A visible-light-driven heterojunction for enhanced

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A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst Yuanzhi Hong a, Zhenyuan Fang b, Bingxin Yin b, Bifu Luo b, Yong Zhao b, Weidong Shi b,*, Changsheng Li a,** a b

School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, PR China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China

article info

abstract

Article history:

The photocatalytic water splitting for generation of clean hydrogen energy has received

Received 15 October 2016

increasingly attention in the field of photocatalysis. In this study, the Ta2O5/g-C3N4 het-

Received in revised form

erojunctions were successfully fabricated via a simple one-step heating strategy. The

11 December 2016

photocatalytic activity of as-prepared photocatalysts were evaluated by water splitting for

Accepted 12 December 2016

hydrogen evolution under visible-light irradiation (l > 420 nm). Compared to the pristine g-

Available online 28 December 2016

C3N4, the obtained heterojunctions exhibited remarkably improved hydrogen production performance. It was found that the 7.5%TO/CN heterojunction presented the best photo-

Keywords:

catalytic hydrogen evolution efficiency, which was about 4.2 times higher than that of pure

g-C3N4

g-C3N4. Moreover, the 7.5%TO/CN sample also displayed excellent photochemical stability

Ta2O5

even after 20 h photocatalytic test. By further experimental study, the enhanced photo-

Heterojunction

catalytic activity is mainly attributed to the significantly improve the interfacial charge

Water splitting

separation in the heterojunction between g-C3N4 and Ta2O5. This work provides a facile

Photocatalytic activity

approach to design g-C3N4-based photocatalyst and develops an efficient visible-light-

Hydrogen evolution

driven heterojunction for application in solar energy conversion. © 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Introduction Semiconductor-mediated photocatalysis has been widely recognized as the most prospective technology to resolve the increasing serious energy and environmental concerns. Particularly, hydrogen fuel from photocatalytic water spilling by utilizing solar light as renewable energy is the research focus in the area of photocatalysis [1e3]. Currently, graphitic carbon nitride (g-C3N4) has emerged as promising visible-light photocatalyst for hydrogen production owing to its abundance, good chemical stability, and easy preparation [4e9]. It

is believed that g-C3N4-based photocatalysts will play an increasingly significant role in development of a sustainable future [10e14]. However, pristine g-C3N4 prepared by thermal polymerization has poor charge carrier mobility, leading to the photocatalytic hydrogen evolution efficiency is severely restricted [11,15e17]. In recent years, the none-noble metal such as Ni [18e20], Cu [21,22], and Se [23] modified g-C3N4 photocatalysts were reported for the improvement of charge separation and photoactivity. Very recently, the latest reviews by Ong et al. [13], Mamba et al. [24], and Wen et al. [25] have shown that constructing of g-C3N4-based semiconductor

* Corresponding author. Fax: þ86 511 8879 1108. ** Corresponding author. Fax: þ86 511 8879 1108. E-mail addresses: [email protected] (W. Shi), [email protected] (C. Li). http://dx.doi.org/10.1016/j.ijhydene.2016.12.055 0360-3199/© 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.


heterojunction system is considered as a more effective and feasible strategy to enhance photocatalytic activity. When two semiconductors successfully formed a heterojunction, the contacting interface can greatly accelerate the photogenerated electronehole pair's separation, resulting in further optimizing photocatalytic efficiency [26]. So far, various heterostructured composites have been fabricated to improve photocatalytic activity for hydrogen production from water splitting, including g-C3N4/TiO2 [27e30], g-C3N4/CdS [31e33], g-C3N4/MoS2 [34,35], g-C3N4/Cu2O [36], g-C3N4/WO3 [37,38], gC3N4/Ni(OH)2 [39], g-C3N4/ZnIn2S4 [40], g-C3N4/Cd0.5Zn0.5S7 [41], g-C3N4/SrTiO3 [42e44], g-C3N4/Sr2Nb2O7 [45], g-C3N4/ ZnFe2O4 [46], etc. That is to say, designing of g-C3N4-based heterojunction has been regarded as a common approach to enhance the photocatalytic performance. Tantalum pentoxide (Ta2O5), an important transition metal oxide semiconductor, is largely used in gas sensor, photovoltaic devices, and electronic industries because of its high dielectric and refractive coefficient as well as good stability [47e50]. Recently, Ta2O5 has been regarded as a typical active photocatalyst for water splitting due to its conduction band not only higher than the reduction potential of water, but also is relatively higher than the traditional TiO2 [51e55]. It has been experimentally shown that even the commercial Ta2O5 also possesses photocatalytic activity for hydrogen production [56,57]. What's more, the conduction band edge of g-C3N4 (ECB ¼ 1.1 eV) is higher than the Ta2O5, thus Ta2O5 could be as a good candidate for capture of the photoexcited electrons from g-C3N4, resulting in improvement charge carrier separation and enhancement of photocatalytic hydrogen evolution. To best of our knowledge, the fabrication of Ta2O5 modified g-C3N4 heterojunction and application in photocatalytic water splitting has never been reported. In the present study, a series of different amount Ta2O5 modified g-C3N4 heterostructured photocatalysts were firstly prepared by direct calcination of melamine and Ta2O5 as starting materials. The photocatalytic activity of as-prepared heterojunctions were evaluated for hydrogen production from water splitting under visible-light illumination (l > 420 nm). Compared to the pure g-C3N4, the obtained Ta2O5/g-C3N4 heterojunctions could dramatically enhance photocatalytic activity for hydrogen evolution. Meanwhile, the 7.5%TO/CN heterojunction exhibited the highest hydrogen evolution rate, which was 4.2-fold higher than that of pristine g-C3N4. In addition, the synthesized 7.5%TO/CN sample also possessed excellent stability even after 20 h photocatalytic recycling tests.

Experimental section Synthesis of photocatalysts Ta2O5 and melamine were analytical grade agents and purchased from Aladdin (China) without further purification. A series of Ta2O5/g-C3N4 heterojunctions were synthesized through a facile one-step heating strategy. Typically, 2 g melamine and different amount of Ta2O5 (50, 100, 150, and 200 mg) were putted into an agate mortar, and well grounded together for 5 min. After that, the milled powders were

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transferred to a 50 mL covered alumina crucible, then calcined at 550 C for 4 h with a heating rate of 2.3 C/min under air atmosphere. After being cooled to room temperature, the resulting products were milled into powder and collected for further using. The added contents of Ta2O5 were 2.5, 5, 7.5, and 10 wt%, which are named as 2.5%TO/CN, 5%TO/CN, 7.5%TO/ CN and 10%NO/CN, respectively. For comparison, bare g-C3N4 was also similarly prepared but without the addition of Ta2O5 during the thermal polymerization.

Characterization The crystal structures of the as-prepared samples were determined by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.54178 A), (D/MAX-2500 diffractometer, Rigaku, Japan). Infrared spectra were obtained on KBr pellets on a Nicolet NEXUS470 FTIR in the range of 400e4000 cm1. The X-ray photoelectron spectroscopy (XPS) was determined by a Thermo ESCALAB 250X (America) electron spectrometer using 150 W Al Ka X-ray sources. The transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images, and selected area energy dispersive X-ray spectra (EDX) were observed by field emission electron microscopy (JEM-2100F, Japan) with an accelerating voltage of 200 kV. The UVevis diffused reflectance spectra (DRS) of the samples were measured on an UVevis spectrophotometer (UV-2450, Shimadzu, Japan). The electrochemical impedance spectra (EIS) tests were performed by using a CHI 660 C (Chenhua Instruments, China) electrochemical workstation in a standard three-electrode configuration. The photoluminescence (PL) spectra were analyzed with a Perkin-Elmer LS 55 at room temperature using a fluorescence spectrophotometer.

Photocatalytic hydrogen evolution test The photocatalytic water splitting tests were implemented in a Lab-H2 photocatalytic H2 production system. A 300 W Xenon arc lamp equipped with a 420 nm cut-off filter was chosen as a visible-light source in the system. Typically, 100 mg photocatalysts were well dispersed in 200 mL aqueous solution containing 20 vol% methanol scavenger. Subsequently, a certain amount of H2PtCl6$6H2O aqueous solution was added for the in-situ formation of Pt as the cocatalyst, which yields a loading of around 0.5 wt%. The oxygen dissolved in the aqueous solution was removed by pumping vacuum and the temperature was carefully maintained below 5 C under the whole experiment. The above solution was constantly stirred to keep the uniformity of the suspension under photocatalytic reaction. The quantitative gas was collected at the given time intervals and was analyzed to get the specific content of hydrogen by gas chromatograph (GC-14C, Shimadzu, Japan, TCD, Nitrogen as a carrier gas).

Results and discussion The phase and crystal structures of the as-prepared samples were analyzed by the XRD technique. As shown in Fig. 1, pristine g-C3N4 exhibits two typical peaks at 13.1 (100) and 27.4 (002), corresponding to the in-plane structural packing

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motif and the stacking of the conjugated aromatic systems, respectively [4,7,11]. For pure Ta2O5, all of those peaks can be readily indexed to the orthorhombic phase (JCPDS 71-0639) of Ta2O5. Compared to the bare g-C3N4 and Ta2O5, the assynthesized TO/CN heterojunctions display the combination characteristic peaks of g-C3N4 and Ta2O5. In addition, no other impurity phases are discovered, indicating the as-obtained samples are well crystallized. Moreover, with the mass ratio of Ta2O5 increased from 2.5 to 10%, the two peaks intensity of g-C3N4 become weaker and the peaks of Ta2O5 became stronger, evidencing the fact that more Ta2O5 coupling with gC3N4 to formation the heterojunction [58]. The crystal structure of as-obtained samples can be further confirmed by FTIR spectroscopy. As illustrated in Fig. 2, bare gC3N4 presents the typical several peaks at 3000e3500, 1200e1700, and 810 cm1, which are attributed to the stretching of amino groups, aromatic stretching modes of CN heterocycles, and the breathing vibration of tri-s-triazine units, respectively [7,15,17]. The characteristic FTIR spectrum of the TO/CN heterojunctions are very similar to that of g-C3N4, suggesting their same chemical structures. That is to say, the Ta2O5 introducing have no influence on the g-C3N4 crystallization. Meanwhile, it is not found the Ta2O5 peaks in those heterojunctions, further demonstrating that the relatively lower amount of Ta2O5 loading on the g-C3N4. The morphologies and microstructures of as-synthesized samples were recorded by TEM and HRTEM. As shown in Fig. 3a, the TEM image of Ta2O5 exhibits some agglomeration with an average particle size of around 150 nm. From Fig. 3b, it can be observed that pure g-C3N4 presents the stacking layers structure with different nanosizes and smooth surface. After introducing the Ta2O5, the 7.5%TO/CN sample shows twodimensional sheet-like structure which combined with two black Ta2O5 nanoparticles in Fig. 3c. Meanwhile, the insert selected area EDX spectra further demonstrating that the Ta2O5 has been successfully modified on the g-C3N4 surface. From Fig. 3d, the HRTEM image clearly reveals that the lattice spaces of Ta2O5 crystallite is determined to be 0.245 nm, corresponds to the (1 11 1) plane of the orthorhombic phase Ta2O5 as well as agrees with the XRD analysis. In addition, the clearly

Fig. 1 e XRD patterns of as-prepared photocatalysts.

Fig. 2 e FTIR spectra of as-prepared photocatalysts.

interface between g-C3N4 and Ta2O5 could be observed, implying the heterojunction has been constructed. The compositions and chemical states of the synthesized 7.5%TO/CN sample were investigated by XPS. As illustrated in Fig. 4a, it can be obviously observed that the survey spectra composed of C, N, Ta and O elements, revealing that both gC3N4 and Ta2O5 are existing on this heterojunction. From the high-resolution C 1s spectrum in Fig. 4b, it can be resolved into two peaks centered at 285.1 and 288.4 eV, and these are respectively attributed to the sp2-bonded carbon in CeC and NeC]N bonds [11,15]. From the high-resolution N 1s spectrum in Fig. 4c, three peaks centered at binding energies of 398.6, 399.2, and 400.5 eV are obtained after deconvolution of the N 1s spectra, which arise from sp2-hybridized nitrogen in CeN]C groups, tertiary nitrogen Ne(C)3 and the NeH bonds, respectively [15,59,60]. From the Ta 4f high-resolution XPS spectrum in Fig. 4d, the doublet peaks centered at 26.4 eV and 28.5 eV are respectively corresponding to the Ta 4f7/2 and Ta 4f5/2 orbitals of Ta2O5 [61]. From the O 1s high-resolution XPS spectrum in Fig. 4e, the two peaks centered at 530.4 eV and 532.3 eV are assigned to the O1s species in OeTaeO and Tae OH species in the Ta2O5 network, respectively [53,61]. The optical properties of as-prepared samples were recorded by the UVevis diffuse reflectance spectra (DRS). As shown in Fig. 5a, it can be observed that pure Ta2O5 exhibits a sharp absorption edge rising at around 320 nm, whereas bare g-C3N4 displays absorption wavelengths from UV to visible-light region at about 460 nm. Compared to the bare g-C3N4, the synthesized TO/CN heterojunctions show a little systematic slight red-shift, suggesting that those photocatalysts also possess good visible-light absorption and can be used as promising visible-light-active materials for application in photocatalysis reaction. Additionally, the intrinsic band gap energies were obtained by using the transformed KubelkaeMunk function vs. the energy light. As depicted in Fig. 5b, the band gap energies of g-C3N4 and Nb2O5 were determined to be 2.68 and 3.96 eV, respectively. The photocatalytic hydrogen evolution performance of the resultant samples are evaluated for water splitting under visible-light irradiation (l > 420 nm). As shown in Fig. 6a, bare


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Fig. 3 e Typical (a, b) TEM images of the Ta2O5 and g-C3N4 samples; (c, d) TEM and HRTEM images of the 7.5%TO/CN heterojunction.

Fig. 4 e XPS spectra of the 7.5%TO/CN heterojunction photocatalyst: (a) survey spectra; (b) C1s; (c) N1s; (d) Ta 4f; (e) O1s.

Ta2O5 exhibits negligible photocatalytic activity for hydrogen evolution, which is attributed to the fact that Ta2O5 cannot be initiated by visible-light irradiation due to its larger band gap. Pure g-C3N4 shows a poor photocatalytic hydrogen production, whereas the photocatalytic activity of synthesized TO/ CN heterojunctions gradually increased by increasing the Ta2O5 amount from 2.5 to 7.5 wt%. When further increasing the loading amount to 10%, the photocatalytic activity is decreased. That is to say, the hydrogen evolution rate (HER) of the as-prepared heterojunctions are strongly affected by the Ta2O5 introducing amount. As given in Fig. 6b, compared to the pure g-C3N4 (HER ¼ 8.7 mmol h1 g1), the 2.5%TO/CN sample shows a slightly increased HER of 20.1 mmol h1 g1.

The 5%TO/CN and 10%TO/CN samples exhibit further improved the HER to 29.8 mmol h1 g1 and 33.9 mmol h1 g1, respectively. In particular, the 7.5%TO/CN sample presents the highest HER (36.4 mmol h1 g1), which is about 4.2-folds higher than that of pristine g-C3N4. Additionally, the repeated photocatalytic test of the 7.5%TO/CN sample for hydrogen evolution was investigated. As illustrated in Fig. 7, the hydrogen production reaction of this heterojunction was sustained for over 20 h without noticeable deactivation, demonstrating its excellent high photochemical stability. The electrochemical impedance spectra (EIS) of asprepared samples were measured to investigate their charge transfer capability [45,62]. As shown in Fig. 8a, the EIS Nyquist

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Fig. 5 e (a) DRS of as-prepared photocatalysts; (b) plots of the (ahn)1/2 vs. photon energy (hn) for g-C3N4 and plots of the (ahn)2 vs. photon energy (hn) for Ta2O5.

plot of bare g-C3N4 exhibits the largest semicircles, indicating its low photoinduced charge carrier's separation and migration efficiency. Compared to the pure g-C3N4, the TO/CN samples display much smaller semicircles, revealing a significant decrease of the charge transfer resistance after formation heterojunctions. Meanwhile, the 7.5%TO/CN presents the smallest semicircle, suggesting its best charge transfer capability and photocatalytic hydrogen evolution activity. In addition, the photoinduced charge migration, transfer, and recombination abilities of the synthesized samples were further estimated by the photoluminescence (PL) analysis [6,17,36]. As depicted in Fig. 8b, it can be clearly found that the PL emission intensities of the TO/CN samples are much lower than that of pristine g-C3N4, indicating that the recombination of photogenerated electronehole pairs is greatly inhibited. Moreover, the 7.5%TO/CN photocatalyst shows the lowest PL emission intensity, implying its fastest charge transfer ability and corresponding to the EIS result. Thus, the results indicate that the Ta2O5 modified g-C3N4 heterojunction can efficiently improve the photogenerated electronehole pair's separation, which is indeed beneficial for their photocatalytic hydrogen evolution activity. The position of band gap edges of g-C3N4 and Ta2O5 were further confirmed to detail explaining the photocatalytic mechanism. For a semiconductor, the valence band (VB) and conduction band (CB) can be calculated according to the empirical equation:

Fig. 6 e (a) Photocatalytic hydrogen evolution amount of as-prepared samples and (b) comparison of the hydrogen evolution rate for different photocatalysts.

Fig. 7 e Stability test of hydrogen production (evacuation every 5 h) for 7.5%TO/CN heterojunction under visible-light irradiation.

1 ECB ¼ X Ee Eg 2

(1)

EVB ¼ ECB þ Eg

(2)

where ECB is the conduction band potential, EVB is the valence band potential; X is the absolute electronegativity of the semiconductor; Ee is the energy of free electrons on the hydrogen scale (Ee is about 4.5 eV). The X values for g-C3N4


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mechanisms of TO/CN heterojunction was proposed in Fig. 9. It is found that g-C3N4 can be initiated to generate photogenerated electronehole pairs under visible-light irradiation, whereas the Ta2O5 cannot be excited due to its larger band gap. Since the potential of CB of g-C3N4 is more negative than that of Ta2O5, the photoinduced electrons on the CB of g-C3N4 could be easily transferred on the CB of Ta2O5 through the heterojunction interface. Finally, the accumulated electrons on the CB of Ta2O5 can be involved in the photochemical reduction of water to generate hydrogen. Simultaneously, the holes in the VB of g-C3N4 are trapped by the methanol as scavengers. In such a way, the heterojunction could be inhibit the recombination rate of photogenerated electronehole pairs, resulting in remarkably enhanced photocatalytic activity for hydrogen evolution than that of pure g-C3N4.

Conclusions

Fig. 8 e (a) EIS spectra and (b) PL emission spectra of asprepared samples.

In summary, we have been successfully constructed the novel Ta2O5/g-C3N4 heterojunction via a simple one-step heating process for the first time. The as-prepared heterojunctions exhibited superior visible-light photocatalytic activity for hydrogen production than the pure g-C3N4. The 7.5% TO/CN heterojunction not only showed the best photocatalytic performance, but also possessed high photochemical stability. The heterojunction interface between g-C3N4 and Ta2O5 with suitable band positions could be efficiently promote the charge transfer, leading to significantly enhanced photocatalytic activity. Additionally, the asprepared heterojunctions are cost saving, facile fabrication, high visible-light activity, and excellent stability, which are indeed benefit for its practical application.

Acknowledgements We would like to acknowledge the National Natural Science Foundation of China (21276116, 21477050, 21301076, 21303074, 21522603 and 21576121), the Chinese-German Cooperation Research Project (GZ1091), the China Postdoctoral Science Foundation (2015M571689), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), the Program for New Century Excellent Talents in University (NCET-13-0835), the Jiangsu Postdoctoral Science Foundation (1402100C), the Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).

references Fig. 9 e Schematic of the charge carriers separation and transfer over the Ta2O5/g-C3N4 heterojunction under visible-light irradiation. and Ta2O5 are 4.73 and 6.23 eV, respectively. Thus, the ECB of g-C3N4 and Ta2O5 are calculated to be 1.11 and 0.25 V vs. NHE, and the EVB of them are determined to be 1.57 and 3.71 V vs. NHE, respectively. The enhancement of photocatalytic

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