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Abstract
In this paper, the photocatalytic activity enhancement of TiO2 thin films was realized by laser irradiation. The H2 yield of the as-irradiated film is 79 μmol/(h*m2), which is 33% more than that of the as-deposited TiO2 film. Spectrophotometer, X-ray diffraction and Raman system were employed to characterize the samples. The results showed that both the scanning rate and line spacing of the laser modification have effects on photocatalytic activity. It suggests that a phase junction is formed between the amorphous and rutile phases. The increment of H2 generation could be attributed to the alignment of Fermi levels in the phase junction.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the exhaustion of fossil energy and pollution of environment getting worse, more and more countries and regions begin to develop renewable energy sources for sustainable development. Due to its recyclability, safety and sustainability [1,2], hydrogen energy technology is expected to become a key technology in realizing a low-carbon emission society. However, pure hydrogen does not occur naturally on Earth in large quantities. The industrial production of hydrogen is also limited by its substantial energy requirement. In the early 1970s, Honda and Fujishima [3] reported H2 generation induced by UV light on a TiO2 electrode. Since then, the photocatalytic water splitting has been studying extensively. Among all semiconductor photocatalysts, TiO2 [4,5] appears to be the most promising and suitable material because of its superior photocatalytic activity, chemical stability, low cost, and nontoxicity [6]. ZnO [7], ZrO2 [8], Ta2O5 [9] and other transition metal oxides [5] were also studied as the photocatalysts in a great extent.
The photocatalytic activity is partially determined by the amount of working electrons and holes on the catalyst surface. So it is important to suppress the rapid combination of photogenerated electrons and holes. Many methods such as ion doping [10,11], dye sensitization [12] and use of cocatalyst [13] or sacrifice agent [14] were widely studied. However, some of these methods are not suitable for the photocatalytic water splitting, while the others dramatically raise the costs. Recently, the correlation between the phases of TiO2 and its photocatalytic performance draws lots of attentions [15]. There are three principal crystal phases for TiO2: anatase, rutile and brookite, and many other minor phases such as monoclinic phase. It is commonly believed that anatase phase is more active than the rutile phase in photocatalytic reactions [16]. However, an interesting result is that the photocatalytic activity of TiO2 with mixed phase structure is more active than either of the individual phase. Yuying Gao et al. [17] reported the working mechanism of phase junction at the anatase-rutile interface, which could be attributed to a built-in electric field that directs from anatase phase to rutile phase.
In this paper, amorphous TiO2 film was irradiated by the CO2 laser to obtain an enhancement of photocatalytic activity. Both scanning rate and line spacing (LS) of laser beam were varied to control the ratio of irradiated surface. X-ray diffraction (XRD) and Raman spectroscopy were employed to analyze the crystal structure of the as-irradiated TiO2 film. The confocal microscopy and spectrophotometer were also employed to study the properties of samples. According to the results of the photodegradation and photocatalytic water splitting, the photocatalytic activity of TiO2 is found to be directly related to its phase. On the basis of the understanding thus obtained, we gave a possible explanation for the charge transfer process in the rutile and amorphous phase junction.
2. Experimental
TiO2 thin films with a thickness of 50 nm were deposited on quartz substrates by electron beam evaporation at 200 °C. The commercial available CO2 laser equipment at a maximum laser power of 10 W was used for irradiation. The CO2 laser has a wavelength of 10.6 μm. In this work, the laser power was set at 2 W, the focal length is about 12.5 cm and the beam spot size, which is defined as the diameter at which the intensity drops to 1/e2 of their maximum values, was 0.01 mm. To verify the feasibility of irradiation-induced phase transformation, both the preliminary and formal experiment were carried in this work. In the preliminary experiment, the scanning speed was changed from 0.1 mm/s to 10 mm/s and the line spacing was 0.01mm. In the formal experiment, the line spacing was varied from 0.01 mm to 0.1 mm and the scanning rate is fixed at 0.1mm/s.
The photocatalytic reaction was carried out in a reaction container connected to a closed gas circulation and evacuation system. A 300 W Xe lamp with an ultraviolet band-pass filter cut at 420 nm was used as the excitation source and placed above a glass container. This glass container was filled with 50 ml water (no sacrificial reagent was added) and the sample was placed in it. The distance between the sample and the light source was 10 cm. Prior to light irradiation, the system was pumped to vacuum to remove hydrogen completely and then kept closed so as to eliminate the influence of hydrogen. The amount of H2 was measured using on-line gas chromatography. Nitrogen gas was used as carrier gas. The crystal structure of samples was characterized by X-ray diffraction (XRD) using a Bruker AXS/D8 Advance system, with CuKα radiation (λ = 0.15408 nm). Band gap was calculated from optical transmittance which was measured by a dual-beam UV-VIS-NIR spectrophotometer (Lambda 1050, Perkins Elmer) at normal incidence. The scanning step and integration time was 2 nm and 0.24 s, respectively. Raman spectrum was studied by a confocal microprobe Raman system (inVia Raman Microscope, Renishaw) operated with a 633 nm laser. All the measurements were carried out at room temperature.
3. Results and discussion
In general, the amorphous TiO2 film can be turned into anatase and rutile phase through annealing at about 600°C and 1000°C, respectively. Since the conventional thermal annealling process usually takes for a few hours, laser irradiation may become a cost-effective method to obtain the mixed phase on TiO2 film surface.
The preliminary experiment was carried out to verify the feasibility of irradiation-induced phase transformation. The scanning rate of laser beam was varied from 0.1 mm/s to 10 mm/s while the other parameters maintain constant. Herein, the scanning rate is a key factor because the temperature of the irradiated surface is dependent on it. As shown in Fig. 1(a), the slower scanning rate will prolong the contact time of laser beam and film surface, which finally brings a higher temperature. The crystal structures of the samples were characterized by XRD and Raman spectra. As shown in the Fig. 1(b), the XRD diffraction peak at around 27.64° is corresponding to the (1 1 0) crystallographic plane of rutile (JCPDS 86-0148). It means a few samples (scanning rate = 0.1, 0.5 and 1 mm/s) were transformed into rutile phase while the remaining samples are still amorphous. According to the Bragg equation, the interplanar spacing (d) could be calculated. For the film samples, it is approximately 0.322 nm, which is less than the standard cards (0.324 nm). The discrepancy in the value is due to the residual stress developed in the films.
Fig. 1 Preliminary Experiment: (a) Schematic drawing of the relationship between laser scanning rate and localized temperature on film. (b) XRD spectra, (c)Raman spectra and (d) Absorption spectra of samples which were irradiated by a CO2 laser under different scanning rates. (e) Photocatalytic decomposition rate for the methylene blue of the TiO2 films.
In Raman spectra pattern (Fig. 1(c)), one strong Raman band at about 143 cm−1, typically assigned to the B1g mode of rutile [18], was observed for all samples except that irradiated under a 10 mm/s scanning rate. Although the phase transition temperature of the anatase phase is lower than rutile phase, the crystallographic plane or Raman peak of the anatase phase was not found thoroughly. It should be attributed to the high localization of the laser induced temperature field. Most of the energy is limited in a small area, and the rest radiates only a little distance [19]. According to the results of XRD and Raman spectra, the phase transition of TiO2 films through laser irradiation technology has been confirmed. Moreover, the relationship between the laser scanning rate and phase transition is also verified. The lower scanning rate brings higher energy which induces a greater phase transition on film.
In the preliminary experiment, the optical properties are also measured. In Fig. 1(d), the optical absorption spectra show an apparent difference. With the decrease of laser scanning rate, the absorption intensity among the region from 320 nm to 500 nm is increased a lot. The optical band gap is calculated from transmittance value by the relation:
where hν is the photon energy, Eg is the optical band gap and A is a constant. It reveals that the band gap of amorphous TiO2 thin film was approximately 3.25 eV. With the scanning rate increasing, the band gap of the as-irradiated TiO2 film is narrowed down to about 3.0 eV, equaling to that of the rutile TiO2 [20]. Compared with amorphous TiO2, the rutile TiO2 oughts to have a better photocatalytic activity. The photocatalytic activity of the as-irradiated TiO2 film was measured by monitoring the change in optical absorption of the methylene blue (MB) solution (concentration = 1*10−4 M) at ~660 nm during its photocatalytic decomposition process. The absorption data was dealt with normalization. The MB solution was completely photodegraded after 3 hours for the as-irradiated TiO2 film (0.1 mm/s), while more than 4 hours was taken by the pure amorphous TiO2 film under the same testing condition (Fig. 1(e)). This improvement should be attributed to the better catalyst activity from the rutile TiO2. The preliminary experiment above certifies the feasibility of laser irradiation induced phase transformation. It is a meaningful technology to prepare a mixed phase TiO2 thin film.The formal experiment was carried out to study the mixed phase structure. In this experiment, the value of line spacing (LS) was changed to control the surface proportions of amorphous and rutile phase (Fig. 2(a)). The scanning speed was fixed at 0.1 mm/s while the line spacing was varied from 0.1 mm to 0.01 mm. By adjusting the LS, the irradiated area on the film surface can be chosen freely and the phase junction can be formed. The amount variation of rutile phase was revealed in Fig. 2(b). With the line spacing decreasing, the intensity of diffraction peak increases. It indicates that the TiO2 films are partly transformed into rutile phase. The width of line spacing is related to the number of lines, and further influences the rutile content. In order to obtain an intuitive characterization result, the confocal microscopy was used to observe the sample surface. As shown in Fig. 3, the microscopy pictures indicate an apparent modifying effect on the surface of TiO2 thin film via the laser irradiation. The actual line spacing width is somewhat larger than the value which was set in the software. It is an acceptable error from a commercial instrument, which will not influence the reliability of this experiment. For convenience, the set value of line spacing will still be used in following to mark the samples. The width of central region which was irradiated by laser is almost 20 μm while the set beam spot size was 100 μm. In Fig. 3(d), the scanning path is shown more clearly. The area of obvious color change is just 20 μm wide, but the whole laser affected area is about 70 μm wide which is close to the set beam spot size. As above, by adjusting the LS, the laser irradiation technology prepares mixed phases which consists of amorphous and rutile phases on the surface of TiO2.
Fig. 2 Formal Experiment: (a) Schematic drawing of the mixed phase junction and the carrier migration. The difference of laser beam spot size and line spacing determines the junction activity and amount. (b) XRD patterns of the samples with different line spacing (LS).
Fig. 3 Confocal microscopy pictures of samples with different line spacing (LS). (a) LS = 0.2 mm, (b) LS = 0.03 mm, (c) LS = 0.05 mm, and (d) LS = 0.1 mm.
As shown in Fig. 4, the photocatalytic water splitting activity of amorphous-rutile mixed phase structure was characterized. The amount of H2 generation as a function of time is shown in Fig. 4(a). Although the amount of the rutile phase in the film is increased with the LS decreasing (Fig. 2(b)), the rate of H2 evolution does not change with the same trend. A maximum activity is observed on TiO2 film which was irradiated with the LS = 0.1mm. Compared with the pure amorphous TiO2 substrate, the photocatalytic activity is remarkably increased by introducing a small amount of rutile phase. This exciting increase in the photocatalytic activity can be attributed to the formation of the surface amorphous/rutile phase junction. The phase junction could promote spatial charge separation in the surface region. The H2 generation per hour per square meter for different LS was calculated (Fig. 4(b)). The largest yield is almost 33% more than that of the pure amorphous TiO2 film. However, for a few samples (LS = 0.01 mm, 0.02 mm), the photocatalytic water splitting efficiency is less than that of the pure amorphous film, although they have more rutile phase content.
Fig. 4 (a) The H2 generation under UV-light irradiation as a function of time with different line spacing (LS). (b) The calculated H2 generation per hour per square meter.
The Fermi levels (EF) are varied for different phases of TiO2. When two different phases contact, an alignment of EF in phase junction will result in local vacuum level shifting [21]. The defects in the amorphous layer (e.g. disorder and oxygen vacancies) form a narrow band of states close to the bottom of the conduction band (CB) where the EF is pinned. In other words, EF in the amorphous layer is closer to CB than it is in the crystalline layer [22]. The phase junction may facilitate the transfer of the photogenerated electrons from the conduction band of the rutile phase to the amorphous, thereby improving the charge separation efficiency and enhancing the photocatalytic activity (Fig. 5). The degradation of the photocatalytic activity with decreasing the line spacing is also interesting and meaningful. The phase transform was induced by the laser irradiation. While the line spacing is too small, the rutile content on film surface will be much more than the amorphous content. It will finally lead to the exhaustion of the carriers in amorphous part. So the small line spacing will play an opposite role during the catalysis process. As shown in Fig. 4(b), the hydrogen generation of samples whose line spacing is 0.02 mm and 0.01 mm is less than the pure amorphous TiO2, which is consistent with our supposition.
Fig. 5 The schematic of photo-generated electrons and holes migration in phase junction of the TiO2 thin film.
4. Conclusion
This work shows the enhancement of photocatalytic activity of TiO2 thin film. The laser irradiation was employed to modify the crystal phase structure of thin film. The formation of a phase junction between the rutile phase and amorphous phase was certified by XRD and Raman spectroscopy. The as-irradiated thin film is able to greatly improve the photocatalytic yield of H2. Instead, the as-deposited amorphous sample and the sample contains excessive rutile phase showed a worse photocatalytic activity. From this work, it can thus be concluded that the best proportion of amorphous and rutile TiO2 is a key factor to develop high performance photocatalytic activity for semiconductor film catalysts. In other words, the creation of phase junction is a valuable and good strategy to get high-efficiency photocatalysts.
Funding
The National Key Research and Development Program of China (2016YFB1102303); National Natural Science Foundation of China (NSFC) (61775140, 61775141).
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