Biology and Chemistry Research
Synthesis of a Novel Titanium Dioxide and Monolayer Molybdenum Disulfide Photocatalyst for Applications in Hydrogen Generation Margaret Tian ABSTRACT The depletion of fossil fuel reserves as well as the environmental and human health hazards posed by combustion of nonrenewable fuels have created a push for clean, renewable energy. Hydrogen fuel produced by solar-powered photocatalysis of the water splitting reaction provides a source of sustainable and unlimited energy. In this study, a novel titanium dioxide and monolayer molybdenum sulfide co-catalyst was engineered for applications in photocatalytic hydrogen production. Chemical vapor deposition was used to create a monolayer MoS2 thin film. Mixed-phase TiO2 powder was synthesized using the sol-gel method, and a thin TiO2 layer was deposited on the MoS2 films using an atomizer. Raman and AFM measurements confirmed the presence of mixed-phase TiO2 and unprecedented uniformity of the monolayer MoS2 films. Hydrogen production testing showed that the produced co-catalysts were significantly more effective at water splitting than TiO2 systems. This co-catalyst shows potential in efficient and low cost production of renewable hydrogen fuel using solar energy and water.
Introduction As industries grow and global population rises, the world’s energy consumption is projected to increase by 56% by 2040 [18]. Nonrenewable energy sources, such as the fossil fuels and natural gas that currently supply over 82% of our energy, are becoming increasingly limited and produce environmentally harmful byproducts, leading to global warming [18]. The development of a long-term, clean, and renewable energy source is essential to power our growing needs [1]. Hydrogen fuel (H2) produced from water splitting using solar energy has emerged as an entirely sustainable fuel that utilizes abundant and renewable resources – water and sunlight [1, 2]. To realize hydrogen fuel as a commercially viable energy source, a low-cost and efficient photocatalyst must be engineered to drive its production. This technology would provide a clean and virtually unlimited source of renewable energy with just sunlight, water, and a photocatalyst. 1.1 Photocatalysis Photocatalysts are materials, typically solid semiconductors, that harness light energy to drive chemical reactions. There are two types of photocatalyst bandgaps: direct and indirect. In a direct bandgap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum, while in an indirect bandgap semiconductor, they occur at different momentums. When light with sufficient energy hits a photocatalytic semiconductor, the negatively charged electrons (e–) within the valence band are excited and bridge the bandgap to the conduction band, resulting in charge 4 | 2013-2014 | Volume 3
separation by leaving behind a positively charged hole (h+) [3]. It is easier for an electron-hole pair to be produced with a direct bandgap because less momentum is required to excite the electron to the desirable in semiconductors. During water splitting, the photogenerated electrons are donated to create hydrogen gas while the holes are filled with the electrons given up by oxygen production [4]. The goal of this project is to engineer a photocatalyst with an appropriate bandgap to absorb visible light and catalyze the oxidation and reduction of water to produce hydrogen fuel. To engineer an efficient and effective photocatalyst for water splitting, two important criteria must be met. Firstly, the catalyst must have a larger bandgap than that required for the oxidation and reduction of water into oxygen and hydrogen (~1.23eV) [4]. Secondly, the bandgap must range from a higher potential than the reduction of water to a lower potential than the oxidation of water as illustrated in Figure 2. Many photocatalysts have been engineered to
Biology and Chemistry Research absorb UV light. However, UV light only constitutes ~3% of the light energy that hits the Earth’s surface, whereas visible light is more abundant and can be more efficiently used [1, 3, 5]. Combining two photocatalysts with suitable bandgaps to create a co-catalyst can promote efficient charge separation and light absorption in the visible range [1]. In this study, titanium dioxide (TiO2) and monolayer molybdenum disulfide (MoS2) were combined to create an efficient water splitting co-catalyst.
1.2 Nanomaterial Research Over the past decade, there has been an exponential growth in nanoscience and nanotechnology research [6]. The discovery of graphene, a single atomic layer or monolayer of carbon, and its fascinating functionalities has prompted an intensive search for other a2D singlelayer materials [1, 6, 7]. New and unique physical and chemical properties emerge when materials are reduced to a nanometer scale due to quantum confinement [6]. Quantum confinement refers to the transition from the continuum of energy states found in parent bulk materials to narrow, discrete energy states in nanoscale materials due to the trapping or confinement of electron-hole pairs in potential wells, areas surrounding an energy minimum. Potential wells can be visualized as a deep valley on a flat terrain that traps objects inside of it. In bulk materials, particles behave as if they are free. However, when materials are in the nanoscale, the small potential wells have a significant confining effect on electron-hole pairs. Changing the size of a material impacts the dimensions of its potential wells and available energy states of its electronhole pairs. This effectively tunes the material’s bandgap, making it possible for scientists to engineer materials for specialized uses [6, 9]. This ability to create materials with specific desirable properties makes 2D nanomaterials ideal for use in catalysis. 1.3 Titanium Dioxide Since its photocatalytic abilities were discovered by Fujishima and Honda in 1972 [6], TiO2 has been
extensively studied as a photocatalyst and semiconductor material for usage in solar cells [10], air and water purification [9, 12], self-cleaning surfaces, and hydrogen generation [6]. TiO2 is one of the most widely used photocatalysts due to its chemical and photocatalytic stability, nontoxicity, high catalytic ability, and low cost of production [3, 12, 13]. The synthesis of TiO2 nanomaterials has been carried out using various methods, including sol-gel, hydrothermal, solvothermal, chemical vapor deposition, physical vapor deposition, and electrodeposition [6]. For applications in photocatalysis, high surface area, mixed-phase anatase and rutile TiO2 is most desirable. High surface area allows for more catalytic interactions and mixed phase TiO2 has been shown to possess the highest photocatalytic ability [6]. However, for uses in water splitting, TiO2 suffers from charge recombination and absorption only in the UV range [1]. In addition, while TiO2 has a large bandgap (3.2eV) and is effective in the oxidation of water, it cannot reduce water efficiently due to its incompatible bandgap and undesirable charge recombination [6]. TiO2/platinum systems have been created to overcome this problem, but the high cost and scarcity of platinum prevents this technology from being commercialized [1]. To create a commercially viable TiO2 co-catalyst system, it is essential to combine TiO2 with a low-cost photocatalyst to achieve more efficient water splitting by tuning the bandgap and promoting charge separation. 1.4 Molybdenum Disulfide The transition metal semiconductor molybdenum disulfide (MoS2) has recently gained much attention due to its high catalytic activity [5], thermal and chemical stability [8], strong photoluminescence [13], and lack of unsatisfied valence shells [8]. Molybdenum disulfide is made up of layers of covalently bonded S-Mo-S held together by weak van de Waals forces [9]. It has found applications in dry lubrication [9], hydrodesulfurization [5], photoelectrochemical electrodes [5], phototransistors [1], and photocatalysis [9]. However, MoS2 is an indirectbandgap semiconductor with a bandgap of 1.29 eV [9] and therefore cannot absorb and utilize light energy efficiently. Furthermore, its small bandgap prevents it from being an effective water splitting photocatalyst. When MoS2 is synthesized on a nanoscale, however, quantum confinement effects emerge to give the material unique and desirable properties. Two dimensional monolayer and few-layer MoS2 show increased photoluminescence and an extended direct bandgap of 1.90eV in monolayer films [9, 15]. While bulk MoS2 does not have an adequate bandgap, the larger 1.90eV direct bandgap of monolayer MoS2 matches well with the reduction of water to hydrogen (Fig. 2), making it an excellent catalyst to combine with TiO2. Unfortunately, current methods employed in making monolayer MoS2, such as mechanical and chemical exfoliation [9, 14] or sulfurization of MoO3 Volume 3 | 2013-2014 | 5
Biology and Chemistry Research
[14], cannot produce centimeter scale films or precisely control film thickness, thereby preventing the production of monolayer MoS2/TiO2 photocatalysts. In this project, a new self-limiting MoS2 chemical vapor deposition procedure was used to develop high quality, large area, and uniform monolayer films [14]. By combining TiO2 nanoparticles and monolayer MoS2, an effective co-catalyst for solar-powered water splitting can be engineered (Fig. 3). In the past, MoS2/ TiO2 co-catalysts have been created by depositing TiO2 nanoparticles on MoS2 bulk structures or vice versa [1, 5]. Few-layer MoS2 coupled with TiO2 has already been shown to have markedly higher water splitting abilities [1] and absorbance in the visible light range as compared to TiO2 due to the more suitable bandgap [5]. These catalysts could be further improved with the use of atomic-scale monolayer, rather than few-layer, MoS2 with its even larger direct bandgap and more prevalent quantum confinement effects. Not only does a monolayer MoS2/TiO2 cocatalyst offer advantages in a more suitable bandgap, but also the heterostructure interface between the catalysts promotes better charge separation. This project aims to create a uniform MoS2 monolayer film, synthesize rough and catalytically efficient mixed phase TiO2, and produce a monolayer MoS2/TiO2 co-catalyst for applications in hydrogen production. 2. Methods and Materials 2.1 Preparation of TiO2 The synthetic procedure for titanium dioxide was selected with two important properties in mind: mixed phase crystalline structure [15] and high surface area [16]. TiO2 can be found in three crystalline phases: anatase, rutile, and brookite. Though pure anatase is the most catalytically active crystalline phase of TiO2, it has been shown that mixed-phase anatase and rutile, like commercial Degussa P25 TiO2 (80:20), outperforms individual polymorphs with the highest photocatalytic ability [12]. Although there is no comprehensive explanation for this, it is hypothesized to be due to rapid electron transfer from rutile to lower energy anatase trapping sites, which leads to stable charge separation [15]. The surface morphology and catalytic ability of TiO2 can also be modified. Additives such as polyethylene glycol have been shown to increase surface roughness of TiO2 and enhance overall catalytic ability [2]. 6 | 2013-2014 | Volume 3
The sol-gel method was used to synthesize TiO2 powder due to its minimal equipment requirements, low production cost, and ability to produce high quality powders and large area, homogenous thin films [12, 17]. Mixed-phase anatase and rutile TiO2 was synthesized using a sol-gel method adapted from Šegota et al [11]. Titanium (IV) isopropoxide (TTIP, Aldrich 97%) was used as a titanium precursor and dissolved into 200-proof ethanol in a 1:40 molar ratio. Glacial acetic acid (to catalyze the reaction), acetylacetone (for peptization), and distilled water (for gelation) were then added to the solution in a 0.9:1.3:12.5 molar ratio. Polyethylene glycol (PEG, Mw = 4500) was added to produce rougher and more catalytically active TiO2. The solution was stirred vigorously at room temperature for 2 hours and sonicated for 30 minutes. The resulting sol-gel was dried at 60°C for 24 hours and then calcined at 550°C in open air to form crystalline anatase and rutile mixed-phase TiO2. The crystalline product was ground into a fine TiO2 powder using a mortar and pestle. Films of TiO2 were produced as controls. The prepared TiO2 powder was added to water (0.2g/25mL) and sonicated to produce a dilute solution. Clean borosilicate glass slides (~1cm2) were heated on a hot plate to 100°C. The TiO2 solution was then sprayed onto the slides using an atomizer. The high temperature instantly vaporized the water, leaving behind a thin layer of TiO2. The TiO2 slides were annealed in air at 200°C for 30 min. 2.2 Dip Coating Setup To improve upon the atomizer thin film procedure, a dip coating synthetic setup was created to produce more uniform TiO2 thin films [17] (Fig. 4). Rather than manually dipping the substrates into the sol, a setup was created that would allow for consistent, controllable dip and withdrawal rates to improve uniformity of the thin films. Two open syringes were used to transfer TiO2 sols at a steady rate determined by the diameter of the needle. The glass substrate was placed inside the barrel of the lower syringe and the sol was dripped inside, essentially “dipping” the substrate as the liquid’s surface level rose (~2 cm/min). Once the substrate was entirely covered, it was allowed to soak in the solution for 10 minutes. The solution was subsequently dripped out of the syringe at the same rate, and the substrate was allowed to cure for 10 minutes before being dried at 100°C for 1 hour and calcined at 550°C for 3 hours. In future work, this method will be adapted to deposit uniform, thin layers of TiO2 upon the monolayer MoS2.
Biology and Chemistry Research 2.3 Synthesis of monolayer MoS2 High-quality, uniform monolayer molybdenum disulfide (MoS2) films were deposited onto sapphire substrates (~1 cm2) using a new self-limiting chemical vapor deposition (CVD) technique developed by Yu et al [14]. MoCl5 (>99.99% Aldrich) and sulfur powder (Aldrich) which were used as precursors, were heated up to 850°C (28°C/ min) at low pressure (2 torr) in a tube furnace under argon flow (50 sccm). Gaseous MoS2 was formed under the high temperature and was deposited as a solid film onto a substrate downstream (Fig. 5a).The synthesis was based on a self-limiting process in order to accurately control the number of MoS2 layers deposited. The partial pressure of MoS2 gas (PMo) must be larger than the vapor pressure of the film (P°Mo) for the reaction to move forward. The film vapor pressure increases with layer number. Therefore, to guarantee the formation of a smooth, single atomic monolayer, the MoS2 partial pressure during synthesis was adjusted to remain between the vapor pressure of monolayer and bilayer MoS2, with the larger vapor pressure of bilayer film preventing further deposition (Fig. 5b). Unlike current exfoliation techniques, this synthesis procedure produced high quality, uniform, centimeterscale MoS2 monolayer films.
an argon environment to allow the TiO2 to better bind to the monolayer MoS2. Two different co-catalysts were synthesized and compared: one using the synthesized TiO2 with PEG additive and the other with Degussa P25 TiO2. 2.5 Data Collection Raman mapping measurements were carried out using a Horiba Labram HR800 Raman Microscopy with an excitation wavelength of 532 nm. A Shimadzu 1700 UVvis was used to carry out UV-vis spectrum and methyl orange dye degradation tests. The thickness and surface topology of the produced substrates were measured using an Atomic Force Microscope (AFM, Veeco Dimension-3000). The photocatalytic ability of the synthesized TiO2 powders were compared to that of commercial Degussa P25 by examining each powder’s degradation rate of methyl orange dye. Methyl orange powder (33mg/L) was added to a solution of 0.020M citric/citrate buffer (pH = 3.0) and DI water. TiO2 powder (0.2g) and 30 mL of the methyl orange solution were combined in a beaker and stirred constantly under illumination with a 100W light source. Samples of methyl orange were drawn out every 5 minutes using a syringe and nylon filter to filter out TiO2 particles and were analyzed using a spectrophotometer. The absorbance at a chosen wavelength (504.7nm) was monitored for 45 minutes. By examining the rate of decrease in absorbance, photocatalytic ability of the individual TiO2 powders was compared. The catalysts’ water-splitting abilities were tested using a custom-designed experimental setup (Fig. 6b). The catalyst substrates were secured inside short sections of dialysis tubing (Fig. 6a), which were used to allow the movement of water and trap the produced gas. The catalysts were then placed in small glass containers and immersed completely in a methanol and water solution (1:4 by volume). A 100W light source was used to illuminate the samples with a water tank placed in between to absorb heat and prevent evaporation. Images were taken at different times to monitor gas production.
2.4 Synthesis of monolayer MoS2/TiO2 co-catalyst The monolayer MoS2/TiO2 co-catalyst was produced by depositing a thin layer of TiO2 on top of the existing monolayer MoS2 film. In depositing the TiO2 layer, it was important to create a thin layer so as to not obscure the underlying MoS2 film. The same procedure used for producing TiO2 films with an atomizer mentioned above (Section 2.1) was carried out on a monolayer MoS2 substrate. After the TiO2 layer was deposited, the co-catalyst was heated at 200°C for 30 minutes in Volume 3 | 2013-2014 | 7
Biology and Chemistry Research vibrations of Mo and S atoms [14]. The frequency difference between the two modes is known to correspond with layer number and can be used to determine the thickness of the MoS2 film. Monolayer MoS2 with its single layer of MoS2 units has fewer possible vibrational modes as compared to bulk MoS2 material, resulting in a smaller frequency difference (20 – 22 cm-1 versus 26 cm-1) between the two modes. This is clearly illustrated in Figure 8, which shows the Raman spectra of bulk MoS2 (b) as well as the synthesized MoS2 film (a) with an extra peak in the monolayer spectrum that corresponds to the sapphire substrate used. Numerous Raman spectra were taken at different locations on the substrate. The Ag and E12g peak positions were identical for these spectra, indicating that the entire MoS2 film was a homogenous monolayer.
3. Results 3.1 Characterization of TiO2 powder and MoS2 thin films The composition of synthesized TiO2 powder and uniformity and thickness of MoS2 thin films were analyzed using Raman spectroscopy and Atomic Force Microscopy (AFM). 3.1.1 Raman Spectroscopy The synthesized TiO2 powder and MoS2 thin films were characterized using Raman spectroscopy. The composition of the mixed-phase TiO2 powder was confirmed by the anatase and rutile peaks present in the Raman spectrum (Fig. 7) and were consistent with the peaks for mixedphase TiO2 found in literature [10]. The MoS2 thin films exhibited excellent uniformity as indicated by Raman measurements. Two characteristic modes can be found in the spectrum: the Ag mode associated with out-ofplane sulfur atoms and the E12g mode related to in-plane
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3.1.2 Atomic Force Microscopy Atomic force microscopy (AFM) was used to characterize the thickness and surface topology of the MoS2 films (Fig. 9). Intentional scratches were introduced to the thin film to measure the difference in thickness between the bare substrate and the MoS2 film. The data showed a smooth (roughness < 0.2nm) and continuous MoS2 surface with no steps or voids. A 0.68nm difference in thickness was recorded between the sapphire substrate and MoS2 film, which matches literature values for the thickness of monolayer MoS2 [14]. 3.1.3 UV-vis analysis The absorbance of TiO2, monolayer MoS2, and monolayer MoS2/TiO2 thin films were analyzed using UV-vis. The spectrum of the MoS2/TiO2 co-catalyst showed higher absorption in the visible range (400 – 700 nm) over TiO2. Spectra of TiO2 and MoS2 were taken as controls (Fig. 10). The results also confirm the presence of both TiO2 and MoS2 in the co-catalyst. The co-catalyst curve’s shape strongly follows that of TiO2, but two characteristic MoS2 peaks also appear in the 600 – 650 nm region.
Biology and Chemistry Research surface parameters and higher catalytic ability due to the PEG additive, these properties were not reflected in the data. This may be due to the commercial processing that Degussa P25 TiO2 undergoes to achieve particles sizes of 20 – 30nm, thus giving it a much larger effective surface area than that of the synthesized TiO2 powders that were ground by hand with a mortar and pestle and had grain sizes of ~1mm. 3.2.2 Water-splitting Hydrogen Production Tests
3.2 Photocatalytic Ability 3.2.1 Methyl Orange Dye Degradation Photocatalytic ability was assessed by examining the degradation rate of methyl orange (MO) dye. Three different TiO2 powder samples were tested: Degussa P25 and two samples of TiO2synthesized with PEG additive (pure anatase and mixed-phased). After 45 minutes of illumination, Degussa P25 TiO2 showed the largest amount of degradation (12.8%) with the synthesized mixed-phase and pure anatase TiO2 powders degrading 2.06% and 1.55% of the methyl orange dye, respectively (Table 1). The larger amount of degradation in the mixedphase TiO2 supports the literature findings that mixedphase anatase and rutile is more photocatalytically active than phase-pure anatase TiO2. Although the synthesized TiO2 powders had rougher
The final step was to examine the MoS2/TiO2 cocatalyst’s efficiency by assessing its ability to produce hydrogen gas in water. Four catalysts were tested: two monolayer MoS2/TiO2 co-catalysts made with either synthesized TiO2 or Degussa P25, one plain Degussa P25 TiO2 substrate, and one glass substrate as a control. Hydrogen production was evaluated by taking images of the water-splitting reactions after 30 minutes, 1 hour, and 24 hours of illumination (Fig. 11). After 24 hours, the catalysts were agitated to remove the bubbles on the surface and illuminated for another few hours to observe whether gas would continue to be produced. The images of the reactions show a clear difference in the amount of hydrogen produced by the different substrates. After 30 minutes of illumination, pure TiO2 substrates (Fig. 11b) produced a small amount of gas bubbles, while both MoS2/TiO2 co-catalysts (Fig. 11c, d) produced significantly larger amounts of gas. The control slide showed only a single bubble after 24 hours of illumination (not shown), indicating that the gas observed was not a result of evaporation or gas coming out of solution. Out of the two co-catalysts tested, the co-catalyst with Degussa P25 TiO2 showed more hydrogen production, as evidenced by the higher bubble density on the catalyst’s surface. This is consistent with the previous methyl orange degradation results that showed higher photocatalytic ability in Degussa P25. After 30 minutes of illumination, the catalysts did not appear to continue producing significant amounts of gas, which may have been due to the fact that the catalyst surface was covered with bubbles, thus limiting its contact with water molecules. After agitation and further illumination, the catalysts continued to produce hydrogen gas. 4. Discussion
Fig. 11: Images after 30 minutes of illumination of a) plain glass, b) TiO2, c) MoS2/TiO2 co-catalyst with synthesized TiO2, and d) MoS2/TiO2 co-catalyst with commercial Degussa P25.
The foremost achievement of this research was the synthesis of a monolayer MoS2 and mixed-phase TiO2 thin film photocatalyst for use in water-splitting hydrogen production. This was accomplished by first creating a uniform monolayer MoS2 film, then synthesizing mixedphase TiO2 powder, and finally depositing a thin layer of TiO2 on top of the monolayer MoS2 film to form a cocatalyst. Significant testing was performed on the synthesized cocatalyst and its components to show their potentials Volume 3 | 2013-2014 | 9
Biology and Chemistry Research in photocatalytic water splitting. The monolayer MoS2 films made using a new CVD self-limiting procedure were found to be of high quality. AFM measurements and Raman spectra taken at several locations on a single film showed excellent monolayer uniformity across the entire film. Compared to current methods of mechanical or chemical cleavage of bulk material, this CVD procedure was confirmed to produce larger area films and offer better control over the thickness or layer number of the deposited film. Raman spectra were used to confirm that the TiO2 powder synthesized was indeed mixed-phase. Methyl orange degradation tests performed on three different TiO2 powders showed that commercial Degussa P25 had the highest photocatalytic ability, which may have been due to the larger particle size, and thus smaller surface area, of the synthesized powders. However, between the two synthesized powders, mixed-phase anatase and rutile was more catalytically active than pure anatase TiO2, which was supported by the literature. Tests performed on the monolayer MoS2/TiO2 co-catalyst indicated its improved absorption and higher water-splitting abilities as compared to plain TiO2 systems. UV-vis spectra taken of the co-catalyst showed increased absorption in the visible range of light, a property that allows the co-catalyst to better harness solar energy in the visible range. The cocatalyst also showed significant improvement over TiO2 in hydrogen production when illuminated in a methanolwater solution. During the testing, it was found that gas production stopped after the catalysts’ surfaces were covered, indicating that frequent agitation is needed to allow for continued hydrogen production. To the best of my knowledge, this project is particularly unique because TiO2 has never before been combined with monolayer MoS2; it has only been coupled with bulk MoS2 and fewlayer MoS2. Monolayer MoS2 compared to few-layer or bulk MoS2 has advantages in a larger bandgap and higher surface to volume ratio, which results in a more efficient water-splitting photocatalyst films and can be adapted to deposit MoS2 with different dimensions and parameters. 5. Conclusions and Future Work Through my research, I was able to synthesize a new lowcost monolayer MoS2/mixed-phase TiO2 photocatalyst that shows potential in improving solar-powered hydrogen production through splitting water. This technology can supply a source of clean, abundant, and sustainable energy to meet the world’s growing energy needs. Future work includes: •Further processing of TiO2 powders to achieve particle sizes comparable to Degussa P25. •Dissolving the sapphire substrate using concentrated NaOH to increase contact between the co-catalyst layers and water. •Increase control over TiO2 deposition using dip-coating setup shown above (Section 2.2) •Quantifying hydrogen production rate in pure water 10 | 2013-2014 | Volume 3
•Testing co-catalyst water-splitting performance under natural sunlight. References [1] W. Zhou, Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, H. Liu, J. Wang, and H. Zhang, “Synthesis of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures for enhanced photocatalytic activities.,” Small (Weinheim an der Bergstrasse, Germany), vol. 9, no. 1, pp. 140–7, Jan. 2013. [2] T. D. Silipas, E. Indrea, S. Dreve, R.-C. Suciu, M. C. Rosu, V. Danciu, V. Cosoveanu, and V. Popescu, “TiO 2 – based systems for photoelectrochemical generation of solar hydrogen,” Journal of Physics: Conference Series, vol. 182, p. 012055, Aug. 2009. [3] U. G. Akpan and B. H. Hameed, “Solar degradation of an azo dye, acid red 1, by Ca–Ce–W–TiO2 composite catalyst,” Chemical Engineering Journal, vol. 169, no. 1–3, pp. 91–99, May 2011. [4] A. L. Linsebigler, G. Lu, and J. T. Yates, “Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results,” pp. 735–758, 1995. [5] K. H. Hu, X. G. Hu, Y. F. Xu, and J. D. Sun, “Synthesis of nano-MoS2/TiO2 composite and its catalytic degradation effect on methyl orange,” Journal of Materials Science, vol. 45, no. 10, pp. 2640–2648, Jan. 2010. [6] X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications.,” Chemical reviews, vol. 107, no. 7, pp. 2891– 959, Jul. 2007. [7] Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, and and H. Z. , Yinghui Sun, Gang Lu, Qing Zhang, Xiaodong Chen, “Single-Layer MoS 2 Phototransistors,” ACS Nano, no. 1, pp. 74–80, 2012. [8] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS 2 transistors,” vol. 6, no. March, 2011. [9] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor.,” Physical review letters, vol. 105, no. 13, p. 136805, Sep. 2010. [10] O. Frank, M. Zukalova, B. Laskova, J. Kürti, J. Koltai, and L. Kavan, “Raman spectra of titanium dioxide (anatase, rutile) with identified oxygen isotopes (16, 17, 18).,” Physical chemistry chemical physics : PCCP, vol. 14, no. 42, pp. 14567–72, Nov. 2012. [11] S. Šegota, L. Ćurković, D. Ljubas, V. Svetličić, I. F. Houra, and N. Tomašić, “Synthesis, characterization and photocatalytic properties of sol–gel TiO2 films,” Ceramics International, vol. 37, no. 4, pp. 1153–1160, May 2011. [12] D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. a Shevlin, A. J. Logsdail, S. M. Woodley, C. R. a Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, G. W. Watson, T. W. Keal, P. Sherwood, A. Walsh, and A. a Sokol, “Band alignment of rutile and anatase TiO2.,” Nature materials,
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