of Co-Pi does not change band position and band gap of the TiO2@g-C3N4 electrode. Hence, we



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believe that the high PEC performance may be due to the fast charge migration on the interface between photoanode and electrolyte. Co-Pi just acting as a cocatalyst quickly imports holes into electrolyte for the oxidation of water.

Fig. 4 Mott-Schottky image of (A) TiO2 (B) TiO2@g-C3N4 NRs and CCNRs. (The mass ratio of N to Ti is 0.12 in hybrid

samples).

3.6 Stable performance of the CCNRs photoelectrode for seawater splitting

Fig. 5 Chronoamperometry (i–t) of CCNRs measured in natural seawater with a three-electrode system at 1.23 VRHE for 10 h.

(The mass ratio of N to Ti is 0.12).

Fig. 5 displayed the photoresponse of CCNRs by photocurrent-time (I-t) measurement in seawater at 0.63 V (VRHE=1.23 V) under continuous illumination (100 mW/cm2, AM 1.5G) for ten hours 63. At the beginning period without light irradiation, the photocurrent value is almost zero. When applied irradiation the photocurrents sharply increased to the maximum of


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1.82 mA/cm2, then recovered to about 1.64 mA/cm2 in a short period of time. The rapidly rise of photocurrent in the first-stage was caused by the charging current of the electric double-layer on the anode surface

64

. After that the

photocurrent density was kept undiminished even after 10 hours of continuous irradiation in natural seawater. The achieved high efficiency and good stability confirmed that the CCNRs composite electrode is an ideal candidate for seawater splitting.

3.6 Mechanism analysis Based on the results of the photocatalytic activity measurements of the as-prepared samples, a possible mechanism for the photoelectrochemical performance improvement over CCNRS is proposed, as showed in Fig. 6A. Because the valance band and conduction band of g-C3N4 are higher than those of TiO2, respectively 65, a well-matched heterojunction was formed. Under simulated light irradiation, the photon-generated electrons are excited from valence band (VB) of g-C3N4 to its conduction band (CB) then transferred to the CB of TiO2, leaving the hole in the VB of g-C3N4. Finally, electron arrived at the counter electrode through the external circuit and consumed by H+ for the generation of H2. This migration direction of the charge is consistent with result of Mott-Schottky measurement. Correspondingly, holes transferred from VB of TiO2 to VB of g-C3N4 then consumed by the process that Co2+ is oxidized to Co3+. During the oxidation process of water, the existence of Co-Pi particles are regarded as a fast redox mediator that can not only reduce the needed reaction activation energy of PEC water oxidation, but also promote effective charge separation. To further elucidate effective charge separation in the CCNRS composite, open circuit voltage decay measurement was performed. At open-circuit conditions, the longer lifetime of excited electrons can be proved by the lower decay rate of photogenerated voltage which was evaluated by using model equation

66

. As shown in Fig. 6, the illumination of

simulated light will results in a negative voltage which means these photo electrodes is typical n-type semiconductor. When cutoff the irradiation, the saturated photogenerated voltage gradually decreased and offers a continuous reading of the survivability of electron. The CCNRs display higher value of lifetime than the pristine TiO2 and TiO2@g-C3N4



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nanorod arrays. This phenomenon strongly illustrate that the reasonable composite structure of CCNRs indeed played an effective role in suppressing the charge recombination. At the same time, the slowest decay rate of CCNRs is a powerful evidence for the existence of effective charge separation and this result also strongly supports the proposed photocatalytic mechanism.

Fig. 6 (A) Schematic diagram for transfer and separation of photogenerated charges and holes in the CCNRs heterostructure; (B) Comparison of the lifetime of electrons derived from the model equation as a function of open circuit voltage decay. (The mass ratio of N to Ti is 0.12 in hybrid samples).

4. Conclusions In conclusion, a series of TiO2@g-C3N4 heterojunction nanorod arrays with different mass ratios of g-C3N4 have been fabricated. It not only expands the optical absorption range of TiO2, but also significantly improves PEC performance of TiO2 NRs. The outermost modification of Co-Pi further accelerates the migration of charge carriers and improves the


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effective separation of the charge. Meanwhile, the examined photocatalyst for seawater splitting exhibits high photoelectrocatalytic activity and stable performance. All the conclusions discussed above will provide convenient guidance for researchers to study photoelectrochemical process converting water to environmentally-friendly hydrogen energy with semiconductor photoelectrode.

Acknowledgement We thank financial supports from the National Natural Science Foundation of China (21103134, 21473110). We do also thank Mr Xiangyang Yan (School of Chemistry and Chemical Engineering, Shaanxi Normal University) for his technical support in XPS measurements. Supporting Information HRTEM images of TiO2@g-C3N4 nanorod, TEM image of the CCNRs (Co-Pi particles are marked by white arrows), XPS spectrum of P 2p for CCNRs, XPS spectrum of Co 2p for CCNRs, XPS spectrum of O 1s for pure TiO2 NRs, TiO2@g-C3N4 NRs, CCNRs, Photocurrent density versus potential curves for TiO2@g-C3N4 NRs with different mass ratios were measured in a 0.2 M Na2SO4 aqueous solution (pH=6.8) under AM 1.5G (100 mW/cm2) illumination, LSV curves for pristine TiO2 NRs, TiO2@g-C3N4 NRs and CCNRs in seawater under 100 mW/cm2 (AM 1.5G) illumination, electrochemical impedance spectra (Nyquist plots) for pure TiO2 NRs, TiO2@g-C3N4 NRs, CCNRs in a solution containing 0.5 M K3Fe(CN)6 and 0.5 M K4Fe(CN)6, open circuit voltage decay with time for CCNRs after illumination interruption, SEM image of the TiO2@g-C3N4 NRs. This information is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Image for Manuscript

Efficient and Stable Photoelectrochemical Seawater Splitting with TiO2@g-C3N4 Nanorod Arrays Decorated by Co-Pi Yuangang Li*1, Rongrong Wang1, Huajing Li1, Xiaoliang Wei1, Juan Feng1, Kaiqiang Liu2, Yongqiang Dang1 and Anning Zhou1

1 College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China.

2 Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China

Corresponding author, E-mail: [email protected] (Yuangang Li), Tel. /Fax: 0086 29 85583183


Efficient and Stable Photoelectrochemical Seawater Splitting with TiO2

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