rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation

international journal of hydrogen energy xxx (xxxx) xxx

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Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation Xiang Xie a, Ruimiao Wang a, Enzhou Liu a,*, Jun Fan a, Bo Chen a,**, Xiaoyun Hu b a b

School of Chemical Engineering, Northwest University, Xi'an, 710069, PR China School of Physics, Northwest University, Xi'an, 710069, PR China

highlights
Cu2-xSe/rGO

heterojunction

graphical abstract is

synthesized by a simple in-situ hot-injection method.
Cu2-xSe/rGO

heterojunction

is

applied to photocatalytic H2 production performance for the first time.
Cu2-xSe/rGO heterojunction shows superior photocatalytic H2 production performance than pure Cu2-xSe.
Cu2-xSe/rGO exhibits good stability after cycling runs.

article info

abstract

Article history:

The present study has successfully fabricated a Cu2-xSe/rGO heterojunction for the first

Received 22 May 2019

time using an in situ hot-injection method and employed it to produce photocatalytic

Received in revised form

hydrogen. The optimal Cu2-xSe/3%rGO can achieve an efficient photocatalytic H2 produc-

21 September 2019

tion at the rate 3123.48 mmol g1 h1, nearly 3.46 times higher than that of the pure Cu2-xSe.

Accepted 17 October 2019

The enhanced activity can be attributed to facilitated light absorption, up-regulated charge

Available online xxx

density, lower interfacial transfer resistance as well as a longer electron decay lifetime. In the meantime, the expanded specific surface area can create more active reaction sites,

Keywords:

leading to the enhancement of photocatalytic peropeties. Besides, the mechanism of the

Cu2-xSe

Cu2-xSe/rGO heterojunction's hydrogen production is proposed.

rGO

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Heterojunction in situ hot-injection method Photocatalytic H2 generation

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E. Liu), [email protected] (B. Chen). https://doi.org/10.1016/j.ijhydene.2019.10.148 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Introduction In recent years, sustainable development has aroused rising attention for the progressively severe environmental pollution and energy shortages. Hydrogen energy, a clean renewable energy source, is critical to address the above issues [1e3]. Since Honda and Fujishima discovered water splitting over TiO2 under ultraviolet light irradiation to generate H2 in 1972 [4], semiconductor photocatalysis has stimulated growing attention in tackling down the energy shortage and environmental problems due to its promising applications in splitting of water, removal of heavy metals and organics, reduction of carbon dioxide, disinfection of bacteria, etc. [5e12]. Nevertheless, most of the developed photocatalysts exhibit unsatisfactory efficiency and stability. For instance, TiO2 and SnO2 are wide-bandgap (>3.0 eV) semiconductors capable of absorbing ultraviolet light only, causing low utilization of sunlight [13e18]. CdS and g-C3N4 exhibit a visiblelight photoresponse ability for their relatively narrow band gap (~2.4 eV) [19], but CdS suffers from photocorrosion and the absorption edge of g-C3N4 is nearly 500 nm with insufficient utilization of visible-light [20e25]. Currently, the exploration and design of novel and efficient photocatalysts is critical and expected. Copper selenide (Cu2-xSe) is generally known as a p-type semiconductor and has hugely potential applications in solar cells, optical filters, superionic materials, thermoelectric converters and ablation of tumor cells [26e31]. However, Cu2xSe, as a photocatalyst for hydrogen production, has rarely been investigated. In practice, Cu2-xSe exhibits visible-light absorption ability due to its narrow bandgap in the range of 1.4e2.2 eV, which is suitable for the photocatalytic process [32e34]. It has attracted widespread attention to delving into the photocatalytic performance of Cu2-xSe because of its wide range of light absorption, excellent stability and ecofriendliness. However, the high recombination rate and the poor migration capability of photogenerated electrons still limit its large-scale application [35,36]. To overcome the abovementioned weaknesses, coupling two important semiconductors has been extensively applied to construct heterojunction photocatalysts, which can enhance the light capture ability and effective separation of photoexcited electron-hole pairs, thereby achieving remarkable photocatalytic activity [37,38]. Liu et al. employed solvothermal method to prepare Cu2O/Cu2Se multilayer heterostructure nanowires in ethanol amine, and the amount of H2 evolution was approximately 115 mmol g1 over 4 h due to the effective separation of photogenerated electrons [39]. Clearly, the H2 production rate of the Cu2O/Cu2Se heterostructure is lower. Reduced graphene oxide (rGO) refers to an excellent electron transition mediator because of its two-dimensional structure with a large specific surface area, a superior electron transfer property and a low cost [40,41]. Meanwhile, rGO exhibits prominent synergetic catalytic abilities to construct large contact interfaces with other materials as cocatalysts [42e44]. As Quan et al. reported, the a TiO2/MG (MoS2 and GO) photocatalyst was fabricated following a hydrothermal process. The results of investigations indicated that the H2

production rate of TiO2/MG was 39 and 4 times than that of TiO2 and TiO2/MoS2, respectively, due to the enhanced transfer of interfacial electrons and more active reaction centers [45]. Wan et al. found that g-C3N4/rGO was prepared using a dissolution-precipitation strategy, and the g-C3N4/2 wt %rGO demonstrated the optimal photocatalytic H2 production rate (715 mmol g1 h1) because of the fast transfer of photogenerated charges and an extended charge carrier lifetime, which was approximately 13 times than that obtained on CN [46]. To the best of knowledge, there is no previous report about Cu2-xSe/rGO heterojunction photocatalysts for photohydrolysis processes. Thus far, several methods have been adopted to synthesize Cu2-xSe nanomaterials, covering template reactions [47], pulsed laser deposition technology [48], solvothermal methods [49], sonochemical treatments [50], as well as chemical vapor deposition techniques [51]. However, these methods have high costs, high-energy demands, high-level equipment requirements and require complex operational technological processes. In addition, it is well known that CueSe systems have different stoichiometric (Cu2Se, CuSe, CuSe2, Cu3Se2, CuSe2, Cu5Se4 and Cu7Se5) and nonstoichiometric (Cu2-xSe) compounds and numerous phases [52]. Hence, the fabrication of the Cu2-xSe using an easy and eco-friendly method remains challenging. In this study, a Cu2xSe semiconductor was prepared by a facile hot-injection method. In this work, Cu2-xSe/rGO heterojunction photocatalysts were synthesized via an in situ hot-injection method. It was discovered that the as-prepared Cu2-xSe/rGO exhibits an enhanced activity for photocatalytic H2 generation compared with Cu2-xSe due to the existence of a synergistic effect between Cu2-xSe and rGO. The probable photocatalytic mechanism was proposed and explained according to the BET surface area, UVevis diffuse reflectance spectra, electrochemical characterizations and time-resolved fluorescence decay spectra.

Experimental section Synthesis Preparation of graphene oxide A certain amount of graphite oxide (XF NANO Inc.) was dispersed in triethylene glycol (TEG, AR, Macklin) to form a solution of 0.5 mg ml1 using an ultrasonic exfoliation method for 9 h to obtain gaphene oxide.

Synthesis of Cu2-xSe/rGO The Cu2-xSe/rGO composites with different rGO contents were prepared using a facile in situ hot-injection method. The composites are expressed as Cu2-xSe/X % rGO, where X represents the mass fraction of rGO and X ¼ 1.5, 3, 4.5 or 6. Fig. 1 shows a schematic diagram for the preparation process of the Cu2-xSe/rGO composites. To be specific, 1.6 mmol Cu(NO3)2$3H2O (AR, Macklin) and 0.08 g ascorbic acid (AA, 99.7%, Sinopharm chemical Reagent Co., Lid) were dissolved into 30 mL of TEG in a beaker at 80  C. Subsequently, a certain

Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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volume of GO solution and Cu(NO3)2 solution were fully mixed to obtain the precursor and a stoichiometric amount of Se (AR, Kermel) as affected by the copper was well dispersed in 40 mL of TEG in a three-neck flask. When the Se was completely dissolved, the former was added to the latter by injector, reacted for 1 h at 230  C and shaken at 1000 rpm to produce the mixed solution. Lastly, the three-neck flask was slowly cooled to room temperature and the products were centrifuged at 9000 rpm for 10 min, cleaned with anhydrous ethanol 6 times and dried at 60  C for 7 h.

Synthesis of Cu2-xSe The Cu2-xSe was synthesized by the hot-injection method. The only difference in the synthesis process of Cu2-xSe was the absence of the GO solution. In the previous work, it was discovered that the modifiers and temperature significantly affect the micro-structure of Cu2-xSe. (Fig. S1 and Fig. S2).

3

calculated per 30 min by a gas chromatograph with TCD (thermal conductive detector) with N2 as the carrier gas.

Results and discussion The characterization of the samples Fig. 2 displays the XRD patterns of the GO, only Cu2-xSe and Cu2-xSe/rGO with different rGO contents, respectively. The characteristic diffraction peaks centered are at 26.8 , 44.6 , 53.0 , 64.9 , 71.6 and 82.3 , which can be assigned to the (111), (220), (311), (400), (331) and (422) crystal face of the cubic phase

Characterization The details of the physical characterizations, fabrication of the photoelectrodes and the photoelectrochemical measurements are presented in the supporting information.

Test of photocatalytic H2 evolution The H2 production from water splitting was conducted to estimate the photocatalytic performances of the samples in a Pyrex flat-bottomed reaction vessel attached onto a glassclosed gas circulation system under 300 W Xe lamp. The prepared sample (20 mg) was suspended in 100 ml scavenger containing 0.35 M Na2S and 0.15 M Na2SO3. The dissolved O2 was removed by creating a vacuum in the photocatalytic reactor before light irradiation. Additionally, vigorous magnetic stirring was adopted to keep the photocatalysts in a homogeneous status during the reactions. The H2 amount was

Fig. 2 e XRD patterns of GO, Cu2-xSe, Cu2-xSe/1.5%rGO, Cu2-xSe/3%rGO, Cu2-xSe/4.5%rGO and Cu2-xSe/6%rGO.

Fig. 1 e Schematic diagram of the fabrication of the Cu2-xSe/rGO heterojunction. Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Cu2-xSe (Fig. 2 black curve), and the composite samples display no other phase diffraction peaks of Cu2-xSe. The mentioned results reveal that the crystal structures of Cu2-xSe are not influenced by the incorporation of rGO. With the increase in the rGO content, the intensity of the diffraction peak increases, indicating that crystallization of Cu2-xSe is facilitated. The peak centered at 10.1 could be attributed to the (001) lattice plane of GO. This characteristic diffraction peak is absent in the Cu2-xSe/rGO because of fewer GO and reduction reaction of GO, leading to the decrease in the oxygencontaining group [53]. the SEM patterns of the Cu2-xSe and Cu2-xSe/3%rGO heterojunction are shown in Fig. 3. Fig. 3a suggests that the Cu2is composed of inconsistent nanoparticles with xSe 40e200 nm diameters, and there are numbers of small pores. rGO exhibits a two-dimensional layered structure with crumpled edges in Fig. 3b and Cu2-xSe nanoparticles distribute evenly on its surface, which suggests the Cu2-xSe/ rGO heterojunction has been prepared successfully. Furthermore, the BET specific surface area of Cu2-xSe/3% rGO (15.91 m2 g1) is larger than that of Cu2-xSe (5.67 m2 g1) in Fig. 4, creating more abundant active sites to produce charge carriers, which is also beneficial to its photocatalytic properties. Fig. 5 shows TEM images of Cu2-xSe/3%rGO. Cu2-xSe is anchored on the surface of rGO in Fig. 5a and b (Fig. S4), forming the intimate contact interface (Fig. 5c), which would facilitate the separation and migration of electrons, causing the improved the photocatalytic activity. As shown in Fig. 5d (high-resolution TEM), the interplane distance of the Cu2-xSe is 0.20 nm, which is most consistent with the (220) crystalline plane of Cu2-xSe. The elemental composition of Cu2-xSe/3%rGO was tested by EDS and elemental mapping, which indicates the coexistence of Cu, Se, C and O in the samples in Fig. 3c to f. The EDS results of the Cu2-xSe and Cu2-xSe/3%rGO are listed in Table 1 (the EDS spectrum is shown in Fig. S3). The atomic ratios of Cu/Se are 1.91/1 and 1.95/1, respectively.

Volume adsorbed (cm 3 STP/g)

4

S=5.67 m2/g

Cu2-xSe

0.0

Cu2-xSe/3% rGO

S=15.91 m2/g

0.2

0.6

0.4

0.8

1.0

P/P0 Fig. 4 e N2 adsorptionedesorption isotherms of Cu2-xSe, and Cu2-xSe/3%rGO. The XPS spectra of the product are shown in Fig. 6. All of the data were regulated according to the C 1s at 284.6eV. The survey spectra of Cu2-xSe/rGO are illustrated in Fig. 6a, which suggests the existence of Cu, Se, C and O in the composites. In Fig. 6b, the peaks at 284.6, 286.2 and 288.1 eV for the Cu2-xSe/ 3%rGO can be assigned for C]C, CeO and C]O bonds of rGO. However, it is obvious that the peak at 290.3eV (O]CeOH) obviously disappears for Cu2-xSe/3%rGO, and the intensity of the peak for CeO is clearly lowered compared with GO, revealing that GO might be reduced and illustrating the existence of rGO in the composites [46,54]. Furthermore, the two peaks at 932.3 and 952.4eV in Fig. 6c are distributed to Cu 2p3/2 and Cu 2p1/2 in Cu2-xSe/3%rGO, respectively, corresponding to Cuþ. Meanwhile, there are two satellite peaks (Sat) at binding energies of 943.9 eV and 962.8 eV, which illustrates the existence of Cu2þ [42,55]. The XPS spectrum of Se 3d is displayed in

Fig. 3 e SEM images of (a) Cu2-xSe and (b) Cu2-xSe/3%rGO, Elemental mapping images of (c) Cu, (d) Se, (e) C, (f) O in Cu2-xSe/3% rGO. Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Fig. 5 e The (a and b) TEM images and (c and d) HRTEM images of the Cu2-xSe/3%rGO heterojunction. Fig. 6d. For the Cu2-xSe/3%rGO, the peak at 54.1 eV is attributed to binding states of Se 3d and a peak at 58.6 eV is due to the Se4þ (possibly SeO2) [56,57]. In addition, as for Cu2-xSe/3%rGO, the peaks of Se 3d, Cu 2p slightly move to lower binding energies compared with those of pure Cu2-xSe. Meanwhile, the binding energy of C 1s slightly shifts to higher binding energies, probably because of the partial electron transmission. The results indicate that an intensive electronic interaction was generated between the interfaces of the Cu2-xSe and rGO during the inverse flow process [58]. To further verify whether GO was combined with Cu2-xSe nanoparticles and GO was reduced to rGO at the elevated temperature condition, the Raman spectra of GO, pure Cu2xSe, Cu2-xSe/3%rGO were performed. As shown in Fig. 7, the D band at 1357 cm1 originates from the disorganized aromatic structure of GO and the G band appears at 1597 cm1 due to the oscillation of the sp2 C atoms [59]. Meanwhile, the

Table 1 e Chemical stoichiometries of the as-prepared products by EDS and the Cu/Se ratio. Samples

Cu(at Se (at C (at O (at %) %) %) %)

Cu2-xSe Cu2-xSe/3% rGO

65.67 50.66

34.33 26.06

0 19.45

0 3.83

Atomic ratio of Cu/Se 1.91/1 1.95/1

two characteristic main bands can be seen in the Cu2-xSe/3% rGO composite, whereas the two bands are not observed in the pure Cu2-xSe nanoparticle, indicating GO was successfully introduced into the Cu2-xSe/rGO heterojunction. In addition, the intensity ratio of the D and G band (ID/IG) denotes the disordered degree of the carbon materials structure [60]. It can be observed from Fig. 7 that the ID/IG ratios of GO is 0.96. After the reaction, the ID/IG are decreased to 0.83 for Cu2-xSe/3%rGO, which indicates that GO has been reduced to rGO [42,61].

Photocatalytic hydrogen generation and stability To investigate the photocatalytic performance of the Cu2-xSe and Cu2-xSe/rGO heterojunction, an experiment of photocatalytic water splitting into H2 was executed under Xe lamp irradiation with a Na2SeNa2SO3 solution as the hole scavenger. There was no H2 produced without light or the photocatalyst, indicating that H2 production is attributed to by the photocatalytic reaction process. As shown in Fig. 8a and b, the photocatalytic hydrogen evolution reaction performance of Cu2-xSe with different content of rGO (0, 1.5, 3, 4.5, 6 wt%) shows an almost linear increase of the amount of H2 throughout the entire light irradiation time, suggesting their superior photostabilities during the photocatalytic H2 production reaction process. The pure Cu2-xSe exhibits a relatively low H2 generation rate of 901.93 mmol g1 h1, indicating

Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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(b) intensity (a.u)

O 1s

Se 3d

C 1s

Intensity (a.u)

Cu 2p

(a)

0

284.6 eV 288.1 eV C=O

Cu2-xSe/3%rGO

287.5 eV

290.3 eV O=C-OH

GO

150

300

450

600

750

900

1050 1200

280

282

Cu 2p1/2 954.8 eV

Cu 2p Sat

Sat Cu2-xSe

Cu 2p3/2 932.3 eV

Cu 2p1/2 952.4 eV Sat

284

286

288

290

292

Binding energy (eV)

Sat

(d)

54.4 eV

Se 3d Cu2-xSe

Intensity (a.u)

Cu 2p3/2 934.6 eV

Intensity (a.u)

286.2 eV C-O

284.6 eV 285.6 eV

Binding energy (eV)

(c)

C 1s

54.1 ev

Se 3d Cu2-xSe/3%rGO

58.6 ev Cu2-xSe/3%rGO

930

935

940

945

950

955

960

965

Binding energy (eV)

50

52

54

56

58

60

62

Binding energy (eV)

Fig. 6 e (a) XPS survey spectra of Cu2-xSe/3%rGO, and high resolution XPS spectra of (b) C 1s for GO and Cu2-xSe/3%rGO, (c)Cu 2p and (d) Se 3d for Cu2-xSe and Cu2-xSe/3%rGO.

Intensity (a.u)

a weak photocatalytic activity, which is presumably caused by the low surface area and photogenerated charge carriers rapid recombination. However, the Cu2-xSe/rGO heterojunction shows a superior photocatalytic property, demonstrating that the construction of the heterojunction is beneficial to the advancement of the photocatalytic property. When the content of rGO increases to 3% wt, the Cu2-xSe/rGO heterojunction exhibits the best H2 production rate of 3123.48 mmol g1 h1, which is approximately 3.46 times higher than that of the Cu2xSe. Moreover, this performance is favorably comparable to

GO Cu2-xSe/3% rGO Cu2-xSe 1597 1357

ID/IG=0.96 ID/IG=0.83

1000

1200

1400

1600

1800

2000

Raman Shift (cm ) -1

Fig. 7 e Raman spectra of GO, pure Cu2-xSe and Cu2-xSe/3% rGO.

those of as-prepared semiconductor photocatalysts for H2 production, for instance In2Se3 homojunction (1347.6 mmol g1 h1) [62] and FeSeFeSe2 nanosheet (2071.1 mmol g1$h1) [63] et al. (Table S1), which indicates the excellent photocatalytic properties of Cu2-xSe/rGO as a H2 production photocatalyst. The obviously enhanced photocatalytic performances of Cu2-xSe/rGO are probably because rGO can provide more active sites. A notable synergetic effect arising from the intimate interactions between Cu2-xSe and rGO appears, thus accelerating the transfer of electrons in the H2 production reaction process [46,64]. However, a relative decrease in photocatalytic H2 generation activity arises from the excess rGO (4.5% and 6%) in the heterojunction, because of superfluous rGO will impede the utilization light and hinder the reduction of protons by the active site [65,66]. Accordingly, using the proposed method can effectively and feasibly construct Cu2-xSe/rGO heterojunction with a suitable content of rGO for high-efficiency photocatalytic H2 generation. To reveal the stability of the Cu2-xSe/3%rGO, four recycling experiments for photocatalytic H2 production were performed under similar experimental conditions. Fig. 9a shows the curves of H2 evolution for Cu2-xSe/3%rGO, suggesting that the amount of H2 generation nearly remains unchanged after the four recycling experiments. Furthermore, the XRD patterns of Cu2-xSe/3% rGO before and after the recycling experiments are almost identical in Fig. 9b. These results reveal that Cu2-xSe/3%rGO is extremely stable during the H2 production process.

Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Fig. 8 e (a) The amount of H2 production for all samples (b) Comparison of H2 production rates for all of the synthesized products.

Fig. 9 e (a) Cyclic H2 production curves for the Cu2-xSe/3% rGO composite, (b) XRD patterns of Cu2-xSe/3% rGO before and after recycling runs.

The mechanism of the improved photocatalysis of the Cu2-xSe/rGO heterojunction It is well generally that the light absorption properties and band structures of photocatalysts significantly impact their photocatalytic properties. The UVevis of the samples are exhibited in Fig. 10a. Note that the absorption edge of Cu2-xSe lies at 704 nm in Fig. 10a, representing a band gap of 1.76 eV. For the Cu2-xSe/3%rGO, the absorption edge exhibits an obvious redshift, which arises from the black rGO and violent

interfacial interaction existing between rGO and Cu2-xSe [2,67]. Clearly, with the incorporation of rGO, it can be observed that the visible light absorption of the Cu2-xSe/rGO composite is facilitated compared with that of Cu2-xSe, which is beneficial for improving its photocatalytic properties. To further explore the mechanism for the promoted photocatalytic properties of the Cu2-XSe/rGO heterojunction, the energy band structures of the as-prepared photocatalysts were investigated via Mott-Schottky plots. In Fig. 10b, the linear region of the plot exists in a negative slope, which

Fig. 10 e (a) UVevis diffuse reflectance spectra, and (b) the Mott-Schottky plots of Cu2-xSe and Cu2-xSe/3%rGO. Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Fig. 11 e (a) Transient photocurrent response and (b) electrochemical impedance spectroscopy measurement for all samples measured at the bias potential for 0.5 V (vs. SCE) in 0.5 M Na2SO4 aqueous solution.

suggests that the as-prepared Cu2-xSe is a p-type semiconductor material. According to the intersection of the tangent line and the abscissa axis, the flat-band potential (EFB) of Cu2-xSe could be ascertained as nearly 1.22 V vs SCE. The valence band (VB) potential (EVB) of a p-type semiconductor is generally considered to be approximate to (0.1 eV more positive) the EFB, so the EVB of Cu2-xSe is 1.56 eV vs NHE (by the equation: E (vs NHE) ¼ E (vs SCE)þ0.24 eV). Combining with the results of UVevis, the conduction band (CB) of Cu2-xSe lies around 0.20 eV vs NHE. In addition, compared with Cu2-xSe, the flat-band of Cu2-xSe/3%rGO (1.46 V vs SCE) obviously shifts to the positive position, which can qualitatively demonstrate that an effective heterojunction has been successfully constructed. The energy bands of p-type Cu2-xSe move downward along its Fermi level until arriving at equilibration upon the electric field forming at the interface between Cu2-xSe and rGO [46,68,69]. As a result, the built-in electric field would noticeably facilitate the effective separation and transfer of the photoinduced charges at the heterojunction, which promotes photocatalytic H2 evolution properties. To further ascertain the separation and migration capability of the photoinduced electron-hole pairs, the transient

photocurrent responses and electrochemical impedance spectroscopy (EIS) were detected under a typical threeelectrode cell system. The transient photocurrent of Cu2-xSe and Cu2-xSe/rGO with different rGO contents under intermittent light irradiation is illustrated in Fig. 11a. The value of the photocurrent quickly decreased and regressed to the same value with the light turned off and on, which indicates an excellent reproducibility. It is noteworthy that pure Cu2-xSe exhibits a relatively low photocurrent density, while the Cu2xSe/3%rGO displays the maximum photocurrent density and the most robust photocurrent response, indicating a higher efficient separation of photogenerated charge carriers, which will enhance its photocatalytic properties. These results demonstrate that the rGO could efficiently inhibit the photogenerated electrons recombination, facilitate the photoinduced electron separation, and enhance electron transfer capability [70]. The EIS plots for the as-prepared samples are displayed in Fig. 11b. A smaller arc radius of the EIS plots implies a weaker electron transfer impedance, meaning a higher effective separation of charge carriers and fast charge carriers migration at the photoelectrode interface [71]. Compared with pure Cu2-xSe, all Cu2-xSe/rGO samples exhibit a smaller semidiameter. Besides, obviously the Cu2-xSe/3% rGO has the smallest semidiameter, revealing the minimum electron-transfer impedance and the maximum efficient separation and transfer of interfacial charge [72]. The results of the photoelectrochemical measurements prove the higher effective separation and immigration of charge carriers in the heterojunction. In addition, time-resolved fluorescence was used to measure the electron lifetimes for the Cu2-xSe and Cu2-xSe/3% rGO decay spectrum as shown in Fig. 12 to confirm the effective separation of the photogenerated electron-hole pairs. The decay lifetime and corresponding percentages were

Table 2 e Fitting parameters for the fluorescence decay of the samples. Samples Fig. 12 e Time-resolved fluorescence decay spectra and calculated average lifetime for Cu2-xSe and Cu2-xSe/3% rGO.

Cu2-xSe Cu2-xSe/3%rGO

t1 (ns)

A1 (%)

t2 (ns)

A2 (%)

t(ns)

1.63 2.25

97.9 90.8

6.22 6.86

2.1 9.2

1.72 2.67

Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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Fig. 13 e Proposed photocatalytic H2 production mechanisms over the Cu2-xSe/rGO heterojunction.

calculated by fitting the decay spectra as shown in Table 2. The Cu2-xSe/3% rGO displays a longer radiative lifetime (2.67 ns) compared with pure Cu2-xSe (1.72 ns). A prolonged decay lifetime means a lower recombination rate of photogenerated charge carriers, rising the possibility of participation in the reaction and the participation of more photogenerated electrons in the H2 evolution reaction [46,73]. According to these results, it can be determined that rGO modification obviously facilitates the effective separation and transfer of photoinduced electron in photocatalysts. By considering the morphology and strengthened optical absorption, the Cu2-xSe/ rGO heterojunction achieves a high-efficiency photocatalytic H2 production. Based on the mentioned characterization results and analyses, the mechanism for Cu2-xSe/rGO heterojunction in the photocatalytic H2 production reaction process was ascertained, as shown in Fig. 13. Under light irradiation, the electron in the valence band of the Cu2-xSe obtains energy and is excited to the conduction band. In the meantime, holes still remain in the valence band. Because the CB of Cu2-xSe (0.20 eV vs NHE) is higher than the Fermi level of rGO (0.08 eV) [2,74], the photoexcited electrons from the CB of Cu2-xSe could rapidly migrate to the rGO. Subsequently, the electrons further participate in the reduction reaction of Hþ to produce H2, and the holes with stronger oxidizability are

consumed by the S2/SO2 3 sacrificial agent. Consequently, the photogenerated charge carriers recombination process could be effectively suppressed, leading to the improved photocatalytic activities. In brief, the enhanced photocatalytic performance of the Cu2-xSe/rGO heterojunction can be assigned to the following synergetic effects: (i) the expanded specific surface area providing more active sites; (ii) a narrow band gap, enhanced light absorption and higher effective separation and transfer of charge carriers; and (iii) an higher charge carrier density, lower interfacial charge transfer resistance, as well as longer decay lifetimes.

Conclusions In summary, the Cu2-xSe/rGO heterojunction has been successfully prepared using an in situ hot-injection method to achieve highly effective photocatalytic H2 production (3123.48 mmol g1 h1). During a continuous test of H2 evolution for 12 h, the Cu2-xSe/rGO heterojunction obviously exhibits excellent photocatalytic properties and superior stability. The enhanced photocatalytic property is attributed to the expanded specific surface, the enhanced light absorption, the extended electron lifetime, the lower interfacial migration impedance, and effective separation and migration

Please cite this article as: Xie X et al., Fabrication of a Cu2-xSe/rGO heterojunction photocatalyst to achieve efficient photocatalytic H2 generation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.148

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of photogenerated charges. The appropriate proportions of Cu2-xSe and rGO in the heterojunction are critical to the photocatalytic performance of the heterojunction. This study will be of high implication to facilitating the photocatalytic applications of Cu2-xSe semiconductors.

Acknowledgement This work was supported by the National Natural Science Foundation of China (21676213, 21476183, and 51372201), the China Postdoctoral Science Foundation (2016M600809) and the Natural Science Foundation of Shaanxi Province (2017JM2026).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.148.

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