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Effects of photocatalytic activity of metal and non-metal doped Tio2 for Hydrogen production enhancement - A Review

Titanium dioxide TiO2 is well-known materials that has become an efficient photocatalyst for environmental sustainability. Known as solar driven catalysis, TiO2 is considered as the most promising way to alleviate environmental issues caused by the combustion of fossil fuels and to meet worldwide demands for energy. Much effort has been concerned on TiO2 band gap modification to become a visible-light-activated photocatalysts of TiO2 because it can only be excited by UV light irradiation due to its large band gap. Modifications like metals and nonmetals doping has been proposed in the past decades. This reviews survey recent advanced preparation methods of doped-TiO2 including various types of doping methods for various types of dopants and provides general review on further modifications. The characterizations techniques used in order to determine the structural, morphological and optical properties of modified TiO2 is also discussed. Further, a new method of TiO2 modification is proposed in this mini review paper.


Introduction
Titanium dioxide (TiO2) or commonly known as titania is the most effective photocatalyst for air and water purification and self-cleaning surfaces, and also widely used for energy-related fields especially in water splitting under visible light irradiation [1][2][3]. According to previous research, TiO2 has been proven to be the most efficient and economic photocatalyst due to its capability to completely mineralize contaminants [4][5], low cost material, superhydrophilicity [6,8] for self-cleaning function and antibacterial agent [9], non-toxicity, and has high oxidative abilities [10][11][12].
Photocatalysis was first discovered by Fujishima in 1970's and later called as Honda-Fujishima Effect. A strong oxidation and reduction power of TiO2 was realized through a simple experiment of TiO2 electrode in an aqueous solution exposed to strong light. Photoinduced decomposition of water on TiO2 electrodes are reported as a gas bubbles evolved from the surface of electrodes after being exposed to strong light. Analogically, photocatalytic reaction on TiO2 was found to be similar with photosynthesisas illustrated in Figure 1; light is used as an energy that enables to break water molecule into hydrogen and oxygen [13]. Light is absorbed by (a) titania to break water molecule into hydrogen and oxygen, and (b) chlorophyll during photosynthesis to produce oxygen gas [13].
Despite all the interesting features in TiO 2 , its large energy band gap is in range 3.0 eV (rutile) to 3.5 eV (anatase) corresponding to wavelengths of 385 and 410 nm respectivelywhich allows the absorption of light in ultraviolet range only [28]. Besides, fast recombination of electrons and holes, slow charge carrier transfer and also requires high reproduction cost limit the efficiency of photocatalytic activity of TiO 2 [29]. Due to this flaw, various attempts has been done in order to modified the band gap of TiO 2 ; with such it will broaden the light absorption range mainly in visible light range that corresponds about 40 % of light spectrum [28,[30][31][32][33]. The photocatalytic activity under simulated sunlight irradiation is determine from the band gap, the valence band VB and covalence band CB edge positions of photocatalyst [17].

Preparation of Doped TiO 2
There are various chemical and physical methods available on preparing doped-TiO 2 -either anatase or rutile structure. Those methods including ion-assisted sputtering, plasma, ion-implantation, chemical vapor deposition (CVD) and sol-gel to enhance the photocatalytic effect in the visible light region. A list of doped TiO 2 and methods of its preparation are shown in Table 1

TiO 2 Doped with Metal Species
Metal doped TiO2 especially transition metal has been extensively investigated over the past decades for enhancing its photocatalytic performance on the degradation of various organic pollutants under visible irradiation. This modification is done to extend the optical absorption of TiO2 to visible light. [39] Fe Titanium tetraisopropoxide, 97% pure, ethanol, diethanolamine and iron nitrate, 98% pure, were used for preparation of thin films. Indium doped tin oxide (SnO 2 :In) conducting glass was used as substrate. A transparent gel solution of doped/ undoped TiO 2 was prepared by mixing of 3 ml TTIP and calculated amount of iron nitrate in 20 ml ethanol in the presence of diethanolamine. The solution was stirred for 4 hours at room temperature to enhance the reaction rate between diethanolamine and TTIP and finally it gets converted into gel. This gel solution was applied on nearly two third area of the conducting glass substrate and uniformly coated with spin coating unit. After each successive layer the film was allowed to dry for 10 min at 80 °C. Final sintering was done at 500 °C for 2 h. [40] It is reported that metal doping has shown photocatalytic activities at both UV and visible light range [43]. Furthermore, metal-doped TiO 2 will caused many drawbacks including dopant insolubility, thermal instability during the process, phase transformation among the titania polymorphs, the increase of carrier-recombination centers, and the alteration of charge carrier diffusion length.

TiO 2 With Non-metal Species
Nonmetal-doped TiO 2 is reported as the most promising photocatalysts candidates [44]. An extensive research of nonmetal-doped TiO 2 photocatalysts was widely carried out since the early 1990s [5,8]. Among all the nonmetal-doped TiO 2 photocatalysts, nitrogen-doped TiO 2 is the most studied material [45]. In addition, several other visible-light active materials, such as carbon C, F and S, doped TiO2 were reported [19].  The three components were used in the preparation of the TiO 2 Sol-Gel films it can be explained as follows: Titanium Isopropoxide (TIP), Acetic acid and Ethanol. TiO 2 thin films are prepared by Sol-Gel spin-coating method using 50 ml of Ethanol and allowed to mix for 50 min, measure 5 ml of acetic acid with a pipette into Ethanol and stir by using a magnetic stirrer it for 5 min or at least 3 min, and measure 6.3 ml of TIP was added by pipette to a beaker containing a mixture of glacial acetic acid and ethanol that had been mixed for five minutes. The mixture was continually stirred using a magnetic stirrer addition and for a further two minutes after addition of the precursor. The concentration was altered in order to assess the effects of ratios and concentration. Glass slide coated with transparent conducting indium tin oxide (ITO) films were used as substrates. [10] Zz N, S, C Doping was performed by an incipient wet impregnation method as the N, C and S-sources. TiO 2 Degussa P25 and appropriate amounts of N, C and S were mixed with specific volumes of doubly distilled water and stirred for 1 hour. During this period, the mixture changed color into a yellowish/ greyish white depending upon the anion concentration. Then, the prepared photocatalysts were washed with water three times, heat-treated at 373 K for 24 hours to eliminate water, calcined at three different temperatures in the range 623-973 K for 1, 3 and 5 hours, ground and sieved.
[41] N TiO 2 nanostructures were prepared using LPD method. ITO substrates underwent standard cleaning procedure where they were cleaned using acetone and ethanol in sequence. The growth process was carried out under room temperature. The as prepared nanostructured TiO 2 was rinsed and dried before it was annealed at 400 °C for 30 minutes. Growth of the TiO 2 nanoparticles was started with hydrolysis of the metal flouro cation used ((NH 4 ) 2 TiF 6 ) with boric acid was used to scavenge the flouro cation left. TiO 2 nanoparticles film formed on the surface was then subjected towards annealing process to purify the metal oxide nanoparticles produce and to remove organic materials left during the hydrolysis process. The treated samples were then subjected towards heating at 100 °C for 15 minutes to remove any hydroxyl compounds. [42] The three components were used in the preparation of the TiO 2 Sol-Gel films it can be explained as follows in Figure 2 TiO 2 thin films are prepared by sol-gel spin-coating method using 50 ml of ethanol and allowed to mix for 50 min, measure 5 ml of acetic acid with a pipette into ethanol and stir by using a magnetic stirrer it for 3 to 5 min. The mixture was continually stirred using a magnetic stirrer addition and for a further two minutes after addition of the precursor. The sol-gel concentration was altered in order to assess the effects of ratios and concentration. The effects of altering molar ratios of acid are investigated. The Sol-Gel manufactured due to the method described above was immediately used to produce coating on the various substrates. Thin films of TiO 2 were deposited by spin coating method using a standard photoresist spinner [1,10,46].
In short, the Sol-Gel process is one of the versatile methods in preparing Nano-size materials. This technique does not require a complicated instrument which provides a simple and easy means for preparing Nano-size particles. The incorporation of an active dopant in the Sol-Gel during the gelation stage allows the doping elements to have a direct interaction with support, therefore, the material possesses catalytic or photocatalytic properties. Titanium precursor, such as titanium isoprooxide (TIP), tetrabutyl orthotitanate (TBOT), titanium tetra chloride, is mixed with dopant precursor dissolved in alcohol, followed by hydrolysis performed at the room or elevated temperature. The precipitate is dried usually at temperature range from 80 to 110°C, pulverized to obtain xerogel and calcinated in air at temperature from 200 to 600°C [46].

Morphological Surface and Photocatalytic Analysis
Among metal dopants as mentioned previously, Fe (III) ions as the most favourable dopant due to its unique electronic structure and its similar size with titanium (IV), Ti 4+ ions [40]. The energy level of the Fe 4+ /Fe 3+ couple is only just greater than the titania conduction band and the energy level of the Fe 3+ / Fe 2+ couple is just above the valance band [47]. This useful electronic states of iron ions in titania results in formation of efficient trapping sites for electrons and holes [48].

Crystalline structure of doped-TiO2
Aside of types of dopant substitute into TiO2 lattice structure, the annealing temperature of TiO 2 thin films also plays a crucial roles in order to produce a rutile or anatase semiconductor. A pure TiO 2 films is prepared at various annealing temperature from 300 to 800 °C. Figure 3 shows the X-ray diffraction XRD patterns of the prepared samples. It was found that the pure TiO 2 films with anatase phase increase when annealing temperature increased. The pure TiO 2 films formed of anatase phase show a very high efficiency photocatalytic activity due to their large internal surface. Both the optical properties and the photocatalytic activity of TiO 2 coatings depend strongly on the crystalline phase, the crystallite size and the porosity of the coatings. Moreover, the TiO 2 films formed of rutile phase when the sample annealed at a temperature more than 700 °C [6]. Figure 4 shows the XRD pattern of pure TiO 2 and 0.2 at.% Fe doped TiO 2 films at final sintering temperature at 500 °C for 2 h. . The obtained patterns shows a predominance of sharp, intense peak corresponding to underlying SnO2:In layer on the substrate. From the pattern, weak intensity peaks corresponding to anatase phase of TiO 2 is obtained for all samples of Fe 3+ doped TiO 2 . Besides, a high crystalline material was obtained for 0.2 at.% Fe doped TiO 2 . By using Scherrer's equation (D = Kλ/ βcos θ), the crystallite size was calculated and the sizes of TiO 2 particles are confirmed in nanosize for different concentration of dopant [40].
As for non-metal doped TiO 2 , a carbon-doped TiO 2 thin films were prepared by hydrothermally synthesized using Ti(SO 4 ) 2 and glucose as titania and carbon resources for 12 to 48 hours [19]. The obtained carbon-TiO 2 nanocrystals display higher energy conversion efficiencies than pure TiO 2 when applied as the photo-anode materials in dye-sensitized solar cells DSSCs. Based on the combinational analyses on dye adsorption efficiency, photoluminescence and electron lifetime of TiO 2 , the retarded hole-electron recombination rate and the negatively shifted flat band potential were revealed as the main reasons for the improved energy conversion efficiency.
XRD pattern in Figure 5 shows an obvious anatase phase peaks in each sample. This proves that the presence of glucose does not cause the change of crystalline phase. The calculated crystallite size of carbon-TiO 2 is in the range of 14 to 19 nm that is smaller than that of pure TiO 2 (24 nm). Smaller size of TiO 2 nanoparticles will improved particle dispersity for conversion efficiency advancement.  Figure 3. X-ray diffraction patterns of pure TiO 2 films after annealed at different temperature [6].  The band gap of the TiO 2 :Zn 2+ is decreased with increasing Zn 2+ dopant. Since the valence band edge of TiO 2 material is dominated by O 2p, and the conduction band edge is formed from Ti 3d, some of the new occupied molecular orbitals located below the conduction band of TiO 2 would be formed when Zn 2+ ions were doped into TiO 2 lattice. The decreased of band gap would likely cause to the charge transfer from the dopant energy level Zn 2+ to the conduction band of TiO 2 [47]. Besides, other dopant that shows an interesting result is carbon. C-doped-TiO 2 proved that the particles are crystalline with spherical morphology, high surface area and an optical band gap of 2.79 eV for C-TiO 2 annealed at 400 °C as reported by Warkhade et. al. [11].

Conclusion
The modification methods of x-doped-TiO 2 have been discussed in this review paper. Between two types of dopant elements that are metal and non-metal, non-metal (anions) shows a significance role in order to enhance the performance of TiO 2 photocatalyst. The biggest challenge involving TiO2 as photocatalysts is to increase its spectral sensitivity to visible light. A new dopants and new method in preparing modified structure of TiO 2 would be the next approach of future research. With such, a new applications of TiO 2 in various sector would be revealed. Currently, the major problem with doped TiO 2 may be the loss of photoactivity during recycling and long-term storage. It is seems that the efficiency of doped-TiO 2 under visible light strongly depended on the preparation method used.

Acknowledgement
The author would like to express their thanks to government Malaysia for the financial support in this project through FRGS grant 1620, and also UTHM through RMC for their management of the grant.