A brief Review on the Applications of Zn and Tio₂ in Photocatalysis and their Modification with β-Cyclodextrin

N. Uma Sangari*

The Standard Fireworks Rajaratnam College For Women, Sivakasi

*Corresponding Author E-mail: umasangariselvakumar@gmail.com

ABSTRACT:

Waste water effluents coming from textile and other dyestuff industries create increasing environmental hazard. These effluents will pose serious effects to the aquatic ecosystem, if they haven’t properly treated before discharging into water stream. Hence, it is necessary to evolve immediate steps to control water pollution and safeguard our environment and mankind. Heterogeneous photocatalysis offers several advantages as it requires no special reagents, critical experimental set up and reaction conditions. Zinc oxide and titanium dioxide are the most widely used photocatalysts. They are photo-stable, non-toxic and commercially available at low-cost. Hence we reviewed the literature available on the various methodologies applied to the synthesis of ZnO and TiO₂, the different attempts made to enhance the catalytic activity by the modification of catalyst’s surface through doping of metal and non-metal, metal coating, surface sensitization and their application in the removal of dyes, pesticides and metal ions, mechanism of TiO₂/UV photocatalysis and surface modification of photocatalysts by β-Cyclodextrin (β-CD) to enhance their UV, visible and solar light driven catalytic activity.

KEYWORDS: β-Cyclodextrin ; photocatalysts; pollution ; enhance.

INTRODUCTION:

The increasing population and industrialization has posed a major threat to the environment.  The gaseous and/or the liquid effluents produced by various industries, laboratories, factories etc. increase the concentration of recalcitrant organic pollutants in air and water streams. The effluents containing dyes when discharged into the water streams are disastrous as they are toxic, non-biodegradable, and visible.  They are known to produce adverse effects on aquatic flora, fauna and human beings.  Zollinger reported that about 1-20 % of the world production of dyes is lost during the dyeing processes and are discharged as effluents¹. The discharged toxic organic dyes yield various poisonous bi-products through oxidation, hydrolysis or other chemical reactions taking place in the water system². The existence of dyes can present serious problems to aquatic biota by reducing light penetration into the water phase³. They also reduce the concentration of dissolved oxygen which is most essential for the life of microorganisms in the water streams⁴. Hence, degradation of dyes present in waste water has received increasing attention. Various efforts have been undertaken to overcome the waste water pollution problems. Traditional physical methods of remediation such as adsorption on activated carbon, reverse osmosis, coagulation by chemical reagents, ultrafiltration, adsorption on synthetic ion- exchange resins, etc. have been found to cause secondary pollution by transferring the pollutants from water phase to the solid phase^{5, 6}. Conventional biological methods are also proven to be ineffective for the degradation and decolorisation of dyes as the dyes are highly stable⁷. Therefore the development of eco-friendly methods for degrading the dye pollutants has become an imperative task.  Over the last few decades several new eco-friendly technologies in the remediation of organic pollutants with high performance have been developed. 

Among them, Advanced oxidation processes (AOP) such as Fenton, photo-Fenton catalytic reactions, H₂O₂ / UV, UV/H₂O₂/TiO₂ processes, semiconductor photocatalysis^8-12 have been broadly studied due to their compliance with green chemistry concepts in promoting innovative technologies to reduce or eliminate the use or generation of hazardous substances in the design, manufacturing and use of chemical by-products¹³.  Recent studies have been devoted to the application of heterogeneous photocatalytic processes for the complete mineralization of dyes into CO₂ and H₂O^14-17.  Khan and Adil have described the technological importance of metal oxides such as ZnO, TiO₂, SnO₂ and CeO₂ in environmental remediation and electronics. In this article the author discussed about the structural characteristics, requirement of photocatalysts, classification of photocatalysts and the mechanism of photocatalysis¹⁸.

This article is slated to review the photocatalytic degradation of dyes and organic compounds using ZnO and TiO₂.  A brief review of various methods developed for the increment of overall photocatalytic activities of these semiconductors is also discussed.

Doped and Undoped TiO₂ Systems for Photocatalysis

TiO₂ has been widely used as photocatalyst for the mineralization of dyes because of its photochemical stability, non-toxicity and low cost^19-21.  The fundamental photochemical and photophysical processes underlying heterogeneous photocatalysis involving TiO₂ are well established and have been cited in many literatures^{22, 23}.  The photocatalytic reactions are initiated by the absorption of a photon with energy E ≥ Eg, the band gap energy of the photocatalyst.  As a result of absorption of a photon, the electrons from the valence band (VB) are promoted to the conduction band (CB) leaving behind an equal number of holes in the VB²⁴.  The mechanism of mineralization of dyes by TiO₂ photocatalysis could be summarized as follows:

h⁺_VB + H₂O      _{--------UV-----------à}                       H⁺+ °OH

h⁺_VB + OH^─      _{----------UV---------à} °OH

e^─ _CB + O_{2           -------------------à} O₂° ^─

O₂° ^─ + H⁺         _{-------------------à} H₂O°

Dye + °OH _{-------------------à} mineralized products

Dye + h⁺_{VB --------------------à} oxidized products

Dye + e^─_{CB ---------------à} reduced products

The photoproduced electrons in the conduction band could reduce the pollutants or react with adsorbed O₂ to yield superoxide radical anion. The superoxide radical anions react with protons to form peroxide radicals which are strong oxidizing species. The photogenerated holes are also very good oxidizing species and form   dye ⁺ ions or they react with chemi-adsorbed OH^¯ ions to form °OH radicals.  The resultant °OH radicals oxidize the dyes and mineralize them to CO₂ and H₂O²⁵.

Liu et al. investigated the photocatalytic decomposition of the synthetic dye C. I. Acid Yellow 17 by UV/TiO₂ process. They obtained the anatase form of TiO₂ on annealing the samples at 550° C for 24 hours. They studied the effect of pH of solution, flow rate, initial concentration of dye and light intensity. They also reported that pH of the solution decreased during the photocatalytic process that might be due to the mineralized HCl, H₂SO₄ and other organic acid intermediates produced in the process²⁶.

Baiju et al. synthesized nanotube of anatase –titania by hydrothermal method using the as-received titania powder. They successfully increased the specific surface area of the anatase –titania. They examined the dye adsorption and photocatalytic activity of the as-received titania powder and the synthesized nanotube of anatase –titania using the Methylene Blue (MB) as their model catalytic dye agent. Their study has shown that the dye removal from the aqueous solution is highly morphology dependent²⁷.

Valencia et al. evaluated the photocatalytic activity of nano-titanium dioxide synthesized by sol-gel method coupled with solvothermal technique. Titanium dioxide material with highest anatase phase crystallinity showed highest photocatalytic activity for the degradation of Methyl Orange (MeO)²⁸.

Krengvirat and his co-workers have demonstrated the synthesis and photocatalytic properties of TiO₂ nanotube arrays for the degradation of MB dye under visible light irradiation.  They found that hot water treatment of TiO₂ nanotube arrays significantly increased the photocatalytic oxidation rate than heat treatment.  They suggested that hot water treated nanotube array possess high BET specific surface area and led to high photo- catalytic oxidation²⁵.

Konstantinou and Albanis have carried out an exhaustive review of the TiO₂ assisted photocatalytic degradation of azo dyes in aqueous solutions under solar and UV irradiation.  This review focuses on the effects of several parameters such as pH, catalyst concentration, substrate concentration and presence of electron acceptors, inorganic ions, humic acids and solvents.  The authors also gave valuable information about the determination of the nature of the principal organic intermediates and evolution of the mineralization as well as the degradation path ways followed during the process. They have concluded that the evaluation of by-products evaluation and toxicity measurements are the key-actions in order to assess the overall process²⁹.

Mahavi et al. examined the photocatalytic oxidation of Reactive Orange 16 aqueous solution by TiO₂ nano particles under UV light irradiation. They investigated the effects of initial dye concentration, pH, TiO₂ loading and the presence of anions such as carbonate, bicarbonate, sulphate and chloride on the dye degradation. Their study revealed that the presence of sodium carbonate and sodium bicarbonate have tardiest effect on the photocatalytic processes³⁰.

Though most of the experiments utilized TiO₂ particles suspended in water as the photocatalyst, there are some disadvantages in titania based photocatalysis such as optical band gap that can be active only in presence of UV irradiation and high degree of e¯- h⁺ pair recombination process³¹. To achieve 100 % efficiency, the formed holes and electrons must be prevented from recombination. In order to overcome these problems, several attempts have been made for the modification of catalyst surface by doping, metal coating, surface sensitization and increase in surface area³². The doping of metal ions can result in the shifting of TiO₂ band gap into the visible region by introducing a dopant energy level between the conduction band and valence bands of TiO₂. 

The study of Mesgari et al. revealed that the phthalocyanine (Pc) and Fe³⁺ ions caused significant of shift of TiO₂ absorption to visible region. The Pc / Fe -TiO₂ nano composites exhibited higher photocatalytic activity than pure TiO₂ for the degradation of MeO under visible light irradiation.  The authors proposed that the increased photocatalytic activity is due to the higher efficiency for the e^--h⁺ pair generation and lower recombination rate of them². 

Saepurahman et al. evaluated the effect of TiO₂ modification with tungsten trioxide using impregnation method on the photocatalytic decolorisation of MB, Methyl Violet (MV) and MeO dyes. The effects of operating conditions, such as initial dye concentration, catalyst loading and initial pH have been studied.  It was observed that the loading of TiO₂ with tungsten could stabilize the anatase phase from transforming into inactive rutile phase and did not shift the absorption to the visible region.  The tungsten loaded TiO₂ showed two fold increase in degradation rate of MB than that of the unmodified TiO₂ [13].

Tobaldi and his research group assessed the photocatalytic activity of tungsten, silver and tungsten/silver co-doped titania powders synthesized via aqueous sol-gel method by monitoring the degradation of MB under UVA and visible light irradiation.  They discussed the influence of phase composition, optical properties, dimensions and specific surface area of the samples on photocatalytic activity.  They reported that tungsten doped and co-doped titania powders fired at 450°C have shown less photocatalytic activity than undoped TiO₂ which is probably due to electron-hole recombination³³.

Nogawa et al. investigated the photocatalytic degradation of 2-propanol by visible light irradiated TiO₂ and Au modified TiO₂ synthesized with and without ultra sonication. The modified powders possessed almost identical Au concentrations. They observed that the sonication has red-shifted the sensitive wavelength for the photocatalysis under visible light to 46 nm³⁴.

Mihai et al. described the synthesis of bioinspired morpho TiO₂ decorated with gold nanoparticles and investigated their photocatalytic properties by the degradation of MB dye at ambient temperature. The superior photocatalytic activity is ascribed to the fine structure and the presence of plasmonic gold³⁵.

Huang et al. described enhanced photocatalytic degradation of gaseous benzene under vacuum ultraviolet (VUV) irradiation over TiO₂ and a series of transition metal modified TiO_2. Among the synthesized catalysts, Mn / TiO₂ showed best photocatalytic activity due to its superior capacity for ozone decomposition. It was noted that the photocatalytic oxidation efficiency of benzene was 20 times higher under VUV than 254 nm UV irradiation³⁶.

The visible light driven photocatalytic activity of iodine doped Titania with different iodine/titanium molar ratios synthesized by sol-gel technique was investigated by Vereb et al. They compared the photocatalytic efficiencies of iodine doped TiO₂ for the degradation of phenol and inactivation of E. coli bacteria with aeroxide P₂₅ and aldrich anatase.  The iodine doped TiO₂ showed higher efficiency. It was proved that elemental iodine was produced by visible light irradiated TiO₂.  This element iodine could be participated in the oxidation of phenol.  They were also confirmed the formation of °OH radicals by ESR measurement.  They performed experiments and proved that no singlet oxygen and superoxide radical ions are detected³⁷.

The study of Pang and Abdullah showed that the carbon and nitrogen co-doping with titania nanotubes resulted in higher surface area, lower band gap energy and creation of surface oxygen vacancies compared to undoped TiO₂ nano tubes.  The C-N co-doped TiO₂ nanotubes exhibited about 2.3 times higher sonophotocatalytic activity for Rhodamine B (RhB) degradation than undoped nano tubes³⁸. 

Monterio et al. prepared nitrogen modified TiO₂ by grinding TiO₂ with different amounts of urea and applying the calcination temperatures between 340 and 240°C. It was identified that the nitrogen modification did not affect the crystalline phase of TiO_2. The material with urea: TiO₂ weight ratio of 1:2 calcined at 380°C exhibited highest photocatalytic activity under visible light irradiation (λmax = 430 nm) towards the degradation of diphenylamine³⁹.

The visible light photocatalytic activity of TiO₂ catalysts self-doped with Ti³⁺ was investigated by Xing et al.  They found that visible light photocatalytic activity of Ti³⁺ self-doped TiO₂ was proportional to the concentration of Ti³⁺ and they showed higher photocatalytic activity than pure TiO₂ after HCl washing⁴⁰.

Liu et al. synthesized Ti⁺³ doped TiO₂ via hydrothermal reaction between the titanium powder, HCl and HF. This photocatalyst demonstrated better visible light absorption compared to TiO₂-HCl and pure TiO₂ samples. They exhibited better photocatalytic decomposition rate for MB dye than other two catalysts. This is attributed to the improved light absorption ability and the concentration of photo-generated e¯─ h⁺ pairs⁴¹.

Gurkan et al. studied the kinetics of photocatalytic activity of TiO₂ doped with Se (IV) ions under UVA and solar light irradiation for the degradation of 4-nitrophenol. They observed that Se (IV) doped TiO₂ exhibited substantial photocatalytic activity under direct sunlight irradiation. By utilizing the experimental and quantum mechanical methods they determined the electronic, optical and photocatalytic properties of Se (IV) doped TiO₂. The results of their study revealed that Se (IV) doping of TiO₂ enhances the visible-light photocatalytic activity by the introduction of additional electronic states origination from the selenium 3p orbitals in the band gap⁴².

Pawar and Nimbalkar have investigated the effect of Zn²⁺ and Cr³⁺ ions doping on the activity of TiO₂ for degradation, synthesized by combustion method.  The results of their study showed that doped TiO₂ is more efficient than pure TiO₂. TiO₀.₉ CrO₀.₁ O₂ showed enhanced activity which was attributed to smaller crystallite size, synergistic effect between anatase and rutile phase and larger specific surface area⁴³.

Rajamanickam and Shanthi have investigated photocatalytic activity and promoting effect of TiO₂ by commercial activated carbon (CAC) for removing the model pollutant Sunset Yellow. The photodegradation efficiency of coupled TiO₂/CAC was found to be more than synthesized bare TiO₂ and TiO₂-P₂₅ at pH 7.  They analyzed the effects of operational parameters such as amount of catalyst, dye concentration and initial pH on the mineralization of Sunset Yellow dye and confirmed the mineralization by COD measurements.  They have reported that the loading of CAC suppressed the recombination of photo-generated electron-hole pairs⁴⁴.

Gomez et al. reported reusable photocatalytic materials based on TiO₂ supported on zeolite matrices (HBETA, HY and HZSM5) for the degradation of dichlorvos. The results of this study indicate that the most convenient catalyst is TiO₂/ HBETA (20 %). It was observed that the mineralization degree was lower compared to the degradation percentage due to the formation of organophosphorus intermediate which is less toxic than the starting material. They also reported that these materials offer certain advantages like easy separation and reusability⁴⁵.

Doped and Undoped ZnO Systems for Photocatalysis

Zinc oxide is also recognized as one of the promising photocatalysts because of its wide band gap of 3.37 eV which is comparable to that of TiO₂ and large exciton binding energy (60 meV)⁴⁶.

Chakrabarti and Dutta have investigated the photocatalytic degradation of two model textile dyes namely MB and Eosin in aqueous solutions by ZnO under UV and visible light irradiations. The effects of operational parameters like catalyst loading, initial concentration of the dye, air flow rate, UV light intensity and pH on the photocatalytic degradation have been investigated. They estimated in addition to color removal percentage, a reduction in COD. Of these two dyes, MB was degraded faster. The photocatalytic process followed pseudo- first order kinetics ⁴⁷.

Pare et al. have observed the complete mineralization of Lissamine Fast Yellow dye with ZnO under artificial radiations (AR) and they confirmed the mineralization by COD removal and UV-visible spectral analysis. They also validated various photocatalytic processes such as Fe⁺³/H₂O₂/ZnO/AR, H₂O₂/ZnO/AR, Fe⁺³/ZnO/AR and ZnO/AR for the efficient degradation of the dye. It was found that Fe⁺³/H₂O₂/ZnO/AR system was most efficient among all other processes⁴⁸.

Lai et al. reported the effect of relative content of oxygen vacancies on the photocatalytic degradation of RhB over hydrothermally synthesized ZnO with or without ultrasonic assistance.  Their results indicated that employment of ultrasonic treatment during the preparation process decreased the relative content of oxygen vacancy, suppressed the green emission and reduced the photocatalytic activity⁴⁹.

Vaishampayam et al. studied on the effects of particle size, surface area and surface defeect on the photocatalytic activity of flower-like ZnO nanostructures synthesized via facile solution route. They have investigated the photocatalytic degradation of MB over these ZnO catalysts with different zinc acetate: ethanolamine molar ratio⁵⁰.

Khezrianjoo and Revenasiddappa have examined the effects of some parameters such as pH, catalyst loading and ethanol concentration on the photodegradation of Acid Yellow 36 in the aqueous heterogeneous medium containing ZnO as photocatalyst under UV light irradiation.  From the experiment with ethanol, they confirmed that hydroxyl radicals played the major role in the degradation⁵¹.

The study of Zhang et al. on the photocatalytic activity of ZnO rods for the photo-assisted degradation of MB under high-pressure sodium lamp irradiation revealed that ZnO nano rods with higher aspect ratio exhibited high photocatalytic activity.  They reported that the higher photocatalytic activity may be attributed to the higher amount of surface defects and visible hydroxyl groups. They have confirmed their results with XPS and photoluminescence spectra⁵².

Sanjay et al. reported the synthesis, characterization and photocatalytic activity of lamellar porous ZnO for the degradation of Bromophenol Blue in presence of UV light irradiation.  They found that the presence of oxygen is profoundly important for the effective photodegradation of dyes. They reported that the dioxygen scavenged the conduction band electrons and thus prevented the exciton recombination⁵³.

Muslim et al. prepared nano ZnO by heating (300°C) ZnCO₃ which was obtained as precipitate by mixing ZnSO₄ and (NH₄)₂CO₃ solutions. It has been used to catalyze the decolorization of Ponceau S (PS), a model diazo dye, in an aqueous suspension under visible light (Intensity ≈1.8 × 10⁻⁴ Wcm⁻²). This ZnO was found to be more efficient as a photocatalyst compared to pristine ZnO. ZnO samples with higher temperatures (500°C and 700°C) showed less catalytic activity. This was attributed to the increase in particle size with the increase in calcined temperature of ZnO through agglomeration that resulted in a decrease in surface area⁵⁴.

Ma et al. prepared ZnO nanowires by modified carbothermal reduction method. They demonstrated the photocatalytic activity of the nanowires toward the photodegradation of MB dye under UV irradiation. The photodegradation of MB over the prepared ZnO nanowires proceeded through the pseudo-first-order kinetics⁵⁵.

Sanna et al. fabricated ZnO nanoparticles by a simple and low cost precipitation method as photocatalysts for the decomposition of MeO under solar light irradiation. They determined the effects of catalyst concentration, pH and initial MeO concentration. They studied the reaction kinetics and found the rate constant values⁵⁶.

The main factor influencing the photocatalytic activity of ZnO is the quick recombination of charge carriers (e¯─ h⁺).  In order to improve the photocatalytic efficiency of ZnO, different attempts have been made to reduce the electron-hole pair recombination. Organic molecules, inorganic metal ions or other semiconductor metal oxides have been added as dopants. The added dopants can act as the charge trapping centers and reduces the e¯ ─ h⁺ recombination and hence enhances the photocatalytic activity⁵⁷.

Rezaei and Yangjeh have proposed microwave assisted preparation of Ce-doped ZnO nanostructures and investigated the photocatalytic activity for the degradation of MB under UV irradiation.  They found that doping of ZnO with 0.1 mole fraction of Ce⁴⁺ ions increased the rate constant by about seven fold.  This has been attributed to the decreasing electron hole recombination⁵⁸.

The study of structure and photocatalytic activity of Ni-doped ZnO nano rods made by Zhao et al. confirmed that the excessive doping of Ni from 2 % to 5 % had decreased the band gap energy significantly and caused the energy of photo induced electron decreased.  Thus, the Zn_0.95 Ni_0.05 O exhibited highest photocatalytic activity⁵⁹.

Karunakaran et al. have reported the antibacterial and photocatalytic activities of sonochemically prepared ZnO and Ag-doped ZnO. They compared the photocatalytic efficiencies of the prepared ZnO and Ag-ZnO by employing MeO (azo dye), RhB (xanthene dye) and MB (heterocyclic dye)⁶⁰. In contrast to the other reports of enhanced photocatalytic efficiencies of Ag-doped ZnO^{61, 62}, Karunakaran et al. had observed inhibition of photocatalytic activity of ZnO on doping with Ag by sonochemical method. They also have reported the possible reasons for these observed results that photocatalysis is also substrate specific and the nanoparticles agglomerates in dye solution.

Yayapao et al. evaluated the photocatalytic efficiencies of the 0, 0.5 and 1% Neodymium-doped ZnO nano needle and undoped ZnO by the degradation of MB under UV light. They found that 1% Nd-doped ZnO was the best photocatalyst with photocatalytic efficiency which is 2.5 times greater than that of the undoped ZnO.  They reported that during photocatalysis, Nd³⁺ ions have acted as electron scavengers and suppressed the electron-hole recombination⁶³.

Ullah and Dutta have synthesized Mn-doped and undoped ZnO via wet-chemical techniques and evaluated the photo-reduction activities using Methylene Blue dye under visible light irradiation. Their experiments demonstrated that the photodegradation efficiency of Manganese-doped ZnO irradiated with visible light was significantly higher than undoped ZnO irradiated with visible light and ZnO irradiate with UV light.  From the experimental results, they concluded that upon illumination with visible light, Mn-doped ZnO generates electron-hole pair at the tail states of the conduction and valence bands respectively⁶⁴.

Surface Modification of Photocatalysts by β-cyclodextrin

Heterogeneous photocatalytic reactions using semiconductors such as ZnO and TiO₂ have been extensively studied for photodegradation / decolorisation of organic dye pollutants.  Although these two semiconductors are widely used, they are also proven to be inefficient due to the rapid recombination of electron-hole pairs generated upon photoexcitation.  Due to rapid recombination, electron acceptors adsorbed or close onto the semiconductor surface alone can trap the electrons.

Hence, in order to achieve effective photodegradation of dyes, the dyes should reach the near surface of the photocatalysts.  However, it is quite difficult to make the dyes to get adsorbed on to the surface of photocatalysts without chemical modification⁶⁵. At present many methods have been adopted towards the increasing the adsorption capacity and improving the photocatalytic efficiencies of TiO₂ and ZnO. 

Damardji et al. have synthesized TiO₂ pillared montmorillonite by microwave and conventional heating methods. The use of these photocatalysts was studied with the photodegradation of Solophenyl Red3BL dye. The photodegradation of the dye at acidic pH was higher for TiO₂ –PIM/MW which is in agreement with the optimum adsorption of the dye on the PIM.  But, for a same amount of TiO₂, the apparent rate constants were higher with commercial TiO₂-P25 than with TiO₂-PIM/MW. This has been attributed to the competitive adsorption of the dye between TiO₂ pillars and montmorillonite which decreased the probability for dye molecules to be close to the active sites of °OH radical production. This confirms that adsorption phenomenon did not govern exclusively the degradation process⁶⁶.

Chen et al. have demonstrated the photocatalytic performance of visible light TiO₂ after surface chemical modification with organics such as toluene 2,4-diisocyanate. This photocatalyst exhibited satisfactory photostability and high photocatalytic performance for the degradation of phenol, 2, 4-dichlorophenol, Fluorescein and MeO⁶⁷.

Fatimah et al. performed experiments to enforce the heterogeneous photocatalytic activity of ZnO by immobilization into montmorillonite clay through sol-gel method.  The results of their investigations suggested that photocatalytic activity of the ZnO/montmorillonite for the degradation of MB was increased.  The increased adsorption capacity of ZnO/montmorillonite resulted in enhancement of photodegradation⁶⁸.

Physical adsorption of dye molecules to the semiconductor surface by means of hydrophobic or electrostatic interactions facilitates charge transfer.  However, intermolecular interactions in dye aggregates upon excitation resulted in quenching of electron transfer⁶⁹. Hence, number of methods such as ion doping, dye sensitization etc. has been utilized to improve the photocatalytic efficiencies of these semiconductors⁷⁰.  One of the possibilities is to modify the surface of TiO₂ and ZnO with β-CD. β-CD adsorbed onto the surface of photocatalyst plays important function: It forms inclusion complex with the dye pollutants and provides the close proximity for the dyes and the photocatalysts⁷¹.  The encapsulation of dye also prevents the formation of dye aggregates⁷².  The β-CD can capture the photogenerated holes and thus degrades the electron-hole recombination which leads to enhanced photocatalytic activities of photocatalysts⁷³.  The physical adsorption of β-CD onto photocatalyst surface results the aggregation of photocatalysts⁷⁴. 

Zhang et al. performed modification of TiO₂ with β-CD for enhancing the selective degradation of RhB under visible light irradiation. They have correlated the reactivity of TiO₂/ β-CD with specific surface area.  They also tested the photocatalytic degradation bisphenol-A (BPA) over TiO₂/β-CD hybrid materials synthesized by physical adsorption method.  They found the BPA degradation was not accelerated. On the basis of above two observations they concluded that, synthesis method and TiO₂ surface area have great effect on the photoactivity of TiO₂ modified with cyclodextrin⁷⁵.

Wang et al. have investigated the enhanced photodegradation of BPA in aqueous solution by β-cyclodextrin under a 30W UV disinfection lamp.  They found that photodegradation rate of BPA is a function of β-CD concentration, pH and BPA initial concentration.  The enhanced photodegradation of BPA was resulted mainly from the lowering of bond energy between the some atoms of BPA molecule due to inclusion interaction with β-CD⁷⁶.

They have also investigated the photocatalytic degradation of  Bisphenol F (BPF) by β-cyclodextrin in aqueous TiO₂ dispersion under 250 W metal halide lamp (λ ≥ 365nm).  The photodegradation rate of BPF in aqueous solutions containing β-CD and TiO₂ was obviously enhanced than fuse containing only TiO₂.  The observed enhancement go in hand with the enhancement of adsorption of BPF on TiO₂ surface and moderate inclusion-depth of BPF in β-CD cavity⁷⁷.

The role of β-cyclodextrin on the enhancement of TiO₂ photocatalytic oxidation of four azo dyes and reduction of Cr (IV) in aqueous solutions were investigated by Lu et al. The photodecolorisation rates for azo dyes are increased seven fold in solutions containing 0.8 g L^-1 of TiO₂ and 4×10^-4 mg L^-1 of β-CD.  They also proposed the mechanism of the enhancement⁷⁸.

The effect of cyclodextrin on the photodegradation of organo phosphorous pesticides in humic water was monitored by Kamiya et al. They found a remarkable increase in the rate constant for the photodegradation in presence of humic water. This enhanced effect was mainly assigned to the inclusion effects of cyclodextrin to catalyze interactions of pesticides with reactive radicals generated by the humin photosensitizer and inclusion-trapped in cyclodextrin⁷⁹.

Zhang et al. have synthesized a novel β-CD grafted titanium dioxide and compared the photocatalytic degradation of Orange-II with Degussa P₂₅, anatase TiO₂ and TiO₂/β-CD under visible light illumination. They also performed the photocatalytic removal of Orange-II with TiO₂/β-CD under different simulated irradiation sources. The TiO₂/β-CD had exhibited high photocatalytic performance under visible and simulated solar irradiation. It was confirmed that the lifetime for the exited states of unreactive dyes is prolonged when incorporated inside cyclodextrins and facilitated electron transfer from the excited dye to TiO₂ conduction band, which enhanced the dye pollutant degradation⁸⁰.

Mukerji et al. have reported an approach to effect the selective photocatalysis by means of molecular recognition site constructed on inert domains located in the vicinity of TiO₂. Thiolated β-cyclodextrin (TCD) was chosen as the molecular recognition site.  They have demonstrated the photocatalytic measurements with 2-methyl-1, 4-naphthoquinone (2MNQ).  They confirmed that the enhancement in the photodegradation was due to molecular recognition sites and TCD⁸¹.

Anandan and Yoon have established an efficient and complete degradation of Nile Red dye using TiO₂ – β-cyclodextrin colloids (TiBCD) in aqueous solution under visible light irradiation.  The influence of fundamental parameters such as catalyst amount and concentration of dye have been studied.  They also compared the photocatalytic efficiency of TiBCD colloids with that of TiO₂ and found that the former had 1.8 times greater efficiency than the later⁸².

Velusamy et al. have modified the photocatalytic activity of TiO₂ with addition of β–CD for the decoloration of Ethyl Violet (EV) under UV-A light irradiation. They have investigated the different operating parameters like initial concentration of dye, illumination time, pH and amount of catalyst on the photocatalytic decoloration efficiency. The complexation patterns were confirmed with UV–visible and FT-IR spectral data. The interactions between TiO₂ and β -CD were characterized by field emission scanning electron microscopy, X-ray powder diffraction analysis and UV–visible diffuse reflectance spectroscopy⁸³.

Rajalakshmi et al. assessed the photodecoloration efficiency of Acid Violet (AV) dye under UV light irradiation by comparing TiO₂, ZnO, TiO₂/ β -CD and ZnO/ β-CD. The effects of concentration of dye, pH and dose of the catalyst on photodecoloration efficiency were investigated. It was found that photodecoloration efficiency was doubled with TiO₂/ β -CD and ZnO/ β -CD than the pure TiO₂ and ZnO. Kinetics results showed that the photocatalytic decoloration of AV dye follows pseudo-first-order kinetics. Photodecoloration efficiency of ZnO/β-CD was higher than TiO₂/ β –CD⁸⁴. These authors have proposed the mechanism for photodecoloration of AV dye under UV light irradiation. They also studied the photocatalytic decoloration of RhB dye with pure TiO₂, ZnO and TiO₂/ β -CD and ZnO/β–CD composites under solar light irradiation. They confirmed the photocatalytic decoloration pathway of RhB dye by GC–MS spectral studies and confirmed that the degradation consists of N-deethylation and cleavage of chromophores which finally lead to mineralization of dye.  They also studied the inclusion of RhB dye into β –CD cavity by H¹NMR⁸⁵.

Pitchaimuthu and Velusamy had made an attempt to enhance the photocatalytic activity of CeO₂ for visible light assisted decoloration of MB dye in aqueous solutions by β-CD. The effects of key operational parameters such as initial dye concentration, initial pH and CeO₂ concentration as well as illumination time on the extent of decolorisation were investigated. Among the processing parameters, the pH of the reaction solution played an important role in tuning the photocatalytic activity of CeO₂. The maximum photodecoloration rate was achieved at basic pH (pH 11). The inclusion complexation patterns between host–guest (i.e., β-CD and MB) have been confirmed with UV– visible spectral data⁸⁶.

Pitchaimuthu et al. have also investigated on the enhanced photocatalytic decoloration of Acid Yellow 99 (AY99) dye by the addition of β-CD to TiO₂. The mineralization of AY99 has been confirmed by the measurement of chemical oxygen demand. The higher photoactivity of TiO₂─β-CD/visible light system than TiO₂/visible light system was ascribed due to the ligand to metal charge transfer from β-CD to Ti ^IV located in an octahedral coordination environment⁸⁷. The authors had also investigated the photodecoloration of AY99 dye by ZnO and the enhanced activity of ZnO by β-CD under solar light radiation⁸⁸.  The enhanced photocatalytic activity of TiO₂ by β –CD under solar light irradiation for the decoloration of Azocarmine G (AZG) dye was also examined by Pitchaimuthu et al. The  TiO₂–β-CD  system  exists  higher  photocatalytic  activity  than  pure  TiO₂ system. This is attributed to their large surface area, absorption enhancement in solar light region and effective separation of electrons and holes⁸⁹.

Velusamy et al. have explained the enhanced  photocatalytic activity of ZnO/TiO₂ composites modified by β-CD  through  photodecoloration  of  RhB  dye  in  aqueous  solution  under  visible  light irradiation. Optical property of ZnO/TiO₂ composites and ZnO/TiO₂-β-CD composites were characterized by UV-DRS analyses. The effects of various operational parameters like concentration of RhB dye, amount of catalysts, irradiation time, initial pH and the ratio of ZnO/TiO₂ have been studied. The maximum removal was obtained at basic pH (pH=12). The kinetic data shows that the reactions follow pseudo-first order mechanism.  They compared the photocatalytic efficiency of ZnO/TiO₂-β-CD with ZnO/TiO₂, bare ZnO and bare TiO₂. It was found that ZnO/TiO₂-β-CD system exhibited better photocatalytic efficiency than ZnO/TiO₂, ZnO and TiO₂⁹⁰.

CONCLUSION:

Heterogeneous photocatalytic degradation using ZnO and TiO₂ is a powerful advanced oxidation technology for the waste water remediation. There are certain drawbacks in the technology.

·The photocatalysts are activated only upon illumination with light energy greater than their band gap energy.

·The quantum efficiency of the photocatalyst are very low due to rapid recombination of the photoproduced electron-hole pairs.

In this study we reviewed, certain methodologies adopted to restrict the recombination of the photoproduced electron-hole pairs. Even though this review is non-exhaustive in the scope of the photocatalytic degradation of dyes, some significant parameters that affect the efficiency of the photocatalysts have been explored.

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Received on 18.04.2018                                Modified on 20.04.2018

Accepted on 25.04.2018                              ©AJRC All right reserved

AsianJ.ResearchChem.2018;11(3):681-690.

DOI: 10.5958/0974-4150.2018.00121.9