Accepted Manuscript Title: Hydrogenated TiO2 nanobelts as highly efficient photocatalytic organic dye degradation and hydrogen evolution photocatalyst Author: Jian Tian Yanhua Leng Hongzhi Cui Hong Liu PII: DOI: Reference:

S0304-3894(15)00468-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.06.019 HAZMAT 16874

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

11-4-2015 7-6-2015 9-6-2015

Please cite this article as: Jian Tian, Yanhua Leng, Hongzhi Cui, Hong Liu, Hydrogenated TiO2 nanobelts as highly efficient photocatalytic organic dye degradation and hydrogen evolution photocatalyst, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.06.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrogenated TiO2 nanobelts as highly efficient photocatalytic organic dye degradation and hydrogen evolution photocatalyst Jian Tiana,b, Yanhua Lengb, Hongzhi Cui*a, Hong Liu*b a

School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China. Email: [email protected]

b

State key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China. Email: Highlights ►

[email protected] Highlights

A facile synthesis of hydrogenated TiO2 nanobelts is reported.

Utilizing UV and visible light in photocatalytic degradation and H2 production.

The improved photocatalytic property is owe to Ti3+ ions and oxygen vacancies. Graphical abstractTOC Graph

Abstract

TiO2 nanobelts have gained increasing interest because of its outstanding properties and promising applications in a wide range of fields. Here we report the facile

synthesis of hydrogenated TiO2 (H-TiO2) nanobelts, which exhibit excellent UV and visible photocatalytic decomposing of methyl orange (MO) and water splitting for hydrogen production. The improved photocatalytic property can be attributed to the Ti3+ ions and oxygen vacancies in TiO2 nanobelts created by hydrogenation. Ti3+ ions and oxygen vacancies can enhance visible light absorption, promote charge carrier trapping, and hinder the photogenerated electron-hole recombination. This work offers a simple strategy for the fabrication of a wide solar spectrum of active photocatalysts,

which

possesses

significant

potential

for

more

efficient

photodegradation, photocatalytic water splitting, and enhanced solar cells using sunlight as light source.

Keywords: Hydrogenation; TiO2 nanobelts; Photocatalytic; Hydrogen production; UV-visible 1. Introduction Search for efficient semiconductor photocatalysts utilizing solar energy for degradation of organic contaminants and water-splitting for hydrogen production remains a great challenge [1], [2], [3] , [4] and [5]. Titanium dioxide (TiO2) is regarded as a suitable photocatalyst due to its unique physical and chemical properties, which has attracted much scientific interest and has been widely applied in photocatalysis, solar cells, water splitting, and antibacterial applications [6] and [7]. However, the photoconversion efficiency of TiO2 for photocatalysis is very low due to its wide band gap of TiO2 (3.2 eV for anatase, and 3.0 eV for rutile), which makes it

be activated solely by UV light [8]. UV light makes up only 5% of the total incoming solar radiation [9]. Hence, numerous efforts have been attempted to enhance visible light (43% of total solar energy) absorption through different approaches, including metal or nonmetal element doping, controlling the structures and facets, and introducing defects into nanocrystals [10]. Although these methods have been improved to some degree, their visible light absorption and power conversion still remain insufficient to meet the demand of practical applications [11]. Very recently, Chen and co-workers [12] have demonstrated a novel approach to extend the absorption spectrum of TiO2 crystals into the visible light region through hydrogenation, which allows expansion of their applications into areas beyond photocatalysis, lithium ion batteries, and solar cells [13] and [14]. Hydrogenation of the surface of TiO2 can introduce surface disorder and oxygen vacancies, which lead to the creation of Ti3+ centers or unpaired electrons, and subsequently form donor levels in the electronic structure of TiO2 [15]. Oxygen vacancies are believed to suppress the recombination of photogenerated electrons and holes, thus improving the photocatalytic activity of TiO2 [16]. Among various TiO2 nanostructures, one-dimensional TiO2 nanobelts have low number of grain boundaries, fast charge transfer dynamics and high specific surface area [17]. Therefore, they are quite suitable for photocatalysis applications. However, two bottlenecks hinder the applications of TiO2 nanobelts as photocatalysts. One is the high recombination rate of the photoinduced electron-hole pairs because of the single-phase nanobelt structure, and the other is that pure TiO2 can only absorb UV

light due to its large band gap, which prevents its widespread applications. In this work, the TiO2 nanobelts were subject to a successive hydrogenation under an atmospheric H2/Ar (1:1) flow at 600-1000oC, which resulted in hydrogenated TiO2 (H-TiO2) nanobelts with disordered surface and varying colors. The H-TiO2 nanobelts showed significantly improved UV and visible photocatalytic degradation of methyl orange (MO) and water-splitting for hydrogen production in comparison with P25 and pristine TiO2 nanobelts. Through experiment results and theoretical calculations, Ti3+ and oxygen vacancies produced by hydrogenation could generate a new vacancy below its conduction band, reducing the band gap and enhancing the visible light absorption without the recombination effect from doped impurities, thus the photocatalytic properties are improved. Our results suggest a simple and practical method to improve the photocatalytic activities of one-dimensional TiO2 nanomaterials. 2. Experimental Section 2.1. Materials The chemicals used in this work were of analytical reagent grade. Titania P25 (TiO2; ca. 80 % anatase, and 20 % rutile), sodium hydroxide (NaOH), hydrochloric acid (HCl), and sulfuric acid (H2SO4) were purchased from Sinopharm and used without further treatment. 2.2. Preparation of TiO2 nanobelts TiO2 nanobelts were synthesized by a hydrothermal procedure. Typically, P25 powder (0.1 g) was mixed with an aqueous solution of NaOH (20 mL, 10 M),

followed by a hydrothermal treatment at 180 oC in a 25 mL Teflon-lined autoclave for 48 h. The treated powder was washed thoroughly with deionized water followed by filtration and drying processes. The obtained Na2Ti3O7 nanobelts were then immersed in an aqueous solution of 0.1 M HCl for 48 h and then washed thoroughly with water to produce H2Ti3O7 nanobelts. The H2Ti3O7 nanobelts were added into a 25 mL Teflon vessel, which was filled with an aqueous solution of H2SO4 (0.02 M) up to 80% of the total volume and maintained at 100 oC for 12 h. Finally, the products were isolated from the solution by centrifugation and sequentially washed with deionized water several times, and dried at 70 oC for 10 h. Thermal annealing of the H2Ti3O7 nanobelts by acid corrosion at 600 oC for 2 h led to the production of TiO2 nanobelts with roughened surfaces. 2.3. Preparation of hydrogenated TiO2 nanobelts. The hydrogenated TiO2 (H-TiO2) nanobelts were obtained by annealing the TiO2 nanobelts in hydrogen atmosphere at temperature at 600 °C, 800 °C, and 1000 °C for 120 min (samples are denoted as H600, H800 and H1000). Thermal treatment was performed in tube furnace filled with hydrogen and argon gas mixtures (1:1). 2.4. Characterization X-ray powder diffraction (XRD) pattern of catalysts were recorded on a Bruke D8 Advance powder X-ray diffractometer with Cu Kα (λ = 0.15406 nm). HITACHI S-4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphologies and size of the synthesized samples. The chemical composition was investigated via energy-dispersive X-ray spectroscopy (EDS). High

resolution transmission electron microscopy (HRTEM) images were carried out with a JOEL JEM 2100 microscope. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250. Fourier transform infrared (FTIR) spectra were collected on a Nicolet Avatar 370 infrared spectrometer in the range 400-4000 cm−1 using pressed KBr discs. The KBr disks were formed by mixing 10 mg of each sample with 1000 mg of KBr in an agate mortar. From this stock, 200 mg were then pressed into pellets of 13 mm diameter. UV-Vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV-Vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment within the range of 200 to 900 nm and with BaSO4 as the reflectance standard. 2.5. Photocatalytic activity test The photocatalytic activity of the H-TiO2 nanobelts was investigated by the photodegradation of methyl orange (MO, 20 mg/L) and photocatalytic hydrogen evolution. In a typical photodegradation experiment, 20 mL aqueous suspension of MO and 20 mg of photocatalyst powder were placed in a 50 mL beaker and were conducted in an XPA-photochemical reactor (Xujiang Electromech-anical Plant, Nanjing, China). Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium between the dye and the surface of the catalyst under normal atmospheric conditions. A 350 W mercury lamp with a maximum emission at 356 nm was used as the UV source for photocatalysis. A 300 W Xe arc lamp was used as the visible light source with filter glasses to filter UV light for visible light photocatalysis. At given irradiation time

intervals, aliquots of the mixed solution were collected and centrifuged to remove the catalyst particulates for analysis. The residual MO concentration was detected using a UV-Vis spectrophotometer (Hitachi UV-3100). In a typical photocatalytic hydrogen evolution experiment, all samples were loaded with 1 wt% platinum by a photo-deposition process before photocatalytic production of H2. Briefly, samples and chloroplatinic acid (H2PtCl6•6H2O, c=10 g/L) were suspended in 20:80 v/v mixture of ethanol:water, and were irradiated with a Mercury lamp (300 W) for 30 min. The catalyst (0.05 g) was suspended in 100 mL aqueous solution containing methanol (20% v/v). The reaction temperature was maintained at 5 oC. A 300 W Xe arc lamp (CEL-HXF300, Beijing Aulight Co. Ltd.) was used as the light source. The amount of H2 evolved was determined with a gas chromatograph (Techcomp GC7900) equipped with thermal conductivity detector (TCD). 3. Results and Discussion

Fig. 1. XRD spectra of (a) TiO2 nanobelts and H-TiO2 nanobelts prepared at

hydrogenation temperature of (b) 600 oC, (c) 800 oC, and (d) 1000 oC.

To determine the crystal structures and possible phase changes during the hydrogenation process, X-ray diffraction (XRD) spectra were collected from the pristine TiO2 nanobelts and H-TiO2 nanobelts prepared at various hydrogenation temperatures, as shown in Fig. 1. Strong XRD diffraction peaks at 2θ = 25.28°, 36.95°, 37.80°, 38.58°, 48.05°, 53.89°, 55.06°, 62.69°, 68.78°, 70.31°, 75.03° and 76.02° in curve a are assigned to the (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), (215) and (301) faces of anatase TiO2 (PDF-#21-1272) [18]. After hydrogenation at 600 oC and 800 oC (curve b and c), there is no crystal-phase change, although the TiO2 peak intensity decreases with the increase of hydrogenation temperature. This may be due to the increase of the amount of oxygen vacancies in the TiO2 structure, resulting disorder-induced lattice strains and slightly reduced crystal size [19]. For the XRD of H600 and H800 samples (curve b and c), a slight diffraction peak at 23.82° is observed. This peak can be perfectly indexed to the (012) crystal planes of Ti2O3 (JCPDS 10-0063) [20]. Ti3+ is known to form in H-TiO2 nanobelts. Under thermal hydrogenation, hydrogen atoms interact dramatically with lattice oxygen on the surface of TiO2 nanobelts, the electrons of the H atoms are transferred to the Ti4+ of TiO2, and Ti3+ defects are formed [21]. When the hydrogenation temperature is higher than 1000 oC (curve d), most of the anatase is transformed into the rutile phase. The peaks at 2θ = 27.91°, 36.43°, 39.88°, 41.72°, 44.83°, 55.12°, 57.68°, 63.20° and 66.58° are attributed to the (110), (101), (200), (111), (210), (211), (220), (002) and (221) faces of rutile phases (JCPDS 88-1175), which is the main composition in the sample. Moreover, the Ti2O3 peak at 23.82° is

still found in H-1000 samples.

Fig. 2. SEM images of (a) TiO2 nanobelts and H-TiO2 nanobelts prepared at hydrogenation temperature of (b) 600 oC, (c) 800 oC, and (d) 1000 oC. The morphology and structural changes of the samples was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. Fig. 2a shows a typical SEM image of the TiO2 nanobelts, which are 50-200 nm in width, 20-40 nm in thickness, and have a belt-like structure. In comparison, there is no obvious morphological change for H-600 samples (Fig. 2b), suggesting that there is no structural change between TiO2 nanobelts and H-TiO2 nanobelts. However, the color of the nanobelt powder changes from white (Fig. 3a) to gray (Fig. 3b) after hydrogenation. These results clearly reveal the structure stability of TiO2 nanobelts and successful surface disordered engineering of high crystalline anatase nanobelts after the hydrogenation treatment. Hydrogenation treatment at 800 °C does not destroy the morphologies of the TiO2 nanobelts (Fig. 2c). However, H-800 samples’ surface becomes smooth and the zigzag contour on the TiO2 nanobelts is observed. At 1000 °C, the morphologies of H-1000 samples are destroyed,

which sinter together to form a dendritic morphology with a smooth surface (Fig. 2d).

Fig. 3. Optical photograph of (a) TiO2 nanobelts and H-TiO2 nanobelts prepared at hydrogenation temperature of (b) 600 oC, (c) 800 oC, and (d) 1000 oC. With the increase of hydrogenation temperature from 600 oC to 1000 oC (Fig. 3b-d), the color changes from gray, blue to black. This color remains stable over time with exposure of the nanobelts to air or water. The change in color of the powder indicates that the hydrogenation process occurs at the powder’s surface and hence a surface oxide is being produced. The blue and dark coloration of the samples is due to the formation of Ti3+ ions by electron trapping at the Ti4+ centers. Fig. S1 displays the energy dispersive X-ray spectroscopy (EDS) patterns of H-TiO2 nanobelts. In addition to the Ti and O peaks resulting from TiO2, no other impurities can be clearly observed.

Fig. 4. (a) Low-magnification and (b) high-magnification TEM images of TiO2 nanobelts; (c) low-magnification and (d) high-magnification TEM images of H-600 samples. In order to get the detailed information on the structural changes in the H-TiO2 nanobelts, we performed TEM and HRTEM studies of TiO2 nanobelts and H-TiO2 nanobelts. The TEM image of TiO2 nanobelts shows that the crystal has a diameter about 200 nm (Fig. 4a), which agrees well with the above SEM result (Fig. 2a). The pristine TiO2 nanobelts are very highly crystallized, displaying clearly resolved and well-defined lattice fringes even on the surface of the nanocrystal, which are shown in the HRTEM image (Fig. 4b). The interplanar spacing of 0.23 nm and 0.35 nm corresponds to the (004) and (101) crystal planes of anatase TiO2 [22] and [23]. After hydrogenation, we note that the H-TiO2 nanobelts retain the one-dimensional morphology (Fig. 4c). However, the H-TiO2 nanobelts have a hydrogen-stabilized

amorphous layer surrounding a crystalline core to form a unique amorphous shell/crystalline core structure (Fig. 4d), which is not observed on the surface of the pristine TiO2 nanobelt. The amorphous surface layer with an average thickness of about 8 nm is coated on the crystalline core after the hydrogenation treatment at 600 o

C for 2 h. The amorphous surface layer of the H-TiO2 nanobelt is probably ascribed

to the effect of hydrogenation treatment, in which Ti3+ ions are generated near the surface. Even at the very inner surface of the H-TiO2 nanobelts, the crystal lattice features are still highly ordered and the well-resolved (101) lattice fringes (d=0.35 nm) can be seen (Fig. 4d). X-ray photoelectron spectroscopy (XPS) is a powerful tool to examine the hydrogenation on the chemical composition and oxidation state of TiO2, and understand the oxygen vacancies in the samples. XPS spectra of pure TiO2 nanobelts (Fig. S2) show that only the characteristic peaks attributed to Ti4+ are observed. Fig. 5 displays the Ti2p XPS spectra of H-TiO2 nanobelts. The main two broad peaks of Ti2p3/2 located at 458.8 eV and Ti2p1/2 located at 464.5 eV are assigned to Ti4+ in TiO2 [24]. In addition, two small peaks centered at 457.4 eV and 462.8 eV that correspond to the characteristic Ti 2p3/2 and Ti 2p1/2 peaks of Ti3+ are observed [25], indicating the presence of Ti3+ species on the H-TiO2 nanobelts. The Ti3+/Ti4+ ratio is calculated to be approximately 11.1:89.9 from the peak areas, and the concentration of oxygen vacancies is calculated to be about 3%. The 36-atom super cell of H-600 samples has approximately one O vacancy.

Fig. 5. Ti 2p XPS spectra of H-600 samples.

Fig. 6. UV-Vis diffuse reflectance spectra of TiO2 nanobelts and H-TiO2 nanobelts prepared at hydrogenation temperature of 600 oC, 800 oC, and 1000 oC. UV-Vis diffuse reflectance spectra of all the samples were measured to investigate the light absorption characteristics of as-prepared photocatalysts. Fig. 6 shows UV-Vis diffuse reflectance spectra of the TiO2 nanobelts and H-TiO2 nanobelts prepared at various hydrogenation temperatures. TiO2 nanobelts show strong

absorption in the UV region, and the absorption wavelength is approximately 380 nm (black curve). The hydrogenation has little effect on the light absorption ability of TiO2 nanobelts in the UV region. However, compared to pristine TiO2 nanobelts, the H-TiO2 nanobelts prepared at hydrogenation temperature of 600-1000 oC exhibit a high and broad visible light absorption. The strong absorbance in the visible region is attributed to the generation of Ti3+ ions. The low-energy photon of trapped electrons in localized states of oxygen vacancy associated with Ti3+ just below the conduction band, resulting in the strong response in the visible region. The absorption of visible light indicates that more electrons and holes in the H-TiO2 nanobelts can be generated under visible light irradiation, leading to the enhancement of photocatalytic properties. The UV-Vis diffuse reflectance spectra of H-TiO2 nanobelts prepared at various hydrogenation temperatures give more interesting information. It is clear that the visible absorption efficiency firstly increases to a high level with the increase of the hydrogenation temperature from 600 oC to 800 oC. This can be attributed to the formation of more oxygen vacancies in the higher hydrogenation temperature, which promote the stronger visible absorption. Then, the visible absorption ability decreases gradually when the hydrogenation temperature increases from 800 oC to 1000 oC due to the destruction of structure and change of crystal phase. The absorption edge of TiO2 nanobelts, H-600, H-800, and H-1000 is determined to be 393 nm, 431 nm, 450 nm, and 408 nm, respectively, according to Fig. 6. Therefore, their respective bandgap energy is evaluated to be 3.15 eV, 2.88 eV, 2.76

eV, and 3.04 eV, respectively. We tested the photocatalytic activity of the H-TiO2 nanobelts in the degradation of methylene orange (MO), a standard dye employed for photocatalytic studies in the literature, under UV and visible light irradiation (Fig. 7). For comparison, P25 and TiO2 nanobelts were used as a photocatalytic references to measure under the same experimental conditions. Before the photocatalysis, (1) the solution including MO and photocatalysts was stirred in the dark for the adsorption equilibrium (Fig. 7), and (2) with UV and visible light irradiation in the absence of the photocatalysts (Fig. S3). These results highlight that the samples by themselves exhibit no catalytic activity or absorption on MO in the dark. Furthermore, only a negligible amount of the MO is degraded after 80 min under these conditions. FTIR spectra indicate that the MO de-colorization is due to the photocatalytic reaction of photocatalysts rather than adsorption (Fig. S4). The H-TiO2 nanobelts exhibit an enhanced photodegradation efficiency under UV and visible light irradiation (Fig. 7).

Fig. 7. Photocatalytic degradation of MO in the presence of P25, TiO2 nanobelts, and H-600 samples under (a) visible and (b) UV light irradiation. As shown in Fig. 7a, pristine TiO2 nanobelts and P25 show little ability in decomposing MO under visible light irradiation for 25 min. In contrast, the

photocatalytic efficiency of H-TiO2 nanobelts in the degradation of MO (61.9%) is higher than that of P25 (25%) and pristine TiO2 nanobelts (17%). This is because H-TiO2 nanobelts have a narrow band gap and exhibit enhanced visible light absorption in the region of 400-800 nm (Fig. 6). It is evident from the results that the H-TiO2 nanobelts can absorb more visible light than pristine TiO2 nanobelts, and thus display better visible photocatalytic activity. As shown in Fig. 7b, the H-TiO2 nanobelts exhibit obviously improved UV light photocatalytic activity, which degrade 84% of MO after UV light irradiation for 25 min, whereas P25 and pristine TiO2 nanobelts degrade only 62% and 60% of MO, respectively. Thus, the enhancement of photoactivity under UV light is mainly due to the existence of oxygen vacancies and Ti3+ created by hydrogenation. The role of Ti3+ species can serve as hole scavengers. Also, oxygen vacancies on the surface of TiO2 nanobelt would act as O2 binding sites and electron scavengers, thus favoring the photogenerated electron-hole pair separation and remarkably improving the UV photocatalytic activity of TiO2 nanobelt.

Fig. 8. Photocatalytic degradation of MO in the presence of H-TiO2 nanobelts

prepared at hydrogenation temperature of 600 oC, 800 oC, and 1000 oC under (a) visible and (b) UV light irradiation.

We also tested the UV and visible photocatalytic activities of H-TiO2 nanobelts prepared at different hydrogenation temperatures, and the results are shown in Fig. 8. The samples of H-600 and H-800 have the similar photocatalytic activities. On the one hand, H-800 samples exhibit better UV and visible light absorption than that of H-600 samples (Fig. 6), which can help to improve the photocatalytic activity of H-800 samples. On the other hand, H-600 samples’ surface is coarser that than of H-800 samples, the coarse surface can absorb more pollution molecules and enhance the photocatalytic properties. On the average, such reasons balance out, the similar photocatalytic activities for the samples of H-600 and H-800 can be observed. Compared with the samples of H-600 and H-800, H-1000 samples exhibit the worst photocatalytic activities. Firstly, the H-1000 samples have the lowest light absorption. Secondly, the belt-like structure of H-1000 samples has been destroyed and make the sample present smaller specific surface area.

Fig. 9. Photocatalytic hydrogen generation of 1 wt% Pt loaded TiO2 nanobelts and

H-600 samples under Xe arc lamp (300 W).

To further demonstrate the enhanced photocatalytic property of the H-TiO2 nanobelts, hydrogen evolution was measured for the photocatalytic water-splitting process (Fig. 9). For the TiO2 nanobelts, the rate of H2 production is only 0.65 mmol h−1g−1, because TiO2 nanobelts can only absorb UV light. However, the H-TiO2 nanobelts display the higher photocatalytic activity, and the rate of H2 production is about 6.01 mmol h−1g−1. The enhanced photocatalytic H2 production activities can be attributed to the efficient UV and visible light absorption of H-TiO2 nanobelts. These results have verified that hydrogenation process could effectively promote the photocatalytic activities of TiO2 nanobelts. We also measured the photocatalytic hydrogen generation of H-TiO2 nanobelts prepared at different hydrogenation temperature (Fig. S5). The H-TiO2 nanobelts prepared at hydrogenation temperature of 600 oC, 800 oC and 1000 oC display the high photocatalytic activity, and the rate of H2 production is about 6.01 mmol h−1g−1, 6.32 mmol h−1g−1 and 5.64 mmol h−1g−1, respectively. H-800 samples have better photocatalytic H2 production activities than that of H-600 samples, because H-800 samples exhibit better UV and visible light absorption. Compared with the samples of H-600 and H-800, H-1000 samples exhibit the worst photocatalytic H2 production activities, which can be attributed to the lowest light absorption and the smallest specific surface area. Density functional theory (DFT) simulations show that oxygen vacancies in the TiO2 crystalline structure produce visible absorption due to the formation of additional electronic states below the conduction band (CB) of TiO2 (Fig. 10). In this

case, the gray or black color of H-TiO2 nanobelts with the substantial enhancement of visible light absorption might be attributed to the transitions from the TiO2 valence band (VB) to these additional electronic states or from these additional electronic states to the TiO2 CB (Scheme 1). The dotted green line represent the Fermi level, which is set to 0 for both conditions shown in Fig. 11. The density of states (DOS) of pristine TiO2 shows that the calculated band gap is approximately 2.17 eV (Fig. 11a). However, for the TiO2 with O vacancies (Fig. 11b), two gap states at E = -0.79 eV and -0.57 eV below the Fermi level are induced. The theoretical calculation of absorption spectrum of H-TiO2 nanobelts matches well with the experimental spectrum (Fig. 12).

Fig. 10. Band structures of (a) pure TiO2 and (b) TiO2 with one O vacancy.

Fig. 11. Density of states (DOS) of (a) pure TiO2 and (b) TiO2 with one O vacancy.

Fig. 12. Theoretical light absorption spectrum of TiO2 with one O vacancy. As shown in Scheme 1, there will be two typical modes of photocatalysis in H-TiO2 nanobelts under UV and visible light irradiation. Firstly, under UV light irradiation, the strong transitions still exist at the pristine band gap of roughly 462 nm. H-TiO2 nanobelts can absorb sufficient UV light, and photogenerated electron-hole pairs are formed, which make the H-TiO2 nanobelts have excellent UV light photocatalytic properties. On the other hand, when exposed to visible light, Ti3+ ions in the H-TiO2 nanobelts form sublevel states below the CB of TiO2, which reduce the band gap of TiO2 and make the electrons of H-TiO2 nanobelts can be excited by the

visible light. The oxygen vacancies also promote visible light adsorption and enhance visible photocatalytic activity.

Scheme 1. Schematic diagram of a proposed photocatalytic mechanism of H-TiO2 nanobelts. 4. Conclusions In conclusion, the hydrogenated TiO2 (H-TiO2) nanobelts have been synthesized under the H2 atmosphere in the temperature range of 600-1000oC. The change in crystal phase, band gap, valence change and oxygen vacancies are closely related to the hydrogenation. The H-TiO2 nanobelts possess a remarkable visible absorption, which is attributed to the reduced band gap created by oxygen vacancies and Ti3+ ions after hydrogenation. Oxygen vacancies on the surface of H-TiO2 nanobelts, associating with Ti3+ sites as electron and hole scavengers can also lead to the separation of charge carriers, which is responsible for the enhanced UV and visible light photocatalytic activities and water splitting for H2 production. The findings of this work provide a guideline for the development of TiO2 materials as photocatalysts for water splitting under broad spectrum (UV and visible light) irradiation as well as for other solar energy conversion applications.

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Hydrogenated TiO2 nanobelts as highly efficient photocatalytic organic dye degradation and hydrogen evolution photocatalyst.

TiO2 nanobelts have gained increasing interest because of its outstanding properties and promising applications in a wide range of fields. Here we rep...
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