Effects of Experimental Configuration on the Morphology of Two

crystals

Article Eﬀects of Experimental Conﬁguration on the Morphology of Two-Dimensional ZnO Nanostructures Synthesized by Thermal Chemical-Vapor Deposition

Seok Cheol Choi 1, Do Kyung Lee 2,* and Sang Ho Sohn 3,*

1 Department of Process Development, LG Electronics, Gumi, Gyeongsangbuk-do 3938, Korea; [email protected] 2 School of Advanced Materials Science and Chemical Engineering, Daegu Catholic University, Gyeongsan, Gyeongsangbuk-do 38430, Korea 3 Department of Physics, Kyungpook National University, Daegu 41566, Korea * Correspondence: [email protected] (D.K.L.); [email protected] (S.H.S.)

Received: 20 April 2020; Accepted: 14 June 2020; Published: 17 June 2020

Abstract: Using two experimental configurations, self-assembled zinc oxide (ZnO) nanostructures including nanoplates, nanosaws, and nanobelts were synthesized by thermal chemical-vapor deposition (CVD), and their morphological properties were investigated. ZnO nanostructures grown on Au-coated Si substrates in a parallel setup revealed highly defined ZnO nanoplates and branched nanowires. ZnO nanostructures grown in a perpendicular setup using Si substrates with and without the Au catalyst exhibited vertically oriented ZnO nanosaws and randomly aligned nanobelts, respectively. In the thermal CVD method, experiment conditions such as oxygen-flow rate, growth temperature, and catalyst, and experimental configurations (i.e., parallel and perpendicular setups) were important parameters to control the morphologies of two-dimensional ZnO nanostructures showing platelike, sawlike, and beltlike shapes.

Keywords: experimental conﬁguration; ZnO nanoplates; ZnO nanosaws; ZnO nanobelts; thermal chemical-vapor deposition; two-dimensional nanostructures

1. Introduction Zinc oxide (ZnO) has attracted much attention because of its interesting characteristics such as its near-ultraviolet (UV) emission, transparent conductivity, semiconducting property, and piezoelectricity [1–3]. Self-assembled ZnO nanostructures with different morphologies have promising applications in many fields such as nanolasers [4], piezoelectric-power generation [5], gas sensing [6], and solar cells [7] owing to their high photoelectric reaction, large surface area, cost-effective fabrication, and ecofriendly nature. Unusual material properties of ZnO nanostructures with unique morphologies were reported, including the presence of nanobelts [8], nanocages [9], nanocombs [10], and nanoforests [11]. In particular, two-dimensional ZnO nanostructures, such as nanosheets and nanoplates, are advantageous for catalysis and photocatalysis applications due to their high surface-to-volume ratio [12]. The specific surface areas of ZnO nanostructures can be enhanced by controlling their shapes and morphologies. Various methods were developed to synthesize ZnO nanostructures, including thermal chemical-vapor deposition (CVD) [13], metal-organic CVD [14], the solution process [15], molecular beam epitaxy [16], and laser ablation [17]. Among these methods of growing ZnO nanostructures, the thermal CVD method has received great interest because the growth process is relatively simple,

Crystals 2020, 10, 517; doi:10.3390/cryst10060517 www.mdpi.com/journal/crystals Crystals 2020, 10, 517 2 of 10 and the morphology of the nanostructures can easily be tuned by varying experiment parameters such as temperature and oxygen partial pressure. In most cases of thermal CVD, substrates are placed in a position parallel to the boat with the source powders in a quartz reactor tube. When the temperature of the boat increases, the mixture of ZnO and carbon powders is vaporized, and Zn is formed by a carbothermal reduction process. Zn reacts with oxygen under ambient conditions on the Si substrates, leading to the growth of ZnO nanostructures. Because vapor transport and reactions depend on the position in the reactor tube [18], the location of the substrates has a significant effect on the morphologies of the synthesized nanostructures. In addition, depending on the geometry of the reactor tube, two-dimensional ZnO nanostructures can be observed during thermal CVD growth on Si substrates under specific growth conditions [19,20]. Hence, we examined the influence of experiment setups and selective conditions in thermal CVD on the growth of self-assembled ZnO nanostructures. A metal catalyst on the substrate surface is typically required for the initiation and guidance of ZnO nanostructure growth in thermal CVD, resulting in vapor-liquid-solid (VLS) [21,22] and vapor-solid (VS) [20,23] mechanisms. On the other hand, catalyst-free growth of two-dimensional ZnO nanostructures by thermal CVD is interesting with respect to cost-effectiveness. VS growth or self-catalyzed growth mechanisms [24,25] are most often used to describe the catalyst-free growth of ZnO nanostructures. In this study, we investigated the effects of growth conditions on the morphology of catalyst-mediated ZnO nanostructures synthesized by thermal CVD under different experimental configurations, specifically parallel and perpendicular setups. Two-dimensional ZnO nanostructures were prepared by changing oxygen-flow rate from 0.1 to 2.5 sccm, and growth temperature from 700 to 900 ◦C. Furthermore, the catalyst-free growth of ZnO nanostructures in thermal CVD with a perpendicular setup was investigated in relation to growth temperatures.

2. Experiments Two-dimensional ZnO nanostructures with different morphologies were grown in a horizontal quartz reactor tube by thermal CVD using two types of experimental configurations, as shown in Figure1. Parallel and perpendicular setups were classified by the position of substrate to the gas flow. In the parallel setup, the Zn precursor was placed on the upstream side. Distance between substrate and precursor in the parallel setup was fixed at about 7 cm. The substrate in the perpendicular setup vertically faced the precursor at a distance of 4 mm. Before growing the ZnO nanostructures, for the catalyst-assisted growth of ZnO nanostructures, a gold layer was coated onto the Si substrates by direct-current sputtering. To prepare a Zn precursor, ZnO powder (99.9%, Sigma-Aldrich, Saint Louis, MO, USA) was mixed with graphite powder (99.9%, Sigma-Aldrich, Saint Louis, MO, USA) at a 1:1 weight ratio. The mixed powders were placed in an alumina boat inserted into a quartz tube and placed close to the middle of the furnace. Prior to heating, a furnace system was flushed with inert Ar gas for 1 h to remove contamination and residual gases in the reactor. A heating system was used to increase the temperature inside the furnace. After initially maintaining at room temperature for 1 h, the temperature was raised to the desired growth temperature at a rate of 5 ◦C/min. After the desired temperature was attained, the growth time of ZnO nanostructures was 1 h. Growth temperature was changed from 700 to 900 ◦C. During the growth of the ZnO nanostructures, a mixture of Ar and O2 was introduced toward the right side from the left side in the reactor. The flow rates of Ar and O2 were measured by flowmeter. O2 flow rate was varied from 0.1 to 2.5 sccm, while Ar flow rate was maintained at 100 sccm. Finally, after ZnO nanostructure growth, the furnace was turned off and allowed to cool to room temperature. Crystals 2020, 10, 517 3 of 10 Crystals 2020, 10, x FOR PEER REVIEW 3 of 10

FigureFigure 1.1. SchematicsSchematics of of thermal thermal chemical-vapor chemical-vapor deposition deposition (CVD) (CVD) setups setups in this in this study: study: (a) parallel (a) parallel and (andb) perpendicular (b) perpendicular setups. setups.

TheBefore morphologies growing the ofZnO the nanostructures, ZnO nanostructures for the were catalyst-assisted observed using growth a field-emission of ZnO nanostructures, scanning electrona gold layer microscope was coated (FE-SEM, onto the S-4800, Si substrates Hitachi, by di Tokyo,rect-current Japan). sputtering. To clearly To observe prepare the a Zn shapes precursor, and morphologiesZnO powder (99.9%, of the ZnO Sigma-Aldrich, nanostructures, Saint accelerationLouis, MO, USA) voltage was was mixed measured with graphite by changing powder from (99.9%, 10 to 20Sigma-Aldrich, kV. Room-temperature Saint Louis, photoluminescence MO, USA) at a 1:1 (PL, weight LabRam ratio. HR, The Horiba, mixed Kyoto, powders Japan) were characteristics placed in an ofalumina the ZnO boat nanostructures inserted into a grown quartz on tube the and Si wafer placed were close measured. to the middle In the of the PL measurement,furnace. Prior to a 325heating, nm wavelengtha furnace system He-Cd was Laser flushed (30 mW)with andinert a Ar GaAs gas for photomultiplier 1 h to remove tube contamination were used and for excitationresidual gases and detection,in the reactor. respectively. A heating X-ray system diffraction was used (XRD, to X’Pert increase MPD, the PANalytical, temperature Almelo, inside Netherlands)the furnace. usingAfter Cu-Kinitiallyα radiation maintaining at room at room temperature temperature was performedfor 1 h, the intemperature order to investigate was raised the to crystal the desired structures growth of ZnOtemperature nanostructures. at a rate The of 5 2theta-theta °C/min. After measurement the desired method temperature was used was for attained, crystallinity the growth analysis. time of ZnO nanostructures was 1 h. Growth temperature was changed from 700 to 900 °C. During the 3. Results and Discussion growth of the ZnO nanostructures, a mixture of Ar and O2 was introduced toward the right side from the leftFigure side2 in shows the reactor. FE-SEM The images flow rates of ZnO of Ar nanostructures and O2 were measured grown on by Au-coatedflowmeter. SiO2 substrates flow rate was at avaried constant from growth 0.1 to temperature 2.5 sccm, while of 800 Ar◦ Cflow in the rate parallel was maintained setup while at varying100 sccm. the Finally, oxygen-flow after ZnO rate ofnanostructure the mixed gas growth, with argonthe furnace and oxygen. was turned As shownoff and inallowed Figure to2a, cool the to ZnO room nanostructures temperature. grown at anThe Ar/ Omorphologies2 flow rate of of 120 the/0.1 ZnO sccm nanostructures had many interconnected were observed or using branched a field-emission nanowires and scanning a few nanoplates.electron microscope Au nanoparticles (FE-SEM, were S-4800, observed Hitachi, at theTokyo, top ofJapan). the nanowires, To clearly indicating observe thatthe shapes the growth and processmorphologies of the of ZnO the nanowires ZnO nanostructures, grown at an acceleration Ar/O2 flow voltage rate of was 120 measured/0.1 sccm couldby changing be explained from 10 by to the20 kV. VLS Room-temperature mechanism. However, photoluminescence because no Au (PL, particles LabRam were HR, Horiba, observed Kyoto, in the Japan) FE-SEM characteristics images of theof the ZnO ZnO nanoplates nanostructures grown grown at an Ar on/O the2 flow Si wafer rate ofwere 120 measured./0.1 sccm, ZnOIn the nanoplates PL measurement, could have a 325 been nm synthesizedwavelength byHe-Cd the VS Laser mechanism. (30 mW) Accordingly,and a GaAs fromphotomultiplier the viewpoint tube of were the VS used growth for excitation process, ZnO and nanoplatesdetection, respectively. were grown byX-ray thermally diffraction evaporating (XRD, X’Pert a precursor, MPD, and PANalytical, condensation Almelo, on the Netherlands) Au catalyst. Inusing Figure Cu-K2b,α the radiation ZnO nanostructure at room temperature grown at anwas Ar performed/O2 flow rate in oforder 120 /1.0to investigate sccm exhibited the highlycrystal definedstructures ZnO of ZnO nanoplates nanostructures and branched. The 2theta-theta nanowires. measurement As seen in Figure method2c, thewasZnO usednanostructures for crystallinity grownanalysis. at an Ar/O2 flow rate of 120/2.5 sccm showed almost the same number of ZnO nanoplates and

3. Results and Discussion

Crystals 2020, 10, x FOR PEER REVIEW 4 of 10

Figure 2 shows FE-SEM images of ZnO nanostructures grown on Au-coated Si substrates at a constant growth temperature of 800 °C in the parallel setup while varying the oxygen-flow rate of the mixed gas with argon and oxygen. As shown in Figure 2a, the ZnO nanostructures grown at an Ar/O2 flow rate of 120/0.1 sccm had many interconnected or branched nanowires and a few nanoplates. Au nanoparticles were observed at the top of the nanowires, indicating that the growth process of the ZnO nanowires grown at an Ar/O2 flow rate of 120/0.1 sccm could be explained by the VLS mechanism. However, because no Au particles were observed in the FE-SEM images of the ZnO nanoplates grown at an Ar/O2 flow rate of 120/0.1 sccm, ZnO nanoplates could have been synthesized by the VS mechanism. Accordingly, from the viewpoint of the VS growth process, ZnO nanoplates were grown by thermally evaporating a precursor, and condensation on the Au catalyst. In Figure 2b,Crystals the 2020ZnO, 10 nanostructure, 517 grown at an Ar/O2 flow rate of 120/1.0 sccm exhibited highly defined 4ZnO of 10 nanoplates and branched nanowires. As seen in Figure 2c, the ZnO nanostructures grown at an Ar/O2 flow rate of 120/2.5 sccm showed almost the same number of ZnO nanoplates and nanowires, nanowires, displaying irregularly shaped nanostructures. From this result, it can be proposed that displaying irregularly shaped nanostructures. From this result, it can be proposed that excess oxygen excess oxygen gas prohibited nanoplate synthesis during the VS growth process. Due to diﬀerent gas prohibited nanoplate synthesis during the VS growth process. Due to different oxygen contents oxygen contents in the gas mixtures, diﬀerent morphological features of the ZnO nanoplates were in the gas mixtures, different morphological features of the ZnO nanoplates were created. Therefore, created. Therefore, we suggest that oxygen concentration in the gas mixture was a crucial parameter to we suggest that oxygen concentration in the gas mixture was a crucial parameter to control the control the morphologies of ZnO nanoplates in the parallel setup. morphologies of ZnO nanoplates in the parallel setup.

FigureFigure 2. Field-emissionField-emission scanning-electron-microscope scanning-electron-microscope (FE-SEM) (FE-SEM) images images of ZnO of nanostructuresZnO nanostructures grown grownon Au-coated on Au-coated Si substrates Si substrates at a constant at a constant growth gr temperatureowth temperature of 800 ◦ Cof in800 parallel °C in parallel setup with setup varying with varyingAr/O2 ﬂow Ar/O rates2 flow in rates the gas in mixture;the gas mixture; (a) 120/0.1 (a) sccm,120/0.1 (b sccm,) 120/1.0 (b) sccm,120/1.0 and sccm, (c) 120 and/2.5 (c) sccm. 120/2.5 sccm.

Figure3 shows the PL spectrum of ZnO nanostructures with nanoplates and branched nanowires grown on Au-coated Si substrates in the parallel setup at an Ar/O2 flow rate of 120/1.0 sccm. We noted a narrow UV band at 380 nm (3.26 eV) and a broad green band at 570 nm (2.17 eV). UV band emission was due to a near band-edge transition of ZnO nanorods [26]. Green-band emission is caused by the radial recombination of a photogenerated hole with an electron that belongs to singly ionized oxygen vacancy, which was also observed in the bulk ZnO [27]. It was obvious that the green-band emission of the ZnO nanostructures grown at an Ar/O2 flow rate ratio of 120/2.5 was weaker than that of the ZnO nanostructures grown at an Ar/O2 flow rate ratio of 120/1.0, resulting from the reduction of oxygen vacancies in the ZnO nanostructures due to the increase of O2 content in the gas mixture. Thus, the ZnO nanostructure grown on Au-coated Si substrates in the parallel setup was oxygen-deficient nanoplates, exhibiting similar PL properties to the bulk ZnO. Crystals 2020, 10, x FOR PEER REVIEW 5 of 10

Figure 3 shows the PL spectrum of ZnO nanostructures with nanoplates and branched nanowires grown on Au-coated Si substrates in the parallel setup at an Ar/O2 flow rate of 120/1.0 sccm. We noted a narrow UV band at 380 nm (3.26 eV) and a broad green band at 570 nm (2.17 eV). UV band emission was due to a near band-edge transition of ZnO nanorods [26]. Green-band emission is caused by the radial recombination of a photogenerated hole with an electron that belongs to singly ionized oxygen vacancy, which was also observed in the bulk ZnO [27]. It was obvious that the green-band emission of the ZnO nanostructures grown at an Ar/O2 flow rate ratio of 120/2.5 was weaker than that of the ZnO nanostructures grown at an Ar/O2 flow rate ratio of 120/1.0, resulting from the reduction of oxygen vacancies in the ZnO nanostructures due to the increase of O2 content Crystalsin the 2020gas , mixture.10, 517 Thus, the ZnO nanostructure grown on Au-coated Si substrates in the parallel5 of 10 setup was oxygen-deficient nanoplates, exhibiting similar PL properties to the bulk ZnO.

Figure 3. Photoluminescence (PL) spectrum of ZnO nanoplates and branched nanowires grown on Figure 3. Photoluminescence (PL) spectrum of ZnO nanoplates and branched nanowires grown on Au-coated Si substrates in parallel setup with varying Ar/O2 flow rates of 120/1.0 sccm (dotted line) Au-coated Si substrates in parallel setup with varying Ar/O2 flow rates of 120/1.0 sccm (dotted line) and 120/2.5 sccm (dashed line). and 120/2.5 sccm (dashed line). Figure4 shows FE-SEM images of ZnO nanostructures grown in the perpendicular setup on Au-coatedFigure Si4 shows substrates FE-SEM as aimages function of ZnO of growth nanostruct temperature.ures grown According in the perpendicular to Figure4 setupa, the on ZnO Au- coated Si substrates as a function of growth temperature. According to Figure 4a, the ZnO nanostructures grown at 700 ◦C had many nanosaws and some nanorods that were randomly oriented. nanostructures grown at 700 °C had many nanosaws and some nanorods that were randomly As shown in Figure4c, ZnO nanostructures grown at 900 ◦C had many nanowires and a few nanosaws. oriented. As shown in Figure 4c, ZnO nanostructures grown at 900 °C had many nanowires and a Contrary to these results, as illustrated in Figure4b, ZnO nanostructures grown at 800 ◦C had vertically orientedfew nanosaws. ZnO nanosaws Contrary withto these a tapered results, shape. as illustrated Therefore, in growthFigure 4b, temperature ZnO nanostructures was a key parameter grown at that800 °C could had be vertically used to controloriented the ZnO morphologies nanosaws ofwith ZnO a tapered nanosaws. shape. It is wellTherefore, known growth that ZnO temperature nanosaws arewas synthesized a key parameter by the that catalyzed could growthbe used of to the contro Zn-terminatedl the morphologies (001) surface, of ZnO while nanosaws. the O-terminated It is well (001)known surface that ZnO is relatively nanosaws chemically are synthesized inactive [ 28by]. th Moreover,e catalyzed the growth morphology of the of Zn-terminated ZnO nanostructures (001) issurface, determined while bythe growth O-terminated temperature, (001) whichsurface is is related relatively to surface chemically energy inactive including [28]. polarization Moreover, andthe morphology of ZnO nanostructures is determined by growth temperature, which is related to surface termination [29]. As such, it is suggested that a growth temperature of 800 ◦C resulted in self-catalyzed growth,energy including yielding tapered polarization and vertically and termination aligned nanostructures, [29]. As such, as shownit is suggested in Figure4 b.that a growth temperatureFigure5 showsof 800 FE-SEM°C resulted images in self-catalyzed of ZnO nanostructures growth, yielding grown intapered a perpendicular and vertically setup aligned on Si substratesnanostructures, without as sh theown Au in catalyst Figure 4b. as a function of growth temperature. As shown in Figure5a, ZnO nanostructures grown at 700 ◦C had many nanowires and some nanosheets that were randomly oriented. The ZnO nanostructures grown at 800 ◦C, as illustrated in Figure5b, had many nanowires and some ZnO nanobelts that had widths ranging from 1 to 2 µm. As seen in Figure5c, ZnO nanostructures grown at 900 ◦C had many nanoplates and a few nanowires. Thus, we can confirm that ZnO nanobelts with wide widths can be synthesized in growth process without introducing a catalyst [30]. In all samples, ZnO nanostructures were randomly oriented because the Au catalyst was not used in the growth process. This is related to the formation of a polycrystal ZnO film without the preferred orientation at an initial growth stage [31]. Therefore, growth temperature in the perpendicular setup without the Au catalyst was an important parameter that could be used to control the morphologies of ZnO nanobelts under suitable growth conditions, such as under specific partial oxygen pressure or with the use of specific substrate-setup positions. Crystals 2020, 10, 517 6 of 10 Crystals 2020, 10, x FOR PEER REVIEW 6 of 10

FigureFigure 4. 4. FE-SEMFE-SEM images images of of ZnO ZnO nanostructures nanostructures grown grown in in perpendicular perpendicular setup setup on on Au-coated Au-coated Si Si

substratessubstrates as as function function of ofgrowth growth temperature temperature at Ar/O at Ar2 flow/O2 ﬂowrate of rate 120/1.0 of 120 sccm/1.0 in sccm gas mixture: in gas mixture: (a) 700 °C,(a) ( 700b) 800◦C, °C, (b) and 800 ◦(c)C, 900 and °C. (c) Scale 900 ◦ C.bar Scale is 3 μ barm. is 3 µm.

It is well known that ZnO nanobelts that show flat top and bottom surfaces (2–10), and side Figure 5 shows FE-SEM images of ZnO nanostructures grown in a perpendicular± setup on Si surfaces (001) grow along [010] and occur bending due to strain [30]. Moreover, the morphology of substrates± without the Au catalyst as a function of growth temperature. As shown in Figure 5a, ZnO nanostructuresZnO nanostructures grown grown at 700 without °C had amany catalyst nanowires can be determined and some bynanosheets growth temperature,that were randomly which is oriented.related to The the ZnO chemical nanostructures inactivity ofgrown the surface at 800 °C, and as the illustrated activation in energyFigure of5b, normal had many VS growthnanowires [26]. andFrom some these ZnO facts, nanobelts it is suggested that had that awidths growth ranging temperature from of1 800to 2◦ Cμ inm. aAs perpendicular seen in Figure setup 5c, without ZnO nanostructuresa catalyst led to grown normal at VS 900 growth, °C had yielding many nanopl bent andates randomly and a few aligned nanowires. nanostructures. Thus, we can confirm that ZnOHereafter, nanobelts weconsider with wide catalyst-mediated widths can be synt ZnOhesized nanostructures in growth grown process under without different introducing experiment a catalystsetups at[30]. a growth In all temperaturesamples, ZnO of nanostructures 800 ◦C and an Ar were/O2 flowrandomly rate of oriented 120/1.0 sccm.because The the eff ectAu ofcatalyst the Au wascatalyst not used on ZnO in the nanostructure growth process. growth This duringis relate thermald to the formation CVD with of a a perpendicular polycrystal ZnO setup film under without the thesame preferred growth conditionsorientation is at reviewed. an initial growth stage [31]. Therefore, growth temperature in the perpendicularAs seen in setup Figure without2b, in the the parallel Au catalyst setup, ZnO was nanoplates an important showing parameter rectangular that shapescould werebe used grown to controlusing twothe morphologies growth processes of ZnO consisting nanobelts of two-dimensionalunder suitable growth filling conditions, and one-dimensional such as under branching specific partialyielding oxygen ZnO nanoplatespressure or and with branched the use of nanowires. specific substrate-setup The ZnO nanoplates positions. had widths ranging from 3 to 4 µmIt andis well lengths known ranging that ZnO from nanobelts 4 to 6 µm. that The show thicknesses flat top ofand the bottom nanoplates surfaces and ± the (2–10), lengths and of side the surfacesbranched ± (001) nanowires grow along ranged [010] from and 300 occur to 600 bendin nm andg due 2 to to 4strainµm, respectively.[30]. Moreover, However, the morphology as shown of in ZnOFigure nanostructures4b, ZnO nanosaws grown showing without triangular a catalyst shapescan be weredetermined tapered by from growth the bottom temperature, on the substratewhich is relatedin the CVDto the process, chemical and inactivity had widths of the ranging surface fromand the 200 activation to 400 nm energy at their of basenormal and VS about growth 20 nm[26]. at Fromthe tip. these In facts, contrast, it is suggested as illustrated that a in growth Figure 5teb,mperature ZnO nanobelts of 800 °C grown in a perpendicular in the perpendicular setup without setup a catalyst led to normal VS growth, yielding bent and randomly aligned nanostructures.

Crystals 2020, 10, 517 7 of 10 without the Au catalyst were bent during the CVD growth process and had widths ranging from 1 to 2 µm. The tapered nanosaws exhibited a strongly preferred an orientation with respect to the c plane, as shown in Figure4b. Here, we can tell that the parallel setup led to the growth of randomly oriented plate-shaped ZnO nanostructures due to the temperature gradient between source materials and substrates in the quartz tube. However, the perpendicular setup led to the growth of well-deﬁned saw- and belt-shaped nanostructures due to stable temperature distribution between source materials and substrates in the quartz tube. The perpendicular setup, with and without the Au catalyst, caused Crystalsthe growth 2020, 10 of, x vertically FOR PEER alignedREVIEW ZnO nanosaws and randomly aligned ZnO nanobelts, respectively,7 of on10 Si substrates.

FigureFigure 5. FE-SEM imagesimages of of ZnO ZnO nanostructures nanostructures grown grown in perpendicular in perpendicular setup setup on Si substrateson Si substrates without

withoutAu catalyst Au catalyst as function as function of growth of temperaturegrowth temperature at a Ar/ Oat2 aﬂow Ar/O rate2 flow of120 rate/1.0 of sccm120/1.0 in sccm gas mixture: in gas mixture:(a) 700 ◦ C,(a)( b700) 800 °C,◦ (C,b) and800 (°C,c) 900 and◦ C.(c) 900 °C.

Hereafter,Figure6 shows we theconsider XRD patternscatalyst-mediated of the ZnO ZnO nanoplates nanostructures (Figure2b), grown nanosaws under (Figure different4b), experimentand nanobelts setups (Figure at a5 growthb) grown temperature via the thermal of 800 CVD °C and method. an Ar/O The2 XRDflow patternsrate of 120/1.0 exhibited sccm. a large The effectpeak atof aboutthe Au 69.16 catalyst◦ corresponding on ZnO nanostructure to the Si substrate. growth during All samples thermal exhibited CVD with diff araction perpendicular peaks of setup(100), under (002), the (101), same (102), growth (110), conditions (103), (200), is reviewed. and (112) planes, consistent with the hexagonal wurtzite structureAs seen (Joint in committeeFigure 2b, onin the powder parallel diff ractionsetup, ZnO standards nanoplates (JCPDS) showing no. 89-1397). rectangular This indicatedshapes were that grownmorphological using two changes growth in the processes ZnO nanostructures consisting seldomof two-dimensional affected the crystal filling structure. and one-dimensional The diffraction branchingpeak of the yielding (002) plane ZnO for nanoplates the ZnO nanoplates and branched and nanobelts nanowires. was The weakly ZnO observed nanoplates compared had widths to that rangingof the (101) from plane, 3 to 4 implying μm and alengths weakly ranging preferred from orientation 4 to 6 μm. in The the c-axisthicknesses direction. of the However, nanoplates a strong and the lengths of the branched nanowires ranged from 300 to 600 nm and 2 to 4 μm, respectively. However, as shown in Figure 4b, ZnO nanosaws showing triangular shapes were tapered from the bottom on the substrate in the CVD process, and had widths ranging from 200 to 400 nm at their base and about 20 nm at the tip. In contrast, as illustrated in Figure 5b, ZnO nanobelts grown in the perpendicular setup without the Au catalyst were bent during the CVD growth process and had widths ranging from 1 to 2 μm. The tapered nanosaws exhibited a strongly preferred an orientation with respect to the c plane, as shown in Figure 4b. Here, we can tell that the parallel setup led to the growth of randomly oriented plate-shaped ZnO nanostructures due to the temperature gradient between source materials and substrates in the quartz tube. However, the perpendicular setup led to the growth of well-defined saw- and belt-shaped nanostructures due to stable temperature

Crystals 2020, 10, x FOR PEER REVIEW 8 of 10

distribution between source materials and substrates in the quartz tube. The perpendicular setup, with and without the Au catalyst, caused the growth of vertically aligned ZnO nanosaws and randomly aligned ZnO nanobelts, respectively, on Si substrates. Figure 6 shows the XRD patterns of the ZnO nanoplates (Figure 2b), nanosaws (Figure 4b), and nanobelts (Figure 5b) grown via the thermal CVD method. The XRD patterns exhibited a large peak at about 69.16° corresponding to the Si substrate. All samples exhibited diffraction peaks of (100), (002), (101), (102), (110), (103), (200), and (112) planes, consistent with the hexagonal wurtzite structure (Joint committee on powder diffraction standards (JCPDS) no. 89-1397). This indicated that morphological changes in the ZnO nanostructures seldom affected the crystal structure. The Crystalsdiffraction2020, 10peak, 517 of the (002) plane for the ZnO nanoplates and nanobelts was weakly observed8 of 10 compared to that of the (101) plane, implying a weakly preferred orientation in the c-axis direction. diHowever,ﬀraction a peak strong of thediffraction (002) plane peak and of athe very (002) weak plane peak and of a the very (101) weak plane peak for of the the ZnO (101) nanosaws plane for were the observed,ZnO nanosaws implying were highly observed, (002) implying plane-oriented highly growth. (002) plane-oriented It is known that, growth. in ZnO, It is a known faster (001)that, growthin ZnO, directiona faster (001) is due growth to the direction higher surface is due energy to the higher of the polar surface plane. energy Therefore, of the polar a high plane. (002) Therefore, plane orientation a high indicated(002) plane that orientation most of theindicated ZnO nanosaws that most grew of the along ZnO the nanosaws (001) direction. grew along In addition, the (001) compareddirection. toIn theaddition, catalyst-free compared growth to the of catalyst-free ZnO nanostructures, growth of the ZnO Au nanostructures, catalyst gave rise the toAu the catalyst enhancement gave rise of to a crystallinethe enhancement orientation, of a crystalline as shown inorientation, Figures4– 6as. Thisshown is based in Figures on the 4–6. observation This is based that on catalyst-mediated the observation growththat catalyst-mediated led to crystalline growth nanostructures led to crystalline growing nanostructures in speciﬁc crystallographic growing in specific directions crystallographic [32]. directions [32].

Figure 6. X-ray diffraction (XRD) patterns of ZnO nanoplates, nanosaws, and nanobelts grown under Figure 6. X-ray diffraction (XRD) patterns of ZnO nanoplates, nanosaws, and nanobelts grown under different experiment setups at growth temperature of 800 ◦C and Ar/O2 flow rate of 120/1.0 sccm. different experiment setups at growth temperature of 800 °C and Ar/O2 flow rate of 120/1.0 sccm. 4. Conclusions 4. Conclusions Two-dimensional ZnO nanostructures showing various shapes were grown under different Two-dimensional ZnO nanostructures showing various shapes were grown under different experiment setups and selective conditions using a thermal CVD method. By varying O2 flow rate, ZnOexperiment nanostructures setups and grown selective on Au-coated conditions Si using substrates a thermal in the CVD parallel method. setup By revealed varying highly O2 flow defined rate, ZnO nanoplatesnanostructures and branchedgrown on nanowires, Au-coated likelySi substrates as a result in ofthe two parallel growth setup processes: revealed two-dimensional highly defined ZnO nanoplates and branched nanowires, likely as a result of two growth processes: two- filling and one-dimensional branching. With a growth temperature of 800 ◦C and an Ar/O2 flow rate of 120dimensional/1.0 sccm, filling Au-catalyst-mediated and one-dimensional ZnO nanostructures branching. With grown a growth in the temperature perpendicular of setup800 °C exhibited and an verticallyAr/O2 flow oriented rate ZnOof 120/1.0 nanosaws sccm, that grewAu-catalyst-mediated from the bottom ofZnO the substratenanostructures in a tapered grown shape. in ZnOthe nanostructuresperpendicular setup grown exhibited in the perpendicular vertically oriented setup ZnO without nanosaws the Au that catalyst grew showedfrom the bent bottom randomly of the alignedsubstrate ZnO in a nanobelts.tapered shape. The Zn ZnOO nanostructures nanoplates, nanosaws, grown in the and perpendicular nanobelts had setup hexagonal without wurtzite the Au structures,catalyst showed indicating bent that randomly the morphological aligned ZnO changes nanobelts. of ZnO The nanostructures ZnO nanoplates, seldom nanosaws, affected theirand crystal structures. It can be concluded that experimental configuration (i.e., a parallel or perpendicular setup) and experiment conditions (e.g., oxygen-flow rate, growth temperature, and catalyst) are key parameters that can be used to control the morphologies of two-dimensional ZnO nanostructures that grow in the thermal CVD method.

Author Contributions: Conceptualization, S.C.C., D.K.L., and S.H.S.; investigation, S.C.C.; writing—original-draft preparation, S.C.C.; writing—review and editing, D.K.L. and S.H.S.; funding acquisition, S.H.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1I1A3A01041101). Crystals 2020, 10, 517 9 of 10

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Kang, J.-W.; Kim, B.-H.; Song, H.; Jo, Y.-R.; Hong, S.-H.; Jung, G.Y.; Kim, B.-J.; Park, S.-J.; Cho, C.-H. Radial multi-quantum well ZnO nanorod arrays for nanoscale ultraviolet light-emitting diodes. Nanoscale 2018, 10, 14812–14818. [CrossRef][PubMed] 2. Mahmud, A.; Khan, A.A.; Voss, P.; Das, T.; Abdel-Rahman, E.; Ban, D. A High Performance and Consolidated Piezoelectric Energy Harvester Based on 1D/2D Hybrid Zinc Oxide Nanostructures. Adv. Mater. Interfaces 2018, 5, 1801167. [CrossRef] 3. Wang, Q.; Yang, D.; Qiu, Y.; Zhang, X.; Song, W.; Hu, L. Two-dimensional ZnO nanosheets grown on flexible ITO-PET substrate for self-powered energy-harvesting nanodevices. Appl. Phys. Lett. 2018, 112, 63906. [CrossRef] 4. Huang, M.H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 1897–1899. [CrossRef] 5. Song, J.; Zhou, J.; Wang, Z.L. Piezoelectric and Semiconducting Coupled Power Generating Process of a Single ZnO Belt/Wire. A Technology for Harvesting Electricity from the Environment. Nano Lett. 2006, 6, 1656–1662. [CrossRef] 6. Sadek, A.Z.; Wlodarski, W.; Li, Y.X.; Yu, W.; Li, X.; Yu, X.; Kalantar-Zadeh, K. A ZnO nanorod based layered ZnO/64 YX LiNbO3 SAW hydrogen gas sensor. Thin Solid Films 2007, 515, 8705. [CrossRef] 7. Law, M.; Greene, L.E.; Johnson, J.C.; Saykally, R.; Yang, P. Nanowire dye-sensitized solar cells. Nat. Mater. 2005, 4, 455–459. [CrossRef] 8. Sun, T.; Qiu, J.; Liang, C. Controllable Fabrication and Photocatalytic Activity of ZnO Nanobelt Arrays. J. Phys. Chem. C 2008, 112, 715–721. [CrossRef] 9. Snure, M.; Tiwari, A. Synthesis, Characterization, and Green Luminescence in ZnO Nanocages. J. Nanosci. Nanotechnol. 2007, 7, 481–485. [CrossRef] 10. Wang, J.X.; Sun, X.W.; Wei, A.; Lei, Y.; Cai, X.P.; Li, C.M.; Dong, Z.L. Zinc oxide nanocomb biosensor for glucose detection. Appl. Phys. Lett. 2006, 88, 233106. [CrossRef] 11. Ko, S.H.; Lee, S.; Kang, H.W.; Nam, K.H.; Yeo, J.; Hong, S.J.; Grigoropoulos, C.P.; Sung, H.J. Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowires for a High Efficiency Dye-Sensitized Solar Cell. Nano Lett. 2011, 11, 666–671. [CrossRef] 12. Tan, C.; Zhang, H. Wet-chemical synthesis and applications of non-layer structured two-dimensional nanomaterials. Nat. Commun. 2015, 6, 7873. [CrossRef] 13. Pung, S.-Y.; Choy, K.-L.; Hou, X. Tip-growth mode and base-growth mode of Au-catalyzed zinc oxide nanowires using chemical vapor deposition technique. J. Cryst. Growth 2010, 312, 2049–2055. [CrossRef] 14. Iwan, S.; Zhao, J.; Tan, S.; Sun, X. Enhancement of UV photoluminescence in ZnO tubes grown by metal organic chemical vapour deposition (MOCVD). Vacuum 2018, 155, 408–411. [CrossRef] 15. Wu, L.; Wu, Y.; Lu, W. Preparation of ZnO Nanorods and optical characterizations. Phys. E Low Dimens. Syst. Nanostruct. 2005, 28, 76–82. [CrossRef] 16. Kennedy, O.W.; Coke, M.L.; White, E.R.; Shaffer, M.; A Warburton, P. MBE growth and morphology control of ZnO nanobelts with polar axis perpendicular to growth direction. Mater. Lett. 2018, 212, 51–53. [CrossRef] 17. Son, H.J.; Jeon, K.A.; Kim, C.E.; Kim, J.H.; Yoo, K.H.; Lee, S.Y. Synthesis of ZnO nanowires by pulsed laser deposition in furnace. Appl. Surf. Sci. 2007, 253, 7848–7850. [CrossRef] 18. Menzel, A.; Subannajui, K.; Bakhda, R.; Wang, Y.; Thomann, R.; Zacharias, M. Tuning the Growth Mechanism of ZnO Nanowires by Controlled Carrier and Reaction Gas Modulation in Thermal CVD. J. Phys. Chem. Lett. 2012, 3, 2815–2821. [CrossRef] 19. Wang, Z.L. Zinc oxide nanostructures: Growth, properties and applications. J. Phys. Condens. Matter 2004, 16, R829. [CrossRef] 20. Park, J.H.; Park, J.G. Synthesis of ultrawide ZnO nanosheets. Curr. Appl. Phys. 2006, 6, 1020. [CrossRef] 21. Wagner, R.S.; Ellis, W.C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89. [CrossRef] 22. Huang, M.H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport. Adv. Mater. 2001, 13, 113. [CrossRef] Crystals 2020, 10, 517 10 of 10

23. Sekar, A.; Kim, S.H.; Umar, A.; Hahn, Y.B. Catalyst-free synthesis of ZnO nanowires on Si by oxidation of Zn powders. J. Cryst. Growth 2005, 277, 471. [CrossRef] 24. Wu, C.C.; Wuu, D.S.; Lin, P.R.; Chen, T.N.; Horng, R.H. Three-Step Growth of Well-Aligned ZnO Nanotube Arrays by Self-Catalyzed Metalorganic Chemical Vapor Deposition Method. Cryst. Growth Des. 2009, 9, 4555. [CrossRef] 25. Wu, C.C.; Wuu, D.; Lin, P.R.; Chen, T.N.; Horng, R.H. Realization and Manipulation of ZnO Nanorod Arrays on Sapphire Substrates Using a Catalyst-Free Metalorganic Chemical Vapor Deposition Technique. J. Nanosci. Nanotechnol. 2010, 10, 3001–3011. [CrossRef][PubMed] 26. Kong, Y.C.; Yu, D.P.; Zhang, B.; Fang, W.; Feng, S.Q. Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach. Appl. Phys. Lett. 2001, 78, 407–409. [CrossRef] 27. Vanheusden, K.; Warren, W.L.; Seager, C.H.; Tallant, D.R.; Voigt, J.A.; Gnade, B.E. Mechanisms behind green photoluminescence in ZnO phosphor powders. J. Appl. Phys. 1996, 79, 7983–7990. [CrossRef] 28. Wang, Z.L.; Kong, X.Y.; Zuo, J.M. Induced Growth of Asymmetric Nanocantilever Arrays on Polar Surfaces. Phys. Rev. Lett. 2003, 91, 185502. [CrossRef] 29. Gao, P.X.; Lao, C.S.; Ding, Y.; Wang, Z.L. Metal/Semiconductor Core/Shell Nanodisks and Nanotubes. Adv. Funct. Mater. 2006, 16, 53–62. [CrossRef] 30. Pan, Z.; Dai, Z.R.; Wang, Z.L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947–1949. [CrossRef] 31. Kong, B.H.; Cho, H. Formation of vertically aligned ZnO nanorods on ZnO templates with the preferred orientation through thermal evaporation. J. Cryst. Growth 2006, 289, 370–375. [CrossRef] 32. Martensson, T.; Borgstrom, M.T.; Seifert, W.; Ohlsson, J.; Samuelson, L. Fabrication of individually seeded nanowire arrays by vapour–liquid–solid growth. Nanotechnology 2003, 14, 1255. [CrossRef]

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