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Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting
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Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting Qianqian Hu a,b, Jiquan Huang a,*, Qiaohong Li a, Chong Wang a, Guojing Li a,b, Jian Chen a, Yongge Cao c a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China b University of the Chinese Academy of Sciences, Beijing 100039, China c Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China
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abstract
Article history:
We report for the first time the experimental realization of anatase TiO2 film comprised
Received 25 November 2015
mainly of nanorods exposed with dominant {110} lateral surfaces growing on the quartz glass
Received in revised form
substrate. Based on microstructure and morphology analysis, it is found that, the average
24 June 2016
diameter of nanorods is less than 40 nm, which is favorable for the effective migration and
Accepted 26 July 2016
separation of photogenerated carriers, while their lateral {110} facets contain abundant un-
Available online xxx
saturated 4c-Ti atoms which act as active sites for water splitting. The H2 generation rate by
Keywords:
without the assistance of metal cathode, bias or loading of co-catalyst, which is about three
TiO2 nanorods
orders of magnitude higher than that of anatase TiO2 nanoparticles film, and even higher
this bare anatase TiO2 nanorods film is 8.4 mmol g1 h1 under full-arc light irradiation
Preferred orientation
than the reported values of some noble metal loaded TiO2 photocatalysts. This work ex-
Efficient water splitting
emplifies that the controlled growth of photocatalysts with specified active facets and desired architectures is an effective strategy to achieve excellent photocatalytic properties. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Direct photocatalytic water splitting for hydrogen generation has been widely considered as a promising solution for the ever-growing global energy crisis and environmental pollution. On the past, much research focuses on the development of various semiconductor photocatalysts. Nowadays, hundreds of materials have been characterized in terms of their ability to split water [1e3], nevertheless, none of them is deemed to be free of limitations such as low conversion
efficiency, low chemical stability, and environmental impact in their manufacture and/or use [3e6]. Therefore, except for developing new kinds of photocatalysts, it is also essential to overcome the limitations of the existing photocatalysts to enhance their catalytic activity. Among all the discovered photocatalysts for water splitting, anatase TiO2 is one of the most prominent candidates due to its superior catalytic activity, high corrosion resistance, low cost, and nontoxicity [2,7,8]. However, bare TiO2 is nearly not reactive for splitting water into H2 and O2, and the energy conversion efficiency from incident light to generated H2, even
* Corresponding author. Fax: þ86 591 83721039. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.200 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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considering only the irradiation of UV light, is still very low, mainly due to the easy recombination of photoexcited electronehole pairs, and the quick back-reaction of evolved H2 and O2 into water [2,7]. The mostly adopted methods to avoid these drawbacks are chemical modifications such as introduction of cocatalyst, particularly noble metals. It has been widely reported that the loading of noble metals such as Pt and Pd on the surface of TiO2 photocatalyst can suppress the electronehole recombination and the back-reaction and thus greatly enhance the hydrogen generation efficiency [9]. However, noble metals are rare and expensive. And moreover, the performance of noble metal loaded TiO2 is still unsatisfactory. Another strategy is the morphological modification. In this respect, researchers usually focus on the increase of surface area and porosity rather than the tailor of specific nanostructure exposed with optimal crystallographic plane [10,11]. It is well-known that photocatalytic ability of TiO2 crystals depends greatly on its crystallinity and morphology (e.g., the exposed facets, particle size and shape). In general, different crystallographic orientations exhibit distinctive surface atomic arrangement and electronic structure [12,13], and consequently, the photocatalytic activity of anatase TiO2 crystals can be greatly enhanced through increasing the proportion of highenergy surfaces, such as {001}, {100}, {211}, and {111} surfaces [14e17]. Besides, one-dimensional (1-D) nanostructures, such as nanowires, nanorods, and nanotubes array, may exhibit higher photocatalytic activity than particles network, due to their enhanced charge transport and other characterizations concerned with the particular structure with high single crystallinity [18e21]. Therefore, nanostructured photocatalysts with well-defined morphology is expected to be a future trend for photocatalytic application [22]. Herein, we fabricate anatase TiO2 film mainly comprised of nanorods exposed with dominant {110} lateral surfaces growing on the quartz glass substrate, and investigate its water splitting ability by performing measurements directly in an aqueous methanol solution under the irradiation of Xe lamp. It is found that the H2 evolution rate is higher than 8 mmol g1 h1, which is about three orders of magnitude higher than that of the anatase TiO2 film composed of quasispherical nanoparticles, as well as the commonly reported values for bare TiO2 [12,23e25]. This experimental result demonstrates that bare anatase TiO2 without chemical modification can also be an efficient photocatalyst for hydrogen generation by controlling its morphology and preferential growth orientation. It is also worthy to note that, up to date, the synthesis and photocatalysis characterization of anatase TiO2 with dominant {110} surfaces have rarely been reported as it is difficult to be synthesized resulting from the super-high surface energy.
Experimental section Fabrication of TiO2 thin films TiO2 thin films were deposited on quartz glass substrates (2 cm 4 cm) by RF reactive magnetron sputtering. The working gas was a mixture of O2 (99.99% pure) and Ar (99.999%
pure). The target was a 2-inches metallic plate of Ti (99.999% pure) and the base vacuum was 4.9 103 Pa. Before the deposition, the target was pre-sputtered by argon plasma for 20 min. For all the samples, the total working pressure, O2 flow rate, and Ar flow rate, were set at 0.3 Pa, 6 sccm (standard cubic centimeter per minute) and 40 sccm respectively. During the deposition process, the substrate was rotated at 8 rotations per minute along its axis. Four samples, namely, T1, T2, T3 and T4, were deposited for 4, 4, 5, and 4 h, with the targetsubstrate distances of about 120, 110, 130, and 120 mm, respectively. The RF power was set at 140 W for T1 and T4, and 130 W for T2 and T3. The temperature of the substrates was maintained at 450 C during the whole deposition processes for T2 e T4, while T1 was deposited at room temperature and then annealed in situ at 450 C for 4 h to transform the amorphous TiO2 into crystalline anatase phase.
Characterizations of TiO2 thin films The crystalline structure of the TiO2 thin films was identified by X-ray Diffraction (XRD) (Miniflex600, Rigaku, Japan), and the morphology were characterized by Scanning Electron Microscopy (SEM) (SU8010, Hitachi, Japan) and TEM (Transmission Electron Microscopy) (JEM-2010, JEOL, Japan). The transmittance spectra of the films were measured by a UVevis spectrophotometer (Lambda 900, PerkinElmer, USA).
Photocatalytic activity testing The hydrogen generation was tested using a photocatalytic water splitting testing system (CEL-SPH2N, AULTT, China). The samples (T1 e T4) were successively dipped in 100 mL aqueous solution containing 10 vol% methanol in the same quartz cell. The solution was purged with argon gas for 10 min, and then the gas in the quartz cell was removed by mechanical pumping for 3 h. During this duration, the quartz cell was irradiated by a 300 W Xe lamp fixed at 15 cm above the film. Thereafter, the amount of H2 generated under the continuous irradiation of Xe lamp was determined by a gas chromatograph using N2 as carrier gas.
Results and discussion Microstructure and morphology characterization of TiO2 thin films The crystallographic and texture characterization of the TiO2 films was identified by XRD and the results are shown in Fig. 1. It is found that all the samples were of pure anatase phase. For sample T1, (101), (004), (112), (200), (105), (211), (204), (220) and (215) reflections of anatase phase were observed at the expected 2q values (according to JCPDS Card No. 21-1272) and the most intense peak was (101) reflection. However, for samples T2 e T4, only (101), (112), (211), and (220) reflections were observed and the intensity of the (112) peak turned to be the strongest one, indicating the preferred orientation of these samples. It was proposed that the intensity ratio of two selected peaks (or one peak to the sum of several peaks) from the
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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and c displays the typical TEM morphology of the two kinds of anatase nanorods existing in samples T2 e T4. Although they had similar appearance, their growth directions were different, as reveled by HRTEM images and the corresponding fast Fourier transition (FFT) images of the selected areas. For the nanorods shown in Fig. 3a (denoted as NR-112), the lateral surfaces were {112} facets, and the growth was along [110] direction; whereas for the nanorods shown in Fig. 3c (denoted as NR-110), the lateral surfaces were {110} facets and the growth direction was perpendicular to {112} facets, and highly ordered crystal lattices, without dislocations or stacking faults, were observed. Moreover, by performing TEM observation, it is also noticed that, for samples T2 - T4, NR-110 (Fig. 3c) were the major product while NR-112 (Fig. 3a) were the subordinate product, and the percentage of NR-110 in sample T3 was the highest among the three samples. Obviously, The TEM observation is well consistent with the XRD patterns.
Fig. 1 e XRD patterns of the thin films (T1 e T4).
diffraction pattern of the sample comparing with the corresponding ratio from a reference can be used to characterize the degree of crystallographic preferred orientation (CPO) [14,26]. Based on this definition, herein, CPO is specified as the intensity ratio of a selected peak to the sum of all peaks within the 2q range of 10 e80 , and is stated as the following equation: X I CPO total ¼ I
IX film Itotal film
! IX film
IX ref
!
Itotal ref
!
3
(1)
Itotal film
where IXfilm andItotal film refer to the intensity of the selected (X) peak and that of the sum of all peaks within the 2q range of 10 e80 from the diffraction pattern of the anatase TiO2 films (the broadened peak originated from the quartz glass substrate around 2q ¼ 22 is excluded) respectively, whereas IXref and Itotal ref indicate the intensity of (X) peak and the sum intensity of all peaks within the 2q range of 10 e80 from the powder diffraction pattern of a reference (herein, JCPDS Card No. 211272 is selected), respectively. Using Eq. (1), the CPO values of (112) peak are calculated to be 0.16, 0.87, 0.93, and 0.91 for samples T1, T2, T3, and T4, respectively. It means that the growth direction was mainly perpendicular to {112} facets for T2eT4, which will be discussed later in detail. The surface microstructures of the samples are shown in Fig. 2aed. In comparison to the compacted surfaces of samples T1 and T4, a large number of pores were observed on the surface of samples T2 and T3. The cross-section images shown in Fig. 2eeh reveals that the film thickness of samples T1, T2, T3, and T4 was approximately 1.05, 1.05, 1.10 and 1.15 mm, respectively. Sample T1 was consisted of randomly interconnected quasi-spherical TiO2 grains (Fig. 2e), while the other three samples (T2 e T4) were composed of nanorods growing on the glass substrates and a thin layer of nanoparticles (basically, less than 100 nm in thickness) on the surface (Fig. 2, panels feh). As mentioned above, samples T2 e T4 exhibited strong preferred orientation. This texture characterization is thought to reflect the crystallographic information of the nanorods, which can be further confirmed by TEM observation. Fig. 3a
Photocatalytic water splitting by TiO2 thin films Fig. 4 shows the photocatalytic activity of T1 e T4. It is found that the amount of H2 evolution increased almost linearly with the irradiation time for all the samples. The H2 evolution rate was ~6.0 mmol g1 h1 for T1, 5.3 mmol g1 h1 for T2, 8.4 mmol g1 h1 for T3, and 1.7 mmol g1 h1 for T4, respectively. In other words, the photocatalytic activity of the nanorods film T3 was three orders of magnitude higher than that of nanoparticles film T1, and even higher than that of previously reported chemically modified TiO2, such as Ndoped TiO2 [16,27,28], Cu-doped TiO2 [29,30], and Pt-loaded TiO2 [12,31,32]. The divergence in the H2 evolution rate of the four samples is studied based on the aforementioned experimental results. Sample T1 was almost incapable to generate hydrogen, which is consistent with the previously reported values (generally, <10 mmol g1 h1) for bare TiO2 without chemical modification [12,23,24]. Such poor performance for H2 evolution of sample T1 can be attributed to the combined effects of the following factors: (i) the film was consisted of randomly interconnected quasi-spherical anatase TiO2 nanoparticles with dominant {101} facets, which is the most thermodynamically stable facets with low activity; (ii) the compact construction of the nanoparticles leaded to the difficult diffusion of water molecules inside the film; and (iii) undesirable recombination of electrons and holes resulting from the particle-to-particle hopping of the photogenerated carriers. In many literature, the easy and rapid recombination of electrons in the conduction band and holes in the valence band was considered to be responsible for the negligible H2 evolution rate of bare TiO2 photocatalyst. In contrast to sample T1, samples T2 e T4 had high H2 evolution rate, which can be attributed to their special morphologies since all the four samples had the similar optical bandgap of about 3.2 eV (Fig. 5). Firstly, these samples were mainly composed of nanorods. By virtue of the single crystallinity of the nanorods, grain boundaries which exist widely in the nanoparticle network (e.g., sample T1) can be avoided, and therefore the bulk recombination of electronhole pairs can be depressed dramatically. Secondly, the average diameter of nanorods existing in these samples was
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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Fig. 2 e SEM images of T1 (a, e), T2 (b, f), T3 (c, g), and T4 (d, h). (aed) top-down view, (eeh) cross-section images.
Fig. 3 e (a, c) Typical TEM images of anatase TiO2 nanorods. (b) and (d) HRTEM images taken from the white circles in (a) and (c), respectively. The insets in (b) and (d) show the corresponding fast Fourier transition (FFT) images. Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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Fig. 4 e Time course of H2 evolution for the thin films with methanol as the sacrificial agent.
Fig. 5 e Absorption spectra for samples T1 e T4.
less than 40 nm, which is short enough for the effective migration of photogenerated carriers from where they are generated to reaction sites on the lateral surfaces before recombination. Lastly and most importantly, the lateral surfaces of nanorods were mainly {110} facets, whose 4c-Ti atom and 2c-O atom are capable to support bonding sites for the adsorption of O atom and H atom of water molecule. And the adsorption energy of the {110} facets was reported to be higher than that of {101}, {100} and {001} facets [33]. Therefore, the infused water molecules can be easily chemisorbed on the surfaces of nanorods for reacting with photogenerated carriers. It is known that the surface energy of {110} facets is the highest among all the low-index surfaces of anatase TiO2. Generally, higher surface energy means higher density of dangling bonds on the surfaces and thereby higher surface reactivity [17]. From the perspective of atomic structure, the Ti atoms on the top layer of {110} surface are undercoordinated 4c-Ti atoms (Fig. 6), which can act as active sites for water splitting. Doubtlessly, the high-reactive {110} facets with abundant 4c-Ti atoms are vital for the ultra-high H2 generation rate of anatase TiO2. Among samples T2eT4, sample T3 showed the highest H2 evolution rate, which can be ascribed to the maximum
Fig. 6 e Models of anatase TiO2 {101} (a), {110} (b), and {112} (c) surfaces.
extent of preferred orientation. As mentioned above, T3 shows the highest CPO value of (112) peak and consequently the highest percentage of NR-110 among these three samples. However, the H2 evolution rate of T4 was far lower than that of T2 (only about 32% of T2), though its CPO value of (112) peak is higher than that of T2. It is considered to be an aftermath of the compact surface of sample T4 (although the surfaces of samples T2 and T3 was dense, their densification
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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were far below that of sample T4) since samples T2 and T4 had the similar cross-section morphology, as shown in Fig. 2. Generally, the looser surfaces of samples T2 and T3 are conducive to the infiltration and infusion of water molecules inside the film to soak the lateral surfaces of the nanorods, while the compact surface of sample T4 may inhibit the infiltration of water molecules inside the film and thereafter their chemisorption by the lateral surfaces of the nanorods, and unavoidably decreases the effective oxidation/reduction reaction at the active sites on the surfaces of the nanorods. Thus, the compact surface is a crucial factor that leads to limited contact area between the film and water molecules. That is, it is mainly the nanorods rather than the surface nanoparticles (see Fig. 2) that are responsible for the high photocatalytic reactivity of these bare anatase TiO2 samples.
[3] [4]
[5]
[6]
[7]
[8]
Conclusions [9]
Anatase TiO2 film (sample T3) consisted of nanorod arrays with mainly lateral {110} surfaces growing on the glass substrate and a thin layer of quasi-spherical nanocrystals covering on the tips of the nanorods was deposited by RF reactive magnetron sputtering. This TiO2 sample displayed ultra-high H2 generation rate of 8.4 mmol g1 h1, while only 6.0 mmol g1 h1 was achieved for the reference sample that composed of randomly interconnected quasi-spherical particles without preferred orientation (sample T1). The largely enhanced photocatalytic activity can be attributed to its onedimensional (1-D) nanostructures and exposed high reactive {110} facets. This work further demonstrates the potentiality of TiO2 as an efficient photocatalyst for hydrogen generation, and highlights the importance of crystallographically preferred orientation and morphology control of photocatalysts in achieving excellent photocatalytic performance. Within this context, it is believed that, through controlling the growth of {110}-exposed anatase TiO2 nanorod arrays with lower density and higher specific surface, as well as the concentration of oxygen vacancies and titanium vacancies (or Ti3þ) on {110} facets, the photocatalytic efficiency of TiO2ebased photocatalysts can be further enhanced for industrial application.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgments This work was supported by Chunmiao Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2014-005), the Natural Science Foundation of Fujian Province (2015j01231), and the National High Technology Research and Development Program of China (2013AA03A116).
[17]
[18] [19]
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Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting Qianqian Hu a,b, Jiquan Huang a,*, Qiaohong Li a, Chong Wang a, Guojing Li a,b, Jian Chen a, Yongge Cao c a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China b University of the Chinese Academy of Sciences, Beijing 100039, China c Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China
article info
abstract
Article history:
We report for the first time the experimental realization of anatase TiO2 film comprised
Received 25 November 2015
mainly of nanorods exposed with dominant {110} lateral surfaces growing on the quartz glass
Received in revised form
substrate. Based on microstructure and morphology analysis, it is found that, the average
24 June 2016
diameter of nanorods is less than 40 nm, which is favorable for the effective migration and
Accepted 26 July 2016
separation of photogenerated carriers, while their lateral {110} facets contain abundant un-
Available online xxx
saturated 4c-Ti atoms which act as active sites for water splitting. The H2 generation rate by
Keywords:
without the assistance of metal cathode, bias or loading of co-catalyst, which is about three
TiO2 nanorods
orders of magnitude higher than that of anatase TiO2 nanoparticles film, and even higher
this bare anatase TiO2 nanorods film is 8.4 mmol g1 h1 under full-arc light irradiation
Preferred orientation
than the reported values of some noble metal loaded TiO2 photocatalysts. This work ex-
Efficient water splitting
emplifies that the controlled growth of photocatalysts with specified active facets and desired architectures is an effective strategy to achieve excellent photocatalytic properties. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Direct photocatalytic water splitting for hydrogen generation has been widely considered as a promising solution for the ever-growing global energy crisis and environmental pollution. On the past, much research focuses on the development of various semiconductor photocatalysts. Nowadays, hundreds of materials have been characterized in terms of their ability to split water [1e3], nevertheless, none of them is deemed to be free of limitations such as low conversion
efficiency, low chemical stability, and environmental impact in their manufacture and/or use [3e6]. Therefore, except for developing new kinds of photocatalysts, it is also essential to overcome the limitations of the existing photocatalysts to enhance their catalytic activity. Among all the discovered photocatalysts for water splitting, anatase TiO2 is one of the most prominent candidates due to its superior catalytic activity, high corrosion resistance, low cost, and nontoxicity [2,7,8]. However, bare TiO2 is nearly not reactive for splitting water into H2 and O2, and the energy conversion efficiency from incident light to generated H2, even
* Corresponding author. Fax: þ86 591 83721039. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.200 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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considering only the irradiation of UV light, is still very low, mainly due to the easy recombination of photoexcited electronehole pairs, and the quick back-reaction of evolved H2 and O2 into water [2,7]. The mostly adopted methods to avoid these drawbacks are chemical modifications such as introduction of cocatalyst, particularly noble metals. It has been widely reported that the loading of noble metals such as Pt and Pd on the surface of TiO2 photocatalyst can suppress the electronehole recombination and the back-reaction and thus greatly enhance the hydrogen generation efficiency [9]. However, noble metals are rare and expensive. And moreover, the performance of noble metal loaded TiO2 is still unsatisfactory. Another strategy is the morphological modification. In this respect, researchers usually focus on the increase of surface area and porosity rather than the tailor of specific nanostructure exposed with optimal crystallographic plane [10,11]. It is well-known that photocatalytic ability of TiO2 crystals depends greatly on its crystallinity and morphology (e.g., the exposed facets, particle size and shape). In general, different crystallographic orientations exhibit distinctive surface atomic arrangement and electronic structure [12,13], and consequently, the photocatalytic activity of anatase TiO2 crystals can be greatly enhanced through increasing the proportion of highenergy surfaces, such as {001}, {100}, {211}, and {111} surfaces [14e17]. Besides, one-dimensional (1-D) nanostructures, such as nanowires, nanorods, and nanotubes array, may exhibit higher photocatalytic activity than particles network, due to their enhanced charge transport and other characterizations concerned with the particular structure with high single crystallinity [18e21]. Therefore, nanostructured photocatalysts with well-defined morphology is expected to be a future trend for photocatalytic application [22]. Herein, we fabricate anatase TiO2 film mainly comprised of nanorods exposed with dominant {110} lateral surfaces growing on the quartz glass substrate, and investigate its water splitting ability by performing measurements directly in an aqueous methanol solution under the irradiation of Xe lamp. It is found that the H2 evolution rate is higher than 8 mmol g1 h1, which is about three orders of magnitude higher than that of the anatase TiO2 film composed of quasispherical nanoparticles, as well as the commonly reported values for bare TiO2 [12,23e25]. This experimental result demonstrates that bare anatase TiO2 without chemical modification can also be an efficient photocatalyst for hydrogen generation by controlling its morphology and preferential growth orientation. It is also worthy to note that, up to date, the synthesis and photocatalysis characterization of anatase TiO2 with dominant {110} surfaces have rarely been reported as it is difficult to be synthesized resulting from the super-high surface energy.
Experimental section Fabrication of TiO2 thin films TiO2 thin films were deposited on quartz glass substrates (2 cm 4 cm) by RF reactive magnetron sputtering. The working gas was a mixture of O2 (99.99% pure) and Ar (99.999%
pure). The target was a 2-inches metallic plate of Ti (99.999% pure) and the base vacuum was 4.9 103 Pa. Before the deposition, the target was pre-sputtered by argon plasma for 20 min. For all the samples, the total working pressure, O2 flow rate, and Ar flow rate, were set at 0.3 Pa, 6 sccm (standard cubic centimeter per minute) and 40 sccm respectively. During the deposition process, the substrate was rotated at 8 rotations per minute along its axis. Four samples, namely, T1, T2, T3 and T4, were deposited for 4, 4, 5, and 4 h, with the targetsubstrate distances of about 120, 110, 130, and 120 mm, respectively. The RF power was set at 140 W for T1 and T4, and 130 W for T2 and T3. The temperature of the substrates was maintained at 450 C during the whole deposition processes for T2 e T4, while T1 was deposited at room temperature and then annealed in situ at 450 C for 4 h to transform the amorphous TiO2 into crystalline anatase phase.
Characterizations of TiO2 thin films The crystalline structure of the TiO2 thin films was identified by X-ray Diffraction (XRD) (Miniflex600, Rigaku, Japan), and the morphology were characterized by Scanning Electron Microscopy (SEM) (SU8010, Hitachi, Japan) and TEM (Transmission Electron Microscopy) (JEM-2010, JEOL, Japan). The transmittance spectra of the films were measured by a UVevis spectrophotometer (Lambda 900, PerkinElmer, USA).
Photocatalytic activity testing The hydrogen generation was tested using a photocatalytic water splitting testing system (CEL-SPH2N, AULTT, China). The samples (T1 e T4) were successively dipped in 100 mL aqueous solution containing 10 vol% methanol in the same quartz cell. The solution was purged with argon gas for 10 min, and then the gas in the quartz cell was removed by mechanical pumping for 3 h. During this duration, the quartz cell was irradiated by a 300 W Xe lamp fixed at 15 cm above the film. Thereafter, the amount of H2 generated under the continuous irradiation of Xe lamp was determined by a gas chromatograph using N2 as carrier gas.
Results and discussion Microstructure and morphology characterization of TiO2 thin films The crystallographic and texture characterization of the TiO2 films was identified by XRD and the results are shown in Fig. 1. It is found that all the samples were of pure anatase phase. For sample T1, (101), (004), (112), (200), (105), (211), (204), (220) and (215) reflections of anatase phase were observed at the expected 2q values (according to JCPDS Card No. 21-1272) and the most intense peak was (101) reflection. However, for samples T2 e T4, only (101), (112), (211), and (220) reflections were observed and the intensity of the (112) peak turned to be the strongest one, indicating the preferred orientation of these samples. It was proposed that the intensity ratio of two selected peaks (or one peak to the sum of several peaks) from the
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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and c displays the typical TEM morphology of the two kinds of anatase nanorods existing in samples T2 e T4. Although they had similar appearance, their growth directions were different, as reveled by HRTEM images and the corresponding fast Fourier transition (FFT) images of the selected areas. For the nanorods shown in Fig. 3a (denoted as NR-112), the lateral surfaces were {112} facets, and the growth was along [110] direction; whereas for the nanorods shown in Fig. 3c (denoted as NR-110), the lateral surfaces were {110} facets and the growth direction was perpendicular to {112} facets, and highly ordered crystal lattices, without dislocations or stacking faults, were observed. Moreover, by performing TEM observation, it is also noticed that, for samples T2 - T4, NR-110 (Fig. 3c) were the major product while NR-112 (Fig. 3a) were the subordinate product, and the percentage of NR-110 in sample T3 was the highest among the three samples. Obviously, The TEM observation is well consistent with the XRD patterns.
Fig. 1 e XRD patterns of the thin films (T1 e T4).
diffraction pattern of the sample comparing with the corresponding ratio from a reference can be used to characterize the degree of crystallographic preferred orientation (CPO) [14,26]. Based on this definition, herein, CPO is specified as the intensity ratio of a selected peak to the sum of all peaks within the 2q range of 10 e80 , and is stated as the following equation: X I CPO total ¼ I
IX film Itotal film
! IX film
IX ref
!
Itotal ref
!
3
(1)
Itotal film
where IXfilm andItotal film refer to the intensity of the selected (X) peak and that of the sum of all peaks within the 2q range of 10 e80 from the diffraction pattern of the anatase TiO2 films (the broadened peak originated from the quartz glass substrate around 2q ¼ 22 is excluded) respectively, whereas IXref and Itotal ref indicate the intensity of (X) peak and the sum intensity of all peaks within the 2q range of 10 e80 from the powder diffraction pattern of a reference (herein, JCPDS Card No. 211272 is selected), respectively. Using Eq. (1), the CPO values of (112) peak are calculated to be 0.16, 0.87, 0.93, and 0.91 for samples T1, T2, T3, and T4, respectively. It means that the growth direction was mainly perpendicular to {112} facets for T2eT4, which will be discussed later in detail. The surface microstructures of the samples are shown in Fig. 2aed. In comparison to the compacted surfaces of samples T1 and T4, a large number of pores were observed on the surface of samples T2 and T3. The cross-section images shown in Fig. 2eeh reveals that the film thickness of samples T1, T2, T3, and T4 was approximately 1.05, 1.05, 1.10 and 1.15 mm, respectively. Sample T1 was consisted of randomly interconnected quasi-spherical TiO2 grains (Fig. 2e), while the other three samples (T2 e T4) were composed of nanorods growing on the glass substrates and a thin layer of nanoparticles (basically, less than 100 nm in thickness) on the surface (Fig. 2, panels feh). As mentioned above, samples T2 e T4 exhibited strong preferred orientation. This texture characterization is thought to reflect the crystallographic information of the nanorods, which can be further confirmed by TEM observation. Fig. 3a
Photocatalytic water splitting by TiO2 thin films Fig. 4 shows the photocatalytic activity of T1 e T4. It is found that the amount of H2 evolution increased almost linearly with the irradiation time for all the samples. The H2 evolution rate was ~6.0 mmol g1 h1 for T1, 5.3 mmol g1 h1 for T2, 8.4 mmol g1 h1 for T3, and 1.7 mmol g1 h1 for T4, respectively. In other words, the photocatalytic activity of the nanorods film T3 was three orders of magnitude higher than that of nanoparticles film T1, and even higher than that of previously reported chemically modified TiO2, such as Ndoped TiO2 [16,27,28], Cu-doped TiO2 [29,30], and Pt-loaded TiO2 [12,31,32]. The divergence in the H2 evolution rate of the four samples is studied based on the aforementioned experimental results. Sample T1 was almost incapable to generate hydrogen, which is consistent with the previously reported values (generally, <10 mmol g1 h1) for bare TiO2 without chemical modification [12,23,24]. Such poor performance for H2 evolution of sample T1 can be attributed to the combined effects of the following factors: (i) the film was consisted of randomly interconnected quasi-spherical anatase TiO2 nanoparticles with dominant {101} facets, which is the most thermodynamically stable facets with low activity; (ii) the compact construction of the nanoparticles leaded to the difficult diffusion of water molecules inside the film; and (iii) undesirable recombination of electrons and holes resulting from the particle-to-particle hopping of the photogenerated carriers. In many literature, the easy and rapid recombination of electrons in the conduction band and holes in the valence band was considered to be responsible for the negligible H2 evolution rate of bare TiO2 photocatalyst. In contrast to sample T1, samples T2 e T4 had high H2 evolution rate, which can be attributed to their special morphologies since all the four samples had the similar optical bandgap of about 3.2 eV (Fig. 5). Firstly, these samples were mainly composed of nanorods. By virtue of the single crystallinity of the nanorods, grain boundaries which exist widely in the nanoparticle network (e.g., sample T1) can be avoided, and therefore the bulk recombination of electronhole pairs can be depressed dramatically. Secondly, the average diameter of nanorods existing in these samples was
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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Fig. 2 e SEM images of T1 (a, e), T2 (b, f), T3 (c, g), and T4 (d, h). (aed) top-down view, (eeh) cross-section images.
Fig. 3 e (a, c) Typical TEM images of anatase TiO2 nanorods. (b) and (d) HRTEM images taken from the white circles in (a) and (c), respectively. The insets in (b) and (d) show the corresponding fast Fourier transition (FFT) images. Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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Fig. 4 e Time course of H2 evolution for the thin films with methanol as the sacrificial agent.
Fig. 5 e Absorption spectra for samples T1 e T4.
less than 40 nm, which is short enough for the effective migration of photogenerated carriers from where they are generated to reaction sites on the lateral surfaces before recombination. Lastly and most importantly, the lateral surfaces of nanorods were mainly {110} facets, whose 4c-Ti atom and 2c-O atom are capable to support bonding sites for the adsorption of O atom and H atom of water molecule. And the adsorption energy of the {110} facets was reported to be higher than that of {101}, {100} and {001} facets [33]. Therefore, the infused water molecules can be easily chemisorbed on the surfaces of nanorods for reacting with photogenerated carriers. It is known that the surface energy of {110} facets is the highest among all the low-index surfaces of anatase TiO2. Generally, higher surface energy means higher density of dangling bonds on the surfaces and thereby higher surface reactivity [17]. From the perspective of atomic structure, the Ti atoms on the top layer of {110} surface are undercoordinated 4c-Ti atoms (Fig. 6), which can act as active sites for water splitting. Doubtlessly, the high-reactive {110} facets with abundant 4c-Ti atoms are vital for the ultra-high H2 generation rate of anatase TiO2. Among samples T2eT4, sample T3 showed the highest H2 evolution rate, which can be ascribed to the maximum
Fig. 6 e Models of anatase TiO2 {101} (a), {110} (b), and {112} (c) surfaces.
extent of preferred orientation. As mentioned above, T3 shows the highest CPO value of (112) peak and consequently the highest percentage of NR-110 among these three samples. However, the H2 evolution rate of T4 was far lower than that of T2 (only about 32% of T2), though its CPO value of (112) peak is higher than that of T2. It is considered to be an aftermath of the compact surface of sample T4 (although the surfaces of samples T2 and T3 was dense, their densification
Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200
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were far below that of sample T4) since samples T2 and T4 had the similar cross-section morphology, as shown in Fig. 2. Generally, the looser surfaces of samples T2 and T3 are conducive to the infiltration and infusion of water molecules inside the film to soak the lateral surfaces of the nanorods, while the compact surface of sample T4 may inhibit the infiltration of water molecules inside the film and thereafter their chemisorption by the lateral surfaces of the nanorods, and unavoidably decreases the effective oxidation/reduction reaction at the active sites on the surfaces of the nanorods. Thus, the compact surface is a crucial factor that leads to limited contact area between the film and water molecules. That is, it is mainly the nanorods rather than the surface nanoparticles (see Fig. 2) that are responsible for the high photocatalytic reactivity of these bare anatase TiO2 samples.
[3] [4]
[5]
[6]
[7]
[8]
Conclusions [9]
Anatase TiO2 film (sample T3) consisted of nanorod arrays with mainly lateral {110} surfaces growing on the glass substrate and a thin layer of quasi-spherical nanocrystals covering on the tips of the nanorods was deposited by RF reactive magnetron sputtering. This TiO2 sample displayed ultra-high H2 generation rate of 8.4 mmol g1 h1, while only 6.0 mmol g1 h1 was achieved for the reference sample that composed of randomly interconnected quasi-spherical particles without preferred orientation (sample T1). The largely enhanced photocatalytic activity can be attributed to its onedimensional (1-D) nanostructures and exposed high reactive {110} facets. This work further demonstrates the potentiality of TiO2 as an efficient photocatalyst for hydrogen generation, and highlights the importance of crystallographically preferred orientation and morphology control of photocatalysts in achieving excellent photocatalytic performance. Within this context, it is believed that, through controlling the growth of {110}-exposed anatase TiO2 nanorod arrays with lower density and higher specific surface, as well as the concentration of oxygen vacancies and titanium vacancies (or Ti3þ) on {110} facets, the photocatalytic efficiency of TiO2ebased photocatalysts can be further enhanced for industrial application.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgments This work was supported by Chunmiao Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2014-005), the Natural Science Foundation of Fujian Province (2015j01231), and the National High Technology Research and Development Program of China (2013AA03A116).
[17]
[18] [19]
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Please cite this article in press as: Hu Q, et al., Anatase TiO2 film composed of nanorods with predominant {110} active facets as an excellent photocatalyst for water splitting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.200