TiO2(B) Heterophase

Subscriber access provided by READING UNIV

Article

Hierarchical nanotubular anatase/rutile/TiO2(B) heterophase junction with oxygen vacancies for enhanced photocatalytic H2 production Xiaoqiang An, Chengzhi Hu, Huijuan Liu, and Jiuhui Qu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03745 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Hierarchical

nanotubular

anatase/rutile/TiO2(B)

heterophase junction with oxygen vacancies for enhanced photocatalytic H2 production Xiaoqiang An†, Chengzhi Hu†,‡,*, Huijuan Liu#,‡, and Jiuhui Qu†,‡

† Key Laboratory of Drinking Water Science and Technology, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. # State Key Laboratory of Environmental Aquatic Chemistry, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China KEYWORDS: oxygen vacancy, phase junction, titanium dioxide, hydrogen evolution, photocatalyst

ABSTRACT: Oxygen vacancies have been demonstrated to enhance the interfacial charge separation in TiO2-based photocatalysts. In this report, we explored a facile route to synthesize hierarchical nanotubular anatase/rutile/TiO2(B) nanostructures with high surface area and defective electronic structure. The formation of oxygen vacancies in the heterophase junction was analyzed by UV-Vis absorption spectra, electron spin resonance and X-ray photoelectron

ACS Paragon Plus Environment

1

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

spectroscopy. The enhanced interfacial charge separation and transportation ensured the excellent photoactivity of oxygen-deficient junctions for the photocatalytic production of hydrogen. As a result, defective anatase/rutile/TiO2(B) junction showed a high hydrogen evolution rate of 2.79 mmol/h, which was 19 times higher than blank TiO2 nanotubes. The results demonstrate that defect modulation is a powerful tool to enhance the catalytic performances of TiO2-based photocatalysts.

1. Introduction Due to the abundant availability, nontoxicity, and high chemical stability, TiO 2 has been recognized as one of the most promising photocatalytic materials for energy conversion and environmental remediation. However, photoinduced electrons and holes in TiO2 are easy to combine, resulting in its moderate photocatalytic activity. Numerous strategies have been explored to enhance the separation of charge carriers, while mixed-phase of TiO2 has aroused substantial interest.1,2 It was suggested that the differences in the band structures of phase junction can provide driving force for charge transfer across the junction interface, which is beneficial for improving the photocatalytic performance. Biphasic TiO2 based on anatase and rutile, anatase and brookite, brookite and rutile, or even three mixed phases of TiO2 have been explored.3-5 A typical example of TiO2-based phase junction is Degussa p25, the solids with mixed anatase and rutile which are often used as a benchmark model photocatalyst. Nevertheless, photocatalytic activity of p25 is still restricted by the small specific surface area and limited surface reaction sites. In this respect, developing TiO2-based heterophase junctions with high porosity and long aspect ratio is of great importance. Especially, tubular titania derived from hydrogen titanate, a promising 1-D nanomaterials with high surface area, is a strong candidate for developing more efficient photocatalysts.6


2

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Besides material morphology, it should be pointed out that the activity of photocatalysts is highly dependent on their electronic structures. At present, modulating the electronic structure of TiO2 through oxygen vacancy control has become a research hotspot.7,8 Although the fundamental mechanism remains somewhat controversial, it has been identified that the structural rearrangement of crystal surface in oxygen-deficient semiconductors can create localized state in the band gap.9 This provides an ideal platform for improving the electrical conductivity and donor density in TiO2. Accordingly, significantly enhanced photoactivity can be achieved, because of the inhibited recombination of photogenerated charge carriers. Bearing these facts in mind, a series of non-stoichiometric heterostructures with excellent photocatalytic activities have been developed in our groups, through controlling the concentration and distribution of interfacial oxygen vacancies.10,11 However, the strategically modulation of oxygen vacancies in 1-D mixed-phase nanostructures is still an unexplored field. To exert the great potential of TiO2-based photocatalyst, it is highly desirable to develop more efficient heterophase junction with high surface area and oxygen-deficient electronic structure, enhancing the adsorption and uniform dispersion of reactant. In this paper, hierarchical anatase/rutile/TiO2(B) nanostructures with high surface area were fabricated through coupling TiO2 nanotubes with p25 nanoparticles. A subsequent heat-treatment procedure was used to generate oxygen vacancies in the mixed-phase junctions. Due to the enhanced charge separation around heterophase interfaces and the decreased charge transfer resistance, as-synthesized oxygen-deficient heterophase junctions exhibit significantly improved activity for photocatalytic hydrogen evolution. Compared to pristine TiO2 nanotubes, the 17-fold increased hydrogen evolution rate well demonstrates the contribution of oxygen vacancies to the photocatalytic performance of TiO2-based nanomaterials.


3

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

2. Experimental section 2.1 Fabrication of titania nanotues The fabrication procedures of anatase/rutile/TiO2(B) heterophase junctions is illustrated in Scheme 1. To fabricate hydrogen titanate nanotubes, 2 g of p25 TiO2 was firstly mixed with 30 mL of 10 M NaOH solution. After stirring for 30 min, the suspension was transferred into a Teflon-lined autoclave with the capacity of 50 mL. After a hydrothermal reaction at 150℃ for 48 h, the precipitate was collected by filtration and washed with water until the pH value was close to 7 (Step I). As-synthesized sodium titanate was neutralized using 0.1 M HCl solution, which was subsequently washed with DI water until the neutral pH value. The products were separated by filtration and dried at 80 ℃ in a vacuum oven (Step II). As-synthesized hydrogen titanate nanotubes were calcined at 500 ℃ in air for 4 h to form TiO2 nanotubes (Step III, denoted as TiO2 NTs). 2. 2 Fabrication of anatase/rutile/TiO2(B) mixed-phase junctions To fabricate anatase/rutile/brookite heterophase junctions, TiO2 NTs/p25 was firstly synthesized by mixing TiO2 NTs and p25 with the weight ratio of 10:3 in 30 mL of ethanol. Under strong stirring, the solution was dried on a hot plate (Step IV). The grinded powders were placed in the middle of a quartz tube furnace. A hydrogen treatment was carried out at 450 °C for 4 h under Ar/H2 atmosphere, at a flow rate of 50 mL/min. Finally, oxygen-deficient anatase/rutile/TiO2(B) heterophase junctions (Step V) TiO2 NTs/p25-H2) were obtained. For comparison, TiO2 NTs and p25 were directly heat-treated by hydrogen gas. The corresponding samples were denoted as TiO2 NTs-H2 and p25-H2, respectively.


4

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Scheme 1. Scheme of the fabrication processes of the anatase/rutile/TiO2(B) heterophase junctions. 2.3 Materials characterization Phase structure of samples was studied by X-ray diffraction (XRD, Rigaku RINT 2100). Field emission scanning electron microscope (FE-SEM, JSM-7600F, JEOL) and transmission electron microscopy (TEM, JEOL-2100) were used to characterize the morphology of samples. The surface elemental composition and bonding environment was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALab220I-XL). The Raman spectra were recorded by a HORIBA LabRAM HR confocal microscope spectrograph with 532 laser excitation. Optical property of samples was studied by UV-Vis-NIR diffusion reflectance spectra (DRS, Cary 5000), using BaSO4 as reference. N2 adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010N analyzer. Electron spin resonance (ESR) analysis was operated at a Bruker E500 spectrometer. 2.4 Photocatalytic reaction measurements Photocatalytic hydrogen generation reactions were carried out in a closed gas-circulation system. The system was evacuated several times prior to irradiation. In a typical procedure, 30 mg


5

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

photocatalysts were dispersed in 100 mL of DI water, which contains 10 vol % methanol as the scavenger and 1 wt % Pt as the cocatalyst. H2PtCl6 was used as precursor for the photodeposition of Pt on the catalyst. The evolved gas in the photocatalytic reactor was extracted and analyzed by a GC-14C gas chromatograph equipped with a column of 5 Å molecular sieves. The photocatalytic activity of different samples was evaluated by monitoring the hydrogen evolution under UV-Vis irradiation from a 300 W Xe lamp (Beijing Trusttech Co. Ltd, PLS-SXE-300 UV). 2.5 Photoelectrochemical experiment The working electrode was prepared on fluoride tin oxide (FTO) glass substrates. 5 mg of catalysts and 10 μl of Nafion solution (5 wt%) were dispersed in 1 ml water/isopropanol mixed solvent (3:1 v/v) by sonication to get a slurry. Then, the slurry was spread onto FTO and airdried. The electrochemical measurements were performed in a conventional three-electrode cell, using an electrochemical analyzer (Gamry, Interface 1000). 0.5 M Na2SO4 was used as electrolyte solution. A Pt sheet and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrical impedance spectroscopy (EIS) and Mott-Schottky plot were measured using a electrochemical workstation (Interface 1000). For Mott−Schottky measurements, the perturbation signal was 10 mV with the frequency of 1 kHz. For electrochemical impedance spectroscopy (EIS) measurements, the perturbation signal was 5 mV, over the frequency range of 1 MHz to 100 mHz. 3. Results and discussion The crystallinity and phase composition of different TiO2-based samples were characterized by XRD. As shown in Figure 1, TiO2 NTs presents strong diffraction peaks at 2θ=25.5, 37.9, 48.4, 54.2, 55.3 and 62.9o, corresponding to the reflection planes of (101), (004), (200), (105), (211)


6

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

and (204) of anatase TiO2 (JCPDS 21-1272), respectively. The characteristic peaks of crystalline rutile phase (JCPDS 21-1276) are also observed at 2θ=27.7, 36.2 and 41.4o, indicating that the sample contains mixed phases of anatase and rutile. Compared to TiO2 NTs, the XRD profiles of p25 and TiO2 NTs/p25 does not present any noticeable change. It proves that the hybridization procedure exhibits negligible effect on the composition and phase structure of products. In contrast, the subsequent heat-treatment in hydrogen flow results in the decreased peaks of rutile and the appearance of foreign diffraction peaks at 29.8 and 44.4o, which correspond to those of TiO2(B) phase (JCPDS 74-1940).12 According to the above results, TiO2-based heterophase junction, with three mixed phases of anatase, rutile and TiO2(B), has been successfully prepared. The content of each phase can be determined by integrating intensities of anatase (101), rutile (110) and brookite (121).13,14 Calculation indicates that as-prepared composite is consisted of anatase (93 %), rutile (3 %) and brookite (4 %), respectively.

Figure 1. XRD patterns of TiO2 NTs, p25, TiO2 NTs/p25 and TiO2 NTs/p25-H2. The morphology of different samples was studied by SEM. As shown in Figure S1a, TiO2 NTs are composed of 1-D nanostructures, which are further self-assembled into a 3D fibrous network.


7

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 21

Each fiber-like nanostructure possesses a typical diameter of 20 nm and a length up to several micrometers. Figure S1b shows the SEM image of commercial p25 TiO2. Large amount of nanoparticles with the average diameter of 30 nm are observed. The hybridization and heattreatment procedures exhibit negligible influence on the 3-D fibrous network structure. The adherence of nanoparticles to the surface of 1-D nanostructures indicates the formation of TiO2 NTs/p25 composites.

Figure 2. TEM and HR-TEM images of TiO2 NTs (a, b), p25 (c, d), TiO2 NTs/p25 (e, f) and TiO2 NTs/p25-H2 (g, h).


8

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

TEM image in Figure 2a shows that the 1-D nanostructure consists of nanotubes with open edges, a structure which can provide extremely high surface area for photocatalytic reactions. The coexistence of (101) plane of anatase and (110) plane of rutile indicates the formation of mixed-phase TiO2-based junctions (Figure 2b). When the wet impregnation method was used to load p25 TiO2 (Figure 2c and 2d) onto the surface of as-synthesized nanotubes, nanoparticle/nanotube hybrid nanostructures were obtained. HR-TEM image of TiO2 NTs/p25 (Figure 2f) proves the intimate contact between different phases of TiO2, which is considered to facilitate interfacial charge transfer. The following heat-treatment under hydrogen atmosphere not only results in the partial fracture of nanotubes, but also contributes to the phase transformation of TiO2 (Figure 2g). The lattice spacing of d = 0.35 nm can be ascribed to the (101) plane of anatase TiO2. The appearance of interplanar distance of d =0.62 nm confirms the formation of metastable brookite TiO2(B) with corresponding (001) plane.15 The coexistence of clear lattice space and amorphous shell with a thickness of ~1 nm indicates the defective structure of TiO2(B) (Figure 2h), which agrees well with that of oxygen-deficient TiO2.16,17 Different from TiO2/p25 composites, lattice fringes corresponding to rutile TiO2 are difficult to be observed in TiO2 NTs/p25-H2. It indicates that hydrogen treatment remarkably interrupts the bond structure of most rutile nanoparticles. This phenomenon is consistent with the decreased peak intensity of rutile TiO2 in the XRD measurements. The above results indicate that phase structure presents significant influence on the formation of structural defects. Oxygen vacancies are preferentially generated in rutile TiO2, which agrees well with that reported in the literatures.18 Accordingly, either disordered nanoparticles or amorphous layer wrapped rutile TiO2 are observed in the HR-TEM observation (Figure S2). In contrast, anatase TiO2 is relatively stable in the H2 atmosphere. It has been reported that this amorphization process can


9

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

significantly increase the conductivity of semiconductors, which contributes to the interfacial separation of charge carriers.19,20 Therefore, hydrogen treatment of TiO2 NTs/p25 composites should result in the formation of anatase/rutile/TiO2(B) phase junctions with superior photoactivityies. The phase structure of different TiO2 samples was studied by Raman spectroscopy. As shown in Figure 3, three strong peaks at the wavenumber of 396 cm-1, 514 cm-1 and 636 cm-1 can be easily observed, which are attributed to the vibration of anatase TiO2. The existence of rutile phase in TiO2 NTs can be evidenced by the weak peak at 443 cm-1. For TiO2 NTs/p25-H2, hydrogen treatment leads to the decreased peak intensity of rutile and the emergence of TiO2(B) peak at 279 cm-1.21 Thus, Raman spectra provide more evidence for the amorphization of rutile phase and the formation of brookite TiO2 phase. The typical Raman peaks of anatase and TiO2(B) was integrated to estimate the weight fraction of TiO2(B) in the composites (Figure S3).22 The weight ratio of anatase/TiO2(B) is calculated to be 22, which is consistent with the XRD results.

Figure 3. Raman spectra of TiO2 NTs, p25 and TiO2 NTs/p25-H2. The influence of oxygen vacancy formation on the optical absorption of samples was studied by UV-vis DRS. In Figure 4a, TiO2 NTs shows a strong absorption in the UV region with the absorption edge at about 400 nm. The hybridization of TiO2 NTs with p25 shows negligible


10

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

influence on the light absorption of photocatalysts. Differently, heat-treatment of TiO2-based photocatalysts under hydrogen atmosphere results in the obvious change of sample color from white to gray (inset of Figure 4b). Both TiO2-H2 and TiO2/p25-H2 show a wide strong absorption in the visible light region. Especially, the considerable large absorption tail provides indisputable evidence that TiO2 NTs/p25-H2 contains a large number of oxygen vacancies.23 Due to the formation of oxygen–deficient structure, bandgap energy of heterophase junction decreases from 3.0 to 2.3 eV.

Figure 4. (a) UV−vis diffuse reflectance spectra of TiO2 NTs, TiO2 NTs/p25, TiO2 NTs-H2 and TiO2 NTs/p25-H2; (b) The corresponding Kubelka−Munk transformed diffuse reflectance spectra. The surface chemical bonding and element binding energy of different TiO2-based samples were characterized by XPS. In the Ti 2p spectra (Figure 5a), the peaks centered at 458.5 and 464.3 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 in TiO2 NTs. In contrast, the Ti 2p peaks of TiO2 NTs/p25-H2 shift to low binding energies, indicating the formation of Ti3+ caused by oxygen vacancies.24 No obvious shift can be detected for the Ti 2p spectra of TiO2 NTs and TiO2 NTs/p25, indicating that heat-treatment in hydrogen atmosphere induces changed chemical bonding of titanium ions. The formation of oxygen vacancy-induced band-gap state can be


11

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

further confirmed by the notable band tail in the valence band spectrum of TiO2 NTs/p25-H2 (Figure S4).25 Besides the XPS measurement, ESR spectra were used to analysis the electronic structure of heterophase junctions. As shown in Figure 5b, no obvious signal can be detected for TiO2 NTs and TiO2 NTs/p25. However, symmetrical peaks centered at g = 2.003 are observed for TiO2 NTs-H2 and TiO2 NTs/p25-H2. According to the earlier studies, this signal is referenced to electrons trapped on the oxygen vacancies.26 All these results confirm the formation of anatase/rutile/TiO2(B) phase junctions with defective structures.

Figure 5. (a) Ti 2p XPS spectra of TiO2 NTs, TiO2 NTs/p25 and TiO2 NTs/p25-H2; (b) ESR spectra of TiO2 NTs, TiO2 NTs/p25, TiO2 NTs-H2 and TiO2 NTs/p25-H2. After the thorough characterizations, as-synthesized heterophase junctions were tested for the photocatalytic hydrogen evolution in the presence of sacrificial electron donor under irradiation. Only a trace amount of H2 is detected for TiO2 NTs, with the production rate of 0.14 mmol/h. Oxygen-deficient TiO2 NTs-H2 and p25-H2 were fabricated by heat-treating TiO2 NTs and p25 in hydrogen atmosphere. TiO2 NTs/p25-H2 with oxygen vacancies exhibits significantly improved photoactivity for hydrogen production. The value of 2.79 mmol/h is 19, 3, and 2 times


12

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

higher than TiO2 NTs, TiO2 NTs-H2 and p25-H2, respectively. This well demonstrates the significant impact of oxygen vacancies on the photoactivity of photocatalysts. TiO2 NTs/p25-H2 also exhibits superior activity than the mixture of TiO2 NTs-H2 and p25-H2 (Figure S5). It indicates that the interfacial interactions between different TiO2 phases should contribute to the photoactivity of photocatalysts. In Figure 6b, the activity of TiO2 NTs/p25-H2 was compared with benchmark p25 and its mixture with TiO2 NTs. Among them, hydrogen evolution over three-phase interfaces of TiO2 NTs/p25-H2 is much more efficient than those without oxygen vacancies. Therefore, it is not difficult to understand the component-dependent photoactivity of anatase/rutile/TiO2(B) heterophase junctions. When the ratio of TiO2 NTs and p25 was 10:3, assynthesized TiO2 NTs/p25-H2 possesses the highest hydrogen evolution rate (Figure S6a). It indicates that the photoactivity of junction is highly dependent with the ratio of different TiO2 phases, as higher amount of p25 leads to the increased ratio of rutile and decreased ratio of brookite phase (Figure S7). Moreover, temperature of heat-treatment shows obvious impact on the activity of photocatalysts. As shown in Figure S8, the concentration of oxygen vacancies gradually increases accompanied with the increase of temperature from 400 to 500℃. In the control experiments, 450 ℃ is determined to be the optimal temperature for the fabrication of high-performance three-phase junctions (Figure S6b).


13

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

Figure 6. (a) Photcatalytic hydrogen evolution of TiO2-based samples with oxygen vacancies; (b) Photocatalytic hydrogen evolution of different TiO2-based phase junctions. The possible reasons for the significantly improved photoactivity of TiO2 NTs/p25-H2 were deliberately considered. The hydrogen evolution is negligible under visible light irradiation, which demonstrates that the intense background absorption at wavelengths longer than 400 nm is not the origin of the enhanced photoactivity.18 One may expect that high specific surface area of nanotubular nanostructures can contribute to the reactivity of photocatalysts. However, according to the BET measurements, heat-treatment in hydrogen atmosphere results in the decrease of surface area from 267.8 m2/g to 156.6 m2/g (Figure S9). Thus, surface area is not the dominant reason for the improved photoactivity of catalysts. Oxygen vacancy, one of the most important defects in metal oxides, plays an important role in enhancing the photoactivity of anatase/rutile/TiO2(B) heterophase junctions. On one hand, oxygen vacancy can act as electron donor to improve the electron transfer in the heterostructured photocatalysts. It should be pointed out that anatase TiO2 possesses more positive conduction band than rutile and brookite.27,28 Thus, more electrons can migrate from rutile and brookite to anatase, as confirmed by the selective deposition of Pt nanoparticles on anatase nanoparticles (Figure S10).29 As a result, electrons accumulated in anatase can efficiently trigger the photoreduction of water into hydrogen. On the other hand, the formation of oxygen vacancies can result in the distortion of surface lattice, which benefits the transfer of electrons from the trapping sites to the photocatalytic reaction sites.18 To validate our postulate that the interfacial behavior of photogenerated charge carriers contributes to the better performance, EIS and Schottky measurements were carried out. The radius of the arcs of Nyquist plots is associated with the charge transfer process. Obviously, TiO2 NTs/p25-H2 with oxygen-deficient structure exhibits the smallest radium, suggesting less charge


14

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

transfer resistance and thus a more efficient separation of photogenerated charge carriers (Figure 7a).30 Mott-Schottky measurements were carried out to determine the donor density and flat band potential (Efb) at semiconductor/electrolyte interface. In Figure 7b, the generation of oxygen vacancies in TiO2 NTs/p25-H2 leads to an obvious negative shift of Efb. In principle, the upward shift of Efb indicates the higher band bending and the larger driving force for interfacial charge transfer. Therefore, much higher donor density is achieved in defective anatase/rutile/TiO2(B) heterophase junctions, which can be confirmed by the decreased slope of the Mott-Schottky plot.31 It is believed that the improved charge separation around heterophase interfaces and the decreased charge transfer resistance contribute to the photoactivity of catalysts (Figure S11). Therefore, anatase/rutile/TiO2(B) phase junctions with oxygen vacancy defects present the highest activity for photoreduction reaction to produce hydrogen.

Figure 7. (a) Nyquist spectra of TiO2 NTs, TiO2 NTs/p25 and TiO2/p25-H2; (b) Mott-Schottky plots of TiO2 NTs and TiO2 NTs/p25-H2. 4. Conclusion In summary, a new kind of hierarchical nanotubular anatase/rutile/TiO2(B) photocatalysts with high surface area and defective structure has been successfully fabricated through coupling TiO 2


15

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

nanotubes with p25 nanoparticles. The various physical and electrochemical characterizations demonstrate the significant contribution of oxygen vacancies to interfacial charge separation and transportation. As a result, as-synthesized oxygen-deficient heterophase junction exhibits drastically increased photoactivity for hydrogen evolution, which is 19 times higher than pristine TiO2 nanotubes. Our work provides a promising way to develop more efficient TiO2-based junctions for photcatalytic applications. ASSOCIATED CONTENT Supporting Information Additional SEM, TEM, XPS, BET, transient photocurrent characterizations, and data for photocatalytic reactions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Prof. Chengzhi Hu. E-mail: [email protected]. Tel.: +86 10 62918589 ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51578531, 51438011, 51538013). REFERENCES


16

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(1) Wang X.; Xu Q.; Li M.; Shen S.; Wang X.; Wang Y.; Feng Z.; Shi J.; Han H.; Li C. Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3. Angew. Chem. Int. Ed. 2012, 51, 13089. (2) Zhu Y.; Liu Y.; Lv Y.; Ling Q.; Liu D.; Zhu Y. Enhancement of photocatalytic activity for BiPO4 via phase junction. J. Mater. Chem. A 2014, 2, 13041. (3) Li A.; Wang Z.; Yin H.; Wang S.; Yan P.; Huang B.; Wang X.; Li R.; Zong X.; Han H.; Li C. Understanding the anatase–rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting. Chem. Sci. 2016, 7, 6076. (4) Yan P.; Wang X.; Zheng X.; Li R.; Han J.; Shi J.; Li A.; Gan Y.; Li C. Photovoltaic device based on TiO2 rutile/anatase phase junctions fabricated in coaxial nanorod arrays. Nano Energy 2015, 15, 406. (5) Li R.; Weng Y.; Zhou X.; Wang X.; Mi Y.; Chong R.; Han H.; Li C. Achieving overall water splitting using titanium dioxide-based photocatalysts of different phases. Energy Environ. Sci. 2015, 8, 2377. (6) Cai J.; Wang Y.; Zhu Y.; Wu M.; Zhang H.; Li X.; Jiang Z.; Meng M. Defect Engineering and Phase Junction Architecture of Wide-Bandgap ZnS for Conflicting Visible Light Activity in Photocatalytic H2 Evolution. ACS Appl. Mater. Interfaces 2015, 7, 13915. (7) Nowotny J.; Alim M.; Bak T.; Idris M.; Ionescu M.; Prince K.; Sahdan M.; Sopian K.; Teridie M.; Sigmund W. Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy Conversion. Chem. Soc. Rev. 2015, 44,8424.


17

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

(8) Chen X.; Liu L.; Yu P.; Mao S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746. (9) Tian M.; Mahjouri-Samani M.; Eres G.; Sachan R.; Yoon M.; Chisholm M.; Wang K.; Puretzky A.; Rouleau C.; Geohegan D.; Duscher G. Structure and Formation Mechanism of Black TiO2 Nanoparticles. ACS Nano 2015, 9, 10482. (10) An X.; Li T.; Wen B.; Tang J.; Hu Z.; Liu L.; Qu J.; Huang C.; Liu H. New Insights into Defect-Mediated Heterostructures for Photoelectrochemical Water Splitting. Adv. Energy Mater. 2016, 6, 1502268. (11) An X.; Zhang L.; Wen B.; Gu Z.; Liu L.; Qu J.; Liu H. Boosting photoelectrochemical activities of heterostructured photoanodes through interfacial modulation of oxygen vacancies. Nano Energy 2017, 35, 290. (12) Yang D.; Zhao J.; Liu H.; Zheng Z.; Adebajo M.; Wang H.; Liu X.; Zhang H.; Zhao J.; Bell J.; Zhu H. Enhancing Photoactivity of TiO2(B)/Anatase Core–Shell Nanofibers by Selectively Doping Cerium Ions into the TiO2(B) Core. Chem. Eur. J. 2013, 19, 5113. (13) Kaplan R.; Erjavec B.; Drazic G.; Grdadolnik J.; Pintar A. Simple synthesis of anatase/rutile/brookite TiO2 nanocomposite with superior mineralization potential for photocatalytic degradation of water pollutants. Appl. Catal. B-Environ. 2016, 181, 465. (14) Liu Y.; Wang Z.; Wang W.; Huang W. Engineering highly active TiO2 photocatalysts via the surface-phase junction strategy employing a titanate nanotube precursor. J. Catal. 2014, 310, 16.


18

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(15) Liu Y.; Wang Z.; Wang W.; Huang W. Engineering highly active TiO2 photocatalysts via the surface-phase junction strategy employing a titanate nanotube precursor. J. Catal. 2014, 310, 16. (16) Wang Z.; Yang C.; Lin T.; Yin H.; Chen P.; Wan D.; Xu F.; Huang F.; Lin J.; Xie X.; Jiang M. H-Doped Black Titania with Very High Solar Absorption and Excellent Photocatalysis Enhanced by Localized Surface Plasmon Resonance. Adv. Funct. Mater. 2013, 23, 5444. (17) Yang Y.; Liu G.; Irvine J.; Cheng H. Enhanced Photocatalytic H2 Production in Core–Shell Engineered Rutile TiO2. Adv. Mater. 2016, 28, 5850. (18) Zhang K.; Wang L.; Kim J.; Ma M.; Veerappan G.; Lee C.; Kong K.; Lee H.; Park J. An order/disorder/water junction system for highly efficient co-catalyst-free photocatalytic hydrogen generation. Energy Environ. Sci. 2016, 9, 499. (19) Wang Z.; Yang C.; Lin T.; Yin H.; Chen P.; Wan D.; Xu F.; Huang F.; Lin J.; Xie X.; Jiang M. Visible-light photocatalytic, solar thermal and photoelectrochemical properties of aluminiumreduced black titania. Energy Environ. Sci., 2013, 6, 3007. (20) Lü X.; Chen A.; Luo Y.; Lu P.; Dai Y.; Enriquez E.; Dowden P.; Xu H.; Kotula P.; Azad A.; Yarotski D.; Prasankumar R.; Taylor A.; Thompson J.; Jia Q. Conducting Interface in Oxide Homojunction: Understanding of Superior Properties in Black TiO2. Nano Lett. 2016, 16, 5751. (21) Wang C.; Zhang X.; Liu Y. Coexistence of an anatase/TiO2(B) heterojunction and an exposed (001) facet in TiO2 nanoribbon photocatalysts synthesized via a fluorine-free route and topotactic transformation. Nanoscale 2014, 6, 5329.


19

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

(22) Wang C.; Zhang X.; Liu Y. Coexistence of an anatase/TiO2(B) heterojunction and an exposed (001) facet in TiO2 nanoribbon photocatalysts synthesized via a fluorine-free route and topotactic transformation. Nanoscale, 2014, 6, 5329. (23) Yang C.; Wang Z.; Lin T.; Yin H.; Lü X.; Wan D.; Xu T.; Zheng C.; Lin J.; Huang F.; Xie X.; Jiang M. Core-Shell Nanostructured “Black” Rutile Titania as Excellent Catalyst for Hydrogen Production Enhanced by Sulfur Doping. J. Am. Chem. Soc. 2013, 135, 17831. (24) Sinhamahapatra M.; Jeon J.; Yu J. A new approach to prepare highly active and stable black titania for visible light-assisted hydrogen production. Energy Environ. Sci. 2015, 8, 3539. (25) Wang S.; Zhao L.; Bai L.; Yan J.; Jiang Q.; Lian J. Enhancing photocatalytic activity of disorderengineered C/TiO2 and TiO2 nanoparticles. J. Mater. Chem. A 2014, 2, 7439. (26) Su T.; Yang Y.; Na Y.; Fan R.; Li L.; Wei L.; Yang B.; Cao W. An Insight into the Role of Oxygen Vacancy in Hydrogenated TiO2 Nanocrystals in the Performance of Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 3754. (27) C. J.; Wang Y.; Zhu Y.; Wu M.; Zhang Hao.; Li X.; Jiang Z.; Meng M. In Situ Formation of Disorder-Engineered TiO2(B)-Anatase Heterophase Junction for Enhanced Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 24987. (28) Scanlon D.; Dunnill C.; Buckeridge J.; Shevlin S.; Logsdail A.; Woodley S.; Catlow R.; Powell M.; Palgrave R.; Parkin I.; Watson G.; Keal T.; Sherwood P.; Walsh A.; Sokol A. Band alignment of rutile and anatase TiO2. Nat. Mater. 2013, 12, 798.


20

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(29) Kaplan R.; Erjavec B.; Drazic G.; Grdadolnik J.; Pintar A. Simple synthesis of anatase/rutile/brookite TiO2nanocomposite with superior mineralization potential for photocatalytic degradation of water pollutants. Appl. Catal. B-Environ. 2016, 181, 465. (30) Mao C.; Zuo F.; Hou Y.; Bu X.; Feng P. In Situ Preparation of a Ti3+ Self-Doped TiO2 Film with Enhanced Activity as Photoanode by N2H4 Reduction. Angew. Chem. Int. Ed. 2014, 53, 10485. (31) Yu X.; Du R.; Li B.; Zhang Y.; Liu H.; Qu J.; An X. Biomolecule-assisted self-assembly of CdS/MoS2/graphene hollow spheres as high-efficiency photocatalysts for hydrogen evolution without noble metals. Appl. Catal. B-Environ. 2016, 182, 504.

Table of Contents


21

TiO2(B) Heterophase

Recommend Documents