Review Article An Overview: Recent Development of Titanium ...An Overview: Recent Development of Titanium Oxide Nanotubes as Photocatalyst for Dye Degradation ChinWeiLai, 1 JoonChingJuan,

Review ArticleAn Overview Recent Development of Titanium OxideNanotubes as Photocatalyst for Dye Degradation

Chin Wei Lai1 Joon Ching Juan1 Weon Bae Ko2 and Sharifah Bee Abd Hamid1

1 Nanotechnology amp Catalysis Research Centre (NANOCAT) Institute of Postgraduate Studies (IPS) University of Malaya3rd Floor Block A 50603 Kuala Lumpur Malaysia

2 Department of Chemistry Sahmyook University Seoul 139-742 Republic of Korea

Correspondence should be addressed to Chin Wei Lai cwlaiumedumy and Weon Bae Ko kowbsahmyooksyuackr

Received 25 December 2013 Revised 14 January 2014 Accepted 15 January 2014 Published 27 February 2014

Academic Editor Jiaguo Yu

Copyright copy 2014 Chin Wei Lai et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Today organic dyes are one of the largest groups of pollutants release into environment especially from textile industry It is highlytoxic and hazardous to the living organism thus the removal of these dyes prior to discharge into the environment is essentialVarieties of techniques have been employed to degrade organic dyes and heterogeneous photocatalysis involving titanium dioxide(TiO2) appears to be the most promising technology In recent years TiO

2nanotubes have attracted much attention due to their

high surface area and extraordinary characteristicsThis paper presents a critical review of recent achievements in the modificationof TiO

2nanotubes for dye degradation The photocatalytic activity on dye degradation can be further enhanced by doping with

cationic or anionic dopant

1 Introduction

11 Overview of Organic Dye Degradation Nowadays globalwarming poses one of the most serious threats to the globalenvironment ever faced in human history The environmentpollutions will cause an unprecedented onslaught of deadlyand costly weather disasters such as severe storms droughtsheat waves and rising seas and floods all over the world [1ndash3] One of the major water pollutions is the residual dyesfrom different sources (eg textile industries paper and pulpindustries dye and dye intermediates industries pharmaceu-tical industries tannery and craft bleaching industries etc)which are considered a wide variety of persistent organicpollutants introduced into the natural water resources orwastewater treatment systems [4] Dyeing and finishingindustry produces as much as 50ndash100 L wastewaterkg offinished textile products [5] As shown in Figure 1 WorldBank has estimated that textile industries in many countriescontribute to 17 to 20 of global industrial water pollution[6] There are about 10ndash15 of 700 thousand tons of 10thousand types of dyes that have been dischargedwithout anyproper physicochemical treatment [7]

There are more than 100 000 commercially availabledyes and an annual worldwide production of synthetic dyesof nearly 1 million tons [8ndash10] Reactive dyes have beenestimated to contribute the most to the discharge of dyewastewater as much as 57 thousand tons (Figure 2) becauseof their low fixation rate As shown in Table 1 the reactive dyehas a very poor fixation rates The fixation rate is influencedby the extent of hydrolysis of the dye [11] signifying thevulnerability of reactive dyes to hydrolysis in dye bath

The release of the persistent organic dyes structure withtoxicity property may cause negative effects on the envi-ronment and human health These negative impacts mainlyattributed to the nonbiodegradable nature of the persistentorganic dyes as well as their high colour intensity and are ableto reduce aquatic diversity by blocking the passage of sunlightthrough the water [12] In particular the discharge of persis-tent organic dye effluents into the natural water resources orwastewater treatment systems is undesirable because most ofthese organic dyes released and their breakdown products arevery toxic carcinogenic or mutagenic to life forms mainlydue to the carcinogens such as benzidine naphthalene andother aromatic compounds in the structure of persistent

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2014 Article ID 524135 14 pageshttpdxdoiorg1011552014524135

2 International Journal of Photoenergy

Figure 1 Water pollution caused by textile industrial from variouscountries The darker icon signifies higher level of water pollutionwhich is based on percentage of total biochemical oxygen demand(BOD) emissions [20]

0

10

20

30

40

50

60

Mas

s of d

ye w

ater

(1000

tons

)

Acid

Reac

tive

Disp

erse

Dire

ct Vat

Basic

Sulfu

r

Types of textile dyes

Figure 2 Mass loss of global dyes into the wastewater [21]

organic dyes [5 13 14] These pollutants can cause irritationof the eyes skin respiratory tract sore throat asthma andallergic contact dermatitis [15] The persistent organic dyeeffluent will remain in the environment for a long period oftime if incomplete or inadequate treatments during the watercleaning stage [5] For instance the half-life of hydrolyzedReactive Blue 19 is about 46 years at pH 7 and 25∘C [16]

Wastewater containing dyes is very difficult to treatsince the dyes are recalcitrant organic molecules resistant toaerobic digestion and are stable to light Many traditionaltextile wastewater treatment studies have been applied atmany textilemillsHowever a synthetic dye inwastewater cannot be efficiently decolorized by these traditional methodsThis is because of the high cost and disposal problems fortreating dye wastewater at large scale in the textile andpaper industries [17] As such several wastewater treatmentsfor colour removal can be divided into 3 categories whichinclude physical chemical and biological treatments [1819] Nevertheless most of the conventional dye removaltechniques will have their respective limitations in textileindustry

Table 1 General fixation rates for different dye classes [22]

Dye class Fixation rate () FibersAcid 83 to 93 Wool nylonBasic 90 to 96 AcrylicDirect 70 to 95 CelluloseDisperse 80 to 92 SyntheticPigments 95 to 98 WoolReactive 50 to 80 CelluloseSulphur 60 to 70 CelluloseVat 80 to 95 Cellulose

12 Dye Removal Techniques The most popular method forthe removal of pollutants from wastewater is using liquid-phase adsorption In fact liquid-phase adsorption providesan attractive alternative for the treatment of contaminatedwaters especially if the sorbent is cheap and does not requirean additional pretreatment step before its application Theadsorption is a well-known equilibrium separation processand an effective method for water decontamination appli-cations due to the water reuse in terms of initial costflexibility and simplicity of design ease of operation andinsensitivity to toxic pollutants [23] Adsorption and ionexchange reactions play an important role in decolonization[24 25] This decolorization process is strongly dependenton the physiochemical factors (eg interaction between dyeand sorbent active surface area of sorbent particle sizereaction time environment temperature and pH) [26] Themost important aspect of the adsorption technique is that noharmful substance will be generated and released throughoutthe reactions Besides the adsorption process using activatedcarbons has been widely used to remove persistent organicdyes from wastewaters [27 28] However commerciallyavailable activated carbon is still far from being a potentialcandidate in dyes removal applications due to the high pro-duction cost and regeneration problems [2]The regenerationactivated carbons involve restoring the adsorptive capacity ofsaturated activated carbon by desorbing adsorbed dyes on itssurface In addition the adsorption involves a phase transferof pollutants to other secondary wastes that subsequentlyrequire additional treatment of proper disposal procedure

In general several types of filtration technology (physicalmethods) have been widely used for wastewater and textileeffluents treatment such as membrane-filtration processes(microfiltration ultrafiltration nanofiltration and reverseosmosis) and adsorption techniques [23 29ndash33] Howevermajor disadvantages of the membrane-filtration processesare that they are only applied in small wastewater flow raterequire higher production cost of membrane and cost ofperiodic replacement require high working pressure to pushthe wastewater flow through membrane filtration and areunable to reduce dissolved solid content In addition thistechnique requires frequent cleaning and replacement of themodules to maintain effectiveness in removing organic dyes

Besides biodegradation of organic dyes is often themost economical alternative as compared to physical andchemical processes The possible microorganisms used for

International Journal of Photoenergy 3

Table 2 Oxidation potential of several powerful oxidants [34]

Oxidant ∙OH (hydroxyl radical) O3 (ozone) H2O2 (hydrogen peroxide)Oxidation potential (V) 280 207 177

TiO2

nanoparticles

H2O (OHminus)

eminus eminus

O2

h+ h+

+OH

(CO2 + H2O)

h (photonic energy)

Degradation product

+O2

minus

Figure 3 Overall mechanism of the photocatalytic degradation of organic dye compounds using TiO2-based nanomaterial

biodegradation dyes are fungi bacteria yeasts and algaeThey are involved in fungal decolourizationmicrobial degra-dation and adsorption by (living or dead) microbial biomassand bioremediation systems that are commonly applied tothe treatment of industrial effluents [9 35] However thelimitations of this technique are often restricted because oftechnical constraint The high toxicity of some chemicalscould decrease the numbers of active organisms used in theaerobic biological process and long hydraulic retention time[36] In addition to that biodegradation of organic dyes nor-mally is incapable of obtaining satisfactory colour eliminationwith current conventional biodegradation processes [37] Inthis manner many other organic dyes are recalcitrant dueto their complex chemical structure and synthetic organicorigin [38] In this case azo dyes are not totally degradedor show slow degradation because of their xenobiotic nature[39]

The chemical treatment includes coagulation and floc-culation for separation of dyes The major disadvantages ofthese techniques are high cost of coagulating or flocculatingagent and pH dependency for effective dye removal as wellas disposal problem resulted from accumulation of concen-trated sludge In addition secondary pollution problems willarise due to the excessive chemical used Meanwhile themain reaction involved in chemical wastewater treatmentis ion exchange which is ineffective in removing severaltypes of persistent organic dyes The low removal in thecase of anionic dyes (disperse dyes) diffusion limitation thatcan affect reaction rate while the use of organic solventfor regeneration is very expensive [18] and thus they arecommercially unattractive

13 Photocatalytic Degradation of Organic Dye by TitaniumDioxide In recent years photocatalysis system has attractedgreat interest from science community as themost promisingway to solve the environmental problems especially gettingrid of residual dyes pollutants fromwastewater stream [2 40ndash42] In this manner photocatalysis system is considered to

be ideal green environmental solution to realize our greeneconomy future In this case TiO

2-based nanomaterials have

been broadly studied as the most promising photocatalystfor environmental remediation such as air purification waterpurification heavy metals degradation and hazardous wasteremediation [3 43ndash49] The reasons mainly attributed tothe nontoxicity cost effective long-term stability widespreadavailability and high stability against photocorrosion withgreat capacity for oxidation and high photocatalytic propertyThe development of the nanoarchitecture of TiO

2assemblies

with precisely controllable nanoscale features has gainedsignificant scientific interest [42ndash54]

According to Ibhadon and Fitzpatrick (2013) [34] theprinciple behind the photocatalysis involves the photoexci-tation of the metal oxides with light energy (Figure 3) Inthis case only UVL (120582 le 387 nm) greater than the bandgap of the anatase TiO

2(119864bg = 32 eV) generates electron-

hole pairs as shown in (1) The photoinduced electrons andholes diffuse to the particle surface of the TiO

2 in which

they can be exploited for various redox processes analogousto those of an electrochemical cell The positive holes reactwith water an electron donors to produce the most crucialand powerful oxidizing free radicals ∙OH ((2) and Table 2)Thus any organic compound adsorbed on the surface ofthe photocatalysts is oxidized by ∙OH (3) The photoexcitednegative electrons react with oxygen to become superoxideradicals O

2

minus∙ and perform the similar oxidative attack onorganic compounds Consider

TiO2+ hV 997888rarr TiO

2(h+VB) + TiO2 (e

minus

CB) (1)

where TiO2(h+VB) is the positive hole in valence band (VB)

and TiO2(eminusCB) is the photoexcited electron in conduction

band (CB) (Figure 4) Consider

TiO2(h+VB) +H2Oads 997888rarr TiO

2+∙OH +H+ (2)

∙OH + dye 997888rarr CO2+H2O (3)


3D2D

1D

Figure 4 A schematic model for the formation of TiO2nanotubes

[55]

TiO2(eminusCB) +O2ads 997888rarr TiO

2+O2

∙minus

ads (4)

O2

minus∙+ dye 997888rarr CO

2+H2O (5)

In this manner TiO2can act as an ideal semiconductor

material for photocatalysis because it is nontoxic relativelycheap and highly photostable It is interesting to notethat TiO

2exists in six different polymorphs namely rutile

(tetragonal) anatase (tetragonal) brookite (orthorhombic)TiO2(B) (monoclinic) TiO

2(II) (alpha-PbO

2) and TiO

2(H)

(hollandite) [56 57] TiO2nanoparticle has been widely used

as an active photoelectrode applied in variety of environmentand energy system since the discovery by Fujishima andHonda [58] In particular the particulate TiO

2in the form of

suspension requires appropriate substrates as support to formcompacted film [44 59] TiO

2possessing a large bandgap

energy of 32 and 302 eV for anatase and rutile respectivelyThus they only can be excited under UV light irradiationWhen TiO

2is excited by UV light irradiation electrons

from valence band will jump to the conduction band leavingbehind positive holes (h+) [34] The electrons and holesdiffuse to the particle surface of the TiO

2in which they can be

exploited for various redox processes analogous to those of anelectrochemical cell The holes react with water an electrondonor to produce the most crucial and powerful oxidizingfree radicals capable of degrading the organic dyes

However the major drawback of such nanoparticle TiO2

is the random pathway of photoinduced electrons duringphotocatalytic reactions which will eventually lead to therecombination through trappingdetrapping of electronholepairs as well as longer electron transporting time in theparticulate TiO

2 In addition to that the probability of

recombination losses of photoinduced charge carriers willbe increased significantly due to the presence of defectsor trapping sites more grain boundaries and disorderedcontact areas [50 60ndash63] Meanwhile it was found that two-dimensional TiO

2thin film is able to eliminate the reusability

of photocatalyst issues and ease the filtration procedure afterphotoreaction [47 50] In order to further maximize specificsurface area of TiO

2thin film for better photons absorption

from illumination design and development of TiO2-based

nanostructure assemblies have gained significant interest andtriggered enormous effort in physics chemistry andmaterialscience [45ndash47 50 51 60] In this paper the nanotubularstructure of TiO

2is extensively reviewed This is because

TiO2nanotubes are becoming more popular due their high

photocatalytic activity for various types of dye degradation

2 Preparation of TiO2 Nanotubes

TiO2nanotubes can be produced by several methods such as

using a template of nanoporous alumina [64] sol-gel tran-scription process using organogelator templates [65] seededgrowth mechanisms [66] and hydrothermal techniques [67ndash69] However none of these methods offer superior con-trol on the nanotube dimensions than the electrochemicalanodization technique [52 70 71] In the following sectionsseveral techniques to prepare TiO

2nanotubeswere discussed

21 Sol-Gel Transcription Synthesis Sol-gel transcriptionprocess is a wet-chemical technique used to synthesize oxidesat relatively low temperature The process consists of hydrol-ysis of precursor species and subsequently polycondensationto form solid network which is known as gel The commonprecursors employed in sol-gel processing aremetal alkoxidesand metal chloridesThis sol-gel template processing makinguse of porous alumina polymer fibers or super molecularcompound as a template and their diameters was normallylarger than 50 nm [72ndash74] The walls of the TiO

2nanotubes

prepared by deposition in porous alumina membrane con-sisted of anatase nanoparticles and contained mesoporesarising from the spaces between the anatase particles [7576] Hoyerrsquos research team firstly reported the formation ofthe pore diameter of 70ndash100 nm TiO

2nanotubes via sol-gel

transcription process in 1996Sol-gel transcription process in formation of high-

ordered TiO2nanotubes was reported by Kang et al (2009)

[77] In this report titanium isopropoxide Ti(OC3H7)4 in

ethanol was used as precursor The TiO2nanotubes were

grown on an alumina template Then the resultant samplewas dried at room temperature for 12 hours in an ambientenvironment and calcined at 500∘C to convert into crystallinephase After that the isolated TiO

2nanotubes were obtained

by removing the alumina template However the main draw-back of sol-gel method is that it is difficult to form alignedand well-ordered nanotubular structure Besides that thismethod is time consuming because it is relatively difficult toremove the solvent and organic by-products and not practicalfrom the economic point of view due to high chemical andenergy consumption Furthermore rawmaterials used in thisexperimental works are expensive and caused many researchgroups to try to find an alternative way to produce TiO

2

nanotubes

22 Hydrothermal Synthesis Hydrothermal synthesis canbe defined as a heterogeneous reaction in the presence ofaqueous solvents or mineralizers under high pressure andtemperature This is an important technique to produce bigpure and dislocation-free single crystals [78ndash80]The crystalgrowth is performed in a steel pressure vessel known asautoclave where nutrient is supplied along with water Atemperature gradient needs to be maintained in the growthchamber so that the hotter end dissolves the nutrient whereasthe cooler end causes seeds to take additional growthThe growth rate of the seeds will increase with increasing


Table 3 Comparison of the most widely used synthesis methods for preparing TiO2 nanotubes

Synthesis method Characteristics

Sol-gel transcription process(a) Ordered arrays(b) Advantages the scale of nanotubes can be controlled by the applied template(c) Disadvantages the nanotubular structure can be damaged or destroyed during the afterremoval of the templates raw materials used are expensive

Hydrothermal synthesis(a) Random alignment(b) Advantages easy route to obtain nanotubes in relatively large amount(c) Disadvantages difficulty in producing uniform size of nanotubular long reactionduration and random distribution of nanostructured tubes

Electrochemical anodization

(a) Oriented arrays with high aspect ratio(b) Advantages self-organized anodic oxides in the form of nanotubular structures withalmost perfect vertical relatively simple alignment and can be adopted for large-scaleindustrial production(c) Disadvantages fabrication apparatuses are costly massive organic waste will beproduced after the anodization process

concentration of solvent crystallization temperature andtemperature gradient [76 81 82]

In a typical hydrothermal experiment the TiO2nanopar-

ticle powder is processed in alkaline solution such as NaOHin a Teflon-lined autoclave at a temperature up to 150∘C fortime range from 15 h and above [69] TiO

2nanoparticles

will transform to nanotubes when the reactive Na+ andOHminus species react with TiO

2and forms TindashOndashNa and Tindash

OH bonds At this stage the particles will be transformedto lamellar structure and finally roll up to form tubularstructure to reduce the high energy [68 83] Figure 4 showsthe schematic model for the formation of TiO

2nanotubes

However the main drawback of hydrothermal technique isthe inability to form nanostructured tubes which are wellaligned and ordered Besides that hydrothermal treatmentrequired long reaction duration and addition of highlyconcentrated NaOH having difficulty in obtaining uniformsize of TiO

2nanotubes

23 Electrochemical Anodization Synthesis Highly orderedand vertically oriented TiO

2nanotubes can be fabricated by

potentiostatic anodization of Ti metal under suitable elec-trolyte and processing conditions TiO

2nanotubes are gained

after the process which initially involves the formation ofbarrier layer and followed by rather well-defined nanoporousstructure [51 53 84 85] The Ti metal was used as substratefor TiO

2nanotubes to growThe amorphous TiO

2nanotubes

can be obtained at the end of the anodization process Thisamorphous phase of nanotubes needs to be crystallized viahigh temperature annealing process [70 86 87]

Electrochemical anodization is an electrolytic processthat creates a protective or decorative oxide layer over ametallic surface [88] Synthesis of TiO

2nanotubes using

electrochemical anodization method is preferred due to thesimplicity in preparation and handling and being morecontrollable than the othermethods [58]Generally verticallyoriented nanotubes offer large specific surface area whichhave tube-like structures with circular nanotubular openingthat serve as a scaffold to anchor light-harvesting assemblies[50 51] The diameter of the opening ranges from 20 nm to

350 nm and the length of the tube can vary from 02 120583m to1000 120583mdepending on the processing parameterThe bottompart of the nanotubes which are in the form of domes is calledbarrier layer with typical shape of hexagonal or pentagonal[89ndash91]

In anodizing cell Ti is used as an anode and it isconnected to positive terminal of power source whereasplatinum is used as cathode being connected to negativeterminal of power source [84 92] There are few othercandidates for cathode which consist of carbon lead nickelor stainless steel The cathode has to be an inert electrodeand nonreactive in the electrolyte bath [90 93] Generally thegeometrical feature of the nanotubes is controlled by a varietyof parameters such as anodization potential electrolyte com-position and properties such as conductivity and viscosity aswell as anodization time and temperature From theoreticalperspective an anodic oxide layer growth involved field-assisted oxidation of anodic Ti film The high electric fieldacross the anodic oxide layer will induce the TindashO bondpolarization Then TindashO polarization will lead to the pitformationThese random pits will then etch into nanotubularstructure in the presence of fluoride ions via chemical disso-lution reactionThe comparison between the unique featuresof sol-gel transcription process hydrothermal synthesis andelectrochemical anodization techniques is summarized inTable 3

3 Titanium Dioxide Nanotubes forDye Degradation

In this manner one-dimensional TiO2nanotubes are con-

sidered a promising candidate because of their inner andouter wall surface area of nanotube that greatly increasesthe active sites available for photon absorption [43 48 5080 94] One-dimensional TiO

2with well-aligned nanotube

arrays provides unique electronic properties such as highelectronmobility low quantum confinement effects and highmechanical strength [95ndash97] Furthermore vertical chargetransport in well-aligned TiO

2nanotube structure greatly

contributes to better photocatalytic efficiency due to the


E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping

Figure 5 Band structure of a cationic loaded on wide band gap ofsemiconductor photocatalyst with visible light response [105]

grains of TiO2that are stretched in the tube growth direction

and low recombination losses at grain boundaries as com-pared to randomly oriented particulate TiO

2[47 50 63 97]

Quan et al (2005) [98] have reported that TiO2nanotubes

give a higher kinetics constant than TiO2nanoparticles

for photoelectrochemical degradation of pentachlorophenolIn terms of dye degradation in TiO

2nanotubes it is also

performed far much better as compared to that of TiO2

nanoparticles Xu et al (2011) [99] show that TiO2nanotubes

efficiency was two times much better than nano-TiO2in

degrading Acid Orange 7 dye

31 Cationic Doping on Titanium Dioxide Nanotubes TiO2

nanotube is considered the noble photocatalyst in organic dyedegradation because of itsmany intrinsic properties [88 100]but it has its own limitations which are critical to photocat-alytic techniques such as recombination of photogeneratedelectron-hole pairs fast backward reaction and inefficientvisible light utilization These drawbacks are very muchrelated to the wide band gap of TiO

2nanotubes In fact TiO

2

nanotube can only effectively function under the UV region(120582 lt 400 nm) and it only contains about 4-5 of UV raysfrom our solar energy Thus the efficient use of visible lightfrom our solar energy is essential for effectively degradingorganic dye These drawbacks can not be overcome by onlyoptimizing the dimensions of TiO

2nanotube itselfTherefore

considerable efforts that have been exerted to solve thesedrawbacks by modifying TiO

2nanotubes with the addition

of electron donors (hole scavengers) carbonates salts metalion doping [101] anion doping [102] and dye sensitization[62] have been investigated Among these doping of TiO

2

using metal ions (Fe Cr V Nb Si Al Li Na K Cu MnCo and Gd) and nonmetal (C N S and P) has reported toshow a promising result under visible light irradiation thoughthe mechanism under which this process occurs still remainscontroversial The following are some of the attempts madeto modify the electronic structures of TiO

2nanotubes to

improve their effectiveness under visible lightIn 2004 Komornicki et al [103] proposed that binary

oxide photocatalysts are promising candidates with enhancedproperties for photocatalytic system Such photocatalystsmay benefit from the combination of the best properties oftheir pure components and modification of the electronicstructure of the system could be expected Later in 2006

Higashimoto et al [104] suggested that binary oxide pho-tocatalyst can promote better charge carriers separation byaccumulating electrons and holes in two different semicon-ductor layers with suppression of charge carriers recombi-nation The useful features of the individual components inthe mixed oxide systems can be complementary with eachother while alleviating the disadvantages at the same timeGenerally numerous studies have proved that visible lightabsorption of TiO

2photocatalyst could be improved and

recombination of charge carriers could be suppressed bycoupling with another cationic species [43 105ndash107] Thereplacement of cationic ions within the crystal lattice maycreate impurity energy levels that facilitate better absorptionin the visible light region as depicted in Figure 5 Further-more different redox energy levels for their correspondingconduction and valence bands provide another attractiveapproach to achieve much more efficient charge carrierseparation tominimize the recombination lossesWhenTiO

2

photocatalyst is loaded with sufficient content of cationicspecies in lattice the electrons would be injected fromthe semiconductor with a more negative CB level to thepositive one while holes would be transferred from thesemiconductor with a more positive VB level to the negativeone Thus separation of charge carriers could be achievedas a consequence the lifetime of the charge carriers andthe efficiency of the interfacial charge transfer to water areincreasing significantly [49 100 108]

Based on the literature majority of the cationic species-loaded TiO

2photocatalysts studied were prepared using the

coprecipitation incipient wet impregnation method and sol-gel method Details of the cationic species-loaded TiO

2pho-

tocatalysts by several researchers and their findings are sum-marized in Table 4 Transitional metal and rare earth metalions have been tried as dopants to improve the photocatalyticefficiency of TiO

2in the visible light region However per-

formances of those binary oxide photocatalysts were stronglydependent on the content of cationic dopants The impuritylevels created by cationic dopants in the binary oxide photo-catalyst are usually discrete which would appear disadvanta-geous for the migration of the charge carriers if the contentof cationic dopants exceeds the optimum value [43 47 109]Furthermore there are a few drawbacks of cationic speciesdoping such as different morphological and crystalline prop-erties of the photocatalyst due to different methods of dopingand the shift of the absorption edge in the metal ion dopedsystem may have a complex origin due to either homoge-neous substitution of Ti4+ or segregated M

119909O119910clusters

Although cationic species doping can improve the visiblespectrum response the photogenerated electron-hole pairlifetime decreases causing the overall efficiencies to be atsimilar level [126] This reason might be attributed to thedoped sites that often behave as recombination centers forphotogenerated charged carriers due to the substitution ofTi4+ with metallic ions Also transition metal species-loadedTiO2samples often suffer from thermal instability These

cause the photocatalytic activity of cationic species-loadedTiO2to often decrease because of the thermal instability or

an increase in carrier recombination centers [127 128] Thus


Table 4 Summary of the works reported on the different cationic-loaded TiO2 photocatalyst

Cationic Author (year) Findings Reference

Cu Li et al (2008)

Cu ions on TiO2 nanotubes effectively acted as electrontrappers prohibiting the recombination of photoexcitedelectrons and holes in Cu-loaded TiO2 nanotubes henceimproving the photocatalytic degradation of Rhodamine Bunder UV light irradiation within 50min The catalytic

performance is also better than the undoped TiO2nanotubes

[110]

Pd Mohapatra et al(2008)

Self-organized TiO2 nanotubes with Pd nanoparticles werefound to be an excellent photocatalyst that can decompose

nonbiodegradable azo dyes (methyl red and methylorange) due to the fact that lifetime of the charge carriers

has been increased significantly

[111]

Ru Khan et al(2009)

Doping of Ru by the ion exchange method to thehydrothermally synthesized TiO2 nanotube was found tobe an effective photocatalyst active under visible light for

degradation of methylene blue dye

[112]

Fe Deng et al(2009)

The methyl orange was completely degraded using 05Fe-doped TiO2 nanotubes under UV light irradiation

within 3 h[113]

Nd Xu et al (2009)Nd-doped TiO2 nanotubes showed a high photocatalyticactivity by degrading 996 of methyl orange within 20

min under UV irradiation[114]

W Xiao et al(2009)

5 wt of W-loaded TiO2 nanotubes enhancedphotocatalytic activity of RhB degradation compared to

the pure TiO2 nanotubes due to the better chargeseparation efficiency and the extension of the wavelength

range of photoexcitation

[115]

Co Hsieh et al(2009)

Codoped TiO2 nanotubes showed a promising candidatein a photocatalysis degradation of basic Violet 10 Dye

under visible light irradiation[116]

W Paramasivam etal (2010)

The improvement of the photocatalytic degradation ofRhodamine B was observed for the addition of lowWO3

content (02 at W) to the Ti nanotubes[59]

W Sajjad et al(2010)

4WOx-TiO2 nanotubes composite demonstrated thebest reactivity under visible light in degradation of acid

Orange 7 Dye[117]

La Wu et al (2010) La-doped TiO2 nanotubes have been tested by usingmethyl orange dye under UV light irradiation [118]

Cu Ma et al (2010)

Cu-doped TiO2 nanotube has been employed to degrademethyl orange However only 21 of methyl orange wasremoved using Cu-doped TiO2 nanotube arrays after

240min of UV illumination

[119]

W Das et al (2011) The high efficiency of photodegradation of Rhodamine Bis observed for the 02 at W content [120]

Pt Su and Deng(2011)

The Pt-doped TiO2 nanotubes possess high photocatalyticactivity for degrading methyl orange under UV and visible

light irradiation[121]

Fe Pang andAbdullah (2012)

The Fe-doped TiO2 nanotubes were evaluated based onoxidation of Rhodamine B under ultrasonic irradiation [122]

Fe Wu et al (2012)The incorporation of Fe into TiO2 nanotubes has increasethe efficiency of methylene blue degradation under visible

light region[123]

Cu Sreekantan et al(2014)

Cu loaded on TiO2 nanotube arrays demonstrates thehighest photocatalytic activity with 84 degradation of

methyl orange under visible light[124]


Table 4 Continued


Bi Natarajan et al(2013)

Bismuth-doped TiO2 nanotubes were successfullysynthesized and capable of degrading Rhodamine B

completely under direct sunlight[125]

making intuitive prediction is impossible and conductinga focused research is challenging It is important to fine-tune the content of the cationic substitution into TiO

2

photocatalysts to develop an efficient visible light drivenphotocatalyst

32 Anionic Doping on Titanium Dioxide Nanotubes Aniondoping is another type of elemental doping to modifythe band gap of TiO

2nanotubes It has been proven that

anion-doped TiO2 such as nitrogen- carbon- phosphorus-

or sulphur-doped TiO2exhibits visible photoresponse and

reduced the band gap of native TiO2 According to Tang and

Li (2008) this is because the atomic orbitals of nonmetalelements (eg N

2119901 S3119901

andC2119901) have higher potential energy

than O2119901

[129] This will form a new VB instead of a pureO2119901atomic orbital which reduce the band gap energy without

affecting the conduction band levelThis doping required thecreation of surface oxygen vacancies where the anions willincorporate into the anatase as well as rutile by substitutionin oxygen lattice sites [130]

The nonmetal ion doped catalysts are better for extend-ing the photocatalytic activity of TiO

2into visible region

compared to metal cationic species loading methods becausetheir impurity states are near the VB edge and their rolesas recombination centers may be minimized Table 5 showsthe characteristics of anion-doped TiO

2 However the photo-

catalytic activity of conventional TiO2doped with nonmetal

elements is still limited or at early stage It is expected thata combination of doping elements could result in a markedincrease in the photocatalytic activity of TiO

2under both

UV and visible light Most of the researchers have doped theTiO2with nitrogen in order to enhance the photocatalytic

activity and expand the photoactivity response toward thevisible light region In addition codoped species may bemore appropriate for extending the photocatalytic activityof TiO

2into the visible light region For example nonmetal

codoping of TiO2such as F-B-codoping C-N-codoping and

S-N-codopingwas studied to further enhance the visible lightactivity

The purpose of the modification of TiO2is believed to

improve the performance of TiO2nanotubes by introducing

new energy level into the forbidden gap therefore causing theeffect of band gap narrowing and provide site that slow downthe recombination process This improved the performanceof the TiO

2nanotubes in photocatalytic degradation of

organic dyes [131] The following sections are some of theattempts made to modify the electronic structure of TiO

2

nanotube to improve its effectiveness under visible light Theelectronic structure of TiO

2by the substitution of metal

O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+

Metal ions implantation andmetal ions substitution

Chemically anionsdoping and oxygen

defectsTi4

Ti4+

Anions or defectsMetal ions

Figure 6 Electronic structure of TiO2by the substitution of metal

ions or anions to harvest visible light response [101]

(a) (b) (c) (d) (e)

Figure 7 Various schemes illustrating the possible band gap elec-tronic structure and excitation processes of visible-light-responsiveTiO2materials (a) pure TiO

2 (b) band gap narrowing model for

nonmetal-dopedTiO2 (c) oxygen-deficient TiO

2 (d) localizedmid-

gap level model for nonmetal-doped TiO2 (e) oxygen vacancy and

nonmetal-doped mid-gap levels are considered together [41]

ions or anions to harvest visible light response is shown inFigure 6 The various schemes illustrating the possible bandgap electronic structure and excitation processes of visible-light-responsive TiO

2materials are shown in Figure 7


Table 5 Summary of the works reported on the different anionic-doped TiO2 photocatalysts

Anionic Authors Findings Reference

N Dong et al(2009)

N-doped TiO2 nanotubes exhibit enhanced photocatalyticdegradation of methyl orange under visible lightirradiation by narrowing the band gap energy

[132]

N and C Liu et al (2009)

The CndashN doping shifts the absorption edge of TiO2nanotubes to the visible light regions and promotes thecharge carrier transfer from the TiO2 surface to the

electrolyte The CndashN doped TiO2 higher photocatalyticactivity in the degradation of methyl orange than the

undoped TiO2 nanotubes

[133]

N Lai et al (2010)The N-doped titanium dioxide nanotubes exhibited higherphotocatalytic activity in degrading methyl orange under

UV and simulated sunlight irradiation[134]

N Peng et al(2010)

N-doped titanate nanotubes completely degraded methylorange within 240 minutes under 15W fluorescent lamps [135]

N and Zr Liu et al (2010)NZr-codoped TiO2 nanotubes significantly improved thephotocatalytic efficiency of TiO2 nanotube arrays under

both UV and visible light irradiation[136]

N Z He and H YHe (2011)

The N-doped TiO2 nanotubes with 257 at nitrogencontent were capable of degrading methyl orange under

visible and sunlight irradiation[137]

N and C Wang et al(2011)

The photocatalytic activity of the codoped TiO2 nanotubeswith C and N shows a superior photocatalytic activity in

degradation of Rhodamine B under visible lightirradiation

[102]

N Wang et al(2012)

N-doped TiO2 nanotubes associated with iron oxidephotocatalyst successfully decomposed methyl orange in

air under visible light irradiation[138]

N Lv et al (2012)N-doped TiO2 nanotubes showed further enhancement inphotodegradation activity of methylene blue in the visible

region[139]

N and F Lee at al (2012)FndashN-codoped TiO2 nanotubes effectively photodegradedthe aniline blue dye within 200min under visible light

irradiation[140]

F and S Liao et al (2012)The binary F- and S-doped TiO2 nanotubes were capableof degrading methyl orange using xenon lamp to simulate

natural sunlight[141]

N Dang et al(2012)

N-doped TiO2 nanotube array films effectively degraded81 of methyl orange in 150min under visible light

irradiation[142]

N and S Wang et al(2013)

Thiourea-doped TiO2 nanotubes composed of N and Ssuccessfully decomposed 815 of methyl orange under

visible light illumination within 90 min[143]

N Yuan et al(2013)

The N-doped TiO2 exhibited higher photocatalyticdegradation of methyl orange as compared to undoped

TiO2 under UV and visible light irradiation[144]

4 Conclusion and Future Works

Nanosize TiO2especially TiO

2nanotubes seems to be the

most promising photocatalyst for organic dye degradationIt is very obvious that many researchers have been tryingto synthesize not only higher active surface area of nan-otubes but also sunlight-driven photocatalyst for effective dyedegradation The industrial application of TiO

2nanotubes

is anticipated for the future due to its high photocatalytic

activity photostability and nontoxicity Nevertheless furthermodifications are essential to improve its photocatalyticactivity under the direct sunlight irradiation Thus variouspotential cationic and anionic elements have been employedto increase TiO

2nanotubes efficiency for organic dye degra-

dation under sunlight irradiation It is believed that TiO2

nanotubes still permit extensive and in-depth study on theelectronic and lattice structure in order to truly understandthe effect of different doping method and elements on TiO

2


nanotubes This is crucial because it is obvious that sameelements with different doping methods demonstrated adifferent photocatalytic activity on dye degradation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This research is supported by High Impact Research Chan-cellory Grant UMC6251HIR228 (J55001-73873) from theUniversity of Malaya In addition authors would like tothank University of Malaya for sponsoring this work underEScience MOSTI (06-02-10 SF0130)

References

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[2] D Mohan and C U Pittman Jr ldquoArsenic removal fromwaterwastewater using adsorbentsmdasha critical reviewrdquo Journalof Hazardous Materials vol 142 no 1-2 pp 1ndash53 2007

[3] H-J Oh J-H Lee Y-J Kim S-J Suh J-H Lee and C-SChi ldquoSynthesis of effective titania nanotubes for wastewaterpurificationrdquo Applied Catalysis B vol 84 no 1-2 pp 142ndash1472008

[4] C Zaharia and D Suteu ldquoTextile organic dyesmdashcharacteristicspolluting effects and separationelimination procedures fromindustrial effluentsmdasha critical overviewrdquo in Organic PollutantsTen Years after the Stockholm ConventionmdashEnvironmental andAnalytical Update T Puzyn Ed InTech 2012

[5] B Manu and S Chaudhari ldquoAnaerobic decolorisation ofsimulated textile wastewater containing azo dyesrdquo BioresourceTechnology vol 82 no 3 pp 225ndash231 2002

[6] S H S Chan T Y Wu J C Juan and C Y Teh ldquoRecentdevelopments of metal oxide semiconductors as photocatalystsin advanced oxidation processes (AOPs) for treatment of dyewaste-waterrdquo Journal of Chemical Technology and Biotechnologyvol 86 no 9 pp 1130ndash1158 2011

[7] R Vinu and G Madras ldquoKinetics of sonophotocatalytic degra-dation of anionic dyes with nano-TiO

2rdquo Environmental Science

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from textile wastewater using whole bacterial cells a reviewrdquoDyes and Pigments vol 58 no 3 pp 179ndash196 2003

[9] G McMullan C Meehan A Conneely et al ldquoMicrobialdecolourisation and degradation of textile dyesrdquoAppliedMicro-biology and Biotechnology vol 56 no 1-2 pp 81ndash87 2001

[10] A K Sinha M Pradhan S Sarkar and T Pal ldquoLarge-scalesolid-state synthesis of SnndashSnO

2nanoparticles from layered

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[11] W J Epolito Y H Lee L A Bottomley and S G PavlostathisldquoCharacterization of the textile anthraquinone dye ReactiveBlue 4rdquo Dyes and Pigments vol 67 no 1 pp 35ndash46 2005

[12] R Koswojo R P Utomo Y-H Ju et al ldquoAcid green 25 removalfrom wastewater by organo-bentonite from Pacitanrdquo AppliedClay Science vol 48 no 1-2 pp 81ndash86 2010

[13] D Suteu C Zaharia and T Malutan ldquoRemoval of orange 16reactive dye from aqueous solutions by waste sunflower seedshellsrdquo Journal of the Serbian Chemical Society vol 76 no 4pp 607ndash624 2011

[14] C Zaharia D Suteu A Muresan R Muresan and A PopesculdquoTextile wastewater treatment by homogeneous oxidation withhydrogen peroxiderdquo Environmental Engineering and Manage-ment Journal vol 8 no 6 pp 1359ndash1369 2009

[15] S Merouani O Hamdaoui F Saoudi M Chiha and C PetrierldquoInfluence of bicarbonate and carbonate ions on sonochemicaldegradation of Rhodamine B in aqueous phaserdquo Journal ofHazardous Materials vol 175 no 1ndash3 pp 593ndash599 2010

[16] O J Hao H Kim and P-C Chiang ldquoDecolorization ofwastewaterrdquo Critical Reviews in Environmental Science andTechnology vol 30 no 4 pp 449ndash505 2000

[17] S M Ghoreishi and R Haghighi ldquoChemical catalytic reactionand biological oxidation for treatment of non-biodegradabletextile effluentrdquo Chemical Engineering Journal vol 95 no 1 pp163ndash169 2003

[18] N J Robinson S K Whitehall and J S Cavet ldquoMicrobialmetallothioneinsrdquoAdvances inMicrobial Physiology vol 44 pp183ndash213 2001

[19] D Acemoglu S Johnson and J A Robinson ldquoThe colonial ori-gins of comparative development an empirical investigationrdquoAmerican Economic Review vol 91 no 5 pp 1369ndash1401 2001

[20] The World Bank ldquoWater pollution textile industry ( oftotal BOD emissions)rdquo 2013 httpdataworldbankorgindica-torEEBODTXTLZScountries1Wdisplay=map

[21] S Vanhulle M Trovaslet E Enaud et al ldquoDecolorizationcytotoxicity and genotoxicity reduction during a combinedozonationfungal treatment of dye-contaminated wastewaterrdquoEnvironmental Science and Technology vol 42 no 2 pp 584ndash589 2008

[22] E Matyjas and E Rybicki ldquoNovel reactive red eyesrdquo AutexResearch Journal vol 3 no 2 pp 90ndash95 2003

[23] A Dbrowski ldquoAdsorptionmdashfrom theory to practicerdquo Advancesin Colloid and Interface Science vol 93 no 1ndash3 pp 135ndash2242001

[24] YM Slokar andAM leMarechal ldquoMethods of decoloration oftextile wastewatersrdquo Dyes and Pigments vol 37 no 4 pp 335ndash356 1998

[25] A Kandelbauer A Erlacher A Cavaco-Paulo and G MGuebitz ldquoLaccase-catalyzed decolorization of the synthetic azo-dye Diamond Black PV 200 and of some structurally relatedderivativesrdquo Biocatalysis and Biotransformation vol 22 no 5-6 pp 331ndash339 2004

[26] V Kumar L Wati P Nigam et al ldquoDecolorization andbiodegradation of anaerobically digested sugarcane molassesspent wash effluent from biomethanation plants by white-rotfungirdquo Process Biochemistry vol 33 no 1 pp 83ndash88 1998

[27] B H Hameed A T M Din and A L Ahmad ldquoAdsorption ofmethylene blue onto bamboo-based activated carbon kineticsand equilibrium studiesrdquo Journal of Hazardous Materials vol141 no 3 pp 819ndash825 2007

[28] K Selvi S Pattabhi and K Kadirvelu ldquoRemoval of Cr(VI)from aqueous solution by adsorption onto activated carbonrdquoBioresource Technology vol 80 no 1 pp 87ndash89 2001

[29] V Kumar N Talreja D Deva N Sankararamakrishnan ASharma and N Verma ldquoDevelopment of bi-metal dopedmicro- and nano multi-functional polymeric adsorbents forthe removal of fluoride and arsenic(V) from wastewaterrdquoDesalination vol 282 pp 27ndash38 2011


[30] A Aouni C Fersi B Cuartas-Uribe A Bes-Pia M I Alcaina-Miranda and M Dhahbi ldquoReactive dyes rejection and textileeffluent treatment study using ultrafiltration and nanofiltrationprocessesrdquo Desalination vol 297 pp 87ndash96 2012

[31] E Ellouze N Tahri and R B Amar ldquoEnhancement of textilewastewater treatment process using Nanofiltrationrdquo Desalina-tion vol 286 pp 16ndash23 2012

[32] H K Shon S Phuntsho D S Chaudhary S Vigneswaran andJ Cho ldquoNanofiltration for water and wastewater treatmentmdashamini reviewrdquoDrinkingWater Engineering and Science vol 6 pp47ndash53 2013

[33] R A Al-Juboori and T Yusaf ldquoBiofouling in RO systemmechanisms monitoring and controllingrdquo Desalination vol302 pp 1ndash23 2012

[34] A O Ibhadon and P Fitzpatrick ldquoHeterogeneous photocataly-sis recent advances and applicationsrdquo Catalysts vol 3 pp 189ndash218 2013

[35] Y Fu and T Viraraghavan ldquoFungal decolorization of dyewastewaters a reviewrdquo Bioresource Technology vol 79 no 3 pp251ndash262 2001

[36] K G Bhattacharyya and A Sarma ldquoAdsorption characteristicsof the dye Brilliant Green on Neem leaf powderrdquo Dyes andPigments vol 57 no 3 pp 211ndash222 2003

[37] T Robinson G McMullan R Marchant and P Nigam ldquoReme-diation of dyes in textile effluent a critical review on currenttreatment technologieswith a proposed alternativerdquoBioresourceTechnology vol 77 no 3 pp 247ndash255 2001

[38] MN V R Kumar T R Sridhari K D Bhavani and P K DuttaldquoTrends in color removal from textile mill effluentsrdquo Colouragevol 45 no 8 pp 25ndash34 1998

[39] W S Pereira and R S Freire ldquoAzo dye degradation byrecycled waste zero-valent iron powderrdquo Journal of the BrazilianChemical Society vol 17 no 5 pp 832ndash838 2006

[40] D A Tryk A Fujishima and K Honda ldquoRecent topics inphotoelectrochemistry achievements and future prospectsrdquoElectrochimica Acta vol 45 no 15-16 pp 2363ndash2376 2000

[41] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis

and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008

[42] A Kubacka M Fernandez-Garcıa and G Colon ldquoAdvancednanoarchitectures for solar photocatalytic applicationsrdquo Chem-ical Reviews vol 112 no 3 pp 1555ndash1614 2012

[43] Y-C Nah I Paramasivam and P Schmuki ldquoDoped TiO2and

TiO2nanotubes synthesis and applicationsrdquo ChemPhysChem

vol 11 no 13 pp 2698ndash2713 2010[44] C W Lai and S Sreekantan ldquoPreparation of hybrid WO

3ndash

TiO2nanotube photoelectrodes using anodization and wet

impregnation improved water-splitting hydrogen generationperformancerdquo International Journal of Hydrogen Energy vol 38pp 2156ndash2166 2013

[45] X Chen and S S Mao ldquoTitanium dioxide nanomaterialssynthesis properties modifications and applicationsrdquoChemicalReviews vol 107 no 7 pp 2891ndash2959 2007

[46] Z Su andW Zhou ldquoFormation morphology control and appli-cations of anodic TiO

2nanotube arraysrdquo Journal of Materials

Chemistry vol 21 no 25 pp 8955ndash8970 2011[47] A Ghicov and P Schmuki ldquoSelf-ordering electrochemistry a

review on growth and functionality of TiO2nanotubes and

other self-aligned MO119909structuresrdquo Chemical Communications

no 20 pp 2791ndash2808 2009

[48] M Kitano M Matsuoka M Ueshima and M Anpo ldquoRecentdevelopments in titanium oxide-based photocatalystsrdquo AppliedCatalysis A vol 325 no 1 pp 1ndash14 2007

[49] C W Lai and S Sreekantan ldquoOptimized sputtering powerto incorporate WO

3into C-TiO

2nanotubes for highly visible

photoresponse performancerdquo NANO vol 7 no 6 Article ID1250051 2012

[50] L Sun S Zhang X Sun and X He ldquoEffect of the geometry ofthe anodized titania nanotube array on the performance of dye-sensitized solar cellsrdquo Journal of Nanoscience and Nanotechnol-ogy vol 10 no 7 pp 4551ndash4561 2010

[51] C A Grimes ldquoSynthesis and application of highly orderedarrays of TiO

2nanotubesrdquo Journal of Materials Chemistry vol

17 no 15 pp 1451ndash1457 2007[52] S Sreekantan L C Wei and Z Lockman ldquoExtremely fast

growth rate of TiO2nanotube arrays in electrochemical bath

containing H2O2rdquo Journal of the Electrochemical Society vol

158 no 12 pp C397ndashC402 2011[53] C W Lai and S Sreekantan ldquoEffect of applied potential on

the formation of self-organized TiO2nanotube arrays and its

photoelectrochemical responserdquo Journal of Nanomaterials vol2011 Article ID 142463 7 pages 2011

[54] C W Lai and S Sreekantan ldquoPhotoelectrochemical perfor-mance of smooth TiO

2nanotube arrays effect of anodization

temperature and cleaning methodsrdquo International Journal ofPhotoenergy vol 2012 Article ID 356943 11 pages 2012

[55] Y Q Wang G Q Hu X F Duan H L Sun and Q K XueldquoMicrostructure and formation mechanism of titanium dioxidenanotubesrdquo Chemical Physics Letters vol 365 no 5-6 pp 427ndash431 2002

[56] M Latroche L Brohan R Marchand and M Tournoux ldquoNewhollandite oxides TiO

2(H) and K006TiO

2rdquo Journal of Solid

State Chemistry vol 81 no 1 pp 78ndash82 1989[57] DWMeng X LWu F Sun et al ldquoHigh-pressure polymorphic

transformation of rutile to 120572-PbO2-type TiO

2at 0 1 1R twin

boundariesrdquoMicron vol 39 no 3 pp 280ndash286 2008[58] A Fujishima and K Honda ldquoElectrochemical photolysis of

water at a semiconductor electroderdquo Nature vol 238 no 5358pp 37ndash38 1972

[59] I Paramasivam Y-C Nah C Das N K Shrestha and PSchmuki ldquoWO

3TiO2nanotubes with strongly enhanced pho-

tocatalytic activityrdquoChemistrymdashA European Journal vol 16 no30 pp 8993ndash8997 2010

[60] J Yan and F Zhou ldquoTiO2nanotubes structure optimization for

solar cellsrdquo Journal of Materials Chemistry vol 21 no 26 pp9406ndash9418 2011

[61] A Kudo and YMiseki ldquoHeterogeneous photocatalyst materialsfor water splittingrdquo Chemical Society Reviews vol 38 no 1 pp253ndash278 2009

[62] B-X Lei J-Y Liao R Zhang J Wang C-Y Su and D-BKuang ldquoOrdered crystalline TiO

2nanotube arrays on transpar-

ent FTO glass for efficient dye-sensitized solar cellsrdquo Journal ofPhysical Chemistry C vol 114 no 35 pp 15228ndash15233 2010

[63] A E R Mohamed and S Rohani ldquoModified TiO2nanotube

arrays (TNTAs) progressive strategies towards visible lightresponsive photoanode a reviewrdquo Energy and EnvironmentalScience vol 4 no 4 pp 1065ndash1086 2011

[64] A Michailowski D Almawlawi G Cheng and M MoskovitsldquoHighly regular anatase nanotubule arrays fabricated in porousanodic templatesrdquo Chemical Physics Letters vol 349 no 1-2 pp1ndash5 2001


[65] S Kobayashi N Hamasaki M Suzuki M Kimura H Shiraiand K Hanabusa ldquoPreparation of helical transition-metaloxide tubes using organogelators as structure-directing agentsrdquoJournal of the American Chemical Society vol 124 no 23 pp6550ndash6551 2002

[66] Z R Tian J A Voigt J Liu et al ldquoComplex and oriented ZnOnanostructuresrdquo Nature Materials vol 2 no 12 pp 821ndash8262003

[67] DV Bavykin VN ParmonAA Lapkin and F CWalsh ldquoTheeffect of hydrothermal conditions on the mesoporous structureof TiO

2nanotubesrdquo Journal of Materials Chemistry vol 14 no

22 pp 3370ndash3377 2004[68] H-K Seo G-S Kim S G Ansari et al ldquoA study on the struc-

turephase transformation of titanate nanotubes synthesizedat various hydrothermal temperaturesrdquo Solar Energy Materialsand Solar Cells vol 92 no 11 pp 1533ndash1539 2008

[69] S Sreekantan and L C Wei ldquoStudy on the formation andphotocatalytic activity of titanate nanotubes synthesized viahydrothermal methodrdquo Journal of Alloys and Compounds vol490 no 1-2 pp 436ndash442 2010

[70] G K Mor O K Varghese M Paulose and C A GrimesldquoTransparent highly ordered TiO

2nanotube arrays via anodiza-

tion of titanium thin filmsrdquoAdvanced Functional Materials vol15 no 8 pp 1291ndash1296 2005

[71] C W Lai S Sreekantan and Z Lockman ldquoPhotoelectrochem-ical behaviour of uniform growth TiO

2nanotubes via bubble

blowing synthesised in ethylene glycol with hydrogen peroxiderdquoJournal of Nanoscience and Nanotechnology vol 12 pp 4057ndash4066 2012

[72] J H Jung H Kobayashi K J C van Bommel S Shinkaiand T Shimizu ldquoCreation of novel helical ribbon and double-layered nanotube TiO

2structures using an organogel templaterdquo

Chemistry of Materials vol 14 no 4 pp 1445ndash1447 2002[73] J H Jung T Shimizu and S Shinkai ldquoSelf-assembling struc-

tures of steroidal derivatives in organic solvents and theirsol-gel transcription into double-walled transition-metal oxidenanotubesrdquo Journal of Materials Chemistry vol 15 no 35-36pp 3979ndash3986 2005

[74] Q Ji R Iwaura and T Shimizu ldquoRegulation of silica nan-otube diameters sol-gel transcription using solvent-sensitivemorphological change of peptidic lipid nanotubes as templatesrdquoChemistry of Materials vol 19 no 6 pp 1329ndash1334 2007

[75] H Imai Y Takei K Shimizu M Matsuda and H HirashimaldquoDirect preparation of anatase TiO

2nanotubes in porous

alumina membranesrdquo Journal of Materials Chemistry vol 9 no12 pp 2971ndash2972 1999

[76] Z-Y Yuan and B-L Su ldquoTitanium oxide nanotubes nanofibersand nanowiresrdquo Colloids and Surfaces A vol 241 no 1ndash3 pp173ndash183 2004

[77] T-S Kang A P Smith B E Taylor and M F DurstockldquoFabrication of highly-ordered TiO

2nanotube arrays and their

use in dye-sensitized solar cellsrdquo Nano Letters vol 9 no 2 pp601ndash606 2009

[78] M Yoshimura and K Byrappa ldquoHydrothermal processing ofmaterials past present and futurerdquo Journal of Materials Sciencevol 43 no 7 pp 2085ndash2103 2008

[79] M Yada Y Inoue M Uota et al ldquoFormation of sodium titanatenanotube films by hydrothermal transcriptionrdquo Chemistry ofMaterials vol 20 no 2 pp 364ndash366 2008

[80] A Nakahira T Kubo and C Numako ldquoFormation mechanismof TiO

2-derived titanate nanotubes prepared by the hydrother-

mal processrdquo Inorganic Chemistry vol 49 no 13 pp 5845ndash58522010

[81] F Huang H Zhang and J F Banfield ldquoThe role of orientedattachment crystal growth in hydrothermal coarsening ofnanocrystalline ZnSrdquo Journal of Physical Chemistry B vol 107no 38 pp 10470ndash10475 2003

[82] J-N Nian and H Teng ldquoHydrothermal synthesis of single-crystalline anatase TiO

2nanorods with nanotubes as the pre-

cursorrdquo Journal of Physical Chemistry B vol 110 no 9 pp 4193ndash4198 2006

[83] N Viriya-Empikul N Sano T Charinpanitkul T Kikuchiand W Tanthapanichakoon ldquoA step towards length control oftitanate nanotubes using hydrothermal reactionwith sonicationpretreatmentrdquoNanotechnology vol 19 no 3 Article ID 0356012008

[84] J M Macak and P Schmuki ldquoAnodic growth of self-organizedanodic TiO

2nanotubes in viscous electrolytesrdquo Electrochimica

Acta vol 52 no 3 pp 1258ndash1264 2006[85] J M Macak H Tsuchiya A Ghicov et al ldquoTiO

2nanotubes

self-organized electrochemical formation properties and appli-cationsrdquo Current Opinion in Solid State and Materials Sciencevol 11 no 1-2 pp 3ndash18 2007

[86] V K Mahajan M Misra K S Raja and S K Mohapatra ldquoSelf-organized TiO

2nanotubular arrays for photoelectrochemical

hydrogen generation effect of crystallization and defect struc-turesrdquo Journal of Physics D vol 41 no 12 Article ID 1253072008

[87] C W Lai and S Sreekantan ldquoHigher water splitting hydrogengeneration rate for single crystalline anatase phase of TiO

2nan-

otube arraysrdquo The European Physical JournalmdashApplied Physicsvol 59 no 2 Article ID 20403 2012

[88] C W Lai and S Sreekantan ldquoPhotoelectrochemical propertiesof TiO

2nanotube arrays effect of electrolyte pH and annealing

temperaturerdquo Journal of Experimental Nanoscience vol 9 no 3pp 230ndash239 2014

[89] M Paulose H E Prakasam O K Varghese et al ldquoTiO2

nanotube arrays of 1000 120583m length by anodization of titaniumfoil phenol red diffusionrdquo Journal of Physical Chemistry C vol111 no 41 pp 14992ndash14997 2007

[90] G K Mor O K Varghese M Paulose K Shankar and C AGrimes ldquoA review on highly ordered vertically oriented TiO

2

nanotube arrays fabrication material properties and solarenergy applicationsrdquo Solar EnergyMaterials and Solar Cells vol90 no 14 pp 2011ndash2075 2006

[91] S K Mohapatra M Misra V K Mahajan and K S RajaldquoDesign of a highly efficient photoelectrolytic cell for hydrogengeneration by water splitting application of TiO

2minus119909C119909nan-

otubes as a photoanode and PtTiO2nanotubes as a cathoderdquo

Journal of Physical Chemistry C vol 111 no 24 pp 8677ndash86852007

[92] J M Macak H Hildebrand U Marten-Jahns and P SchmukildquoMechanistic aspects and growth of large diameter self-organized TiO

2nanotubesrdquo Journal of Electroanalytical Chem-

istry vol 621 no 2 pp 254ndash266 2008[93] N K Allam andCAGrimes ldquoEffect of cathodematerial on the

morphology and photoelectrochemical properties of verticallyoriented TiO

2nanotube arraysrdquo Solar Energy Materials and

Solar Cells vol 92 no 11 pp 1468ndash1475 2008[94] S So K Lee and P Schmuki ldquoUltrafast growth of highly

ordered anodic TiO2nanotubes in lactic acid electrolytesrdquo


Journal of the American Chemical Society vol 134 pp 11316ndash11318 2012

[95] S K Mohapatra M Misra V K Mahajan and K S RajaldquoA novel method for the synthesis of titania nanotubes usingsonoelectrochemical method and its application for photoelec-trochemical splitting of waterrdquo Journal of Catalysis vol 246 no2 pp 362ndash369 2007

[96] D R Baker and P V Kamat ldquoDisassembly reassembly andphotoelectrochemistry of etched TiO

2nanotubesrdquo Journal of

Physical Chemistry C vol 113 no 41 pp 17967ndash17972 2009[97] P Roy S Berger and P Schmuki ldquoTiO

2nanotubes synthesis

and applicationsrdquo Angewandte ChemiemdashInternational Editionvol 50 no 13 pp 2904ndash2939 2011

[98] X Quan S Yang X Ruan and H Zhao ldquoPreparation of titaniananotubes and their environmental applications as electroderdquoEnvironmental Science and Technology vol 39 no 10 pp 3770ndash3775 2005

[99] S Xu J Ng X Zhang H Bai and D D Sun ldquoAdsorption andphotocatalytic degradation of acid orange 7 over hydrother-mally synthesized mesoporous TiO

2nanotuberdquo Colloids and

Surfaces A vol 379 no 1ndash3 pp 169ndash175 2011[100] D Y C Leung X Fu C Wang et al ldquoHydrogen production

over titania-based photocatalystsrdquo ChemSusChem vol 3 no 6pp 681ndash694 2010

[101] H Shon S Phuntsho Y Okour et al ldquoVisible light responsivetitanium dioxide (TiO

2)rdquo Journal of the Korean Industrial and

Engineering Chemistry vol 19 no 1 pp 1ndash16 2008[102] J Wang B Huang Z Wang X Qin and X Zhang ldquoSyn-

thesis and characterization of C N-codoped TiO2

nan-otubesnanorodswith visible-light activityrdquoRareMetals vol 30no 1 pp 161ndash165 2011

[103] S Komornicki M Radecka and P Sobas ldquoStructural electricaland optical properties of TiO

2-WO3polycrystalline ceramicsrdquo

Materials Research Bulletin vol 39 no 13 pp 2007ndash2017 2004[104] S Higashimoto M Sakiyama and M Azuma ldquoPhotoelectro-

chemical properties of hybrid WO3TiO2electrode Effect of

structures of WO3on charge separation behaviorrdquo Thin Solid

Films vol 503 no 1-2 pp 201ndash206 2006[105] R M Navarro Yerga M C Alvarez Galvan F del Valle J A

Villoria de la Mano and J L G Fierro ldquoWater splitting onsemiconductor catalysts under visiblelight irradiationrdquo Chem-SusChem vol 2 no 6 pp 471ndash485 2009

[106] J H Bang and P V Kamat ldquoSolar cells by design photoelec-trochemistry of TiO

2nanorod arrays decorated with CdSerdquo

Advanced Functional Materials vol 20 no 12 pp 1970ndash19762010

[107] J Zhang Y Wu M Xing S A K Leghari and S SajjadldquoDevelopment of modified N doped TiO

2photocatalyst with

metals nonmetals andmetal oxidesrdquoEnergy and EnvironmentalScience vol 3 no 6 pp 715ndash726 2010

[108] CW Lai S Sreekantan and S E Pei ldquoEffect of radio frequencysputtering power on W-TiO

2nanotubes to improve photoelec-

trochemical performancerdquo Journal of Material Research vol 27no 13 pp 1695ndash1704 2012

[109] K Maeda A Xiong T Yoshinaga et al ldquoPhotocatalytic overallwater splitting promoted by two different cocatalysts for hydro-gen and oxygen evolution under visible lightrdquo AngewandteChemiemdashInternational Edition vol 49 no 24 pp 4096ndash40992010

[110] H Li XDuanG Liu and L Li ldquoSynthesis and characterizationof copper ions surface-doped titanium dioxide nanotubesrdquoMaterials Research Bulletin vol 43 no 8-9 pp 1971ndash1981 2008

[111] S K Mohapatra N Kondamudi S Banerjee and M MisraldquoFunctionalization of self-organized TiO

2nanotubes with Pd

nanoparticles for photocatalytic decomposition of dyes undersolar light illuminationrdquo Langmuir vol 24 no 19 pp 11276ndash11281 2008

[112] M A Khan D H Han and O-B Yang ldquoEnhanced photore-sponse towards visible light in Ru doped titania nanotuberdquoApplied Surface Science vol 255 no 6 pp 3687ndash3690 2009

[113] L Deng S Wang D Liu et al ldquoSynthesis characterization ofFe-doped TiO

2nanotubes with high photocatalytic activityrdquo

Catalysis Letters vol 129 no 3-4 pp 513ndash518 2009[114] Y-H Xu C Chen X-L Yang X Li and B-F Wang

ldquoPreparation characterization and photocatalytic activity of theneodymium-doped TiO

2nanotubesrdquo Applied Surface Science

vol 255 no 20 pp 8624ndash8628 2009[115] MXiao LWang XHuang YWu andZDang ldquoSynthesis and

characterization of WO3titanate nanotubes nanocomposite

with enhanced photocatalytic propertiesrdquo Journal of Alloys andCompounds vol 470 no 1-2 pp 486ndash491 2009

[116] C-T Hsieh W-S Fan W-Y Chen and J-Y Lin ldquoAdsorptionand visible-light-derived photocatalytic kinetics of organic dyeon Co-doped titania nanotubes prepared by hydrothermalsynthesisrdquo Separation and Purification Technology vol 67 no3 pp 312ndash318 2009

[117] A K L Sajjad S Shamaila B Tian F Chen and J ZhangldquoComparative studies of operational parameters of degradationof azo dyes in visible light by highly efficient WO

119909TiO2

photocatalystrdquo Journal of Hazardous Materials vol 177 no 1ndash3 pp 781ndash791 2010

[118] H-H Wu L-X Deng S-R Wang et al ldquoThe preparation andcharacterization of la doped TiO

2nanotubes and their photo-

catalytic activityrdquo Journal of Dispersion Science and Technologyvol 31 no 10 pp 1311ndash1316 2010

[119] Q Ma S J Liu L Q Weng Y Liu and B Liu ldquoGrowthstructure and photocatalytic properties of hierarchical Cu-Ti-O nanotube arrays by anodizationrdquo Journal of Alloys andCompounds vol 501 no 2 pp 333ndash338 2010

[120] C Das I Paramasivam N Liu and P Schmuki ldquoPhotoelec-trochemical and photocatalytic activity of tungsten doped TiO

2

nanotube layers in the near visible regionrdquo Electrochimica Actavol 56 no 28 pp 10557ndash10561 2011

[121] Y Su and Y Deng ldquoEffect of structure on the photocatalyticactivity of Pt-doped TiO


vol 257 no 23 pp 9791ndash9795 2011[122] Y L Pang and A Z Abdullah ldquoEffect of low Fe3+ doping on

characteristics sonocatalytic activity and reusability of TiO2

nanotubes catalysts for removal of Rhodamine B from waterrdquoJournal of Hazardous Materials vol 235-236 pp 326ndash335 2012

[123] Q Wu J Ouyang K Xie L Sun M Wang and C LinldquoUltrasound-assisted synthesis and visible-light-driven pho-tocatalytic activity of Fe-incorporated TiO

2nanotube array

photocatalystsrdquo Journal of Hazardous Materials vol 199-200pp 410ndash417 2012

[124] S Sreekantan C W Lai and S M Zaki ldquoThe influenceof lead concentration on photocatalytic reduction of Pb(II)ions assisted by Cu-TiO

2nanotubesrdquo International Journal of

Photoenergy vol 2014 Article ID 839106 7 pages 2014[125] T S Natarajan K Natarajan H C Bajaj and R J Tayade

ldquoEnhanced photocatalytic activity of bismuth-doped TiO2nan-

otubes under direct sunlight irradiation for degradation ofRhodamine B dyerdquo Journal of Nanoparticle Research vol 15 no5 article 1669 2013


[126] K Shankar G K Mor A Fitzgerald and C A Grimes ldquoCationeffect on the electrochemical formation of very high aspect ratioTiO2nanotube arrays in formamide-water mixturesrdquo Journal of

Physical Chemistry C vol 111 no 1 pp 21ndash26 2007[127] T Hathway E M Rockafellow Y-C Oh and W S Jenks

ldquoPhotocatalytic degradation using tungsten-modified TiO2and

visible light kinetic and mechanistic effects using multiplecatalyst doping strategiesrdquo Journal of Photochemistry and Pho-tobiology A vol 207 no 2-3 pp 197ndash203 2009

[128] C W Lai and S Sreekantan ldquoSingle step formation of C-TiO2

nanotubes influence of applied voltage and their photocatalyticactivity under solar illuminationrdquo International Journal of Pho-toenergy vol 2013 Article ID 276504 8 pages 2013

[129] X Tang and D Li ldquoSulfur-doped highly ordered TiO2nan-

otubular arrays with visible light responserdquo Journal of PhysicalChemistry C vol 112 no 14 pp 5405ndash5409 2008

[130] Y Zhang W Fu H Yang et al ldquoSynthesis and characterizationof P-doped TiO

2nanotubesrdquoThin Solid Films vol 518 no 1 pp

99ndash103 2009[131] J J Fan L Zhao J Yu and G Liu ldquoThe effect of calcination

temperature on the microstructure and photocatalytic activityof TiO

2-based composite nanotubes prepared by an in situ

template dissolution methodrdquoNanoscale vol 4 pp 6597ndash66032012

[132] L Dong G X Cao Y Ma X L Jia G T Ye and S K GuanldquoEnhanced photocatalytic degradation properties of nitrogen-doped titania nanotube arraysrdquo Transactions of NonferrousMetals Society of China vol 19 no 6 pp 1583ndash1587 2009

[133] S Liu L Yang S Xu S Luo and Q Cai ldquoPhotocatalyticactivities of C-N-doped TiO

2nanotube arraycarbon nanorod

compositerdquo Electrochemistry Communications vol 11 no 9 pp1748ndash1751 2009

[134] Y-K Lai J-Y Huang H-F Zhang et al ldquoNitrogen-doped TiO2

nanotube array films with enhanced photocatalytic activityunder various light sourcesrdquo Journal of Hazardous Materialsvol 184 no 1ndash3 pp 855ndash863 2010

[135] Y-P Peng S-L Lo H-H Ou and S-W Lai ldquoMicrowave-assisted hydrothermal synthesis of N-doped titanate nanotubesfor visible-light-responsive photocatalysisrdquo Journal of Haz-ardous Materials vol 183 no 1ndash3 pp 754ndash758 2010

[136] H Liu G Liu and X Shi ldquoNZr-codoped TiO2nanotube

arrays fabrication characterization and enhanced photocat-alytic activityrdquo Colloids and Surfaces A vol 363 no 1ndash3 pp 35ndash40 2010

[137] Z He and H Y He ldquoSynthesis and photocatalytic propertyof N-doped TiO

2nanorods and nanotubes with high nitrogen

contentrdquo Applied Surface Science vol 258 no 2 pp 972ndash9762011

[138] Y Wang G Zhang F Zhang and X Fan ldquoModified Ndoped TiO

2nanotubes with magnetic 120574-Fe

2O3as visible light

photocatalystsrdquo Advanced Materials Research vol 532-533 pp20ndash24 2012

[139] X Lv H Zhang and H Chang ldquoImproved photocatalyticactivity of highly ordered TiO

2nanowire arrays for methylene

blue degradationrdquoMaterials Chemistry and Physics vol 136 pp789ndash795 2012

[140] S Lee H J Oh and C S Chi ldquoEffects of anodic voltagesof photcatalytic TiO

2and doping in H

2SO4solutions on the

photocatalytic activityrdquo Korean Journal of Materials Researchvol 22 pp 439ndash444 2012

[141] H Liao W Zhang X Sun L Shi and M Qin ldquoSynthesis andphotoelectrocatalytic property of two-nonmetal-codoped TiO

2

nanotube arrays with high aspect ratiordquo Advanced MaterialsResearch vol 412 pp 219ndash222 2012

[142] M Dang Y Zhou H Li and C Lv ldquoPreparation and photocat-alytic activity of N-doped TiO

2nanotube array filmsrdquo Journal

of Materials Science Materials in Electronics vol 23 no 1 pp320ndash324 2012

[143] J Y Wang Z H Zhao J M Fan and L X Zhu ldquoOne-pothydrothermal synthesis of N-(S F) co-doped titanium nan-otubes and its visible light responsive photocatalytic propertiesrdquoJournal of Functional Materials vol 44 pp 1502ndash1506 2013

[144] B Yuan Y Wang H Bian T Shen Y Wu and Z ChenldquoNitrogen doped TiO

2nanotube arrays with high photoelec-

trochemical activity for photocatalytic applicationsrdquo AppliedSurface Science vol 280 pp 523ndash529 2013

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


Figure 1 Water pollution caused by textile industrial from variouscountries The darker icon signifies higher level of water pollutionwhich is based on percentage of total biochemical oxygen demand(BOD) emissions [20]

0

10

20

30

40

50

60

Mas

s of d

ye w

ater

(1000

tons

)

Acid

Reac

tive

Disp

erse

Dire

ct Vat

Basic

Sulfu

r

Types of textile dyes

Figure 2 Mass loss of global dyes into the wastewater [21]

organic dyes [5 13 14] These pollutants can cause irritationof the eyes skin respiratory tract sore throat asthma andallergic contact dermatitis [15] The persistent organic dyeeffluent will remain in the environment for a long period oftime if incomplete or inadequate treatments during the watercleaning stage [5] For instance the half-life of hydrolyzedReactive Blue 19 is about 46 years at pH 7 and 25∘C [16]

Wastewater containing dyes is very difficult to treatsince the dyes are recalcitrant organic molecules resistant toaerobic digestion and are stable to light Many traditionaltextile wastewater treatment studies have been applied atmany textilemillsHowever a synthetic dye inwastewater cannot be efficiently decolorized by these traditional methodsThis is because of the high cost and disposal problems fortreating dye wastewater at large scale in the textile andpaper industries [17] As such several wastewater treatmentsfor colour removal can be divided into 3 categories whichinclude physical chemical and biological treatments [1819] Nevertheless most of the conventional dye removaltechniques will have their respective limitations in textileindustry

Table 1 General fixation rates for different dye classes [22]

Dye class Fixation rate () FibersAcid 83 to 93 Wool nylonBasic 90 to 96 AcrylicDirect 70 to 95 CelluloseDisperse 80 to 92 SyntheticPigments 95 to 98 WoolReactive 50 to 80 CelluloseSulphur 60 to 70 CelluloseVat 80 to 95 Cellulose

12 Dye Removal Techniques The most popular method forthe removal of pollutants from wastewater is using liquid-phase adsorption In fact liquid-phase adsorption providesan attractive alternative for the treatment of contaminatedwaters especially if the sorbent is cheap and does not requirean additional pretreatment step before its application Theadsorption is a well-known equilibrium separation processand an effective method for water decontamination appli-cations due to the water reuse in terms of initial costflexibility and simplicity of design ease of operation andinsensitivity to toxic pollutants [23] Adsorption and ionexchange reactions play an important role in decolonization[24 25] This decolorization process is strongly dependenton the physiochemical factors (eg interaction between dyeand sorbent active surface area of sorbent particle sizereaction time environment temperature and pH) [26] Themost important aspect of the adsorption technique is that noharmful substance will be generated and released throughoutthe reactions Besides the adsorption process using activatedcarbons has been widely used to remove persistent organicdyes from wastewaters [27 28] However commerciallyavailable activated carbon is still far from being a potentialcandidate in dyes removal applications due to the high pro-duction cost and regeneration problems [2]The regenerationactivated carbons involve restoring the adsorptive capacity ofsaturated activated carbon by desorbing adsorbed dyes on itssurface In addition the adsorption involves a phase transferof pollutants to other secondary wastes that subsequentlyrequire additional treatment of proper disposal procedure

In general several types of filtration technology (physicalmethods) have been widely used for wastewater and textileeffluents treatment such as membrane-filtration processes(microfiltration ultrafiltration nanofiltration and reverseosmosis) and adsorption techniques [23 29ndash33] Howevermajor disadvantages of the membrane-filtration processesare that they are only applied in small wastewater flow raterequire higher production cost of membrane and cost ofperiodic replacement require high working pressure to pushthe wastewater flow through membrane filtration and areunable to reduce dissolved solid content In addition thistechnique requires frequent cleaning and replacement of themodules to maintain effectiveness in removing organic dyes

Besides biodegradation of organic dyes is often themost economical alternative as compared to physical andchemical processes The possible microorganisms used for




TiO2

nanoparticles

H2O (OHminus)

eminus eminus

O2

h+ h+

+OH

(CO2 + H2O)

h (photonic energy)

Degradation product

+O2

minus








2assemblies





2 in which


2



2(h+VB) + TiO2 (e

minus

CB) (1)





2+∙OH +H+ (2)



3D2D

1D


[55]


2+O2

∙minus

ads (4)

O2


2+H2O (5)





2(II) (alpha-PbO

2) and TiO

2(H)



2in the form of

























2nanotubes










2

nanotubes












2nanoparticles




2nanotubes


2nanotubes







2nanotubes



2nanotubes using












E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping




2[47 50 63 97]












2






2


2nanotubes to






2





2pho-











Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




TiO2

nanoparticles

H2O (OHminus)

eminus eminus

O2

h+ h+

+OH

(CO2 + H2O)

h (photonic energy)

Degradation product

+O2

minus








2assemblies





2 in which


2



2(h+VB) + TiO2 (e

minus

CB) (1)





2+∙OH +H+ (2)



3D2D

1D


[55]


2+O2

∙minus

ads (4)

O2


2+H2O (5)





2(II) (alpha-PbO

2) and TiO

2(H)



2in the form of

























2nanotubes










2

nanotubes












2nanoparticles




2nanotubes


2nanotubes







2nanotubes



2nanotubes using












E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping




2[47 50 63 97]












2






2


2nanotubes to






2





2pho-











Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


3D2D

1D


[55]


2+O2

∙minus

ads (4)

O2


2+H2O (5)





2(II) (alpha-PbO

2) and TiO

2(H)



2in the form of

























2nanotubes










2

nanotubes












2nanoparticles




2nanotubes


2nanotubes







2nanotubes



2nanotubes using












E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping




2[47 50 63 97]












2






2


2nanotubes to






2





2pho-











Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of











2nanoparticles




2nanotubes


2nanotubes







2nanotubes



2nanotubes using












E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping




2[47 50 63 97]












2






2


2nanotubes to






2





2pho-











Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


E∘ (H+H2)

E∘ (O2H2O)

Vis UV

VB

CBeminus

h+

h

Cation doping




2[47 50 63 97]












2






2


2nanotubes to






2





2pho-











Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




Cu Li et al (2008)



[110]





[111]

Ru Khan et al(2009)



[112]

Fe Deng et al(2009)


within 3 h[113]



W Xiao et al(2009)




[115]









Orange 7 Dye[117]


Cu Ma et al (2010)



[119]








light region[123]





Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Table 4 Continued






2








2119901 S3119901


than O2119901









2under both











2



O2minus

O2minusO2minus

O2minus

O2minus

O2minusO2minus

O2minus

O2minus

O2minus

O2minus

O2minus O2minus

O2minus

O2minus

O2minus

O2minus

Mn+

Mn+



defectsTi4

Ti4+




(a) (b) (c) (d) (e)




2 (d) localizedmid-








N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




N Dong et al(2009)


[132]





[133]



N Peng et al(2010)










[102]

N Wang et al(2012)




region[139]


irradiation[140]



N Dang et al(2012)


irradiation[142]




N Yuan et al(2013)







2nanotubes






2





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





Acknowledgments


References




















































3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



















3ndash









no 20 pp 2791ndash2808 2009



3into C-TiO

















2(H) and K006TiO




2at 0 1 1R twin






















































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of








































2nanotubes






2nan-








2



2minus119909C119909nan-







































2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of






























2photocatalyst with








2nanotubes with Pd













119909TiO2







2








2nanotube array




































2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of






























2O3as visible light






2and doping in H




2


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Documents

Review Article An Overview: Recent Development of Titanium ...An Overview: Recent Development of Titanium Oxide Nanotubes as Photocatalyst for Dye Degradation ChinWeiLai, 1 JoonChingJuan,