Artículo asociado a tesis de Doctorado del estudiante Pedro Estanislao Acuña AvilaMetallic Zn nanodisks with hexagonal morphology were obtained onto glass sub- strate under vacuum thermal evaporation. A thermal characterization of Zn nanodiks showed a lower oxidation temperature than source powder Zn. Different thermal treatment on Zn nanodisks played an important role on the morphology, crystal size and surface vibrational modes of ZnO. The growth of ZnO nanoneedles started at the edge of metallic zinc hexagonal structures according with SEM images, the higher temperature the longer needles were grown. XRD diffractogram confirmed the wurtzite structure of ZnO with metallic nuclei. A wide band between 530 and 580 cm−1 of Raman scattering corresponded at surface vibrational modes not observed at higher temperatur

Acuña Avila, Pedro Estanislao

Antunez Flores, Wilber

CAMACHO LOPEZ, MARCO ANTONIO

HERNANDEZ LOPEZ, SUSANA

LOPEZ RAMIREZ, ROBERTO

ORNELAS GUTIERREZ, CARLOS ELIAS

VIGUERAS SANTIAGO, ENRIQUE

AIP Advances

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Repositorio Institucional de la Universidad Autónoma del Estado de México

Effect of thermal treatment on Zn nanodisksPedro E. Acuña-Avila, Roberto López, Enrique Vigueras-Santiago, Susana Hernández-López, MarcoCamacho-López, Carlos Ornelas-Gutierrez, and Wilber AntunezCitation: AIP Advances 5, 067109 (2015); doi: 10.1063/1.4922214View online: https://doi.org/10.1063/1.4922214View Table of Contents: http://aip.scitation.org/toc/adv/5/6Published by the American Institute of PhysicsArticles you may be interested inA comprehensive review of ZnO materials and devicesJournal of Applied Physics 98, 041301 (2005); 10.1063/1.1992666Photoluminescence and ultraviolet lasing of polycrystalline ZnO thin films prepared by the oxidation of themetallic ZnApplied Physics Letters 75, 2761 (1999); 10.1063/1.125141AIP ADVANCES 5, 067109 (2015)Effect of thermal treatment on Zn nanodisksPedro E. Acuña-Avila,1,a Roberto López,1 Enrique Vigueras-Santiago,1Susana Hernández-López,1 Marco Camacho-López,1Carlos Ornelas-Gutierrez,2 and Wilber Antunez21Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA). Facultad deQuímica de la Universidad Autónoma del Estado de México. Paseo Colón esquina PaseoTollocan C.P. 50120, Toluca, Estado de México, México2Centro de investigación en Materiales Avanzados S. C. (CIMAV). Miguel de CervantesN◦ 120. C.P. 31109. Chihuahua, Chihuahua, México(Received 13 April 2015; accepted 24 May 2015; published online 2 June 2015)Metallic Zn nanodisks with hexagonal morphology were obtained onto glass sub-strate under vacuum thermal evaporation. A thermal characterization of Zn nanodiksshowed a lower oxidation temperature than source powder Zn. Different thermaltreatment on Zn nanodisks played an important role on the morphology, crystalsize and surface vibrational modes of ZnO. The growth of ZnO nanoneedles startedat the edge of metallic zinc hexagonal structures according with SEM images, thehigher temperature the longer needles were grown. XRD diffractogram confirmedthe wurtzite structure of ZnO with metallic nuclei. A wide band between 530and 580 cm−1 of Raman scattering corresponded at surface vibrational modes notobserved at higher temperature. C 2015 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution 3.0 UnportedLicense. [http://dx.doi.org/10.1063/1.4922214]I. INTRODUCTIONIt is well known that a lot of oxide metallic semiconductors like SnO2, TiO2 and ZnO can beuseful in electronic devices as gas sensors1–3 due to changes on electrical conductivity when theyare exposed to reducing or oxidizing gases. ZnO have some advantages like thermal and chemicalstability and band energy gap 3.37 eV.4 Most common synthesis of ZnO have a totally oxidationphase without Zn metallic, this occurs either by chemical methods,5–7 electrochemical8,9 or sputter-ing.10,11 Due to the lack of heterojunctions in a completely oxidized phase, ZnO is usually dopedwith metals to improve sensing performance12–14 because of catalytic effect and oxidation resis-tance. ZnO nanoestructures have a higher surface area for sensing with different morphologies ac-cording with the methods and used conditions like needles,15 belts,4 wires16 or rods17 among others,where very thin structures could have better sensing performance because of greater depletion layereffect.13Furthermore, there are a lot of reports in which Zn metallic thick films are oxidized withthermal treatment, but a lot of them do not take to account that treatments on films upper than 500◦Cinduce fragile layers that break easily due to thermal expansion coefficient,22 useless on sensorsdevices. To our knowledge, there is a lack of information about Zn nanostructures as precursorof ZnO because of the melting point of metallic source (419◦C). We used a lower temperaturerange (150-350 ◦C) with different stages of heating for thermal oxidation than used in other papersthat range between 400 and 900◦C18–21 because melt metallic zinc could form other uncontrollablemorphologies. Finally, when a lower temperature is used is possible to obtain ZnO with goodcristalinity and strong UV emission.22 The aim of this work was to evaluate the effect of thermaloxidation at low temperature on metallic Zn nanostructured film isothermally and with heating rate.aCorresponding author: pacunaa004@alumno.uaemex.mx2158-3226/2015/5(6)/067109/7 5, 067109-1 ©Author(s) 2015067109-2 Acuña-Avila et al. AIP Advances 5, 067109 (2015)II. EXPERIMENTALA. Zn depositionIt was used 250 mg metallic Zn powders from distinct purity (99.995% and 98%, both Aldrich).Zn powders were placed in a molybdenum boat (Mo), once the pressure of the vacuum chamberwas pumped to 3 X10−6 Torr, the Zn powders were heated with electrical current with a rate of 3Amp/min until 124 Amp and maintained for 60 min to generate hot zinc vapor. The hot zinc vaporcondensed onto substrates at 88 mm of distance on glass 15 mm x 25 mm washed with xylene,acetone and ethanol in ultrasonic bath for 10 minutes.B. DSC and TGA of Zn and Zn depositionTthermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were done toanalyze physical and chemical properties of Zn films compared with Zn powders (SDT Q 600 TA)on N2 (99.999 %) and O2 (99.999 %) with heating rate of 20◦C/min with a flow of carrier gas of100 mL/min.C. Thermal treatmentsTwo kinds of heating were used to study the effect of thermal treatment at atmospheric pres-sure, one named as isothermal (ISO) which means that temperature was almost constant from initialto the final of treatment, and the other with a heating rate (HR) because of the furnace on/offfunction induce two stages. In the first stage, temperatures reach a maximum for few seconds andin the second stage temperature were maintained. Then 4 treatments were done, two at 150◦C( 3A150_ISO and 3A150_HR) and at 250◦C (3A250_ISO and 3A250_HR) by 3 hours, all followedby slow cooling.D. CharacterizationThe metallic and oxidized structures were characterized by field emission scanning electronmicroscopy (JSM-7401F) with an a acceleration of 5 kV, structural analysis was carried out usingtransmission electron microscopy (TEM JEM-2200FS) with an acceleration of 200 kV, the crystal-line structure of the films was analyzed by an X-ray diffractometer on Bragg-Brentano geometry(XDR, Bruker D8Advances) with CuKα (1.541 A) and Raman spectroscopy was performed withmicro-Raman system (LabRam HR 800, Jobin-Yvon-Horiba) using the 632 nm line of a He-Nelaser in a backscattering configuration with a 50x objective (Olympus BX-41).III. RESULTS AND DISCUSSIONA nanostructured thick film (1-3 µm) was obtained with hexagonal grain and disks morphology(Fig 1(a)). The disks were of different lengths, from 30 nm to 1000 nm, with different thickness. ItFIG. 1. (a) Top view of Zn metallic film, (b) lateral view and (c) and down or mirror view.067109-3 Acuña-Avila et al. AIP Advances 5, 067109 (2015)FIG. 2. DSC and TGA under N2 of a) Zn powder as source material and b) Zn deposited by thermal evaporation.was observed that disks were grown through very thin disks (10 nm) one after another. The filmshad poor adhesion to the substrate, as indicated in other papers26 it was used a strip of adhesiveand tape off the film and a lateral view were obtained (Fig 1(b)), at down of the film is observedparallel structures to the substrate (not included) which form a view mirror (Fig 1(c)). On the downview is clearly seen the hexagonal morphology of the disks, but along the growth of the structures,most of them collapse to form grains. Described disks were similar to few reports about metallic Znnanostructures.23–25The TGA and DSC under N2 atmosphere was used to found if the melting point of Zn waschanged. As can be observed on Fig 2, the endothermic peak on Zn powder was 416◦C, and thedeposited material reduced the melting point by 4◦C. But, indeed another more surprising size effectwas observed; apparently oxidation occurs under N2 atmosphere because of the gain on weight anda wide exothermic peak that indicate reaction not observed with Zn powder.The zinc was oxidized totally in both samples when an oxidant atmosphere was used (Fig 3).Nevertheless, the start and the end of oxidation temperature were different. Zn powder startedthe oxidation at approximately after the melting point and finished at 800◦C; by the other hand,Zn nanostructures started before the melting point at 314◦C where a small exothermic peak wasobserved, and finished at 597◦C. This difference is related with the size of the particles.18On FESEM micrograph 3A150_ISO (Fig. 4(a)) were observed that hexagons are regular withwell defined angles and thinner than untreated film. This phenomenon have not been well studied,we believe that at this temperature atoms migrate to form a stable structure, other workers23 proposethat is a nonequilibrium process that can help determine the evolution of the surface morphologyand size of the Zn hexagonal nanodisks. Also, have not been studied the change on melting temper-ature of Zn nanodisks, according to the model proposed by Qi27 the reduction of melting point onnanostructures is proportional to the atoms on the surface. Taking into account the geometry ofnanodisks, the thickness of the disks defines a change on melting point.When the treatment included a peak on the heating rate of 300◦C by few seconds (3A150_HR,Fig 4(b)) was observed a crystal growth of very thin nanoflakes with an average length of 100 nmFIG. 3. DSC and TGA under O2 of a) Zn powder as source material and b) Zn deposited by thermal evaporation.067109-4 Acuña-Avila et al. AIP Advances 5, 067109 (2015)FIG. 4. FESEM images of Zn treated thermally by 3 hours with different conditions: 3A150_ISO (a), 3A150_HR (b),3A250_ISO(c) and 3A250_HR (d).with a thickness <20 nm at the edge of the hexagons. The oxidation process started at the edgesbecause it is more stable energetically, this was an epitaxial growth because there were not sterichindrance in other directions,24 but, neither XRD nor Raman was detected the presence of ZnOperhaps by the detection limit of each technique.At higher temperature, oxidation was favored in all directions. On FESEM images of 3A250_ISO(Fig 4(c)) nanoneedles were observed with width of 10 to 100 nm and length from 10 to 500 nm,but when a maximum temperature was reached, even by few seconds (3A250_HR, Fig 4(d)) needleswere grown a microns on length and some nanobelts were formed.FIG. 5. Diffractogram of deposited Zn nanostructures.067109-5 Acuña-Avila et al. AIP Advances 5, 067109 (2015)FIG. 6. Diffractogram of Zn thermally treated 3 hours at 250◦C.An XRD analysis of deposited Zn nanodiks indicate (Fig 5) that crystallites were highly ori-ented to c-axis, a strong signal at 2θ=36.52◦ and a very weak at 2θ=39◦ corresponding to theplanes (0002) and (101̄0) respectively, showing the wurtzite structure with lattice constants ofa=b=0.266 nm, c =0.49 nm, α=β=90◦ and γ=120◦ (JCPDS card no. 65-3358)X-ray diffraction detected ZnO in the thermal treatments at 250◦C. No differences on crys-tal structure was detected between ISO an RH treatments, both had 5 peaks, the first three werebroad and less intense and the other two were sharp and intense (Fig 6.). The first three signals at2θ =31.66, 34.34◦ and 36.15◦ corresponding to the planes (101̄0), (0002) and (101̄1) respectively,showing the wurtzite structures with lattice constants of a=b=0.326 nm, c=0.522, α=β=90◦ andγ=120◦ (JCPDS card no. 36-1451). The sharp and intense peaks at 2θ=39◦ and 43.21◦ corre-sponding to the planes (101̄0) and (101̄1) of metallic Zn respectively (JCPDS card no. 65-3358).Using Sherrer’s equation, the broad signals due to ZnO indicate smaller crystallites than the Zncrystallites, calculating the crystallite of ZnO is approximately 20 nm.FIG. 7. (a) Nanoneedle extracted from 3A250_ISO and (b) zoom at the edge of the nanoneedle where measure is accordingto ZnO(101̄0).067109-6 Acuña-Avila et al. AIP Advances 5, 067109 (2015)FIG. 8. Scattering Raman spectra of films thermally treated at 250◦C.The XRD of nanoflakes, nanoneedles and nanobelts demonstrated that there were ZnO crys-tallites that were grown on preferential direction c-axis, and there still metallic Zn with differentcrystalline arrangement compared with Zn-deposition (Fig 5).TEM was used to examine the crystalline structure of nanoneedles obtained under treatment3A250_ISO. The high magnification TEM image in Fig 7(a) displays a nanoneedle. The portion ofthe edge shown in Fig. 7(b) revealed the atomic planes of ZnO (101̄0). This confirms the crystallinestructure of ZnO obtained under thermal treatment of Zn nanodisks.Raman spectra (Fig. 8) confirmed the presence of nanometric crystals of ZnO due to a scatterband between 530 and 580 cm−1 which corresponds to vibration mode A1(LO), this mode havebeen linked with oxygen vacancies, interstitial Zn, or complex defects with oxygen vacancies andinterstitial Zn on the ZnO lattice.10 The vibrational mode E2(High) is intense when the temperatureof the thermal treatment is higher than 400◦C linked with a higher degree of cristalinity that couldbe confirmed by XRD with larger crystal size. Therefore, the vibrational band 530-580 cm−1 isattributed to the presence of nanometric crystals (<20 nm) on the surface of metallic Zn nuclei.IV. CONCLUSIONMetallic nanostructured Zn film obtained by thermal evaporation had a lower melting point andlower oxidation temperature than Zn powder and then it was possible to obtain ZnO nanostruc-tures with thermal treatments at temperatures lower than the melting point of Zn. ZnO nanoflakes,nanoneedless and nanobelts were obtained, although similar temperature ranges with different heat-ing stages. The higher temperature was reached, the longer ZnO structures were observed. Finally,Zn/ZnO core/shell structures were obtained when thermal oxidation is performed at 250◦C, char-acterized by smaller crystal size and surface vibrational modes. Optical, electrical and sensingproperties are needed to be investigated.1 S Wang, P Wang, C Xiao, Z Li, B Xiao, R Zhao, and T. Yang, Mater Lett 131, 358-60 (2014).2 F Pourfayaz, Y Mortazavi, a. Khodadadi, and S. Ajami, Sens Actuators, B 130, 625-–9 (2008).3 Y Kwon, H Kim, S Lee, IJ Chin, TY Seong, and WI. 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Effect of thermal treatment on Zn nanodisks

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Effect of thermal treatment on Zn nanodisks

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