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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 22 (2014) 76–82

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Optoelectronic properties of solution synthesis of carbonnanotube/ZnO:Al:N nanocomposite and its potential asa photocatalyst

Ian Y.Y. Bu n

Department of Microelectronics Engineering, National Kaohsiung Marine University, 81157 Nanzih District, Kaohsiung City,Taiwan, Republic of China

a r t i c l e i n f o

Available online 22 February 2014

Keywords:Carbon nanotubesZinc oxideCompositePhotocatalystCo-doping

x.doi.org/10.1016/j.mssp.2014.01.04301 & 2014 Elsevier Ltd. All rights reserved.

þ886 972506900; fax: þ886 73645589.ail address: ianbu@mail.nkmu.edu.tw

a b s t r a c t

A novel multiwalled carbon nanotubes/aluminum–nitrogen co-doped ZnO (MWCNT/ZnO:Al:N) nanocomposite film was synthesized by a sol–gel deposition method and used forthe photodegradation of organic dye under UV light illumination. The MWCNT/ZnO:Al:Ncomposite film was extensively characterized through scanning electron microscope(SEM), Energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission elec-tron microscopy (TEM), photoluminescence measurements (PL) and UV–vis spectroscopy.Photocatalytic measurements reveal that the addition of MWCNT enhances photocatalyticdegradation of MB by providing conduction path for electron transfer and reactive oxygengroups.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, nanostructured zinc oxide (ZnO) materials haveattracted considerable research interests due to its wide directband gap (3.37 eV) and a large exciton binding energy(60 meV) [1,2]. One of the key processing advantages thatZnO possesses over other materials is the numerous geome-tries that it can adapt. Successful synthesis of ZnO nanoma-terials include nanowire [3], nanobelts [4], nanotube [5],nanoflake [6] and nanoflower [7]. As-deposited ZnO tends toexhibit n-type conduction behavior due to defects such asoxygen vacancies and zinc interstitials. Comparatively, it isrelatively difficult to obtain stable p-type ZnO due to thewell-documented compensation effects [8–10].

Theoretically, it has been shown that p-type ZnO canbe obtained through nitrogen doping [11,12]. However,p-type ZnO obtained by atomic nitrogen doping is often

metastable and tends to revert to n-type conductionbehavior over time [13,14]. A much more successfulstrategy to obtain p-type ZnO is by co-doping with bothdonor and acceptor as proposed by Yamanoto [15].Recently, several groups reported co-doping of ZnOthrough combination of Ga/N [16], Li/N [17], In/N [18]andAl/N [19] and so on. Among the possible combinations, Al/N possesses the distinct processing advantages due to itsnon-toxicity and natural abundance [19]. Although N/Alco-doped p-type ZnO (NZO) can be prepared by sputteringZnO:Al under N or NH3 environment [20], it suffers fromlow material usage and requires expensive vacuum set-ups. Alternatively, p-type ZnO can be synthesized throughlow-cost, solution-based process, which only involvessimple set-up and precise control of chemical composition[2,19,21,22]. In terms of application, ZnO has been exten-sively investigated as a photocatalyst [23–28], owing toits high photocatalytic activities and non-toxic feature.Currently, the photocatalytic activities of ZnO are limitedby its large band gap that fails to utilize all the incominglight and defect induced electron–hole recombination. One

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of the proven approaches to increase photocatalytic activ-ity of ZnO is to alter the optoelectronic properties of ZnOby addition of dopants [29–32]. In the past, several studieshave reported that N-doped ZnO exhibited enhancedphotocatalytic performances against organic dyes in com-parison with undoped ZnO [33–35]. Another key issueassociated with N-doped ZnO is its inherent high electricalresistivity that limits its use in semiconductor applications[36]. Carbon-based conductive fillers such as carbon black,graphite and carbon nanotubes have often been incorpo-rated into polymers [37,38] and ceramics [39] to enhanceits overall conductivity. Previous studies have demon-strated that when incorporated into other compounds,MWCNT composite can enhance the optical, electrical andmechanical properties of the original material [40–42].Successful demonstrations of such enhancements includeincorporation of MWCNTs into metal oxide such as TiO2

[43], FeO2 [44] and ZnO [45]. To the best of our knowledge,the photocatalytic properties of NZO/MWCNT compositehave not been studied.

In this study, sol–gel deposited, p-type, NZO/MWCNTcomposite films for photocatalytic applications are investi-gated. The optoelectronic and photocatalytic properties of theNZO/MWCNT composite films were examined by X-ray dif-fraction (XRD), scanning electron microscope (SEM), Hall-effect measurements, UV–vis spectroscopy and photocatalyticactivity measurements.

2. Experimental

All chemicals used during this study were of reagentgrade, purchased from Sigma-Aldrich and used withoutfurther purification. Commercial p-type MWCNTs withmean diameter �50 nm (Xinnano, Taiwan) were sonicatedin nitric acid at 130 1C for 3 h to functionalize its surfaceand remove impurities. Then washed with DI water andredispersed into isopropanol (IPA) solution.

2.1. Sol–gel NZO

NZO films were synthesized under the condition aspreviously described [2]. Briefly, sol–gel precursor was pre-pared by dissolving 1 M zinc acetate (Zn(CH3COO)2 �H2O) inisopropanol (IPA). The solution was magnetically stirred for1 h at 70 1C and left to age for at least 24 h. 1 wt% aluminumchloride hexahydrate and 2.5 wt% ammonium hydroxidewere added as sources of co-dopant. For comparison, anintrinsic ZnO thin film was deposited using the exactexperimental condition but without the addition of dopant.

The purified MWCNTs were dispersed in IPA, whichresulted in concentration of MWCNTs in IPA of around30 ppm. Then, the IPA/MWCNT solution was mixed intothe NZO sol via magnetic stirrer which yielded a semi-transparent, yellow solution without visible precipitation.Subsequently, the NZO/MWCNTs precursor was spincoated onto the glass substrates and sintered at 550 1C.For comparative purposes, an NZO sample without theaddition of CNT was also deposited on another piece ofglass substrate. The thicknesses of the deposited sampleswere around 250 nm. In addition, CNTs were spin coated

onto a glass substrate and treated with ammonia, for Halleffect measurement, to confirm its conductivity type.

The structural properties of the deposited sampleswere determined using a FEI Quanta 400F EnvironmentalScanning Electron Microscope (ESEM). Energy dispersivespectroscopy (EDS) was also conducted within the samechamber to determine the chemical composition of thefilms. The detailed morphology of the synthesized NZO/MWCNT sample was evaluated using a Philips CM-200TWIN transmission electron microscope (TEM).

The crystal orientation was investigated by a SiemensD5000 X-Ray diffractometer using Cu Kα radiation. Photo-luminescence of the NZO films was performed by theexcitation from 325 nm He–Cd laser at room temperature.Hall measurements were performed by four-point probe inthe Van der Pauw configuration in order to determineresistivity and carrier concentration. The photocatalyticactivities of pure NZO and NZO/MWCNT samples weredetermined through time-dependent photodegradationmeasurements of an aqueous dye (methylene blue, MB)solution. The dye stock solution was prepared by dissol-ving 50 mg MB in 50 ml DI water. In a typical photocata-lytic activity measurement, 50 mg NZO or NZO/MWCNTcoated on 1 cm2 glass substrate sample was added to theMB stock solution and continuously stirred under UVillumination generated by a 100 W high-pressure Hg lamp(Beijing Changtuo) passed through a glass filter (ZUL0422,Asahi spectra) to allow visible light to pass (wave-length44420 nm), enclosed in a black box. Then, 5 mlof the sample solution was collected at every 10, 20, 40and 60 min period. The samples were filtrated throughWhatman anopore membranes and the aliquots wereanalyzed using the aforementioned UV–vis spectrophot-ometer. The absorption spectra were recorded in the rangeof 500–700 nm and photocatalytic degradation of the MBwas monitored based on the intensity of the characteristicabsorbance band of the dye at 668 nm. The percentage ofthe remaining MB dye as a function of UV-illuminationtime was determined by normalizing the measured absor-bance intensity (at 668 nm of MB dye) to the initial (pre-irradiation) MB stock solution.

3. Results and discussion

The structural properties of the NZO and NZO/MWCNTsample were evaluated by using an SEM and presentedin Fig. 1(a) and (b), respectively. Fig. 1(a) shows the NZOnanocomposite film deposited with MWCNTs. The insetof Fig. 1(a) shows the same sample at an intentionallyscratched area to confirm the presence of the MWCNTs.Clearly, it can be observed that straight, individualMWCNTs are encrusted within the NZO film. The MWCNTsappeared to have bundled together and horizontallyaligned to the substrate. Several groups have reported aMWCNT bundling effect and have attributed to the strongcapillary force that densify the MWCNTs [46,47].

Fig. 1(b) shows the NZO thin film without MWCNTaddition. The sample consists of small grains on ZnOsurface which is a typical feature of sol–gel depositedZnO [36]. Upon heating, the sample results in evaporationof IPA and formation of Zn–OH–Zn bonds through the

Fig. 1. (a) The SEM image of the NZO/CNT sample (scale bar 5 μm) with the inset showing the scratched area of the same sample (scale bar 5 μm). (b) SEMimage of the NZO sample with scale bar 1 μm. (c) TEM image of the NZO encrusted carbon nanotube. d) EDS analysis of the sample. (Au, Si and Ca wereexcluded from the composition analysis and do not affect the analysis.)

Fig. 2. XRD pattern of the NZO and NZOþMWCNT samples.

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dehydroxylation process. Further increase of the annealingtemperature to 550 1C resulted in ZnO grain nucleationdue to faster diffusion of the atoms. Fig. 1(c) shows therepresentative TEM image of MWCNT/NZO. The TEMimage shows a CNT encrusted with NZO nanoparticles.Fig. 1(d) shows the compositional EDS analysis of theNZO/MWCNT sample. Zn (40.7 at%), O (47.8 at%), N (2.3at%), Al (2.1 at%), and C (7.1 at.%) peaks were detected.Thus, it confirms the successful Al/N co-doping into ZnO.Peaks such as Au (2.1 ev), Si (1.8 ev) and Ca (3.7 eV) wereexcluded from the compositional analysis.

Fig. 2 shows the XRD pattern of spin coated NZO andNZO/MWCNTs nanocomposites. Only a single diffractionpeak corresponding to the (002) plane was observed forboth samples, suggesting a high crystallinity film witha strong preferential growth in the c-axis orientation.Generally, thin films tend to grow along the orientationwith the lowest surface energy [48]. For ZnO, Fujimuraet al. [49] concluded that the surface energy density of the(002) orientation is the lowest and hence the preferentialgrowth.

It is also evident from the XRD peak that the additionof MWCNT into the NZO precursor solution has resultedin decrease in the diffraction intensity. The weaker XRD

Fig. 3. Photoluminescence emission of the NZO and NZOþMWCNTsamples.

Fig. 4. (a) Optical transmittance of the NZO and NZO/CNT samples and(b) (αhν)2 versus energy plot for Tauc gap extraction.

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intensity implies that the crystal quality of the NZO/CNTthin film is inferior than that of NZO thin film. Theincorporation of MWCNT induces additional intrinsicstresses within the bulk, which disrupts c-axis orientationgrowth.

Fig. 3 shows the room temperature photoluminescence(PL) emission measurement of the NZO and NZO/CNTs thinfilms. Clearly, NZO thin films show emission peaks centeredaround 400 nm and 680 nm, whereas NZO/MWCNT exhib-ited two peaks centered at 450 and 680 nm, respectively.

In the past, two PL emission peaks have been observedfor high quality ZnO centered at 380 nm (near band emis-sion) and 550 nm (green) [36,50]. The near band emissionpeak is believed to have originated from recombination offree excitons and the green emission is associated withdefect within the ZnO and ascribed to zinc interstitials andoxygen vacancies in the ZnO film [51]. From Fig. 3, it can beobserved that the commonly observed near band edgeemission peak of ZnO is absent. Instead the emission peakappeared at 400 (NZO) and 450 nm (NZO/MWCNT). The co-doping of Al and N seems to have induced defects (oxygenvacancies and zinc interstitials) within the ZnO structure.Furthermore, the MWCNTs addition into the ZnO appearedto have resulted in defect creation and a strong shift of theemission peak. Based on theoretical calculations, the PLemission peak at 400 nm has been assigned to zinc inter-stitials that is located 0.22 eV below the conduction band[52,53]. Previous study has attributed the broad emissionpeak at 450 nm to the recombination of photogeneratedholes with singly ionized oxygen vacancies [54]. Althoughthis is a probable cause in the case of ZnO without MWCNTincorporation, it is believed that the addition of CNTcauses the formation of NZO–MWCNT heterojunction thatresults in electron transfer from ZnO to MWCNT andin PL emission at 450 nm. Similar mechanism has beenobserved in ZnO coated MWCNT [55].

Fig. 4(a) shows the optical transmittance of thedeposited NZO and NZO/MWCNT film as determined byUV–vis spectroscopy. The measured optical transmittancerevealed that the NZO films are highly transparent in thevisible region (550 nm)�82%. It can also be observed inFig. 4(a) that the MWCNT addition has a profound effect onthe film's optical transparency; with decrease in transpar-ency down to 69% for NZO/MWCNT sample. The reductionin optical transmittance is due to the fact that the CNTs arenot transparent and absorb the incoming light [56] andincrease the light scattering. The optical band gap of theNZO and NZO/MWCNT thin films was determined byassuming that ZnO is a direct band-gap semiconductor,from the plotted graph of (αhν)2 versus the incidentphoton energy (hν), where α is the absorption coefficient.From the graph, one can estimate the optical band gap byextrapolation of the straight line region into the x-axis(αhν)2. For NZO/MWCNT composite, band gap of around3.15 eV was obtained from the film, whereas for NZO thinfilm the extracted band gap is around 3.25 eV. Theobtained band gap deviates from the pure ZnO due totwo possible mechanisms. Firstly the addition of MWCNTsinfluences the overall carrier concentration and reducesthe band gap [57]. Secondly, the band gap narrowing effecthas been observed in ZnO films co-doped with Al and Nand has been attributed to enhanced N incorporation intothe ZnO thin film [58]. In fact, band gap reduction isbeneficial towards the photocatalytic efficiency of wideband semiconductors such as ZnO as it is more effective inutilizing the visible light.

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NZO and NZO/MWCNT nanocomposites were electri-cally characterized by Hall effect measurements and con-firmed that both samples are p-type with carrier density of7.898�1017 cm3 and resistivity of around 89.7 Ω cm forNZO. In the case of NZO/MWCNT the extracted carrierconcentration is around 8.675�1016 cm3 and resistivityvalue is around 9.5 Ω cm. Evidently, the MWCNT additionyielded films with a much lower resistivity and reducedhole carrier concentration. The difference can be attributedto two causes; Firstly, the insertion of MWCNTs resultsin conductive networks within the NZO film whichtranslates into lower resistivity of the film. Secondly, theincorporation of ammonia (a source of nitrogen) into the

Fig. 5. (a) UV–vis absorbance spectra of mythelene blue as a function of time ufunction of time using NZO/CNT, (c) UV–vis absorbance spectra of mythelene bluesolution with time at 665 nm and (e) reusability measurements.

MWCNTs may have resulted in enhanced n-type doping inMWCNTs and hence lower hole carrier concentration dueto electron–hole recombination. Hall effect measurementperformed on ammonia treated CNTs confirmed its n-typeconduction behavior. Previous studies have revealedthat raw MWCNTs exhibit metallic characteristics [59,60].However, our NZO/MWCNT nanocomposite sample exhi-bits semiconducting behavior. Such result correlates withZhang's study on ZnO quantum dot coated MWCNTs,which exhibit semiconductor transport behavior at lowtemperature [61].

Photocatalytic measurements were performed andplotted in Fig. 5(a)–(c). Fig. 5 (a)–(c) shows the change in

sing NZO thin film, (b) UV–vis absorbance spectra of mythelene blue as aas a function of time using ZnO, (d) absorbance change in methylene blue

Fig. 6. Schematic diagram of the energy level and dye degradationmechanism of the NZO/CNT heterojunction.

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absorption of methylene blue solution with time for NZO,NZO/MWCNT and ZnO samples under UV irradiation.Change in absorption could be correlated with the dyeconcentration change and plotted in Fig. 5(c). It can beseen from Fig. 5(d) that the pure NZO sample has muchlower effect on the concentration change of methyleneblue solution than the NZO/MWCNT samples. These resultsconfirm that the addition of MWCNTs into NZO improvesdecomposition of methylene blue under UV irradiation.As mentioned in the experimental section, only a smallarea (1 cm2) piece of glass substrate was used for thedecomposition of 50 ml of highly concentrated methyleneblue solution. Therefore, the rate of change of methyleneblue decomposition is slow. Nevertheless, it clearlydemonstrates enhanced photocatalytic activity of NZO/MWCNT samples over NZO samples. The enhanced photo-catalytic activity is likely due to the increase surfacedefects (especially oxygen) and reduced band gap thatfacilitate the photocatalytic process. As a comparison, anundoped ZnO sample was also tested for its photocatalyticperformances and is plotted in Fig. 5(c). Clearly, undopedZnO exhibits lower photocatalytic degradation of the dyethan both the NZO and the NZO/MWCNT samples. Pre-vious studies revealed that N doped ZnO showedimproved photocatalytic performances in comparison withthe undoped ZnO due to the narrowing of the band gap[62]. The reusability of the ZnO, NZO and NZO/MWCNTsamples was performed and is plotted in Fig. 5(e). All theinvestigated catalysts were used repeatedly in 10 cycles,under the identical experimental condition and at a reac-tion time of 60 min. It can be observed that the reusabilityof NZO/MWCNT is excellent, with only around 5% decreasein photocatalytic performance as the number of cyclesincreased. Both the NZO and the ZnO samples show lowerreusability with decrease in photocatalytic performance ofaround 22% and 30%, respectively. The obtained resultcorrelates with previous study that confirms the incor-poration of MWCNT into ZnO which improves the reusa-bility of the sample [63]. This is due to the increasedphotocatalytic degradation of the dye, which meansdecreased absorption of organic matters on the photoca-talyst's surface. It is also interesting to note that NZO ismore chemically stable than undoped ZnO due to thedoping process [64].

The photocatalytic activity of a catalyst is determinedby the photogenerated holes and electrons, which can beproduced in p-type NZO and n-type MWCNT upon irradia-tion. Considering the wide band-gap (3.25 eV) of NZO/MWCNT thin film, degradation of MB under UV light in thepresent study should be conducted following the dyeexcitation mechanism, as illustrated in Fig. 6. Acid treatedMWCNTs with large specific surface area and numerousoxygenic groups allow dye molecules access to theirsurfaces for photocatalytic degradation. When the p–njunction (NZO/MWCNT) is irradiated by light, the photo-generated holes flow to the valence band of the NZO, whilethe electrons flow to the conduction band of CNTs. There-fore, the electrons on the MWCNT's surface are scavengedby oxygen (O2) adsorbed on its surface or dissolved oxygenfrom the MB solution to produce superoxide radical anionO2●_[65]. Meanwhile, the photogenerated holes in valence

band of NZO oxidize the organic molecule in MB solutionor react with OH� or H2O and then oxidize them into OH●

radicals.

4. Conclusion

In summary, the effects of MWCNT addition into NZOon the optical, electrical, morphological and photocatalyticperformances, and transmittance were investigated. It wasfound that the MWCNT addition has a significant effect onthe films structure. SEM images revealed the behaviorof the MWCNTs bunches and were horizontally aligned.XRD patterns confirm that all the films preferentially growin the c-axis orientation. The introduction of MWCNT alsomodifies the PL spectra of the NZO by surface relatedrecombination within the MWCNT. UV–vis spectroscopymeasurements indicate that the as deposited NZO filmshows good transmittance �80% that reduces as CNTs areincorporated due to increased light absorption and scatter-ing. Photocatalytic measurements confirm that the addi-tion of MWCNT enhances photocatalytic degradation ofMB by forming a p–n heterojunction that dissolve oxygenfrom the MB solution and oxidize the organic molec’ule.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.mssp.2014.01.043.

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