Clay supported TiO2 nanoparticles for photocatalytic degradation of environmental pollutants: A review

Accepted Manuscript Title: Clay supported TiO2 nanoparticles for photocatalytic degradation of environmental pollutants: A Review Authors: Amit Mishra, Akansha Mehta, Soumen Basu PII: DOI: Reference:

S2213-3437(18)30562-1 https://doi.org/10.1016/j.jece.2018.09.029 JECE 2650

To appear in: Received date: Revised date: Accepted date:

26-4-2018 15-8-2018 17-9-2018

Please cite this article as: Mishra A, Mehta A, Basu S, Clay supported TiO2 nanoparticles for photocatalytic degradation of environmental pollutants: A Review, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.09.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Clay supported TiO2 nanoparticles for photocatalytic degradation of environmental pollutants: A Review Amit Mishra, Akansha Mehta, and Soumen Basu*

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School of Chemistry and Biochemistry, Thapar Institute of Engineering & Technology, Patiala147004, India.

*Corresponding author. E-mail:[email protected] Abstract

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Pure air and water are essential requirements for sustainability of human civilization and wildlife

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on earth. However, due to illicit industrial practices of waste disposal, the presence of harmful

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effluents has increased in both air and water bodies which are harmful to both humans and

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wildlife. Heterogeneous photocatalysis can be a promising measure for the removal of these pollutants from both water and air. TiO2 is a highly investigated photocatalyst for such a purpose

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but it suffers from few demerits which hamper its practical application. Commercially available

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TiO2 (Degussa P25) has low photocatalytic efficiency owing to its low surface area (50 m2/g) and porosity and it is difficult to separate it from the reaction mixture which makes it less reusable. In

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order to overcome these limitations a variety of materials have been used as catalytic supports

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for TiO2. Among these clays have gained immense attention since they are cheap, highly available in the earth’s crust and possess thermal, chemical and mechanical stability. Clays

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provide TiO2 with the high surface area, porosity, high number of surface active sites which makes TiO2/clay nanocomposites highly active photocatalyst than pure TiO2. The review represents different methodologies for TiO2/clay synthesis and the impact of clay on the physical and photocatalytic activity of TiO2. Also, the role of different clay supports for TiO2 and comparison of their effect on the photocatalytic activity of TiO2 has been covered. 1

Keywords: TiO2; clay; nanocomposites; photocatalysis; degradation. 1.

Introduction

The rapid development of human civilization in the late 19th century has occurred as a

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result of the industrial revolution. However, the increase in urbanization and industrialization over the years has degraded the quality of the environment. As a consequence of traditional disposal practices of industrial waste materials, there has been an increase in the toxicity of groundwater and air which in turn is affecting both human health and wildlife. Due to all such malpractices, human society in the twenty-first century is facing major environmental issues

According to recent reports by WHO, nearly 3.7 million people have died

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ecosystem[1].

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such as global climate change andpollution which can pose a great threat to human health and

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globally due to environmental pollution in the twenty-first century and nearly 92% of the world's

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population lives in the regions where there is a high level of air and water pollution [2]. Hence,

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remediation of hazardous waste materials from water and air has become a topic of high national

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and international priority. Among the industrial waste materials, dyes and volatile organic compounds (VOCs) are one of the prime sources of water and air contamination. Dyes are

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colored organic pollutants which are released from textile and printing industries into water bodies. Most of the dyes are non-biodegradable and possess high resistivity to physiochemical

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degradation processes [3, 4]. Presence of dyes in water blocks the penetration of sunlight and

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oxygen and affects the aquatic life. Some of the dyes undergo anaerobic decolonization to carcinogenic amines making water unfit for human consumption[5]. On the other hand, VOCs are carbon-based compounds which are volatile at relatively low temperature. Most of the industrial processes involve the emission of volatile organic compounds. Vehicular emissions and day to day activities such as varnishing and painting also result in the emission of VOCs in

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the air. Most of the VOCs are hazardous to the human health and cause several types of cancers [6]. Exposure to these harmful pollutants also results in neurotoxicity and many problems related to skin and lungs [7]. There have been several measures taken for remediation of pollutants such

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as adsorption on mesoporous materials, biological and chemical treatment. However, most of these means of purification are inefficient, time-consuming, costly and require high energy inputs. Hence, development of a new and eco-friendly process for remediation of such toxic organic pollutants is desirable. For this purpose, more stress is being laid upon heterogeneous photocatalytic process for degradation of organic pollutants. The heterogeneous photocatalytic

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process takes place on the surface of a semiconductor photoactive material in the presence of

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sunlight. Therefore, the heterogeneous photocatalytic process can be highly promising for

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treatment of air and water pollution since it relies upon sunlight which is cleaner, cheap and an

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ultimate source of energy.

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1.1. Heterogeneous photocatalysis for remediation of organic pollutants:

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The basic principle behind heterogeneous photocatalysis is the in-situ generation of electrons and holes when the light of photon energy greater than or equal to the band gap of semiconductor

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falls on its surface. These electrons and holes initiate the generation of active free radicals on semiconductor surface (Fig. 1) [8, 9]. These radicals then oxidize harmful organic molecules to

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non-toxic CO2 and H2O[9]. Hence, heterogeneous photocatalysis can be considered as an

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efficient and eco-friendly process for remediation of organic pollutants[10]. Many photocatalysts such as Fe2O3, Cu2O, and TiO2 occur naturally while a large number of photocatalysts such as CdS, CdTe, CdSe, V2O5, Sb2S3, Sn2S3, etc can be prepared in the laboratory.

1.2.

TiO2 as a highly investigated photocatalyst: 3

Among the semiconductor photocatalysts, TiO2 is highly popular due to its high chemical and physical stability, low toxicity, low cost and ability to photodegrade a variety of organic pollutants in water and air [11]. Like many semiconductors, TiO2 constitutes a band gap

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and its conduction and valence bands are separated by 3.2 eV in anatase and 3.0 eV in rutile [12]. The band gaps of anatase and rutile correspond to their absorbance at 387.5 and 400 nm in the ultraviolet region of the solar spectrum[13]. When light under this region falls upon the TiO2 surface, then electrons and holes are generated (Fig. 2)

in conduction and valence band

respectively. These charge carriers then generate a number of free radicals[14] by series of

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mechanisms given in Fig. 3. Photocatalytic processes for the degradation of organic pollutants

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are initiated by the formation of reactive free radicals by photo-generated holes and electrons on

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semiconductor surface like TiO2. The free radicals are generated by a chain reaction which starts

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from the formation of hydroxyl radical (OH.) and superoxide radicals (O2-.) by holes and electrons. The O2-. further reacts with H+ to form peroxohydroxl (OOH.) radicals and H2O2 which

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participate in the photocatalytic process. TiO2 often exists in the form of three main phases such

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as anatase, rutile, and brookite among which rutile is the most stable phase, anatase is moderately

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stable and brookite is least stable. The three polymorphs of TiO2 arise due to three different ways of the arrangement of TiO6 octahedra [15, 16]. Among the three phases, anatase and rutile are

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highly investigated. Anatase is found to be highly photoactive mainly due to its indirect band gap which results in high electron-hole separation lifetime. Rutile, on the other hand, has low

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electron-hole separation lifetime due to direct band gap which makes it less photoactive than anatase[17].

1.3.

Limitations of TiO2 and their countermeasures: 4

Despite all merits, TiO2 has some limitations which tend to hamper its practical application. These are: i.

Due to its wide band gap, it is active only in the ultra-violet region of solar spectrum

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which comprises only 4% of the total solar spectrum. ii.

It has low adsorption capability due to the relatively low surface area and porosity.

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It has high aggregation tendency.

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It tends to form a colloidal suspension in an aqueous medium which makes its recovery difficult.

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A number of measures have been taken to overcome the above limitations. To enhance the

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activity of TiO2 towards the visible region of the solar spectrum, numerous methods have been

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carried out such as elemental doping[18-20], metal[21] and semiconductor loading[22] and dye

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sensitization[23]. Nowadays, carbon-based materials such as carbon quantum dots (CQDs)[24]

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and g-C3N4 [25] have also been used to sensitize TiO2 to expand its activity towards visible light.

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In order to enhance its adsorption capability and to reduce its aggregation tendency a wide variety of materials such as silica[26], activated carbon[27], zeolites[28] and clays have been

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used as supports for TiO2. The role of these support materials is to provide high surface area, porosity and reactive sites to TiO2. The support material also hinders the aggregation of TiO2 and

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enhances reusability by making it recoverable from the reaction mixture. Among these, clays

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have been highly investigated as supports since they are cheap, easily available in the earth's crust, have low toxicity, have high chemical stability, mechanical strength and porosity [29, 30]. 1.4. Clay as promising supports for TiO2 photocatalyst:

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Clays are layered phyllosilicate minerals which occur naturally in the earth’s crust and are important constituents of soils [31, 32]. Clay minerals show plasticity depending upon the water content and harden up when dried [33]. Clays possess extraordinary physiochemical

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properties such as high surface reactivity, high adsorption capability, cation exchange capacity, swelling property, and biocompatibility which make them suitable for a wide variety of applications such as pharmaceutics, cosmetics, catalysis, medicine, and sensors. Clay minerals consist of tetrahedral (T) silica sheet and an octahedral sheet of either (O) gibbsite (Al(OH)3) or brucite (Mg(OH)2) stacked upon each other (Fig. 4). Depending upon the number of silica sheets

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stacked to either octahedral gibbsite or brucite the clay minerals are classified into two types 1:1

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and 2:1 (Fig. 5)[34]. The 1:1 clay consists of one tetrahedral silica sheet stacked to octahedral

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gibbsite or brucite sheet. Kaolinite is 1:1 clay consisting of tetrahedral silica and octahedral

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gibbsite sheet. Montmorillonite (Mt) on the other hand is 2:1 clay having two tetrahedral silica sheets and one octahedral gibbsite sheet. Isomorphic substitution of Al3+ for Si4+ in tetrahedral

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silica sheet and Mg2+ for Al3+ in octahedral sheet which gives rise to negative charge on clay

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surface which is balanced by the exchangeable catins in the interlayer space. Based upon the

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charge possessed by their surface clays can be classified as cationic or anionic. Cationic clay minerals consist of negatively charged aluminosilicate surface and contain positively charged

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cations in their interlayer space to balance the surface charge and also have interstitial water molecules[34, 35]. On the other hand, anionic clays such as layered double hydroxides possess

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positively charged surface, are mostly synthetic and not occur as crude forms in nature[34]. Clays have gained a lot of popularity as supports for TiO2. It has been found that clays tend to enhance the photocatalytic activity of TiO2 NPs. Pristine TiO2 NPs such as commercial Degussa P25 are less photoactive than clay supported TiO2. The enhancement of photocatalytic activity

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can be due to the high surface area, adsorption capability, porosity and presence of surface active sites in TiO2/clay nanocomposites[36]. The increase in photocatalytic activity is also due to lower charge recombination rate in TiO2/clay nanocomposites. The decrease in charge

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recombination in TiO2/clay nanocomposites by clay particles can be due to the presence of interlayer cations in clay which tend to trap electrons and let the holes free for oxidation[37]. Clay also enhances the reusable efficiency of TiO2 by making it separable from the reaction mixture. Clay minerals such as montmorillonite (Mt), bentonite, kunipia, kaolinite, smectite, rectorite, hectorite, laponite, palygorskite, halloysite, attapulgite, diatomite and layered double

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hydroxides (LDH) have been utilized as TiO2 supports. For the preparation of TiO2/clay

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nanocomposites, impregnation of TiO2 either on clay surface or between its layers is highly

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preferred. For this purpose titanium (IV) alkoxides such as titanium isopropoxide and titanium

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(IV), butoxides and low-cost titanyl sulphate (TiOSO4) and titanium tetrachloride (TiCl4) are most common precursors. The present review focuses upon the photocatalytic application of

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TiO2/clay nanocomposite regarding degradation of organic pollutants, however there are some

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reports explaining the applications such as sunscreens[38], CO2 reduction[37], antibacterial

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activities, packaging[39] etc. However some clays possess pozzolanic and cement like properties, so the corresponding nanocomposites have highly promising applications in building

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materials. Hence, it can be said that the TiO2/clay nanocomposite have positive future perspective for both industrial and day to day application. The present review emphasize upon

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the properties, preparation methods as well as photoactivity of TiO2/clay nanocomposites prepared from different clay minerals which have been discussed in sections below. 2. Nanocomposites of TiO2 with different clays

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2.1.TiO2/Mt nanocomposites: TiO2 pillared Mt nanocomposites were synthesized by Ding et al. [40]using sol-gel method followed by different drying processes such as air drying, ethanol extraction drying and

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supercritical drying. It was found that drying processes had significant impacts upon their photocatalytic activities against phenol degradation. The nanocomposite obtained by supercritical drying process showed high photoactivity due to the high surface area and crystallinity of TiO2.

Ooka et al. [41] prepared TiO2 pillared Mt nanocomposites by the

hydrothermal method which led to the formation of highly crystalline anatase TiO2 in the size

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range 40-60 Å. The as-synthesized nanocomposites had high photocatalytic activity regarding

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trichloroethylene degradation in water. Jagtap et al. [42] carried out pillaring of Mt with TiO2 via

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conventional stirring and ultrasonic agitation followed by hydrothermal treatment. Ultrasonic

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treatment resulted in the fast formation of TiO2 pillared Mt nanocomposite within 20 minutes.

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Nanocomposites prepared by ultrasonic as well as conventional stirring methods had TiO2 in

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both anatase and rutile phase. However, the rutile phase was highly prominent in nanocomposites prepared by conventional stirring. The rutile content also increased upon

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increasing the duration of hydrothermal treatment. The nanocomposites were then employed for degradation of aniline which was selectively oxidized to azoxybenzene at ambient temperature.

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Nanocomposites prepared in shorter duration by ultrasonic treatment were highly active in

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aniline photo-oxidation. TiO2 pillared Mt was also prepared by the solvothermal process using ethanol and water as solvents, hexamethylenetetramine as precipitant and TiCl3 as a precursor. The resultant nanocomposite possessed the mesoporous structure with pore diameters 6-10 nm and enhanced surface area. The photocatalytic activity was investigated by degradation of methylene blue (MB) dye in the aqueous medium and it was found to be dependent upon the 8

ratio of TiCl3 and Mt. The nanocomposite by taking TiCl3 and Mt in the ratio 0.2:1 showed the highest photoactivity due to

high surface area and porosity[43]. TiO2/pillared Mt

nanocomposites were prepared at lower temperatures (30-80oC) and without calcination by

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Zhang et al. [44]. Pillaring of TiO2 on Mt was carried out by impregnating TiO2 sol prepared form low cost TiCl4 between layers of Mt. Pillaring of TiO2 destructed the layered structure of Mt and led to the formation of TiO2 nanoparticles (TiO2 NPs) on the surface and in inter-layer space of Mt. Djellabi et al. [45] prepared TiO2/Mt nanocomposites by impregnation of TiCl4 on Mt followed by calcination at 350oC which resulted in the formation of crystalline anatase TiO2

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on clay surface. The photoactivity was carried out under UV-A radiation and was evaluated with

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respect to the degradation of five cationic dyes such as crystal violet, MB , RhB, Methyl orange

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(MO) and congo red. The degradation rates were found to be in the order: crystal violet (97.1%)

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> MB (93.2%), RhB (79.8%) > MO (36.1%) > congo red (22.6%). The comparative study indicated that the photoactivity depended upon the contact between the TiO2 NPs on Mt surface

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and dye molecules.

Apart from above mentioned methods, the intercalation of TiO2 NPs on Mt can also be

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carried out by using long chain surfactants such as polyoxyproplylene (POP), Cetyl trimethyl ammonium bromide (CTAB) etc. The colloidal TiO2 particles were intercalated into Mt layers

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by using POP as expanding agent to prepare TiO2 pillared Mt nanocomposites by Chen et al.

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[46]. The introduction of POP not only led to the formation of delaminated structure (Fig. 6) but also incremented the surface area and porosity of the nanocomposite. Photocatalytic degradation of methylene blue dye by these nanocomposites indicated the surface area to be an important parameter influencing the photoactivity of the nanocomposites due to increased contact between dye and catalyst. Chen et al.[47] used CTAB as expanding agent to intercalate Ti alkoxide 9

precursor between Mt layers. Here, CTAB played a similar role as that of POP discussed above in increasing the surface area and porosity of the nanocomposites. It led to the homogeneous distribution of TiO2 NPs on Mt. The intercalation of Ti precursor led to the formation of anatase

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TiO2 NPs in the size range 5-10 nm. The nanocomposite was found to possess good thermal stability since there was no phase transformation from anatase to rutile even after calcination at 900oC. The photocatalytic activity was investigated by methylene blue degradation and nanocomposites exhibited better photoactivity than commercial P25 with maximum degradation efficiency up to 99% within 60 minutes. Similarly, in an another investigation carried out by

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Dvininov et al. [48] cetyl trimethyl cations (CTA+) were used for expanding Mt layers so as to

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adsorb titanium alkoxide precursors between them under acidic conditions. TiO2 was formed by

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subsequent calcination of titanium alkoxide by calcination. The size of TiO2 pillars was directly

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proportional to the size of Ti-polycationic species interlayer space. The photoactivity of the nanocomposites was investigated by degrading congo red dye under UV light. The photoactivity

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was highly dependent upon TiO2 pillar size and increase in pillar size led to enhanced contact

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between the dye molecules and photocatalyst leading to high photoactivity.

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Apart from these there are some other techniques developed to impregnate TiO2 between Mt interlayers such as hetero-coagulation[49] and pH controlled hydrothermal

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process[50]. Mogyorosi et al. [49] prepared TiO2/Mt nanocomposites by two distinct methods. In

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the first method, the titanium alkoxides were adsorbed on to Mt and subsequently hydrolyzed to TiO2. In the second method hetero-coagulation of TiO2 NPs between Mt interlayer spaces was carried out. In both these processes, the TiO2 NPs were found to successfully intercalate between Mt layers. Highly crystalline anatase TiO2 NPs of size less than 5 nm were found to exist in the nanocomposites prepared by both the methods. The nanocomposites prepared by hetero10

coagulation method were found to possess high surface area. Hetero-coagulation method was also carried out by Kun et al. [51] to prepare TiO2/Mt nanocomposites under highly acidic (pH~1) and weakly acidic conditions (pH~4). Pure anatase phase of TiO2 was obtained in the

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nanocomposites and TiO2 content was varied from 20 to 75%. The samples prepared under highly acidic conditions had high surface areas ranging from 171-284 m2/g. The photocatalytic activity of the nanocomposites was investigated by oxidation of phenol which enhanced upon intercalation of TiO2 in Mt interlayer spaces. In a recent study, the self assembling of Mt microlayers was carried to form a sandwiched layered structure of TiO2 intercalated on Mt by

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Huo et al. [50]. The synthesis was carried out under pH controlled hydrothermal process. The

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size of intercalated TiO2 was around 15 nm. The nanocomposite showed much higher

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photocatalytic activity and recycling ability regarding the degradation of MO dye due to

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generation of Ti3+ active sites and high surface area and pore volume. The nanocomposite also

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overcame the limitations such as light barrier and limited mass transfer efficiency.

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Apart from enhanced surface area and porosity immobilization of Fe3+ ions on nanocomposites can lead to faster and efficient photocatalysis due to combined Fenton and

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photocatalytic oxidation as reported by Munawwarah et al. [52]. To further investigate the role of Fe species loading of Fe on TiO2/Mt nanocomposites were carried out by Okte et al. [53] via

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in-situ growth of TiO2 and Fe species on Mt surface. Presence of mixed valence of Fe played an

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important role in electron transfer processes which enhanced the photocatalytic activities of Mt supported TiO2 regarding β-Naphthol degradation. The nanocomposites had higher surface area, pore volume and extended optical absorption profiles through longer wavelengths. Due to their mesoporous nature, the nanocomposites had better adsorption and degradation capabilities. The Fe species present in the nanocomposites mediated the electron transfer processes during 11

photocatalytic reactions. The degradation kinetics of β-Naphthol obeyed the LangmuirHinshelwood mechanism. Although, TiO2/clay nanocomposites can be easily separated from the reaction mixture and have high reusability than commercial P25, but complete separation is still

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a challenge as some amount of the catalyst is lost during commonly used separation processes such as decantation, centrifugation, filtration etc. In order to overcome this issue magnetically separable nanocomposites can be a highly promising strategy to obtain much higher reusability. In this regard, TiO2/Mt/Fe3O4 nanocomposites were prepared by hydrolysis of Fe3O4-tetra-nbutyl titanate micro-emulsion to form Fe3O4 loaded TiO2 NPs in the interlayer space of Mt (Fig.

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7) [54]. The size of as obtained TiO2 NPs were found to be in the range of 10-20 nm whereas

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size of Fe3O4 NPs were in the range of 40-60 nm. The nanocomposite was able to degrade about

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94% of MB dye in comparison to 85% by pristine TiO2 NPs. The nanocomposite retained high

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photoactivity even after being used for six consecutive runs which reveals its high reusable

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efficiency.

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Although TiO2/Mt nanocomposites tend to have high photoactivity compared to commercial and unsupported TiO2, it is inactive in the visible region of solar spectrum due to

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wide band gap of TiO2 (3.2 eV). In order to make it active in the visible light, several attempts have been made which are almost similar to those of TiO2 alone. Liu et al. [55] prepared silver

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(Ag) metal loaded TiO2/Mt nanocomposites by hydrolysis of TiCl4 between Mt interlayer spaces

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and subsequently loading AgNPs by reduction of silver nitrate. According to their report, silver loading enhanced the light absorption of nanocomposites due to LSPR effect which resulted in high photocatalytic activity of Ag-TiO2/Mt when compared to TiO2/Mt and commercial P25 under UV light. Elemental doping can be another option for obtaining visible light active nanocomposite. For this purpose, Zhang et al. [56, 57] have synthesized nitrogen and sulphur co12

doped TiO2/Mt nanocomposites by impregnating doped-TiO2 sol into layers of Mt. The doping of nitrogen and sulphur led to red shift in the absorption edge of UV-Visible diffuse reflectance spectra of (Fig. 8) TiO2/Mt nanocomposites. The nanocomposites were found to successfully

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degrade 4 BS and Acid red-G dye under visible light irradiation. Solophenyl-3BL dye was degraded by TiO2/pillared Mt prepared by microwave synthesis under both visible and UV light irradiation[58]. It was proposed that the electron transfer from excited dye molecule to conduction band of TiO2 resulted in photosensitization of TiO2. However, the process represented about 25 % of total photoactivity. It was also found by total organic carbon (TOC)

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analysis that only 6% of the dye was photo-mineralized under visible light illumination with a

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low rate constant. Carbon (C) and Vanadium (V) doped TiO2/Mt nanocomposites were prepared

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by Chen et al. [59, 60] and were employed for degradation of sulphorhodamine-B dye under UV

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as well as visible light. The photocatalytic activities of V-TiO2/Mt and C-V-TiO2/Mt were higher than that of V-TiO2 and C-V-TiO2 in both UV and visible light. However, C-TiO2/Mt

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nanocomposite had lower photoactivity than C-TiO2. The C-TiO2 had higher photoactivity under

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both UV and visible light than V-TiO2 and C-V-TiO2 which reveals elemental carbon to be a

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better photo-sensitizer. However, low photoactivity of C-TiO2/Mt than C-TiO2 was attributed to the fact that Mt mediated the transfer of electrons from excited carbon species to vacant d orbital

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of transition metals present on Mt and hampered the photoactivity of C-TiO2/Mt. Table 1 shows the comparative overview of different TiO2/Mt nanocomposites regarding photodegradation of

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different organic pollutants. 2.2.TiO2/bentonite nanocomposites: Bentonite is an impure Mt containing quartz, cristobalite, feldspar, pyrites, carbonates, mica and illite as impurities and it is naturally found in the earth’s crust[61, 62]. TiO2/bentonite 13

nanocomposites prepared by acid catalyzed sol-gel process by Sun et al. [63] were employed for degradation of cationic red GTL dye. The photocatalytic activity of the nanocomposites was higher than the pristine TiO2 with a rate constant almost 2.57 times that of pristine TiO2. The

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photocatalytic degradation of cationic red GTL dye was pH dependent and was highly effective in alkaline medium. However, the above mentioned acid catalyzed sol-gel process is more time taking and complex, hence, Mishra et al. [64] synthesized TiO2/bentonite nanocomposites under microwave conditions (180oC) leading to the formation of anatase TiO2 NPs on bentonite surface within 10 minutes. The effect of TiO2 content on photocatalytic activity was investigated and it

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was found that TiO2/bentonite nanocomposites containing 50% of TiO2 by weight showed high

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photoactivity for methylene blue degradation due to its large surface area (70 m 2/g) and pore

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volume (0.3455 cm3/g).

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There have been some another attempts to prepare visible light active TiO2/bentonite nanocomposites. Li et al. [65] have synthesized gold (Au) loaded TiO2/bentonite

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nanocomposites by the deposition-precipitation method followed by calcination at different

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temperatures. Au-TiO2/bentonite nanocomposites exhibited higher photocatalytic activity

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regarding photodegradation of sulforhodamine B dye under UV and visible light. In terms of carbon-oxygen demand (COD) changes the Au-TiO2/bentonite nanocomposites were highly

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efficient in mineralizing the dye than commercial TiO2. Further Ag loaded TiO2/bentonite was prepared by impregnating Ag-TiO2 sol onto bentonite [75]. The Ag-TiO2/bentonite

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nanocomposite exhibited higher photocatalytic activity under the simulated solar light. The phenol removal efficiency for the nanocomposite was higher than TiO2 and Ag-TiO2[66]. Different weight % of AgNPs (0.5-3%) was loaded by wet impregnation process on TiO2/bentonite nanocomposite synthesized by microwave irradiation method and their effect on

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photoactivity of the nanocomposite was investigated[67]. The as-prepared Ag-TiO2/bentonite nanocomposites were found to be effective for chlorobenzene degradation under UV and visible light. In order to further investigate effect of plasmonic metal NPs loading, different plasmonic

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metal NPs (Ag, Au, Pd) were loaded on TiO2/bentonite nanocomposite by an in-situ reduction of metal salt adsorbed on its surface by NaBH4[68]. Almost 1 weight % each of Ag, Au, and Pd was loaded upon TiO2/bentonite to examine their effect on photoactivity of the TiO2/bentonite nanocomposite. The LSPR peaks corresponding to different metal NPs (502, 503 and 541 nm for Pd, Ag and Au respectively) were observed from UV–Visible DRS spectra of as-prepared M-

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TiO2/bentonite nanocomposites. Metal NPs acted as electron sinks suppressing electron-hole

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recombination as depicted from increased exciton lifetime (2.50 ns to 2.60 ns) from time-

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resolved fluorescence spectroscopy (Fig. 9). The M-TiO2/bentonite nanocomposites possessed

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the higher surface area (119-125 m2/g) than TiO2/bentonite (112m2/g). The nanocomposites showed higher photoactivity for benzaldehyde and chlorobenzene degradation. The Ag-

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TiO2/bentonite nanocomposite was highly active in both chlorobenzene and benzaldehyde

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degradation under visible and UV light due to its high exciton lifetime (2.60 ns) which mediated

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the formation of highly oxidative superoxide and hydroxyl radicals and in turn enhanced the photoactivity. The high exciton lifetime in Ag-TiO2/bentonite can be due to Fermi level position

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of Ag which is very near to conduction band of TiO2 and enables quick transfer of electrons between AgNPs and TiO2. Similar results were obtained by Kaur et al. [21] in case of

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photodegradation of benzaldehyde and nitrobenzaldehyde by metal loaded TiO2 (M-Ag, Au, Cu). Recently, g-C3N4/TiO2/bentonite nanocomposites were prepared by wet impregnating

g-C3N4 on TiO2/bentonite nanocomposites[69]. It led to the formation of highly porous nanocomposites with uniform pore distribution. The g-C3N4/TiO2/bentonite nanocomposites

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were highly active in degradation of an industrial dye, reactive brilliant red-X3BS. In these nanocomposites g-C3N4 played an active role in making TiO2/bentonite nanocomposite visibly active and electrostatic interaction[70] between g-C3N4 and bentonite led to an effective charge

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separation and led to enhancement in photocatalytic activity. The morphological and photophysical properties of g-C3N4 depend upon the precursor from which it is prepared which also further affects its sensitizing abilities for TiO2 [71, 72]. The g-C3N4 was prepared from three different precursors such as urea, thiourea and mixture of urea and thiourea and was loaded on TiO2/bentonite by wet impregnation method[71]. Surface area, morphology and porosity were

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the important parameters that played an important role in high photocatalytic activity of the

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nanocomposites. The g-C3N4 prepared from urea was found to consist of thin and long sheets and

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it was observed to effectively interact with TiO2/bentonite nanocomposites and led to enhanced

M

photoactivity. A comparative study of photodegradation of different pollutants by different

D

TiO2/bentonite nanocomposites has been represented in Table 2.

TE

2.3.TiO2/kaolinite nanocomposites

EP

The TiO2 NPs were impregnated on pretreated kaolinite by a two-step method involving sol-gel and heterocoagulation process[73]. The pretreatment of kaolinite was carried

CC

out by consecutive processes which involve acidification-alkalinization and thermal treatment. This led to the formation of flat delaminated surfaces which mediated the homogeneous growth

A

of TiO2 nanoparticles upon them. The chemical interaction between TiO2 nanoparticles and SiO2 of kaolinite via Si-O-Ti bonding led to the stability of as-prepared photocatalysts. Uniform distribution of TiO2 nanoparticles in the kaolinite surface led to enhanced photoactivity for degradation of congo red dye. During the thermal regeneration process of the photocatalyst, the

16

microporous structure of TiO2 transformed to macroporous without any phase transformation from anatase to rutile as observed from X-ray diffraction pattern (Fig. 10). The process was accompanied by the slight decrement in the surface area followed by an increment in particle

SC RI PT

size. Photocatalytic degradation of congo red dye by the as-prepared TiO2/kaolinite nanocomposites indicated that their degradation efficiency was comparable after six thermal regenerative cycles. A similar process was adopted by Chong et al. [74] for the synthesis of TiO2/kaolinite nanocomposites. In their method nitric acid (HNO3) was used for acidification. The variation in synthesis temperature and subsequent thermal treatment resulted in the

U

formation of TiO2/kaolinite nanocomposites with different physical properties such as particle

N

size, surface area, and photoactive phase. The as-prepared TiO2/kaolinite had a rigid porous

A

layered structure and nanosize properties which led to its high photoactivity against congo red

M

photodegradation. The nanocomposites had high reusable efficiency since they could be easily separated from the reaction mixture after congo red degradation.

D

To further enhance the photocatalytic activity of TiO2/kaolinite nanocomposites

TE

Zhang et al. [75] prepared the nanocomposite consisting of mixed phase TiO2 NPs by a simple

EP

method which involves stirring and aging the sample for 12-24h at low temperature. The layered structure of kaolinite was destroyed to some extent during the synthesis leading to the formation

CC

of a dual mesoporous structure due to the embedding of TiO2 on kaolinite. The HRTEM (Fig. 11) showed different lattice fringes which allowed the identification of different TiO2 phases.

A

The lattice fringes with a fringe spacing of 0.35 nm were ascribed to (101) plane of anatase TiO2 and those with spacing 0.32 nm and 0.34 nm were related to (110) plane of rutile and (111) plane of brookite phase, respectively. The nanocomposites showed excellent photoactivity than commercial P25 regarding degradation of acid red-G and 4-Nitrophenol (4-NP). The mesoporous

17

structure, high surface area and formation of a heterojunction between anatase-brookite and anatase-rutile TiO2 led to high photocatalytic activity in TiO2/kaolinite nanocomposites. Metakaolinite is a calcined form of kaolinite prepared at calcination

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temperatures upto 500-600oC. It is well known for its pozzolanic properties and is often used in cements and concretes[76]. Kutlakova et al. [77] prepared TiO2/kaolinite nanocomposites by hydrolysis of titanyl sulphate on kaolinite surface. The ratio of titanyl sulphate and kaolinite was varied to obtain desired TiO2 amount in the nanocomposites. Calcination of the nanocomposite at 600oC led to the formation of TiO2/metakaolinite. The structural modeling of TiO2 NPs on

U

kaolinite surface was modeled through empirical force field atomistic simulations in material

N

studio modeling environment. Photocatalytic activity of the nanocomposites was evaluated by

A

the degradation of acid orange-7 dye and it was found that the photocatalytic activity of the

M

calcined TiO2/metakaolinite samples was higher than that of uncalcined nanocomposites. However, the higher photoactivity of calcined samples was not due to the high surface area since

D

it was low for calcined nanocomposites but due to the decrease of sulphur content (sulphuric acid

TE

was adsorbed on TiO2). In an another similar attempt, Barbasoa et al. [78] carried out the

EP

functionalization of kaolinite with titanium (IV) isopropoxide by hydrolytic sol-gel process (Fig. 12) followed by calcination at temperatures between 100-1000oC for 24 hours which resulted in

CC

the formation of TiO2/kaolinite nanocomposites. As the samples were calcined at a higher temperature, kaolinite transformed into metakaolinite. Methylene blue and MO were used as

A

model dye pollutants to investigate the photoactivity of the nanocomposites. Anatase containing TiO2/kaolinite formed by calcination at 400oC showed the highest photoactivity among the nanocomposites. The high photocatalytic activity was attributed to better dispersion of anatase

18

TiO2 on kaolinite surface which was promoted due to low aggregation between TiO2 and kaolinite particles. In a recent report, a visible light active novel 3-D g-C3N4/TiO2/kaolinite nanocomposites

SC RI PT

were prepared by mild sol-gel synthesis approach which is related with chemical stripping and self assembly[79]. This led to the formation of ‘sandwitch’ like structure with better dispersion of TiO2 and also led to enlarge surface area of the nanocomposites. The evaluation of photocatalytic activity was carried out by photocatalytic degradation of ciprofloxacin (CIP). The apparent rate constants of CIP degradation on the surface of g-C3N4/TiO2/kaolinite were almost

U

5.35, 6.35 and 4.49 times higher than that of pure TiO2, P25 and g-C3N4. The enhanced

N

photocatalytic activity was attributed to the improved light harvesting and charge transfer and

A

separation. Table 3 represents the summary regarding photodegradation of different pollutants by

M

TiO2/kaolinite nanocomposites. 2.4.TiO2/halloysite nanocomposites

D

Halloysite is 1:1 dioctahedral clay composed of octahedral Al-OH, alumina and

TE

tetrahedral Si-OH sheets with a general formula Al2(OH)4Si2O5.nH2O[80-82]. Although, it is

EP

structurally and chemically similar to kaolinite there is a layer of water molecules present between interlayer spaces[80, 82]. The tubular morphology of halloysite results from the

CC

wrapping of 1:1 mineral layers under favorable geological conditions which are mediated by oxygen sharing octahedral and tetrahedral sheets in 1:1 layer[80]. The tubular structure prevents

A

the aggregation of TiO2 NPs which leads to improvement in their dispersibility and photocatalytic activity[81]. Wang et al. [83] prepared TiO2/halloysite nanotube composites by one step solvothermal method. The nanocomposites were pH sensitive and showed high photocatalytic

19

activity regarding methanol and acetic acid degradation. The rate constant of acetic acid degradation was higher than that for methanol since fewer radicals were required to get consumed during the reaction. The pH sensitivity toward the photocatalytic activity of the as-

SC RI PT

prepared TiO2/halloysite nanocomposite was such that at pH 4 it showed high photoactivity. Papulis et al. [84] synthesized TiO2/halloysite nanocomposites using halloysites bought from two different geographical regions one from the USA and other from Greece. Both the clays had better dispersion of TiO2 NPs in their respective nanocomposites and both had interparticle mesopores of about 5.7 nm. The nanocomposites showed higher photocatalytic activity in

U

decomposing gaseous NOx and toluene compared to P25. The TiO2/halloysite nanocomposite

N

synthesized from halloysite bought from the USA showed higher photoactivity of gaseous

A

pollutants due to its high specific surface area (187 m2/g). Du et al. [85] deposited TiO2

M

nanoparticles on halloysite nanotubes by hydrolysis of tetraethyl titanate on their surface followed by calcination at different temperatures. Increasing the calcination temperature

D

produced crystalline anatase TiO2 but destroyed the halloysite nanotube structure. The

TE

TiO2/halloysite nanocomposite prepared by calcination at 300oC exhibited high photocatalytic

EP

efficiency for MB degradation which could be ascribed to the combined effect of adsorption of dye on halloysite surface and photoactivity of TiO2.

CC

Amylose, a biomolecule which is separated from starch-rich yam, Dioscorea opposita thunb has been used for non-covalent fictionalization of halloysite and their supramolecular

A

structure can be obtained just by applying mechanical force[86]. The amylose-halloysite supramolecular assembly was used as support for TiO2 NPs by Zheng et al. [87] where amylose acted as template for growth of TiO2 NPs and also led to its uniform distribution in the nanocomposite. Amylose assisted growth and uniform distribution of TiO2 on halloysite surface

20

was attributed to the strong non-covalent interaction between metal ions in halloysite and hydroxyl-glucoside groups in amylose. Due to high surface area and uniform distribution of TiO2 NPs TiO2/amylose-halloysite nanocomposites were highly effective for photocatalytic

SC RI PT

degradation of MB and 4-NP under UV light. There are a number of articles which lay emphasis on visible light photoactivity of TiO2/haloysite nanocomposites. One dimensional, TiO2/halloysite nanotube composites were prepared by in-situ low-temperature synthesis of TiO2 NPs on halloysite nanotubes[88]. The adsorption of the TiO2 precursor on halloysite nanotube led to the growth of TiO2 NPs upon its

U

surface which was observed from the TEM image of the nanocomposite (Fig. 13). The

N

crystallinity of TiO2 as well as its phase content was tuned by adjusting the acidity of TiO2 sol.

A

The photoactivity of nanocomposites was tested for degradation of RhB and gelatin violet and

M

the nanocomposites were highly photoactive than P25 and unsupported heterogeneous TiO2 under visible light illumination. The visible light activity in TiO2/hlloysite nanotubes was due to

D

the presence of Al2O3/SiO2 structure in halloysite which was similar to that of Al2O3 doped SiO2

TE

having optical absorption around 500 nm in visible region which disappeared after removal of

EP

Al2O3 by HCl. Hence, it can be concluded that the weak visible light absorption around 500 nm was due to the presence of Al2O3 in halloysite. Also, the presence of doped nitrogen and carbon

CC

as observed from XPS may have led to visible light absorption in halloysite. The TiO2/halloysite nanotubes containing mixed phase anatase/rutile TiO2 were highly photoactive due to stabilized

A

charge separation by electron transfer from anatase to rutile. Nitrogen (N) doped TiO2/halloysite nanocomposites were prepared by chemical vapor deposition in an autoclave by Cheng et al. [89]. Nitrogen doping led to a further red-shift in UV-Visible spectra of N-TiO2/halloysite nanocomposites when compared to that of N-TiO2. The photocatalytic activity was evaluated

21

regarding degradation of phenol. The high photoactivity of N-TiO2/halloysite nanocomposites was attributed to the combined effect of high photoactivity of the N-TiO2 and unique structure of halloysite. Li et al. [90] synthesized a series of one-dimensional polyaniline(PANI)-crystalline-

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TiO2/halloysite nanocomposites by a facile low-temperature synthesis with different mass ratios of PANI toTiO2. The synthesis procedure was a two-step process of either growing PANI and TiO2 or TiO2 and PANI consecutively on halloysite. By simply adjusting the acidity of TiO2 sol at a low temperature of around 65oC nanocomposites having the anatase, mixed phase TiO2 and PANI of different redox states were obtained. The photocatalytic activity of the nanocomposites

U

was evaluated by degradation of RhB dye in the aqueous medium under visible light. It was

N

found that the photocatalytic activity was dependent on the pH and volume ratio of aniline (ANI)

A

to titanium isopropoxide (TTIP) during nanocomposite preparation. The high photoactivity was

M

obtained from the nanocomposite prepared at pH 0.5 and with 1% volume ratio of ANI and TTIP due to sensitizing effect of PANI leading to electron transfer from photoexcited PANI to TiO2. In

D

another such work by Li et al. [91], different acid dopants such as HNO3, HCl, H2SO4, and

TE

H3PO4 were employed to tune up the acidity of TiO2 sol during the synthesis of the polyaniline-

EP

TiO2/halloysite nanocomposite. The nanocomposite synthesized by HCl at pH 0.5 showed superior photoactivity regarding RhB degradation than those prepared from HNO3, H2SO4, and

CC

H3PO4 at same pH condition. Table 4 gives the general summary of photodegradation of different pollutants by TiO2/halloysite nanocomposite.

A

2.5.TiO2/palygorskite and TiO2/attapulgite nanocomposites Palygorskite and attapulgite are magnesium aluminum silicates with general formula

[(H2O)4(Mg.Al.Fe)5(OH)2Si8O20.4H2O] having high crystallinity, fibrous morphology and porous structure composing of alternative talc-like units or tetrahedral layers alloyed together

22

and aligned along longitudinal side chains generating tunnels of dimension 3.76.4Å along caxis of the fiber[92]. Naturally occurring palygorskite and attapulgite can absorb exchangeable cations owing to isomorphic substitution during their formation. High porosity and absorbed

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cations endow it with a large surface area and moderate cation exchange capacity which was useful for adsorption of heavy metals [93, 94].

Zhao et al. [95] prepared TiO2 coated copper (Cu2+) palygorskite (Cu2+palygorskite) by hydrolysis of titanium tetrachloride on its surface. This led to the formation of relatively small anatase TiO2 NPs with size >5 nm well dispersed on the Cu2+-palygorskite

U

surface The palygorskite structure remained unaltered after modification with Cu2+. The

N

TiO2/Cu2+-palygorskite nanocomposite containing 0.6% Cu2+ and 25% TiO2 had a high surface

A

area of about 198 m2/g. The nanocomposites exhibited high photocatalytic activity for MB

M

degradation in UV light than pure TiO2. Zhang et al. [93] carried out the photodegradation of phenol by SnO2-TiO2/palygorskite nanocomposites prepared by attaching SnO2-TiO2 by in-situ

D

sol-gel technique. When compared to TiO2/palygorskite, SnO2/palygorskite and P25 the SnO2-

TE

TiO2/palygorskite nanocomposite exhibited higher photoactivity for phenol degradation with the

EP

highest rate constant of 0.03435 min-1. The decrease in COD from 220.2mg/l to 0.2 mg/l indicated almost complete photo-mineralization of phenol by as synthesized nanocomposites.

CC

The TiO2/palygorskite were synthesized by Bouna et al. [96] through a colloidal route using a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB) as a template for providing

A

an organic environment for TiO2 formation on clay surface followed by calcination at different temperatures. The TiO2 was crystallized to anatase above 450oC and remained stable up to 900 o

C in contrast to pure TiO2 xerogel which transformed to rutile at about 600 oC. The remarkable

stability of anatase TiO2 supported on palygorskite was due to hindrance in particle growth by

23

sintering. The nanocomposite calcined at 600oC during synthesis showed the highest photocatalytic activity towards orange-G degradation compared to other nanocomposites. The TiO2/palygorskite nanocomposites active in both visible and UV light were

SC RI PT

synthesized by Papoulis et al. [97] via sol-gel method with titanium isopropoxide precursor under hydrothermal treatment at 180oC. Nanocomposites containing different mass ratios of palygorskite and TiO2 were prepared and TiO2 NPs (3-10 nm) were well dispersed on palygorskite surface. Increasing the amount of TiO2 led to its aggregation while a decrease in its amount led to aggregation of palygorskite. The nanocomposite containing TiO2 and palygorskite

U

in the ratio of 70:30 showed high photoactivity regarding degradation of NO gas and toluene

N

under visible and UV light which could be due to better dispersion of TiO2 in the nanocomposite.

A

The visible light activity can be ascribed to the fact that nanocomposite showed absorption in

M

visible region as observed from UV-Visible diffuse reflectance spectra and it slightly increased with the increase of palygorskite content and hence visible light activity could be linked to the

D

presence of palygorskite and carbon doping during preparation[98]. Visible light active

TE

TiO2/CdS-palygorskite were synthesized by Chen et al. [99] via a colloidal route (Fig. 14) based

EP

on controlled hydrolysis of alkoxide on palygorskite in the presence of CTAB as template which led to uniform distribution of CdS and TiO2 NPs on palygorskite surface as seen from its TEM

CC

images (Fig. 15). The presence of CdS in the nanocomposite extends its photoresponse to the visible region of the solar spectrum. The photocatalytic activity was evaluated by MB

A

degradation and TiO2/CdS-palygorskite exhibited higher photoactivity than TiO2/palygorskite and CdS/palygorskite due to sensitization of TiO2 by CdS. Apart from palygorskite, attapulgite has also been used as TiO2 support. Zhang et al. [92] prepared SnO2-TiO2/attapulgite nanocomposites by deposition of SnO2-TiO2 NPs on

24

attapulgite surface by in-situ sol-gel technique. Photocatalytic degradation of MO dye was carried out in UV light. The molar ratio of TiO2 and SnO2 was the critical factor influencing the photoactivity of the nanocomposite. The nanocomposite containing TiO2 and SnO2 in the ratio

SC RI PT

0.82 showed the higher photocatalytic activity degrading 99% of MO in 30 minutes time. Attapulgite was also used as sorbent and as a carrier for BiOBr-TiO2 hybrid photocatalyst reported by Zhang et al. [100]and the results showed that BiOBr-TiO2 NPs of average size 10 nm were successfully induced on to the attapulgite layers. The BiOBr-TiO2/attapulgite nanocomposite having BiOBr and TiO2 in the molar ratio of 0.83 was able to degrade 96% of

U

MO dye within 2 hours of visible light illumination. The summary of TiO2/plygorskite and

N

TiO2/attapulgite nanocomposites regarding photodegradation of pollutants has been given in

A

Table 5.

M

2.6.TiO2 nanocomposites with other clay mineral supports

D

Rectorite is a clay mineral which consists of alternating pairs of the non-

TE

expandable dioctahedral mica-like layer and expandable dioctahedral smectite layer in the ratio of 1:1[101]. Rectorite can be easily cleaved between smectite inter-layers to form 2 nm thick

EP

monolithic rectorite [101, 102]. The interlayer cations present in smectite layers in rectorite can be easily exchanged by organic or inorganic cations and hence rectorite can be used as an

CC

adsorbent for harmful metallic cations and cationic dyes[102]. Zhang et al. [103] prepared stable

A

TiO2/rectorite nanocomposites by intercalating TiO2 NPs on rectorite at low temperature (70oC) which led to the formation of anatase TiO2 (10 nm) on rectorite surface. The UV absorption edge of TiO2/rectorite was slightly blue shifted compared to pure TiO2.

The as-synthesized

nanocomposite showed high photoactivity for five consecutive runs which was due to high adsorption capability of rectorite and photocatalytic activity of TiO2. The TiO2/Fe3O4/rectorite 25

nanocomposite was prepared by orderly depositing anatase and magnetic Fe3O4 on rectorite. Introduction of TiO2 led to partial exfoliation of rectorite layers and deposition of Fe3O4 completely exfoliated the rectorite layers and Fe3O4 appeared on rectorite, rather on TiO2

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surface. The as-prepared nanocomposite had a high capability of adsorption and photocatalytic degradation of MB and 4-NP and was magnetically separable from the reaction mixture[102].

Diatomite is a porous natural clay mineral derived from the deposition of singlecelled aquatic algae having a large number of diatom units of various shapes and sizes[104, 105]. Due to its high porosity, chemical stability, thermal resistance and low cost, it is widely

U

employed as a catalytic carrier[105]. TiO2/diatomite nanocomposite was synthesized by Sun et

N

al. [105] using a modified hydrolysis deposition method at low temperature taking TiCl4 as a

A

precursor. The as- synthesized TiO2 was uniformly dispersed in the nanocomposite which led to

M

high photoactivity of TiO2/diatomite in degrading 95% of RhB in 20 minutes under UV light.

D

Zhang et al. [106] immobilized TiO2 NPs on diatomite clay by hydrolysis deposition method

TE

using TiCl4 as a precursor. The impact of sulphate ions on crystallinity and photoactivity of TiO2/diatomite nanocomposites were investigated and it was observed that addition of small

EP

amount of sulphate ions promoted the formation of anatase and inhibited the transformation of anatase to rutile phase. Sulphate ions had a strong affinity for electrons and also inhibited the

CC

charge recombination by capturing the photo-generated electrons during photocatalytic

A

degradation of MB. Xia et al. [107] prepared TiO2/diatomite nanocomposites by sol-gel method using pre-modified diatomite clay. Modification of diatomite was carried out by two different approaches such as calcination and phosphoric acid treatment. The anatase to rutile transition temperature of TiO2 increased to 900oC after its loading on diatomite. The photocatalytic activity was evaluated by MO degradation under UV and visible light. The samples prepared by using 26

phosphoric acid treated diatomite possessed high photocatalytic activity. About 90% of MO was degraded under UV light in 90 minutes and 60% decomposed in 8 hours under visible light by TiO2/diatomite prepared from phosphoric acid treated diatomite. Further investigations revealed

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that phosphoric acid treatment changed the surface features of the diatomite support which led to increase in the binding strength between TiO2 and diatomite as well as increase the formation of mesoporous structure and improvement in the mass and charge transfer during the photocatalytic process.

The g-C3N4/TiO2@diatomite nanocomposites were prepared by in-situ deposition of g-

U

C3N4/TiO2 on diatomite surface. The g-C3N4/TiO2 heterojunction was formed on the surface and

N

on the pores of diatomite as observed from HRTEM image of the nanocomposite (Fig. 16). The

A

as-prepared nanocomposite was 2.5 and 3.5 times more active than pristine g-C3N4 in visible and

M

simulated solar light for RhB degradation. The high photoactivity was attributed due to

TE

growth of TiO2 grains[108].

D

synergetic effect between diatomite and g-C3N4/TiO2 hetero-junction and inhibition of the

Laponite and hectorite are 2:1 clays consisting of two tetrahedral silica layers over

EP

one octahedral magnesia (Mg(OH)2, brucite) layer. Laponite is synthetic hectorite and both

CC

form exfoliated silica when dispersed in water[109, 110]. Daniel et al. [110] prepared TiO2/laponite nanocomposites by sol-gel synthesis process followed by microwave

A

hydrothermal treatment. The photocatalytic activity of the nanocomposite regarding Sulforhodamine B degradation was found to increase by increasing the TiO2/laponite ratio which was attributed to a greater number of active crystalline anatase sites. The adsorption and removal efficiency of formaldehyde by TiO2/hectorite were investigated by Kibanova et al.[111]. The formaldehyde uptake capacity of TiO2/hectorite was 4.1 times more than 27

commercial P25. Under UV illumination the removal efficiency increased with TiO2 content and contact time. Sepeolite is a 2:1 type of layered clay just like Mt, bentonite and hectorite having

SC RI PT

fiberous minerals with molecular sized channels [112]. It is lightweight, non-swelling and has high porosity with large surface area. Zhou et al. prepared highly photoactive TiO2/sepeolite nanocomposites (Sep-TiO2) for Orange-G degradation with different ratios of Ti/Sep. the catalyst with Ti/Sep ratio of about 40 mmol/g showed the highest photocatalytic activity. In an another similar report TiO2/sepeolite nanocomposites were prepared by Zhou et al. [113] by

U

using two methods. The first method comprises of conventional calcination at 500oC for 3

N

hours and the other is based upon microwave hydrothermal treatment (M-H) at 200 oC for 40

A

minutes process. Conventional calcination process led to reduction in surface area and

M

destruction of sepeolite structure. The samples prepared by M-H treatment had high specific surface area and were highly photoactive regarding Orange G degradation. The intact surface

D

of sepeolite, homogenious dispersion of TiO2 on sepeolite surface and high surface area led to

TE

high photoactivity in M-H treated samples.

EP

LDH consist of positively charged metal hydroxide sheets having intercalated ions and water molecules. Structurally these are derived from brucite where the divalent cations are

CC

isomorphically substituted by trivalent cations [114]. A series of TiO2/LDH nanocomposites have been prepared by Seftel et al. [114] with TiO2 as anatase crystalline phase. The

A

modifications in LDH structure were created by inserting different divalent, trivalent or tetravalent cations within brucite sheet after which anatase TiO2 was deposited on LDH to obtain nanocomposite with tunable photocatalytic activity. Insertion of cations shifted the photo-response of the nanocomposites from UV to the visible range of the solar spectrum. The

28

photocatalytic activity was investigated upon the degradation of phenol and MB under UV and visible light. The di-, tri- and tetra-valent cations played an important role as charge separation centers by enabling the hetero-junction formation between TiO2 and LDH sheets and as

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dopants for TiO2 which made it photo-responsive to visible light. Table 6 provides the comparative summary of TiO2/clay nanocomposites prepared from different clays discussed above like rectorite, diatomite, LDH, and hectorite.

3. Role of clay in enhancing the photoactivity of TiO2

U

TiO2/clay nanocomposites were prepared to form different clays such as bentonite,

N

kaolinite and kunipia-F having textural differences (1:1 and 2:1) by simple microwave synthesis

A

method within 10 minutes of time [29]. Anatase NPs of TiO2 with crystallite size 10-20 nm were

M

formed on the clay surface. The nanocomposites exhibited high surface area and pore volume compared to pure clays and commercial TiO2 (P25). The photocatalytic activities of the

D

nanocomposites were found to depend upon the clay texture (1:1 and 2:1) and optical properties

TE

apart from surface area. The 2:1 clays (bentonite and kunipia-F) were found to act as better

EP

supports for TiO2 than the 1:1 clay (kaolinite) regarding the photocatalytic degradation of MB and chlorobenzene. The optical properties of the clay also played a crucial role in photoactivity

CC

since bentonite and kunipia had high absorbance for UV light which was due to the electron transfer mechanisms involving the Fe3+ cations in their octahedral sites. The extent of UV

A

absorption was directly proportional to structural Fe3+ concentration and inversely to clay tactoid size. Hence TiO2/bentonite and TiO2/kunipia-F had higher activity than TiO2/kaolinite. Influence of optical transparency of the clay on photocatalytic activity was investigated by Yang et al. [115]. The layer charge density and optical transparency of the Na-Mica (Na-M) were tuned by

29

incorporating Mg2+ and Fe3+ cations into empty octahedral sites of Na-M by Hofmann-Klemen (HK) effect [116], leading to the formation of Mg-M and Fe-M. The TiO2 NPs were successfully intercalated into Mg-M, Fe-M and kunipia-G by ion exchange reaction followed by calcination.

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From UV-Visible DRS spectra of the clays, it was found that Fe-M and kunipia-G had a high absorbance in the range of 300-350 nm when compared to Mg-M and Na-M due to stabilization of Fe2+ in the octahedral site of it lattice. Hence, Na-M and Mg-M were considered as better supports due to its high optical transparency and this fact was supported by high photoactivity of TiO2/Mg-M and TiO2/Na-M when compared to TiO2/Fe-M and TiO2/Kunipia-G regarding

U

methylene orange degradation.

N

Some reports suggest that interaction of clay surface with photogenerated electrons and

A

holes play critical role in mechanism of photocatalytic degradation taking place on surface of

M

TiO2/clay nanocomposites. In this regard Okte et al. [53] have stated that after illumination of TiO2, reactive free radicals are generated by electrons and holes which interact with Fe3+ ions

D

present in clay such as Mt. These Fe3+ ions act as trapping agents for both electrons and holes

TE

and reduce recombination rate. Some of the articles also proposed that electrostatic interactions

EP

of clay with photogenarated charge carriers tend to reduce recombination, thereby increasing its photoactivity [37, 69, 79]. There are some few recent reports which explain about the role of

CC

interaction between photogenerated charge carriers and clay surface in generation of reactive free radicals involved in the degradation. In a recently published article the photodegradation of

A

coprofloxzcin (CIP) was carried out with g-C3N4/TiO2/kaolinite photocatalyst in the presence of isopropanol (IPA), 1,4 benzoquinine (BQ), EDTA-2Na and AgNO3 which were employed as scavengers for OH●, O2●-, holes h+ and e- [79]. It was found that the photodegradation of CIP was highly inhibited by the BQ and AgNO3 which revealed that e- and O2●- were the active

30

species highly involved the reaction. From these experiments the following mechanism was proposed according to which photo-generated electrons from g-C3N4 were repelled by the negatively charged kaolinite surface and transferred to CB of TiO2 which might have

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participated in O2●- radical formation and dye degradation. The photocatalytic degradation of organic pollutants on TiO 2/clay surface has been explained on the basis of most commonly used pseudo first-order rate law according to which 𝐶

ln (𝐶 ) = −𝑘𝑡 , where 𝑘 is the pseudo-first order rate constant (min-1), 𝐶0 is the initial 0

concentration (mol l-1) and 𝐶 is the concentration(mol l-1) of the organic pollutant at time 𝑡(min). 𝐶

U

Straight line fitting of the plot between ln (𝐶 ) and 𝑡 signifies the extent to which the reaction is

N

0

A

obeying the pseudo first order law. Another equation commonly used for explaining the

which 𝑅 =

𝑑𝐶 𝑑𝑡

M

photodegradation mechanism is the Langmuir Hinshelwood kinetic model, according to 𝒌𝑲𝐶

= − 1+𝑲𝐶, where 𝐶 is the dye concentration (mol l-1), 𝑅 is the rate of reaction (mol

D

l-1 min-1) and it depends upon pollutant concentration 𝐶 and time 𝑡 , 𝑲 is the Langmuir

TE

Hinshelwood adsorption coefficient (l mg-1) and 𝒌 is the rate constant (mg l-1 min-1). At lower

EP

concentrations 𝑲𝐶 ≪ 1, and can be neglected upon which the equation reduces to 𝑅 =

𝑑𝐶 𝑑𝑡

=

𝐶

−𝒌𝑲𝐶. Upon solving this differential equation we can obtain an expression ln (𝐶 ) = −𝒌𝑲𝑡,

CC

0

which is similar to the first order rate equation. Here, 𝐶0 is the equilibrium concentration of

A

pollutant. Hence, the pseudo first-order rate constant 𝑘 can be considered to be equal to 𝒌𝑲. Since the reaction rate R is dependent upon the concentration so by putting 𝐶 = 𝐶0 in Langmuir 𝒌𝑲𝐶

Hinshelwood equation it can be written as 𝑅0 = − 1+𝑲𝐶0 , where 𝑅0 is the initial rate of the 0

reaction. The above equation can be simplified to

31

1 𝑅0

1

1

= 𝒌𝑲𝐶 + 𝒌 which means that reciprocal of 0

1

1

initial rate 𝑅 varies linearly with reciprocal of initial concentration 𝐶 of the pollutant and degree 0

0

of linearity of plot between

1 𝑅0

1

and

𝐶0

signifies that the process is obeying the Langmuir 𝐶

Hinshelwood kinetic model. The pseudo first order equation ln (𝐶 ) = −𝑘𝑡 has been widely used

SC RI PT

0

to study the photodegradation kinetics of TiO2/clay nanocomposites. In an article by Okte et al. 1

1

[53], linear fitting of plot between 𝑅 and 𝐶 obtained after photodegradation of β Naphthol on 0

0

1%Fe 25%TiO2/montemorilonite sample indicated that reaction mechanism is obeying Langmuir Hinshelwood kinetic model. Similar results were obtained by Zhou et al. [112] for Orange-G

Conclusions and outlook

N

4.

U

degradation by TiO2/sepeolite nanocomposites.

A

This review highlights the properties of TiO2/clay nanocomposites and the role of

M

different clay supports on photocatalytic activity of TiO2 NPs. It also presents the properties and

D

features of clay supported heterostructured TiO2 photocatalysts. The clay plays significant role in

TE

photocatalytic activity. A number of clay minerals have been used for TiO2/clay nanocomposite synthesis mainly montmorillonite, bentonite, kaolinite, haloysite, palygorskite, attapulgite,

EP

rectorite, diatomite, laponite/hectorite and LDH as discussed in the present review. Titanium (IV) butoxide, titanium isopropoxide, tetrabutyl titanate and TiCl4 are commonly used precursors

CC

for TiO2 nanocomposite synthesis. There have been a number of methods developed for the

A

preparation of TiO2/clay nanocomposites mainly, sol-gel, hydrothermal, solvothermal, microwave assisted, ultrasonic and heterocoagulation etc. The use of different surfactants has enhanced the photocatalytic activity of nanocomposites due to increment in specific surface area and porosity. Pore size, pore volume and interlayer space of clay minerals can vary by the use of

32

surfactants such as CTAB, POP. The variation in pore size, interlayer space and pore volume depends upon surfactant chain length and concentration. The TiO2/clay nanocomposites are endowed with high photocatalytic efficiency than

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pure commercial TiO2 (P25) due to the combined effect of high adsorption capacity of clay and photoactivity of TiO2. TiO2/clay nanocomposites possess high surface area, porosity, reusability and chemical and mechanical stability than pristine TiO2 (P25). In many cases, it has been found that clays prevented transformation of anatase to rutile even after calcination at 1000oC. Clay tends to provide TiO2 NPs with surface active sites, prevents their agglomeration and their

U

release to the environment. The layered structure of clay, the presence of exchangeable cations

N

between interlayer space, and presence of negative charge on its surface play an important role in

A

shaping the photocatalytic properties of TiO2/clay nanocomposites. The negatively charged

M

surface of clay and exchangeable cations present in interlayer spaces tend to lower the charge recombination by trapping either photo-generated electron or hole from the photoactive material.

D

The photocatalytic activity of TiO2/clay nanocomposites also depends upon the clay texture (2:1

TE

or 1:1) and morphology. From recent comparative studies, it has been observed that optical

EP

aspects such as light absorption and transparency of clay play significant role in photoactivity of TiO2/clay nanocomposite.

CC

Moreover, recent trends lay more stress upon the modification of either clay or TiO2 surface or both for obtaining high photocatalytic efficiency in solar light and recent articles

A

emphasize on influence of negatively charged clay surface on photocatalytic mechanism of TiO2/clay based nanocomposites. Elemental doping has also been carried out to extend the activity of TiO2/clay nanocomposites towards visible light and elements such as nitrogen, sulphur, carbon and vanadium etc. have been used for this purpose. Apart from this few reports

33

also introduced some new strategies such as modification of clay surface by phosphoric acid, PANI and acid doping. Loading of noble metal NPs upon TiO2/clay nanocomposite has been helpful in reducing charge carrier recombination and also make the nanocomposite visible light

SC RI PT

active due to LSPR effect. Some of the semiconductors NPs such as V2O5, BiOBr, CdS, gC3N4 etc. have been incorporated onto TiO2/clay nanocomposites for better visible light photoactivity. Incorporation of g-C3N4 on TiO2/clay nanocomposites can be also found in few recent articles. The g-C3N4 is easy to prepare by pyrolisis of low cost precursors such as urea and can be easily incorporated on TiO2/clay nanocomposite and it can be found to act as an effective

U

sensitizer. Hence, TiO2/clay nanocomposites can be highly promising materials for their

N

application in water purification at industrial levels. Hence, it can be concluded that clays can be

A

highly promising supports for TiO2 and TiO2/clay nanocomposites can effectively degrade a

M

number of organic pollutants with high reusable efficiency.

D

Conflict of interests:

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The authors declare that there is no conflict of interests regarding the publication of this

EP

manuscript.

Acknowledgements:

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Authors are thankful to DST (Grant No: SB/FT/CS-178/2013), New Delhi for

fellowship as well as other financial assistance.

A

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[94] H. Chen, A. Wang, Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay, Journal of Colloid and Interface Science, 307 (2007) 309-316. [95] D. Zhao, J. Zhou, N. Liu, Characterization of the structure and catalytic activity of copper modified palygorskite/TiO 2 (Cu 2+-PG/TiO 2) catalysts, Materials Science and Engineering: A, 431 (2006) 256-262. [96] L. Bouna, B. Rhouta, M. Amjoud, F. Maury, M.-C. Lafont, A. Jada, F. Senocq, L. Daoudi, Synthesis, characterization and photocatalytic activity of TiO 2 supported natural palygorskite microfibers, Applied Clay Science, 52 (2011) 301-311. [97] D. Papoulis, S. Komarneni, D. Panagiotaras, A. Nikolopoulou, H. Li, S. Yin, S. Tsugio, H. Katsuki, Palygorskite–TiO 2 nanocomposites: part 1. Synthesis and characterization, Applied Clay Science, 83 (2013) 191-197. [98] D. Papoulis, S. Komarneni, D. Panagiotaras, A. Nikolopoulou, K. Christoforidis, M. Fernández-Garcia, H. Li, Y. Shu, T. Sato, Palygorskite–TiO 2 nanocomposites: Part 2. photocatalytic activities in decomposing air and organic pollutants, Applied Clay Science, 83 (2013) 198-202. [99] D. Chen, Y. Du, H. Zhu, Y. Deng, Synthesis and characterization of a microfibrous TiO 2– CdS/palygorskite nanostructured material with enhanced visible-light photocatalytic activity, Applied Clay Science, 87 (2014) 285-291. [100] J. Zhang, L. Zhang, S. Zhou, H. Chen, Y. Zhao, X. Wang, Exceptional visible-light-induced photocatalytic activity of attapulgite–BiOBr–TiO2 nanocomposites, Applied Clay Science, 90 (2014) 135140. [101] X. Bu, B. Wu, T. Long, M. Hu, Preparation, characterization and enhancement of the visible-light photocatalytic activity of In 2 O 3/rectorite composite, Journal of Alloys and Compounds, 586 (2014) 202-207. [102] Y. Lu, P.R. Chang, P. Zheng, X. Ma, Rectorite–TiO 2–Fe 3 O 4 composites: Assembly, characterization, adsorption and photodegradation, Chemical Engineering Journal, 255 (2014) 49-54. [103] Y. Zhang, Y. Guo, G. Zhang, Y. Gao, Stable TiO 2/rectorite: preparation, characterization and photocatalytic activity, Applied Clay Science, 51 (2011) 335-340. [104] P. Yuan, D. Liu, M. Fan, D. Yang, R. Zhu, F. Ge, J. Zhu, H. He, Removal of hexavalent chromium [Cr (VI)] from aqueous solutions by the diatomite-supported/unsupported magnetite nanoparticles, Journal of hazardous materials, 173 (2010) 614-621. [105] Z. Sun, Z. Hu, Y. Yan, S. Zheng, Effect of preparation conditions on the characteristics and photocatalytic activity of TiO2/purified diatomite composite photocatalysts, Applied Surface Science, 314 (2014) 251-259. [106] J. Zhang, X. Wang, J. Wang, J. Wang, Z. Ji, Effect of sulfate ions on the crystallization and photocatalytic activity of TiO2/diatomite composite photocatalyst, Chemical Physics Letters, 643 (2016) 53-60. [107] Y. Xia, F. Li, Y. Jiang, M. Xia, B. Xue, Y. Li, Interface actions between TiO2 and porous diatomite on the structure and photocatalytic activity of TiO2-diatomite, Applied Surface Science, 303 (2014) 290-296. [108] Z. Sun, C. Li, G. Yao, S. Zheng, In situ generated g-C3N4/TiO2 hybrid over diatomite supports for enhanced photodegradation of dye pollutants, Materials & Design, 94 (2016) 403-409. [109] L.M. Daniel, R.L. Frost, H.Y. Zhu, Laponite-supported titania photocatalysts, Journal of colloid and interface science, 322 (2008) 190-195. [110] L.M. Daniel, R.L. Frost, H.Y. Zhu, Synthesis and characterisation of clay-supported titania photocatalysts, Journal of colloid and interface science, 316 (2007) 72-79. [111] D. Kibanova, M. Sleiman, J. Cervini-Silva, H. Destaillats, Adsorption and photocatalytic oxidation of formaldehyde on a clay-TiO2 composite, Journal of hazardous materials, 211 (2012) 233-239. [112] F. Zhou, C. Yan, T. Liang, Q. Sun, H. Wang, Photocatalytic degradation of Orange G using sepioliteTiO2 nanocomposites: Optimization of physicochemical parameters and kinetics studies, Chemical Engineering Science, 183 (2018) 231-239. 40

A

CC

EP

TE

D

M

A

N

U

SC RI PT

[113] F. Zhou, C. Yan, H. Wang, S. Zhou, S. Komarneni, Sepiolite-TiO2 nanocomposites for photocatalysis: Synthesis by microwave hydrothermal treatment versus calcination, Applied Clay Science, 146 (2017) 246-253. [114] E. Seftel, M. Niarchos, C. Mitropoulos, M. Mertens, E. Vansant, P. Cool, Photocatalytic removal of phenol and methylene-blue in aqueous media using TiO2@ LDH clay nanocomposites, Catalysis Today, 252 (2015) 120-127. [115] J.-H. Yang, H. Piao, A. Vinu, A.A. Elzatahry, S.-M. Paek, J.-H. Choy, TiO 2-pillared clays with wellordered porous structure and excellent photocatalytic activity, RSC Advances, 5 (2015) 8210-8215. [116] E. Srasra, F. Bergaya, J. Fripiat, Infrared spectroscopy study of tetrahedral and octahedral substitutions in an interstratified illite-smectite clay, Clays and Clay Minerals, 42 (1994) 237-241.

41

U

SC RI PT

Figures

N

Fig. 1. Schematic diagram showing generation of electron hole pairs on semiconductor

A

CC

EP

TE

D

M

A

surface during heterogeneous photocatalysis.

Fig. 2. Schematic diagram showing different processes taking place on TiO2 surface during UV light irradiation.

42

TiO 2 + h

TiO 2 (e-CB + h+VB) H+ + OH

H 2O

O 2 + e CB

O 2•-

O 2•- + H +

OOH•

2 OOH • + O 2

U

H 2O2

N

OH• O 2•-

A

Organic Pollutant

CO2 + H 2O

D

M

OOH • H 2O2

SC RI PT

OH•

OH- + h+VB

TE

Fig. 3. Generation of different free radicals on TiO2 surface and subsequent mineralization

A

CC

EP

of organic pollutants by these free radicals.

43

SC RI PT

A

CC

EP

TE

D

M

A

N

U

Fig. 4 Different components of a basic clay structure[1].

Fig. 5. Expanding and non-expanding clays and arrangement of tetrahedral and octahedral layers in different clay types [1].

44

SC RI PT U

N

Fig. 6. Schematic representation of delamination of Mt by POP and subsequent TiO2

A

CC

EP

TE

D

M

A

formation on its surface [2]

Fig. 7. Schematic representation for the formation of TiO2/Mt/Fe3O4 nanocomposite[3].

45

SC RI PT U N

A

Fig. 8. UV-Visible diffuse reflectance spectra of TiO2 and N-S co-doped TiO2/Mt

A

CC

EP

TE

D

M

nanocomposite (sample B)[4]

46

SC RI PT U N A

A

CC

EP

TE

D

M

Fig. 9. Time-resolved spectra of M-TiO2/bentonite nanocomposites[5].

47

Fig. 10. X-ray diffraction pattern of recycled TiO2/kaolinite nanocomposites after cycles of

A

N

U

SC RI PT

thermal treatment[6] .

A

CC

EP

TE

D

M

Fig. 11. HRTEM image of TiO2/kaolinite nanocomposites containing mixed phase TiO2[7]

48

SC RI PT U N A M D TE

via sol-gel method[8]

A

CC

EP

Fig. 12. Schematic representation of preparation of TiO2/kaolinite nanocomposite prepared

49

SC RI PT

U

Fig. 13. TEM images of TiO2/halloysite nanocomposites prepared by in-situ low

A

CC

EP

TE

D

M

A

N

temperature synthesis of TiO2 on halloysite nanotubes [9].

Fig. 14. Schematic representation of CdS-TiO2/palygorskite nanocomposites[10].

50

SC RI PT U N A M D

A

CC

EP

TE

Fig. 15. TEM images of palygorskite (a) and CdS-TiO2/ palygorskite nanocomposite [10].

51

SC RI PT U N A M

A

CC

EP

TE

D

Fig. 16. HRTEM images of g-C3N4-TiO2/diatomite nanocomposite [11].

52

N U SC RI PT Tables

Table 1 Comparative overview of different TiO2/Mt nanocomposites for photodegradation of different pollutants

Catalyst

Concentrati

osite

organic

amount

on of

pollutant

(mg/ml)

pollutant

Reaction conditions

A

Model

Fe-

β

TiO2/Mt

Naphthol

ED

M

Nanocomp

CC E Methylen

minum

e Blue

A

TiO2/Alu

Pillard Mt

dye

TiO2/Mt

Methylen e blue

1

Rate

Reac

on

consta

tion

efficiency

nt

time

(%)

(min-1)

(min

R ef.

utes)

3-25 ppm

β Naphthol Solution was kept in irradiation

(mgL-1)

chamber equipped with 8 lamps each of 15W

PT

1

Degradati

97

N.A

300

[1 ]

N.A.

16.741

180

[2 ]

60

[3 ]

emitting UV-A light(λ=365 nm) with incident flux of 4.51015 photons per second 510-5 M

Methylene Blue was kept in pyrex glass reactor

10-3

equipped with 4 UV tube lamps of 10W each emitting UV-B light 4.1

100 ppm

200W high pressure Hg Lamp was used as UV light source( λ=340 nm)

dye 53

80

N.A.

Methylen

N U SC RI PT

TiO2/Mt

1

310-5 M

250 W Hg lamp with wavelength range 250-365 nm

N.A.

810-4

60

[4 ]

2.5

100 ppm

250W high pressure Hg lamp emitting light in

95

0.01

220

[5 ]

>99

N.A

60

[6 ]

67.3

N.A

99

N.A.

e Orange dye Solophen

wavelength range λ≥310 nm providing radiant flux

A

TiO2/Mt

3BL dye Methylen

dye

Sulforho

1.210-4M

0.2

210-5M

CC E

Vanadium

0.2

PT

e blue

of 9.2 mW cm-2

ED

TiO2/Mt

M

yl red

A

- TiO2/Mt

TiO2/Mt

High Pressure Hg lamp (500W, 365 nm) kept 15 cm above the reaction mixture enclosed in a dark box

500 W bromine tungstun lamp was used as visible

damine B

light source with a cut-off filter to remove the

dye

wavelengths shorter than 450 nm, distance between

Methylen

18 hours [7 ]

reactor and light source was kept 12 cm. 0.2

30 ppm

Reaction was carried out in pyrex reactor equipped

e blue

with 250 W High pressure Hg Lamp as UV light

dye

source having thermo-stated water flowing between

54

60

[8 ]

N U SC RI PT

reaction chamber and lamp

Acid Red

1

30 ppm

The sample was irradiated with 20W (λ=253.7 nm)

N.A.

5102

60

[9 ]

95.6

N.A.

60

[1 0]

94

N.A.

150

[1 1]

The photodegradation was performed in modified

Almost

N.A.

60

[1 2]

domestic microwave oven (700 W) equipped with

100

A

TiO2/Mt

UV lamp 8 cm above with light intensity 0.524

M

G dye

mW/cm2

4BS

TiO2/Mt

Methylen

CC E

Fe3O4-

PT

codeped

TiO2/Mt

A

TiO2/Mt

0.4

30 ppm

ED

N-S

1

10ppm

orange

equipped with light filter ( λ≥400 nm) kept 25cm above the sample surface 100 W medium pressure Hg lamp was used as light source(λ=365nm).Temperature of the reactor was

e Blue

Methyl

The sample was irradiated by 300 W Dy lamp

maintained by circulating water. 0.5

100ppm

microwave electrode less lamp fixed in glass reactor

55

Methylen

TiO2/Mt

e blue

0.2

20ppm

N U SC RI PT

Ag-

The samples were irradiated with 500 W high

>99

N.A.

20

[1 3]

18.64

140

[1 4]

pressure Hg Lamp kept 15 cm above the reaction mixture

Methylen

0.2

30ppm

1,4

CC E

TiO2/Mt

PT

e blue

0.2

0.2

50ppm

Reaction was carried out in pyrex reactor equipped

10-3 83

N.A.

90

[1 5]

N.A

2.410

300

[1 6]

with 250 W High pressure Hg Lamp as UV light source having thermo-stated water flowing between reaction chamber and lamp The Solution was UV-irradiated by Xe-Hg lamp (λ=365 nm) at intensity of 125 mW/cm-.2

A

Dioxane

30ppm

ED

Methylen

evaluated by irradiating with 250 W Hg Lamp

M

e blue TiO2/Mt

The photoactivity of the photocatalysts was

A

TiO2/Mt

56

-4

Crystal

0.16

10-4 M

N U SC RI PT

TiO2/Mt

The photoactivity was carried out in a static reactor

Crystal

violet,

equipped with cold finger to avoid thermal

Violet

Rhodami

reactions. A UV-A lamp (λ=365 nm, 100 W/m2)

97.1%,

was placed 10 cm next to the reactor.

A

ne B,

e Blue,

Orange,

Orange

50

[1 8]

Methyl Orange 36.1%,

CC E A

Methyl

N.A.

e 79.8%,

Red

TiO2/Mt

Methylen

Rhodamin

PT

Congo

[1 7]

93.2%,

ED

Methyl

350

e blue

M

Methylen

N.A

Congo red 22.1% 4

20 ppm

The photocatalytic reaction was carried out under 30W UV lamp (254 nm) kept 15 cm above beaker containing the dye catalyst mixture.

57

>99

N U SC RI PT

ED

M

A

Table 2. Comparative overview of TiO2/bentonite nanocomposites for photodegradation of different pollutants

Model

Catalyst

Concentra

ite

organic

amount(

te

Methylene

Degradati

Rate constant

Reacti

tion of

on

(min-1)

on

pollutant

efficiency

time

(%)

(minut

0.25

Ag-

10 ppm

TiO2/bentoni

The photodegradation experiment was carried out by

>99

N.A.

60

[1 9]

98.94

N.A.

60

[2 0]

250 W Hg lamp (λ=365 nm) in quartz reactor equipped

blue dye

phenol

Re f.

es)

A

TiO2/bentoni

mg/ml)

CC E

pollutant

Reaction conditions

PT

Nanocompos

with cold finger to avoid thermal reactions 0.16

100 ppm

300 ml of phenol was stirred in natural solar light or solar simulator

te 58

Chloroben

TiO2/bentoni

zene

0.1

400 ppm

The reaction was carried out in closed vessel in an ice

>99%

0.03963 under

16

bath to maintain the temperature of the vessel less than

under UV

visible light

minutes

20oC. The sample was irradiated by 100W UV lamp

and 92%

under UV

(λ=365 nm). To investigate visible light activity the

in visible

light and

light

60

A

te

N U SC RI PT

Ag-

M

sample was irradiated by 65W CFL lamp as visible light source. The distance between the lamps and vessel

minutes under

TiO2/bentoni

zene and

te (M-Au,

visible light

400 ppm

The reaction was carried out in closed vessel in an ice

For

For

For

chloroben

bath to maintain the temperature of the vessel less than

benzaldeh

chlorobenzene

benzaldeh

benzaldehy

zene and

20oC. The sample was irradiated by 100W UV lamp

yde- 96%

- 0.0178 under

yde- 220

de

0.1 mM

(λ=365 nm, 46-47W/m2 flux density). To investigate

under UV

visible light

minutes

benaldehy visible light activity the sample was irradiated by 65 W

and 78%

and 0.055

under

CFL lamp (125 W/m2 flux density) as visible light

in visible

under UV

visible

source. The distance between the lamps and vessel was

light

light.

light and

A

Ag, Pd)

0.1

CC E

Chloroben

PT

ED

was kept 11 cm.

M-

de

[2 1]

59

[2 2]

N U SC RI PT

RBR-

C3N4/TiO2/b

X3BS

40 ppm

For

For

140

chloroben

benzaldehyde-

minutes

zene- 99%

0.004 under

under UV

visible light

light and

and 0.027

88%

under UV

under

light.

visible light To investigate visible light photocatalytic activity the sample was irradiated by 65 W CFL lamp (125 W/m2 flux density) as visible light source. The distance between the lamps and vessel was kept 11 cm.

A

entonite

1

CC E

g-

PT

ED

M

A

kept 11 cm.

60

90

N.A.

100

[2 3]

N U SC RI PT

Catalyst

Concentrat

osite

organic

amount

ion of

pollutan

(mg/ml)

pollutant

t(s)

nite

red dye

A

TiO2/kaoli nite

4-12

Acid

Orange 7

57.4 µM

PT

Congo

CC E

TiO2/Kaoli

0.76

Reaction conditions

M

Model

Degradation

Rate

Reacti

Re

efficiency

constant

on

f.

(%)

(min-1)

time (minu

ED

Nanocomp

A

Table 3 Comparative overview of TiO2/kaolinite nanocomposites for photodegradation of different pollutants

tes)

11 W UV-C light was positioned centrally to magnetically

N.A.

0.987 h-1

120

stirred beaker containing reaction solution. The linear

[24 ]

attenuation of UV light at the solution surface was determined to be 300µW/m2 6.210-4 M The sample was stirred for 60 minutes in dark before irradiating it with UV light from UVP pen ray lamp (λ=365 nm).

61

70

N.A.

60

[25 ]

Methyle

nite

ne blue

10

N U SC RI PT

TiO2/kaoli

25 ppm of

The photocatalyst was dispersed in aqueous solution of

100 for

both dyes

dyes and the reaction mixture was exposed to UV light (30

both dyes

N.A

60

]

W, λ=365 nm) for 24 h under constant stirring

and

A

methyl

30-50 ppm

5.49810

dye->99%,

2

Acid

1.5 for

nite

Red G

Acid red

and 4

G dye

dye and 10

4

nitrophe

and 0.1

ppm for 4

Nitrophenol

4

nol

for 4

- 90%

Nitrophe

UV lamp(λ=253.7 nm) as light source

CC E

PT

Acid red G

The photocatalytic activity was evaluated by using 20 W

Acid red G

TiO2/kaoli

ED

II

M

orange

Nitropheno

Acid red

[27

for acid

G dye- 90

]

red G

minutes,

Nitrophe l

nol- 240

nol

A

[26

2

minutes

g-

Ciprofo

10 ppm

The photocatalytic degradation was carried out in Xe lamp

C3N4/TiO2/

xacin

with average light intensity of 90 mWcm-2 equipped with

kaolinite

(CIP)

400 nm cut-off filter.

62

92

0.00813

240

[28 ]

N U SC RI PT

ED

M

A

Table 4 Comparative overview of different TiO2/halloysite nanocompoites for photodegradation of different pollutants

Model

Catalyst

Concentrat

e

organic

amount

ion of

PT

Nanocomposit

CC E

pollutant

Degradation

Rate

efficiency (%)

constant

Reacti

(min-1)

on

pollutant

(minu tes)

800W Xe lamp (290<λ<800 nm) equipped

B and

Rhodamin

with filter to eliminate the UV radiation Rhodamine B

Gentian

e and 20

below 400 nm was used as light source.

and 60% of

violet dye

ppm of

During the reaction the suspension was

Gentian

Gentian

purged with inert gas and air. The emission

violet dye

violet dye

intensity at surface of suspension was 58

Rhodamine

e

Ref .

time

10 ppm of

TiO2/halloysit

A

(mg/ml)

Reaction conditions

0.5

63

mW/cm-2.

88% of

N.A.

6 hours

[29 ]

Rhodamine

TiO2/halloysit

B

0.5

N U SC RI PT

polyaniline–

800W Xe lamp (290<λ<800 nm) equipped

10 ppm

76.74

N.A.

6 hours

[30 ]

90

N.A.

6 hours

[31 ]

70

N.A

150

[32 ]

with filter to eliminate the UV radiation

e

below 400 nm was used as radiation source.

A

During the reaction the suspension was

Rhodamine

TiO2/halloysit

B

10 ppm

TiO2/halloysit

mW/cm-2. 800W Xe lamp (290<λ<800 nm) equipped with filter to eliminate the UV radiation

During the reaction the suspension was purged with inert gas and air. The emission intensity at surface of suspension was 24

A N-doped

intensity at surface of suspension was 28

below 400 nm was used as light source.

CC E

e

0.5

PT

polyaniline–

ED

M

purged with inert gas and air. The emission

mW/cm-2. Phenol

0.3

10 ppm

The photocatalytic activity as investigated under simulated solar light (300W)

64

N U SC RI PT

e

TiO2/halloysit

Methylene

e

blue

0.5

0.1 mM

The samples were irradiated by 12 W lamp

81.6

N.A

4 hours

[33 ]

N.A

4 hours4

[34 ]

(λ=365 nm) with irradiation intensity of

4

1

TiO2/halloysit

nitrophenol

e

and

The catalyst was suspended in 4 nitrophenol

90% of 4

nitropheno

solution and exposed to UV light by 12W

nitrophenol

nitrophen

lamp (λ=253 nm) with irradiation intensity of

and 91% of

ol and 10

350µW/cm-2.

Methylene

hours for

blue

Methylen

l and 32 ppm

Methylene blue

For

methylene

blue

degradation 12 W lamp emitting high intensity at 365 nm was used

A

CC E

PT

Methylene blue

10 ppm 4

ED

Amylose-

M

A

350µW/cm-2.

65

e blue

N U SC RI PT

e

pollutant

Catalyst

Concentra

Reaction conditions

ED

Model organic

Degradatio

Rate

Reacti

Re f.

amount(

tion of

n efficiency

constan

on

mg/ml)

pollutant

(%)

t (min-

time

1

(minut

CC E

PT

Nanocomposit

M

A

Table 5 Comparative overview of different TiO2/palygorskite and TiO2/attapulgite nanocomposites for photodegradation of different pollutants.

TiO2-SnO2

Phenol

0.2-0.8

)

es)

20-120

/Pallygorskite

Photoactivity irradiation

in

was

carried

XPA

out

under

photochemical

UV

99.8

0.035

90

reactor

[3 5]

A

equipped with 300W high pressure Hg lamp. The reaction vessel was bubbled with air at the rate 20 ml/min

TiO2/Pallygor skite

Orange G dye

1

10-5M

The photocatalytic reaction was carried out in a batch quartz reactor (402036) placed in 66

thermostatic chamber under UV lamp (125 W, 365 nm) emitting photon flux 1mWcm-2.

89

N.A.

60

[3 6]

Methyl

TiO2/attapulgi

orange

1

20 ppm

N U SC RI PT

BiOBr–

Before

illumination

the

sample

was

96.80

N.A.

120

ultrasonicated for 10 minutes and stirred in dark

te

[3 7]

for 30 minutes. The sample was then irradiated

Methyl

TiO2/attapulgi

orange

1

Methylene

CdS/palygors

Blue

CC E

photochemical reactor equipped with 300W high

20 ppm

99

N.A.

30

[3 8]

pressure Hg lamp. Air was bubbled in the reaction vessel at the rate of 20 mL/min. Suspension containing the dye and photocatalyst was illuminated with 150 W high pressure Hg lamp with cut off filter (420 nm>λ>700 nm). During the reaction the reaction was continuously stirred and cooled by cold water.

A

kite

N.A.

PT

TiO2-

The photocatalytic activity was evaluated in XPA

ED

te

20 ppm

M

SnO2–

A

with 500 W Xe lamp.

67

N.A.

310-3

150

[3 9]

N U SC RI PT

Table 6. Comparative overview of TiO2/clay nanocomposites prepared from other different clays for photodegradation of different pollutants

Model

Catalyst

Concentrati

e

organic

amount(mg

on of

pollutant

/ml)

1

M

Reaction conditions

Degradation

Rate constant

Reactio

Re

efficiency (%)

(min-1)

n time

f

pollutant

ED

Acid red G

PT

TiO2/rectorite

A

Nanocomposit

30 ppm

(minut es)

The sample was irradiated with 20 W 98% for acid

N.A.

120

[40

Acid red G

UV lamp (λ=253.7 nm) with light red

nitrophenol

dye and 10

intensity 0.524 mW/cm2. The distance and 70% for

for

ppm 4

between the reaction mixture and light 4 nitrophenol

red G dye

CC E

dye and 4

G

dye

minutes

nitrophenol source was kept 8cm.

and

]

acid

240

A

minutes for

4

nitropheno l TiO2/Zn-LDH

Phenol

2

0.425 mM

The solutions were stirred in batch 68

reactor for 40 minutes in order to reach adsorption/desorption equilibrium. The

85

N.A.

240

[41 ]

N U SC RI PT

samples were then illuminated with

UV lamp (λ=254 nm) for 1-5 hours with an intensity 4400 mW/cm-2. Rhodamine

0.5

25 ppm

CC E

PT

ED

M

B, phenol

Zr doped

A

TiO2/Cloisite

Antipyrine

The experiments were carried out in

N.A.

A

TiO2/Cloisite

0.25

0.0101 for

solar simulator equipped with 765-250

Rhodamine B

W/m2 Xe lamp (61-24 W/m2 from

and 0.023 for

300> λ>700 nm, 1.41020-5.51019)

phenol

600

[42 ]

and day light filter cutting of 290 nm. The intensity was fixed to 550 W/m2 for all experiments

10 ppm

The experiments were carried out in solar simulator equipped with 765-250 W/m2 Xe lamp (61-24 W/m2 from 300> λ>700 nm, 1.41020-5.51019) and day light filter cutting of 290 nm. The intensity was fixed to 450 W/m2

69

90

N.A.

360

[43 ]

N U SC RI PT

for all experiments

Phenol and

1

10 ppm phenol and

blue

110-4M

After stirring in dark for 30-60 minutes For

Methylene

100 W Hg lamp as UV light source. To visible

and

investigate visible light activity 150 W UV light at halogen lamp was used.

ED

blue

phenol

the suspensions were illuminated by 90% in both

A

Methylene

M

TiO2/LDH

PT CC E

[44

Phenol 5

]

hours and for mehtylene

for

blue 100

blue 70% in UV and 55% in

visible

light at PH

A

For

PH 10 and

Methylene

10

70

N.A.

minutes

Orange G

0.8

N U SC RI PT

TiO2/beidellit

10-4M

The photocatalytic experiment was

e

>99

N.A.

45

performed in batch quartz reactor

[45 ]

placed in thermostatic reactor (25oC)

A

under UV lamp (125W) emitting 365

Methylene

PT

blue

Rhodamine

CC E

g-C3N4-

0.5

110-4M

ED

TiO2/hectorite

M

nm with photon flux of about 1 mW

1

10 ppm

cm-2.

The aqueous suspensions of Methylene

99.85

N.A.

blue irradiated with 450 W high

35

[46 ]

pressure Hg lamp. The photocatalytic activities were

N.A.

For Rhodamine

[47 ]

B,

evaluated by photodegradation of dyes

B-0.00409

e

Methylene

under 500 W Xe lamp with or without

under visible

blue

420 nm cut off filter.

light and

A

TiO2/diatomit

0.01330 under solar light and for Methylene

71

N U SC RI PT

blue 0.00098 under visible light and

Methyl

e

Orange

0.6

20 ppm

CC E

PT

ED

TiO2/diatomit

M

A

0.00961 under

A

TiO2/sepeolite

Orange G

solar light.

The sample was illuminated by 250 W

99% in UV

Hg high pressure lamp in tubular

N.A.

90

[48

light and

minutes

]

quartz reactor cooled by circulating

60% in

under UV

water and to investigate visible light

visible light

light and 8

activity 300 W white light lamp was

hours in

used as visible light source for sample

visible

illumination. 0.3

10 ppm

light

The photocatalytic reaction was carried out under 300W Xe lamp equipped with 365 nm pass filter kept 15 cm above beaker containing the dye

72

90

N.A.

120

[49 ]

N U SC RI PT

A

CC E

PT

ED

M

A

catalyst mixture.

73