1. Introduction

Conserving the environment is becoming an ever-growing issue as new materials are being synthesized. These new materials have complex structures, making it difficult for them to be decomposed naturally. Using photocatalysis, it is possible to degrade organic dyes under irradiation of light. Transition metal oxides such as titanium dioxide or tungsten oxide are used for this because their energy levels can allow (i) absorption in the UV and visible light region and (ii) the use of this absorbed energy for reduction of oxygen. Thus, facilitating the decomposition of organic compounds.

Although Titanium dioxide is most widely used in photocatalysis studies, because it has a band gap energy of 3.2 eV (anatase), it functions only in UV light conditions, only being spread into the visible light region when treated with by methods such as doping. Tungsten oxide has the advantage of working better in visible light conditions than titanium dioxide because it has a relatively smaller band gap of 2.4~2.8 eV[1]. This is important because only about 4 to 5% of solar radiation is UV light while approximately 40% is visible light[2].

However, even with the smaller band gap, tungsten oxide does not show a high activity in visible light because the conduction band has an energy level that is higher than the O₂/O₂^- reduction potential[3]. There are various methods to improve the photocatalytic degradation efficiency of tungsten oxide in visible light, with the simplest being changing the morphology of tungsten oxide. Other methods would be using noble metal composites, carbon composites, or coupling with other semiconductors, along with other more complex methods. Here the advantages and disadvantages of each method are examined in more detail.

2. Tungsten Oxide (WO₃)

For most of the recent methods for synthesizing 3D nanostructures from tungsten oxide, a special template is needed[4]. However, such structures can also be achieved without a template in a hydrothermal process where the pH of the precursor solution is modified. The solution consists of Na₂WO₄⋅2H₂O, NaCl, HCl mixed in deionized water. In this article, tungsten oxide was fabricated into a 3D architecture consisting of 1D nanorods, which is efficient in degradation of rhodamine B (RhB) under UV-light. Measurements were conducted for samples WO₃-1.5, 2.0, 2.5 and 3.0. FESEM of the samples with highest photodegradation efficiency, the samples with precursor solution pH of 1.5, showing nanorods with smooth surfaces that make flower-like structures which are 3~5 μm in diameter, and of pH 2.0, showing sandwich structures with two outer layers consisting of a bundle of nanorods, one thin interlayer consisting of horizontal nanorods (Fig. 1). The photodegradation efficiency was tested for RhB in UV-light and divided by the BET surface area, so as to eliminate the effects of surface area and make a comparison based on the effects of photoinduced charges. Photodegradation efficiency of WO₃-1.5 and WO₃-2.0 is higher than WO₃-2.5 and WO₃-3.0. Due to having a partly monoclinic structure, the result for WO₃-1.5 stretched into the range of visible light. WO₃ synthesized at lower pH showed higher charge separation with lower charge carrier recombination resulting in longer life time and higher photocatalytic efficiency. This mechanism can be extended to other areas like the purification of water, gas sensors, and solar energy cells.

2.1. Nanocomposite of noble metal and tungsten oxide

WO₃ is effective as a visible light photocatalyst in the decomposition of organic compounds, especially when loaded with Platinum[5]. While decomposition by WO₃ was believed to be ineffective in air due to the conduction band (CB) level being more positive than the reduction potentials of O₂, because Pt catalyzes the more positive multi-electron reduction of O₂, high efficiency in PtWO₃ was observed. A photodeposition method was used to load Pt on WO₃ of particle size 50~200 nm, using H₂PtCl₆⋅6H₂O and visible light irradiation, and resulted in platinum particles of average size 5 nm, uniformly distributed on the surface of WO₃. Nitrogen-doped TiO₂ (N-TiO₂) was prepared as a reference sample. With full-arc irradiation in an aqueous acetic acid (AcOH) solution, Pt-WO₃ showed a higher rate of CO₂ generation than TiO₂ and presented a rate nearly as high as in full-arc for visible light. Comparing the action spectra with the photoabsorption data for pure WO₃ leads to the conclusion that the band gap of WO₃ is responsible for the decomposition. Isopropyl alcohol (IPA) decomposition in the visible light region for WO₃ with an optimal Pt loading (0.5 wt%) showed a 100-fold enhanced CO₂ generation rate (from 0.4 to 48 μmol/h), while for N-TiO₂ with optimal Pt loading showed only a 4-fold increase. Another group reported that when platinum is loaded on WO₃, it acts as a co-catalyst and facilitates multielectron O₂ reduction, improving photodegradation efficiency[6]. WO₃ nanoparticles with an original size of 7.3 ± 1.8 nm was annealed at temperatures 300, 350, 400, 450, 500, 550, and 600°C. The higher the annealing temperature, the larger the resulting particle size. XRD peaks of WO₃ corresponded to the monoclinic phase. A photoreduction technique was used to load WO₃ with Pt. 1 wt% Pt was used because of its activeness in amaranth degradation. Crystallite size, and therefore particle size, grew with higher annealing temperature. Amaranth was used in calculating the rate of photocatalytic activity because it does not degrade under visible light without a photocatalyst. For the amaranth degradation by Pt-WO₃ under visible light when WO₃ particle size range was 18 to 26.4 nm, the initial rate was approximately 3.5 times higher than at 7.3~9.8 and 42.4 nm. Bare WO₃ showed similar tendencies. Pt-WO₃ was 6 times higher than bare WO₃ of same particle size. The initial rate for N-doped TiO₂ was lower than bare WO₃. When particles are at their optimal size of around 19.2 nm, the balance between surface area and surface recombination rate is most optimal. Photoluminescence (PL) spectra indicated that both the recombination of carriers and number of defects are minimum at 500°C where particle size was 19.2 ± 5.4 nm. It was also the most active for the degradation of rhodamine-B.

2.2. Nanocomposite of carbon and tungsten oxide

The graphitic carbon nitride (g-C₃N₄) sheets were synthesized by decomposition of melamine[7]. The synthesis of W₁₈O₄₉ nanorods (NRs) involved a solvothermal process including ethanol and heating at 200 °C for 6~8 h. g-C₃N₄/W₁₈O₄₉ nanocomposite (NC) was synthesized in a solvothermal process similar to W₁₈O₄₉ NRs. The TEM image show a plate shape with pores for g-C₃N₄ sheets. According to HRTEM, few-layered g-C₃N₄ sheets are strongly coupled with well crystallized W₁₈O₄₉ NRs of diameter 8~10 nm that are uniformly distributed on them. The g-C₃N₄/W₁₈O₄₉ NC showed higher adsorption and photodegradation efficiency for methylene blue (MB) and methyl orange (MO) than for the g-C₃N₄ sheets and W₁₈O₄₉ NRs, with a removal efficiency of 99.14% in the dark and 99.91% including photocatalysis in the case of MB. The higher adsorption capacity of W₁₈O₄₉ NRs and g-C₃N₄/W₁₈O₄₉ NC for MB than g-C₃N₄ is attributed to the existence of unique crystal defects in W₁₈O₄₉. While the high photocatalytic efficiency stems from the Z-scheme photocatalytic mechanism operating in heterojunction between the two materials.

Kim et al. reported loaded of nanodiamond (ND) powder on WO₃ using dehydration condensation to make ND/WO₃[8]. HR-TEM images show the surface of the ND/WO₃ was covered by ND 2~10 nm in size with a carbon layer of thickness ≤ 1 nm on their surfaces. WO₃ showed the highest activity for the visible light photocatalytic degradation of acetaldehyde among various photocatalysts including TiO₂, TaON, C60-TiO₂, TiO₂/WO₃, N-doped TiO₂ (Fig. 2). WO₃ was considerably covered for ND (16 wt%)/WO₃ but only partially covered for ND (8 wt%)/WO₃. Diffuse reflectance UV/visible absorption spectra (DRS) showed insignificant change due to diamond having high transparencies as opposed to other carbon composites such as carbon nanotubes or GO. ND/WO₃ showed a higher efficiency than WO₃ in the visible light photodegradation of organic compounds comparable to Pt/WO₃. NDs function as a co-catalyst, similar to Pt, by facilitating the reduction of O₂. Interfacial electron transfer is enhanced by the sp² and sp³ orbitals each from the shell of graphitic carbon and the diamond core. The photocatalytic activity responds closely to the graphitic layer content and the surface functional groups. Noble metals (e.g. Pt, Pd), often used as a co-catalyst for WO₃ are an expensive type of material, while carbon is one of the most abundant elements on earth. That is why NDs are suggested as an alternative co-catalyst to facilitate visible light photocatalytic purification of air.

Another group used a porous tungsten oxide nanoframework with graphene (GWF) which is a porous tungsten oxide nanoframework with graphene to degrade pollutants under visible light[9]. GWF has significant adsorptivity of aromatic molecules, large range of light absorption, considerable light trapping, and efficiently separated charge carriers. These properties increased the photocatalytic efficiency. Compared to bare WO₃ and TiO₂ nanomaterials, the degradation of Rhodamine B showed a substantial photocatalytic improvement under visible light irradiation. After 15 minutes of visible light irradiation, almost 100% of the Rh B molecules were degraded. However, only 48.6 and 82.3% Rh B were degraded with bare WO₃ nanofibers and TiO₂ under the same condition. This research is important since they used graphene film. It was powder in previous researches that showed significant photocatalytic ability. To overcome the prob-lem of graphene, it is important to coat on the graphene’s surface with WO₃. To synthesis GWF, they used electrospinning technique. After the calcination, we can see the WO₃ nanofibers coated on the graphene from the SEM image and they increased the integrality of the GWF. Also, the TEM image showed the porous surface of GWF. The pores increased the surface area (56.4 m²g^-1) and the photocatalytic ability from the multiple reflection of visible light. Ismail et al. compared photodegradation efficiency of Methylene Blue dye using WO₃, WO₃-graphene oxide (GO), Pt/WO₃ and Pt/WO₃-GO. GO was produced by Hummers’ methods [10]. WO₃-GO was synthesized by one-pot synthesis with mixing surfactant, WO₃ and GO. Pt/WO₃ and Pt/ WO₃-GO were synthesized by solvothermal process and dried at 110°C overnight. The TEM images of Pt/ WO₃-GO showed the lumps of WO₃ obviously at various sizes. Also, the particle sizes of the Pt were up to 10 nm and WO₃’s particle sizes were 20~50 nm. The result of the experiment showed that the photodegradation ability of mesoporous Pt/WO₃, WO₃-GO and Pt/WO₃-GO were much greater than bare WO₃. Especially, mesoporous Pt/WO₃-GO had 3, 2 and 1.15 times better photodegradation efficiency compared to mesoporous WO₃, WO₃-GO and Pt/WO₃, respectively. This improved efficiency is because of the GO sheets by helping free electrons to transfer easily. In another report, graphene oxide (GO), tungsten oxide (W₁₈O₄₉) nanowires, and tungsten oxide-reduced graphene oxide (W₁₈O₄₉-RGO) aerogel was used to compare the degradation of six organic dyes, Rhodamine B, reactive black 39, reactive yellow 145, weak acid black BR, methyl orange (MO), and weak acid yellow G[11]. GO was synthesized by a modified Hummers method. W₁₈O₄₉ nanowires and W₁₈O₄₉-RGO were synthesized by solvothermal process with heating them at 200°C for 12 hr. SEM and TEM images showed that the W₁₈O₄₉ samples were composed of several bundles of nanowires which looks like flowers. The diameters of nanowires were about 10~20 nm. The surface area, measured by BET, was 228 m²/g and the pore volume, measured by BJH, was 0.50 cm³/g. From the result of Photodegradation of MO under visible light, the W₁₈O₄₉-RGO showed significant photodegradation ability compared to RGO and W₁₈O₄₉. While bare W₁₈O₄₉ nanowire degraded 68% of MO, W₁₈O₄₉-RGO decomposed MO almost completely in 25 minutes. Finally, pure WO₃, multi-walled carbon nanotubes (MWCNTs) and multi-walled carbon nanotube/tungsten trioxide (MWCNT/WO₃) were used for the new photocatalysis of methylene blue (MB) dye under UV-Vis light[12]. For MWCNT, commercial material was used and MWCNT/WO₃ was synthesized by solvothermal process. From the FE-SEM image, the surface of MWCNT were magnified. The diameter of MWCNTs was measured as 15~30 nm and the spherical WO₃ particles were shown on planar WO₃ and tubular MWCNTs. The TEM showed the circular WO₃ and tubular MWCNTs. The result showed the pure MWCNTs had poor photodegradation ability. Unlike MWCNTs, hybrid photocatalyst, MWCNT/WO₃, showed better photodegradation efficiency. This is because of the larger surface area, isolation of electron/hole pairs and high oxidation ability. Therefore, 2% of MWCNT/WO₃ showed the most efficient photodegradation ability.

2.3. Metal oxide nanocomposites

Adding TiO₂ on WO₃ can increase the device life time five times compared to pure WO₃, with the side effect of lowering the coloration efficiency[13]. Here, thin films of tungsten oxide doped with titanium (Ti : WO₃) were made to prepare dendrite shaped surfaces using reactive co-sputtering of titanium and tungsten in an argon and oxygen atmosphere. Films synthesized at different oxygen flow rates was conducted for pressures of 1.0 × 10^-3, 2.0 × 10^-3, 3.0 × 10^-3, 4.0 × 10^-3, 5.0 × 10^-3 mbar. With the metastable phase of hexagonal WO₃ being the dominant phase at all pressures, the ratio of the stable phase of monoclinic WO₃ is increased as the pressure of oxygen chamber is increased. Unlike pure tungsten oxide, annealed Ti : WO₃ exhibits a dendrite pattern on the surface, which decreases in size and density as the pressure of oxygen is raised. This is because Heating the substrate during the deposition process forms Ti : WO₃ seed particles and annealing causes free energies of the planes in the crystal to change, resulting in a difference in relative growth between planes. Electrochemical reversibility, optical modulation, and coloration efficiency all increased with increasing oxygen pressure for the Ti : WO₃ thin films. The degrading of methylene blue was also higher for Ti : WO₃ than plain WO₃ (because the heterojunction allows charge carriers to be transferred between TiO₂ and WO₃), and efficiency increased with increasing oxygen pressure. The efficiency of both electrochromism and photocatalysis is positively affected by the increase in surface area, each attributed to enhanced intercalation/de-intercalation and increased generation of electron-hole pairs/hole consumption leading to increased formation of the OH⋅ radicals, respectively. In another report Fe₂O₃@WO₃ with 0D Fe₂O₃ on 3D WO₃ is synthesized in a relatively simple method by mixing Na₂WO₄⋅2H₂O, citric acid dissolved in water with an aqueous HCl solution, heating at 120°C for 24 h, and then calcinating at 400°C for 4 h[3]. Then the WO₃ was mixed with Fe(NO₃)₃⋅9H₂O solution and calcinated at 400°C for 4 h to get the desired result. XPS and XRD both confirms the iron oxide coupled with WO₃ is Fe₂O₃. Pure WO₃ has a monoclinic structure while pure Fe₂O₃ has a rhombohedral structure. FESEM shows the WO₃ structure is maintained while iron oxide is distributed on it. RhB has an alkaline character which makes adsorption of RhB is weaker for Fe₂O₃@WO₃ (28%) than for pure WO₃ (42%) (Fig. 2). The photocatalytic degradation, on the other hand, is accelerated. When H₂O₂ is added to the solution, degradation is improved further. In the case of 5 mM H₂O₂, 85.7% of RhB was degraded after 45 min. While increasing the temperature that Fe(OH)3 is calcinated on WO₃ raises adsorption of RhB, the degradation of RhB rises only at first, due to the increase in crystallinity affecting the transfer of electron-hole pairs, then decreases after the optimum temperature of 400°C as Fe₂O₃ nanoparticles agglomerate and cause a decrease in surface area. RhB degradation in the visible- light photocatalytic reaction by Fe₂O₃@WO₃ is enhanced as concentration of H₂O₂ is increased, for up to 5 mM H₂O₂ concentration, as it helps initiate the reaction by consuming the photogenerated electrons. Taking away this difference caused by H₂O₂ concentration, the activity of Fe₂O₃@WO₃ is increased 68% compared to pure WO₃ when at the optimum value of 1% Fe₂O₃. Anything higher would result in agglomerates decreasing surface area and thus the active sites. Fe₂O₃@WO₃ showed a high recyclability by going through five, 75 minute cycles with only a slight drop in degradation efficiency. Xi et al. reported synthesis of Fe₃O₄/WO₃ with a hierarchical core-shell structure. It was prepared by solvothermal process in presence of binary oleic acid-ethylene glycol (EG) mixture and dispersed in a solution containing anhydrous ethanol, EG, and WCl6 which is heated at 180°C for 24 h to get W₁₈O₄₉ nanocrystals[14]. SEM and TEM images show nanoparticles about 5~10 nm in size form uniform Fe₃O₄ microspheres of diameter 450 nm. Leaf-like W₁₈O₄₉ nanoplates of 10~20 nm thickness surround their surfaces in a radial direction. XRD patterns show the composite contains monoclinic-phase W₁₈O₄₉ with spinel-phase of Fe₃O₄. BET calculations showed specific surface area was 34 m²/g. Using a Barrett-Joyner-Halenda method, it was observed that mesopores and macropores of diameter 3~20 nm were each constructed from Fe₃O₄ and WO₃, respectively. Fe₃O₄/WO₃ core-shell structured photocatalysts showed higher photodegradation for RhB and methylene blue than WO₃ hollow microspheres, with RhB being almost fully degraded by Fe₃O₄/WO₃ after irradiation of visible light for 1.5 h. Using a superconducting quantum interference device magnetometer (SQIDM) ferromagnetic behavior is observed. The highly stable structure of Fe₃O₄/WO₃ lets it be recycled without decrease in photocatalytic activity three times.

In another report W₁₈O₄₉ was synthesized from alcoholysis of WC₁₆ in a solvothermal process where heat was applied for 24 h at 160°C[15]. The mixing with TiO₂ resulted in a W₁₈O₄₉/TiO₂ hybrid, creating a charge- transfer pathway. Using SEM and TEM, urchin-like W₁₈O₄₉ formed from nanowires of length 500~600 nm and diameters of < 20 nm. Images from HRTEM shows the W₁₈O₄₉ nanowires are monocrystalline. XRD shows that W₁₈O₄₉ and WO₃ is each in the monoclinic phase. W₁₈O₄₉ and WO₃ shows urchin-like structure and large BET surface are of 178, and 169 m²g^-1 each. With visible light photodegradation of MO and phenol, there is a negligible amount of degradation activity by TiO₂. W₁₈O₄₉ (90 wt%)/TiO₂ has best photocatalytic efficiency than pure W₁₈O₄₉ for the degradation of methyl orange (MO). Due to the fact that energy levels of the two semiconductors are similar, charge transfer on the interfaces of the two is easy, resulting in better charge separation. Oxygen defects in the W₁₈O₄₉ structure are important for the hybrids because they give W₁₈O₄₉ an advantage in the reduction activity of CO₂ compared to WO₃.

2.4. Mixed metal oxide nanocomposite

WO₃, ZnS nanoparticles (NPs), 20 wt% ZnS-WO₃ and 40 wt% ZnS-WO₃ were used to compare the degradation efficiency of methylene blue (MB) dye under UV-Vis light[16]. WO₃ nanosheets, ZnS NPs and ZnS-WO₃ nanosheet were synthesized by hydrothermal method, precipitation method and wet impregnation method, respectively. The HRTEM images showed the morphology of WO₃. It represented WO₃ as a sheet of paper and the ZnS NPs were embedded on it. 20 wt% ZnS-WO₃ showed the highest photodegradation efficiency compared to WO₃, ZnS NPs and 20 wt% ZnS-WO₃. The photodegradation efficiencies under UV-Vis light after 240 minutes were 68.17, 75.79, 68.74 and 92.40% for ZnS NPs, WO₃ nanosheet, 40 and 20 wt% ZnS-WO₃. Low efficiency of 40 wt% ZnS-WO₃ was expected to be covering the surface of WO₃ nanosheets with ZnS NPs and interrupting photons to reach the WO₃ nanosheets. In another report, various concentration of Sn-WO₃/g-C₃N₄ were used to compare the photodegradation efficiency of anionic methyl orange (MO) and cationic Rhodamine B (Rh B) to bare WO₃, Sn-WO₃, g-C₃N₄ and 8% WO₃/g-C₃N₄ under visible light[17]. Sn-WO₃ solid solution and g-C₃N₄ were synthesized by solvothermal method and calcination method, respectively. WO₃/g-C₃N₄ and Sn-WO₃/ g-C₃N₄ were synthesized by calcination method. The SEM image of Sn-WO₃/g-C₃N₄ showed Sn-WO₃ nanoparticles were dispersed over the g-C₃N₄ sheet. The photocatalysis were conducted with 87% of MO during 120 minutes and 99% of Rh B during 50 minutes under visible light. The result showed 8% of Sn-WO₃/ g-C₃N₄ had the highest photodegradation effi- ciency. Sn-WO₃ and g-C₃N₄ had poor photodegradation efficiency and WO₃/g-C₃N₄ showed better efficiency than previous two materials.

Another group compared pure BiFeO₃, pure Bi₂WO₆ and BiFeO₃-Bi₂WO₆ for photodegradation efficiency of methylene blue (MB) and Rhodamine B (Rh B) under visible light irradiation[18]. BiFeO₂ particles were synthesized by solvothermal method dried at 80°C for 24 h. Pure Bi₂WO₆ was synthesized by hydrothermal method. BiFeO₃-Bi₂WO₆ nanocomposites were synthesized by solvothermal method with 1 : 1 mol ratio. The TEM image showed the size, morphology and lattice fringe of three materials. In the TEM image, the pure BiFeO₃ was a 25~50 nm length rectangular and Bi₂WO₆ was about 50~100 nm plate. BiFeO₃-Bi₂WO₆ had both BiFeO6 and Bi₂WO₆ morphology. BET method measured the surface area of the materials. The surface area of BiFeO₃ was largest with 8.44 m²g^-1 and the surface areas of BiFeO₃-Bi₂WO₆ were small in general. The most efficient photocatalyst was 1 : 1 mol ratio of in both MB and Rh B solutions. After 75 minutes of photodegradation under visible light, BiFeO₃-Bi₂WO₆ decomposed MB and Rh B 54 and 36%, respectively. The pure BiFeO₃ and Bi₂WO₆ showed poor efficiency compared to BiFeO₃-Bi₂WO₆. Although BiFeO₃ had the largest surface area, it showed the lowest photodegradation efficiency. It was predicted as the BiFeO6 and Bi₂WO₆ had synergetic effect together by isolating electrons and holes. Ngigi et al. used CdS-WO₃ to compare the photodegradation efficiency of Ethylparaben under visible light with bare WO₃[19]. CdS-WO₃ nanocomposite was synthesized by hydrothermal process. SEM image showed the lump of square WO₃ nanoparticles and it was shown in TEM image, too. TEM image showed that the spherical CdS nanoparticles were on the square WO₃ structure (Fig. 3). The CdS-WO₃ with the ratio of 3 : 7 showed the most efficient photodegradation compared to the CdS-WO₃ with the ratio of 1 : 1.

In another study, fly ash (FAw) and tungsten oxide- fly ash oxide (FAWO₃) were used to compare the photodegradation efficiency of bemacid blau (BB), bemacid rot (BR) and a heavy metal (ion-Cu²⁺)[20]. Raw fly ash was collected from the electro-filter the plant and WO₃ was commercial material. Tungsten oxide-fly ash oxide was synthesized by hydrothermal process. From the SEM images, the surface of FAw showed the rough surface with many pores on it. However, FAWO₃ showed smooth surface and uniformly dispersed pores because of the hydrothermal process. This experiment showed better photocatalytic efficiency with the novel composites. The novel nanocomposite showed better removal efficiency in combined adsorption and photocatalysis process.

3. Conclusions

WO₃ is a major interest in the area of degrading organic dyes using photocatalysis. Although changing the morphology of pure tungsten oxide can improve degradation efficiency by increasing surface area with 3D structures while at the same time improving charge transport with 1D structures, it has its limits. This is due to the energy level of the conduction band being higher than the O₂/O₂^- reduction potential. This can be made up for by using a co-catalyst. Possible co-catalysts include noble metals such as Pt, Ag, Au, out of which Pt is focused in this review for it is the most commonly used. These co-catalysts act as an electron pool to facilitate the reduction of oxygen. The disadvantage of this methods is that these noble metals are expensive. This has led to the introduction of other less-expensive methods of increasing the efficiency of tungsten oxide. First, by coupling with metal oxide semiconductors such as Fe₂O₃ or TiO₂. Second, by using carbon composites such as graphene, thus increasing interfacial charge transfer. Nanodiamond-loaded tungsten oxide goes further by being transparent as well as showing high charge transfer qualities. Combining the different methods is also used to try to combine the advantages behaviors of each method, with the disadvantage that doing so will increase the number of procedures spent on preparation of the complex photocatalyst. The development of photocatalysis is related to membrane photoreactor and integration of photocatalysis with water purification membrane process.