Journal of Photochemistry and Photobiology A: Chemistry 298 (2015) 49–54

Rapid synthesis of ZnO nano-corncobs from Nital solution and itsapplication in the photodegradation of methyl orange

C. Gomez-Solís a,*, J.C. Ballesteros a,*, L.M. Torres-Martínez a, I. Juárez-Ramírez a,L.A. Díaz Torres b, M. Elvira Zarazua-Morin a, Soo Whon Lee c

aUniversidad Autónoma de Nuevo León (UANL), Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Ciudad Universitaria, San Nicolás delos Garza,Nuevo León C.P. 64451, Mexicob Laboratorio de Fotocatálisis y Fotosíntesis Artificial (F&FA), Grupo de Espectroscopía de Materiales Avanzados y Nanoestructurados (GEMANA), Centro deInvestigaciones en Óptica, A.P. 1-948, León Gto. 37160, Mexicoc Sun Moon University (GRL) Ecointerface LAB 100 Gal-San ri, Asan, Chungnam 336-708, Republic of Korea

A R T I C L E I N F O

Article history:Received 12 August 2014Received in revised form 18 September 2014Accepted 13 October 2014Available online 18 October 2014

Keywords:ZnOPhotodegradationMethyl orangeNital solutionPhotocorrosion

A B S T R A C T

This paper reports the synthesis of ZnO with nano-corncobs morphology; this method consists of twostages of reaction: the first is the formation of ZnO precursor at 70 �C by a mixture formed from a Nitalsolution (ethanol + nitric acid) and zinc acetate; the second stage occurs at 180 �C, where the combustionprocess occurs to obtain the ZnO nanoparticles. XRD data showed the presence of hexagonal single-phaseZnO with the wurtzite crystal structure. FE-SEM images indicated that the synthesized ZnO presentsnano-corncobs morphology. The analysis of photoluminescence spectra shows the presence of oxygenvacancies in the synthesized ZnO, which are related to the ratio of polar and non-polar planes. Thecommercial and synthesized ZnO were used as catalysts for degradation of methyl orange undersimulated sun-light. Results showed that synthesized ZnO is more catalytically active for photo-degradation under simulated sun-light than commercial ZnO. Photocatalytic activity tests showed thatbest activity was obtained with uncalcined ZnO powders and this enhanced activity was attributed to thesynergistic effect found between the material polar plane and oxygen vacancies. Additionally,electrochemical experiments shown that synthesized ZnO by Nital solution are free of photocorrosion.

ã 2014 Published by Elsevier B.V.

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Journal of Photochemistry and Photobiology A:Chemistry

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1. Introduction

Zinc oxide (ZnO) has a hexagonal wurtzite and cubic zinc blendecrystal structure. The wurtzite structure is most stable at roomtemperature and therefore the most common and intensivelyinvestigated for environmental remediation and photovoltaicpower production. A great disadvantage of ZnO is thephotocorrosion as well as the low photocatalytic efficiency becauseof its weak visible light absorption and low photocatalyticquantum efficiency, which are caused from fast recombinationof photo-generated carriers [1–8].

This is more demanding to expand the application of ZnO as aphoto-catalyst to be active in the visible light region. One way is theZnO doped with different metals, non-metals or with the mixtureof other oxides [9–12]. On the other hand, oxygen vacancies play an

* Corresponding authors. Tel.: +52 81 83 29 40 00x7230, 7259/14 42 44 00x5106.E-mail addresses: lienquidremo@hotmail.com (C. Gomez-Solís),

jballesteros_pacheco@yahoo.com.mx (J.C. Ballesteros).

http://dx.doi.org/10.1016/j.jphotochem.2014.10.0121010-6030/ã 2014 Published by Elsevier B.V.

important role to expand the visible photocatalytic activity. Theatomic arrangements on low index planes of the hexagonal prismon ZnO are stoichiometric, with equal numbers of exposed Zn2+ orO2�ions, while the basal planes and the pyramidal planes arestrongly polar, consisting of sheets of Zn2+ or O2�. The polar planesof ZnO play a very important role in solar photocatalytic reactions,because they favor the formation of more oxygen vacancies [13].Xu et al. reported that visible light absorption capability of ZnO canbe enhanced as a result of high concentration of oxygen vacancies.It also helps to produce the H2O2 in photocatalytic processes.Oxygen vacancies can act as trap centers, in addition, oxygenvacancies help in the band gap narrowing as a result of producingan impurity level near the valance band [14,15]. Lv et al. [16] showsthat the enhancement of UV activity of ZnO is attributed to the highseparation efficiency of photogenerated electron–hole pairscaused by the broadening of the valence band (VB) width inducedby surface oxygen vacancies states.

Recently, nanostructures of ZnO have attracted intense atten-tion, since these offer advantages over the bulk ZnO due to theirshort lateral diffusion length and low reflectivity [17]. Zhang et al.

Fig. 1. FT-IR spectra obtained from samples extracted of round-bottom flask withthe syringe during the reaction of formation of ZnO. At 70 �C first stage: (a) 5 min, (b)7 min and at 180 �C second stage: (c) 1 min and (d) 10 min.

50 C. Gomez-Solís et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2015) 49–54

[18] have reported the synthesis of TiO2 nanostructured for thephotodegradation of RhB in aqueous solution, and the resultsshowed a higher activity with respect to commercial anatase-TiO2.

In this work, the novel and simple Nital-solution andcombustion method is used to improve the visible photocatalyticactivity of ZnO by combined effect of ZnO nanostructured andincreasing oxygen vacancies; the increased activity of ZnOnanostructured has been tested for the degradation of methylorange under simulated sun-light irradiation.

2. Experimental

2.1. Synthesis of ZnO nano-corncobs photocatalyst

The synthesis of ZnO nano-corncob was carried out from Nitalsolution [19], which is a solution of alcohol and nitric acid; thereaction time of the formation of ZnO nanostructured was about10 min. The synthesis route was carried out in a typical refluxapparatus as follows: the liquid reaction mixture composed byethanol (95%) and nitric acid (with 10:1 volume ration) wereplaced into a round-bottom flask open only at the top; this flask is atriple-neck type and the other two necks were closed with cork labstoppers. This vessel is connected to a condenser, such that anyvapors given off was cooled back to liquid and fall back into thereaction vessel. Then, the vessel was heated at 180 �C for the start ofthe reaction. When the temperature was 70 �C, then the zincacetate was incorporated into the flask and this mixture was keptunder stirring, heating and refluxing up to 180 �C. The amount ofreagents was used to obtain 1 g of ZnO. After reaction time, theflask is completely open and then the powder obtained ismaintained at the same temperature to achieve completeevaporation. The powder obtained was divided in three partsand during the degradation experiments these were tested withand without annealing at 400 �C and 600 �C, respectively.

With the finality to know the chemical species present in thereaction mixture, each cork stopper was perforated with a syringein order to remove or incorporate reactants. The substancesextracted were analyzed by infrared spectroscopy from Nicolet380 FT-IR Spectrometer.

2.2. Characterization

The ZnO nano-corncob obtained was characterized by X-raypowder diffraction (XRD) using a Bruker D8 Advance diffractome-ter with CuKa radiation (l = 1.5406 Å). Morphology wasdetermined by scanning electron microscope (SEM) from a JEOL6490 LV; prior to the analysis, the powder was stuck to carbon tapeattached to an aluminum sample holder and then placed into theSEM chamber. The energy band gap (Eg) was determined by theKubelka–Munk function using a UV–vis spectrophotometer(Lambda 35 PerkinElmer Corporation) coupled with an integratingsphere. The room temperature photoluminescence (PL) spectra ofZnO were carried out at Cary Eclipse Fluorescence spectropho-tometer (Agilent Technologies) with an excitation wavelength of370 nm. Specific surface area (SBET) was measured by N2

physisorption through the BET method using QuantachromeNOVA 2000e equipment.

The electrochemical characterization was carried out with apotentiostat/galvanostat AUTOLAB PGSTAT302N connected to apersonal computer running the system software (NOVA) forcontrol of experiments and data acquisition. The chemicalcomposition of electrolytic solution was of 1.0 M Na2SO4. Thephotocurrents generated were studied by chronoamperometryunder simulated sun-light illumination with a solar simulator(Sol1A, Newport).

2.3. Photocatalytic evaluation

Photocatalytic tests were carried out in a quartz reactor(500 mL). The solution contained 150 mL of methyl orange withconcentration of 20 ppm and 150 mg of ZnO catalyst were placedinto a quartz reactor under dark condition to reach the adsorptionequilibrium. Prior to irradiation, the mixture was placed inultrasound during 5 min. The photocatalytic reaction was startedwhen the solar-lamp was turn on. Advance of the photocatalyticreaction was followed by means of UV–vis analysis; so, sampleswere taken at different times during 3 h of reaction. Then theparticles of ZnO catalyst were removed by centrifugation at5000 rpm during 30 min and recovered with a 0.22 mm MilliporeGV filter.

3. Results and discussion

3.1. Synthesis of ZnO nano-corncob

The course of the reaction to the synthesis of ZnO from a Nitalsolution can be divided in two stages accordingly to thetemperature of reaction. The stage 1 and stage 2 were associatedwith the chemical reactions involved at 70 �C and 180 �C,respectively. During the first stage occurred the condensationreaction between ethanol and nitric acid to produce ethyl nitrate(Eq. (1)). When the zinc acetate is added to the ethyl nitratesolution at 70 �C, it is assumed that a zinc intermediate is formed[20], Zn(H3C—CH2—O—NO2)2 (Eq. (2)), which quickly decomposesduring the combustion reaction at 180 �C to form the ZnOnanoparticles and release of gases like CO2, CO, NO2, and steam(Eq. (3)) [21].

H3C—CH2—OH + HNO3! H3C—CH2—O—NO2+ H2O (1)

H3C—CH2—O—NO2+ Zn(H3C—COO)2! Zn(H3C—CH2—O—NO2)2 + 2(H3C—COO) (2)

Fig. 2. XRD patterns of the commercial (curve a) and as-synthesized ZnO from Nitalsolution (curve b) and its corresponding annealed ZnO at: 400 �C (curve c) and600 �C (curve d).

C. Gomez-Solís et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2015) 49–54 51

ZnðH3C� � �CH2�O� � �NO2Þ2 þ 2ðH3C�COOÞþ 602 !180

�C2CO2ðgÞ " þ6COðgÞ " þZnOð1�xÞðsÞ þ 2NO2ðgÞ " þ8H2OðgÞ "

(3)

Fig. 1 shows the FT-IR spectra obtained from samples extractedat different times during each stage. For the first stage: (a) 5 min,(b) 7 min and for second stage: (c) 1 min and (d) 10 min. In Fig. 1(a)it is possible to observe the peaks around 824 and 1429 cm�1,which correspond to the nitrate ions, and the signals at 1026 and1298 cm�1 that are assigned to the bond R—CH2—O. Also it isobserved the signals at 1429 and 1631 cm�1 associated to C¼Obond, and the peak at 3458 cm�1 attributed to the —OH group.With this result we assumed that the ethyl nitrate has beenformed. In the spectra of Fig. 1(b), it can be observed theappearance of a signal between 430 and 500 cm�1, which isattributed to the Zn—O bond. Also it is detected signals between1288 and 1480 cm�1, which were associated to the R—CH2—O andC¼O groups, respectively. The presence of the band at 720 cm�1

indicates that there is hydroxochloride in this step of the thereaction.

On the other hand, the FT-IR spectra obtained during the secondstage indicated the diminishing of the signal intensity related tothe organic bonds due to the combustion process that has beeninitiated. While the signal corresponding to the Zn—O bonddetected around 521 cm�1 increases the intensity, Fig. 1(c). Whenthe combustion reaction has been completed (10 min), the FT-IRspectrum shows only the signal associated to the Zn—O bond,which was detected between 720 and 400 cm�1. Accordingly tothese results, ZnO with high purity, using a very short time of

Table 1I(0 0 2)/I(10 0) ratio, surface area and band gap energy (Eg) of commercial and sythesized

ZnO sample I(0 0 2)/I(10 0)

Commercial 0.64

Nital without annealing 0.86

Nital annealed at 400 �C 0.84

Nital annealed at 600 �C 0.83

reaction, was obtained in this work. Similar results has beenreported by Peulon and Lincot [22] in where they have showed thatthe shape of the absorption band depends on the morphology ofthe zinc oxide crystallites.

3.2. XRD analysis

Fig. 2 shows the XRD patterns for ZnO samples, commercial(curve a), as-synthesized from Nital solution (curve b), andannealed at 400 and 600 �C (curves c and d). All samples presentfor hexagonal single-phase ZnO with the wurzite crystal structurewhere the typical peaks appear at 2u = 32.5�, 34.9� and 37.5� (JCPDScard no. 36-1451), corresponding to the diffraction planes (10 0),(0 0 2) and (10 1), respectively.

It is well known that ZnO with a hexagonal lattice ischaracterized by two interconnecting sublattices of Zn2+ andO2�, where each Zn ion is surrounded by tetrahedral of O ions, andvice-versa [23]. This tetrahedral coordination gives rise to polarsymmetry along the hexagonal axis. This polarity is responsible fordefect generation or the presence of oxygen vacancies on thesurface of ZnO. Li et al. [13] have reported that for case of ZnO, thediffraction intensity ratio of (0 0 2) polar plane to (10 0) nonpolarplane (I(0 0 2)/I(10 0)) is directly proportional with the number ofoxygen vacancies. In our case, the I(0 0 2)/I(10 0) values of ZnOsamples were obtained from XRD data shown in Fig. 2. Thesevalues are shown in Table 1. It is observed that ZnO synthesizedfrom Nital solution show higher I(0 0 2)/I(10 0) values thancommercial ZnO; the highest value was for synthesized ZnOwithout annealing.

3.3. SEM analysis

Fig. 3 shows the FE-SEM image of synthesized ZnO from Nitalsolution without annealing. For this oxide, the morphology isclearly similar with nano-corncobs (�100 nm in size). The nano-corncobs present grains of width lower than 100 nm. Also it is veryimportant to mention that when the powder was annealed at400 �C and 600 �C, its morphology remained unchanged.

3.4. TEM analysis

Fig. 4 shows the TEM results of the ZnO synthesized from Nitalsolution and annealed at 600 �C. Similar topographies were seenfor all samples. The semi-quantitative elemental analysis on thewhite and black zones confirmed the presence of Zn in differentstoichiometric ratio. These results indicate that the nano-corncobsobtained in this work have oxygen vacancies.

3.5. Nitrogen physisorption and energy band gap (Eg) studies

Table 1 resumes the energy band gap and specific surface areavalues for each one of the samples. The BET analysis results revealthat specific surface area of the ZnO obtained from Nital solutionand annealed at 400 and 600 �C decrease in comparison to ZnOwithout annealing. According to the band gap results, the materialssynthesized by Nital solution method could be activated efficientlyin photocatalytic processes using sun-light because their Eg values

ZnO from Nital solution.

Surface area (m2 g�1) Band gap energy (eV)

49 3.220 2.917 2.911 2.9

Fig. 3. FE-SEM images of ZnO with morphology of nano-corncobs obtained from Nital solution.

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are below 3.0 eV. The difference observed in band gap values ofcommercial and synthesized ZnO can be associated with thepresence of oxygen vacancies.

3.6. Luminescence measurements

Room-temperature PL spectra are shown in Fig. 5 forcommercial and synthesized ZnO samples. The results in Fig. 5reflect the effect of synthesis method, the characteristic peak at600 nm related to the singly ionized oxygen vacancy results fromthe recombination of a photogenerated hole with a single ionizedcharge state [24]. The ZnO synthesized from Nital solution showsthat the green emission is higher in comparison to the commercialZnO. This means that oxygen vacancies in synthesized ZnO are

Fig. 4. TEM images for ZnO nano-corncobs obtaine

modified and improved by the Nital-combustion method, due tothe evaporation of organic raw material to form the gasesmentioned in Eq. (2). As a consequence of obtain the ZnO fromNital solution and combustion of ethyl nitrate is that the band gapenergy is shifted toward lower values allowing the photocatalyticactivity to take place at visible sun-light wavelengths.

3.7. Photocorrosion test

One of the great problems of semiconductors is the instabilityand that readily becomes deactivated through photocorrosion orself-oxidation, rather than evolving O2. ZnO is also photo corrodedunder band gap excitation and it process is represented by the nextoverall reaction [25,26]:

d from Nital solution and annealed at 600 �C.

Fig. 5. PL spectra of commercial and synthesized ZnO from Nital solution andcombustion method.

0 20 40 60 80 10 0 12 0 14 0 16 0 180 20 00.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Time (minutes)

(a)

(b)

(c)(d)

Fig. 7. Photodegradation curves of methyl orange using the commercial ZnO (curvea) and synthesized ZnO obtained from Nital solution and combustion method,annealed ZnO at 600 �C (curve b), annealed ZnO at 400 �C (curve c) and ZnO withoutannealing (curve d).

C. Gomez-Solís et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2015) 49–54 53

ZnO þ 2hþ ! Zn2þ þ 12O2 (4)

On this basis, we carried out electrochemical photocurrentmeasurements. Fig. 6 shows the photocurrent transient ofcommercial and synthesized ZnO nano-corncobs structuredelectrode generated under simulated sun-light. The photocurrentresponse was measured in on–off cycles at 0.25 V vs. Ag/AgCl in1.0 M Na2SO4 electrolyte solution without sacrificial reagents orco-catalyst. In this figure is possible to observe that when thesun-light is switch on, the photocurrent of ZnO electrodeincreased rapidly and if the sun-light is switch off, thephotocurrent decreases, for both materials. However, it ispossible to observe that synthesized ZnO presents uniformphotocurrent in these transients, indicating that synthesizedZnO is free of photocorrosion. In the case of commercial ZnO,the photocurrents increment and decrement at repeatable levelsover these cycles, indicates that this material has been photocorroded under these conditions.

Fig. 6. Photocurrent response for the (a) synthesized ZnO from Nital solution andcombustion method and (b) commercial ZnO.

From these results, it is possible to propose that oxygenvacancies in synthesized ZnO from Nital solution helps to improvethe chemical stability of this material.

3.8. Photocatalytic test

Fig. 7 shows the curves of photodegradation of the methylorange by using both commercial and synthesized ZnO from Nitalsolution. It is clear to observe that synthesized ZnO samplespresent better photocatalytic activity than commercial ZnO. On theother side, synthesized ZnO without annealing shows the highestactivity, reaching almost 100% of degradation of methyl orange in90 min under solar simulator; lifetime (t1/2) was around 12 min,which was calculated assuming that kinetic reaction follows a firstorder model. In the case of synthesized ZnO samples annealed at400 and 600 �C, lifetime (t1/2) values were 20 and 28 min,respectively; all these samples showed lower lifetime (t1/2) thancommercial ZnO (t1/2 = 107 min). According with our results, thephotocatalytic activity under solar simulator can be attributed tothe presence of oxygen vacancies that help to increase the activityin visible light region, also the presence of polar plane (0 0 2) inZnO, because the ratio (I(0 0 2)/I(10 0)) is higher for the synthesizedZnO without annealing. Additionally, the presence of oxygenvacancies in ZnO decreases the recombination of electron–holepairs and increases the photocatalytic activity for degradationof methyl orange.

4. Conclusions

ZnO photocatalysts with nano-corncobs morphology wereobtained from Nital solution. These materials exhibited betterphotocatalytic activity under solar simulator than that commercialZnO when photodegradation of methyl orange is performed.The photocatalytic activity of synthesized ZnO is attributed to therelationship between polar and non-polar planes and also tothe presence of oxygen vacancies in this material, which provokethat the value band gap energy of this synthesized ZnOdiminished; this change was attributed to the increasing impuritylevels in ZnO. On other hand, photocurrent experiments undersolar simulator excitation indicate that the presence of oxygenvacancies help to prevent the photocorrosion phenomena in ZnO.

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Acknowledgements

Authors want to thank the financial support for this research toCONACYT through projects: FON.INST./75/2012 “FotosíntesisArtificial”, CNPq México–Brasil 2012-174247, CB-2011-168730, aswell as PAICYT-UANL-2012.

References

[1] N.H. Nickel, E. Terukov, Zinc Oxide – A Material for Micro- and OptoelectronicApplications, NATO Science Series, Dordrecht, 2005.

[2] Y. Wang, J. Sun, X. Fan, X. Yu, A CTAB-assisted hydrothermal and solvothermalsynthesis of ZnO nanopowders, Ceram. Int. 37 (2011) 3431–3436.

[3] X. Fang, Y. Li, S. Zhang, L. Bai, N. Yuam, J. Ding, The dye adsorption optimizationof ZnO nanorod-bared-dye-sensitized solar cells, Sol. Energy 105 (2014) 14–19.

[4] R. Garzia, P. Motto, S. Stassi, A. Sacco, A. Virgia, A. Lamberti, G. Canavesic,Photodetection and piezoelectric response from hard and flexible sponge-likeZnO based structures, Nano Energy 22 (2013) 1294–1302.

[5] C.C. Hsiao, Y.C. Hu, R.C. Chang, C.K. Chao, Residual stresses and mechanicalproperties of ZnO pyroelectric sensor, Theor. Appl. Fract. Mech. 52 (2009) 1–6.

[6] S. Xie, X. Lu, T. Zhai, W. Li, M. Yu, C. Liang, Y. Tong, Enhanced photoactivity andstability of carbon and nitrogen co-treated ZnO nanorod arrays for photo-electrochemical water splitting, J. Mater. Chem. 22 (2012) 14272–14275.

[7] N.F. Hsu, M. Chang, K.T. Hsuc, Rapid synthesis of ZnO dandelion-likenanostructures and their applications in humidity sensing and photocatalysis,Mater. Sci. Semicond. Process. 21 (2014) 200–205.

[8] A.R. Khataee, M. Zarei, Photocatalysis of a dye solution using immobilized ZnOnanoparticles combined with photoelectrochemical process, Desalination 273(2011) 453–460.

[9] V. Subramanian, P.V. Wolf, E. Kamat, Semiconductor-metal compositenanostructures. To what extent do metal nanoparticles improve the photo-catalytic activity of TiO2 films? J. Phys. Chem. B 105 (2001) 11439–11446.

[10] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis innitrogen-doped titanium oxides, Science 293 (2001) 269–271.

[11] S.C. Padmanabhan, S.C. Pillai, J. Colreavy, S. Balakrishanan, D.E. McCornak, T.S.Perova, Y. Gun’ko, S.J. Hinder, J.M. Kelly, Synthesis and thermal and wetting

properties of tin/silver alloy nanoparticles for low melting point lead-freesolders, Chem. Mater. 19 (2007) 4482–4485.

[12] R.S. Mane, W.J. Lee, H.M. Pathan, S.H. Han, Nanocrystalline TiO2/ZnO thin films:fabrication and application to dye-sensitized solar cells, J. Phys. Chem. B 109(2005) 24254–24259.

[13] G.R. Li, T. Hu, G.L. Pan, T.Y. Yan, X.P. Gao, H.Y. Zhu, Morphology-functionrelationship of ZnO: polar planes, oxygen vacancies, and activity, J. Phys. Chem.C 112 (2008) 11859–11864.

[14] X. Xu, D. Chen, Z. Yi, M. Jiang, L. Wang, Z. Zhou, X. Fan, Y. Wang, D. Hui,Antimicrobial mechanism based on H2O2 generation at oxygen vacancies inZnO crystals, Langmuir 29 (2013) 5573–5580.

[15] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, Oxygenvacancy induced band-gap narrowing and enhanced visible light photo-catalytic activity of ZnO, Appl. Mater. Interfaces 4 (2012) 4024–4030.

[16] Y. Lv, C. Pan, X. Ma, R. Zong, X. Bai, Y. Zhu, Production of visible activity and UVperformance enhancement of ZnO photocatalyst via vacuum deoxidation,Appl. Catal. B: Environ. 138–139 (2013) 26–32.

[17] G. Wang, X. Yang, F. Qian, J.Z. Zhang, Y. Li, Double-sided CdS and CdSe quantumdot co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogengeneration, Nano Lett. 10 (2010) 1088–1092.

[18] W. Zhang, G. Chen, Z. Yang, C. Zeng, A novel approach to well-aligned TiO2

nanotube arrays and their enhanced photocatalytic performances, Am. Inst.Chem. Eng. J. 59 (2013) 2134–2144.

[19] ASTM E407–07e1, Standard Practice for Microetching Metals and Alloys, ASTMInternational, West Conshohocken, PA, 2007.

[20] J.E. Mc Murry, Organic Chemical, fifth ed., Thomson, 2012.[21] F.H. Pollard, H.S.B. Marshall, A.E. Pedler, The thermal decomposition of ethyl

nitrate, Trans. Faraday Soc. 52 (1956) 59–68.[22] S. Peulon, D. Lincot, Mechanistic Study of cathodic electrodeposition of zinc

oxide and zinc hydroxychloride films from oxygenated aqueous zinc chloridesolutions, J. Electrochem. Soc. 145 (1998) 864–874.

[23] C. Jagadish, S.J. Pearton, Zinc Oxide Bulk, Thin Films and Nanostructures:Processing, Properties, and Applications, first ed., Elsevier, Amsterdam, 2006.

[24] J. Zhou, F. y. Wang, Zhao, Y. Wang, Y. Zhang, L. Yang, Photoluminescence of ZnOnanoparticles prepared by a novel gel-template combustion process, J. Lumin.119–120 (2006) 248–252.

[25] H. Gerischer, Electrochemical behavior of semiconductors under illumination,J. Electrochem. Soc. 113 (1966) 1174–1181.

[26] A.L. Rudd, C.B. Breslin, Photo-induced dissolution of zinc in alkaline solutions,Electrochim. Acta 45 (2000) 1571–1579.