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Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution
Journal of Colloid and Interface Science xxx (xxxx) xxx
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Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution Fuxiang Li, Yanqing Jiao, Jianan Liu, Qi Li, Chuanyu Guo, Chungui Tian ⇑, Baojiang Jiang ⇑ Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, PR China
g r a p h i c a l a b s t r a c t The design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE.
a r t i c l e
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Article history: Received 20 October 2019 Revised 8 November 2019 Accepted 8 November 2019 Available online xxxx Keywords: ZrO2 TiO2 Core-shell heterostructure UiO-66-NH2 Photocatalytic hydrogen evolution
a b s t r a c t The robust photocatalytic hydrogen evolution (PHE) from water needs an effective photogenerated charge spatial separation and enough contact between reactant and catalyst, but the synthesis of catalysts with the characteristics remains a challenge. Herein, we report the design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE. In this system, UiO-66-NH2, one of popular MOF with Zr as metal node, has been chosen as the precursor template due to its plentiful pores, uniform morphology, as well as the rich NH2 groups. Our results show that Ti precursor can uniformly coat on UiO-66-NH2, by means of interaction of tetrabutyl titanate (TBT) with -NH3 in UiO-66-NH2. Followed by the calcination, the Ti precursor and UiO-66-NH2 can be converted into ZrO2 and TiO2, respectively, thus leading to the formation of ZrO2@TiO2 core-shell heterostructure. The ZrO2@TiO2-500 has the high specific surface area of 52.4 m2 g 1. Besides, the intimate contact of TiO2 shell with ZrO2 core facilitates the separation and migration of photoinduced carriers, exposing more active sites for the surface photocatalytic hydrogen evolution reaction. The spectrum and electrochemical characterization further exhibit the extended life of photon-generated carrier and easy mass transfer. The optimized ZrO2@TiO2-500 shows enhanced photocatalytic rate of 39.7 mmol h 1 g 1, much higher than those of ZrO2 (0.8 mmol h 1 g 1) and TiO2 (7.6 mmol h 1 g 1). Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (C. Tian), [email protected] (B. Jiang). https://doi.org/10.1016/j.jcis.2019.11.031 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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1. Introduction The solar photocatalytic water splitting has attracted more and more attention in current world due to its ability for the production of clean hydrogen energy with many advantages of mild condition, simple operation, direct utilization of sunlight and no secondary pollution [1–3]. The development of effective photocatalysis with good activity and stability is constantly pursued, but remains a severe challenge. TiO2, as a stable, nontoxic photocatalyst, is promising in the photocatalytic hydrogen evolution field [4,5]. However, the practical application of TiO2 has been limited due to high recombination probability of charge carriers and low quantum efficiency caused by weak electron mobility and short minority carrier diffusion [6,7]. In this regard, many methods have been developed to improve the photocatalytic performance of TiO2, such as doping [8,9], surface sensitization [10,11], and the construction of the heterostructure [12–14]. By integrating TiO2 with ZrO2 [15], CdS [16], Fe2O3 [17], g-C3N4 [18], Cu2O [19] or BiVO4 [20], a new carrier transport path can be established. The corresponding heterostructure can accelerate charge separation during the photocatalytic process, thus improving the catalytic performance largely. Notably, a core-shell heterostructure has possessed many advantages for the effective separation of photogenerated charge carriers, thus being interesting in the photocatalytic filed [21–24]. In that case, the photogenerated electrons and holes in heterostructure will transfer toward different direction, so their recombination can be suppress. As a kind of wide band gap n-type semiconductor, ZrO2 with the band gap of about 5.0 eV is useful in hydrogen generation with strong resistance to corrosion [25]. Coupling of TiO2 with ZrO2 is effective for the degradation of organic compounds, such as copper supported TiO2/ZrO2, ZrO2/TiO2-xNx and S-doped TiO2/ZrO2 [26–28]. The self-assembly, colloidal aggregation, and precursor templates are the most common approaches for ZrO2@TiO2 core-shell structure [29–31]. Nevertheless, these syntheses are difficult to generate the uniform coating and regular structure with high specific areas, which is not conductive to bring enhanced photocatalytic activity. In order to overcome the above issues, the porous and regular ZrO2 should be chosen as template to support TiO2 uniformly. The selection of the suitable precursor for ZrO2 is a key to realize the goal. The metal organic framework (MOF) is a class of crystal material with micro- and mesoporous structure and large specific surface area, which can be used as a template or precursor for the synthesis of semiconductor oxides [32–33]. The UiO-66-NH2, a typical MOF with Zr ion as the metal node and 2aminoterephthalic acid as the organic ligand, has high porosity, large specific surface area, and network topology [34]. These characterizations make the UiO-66-NH2 promising to produce the Zr-based composites with regular morphology such as C-ZrO2/g-C3N4 [35]. However, up to now, the preparation of ZrO2@ TiO2 core-shell heterostructure from UiO-66-NH2 as Zr source has not been reported yet. In this work, we report a synthetic method to obtain porous core-shell heterostructure consisted of ZrO2 and TiO2. UiO-66NH2 (Zr-MOF) is applied as precursor template and Zr source. Then, the hydrolyzed tetrabutyl titanate (Ti source) is absorbed on its surface through the interaction with the rich ANH2 group in UiO-66-NH2. After thermal treatment, ZrO2@TiO2 core-shell heterostructure has been fabricated, in which thin TiO2 shell covers the ZrO2 core evenly. In the core-shell structure, the matched bang gap of TiO2 and ZrO2 facilitate the separation of photoexcited electron-hole pairs and their migration toward the inner and outer surfaces, thereby decreasing their recombination. As a result, the ZrO2@TiO2 exhibit enhanced photocatalytic activity of 39.7 mmol h 1 g 1, much higher than those of ZrO2 (0.8 mmol h 1 g 1) and TiO2 (7.6 mmol h 1 g 1). This work opens a new avenue to
design MOF-derived core-shell heterostructure photocatalysts for the hydrogen evolution. 2. Experimental section Zirconium chloride (ZrCl4, 99.9%), 2-aminotetrephthalic acid (H2ATA, 98%) and acetic acid (CH3COOH, 99.5%) were purchased from Sigma-Aldrich Trading Co. Ltd. C16H36O4Ti (99%) (TBT) was purchased from Tianjin Kermel Co. Ltd. Ethanol (99.7%) and N, NDimethylformamide (DMF, 99.7%) were purchased from Tianjin Fuyu fine chemical industry Co. Ltd. All chemical reagents were obtained from commercial reagents company without further purification. The deionized water was used in the experiment process. 2.1. Synthesis of UiO-66-NH2 (Zr-MOF) UiO-66-NH2 was synthesized by a solvothermal method [32]. Firstly, 2-aminotetrephthalic acid (H2ATA, 0.18 g) and zirconium chloride (ZrCl4, 0.23 g) was dissolved in 100 mL DMF. The above solution was stirred vigorously for 30 min to form a homogeneous solution. Then 20 mL of CH3COOH was added to the above mixed solution again and stirred for 30 min. The resultant suspension was transferred into a 50 mL Teflon-lined autoclave and heated at 120 °C for 16 h. After naturally cooling to room temperature, the product was washed three times with DMF and methanol, and dried at 60 °C. 2.2. Synthesis of UiO-66-NH2@Ti precursor Zr-MOF was dispersed in a mixture of deionized water (0.1 mL) and ethanol (60 mL), sonicated for 15 min and stirred for 10 min to form a homogeneous dispersion. Then, 25 mL ethanol solution (containing 5 mL of TBT) was added into the dispersion, stirred at room temperature for 4 h. The solution was washed three times with ethanol, and dried overnight at 60 °C to obtain final precursor. The water amount in the synthesis of UiO-66-NH2@Ti precursor was tuned as 0 mL, 0.1 mL, 0.2 mL, 0.3 mL with other reaction conditions same to above typical preparation procedure. 2.3. Synthesis of ZrO2@TiO2 core-shell heterostructure The typical sample ZrO2@TiO2-500 core-shell heterostructure was obtained by calcinating the UiO-66-NH2@Ti precursor at 500 for 2 h with a heating rate of 1 °C/min. The precursor is also calcinated at 450 (ZrO2@TiO2-450), 550 (ZrO2@TiO2-550), 600 (ZrO2@TiO2-600) to study the effect of calcinated temperature on the structure and performance. The ZrO2 was synthesized by calcining the UiO-66-NH2 at 500 for 2 h at a heating rate of 1 /min. The TiO2 was produced as follow: at first, 25 mL ethanol solution (containing 5 mL TBT) was added dropwise to 60 mL ethanol solution (contained 0.1 mL deionized water), stirred at room temperature for 4 h. The obtained solid was washed three times with ethanol, and dried overnight at 80 to get TiO2 precursor. FinialTiO2 was prepared by calcinating the power at 550 for 2 h with a heating rate of 1 /min under air atmosphere. 2.4. Characterization The crystal structures of the synthesized samples were confirmed by X-ray powder diffraction (XRD, Rigaku D/max-IIIB with Cu Ka, k = 1.5406 Å). The structure and morphology of the synthesized samples were characterized by Hitachi S-4800 field emission scanning electron microscope (SEM) operating on 15 KV and JEMF200 transmission electron microscope (TEM) with an acceleration
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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voltage of 200 kV. To analyze the elemental composition, surface chemical environment and valence band position of samples, an X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II) test was performed. The Brunauer-Emmett-Teller (BET) surface area was identified in the range of relative pressure (p/p0) from 0 to 1.0 by using a Tristar II 3020 surface area and porosity analyzer (Micromeritics). Pore size distribution curves were obtained from the adsorption data by using the Barrett-Joyner-Halenda (BJH) method. The light response capacity of the sample was evaluated by using ultraviolet-visible spectrophotometer (Shimadzu UV2550); A fluorescence spectrophotometer (Hitachi F-4600) was used to evaluate the charge separation capability at an excitation wavelength of 380 nm. The corresponding photoluminescence spectrum (PL) was acquired to demonstrate the separation ability of the electron-hole pairs of the catalyst. 2.5. Photocatalytic hydrogen evolution measurement An on-line photocatalytic system (Labsolar-IIIAG, Beijing Perfect Light Co. Ltd) and an online gas chromatograph (GC7900, TCD, Shanghai Tianmei Scientific Instrument Co. Ltd) were used to detect the activity for hydrogen evolution reaction. By using nitrogen as a carrier gas, 50 mg catalyst was mixed with 80 mL of distilled water and 20 mL of methanol in a reaction tank to form a suspension. After ultrasonication for 10 min, 1% (wt) Pt from H2PtCl6, was loaded into the above catalyst as a co-catalyst by known in-situ photodeposition method. 24 Before the light radiation reaction, the mixture in the reactor was degassed by vacuuming to remove the O2 and CO2 dissolved in water. The suspension was illuminated by a Xenon lamp source (PLS-SXE300/300UV, 15 A) equipped with an ultraviolet filter (remain k < 400 nm light). The produced gas was collected and analyzed online. 2.6. Electrochemistry measurement of as prepared products Electrochemistry testing was performed by using an electrochemical workstation (Princeton electrochemical workstation) with three electrodes system. The working electrode is prepared as follows: 0.1 g of catalyst was mixed with 15 mL ethanol, and then the obtained sample was deposited on the FTO glass within a controlled area of 1 cm2 using airbrush. Then, then FTO glass coated by catalyst was heated at 350 °C for 2 h in the air. Pt foil (3 2 cm) and Ag/AgCl were respectively used as the counter electrode and the reference electrode. In this system, 0.2 M Na2SO4 solution electrolyte and ultraviolet light irradiation from 300 W Xenon lamp is selected. 3. Results and discussion A schematic procedure for the porous ZrO2@TiO2 core-shell heterostructure is shown in Scheme 1. Firstly, UiO-66-NH2 with plentiful ANH2 group and pores was employed as a template and Zr source. The tetrabutyl titanate was adsorbed on the surface of UiO-66-NH2 and hydrolyzed to form Ti precursor layer. A precursor contained both Ti and Zr (UiO-66-NH2@Ti) was formed. Finally, a calcination under air (500 ) of UiO-66-NH2@Ti results in the formation of the porous ZrO2@TiO2 core-shell heterostructure. It is noted that the hydrolysis dynamic of TBT can be affected by the amount of the water in the system. Thus, the amount of water in the synthesis was a key for the regulation of morphology and structure (Fig. S1). The agglomerated spheres can be formed when adding 0.30 mL of H2O in the synthesis, and the morphology of UiO-66-NH2 was not remained. In the case of adding 0.2 mL H2O, the UiO-66-NH2@TiO2 shows the octahedronal morphology, being similar to UiO-66-NH2, but with some agglomeration. As further
Scheme 1. The synthetic process of porous ZrO2@TiO2 core-shell structure.
decreasing the H2O amount to 0.1 mL, the UiO-66-NH2@TiO2 with the uniform octahedron morphology was acquired. The results indicate that the control of the hydrolysis dynamics of TBT is important to form uniform coating on UiO-66-NH2. As such, the product prepared by using 0.1 mL H2O was taken as the typical sample for the subsequent experiment and characterizations. The precursors and samples have been characterized by the XRD and the results have been shown in Fig. 1a. As seen in Fig. 1a, the main diffraction peaks of as-synthesized UiO-66-NH2 correspond to the previous reports [36]. After coating by Ti source, the sample shows the diffraction peaks of UiO-66-NH2. The peaks corresponding to Ti species could not be observed, which should be due to weak crystallization of Ti species. Fortunately, FT-IR spectra can provide some useful information about the combination and interaction between two components. As shown in Fig. S2, UiO-66-NH2 displays NAH bending vibration at 1650 cm 1 and CAN stretching absorption of aromatic amines at 1380 cm 1 [36,37]. Compared to UiO-66-NH2, the NAH bending vibration for UiO-66-NH2@Ti precursor is significantly weak, which should be attributed to the strong interaction between the UiO-66NH2 and Ti precursor. After calcination of UiO-66-NH2@Ti precursor, ZrO2@TiO2-500 has been obtained. As shown in Fig. 1a, for ZrO2@TiO2-500, the diffraction peaks located at 25.3° and 30.1° can be attributed to (1 0 1) lattice plane of anatase TiO2 and (1 1 1) lattice plane of ZrO2, respectively. Other diffraction peaks can also be assigned to either TiO2 or ZrO2, meaning the effective contact between ZrO2 and TiO2 in Fig. S3 [38,39]. As shown in Table S1, we can obtain the contents of Ti and Zr element in the ZrO2@TiO2-500 by the ICP-OES test, which are 27.48 and 40.39%, respectively. When further changing the calcination temperature, other samples including ZrO2@TiO2-450, ZrO2@TiO2-550, and ZrO2@TiO2-600 can be acquired (Fig. S4). Meanwhile, in Fig. 1a, reference sample ZrO2, derived from UiO-66-NH2, exhibits serials of diffraction peaks, corresponding to standard ZrO2 (JCPDS 491642) [40]. Reference TiO2 also shows the anatase structure (JCPDS 21-1272) [41]. The microstructure of different product is investigated by the TEM and SEM tests, and shown as Fig. 1b–g. The UiO-66-NH2 shows the uniform octahedral morphology with smooth surface and the size about 400 nm (Fig. 1b). The SEM image of ZrO2@ TiO2-500 shows the similar shape of octahedron, but with slightly rough surface, implying the formation of TiO2 on the surface. The TEM images can provide further information about the
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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Fig. 1. (a) XRD patterns of UiO-66-NH2, UiO-66-NH2@Ti precursor, and ZrO2@TiO2-500. (b) SEM image of UiO-66-NH2 and (c) SEM image of ZrO2@TiO2-500. (d) TEM image of ZrO2@TiO2-500 and partial enlargement HRTEM images of ZrO2@TiO2-500 (e, f). Corresponding elemental maps of Ti, O, and Zr within ZrO2@TiO2-500 (g).
micro-structure of ZrO2@TiO2-500 (Fig. 1d). Compared with UiO66-NH2, the size of ZrO2@TiO2-500 reduces to about 250 nm, which is related to the contraction during the transformation of UiO-66NH2 skeleton to ZrO2 under calcination. It is noted that many pores (2–7 nm) can be found in ZrO2@TiO2-500 based on the partial enlargement of HRTEM image (Fig. 1e). The large specific surface area and superficial porous structure of the core-shell heterostructure of ZrO2@TiO2 are beneficial to the mass transport and can expose more catalytic active sites due to more dangling bonds or defects, which is more favorable for the surface catalytic reaction and improve the catalytic efficiency. The HRTEM shows the external lattice fringes space of 0.351 nm and 0.237 nm, which are pointed to (1 0 1) and (0 0 4) crystal planes of anatase TiO2 [42]. Meanwhile, the space of internal lattice fringes of 0.296 nm can be ascribed to (1 1 1) crystal planes of ZrO2 (Fig. 1f) [43]. Above analysis proves that as-synthesized ZrO2@TiO2-500 is a porous core-shell heterostructure with ZrO2 as the core and TiO2 as the shell. The SAED and other TEM image further indicate the effective combination between ZrO2 and TiO2 (Figure S5). The elemental mapping of ZrO2@TiO2-500 exhibits the uniform distribution of Ti, O, Zr elements, which being consistent with EDX spectra of ZrO2@TiO2-500 (Fig. 1g and Fig. S6), indicating the uniform coating of TiO2 around ZrO2. The formation of core-shell heterostructure with plentiful pores is beneficial to improve the photocatalytic performance. In order to study the pore structure of the sample, the N2 adsorption–desorption measurement has been performed. Fig. 2a,
b show the analysis results of ZrO2, TiO2, and ZrO2@TiO2-500. It is worth noting that a typical IV isotherm can be observed from all samples, which indicates the presence of pores in consideration of H3 hysteresis loop [44]. The specific surface area of ZrO2 is only 5.9 m2 g 1, which should be the collapse and shrink partially of UiO-66-NH2 skeleton during the calcination. The specific surface area of TiO2 is also relatively small (~6.6 m2 g 1). In contrast, the ZrO2@TiO2-500 heterostructure possesses a high specific surface area of 52.4 m2 g 1 and uniform mesopores structure (~16.2 nm), which is consistent with TEM results (Fig. 1e). The obtained ZrO2@ TiO2-500 heterostructure has maintained the octahedral morphology of UiO-66-NH2 and inherited its large specific surface area and porous structure, thanks to the protective effect of TiO2 layer on UiO-66-NH2. The porous structures facilitate carrier migration and surface reaction mass transfer, being conducive to improve photocatalytic performance of products. The surface chemical composition and the oxidation state of ZrO2, TiO2 and ZrO2@TiO2-500 were investigated by XPS. In Fig. 2c, the O1s XPS pattern of ZrO2 shows two main peaks at 529.8 and 531.4 eV, which can be respectively referred to lattice oxygen and surface OH groups [45]. However, we can find that O1s peaks of ZrO2@TiO2-500 have obvious shift toward low binding energy, demonstrating the increase of electronic intensity. Furthermore, in comparison with ZrO2, the Zr XPS peaks for ZrO2@ TiO2-500 shows a shift about 0.9 eV toward high binding energy, indicating the decrease of electronic density around Zr atom (Fig. 2d). Above results prove that building the hetero-interface
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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Fig. 2. Nitrogen adsorption-desorption isotherms (a) and BJH pore size distributions (b) of ZrO2, TiO2, and ZrO2@TiO2-500. XPS O 1s (c) and Zr 3d (d) of ZrO2 and ZrO2@ TiO2-500, respectively.
Fig. 3. (a) UV-vis reflectance spectra of ZrO2, TiO2 and ZrO2@TiO2-500 and (b) the band gap determined from Kubelka-Munk function. (c) XPS valence band spectra of ZrO2, TiO2 and ZrO2@TiO2-500; (d) the photo-induced carrier’s migration process within ZrO2@TiO2.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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of ZrO2@TiO2 can make the effective electronic transfer due to the strong interaction between them. Furthermore, the XPS Ti 2p of TiO2 and ZrO2@TiO2-500 is also confirmed (Fig. S7), in which two major peaks located at 458.5 and 464.2 eV are ascribed to Ti 2p3/2 and Ti 2p1/2, respectively [46]. For photocatalytic reaction, the light absorption capacity of catalyst is a crucial factor. The UV-vis diffuse reflectance spectra test has been performed and the data are displayed as Fig. 3a. We can clearly see that the main optical response ranges of ZrO2, TiO2 and ZrO2@TiO2-500 are located in the ultraviolet region, and their strongest absorption is at 250 nm–450 nm. Compared with ZrO2, it is worthy of note that the absorption band edge of ZrO2@TiO2500 has red-shift toward the long wavelength, indicating the decreases of the band gap of ZrO2@TiO2-500. In Fig. 3b, the band gaps of ZrO2, TiO2 and ZrO2@TiO2-500 are 4.52, 2.98, and 3.15 eV as calculated through Kubelka-Munk function [47], respectively. Compared with ZrO2, the band gap of ZrO2@TiO2-500 is significantly narrow due to the formation of core-shell heterostructure. Meanwhile, it has already a significant enhancement of the ultraviolet spectral region utilization (Fig. 3a). In order to achieve the
valence band position of the catalyst, the XPS valence band spectrum test has also been studied. As shown in Fig. 3c, the valence band positions of ZrO2, TiO2 and ZrO2@TiO2-500 are located at 2.92, 2.56, and 2.68 eV. Based on the band gap and the valence band (VB) position of catalyst, the energy band structure of catalyst can be confirmed. Fig. 3d shows the process about migration of the photogenerated carriers within ZrO2@TiO2-500 heterostructure. Under the ultraviolet light irradiation, both TiO2 and ZrO2 have been excited along with the electron transfer from the ground state to the excited state. The electrons in the VB of TiO2 are excited to the conduction band (CB), leaving holes in the valence band. At the same time, the electrons in ZrO2 VB are also excited to the CB. Subsequently, the electrons in CB of ZrO2 migrate to CB of TiO2 because the VB position of ZrO2 is more positive than that of TiO2 [48–50]. Therefore, the electron migration rate of ZrO2@ TiO2-500 is accelerated, and the recombination of electron-hole pair is suppressed, all of which favor the photocatalytic hydrogen evolution reaction. Fig. 4a shows photocatalytic activity of serials of samples obtained at different calcination temperature. The hydrogen
Fig. 4. (a) Photocatalytic H2 evolution rate of ZrO2, TiO2 and ZrO2@TiO2-500. (b) Photocatalytic H2 evolution rate of ZrO2@TiO2-500 and reference products. (c) Photocatalytic stability measurement. (d) Photoluminescence spectra, (e) time-resolved photo-response under ultraviolet irradiation at +0.6 V with versus saturated calomel electrode, and (f) EIS Nyquist plots of different samples.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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evolution amount of ZrO2@TiO2-450, ZrO2@TiO2-500, ZrO2@TiO2550 and ZrO2@TiO2-600 is 14.4, 39.7, 30.5 and 24.8 mmol h 1 g 1, respectively. The photocatalytic hydrogen evolution performances of ZrO2, TiO2 and ZrO2@TiO2-500 are also exhibited as Fig. 4b. The photocatalytic hydrogen evolution rates of ZrO2 and TiO2 are 0.8 and 7.6 mmol h 1 g 1, respectively. The photocatalytic hydrogen evolution rate of ZrO2@TiO2-500 is about 50 times than that of ZrO2 and 5 times that of TiO2. It can be concluded that the photocatalytic hydrogen evolution performance of as prepared ZrO2@TiO2-500 is superior to any single catalyst. In Fig. 4c, the photocatalytic hydrogen evolution performance of ZrO2@TiO2500 shows almost no decrease after three cycles of 4 h as a period, indicating ZrO2@TiO2-500 is stable during illumination without light corrosion. The cycle stability test results show that ZrO2@ TiO2-500 heterostructure has good stability. We performed XRD tests on samples before of lighting and after 4 h of lighting illumination (Fig. S8). It was found that the XRD diffraction peaks of the samples did not change before and after the observation, indicating that the stability of the catalyst was good. The outstanding photocatalytic performance can be attributed to the core-shell heterostructure, high specific surface area, abundant surface pores structure and active sites of ZrO2@TiO2-500. In order to explore the separation ability of photoelectron-hole pair in the catalyst under photo-excitation, the photoluminescence spectroscopy (PL) has been tested. Fig. 4d shows the PL spectra of three catalysts. These PL spectra have similar peak positions at the excitation wavelength of 360 nm. Generally, the lower PL intensity means the lower recombination rate of photoelectron-hole [51]. Compared with ZrO2, the PL position of TiO2 is much lower, signifying a lower recombination rate of photoelectron-hole pair. It is worth noting that the intensity of PL spectrum of ZrO2@TiO2-500 is weaker in comparison with that of TiO2, meaning the enhanced separation efficiency of photogenerated electron-hole pairs. Therefore, it may be expected that the ZrO2@TiO2-500 heterostructure may show an enhanced photocatalytic activity. Fig. 4e shows the time-resolved photo-response of ZrO2, TiO2 and ZrO2@TiO2-500, reflecting the photo-to-current conversion rate. Once the light is added, a significant increase of current density of the catalyst could be observed. The current densities of ZrO2, TiO2 and ZrO2@TiO2-500 are 15, 78 and 165 lA cm 2, respectively. Among them, ZrO2@TiO2-500 has the highest current density, which is 11 times and 2.1 times as large as ZrO2 and TiO2, respectively. It can be attributed to the formation of heterostructure that can accelerate the separation and migration of photogenerated carriers. We can also see that the photocurrents of ZrO2, TiO2 and ZrO2@TiO2-500 have almost no attenuation as the illumination time increases, indicating that the good photochemical stability of the catalyst. Fig. 4f shows EIS Nyquist plots of as-prepared ZrO2, TiO2 and ZrO2@TiO2-500, in which the resistance of ZrO2, TiO2 and ZrO2@TiO2-500 reduce in order. The resistances of three catalysts have been significantly decreased under light irradiation, indicating a significant increase of the electron transport efficiency during illumination. Among them, ZrO2@TiO2-500 has the lowest resistance, which should be relative with the core-shell heterostructure and larger specific surface area.
4. Conclusion In summary, we have prepared porous ZrO2@TiO2 core-shell heterostructure photocatalytic catalyst by utilizing UiO-66-NH2 (Zr-MOF) as precursor and Zr source. Porous ZrO2@TiO2 heterostructure inherits the characteristics of UiO-66-NH2. Compared with ZrO2 and TiO2, ZrO2@TiO2-500 displays the higher photocatalytic hydrogen evolution rate of 39.7 mmol h 1 g 1. The superior photocatalytic hydrogen evolution properties of
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ZrO2@TiO2-500 can be attributed to its core-shell heterostructure, large specific surface area and porous structure for promoting carrier transport and migration, and inhibiting electron-hole pair’s recombination. The Zr-MOF template route should be indicative to construct other core-shell semiconductor oxide heterostructures. Declaration of Competing Interest The authors declare no competing financial interests. Acknowledgment This work was supported by the National Natural Science Foundation of China (21771061, 21601055) and University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2017118) Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.031. References [1] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, J. Mater. Chem. A. 3 (2015) 2485. [2] S. Chandrasekaran, L. Yao, L. Deng, C. Bowen, Y. Zhang, S. Chen, Z. Lin, F. Peng, P. Zhang, Chem. Soc. Rev. 48 (2019) 4178. [3] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Renew. Sustain. Energy Rev. 43 (2015) 599. [4] Q. Guo, C. Zhou, Z. Ma, X. Yang, Adv. Mater. (2019) 1901997. [5] X. Chen, L. Liu, F. Huang, Chem. Soc. Rev. 44 (2015) 1861. [6] S. Sun, P. Gao, Y. Yang, P. Yang, Y. Chen, Y. Wang, A.C.S. Appl, Mater. Interfaces. 8 (2016) 18126. [7] J. Yan, H. Wu, H. Chen, Y. Zhang, F. Zhang, S.F. Liu, Appl. Catal. B Environ. 191 (2016) 130. [8] S. Sun, J. Zhang, P. Gao, Y. Wang, X. Li, T. Wu, Y. Wang, Y. Chen, P. Yang, Appl. Catal. B Environ. 206 (2017) 168. [9] J. Zhu, M. Zhang, J. Xiong, Y. Yan, W. Li, G. Cheng, Chem. Eng. J. 375 (2019) 121902. [10] Q. Sun, Y. Li, X. Sun, L. Dong, A.C.S. Sustain, Chem. Eng. 1 (2013) 798. [11] C. Ingrosso, C. Esposito Corcione, R. Striani, R. Comparelli, M. Striccoli, A. Agostiano, M.L. Curri, M. Frigione, ACS Appl. Mater. Interf. 7 (2015) 15494. [12] X. Ren, P. Gao, X. Kong, R. Jiang, P. Yang, Y. Chen, Q. Chi, B. Li, J. Coll. Interf. Sci. 530 (2018) 1. [13] Z. Li, L. Shi, D. Franklin, S. Koul, A. Kushima, Y. Yang, Nano Energy. 51 (2018) 400. [14] Y. Gong, Y. Wu, Y. Xu, L. Li, C. Li, X. Liu, L. Niu, Chem. Eng. J. 350 (2018) 257. [15] G. Pacchioni, ACS Catal. 4 (2014) 2874. [16] S.S.M. Bhat, S.A. Pawar, D. Potphode, C.-K. Moon, J.M. Suh, C. Kim, S. Choi, D.S. Patil, J.-J. Kim, J.C. Shin, H.W. Jang, Appl. Catal B Environ. 259 (2019) 118102. [17] P. Zhang, L. Yu, X.W. David Lou, Angew. Chemie - Int. Ed. 57 (2018) 15076. [18] W. Gu, F. Lu, C. Wang, S. Kuga, L. Wu, Y. Huang, M. Wu, A.C.S. Appl, Mater. Interf. 9 (2017) 28674. [19] M.E. Aguirre, R. Zhou, A.J. Eugene, M.I. Guzman, M.A. Grela, Appl. Catal. B Environ. 217 (2017) 485. [20] Z. Tian, P. Zhang, P. Qin, D. Sun, S. Zhang, X. Guo, W. Zhao, D. Zhao, F. Huang, Adv. Energy Mater. (2019) 1901287. 1. [21] F. Li, X. Gao, R. Wang, T. Zhang, G. Lu, N. Barsan, A.C.S. Appl, Mater. Interf. 8 (2016) 19799. [22] Y. Zheng, J. Xu, X. Yang, Y. Zhang, Y. Shang, X. Hu, Chem. Eng. J. 333 (2018) 111. [23] W. He, Y. Sun, G. Jiang, Y. Li, X. Zhang, Y. Zhang, Y. Zhou, F. Dong, Appl. Catal. B Environ. 239 (2018) 619. [24] Y. Guo, J. Tang, Z. Wang, Y.M. Kang, Y. Bando, Y. Yamauchi, Nano Energy. 47 (2018) 494. [25] Y.S. Lai, H.H. Lu, Y.H. Su, A.C.S. Sustain, Chem. Eng. 5 (2017) 7716. [26] S. Rtimi, C. Pulgarin, R. Sanjines, V. Nadtochenko, J.C. Lavanchy, J. Kiwi, A.C.S. Appl, Mater. Interf. 7 (2015) 12832. [27] Y. Yu, Z. Zhou, Z. Ding, M. Zuo, J. Cheng, C. Jing, J. Hazard. Mater. 377 (2019) 267. [28] X. Wang, J.C. Yu, Y. Chen, L. Wu, X. Fu, Environ. Sci. Technol. 40 (2006) 2369. [29] J. Zhang, L. Li, Z. Xiao, D. Liu, S. Wang, J. Zhang, Y. Hao, W. Zhang, A.C.S. Sustain, Chem. Eng. 4 (2016) 2037. [30] H. Hu, Y. Lin, Y.H. Hu, Chem. Eng. J. 375 (2019) 122029. [31] V.P. Prakashan, G. Gejo, M.S. Sanu, M.S. Sajna, T. Subin, P.R. Biju, J. Cyriac, N.V. Unnikrishnan, Appl. Surf. Sci. 484 (2019) 219. [32] Y. Su, Z. Zhang, H. Liu, Y. Wang, Appl. Catal. B Environ. 200 (2017) 448. [33] Z. Liu, J. Chen, Y. Wu, Y. Li, J. Zhao, P. Na, J. Hazard. Mater. 343 (2018) 304.
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Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution Fuxiang Li, Yanqing Jiao, Jianan Liu, Qi Li, Chuanyu Guo, Chungui Tian ⇑, Baojiang Jiang ⇑ Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, PR China
g r a p h i c a l a b s t r a c t The design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE.
a r t i c l e
i n f o
Article history: Received 20 October 2019 Revised 8 November 2019 Accepted 8 November 2019 Available online xxxx Keywords: ZrO2 TiO2 Core-shell heterostructure UiO-66-NH2 Photocatalytic hydrogen evolution
a b s t r a c t The robust photocatalytic hydrogen evolution (PHE) from water needs an effective photogenerated charge spatial separation and enough contact between reactant and catalyst, but the synthesis of catalysts with the characteristics remains a challenge. Herein, we report the design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE. In this system, UiO-66-NH2, one of popular MOF with Zr as metal node, has been chosen as the precursor template due to its plentiful pores, uniform morphology, as well as the rich NH2 groups. Our results show that Ti precursor can uniformly coat on UiO-66-NH2, by means of interaction of tetrabutyl titanate (TBT) with -NH3 in UiO-66-NH2. Followed by the calcination, the Ti precursor and UiO-66-NH2 can be converted into ZrO2 and TiO2, respectively, thus leading to the formation of ZrO2@TiO2 core-shell heterostructure. The ZrO2@TiO2-500 has the high specific surface area of 52.4 m2 g 1. Besides, the intimate contact of TiO2 shell with ZrO2 core facilitates the separation and migration of photoinduced carriers, exposing more active sites for the surface photocatalytic hydrogen evolution reaction. The spectrum and electrochemical characterization further exhibit the extended life of photon-generated carrier and easy mass transfer. The optimized ZrO2@TiO2-500 shows enhanced photocatalytic rate of 39.7 mmol h 1 g 1, much higher than those of ZrO2 (0.8 mmol h 1 g 1) and TiO2 (7.6 mmol h 1 g 1). Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (C. Tian), [email protected] (B. Jiang). https://doi.org/10.1016/j.jcis.2019.11.031 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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1. Introduction The solar photocatalytic water splitting has attracted more and more attention in current world due to its ability for the production of clean hydrogen energy with many advantages of mild condition, simple operation, direct utilization of sunlight and no secondary pollution [1–3]. The development of effective photocatalysis with good activity and stability is constantly pursued, but remains a severe challenge. TiO2, as a stable, nontoxic photocatalyst, is promising in the photocatalytic hydrogen evolution field [4,5]. However, the practical application of TiO2 has been limited due to high recombination probability of charge carriers and low quantum efficiency caused by weak electron mobility and short minority carrier diffusion [6,7]. In this regard, many methods have been developed to improve the photocatalytic performance of TiO2, such as doping [8,9], surface sensitization [10,11], and the construction of the heterostructure [12–14]. By integrating TiO2 with ZrO2 [15], CdS [16], Fe2O3 [17], g-C3N4 [18], Cu2O [19] or BiVO4 [20], a new carrier transport path can be established. The corresponding heterostructure can accelerate charge separation during the photocatalytic process, thus improving the catalytic performance largely. Notably, a core-shell heterostructure has possessed many advantages for the effective separation of photogenerated charge carriers, thus being interesting in the photocatalytic filed [21–24]. In that case, the photogenerated electrons and holes in heterostructure will transfer toward different direction, so their recombination can be suppress. As a kind of wide band gap n-type semiconductor, ZrO2 with the band gap of about 5.0 eV is useful in hydrogen generation with strong resistance to corrosion [25]. Coupling of TiO2 with ZrO2 is effective for the degradation of organic compounds, such as copper supported TiO2/ZrO2, ZrO2/TiO2-xNx and S-doped TiO2/ZrO2 [26–28]. The self-assembly, colloidal aggregation, and precursor templates are the most common approaches for ZrO2@TiO2 core-shell structure [29–31]. Nevertheless, these syntheses are difficult to generate the uniform coating and regular structure with high specific areas, which is not conductive to bring enhanced photocatalytic activity. In order to overcome the above issues, the porous and regular ZrO2 should be chosen as template to support TiO2 uniformly. The selection of the suitable precursor for ZrO2 is a key to realize the goal. The metal organic framework (MOF) is a class of crystal material with micro- and mesoporous structure and large specific surface area, which can be used as a template or precursor for the synthesis of semiconductor oxides [32–33]. The UiO-66-NH2, a typical MOF with Zr ion as the metal node and 2aminoterephthalic acid as the organic ligand, has high porosity, large specific surface area, and network topology [34]. These characterizations make the UiO-66-NH2 promising to produce the Zr-based composites with regular morphology such as C-ZrO2/g-C3N4 [35]. However, up to now, the preparation of ZrO2@ TiO2 core-shell heterostructure from UiO-66-NH2 as Zr source has not been reported yet. In this work, we report a synthetic method to obtain porous core-shell heterostructure consisted of ZrO2 and TiO2. UiO-66NH2 (Zr-MOF) is applied as precursor template and Zr source. Then, the hydrolyzed tetrabutyl titanate (Ti source) is absorbed on its surface through the interaction with the rich ANH2 group in UiO-66-NH2. After thermal treatment, ZrO2@TiO2 core-shell heterostructure has been fabricated, in which thin TiO2 shell covers the ZrO2 core evenly. In the core-shell structure, the matched bang gap of TiO2 and ZrO2 facilitate the separation of photoexcited electron-hole pairs and their migration toward the inner and outer surfaces, thereby decreasing their recombination. As a result, the ZrO2@TiO2 exhibit enhanced photocatalytic activity of 39.7 mmol h 1 g 1, much higher than those of ZrO2 (0.8 mmol h 1 g 1) and TiO2 (7.6 mmol h 1 g 1). This work opens a new avenue to
design MOF-derived core-shell heterostructure photocatalysts for the hydrogen evolution. 2. Experimental section Zirconium chloride (ZrCl4, 99.9%), 2-aminotetrephthalic acid (H2ATA, 98%) and acetic acid (CH3COOH, 99.5%) were purchased from Sigma-Aldrich Trading Co. Ltd. C16H36O4Ti (99%) (TBT) was purchased from Tianjin Kermel Co. Ltd. Ethanol (99.7%) and N, NDimethylformamide (DMF, 99.7%) were purchased from Tianjin Fuyu fine chemical industry Co. Ltd. All chemical reagents were obtained from commercial reagents company without further purification. The deionized water was used in the experiment process. 2.1. Synthesis of UiO-66-NH2 (Zr-MOF) UiO-66-NH2 was synthesized by a solvothermal method [32]. Firstly, 2-aminotetrephthalic acid (H2ATA, 0.18 g) and zirconium chloride (ZrCl4, 0.23 g) was dissolved in 100 mL DMF. The above solution was stirred vigorously for 30 min to form a homogeneous solution. Then 20 mL of CH3COOH was added to the above mixed solution again and stirred for 30 min. The resultant suspension was transferred into a 50 mL Teflon-lined autoclave and heated at 120 °C for 16 h. After naturally cooling to room temperature, the product was washed three times with DMF and methanol, and dried at 60 °C. 2.2. Synthesis of UiO-66-NH2@Ti precursor Zr-MOF was dispersed in a mixture of deionized water (0.1 mL) and ethanol (60 mL), sonicated for 15 min and stirred for 10 min to form a homogeneous dispersion. Then, 25 mL ethanol solution (containing 5 mL of TBT) was added into the dispersion, stirred at room temperature for 4 h. The solution was washed three times with ethanol, and dried overnight at 60 °C to obtain final precursor. The water amount in the synthesis of UiO-66-NH2@Ti precursor was tuned as 0 mL, 0.1 mL, 0.2 mL, 0.3 mL with other reaction conditions same to above typical preparation procedure. 2.3. Synthesis of ZrO2@TiO2 core-shell heterostructure The typical sample ZrO2@TiO2-500 core-shell heterostructure was obtained by calcinating the UiO-66-NH2@Ti precursor at 500 for 2 h with a heating rate of 1 °C/min. The precursor is also calcinated at 450 (ZrO2@TiO2-450), 550 (ZrO2@TiO2-550), 600 (ZrO2@TiO2-600) to study the effect of calcinated temperature on the structure and performance. The ZrO2 was synthesized by calcining the UiO-66-NH2 at 500 for 2 h at a heating rate of 1 /min. The TiO2 was produced as follow: at first, 25 mL ethanol solution (containing 5 mL TBT) was added dropwise to 60 mL ethanol solution (contained 0.1 mL deionized water), stirred at room temperature for 4 h. The obtained solid was washed three times with ethanol, and dried overnight at 80 to get TiO2 precursor. FinialTiO2 was prepared by calcinating the power at 550 for 2 h with a heating rate of 1 /min under air atmosphere. 2.4. Characterization The crystal structures of the synthesized samples were confirmed by X-ray powder diffraction (XRD, Rigaku D/max-IIIB with Cu Ka, k = 1.5406 Å). The structure and morphology of the synthesized samples were characterized by Hitachi S-4800 field emission scanning electron microscope (SEM) operating on 15 KV and JEMF200 transmission electron microscope (TEM) with an acceleration
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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voltage of 200 kV. To analyze the elemental composition, surface chemical environment and valence band position of samples, an X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II) test was performed. The Brunauer-Emmett-Teller (BET) surface area was identified in the range of relative pressure (p/p0) from 0 to 1.0 by using a Tristar II 3020 surface area and porosity analyzer (Micromeritics). Pore size distribution curves were obtained from the adsorption data by using the Barrett-Joyner-Halenda (BJH) method. The light response capacity of the sample was evaluated by using ultraviolet-visible spectrophotometer (Shimadzu UV2550); A fluorescence spectrophotometer (Hitachi F-4600) was used to evaluate the charge separation capability at an excitation wavelength of 380 nm. The corresponding photoluminescence spectrum (PL) was acquired to demonstrate the separation ability of the electron-hole pairs of the catalyst. 2.5. Photocatalytic hydrogen evolution measurement An on-line photocatalytic system (Labsolar-IIIAG, Beijing Perfect Light Co. Ltd) and an online gas chromatograph (GC7900, TCD, Shanghai Tianmei Scientific Instrument Co. Ltd) were used to detect the activity for hydrogen evolution reaction. By using nitrogen as a carrier gas, 50 mg catalyst was mixed with 80 mL of distilled water and 20 mL of methanol in a reaction tank to form a suspension. After ultrasonication for 10 min, 1% (wt) Pt from H2PtCl6, was loaded into the above catalyst as a co-catalyst by known in-situ photodeposition method. 24 Before the light radiation reaction, the mixture in the reactor was degassed by vacuuming to remove the O2 and CO2 dissolved in water. The suspension was illuminated by a Xenon lamp source (PLS-SXE300/300UV, 15 A) equipped with an ultraviolet filter (remain k < 400 nm light). The produced gas was collected and analyzed online. 2.6. Electrochemistry measurement of as prepared products Electrochemistry testing was performed by using an electrochemical workstation (Princeton electrochemical workstation) with three electrodes system. The working electrode is prepared as follows: 0.1 g of catalyst was mixed with 15 mL ethanol, and then the obtained sample was deposited on the FTO glass within a controlled area of 1 cm2 using airbrush. Then, then FTO glass coated by catalyst was heated at 350 °C for 2 h in the air. Pt foil (3 2 cm) and Ag/AgCl were respectively used as the counter electrode and the reference electrode. In this system, 0.2 M Na2SO4 solution electrolyte and ultraviolet light irradiation from 300 W Xenon lamp is selected. 3. Results and discussion A schematic procedure for the porous ZrO2@TiO2 core-shell heterostructure is shown in Scheme 1. Firstly, UiO-66-NH2 with plentiful ANH2 group and pores was employed as a template and Zr source. The tetrabutyl titanate was adsorbed on the surface of UiO-66-NH2 and hydrolyzed to form Ti precursor layer. A precursor contained both Ti and Zr (UiO-66-NH2@Ti) was formed. Finally, a calcination under air (500 ) of UiO-66-NH2@Ti results in the formation of the porous ZrO2@TiO2 core-shell heterostructure. It is noted that the hydrolysis dynamic of TBT can be affected by the amount of the water in the system. Thus, the amount of water in the synthesis was a key for the regulation of morphology and structure (Fig. S1). The agglomerated spheres can be formed when adding 0.30 mL of H2O in the synthesis, and the morphology of UiO-66-NH2 was not remained. In the case of adding 0.2 mL H2O, the UiO-66-NH2@TiO2 shows the octahedronal morphology, being similar to UiO-66-NH2, but with some agglomeration. As further
Scheme 1. The synthetic process of porous ZrO2@TiO2 core-shell structure.
decreasing the H2O amount to 0.1 mL, the UiO-66-NH2@TiO2 with the uniform octahedron morphology was acquired. The results indicate that the control of the hydrolysis dynamics of TBT is important to form uniform coating on UiO-66-NH2. As such, the product prepared by using 0.1 mL H2O was taken as the typical sample for the subsequent experiment and characterizations. The precursors and samples have been characterized by the XRD and the results have been shown in Fig. 1a. As seen in Fig. 1a, the main diffraction peaks of as-synthesized UiO-66-NH2 correspond to the previous reports [36]. After coating by Ti source, the sample shows the diffraction peaks of UiO-66-NH2. The peaks corresponding to Ti species could not be observed, which should be due to weak crystallization of Ti species. Fortunately, FT-IR spectra can provide some useful information about the combination and interaction between two components. As shown in Fig. S2, UiO-66-NH2 displays NAH bending vibration at 1650 cm 1 and CAN stretching absorption of aromatic amines at 1380 cm 1 [36,37]. Compared to UiO-66-NH2, the NAH bending vibration for UiO-66-NH2@Ti precursor is significantly weak, which should be attributed to the strong interaction between the UiO-66NH2 and Ti precursor. After calcination of UiO-66-NH2@Ti precursor, ZrO2@TiO2-500 has been obtained. As shown in Fig. 1a, for ZrO2@TiO2-500, the diffraction peaks located at 25.3° and 30.1° can be attributed to (1 0 1) lattice plane of anatase TiO2 and (1 1 1) lattice plane of ZrO2, respectively. Other diffraction peaks can also be assigned to either TiO2 or ZrO2, meaning the effective contact between ZrO2 and TiO2 in Fig. S3 [38,39]. As shown in Table S1, we can obtain the contents of Ti and Zr element in the ZrO2@TiO2-500 by the ICP-OES test, which are 27.48 and 40.39%, respectively. When further changing the calcination temperature, other samples including ZrO2@TiO2-450, ZrO2@TiO2-550, and ZrO2@TiO2-600 can be acquired (Fig. S4). Meanwhile, in Fig. 1a, reference sample ZrO2, derived from UiO-66-NH2, exhibits serials of diffraction peaks, corresponding to standard ZrO2 (JCPDS 491642) [40]. Reference TiO2 also shows the anatase structure (JCPDS 21-1272) [41]. The microstructure of different product is investigated by the TEM and SEM tests, and shown as Fig. 1b–g. The UiO-66-NH2 shows the uniform octahedral morphology with smooth surface and the size about 400 nm (Fig. 1b). The SEM image of ZrO2@ TiO2-500 shows the similar shape of octahedron, but with slightly rough surface, implying the formation of TiO2 on the surface. The TEM images can provide further information about the
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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Fig. 1. (a) XRD patterns of UiO-66-NH2, UiO-66-NH2@Ti precursor, and ZrO2@TiO2-500. (b) SEM image of UiO-66-NH2 and (c) SEM image of ZrO2@TiO2-500. (d) TEM image of ZrO2@TiO2-500 and partial enlargement HRTEM images of ZrO2@TiO2-500 (e, f). Corresponding elemental maps of Ti, O, and Zr within ZrO2@TiO2-500 (g).
micro-structure of ZrO2@TiO2-500 (Fig. 1d). Compared with UiO66-NH2, the size of ZrO2@TiO2-500 reduces to about 250 nm, which is related to the contraction during the transformation of UiO-66NH2 skeleton to ZrO2 under calcination. It is noted that many pores (2–7 nm) can be found in ZrO2@TiO2-500 based on the partial enlargement of HRTEM image (Fig. 1e). The large specific surface area and superficial porous structure of the core-shell heterostructure of ZrO2@TiO2 are beneficial to the mass transport and can expose more catalytic active sites due to more dangling bonds or defects, which is more favorable for the surface catalytic reaction and improve the catalytic efficiency. The HRTEM shows the external lattice fringes space of 0.351 nm and 0.237 nm, which are pointed to (1 0 1) and (0 0 4) crystal planes of anatase TiO2 [42]. Meanwhile, the space of internal lattice fringes of 0.296 nm can be ascribed to (1 1 1) crystal planes of ZrO2 (Fig. 1f) [43]. Above analysis proves that as-synthesized ZrO2@TiO2-500 is a porous core-shell heterostructure with ZrO2 as the core and TiO2 as the shell. The SAED and other TEM image further indicate the effective combination between ZrO2 and TiO2 (Figure S5). The elemental mapping of ZrO2@TiO2-500 exhibits the uniform distribution of Ti, O, Zr elements, which being consistent with EDX spectra of ZrO2@TiO2-500 (Fig. 1g and Fig. S6), indicating the uniform coating of TiO2 around ZrO2. The formation of core-shell heterostructure with plentiful pores is beneficial to improve the photocatalytic performance. In order to study the pore structure of the sample, the N2 adsorption–desorption measurement has been performed. Fig. 2a,
b show the analysis results of ZrO2, TiO2, and ZrO2@TiO2-500. It is worth noting that a typical IV isotherm can be observed from all samples, which indicates the presence of pores in consideration of H3 hysteresis loop [44]. The specific surface area of ZrO2 is only 5.9 m2 g 1, which should be the collapse and shrink partially of UiO-66-NH2 skeleton during the calcination. The specific surface area of TiO2 is also relatively small (~6.6 m2 g 1). In contrast, the ZrO2@TiO2-500 heterostructure possesses a high specific surface area of 52.4 m2 g 1 and uniform mesopores structure (~16.2 nm), which is consistent with TEM results (Fig. 1e). The obtained ZrO2@ TiO2-500 heterostructure has maintained the octahedral morphology of UiO-66-NH2 and inherited its large specific surface area and porous structure, thanks to the protective effect of TiO2 layer on UiO-66-NH2. The porous structures facilitate carrier migration and surface reaction mass transfer, being conducive to improve photocatalytic performance of products. The surface chemical composition and the oxidation state of ZrO2, TiO2 and ZrO2@TiO2-500 were investigated by XPS. In Fig. 2c, the O1s XPS pattern of ZrO2 shows two main peaks at 529.8 and 531.4 eV, which can be respectively referred to lattice oxygen and surface OH groups [45]. However, we can find that O1s peaks of ZrO2@TiO2-500 have obvious shift toward low binding energy, demonstrating the increase of electronic intensity. Furthermore, in comparison with ZrO2, the Zr XPS peaks for ZrO2@ TiO2-500 shows a shift about 0.9 eV toward high binding energy, indicating the decrease of electronic density around Zr atom (Fig. 2d). Above results prove that building the hetero-interface
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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Fig. 2. Nitrogen adsorption-desorption isotherms (a) and BJH pore size distributions (b) of ZrO2, TiO2, and ZrO2@TiO2-500. XPS O 1s (c) and Zr 3d (d) of ZrO2 and ZrO2@ TiO2-500, respectively.
Fig. 3. (a) UV-vis reflectance spectra of ZrO2, TiO2 and ZrO2@TiO2-500 and (b) the band gap determined from Kubelka-Munk function. (c) XPS valence band spectra of ZrO2, TiO2 and ZrO2@TiO2-500; (d) the photo-induced carrier’s migration process within ZrO2@TiO2.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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of ZrO2@TiO2 can make the effective electronic transfer due to the strong interaction between them. Furthermore, the XPS Ti 2p of TiO2 and ZrO2@TiO2-500 is also confirmed (Fig. S7), in which two major peaks located at 458.5 and 464.2 eV are ascribed to Ti 2p3/2 and Ti 2p1/2, respectively [46]. For photocatalytic reaction, the light absorption capacity of catalyst is a crucial factor. The UV-vis diffuse reflectance spectra test has been performed and the data are displayed as Fig. 3a. We can clearly see that the main optical response ranges of ZrO2, TiO2 and ZrO2@TiO2-500 are located in the ultraviolet region, and their strongest absorption is at 250 nm–450 nm. Compared with ZrO2, it is worthy of note that the absorption band edge of ZrO2@TiO2500 has red-shift toward the long wavelength, indicating the decreases of the band gap of ZrO2@TiO2-500. In Fig. 3b, the band gaps of ZrO2, TiO2 and ZrO2@TiO2-500 are 4.52, 2.98, and 3.15 eV as calculated through Kubelka-Munk function [47], respectively. Compared with ZrO2, the band gap of ZrO2@TiO2-500 is significantly narrow due to the formation of core-shell heterostructure. Meanwhile, it has already a significant enhancement of the ultraviolet spectral region utilization (Fig. 3a). In order to achieve the
valence band position of the catalyst, the XPS valence band spectrum test has also been studied. As shown in Fig. 3c, the valence band positions of ZrO2, TiO2 and ZrO2@TiO2-500 are located at 2.92, 2.56, and 2.68 eV. Based on the band gap and the valence band (VB) position of catalyst, the energy band structure of catalyst can be confirmed. Fig. 3d shows the process about migration of the photogenerated carriers within ZrO2@TiO2-500 heterostructure. Under the ultraviolet light irradiation, both TiO2 and ZrO2 have been excited along with the electron transfer from the ground state to the excited state. The electrons in the VB of TiO2 are excited to the conduction band (CB), leaving holes in the valence band. At the same time, the electrons in ZrO2 VB are also excited to the CB. Subsequently, the electrons in CB of ZrO2 migrate to CB of TiO2 because the VB position of ZrO2 is more positive than that of TiO2 [48–50]. Therefore, the electron migration rate of ZrO2@ TiO2-500 is accelerated, and the recombination of electron-hole pair is suppressed, all of which favor the photocatalytic hydrogen evolution reaction. Fig. 4a shows photocatalytic activity of serials of samples obtained at different calcination temperature. The hydrogen
Fig. 4. (a) Photocatalytic H2 evolution rate of ZrO2, TiO2 and ZrO2@TiO2-500. (b) Photocatalytic H2 evolution rate of ZrO2@TiO2-500 and reference products. (c) Photocatalytic stability measurement. (d) Photoluminescence spectra, (e) time-resolved photo-response under ultraviolet irradiation at +0.6 V with versus saturated calomel electrode, and (f) EIS Nyquist plots of different samples.
Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031
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evolution amount of ZrO2@TiO2-450, ZrO2@TiO2-500, ZrO2@TiO2550 and ZrO2@TiO2-600 is 14.4, 39.7, 30.5 and 24.8 mmol h 1 g 1, respectively. The photocatalytic hydrogen evolution performances of ZrO2, TiO2 and ZrO2@TiO2-500 are also exhibited as Fig. 4b. The photocatalytic hydrogen evolution rates of ZrO2 and TiO2 are 0.8 and 7.6 mmol h 1 g 1, respectively. The photocatalytic hydrogen evolution rate of ZrO2@TiO2-500 is about 50 times than that of ZrO2 and 5 times that of TiO2. It can be concluded that the photocatalytic hydrogen evolution performance of as prepared ZrO2@TiO2-500 is superior to any single catalyst. In Fig. 4c, the photocatalytic hydrogen evolution performance of ZrO2@TiO2500 shows almost no decrease after three cycles of 4 h as a period, indicating ZrO2@TiO2-500 is stable during illumination without light corrosion. The cycle stability test results show that ZrO2@ TiO2-500 heterostructure has good stability. We performed XRD tests on samples before of lighting and after 4 h of lighting illumination (Fig. S8). It was found that the XRD diffraction peaks of the samples did not change before and after the observation, indicating that the stability of the catalyst was good. The outstanding photocatalytic performance can be attributed to the core-shell heterostructure, high specific surface area, abundant surface pores structure and active sites of ZrO2@TiO2-500. In order to explore the separation ability of photoelectron-hole pair in the catalyst under photo-excitation, the photoluminescence spectroscopy (PL) has been tested. Fig. 4d shows the PL spectra of three catalysts. These PL spectra have similar peak positions at the excitation wavelength of 360 nm. Generally, the lower PL intensity means the lower recombination rate of photoelectron-hole [51]. Compared with ZrO2, the PL position of TiO2 is much lower, signifying a lower recombination rate of photoelectron-hole pair. It is worth noting that the intensity of PL spectrum of ZrO2@TiO2-500 is weaker in comparison with that of TiO2, meaning the enhanced separation efficiency of photogenerated electron-hole pairs. Therefore, it may be expected that the ZrO2@TiO2-500 heterostructure may show an enhanced photocatalytic activity. Fig. 4e shows the time-resolved photo-response of ZrO2, TiO2 and ZrO2@TiO2-500, reflecting the photo-to-current conversion rate. Once the light is added, a significant increase of current density of the catalyst could be observed. The current densities of ZrO2, TiO2 and ZrO2@TiO2-500 are 15, 78 and 165 lA cm 2, respectively. Among them, ZrO2@TiO2-500 has the highest current density, which is 11 times and 2.1 times as large as ZrO2 and TiO2, respectively. It can be attributed to the formation of heterostructure that can accelerate the separation and migration of photogenerated carriers. We can also see that the photocurrents of ZrO2, TiO2 and ZrO2@TiO2-500 have almost no attenuation as the illumination time increases, indicating that the good photochemical stability of the catalyst. Fig. 4f shows EIS Nyquist plots of as-prepared ZrO2, TiO2 and ZrO2@TiO2-500, in which the resistance of ZrO2, TiO2 and ZrO2@TiO2-500 reduce in order. The resistances of three catalysts have been significantly decreased under light irradiation, indicating a significant increase of the electron transport efficiency during illumination. Among them, ZrO2@TiO2-500 has the lowest resistance, which should be relative with the core-shell heterostructure and larger specific surface area.
4. Conclusion In summary, we have prepared porous ZrO2@TiO2 core-shell heterostructure photocatalytic catalyst by utilizing UiO-66-NH2 (Zr-MOF) as precursor and Zr source. Porous ZrO2@TiO2 heterostructure inherits the characteristics of UiO-66-NH2. Compared with ZrO2 and TiO2, ZrO2@TiO2-500 displays the higher photocatalytic hydrogen evolution rate of 39.7 mmol h 1 g 1. The superior photocatalytic hydrogen evolution properties of
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ZrO2@TiO2-500 can be attributed to its core-shell heterostructure, large specific surface area and porous structure for promoting carrier transport and migration, and inhibiting electron-hole pair’s recombination. The Zr-MOF template route should be indicative to construct other core-shell semiconductor oxide heterostructures. Declaration of Competing Interest The authors declare no competing financial interests. Acknowledgment This work was supported by the National Natural Science Foundation of China (21771061, 21601055) and University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2017118) Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.031. References [1] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, J. Mater. Chem. A. 3 (2015) 2485. [2] S. Chandrasekaran, L. Yao, L. Deng, C. Bowen, Y. Zhang, S. Chen, Z. Lin, F. Peng, P. Zhang, Chem. Soc. Rev. 48 (2019) 4178. [3] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Renew. Sustain. Energy Rev. 43 (2015) 599. [4] Q. Guo, C. Zhou, Z. Ma, X. Yang, Adv. Mater. (2019) 1901997. [5] X. Chen, L. Liu, F. Huang, Chem. Soc. Rev. 44 (2015) 1861. [6] S. Sun, P. Gao, Y. Yang, P. Yang, Y. Chen, Y. Wang, A.C.S. Appl, Mater. Interfaces. 8 (2016) 18126. [7] J. Yan, H. Wu, H. Chen, Y. Zhang, F. Zhang, S.F. Liu, Appl. Catal. B Environ. 191 (2016) 130. [8] S. Sun, J. Zhang, P. Gao, Y. Wang, X. Li, T. Wu, Y. Wang, Y. Chen, P. Yang, Appl. Catal. B Environ. 206 (2017) 168. [9] J. Zhu, M. Zhang, J. Xiong, Y. Yan, W. Li, G. Cheng, Chem. Eng. J. 375 (2019) 121902. [10] Q. Sun, Y. Li, X. Sun, L. Dong, A.C.S. Sustain, Chem. Eng. 1 (2013) 798. [11] C. Ingrosso, C. Esposito Corcione, R. Striani, R. Comparelli, M. Striccoli, A. Agostiano, M.L. Curri, M. Frigione, ACS Appl. Mater. Interf. 7 (2015) 15494. [12] X. Ren, P. Gao, X. Kong, R. Jiang, P. Yang, Y. Chen, Q. Chi, B. Li, J. Coll. Interf. Sci. 530 (2018) 1. [13] Z. Li, L. Shi, D. Franklin, S. Koul, A. Kushima, Y. Yang, Nano Energy. 51 (2018) 400. [14] Y. Gong, Y. Wu, Y. Xu, L. Li, C. Li, X. Liu, L. Niu, Chem. Eng. J. 350 (2018) 257. [15] G. Pacchioni, ACS Catal. 4 (2014) 2874. [16] S.S.M. Bhat, S.A. Pawar, D. Potphode, C.-K. Moon, J.M. Suh, C. Kim, S. Choi, D.S. Patil, J.-J. Kim, J.C. Shin, H.W. Jang, Appl. Catal B Environ. 259 (2019) 118102. [17] P. Zhang, L. Yu, X.W. David Lou, Angew. Chemie - Int. Ed. 57 (2018) 15076. [18] W. Gu, F. Lu, C. Wang, S. Kuga, L. Wu, Y. Huang, M. Wu, A.C.S. Appl, Mater. Interf. 9 (2017) 28674. [19] M.E. Aguirre, R. Zhou, A.J. Eugene, M.I. Guzman, M.A. Grela, Appl. Catal. B Environ. 217 (2017) 485. [20] Z. Tian, P. Zhang, P. Qin, D. Sun, S. Zhang, X. Guo, W. Zhao, D. Zhao, F. Huang, Adv. Energy Mater. (2019) 1901287. 1. [21] F. Li, X. Gao, R. Wang, T. Zhang, G. Lu, N. Barsan, A.C.S. Appl, Mater. Interf. 8 (2016) 19799. [22] Y. Zheng, J. Xu, X. Yang, Y. Zhang, Y. Shang, X. Hu, Chem. Eng. J. 333 (2018) 111. [23] W. He, Y. Sun, G. Jiang, Y. Li, X. Zhang, Y. Zhang, Y. Zhou, F. Dong, Appl. Catal. B Environ. 239 (2018) 619. [24] Y. Guo, J. Tang, Z. Wang, Y.M. Kang, Y. Bando, Y. Yamauchi, Nano Energy. 47 (2018) 494. [25] Y.S. Lai, H.H. Lu, Y.H. Su, A.C.S. Sustain, Chem. Eng. 5 (2017) 7716. [26] S. Rtimi, C. Pulgarin, R. Sanjines, V. Nadtochenko, J.C. Lavanchy, J. Kiwi, A.C.S. Appl, Mater. Interf. 7 (2015) 12832. [27] Y. Yu, Z. Zhou, Z. Ding, M. Zuo, J. Cheng, C. Jing, J. Hazard. Mater. 377 (2019) 267. [28] X. Wang, J.C. Yu, Y. Chen, L. Wu, X. Fu, Environ. Sci. Technol. 40 (2006) 2369. [29] J. Zhang, L. Li, Z. Xiao, D. Liu, S. Wang, J. Zhang, Y. Hao, W. Zhang, A.C.S. Sustain, Chem. Eng. 4 (2016) 2037. [30] H. Hu, Y. Lin, Y.H. Hu, Chem. Eng. J. 375 (2019) 122029. [31] V.P. Prakashan, G. Gejo, M.S. Sanu, M.S. Sajna, T. Subin, P.R. Biju, J. Cyriac, N.V. Unnikrishnan, Appl. Surf. Sci. 484 (2019) 219. [32] Y. Su, Z. Zhang, H. Liu, Y. Wang, Appl. Catal. B Environ. 200 (2017) 448. [33] Z. Liu, J. Chen, Y. Wu, Y. Li, J. Zhao, P. Na, J. Hazard. Mater. 343 (2018) 304.
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Please cite this article as: F. Li, Y. Jiao, J. Liu et al., Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.031