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Article

Hierarchical Design of High-Surface-Area Zinc Oxide Nanorods Grown on One-Dimensional Nanostructures

1
Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA
2
Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, USA
*
Author to whom correspondence should be addressed.
Sci 2025, 7(3), 114; https://doi.org/10.3390/sci7030114
Submission received: 5 May 2025 / Revised: 22 June 2025 / Accepted: 28 July 2025 / Published: 14 August 2025

Abstract

In this work, ZnO nanorods were grown on vertically aligned and randomly aligned silica nanosprings using the hydrothermal method. The initial step was the deposition of a ZnO seed layer by atomic layer deposition to promote nucleation. For hydrothermal growth, equimolar (0.2 M) solutions of Zinc nitrate hexahydrate and hexamethylene tetraamine prepared in DI water were used. The ZnO NR grown on the VANS were flower-like clusters, while for the RANS, the ZnO NR grew radially outward from the individual nanosprings. The lengths and diameters of ZnO NR grown on VANS and RANS were 175 and 650 nm, and 35 and 250 nm, respectively. Scanning electron microscopy confirmed the formation of ZnO nanorods, while X-ray diffraction and Raman spectroscopy verified that they have a hexagonal wurtzite crystal structure with preferential growth along the c-axis. X-ray photoelectron spectroscopy, in conjunction with in vacuo annealing, was used to examine the surface electronic structure of ZnO nanorods and defect healing. Photoluminescence of the ZnO nanorods indicates high crystal quality, as inferred from the weak defect band relative to strong excitonic band edge emission.

1. Introduction

One-dimensional (1D) ZnO nanostructures such as nanorods (NR), nanoneedles [1], nanowires [2], nanotubes [3], nanoribbons [4], nano springs [5], nano helix [6], nano rings [7], nano combs [8], and nanobelts [9] possess unique optical [10,11], mechanical [12], electrical, and electrochemical [13] properties. Furthermore, they have demonstrated their usefulness in electronic, electrochemical, electromechanical, and photonic devices [14,15]. In addition, ZnO is environmentally friendly, biocompatible, and low-cost [16]. Zinc oxide is an n-type semiconductor due to oxygen vacancies, and has a wide direct band gap of ~3.37 eV, large exciton binding energy (60 meV), is transparent, has high electron mobility, and is thermally, chemically, and mechanically stable at room temperature [17,18]. However, the properties of nanostructured ZnO depend on crystal size, shape, and orientation [19] and their electrical properties on defects and impurities [20], which in turn depend on the synthesis and post-synthesis conditions. Furthermore, the room temperature conductivity of ZnO is highly sensitive to the surface adsorbed species, which makes it a suitable material for gas sensor applications, especially when the high surface-to-volume ratio of ZnO nanostructures is exploited [21]. In addition, ZnO shows better tolerance to exposure to radiation [22], which makes it an excellent semiconductor material for space and nuclear applications [23]. It is these fascinating properties that make nanostructured ZnO so attractive for research and industrial applications.
There are various methods of synthesizing ZnO nanostructures: chemical and physical, such as pulsed laser deposition [24], magnetron sputtering deposition [25], chemical vapor deposition [26], microwave synthesis [27], green synthesis [28], direct precipitation [29], electrochemical deposition [30], hydrothermal method [31], spray pyrolysis [32], and the sol–gel method [33]. In terms of growing ZnO nanorods (NRs), the simplest, most energy-efficient, and most economical method on a large scale is the hydrothermal method, where epitaxial and anisotropic crystal growth occurs [34]. In the hydrothermal method, several parameters can be adjusted to control the growth of ZnO NR such as the concentration of precursor solution, reaction temperature, reaction time, and pH of the solution. However, controlling the shape, size, and orientation of individual ZnO NR in the hydrothermal method remains a challenge [31]. The successful growth of ZnO NR on different flat substrates such as transparent conductive oxide layers [35], glass [36], plastics [37], carbon fabric [38], Al foil [39], woven Kevlar fiber [40], etc., has been reported in the literature. However, there have been no reports of ZnO NR growth on 1D nanostructures such as silica nanosprings.
The different types of architectural designs of nanostructures have been employed because of the properties of the materials or the performance of the device in which it is used. Efforts have been made to fabricate ZnO nanostructures with 1D morphology, and recently, more attention has been paid to the fabrication of lower-dimensional ZnO structures into complex 3D hierarchical structures [41]. For zero-dimensional nanostructures such as quantum dots, fullerenes, polymer dots, nanodots, nanoparticles, etc., surface area accessibility is limited if the particles agglomerate. Similarly, in 1D nanostructures such as nanowires, nanorods, nanotubes, etc., the exposed surface area becomes limited if they form into bundles. In contrast, the hierarchical design of 3D nanostructures, agglomeration, and bundling can be inhibited, thereby providing the highest possible accessible surface area. These structures possess a high surface-area-to-volume ratio compared to monomorphological structures [41]. The hypothesis of this study was that growing ZnO NR on the surface of interconnected silica nanosprings (NSs) will enhance surface area and possibly increase mechanical stability, loading capacity, and charge transport, all of which will improve thermal, electrical conductivity, and catalytic activity. Subsequently, this will improve the performance of light detectors, light emitters, chemical and biological sensors, power nanogenerators [42], photovoltaics, field emitters, photo electrochemical splitting of water [43], and hydrogen storage [44], to name a few.
The reasons for choosing to use silica nanosprings include the following: a high surface area of ~300 m2/g [45]; they can be grown at 370 °C, well below the softening temperature of glass; and the process is atmospheric chemical vapor deposition that greatly increases yield and reduces processing time [46]. Similarly, the hydrothermal growth of ZnO NR occurs at a low temperature (~95 °C), too, and has readily available, low-cost precursors. The ZnO NR possesses high optical transparency, a wide direct band gap, a high surface area to volume ratio, and biocompatibility as compared to NR of oxide of metals like Cu, Ag, etc. Consequently, combining the two 1D nanostructures should facilitate economical mass production. Herein, we outline a method for the hierarchical design of ZnO NR on two types of silica nanosprings: vertically and randomly aligned nanosprings, VANS and RANS, respectively. The morphology and stoichiometry of the ZnO NR on VANS and RANS have been characterized using field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and photoluminescence (PL) spectroscopy.

2. Materials Preparation and Characterization

The side-view FESEM images of VANS and RANS, as shown in Figure 1, show the different morphology of nanosprings. Figure 1a shows the side-view FESEM of VANS where the NS is growing vertically on the glass substrate. It shows that the NSs are close-packed and growing uniformly, forming a mat surface suitable for the growth of ZnO NR. Figure 1b shows the side-view image of RANS where the density of NS is greater near the glass substrate and decreases as it moves farther away. The growth of ZnO NR on VANS and RANS is schematically outlined in Figure 2. The process consists of growing VANS and RANS on glass slides. Next, the VANS and RANS are coated with a ZnO seed layer by ALD, followed by annealing to crystallize the seed layer. In the final step, ZnO NR is grown on the VANS and RANS in an autoclave heated in an oven.

2.1. Substrate Cleaning

The glass microscope slides, 25 mm × 15 mm × 1 mm upon use for the study, were cleaned by first placing them in a water bath with detergent and sonicated for 30 min to remove dust oils. Next, they were sonicated in acetone for 10 min to remove organics, followed by sonication in methanol for 5 min to remove acetone residue. Finally, the glass substrates were rinsed with DI water and dried using a hot air gun.

2.2. Nanospring Growth

VANS and RANS were grown on the cleaned microscope slides in a custom reactor at 370 °C and atmospheric pressure via a vapor–liquid–solid (VLS) mechanism. A detailed explanation of their growth can be found in Refs. [46,47,48,49]. Briefly, for VANS, the substrate is coated with a ~60 nm thin film of ZnO by atomic layer deposition (ALD), where the ALD process is described in detail below. The ZnO layer inhibits the Au diffusion, creating a denser distribution of Au nanoparticles relative to a glass substrate with Au. This produces a dense array of NS that can only grow vertically due to crowding, resulting in vertically aligned nanosprings. For RANS, the glass substrates were pretreated with a silica layer < 1 nm in the NS reactor in order to create surface roughness to promote nucleation of Au nanoparticles, as opposed to Au microparticles. For VANS and RANS, the pretreated surfaces were coated with a 20–30 nm layer of Au by DC sputtering at 10 mTorr and 25 W of power. VANS were grown for 30 min in the reactor, which produces a ~14 μm thick layer. RANS were grown for 120 min, which produces a ~165 μm thick layer. Figure 1a and Figure 1b are side-view FESEM images of VANS and RANS, respectively.

2.3. Plasma Treatment of Nanosprings

As-grown nanosprings are hydrophobic, but they need to be hydrophilic for ALD of the ZnO seed layer for hydrothermal growth of the ZnO NR. This is achieved by plasma treating at 250 m Torr of atmospheric air at 100 W of power for four minutes in a Plasma Perp III plasma system at 100 W for 4 min.

2.4. ZnO Seed Layer Deposition

The ZnO seed layers were deposited by ALD in a system from Okyay Technologies. Deposition was performed at 170 °C under a constant flow of N2 (8 sccm) using diethyl zinc (DEZ) and DI water as the Zn source and oxidizer, respectively. The deposition cycle consists of a 100 ms dose of DEZ, a 20 s purge, and then a 30 ms dose of DI water, followed by a 10 sec purge before beginning the next cycle. The ZnO deposition rate was 1.4 ± 0.01 Å/cycle [50]. Two hundred cycles were used to grow a 28 nm thick ZnO seed layer. Because ZnO deposited at low temperature is amorphous but needs to be polycrystalline to serve as seeds for ZnO NR growth, the ZnO-coated VANS and RANS were annealed in a tube furnace at 300 °C in an ambient atmosphere.

2.5. Hydrothermal Growth and Characterization

The hydrothermal growth of ZnO NR on VANS and RANS was carried out in an autoclave reactor held facing downward at 95 °C for 4 h using a solution of equal molar concentrations of 0.2 M of zinc nitrate hexahydrate and hexamethylenetetramine (HMTA: a highly water-soluble polar, non-ionic tetradentate cyclic tertiary amine) in DI water. The most probable chemical reaction in the hydrothermal growth process can be written as follows [51,52,53]:
Zn NO 3 2 . 6 H 2 O Zn 2 + + 2 NO 3 + 6 H 2 O
CH 2 6 N 4 + 6 H 2 O 6   HCHO + 4 NH 3 NH 3 + H 2 O     NH 4 +   + OH  
2 OH + Zn 2 + Zn OH 2 ZnO s + H 2 O
  4 OH + Zn 2 + Zn OH 4 2 ZnO s + H 2 O + 2 OH  
Briefly, at 95 °C zinc nitrate decomposes into Zn and nitrate ions, and HMTA decomposes into formaldehyde and ammonia. Then ammonia reacts with water to form hydroxide (OH) ions, whereas it reacts with Zn2+ ions forms Zn ( NH 3 ) 4 2 + . Meanwhile, Zn2+ ions react with HMTA forms Zn { CH 2 6 N 4 } 2 + complex. Once the OH ion concentration reaches super saturation, it reacts with Zn2+ to form Zn OH 2 and negatively charged Zn ( OH ) 4 2 ions. The dehydration of intermediate states, Zn ( NH 3 ) 4 2 + , Zn OH 2 , Zn ( OH ) 4 2 , and Zn { CH 2 6 N 4 } 2 + , form ZnO and byproducts. Note, the purpose of the seed layer is to attract negatively charged Zn ( OH ) 4 2 to its positively charged polar surface. The polar nature of the growth leads to preferential growth along the c-axis in the form of NR. All these reactions occur in equilibrium condition and can be controlled by adjusting reaction parameters such as concentrations of precursor, temperature, and PH of the solution [51].

2.6. Zinc Oxide Nanorod Characterization

The growth of ZnO NR was verified using a FEI Quanta 600F field emission scanning electron microscope (FESEM) (Lausanne, Switzerland) equipped with a Bruker energy dispersive X-ray spectrometer (EDS) (Preston, VIC, Australia) for elemental analysis. The crystal structures of ZnO NR were analyzed using a Rigaku (Tokyo, Japan) Miniflex 600 powder X-ray diffractometer using monochromatic CuKα radiation (λ = 1.54056 Å). The sample was scanned from 20° to 80° (2θ) with scanning steps of 0.020°. X-ray photoelectron spectroscopy (XPS) of ZnO NR samples as a function of in vacuo annealing in a custom-built ultra-high vacuum chamber with a base pressure of 5.0 × 10−10 Torr and using the Mg-Kα X-ray emission line. The kinetic energy of the photoelectrons was measured with an Omicron EA 125 hemispherical electron energy analyzer (Melbourne VIC, Australia) with a resolution of 0.02 eV. The XPS spectra were acquired for the as-grown NR and after annealing from 350 to 500 °C in steps of 50 °C. Raman spectroscopy was performed using a WITec alpha300R Raman microscope (Ulm, Germany), employing 532 nm laser excitation and a 100 μm diameter confocal aperture (fiber). Two gratings of 1800 and 600 lines/mm were used for high spectral resolution and broader wavenumber scans, respectively. The signal was integrated for 200 s using a 20× objective lens of 0.4 numerical aperture. The laser power and beam spot size on the samples were set to 4.9 mW and 2 µm, respectively. Under these conditions, the Raman spectra were stable and free of photothermal shifts in peak positions. Doubling the laser power led to a noticeable shift in the major ZnO peak at 437 cm−1 toward lower wavenumbers.

3. Results and Discussion

3.1. Surface Morphology

The top-down FESEM micrograph of ZnO NR grown on VANS in Figure 3a reveals the formation of clusters of NR radiating outward from a common origin (inset in Figure 3a). Not shown is cracking of the VANS films, which is primarily induced by capillary shrinkage governed by the wetting and drying of the VANS film [54,55]. Due to the close packing of nanosprings in VANS (Figure 1a), cross-sectional FESEM was performed and displayed in Figure 3b. It shows that ZnO NR grows on the surface, forming a crust, and below that, ZnO nanoparticles form instead of NR. We hypothesize that the closely packed nature of the VANS is responsible for this morphology. Specifically, the tops of the VANS are in contact with the bulk of the solution during hydrothermal growth and are the primary nucleation points of NR formation. As a consequence, the solution that passes through the top of the VANS has a lower concentration of the reactants, thereby limiting subsurface growth to ZnO nanoparticles. Furthermore, the closely packed morphology of the VANS, in conjunction with the formation of the NR crust, significantly limits diffusion of the reactants into the subsurface region of the VANS.
In contrast to VANS, the open architecture of RANS facilitates NR formation deep into the nanospring layer (Figure 3c), at least until the density of RANS approaches that of VANS, typically, a few microns above the nanospring/substrate interface, where we expect a similar boundary layer to form as in Figure 3b. The ZnO NR radiates outward in all directions from the nanospring (see inset in Figure 3b), thereby retaining the over-helical morphology of the nanosprings.
It is worth noting that densely grown ZnO NR on VANS may be interdigitated, and this will facilitate charge transport through the layer [56], and probably less so on RANS because they grow radially outward. However, inter-digitation of the NR at the crossing of NS in the RANS should have some effect on charge transport through the mat. As for the ZnO NR, they have hexagonal cross-sections consistent with the Wurtzite hexagonal phase of ZnO. The average diameter and length of nanorods on VANS and RANS are approximately 175 nm and 650 nm, and 35 nm and 250 nm, respectively.

3.2. Energy Dispersive X-Ray Spectroscopy

The energy dispersive analysis in Figure 4 reveals that Zn, O, and Si are the major constituents of both samples, while scanning ZnO NR grown on the surface of the VANS (Figure 4a) and the RANS (Figure 4b), and confirms the composition of the NR is ZnO, where the Si peak is from the SiO2 nanosprings. The peaks at 1.0, 8.6, and 9.6 keV correspond to Zn [57,58,59]. The quantitative analysis yielded the elemental compositions of the ZnO NR grown on the surface of VANS are 66% Zn, 17.7% O, and 1.6% Si, and on RANS are 61.7% Zn, 19.2% O, and 5.7% Si. Similarly scanning the cross-section of the VANS (Figure 4c) gives a composition of 59.6% Zn, 21.9% O, 9.3% Au, and 5.6% Si, supporting the conclusion that ZnO is present in the cavities and interstitial spaces of the nanosprings, but not in the form of NR like on the surface. The peak at 2.12 keV in the EDS of the cross-sectional FESEM of ZnO NR on VANS corresponds to the Au catalyst used to grow the nanosprings [60]. Note that the EDS O concentrations are the sum of those of the NR and the nanosprings.

3.3. XRD Analysis

The X-ray diffraction of ZnO NR on VANS is presented in Figure 5, where the 2θ peaks at 31.2°, 33.9°, 35.7°,47.1°, 56.3°, 62.4°, 67.5°, and 68.5° correspond to the ZnO lattice planes (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), and (2 0 1), respectively, which matches with diffraction peaks of standard ZnO (JCPDS PDF # 36-1451) well, and is representative of the hexagonal wurtzite structure of zinc oxide [61]. The peaks at 2θ = 37.7, 43.8, and 76.9 correspond to the (1 1 1), (2 0 0), and (3 1 1) lattice planes of the gold catalyst, respectively [62]. The XRD of ZnO NR on RANS is equivalent to the VANS and is not displayed for the sake of brevity.

Crystallite Size (D) and Strain ( ε )

The size and internal strain of nanocrystals have a significant impact on the physical, chemical, and mechanical properties of nanomaterials [63,64] and, therefore, are of interest here. Multiple methods are employed to extract this information from the XRD data. The Scherrer method is the simplest method for calculating the crystallite size because it ignores peak broadening due to strain [63]. Note, peak broadening of XRD peaks is a combination of instrumental broadening and sample-dependent factors such as crystallite size and strain. Since the Scherrer method requires the use of a corrected broadening ( β ), the instrumental broadening was removed from the measured peak widths [65], using Equation (1).
β = β measured 2     β Instrumental 2 1 2
where βmeasured is the measured full width half maximum (FWHM) of a peak and βinstrumental is the instrumental broadening of the Rigaku XRD system. The average crystallite size (D) was calculated using the Debye–Scherrer Equation (2) [66], where the average crystallite size is inversely proportional to the FWHM of a diffraction peak [67],
D = K λ β Cos θ    
where K (=0.89) is the Scherer constant that depends on crystallite shape, λ is the X-ray wavelength of the incident Cu Kα emission line (0.154 nm), β the corrected broadening, and θ is the Bragg diffraction angle, in radians and degrees, respectively. The average crystallite sizes were calculated to be 24 ± 7 nm and 21.3 ± 6.3 nm for ZnO NR on VANS and RANS, respectively.
The X-ray diffraction was also used to evaluate the crystallite dislocation density, i.e., the number of defects or imperfections present in the NR, that arise from the combination of lattice mismatch with the substrate, the growth process, and impurities [68,69]. A low dislocation density implies high crystalline quality and vice versa [68]. This is important because ZnO nanocrystals having higher dislocation densities exhibit lower luminescence efficiency [70] and impact the free carrier density and carrier transport [71]. The dislocation density (δ) was determined using Equation (3) [67].
δ = 1 D 2    
where D is the crystallite size calculated from Equation (2). The results of the analysis are summarized in Table 1 and Table 2 for ZnO NR on VANS and RANS, respectively, where the highest dislocation densities for the ZnO NR grown on VANS and RANS correspond to (002) and (101) diffraction planes, respectively.
The interplanar spacing (d) of the ZnO NR was calculated using Equation (4) [72],
1 d 2 = 4 3 h 2 + hk + k 2 a 2 + l 2 c 2  
The results of the analysis can also be found in Table 1 and Table 2 for ZnO NR on VANS and RANS, respectively. The ZnO lattice constants ‘a’ and ‘c’ were calculated using Equations (5) and (6),
a = λ 3 sin θ    
c = λ sin θ  
and found to be 3.25 Å and 5.20 Å, respectively, for ZnO NR grown on VANS and 3.33 Å and 5.33 Å, respectively, for ZnO NR grown on RANS or a ratio of c/a = 1.60 Å in both cases. The Zn-O bond length was calculated using Equation (7) using the lattice geometry by Wang et al. [73],
l = a 2 3 + u 1 2 2 c 2
where ‘u’ is the positional parameter corresponding to the amount of displacement of an atom with respect to the next atom along the c-axis and calculated using Equation (8),
u = a 2 3 c 2 + 1 4  
and is equal to 0.38 for both ZnO NR grown on VANS and RANS. The values of the c/a ratio and ‘u’ of the ZnO NR grown on VANS and RANS are very close to the reported values of 1.63 and 0.38 [74], respectively. Any deviations from these standard values occur when the four tetrahedral distances remain constant by distorting tetrahedral angles [75]. The Zn-O bond length for the ZnO NR grown on VANS and RANS using Equation (7) are 1.98 Å, and 2.02 Å, respectively, in agreement with the literature [75].
The volumes of the hexagonal Wurtzite primitive cells of the ZnO NR grown on VANS and RANS were calculated using Equation (9) [76],
V = 3 2 a 2 c  
and equal to 47.5 Å3 and 51.2 Å3, respectively. The value for NR on VANS is very close to the literature value of 47.6 Å3 [77], but the value for NR on RANS is 7.6% larger. The variation for NR on RANS in the volume is due to the variation in lattice parameters due to strain and may be due to the larger surface-to-volume ratio of them relative to NR on VANS. However, we cannot exclude other external factors such as temperature, pressure, growth time, etc. [77], associated with the differences between how they grow on VANS and RANS.
We can improve on the analysis using Scherer’s Equation (7) if we include the Williamson–Hall (W-H) method of analysis that is a simple and effective approach for calculating microstrain from XRD data. Specifically, the analysis of peak width as a function 2-theta is performed with a simplified integral breadth analysis. According to this method, total peak broadening can be written as Equation (10) [78],
β total =   β Lattice   strain +   β Crystallite   size
The induced strain is attributed to several intrinsic and extrinsic factors that lead to lattice imperfections and distortion in the ZnO crystal. The W-H method assumes that the crystal in question is isotropic, crystal strain is uniform along a crystallographic direction, and crystal deformation is uniform and independent of the crystallographic direction of the measurement, strain is small. These assumptions are referred to as the uniform stress deformation model (USDM). Here, microstrain ( ε ) is due to lattice strain and associated with peak broadening. The lattice strain induced peak broadening is be expressed by Equation (11) [78],
β Lattice   strain = 4 ε tan θ hkl
The line breadth ( β hkl cos θ hkl ) was calculated using Equation (12), which is obtained by combining Equations (10) and (11),
β hkl cos θ hkl   = K λ D + 4 ε sin θ hkl
where β hkl is the calculated FWHM in radians of the corresponding hkl diffraction planes. Under the assumption of USDM, Hook’s law generalized for crystal stress is σ   =   ε Y hkl , where Y hkl is Young’s modulus in the direction perpendicular to the crystal lattice plane. For Hexagonal crystals, Young’s modulus is given by Equation (13) [79],
Y hkl = h 2 + h + 2 k 2 3 + al c 2 2 s 11 h 2 + h + 2 k 3 2 2 + s 33 al c 4 + ( 2 s 13 + s 44 ) h 2 + h + 2 k 3 2 2 al c 2
where s 11 , s 13 , s 33 , and s 44 are the elastic compliances of ZnO with values of 7.858 × 10−12, −2.206 × 10−12, 6.940 × 10−12, and 23.57 × 10−12 m2 N−1, respectively [79]. The calculated average value of Young’s modulus for the ZnO NR grown on the VANS and RANS are 104.4 GPa, and 112.2 GPa, respectively. Knowing the Young’s modulus for the ZnO NR allows us to calculate line breaths that accounts for strain and is given by the following equation [72],
β hkl cos θ hkl   = K λ D +   4 σ sin θ hkl Y hkl
Plotting β hkl cos θ hkl vs. 4 sin θ hkl Y hkl yields the values of the uniform stress and crystallite size, respectively. The data and fit for ZnO NR on VANS are plotted in Figure 6, yielding values of microstrain, crystallite size, and stress of –9.29 × 10−4, 16.4 nm, and −87.5 MPa, respectively. The values for NR grown on RANS are −9.11 × 10−4, 15.3 nm, −98.26 MPa, respectively.

3.4. X-Ray Photoelectron Analysis

Figure 7a shows XPS spectra of the Zn 2p core level states of the ZnO NR on VANS as a function of in vacuo annealing. The binding energies of Zn 2p3/2 and 2p1/2 peaks of the as-grown sample are 1023.7 eV and 1046.9 eV, respectively [80]. The binding energy peaks of Zn 2p core level states shift to 1022.7 eV and 1045.9 eV, respectively, after annealing at 350 °C and remain unchanged thereafter. The binding energy shift in Zn 2p core level states agrees with the literature [81] and attributed to desorption of H2O and COx and partial desorption of hydroxyl groups bound to surface Zn (Zn-O-H) [82,83], as well as healing of surface defects. The O 1s core level also shifts with annealing (Figure 7b). Fitting of the O 1s core level before and after annealing in Figure 7 yields binding energies of 533.5 eV to 531.2 eV, respectively. The peaks on the shoulders of the Zn 2p core level states at 537 and 535 eV in Figure 7a of the as-grown NR are assigned to H2O, COx, and hydroxyl groups, as well as surface defects. After annealing at 350 °C, the O 1s core level shifts to a lower binding energy by 2.3 eV. The shoulder at a binding energy of 533 eV is attributed to remaining hydroxyl groups and unhealed surface defects [82,83,84]. In addition, the shifting of B.E. towards lower binding energy indicates the gradual change in surface configuration leading to the increased oxygen vacancy formation where the concentration of electrons increases around the remaining oxygen atom, causing a decrease in the binding energy [85]. Ultimately, XPS analysis demonstrates that the quality of the surface of the ZnO NR improves with annealing.

3.5. Raman Spectroscopy

Further evaluation of the quality of the ZnO NR was performed using Raman spectroscopy [75]. The optical phonons at the high symmetric point Γ of the Brillouin zone of pure ZnO crystal play a key role in first-order Raman scattering and show eight sets of optical phonon modes based on the C4 (P63mc) space group of wurtzite type ZnO as follows [75,86]:
Γ opt   =   A 1 + 2 B 1 + E 1 + 2 E 2  
Here, the A 1 and E 1 modes are polar and splits into transverse optical (TO) and longitudinal optical (LO) components, whereas E 2 modes consist of two modes—low-frequency phonon mode ( E 2 low ) and high-frequency phonon mode ( E 2 high ) corresponding to the vibration of the Zn sublattice and oxygen atoms, respectively. However, the B 1 modes ( B 1 low , and B 1 high ) are Raman silent.
The broad baseline band subtracted Raman spectra of annealed ZnO NR on VANS and RANS are displayed in Figure 8. All samples exhibit a strong characteristic ZnO Raman peak at 437 cm−1, consistent with bulk Wurtize E2 symmetry [87,88]. ZnO NR on VANS exhibit a higher signal intensity relative to NR on RANS. This is attributed to the increased subsurface diffuse scattering of the excitation source associated with the open morphology of the RANS, which is expected to be less for the highly dense layer of ZnO NR on top of the VANS. The ZnO overtone peaks at 208 cm−1 (2TA; 2 E 2 low ) and 1150 cm−1 (2A1(LO), 2E1(LO); 2LO) [87,88] are present for both samples. The peaks at 331 and 332 cm−1 for ZnO NR on VANS and RANS, respectively, are assigned to the multiphonon process ( E 2 high E 2 low ) , and the peaks at 378 cm−1 to the transverse A1(TO) vibration in ZnO [87,88].
Despite the smaller signal-to-noise ratio of the Raman spectrum of ZnO NR on RANS, additional Raman peaks at 580, 700, 775, and 798 cm−1 are observed that are absent for spectrum for VANS. The former is assigned to the A1LO vibrational process, and the latter three to A1(LA + TO) processes at different points/lines of the Brillouin zone [88]. The band at 480 cm−1 in the RANS spectrum corresponds to amorphous silicon [89]. It is also present for VANS (see insert) but much weaker due to close packed morphology of the ZnO NR. Not shown in the Raman spectrum of this sample are strong bands centered at 2907 and 2968 cm−1, which are assigned to symmetric and asymmetric C–H stretching, respectively, as well as a peak at 1412 cm−1 which is tentatively assigned to CH2 bending. A Raman spectrum of a control sample of only bare silica nanosprings exhibited bands at 480, 2907, and 2968 cm−1, confirming their origin is not the ZnO NR.

3.6. Photoluminescence Spectroscopy

The Photoluminescence (PL) spectra of the annealed ZnO NR on VANS and RANS were acquired using the i-line (365 nm) of a mercury-vapor lamp (UVP Black-Ray B-100AP) as the excitation source and a StellarNet BLACK-Comet CXR-SR-50 CCD spectrometer with 2 nm spectral resolution. The signal was collected using a fiber-coupled lens (StellarNet) whose optical axis was aligned 45° to the normal of the sample surface. Displayed in Figure 9 are PL spectra of the ZnO NR grown of RANS and VANS, as well as the control silica nanospring sample. The strong emission at 3.40 eV is the filtered (selected) i-line of the Hg lamp, where only the base of the peak is shown. Additionally, residual Hg-vapor lines at 3.06, and 3.72 eV are present. The PL spectra are interpreted using two spectral regions: (i) violet-UVA region and (ii) the visible region. The ZnO is a strong emitter in the UV (region (i)) due to excitonic or band–band recombination. Region (i) of the PL spectra for ZnO NR in Figure 9a,b is assigned to ZnO band edge emission [90,91]. The sharp peak at 3.2 eV in this region is indicative of lesser intrinsic defects, which implies well-ordered grains enhancing luminous efficiency and good optical quality ZnO. Region (ii) consists of bands at 2.0 and 2.3 eV and a shoulder band at 2.5 eV, which are associated with defect states of ZnO (inset plot for ZnO NR grown on VANS) [90,91] that are significantly weaker compared to the band-edge emission, again, indicative of the high crystalline quality of the ZnO NR. The reasons for visible emission are not fully understood; however, an oxygen vacancy mechanism is widely accepted [92]. For pure ZnO NR, a high number of oxygen vacancies are present due to low formation enthalpy and occur in their charged states such as (i) V 0 states (an oxygen vacancy that has captured two electrons and is neutral relative to the lattice), (ii) V 0 + states (single ionized with respect to lattice and captured one electron), and V 0 + + states (doubly positively charged with respect to lattice and does not capture electrons) [93]. The evolution of band bending at grain boundary plays a crucial role in the variation in defect chemistry. The grain boundary potential develops when the grain boundary has a lower chemical potential than the grains. In the depletion region, in proximity to the grain boundary, the oxygen vacancies are in the V 0 + state. The electron depletion layer will be created due to band bending, where the Fermi level goes below V 0 + / V 0 + + and most of the oxygen vacancies will be in diamagnetic V 0 + + state. Meanwhile, defects in the bulk are expected to be in the paramagnetic V 0 + state [93,94]. Therefore the bands at 2.0 eV, 2.3 eV, and 2.5 eV for ZnO NR on VANS and RANS are attributed to the recombination of V 0 + + trapped centers with electrons near the conduction band, transition of electrons from V 0 + centers to the valance band edge, and electron transitions from V 0 centers to valance band edge, respectively [92].
At room temperature, PL of bulk Wurtzite ZnO exhibits a sharp peak at 3.26 eV [90,91]. The energy shift from the optical gap of 3.37 eV is attributed to the binding energy (60 meV) of the recombining free excitons as well as the creation of LO phonons during recombination. ZnO NR had the expected 3.37 eV peak, as well as two additional peaks and a fourth peak at 3.40 eV corresponding to the Hg i-line of the excitation source. Therefore, region (i) of the Raman spectra has been deconvoluted using four Lorentzian line shapes, where the fits are displayed in Figure 10a,b. Based on the fits, the PL peaks are at 3.26, 3.22, and 3.18 eV, accounting for the different PL band shapes from the bulk. Figure 10c,d show the fits from Figure 10a,b with the Hg i-line eliminated. The PL peaks at 3.22 and 3.18 eV are assigned to weakly bound excitons. There is a downshift in the energies of these peaks relative to the free-exciton peak by 40 and 80 meV, respectively. Similar PL peaks have been observed for nanocrystalline ZnO which were subject to quenching at 400 K [91], which led to their identification as excitons weakly bound to impurity states, such as neutral donor states [91]. The deconvoluted PL peaks for ZnO NR grown on VANS and that of RANS have different relative intensities. This may be due to relative differences in their populations in NR grown on VANS and RANS or an artifact of the differences in their morphologies.
The XRD, FESEM, XPS, and Raman analysis have demonstrated that the ZnO NR grown on VANS and RANS are of high quality, regardless of their vastly different coverages. The method to grow them was borrowed from the two commonly used methods for depositing ZnO on silica nanosprings reported in the literature, e.g., ALD and the sol–gel method [45,50,95]. Furthermore, each method is capable of producing ZnO coatings with different morphologies on NS. Rajabi et al. [50] reported that ALD on the order of tens of cycles produces excellent conformal nanocrytalline (~10–30 nm) ZnO coatings on nanosprings, preserving their helical morphology, while the sol–gel method produces thicker ZnO coatings but similar nanocrystal sizes. Their conclusion was that the surface curvatures of nanospring play a crucial role in producing different morphologies for the two methods [50]. Dobrokhotov et al. [96] demonstrated that hundreds of ALD cycles of ZnO deposited at 400 °C on NS produce large platelet-like crystals on the order of hundreds of nanometers. Herein, we have demonstrated that the use of ALD and sol–gel is capable of producing ZnO NR on silica nanosprings. Together, these works demonstrate that silica nanosprings are capable of producing a wide range of ZnO morphologies. We feel this is important because the morphology of ZnO affects the catalytic and photocatalytic properties [97,98]. The advantage of growing ZnO NR on NS is that the combination of the two produces a composite nanomaterial with an extremely large surface area. As-grown silica nanosprings have a surface area of 300 m2/g [46]. However, upon coating with a thin film, independent of the composition of the coating, the surface is reduced to 150 m2/g [99]. The loss of surface area is attributed to reduced accessibility to the inner region of the NS. While 150 m2/g of effectively 100% accessible surface area is excellent, it is beneficial to restore some of its original surface area which is why ZnO NR is so desirable relative to small nanocrystalline ZnO coatings. RANS are excellent high surface area materials for microreactors [100], which will be greatly increased with the addition of ZnO NR. While the samples prepared for this study were too small to perform Brunauer–Emmett–Teller (BET) surface area measurements, Wu et al. [101] determined with BET that the surface area of ZnO NR ranged from 14–49 m2/g, depending on their average length and width associated with the specifics of the solution used. The average crystallite size from Table 2 is 23.0 ± 7.2 nm and corresponds to ~14 m2/g of increased surface area based on the work by Wu et al. [101], giving an approximate total surface area of ZnO NR grown on RANS of 164 m2/g or an increase in surface area of 9%. Based on an average ZnO NR diameter on RANS of 35 nm and length of 250 nm and the work by Wu et al. [101], the estimated increased surface area of RANS with ZnO NR is ~49 nm2/g, for an approximate total surface area of NR grown on RANS of 199 m2/g, or an increase in surface area of 33%. We have not been successful in measuring the surface area of VANS, although we speculate that their surface area will be roughly the same as RANS based on the equivalency of the morphology of individual NS. Our conclusion is that the addition of ZnO NR to NS is a worthwhile gain in surface area that should positively impact their application in microreactors and sensors. It is worth noting that Hafez [102] reported a surface area of 270 ± 13.6 m2/g for ZnO NR with diameters in the range of 30–50 nm and lengths from 400 to 650 nm. If we set this as the high end of ZnO NR surface area for the sol–gel growth method, we could effectively triple the surface area of RANS with the addition of ZnO NR to 420 m2/g.
In terms of electrical sensors or electrochemical catalysis, a ZnO seed layer used for nucleation of the ZnO NR has the added functionality of serving as a conductive layer between the NR for improved charge transfer [103], where it has been shown that sensors constructed with ZnO ALD-coated nanospring are very sensitive at low analyte concentrations [104]. Due to the excitations of the free carriers, the bulk ZnO becomes more conductive with a mild heating [105] but for the case of ZnO ALD on the nanosprings, the resistance increases from a few KΩ to tens of MΩ due to the healing of defects [105]. For ZnO thin films at elevated temperatures, lattice fluctuations lead to detrapping of electrons, thus enhancing conductivity. Kwon et al. [106] reported that the sensor with ZnO NR exhibited better performance, as compared to the sensor with ZnO nanoparticles, for toluene gas sensing and enhanced sensitivity when heated in an oxygen environment. In contrast, Bastatas et al. [104] reported that the sensitivity of the ZnO-coated NS with two orders of magnitude more surface area than ZnO thin films had poor sensitivity at high analyte concentrations relative to ZnO thin films but significantly better sensitivity for low analyte concentrations. They proposed that the ZnO-coated NS act as an interconnected network of resistors, where locations where NS touch function like field-effect transistors that are modulated by the adsorption of the analyte at the junction. The large portions of ZnO-coated NS network become isolated if conduction through a critical junction or junctions is interrupted, ergo, a select few junctions, which are referred to as super junctions, control the conductivity of the network. The work by Bastatas [104] and Kwon [106] suggests that ZnO NR-coated NS should have unique sensing characteristics relative to ZnO-coated NS or ZnO NR alone. This is a line of research for future study.
The ZnO NR on the nanosprings not only provide a higher surface area but should also serve as fast routes or channels for electron transport to the substrate [107], leading to potential applications for the fabrication of high-efficiency electrochemical/electromechanical systems. In terms of hydrogen storage, hydrogen will bond to the interstitials of the ZnO NR, where the high surface of the ZnO NR on NS translates into higher gravimetric storage relative to ZnO thin film coated NS [44,108]. Lastly, ZnO NR on NS may enhance the electrowetting effect by offering more active sites for the interactions between the liquid and electric fields. Ultimately, an application that could benefit from high surface area could benefit by incorporating ZnO NR coated NS, be that VANS or RANS. Note, this hybrid nanomaterial can be coated with virtually any material and, in effect, create high-surface-area thin films of one’s choosing.

4. Conclusions

Zinc oxide NR has been grown on vertically aligned and randomly aligned silica nanosprings using the hydrothermal crystal growth method. The FESEM showed that the high-aspect-ratio nanorods with hexagonal prismatic structures form on the VANS and RANS, and growth is along the c-axis. The ZnO NR on VANS forms a closely packed surface, where the average diameter and length of NR are 175 nm and 650 nm, respectively. In contrast, the openness of the RANS allows the ZnO NR to grow radially outward from the NS, with an average diameter and length of 35 nm and 250 nm, respectively. The larger size of the ZnO NR on VANS is attributed to the impedance of the sol–gel solution into the subsurface region of the VANS, leading to nucleation of NR only on the top surface where the concentration of the solution is the highest. In contrast, the openness of the RANS geometry allows the sol–gel solution to diffuse unimpeded into the nanospring mat, thereby rendering a lower relative concentration of the active ingredients of the solution, resulting in smaller diameter and shorter ZnO NR relative to those on VANS. The XRD analysis showed that the ZnO NR is polycrystalline with an average crystallite size of 24.0 ± 7.2 nm on VANS and 21.3 ± 6.3 nm on RANS. XPS analysis showed that the as-grown ZnO NR has defects and that mild annealing at 350 °C reduces its concentration. Raman spectroscopy and PL analysis further verified the Wurtzite crystal structure of the ZnO NR. The ZnO NR grown on RANS is longer and thinner than on VANS, where we attribute the thicker and longer NR on VANS to the localization of the solution at the surface, resulting in a higher concentration that leads to a higher volumetric rate of growth. The estimated surface area of ZnO NR on VANS and RANS is 164 m2/g and 199 m2/g, respectively. Lastly, the quality of the PL of ZnO NR on NS suggests that this hierarchical material should have very interesting optoelectronic properties, and the further study of the photocatalytic properties of ZnO NR on VANS and RANS will ultimately reveal the value of this hybrid structure. In addition, the morphology of nanostructures alters the properties of the sample, and the precise control over the reaction process for the ZnO NR on silica NS in the hydrothermal growth method remains a challenge.

Author Contributions

S.P.—Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing-original draft, Visualization; A.K.K.—Validation, Formal analysis, Data curation, Writing-original draft; D.N.M.—Conceptualization, resources, Writing-original draft, Visualization, supervision, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the US Office of Scientific Research (Grant #: N00014-20-1-2433).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Available upon request.

Acknowledgments

The authors also acknowledge the use of the facilities in the Cowboy Microfabrication Center on the campus of Oklahoma State University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Side-view FESEM of (a) vertically aligned nanosprings and (b) randomly aligned nanospring grown on glass.
Figure 1. Side-view FESEM of (a) vertically aligned nanosprings and (b) randomly aligned nanospring grown on glass.
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Figure 2. A schematic diagram of hydrothermal growth of ZnO nanorods on nanosprings.
Figure 2. A schematic diagram of hydrothermal growth of ZnO nanorods on nanosprings.
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Figure 3. FESEM micrographs of (a) ZnO nanorods grown on VANS, (b) cross-sectional view of ZnO nanorods grown on VANS, and (c) ZnO nanorods grown on RANS. The insets are magnified views of the nanorods.
Figure 3. FESEM micrographs of (a) ZnO nanorods grown on VANS, (b) cross-sectional view of ZnO nanorods grown on VANS, and (c) ZnO nanorods grown on RANS. The insets are magnified views of the nanorods.
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Figure 4. EDS spectra of ZnO NR grown on (a) VANS and (b) RANS; (c) cross-sectional of VANS.
Figure 4. EDS spectra of ZnO NR grown on (a) VANS and (b) RANS; (c) cross-sectional of VANS.
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Figure 5. XRD spectra of ZnO nanorods grown on VANS.
Figure 5. XRD spectra of ZnO nanorods grown on VANS.
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Figure 6. The modified form of W-H plot assuming USDM for ZnO nanorods grown on VANS.
Figure 6. The modified form of W-H plot assuming USDM for ZnO nanorods grown on VANS.
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Figure 7. Fitting blue line for adsorbed water, green for hydroxyl group/surface defect, and red for O 1s core level state XPS spectra of ZnO NR grown on VANS; (a) as-grown and (b) annealed at 350 °C.
Figure 7. Fitting blue line for adsorbed water, green for hydroxyl group/surface defect, and red for O 1s core level state XPS spectra of ZnO NR grown on VANS; (a) as-grown and (b) annealed at 350 °C.
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Figure 8. Raman spectra of ZnO NR grown on (a) RANS and (b) VANS after baseline subtraction.
Figure 8. Raman spectra of ZnO NR grown on (a) RANS and (b) VANS after baseline subtraction.
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Figure 9. PL spectra of (a) ZnO NR on RANS, (b) ZnO NR on VANS, and (c) bare silica nanosprings (control). The inset in (b) is an expanded view of the 1.7–2.8 eV region that shows the low-intensity ZnO defect-induced PL.
Figure 9. PL spectra of (a) ZnO NR on RANS, (b) ZnO NR on VANS, and (c) bare silica nanosprings (control). The inset in (b) is an expanded view of the 1.7–2.8 eV region that shows the low-intensity ZnO defect-induced PL.
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Figure 10. Deconvolution of the PL spectra in the range of 2.9–3.5 eV of ZnO NR grown on (a) VANS and (b) RANS, and with subtraction of the Hg-vapor i-line for (c) VANS and (d) RANS. The experimental spectra are red circles, and the fits are the solid black curves. In panels (c,d) the green and blue lines correspond to weakly bound excitons and the purple to PL of bulk Wurtzite ZnO.
Figure 10. Deconvolution of the PL spectra in the range of 2.9–3.5 eV of ZnO NR grown on (a) VANS and (b) RANS, and with subtraction of the Hg-vapor i-line for (c) VANS and (d) RANS. The experimental spectra are red circles, and the fits are the solid black curves. In panels (c,d) the green and blue lines correspond to weakly bound excitons and the purple to PL of bulk Wurtzite ZnO.
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Table 1. X-ray diffraction planes, diffraction angles, FWHM, line breadth, crystallite size, dislocation density, and d-spacing for ZnO NR for VANS.
Table 1. X-ray diffraction planes, diffraction angles, FWHM, line breadth, crystallite size, dislocation density, and d-spacing for ZnO NR for VANS.
Diffraction
Planes
(h k l)

(Degree)
FWHM
(Radian)
Line
Breadth
Crystallite
Size, D (nm)
Dislocation
Density, δ
×103 (nm−2)
d-Spacing
(Å)
(1 0 0)31.20.00830.00817.13.842.8
(0 0 2)33.90.00850.00816.83.982.6
(1 0 1)35.7 0.00830.00817.33.812.5
(1 0 2)47.10.00780.00719.33.241.9
(1 1 0)56.1 0.00660.00623.42.491.6
(1 0 3)62.40.00750.00621.32.971.5
(1 1 2)67.50.00650.00525.42.431.4
(2 0 1)68.60.00530.00431.11.951.4
Table 2. X-ray diffraction planes, diffraction angles, FWHM, line breadth, crystallite size, dislocation density, and d-spacing for ZnO NR on RANS.
Table 2. X-ray diffraction planes, diffraction angles, FWHM, line breadth, crystallite size, dislocation density, and d-spacing for ZnO NR on RANS.
Diffraction
Planes
(h k l)

(Degree)
FWHM (Radian)Line
Breadth
Crystallite
Size, D (nm)
Dislocation Density, δ
×10−3 (nm−2)
d-Spacing
(Å)
(1 0 0)31.00.0079 0.00818.03.102.9
(0 0 2)33.60.00520.00527.61.322.7
(1 0 1)35.40.00960.00915.04.422.5
(1 0 2)46.80.00720.00720.82.322.0
(1 1 0)55.90.00640.00624.41.671.7
(1 0 3)62.10.00820.00719.62.661.5
(2 0 1)68.30.0074 0.00622.32.021.4
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Puri, S.; Kalkan, A.K.; McIlroy, D.N. Hierarchical Design of High-Surface-Area Zinc Oxide Nanorods Grown on One-Dimensional Nanostructures. Sci 2025, 7, 114. https://doi.org/10.3390/sci7030114

AMA Style

Puri S, Kalkan AK, McIlroy DN. Hierarchical Design of High-Surface-Area Zinc Oxide Nanorods Grown on One-Dimensional Nanostructures. Sci. 2025; 7(3):114. https://doi.org/10.3390/sci7030114

Chicago/Turabian Style

Puri, Sharad, Ali Kaan Kalkan, and David N. McIlroy. 2025. "Hierarchical Design of High-Surface-Area Zinc Oxide Nanorods Grown on One-Dimensional Nanostructures" Sci 7, no. 3: 114. https://doi.org/10.3390/sci7030114

APA Style

Puri, S., Kalkan, A. K., & McIlroy, D. N. (2025). Hierarchical Design of High-Surface-Area Zinc Oxide Nanorods Grown on One-Dimensional Nanostructures. Sci, 7(3), 114. https://doi.org/10.3390/sci7030114

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