Metal-oxide nanowires have found widespread application, especially in the electrical detection of molecules. These compounds have been used in catalysts, batteries, and molecular sensors. Given that the metal oxide nanowire’s surface structure determines the interactions with the adsorbed molecules, understanding the associations between the nanowire surfaces and the adsorbed molecules is extremely important. However, the search for this understanding has been challenging. Particularly in the molecular sensing and inhomogeneous catalysis fields, the available knowledge on the surface interactions is relatively scanty. Various researchers have sought to understand the surface structure interactions between the nanostructured metal oxides and adsorbed molecules (Bertuna et al., 2017; Yang & Wöll, 2017) . Various techniques such as X-ray photoelectron spectroscopy (XPS) and infrared spectroscopic methods have been used to seek an understanding of these interactions. Prior investigations have used bulk powders and have given some understanding regarding the interactions, such as the different intermediates of catalytic reactions that are observable through infrared spectroscopic methods (Wang et al., 2017; Zandi & Hamann, 2016). However, the chemical interactions between the molecules and the metal oxide surfaces is not fully explored as it faces some challenges. One of the notable challenges is the complicated crystal surfaces found in standard nanostructured metal oxides, yet chemical reactions are dependent on the crystal planes holding the molecules. Single crystal metal oxide surfaces have been used in these investigations to make up for the complex crystal’s shortcoming.
In view of the knowledge gaps that are prevalent in understanding surface reactions, as elaborated above, Wang et al. (2019) sought to provide insights into this field. Wang et al. investigated the molecular reactions phenomenon by using single-crystalline metal oxide nanowires. The single crystalline metal oxide nanowires have some unique properties that make them preferable for these investigations. The properties include having a large surface -to-volume ratio and distinct crystal planes that enable the spectroscopic and mass-spectrometric measurements even when the adsorbed molecules’ density is low. The research by Wang et al. utilized zinc oxide surface and nonanal due to the possible applicability that it holds as a disease marker. The prevalent zinc oxide-based aldehyde sensors have utilized simple oxidation of the nonanal as their sensing technique. Thus, there is a need to investigate the aldehyde metal oxide reactions to better the sensors’ performance.
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Wang et al. (2019) undertook various experiments geared towards understanding the transformation that occurs on metal-oxide nanowire surfaces. Wang et al. combined different spectroscopic and mass-spectrometric techniques in order to understand the chemical transformation mechanisms. IR pMAIRS was the spectrometric technique deployed in this research. Temperature programmed desorption (TPD) and gas chromatography (GC) were the mass-spectrometric techniques used. Wang et al. (2019) sequentially deployed the TPD/MC technique and IR pMAIRS methods. The elucidation of the results obtained and the discussion for the respective techniques are provided below:
TPD/MS
The researchers undertook the TPD/MS experiment whereby nonanal obtained from zinc oxide (ZnO) nanowires were subjected to thermal pretreatment at different temperatures as follows 400, 500, 600, 700, and 800 °C and exposed to various atmospheres, namely atmospheric air, 10 Pa oxygen (O 2 ) and vacuum. From this experiment, desorption profiles depicted two definite peaks, one occurring at a low temperature of about 120 °C and another at a high temperature of about 230 °C. The results were indicative of the presence of two unique adsorption states on zinc oxide nanowire surface.
From this experiment, the researchers also found out that the ratio of low T peak intensity to the high-T peak intensity reduced as the temperature increased but decreased with the reduction of the thermal pretreatment pressure. These findings provided the understanding that adsorption and the desorption of the nonanal on the zinc oxide nanowires are heavily dependent on the thermal pretreatment conditions.
GC-MS
The researchers’ decision to undertake the GC-MS analysis was guided by the desire to understand the two unique TPD/MS peaks obtained in the first experiment. The GC-MS analysis of the desorbed gas compounds at the different temperatures was undertaken using Shimadzu GC-MSQP 5050A instrument. In each of the temperatures, 100, 200, 300, 400, and 500 °C, the sample was heated for two minutes and straight away cooled to 35 °C. GC-MS technique was used to analyze the desorbed gas. Temperature measurement was undertaken sequentially, starting from 100 and all the way to 500 °C with the sample still connected to the instrument.
The GC-MS analysis provided some interesting findings. At the 100 °C chromatogram, a single peak was evident. The peak is attributable to the nonanal. The increase of the desorption temperature to 200 and 300 °C made the nonanal peak to disappear, and instead, a new peak was observed at 35.7 minutes retention time. These findings indicated that the two unique peaks that were evident in the TPD/MS technique were caused by the reaction of nonanal on the zinc oxide surface. The peak at 35.7 minutes gave an m/z value of 266, a figure equivalent to the molecular weight of the product formed from the condensation reaction of nonanal molecules. The retention time and mass fragment patterns corresponded with those exhibited by pure (E)-2-heptyl2-undecenal. From this analysis, it was clear that (E)-2- heptyl-2-undecenal is formed on the surface. Thus, it was concluded that the aldol condensation of nonanal occurs on the metal oxide surface. From the results of the GC-MS analysis, the researchers picked two interesting conclusions. Firstly, a proportion of the nonanal molecules is instantly changed to an α,β-unsaturated aldehyde through aldol condensation. Secondly, the reaction’s progress is significantly catalyzed by conducting thermal pretreatment at elevated temperatures or decreased pressure states.
SEM, TEM and SAED
Wang et al. (2019) examined source of the significant change in the aldol reaction progress due to altering the thermal pretreatment atmosphere. The change in the reaction was possibly influenced by changes in surface microstructure. Consequently, the researchers sought to unravel the phenomenon by taking SEM and TEM and SAED images. SEM images for zinc oxide nanowire surfaces thermally pre-treated at 400 °C in different atmospheres, namely atmospheric air, 10 Pa oxygen, and in vacuum, did not display any morphological changes in the aldol condensation progress. TEM and SAED images taken at similar pretreatment conditions as aforementioned displayed the nanowire as having definite crystal planes. Thus, it was concluded that the aldol condensation progress is not exclusively influenced by the surface morphological transformations caused by thermal pretreatment.
IR pMAIRS
Fourier- transform infrared (FT- IR) spectra were determined using the Thermo Fisher Scientific Nicolet iS50 spectrometer. Measurements were taken at angles of incidence ranging from 9 to 44° and at 5° intervals. The out of plane (OP) and in-plane spectra were determined using the pMAIRS software.
The pMAIRS techniques were used to investigate the acidic or basic characteristics of the zinc oxide nanowire surfaces. The aldol condensation process is enhanced by the acidic or basic metal oxides. The acidic ability of the zinc oxide surfaces is indicated by hydroxyl groups’ density (-OHs ) since hydroxyls tend to cap bare Zn 2+ ions that are strong Lewis acids. The spectra of FT-IR zinc oxide nanowires that had been pre-treated at different temperatures in air depicted two peaks that are associated with the surface hydroxyl groups. The absorbance of the peak decreased with an increase in the pretreatment temperature.
The number of hydroxyl groups on the zinc oxide surface was confirmed by measuring the XPS. The measurement indicated two peaks, one at ∼ 530.2 and the other at 531.3 eV for the lattice oxygen and surface-adsorbed hydroxyl, respectively. Elevating the temperature or reducing the pretreatment atmosphere resulted in a decreased area ratio between the surface-adsorbed hydroxyl and the lattice oxygen. It was found out that a negative correlation exists between the surface number of hydroxyl groups and the extent of progress of the aldol condensation. However, a positive correlation was evident between the exposed Lewis acid sites and the aldol condensation’ degree of progress. The IR pMAIRS technique was deployed to get data concerning the interactions between nonanal molecules and the zinc oxide nanowire surface. Nonanal adsorbed zinc oxide pre-treated at 400 and 800 °C depicted two distinct peaks attributed to the vibration mode. The researchers linked the two peaks to the nonanal adsorption states. One of the states is where the adsorption occurs through hydrogen bonding with the surface hydroxyl and the other to the nonanal adsorption through the coordination to surface zinc ions. The peak patterns were synonymous with the zinc oxide adsorbed (E)-2-heptyl-2- undecanal. Consequently, the researchers concluded that the peaks were a result of the aldol condensation products.
Wang et al. (2019) went ahead to use FT-IR spectrometry to observe the dynamic variations in the surface molecular states as the molecular desorption processes occur. The observations were made on zinc oxide nanowires pre-treated at 400 and 800 °C. They also tracked the spectral changes arising from the aldol-condensation of (E)-2-heptyl-2-undecenal adsorbed onto zinc oxide nanowire pre-treated at 800 °C. The thermal oxidation of carbonyl compounds into carboxylic acids was one of the findings derived from these experiments.
From the various experiments, Wang et al. (2019) suggest that the chemical changes of nonanal on zinc oxide nanowires possibly occurs in five stages as elaborated below:
The nonanal molecules are brought together at the surface zinc ion sites and become activated.
The activation of the nonanal molecules gives way o their transformation into enolate-type intermediates.
The nucleophilic attack of the activated nonanals leads to the formation of an aldol structure.
Dehydration of the aldol structure through condensation leads to the formation of an α, β-unsaturated aldehyde.
Decomposition of the condensation product through oxidation leads to the formation of carboxylic acids that stick to the zinc oxide surface. The research is well-grounded and presents some useful knowledge in the field of molecular-surface interactions. Specifically, the researchers’ insight through their chemical interactions proposal is helpful in the molecular sensing and inhomogeneous catalysis fields in which there have been limited knowledge regarding surface interactions. One of the paper’s strengths is that the authors have provided an excellent breakdown, starting with the abstract, introduction, results, discussion, and conclusion. Particularly in the results and discussion section, the researchers have presented a comprehensive analysis of the results obtained from the experimental techniques used in the research. More importantly, the researchers have integrated the data obtained into their discussion and precisely explained the theories and science behind these findings.
The researchers have provided wide-ranging data that provides reasonable conclusions for the research. For example, in the analysis involving temperatures such as the TPD/MS and the GC-MS, the researchers have provided widespread data by undertaking the experiments under different temperatures such as 400, 500, 600, 700, and 800 °C. The experiments are also undertaken under different pressure conditions such as, the atmospheric pressure, 10 Pa oxygen, and vacuums. By conducting the experiments under various conditions, it becomes easy to form reasonable and scientific conclusions. The data provided is also of high quality as it addresses relevant surface chemistry issues. The researchers have also scientifically analyzed the data and presented graphical figures that make it easy for readers to synthesize the data. The graphical models are also explained in the word discussion, further enhancing the comprehension of the research. The only weakness that I could identify in the data presentation is that the researchers have not provided a tabular breakdown of the data used to prepare the graphical figures. Such a display would have further enhanced the understanding of the results as it would be relatively easy to follow how the data changes, for example, the changes arising due to temperature variations.
The GC-MS analysis results are critical findings that I would include in a book with limited space dedicated to the separation science topic. This is because the GC-MC analysis conducted Wang et al. (2019) provides two crucial contributions to the field of separation science. The first result that I would include is that the adsorption of nonanal molecules on the zinc oxide nanowires leads to their immediate transformation into α,β-unsaturated aldehyde. The transformation of the nonanals occurs through aldol condensation. This finding provides an understanding of how the processes of desorption and adsorption occur. Another significant result obtained from GC-MC analysis that I would include in the book is that the degree of aldol condensation reaction is promoted by thermal pretreatment at elevated temperatures or decreased pressure conditions. The finding on the enhancement of the aldol condensation process is useful to scientists in the field of separation science who would want to speed up such a reaction.
One of the findings that I would not include in the book is the FT-IR analysis finding that shows zinc ions act as strong Lewis acids that help promote the aldol condensation reaction. The reason for not including this finding is guided by the fact that the result presents some already well-known knowledge. It is an established knowledge that the aldol condensation process is enhanced by the presence of acidic or basic inhomogeneous metal oxides. Thus, it would not add any new knowledge to the separation science field.
References
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