A. Growth mechanism of ZnO nanorods

Due to the surface-passivated oxide layer, the oxidation of zinc by naturally dissolved oxygen in water is very slow.^{Reference Muster, Neufeld and Cole25} However, the presence of formamide can accelerate the spontaneous atmospheric oxidation process at room temperature to release zinc ions into the reaction solution through the formation of zinc–formamide complexes ([Zn(CH₃NO)_n]²⁺).^{Reference Yu, Zhang, Han, Hao and Zhu24} Heated in the oven, [Zn(CH₃NO)_n]²⁺ can be continuously supplied and decomposed into free zinc ions. As a result, zinc complexes (Zn(OH)₂) occur and deposit on the buffer layer to form ZnO nuclei, leading to the growth of ZnO nanorods on the buffer layer/glass substrate. These can be represented by the following reactions:

(1)$${\rm{Zn + {{1 \over 2} O_2}}} + n{\rm HCON{{\rm H}}_2} + \rm{H_2}O \to {[{{\rm{Zn}}{({\rm{HCON{H_2}}}})_n}]^{2 + }} + 2{\rm{O{H^ - }}}\quad ,$$

(2)$${[{\rm{Zn{(HCON{H_2})}}}_n}]^{2 + }} \to {\rm{Z}}{{\rm{n}}^{{\rm{2}} + }} + n\,\,{\rm{HCON{H_2}}}\quad ,$$

(3)$${\rm{Z}}{{\rm{n}}^{2 + }} + 2{\rm{O}}{{\rm{H}}^ - } \to {\rm{Zn(OH}}{{\rm{)}}_{\rm{2}}}\quad ,$$

(4)$${\rm{Zn(OH}}{{\rm{)}}_{\rm{2}}} \to {\rm{ZnO_{(S)}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\quad {\rm{.}}$$

It is well known that the (002) plane of ZnO has the minimum surface energy. On the basis of surface energy minimization, ZnO crystallites from Zn(OH)₂ nucleate along the c-axis and then grow into one-dimensional rodlike crystals.^{Reference Xu, Lu, Xie and Liu26} The nucleation between the ZnO crystal and the Ag layer/glass substrate is ascribed to heteronucleation. The Ag buffer layer has considerably low surface tension^{Reference Trushin, Kokko and Salo27} and can decrease the nucleation barrier by effectively lowering the interface energy between the crystal nuclei and the glass substrate, thus ZnO tiny crystallites can directly adsorb on the substrate and then nucleate along their own c-axis orientation.^{Reference Tong, Liu, Dong, Zhao, Zhang, Lu, Shen and Fan28} It is reported that there is a good lattice and symmetry match between ZnO and Ag in the corresponding planes, which may also be responsible for the preferred growth of ZnO nanorods.^{Reference Fan, Ding, Liu, Tian and Wang29} In fact, for a (111)-oriented Ag layer which has face-centered cubic structure, the (111) planes of Ag surface have a hexagonal two-dimensional lattice and can serve as templates for the close-packed basal plane of ZnO.^{Reference Liu, Wu, Cao and Chang30} Therefore, ZnO nanorods can grow in alignment along the c-axis on (111)-oriented Ag layers.^{Reference Liu, Wu, Cao and Chang30}

It is noteworthy that the Ag layer facing down is also favorable for the oriented growth of uniform ZnO crystals, since the nuclei from homogeneous nucleation in the solution, which will combine into big particles, cannot deposit on the surface of the Ag layer to act as a new heterogeneous nucleation site.^{Reference Liu, Jin, Bu, Zhao and Liu31} Similarly, the impurities in the solution cannot accumulate on the Ag layer to affect the nucleation of ZnO crystals.^{Reference Park, Lee, Kwak, Seo, Kim, Choi and Han32} Moreover, compared with heating in a water bath, heating in the oven can provide a relatively stable environment without interference of external contaminations, resulting in formation of uniform ZnO nanorods.^{Reference de Moura, Lima, Moreira, Volanti, Espinosa, Orlandi, Pizani, Varela and Longo33}

B. Effect of the position of the Ag buffer layer

To investigate the effect of the position of the Ag layer/glass substrate, Zn foils with lengths of 5, 7, 10, 12, and 15 mm and Ag buffer layers with a constant thickness of 60 nm were used. That is, ZnO nanorods were grown on 60-nm-thick Ag buffer layers at heights of 5, 7, 10, 12, and 15 mm, respectively. Figure 2 shows the top-view and cross-sectional SEM images of the vertical ZnO nanorods grown on Ag layers at heights of 5, 10, and 15 mm. It can be noticed that the position (i.e., height) of the Ag layer/glass substrate has a significant influence on the three-dimensional (3D) size of the final ZnO nanorods. As Figs. 2(a) and 2(d) show, the nanorods grown on the Ag layer at a height of 5 mm have relatively larger diameter and length of 600 nm and 4.6 μm, respectively. With an increase in height to 10 mm, as revealed in Figs. 2(b) and 2(e), the average diameter and length of the nanorods decrease to 300 nm and 3.6 μm, respectively. By increasing the height to 15 mm, the diameter of the nanorods drops slightly [Fig. 2(c)], whereas the length decreases down to 2.6 μm [Fig. 2(f)].

FIG. 2.

SEM images [top (a)–(c) and cross-sectional (d)–(f) views] of the ZnO nanorods grown on 60-nm-thick Ag buffer layers at different heights: (a), (d) 5 mm; (b), (e) 10 mm; (c), (f) 15 mm.

The average diameters and lengths of ZnO nanorods grown on Ag layers at different heights were estimated by utilizing the top-view and cross-sectional SEM images, and the result is summarized in Fig. 3. It is indicated that continuous decreases in the diameter and length of ZnO nanorods emerge until the height of the Ag layer is increased to 12 mm. Afterward, the 3D size of nanorods exhibits unobvious changes with an increase in the Ag layer height. It is believed that the vertical concentration gradient of the formamide aqueous solution can be formed under the influence of gravity.^{Reference Chang, Levelt Sengers, Doiron and Jones34} As a result, the growth rate of ZnO crystals gradually becomes slower from bottom to top of the bottle due to the gradually decreased concentration of reaction solution.^{Reference Sun, Chen, Deng and Li35} Thus, as the height of the Ag layer was raised, the diameter and the length of ZnO nanorods got narrower and shorter. The almost constant 3D size of ZnO nanorods grown on the Ag layer at a height of 15 mm may be attributed to a small difference in concentration at a height higher than 12 mm.^{Reference Akguna, Kalaya and Unalana36,Reference Tsai, Shih, Wu and Meen37}

FIG. 3.

Diameter and length of ZnO nanorods grown on 60-nm-thick Ag buffer layers at different heights.

C. Effect of Ag buffer layer thickness

The buffer layer thickness, as reported by Kim et al.,^{Reference Kim, Lee, Jang, Kim, Cho, Park, Jang and Kim38} also affects the growth properties of hydrothermally grown ZnO nanorods. Figures 4(a) and 4(b) show 3D AFM images with a scanned region of 10 × 10 μm for the Ag buffer layers with thicknesses of 20 and 100 nm, respectively. The surfaces of both Ag layers exhibit rugged morphologies with some pyramidal shapes. However, the pyramids on the 100-nm-thick Ag layer are denser and larger than that on the 20-nm-thick one, resulting in a RMS roughness of 59.7 nm, which is almost two times greater than that of 31.1 nm for the 20-nm case. Figure 4(c) shows the variation of RMS roughness with the thickness of the Ag buffer layer. The RMS roughness sharply increased, reaching a maximum for the Ag layer with a thickness of 60 nm. Above 60 nm, no further increase in the RMS roughness was observed.

FIG. 4.

AFM images of Ag buffer layers with thicknesses of (a) 20 nm and (b) 100 nm. (c) The surface RMS roughness as a function of the thickness of the Ag buffer layer.

The Ag buffer layers with thicknesses of 20, 40, 60, 80, and 100 nm were located on two Zn foils with a constant height of 5 mm, and SEM images of the ZnO rods grown on them are presented in Fig. 5. It can thus be stated that the ZnO rods exhibit well-defined and regular growth changes in density and diameter with the thickness of the Ag layer. As shown in Fig. 5(a), a low Ag layer thickness (20 nm) results in formation of randomly inclined nanorods with an average diameter of ∼200 nm, which form flower-like clusters at some scattered sites that may correspond to the locations of the pyramids on the Ag layer shown in Fig. 4(a). When the Ag layer thickness was increased to 40 and 80 nm, ZnO nanorods were converted into dense and regular ones with average diameters of 350 and 800 nm, respectively, as seen in Figs. 5(b) and 5(c). The Ag layer with a high thickness (100 nm), whose surface contains denser and larger pyramidal shapes [Fig. 4(b)], breeds densely packed and vertically oriented ZnO rods with a microsized diameter (∼1.1 μm) as shown in Fig. 5(d). Therefore, it is thought that the growth of the ZnO crystals on the Ag layer proceeds along the surface morphology of the Ag layer as a template.^{Reference Kim, Kong, Cho, Park and Lee39}

FIG. 5.

SEM images of the ZnO rods grown on 5-mm-height Ag buffer layers with different thicknesses: (a) 20 nm, (b) 40 mm, (c) 80 mm, and (d) 100 nm.

The results of the ZnO rod dimension variation with the Ag layer thickness are summarized in Fig. 6. It is clearly demonstrated that an approximate linear relationship exists between the increases of the Ag layer thickness and the rod dimension. Interestingly, the diameter increases gradually, whereas the length becomes constant above an Ag layer thickness of 60 nm. This implies that there is a critical Ag layer thickness, in which further increase will not have any role in the axial growth of ZnO rods.

FIG. 6.

Diameter and length of ZnO rods grown on 5-mm-height Ag buffer layers with different thicknesses.

D. Effect of the annealing temperature of the Ag buffer layer

The effect of annealed ZnO buffer layer on the morphology of hydrothermally grown ZnO films has been reported.^{Reference Zhao, Kim, Lee, Heo, Shin, Ryu, Chang, Lee, Son, Lee, Jung, Tan, Zhao and Sun40–Reference Shim, Chang and Kim42} Here, by using the 60-nm-thick Ag buffer layers, which were annealed at 300, 400, and 500 °C and located on two Zn foils with a constant height of 5 mm, the effect of the Ag layer annealing temperature was evaluated. Figure 7 shows SEM images of ZnO nanorods grown on the annealed Ag layers. It can be noticed that the Ag layer annealing temperature is also an important factor to control the shape and size of the final ZnO nanorods. On the Ag layer annealed at 300 and 400 °C, vertically oriented ZnO nanorods with average diameters of ∼610 nm [Fig. 7(a)] and 730 nm [Fig. 7(b)] were observed. Compared with those on the unannealed Ag layer, as shown in Fig. 2(a), they both display relatively lower density, which may be relevant to the lower surface roughness of the annealed Ag layer. A similar result was explained by Cho et al.^{Reference Cho, Kim, Choi, Yim and Leem43} More interestingly, the randomly inclined nanorods with diameters of both ∼870 and ∼250 nm dispersedly coexist on the Ag layer annealed at 500 °C [Fig. 7(c)]. To understand the differences in the ZnO nanorods grown on different annealed Ag layers, SEM images of the unannealed and annealed Ag layers are shown in Fig. 8. It is indicated that the annealing treatment can integrate small grains to form larger ones, which will reduce the number of the particles, corresponding to the decreasing number of the nucleation sites of ZnO nanorods thereon.^{Reference Weigand, Skåre, Ladam, Grepstad and Weman44} On the other hand, it has been reported that the diameters of the ZnO nanorods depend on the particle size of the buffer layer.^{Reference Song and Lim45} Thus, the phenomenon that the decrease in the density of ZnO nanorods and the increase in their diameter with increasing buffer layer annealing temperature can be explained. The Ag layer annealed at 500 °C shrinked to some sparse island-like shapes, and even the glass substrates became visible [Fig. 8(d)]. It is well known that compared with the buffer layer, the glass slide has smoother surface and fewer defects,^{Reference Liu, Jin, Bu, Zhao and Liu31} which should be responsible for the formation of randomly oriented nanorods with a smaller diameter of ∼250 nm.

FIG. 7.

SEM images of the ZnO nanorods grown on Ag buffer layers (60 nm in thickness and at a height of 5 mm) with different annealing temperatures: (a) 300 °C, (b) 400 °C, and (c) 500 °C.

FIG. 8.

SEM images of (a) the as-deposited Ag buffer layer and the Ag buffer layers annealed at (b) 300 °C, (c) 400 °C, and (d) 500 °C.

The 3D size variation of ZnO nanorods with the annealing temperature of the Ag layer is also summarized in Fig. 9, where the ZnO nanorods with the greater diameter were taken as the evaluative objects for the 500 °C case. The diameter of ZnO nanorods is slightly greater than that grown on the untreated Ag layer at first and then increases greatly with increasing annealing temperature of the Ag layer. The length of the ZnO nanorods remains in a narrow range of 4.6–4.8 μm, indicating that the length of the ZnO nanorods is not strongly influenced by the Ag layer annealing temperatures.

FIG. 9.

Diameter and length of ZnO nanorods grown on Ag buffer layers (60 nm in thickness and at a height of 5 mm) at different annealing temperatures.

Figures 10(a) and 10(b) show the XRD patterns of different Ag buffer layers and ZnO nanorods, respectively. The presence of diffraction peaks can be used to evaluate the structural order at a long range or periodicity of the material.^{Reference Cavalcante, Marques, Sczancoski, Escote, Joya, Varela, Santos, Pizani and Longo46} As shown in Fig. 10(a), Ag phase was confirmed by the comparison between the XRD patterns and the respective Joint Committee on Powder Diffraction Standards (JCPDS) card (No. 65-2871).^{Reference Wang, Shi, Zhao and Du47} All the diffraction peaks can be assigned to the face-centered cubic structure. All the Ag buffer layers had a preferential orientation along the (111) plane and exhibited characteristic diffraction peaks corresponding to an ordered structure at a long range. The obvious broad peak in the range of 2θ from 15 to 35° may be attributed to the amorphous glass substrate.^{Reference Sato, Hara, Kuritani and Nishide48} Compared with the as-deposited Ag layer, the annealed Ag layers exhibited gradually increased (111) peak intensity with increasing annealing temperature, indicating that the crystallinity of the film improved effectively.^{Reference da Silva, Maia, Bernardi, Andres and Mastelaro49} With respect to the ZnO nanorods, as shown in Fig. 10(b), characteristic diffraction peaks corresponding to an ordered structure at a long range were also exhibited. All the diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO (JCPDS No. 36-1451) with lattice parameters a = 3.250 Å and c = 5.207 Å.^{Reference Lu, Wang, Li, Su, Kuang and Tong50} The sharp and strong (002) peaks indicate that the ZnO nanorods have a preferential c-axis orientation on the as-deposited Ag layer and the Ag layers annealed at 300 and 400 °C.^{Reference Unalan, Hirala, Rupesinghe, Dalal, Milne and Amaratunga51} Moreover, the intensity of the (002) peak increases with an increase in the Ag layer annealing temperature, which can be attributed to the better alignment as shown in the SEM images [Figs. 7(a) and 7(b)].^{Reference Liu, Chen, Wan, Ru, Li and Qu16} However, the (002) preferred orientation for the 500 °C case is unremarkable since the relative intensity of (I ₍₀₀₂₎/I _{(100)+(002)+(101)}) is only 0.425, indicating the randomly oriented growth of ZnO nanorods, which is consistent with the SEM results [Fig. 7(c)].

FIG. 10.

(a) XRD patterns of the as-deposited Ag buffer layer and Ag buffer layers annealed at different temperatures; (b) XRD patterns of ZnO nanorods grown on the as-deposited Ag buffer layer and Ag buffer layers annealed at different temperatures.

For the direct transition in a semiconductor, the relationship between the absorbance (α) and the incident photon energy (hν) can be expressed as (αhν)² = A(hν−E _g),^{Reference Pankove52} where A is a constant and E _g is the optical band gap energy. Figure 11 shows the UV-vis absorbance spectra for the ZnO nanorods grown on the as-deposited and annealed Ag buffer layers. The optical band gap values were obtained through the linear fitting of tails from the UV-vis spectra. From the intercepts on the hν axis, the optical band gap values of the ZnO nanorods grown on the as-deposited Ag buffer layer and the Ag buffer layers annealed at 300, 400, and 500 °C were found to be about 3.20, 3.30, 3.37, and 3.40 eV, respectively. It is known that the exponential optical absorption edge and optical band gap are controlled by the structural order–disorder degree in the ZnO lattice.^{Reference Song and Lim45} The increase in the optical band gap value can be ascribed to the reduction of defects or impurities that give rise to the intermediary energy levels into the band gap of disordered ZnO nanocrystals.^{Reference Cavalcante, Simões, Espinosa, Santos, Longo, Varela and Pizani53} The ZnO nanocrystals grown on the as-deposited Ag buffer layer present a more structural disorder confirmed by a relatively smaller optical band gap value. The evolution of optical band gap value for the ZnO nanocrystals grown on the Ag buffer layers annealed at 300, 400, and 500 °C reveals the disappearance of some intermediate levels due to the gradual reduction of the structural disorder.

FIG. 11.

UV-vis absorbance spectra for the ZnO nanorods grown on the as-deposited Ag buffer layer and Ag buffer layers annealed at different temperatures.

Based on above results, by using this simple hydrothermal process and adopting suitable technological parameters, one can obtain well-aligned ZnO nanorods with various densities, sizes, and physical properties. It is well known that highly oriented ZnO nanorods are more valuable in a wide variety of applications. For some photoelectronic devices, e.g., field-emission devices, moderate density and high aspect ratio of ZnO nanorods are required. The simple hydrothermal process proposed here just provides the feasibility of synthesizing ZnO nanorods with various densities and aspect ratios, which are expected to meet the requirements of optoelectronic devices.