Visible light responsive titania-based nanostructures for photocatalytic, photovoltaic and

photoelectrochemical applications

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2012 Adv. Nat. Sci: Nanosci. Nanotechnol. 3 023001

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 3 (2012) 023001 (9pp) doi:10.1088/2043-6262/3/2/023001

REVIEW

Visible light responsive titania-basednanostructures for photocatalytic,photovoltaic and photoelectrochemicalapplicationsVan Hieu Nguyen and Bich Ha Nguyen

Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet,Hanoi, Vietnam

E-mail: nvhieu@iop.vast.ac.vn

Received 9 January 2012Accepted for publication 13 February 2012Published 9 March 2012Online at stacks.iop.org/ANSN/3/023001

AbstractThis work presents a topical review of selected articles on visible light responsivetitania-based nanostructures used for fabricating the photoanodes of the photocatalytic andphotoelectrical cells for hydrogen production by water splitting or fuel decomposition,electricity generation by fuel decomposition and pollutant degradation under illumination bysunlight as well as for fabricating dye-sensitized and quantum dot-sensitized solar cells. Threemain types of related nanostructures are reviewed: anion-doped titania nanomaterials,cation-doped titania nanomaterials and titania-based nanostructures sensitized by dyes andquantum dots. After the presentation of the obtained results, the prospective further researchworks to achieve the successful fabrication of visible light responsive photocatalytic,photoelectrochemical or photovoltaic devices with high performance are discussed.

Keywords: solar cell, doping, nanostructure, nanomaterial

Classification numbers: 4.01, 4.02, 5.04, 5.07

1. Introduction

The celebrated experiment of Fujishima and Honda [1] onpure water splitting in a photoelectrochemical (PEC) cellusing a titania photoanode has inspired widespread interestfrom the scientific community in the study of solar energyconversion into hydrogen energy and/or electricity. A verylarge number of semiconductor materials which can be usedfor fabricating the photoanodes of PEC cells have beenprepared, characterized and used in experimental studieson solar energy conversion. Many comprehensive reviewson semiconductor photocatalytic (PC) materials have beenpublished (see, for example, [2, 3]). Among all PC materials,titania is the most prospective [4, 5], because it is chemicallyinert, non-toxic, photostable and has low cost. However,titania has one disadvantage: due to its large bandgap (3.2 eV

for anatase) it is not sensible with respect to visible light.To overcome this disadvantage many experiments have beendone in which the electronic structure of titania has beenmodified by doping for preparing visible light responsivePC materials, or titania nanomaterial has been sensitizedfor fabricating visible light responsive photoanodes of PECcells. The present article is a comparative review of selectedtypical related works with discussions on the basic physicalcharacteristics of prepared nanomaterials, such as the edges ofelectron energy bands, the presence of deep energy levels inthe bandgap and the evidence of the separation of the excitedcharge carriers.

The PC activities of the prepared nanomaterials as wellas the working abilities of PEC cells are determined byseveral factors: the sensibility of the visible light absorption,the separation of the exited charge carriers to prevent

2043-6262/12/023001+09$33.00 1 © 2012 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 3 (2012) 023001 Van Hieu Nguyen and Bich Ha Nguyen

electron-hole recombination and/or the electron relaxationdue to the presence of defects and impurities, includingdoping elements, the Brunauer-Emmett-Teller (BET) surfaceareas of the PC materials, the catalytic reactions at the surfacesof the PC materials or at the surface of the counter cathodes ofPEC cells, the chemical reactions in the electrolytes etc. Thecomprehensive study of the above-mentioned characteristicsof each type of PC material or PEC photoanode (as well asthe chemical reactions in the electrolyte, if it is necessary)would give the complete scientific data for the design ofthe most effective PC material or PEC photoanode of thecorresponding type. It was well-known that one of theeffective methods to observe the deep impurity centersin the bandgap of a semiconductor is the investigationof the photoluminescence spectrum. Moreover, the chargeseparation in the semiconductor nanostructure would decreaseits luminescence intensity. Therefore the observed decreaseof the luminescence intensity would be the evidence of thecharge separation. Promoting research on this topic is one ofthe aims of the present article.

The review of typical works on anion-doped titaniananomaterials is presented in section 2. Results of differentworks and conclusions of their authors are compared anddiscussed. Section 3 is devoted to the review of the typicalworks on cation-doped titania nanomaterials. In section 4 wereview the main results of study on dye-sensitized solar cells(DSSCs) and similar PEC cells with fuels as organic sacrificedagents for generating electricity—the photofuel cells (PFCs).It was proposed to replace the dyes in DSSCs by quantum dots(QDs). The main results of the study of QD-sensitized solarcells (QDSSCs) are also reviewed in section 4. The conclusionand discussions on the prospective studies of the reviewedissues are presented in section 5.

2. Anion-doped titania nanomaterials

The earliest experimental work to chemically modify titaniafor preparing a visible-light responsive photocatalyst has beenperformed by Fujishima et al [6]. Although these authors didnot investigate the absorption spectrum of the modified titania,the ability of the modified photocatalyst to split water underthe illumination by sunlight, which contains only a very smallfraction of UV light, might signify that the energy bandgapwas narrowed. Different methods to modify the structure oftitania for preparing photocatalysts with bandgaps narrowerthan that of pure titania have been discussed by Asahi et al [7].They noticed that reducing titania creates localized oxygenvacancy states in the interval 0.75–1.18 eV below the titaniaconduction band minimum (CBM), so that the energy levels ofoptically excited electrons are lower than the redox potentialof the hydrogen evolution (H2/H2O) located just below thetitania CBM. Moreover, due to the localization the electronshave a small mobility. Concerning the doped titania, thefollowing conditions are required:

(i) Doped titania should have a narrow energy band gap toabsorb visible light.

(ii) CBM of doped titania should be as high as that of puretitania or higher than the H2/H2O redox potential toensure the photoreduction.

(iii) Wave functions of the states in the bandgap shouldoverlap sufficiently with those of the band states of titaniato transfer photoexcited electron to the reactive sites at thesurface of the photocatalyst.

Because the doping by transition metals (cation-doping) oftencreates localized d states with deep energy levels in the energybandgap which violate conditions (ii) and (iii) and results inthe appearance of the recombination centers of the carriers,Asahi et al [7] prefer doping by non-metallic elements C,N, F, P or S (anion-doping). They have done full-potentiallinearized augmented plane wave (FLAPW) calculation andshown that substitutional doping by N is the most effective.Its p states cause the bandgap narrowing by mixing with O 2pstates. The N-doped titania nanocrystalline films (NCFs) wereprepared by sputtering the titania target in an N(40%)Ar gasmixture. Photocatalytic activity was evaluated by measuringdecomposition rate of methylene blue under the visible light.The active wavelength range of N-doped titania covers themain peak of the solar light spectrum.

Asahi et al [7] measured the absorption spectra of bothpure and N-doped titania NCFs. Although they observed theredshift of the absorption spectrum of N-doped titania upto the wavelength of 500 nm, they could not determine thevalue of the narrowed bandgap. With the resolution of theabsorption spectrum of N-doped titania nanomaterials untilnow one could not also find discrete energy levels inside thebandgap if they do exist. The absorption spectra should bemeasured with better resolutions. The results of subsequentexperimental works [8–11] agreed well with the conclusionof Asahi et al [7] concerning N-doped titania.

In a series of interesting experimental works by Anpoet al [12–20] the doping of nitrogen to titania wasalso performed by the radiofrequency magnetron sputtering(RF-MS) technique to prepare visible-light responsible PCthin films denoted Vis-TiO2. The source material to besputtered was that in a titania target and the sputtering gas wasa mixture of N and Ar gases. As a result of the bombardmentby a large number of high energy sputtered atoms, the O/TiO2

composition in the prepared substrate from the surface todeep inside the film became different from that of the initialsubstrate. It was shown that for the splitting of pure waterinto hydrogen under illumination by visible or solar light theprepared modified titania film has rather high reactivity. ThisPC property might be a consequence of the narrowing ofthe energy bandgap due to the nitrogen doping. It would beworth carrying out spectroscopic investigations of this type ofmodified titania to reveal its electronic structure, in particularto determine its energy bandgap and also to observe the deeplevels inside the energy bandgap due to the defects or theimpurity centers.

In spite of the arguments of Asahi et al [7] in favor ofN-doping to titania, Khan et al [21] investigated C-dopedtitania film prepared by flame pyrolysis of Ti metal sheet in thepresence of combustion products (CO2 and H2O steam) in anatural gas flame. This preparation method was similar to thatused in reference [6]. The x-ray photoelectron spectroscopy(XPS) data of the samples prepared in [21] showed that theydid not contain nitrogen. From the UV–visible absorptionspectra of these samples the authors have derived thevalues 535 and 440 nm of two optical absorption thresholds

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corresponding to two bandgap energies 2.32 and 2.82 eV.They have also measured the maximum photoconversionefficiency under illumination by a 150 W xenon lamp andobtained a value eight times larger than that of the puretitania. Subsequently Khan and co-workers [22–25] continuedto prepare C-doped titania nanoparticles by means of differentmethods and to study the physical characteristic as well asthe photocatalytic activities of the prepared samples. In [22]the authors synthesized C-doped titania nanoparticles bywet chemical process using glucose solution as the carbonsource and reported the extension of the absorption spectrumof prepared samples up to the wavelength of 800 nm aswell as the thirteen fold increase of the catalytic activity incomparison with that of pure titania. By a treatment of theUV–visible absorption spectrum they have shown that theenergy bandgap lowered from 3.20 to 2.78 eV. Moreover, thereappeared a mid-gap band at 1.45 eV. In [25] a comprehensivestudy of visible light active C-doped titania for efficienthydrogen production by PEC splitting of water has beendone. The samples were prepared by thermally oxidizing Timetal sheets in the flame under the controlled natural gasand oxygen flows. The PEC activities of the material wereinvestigated under the illumination by a 150 W xenon lampand the applied potential of 0.24 V. By means of a carefultreatment of the absorption spectrum the authors have shownthe existence of two energy bandgaps: the first bandgap of2.7 eV resulted by reducing the original energy bandgap of3 eV for rutile titania and the mid-band gap of 1.6 eV causedby C-doping. The treatment method of these authors shouldbe applied in all future works on a similar subject. It wouldalso be worth measuring the photoluminescence spectra of thesamples in addition to the absorption ones.

From the results of the experimental works of Khanet al [21–25] it can be concluded that the physicalcharacteristics and PEC activities of C-doped titaniananoparticles depend on the methods which were usedfor preparing the samples, and C-doped titania is also apromising visible light active photocatalytic material. The lastconclusion agreed well with the result of [26] in which theauthors have observed a five fold increase in photocatalyticactivity of C-doped titania compared to N-doped. Thisinteresting experiment of Khan et al [21] inspired manyauthors to continue to study C-doped titania nanocrystallinefilms [27–34]. Their results agreed well with the conclusionsof the recent work of Shahan and Khan et al [25].

Besides the wide interest in studying the PC propertiesof nanoparticles (NPs) and nanocrystalline films (NCFs) ofN-doped [6–20, 35–40] and C-doped [21–34, 41–45] titania,those of S-doped [46–51], F-doped [52–55], P-doped [56–59]and N/P-co-doped [60–62], N/C-co-doped [63, 64],N/S-co-doped [65] titania were also prepared and theirPC properties were investigated. Recently, owing to theprogress of nanotechnology, C-doped titania nanotubeas well as nanotube array [66, 67] and N-doped titaniananotube [68] were fabricated. They have superior PCactivities in comparison with NPs and NCFs of C-doped andN-doped titania, respectively.

In concluding this section we note that, except for a fewabove-presented typical works, in most experimental studieson anion-doped titania nanomaterials the main attention was

paid to the determination of overall PC properties of thefabricated samples. This is necessary for evaluating thefeasible utilization of prepared nanomaterials in practice.However, in order to find the way to improve the PCproperties of these nanomaterials, their electronic energy bandstructures, including the presence of the discrete deep levelsin the energy bandgaps, as well as the dynamical phenomenaand processes determining their PC activities should beinvestigated in detail. This issue will be discussed again inthe last section of this review.

3. Cation-doped titania nanomaterials

The attempts to prepare visible light responsive cation-dopedtitania nanoparticles (NPs) and nanocrystalline films (NCFs)also arose a long time ago [69–71]. At the beginning thedopants were transition metal elements. Subsequently somerare earth elements were also used as the dopants. The threemost attractive transition metal elements for using as dopantswere Cu, Fe and Pt.

Copper may be present in the visible light responsivetitania-based nanostructures either as a dopant in the titaniananocrystal or as the metallic element in the semiconductorcompounds CuO and Cu2O to be used as visible lightsensitizers of titania nanocrystal. In recent years there arose astrong interest in the research on Cu-doped titania as a visiblelight responsive photocatalyst [72–76]. In a recent work Sunand co-authors [76] have obtained valuable results. Theseauthors prepared Cu-doped titania samples by four differentmethods, namely, in situ sol–gel (SG), wet impregnation(WI), chemical reduction of Cu salt by NaBH4 (NR) and insitu photodeposition (PD). SG sample possessed the largestBET surface area of 87.3 m2 g−1, while WI, NR and PDsamples have moderate ones of 36.1, 44.8 and 46.8 m2 g−1,respectively. The PC hydrogen generation over the samplesof all four types was investigated. The UV–visible absorptionspectra of all samples were measured. These spectra havelarge absorption bands between wavelength values 400 and800 nm. However, from these spectra the edges of the energybandgaps that are narrower than those of pure titania cannotbe determined and the discrete deep levels inside the energybandgap of pure titania were not seen.

Titania NCFs and NPs were also doped by Fe3+

ions [77–81]. With Fe3+ ion concentration lower than0.03 mol%, Fe-doped titania is a visible light responsivephotocatalyst. However, at Fe3+ ion concentration exceeding0.03 mol%, oxide molecules Fe2O3 would be formed andbecome the recombination centers of photoexcited chargecarriers, which would prevent the separation of freeelectrons and holes generated by the light absorption of thephotocatalyst.

Since the early days of photocatalysis study it wasknow that, as a co-catalyst, noble metal Pt deposited onthe surfaces of titania NPs or NCFs would significantlyimprove their PC activity. In the role of a scavenger ofelectrons Pt NPs deposited on these surfaces would trap theelectrons of the photogenerated electron-hole pairs insidethe PC nanomaterials and therefore would stimulate thecharge separation and prevent the recombination of theexcited electrons. These trapped electrons would then rapidly

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transfer from the surfaces of the nanomaterials to thesurrounding media (electrolyte, for example) and therefore,the catalytic reactions at these surfaces would be enhanced.Beside the above-mentioned double role of a co-catalystand, at the same time, of a scavenger of electrons, Pt canplay one more role—that of a cation dopant in the titaniananocrystal [82–84]. A series of experimental works onPt-doped titania nanomaterials have been done by Yang andco-workers [85–88]. Very interesting results were obtainedin [88]. In this last work the authors synthesized titaniananotube (TiNT) by hydrothermal method and introducedPt onto TiNT by means of the ion exchange method. Theprepared material was denoted Pt(IE)/TiNT. For comparisonPt was also introduced onto TiNT by means of theimpregnation method and the prepared material was denotedPt(Imp)/TiNT. The samples of TiNT, Pt(Imp)/TiNT andPt(IE)/TiNT have BET surface areas of 98.8 and 92 m2 g−1,respectively, while that of the starting material TiO2(P-25)is 50 m2 g−1. This result demonstrated the privilege ofone-dimensional titania nanostructures in comparison withparticulate ones. While TiNT and Pt(Imp)/TiNT have thesame bandgap energy of 3.1 eV as that of TiO2(P-25),Pt(IE)/TiNT has a lower bandgap energy of 2.48 eV. Thesevalues of the bandgap energies were derived on the basis oftreatment of the UV–visible diffuse reflectance spectroscopy(DRS) spectra of the samples. It would be worth measuringtheir absorption and luminescence spectra. With a goodresolution they would also give information on the discreteenergy deep levels inside the bandgap. The study of the H2 andO2 evolution from the spitting of water and the measurementof photoresponse confirmed the much higher PC activityof Pt(IE)/TiNT under sunlight in comparison with that ofPt(Imp)/TiNT.

Besides Pt as the most typical co-catalyst, two other noblemetals Au and Ag in the form of NPs were also deposited onthe surface of photocatalyst for enhancing catalytic reactionsat these surfaces as the co-catalysts. Moreover, in [89, 90]Au and Au+ were doped to titania nanocrystals to transformthem into visible light responsive nanomaterials. In [91] it wasshown that Ag-doped titania is also a visible light responsivephotocatalyst.

Besides the above-mentioned cation dopants, many othertransition metals were used for doping titania nanomaterialsto prepare visible light responsive nanophotocatalysts. Amongthem, vanadium is a promising element [92, 93]. In [93] notonly V, but other transition metals W, Zr, Cu and Fe werealso doped to titania. Cr3+-doped titania nanomaterials wereprepared in [80, 94], and Co2+-doped ones in [80]. Visiblelight responsive titania-based cation-doped nanomaterials ofvery high quality were fabricated by means of the ionimplantation technique in celebrated experiments of Anpoet al [95–97]. Although this method is expensive and couldnot be used for widespread applications in practice, it isextremely useful for the fundamental experimental studies onthe electronic structures of the related nanophotocatalysts aswell as on the principal physical and chemical phenomenaand processes in these nanomaterials. Modification of titaniananomaterials by doping transition metals to them wasalso performed in recent works [98–104]. Titania-basednanomaterials co-doped by two transition metals Cr and Sb

were prepared and investigated in [105–107]. Co-doping atitania nanomaterial by cation Bi and anion S was also donein [108].

Not only were transition metals doped to titaniananomaterials for transforming them into visible lightresponsive nanophotocatalysts, but rare earth elements werealso used as the cation dopant. For example, in addition tothe use of different transition metals, Nagaveni et al [93]prepared and investigated Ce-doped titania nanomaterial. In aseries of experiments of Xie et al [109–112], modified titaniananomaterials were prepared by doping rare earth ions Nd3+,Eu3+ and Ce4+. Er3+-doping to titania was done in [113] andSm-doped titania was prepared in [114].

It is worth noting that in most of the above-mentionedworks on visible light responsive cation-doped titania-basednanophotocatalysts their overall PC properties wereinvestigated. Although the photoabsorption spectra ofthe prepared samples were measured by a few authors, onlyin very few works was the accuracy of experimental datagood enough for the quantitative determination of the shiftsof the edges of the photoabsorption spectra. However, thepresence of the discrete energy levels inside the bandgap, ifthey really do exist, was never observed. Finally we remarkthat the careful analysis of high resolution photoluminescencespectra of prepared cation-doped titania nanomaterials wouldgive useful information on their electronic structures. Thephotoluminescence spectra of several cation-doped titaniananomaterials were measured in [93]. Experimental worksin this direction for all prepared samples and with higherresolutions would be very desirable.

4. Sensitized titania-based nanostructures

For the fabrication of visible light responsive photoanodesof various PEC devices on the basis of titania-basednanostructures, instead of anion- and cation-doping titaniananomaterials there exists another original approach: tosensitize pure titania nanomaterials by attaching to theirsurfaces different visible light sensible agents calledsensitizers, such as dyes and semiconductor nanocrystals(NCs) playing the role of semiconductor quantumdots (QDs). At the beginning, fabricated visible lightresponsive nanostructures were used as the photoanodes ofcorresponding solar cells to generate electricity from solarenergy—dye-sensitized solar cells (DSSCs) and quantumdot-sensitized solar cells (QDSSCs). Later on it was proposedto use them as the photoanodes of various PEC devices forgreen energy technology as well as for depollution.

The first experiment on dye-sensitized titania NCFwas done in a celebrated work of O’ Regan andGrätzel [115]. Subsequently, Grätzel [116–118] proposed touse them in PEC cells for various other applications. Thesensitizer used in [115] was the following dye: trimericruthemium complex RuL2(µ-(CN)Ru(CNL′

2))2, where L is2, 2′bipyridine-4, 4′-dicarboxylic acid and L′ is 2, 2′bipyri-dine. In order to sensitize a titania nanocrystalline film withrespect to the visible light, a monolayer of the above sensitizerwas deposited directly on this film. The absorption spectrumof this sensitized nanostructure is then enlarged up to 750 nm.The significant increase of its absorbance with respect to the

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sunlight due to the presence of the sensitizer was clearly seenin the absorption spectrum. This sensitized nanostructure wasused for preparing the photoanode of a dye-sensitized solarcell (DSSC). Other sensitizing dyes were used in [116, 117,119]. The research on DSSCs is still ongoing [119–129].

Nanostructures similar to those in photoanodes of DSSCswere also used for preparing photoanodes of PEC cellsfor hydrogen generation by splitting pure water or organicfuels as well as for pollutant degradation under illuminationby sunlight [117, 118, 130]. The nanostructures used forpreparing photoanodes of PEC cells which degrade pollutantsunder the action of sunlight were also sensitized by otherdyes such as thionine, eosin Y, rhodamine B, methylen blue(MB), etc [131–133]. Moreover, the pollutants or fuels tobe degraded or consumed (decomposed) during the workingprocess of PEC cells themselves could also play the roleof sensitizers: the electron excited from the ground stateof a pollutant or a fuel due to the absorption sunlightby this compound is injected into the conduction band oftitania nanomaterial and then loaded to the counter cathode.Subsequent reactions in the sacrificed pollutant or fuel lead toits degradation or consummation (decomposition) [134–139].The presented sensitization ability of pollutants to bedegraded or fuels to be consumed (decomposed) opened newperspectives for the utilization of titania-based nanostructuresin environmental technology as well as for producing PFCs.In [119–123] the titania nanostructures were sensitizednanotube and nanowire arrays, while in other works the hostmaterials were NCFs.

In order to sensitize titania nanomaterials for fabricatingphotoanodes of solar cells it was proposed a long time ago toalso use QDs [140–145]. The study on QDSSCs is a ratheractive ongoing research topic at the present time [146–155].QDSSCs have several advantages in comparison with DSSCs.Because the energy gaps of QDs depend on their sizes, theworking wavelengths of QDSSCs are tunable and their rangemay be wider than those of energy levels of dyes [156]. Thephotoabsorption coefficients of QDs may be higher than thoseof dyes (typically by a factor of 5), and QDs have superiorresistances against photobleaching over organometallic ororganic dyes [157]. CdS is a direct bandgap semiconductorwith high photoabsorption coefficient and its QDs in theform of NCs were used as sensitizer by many authors [142,144, 150, 152, 157–161]. However, it is unstable due to thephotocorrosion caused by photogenerated holes

2h+ + CdS → Cd2+ + S.

CdTe is also photocorroded and less photosensible. For usingas sensitizer in QDSSC, the most preferable QD was thatof CdSe [147, 155, 162–168]. Sensitizing CdSe QDs weredirectly deposited onto the surfaces of titania nanomaterialsin many experimental works. Attempts were made to connectthis type of sensitizing QD with the host nanomaterialsthrough bifunctional linker molecules [162, 167]. The studyof the co-sensitization of titania nanomaterials by CdS andCdSe has shown that the performance of CdSe/CdS/TiO2

nanostructure is higher than that of CdS/TiO2 and CdSe/TiO2,but that of CdS/CdSe/TiO2 nanostructure is lower than thelatter [149, 169, 170]. In a recent interesting work [171] CdSeQD was used to sensitize N-doped titania nanomaterial. This

new QD-sensitized nanostructure has PEC activity higher thanthat using a pure titania one.

The efficiencies of QDSSCs are still lower than thoseof DSSCs, but attempts to improve them are ongoing. Forexample, the change of the ligand for passivating the surfaceof QD, namely the replacement of tri-n-octylphosphineoxide (TOPO) by 4-butylamine (BA), a shorter passivatingligand, leads to a significant enhancement of both theelectron injection efficiency at the QD/titania NP interfaceand the charge collection efficiency at the QD/electrolyteinterface [153]. The performance of QDSSC can besignificantly enhanced by ZnS surface coating [172], SiO2

surface coating [173], or by introducing a dense TiO2 blockinglayer [174]. Another possibility to enhance the photocurrent inQDSSCs is the exploitation of the carrier multiplication [175,176], the inverse Auger recombination [166, 177] as well asthe multiple exciton photogeneration [178–180]. One moreway to improve the performance of QDSSCs is enhancingthe photoabsorption of QD sensitized nanomaterials by meansof the surface plasmon resonance (SPR) of the metallicnanoparticles directly deposited on their surfaces [181–192].The physical mechanism of the SPR effect can be brieflydescribed as follows. Surface plasmons (SPs) are theelementary excitations (quasiparticles) of the collectiveoscillations of itinerant (quasifree) electrons near the surfaceof the metallic NP. Their resonant coupling with incidentphotons causes the strong absorption of the incident light,and the metallic NP operates as a nanoantenna for receivingthe light [184]. It was known that the resonant frequencyis tunable due to its dependence on the size and shape ofthe metallic NP as well as on the dielectric constant ofthe surrounding environment [193]. The resonant absorptionof the incident light results in the enhancement of theelectron-hole pair generation in the best nanostructure. Notethat this enhancement physical mechanism is different fromthe role of co-catalyst played by Au NP in Au/CdS/TiO2

PC metal-semiconductors nanocomposite [194] or PtNP inPt/CdS/TiO2 PC metal-semiconductors nanocomposite [195,196]. Besides the above-presented sensitization by CdS andCdSe QDs nanocrystals of other semiconductors such asPbS [141, 197], InP [145], InAs [198], CdTe [147, 199],Cu2O [200] and CuO [201] can also play the role ofsensitizing QDs. A promising sensitizer might be CuSe [202].Although Si is a semiconductor with indirect bandgap, itsphotoabsorption coefficient is small, but that of SiQD maybecome large due to the electron confinement and it isdesirable to study the sensitization by using Si nanomaterials.The above- presented QD sensitized titania nanomaterialswould be widely used not only in QDSSCs, but also forfabricating photoanodes of various PEC devices (also calledcells) for solar energy conversion into hydrogen energy,electricity generation by photoassisted decomposition offuels (PFC) as well as PC degradation of pollutants underillumination by sunlight [203].

5. Conclusion and discussions

The discovery of the photocatalytic splitting of pure waterat the titania electrode by Fujishima and Honda [1] fourdecades ago has opened a wide and very promising scientific

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field—the photocatalytic conversion of solar energy intoelectricity and hydrogen energy as well as the photocatalyticdegradation of pollutants under sunlight illumination. Theresults of scientific works in this field were published inthousands papers. The present review concerns only a smallnumber of selected works related to visible light responsivetitania-based nanomaterials and nanostructures. The resultsof many other works were included in many publishedreviews [2–5, 203–210] and monographs [211, 212]. Whilephotoinduced hydrophilicity involving surface self-cleaningand antifogging abilities as well as the PC activity of puretitania under the illumination by UV light were widelyapplied in practice, many research works should be doneuntil successful and effective practical application of the PCproperties of visible light responsive photocatalysts, includingtitania-based PC nanomaterials and nanostructures, as apromising effective way to exploit the solar energy.

The main contents of this review are presented in threesections. In section 2 the detailed results of typical workson two most promising anion-doped titania nanomaterials,namely N-doped and C-doped ones, were presented. Inparticular, attention was paid to the careful treatment of theabsorption spectrum of C-doped titania nanomaterial whichpermits clearly determining the values of its energy bandgapsand to reveal a mid-gap band. The latter would signifythe existence of a discrete level inside the main energybandgap [25]. Some other works on anion-doped titaniananomaterials were mentioned. In these last works little oreven no attention was paid to the study of the photoabsorptionspectra of the prepared samples. No photoluminescencespectrum was measured. In section 3 the detailed results ofthe typical experimental works on two types of cation-dopedtitania nanomaterials, namely Cu-doped and Pt-doped ones,were presented. Their photoabsorption spectra as well astheir PC properties depend on the preparation methods. Manyother experimental studies on different cation-doped titaniananomaterials were briefly reviewed. The photoabsorptionspectra of the samples cation-doped by means of the ionimplantation techniques [95–97] had good quality permittingclear observation of the shifts of the energy bandgapsdue to the doping. The photoabsorption spectra of manyother cation-doped titania nanomaterials were also measuredbut with poor quality. In general the resolution of theobserved photoabsorption spectra of all cation-doped titaniananomaterials was not enough to reveal the existence ofthe discrete levels inside the main energy bandgap. It isinteresting to remark that in the photoluminescence spectra ofmany cation-doped titania nanomaterials, namely W-doped,V-doped, Cu-doped, Fe-doped and Zr-doped ones [93], thenew peaks inside the main energy bandgaps were clearly seen.This means that comprehensive experimental studies of theoptical processes would provide new important informationon the electronic structures of the PC nanomaterials, bothcation-doped as well as anion-doped ones. Moreover, thecomparative studies of the photoluminescence spectra of thedoped titania nanomaterial with and without the Pt co-catalystdeposited on the surface-doped titania NPs or the Pt countercathode connected with the doped titania photoanode, whichplay the role of electron scavengers for enhancing the chargeseparation process, would signify its evident demonstration.

The charge carrier transfer across different interfaces shouldalso be investigated in detail. Recently, the technology ofpreparing one-dimensional titania nanomaterials, nanotubes,nanowires and nanorods, was elaborated [213–215]. Theyhave surface areas larger than those of titania NPs andNCFs. The replacement of NPs and NCFs by correspondingnanotube or nanorod arrays would significantly improve thePC properties of the photoanodes.

Research on visible light responsive titania-basednanostructures sensitized by dyes and QDs began a long timeago, is still actively ongoing at the present time and willcontinue to be developed in the near future. Many interestingexperiments have been done and were reviewed in section 4.As sensitizers, many types of dyes and QDs have been used.The sensitizing dyes were usually attached onto the surfacesof the titania nanomaterials, but they might also be thecomponents of the electrolytes surrounding the photoanodes.In the latter case, sensitizing dyes were degraded (as thepollutants) or decomposed (as the fuels) to generate electricityand/or hydrogen. Among sensitizing QDs, the most preferableone is CdSe QD. With respect to photocorrosion, this QD ismore stable than CdS and CdTe QDs, but there still existsthe question: how stable is it, how long is the lifetime of aQDSSC, for example, using a CdSe QD? In general, a QDSSCas well a PC or PEC cell sensitized by a QD can achieve a highefficiency only if it has a suitable complex structure composedfrom different elements whose working mechanisms arebased on different physical phenomena and processes or oncatalytic reactions at the interfaces between the electrolyteand the electrodes. Lots of physical and chemical fundamentalresearch works should be done in order to achieve successin fabricating perfect QD sensitized PEC cells for solarenergy conversion or for pollutant degradation. In general theworking mechanisms of visible light responsive QD sensitizedtitania-based nanostructures are more complicated than thoseof doped titania nanomaterials. In most works reviewedin section 4 the QD sensitized titania-based nanostructureswere prepared mainly on the basis of pure titania NPsor NCFs. Recently the technology for fabricating titaniananotube, nanowire and nanorod arrays was elaborated. Thereplacement of NPs or NCFs by one-dimensional titaniananomaterials would enhance the PC activity of the preparedphotoanodes. The SPR in the metallic NPs deposited ontothe surfaces of titania-based nanomaterials or nanostructureswould significantly enhance their PC activities with respectto the sunlight illumination. In brief, a promising and widefield of fundamental physical and chemical research on visiblelight responsive QD-sensitized titania-based nanostructuresfor photovoltaic, photocatalytic and photoelectrochemicalapplications was being opened.

Acknowledgment

The authors would like to thank Vietnam Academy of Scienceand Technology for the support.

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