Structural, Morphological, and Optical Properties of Nano- and Micro-Structures of ZnO Obtained by the Vapor–Solid Method at Atmospheric Pressure and Photocatalytic Activity

Abstract

Micro- and nano-structures of ZnO were synthesized by the vapor–solid method at 600, 700, and 800 °C in atmospheres of Ar and air, at atmospheric pressure. The structural characterization XRD shows that the nano-structures synthesized in air atmosphere at 600 °C, while diffraction peaks were found due to Zn because the presence of metallic Zn remains on the surface of the pellet. SEM images show that the morphologies range from nano-wires to micro-tubes. When cathodoluminescence is measured in micro-tubes, there is a shift of the near-band edge of the ZnO toward red; this is due to structural defects in the ZnO network. This result is corroborated with panchromatic CL measurements, which exhibit a difference in brightness between the micro-tubes. Furthermore, EDS measurements show an atomic quantity ratio of Zn:O that differs from the stoichiometric composition in the micro-tubes. The photocatalytic activity of three types of structures—nano-wires, micro-tubes, and micro-rods under UV irradiation using methylene blue as a model pollutant—were evaluated. The best response was obtained for nanowires, not only because they have a larger surface area but also because of the present defects.

1. Introduction

The manufacture and study of the properties of one-dimensional structures, such as nano-wires, nano-belts, nano-swords, and nano-tubes, has increased significantly in recent years due to the special properties of these systems, which often differ from those of the material in bulk and are interesting for their possible technological applications. In the specific case of semiconductors, the interest of these structures is largely related to their use as functional components in future devices and connections in nano-electronics and nano-optical devices. In addition, several binary semiconductor oxides, such as ZnO, SnO₂, In₂O₃, Ga₂O₃, or TiO₂, are used in applications like transparent contacts, gas sensors, and photocatalysis, in which low-dimensional structures are often of great interest [1,2,3,4,5]. In general, nano-wires and other nano-structures are considered basic structures within the bottom-up concept of nanotechnology, according to which functional structures can be constructed from these basic units. The use of nano-structures as part of more complex devices is still in a preliminary phase, however, which is why a large part of the research is dedicated to optimizing and functionalizing these nano- and micro-structures [6].

Of these binary semiconductors, one of the most studied has been zinc oxide (ZnO) since it has a large band gap of 3.3 eV and a high transmittance in the visible range of a large exciton bond at room temperature (60 meV) [7]. ZnO is a material that has piezoelectric properties, has high thermal stability, is friendly to the environment, and is a biocompatible material.

Due to these characteristics, ZnO is used in the electronics industry to realize sensors, LEDs, photodetectors [8], solar cells, field-emission devices, actuators, piezoelectric devices, transparent conductive oxide [9], and photocatalysts [7].

There are currently many different methods to obtain ZnO in nanometric size, such as spray pyrolysis [10], MOCVD [11], hydrothermal synthesis [12], microwave synthesis [13], electrochemical growth method [14], carbothermal evaporation [15], electron beam evaporation [16], and the vapor–solid method [17]. Of these synthesis methods, the vapor–solid method has the following advantages: it is inexpensive, the user has control over the morphology of its nano- and micro-structures, and no catalyst is used. In addition, this method has a high yield: that is, a high density of structures can be synthesized [18].

There are different reports on the growth of ZnO nano- and micro-structures via the vapor–solid method, but, in this study, the synthesis of the structures has been carried out cheaply and with shorter synthesis times than in other reports [19].

Having nano- and micrometric-sized structures for applications in detectors or catalysis is desirable since the recombination processes are reduced; in micro- and nanometric structures, the lattice mismatch can be easier to couple, and anti-reflectivity and light capture can be increased by having a high density of structures [20].

Removing contaminants from wastewater is currently a slow and inefficient process to perform conventionally. Heterogeneous photocatalysis occurs when using a semiconductor as a photosensitizer; in this process, highly reactive species, such as hydroxyl radicals, are produced, which are the basis of this process [21]. These photocatalytic reactions occur mainly on the surface of the catalyst; therefore, nano- and micrometric-sized materials must be developed and thus increase the surface–area ratio so that photocatalysis can be carried out more efficiently. Furthermore, when the size of the catalyst reaches nanometric size, the probability of recombination of the photogenerated electron–hole decreases due to the rapid arrival at the reaction site’s surface [21].

The photocatalytic activity of ZnO has been widely studied in recent years, but there are very few reports that have studied the photocatalytic activity of micro-rods, nano-wires, and micro-tubes. Therefore, in this study, we report on the synthesis of nano- and micro-structures of ZnO by the vapor–solid method using metal Zn pellets as a source and as a substrate, sintered at 600, 700, and 800 °C in argon and compressed air atmospheres. Different morphologies were obtained that are dependent on the atmosphere and temperature. The nano- and micro-structures of ZnO obtained were characterized by XRD, SEM, CL, and EDS. In addition, the photocatalytic activity of nano-threads, micro-tubes, and micro-rods is compared.

2. Materials

Metallic zinc was used as a precursor powder with 99.9% purity. No catalyst was used. The powder was compacted in a hydraulic press at 7 tons to form tablets of 7 mm diameter × 3 mm thickness. The pellets were placed in a horizontal furnace “TZF” tubular furnace that reaches 1200 °C, with a heating zone ranging from 180 to 610 mm. All thermal treatments were carried out at atmospheric pressure. In this process, the ceramic pellet acts simultaneously as a source and as a substrate for the growth of elongated micro- and nano-structures. The thermal treatments of the pellets were carried out in two steps. In the first step, they were held at 450 °C for one hour; the temperatures was then raised to 600, 700, and 800 °C for 2 h for each sample, having a controlled gas flow (Ar and compressed air). The Ar gas used was 99.9% pure, and the compressed air was industrial-grade, with 98% purity. All treatments were carried out at atmospheric pressure. XRD measurements were carried out in grazing incidence using a Philips X’Pert Pro diffractometer using Cu Ka radiation. A Leica 440 Steroscan scanning electron microscope (SEM) operated at 10–15 kV was used for the morphological characterization. For the EDS measurements, a Bruker AXS 4010 Quantax system was used in a Leica 440 SEM. CL measurements in the visible range were carried out in a Hitachi S2500 SEM using a Hamamatsu PMA-11 CCD with a detection spectral range of 300–800 nm to an Oriel Cornerstone^TM 260 monochromator. For CL imaging, a Hamamatsu R-928 photomultiplier was used; the KV range used was 15–20 KV. Degradation analysis was carried out using a hermetically sealed chamber equipped with a UV light source ranging from 320 nm to 380 nm with a power of 20 W. The pollutant used was methylene blue in a proportion of 5 ppm. In a reactor that was coated with glass and filled with water, the water temperature was maintained at 25 ± 3 °C. The degradation process during irradiation was reviewed by means of UV–Vis spectroscopy, taking aliquots every 15 min for a period of 90 min. UV–Vis measurements to observe the degradation were carried out with a Thermoscientific Evolution 600 Uv-Vis spectrophotometer in the range of 300 nm to 900 nm.

3. Synthesis and Characterization

Figure 1 shows the representative patterns of X-ray diffraction obtained from nano- and micro-structures in air atmosphere. In X-ray diffraction, peaks of the wurtzite hexagonal phase of ZnO could be observed in the directions (100), (102), (101), and (002), according to the JCPDS 036-145 crystallographic card; for samples treated at 600, 700, and 800 °C, there is no preferential direction, and the intensity of the diffraction peaks is related to the amount of ZnO on the surface of the pellet. In the sample obtained at 600 °C, a diffraction peak of Zn was observed in the hexagonal phase (01-087-0713); this is due to the fact that the surface of the tablet was not completely oxidized, suggesting that ZnO had not formed on the entire surface, as shown by Xu et al. [17,22]. In the samples synthesized in an Ar atmosphere, no Zn diffraction peaks were observed since there was a high density of nano- and elongated micro-structures, and the crystalline structure due to Zn was not present.

Figure 2 shows the images of scanning electron microscopy (SEM) of the nano- and micro-structures synthesized at temperatures of 600, 700, and 800 °C in the air atmosphere. The oxidation temperatures were chosen because they fall between the melting point (420 °C) and the evaporation point (907 °C) of Zn.

In the images, it is observed that the morphology is dependent on the temperature and the environment. At 600 °C (Figure 2a), the growth in wires of the ZnO is observed on the surface; the wires are up to 100 nm in length and 5 μm in diameter.

The reason this morphology appears is due to the experimental process carried out, starting from an oxidation temperature close to the melting point of Zn (450 °C) for one hour and subsequently rising to 600 °C for one hour. Different reports describe the possible mechanisms of the growth of nano- and ZnO micro-structures caused by the vapor–solid method using different precursors. Therefore, the growth of the micro- and nano-structures for this temperature are follows. At 450 °C, there will be two growth mechanisms: the solid–solid mechanism and the liquid–solid mechanism, as mentioned [23]. The first mechanism can be considered dominant (being close to the melting temperature), in which a layer of ZnO forms on the surface of the Zn pellet, as the O₂ species in the environment reacts with Zn. Later, when the oxidation temperature is 600 °C, there will be two growth mechanisms: the liquid–solid mechanism and the vapor–solid mechanism. At this temperature, a thin layer of liquid Zn is formed under the ZnO film; due to the difference in the mole ratio between ZnO and Zn at the interface, there will be compression stress and, as the Zn species diffuses toward the surface, the growth of the nano-wires is favored (Figure 2a) [23]. In the vapor–solid mechanism at this temperature (600 °C), there are few species of vapor phase Zn that react with the O₂ in the environment to form ZnO and deposit on the surface of the tablet.

At 700 °C (Figure 2b), there is a greater amount of Zn in the liquid phase, so there is also a greater amount of Zn that diffuses on the walls of the nano-wires, growing in diameter and length [23]. In this case, the base of the nanowires is 2 μm, and they have a length of up to 10 μm.

At 800 °C, the amount of nano-structures 1D decreases, and, unlike at the lower temperatures mentioned, pores appear on the surface. This may because, at this temperature, a greater amount of Zn evaporates, reacting with O₂ in the gas phase and forming ZnO powders that will be deposited outside the pellet, thus obtaining a surface with disordered pores. A similar result was reported by Feng et al., who showed that, at temperatures close to the evaporation point of Zn, Zn evaporates almost completely [24,25].

The SEM images of the nano- and micro-structures obtained at temperatures of 600, 700, and 800 °C in Ar atmosphere are presented in Figure 3.

At 600 °C, the growth of micro-trumpets can be observed (Figure 3a). These micro-trumpets have a diameter at their base of around 1 μm and at their tip a diameter of around 5 μm and up to 10 μm long. Again, because the nano- and micro-structures are obtained at two stepwise oxidation temperatures (450 and 600 °C), initially a film of ZnO was obtained (as mentioned above) and subsequently micro-hexagonal structures were obtained as well. In these micro-structures, the center has a non-stoichiometric Zn:O ratio: that is, a Zn content higher than O because the environment is poor in oxygen, which causes the Zn to evaporate from the center and form hollow rods or micro-tomes, as shown in Figure 3a. The hexagonal shape at the upper end of the micro-structures can be attributed to the polarity of the crystal directions in ZnO as mentioned by Huang et al. [26].

These micro-trumpets have a diameter at their base of around 1 μm and at their tip a diameter of around 5 μm and up to 10 μm. Again, because the nano- and micro-structures are obtained in two stepwise oxidation temperatures (450 °C and 600 °C), initially a film of ZnO is obtained (as mentioned above). The hexagonal shape at the upper end of the micro-structures can be attributed to the polarity of the crystal directions in ZnO, as mentioned by Huang et al. [26].

At 700 °C (Figure 3b), there is greater diffusion of Zn species from the base to the tip of the micro-structures; therefore, they grow radially and axially, forming micro-tubes with dimensions up to 5 μm in diameter, with 1 μm in width and 15 μm in length.

When the oxidation temperature is 800 °C (Figure 3c), the evaporation of Zn increases, causing an increase in the reactions in the gas phase, so that there are no micro-tubes; in this case, micro-rods are observed. Proof of this is shown in the insert in Figure 3c, where tetra-pods deposited on substrate adjacent to the Zn pellet are observed.

Similar tubular micro-structures have been described in the literature; they have been obtained, however, using a reducing agent that causes the ZnO to be reduced in the center of the structures and evaporated, obtaining the micro-tubes [27,28].

Different authors have mentioned that the tubular morphology could also be due to a growth promoted by screw dislocations, which generate a growth in tubular form [19].

Table 1. EDX results of the samples synthesized in Ar and air.

Sample	Zn % at.	O % at.
600 °C Air and Argon	53.47	46.52
700 °C Air and Argon	51.07	48.93
800 °C Air and Argon	51.32	43.55
Upper base of the micro-tube	64.9	35.1

presents the results of X-ray energy dispersion spectroscopy (EDX) measurements. It is shown that the composition of nano- and micro-structures at different temperatures in the atmosphere of air and Ar has a Zn:O ratio very close to the stoichiometric composition.

In addition, the measurement of EDX was performed in the center of the microtubes (Figure 4), and a higher atomic percentage of Zn was found with respect to O₂; that is, it was partially oxidized, as proposed in the discussion of the formation of the microtubes.

Figure 5 shows the cathodoluminescence spectra of the samples synthesized in air and Ar at 600 °C, 700 °C, and 800 °C (measures in bulk), showing that there are mainly two emission bands. An emission is located around 3.2 eV, also called band-edge emission, which is due to the recombination of the free exciton [29]. Another emission band is located around 2.5 eV, known as emission of deep levels, and is caused by impurities and structural defects, such as dislocations and point defects. This mechanism of emission reported in the literature indicates that the origin of this emission is due to the recombination of electrons in energy levels corresponding to oxygen vacancies with photogenerated holes [29].

In all cases, the emission intensity is higher in 3.2 eV compared to the emission centered in 2.5 eV, and the emission intensity of the nano- and micro-structures is greater when it is obtained in the Ar atmosphere; therefore, the intensity of the near-band edge is greater, while the surface area of the nano-threads, the nano-rod and the microtubes is greater, probably because they have a lower density of defects [17].

Some authors have mentioned that the red shift in the luminescence of ZnO is due to the difference in the sizes of the micro-structures [30]. Han et al. estimated the emission at 415 nm in ZnO nanoparticles to Zn_i. The blue emission obtained was assigned to electron transition from a shallow donor level of Zn_i to the top level of the balance band [31]. This was corroborated by EDS measurements, which showed that the atomic content of Zn-O along the tube differed from the stoichiometric composition [17].

Panchromatic images and the CL spectrum of a micro-tube are presented in Figure 6a. In it, an increase in emission at the bases of the tubes can be seen, decreasing toward the tip of the tube. Figure 6b shows that the axial surface of the micro-tube contributes to the emission of the near-band edge, as mentioned, due to a lower density of defects, and the upper base of the micro-tube contributes to the broadband related to intrinsic defects; as mentioned, this surface is partially oxidized.

4. Degradation Analysis

Figure 7 shows the typical behavior of a photocatalytic degradation process followed by absorbance in UV–vis spectroscopy. In this case, the degradation of methylene blue in a solution at 5 ppm is shown. As can be seen, the intensity of the maximum absorption peak of methylene blue (664 nm) decreases as the photocatalytic process advances in time, which indicates the elimination of the methylene blue molecule.

In the photocatalytic degradation process, two stages can be distinguished: the first corresponds to the establishment of the adsorption–desorption equilibrium of the dye on the surface of the photocatalyst, which is achieved by placing the solution to be degraded in darkness once the photocatalyst has been dispersed until that the concentration remains constant. In this case, equilibrium was reached at 25 min, as seen in Figure 8. The second stage is the degradation of the dye, which begins once it is irradiated with light of adequate wavelength for the activation of the photocatalyst.

Figure 9 compares the photocatalytic activity of the three types of structures in the degradation of methylene blue (nano-wires, micro-tubes, and micro-rods). As can be seen, changing the type of structure increases the percentage of degradation by approximately 48%. This increase in photocatalytic activity, however, is reduced by changing the structures of the nano-wires to micro-tubes. This is because the micro-tubes have a smaller surface area, in addition to a greater number of defects, as shown in the CL and EDS measurements. In the micro-tubes, notably, there is a shift of the near-band edge of the ZnO toward the visible, and it is likely that its photocatalytic activity will improve with visible light.

Figure 10 shows that the degradation reactions obey pseudo-first-order kinetics [32,33].

Table 2. Results of the methylene blue (5 ppm) photocatalytic process.

Sample	Absorption AM (%)	Degradation AM (%)	Constant Reaction, k (min−1)	Total Elimination AM (%)
Nano-wires	6.4	97.2	0.04063	97.4
Micro-tubes	5.5	49.8	0.00745	52.5
Micro-rods	13.5	94.5	0.03246	95.2

summarizes the results obtained in the photocatalysis process. The total elimination of methylene blue takes into account the absorption and degradation of methylene blue.

It can be observed that 90 min for the degradation of 97.4% of the nano-wires is very good—better than those reported by other authors. This result is still under study, but it is a good start from which to obtain better results in photocatalytic degradation by this method of growth.

5. Conclusions

Many ZnO nano- and micro-structures were synthesized with different morphologies by the vapor–solid method at atmospheric pressure. Morphology, temperature, and the environment play important roles in obtaining these structures. Regarding the optical characteristic, in the CL, measurements can be seen in two emission bands that are obtained mainly in the band; this is due to the band edge of the ZnO, which has a redshift due to defects in the micro-tubes. Furthermore, EDS corroborates that the band runs toward the visible since the amount of ZnO in the tube differs from the stoichiometric compositions.

Performing the degradation tests shows that all the structures had great photocatalytic activity. The difference between them is the number of defects each has in addition to the surface area.