A review on the formation of titania nanotube photocatalysts by hydrothermal

Review

A review on the formation of titania nanotube photocatalysts by hydrothermal treatment

Chung Leng Wong,Yong Nian Tan,Abdul Rahman Mohamed *

School of Chemical Engineering,Engineering Campus,Universiti Sains Malaysia,14300Nibong Tebal,Pulau Pinang,Malaysia

a r t i c l e i n f o

Article history:

Received 14August 2010Received in revised form 15January 2011

Accepted 6March 2011

Available online 29March 2011Keywords:

Titania nanotubes

Hydrothermal method Mechanism

Starting material

Sonication pretreatment Hydrothermal temperature

a b s t r a c t

Titania nanotubes are gaining prominence in photocatalysis,owing to their excellent physical and chemical properties such as high surface area,excellent photocatalytic activity,and widespread avail-

ability.They are easily produced by a simple and effective hydrothermal method under mild temperature and pressure conditions.This paper reviews and analyzes the mechanism of titania nanotube formation by hydrothermal treatment.It further examines the parameters that affect the formation of titania nanotubes,such as starting material,sonication pretreatment,hydrothermal temperature,washing process,and calcination process.Finally,the effects of the presence of dopants on the formation of titania nanotubes are analyzed.

ó2011Elsevier Ltd.All rights reserved.

1.Introduction

In the environmental technology sector,industrial wastewater treatment is gaining importance for the removal of organic pollut-ants (Neves et al.,2009).Large amounts of organic pollutants consumed in the industries are being released into the eco-system over the past few decades and they constitute a serious threat to the environment (Mahmoodi and Arami,2009).As chemical and agri-cultural wastes,these contaminants are frequently carcinogenic and toxic to the aquatic system because of their aromatic ring structure,optical stability and resistance to biodegradation (Mahmoodi and Arami,2009).

Catalytic technologies are gaining recognition in the ?eld of environmental protection (Yu et al.,2007b ).In past decades,the traditional physical techniques for the removal of organic pollut-ants from wastewaters have included adsorption,biological treat-ment,coagulation,ultra ?ltration and ion exchange on synthetic resins (Mahmoodi and Arami,2009and Sayilkan et al.,2006).Those methods have not always been effective and they may not actually break down the pollutants in wastewater.For example,adsorption technology does not degrade the contaminants,but essentially transfers the contaminants from one medium to another,hence,

contributing to secondary pollution (Mahmoodi and Arami,2009and Sayilkan et al.,2006).Moreover,such operations are expen-sive because the pollutants are treated before the adsorption process while the adsorbent medium has to be regenerated for re-use (Mahmoodi and Arami,2009).Traditional biological treatments are often ineffective in removing and degrading pollutants because the molecules,being mostly aromatic,are chemically and physically stable (Xu et al.,2009).Hence,biodegradation of organic pollutants is usually incomplete and selective (Xu et al.,2009).In fact,some of the degradation intermediates may be even more toxic and carci-nogenic than the original pollutants (Xu et al.,2009).Chlorination and ozonation have also been used in contaminant removal,but their operating costs are high compared with other methods (Sayilkan et al.,2006).Finally,although coagulation treatments using alums,ferric salts,or limes are inexpensive,they often pose waste disposal problems of their own (Mahmoodi and Arami,2009).

With the discovery of photocatalytic splitting on TiO 2electrodes by Fujishima and Honda in 1972,heterogeneous photocatalysis has attracted much attention as a new puri ?cation technique for air and water (Fujishima and Honda,1972;Yu et al.,2006,Yu et al.,2007a and Yu et al.,2007b ).Heterogeneous photocatalysis has been successfully used in the oxidation,decontamination or minerali-zation of organic and inorganic contaminants in wastewater without generating harmful byproducts (Thiruvenkatachari et al.,2008).This approach has attracted much attention for its ease of

*Corresponding author.Tel.:t60045996410;fax:t60045941013.E-mail address:chrahman@https://www.doczj.com/doc/b515599565.html,m.my (A.R.Mohamed).Contents lists available at ScienceDirect

Journal of Environmental Management

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Journal of Environmental Management 92(2011)1669e 1680

application in oxidizing and degrading organic pollutants at rela-tively lower costs(Sayilkan et al.,2006).

Currently,many semiconductors have been applied in hetero-geneous photocatalysis(Costa and Prado,2009).Semiconductor photocatalysts such as CdS,SnO2,WO3,TiO2,ZrTiO4,and ZnO (Nawin et al.,2008;Seo et al.,2001and Seo et al.,2009),titania (TiO2)semiconductor photocatalysts have demonstrated advan-tages that include transparency(Nawin et al.,2008),wide band gap (Wang et al.,2008),biological and chemical inertness(Lee et al., 2007;Mahmoodi and Arami,2009;Yu et al.,2006,Yu et al., 2007a,Yu et al.,2007b and Zhang et al.,2010),strong oxidizing power(Lee et al.,2007;Yu et al.,2006and Yu et al.,2007a)and non-toxicity(Lee et al.,2007;Mahmoodi and Arami,2009;Neves et al., 2009;Yu et al.,2006,2007a and Zhang et al.,2010).

Considerable effort has been made to develop TiO2semi-conductor photocatalysts for environmental protection procedures such as air and water puri?cation(Lee et al.,2007;Nawin et al., 2008;Yu et al.,2006and Yu et al.,2007b),antibacterial protec-tion(Vuong et al.,2009),water disinfection(Yu et al.,2006and Yu et al.,2007b),treatment of harmful gas emission(Nawin et al., 2008)and hazardous water remediation(Yu et al.,2006and Yu et al.,2007b).TiO2has been used in many areas such as in photo-catalysis,in the generation of hydrogen from water(photocatalytic water splitting),in photocatalytic oxidation of organic or inorganic compounds,and in solar cells.Despite the enormous potential of TiO2semiconductor photocatalysts,its low ef?ciency limits its role in present day photooxidation technology(Lee et al.,2007;Yu et al., 2006and Yu et al.,2007a).Thus,signi?cant improvements and optimizations to TiO2semiconductor photocatalysts are needed before their many promising applications can be realized(Yu et al., 2006and Yu et al.,2007a).With appropriate modi?cation and optimization,TiO2semiconductor photocatalysts can be develop to generate active charge carriers to degrade organic pollutants into the harmless products(Yu et al.,2006).

The performance of TiO2semiconductor photocatalysts is strongly in?uenced by the physical and chemical properties that determine its morphology,dimension and crystallite phase(Vuong et al.,2009).From research carried with TiO2nanocrystals,it has been shown that the smaller particle size(giving rise to larger surface area-to-volume ratio)of TiO2nanocrystals increases pho-tocatalytic ef?ciency(Wang et al.,2008and Yu et al.,2007a).The disadvantages of TiO2semiconductor photocatalysts include the requirement of large amounts of TiO2semiconductor photo-catalysts,dif?culty in re-cycling TiO2semiconductor photocatalysts, problems encountered in its recovery by?ltration or centrifugation, and the problematic agglomeration of TiO2nanocrystals into large particles(Costa and Prado,2009;Gupta et al.,2006and Ribbens et al.,2008).The separation processes required to recover the TiO2 semiconductor photocatalysts at the end of the photocatalytic treatment are dif?cult to perform because of the small size of the TiO2semiconductor photocatalysts and the high stability of the TiO2 semiconductor photocatalyst hydrocolloid(Costa and Prado,2009). TiO2semiconductor photocatalysts also have the tendency to lose ef?ciency when they agglomerate into larger particles(Ribbens et al.,2008).This also adds to the complications in their disposal. While immobilized TiO2nanocrystals are an option for large scale application,the overall photocatalytic ef?ciency is compromised due to the reduction in surface area and the limitation in mass transfer(Yu et al.,2007a).To overcome these dif?culties,different titanate nanostructure preparations are being investigated with the aim of increasing photocatalytic ef?ciency(Costa and Prado,2009 and Okour et al.,2009).

Nanostructured materials such as nano?bers,nanoparticles, nanorods,nanospheres,nanotubes and nanowires present new features and opportunities for enhanced performance in many promising applications(Costa and Prado,2009;Seo et al.,2009; Wang et al.,2007and Wang et al.,2008).Nanostructured mate-rials are of special signi?cance owing to their excellent physi-cochemical properties that are catalytic,electronic,magnetic, mechanical and optical in nature(Poudel et al.,2005).They are widely applied in air and water puri?cation technologies,photo-catalysis,gas sensors,high effect solar cells and microelectronic devices(Nakahira et al.,2004and Yu and Yu,2006).For example, O’Regan et al.stated that titania nanotubes have been used in high quality,ef?cient solar cells(Nakahira et al.,2004).Nanostructured materials with different morphologies have varying speci?c prop-erties and,hence,the new applications of such materials are related to the shape and size of the nanostructured materials(Wang et al., 2007).The synthesis of nanostructured materials with speci?c shape and size,as well as the understanding of their formation mechanism are two important research aspects in material science and technology(Wang et al.,2007).Nanowires,nanotubes and nano?bres of TiO2have been successfully prepared by electro-chemical synthesis,template based synthesis and a chemical based route although the effectiveness and practicality of some of these materials as photocatalysts are still being evaluated(Okour et al., 2009and Yu and Yu,2006).

The discovery of the carbon nanotubes in the1990s by Iijima opened new?elds in the material science sector(Costa and Prado, 2009).Nanotubular materials are considered important in photo-catalysis owing to their special electronic and mechanical properties, high photocatalytic activity,large speci?c surface area and high pore volume(Idakiev et al.,2005and Yu and Yu,2006).Several studies have shown that titania nanotubes have better physical and chemical properties in photocatalysis compared with other forms of titanium dioxide.For example,titania nanotubes have a relatively higher interfacial charge transfer rate and surface area compared with the spherical TiO2particles(Colmenares et al.,2009).The transfer of the charge carriers along the length of titania nanotubes can reduce the recombination of positive hole and electron(Colmenares et al., 2009).Li et al.(2009)found that hollow titania nanotubes were highly ef?cient in the photocatalytic decomposition of methyl orange compared with rutile phase TiO2nanopowders.Xu et al.(2006)stated that titania nanotubes were excellent photocatalysts,which were more reactive than TiO2nanopowder(anatase P25)in long cycles. Thus,titania nanotubes have raised expectations in what nanotech-nology can achieve because of their interesting microstructure and potential photoelectrochemical applications in dye-sensitized solar cells,gas sensors,organic light-emitting diodes and photocatalysts (Yu et al.,2007a).Considerable effort is now being devoted to the production of well-structured TiO2nanotubes with novel properties such as high surface area and pore volume(Idakiev et al.,2005).

The approaches in developing TiO2nanotubes include chemical vapor deposition(CVD),anodic oxidation,seeded growth,the wet chemical(hydrothermal method and the sol gel method)(Guo et al., 2008;Morgan et al.,2008and Wang et al.,2007).Among these, hydrothermal method is often the method of choice because of its many advantages like cost-effectiveness,low energy consumption, mild reaction condition and simple equipment requirement(Guo et al.,2008).This method also allows for the manipulation of a large number of variable factors by which the morphology of TiO2 nanotubes that are produced is controlled.

Photocatalysis is a“green”technology with promising applica-tions in a wide assortment of chemical and environmental tech-nologies(Colmenares et al.,2009).It has been singled out as a particularly attractive means by which to oxidize and remove toxic compounds,including carcinogenic chemicals,from industrial ef?uent.The pollutants are chemically transformed and completely mineralized to harmless compounds such as carbon dioxide,water and salts(Gupta et al.,2006).

C.L.Wong et al./Journal of Environmental Management92(2011)1669e1680 1670

This process is an example of an advanced oxidation process (AOP),which is de?ned as the chemical treatment process designed to produce strong hydroxyl radicals to oxidize and remove the organic and inorganic materials in the wastewater(Thiruvenkatachari et al., 2008).Photocatalysis is a chemical process that uses light to acti-vate a catalyst that alters the reaction rate without being involved itself.In heterogeneous photocatalysis,three main components are essential for photocatalytic reaction to take place:catalyst,light source and reactant(Thiruvenkatachari et al.,2008).

Heterogeneous photocatalysis using titania nanotubes as pho-tocatalysts has recently gained importance in wastewater treat-ment.It has several advantages compared with other processes:(a) no mass transfer limitations,(b)complete mineralization of organic compounds to carbon dioxide,salts and water,(c)no addition of chemicals(d)no waste-solids disposal problems,(e)utilization of sunlight or near-UV light for irradiation and(f)only mild temper-ature and pressure conditions(atmospheric oxygen is used as oxidant)are necessary(Gogate and Pandit,2004;Konstantinou and Albanis,2004and Mahmoodi and Arami,2009).

TiO2nanotubes have high cation-exchange ability that allows for large active catalyst loadings with high and uniform spreading. In the photocatalytic reaction,access to active sites is optimized by three main characteristics:high speci?c surface area,unavailability of micropores of reactants and open mesoporous morphology of TiO2nanotubes.The high performance of TiO2nanotubes in pho-tocatalysis is due to the semiconducting behavior of titania that produces a powerful electronic interaction between titania nano-tubes and their supports to increase the catalytic activity.As titania nanotubes are very resilient during heat treatment,their applica-tion as catalysts in photocatalysis is attractive.

2.Fabrication of titania nanotubes

The fabrication of titania nanotubes is achieved by one of several methods:the surfactant-directed method,alumina templating synthesis,microwave irradiation,electrochemical synthesis and the hydrothermal method.

Alumina templating synthesis has been a popular method to produce TiO2nanotubes over the last decades(Bavykin et al.,2006). TiO2nanotubes produced by template replication are uniform and well-aligned(Bavykin et al.,2006).However,this method is not suitable for the preparation of smaller nanotubes because of the limitation of the pore size of the mold prepared from porous alumina(Ma et al.,2006).TiO2nanotubes produced by the alumina templating method normally have large diameters(greater than 50nm)and the walls of TiO2nanotubes are composed of nano-particles(Costa and Prado,2009and Nawin et al.,2008).They are dif?cult to split from their template components that are then destroyed and discarded,thus adding to operating cost(Bavykin et al.,2006and Nawin et al.,2008).

TiO2nanotubes produced by the surfactant-directed method have smaller diameters and thinner walls as compared with products fabricated using the other methods(Ma et al.,2006).In addition, there are two main disadvantages of the surfactant-directed method: it is elaborate and time consuming to undertake(Ma et al.,2006).

With the direct anodizaiton method,titania nanotubes are not split in an organized manner and the tubes do not have well-developed gaps in between(Bavykin et al.,2006).Besides,during the preparation of titania nanotubes,they have been immobilized on the surface of titanium effectively(Bavykin et al.,2006).Thus, they can be applied in different applications such as photocatalysis, hydrogen sensors,photoanodes for water splitting,and so on (Bavykin et al.,2006).

The microwave irradiation method is another means by which TiO2nanotubes are synthesized.Wu et al.(Zhao et al.,2009)synthesized TiO2nanotubes by treating TiO2(anatase or rutile) with8e10M sodium hydroxide(NaOH)aqueous solution.On applying195W microwave power,TiO2nanotubes(8e12nm in diameter and100e1000nm in length,with multi-wall and open-ended structure)were produced.

Notwithstanding the methods mentioned above,high quality of TiO2nanotubes with small diameters of about10nm are normally produced via a simple hydrothermal treatment of crystalline tita-nium dioxide nanoparticles with highly concentrated sodium hydroxide(Bavykin et al.,2006;Costa and Prado,2009and Nawin et al.,2008).Alkali titanate nanotubes are generated in hydro-thermal treatment where alkali ions are exchanged with photons to form the H-titanates.In order to produce TiO2nanotubes with different crystallographic phases such as anatase,rutile and broo-kite,thermal dehydration reactions in air are carried out at high temperatures.

Kasuga et al.reported the?rst evidence for the production of small sized titania nanotubes through the hydrothermal process in the absence of molds for template and replication(Idakiev et al., 2005and Kasuga et al.,1998).The hydrothermal synthesis method is widely regarded as a convenient and inexpensive method to produce high quality TiO2nanotubes(Yu et al.,2006).In general,TiO2nanotubes show vast pore structure and high aspect ratio owing to their unique nanotubular structure(Yu et al.,2006). This renders the nanotubes attractive candidates for photocatalytic and photoelectrochemical systems.

3.Hydrothermal method:formation mechanism of titania nanotubes

The fabrication of titania nanotubes by hydrothermal synthesis is performed by reacting titania nanopowders with an alkaline aqueous solution.While the hydrothermal method of titania nan-otube production has been comprehensively investigated in the past decade,the formation mechanisms,compositions,crystalline structures,thermal stabilities and post-treatment functions still remain areas of debate(Guo et al.,2007and Qamar et al.,2008).

As a low temperature technology,hydrothermal synthesis is environmentally friendly in that the reaction takes place in aqueous solutions within a closed system,using water as the reaction medium(Sayilkan et al.,2006;Wang et al.,2008and Yu et al., 2007b).This technique is usually carried out in an autoclave (a steel pressure vessel)under controlled temperature and/or pressure.The operating temperature is held above the water boiling point to self-generate saturated vapor pressure(Chen and Mao,2007and Wang et al.,2008).The internal pressure gener-ated in the autoclave is governed by the operating temperature and the presence of aqueous solutions in the autoclave(Chen and Mao, 2007).TiO2nanotubes are obtained when TiO2powders are mixed with2.5e20M sodium hydroxide aqueous solution maintained at 20e110 C for20h in the autoclave(Chen and Mao,2007).

The hydrothermal method is widely applied in titania nanotubes production because of its many advantages,such as high reactivity, low energy requirement,relatively non-polluting set-up and simple control of the aqueous solution(Lee et al.,2007).The reaction pathway is very sensitive to the experimental conditions,such as pH, temperature and hydrothermal treatment time,but the technique achieves a high yield of tinania nanotubes cheaply and in a relatively simpler manner under optimized conditions.There are three main reaction steps in hydrothermal method:(a)generation of the alka-line titanate nanotubes;(b)substitution of alkali ions with protons; and(c)heat dehydration reactions in air(Hafez,2009and Wang et al.,2008).The hydrothermal method is amenable to the prepa-ration of TiO2nanotubes with different crystallite phases such as the anatase,brookite,monoclinic and rutile phases(Wang et al.,2008).

C.L.Wong et al./Journal of Environmental Management92(2011)1669e16801671

Kasuga et al.(1998)carried out preliminary studies on titania nanotube formation using crystalline TiO 2nanoparticles as pre-cursors.The crystalline TiO 2nanoparticles were reacted with highly concentrated NaOH solution to form titania nanotubes by the hydrothermal https://www.doczj.com/doc/b515599565.html,ing this simple technique,titania nano-tubes were produced with uniform diameter (8e 10nm),speci ?c surface area (380e 400m 2/g)and length (50e 200nm)(Okour et al.,2009;Yu et al.,2007a ).Titania nanotubes produced in this manner were initially considered anatase phase products.When some of the Ti e O e Ti bonds were interrupted by the addition of NaOH solutions,some Ti tions were exchanged with Na tions to form Ti e O e Na bonds (Chen and Mao,2007).In this situation,the anatase phase existed in a metastable condition that had resulted from the “soft-chemical reaction ”at low temperature.The presence of Na tions in ?uenced the subsequent photocatalytic activity of the titania nanotubes (Sreekantan and Lai,2010).

Kasuga and his workers subsequently introduced an acid washing treatment step following the hydrothermal process to form tri-titanate nanotubes (Kasuga et al.,1999).The purpose of the acid treatment was to

remove the Na tions from the samples and to form new Ti e O e Ti bonds that would improve photocatalytic activity of the titania nanotubes (Kasuga et al.,1999).When the samples were treated with hydrochloric acid,the electrostatic repulsions disappeared immediately (Kasuga et al.,1999).The charged components were only gradually removed upon further washing with deionized water (Kasuga et al.,1999).The Na tions were displaced by H tions to form Ti e OH bonds in the washing process (Chen and Mao,2007).Next,the dehydration of Ti e OH bonds produced Ti e O e Ti bonds or Ti e O .H e O e Ti hydrogen bonds (Chen and Mao,2007and Kasuga et al.,1999).The bond distance between one Ti and another on the photocatalyst surface conse-quently decreased,facilitating the sheet folding process (Chen and Mao,2007and Kasuga et al.,1999).The electrostatic repulsion from Ti e O e Na bonds enabled a joint at the ends of the sheets to form the tube structure (Chen and Mao,2007and Kasuga et al.,1999).Kasuga et al.concluded that washing with acid and with deionized water were two principal crucial steps to produce high activity of titania nanotubes (Chen et al.,2002).The simple formation mechanism for titania nanotubes is shown in Fig.1.

Yuan and Su (2004)proposed a mechanism for the fabrication of titanate nanotubes that was roughly similar to Kasuga ’s.They postulated that the crystalline structure of TiO 2was represented as TiO 6octahedral and that the crystalline structure of TiO 2shared vertices and edges to form in the three-dimensional structure.Ti e O e Ti bonds were broken by a cauterization process to produce the layered titanates.The titanate sheets were then peeled off into

nanosheets and subsequently folded into nanotubes.The Na tions were exchanged and eliminated after washing with acid and then with deionized water.The major difference between Yuan and Su ’s titanate nanotubes and Kasuga ’s nanotubes was the dif ?culty in rolling up the former product completely.There were several tri-titanate layers produced simultaneously in the hydrothermal process to form three-dimensional nanosheets,resulting in the nanosheet edges being bent at the end of the hydrothermal process.Some researchers contend that the hydrothermal treatment is signi ?cantly the more important step as compared with the washing process in the mechanism of nanotubes formation.For eters and lengths of a few hundred nanometers by the hydro-thermal process using NaOH (10M)at 130 C,but without HCl washings.Wang et al.(Poudel et al.,2005)reported that the acid washing procedure was not necessary for the formation of titania nanotubes,essentially contradicting the assumptions of Kasuga et al.

The foregoing notwithstanding,there are other researchers who are of the view that the acid washing practice is requisite for the titanate precursor sheets rolling into nanotubes (Liu et al.,2009).For instance,in 2007,Li and his workers (Liu et al.,2009)found that some partially curled nanofoils were formed after brie ?y washing with nitric acid and water.The nanofoils were transformed to nanotubes only after a more thorough washing with a large quantity of nitric acid and water (Liu et al.,2009).Yao et al.(Poudel et al.,2005)observed the formation of titania nanotubes after acid washing.Sun and Li (Poudel et al.,2005)observed that the washing step was the main step to the formation of both titania and sodium titanate nanotubes.

There are also researchers who think that regardless of whether the titanate nanotubes are washed or unwashed,they are capable of removing organic or inorganic pollutants ef ?ciently.Thus,Nawin et al.(2009)stated that the presence of Na had little bearing on the ef ?ciency of the rate at which the pollutants decomposed,as this was in ?uenced more by the rate of nanotube sedimentation,available surface area and tubular structure.

Wang et al.(2002)carried out an in-depth probe of the titania nanotube formation mechanism.They observed that the three-dimensional titanium dioxide structures (anatase phase),which

Fig.1.Formation mechanism of TiO 2nanotubes using hydrothermal method (Chen and Mao,2007and Kasuga et al.,1999).

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were?rst reacted with sodium hydroxide aqueous solution,were transformed into2-dimensional layered structures.These lamellar structures then scrolled or wrapped to form titania nanotubes.In their opinion,the two-dimensional lamellar structure was essential in the formation of titania nanotubes.

A formation of titania nanotubes was proposed by Wang et al. (Chen and Mao,2007).During the hydrothermal reaction with NaOH,the Ti e O e Ti bonds were broken.The free octahedral shapes shared edges between the Ti ions to form hydroxyl bridges,then, a zigzag structure was formed.Thus,the crystalline sheets rolled up in order to saturate these dangling bonds from the surface.This lowered the total energy and hence TiO2nanotubes were formed.

There have been recent debates over the crystal structures of TiO2e based nanotubes,possible forms of which can be summa-rized as follows:(a)anatase/rutile/brookite TiO2,(b)lepidocro-cite H x Ti2àx/4[]x/4O4(x w0.7,[]:vacancy),(c)H2Ti3O7/Na2Ti3O7/ Na x H2àx Ti3O7and(d)H2Ti4O9(Guo et al.,2008;Ou and Lo,2007 and Qamar et al.,2008).Some researchers were of the opinion that anatase TiO2powder formed nanotubes more readily as compared with the rutile TiO2powder due to better surface energy of the former.In this connection,Wang et al.(Poudel et al.,2005) reported that the crystallinity of nanotubes was slightly better using the anatase TiO2phase as precursor.On the other hand,Chen et al.showed that nanotubes were obtained regardless of the size and the structure of the precursor or the starting materials(Papa et al.,2009).

Ma et al.(Idakiev et al.,2005)contended that the nanotubes were formed from lepidocrocite H x Ti2àx/4[]x/4O4(x w0.7,[]:vacancy) sheets rather than the other structures.When the duration of sonication was increased,the reactants formed rolled structures that transformed to rod-like structures.The growth of the length of rod-like structures was increased during the hydrothermal treatment. The sodium ions could then be washed off by the hydrochloric acid solution.

Yoshikazu et al.(Poudel et al.,2005)reported that the nanotubes were hydrated hydrogen titanate(H2Ti3O7?n H2O(n<3)).Du et al. (Poudel et al.,2005)found that the nanotube crystalline phase was not TiO2anatase but H2Ti3O7(a monoclinic system)and that the tubes had multi-wall morphology with interlayer spacing of 0.75e0.78nm.Kukovecz et al.(2005)tried to roll the tri-titanate nanosheets directly from the assumed Na2Ti3O7intermediate in their experiment.However,since sodium tri-titanate was quite stable under the reaction conditions(10M NaOH,130 C),it was not destroyed by NaOH,and nanotube formation did not occur.With the same alkaline hydrothermal treatment,the researchers then used the assumed Na2Ti3O7intermediates as seeding materials to convert anatase TiO2into titania nanotubes.In this instance,the yield of titania nanotubes was almost100%.Fluctuation in the local concentrations of the intermediates might have initiated the formation of nanoloops from the anatase starting materials,and these served as the seeds for titania nanotube formation.

Peng et al.(Idakiev et al.,2005and Ou and Lo,2007)were of the view that H2Ti3O7was produced in the hydrothermal treatment. They proposed the two most likely mechanisms in tri-titanate nanotubes formation.In the initial stage of the hydrothermal process,the reaction between titanium dioxide and concentrated sodium hydroxide solution produced a highly disordered inter-mediate,which contained Ti,O and Na.In the?rst proposed mechanism,single sheets of the tri-titanate(Ti3O7)2àstarted to grow at a slow rate within the disordered intermediate phase.The slow growth was mainly due to the high concentration of NaOH that was present.As the tri-titanite sheets grew two-dimensionally, they rolled up into nanotubes.In the process,however,some H2Ti3O7plates failed to wrap up successfully,resulting in the edges of H2Ti3O7plates having a tendency to twist.In the second proposed mechanism,the lepidocrocite Na2Ti3O7was postulated to assume the form of disorder-phase nanocrystals that was not stable in the boiling water.With the excessive of Natcations intercalating in the interlayer spaces,single layers of tri-titanate were either ?aked off to form nanocrystals or they were curled like wood shavings into nanotubes.

Yang et al.(Idakiev et al.,2005and Ou and Lo,2007)stated that titanium dioxide particle swelling was indicative of Na2Ti2O4(OH)2 formation.After the addition of concentrated NaOH solution,the shorter Ti e O bonds split and swelled.The linear portions(one-dimensional)connected to each other in the presence of Oàe Nate Oàbonds to produce planar fragments(two-dimensional).Titania nanotubes would then be formed by the formation of covalent bonds within the end groups.

Seo et al.(2009)studied the formation mechanism of titania nanotubes by the hydrothermal process at a higher temperature of 230 C TiO2was?rstly reacted with NaOH solution to form Na2Ti2O5$H2O.Next,the hydrochloric acid washing step was carried out to remove Nations and to form the exfoliated sheets that then curled up to produce more uniform nanotube structures.

At the present time,the formation mechanism is still ambiguous despite efforts that have been made to come up with an acceptable and veri?able explanation(Bavykin et al.,2006and Ribbens et al., 2008).Even though a complete understanding has yet to be ach-ieved,it is generally accepted that titanium dioxide(amorphous, anatase,brookite and rutile)under alkaline circumstances are converted to intermediates(single layer and multi-layered titanate nanosheets)that roll or curl into nanotubular structure(Bavykin et al.,2006).The putative driving force that is responsible for the rolling or curving nanotubular structure has been suggested by some groups(Bavykin et al.,2006).

Table1shows the major steps of titania nanotube formation proposed by various researchers.

4.Factors in?uencing the formation of titania nanotubes

There are several considerations affecting the formation of titania nanotubes(Fig.2).These include(a)the starting materials (commercial,self-prepared,amorphous,crystalline,anatase,rutile and brookite),(b)sonication pretreatment,(c)hydrothermal temperature,(d)treatment time and(e)post-treatments(washing, calcinations)(Morgan et al.,2008and Wang et al.,2008).The characteristics and morphology of titania nanotubes such as the speci?c surface area,crystal structure and others are dependent on the hydrothermal conditions selected.

4.1.Effect of the starting materials

The hydrothermal synthesis of titania nanotubes can start with different titania powders such as rutile or anatase TiO2,Degussa TiO2(P25)nanoparticles,layered titanate Na2Ti3O7,Ti metal, TiOSO4,molecular Ti IV alkoxide,doped anatase TiO2,or SiO2e TiO2 mixture(Wang et al.,2008).Alternatively,titania sol can also be utilized as the starting reactant in the hydrothermal process.It has been observed that the structural properties of the nanostructured TiO2products are markedly dependent on different starting TiO2 materials.Basically,nanotubes with outer diameters between10 and20nm can be obtained by the hydrothermal method when starting with titania powder of relative large particle size,such as rutile TiO2,Degussa TiO2(P25),and SiO2e TiO2mixture(Wang et al., 2008).

Saponjic et al.(2005)used different starting materials to produce titania nanotubes using hydrothermal method,which are Degussa TiO2P25nanoparticles,TiO2colloids,and molecular Ti IV alkoxide.Titania nanotubes obtained from those starting materials

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were 12nm in diameter.The products were typically similar in their length.The lengths of the scrolled titania nanotubes changed consistently while the diameters remained the same as the starting material concentration increased from 0.1to 0.5M.

Lan et al.(2005)used

rutile particles (particle size z 120e 280nm)as starting materials to react with 10M concentrated sodium hydroxide solution at 150 C for 48h.The inner diameter and length of the titania nanotubes produced were 2e 3nm and 50e 200nm respectively.The outer diameter was about 10nm.The open-ended nanotubes exhibited uniform inner and outer diame-ters and unbent edges.

Yu and his colleagues (Yu et al.,2006)prepared titania nano-tubes using a hydrothermal treatment similar to that adopted by Kasuga and his https://www.doczj.com/doc/b515599565.html,mercial-grade TiO 2powder (P25,Degussa AG,Germany)was used as starting material to produce titania nanotubes several hundred nanometers in length and 7e 15nm in diameter.The prepared nanotubes possessed uniform inner and outer diameters and they were multi-layered and open-ended.After hydrothermal treatment in concentrated alkaline solution for two days,titania nanotubes no longer conformed to the brookite,anatase or rutile states.As they have been derived from the same layered titanate group,the probable crystallite structures of the titania nanotubes were H 2Ti 3O 7(Na 2Ti 3O 7)and Na x H 2àx Ti 3O 7.As the nanotubes did not contain sodium ions,they were essen-tially composed of hydrogen titanate.

Ma et al.(2006)examined another two different types of the starting materials in the formation of titania nanotubes:TiO 2Hombikat UV100and TiO 2BCC100.The inner and outer diameter of the titania nanotubes prepared from TiO 2Hombikat UV100were 3e 6nm and 7e 10nm respectively.The length of the nanotubes was up to 400nm.With TiO 2BCC100as the starting materials,a large amount of nanosheets with scrolled edges were obtained.

Fig.2.Parameters affecting the formation of TiO 2nanotubes.

Table 1

Summary of the formation of titania nanotubes.Author(s)Hydrothermal step Acid washing step Comment(s)

Reference(s)Kasuga et al.

Yes

No

TiO 2nanotubes with the length of about 100nm and the diameter of around 8nm were formed via hydrothermal treatment with 10M NaOH aqueous solution at 110 C for 20h.

Kasuga et al.(1998)

Kasuga et al.Yes Yes The speci ?c surface area of TiO 2nanotubes decreased after hydrothermal treatment with NaOH aqueous solution,but it increased steadily after acid washing step.

Kasuga et al.(1999)

Yuan and Su Yes Yes The nanotubes were dif ?cult to roll up completely.The edges of nanosheets were being bent after hydrothermal process.

Yuan and Su (2004)Ma et al.Yes Yes TiO 2nanotubes were formed from lepidocrocite sheets.

Idakiev et al.(2005)Li et al.Yes Yes TiO 2nanotubes were successfully synthesized although nitric acid was used in acid washing step.

Papa et al.(2009)Seo et al.

Yes

Yes

Uniform TiO 2nanotubes were produced by the hydrothermal process at a higher temperature of 230 C.

Seo et al.(2009)

Na 2Ti 2O 5?H 2O was formed when TiO 2was reacted with NaOH solution.

Acid washing step was carried out to remove Na tions and to form the exfoliated sheets that then curled up to produce tube-like structures.

Wang et al.Yes Yes Based on XRD observations,TiO 2nanotubes were produced with slightly better crystallinity in the anatase phase.

Poudel et al.(2005)Yao et al.Yes Yes Formation of titania nanotubes was success in the presence of acid washing step.Poudel et al.(2005)Sun and Li Yes Yes Acid washing step was the main factor to form both titania and sodium titanate nanotubes.

Poudel et al.(2005)Weng et al.

Yes

No

TiO 2nanotubes with a wall thickness of ca.1nm and an external diameter of ca.8nm were synthesized

by hydrothermal treatment.

Zhao et al.(2009)

Bavyki et al.Yes No Hydrothermal treatment is signi ?cantly the more important step as compared with the washing process.

Bavykin et al.(2004)Du et al.

Yes

No

Titania nanotubes of 8e 10nm diameters and lengths of a few hundred nanometers by the hydrothermal process.

Chen and Mao (2007)

C.L.Wong et al./Journal of Environmental Management 92(2011)1669e 1680

1674

Owing to the steric hindrance effect of the larger particles,the nanosheets structure would not transform into nanotubular structure upon addition of the HCl aqueous solution.At the end of hydrothermal process,spherical nanoparticles remained in the ?nal sample.

Ribbens et al.(2008)studied the formation of titanate nano-tubes using hydrothermal technique of commercial ultra?ne tita-nium dioxide nanopowders(Riedel De Haen,Honeywell).At the end of the hydrothermal process,a uniform nanotubular structure was formed that was multi-layered and possessed different numbers of shells.The interlayer distance was0.74nm and the inner pore diameter of the nanotubes was4e4.2nm.The nano-tubular structure was not uniform or symmetrical due to the nanosheets scrolling up during the hydrothermal process.The inner pore of titanate nanotubes was accessible because of the open-ended behavior of titanate nanotubes.

Lee et al.(2008a)used commercial-grade TiO2powder (Sigma e Aldrich,99.999%purity)to prepare titania nanotubes by the hydrothermal treatment.The diameter of titania nanotubes obtained was10e30nm and the products were up to several hundred nanometers in length.The nanotubes that had consistent inner and outer diameters were multi-layered and open-ended.

Vuong and his partners(Vuong et al.,2009)investigated the effect of starting materials on the formation of titanate nanotubes. Three different starting materials were tested:(a)wet gel,(b) commercial TiO2nanopowders(P25-anatase)and(c)in-house TiO2 after calcinations at500 C.All of these samples were subjected to the hydrothermal process at150 C for20h.Titania nanotubes produced from wet gel were50e70nm in length with diameters of 10nm.In comparison,titania nanotubes obtained from commercial TiO2nanopowders were several micrometers long and50e300nm in diameter.This product was typically?brillar in its structure.The third starting material,in-house TiO2nanopowder,produced nanotubes that were15nm in diameter and several hundred nanometers long.Titanate nanotubes prepared from in-house TiO2 nanopowders had more uniform diameters and were hence supe-rior to the two aforementioned products.

Table2summarizes the in?uence of various precursors on the formation of titania nanotubes.The table reveals that different starting TiO2materials generate different structural properties of the TiO2nanotubes.

4.2.Effect of sonication pretreatment

The sonication treatment is commonly used in nanotechnology to disperse nanoparticles evenly in liquids.Thus,it plays a signi?-cant part in titania nanotube formation.Nawin and his colleagues (Nawin et al.,2008and Nawin et al.,2009)analyzed the effect of sonication pretreatment on the control of titanate nanotube length in the hydrothermal reaction.Sonication speeds up the dispersion of nanoparticles by breaking the intermolecular interactions between titanium dioxide particles and concentrated sodium hydroxide solution in the hydrothermal process.When the samples are irradiated with(often ultra)sound waves,the milky mixture becomes a smoother and more uniform starting material.Sonica-tion may also be used to provide energy to sustain certain chemical reactions.

The sonication step has a strong in?uence on the length distri-bution and the length of the synthesized product.Thus,the average length of titanate nanotubes without sonication treatment is much shorter than with such a step in the hydrothermal process.Soni-cation aids the migration of the OHàand Nations across the restricted interparticle gaps of clustered titania precursors that otherwise retard the mechanism of nanotubes formation.Nawin et al.(2008)found that the Brunauer Emmett Teller(BET)areas and average nominal lengths of the titanate nanotubes obtained with the sonication pretreatment were about1.4and8e9times those of the titanate nanotubes obtained without the sonication pretreatment.Thus,the sonication pretreatment increases the overall rate of reaction and the yield of titanate nanotubes while enhancing the lengths and speci?c surface areas of the product.

Concurring with previous researchers,Ma et al.(2006)stated that longer titania nanotubes with smaller diameters were pro-duced when the sonication pretreatment was incorporated into the process.As sonication was increased from100W to280W,and then to380W,different morphologies of titania nanotubes were obtained.At100and280W,sheet-like and?ber-like structures were observed respectively whereas tube-like structures of 9e14nm diameter and100e600nm length were obtained with 380W sonication.The sonication treatment promoted intercalating Nations into lattices of titania by breaking the Ti e O e Ti bonds.The spherical shaped titania were transformed into nanorods during the treatment and the subsequent growth of the nanorods led to the formation of longer nanotubes.

The use of sonication method during preparation of titania nanotubes helps to avoid or reduce the crystallite growth.In conclusion,smaller homogeneous titania nanotubes are formed with the use of ultrasonic pretreatment.Longer titania nanotubes with smaller diameters and higher surface area are formed effec-tively with the incorporation of the sonication pretreatment.

4.3.Effect of hydrothermal temperature

The hydrothermal temperature is one of the factors in?uencing morphological characteristics of the titania nanotubes(Wang et al., 2008).Titanate nanotubes could be formed at temperatures ranging from100to180 C,where the starting precursors were powders of TiO2(anatase and/or rutile)or commercial P25(Yuan and Su,2004).On the other hand,nanotubes formation decreased when the temperature fell outside of the range of100e180 C.Yuan and Su(2004)stated that the yield of nanotubes(up to80e90%) increased with the hydrothermal temperature(100e150 C),based on TEM observations.Wang et al.(2008)selected a temperature between100and150 C for the hydrothermal treatment of titania nanoparticles with a10M NaOH solution.Seo et al.(Ou and Lo, 2007)found that the amount and length of titania nanotubes increase with the hydrothermal temperature of100e200 C.The higher inner diameter and speci?c area of titania nanotubes produced at hydrothermal temperature of150 C.Tsai and Teng (Ou and Lo,2007)reported that the largest surface area and pore volume of titania nanotubes was found at temperature of130 C in the case of hydrothermal temperature ranging from110to150 C.

Hydrothermal temperature played an important role in promoting the nucleation and crystal growth of the titanate nano-tubes(Lan et al.,2005).The degree of crystallinity in the?nal product increased with raising the hydrothermal temperature. Thorne et al.(2005)found that the production of nanotubes increased from0to80%when the duration of hydrothermal treat-ment increased from2to72h at150 C.The yield of nanotubes increased with the hydrothermal temperature within this temper-ature range.

Many researchers have analyzed the structure features of the formation of titania nanotubes at various hydrothermal treatment temperatures.For temperatures lower than100 C,almost all researchers concluded that nanotubes did not form(Seo et al.,2008 and Sreekantan and Lai,2010).Sreekantan and Lai found that the delaminating step of spherical-shape precursors led to the forma-tion of nanosheets at90 C Seo et al.(2008),on the other hand, reported that the delaminating step occurred in the presence of sodium ions at an initial temperature of70 C,and this led to the

C.L.Wong et al./Journal of Environmental Management92(2011)1669e16801675

formation of two-dimensional nanosheets.Further increasing the temperature to 90 C resulted in the conversion of nanosheets to nano ?bres.Nawin et al.(2009)suggested that nanosheets,nano-tubes and some crystals of titanate were formed even at the rela-tively low reaction temperature of 90 C.

Ma et al.(2005)found that the combination of a hydrothermal temperature 130e 150 C over a duration of 24e 72h gave the best yield and purity of titanate https://www.doczj.com/doc/b515599565.html,n et al.(2005)also found that optimum operating temperatures between 125and 150 C produced high yield of titanate nanotubes.

In other studies on the effects of the hydrothermal temperature (Seo et al.,2008and Sreekantan and Lai,2010),titanate nanosheets grew and curled up to form nanotubes as hydrothermal tempera-ture increased from 100to 150 C.At 150 C,the tubular structures started to increase in length to a few micrometers while main-taining uniform diameter.Sreekantan and Lai (2010)pointed out that the presence of thermal energy was enough for the nanosheets structure to scroll-up and thereafter convert to nanotubes in the hydrothermal process.The formation of nanotubes began to decrease at temperatures higher than 180 C with the formation of nanorods.This ?nding is in agreement with the ?ndings of Lan et al.(2005)which reported that even at 180 C the morphology of the products was changed dramatically from nanotubes to isolated straight nanorods.However,Lee et al.(2008a)observed that the nanorod formation phenomenon occurred even at 160 C,a temperature lower than that reported by Lan et al.With the hydrothermal temperature increasing to 160 C,the pore volume and speci ?c surface area of the titanate nanotubes were reduced due to (a)the limitation of the interlayer spaces;and (b)sodium ions not being fully replaced by protons after washing with acid and deionized water.Lee et al.(2009)investigated the nanorod

formation phenomenon at a temperature of 170 C.They found microstructures of titanate nanotubes to be signi ?cantly changed at 170 C.The interlayer spaces started to decrease and the pore volume and surface area of the sample became smaller when the temperature was higher than 170 C.

In another study,Seo and his colleagues (Seo et al.,2009)investigated the in ?uence of various hydrothermal temperatures (160,200and 230 C)on the formation of titanate nanotubes.They found that the TiO 2particles were aggregated with heat treatment at 160 C.Tubular-shaped TiO 2structures of 1m m length and 50nm diameter were formed at 200and 230 C.When hydrothermal temperature was raised,the tube length increased while the diameter of the nanotubes became smaller.They also con ?rmed that TiO 2nanotubes were more permeable and porous than TiO 2nanoparticles.The researchers concluded that the TiO 2nano-particles would transfer from the anatase-rutile phase to the anatase-H 2Ti 3O 7in the hydrothermal process regardless of the hydrothermal temperature.

Table 3presents a summary of the in ?uence of hydrothermal temperature on the formation of titania nanotubes.4.4.Effect of the washing process

The morphology and dimensions of the nanostructures are generally determined by the hydrothermal treatment,rather than by the washing process (Wang et al.,2008).Nevertheless,the acidity of the washing agent does have a signi ?cant in ?uence on other important properties of the ?nal nanostructures (Wang et al.,2008).These are the elemental composition,annealing behavior and speci ?c surface area of the nanotubes (Wang et al.,2008).The peak intensities in X-ray diffraction (XRD)patterns were

Table 2

In ?uence of starting materials on titania nanotube formation.Type of starting materials Particle size of

starting materials (nm)Operating

condition ( C,h)Diameter of TiO 2nanotubes (nm)Length of TiO 2nanotubes (nm)Reference(s)

Commercial-grade TiO 2powders (P25,Degussa)25150,2012

e Saponjic et al.(2005)TiO 2colloids

4.5150,2012e Saponjic et al.(2005)Molecular Ti IV alkoxide e

150,2012

e

Saponjic et al.(2005)Rutile powders

120e 280150,482e 3(inner diameter)50e 200

Lan et al.(2005)50e 200(outer diameter)Commercial-grade TiO 2powders (P25,Degussa AG,Germany)(crystalline structure of ca.20%rutile and ca.80%anatase)30

150,48

7e 15

Several hundred

Yu et al.(2006)

Commercial-grade TiO 2powders (P25,Degussa AG,Germany)(crystalline structure of ca.30%rutile and ca.70%anatase)a

30110,49e 14100e 600Ma et al.(2006)

TiO 2Hombikat UV100Sachtleben Chemie GmbH,Germany)anatase 10110,43e 6(inner diameter)7e 10(outer diameter)Up to 400Ma et al.(2006)BCC100(Beijing Fine Chemicals Co.,Ltd.,China)anatase b

200110,4e e Ma et al.(2006)Commercial ultra ?ne TiO 2powders (Riedel De Haen,Honeywell)e 150,484e 4.2e

Ribbens et al.(2008)Commercial-grade TiO 2powders (Sigma Aldrich)

(crystalline structure of ca.25%rutile and ca.75%anatase)<50

110e 270,24

10e 30

Several hundred

Lee et al.(2008a)

Wet gel

e 150,2010

50e 70

Vuong et al.(2009)Commercial-grade TiO 2powders (P25,Degussa AG,Germany)(crystalline structure of ca.100%anatase)

e 150,2050e 300Several micrometers Vuong et al.(2009)

In-house TiO 2after calcinations at 500 C for 3h

e 150,2015Several hundred Vuong et al.(2009)

a Diameter and length of the products were obtained based on sonication treatment of 380W for 1h.

b

Sheet-like structure with rolled edges and a number of spherical particles were remained in the products.

C.L.Wong et al./Journal of Environmental Management 92(2011)1669e 1680

1676

signi?cantly vitiated due to the structural changes in titanate nanotubes after washing with hydrochloric acid solution.In the process,the hydrochloric acid exchanges its own protons with the sodium ions in the titanate nanotubes.Apparently the removal of sodium ions rendered the interlayer space more accessible to N2 molecules during the Brunauer Emmett Teller(BET)measurement, indicating a higher surface area of the product without altering its external morphology(Wang et al.,2008).

Poudel et al.(2005)established that acid washing was required to produce high yield and purity in the nanotubes produced.The optimal concentration of the acid was between0.5and1.5M.Acid concentrations below0.5M were ineffective in eliminating the sodium ions,whereas concentration above2M would destroy the nanotube structure with the formation of clumps of w100nm.

Lee and his co-workers(Lee et al.,2007)produced titanate nanotubes via the hydrothermal technique in highly concentrated sodium hydroxide solution at150 C for one day,following this with washing using different concentrations of hydrochloric acid.For titanate nanotubes washed in0.1M HCl,the titanate nanotubes became buoyant and the lengths of nanotubes were shortened.It was observed that the tubular structure of the titanate nanotubes was destroyed only in the absence of sodium.The lower the sodium ions in the titanate nanotubes,the smaller the pore volume and speci?c surface area were.On the other hand,for titanate nanotubes washed in either0.01or0.001M hydrochloric acid,the nanotube length obtained was about a hundred nanometers and the diameter was10e30nm.Generally,nanotubes prepared in this manner possessed uniform inner and outer diameters and they were almost open-ended and multi-layered.Lee et al.(2008b)cautioned that the nanotubular structure might be destroyed if the HCl concentration used in the washing step was greater than0.01M.This was because Nations in titanate nanotubes decreased as the concentration of HCl solution increased and hydrogen displaced sodium when the titanate nanotubes were washed in0.1M acid.

Papa et al.(2009)used rutile raw material to analyze the effect of acid treatment on the formation of titanate https://www.doczj.com/doc/b515599565.html,ing a vibrating table instead of magnetic stirring,nanoribbons were ?rst synthesized from rutile materials.These were converted into nanotubes after the acid washing(0.1M HCl).However,nano-ribbons did not always change into tube-like structure if the P25 precursor was used.The tubes always contained a small amount of sodium while intermediaries such as sheets and ribbons contained more sodium.Hence,acid washing was essential to eliminate the sodium ions in these structures prior to tube formation.

In conclusion,the acidity of the washing agent does have a signi?cant effect on titanate nanotubes in terms of elemental composition,speci?c surface area and annealing behavior.4.5.Effect of the calcination process

Various morphologies of titanate products fabricated via the hydrothermal treatment are the result of the post-treatment(Wang et al.,2008).The calcination process that takes place during post-treatment is believed to affect the phase structures and micro-structure of titanate nanotube products(Wang et al.,2008).Heat treatment can advance the phase transformation of titanate nanotubes to the anatase phase,and it is also conducive to the elimination of sodium ions in the samples(Wang et al.,2008).

Various researchers have investigated phase transformation of titanate nanotubes in the calcination https://www.doczj.com/doc/b515599565.html,n and his group (Lan et al.,2005)found that short,solid nanotubes were obtained at a calcination temperature of500 C.The radius of the titanate nanotubes was8e22nm and the calcined product was consistent with the structure of a pure anatase phase.

Weng et al.(2006)stated that the crystallinity of the titanate nanotubes increased with the calcination temperature.The TiO2 powders assumed a thin wall nanotubular structure when treated at400 C while its microstructure was comparable to samples that were not subjected to the calcination process.At600 C,the nanotubes amalgamated and agglomeration of nanoparticles was seen when the calcination temperature rose to800 C.

Yu et al.(2006)explored the microstructure and crystalline structure of titanate nanotubes in various calcination temperatures. The crystallite size of the anatase increased from5.3to28.7nm within the temperature range of300e600 C.The particle size distribution ranged from hundreds of nanometers to several micrometers while the average pore size was18.1e31.5nm as the calcination temperature increased from400to600 C.Over this temperature range,the pore size of nanotubes increased steadily with a sharp decrease in surface area(from219.2to64.3m2/g)and pore volume(from0.992to0.347cm3/g).The aggregation of the nanotubes was attributed to the increase of the average pore size. Above600 C,titanate nanotubes started to collapse while titania nanocrystals began to grow.Nevertheless,further investigations on the morphology and crystalline structure of samples became dif?cult at700 C.Some sample aggregation was observed with the phase transformation of the anatase phase to the rutile phase.

Qamar et al.(2008)did not?nd signi?cant differences in the diameter or length of the nanotubes regardless of whether they had been washed in water or in hydrochloric acid.The inner and outer diameters of the open-ended titanate nanotubes were6e8nm and 4e6nm respectively,and the lengths were up to several hundreds of nanometers.A reduction in surface area and pore size distribu-tion was also observed with increasing calcination temperature. The morphology of the products was in?uenced by the calcination

Table3

Effect of hydrothermal temperature on the formation of titania nanotubes.

Type of starting materials Particle size of

starting materials(nm)Operating

temperature( C)

Operating

time(h)

Optimum

temperature( C)

Reference(s)

Commercial-grade TiO2powders(P25,Degussa AG,Germany) (crystalline structure of ca.100%anatase)a e110e19012e168130e150

(24e72h)

Ma et al.(2005)

Rutile powders120e28060e18048125e150Lan et al.(2005) Commercial-grade TiO2powders(P25,Degussa AG,Germany)2570e15048150Seo et al.(2008) Commercial-grade TiO2powders(Sigma Aldrich,99.999%purity)

(crystalline structure of ca.25%rutile and ca.75%anatase)

<50110e27024160Lee et al.(2008a)

Commercial-grade TiO2powders(P25,Degussa AG,Germany)

(crystalline structure of ca.20%rutile and ca.80%anatase)

30140e17024160Lee et al.(2009) Commercial-grade TiO2powders(P25,Degussa AG,Germany)20160e23024230Seo et al.(2009) Anatase powders(020-78675,KISHIDA,Japan,

BET surface area?8.4m2gà1)

40090e18072150Seo et al.(2008)

Commercial-grade TiO2powders(99.7%Sigma Aldrich) (crystalline structure of ca.14%rutile and ca.86%anatase)1090e15024150Sreekantan and

Lai(2010)

a The combination of a hydrothermal temperature130e150 C over a duration of24e72h gave the best yield and purity of titanate nanotubes.

C.L.Wong et al./Journal of Environmental Management92(2011)1669e16801677

temperature(especially the higher temperatures)after washing with water.The surface morphology was unchanged from300to 500 C,but nanotubes were transformed at600 C into an inter-mediate structure that was neither tube-like nor were they nano-rods.After further increasing the calcination temperature to700 C, the samples fused together and then changed into nanorods or one-dimensional nanostructures.Above800 C,the length of the nanorods became smaller but thicker in width,with a portion of these structures transformed into nanostructures.

Upon washing with hydrochloric acid,the nanotubes changed in morphology and shape at500 C.Nanorods and one-dimensional nanostructures were not observed.During the calcination process, the protons in the samples facilitated the acid catalyzed conden-sation of OH groups,and the samples started to change into nanoparticles even at lower temperatures.

Lee et al.(2007)found that as the temperature of the heat treatment increased,different levels of sodium ions in titanate nanotubes led to the evolution of different phases.At0.1M HCl concentration,the intensity of the anatase and rutile phases in XRD patterns increased with increasing calcination temperature from 200to500 C.At0.01M HCl concentration,the anatase phase appeared?rst,and nanotubes as well as nanorods were found at 300 C.Further temperature increment to600 C resulted in the anatase phase declining as the rutile phase became more prevalent. The latter assumed dominance when the temperature was raised to 800 C.At0.001M HCl concentrations and at500 C,Na2Ti6O13 compounds emerged,signaling the beginning of the formation of nanotube structures.It was observed that the intensity of Na2Ti6O13 increased in x-ray diffraction after600 C,and the anatase and rutile phases were not discovered during the annealing process at 0.001M HCl concentration.

Sreekantan and Lai(2010)who investigated the effect of the calcinations process on titanate nanotubes formation found that crystallinity of the anatase phase increased at300 C.The growth of nanotubular structure was improved by increasing the annealing temperature,and whiskers-shaped particles started to form at temperatures up to300 C.Tube-shaped particles that were present at500 C converted fully to nanoparticles at600 C.The researchers noted that titanate nanotubes easily deteriorated at high temperature due to the desiccation of the inter-layered hydroxyl group.It was surmised from this that the hydrogen bonds (H2O or e OH)had been eliminated from the samples,resulting in the tube-shaped structure being transformed to nanoparticles after heat treatment.

Normally,the anatase phase is observed at an annealing temperature of300 C,although Lee et al.(2009)reported its app-earance at400 C.It was noteworthy that the titanate nanotubes surface area at400 C was higher than the surface area of titanium dioxide precursor P25.By increasing calcination temperature from 400to700 C,the crystallization and X-ray diffraction peak intensity of the anatase phase increased as peak width of the anatase phase decreased.The appearance of the rutile phase was recorded at 700 C.With further increase in the calcination temperature,the pore volume and surface area of titanate nanotubes started to decrease.However,the average pore size of the titanate nanotubes increased upon further thermal treatment.

Vuong et al.(2009)concluded that the crystallinity of the nanotubes increased with increasing calcination temperature. However,the tube-shaped structures were damaged and were reduced to particle-shaped structures as the temperature increased.

Many researchers have con?rmed that the anatase phase is present after the thermal treatment process and that the crystalli-zation,morphology,pore structure and speci?c surface area of titania nanotubes are strongly dependent on the calcination temperature.It can be seen that dissimilar optimum annealing temperatures have been reported by different researchers.Such differences in the pre-paration and post-treatment conditions contribute towards different phase structures of the samples.

5.Modi?cation of titania nanotube photocatalysts

Various approaches have been attempted to limit the recombi-nation of the photogenerated electron e hole pairs in the photo-catalysis(Sun et al.,2009).These include non-metal anions doping, surface improvement with noble metal and transition metal cation doping(Sun et al.,2009).Non-metal dopants have been applied in photocatalysis to broaden the photocatalytic activity from the ultra-violet(UV)region to the visible light region(Okour et al., 2009and Xu et al.,2009).Asahi and his colleagues(Okour et al., 2009and Xu et al.,2009)analyzed the use of non-metal dopants (nitrogen atoms)in the photocatalysis.The presence of nitrogen(N) atoms substitutes for oxygen(O)atoms in the titanium dioxide (TiO2)lattice and shifts the titanium dioxide band gap into the visible light region.Similarly,sulfur(S)atoms have been doped into TiO2by Ohno et al.(Okour et al.,2009)in photocatalysis.Sulfur has been reported to have better photo-absorption when used to dope TiO2as compared with nitrogen-doped TiO2.The enhancement of the photocatalytic activity of TiO2in the visible region by dopants has been employed in research on the use of dopants-TiO2nano-tubes in the photocatalytic degradation of pollutants.

Besides applications in photocatalysis,metal and non-metal dopants have been exploited widely in other functions,for exa-mple,in the areas of catalysis and gas sensors.Transition metal cation doping narrows the band gap of the catalyst,and thus provides better photocatalytic activity of the catalyst in shifting the activity from the UV region to the visible light region.In previous

Table4

Effect of the presence of dopants on the formation of titania nanotubes.

Type of dopants Outer diameter

(nm)Inner diameter

(nm)

Length Speci?c surface

area(m2/g)

Pore volume

(cm3/g)

Reference(s)

Nickel(Ni)10e115e6Several hundred

nanometers

e e Kim et al.(2008)

Carbon(C)10e Several hundred

nanometers

e e Xu et al.(2008)

Nitrogen(N)80e10060e80e e e Dong et al.(2009) Nitrogen(N)104200e400e e Geng et al.(2009) Cobalt(Co)10e155e10Several micrometers289e3790.685e0.934Hsieh et al.(2009) Aurum(Au)17.66e e92.390.31Zhao et al.(2009) Platinum(Pt)17.95e e90.990.31Zhao et al.(2009) Iodine(I)e e e205e227 1.11Song et al.(2010)

co-doped with

Gadrium(Gd)

and Nitrogen(N)115e210.34e211.63e Liu et al.(2011)

C.L.Wong et al./Journal of Environmental Management92(2011)1669e1680

1678

studies,a series of metal and non-metal dopants,including chro-mium(Cr),iron(Fe),manganese(Mn),nickel(Ni),platinum(Pt) and vanadium(V),have been investigated for photocatalysis applications(Colmenares et al.,2009).However,they are con?rmed to have higher activity and ef?ciency in the photocatalytic removal of pollutants under visible light up to certain optimum doping level (Colmenares et al.,2009).Beyond the optimum doping level,the photocatalytic activity of TiO2nanotubes is observed to decrease (Colmenares et al.,2009).

Kim et al.(2008)were of the view that nickel(Ni)-doped tita-nate nanotubes were produced by the simple and effective hydro-thermal treatment.The inner diameter and length of the titania nanotubes produced were5e6nm and several hundred nanome-ters respectively.The outer diameter was about10e11nm.The open-ended nanotubes exhibited5e6layers.

Xu et al.(2008)observed that carbon(C)-doped titania nanotubes are long slim tube-like structure,whose the length is several hundred nanometer and diameter is of10nm.The presence of carbon dopants in?uences the light absorption ability of titania nanotubes by nar-rowing the band gap of TiO2nanotubes,and thus shifts the catalyst activity from the UV region to the visible light region.

Dong et al.(2009)synthesized highly ordered nitrogen(N)e doped TiO2nanotubes by the calcination process of anodized TiO2nanotubes with the ammonia.The diameter and wall thickness of the products are60e80nm and20nm respectively.The presence of N dopants decreases the phase transformation temperature of TiO2nanotubes from anatase phase to rutile phase.

Nitrogen(N)e doped titania nanotubes were also produced by a facile wet chemistry method using titania nanotubes as starting materials(Geng et al.,2009).At the end of the doping process, a uniform tube-like structure was formed.The length was400nm and the inner and outer diameters of the nanotubes were4nm and 10nm.The N dopants could incorporate with the TiO2lattice to induce a new band state in the TiO2band gap,which lead to the absorption edges shifting to the visible light region.

Hsieh et al.(2009)used cobalt(Co)-doped titanate nanotubes to degrade the basic violet10(BV10).The band gap of Co e TiO2 nanotubes,being only2.14eV,is smaller than the commercial Degussa P25band gap(3.20eV).A vital discovery in this regard is that the band gap of the photocatalyst becomes smaller due to the addition of Co atoms.Co-doped TiO2nanotubes have hence encouraging prospects in photocatalysis because of the photo-catalyst’s high porosity and its functionality in visible light.

Zhao et al.(2009)who investigated the photodegradation rate of methyl orange(MO)found its removal greatly enhanced by the addition of gold(Au)or Pt dopants.The noble metal dopants (Au and Pt)have higher potential gradient to form Schottky junc-tions as compared with TiO2nanotubes and TiO2P25.This explains their higher photocatalytic performance in the degradation of MO.

Song et al.(2010)who investigated the photodegradation ability of iodine(I)-doped TiO2nanotubes found its surface area greatly larger than that of iodine doped TiO2nanoparticles.The photo-catalytic ability of I-doped TiO2nanotubes was enhanced due to the enhancement of mass transfer and its increase of reactive sites.The large surface areas of I-doped TiO2nanotubes increased the mass transfer and also improved their photocatalytic activity.

Co-doping technique inhibits the particle growth of the catalyst, and thus gives larger surface area in leading to higher photo-catalytic activity.Lower particle growth means smaller particle size, which decrease the recombination rate of electron and hole and increase the photocatalytic ef?ciency.Higher crystallinity could be produced by Gd,N-co-doping into TiO2nanotubes(Liu et al.,2011). Gd3tacted as a sensitizer to absorb the irradiated light and transfer the energy to the surface of TiO2nanotubes in order to enhance the photocatalytic activity.

Table4summarizes the in?uence of the dopants on the formation of titania nanotubes.The table reveals that different dopants give different morphology of TiO2nanotubes.

6.Conclusion

A comprehensive series of titania nanotubes are discussed in this review.There are three main sections included:(a)the mechanism of titania nanotube formation by hydrothermal treatment,(b) factors affecting the formation of titania nanotubes,and(c)effects of the presence of dopants on the formation of TiO2nanotubes. Judging from the referenced literatures,the alkaline hydrothermal technique,also known as soft-chemical hydrothermal treatment,is a simple and environmentally friendly method used in various applications,particularly in photocatalysis.The formation mecha-nism of titania nanotubes is still unclear and ambiguous.In order to enhance the photocatalytic activity of titania nanotubes,optimum conditions are needed to focus on the relationship between the photocatalytic activity and physical properties of TiO2nanotubes. Acknowledgment

The authors would like to acknowledge the economic support received from Universiti Sains Malaysia(USM)through the Fundamental Research Grant Scheme(1001/PJKIMIA/811068)and through the Postgraduate Research Grant Scheme(1001/PJKIMIA/ 8033054).

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英语造句

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学生造句--Unit 1

●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the

英语句子结构和造句

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