CdSe-DETA nanojunction for enhancing visible-light-driven photocatalytic H2 evolution

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Construction of flourinated-TiO2 nanosheets with exposed {001} facets/ CdSe-DETA nanojunction for enhancing visible-light-driven photocatalytic H2 evolution Xiaochun Kea,1, Jinfeng Zhanga,1, Kai Daia,∗, Changhao Liangb,∗∗ a

College of Physics and Electronic Information, Key Laboratory of Green and Precise Synthetic Chemistry and Application, Ministry of Education, Huaibei Normal University, Huaibei, 235000, PR China b Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Photocatalytic H2 evolution F–TiO2 CdSe Diethylenetriamine Heterostructure

Although much significant efforts have been devoted to design highly efficient photocatalysts for addressing energy crisis and environmental issues, it remains challenging to construct a promising and efficient heterostructure among semiconductors for photocatalytic hydrogen (H2) production under visible-light excitation. Herein, the novel structure with 2D fluorinated-TiO2 nanosheets (F–TiO2 NSs) decorated CdSe-diethylenetriamine nanoflowers (CdSe-DETA NFs) was designed and fabricated by a simple and fast microwave-assisted hydrothermal method. The F–TiO2/CdSe-DETA heterojunction displays enhanced photocatalytic activity in splitting water to produce H2. Among them, 20%F–TiO2/CdSe-DETA exhibited the highest H2 evolution rate (12381 μmmol g−1 h−1), which is 2.92 and 6.44 times greater than that of pristine CdSe-DETA and CdSe, respectively. Meanwhile, 20%F–TiO2/CdSe-DETA composite shows a good stability in the cyclic runs for photocatalytic H2 evolution. The enhanced activity and reusability are primarily attributed to rapid separation of charge carriers, enrich catalytic active sites as well as strong light absorption capability. Besides, the in-situ growth of a certain amount F–TiO2 NSs with exposed {0 0 1} high-energy facets on surface of inorganic-organic CdSe-DETA NFs greatly helps to strengthen intimate interfacial contact. These results may furnish a reference to fabricate the heterostructures on the basis of CdSe that is extremely insightful for conversion of solar power to H2 energy.

1. Introduction The increasing environmental pollution, energy crises and fossil fuels shortage have been extensively attracted significant attention [1–7]. Photocatalytic hydrogen (H2) evolution from water splitting on semiconductors is the most promising and superior strategy to solve these environmental and energy issues [8–10]. Up to now, numerous photocatalysts including metal sulfides, metal oxides, metal selenides and polymer semiconductors, have been explored for producing H2 [11–14]. Cadmium selenide (CdSe), as an ideal candidate for converting solar light to H2, has been investigated owing to its appropriate negative conduction band (CB) position for reducing H2O molecules as well as low materials cost [15,16]. Nevertheless, low solar energy utilization and rapid recombination rate of photoinduced charged carriers

still limit the high-efficient catalytic activity of CdSe [17,18]. Aiming to settle these limitations, various solutions have been put forward, such as vacancy engineering, construction of heterojunction, ion doping, surface sensitization and so forth, which can promote the charge migration and enhance visible-light absorption capacity for CdSe-based catalysts [19–21]. Among these methods, building heterostructures for CdSe by coupling other phases has been thought as an excellent way for boosting photocatalytic performance [22,23]. Currently, inorganic-organic hybrid nanomaterials, as a functional material that combines the benefits of organic and inorganic materials, have attracted tremendous interest for their unique chemical/physical properties and expanded applications in energy storage, supercapacitors, catalysis, batteries, electronics and many other fields [24–27]. For example, Yu's group firstly reported special ultrathin

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Dai), [email protected] (C. Liang). 1 These authors contributed equally to this work. ∗∗

https://doi.org/10.1016/j.ceramint.2019.09.044 Received 24 August 2019; Received in revised form 3 September 2019; Accepted 4 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Xiaochun Ke, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.044

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hydrothermal processing. While CdSe-DETA nanoflowers (NFs) were obtained through a novel microwave-assisted hydrothermal route. The microwave hydrothermal technique has received more interest because of many advantages such as shorten reaction time, save energy and inhibit side reaction [55–57]. Notably, the F–TiO2 NSs with {0 0 1} facets promoted the separation efficiency of photo-excited carriers. And the special template DETA molecules can easily form CdSe-DETA hybrids with adjustable band gap, structure and surface area, providing more photocatalytic reaction active sites. Moreover, the in-situ growth method enables the formation of high-quality interface contact between F–TiO2 NSs and CdSe-DETA NFs and large-area heterojunction regions provide more channels for charged carriers’ migration. Thus, F–TiO2/ CdSe-DETA system exhibits superior photoactivity and stability for H2 evolution. This work not only provide a novel strategy to design CdSebased catalysts with outstanding performance and photostability, but deepen our understandings on the wide application of photocatalysts in environment protection and energy.

CoSe2-amine nanobelts with many stacked layers that have been prepared in a solvothermal method [28]. Zhang and his partners also demonstrated that hollow CdxZn1-xSe nanoframes exhibited excellent photocatalytic H2 activity due to selective cation-exchange reaction of inorganic-organic materials [29]. Tang et al. developed MoOx-based inorganic-organic nanohybrids with outstanding photochromic properties depending on their nanowire structures and components [30]. Our group has reported that CdS-diethylenetriamine (CdS-DETA) hybrids with unique nanobelt structures display high photocatalytic H2evolution activity and excellent stability in contrast to pure inorganic CdS nanoparticle [31]. On the basis of this research, we also synthesized flower-like CdSe-DETA hybrids, which exhibited enhanced H2production efficiency under visible-light illumination [32]. Specially, DETA, an organic amine small molecule, play a key role in controlling morphology, adjusting band gap, increasing specific surface area and so on, which may directly contribute to improve photocatalytic performance [33–35]. However, single CdSe-DETA photocatalyst cannot reach excellent activity and high stability because of fast recombination of photogenerated charge carriers and serious photo-corrosion. Constructing heterojunction system with other appropriate catalysts is an effective solution for solving these issues [36–39]. With the widespread research for crystalline materials in recent years, crystal facet engineering of semiconductors has aroused attention [40,41]. This method is mainly to design the exposed crystal facet of materials that is favorable for improving charge carriers movement and providing active sites, resulting in the superior performance through photocatalytic water splitting [42,43]. Yao et al. successfully obtained {1 1 0} facet-exposed Pt3Sn nanocubes/CdS composite via a typical hydrothermal treatment which has the enhanced efficiency of H2 production and high stability [44]. Sun et al. fabricated stable and efficient BiOI(001)/BiOCl(010) heterojunction photocatalyst via a crystal facet engineering method [45]. While Zhou et al. demonstrated that synthesis of BiVO4 polyhedra with (132), (321) and (121) high-index facets through surface engineering exhibited high photoactivity [46]. Titanium dioxide (TiO2), the conventional photocatlyst, has been widely studied in the past few decades due to its low cost, non-toxic, environmental friendly and good chemical stability [47–52]. Since Yang et al. designed anatase TiO2 sheets with high exposed {0 0 1} crystal plane for the first time, crystal facet engineering is becoming an significant way to get high-activity catalysts [53]. Zhao et al. prepared a core-shell nanostructure TiO2 with exposed {1 0 1} facets via a facile approach, displayed excellent photoactivity [54]. It was easily observed that catalytic performance of anatase TiO2 could be modulated by decorating various exposed crystal facets. However, crystal facet engineering technique is still confronted with some difficulties for highly efficient and stable photocatalysts. Herein, we present a facile in-situ generation approach to prepare F–TiO2/CdSe-DETA heterojunction. The F–TiO2 nanosheets (NSs) with high energy exposed {0 0 1} facets were firstly prepared via a typical

2. Experimental section 2.1. Preparation of F–TiO2 NSs Firstly, F–TiO2 NSs were synthesized through a mild solvothermal method. In brief, HF (3.1 mL) and Ti(OC4H9)4 (25 mL) were initially added into 50 mL of autoclave, which was vigorously stirred for 30 min and then heated at 180 °C for one day. F–TiO2 NSs were separated with water (18.25 MΩ) for several times and dried at 333 K. 2.2. Fabrication of F–TiO2/CdSe-DETA In a typical synthetic process of F–TiO2/CdSe-DETA sample, a certain amount of F–TiO2 NSs and CdCl2⋅2.5H2O (0.457 g) were dissolved into the mixture solution containing 24 mL DETA and 12 mL N2H4⋅H2O. Subsequently, Na2SeO3 (0.346 g) was added into the solution and heated at 140 °C for 20 min in microwave autoclave. Then, the product was washed by distilled water and treated in a freeze-dryer at 213 K. Specially, the composites prepared by adding different F–TiO2 contents were referred to as x%F–TiO2/CdSe-DETA (x = 10, 20 and 30). Besides, CdSe-DETA was also synthesized by the similar processing without F–TiO2. As comparison, the samples of CdSe and 20%F–TiO2/CdSe were also fabricated by above the same procedure except for the absence of DETA. 3. Results and discussion The fabrication procedure for F–TiO2 decorated CdSe-DETA phtocatalysis system is illustrated in Scheme 1. Firstly, F–TiO2 NSs were facilely designed by solvothermal pathway. Then, F–TiO2 NSs, SeO32− and Cd2+ were added into the mixture solution of DETA and N2H4 by

Scheme 1. Scheme representation of the preparation of F–TiO2/CdSe-DETA hybrids. 2

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Fig. 3. TEM images of (a) F–TiO2, (b) CdSe-DETA and (c) 20%F–TiO2/CdSeDETA; (d) HRTEM image of 20%F–TiO2/CdSe-DETA composite.

many small CdSe nanorods to form CdSe-DETA NFs with large surface area under high temperature and microwave assistance. At last, F–TiO2/CdSe-DETA composites were successfully obtained. XRD technique has been employed to analyze the crystalline phase of as-prepared samples [58]. Fig. 1 depicts the XRD patterns of CdSeDETA, x%F–TiO2/CdSe-DETA composites and F–TiO2. The main diffraction peaks at 2θ values of 24.04°, 25.59°, 27.38°, 35.99°, 42.43°, 46.15°and 50.14° can be respectively indexed to (1 0 0), (0 0 2), (1 0 1),

Fig. 1. XRD patterns of CdSe-DETA NFs, x%F–TiO2/CdSe-DETA (x = 10, 20, 30) and F–TiO2 NSs.

using a novel microwave-assist hydrothermal method at 413 K for 20 min. During the process, the N2H4 act as a reducing agent, reducing the Se4+ in Na2SeO3 to Se2− and then react with Cd2+ to generate CdSe nanorods. Afterwards, the protonated DETA organic molecules linked

Fig. 2. FESEM images (a) F–TiO2, (b) CdSe-DETA, (c) 20%F–TiO2/CdSe-DETA composite and (f) CdSe; (d) and (e) the EDS spectra and elemental mapping image of 20%F–TiO2/CdSe-DETA, respectively. 3

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Fig. 4. XPS survey spectra of (a) F–TiO2, CdSe-DETA and 20%F–TiO2/CdSe-DETA; and high-resolution spectra of (b) Cd 3d, (c) Se 3d, (d) Ti 2p, (e) O 1s and (f) F 1s.

F–TiO2, the main characteristic peaks at 25.21°, 37.81°, 47.96°, 53.74°, 54.89°, 62.48° and 70.20° coincide with (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (2 2 0) planes of anatase TiO2 (No. 21–1272, JCPDS). No impure peak can be observed, suggesting the high purity of CdSe-DETA and F–TiO2. Additionally, the peaks of TiO2 can still be found in all composites, and the intensity of peak at 47.96° corresponding to (2 0 0) plane of TiO2 gradually ascends with enlarging the F–TiO2 content in the composite photocatalysts. The XRD results clearly demonstrate that as-prepared F–TiO2/CdSe-DETA hybrids are composed of F–TiO2 and CdSe-DETA. The intensity of some diffraction peaks of the composite decrease slightly and broaden in comparison to pure CdSe-DETA, indicating the presence of some crystal defects [59]. Specially, compare with bare CdSe-DETA, the peak of (0 0 2) facet in composite has a slightly shift which demonstrate that the lattice distortion may be generated [60,61]. The crystal defects and lattice distortion might be beneficial to photocatalytic reaction from water. All as-prepared photocatalysts are obtained by a facile hydrothermal, in which the morphologies and structures of samples are observed by FESEM in Fig. 2. The pure F–TiO2 sample with the length of 40–65 nm has a clearly 2D sheet-like structure accompanying smooth surface (Fig. 2a). Inorganic-organic CdSe-DETA hybrid shows a flowerlike morphology with many nanobelts of non-uniform sizes (Fig. 2b). As comparsion, surface rough CdSe nanorods with the sizes of 100–200 nm are also prepared (Fig. 2f). Furthermore, 20%F–TiO2/CdSe-DETA

Fig. 5. FT-IR spectra of CdSe-DETA, F–TiO2/CdSe-DETA composites and pure F–TiO2.

(1 0 2), (1 1 0), (1 0 3) and (1 1 2) facets of hexagonal CdSe (JCPDS No. 72–1524). By comparison, the XRD characterization of pure CdSe with no DETA is also recorded (Fig. S1, Supporting Information). As for 4

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Fig. 6. (a) UV–Vis DRS spectra of F–TiO2, CdSe-DETA and F–TiO2/CdSe-DETA composites; (b) Eg values of F–TiO2 and CdSe-DETA.

Fig. 7. (a) N2 adsorption-desorption isotherms for CdSe-DETA, F–TiO2/CdSe-DETA composites and F–TiO2; (b) SBET for as-fabricated samples.

Cd–Se bond of CdSe and the corresponding peaks exhibit minor decrease to 53.70 and 53.34 eV for 20%F–TiO2/CdSe-DETA (Fig. 4c) [32]. In addition, the two characteristic peaks of 20%F–TiO2/CdSe-DETA are Ti 2p1/2 and Ti 2p3/2 with the measured binding energies of 463.45 and 457.72 eV, respectively, consisting with the Ti4+ species in F–TiO2 (Fig. 4d) [63,64]. Moreover, the characteristic peak at 529.46 eV with high-resolution XPS spectrum of O 1s in Fig. 3e is usually attributed to Ti–O bonds of bare F–TiO2, and its binding energy in the hybrid shifts to 529.24 eV. For pure CdSe-DETA, the O 1s peak centered at 529.16 eV is corresponding to lattice oxygen due to the existence of DETA [65]. As illustrated in Fig. 4f, the peak at 683.16 eV is identified from F1s in 20% F–TiO2/CdSe-DETA composite, which is assigned to Ti–F species from the defects on the surface of TiO2. Otherwise, the high-resolution N 1s and C 1s XPS spectrums were observed in Fig. S4. Thus, all shifts of binding energies may be attributed to the strong chemical interaction between F–TiO2 NSs and CdSe-DETA NFs owing to the formation of heterojunction, which favor photogenerated charge carriers’ separation and enhance the photocatalytic activity [66,67]. To further confirm the role of DETA molecules absorbed on the surface of various photocatalysts, the obtained FT-IR spectra of samples was indicated in Fig. 5. As for pure CdSe-DETA, the special vibration bands of –CH, –CH2, –NH2 and –NH can be clearly found, demonstrating the presence of DETA organic amine molecules [68]. Similarly, these characteristic bands are also observed in all F–TiO2/CdSe-DETA hybrids. Otherwise, for F–TiO2, only the weak intensity of vibration bands appeared due to the absence of DETA. As a consequence, it is believed that the DETA molecules not only can promote the formation of inorganic-organic hybrid materials, but may bring extraordinary benefits in photocatalysis. The light absorption properties of as-synthesized catalysts are examined by diffused reflectance UV–vis spectroscopy. As shown in Fig. 6a, F–TiO2 presents a band-edge absorption at around 400 nm. For pure CdSe-DETA and F–TiO2/CdSe-DETA composites, very few differences about the light absorption curves have been observed, when the

catalysts exhibit the similar morphology to pristine CdSe-DETA. F–TiO2 NSs are obviously detected in the composite (Fig. 2c). The FESEM image of 20%F–TiO2/CdSe can be directly investigated (Fig. S2). It is evidence that DETA has a special effect in manipulating the morphologies and chemical composition of nanaomaterials. As displayed in Fig. 2d and e, the EDS spectra and elemental mapping confirm the existence of Cd, Se, Ti, O, C, N and F elements without other impurities, which indicates the formation of F–TiO2/CdSe-DETA hetero-junction. As shown in Fig. 3, the morphologies and lattice structure of different photocatalysts are further investigated by TEM and HRTEM. Specially, for the 20%F–TiO2/CdSe-DETA hybrid, its flower-like nanostructure is further confirmed by the TEM picture (Fig. 3c). As presented in HRTEM image (Fig. 3d), the regular lattice structure with the d spacing of 0.354 nm is found, which is assigned to the (0 0 1) plane of TiO2. Additionally, the interplanar distance value of 0.332 nm can be ascribed to the (1 0 1) plane of CdSe. Meanwhile, TEM elemental mapping scan is performed to confirm the composition of F–TiO2/CdSeDETA (Fig. S3), which is also consistent with FESEM and EDS results. Accordingly, the high-quality interfacial contacts are expressed in F–TiO2/CdSe-DETA hetero-phase junction, in which F–TiO2 NSs interlaced with CdSe-DETA NFs. Such heterojunction is achieved by covalent bonds at the atomic level rather than the role of van der Waals force, enlarging the transfer and separation of photo-generated electron and hole pairs [62]. The surface chemical situation and elemental composition of samples are analyzed by using XPS. As shown in Fig. 4a, the presence of Ti, O, F, C, N, O elements in F–TiO2 and Cd, Se, C, N, O elements in CdSeDETA can be found in 20%F–TiO2/CdSe-DETA hybrid from the survey spectra. The high-resolution XPS spectrum of Cd 3d appears two strong peaks at 410.85 and 404.10 eV, which are characteristic peaks of Cd 3d3/2 and Cd 3d5/2 for CdSe-DETA in Fig. 4b, [31]. The binding energy of Cd 3d for 20%F–TiO2/CdSe-DETA is more positive than that of pure CdSe-DETA. The Se 3d peaks locating at 53.70 and 52.83 eV can be assigned to Se 3d3/2 and Se 3d5/2 spin orbits, which is ascribed to the 5

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Fig. 8. Optimized models of (a) TiO2 (0 0 1) facet and (b) CdSe (2 0 3) facet.(The Ti, O, Cd, Se atoms are denoted by the white, red, blue and yellow sphere, respectively). Calcualted electronic potential of (c) TiO2 (0 0 1) facet and CdSe (2 0 3) facet. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

region compare with pure CdSe (Fig. S5), which reveals the unique role of DETA in increasing the absorption band edge of photocatalyst. In brief, the subsequent photocatalytic performance test showed that the visible-light absorption property of catalyst is a key factor affecting H2 evolution activity. As indicated in Fig. 6b, the band gap energies (Eg) can be calculated by the following Eq. (1) [69–72]:

αhν = A (hν − Eg )1/2

(1)

Where α is the absorption coefficient, A is a constant, h and ν stand for the Planck constant and light frequency, respectively. Thus, the Eg values of pristine F–TiO2 and CdSe-DETA are 3.12 and 1.74 eV. ECB and EVB can be calculated by using Eqs. (2) and (3) [73,74]:

EVB = X − E e + 0.5Eg

(2)

ECB = EVB − Eg

(3)

Here: ECB and EVB are the potential of conduction band and valence band (VB). Ee is constant (4.5 eV). The X values of CdSe-DETA and F–TiO2 are 4.20 and 5.25 eV, respectively. Thus, ECB and EVB of F–TiO2 are calculated as −0.81 and 2.31 eV, respectively. While ECB and EVB of CdSe-DETA can be determined to −1.17 and 0.57 eV. The measured specific surface area (SBET) and N2 adsorption-desorption isotherms of samples are displayed in Fig. 7. F–TiO2 exhibits typical IV-type isotherms with H3-type hysteresis loop, demonstrating the existence of slit-shaped pores in the catalyst, which may be formed by aggregated F–TiO2 NSs [75]. As for bare CdSe-DETA and F–TiO2/ CdSe-DETA composites, they all show type-IV isotherms with typical H2 hysteresis loops in the relative high pressure region, which also denote that mesoporous structures are formed [76]. These results suggest that the mesoporous structure in the samples is beneficial for

Fig. 9. PL spectra of pure CdSe-DETA, F–TiO2/CdSe-DETA composite photocatalysts.

F–TiO2 content increased, the optical edges of F–TiO2/CdSe-DETA samples slightly blue-shifted in comparison to the bare CdSe-DETA. The observations disclose that the F–TiO2 NSs decorated CdSe-DETA NFs can have superior light absorption ability. The colour of the pure F–TiO2 is white and the CdSe-DETA is dark red, while the colour of x% F–TiO2/CdSe-DETA composites changes little with increased concentrations of F–TiO2. After the formation of inorganic-organic hybrid material, the obtained CdSe-DETA shows a redshift to visible-light 6

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Fig. 10. (a) Transient photocurrent spectra, (b) Nyquist plots of CdSe-DETA, F–TiO2/CdSe-DETA composite photocatalysts and F–TiO2.

hybrids can provide more active sites, which promote the photocatalytic performance and stability of catalysts. The superior photocatalytic activity of F–TiO2/CdSe-DETA can be assigned to the formation of direct heterojunction system between F–TiO2 and CdSe-DETA. The electronic transfer in the interface of F–TiO2/CdSe-DETA composite was further explored by DFT calculation. Fig. 8a and b shows the geometric structure of TiO2 (0 0 1) and CdSe (2 0 3) surfaces. The Fermi levels (EF) of the primarily exposed TiO2 (0 0 1) and CdSe (2 0 3) surfaces were determined as follows [77]:

EF = Evac − Φ

Where Φ is the work function, which was calculated to be 5.76 and 5.08 eV for TiO2 (0 0 1) and CdSe (2 0 3) surfaces, respectively (Fig. 8c and d), and Evac is the energy of a stationary electron at the vacuum level. Thus, the EF of TiO2 (0 0 1) surface is more negative than that of the CdSe (2 0 3) surface. When the TiO2 (0 0 1) surface and CdSe (2 0 3) surface get into contact, the electrons migrate from CdSe to TiO2 for achieving aligned Fermi levels at their interface [78,79]. Furthermore, the possible photocatalytic mechanism can be also confirmed by the above calculation results. At the interface of CdSe and TiO2, the electrons on the CdSe will transfer to TiO2 surface under visible-light excitation, accelerating the separation of photogenerated charge carriers of CdSe, which greatly affect its performance of this photocatalytic system. The spectroscopy of steady-state PL was used to uncover the migration and separation of photo-induced holes and electrons for photocatalyst samples [80–82]. Fig. 9 exhibits the PL spectra of x%F–TiO2/ CdSe-DETA composites under an excitation wavelength of 325 nm. As seen, pure CdSe-DETA displays the strongest PL intensity, which represents the fastest recombination of photo-excited charged carriers. Noticeably, 20%F–TiO2/CdSe-DETA shows a distinctly decreased PL peak intensity as compare to other remaining photocatalysts, indicating a lower recombination rate of photogenerated charge carriers, which is consistent with EIS and photocurrent responses tests. The higher charged carriers transfer and separation ability is in favor of enhancing the photocatalytic performance, thereby 20%F–TiO2/CdSe-DETA could obtain the highest activity compare with others. The transient photocurrent responses and EIS studies of as-obtained samples were carried out to further verify the charge carriers’ separation efficiency and understand photocatalysis mechanism. As can be seen in Fig. 10a, 20%F–TiO2/CdSe-DETA hybrid exhibits the highest photocurrent density among all samples, which is nearly two times stronger intensity than that of pure CdSe-DETA under visible-light excitation. After decorating F–TiO2 and CdSe-DETA, the improved photocurrent density demonstrates that photo-induced hole-electron pairs are more efficiently separated. Additionally, it can be clearly found that the semicircle radius of 20%F–TiO2/CdSe-DETA nanocomposite photocatalyst show the smallest arc radius under irradiation, elucidating a

Fig. 11. Comparision of photocatalytic H2-evolution rates of various catalysts. Table 1 Summarized photocatalytic activities of different samples. Sample

Co-catalyst

Light source

Sacrificial agent

H2 production (μmol g−1 h−1)

Ref.

ZnO/CdSe

2 wt% Pt

[83]

/

7120

[84]

Bi2MoO6/ CdSeDETA TiO2/CdSe

0.6 wt% Pt

5920

[32]

3650

[85]

CdSe/ CaTiO3

0.6 wt% Pt

2657

[86]

F–TiO2/ CdSe

0.6 wt% Pt

3146

This work

F–TiO2/ CdSeDETA

0.6 wt% Pt

0.1 M Na2S+0.1 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3 0.35 M Na2S+0.25 M Na2SO3

6469

MoSe2/ CdSe

300 W Xe-lamp λ ≥ 420 nm 300 W Xe-lamp λ ≥ 420 nm 300 W Xe-lamp λ ≥ 420 nm 300 W Xe-lamp λ ≥ 400 nm 300 W Xe-lamp

12381

This work

/

300 W Xe-lamp λ ≥ 420 nm 300 W Xe-lamp λ ≥ 420 nm

(4)

distracting of photoexcited charge carriers during the photocatalytic process. SBET of F–TiO2 is 90.1 m2/g, which is much larger than that of CdSe-DETA (60.2 m2/g). Apparently, after combining CdSe-DETA with F–TiO2, an obvious increase for SBET can be observed for this heterojunction structure. DETA organic small molecules act as a connector in the inorganic-organic hybrids, which can lead to form the ribbon structure, thereby increasing the SBET of catalysts. In general, the 7

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Fig. 12. (a) Time cycles of photocatalytic H2 evolution for 20%F–TiO2/CdSe-DETA and CdSe-DETA; (b) XRD patterns over 20%F–TiO2/CdSe-DETA composite before and after reaction.

F–TiO2, causing a large thermodynamic driving force in the procedure of photocatalytic reduction of H2 proton. While overweight loading F–TiO2 on CdSe-DETA, the H2 evolution activity of catalyst obviously decreased because much generated defects can lead to the poor contact between them and further block the separation of photoinduced carriers. Moreover, 20%F–TiO2/CdSe sample was also treated by using equivalent experimental conditions as a comparison. The activity of the 20%F–TiO2/CdSe exhibits an insignificant decrease in the H2 evolution rate (3146 μmmol g−1 h−1), illustrating the important role of DETA and its potential application to promote photocatalytic activity. Table 1 lists photocatalytic activities of some reported photocatalysts. The catalytic photostability test of F–TiO2, CdSe-DETA and 20% F–TiO2/CdSe-DETA hybrid was investigated in a long-time photocatalytic process, as depicted in Fig. 12a. It is obvious that only a slight decrease in H2 evolution for 20%F–TiO2/CdSe-DETA can be observed after 16 h of the cycling tests, which demonstrates that the 20%F–TiO2/ CdSe-DETA photocatalyst possesses high-stability activity for the H2 production. More importantly, after the photocatalytic reactions of 16 h, the crystalline of 20%F–TiO2/CdSe-DETA is further characterized by XRD (Fig. 12b). No noticeable change is found in the XRD pattern before and after the reaction, indicating highly structural stability for 20%F–TiO2/CdSe-DETA. It is worth noting that although the performance of pure CdSe-DETA is not as pretty as that of 20%F–TiO2/CdSeDETA, it also has good stability because of the unique effect of DETA in both enhancing activity and maintaining stability. The enhanced activity and superior stability reveal that F–TiO2/CdSe-DETA is a promising catalyst in remarkably photocatalytic H2 evolution and keep consistent with the results of UV, PL, photocurrent response and EIS tests. Based on the above analysis and discussion, a tentative photcatalytic reaction mechanism for F–TiO2/CdSe-DETA system has been proposed, as shown in Fig. 13. Irradiated by visible-light (λ > 420 nm), CdSe-DETA can be photoexcited to produce carriers but F–TiO2 cannot work [87,88]. When CdSe-DETA can be excited, the electrons on CdSe-DETA CB transfer to F–TiO2 CB owing to the intimate interfacial contact between them and suitable energy band position. Pt was regarded as electron acceptor, and it allowed the electrons on the F–TiO2 migrate rapidly to its surface, resulting in the combination rate of photoexcited carriers reduced and significant promote H2 evolution [89]. The as-obtained F–TiO2 NSs will have much electrons to participate in photocatalytic reactions. Meanwhile, the holes on CdSe-DETA VB can be consumed by the sacrificial agent. Basically, when the heterostructure between F–TiO2 and CdSe-DETA formed, it can induce charge carriers migration and enhance photoactivity and stability for F–TiO2/CdSe-DETA composite. With the photo-excited charges

Fig. 13. Schematic illustration of F–TiO2/CdSe-DETA heterojunction system.

decreased resistance for migration of interfacial holes and electrons, which is also in accordance with the result of photocurrent test. Consequently, it is adequately believed that construction of F–TiO2/CdSeDETA heterojunction can offer more photo-induced charge carriers to participate in surface redox reactions and a better photocatalytic activity can be expected. The photocatalytic activity of H2 production from water reduction over all the prepared catalysts was presented in Fig. 11. For bare F–TiO2 NSs, the photocatalytic H2 evolution rate is only 59 μmmol g−1 h−1 with visible light excitation (λ > 420 nm) owing to its large bandgap (3.12 eV). But CdSe-DETA displays a moderate H2-evolution rate of 4247 μmmol g−1 h−1 under identical condition. When CdSe-DETA NFs was combined with F–TiO2 NSs via an in-situ growth process, the H2 evolution rate of photocatalysts can be further increased sharply. In particular, the maximum H2 formation rate of 12381 μmmol g−1 h−1 is achieved by 20 wt% F–TiO2 loading on CdSe-DETA, exceeding that of other ratio composites, up to 2.92 and 6.44 times in contrast to that of pristine CdSe-DETA (4247 μmmol g−1 h−1) and CdSe (1923 μmmol g−1 h−1), respectively. This improved H2 production performance can be substantially attributed to a complicated synergistic effect arising from the high reactivity of F–TiO2 with exposed {0 0 1} high-energy facets and large contact interface between CdSe-DETA and 8

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constantly accumulated, interpolar electronic field produced in interface by polarization can enhance the migration of electrons and holes and then rapidly move them to surface. The large interfacial contact area can further accelerate the separation and transfer of photo-induced charges. Of course, according to the calculation results of the work function (Fig. 8), we can also demonstrated that the electrons would flow from CdSe-DETA with higher EF to F–TiO2 with lower EF across the intimate interface until they have same EF. Hence, the heterojunction system constructed by the lattice binding between CdSe-DETA and F–TiO2 is responsible for high-efficiency and stable photocatalytic H2 evolution.

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4. Conclusion In conclusion, the F–TiO2/CdSe-DETA heterojunction was obtained via a facile microwave-assisted hydrothermal process, among which the 2D F–TiO2 NSs was firstly used to decorate the CdSe-DETA NFs. Result demonstrates that DETA molecules have a notable impact on changing band gap, surface area and morphology of catalyst. The photocatalytic efficiency of various structures has been clearly compared. 20%F–TiO2/ CdSe-DETA triggered a significantly improved photocatalytic H2 evolution rate (12381 μmmol g−1 h−1) under visible light illumination, which is 2.92 and 6.44 folds greater than that of CdSe-DETA and CdSe, respectively. Furthermore, 20%F–TiO2/CdSe-DETA also displayed a superior photostability after 16 h test. The high photocatalytic activity and long-term stability can be attributed to the formation of F–TiO2/ CdSe-DETA heterojunction, rapid separation and transport of photoinduced carriers, as well as the unique structure of F–TiO2 with high exposed {0 0 1} facets, which provided more active sites for water reduction reaction. This work not only offers a new high-performance and stable photocatalyst, but presents an efficient and facile strategy to construct the F–TiO2/CdSe-DETA heterostructure for water splitting. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572103 and 51502106), the Distinguished Young Scholar of Anhui Province (1808085J14), and the Key Foundation of Educational Commission of Anhui Province (KJ2019A0595). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.044. References [1] Q. Xu, L. Zhang, J. Yu, S. Wageh, A.A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: principles, synthesis, and applications, Mater, Today 21 (2018) 1042–1063. [2] G. Zhou, Y. Shan, Y. Hu, X. Xu, L. Long, J. Zhang, J. Dai, J. Guo, J. Shen, S. Li, L. Liu, X. Wu, Half-metallic carbon nitride nanosheets with micro grid mode resonance structure for efficient photocatalytic hydrogen evolution, Nat. Commun. 9 (2018) 3366. [3] M. Ghiyasiyan-Arani, M. Salavati-Niasari, S. Naseh, Enhanced photodegradation of dye in waste water using iron vanadate nanocomposite; ultrasound-assisted preparation and characterization, Ultrason. Sonochem. 39 (2017) 494–503. [4] S. Kundu, A. Patra, Nanoscale strategies for light harvesting, Chem. Rev. 117 (2017) 712–757. [5] Z. Li, X. Wang, J. Zhang, C. Liang, L. Lu, K. Dai, Preparation of Z-scheme WO3(H2O)0.333/Ag3PO4 composites with enhanced photocatalytic activity and durability, Chin. J. Catal. 40 (2019) 326–334. [6] W. Wang, S. Zhu, Y. Cao, Y. Tao, X. Li, D. Pan, D.L. Phillips, D. Zhang, M. Chen, G. Li, H. Li, Edge-enriched ultrathin MoS2 embedded yolk-shell TiO2 with boosted charge transfer for superior photocatalytic H2 evolution, Adv. Funct. Mater. (2019) 1901958. [7] M. Salavati-Niasari, F. Soofivand, A. Sobhani-Nasab, M. Shakouri-Arani, A. Yeganeh Faal, S. Bagheri, Synthesis, characterization, and morphological control of ZnTiO3 nanoparticles through sol-gel processes and its photocatalyst application, Adv. Powder Technol. 27 (2016) 2066–2075.

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