De la motivación en los estudiantes del cuarto grado de secundaria de la I.E.

In order to improve efficiency of photocatalysis, several techniques of material modification have been explored. As physical properties of materials influence the photoactivity, different physical properties of surface or structure such as nanoparticle, nanotube, Z scheme, defect disorder, including layered and core-shell structure would be useful.

- Influence of particle size.

Nanoparticle size can accelerate photoactivity as can be seen by the observation that nanocrystal BiVO4 could decompose methyl orange around 8% better than sample prepared by solid state method.43

- Influence of high surface area materials

It is widely known that surface area plays a vital role on photoactivity. There are many attempts to increase photoactivity by configuring surface properties. High surface area 200-300 m2/g of coloured Cu or Ni incorporated mesoporous tantalum oxide is found to be helpful in acceleration of photoactivity of tantalum oxide for water splitting.44

25 - Influence of Z scheme construction.

Z scheme has been confirmed in assisting photoactivity. Construction of Z scheme between Rh doped SrTiO3, which produced only hydrogen, and BiVO4 or WO3 can generate both H2 and O2.45 In addition, mimicking of Z-scheme by using redox mediator of I-/IO3- in the combination of Pt–WO3 and the Pt loaded on Cr-Ta doped SrTiO3 can split water.27

- Influence of structure character : nanotube, nanosheet, layered, core-shell

Since nanotube and nanosheet structure of several materials can be prepared, the utilisation of these structures in photocatalysis has been studied extensively. It has been reported that single wall carbon nanotubes irradiated with visible light from camera flash in vacuum can split water into hydrogen about 80 mol % in one flash 0.1-0.2J/cm2, 8 ms) and can reach 900 ppm of the mass.46 Whereas titanium nanosheet coated on CdS photoelectrodes have been revealed for visible light photooxidation of water. Unfortunately, titanium nanosheet coating is not useful for photooxidation of water, probably photogenerated holes could not transfer across titanium nanosheet.47

Layered perovskite Sr2(Ta1−XNbX)2O7 controlling band gap energy by solid solution were studied to explore the photocatalytic water splitting. A Sr2(Ta0.75Nb0.25)2O7 photocatalyst showed a relatively high activity among the solid solutions of Sr2(Ta1−XNbX)2O7 (0 < X < 1).48 The results of layered perovskite Sr2(Ta1−XNbX)2O7

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is consistent with Sr3Ti2O7, layered perovskite Ruddlesden–Popper phase. Sr3Ti2O7

also exhibits a promising photoactivity under UV light without co-catalyst.49

Ikeda et al. studied the effect of core-shell structure on photoactivity. Partially modified core-shell structure of Pt loaded SrTiO3-silica with fluoroalkylsilylation agent has been compared with conventional suspension system in overall water splitting. The former has higher photoactivity than the latter, probably, indicating that different properties of photocatalyst can be obtained by different synthetic method.50

- Influence of defect disorder

Defect disorder of cerium titaniate compounds (Figure 1-15) have been getting attention on photoactivity under visible light because of their colours, such as yellow, red, and grey. Defect disorder of oxygen vacancies(V_o••, V_o•, V_ox), interstitial sites (O_i′ O_i′′ O_ix) and holes on lattice oxygen(O_o• ) might act as colour centers.51 Ce2Ti2O7 (red-brown), Ce2/3TiO3 (black, metal like behaviour), and Ce4Ti9O24(brown), with mainly Ce3+, and CeTiO4 (yellow) and CeTi2O6(pale yellow), with mainly Ce4+, showed some activity on photooxidation of MB under visible light but composite powder of CeTiO4 with SrTiO3 has higher photocatalytic activity.51 Moreover, Ce4+-TiO2 powder, predominant anatase titania and crystalline cerium titanate (11.18 wt.% CexTi1−xO2), has shown higher efficiency than nanocrystallites in respect of photocatalytic activity.52

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Figure 1-15 :Diffuse reflectance spectra of the cerium titanates (reproduced from [51]).

- Influence of the incorporation of unique structure.

Incorporation of photocatalyst material with other unique structure material has been investigated in many ways in order to improve efficiency. Shangguan53 has reviewed photoactivity rendered by modification of materials and found that pillared layered structure might be interesting for hydrogen production. Pillared layered compounds prepared of interlayer compound host such as KTiNbO5, K[Ca2Nax-3NbxO3x+1] and pillar metal oxide has been investigated for photoactivity improvement. Silica clusters-pillared Ca2Nb2.9Cr0.1O10 layers produced the surface area of 239 m2/g. Intercalation of CdS in K2Ti3.9Nb0.1O9 evolved hydrogen higher than CdS because photogenerated electrons can be quickly move.53

Nafion was also used for this purpose as well. Coating TiO2 is one of the techniques to activate TiO2 under visible light. Nafion, perfluorinated polymer, has been successfully coated on TiO2 and active under visible light for photocatalytic

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degradation of dye such as methylene blue and rhodamine. Nafion layer might retard the charge recombination.54

In addition to Nafion, fullerene is inspired by scientists. Photodegradation rates could be enhanced by increase charge separation. High migration efficiency at the surface could be modified by adsorbed fullerene, C60, on the surface. C60-modified Bi2WO6 photocatalyst has been reported for improvement of photodecomposition of dyes. Conjugation of π system at the interface is thought to be affected at the surface.55

Photocatalytic decomposition of NOx(NO,N2O) into N2 and O2 and photoreduction of CO2 in water to produce CH4 and CH3OH has been achieved by using selective and unique framework structure of zeolites containing transition metal oxide (Ti, V, Mo, Cr) or incorporation of transition metal ions (Cu+, Ag+, Pr3+)(Figure 1-16). Rapid separation of photogenerated electrons and photogenerated holes is the advantage of this system.56

Figure 1-16 : Illustration of photoactivity of zeolite containing photocatalyst (reproduced from [56]).

29 - Influence of charge migration

As mention earlier about the improvement of photoactivity by increase of surface area and incorporation of other unique character materials, charge separation might be another factor for that improvement. Charge migration was confirmed as a useful factor for efficiency improvement by the photoactivity of heterogeneous microjunctions of TiO2-Sr0.95La0.05TiO3+δ on photodegradation of methylene blue. The photodecolourisation of methylene blue was several times better than naked TiO2.57 Omata and Matsuo also found that the p-n junction of composite between n-type material and p-type material was helpful in photocatalysis process. Composite materials of n-type conductor, with p-type conductor, SrZr0.95Y0.05O3−δ,

CaZr0.95Y0.10O3−δ and CaZr0.95Ga0.05O3−δ have been revealed on photodegradation of

methylene blue and formic acid. Results confirmed the higher photoactivtiy was obtained via separation of photogenerated electrons and photogenerated holes through the p-n junction.58