Review Article A Review of the Synthesis and ...downloads.hindawi.com/journals/jnm/2016/5320625.pdfhave used electrochemical deposition of ZnO nanorods on the rGO where the conductivity

Review ArticleA Review of the Synthesis and Photoluminescence Properties ofHybrid ZnO and Carbon Nanomaterials

Protima Rauwel1 Martin Salumaa1 Andres Aasna1

Augustinas Galeckas2 and Erwan Rauwel1

1Tartu College Tallinn University of Technology Puiestee 78 51008 Tartu Estonia2Department of Physics University of Oslo PO Box 1048 Blindern 0316 Oslo Norway

Correspondence should be addressed to Protima Rauwel protimarauwelttuee

Received 8 July 2016 Accepted 29 August 2016

Academic Editor Stefano Bellucci

Copyright copy 2016 Protima Rauwel et alThis is an open access article distributed under theCreative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Photoluminescent ZnO carbon nanomaterials are an emerging class of nanomaterials with unique optical properties They eachZnO and carbon nanomaterials have an advantage of being nontoxic and environmentally friendlyTheir cost-effective productionmethods along with simple synthesis routes are also of interest Moreover ZnO presents photoluminescence emission in the UVand visible region depending on the synthesis routes shape size deep level and surface defects When combined with carbonnanomaterialsmodification of surface defects inZnOallows tuning of these photoluminescence properties to produce for examplewhite light Moreover efficient energy transfer from the ZnO to carbon nanostructures makes them suitable candidates not only inenergy harvesting applications but also in biosensors photodetectors and low temperature thermal imagingThis work reviews thesynthesis and photoluminescence properties of 3 carbon allotropes carbon quantum or nanodots graphene and carbon nanotubeswhenhybridizedwithZnOnanostructures Various synthesis routes for the hybridmaterialswith differentmorphologies of ZnOarepresented Moreover differences in photoluminescence emission when combining ZnO with each of the three different allotropesare analysed

1 Introduction

ZnO has been one of the most studied materials for over twodecades Being a semiconductor material with a band gap of337 eV it luminesces in the blue and ultraviolet region of thevisible spectrum ZnO is an attractive material due to easyaccessibility of raw materials low production costs and itsnontoxicity properties On the other hand ZnO has a supe-rior advantage in nanotechnology as it is easy to synthesize asnanostructures [1 2] During the last decade many reportshave been published on the synthesis of ZnO nanoparticles[3] nanoflakes [4] hexagonal prismatic rods [5] nanoflowers[6] nanorods [7] nanopillars or nanosheets using multiplemethods of synthesis [8] These studies have shown that themorphology of ZnO also plays a very important role in deter-mining the physical properties of the material Presently themodification of the optoelectronic properties is the currentarea of focus and one of these methods includes doping withrare earths as well as cationic or anionic substitutions [9 10]

However the optical properties also vary as a function ofthe morphology Furthermore the optical properties of bulkZnO can be modified by nanostructuring via the increase ofnative (intrinsic) defects in the crystalline structure or viasize reduction generating in turn a substantial increase of thesurface to volume ratio thereby producing a stronger surfacedefect related emission For example rose like ZnO exhibitsan enhancement in fluorescence properties [11] Rhombusshaped ZnO nanostructures applied to dye sensitized solarcell have manifested an enhancement in the short circuitcurrent [12] A recent study has also shown that ZnOnanoparticles can exhibit interesting optical properties withappropriate choice of precursors [13] therefore justifyingthat by engineering O and Zn vacancies it is possible to varythe photoluminescence emission of ZnO However the largeband gap of ZnO limits its use as a UV light photocatalyst asonly 5of sunlight is captured byZnO So shifting the opticalresponse to the visible range should increase its photocat-alytic efficiencies These methods include doping with metal

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2016 Article ID 5320625 12 pageshttpdxdoiorg10115520165320625

2 Journal of Nanomaterials

nonmetals and the synthesis of heterostructures [14 15]However ZnO nanoparticles or quantum dots on their ownmay not serve very well for electron transport mainly dueto two reasons firstly there may not be sufficient contactbetween the particles themselves and secondly even if thereis contact between them the formation of grain boundarieswould hinder electron transport and deteriorate possibledevice fabrication However due to its intrinsic propertiesand abundance ZnO is nevertheless widely investigatedfor large-scale activities such as purifying water and air[8 16]

Another means to enhance the photoluminescent prop-erties involves the hybridization of the material with carbonbased nanomaterials such as carbon nanotubes (CNT) car-bon quantum dots (CQDs) or graphene among the carbonallotropes [17 18] In fact carbon based structures such asfullerenes graphene CNT and CQDs have been investigatedfor the enhancement of performances of photodetectors andphotovoltaic devices [19ndash23] In recent years carbon basednanostructures have sparked tremendous research interestdue to their superior chemical physical mechanical andelectronic properties [24] Since they are abundant and havebeen successfully shown to be biocompatible they have beenintegrated into bioimaging medical diagnosis catalysis andoptoelectronic applications [25ndash28]

Among the various carbon allotropes CQD or carbonnanodots have emerged as a rising candidate for manyapplications Due to their high quantum yield [29] lowcytotoxicity [30] high photostability [31] and nonblinkingcharacter [32] they are seen as potential candidates forapplications in photovoltaics photocatalysis [33] and lightemitting diodes [34] The significantly large amount of 120587electrons moreover makes them viable for hot carrier solarcells [35] Yu et al have recently demonstrated that graphene-CQDs have this potential contrary to CNT-CQD or TiO

2-

CQDs where charge transfer in the latter two was absent [36]Graphene on the other hand has shown enormous poten-

tial in the fabrication of solar cells In one instance grapheneis used as a charge conducting layer in a solar cell device[37] Some other studies have reported that graphene can beused as an electrode for dye-sensitized and organic solar cells[38ndash41] Similarly carbon nanotubes (CNT) or cylindricalfullerenes have interesting optical properties because theirexcitation gives rise to strongly bound excitons [42] CNTare classified into single-wall carbon nanotube (SWCNT)and multiwall carbon nanotube (MWCNT) SWCNT findsapplications in the electronic industry considering theirhigh electron mobility and also in biomolecule sensors [43]SWCNT have also been studied as a possible solar cellmaterial [44] Additionally MWCNT find applications ascounterelectrodes in dye sensitized solar cells [45 46]

This review article aims at bringing together variousZnO-nanocarbon hybrid materials Three major allotropesof carbon nanomaterials namely CNT graphene and CQDhybridizing with nano-ZnO are analysed This work alsoevaluates and compares the photoluminescence propertiesof these hybrid materials in terms of their defects and mor-phologies determined by various growth techniques whichare also reviewed and assessed

2 Synthesis and MorphologicalCharacterization of ZnO CarbonBased Nanostructures

21 ZnO-CQD Synthesis ZnO-CQD nanohybrids have beenprepared by various groups Yu et al have used hydrothermalsynthesis by introducing Zn(Acetate)

2 2H2O (025M) in an

alcoholic solution containing CQD for 8H at 100∘C [47]Theresulting nanohybrids shown in Figure 1 are indeed ZnO-CQD nanocomposite with the CQD encapsulating the ZnOnanoparticle Different morphologies of ZnO nanoparticleshave been combined with CQD Zhang et al have usedelectrospinning hydrothermal synthesis and have obtainedZnO nanoflowers They subsequently dispersed these ZnOnanoflowers in a solution containing CQD The nanohybridmaterial was then dried at 80∘C [48] In their study theCQDs were themselves prepared via a green synthesis routeusing sucrose On the other hand the ZnO nanoflowerswere prepared via electrospinning of zinc acetate dehydratedat 470∘C followed by hydrothermal synthesis of the Znnanospheres using a solution of zinc nitrate hexahydrate andhexamethylenetetramine at 95∘C The nanocomposite wassubsequently synthesized by dispersing CQD dots in wateralong with different concentrations of ZnO nanoflowers

Another green synthesis approach consists of using D-fructose and NaOH The nanocomposite was then preparedby hydrothermal synthesis of Zn(CH

3COO)

2sdot2H2O in

the presence of CQD at 80∘C [49] Other approachesof combining ZnO and CQD using sol-gel methodsalso exist ZnO mesoporous films have been studied bySuzuki et al but contrary to the above-mentioned studiesthe CQDs were synthesized by less environmentallyfriendly methods such as ArgininesdotHCl and 12-ethyl-endiamine The solution for ZnO was produced usingZn(CH

3COO)

2sdot2H2OEtOHH

2OZonyl and then mixed

with various concentrations of CQD [50] The morphologyof the hybrid nanocomposite consisted of agglomeratednanocrystals of ZnO blended with CQDs

Other methods to grow CQD comprise reacting graphitewith HSO

4for a longer period of 6 days and subsequently

combining them with ZnO prepared by sol-gel methodsusing Zn(NO

3)2sdot6H2O [51] Muthulingam et al have used N

doped ZnO via low temperature hydrothermal synthesis at60∘CTheCQDswere isolated by combining carbon black inkH2SO4andHNO

3at 240∘C for 2 hours [52] Pyrolysis has also

been used byMa et al for the synthesis of these nanocompos-ites which consisted of hydrothermal synthesis of ZnO andpyrolysis of a metal organic framework as precursor Hereoxybisbenzoic acid zinc nitrate hexahydrate 441015840-bipyridineandNN-dimethylformamidewere combined [53] Currentlyporous materials are regarded as attractive materials forphotocatalytic activity due to their large surface areas Veryrecently Ding et al have combined ZnO foam with CQD[54] In their work they synthesized the ZnO foam witha one-step combustion method where Zn(NO

3)2sdot6H2O was

dissolved into ethylene glycol monomethyl ether and burntat 120∘C They also obtained CQD via a green hydrothermalsynthesis route using sucrose The nanocomposite was thenformed by dispersing the ZnO foam into the CQD solution

Journal of Nanomaterials 3

(a) (b)

(c) (d)

Figure 1 (a) SEM and (b) TEM images of the nanohybridmaterial (c) and (d) TEM images of pure ZnO and pure CQD respectively (partiallyreproduced from [46] copyright licence number 3940681292519)

22 ZnO-Graphene Synthesis For the synthesis of reducedgraphene oxide (rGO) ZnO nanocomposites the first stepinvolves the reduction of the graphene oxide (GO) withhydrazine also known as one-step reduction process Whenfollowed by thermal annealing on a quartz substrate it isknown as a two-step reduction process [55] The differencein the morphology of the reduced graphene oxide (rGO)via a one-step or two-step process is mostly depicted in theroughness of the films where a two-step reduction processproduces a smoother surface with a few cracks and folds[56 57] Other than reducing graphene oxide graphene canbe produced by other methods described in this sectionThe synthesis of ZnO-graphene composites is very wellstudied in the literature Synthesis techniques as simple asultrasonication of ZnO with rGO are available [58] Liuet al have used microwave assisted synthesis [59] wherecommercial graphitewas first oxidized viaHummersrsquomethod[60] GO suspension was added to ZnSO

4and sonicated

to homogenise the solution NaOH was further added toincrease the pH to 9 and was subject to microwaves ThepH of 9 was found to be optimal for the precipitation ofZnO

Additionally different morphologies of ZnO have beenused to fabricate ZnO-graphene nanocomposites Yin et alhave used electrochemical deposition of ZnO nanorods onthe rGO where the conductivity of the rGO determined thegrowth rate of ZnO nanorods In effect the conductivity

of graphene is instrumental in producing hydroxyl ionswhich promote the growth of ZnO rods [57] CVD growthof graphene on Ni and Cu substrates is also commonlypractised Dong et al have obtained ZnO nanorod-graphenenanocomposite via hydrothermal synthesis of ZnCl

2at 120∘C

[61] Yi et al have used hydrophilic plasma-etched ZnOnanorods that were subsequently dispersed in a solutioncontaining distilled water and CVD grown graphene [62]Other ZnO-graphene nanocomposites have also been syn-thesized where ZnO either was deposited by CVD [63] orwas hydrothermally grown as nanorods [64 65] onto GOobtained via Hummers method Sol-gel methods have beenused to grow ZnO nanoparticles that were subsequentlyspin coated onto graphene and thermally annealed [66] Thespin coating and annealing were performed twice before thesubstrate with ZnO seeds on the surface was dipped into zincnitrate hexahydrate and hexamethylenetetramine at 90∘C for90min thereby producing nanorods

Flower shaped ZnO-graphene nanocomposites have alsobeen prepared by various groups Xu et al have reducedGO with hydrazine and hydrothermally synthesized ZnOnanoflowers in the presence of graphene [67] Hummersrsquomethod to prepare graphene followed by hydrothermalsynthesis of ZnO nanoflowers has also been conducted byvarious groups [68] CVD growth of graphene followed byelectrochemical deposition of ZnOnanoflowers has also beencarried out [69]


23 CNT-ZnO Synthesis Carbon nanotubes and ZnOnanocomposites have been prepared by various techniquesIn the literature reports of CNT-ZnO hybrids prepared byball milling are available [70] Zhang et al deposited ZnOnanodots onto the CNT films by ultrasonic spray for applica-tions as electrodes [71] 3D structures of ZnO CNT have alsobeen grown by many groups [72] Chemical precipitation ofZnO on SWCNT was carried out by Paul et al [73] Pulsedlaser deposited (PLD) ZnO on MWCNT was synthesizedvia a hydrothermal method by Saleh et al [74 75] A largemajority of the synthesis techniques use a functionalizingagent to prepare the CNT for decoration The acid treatmentdebundles the nanotubes and disposes functional groups ontheir sidewalls which serve as anchorage points for the ZnOnanoparticles Aziz et al have dispersed CNT in sodiumdodecyl sulphate followed by ball milling the ZnO that waspreviously obtained by hydrothermal synthesis [76] CNThave also been grownby spray pyrolysismethods and purifiedthereafter to get rid of the iron impurities they were thensintered at 700∘C with ZnO prepared by oxidizing Zn dust atvery high temperatures under oxygen rich conditions [77]Another type of ZnO-CNT hybrid structure has been syn-thesized via Atomic Layer Deposition (ALD) Here coatingof ZnO is applied to the outer wall and also to the inner wallsof the CNT Depending upon the inert or functional groupspresent on the CNT the morphology of the ZnO coating canbe varied Li et al have obtained a smooth conformal coatingof ZnO on vertically aligned CNT using diethyl zinc andwater as Zn and O sources respectively [78] Kim et al haveobtained nanoparticle like ZnO coating on the CNT givinga rough appearance to the nanohybrid material [79]

3 Photoluminescence of ZnO Carbon BasedStructures

31 Photoluminescence of ZnONanoparticles ZnOon its ownluminesces in the visible andUV regions In the visible regionthe emission is attributed to intrinsic defects induced duringthe synthesis itself The UV emission is due to excitonicrecombination [80 81] Various luminescence emissions inthe visible region are possible depending upon the synthesisprocedure morphology vacancies and surface defects [82]Rauwel et al have studied the influence of hydrate precursorin nonaqueous sol-gel methods and have shown that the PLemission can be tuned by using the appropriate precursor(Figure 2) [13] Moreover they have also shown that theembedment of ZnO nanoparticles in a metal oxide matrixproduces differences in the PL response due to the passivationof the surface defects [83] Various colored emissions havebeen obtained for ZnO orange [84] blue [85] green and red[86]

32 Photoluminescence of ZnO and NanocarbonHybrid Materials

321 ZnO-Graphene Nanohybrids Combining ZnO withgraphene has produced various photoluminescence emis-sions Blue and red emissions have been observed in graphene

Photon energy (eV)

DRS 300 K

320 eV 328 eV

500 600 700 800400

Wavelength (nm)

161822224262833234

101

102

103

104

105

106

107

PL in

tens

ity (c

ount

ss)

Nor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530

Photon energy (eV)[Zni]

[VZn] [VO]

0102Reference bulk ZnO

DRS 300 K

320 eVe 328 eVeNor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]

PL 8K

Figure 2 PL spectra at 8 K from ZnO nanoparticles producedusing zinc acetylacetonate hydrate (curve 01 type-A NPs) and zincacetate (curve 02 type-B NPs) along with the reference spectrumof bulk ZnO (grey curve) Inset shows optical band edges of thecorresponding nanoparticles estimated from the diffuse-reflectancespectra (DRS) at 300K (reproduced with permission from [13])

alone [87 88] Khenfouch et al [89] have succeeded inobtainingwhite light emission fromZnOnanorodhybridizedwith few layers of graphene as shown in Figure 3 At roomtemperature (RT) excitation of 280 nm in the deep UV theyhave observed several PL emission wavelengths and havebeen able to attribute them to various defects For examplethe green luminescence at 524 nm has been attributed toradiative recombination of simple ionized oxygen vacanciesThis is very commonly observed in oxygen deficient ZnO[13] The yellow-orange luminescence is typical of sol-gel orhydrothermal methods [90] They have attributed the emis-sions at 482 nm and 498 nm to isolated sp2 clusters withinthe carbon-oxygen sp3 matrix Moreover poor dispersionor aggregation of graphene flakes gives rise to emissionsat 684 nm and 686 nm Kwon et al have illustrated anincrease in the PL intensity corresponding to the free excitontransition (FEE) at 375 nmwith increase inGO concentrationin their sample Biroju et al have grown vertically alignedZnO on graphene and have compared the PL emission beforeand after annealing the hybrid material [91] They noticeda significant improvement in the UV and visible PL onannealing The reduction in the green and yellow emissionswas attributed to the reduction in deep level defects Ozn orneutral Oi The yellow emission was attributed to a reductionin ionized Oi

Band gap tuning of graphene-ZnO hybrid structures wasstudied by Singh et al [92]They observednot only a blue shiftin their photoluminescence emission but also a quenchingeffect on increasing the rGO concentration They attribute


PL in

tens

ity (a

u) 382

482

610

684

686

400 450 500 550 600 650 700 750 800350

Wavelength (nm)

RGO (visible)FLGZnO (UV)FLGZnO (visible)

524498

Figure 3 White light emission from CQD-ZnO hybrid structuresKhenfouch et al (reproduced from [89] copyright licence number3940721191296)

both phenomena to a depletion region being formed at theinterface of the nanocomposite arising due to the p-typeconductivity of ZnO and n-type conductivity of grapheneIn fact a green light emitting diode has been fabricated byWu et al [93] where they combine ZnO nanowires andgraphene Under forward bias conditions at 6V they observea green emission and a reduced UV emission Here theinjected holes from graphene combine with electrons locatedin defects in ZnO Since a nanowire has a large surface tovolume ratio implying that the majority of defects are onthe surface this suggests that the interface between grapheneand ZnO plays an important role in the emission propertiesof the material Quenching of emission from surface statesof ZnO by passivating it with a layer of rGO has also beenstudied by Han et al [94ndash96] Nevertheless the contrary wasobserved for the UV emission which was enhanced owingto the increase in sp2 carbon This phenomenon is furtherreinforced by Lee et al who used nonoxidized graphene withZnO and studied their photoluminescent properties [97]Their study provides fundamental understanding of the effectof oxygen filling effect of graphene when hybridized withZnOThe FEE was decreased for ZnO nonoxidized graphenesamples on increasing the nonoxidized graphene proportionThey also observed a deep level emission at 530 eV whichwas attributed to interband transitions from Zn interstitialsto O vacancies Another possible effect was also the excitonicrecombination from the conduction band to oxygen defectband gap state However for the GO-ZnO the effect on FEEemission was the opposite Moreover other than the forma-tion of a p-n junction another possible explanation to theblue shift was a reduction in oxygen defects when combinedwith GO which was also translated by a reduction of thegreen visible luminescence Furthermore indirect excitation

of zinc commonly termed as sensitization or ldquoantennardquo effect[98] has also been observed when combining ZnO withgraphene In fact Han et al have observed an increase inthe PL luminescence coming from the hybrid material dueto an antenna effect compared to ZnO alone They attributeit to the resonant excitation of graphene plasmon and theirconversion into photons that propagate to the conjugatedZnO surface [94 99]

322 ZnO-CQD Nanohybrids Carbon quantum dots orgraphene quantum dots emanate a wide range of lumines-cence in the visible region depending upon their size andsurface defects Moreover their photoluminescence emissionvaries as a function of the excitation wavelength [100ndash102]In the PL spectra presented in Figure 4(a) a blue shift isobserved when excited with higher wavelengths [103] Thisalso indicates that the CQD can be excited with visiblewavelengths As presented above nanosized ZnO presents adefect related emission in the blue-green part of the visiblespectrum Such a hybrid structure therefore becomes a goodcandidate for white light emission [104] Overlapping ofenergy bands of the two materials has been successful inproducing a charge transfer between them when the hybridmaterial is excited in the visible or UV regions [47 104]

In Figure 4(b) a significant peak at 550 nm correspondingto deep level defects in ZnO is visible along with its bandedge emission at 375 nm However the emission intensitiesof ZnO-CQD hybrid show a heightened intensity implyingenergetic interactions in such systems It also shows quench-ing in the defect related emission when the ratio of ZnO C isat 4 1 Suzuki et al [105] have explained that the quenchingof the FEE as a function of CQD increase in the sample couldbe attributed to the internal absorption with higher CQDconcentration In their samples they also present orangeluminescence at 185 eV emanating from deep level defectsOzn or neutral Oi and it is a result of their synthesis routeThey also observe a blue shift in the defect level emissionfrom 18 eV to 22 eV with increase in CQD concentrationThis implies that the oxygen interstitial related defects nolonger emit the orange luminescence and the contributionswere mostly from surface defects of ZnO and CQD asindicated by the green luminescence This also implies thatthe addition of CQD increases the oxygen vacancies inZnO by the reduction reaction In a previous work Heet al have explained the quenching of luminescence withincrease in CQD by the lack of dispersion and formationof CQD aggregates which reduces contact between ZnOand CQD thereby quenching the PL spectrum [104] Themechanisms for the various colored emissions in the visibleregion are provided in Figure 5 Moreover they also observedwhite light emission for intermediate concentrations of CQDalong with a nonlinear increase in CQDZnO PL ratio withthe incorporation of CQD Such a nonlinear interaction isindicative of excitation transfer through dipole interactionsUsing the energy level structure of N doped CQD of Tanget al [106] they have compared the band gap of ZnO to theHOMO-LUMO states in CQD and have suggested a chargetransfer from conduction band of ZnO to the O 120587lowast LUMO ofCQD via dipole resonance mechanisms


450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)

490nm470nm450nm430nm

420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO

ZnOC = 3 1ZnOC = 2 1

(b)

Figure 4 (a) Photoluminescence emissions from CQD when excited at different wavelengths (b) PL emission spectra of ZnO-CQD withdifferent ZnO CQD proportions (reproduced from [103] under Creative Free Commons)

Defect related deep level emissions

Orange emission

Orange emission Orange + green emissions Green emission

Near-edge band of ZnO and C-dot emissionsZnO

ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg

C-dots concentration5ndash7mg


C-dots concentration

NH2

eminus

Figure 5 Defect related deep level emissions and various visible colored luminescent emissions as a function of CQD concentration(reproduced from [105] with copyright licence number 3940691218874)

323 ZnO-CNT Nanohybrids There have been several theo-retical and experimental studies related to the defect relatedluminescent properties enhancement of CNT-ZnO structure[107 108] Depending on the method of the ZnO grown withregard to the defects present [109] determined also by itsmorphology namely flower rod and dots the characteristicgreen emission of ZnO is either intensified or quenchedFor example Gupta et al noticed enhancement of thephotoluminescence emission in their hybrid structure at510 nm compared to ZnO alone [77] This enhancement was

explained by enhancement of oxygen vacancy related defectsat the interface of ZnO-CNT Considering the large surfaceto volume ratio the green emission becomes dominantOn the other hand in lily-like-ZnO a broad yellow-greenemission (ionized oxygen vacancy oxygen vacancy or Zninterstitial) band alongwith a strong near band edge emissionis indicative of a high quality ZnO sample [110] Whencombined with ZnO the red shift of the FEE band wasattributed to excitonic recombination in shallow traps orsurface states created on hybridizingMoreover quenching of


(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)

Figure 6 (a) TEM image of ZnO nanoparticles synthesized using zinc acetate at 300∘C in the presence of CNT and (b) PL spectra at RT

the visible luminescence or filling of defects when combinedwith CNT along with a transfer of photoexcited charges fromZnO to CNT empty states has also been reported Stabilityof these hybrid structures is also of importance as studied byZhu et al [111] They studied ZnO nanoparticles with a highdensity of stacking faults hybridized to CNT on growth andafter 24 days In both cases their PL spectra were identicalindicating the stability of these structures They both con-sisted of emissions at 376 nm and 490 nm After annealingthe peak at 490 nm became more intense and blue shiftedto 485 nm indicating that new defects such as O and Zn(peak at 632 nm) vacancies were formed through a probableconversion of stacking faults into oxygen vacancies All of theabove were studied under band gap excitation In generaleach peak has its significance and a band edge emissionintensity helps identify the crystalline quality of ZnO usedYellow-green emission corresponds to a recombination of aphotogenerated hole and an ionized oxygen vacancy [112]The FEE emission enhancement not only is a function ofcrystallinity but is also enhanced by the surface plasmonemission effects of CNT [113] Moreover ZnO-CNT systemsexhibit charge transfer from ZnO to CNT manifested byenhanced quenching of the luminescence and very shortlifetime as compared to bulk ZnO alone [114] as determinedby time resolved PL In order to better understand all thephenomena that occur at the interface between ZnO NPsand CNT theoretical calculations have been performed byChai et al to model the charge transfer from ZnO to metallicCNT [115] More recently ZnO carbon based nanohybridshave been synthesized by nonaqueous sol-gel method Oneobserves that ZnOnanoparticles with sharp edges are directlyconnected to the CNT network (Figure 6(a)) The directsynthesis of ZnO nanoparticles in the presence of CNTand their bonding to CNT promote the green emissionusually exhibited by oxygen vacancies (Figure 6(b)) Thecomplete result of these investigations will be published in aforthcoming paper and cannot be discussed in this review

4 Conclusions

The technological advancement in the synthesis and charac-terization of metal oxide nanomaterials is a rapidly growingfield Zinc oxide is a very versatile optical material having arelatively wide range of UV absorption and demonstratingphotostability Moreover being nontoxic therefore makes itbiocompatible ZnO nanomaterials can be synthesized viaa broad range of chemical of physical methods that enableobtaining different morphologies Defect engineering orsurface modification by choosing appropriate synthesis tech-niques is a way of tailoring the photoluminescence propertiesof the material that has been demonstrated in numerousreports ZnO has already been exploited by industry withapplications in photocatalysis or in sunscreens which aresolar filters that block all ultraviolet radiations

The interest in ZnO has not only grown but withthe advent of the abundant and ecofriendly carbon basednanomaterials hybrid ZnO carbon based materials havearoused the curiosity of several researchers This work hasfocused on the three widely used carbon based materialsthat combine with ZnO nanoparticle graphene CNT andcarbon quantum dots However the synthesis of ZnO hybridfor each carbon based material is different The synthesisis a minimum 2- or 3-step procedure with synthesizing thecarbon nanomaterial as a first step The second step consistsusually of growing the ZnO nanomaterial and the third stepinvolves combining them Sometimes a fourth step suchas postsynthesis annealing is necessary when passivatingdefects A two-step method reduces the number of steps bygrowing the ZnO directly on the carbon based material as asecond step

CNT are usually grown by CVD methods on metalcatalyst nanoparticles such as Ni Furthermore as-grownCNT contain a large number of impurities Treating theCNT with acid or heat has a twofold advantage firstly itgets rid of the residual impurities in the as-grown sample


and secondly it creates functional groups on the side wallof the CNT which facilitate their decoration with variousnanoparticles A more environmentally friendly technique topurify the CNT is sonication ZnO can also be directly grownvia for example an Atomic Layer Deposition (ALD) coatingon CNT among other techniques CQD used in ZnO-CQDhybrid materials on the other hand can be synthesized bygreen synthesis routes for instance by reacting D-fructosewith NaOH or via sucrose Green synthesis routes have beendeveloped as a more economical and valuable alternative forthe large-scale production of metal nanoparticles to avoidthe use of the generation of hazardous substances and canbe based on the utilization of plant extracts or moleculesproduced by plants thatmay act as both reducing and cappingagents for example Less ecofriendly techniques by acidtreating graphite are also used in the production of CQDIn graphene-ZnO hybrid material rGO is obtained fromgraphene oxide in one or two steps GO is usually reducedvia hydrazine and could be followed by a thermal treatmentGO is usually obtained fromHummersrsquo method CVD growngraphene on Ni and Cu is also used in the hybrid mixOther techniques combine the carbon nanomaterial withZnO with the latter having been prepared by spray pyrolysisball milling hydrothermal synthesis or spin coating

These hybrid materials present photoluminescence prop-erties which vary as a function of the carbon based materialsused In fact the emission from ZnO either UV or visiblecan be engineered by passivating or generating defects atthe interface of the hybrid materials considering their largesurface to volume ratios Characteristic emissions from thesematerials such as red blue green yellow and orange cantherefore be enhanced or quenched In some cases white lightemission has also been obtainedMoreover charge transfer inthese materials due to overlapping bands in the excited statemakes them highly suitable for photocurrent generation andnext generation of solar cells including flexible transparentphotovoltaics Carbon and ZnO are ecofriendly biodegrad-able and omnipresent Even though more ecofriendly syn-thesis routes exist for the carbon basedmaterials nonethelessvery few reports where the former are applied to the synthesisof hybrid ZnO-carbon based nanomaterials are available Forexample presently acid treatments of CNT and graphite andthe use of hydrazine which are all very toxic are of commonusage Nevertheless being of very high potential in variousoptoelectronic applications new and green routes for theirsynthesis are the need of the hour

Competing Interests

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

Acknowledgments

The authors wish to acknowledge the Estonian ResearchCouncil (Grant PUT431) the European Regional Develop-ment Fund Project TK134 (TAR16019) MENESR MAEDIFrench Ministries (Parrot Program no 33787YJ) and theNorwegian Research Council The NAMUR Project of the

European Regional Development Funds is acknowledged foraccess to the FEI Titan G2 80-200

References

[1] X-Y Liu C-X Shan C Jiao S-P Wang H-F Zhao and D-Z Shen ldquoPure ultraviolet emission from ZnO nanowire-basedp-n heterostructuresrdquoOptics Letters vol 39 no 3 pp 422ndash4252014

[2] N Gogurla A K Sinha S Santra S Manna and S KRay ldquoMultifunctional Au-ZnO plasmonic nanostructures forenhanced UV photodetector and room temperature NO sens-ing devicesrdquo Scientific Reports vol 4 article 6483 2014

[3] R Jalal E K Goharshadi M Abareshi MMoosavi A Yousefiand P Nancarrow ldquoZnO nanofluids green synthesis charac-terization and antibacterial activityrdquo Materials Chemistry andPhysics vol 121 no 1-2 pp 198ndash201 2010

[4] A Singh and P Kumar ldquoStructural morphological and opticalproperties of sol gel processed CdZnO nanostructured filmseffect of precursor solventsrdquo International Nano Letters vol 3article 57 2013

[5] A Stankovic S Dimitrijevic and D Uskokovic ldquoInfluence ofsize scale and morphology on antibacterial properties of ZnOpowders hydrothemally synthesized using different surfacestabilizing agentsrdquo Colloids and Surfaces B Biointerfaces vol102 pp 21ndash28 2013

[6] N Talebian S M Amininezhad and M Doudi ldquoControl-lable synthesis of ZnO nanoparticles and their morphology-dependent antibacterial and optical propertiesrdquo Journal ofPhotochemistry and Photobiology B Biology vol 120 pp 66ndash732013

[7] E E Hafez H Shokry Hassan M F Elkady and E SalamaldquoAssessment of antibacterial activity for synthesized zinc oxidenanorods against plant pathogenic strainsrdquo International Jour-nal of Scientific amp Technology Research vol 3 no 9 pp 318ndash3242014

[8] A Kołodziejczak-Radzimska and T Jesionowski ldquoZinc oxidemdashfrom synthesis to application a reviewrdquoMaterials vol 7 no 4pp 2833ndash2881 2014

[9] S Major A Banerjee and K L Chopra ldquoOptical and electronicproperties of zinc oxide films prepared by spray pyrolysisrdquoThinSolid Films vol 125 no 1-2 pp 179ndash185 1985

[10] J Hu andR G Gordon ldquoTextured fluorine-doped ZnOfilms byatmospheric pressure chemical vapor deposition and their usein amorphous silicon solar cellsrdquo Solar Cells vol 30 no 1ndash4 pp437ndash450 1991

[11] Y Gong C Zou Y Yao et al ldquoA facile approach to synthesizerose-like ZnOreduced graphene oxide composite fluorescenceand photocatalytic propertiesrdquo Journal of Materials Science vol49 no 16 pp 5658ndash5666 2014

[12] Y Zhu F Deng L Feng H Ding S Ismat Shah and C NildquoHierarchical rhombus-shapedZnO array synthesis formationmechanism and solar cell applicationrdquo Journal of Alloys andCompounds vol 607 pp 132ndash138 2014

[13] E Rauwel A Galeckas P Rauwel M F Sunding and H Fjel-lvaag ldquoPrecursor-dependent blue-green photoluminescenceemission of ZnO nanoparticlesrdquoThe Journal of Physical Chem-istry C vol 115 no 51 pp 25227ndash25233 2011

[14] S G Kumar and K S R K Rao ldquoZinc oxide based photo-catalysis tailoring surface-bulk structure and related interfacialcharge carrier dynamics for better environmental applicationsrdquoRSC Advances vol 5 no 5 pp 3306ndash3351 2015


[15] F Barka-Bouaifel B Sieber N Bezzi et al ldquoSynthesis and pho-tocatalytic activity of iodine-doped ZnO nanoflowersrdquo Journalof Materials Chemistry vol 21 no 29 pp 10982ndash10989 2011

[16] A B Djurisic X Chen Y H Leung and A Man ChingNg ldquoZnOnanostructures growth properties and applicationsrdquoJournal of Materials Chemistry vol 22 no 14 pp 6526ndash65352012

[17] D Eder ldquoCarbon nanotube-inorganic hybridsrdquo ChemicalReviews vol 110 no 3 pp 1348ndash1385 2010

[18] P Rauwel A Galeckas M Salumaa F Ducroquet andE Rauwel ldquoRevealing photocurrent generation in hybridnanocomposite combining carbon nanotubes and HfO2 cubicphase nanoparticlerdquo Beilstein Journal of Nanotechnology vol 7pp 1075ndash1085 2016

[19] S-H Cheng T-M Weng M-L Lu W-C Tan J-Y Chenand Y-F Chen ldquoAll carbon-based photodetectors an eminentintegration of graphite quantum dots and two dimensionalgraphenerdquo Scientific Reports vol 3 article 2694 2013

[20] G Konstantatos M Badioli L Gaudreau et al ldquoHybridgraphene-quantum dot phototransistors with ultrahigh gainrdquoNature Nanotechnology vol 7 no 6 pp 363ndash368 2012

[21] K Tvrdy P A Frantsuzov and P V Kamat ldquoPhotoinducedelectron transfer from semiconductor quantum dots to metaloxide nanoparticlesrdquo Proceedings of the National Academy ofSciences of the United States of America vol 108 no 1 pp 29ndash342011

[22] I V Lightcap and P V Kamat ldquoFortification of CdSe quantumdots with graphene oxide Excited state interactions and lightenergy conversionrdquo Journal of the American Chemical Societyvol 134 no 16 pp 7109ndash7116 2012

[23] W Zhang C-P Chuu J-K Huang et al ldquoUltrahigh-gainphotodetectors based on atomically thin graphene-MoS

2het-

erostructuresrdquo Scientific Reports vol 4 article 3826 2014[24] O Kozak M Sudolska G Pramanik P Cıgler M Otyepka

and R Zboril ldquoPhotoluminescent Carbon NanostructuresrdquoChemistry of Materials vol 28 no 12 pp 4085ndash4128 2016

[25] W U Huynh J J Dittmer and A P Alivisatos ldquoHybridnanorod-polymer solar cellsrdquo Science vol 295 no 5564 pp2425ndash2427 2002

[26] J K Jaiswal E R Goldman H Mattoussi and S M SimonldquoUse of quantum dots for live cell imagingrdquo Nature Methodsvol 1 no 1 pp 73ndash78 2004

[27] Y Li Y Hu Y Zhao et al ldquoAn electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaicsrdquo Advanced Materials vol 23 no 6pp 776ndash780 2011

[28] V Gupta N Chaudhary R Srivastava G D Sharma RBhardwaj and S Chand ldquoLuminscent graphene quantum dotsfor organic photovoltaic devicesrdquo Journal of the AmericanChemical Society vol 133 no 26 pp 9960ndash9963 2011

[29] S Zhu Q Meng L Wang et al ldquoHighly photoluminescent car-bon dots for multicolor patterning sensors and bioimagingrdquoAngewandte ChemiemdashInternational Edition vol 52 no 14 pp3953ndash3957 2013

[30] QWang X Huang Y Long et al ldquoHollow luminescent carbondots for drug deliveryrdquo Carbon vol 59 pp 192ndash199 2013

[31] L Cao X Wang M J Meziani et al ldquoCarbon dots formultiphoton bioimagingrdquo Journal of the American ChemicalSociety vol 129 no 37 pp 11318ndash11319 2007

[32] Y-P Sun B Zhou Y Lin et al ldquoQuantum-sized carbon dotsfor bright and colorful photoluminescencerdquo Journal of the

American Chemical Society vol 128 no 24 pp 7756ndash77572006

[33] H Zhang H Huang H Ming et al ldquoCarbon quantumdotsAg

3PO4complex photocatalysts with enhanced photo-

catalytic activity and stability under visible lightrdquo Journal ofMaterials Chemistry vol 22 no 21 pp 10501ndash10506 2012

[34] F Wang Y-H Chen C-Y Liu and D-G Ma ldquoWhite light-emitting devices based on carbon dotsrsquo electroluminescencerdquoChemical Communications vol 47 no 12 pp 3502ndash3504 2011

[35] M LMueller X Yan BDragnea and L-S Li ldquoSlowhot-carrierrelaxation in colloidal graphene quantum dotsrdquo Nano Lettersvol 11 no 1 pp 56ndash60 2011

[36] P Yu X Wen Y-R Toh et al ldquoEfficient electron transfer incarbon nanodot-graphene oxide nanocompositesrdquo Journal ofMaterials Chemistry C vol 2 no 16 pp 2894ndash2901 2014

[37] Z Liu Q Liu Y Huang et al ldquoOrganic photovoltaic devicesbased on a novel acceptor material graphenerdquo Advanced Mate-rials vol 20 no 20 pp 3924ndash3930 2008

[38] X Wang L Zhi and K Mullen ldquoTransparent conductivegraphene electrodes for dye-sensitized solar cellsrdquoNano Lettersvol 8 no 1 pp 323ndash327 2008

[39] J Wu H A Becerril Z Bao Z Liu Y Chen and P PeumansldquoOrganic solar cells with solution-processed graphene transpar-ent electrodesrdquoApplied Physics Letters vol 92 no 26 Article ID263302 2008

[40] G Eda Y-Y Lin S Miller C-W Chen W-F Su and MChhowalla ldquoTransparent and conducting electrodes for organicelectronics from reduced graphene oxiderdquo Applied PhysicsLetters vol 92 no 23 Article ID 233305 2008

[41] X Wang L Zhi N Tsao Z Tomovic J Li and K MullenldquoTransparent carbon films as electrodes in organic solar cellsrdquoAngewandte ChemiemdashInternational Edition vol 47 no 16 pp2990ndash2992 2008

[42] P Avouris M Freitag and V Perebeinos ldquoCarbon-nanotubephotonics and optoelectronicsrdquo Nature Photonics vol 2 no 6pp 341ndash350 2008

[43] A Bianco and M Prato ldquoCan carbon nanotubes be considereduseful tools for biological applicationsrdquo Advanced Materialsvol 15 no 20 pp 1765ndash1768 2003

[44] C Klinger Y Patel and H W C Postma ldquoCarbon nanotubesolar cellsrdquo PLoS ONE vol 7 no 5 Article ID e37806 2012

[45] W J Lee E Ramasamy D Y Lee and J S Song ldquoEfficient dye-sensitized solar cells with catalytic multiwall carbon nanotubecounter electrodesrdquo ACS Applied Materials amp Interfaces vol 1no 6 pp 1145ndash1149 2009

[46] D Benetti K T Dembele J Benavides et al ldquoFunctionalizedmulti-wall carbon nanotubesTiO

2composites as efficient pho-

toanodes for dye sensitized solar cellsrdquo Journal of MaterialsChemistry C vol 4 no 16 pp 3555ndash3562 2016

[47] H Yu H Zhang H Huang et al ldquoZnOcarbon quantum dotsnanocomposites one-step fabrication and superior photocat-alytic ability for toxic gas degradation under visible light atroom temperaturerdquoNew Journal of Chemistry vol 36 no 4 pp1031ndash1035 2012

[48] X Zhang J Pan C Zhu et al ldquoThe visible light catalytic prop-erties of carbon quantum dotsZnO nanoflowers compositesrdquoJournal of Materials Science Materials in Electronics vol 26 no5 pp 2861ndash2866 2015

[49] H Bozetine Q Wang A Barras et al ldquoGreen chemistryapproach for the synthesis of ZnO-carbon dots nanocompositeswith good photocatalytic properties under visible lightrdquo Journalof Colloid and Interface Science vol 465 pp 286ndash294 2016


[50] K Suzuki L Malfatti D Carboni et al ldquoEnergy trans-fer induced by carbon quantum dots in porous zinc oxidenanocomposite filmsrdquoThe Journal of Physical Chemistry C vol119 no 5 pp 2837ndash2843 2015

[51] Y Li B-P Zhang J-X Zhao Z-H Ge X-K Zhao and L ZouldquoZnOcarbon quantum dots heterostructure with enhancedphotocatalytic propertiesrdquo Applied Surface Science vol 279 pp367ndash373 2013

[52] S Muthulingam I-H Lee and P Uthirakumar ldquoHighly effi-cient degradation of dyes by carbon quantum dotsN-dopedzinc oxide (CQDN-ZnO) photocatalyst and its compatibilityon three different commercial dyes under daylightrdquo Journal ofColloid and Interface Science vol 455 pp 101ndash109 2015

[53] Q Ma Z Zhang and Z Yu ldquoSynthesis of carbon quantum dotsand zinc oxide nanosheets by pyrolysis of novel metal-organicframework compoundsrdquo Journal of Alloys and Compounds vol642 pp 148ndash152 2015

[54] D Ding W Lan Z Yang et al ldquoA simple method for preparingZnO foamcarbon quantum dots nanocomposite and theirphotocatalytic applicationsrdquoMaterials Science in SemiconductorProcessing vol 47 pp 25ndash31 2016

[55] X Zhou X Huang X Qi et al ldquoIn situ synthesis of metalnanoparticles on single-layer graphene oxide and reducedgraphene oxide surfacesrdquo The Journal of Physical Chemistry Cvol 113 no 25 pp 10842ndash10846 2009

[56] D K L Tsang B J Marsden S L Fok and G HallldquoGraphite thermal expansion relationship for different temper-ature rangesrdquo Carbon vol 43 no 14 pp 2902ndash2906 2005

[57] Z Yin S Wu X Zhou et al ldquoElectrochemical depositionof ZnO nanorods on transparent reduced graphene oxideelectrodes for hybrid solar cellsrdquo Small vol 6 no 2 pp 307ndash312 2010

[58] T Xu L Zhang H Cheng and Y Zhu ldquoSignificantly enhancedphotocatalytic performance of ZnO via graphene hybridizationand the mechanism studyrdquo Applied Catalysis B Environmentalvol 101 no 3-4 pp 382ndash387 2011

[59] X Liu L Pan T Lv et al ldquoMicrowave-assisted synthesisof ZnO-graphene composite for photocatalytic reduction ofCr(vi)rdquo Catalysis Science amp Technology vol 1 no 7 pp 1189ndash1193 2011

[60] H Li T Lu L Pan Y Zhang and Z Sun ldquoElectrosorptionbehavior of graphene in NaCl solutionsrdquo Journal of MaterialsChemistry vol 19 no 37 pp 6773ndash6779 2009

[61] X Dong Y Cao J Wang et al ldquoHybrid structure of zincoxide nanorods and three dimensional graphene foam forsupercapacitor and electrochemical sensor applicationsrdquo RSCAdvances vol 2 no 10 pp 4364ndash4369 2012

[62] J Yi J M Lee andW I Park ldquoVertically aligned ZnOnanorodsand graphene hybrid architectures for high-sensitive flexible gassensorsrdquo Sensors and Actuators B Chemical vol 155 no 1 pp264ndash269 2011

[63] CWu F Li Y Zhang andTGuo ldquoImproving the field emissionof graphene by depositing zinc oxide nanorods on its surfacerdquoCarbon vol 50 no 10 pp 3622ndash3626 2012

[64] Z Chen N Zhang and Y-J Xu ldquoSynthesis of graphene-ZnO nanorod nanocomposites with improved photoactivityand anti-photocorrosionrdquo CrystEngComm vol 15 no 15 pp3022ndash3030 2013

[65] A R Marlinda N M Huang M R Muhamad et al ldquoHighlyefficient preparation of ZnO nanorods decorated reducedgraphene oxide nanocompositesrdquoMaterials Letters vol 80 pp9ndash12 2012

[66] WM Choi K-S Shin H S Lee et al ldquoSelective growth of ZnOnanorods on SiO

2Si substrates using a graphene buffer layerrdquo

Nano Research vol 4 no 5 pp 440ndash447 2011[67] S Xu L Fu T S H Pham A Yu F Han and L Chen

ldquoPreparation of ZnO flowerreduced graphene oxide compositewith enhanced photocatalytic performance under sunlightrdquoCeramics International vol 41 no 3 pp 4007ndash4013 2015

[68] H R Pant C H Park P Pokharel L D Tijing D S Leeand C S Kim ldquoZnO micro-flowers assembled on reducedgraphene sheets with high photocatalytic activity for removalof pollutantsrdquo Powder Technology vol 235 pp 853ndash858 2013

[69] N S A Aziz T Nishiyama N I Rusli M R Mahmood KYasui andAMHashim ldquoSeedless growth of zinc oxide flower-shaped structures on multilayer graphene by electrochemicaldepositionrdquo Nanoscale Research Letters vol 9 no 1 pp 1ndash92014

[70] O Guler S H Guler F Yo et al ldquoElectrical and optical prop-erties of carbon nanotube hybrid zinc oxide nanocompositesprepared by ball mill techniquerdquo Fullerenes Nanotubes andCarbon Nanostructures vol 23 no 10 pp 865ndash869 2015

[71] Y Zhang X Sun L Pan et al ldquoCarbon nanotubendashZnOnanocomposite electrodes for supercapacitorsrdquo Solid State Ion-ics vol 180 no 32ndash35 pp 1525ndash1528 2009

[72] X-B Yan B-K Tay Y Yang and W Y K Po ldquoFabricationof three-dimensional ZnOminusCarbon Nanotube (CNT) hybridsusing self-assembled CNT micropatterns as frameworkrdquo TheJournal of Physical Chemistry C vol 111 no 46 pp 17254ndash172592007

[73] R Paul P Kumbhakar and A K Mitra ldquoBluegreen lumines-cence by SWCNTZnO hybrid nanostructure synthesized bya simple chemical routerdquo Physica E Low-Dimensional Systemsand Nanostructures vol 43 no 1 pp 279ndash284 2010

[74] T A Saleh M A Gondal and Q A Drmosh ldquoPreparationof a MWCNTZnO nanocomposite and its photocatalyticactivity for the removal of cyanide from water using a laserrdquoNanotechnology vol 21 no 49 Article ID 495705 2010

[75] B Aıssa C Fauteux M A E Khakani and D TherriaultldquoStructural and photoluminescence properties of laser pro-cessed ZnOcarbon nanotube nanohybridsrdquo Journal of Materi-als Research vol 24 no 11 pp 3313ndash3320 2009

[76] S S S A Aziz N I Harun N Salleh et al ldquoMechanochemicalsynthesis of CNTZnO hybrid materialsrdquo Materials ScienceForum vol 846 pp 479ndash483 2016

[77] B K Gupta V Grover G Gupta and V Shanker ldquoHighlyefficient luminescence from hybrid structures of ZnOmulti-walled carbon nanotubes for high performance display applica-tionsrdquo Nanotechnology vol 21 no 47 Article ID 475701 2010

[78] X L Li C Li Y Zhang D P Chu W I Milne and H J FanldquoAtomic layer deposition of ZnO on multi-walled carbon nan-otubes and its use for synthesis of CNT-ZnO heterostructuresrdquoNanoscale Research Letters vol 5 no 11 pp 1836ndash1840 2010

[79] D S Kim S-M Lee R Scholz et al ldquoSynthesis and opticalproperties of ZnO and carbon nanotube based coaxial het-erostructuresrdquo Applied Physics Letters vol 93 no 10 Article ID103108 2008

[80] CHWang A SWWong andGWHo ldquoFacile solution routeto vertically aligned selective growth of ZnO nanostructurearraysrdquo Langmuir vol 23 no 24 pp 11960ndash11963 2007

[81] Z LWang ldquoZinc oxide nanostructures growth properties andapplicationsrdquo Journal of Physics Condensed Matter vol 16 no25 pp R829ndashR858 2004


[82] A B Djurisic Y H Leung K H Tam et al ldquoGreen yellow andorange defect emission from ZnO nanostructures influence ofexcitation wavelengthrdquo Applied Physics Letters vol 88 no 10Article ID 103107 2006

[83] E Rauwel A Galeckas P Rauwel et al ldquoMetal oxide nanoparti-cles embedded in rare-earthmatrix for low temperature thermalimaging applicationsrdquo Materials Research Express vol 3 no 5Article ID 055010 2016

[84] L Wu Y Wu X Pan and F Kong ldquoSynthesis of ZnO nanorodand the annealing effect on its photoluminescence propertyrdquoOptical Materials vol 28 no 4 pp 418ndash422 2006

[85] L Dai X L Chen W J Wang T Zhou and B Q Hu ldquoGrowthand luminescence characterization of large-scale zinc oxidenanowiresrdquo Journal of Physics CondensedMatter vol 15 no 13pp 2221ndash2226 2003

[86] A El Hichou M Addou J Ebothe and M Troyon ldquoInfluenceof deposition temperature (Ts) air flow rate (f) and precursorson cathodoluminescence properties of ZnO thin films preparedby spray pyrolysisrdquo Journal of Luminescence vol 113 no 3-4 pp183ndash190 2005

[87] X Sun Z Liu K Welsher et al ldquoNano-graphene oxide forcellular imaging and drug deliveryrdquo Nano Research vol 1 no3 pp 203ndash212 2008

[88] Z Luo P M Vora E J Mele A T C Johnson and J MKikkawa ldquoPhotoluminescence and band gap modulation ingraphene oxiderdquo Applied Physics Letters vol 94 no 11 ArticleID 111909 2009

[89] M Khenfouch M Baıtoul and M Maaza ldquoWhite photo-luminescence from a grown ZnO nanorodsgraphene hybridnanostructurerdquo Optical Materials vol 34 no 8 pp 1320ndash13262012

[90] Z H Lim Z X Chia M Kevin A S W Wong and G W HoldquoA facile approach towards ZnO nanorods conductive textilefor room temperature multifunctional sensorsrdquo Sensors andActuators B Chemical vol 151 no 1 pp 121ndash126 2010

[91] R K Biroju P K Giri S Dhara K Imakita and M FujiildquoGraphene-assisted controlled growth of highly aligned ZnOnanorods and nanoribbons growth mechanism and photolu-minescence propertiesrdquoACSAppliedMaterials amp Interfaces vol6 no 1 pp 377ndash387 2014

[92] G Singh A Choudhary D Haranath et al ldquoZnO decoratedluminescent graphene as a potential gas sensor at room tem-peraturerdquo Carbon vol 50 no 2 pp 385ndash394 2012

[93] Z Wu Y Shen X Li Q Yang and S Lin ldquoGreen light-emitting diode based on graphene-ZnO nanowire van derWaals heterostructurerdquo Frontiers of Optoelectronics vol 9 no1 pp 87ndash92 2016

[94] F Han S Yang W Jing et al ldquoSurface plasmon enhancedphotoluminescence of ZnO nanorods by capping reducedgraphene oxide sheetsrdquoOptics Express vol 22 no 10 pp 11436ndash11445 2014

[95] H Zeng Y Cao S Xie et al ldquoSynthesis optical and elec-trochemical properties of ZnO nanowiresgraphene oxide het-erostructuresrdquo Nanoscale Research Letters vol 8 article 1332013

[96] C Zhang J Zhang Y Su M Xu Z Yang and Y Zhang ldquoZnOnanowirereduced graphene oxide nanocomposites for signifi-cantly enhanced photocatalytic degradation of Rhodamine 6GrdquoPhysica E Low-Dimensional Systems and Nanostructures vol56 pp 251ndash255 2014

[97] E Lee J-Y Kim B J Kwon E-S Jang and S J An ldquoVacancyfilling effect of graphene on photoluminescence behavior of

ZnOgraphene nanocompositerdquo Physica Status Solidi (RRL)mdashRapid Research Letters vol 8 no 10 pp 836ndash840 2014

[98] S I Weissman ldquoIntramolecular energy transfer the fluores-cence of complexes of Europiumrdquo The Journal of ChemicalPhysics vol 10 no 4 pp 214ndash217 1942

[99] S W Hwang D H Shin C O Kim et al ldquoPlasmon-enhanced ultraviolet photoluminescence from hybrid struc-tures of grapheneZnO filmsrdquo Physical Review Letters vol 105no 12 Article ID 127403 2010

[100] S Qu X Wang Q Lu X Liu and L Wang ldquoA biocompatiblefluorescent ink based on water-soluble luminescent carbonnanodotsrdquo Angewandte ChemiemdashInternational Edition vol 51no 49 pp 12215ndash12218 2012

[101] W Wei C Xu L Wu J Wang J Ren and X Qu ldquoNon-enzymatic-browning-reaction a versatile route for productionof nitrogen-doped carbon dots with tunable multicolor lumi-nescent displayrdquo Scientific Reports vol 4 article 3564 2014

[102] S Qu H Chen X Zheng J Cao and X Liu ldquoRatiometricfluorescent nanosensor based onwater soluble carbon nanodotswith multiple sensing capacitiesrdquo Nanoscale vol 5 no 12 pp5514ndash5518 2013

[103] D-Y Guo C-X Shan S-N Qu and D-Z Shen ldquoHighly sensi-tive ultraviolet photodetectors fabricated from ZnO quantumdotscarbon nanodots hybrid filmsrdquo Scientific Reports vol 4article 7469 2014

[104] L He S Mei Q Chen et al ldquoTwo-step synthesis of highlyemissive CZnO hybridized quantum dots with a broad visiblephotoluminescencerdquo Applied Surface Science vol 364 pp 710ndash717 2016

[105] K Suzuki M Takahashi L Malfatti and P Innocenzi ldquoCarbondots in ZnOmacroporous films with controlled photolumines-cence through defects engineeringrdquo RSC Advances vol 6 no60 pp 55393ndash55400 2016

[106] L Tang R Ji X Li K S Teng and S P Lau ldquoEnergy-levelstructure of nitrogen-doped graphene quantum dotsrdquo Journalof Materials Chemistry C vol 1 no 32 pp 4908ndash4915 2013

[107] Y H Lu Z X Hong Y P Feng and S P Russo ldquoRoles of carbonin light emission of ZnOrdquo Applied Physics Letters vol 96 no 9Article ID 091914 2010

[108] Y Hu and H-J Chen ldquoEnhancement of green luminescenceof ZnO powders by annealing with carbon blackrdquo MaterialsResearch Bulletin vol 43 no 8-9 pp 2153ndash2159 2008

[109] C Jin S Lee C-W Kim S Park C Lee and D LeeldquoComparison of photoluminescence of carbon nanotubeZnOnanostructures synthesized by gas- and solution-phase trans-portrdquo Applied Physics A vol 118 no 2 pp 733ndash738 2015

[110] R Zhang L Fan Y Fang and S Yang ldquoElectrochemical route tothe preparation of highly dispersed composites of ZnOcarbonnanotubes with significantly enhanced electrochemilumines-cence from ZnOrdquo Journal of Materials Chemistry vol 18 no 41pp 4964ndash4970 2008

[111] Y Zhu X Zhang R Li and Q Li ldquoPlanar-defect-rich zincoxide nanoparticles assembled on carbon nanotube films asultraviolet emitters and photocatalystsrdquo Scientific Reports vol4 article 4728 2014

[112] K Vanheusden W L Warren C H Seager D R TallantJ A Voigt and B E Gnade ldquoMechanisms behind greenphotoluminescence in ZnO phosphor powdersrdquo Journal ofApplied Physics vol 79 no 10 pp 7983ndash7990 1996

[113] Y H Ko M S Kim and J S Yu ldquoStructural and opti-cal properties of zno nanorods by electrochemical growth


using multi-walled carbon nanotubecomposed seed layersrdquoNanoscale Research Letters vol 7 article 13 2012

[114] A-A E Mel M Buffiere C P Ewels et al ldquoZn basednanoparticlendashcarbon nanotube hybrid materials interactionand charge transferrdquo Carbon vol 66 pp 442ndash449 2014

[115] G-L Chai C-S Lin and W-D Cheng ldquoFirst-principles studyof ZnO cluster-decorated carbon nanotubesrdquo Nanotechnologyvol 22 no 44 Article ID 445705 2011

Submit your manuscripts athttpwwwhindawicom

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Journal ofNanomaterials


nonmetals and the synthesis of heterostructures [14 15]However ZnO nanoparticles or quantum dots on their ownmay not serve very well for electron transport mainly dueto two reasons firstly there may not be sufficient contactbetween the particles themselves and secondly even if thereis contact between them the formation of grain boundarieswould hinder electron transport and deteriorate possibledevice fabrication However due to its intrinsic propertiesand abundance ZnO is nevertheless widely investigatedfor large-scale activities such as purifying water and air[8 16]

Another means to enhance the photoluminescent prop-erties involves the hybridization of the material with carbonbased nanomaterials such as carbon nanotubes (CNT) car-bon quantum dots (CQDs) or graphene among the carbonallotropes [17 18] In fact carbon based structures such asfullerenes graphene CNT and CQDs have been investigatedfor the enhancement of performances of photodetectors andphotovoltaic devices [19ndash23] In recent years carbon basednanostructures have sparked tremendous research interestdue to their superior chemical physical mechanical andelectronic properties [24] Since they are abundant and havebeen successfully shown to be biocompatible they have beenintegrated into bioimaging medical diagnosis catalysis andoptoelectronic applications [25ndash28]

Among the various carbon allotropes CQD or carbonnanodots have emerged as a rising candidate for manyapplications Due to their high quantum yield [29] lowcytotoxicity [30] high photostability [31] and nonblinkingcharacter [32] they are seen as potential candidates forapplications in photovoltaics photocatalysis [33] and lightemitting diodes [34] The significantly large amount of 120587electrons moreover makes them viable for hot carrier solarcells [35] Yu et al have recently demonstrated that graphene-CQDs have this potential contrary to CNT-CQD or TiO

2-

CQDs where charge transfer in the latter two was absent [36]Graphene on the other hand has shown enormous poten-

tial in the fabrication of solar cells In one instance grapheneis used as a charge conducting layer in a solar cell device[37] Some other studies have reported that graphene can beused as an electrode for dye-sensitized and organic solar cells[38ndash41] Similarly carbon nanotubes (CNT) or cylindricalfullerenes have interesting optical properties because theirexcitation gives rise to strongly bound excitons [42] CNTare classified into single-wall carbon nanotube (SWCNT)and multiwall carbon nanotube (MWCNT) SWCNT findsapplications in the electronic industry considering theirhigh electron mobility and also in biomolecule sensors [43]SWCNT have also been studied as a possible solar cellmaterial [44] Additionally MWCNT find applications ascounterelectrodes in dye sensitized solar cells [45 46]

This review article aims at bringing together variousZnO-nanocarbon hybrid materials Three major allotropesof carbon nanomaterials namely CNT graphene and CQDhybridizing with nano-ZnO are analysed This work alsoevaluates and compares the photoluminescence propertiesof these hybrid materials in terms of their defects and mor-phologies determined by various growth techniques whichare also reviewed and assessed

2 Synthesis and MorphologicalCharacterization of ZnO CarbonBased Nanostructures

21 ZnO-CQD Synthesis ZnO-CQD nanohybrids have beenprepared by various groups Yu et al have used hydrothermalsynthesis by introducing Zn(Acetate)

2 2H2O (025M) in an

alcoholic solution containing CQD for 8H at 100∘C [47]Theresulting nanohybrids shown in Figure 1 are indeed ZnO-CQD nanocomposite with the CQD encapsulating the ZnOnanoparticle Different morphologies of ZnO nanoparticleshave been combined with CQD Zhang et al have usedelectrospinning hydrothermal synthesis and have obtainedZnO nanoflowers They subsequently dispersed these ZnOnanoflowers in a solution containing CQD The nanohybridmaterial was then dried at 80∘C [48] In their study theCQDs were themselves prepared via a green synthesis routeusing sucrose On the other hand the ZnO nanoflowerswere prepared via electrospinning of zinc acetate dehydratedat 470∘C followed by hydrothermal synthesis of the Znnanospheres using a solution of zinc nitrate hexahydrate andhexamethylenetetramine at 95∘C The nanocomposite wassubsequently synthesized by dispersing CQD dots in wateralong with different concentrations of ZnO nanoflowers

Another green synthesis approach consists of using D-fructose and NaOH The nanocomposite was then preparedby hydrothermal synthesis of Zn(CH

3COO)

2sdot2H2O in

the presence of CQD at 80∘C [49] Other approachesof combining ZnO and CQD using sol-gel methodsalso exist ZnO mesoporous films have been studied bySuzuki et al but contrary to the above-mentioned studiesthe CQDs were synthesized by less environmentallyfriendly methods such as ArgininesdotHCl and 12-ethyl-endiamine The solution for ZnO was produced usingZn(CH

3COO)

2sdot2H2OEtOHH

2OZonyl and then mixed

with various concentrations of CQD [50] The morphologyof the hybrid nanocomposite consisted of agglomeratednanocrystals of ZnO blended with CQDs

Other methods to grow CQD comprise reacting graphitewith HSO

4for a longer period of 6 days and subsequently

combining them with ZnO prepared by sol-gel methodsusing Zn(NO

3)2sdot6H2O [51] Muthulingam et al have used N

doped ZnO via low temperature hydrothermal synthesis at60∘CTheCQDswere isolated by combining carbon black inkH2SO4andHNO

3at 240∘C for 2 hours [52] Pyrolysis has also

been used byMa et al for the synthesis of these nanocompos-ites which consisted of hydrothermal synthesis of ZnO andpyrolysis of a metal organic framework as precursor Hereoxybisbenzoic acid zinc nitrate hexahydrate 441015840-bipyridineandNN-dimethylformamidewere combined [53] Currentlyporous materials are regarded as attractive materials forphotocatalytic activity due to their large surface areas Veryrecently Ding et al have combined ZnO foam with CQD[54] In their work they synthesized the ZnO foam witha one-step combustion method where Zn(NO

3)2sdot6H2O was

dissolved into ethylene glycol monomethyl ether and burntat 120∘C They also obtained CQD via a green hydrothermalsynthesis route using sucrose The nanocomposite was thenformed by dispersing the ZnO foam into the CQD solution


(a) (b)

(c) (d)



4and sonicated




2at 120∘C









Photon energy (eV)

DRS 300 K

320 eV 328 eV

500 600 700 800400

Wavelength (nm)

161822224262833234

101

102

103

104

105

106

107

PL in

tens

ity (c

ount

ss)

Nor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]


DRS 300 K

320 eVe 328 eVeNor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]

PL 8K





PL in

tens

ity (a

u) 382

482

610

684

686

400 450 500 550 600 650 700 750 800350

Wavelength (nm)


524498







450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)


420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO


(b)



Orange emission



ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg




NH2

eminus





(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)



4and sonicated




2at 120∘C









Photon energy (eV)

DRS 300 K

320 eV 328 eV

500 600 700 800400

Wavelength (nm)

161822224262833234

101

102

103

104

105

106

107

PL in

tens

ity (c

ount

ss)

Nor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]


DRS 300 K

320 eVe 328 eVeNor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]

PL 8K





PL in

tens

ity (a

u) 382

482

610

684

686

400 450 500 550 600 650 700 750 800350

Wavelength (nm)


524498







450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)


420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO


(b)



Orange emission



ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg




NH2

eminus





(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









Photon energy (eV)

DRS 300 K

320 eV 328 eV

500 600 700 800400

Wavelength (nm)

161822224262833234

101

102

103

104

105

106

107

PL in

tens

ity (c

ount

ss)

Nor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]


DRS 300 K

320 eVe 328 eVeNor

mal

ized

[F(R

)h]2

00

05

10

31 32 33 34 3530


[VZn] [VO]

PL 8K





PL in

tens

ity (a

u) 382

482

610

684

686

400 450 500 550 600 650 700 750 800350

Wavelength (nm)


524498







450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)


420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO


(b)



Orange emission



ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg




NH2

eminus





(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




PL in

tens

ity (a

u) 382

482

610

684

686

400 450 500 550 600 650 700 750 800350

Wavelength (nm)


524498







450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)


420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO


(b)



Orange emission



ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg




NH2

eminus





(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




450 500 550 600 650 700400

Wavelength (nm)

Nor

mal

ized

inte

nsity

(au

)


420 nm410 nm390nm350nm

(a)

Inte

nsity

(au

)

400 450 500 550 600350

Wavelength (nm)

ZnOC = 4 1ZnO


(b)



Orange emission



ZnO ZnO ZnO ZnO

ZnOZnOC-dots

C-dots C-dots

Oi

Oi

Oi

Oi

Oi

OiOi Oi

OiOi

Oi

Oi

Oi

Oi

Oi

Oi

OiOi

Oi

Oi

Oi

OiVO VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

VO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OHHO

HO

H2N

H2N

H2N

H2N

H2N

H2NNH2

NH2

NH2

NH2

NH2

NH2HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

HO

HOH2N

H2N OH

OHNH2

NH2

Bare ZnO1ndash4mg




NH2

eminus





(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a)

Photon energy (eV)

ETCZn005CNT

225335

500 600 700400

Wavelength (nm)

0

10

PL in

tens

ity (times

103

Cnts

s)

PL 300KAIR

(b)



4 Conclusions







Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






Competing Interests


Acknowledgments



References

























2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













2het-

















































































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



















































































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
















































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Review Article A Review of the Synthesis and ...downloads.hindawi.com/journals/jnm/2016/5320625.pdfhave used electrochemical deposition of ZnO nanorods on the rGO where the conductivity