The effect of NaBH4 on plasmonic properties of WO3-x for optical dissolved oxygen(DO) sensor
Kazemi
Department of Physics
Abstract
Introduction
LSPR
Because free charge carriers in metals are so important, it is also important to study their behavior. So that when they are paired with the frequency of light radiation, they can oscillate collectively and induce a phenomenon called surface plasmon resonance in the metal. However, the smaller the particle size of the metal, the different their behavior and, consequently, their physical and optical properties change. Also, by changing the size and shape of metal nanoparticles, the local plasmon surface resonance peak (LSPR) can be controlled and tuned. This phenomenon can have various applications in biosensors1, medical imaging, thermal light effects, etc. with high potential. Also, surface plasmons, which are coherent oscillations of free charge carriers, interact strongly with incident light, which allows them to be used in photonics and electronics. There have been many reports of surface plasmon intensification in metals due to the high concentration of their free charge carriers, but LSPRs are also shown in transition metal oxide (TMOs) that can be produced by outer-d valence electrons. Among metal oxides, tungsten oxide is very suitable for absorbing light and thus creating the phenomenon of surface plasmon resonance due to its high stability under different conditions and band gap of 2.6 eV. In general, tungsten oxide can be displayed in dark blue due to the defect of oxygen in its structure. Therefore, by engineering the amount of oxygen deficiency in the structure of WO3-x, different stoichiometries with different colors can be achieved. Due to the oxygen vacancy, the electronic structure of WO3 is modified. As a result, WO3-x can be widely used in gas sensors, water splitting, photovoltaic light absorption, photothermal effect and electrochromic devices. To date, plasmonic WO3-x has been synthesized by various methods. For example, solvothermal treatment of the ethanol solution of W(CO)6 2, or utilization of WO2(O2)H2O 3 as precursors in the presence of surfactants. Also, the hydrogen reduction method has become an ideal and simple route for the preparation of substoichiometric WO3-x 4,5. In this article, we have prepared WO3-x in a very simple and easy rout. First, a simple electrochemical anodizing method was used to synthesize WO3 nanoparticles and then NaBH4 solution was used to achieve sub-stoichiometric WO3-x . The solution of WO3-x obtained under ambient conditions remains stable in the reduction state for 10-12 minutes but then returns to its oxidation state. Therefore, we first investigated the stability of wo3-x in the reduced state, and since Pd nanoparticles are known as hydrogen catalysts, PdCl2 solution was used to evaluate the stability time of sub-stoichiometric tungsten oxide. Experimental results showed that in the presence of palladium chloride salt, WO3-x can be completely stable in ambient conditions for up to 2 hours. As a result, the presence of PdCl2 can provide significant stability to sub-stoichiometric WO3-x in the reduced state. Also, Our experimental results show that plasmonic WO3-x shows a strong absorption in the visible and near infrared region and also the surface plasmon resonance can be controlled and tuned within an acceptable range by changing the anodizing potential. It indicates that the location of the WO3-x plasmonic absorption peak can be easily controlled only by changing the anodizing potential. In addition, due to the high sensitivity of WO3-x to the amount of oxygen around it, it can be used for the first time as a dissolved oxygen sensing and food freshness indicators in smart packaging. In the present study, WO3-x fabricated using XRD, TEM, XPS, Raman scattering and UV-Vis spectrophotometry were investigated.
Review on papers related to our work
To date, substoichiometric WO3-x has been synthesized via various approaches. For example, solvothermal treatment of the ethanol solution of W(CO)6 8, or utilization of WO2(O2)H2O 9 as precursors in the presence of surfactants. However, the tedious post-treatment process limits its further applications. In recent years, hydrogen reduction method has become an ideal route to prepare such substoichiometric WO3-x 10,11, and the plasmonic WO3-x has been exploited in various application, such as photocatalysis12, gas sensors13 and photothermal therapy as well14,15.Most of the literature reports that focus their attention on the optical properties of colored films, declare that the NIR absorption includes a broad asymmetric peak at 1–1.5 eV range and various attempts have been made to find a valid explanation for it via peak-deconvolution by two or three multi peaks. The colloidal tungsten oxide nanoparticles as a new chromic system which has unique NIR absorption bands. The scope of this study is to extend the subject of gasochromic liquids into simpler, chipper and more productive synthesis methods. Moreover, to push the criteria toward understanding of fundamentals of the gasochromic mechanism, it is noteworthy that hydrate forms of tungsten oxide can be good candidates owing to their property of high proton transportation. For example, tungsten oxide dihydrate (WO3.2(H2O)) lies in the series of good proton conductors at low operating temperatures and high humidity conditions16. In recent years, a variety of defect-induced photoelectric performance variations in WO3-x have been proposed. Cong suggested that the presence of oxygen deficiencies in WO2.72 could help amplify the spectroscopic signatures in stimulated Raman scattering.17 Cheng reported that the oxygen vacancies in WO2.83 played a critical role in generating exceptional plasmon resonance and contributed to its dramatically enhanced hydrogen evolution reaction (HER) activity relative to that of Meso-WO3.18 Wang found that oxygen-deficient WO2.92 nanoflakes showed excellent photostability for photoelectrochemical (PEC) water splitting.19 Wen et al reported that the ultrasmall WO 3−x nanodots exhibit strong pH- and oxygen-dependent LSPR in the NIR window, which is well aligned with the weak acidity and hypoxia microenvironment of tumors for sensitive photoacoustic imaging of tumors. The excellent photothermal conversion efficiency.15
The main novelty (The main idea of our paper is to fabricate LSPR WOx from WO3 by a simple rout.
What application we predict…
Smart packaging
Intelligent packaging communicates to packaging converters, food manufacturers, distributors, retailers, consumers, and post-consumer package handlers. Freshness indicators are used in some intelligent packages to communicate the shelf life of products within the value chain. However, because much food waste occurs after the purchase of food products, it is essential to ensure that freshness indicators also communicate to consumers. The use of freshness indicators that indicate shelf life after opening and freshness can be expanded to decrease food waste and increase the value of packaging. In-package freshness prior to opening is determined by different markers that indicate the onset of spoilage. These markers include pathogens, amines, degradation byproducts, carbon dioxide, and oxygen. There are various intelligent packaging technologies that detect and communicate product freshness. Oxygen indicators are used to monitor integrity throughout the supply chain. For example, they are used in modified atmosphere packaging to detect leaks in the seals and structure of packages.
Dissolved oxygen
Dissolved oxygen refers to the level of free, non-compound oxygen present in water. It is an important parameter in assessing water quality because of its influence on the organisms living within a body of water. Hence the determination of DO is very important in environmental monitoring and physiological and biological applications. The dissolved oxygen (DO) concentrations are crucial parameter for optical sensing technologies, which are utilized in the food industry, as well as in clinical and environmental applications. Recently, fluorescent materials have been developed for the simultaneous sensing of multiple parameters. An optical sensing technology is particularly advantageous over other sensing techniques as it permits virtually non-invasive sensing, e.g., observation through the glass window of a reaction vessel or a bioreactor. Compared to different DO sensors, optical DO sensors exhibit several advantages, including no oxygen consumption during sensing; no requirement for a reference electrode; and immunity to electromagnetic field interference. Typically, these optical DO sensors are based on collisional quenching by molecular oxygen of different fluorescent dyes embedded in a support matrix, such as a polymer or a sol–gel matrix.
Models and Theory….
How we performe it and how characterize it and what we found in brifly
Experimental
Materials
Preparation
Nanoparticles of tungsten oxide dihydrate were fabricated by anodizing tungsten rods in diluted HCl. For this purpose, two tungsten rods were put 1 cm parallel to each other into a 0.02 M HCl electrolyte. Then a 60V DC bias voltage were applied to the two ends of rods for 60 min. By applying voltage, the anode surface began to corrode and was released gradually into the electrolyte. A PdCl2 solution was prepared by adding 0.02 g of PdCl2 powder (99.99% purity) into a mixture of 99.9 cc DI water and 0.1 cc HCl. This composition was kept in ultrasonic bath until PdCl2 was dissolved and a uniform yellowish solution of 0.2 g/l PdCl2 was obtained after 2 h. Then, constant amounts of this solution including 0.2cc were added to 10 cc of as prepared colloidal solution of tungsten dihydrate. A solution of NaBH4 in ice water with a concentration of 0.05 M was prepared and used in all experiments freshly. Then a specific amount of 0.2cc NaBH4 was added to these samples.
Quantification of Dissolved Oxygen Concentration in Solution:
The oxygen concentration in the NP solutions were varied by using sodium sulfite (Na2SO3), a commonly used scavenger of molecular oxygen. Different concentrations of sodium sulfite were freshly prepared in DI water and added to NP solution for the measurement of oxygen concentration. The measurements were carried out using a pre calibrated Vernier Optical DO probe by placing the tip of the probe into the sample of interest. The measurement has been recorded each time at 21 C after giving a certain delay (ca. 2-3 min) to compensate the temperature and pressure. The oxygen concentration was determined in the units of mg/L or percent saturation (%) for the different samples. The concentration values were taken as an average of three independent measurements.
Table 1, Synthesis parameters
Preparation conditions
Sample name Electrolyte solution concentration(M) WO3 volume (cc) NaBH4 volume (cc) PdCl2 volume (cc) Volume of bubbled air (cc)
WO0.2 0.005 10 0.2 0.2 0.2
WO0.3 0.005 10 0.2 0.2 0.3
WO4.4 0.005 10 0.2 0.2 4.4
WO10 0.005 10 0.2 0.2 10
WO15 0.005 10 0.2 0.2 15
Characterization
In order to perform some characterizations, samples were prepared by drop-drying the nanoparticles from their colloidal solution onto silicon or glass substrates. The crystalline structures were analyzed by X-ray diffraction (Cukα, λ¼0.1544 nm, model Philips XPERT). Chemical bonds of the samples were obtained by FTIR spectroscopy in the mid-infrared range (600–4000 cm1) using Bruker FTIR (model Tensor27) system. The XPS analysis was done in an ESCA/AES system. The system is equipped with a concentric hemispherical analyzer (CHA, Specs model EA10 plus) suitable for auger electron spectroscopy and XPS. For exciting the X-ray photoelectrons, an AlKα line at 1486.6 eV was used. The energy scale was calibrated against the carbon binding energy (284.8 eV). Optical properties of liquids before and after hydrogen intercalation were measured in the 190–1100 nm wavelength range using Perkin Elmer spectrophotometer (Lambda 25).
Results and discussion
Microstructural characteristics
Figure 7 shows the XRD patterns of the prepared sample (WO3-voltage) at different voltages of 27, 30, 40, 50 and 60 volts for 30 minutes of anodizing time. Most WO3 sample diffraction peaks correspond to Orthorhombic tungsten oxide dihydrate (JCPDS 96-900-4175), WO3. (2H2O). The main peaks located at 16.5 16, 25.6% in all samples are constant with different voltages and new peaks appear with increasing voltage. The 12.8 degree peak, which appears in the 40V, 50V and 60V models, gets sharper with increasing voltage. The peak intensity at 16.5 degrees with increasing voltage from 27 volts to 60 volts shows an increasing trend. As the voltage increases, the peaks do not change much. According to previous knowledge, as the voltage increases, the grain size becomes larger. In all of our samples except WO3-30 there is a regular increase in particle size. The grain size increased from 27v to 60v, which was calculated to be 18.5nm, 32.2nm, 23nm, 26.6nm and 27nm, respectively. Also, the main diffraction peaks are at 2θ = 12.8, 14.4, 16.53, 23.7, 25.6, 30.4 and 34.2 degrees.
Figure 7: XRD patterns of different voltages samples in 0.02 M HCl solution, all XRD samples were taken from drying of precipitated nanoparticles
TEM
¬¬XPS
Investigation the different reaction conditions
PdCl2 effect on the stability of reduced W oxide
Although NaBH4 has been used to reduce tungsten oxide, the results show that the plasmonic solution is not very stable. In Figure 1. A) After adding NaBH4, the color of the solution immediately turns dark blue and appear in the absorption spectra of certain peaks at 966 nm and 992 nm. However, after only 4 minutes, the absorption drops sharply and after 12 minutes, it becomes completely zero. So WO3-x made in our conditions is stable for about 12 minutes. However, based on our previous experience, we have found that the addition of palladium chloride solution to colloidal tungsten oxide significantly helps to stabilize the reduced state (WO3-x). This is shown in Figure 1b. First, a solution of palladium chloride with a concentration of 0.2 gr/l and a volume of 0.2 ml was added to colloidal tungsten oxide, and then NaBH4 solution was added under the same conditions as in Part A. Figure 1b not only shows that the intensity of absorption is more than part A, but after 2 hours, almost no noticeable decrease in the amount of absorption occurs. NaBH4 has sufficient reducing power for both existing phases of the mixture, which is expected to form palladium nanoparticles according to the following reactions:
〖PdCl〗_2 □(→┴NaBH4 ) Pd+2HCl
Typical TEM images confirm the presence of particles that can be attributed to palladium. This suggests that the plasmonic stability trend is due to the role of palladium nanoparticles as a source of hydrogen storage and its continuous release. In other words, palladium nanoparticles pump excess H2 molecules into the WO3-x solution when hydrogen is deficient.
Figure 1: UV-Vis spectra of WO3-x (a) in the absence of PdCl2 and (b) in the presence of PdCl2 and the resulting significant stability (c) TEM image of WO3 with Pd nanoparticles
Effect PdCl2 volume
Due to the fact that the formation of palladium nanoparticles was confirmed in the previous section, now to further investigate the effect of the volume ratio of PdCl2, different volumes of it were added to the WO3 solution before reduction. Figure 2 (a) shows an overall increase in absorption intensity as the volume ratio of PdCl2 increases. The effect of PdCl2 on LSPR wavelength and absorption intensity are shown in Figures 2.b and c, respectively.
As can be seen, with the increasing relative volume of PdCl2, the wavelength shifts linearly from 965 to 958 nm (blue shift). These changes indicate that the formation of palladium nanoparticles along with tungsten oxide nanoparticles has a significant effect on the plasmonic peak.
In addition, the optical absorption intensity at the plasmonic peak increases almost linearly with increasing PdCl2 volume. This increase can be explained by the presence of palladium nanoparticles within WO3-x and its role as hydrogen storage.
Figure 2: UV-Vis spectra of WO3-x (a) Different volumes of PdCl2 solution (b) Effect of PdCl2 on plasmonic peak shifts and (c) absorption intensities
Effect of anodizing parameters on LSPR
Effect of electrolyte concentration (pH)
Different parameters can affect particle formation in the anodizing method. The most important of these are electrolyte concentration, anodizing potential and anodizing time. In the following, we try to examine their effects separately. First, we fixed the potential and time of anodizing and prepared solutions with different concentrations of hydrochloric acid. Then, under the same conditions (NaBH4 0.05 M and PdCl2 0.2 g/l) we reduced them to create a plasmonic state (blue). The spectra obtained from the sub-stoichiometric tungsten oxide are shown in Figure 3. It should also be noted that we allowed the resulting solution to stabilize, resulting in parts settling with larger particles. In other words, Figure 3 examines only the stable part of the solutions, which are mainly composed of smaller particles (colloidal state). As can be seen, with increasing HCl concentration, the absorption intensity decreases due to the deposition of most of the WO3 particles that are not in the absorption spectrum. Therefore, as the HCl concentration increases, larger particles are released that cannot remain colloidal and settle. Therefore, in order to have a stable solution consisting of smaller and more nanoparticles, it is better to keep the acid concentration to a minimum. On the other hand, at very low concentrations, the production rate (total suspended and precipitated nanoparticles) may decrease. In general, our results show that the concentration of 0.02 M HCl can be appropriate and we will work with the same concentration in the following sections.
Figure 3: UV-vis spectra of samples at different concentrations (0.06, 0.04, 0.02, 0.01 M HCl)
Effect of potential
Now we examine another parameter in the anodizing process, namely the applied potential between the two W electrodes. For this purpose, anodizing was performed in five different electrical potentials including (27, 30, 40, 50, 60 V). After reduction with NaBH4 and in the presence of PdCl2, their optical absorption spectra were compared (Figure 3). As can be seen, as the potential increases, more nanoparticles are released and the spectra change. Figure 3 shows the plasmonic peak changes in terms of anodizing potential, in which shifts to larger wavelengths are observed. According to the reported articles, as the potential changes, the size of the nanoparticles changes, which can be seen to have an effect on the behavior of the plasmonic peak. As shown in Figure 3 (b), the plasmonic peak shifts to more energies as the potential increases.
Figure 4: UV-Vis spectra of WO3-x with different anodizing potentials
The evolution of the crystal structure of colloidal nanoparticles was investigated by XRD (Figure 4). The diffraction peaks of WO3 sample are well compatible with Orthorhombic tungsten oxide monohydrate (JCPDS 96-900-4175), WO3.H2O. The main peaks located at 16.5, 25.6 degrees are fixed at different potentials, but with the increase of the potential, a new peak at 12.8 ᵒ appears at voltages of 40 V, 50 v, 60 V, which may be the change in the intensity of absorption of the samples due to An additional phase in the 40 V and 50 V samples. Also, the intensity of diffraction peak at 16.5 ᵒ with increasing potential from 27 to 60 V shows an increasing trend. The direction of grain growth at 16.5 degrees, which is related to plate (002), has changed from 40 volts to 50 and 60 volts, and the direction (111) is the preferred direction for grain growth at higher potentials.
XRD patterns of samples with different potentials
Effect of anodizing time (new data)
In order to investigate the effect of anodizing time on plasmonic peaks, 6 samples were made with different times at 60 V potential and 0.02 M HCl electrolyte concentration. The UV-Vis spectra of the samples at different anodizing times are shown in Figure 4. According to the UV-Vis spectra, more nanoparticles are produced with increasing time, and as a result, the adsorption intensity increases. In this experiment, tungsten oxide prepared in 50 minutes showed irregular behavior and its adsorption intensity was less than that prepared in 30 minutes. Figure 5 (b) shows that the anodizing time affects the LSPR peak and shifts to red as time increases. There are also obvious changes in the absorption intensity that you see in Figure 5 (b).
Figure 5: UV-Vis spectra of samples prepared with different anodizing times
Mechanism of DO-induced WO3-x instability
WO3-x can be easily oxidized by dissolved oxygen (DO) in the presence of PdCl2. By trapping hydrogen molecules, the stability of WO3-x increases and no color change is observed in the solution. Now, if hydrogens is removed from the solution and replaced by oxygen molecules, WO3-x is rapidly converted to WO3. As a result, it can be said that WO3-x is very sensitive to oxygen. For further investigation, after preparation of WO3-x, the UV-vis spectrum of the sample was recorded and then 0.1 ml of air was injected into the solution by syringe, the effect of which is shown in Figure 6 (b). As can be seen, after the air injection, a sharp decrease in absorption occurs in the spectra. The spectra were then sequenced for 16 min and it was observed that the WO3-x sample gradually returned to its original absorption intensity. As a result, we can say that WO3-x has good reversibility. In order to further investigate, different volumes of air were injected into the solution, the absorption spectra of which are shown in Figure 6 (a). As it is known, with increasing air volume, the absorption intensity decreases regularly, but with the injection of 15 ml of air, WO3-x is converted to WO3 and does not show any absorption peak. In this case, it can be said that WO3-x can withstand a certain amount of oxygen molecules in its network, and in exchange for larger amounts, O2 molecules enter the crystal structure of WO3-x and convert to WO3.
Figure 6 (d) shows that different volumes of oxygen have a significant effect on plasmonic peaks. Because it has been reported in the literature that oxygen molecules cause corrosion of nanoparticles and thus directly affect the plasmonic peak. Figure 6 (d) shows the evolution of the plasmonic peak and the exponential behavior of WO3-x as the volume of oxygen increases and the LSPR energy changes to the red region.
As the volume of oxygen increases, the intensity of absorption decreases linearly, that the effect of different volumes of oxygen on the intensity of absorption of WO3-x is shown in Figure 6 (d). In order to investigate the effect of oxygen on WO3-x instability, the dissolved oxygen concentration was measured by the WTW Portable Dissolved Oxygen Meter Pro-fiLine Oxi 3205. As shown in Figure 7, the oxygen concentration of WO3 solution containing PdCl2 is about 5 mg/l. The addition of 0.1 M NaBH4 solution causes a sharp drop in the oxygen concentration of the solution due to the replacement of oxygen with hydrogen molecules. After 4 minutes, the concentration of dissolved oxygen in the substance becomes almost zero and becomes so-called Oxygen free. Therefore, it can be said that the cause of instability of WO3-x is dissolved oxygen because WO3-x absorbs oxygen and becomes WO3 after 2 hours in the open air.
Figure 6: UV-Vis spectra of WO3-x showing (a) volumes of 0.3 ml, 4.4 ml, 10 ml, 15 ml of air bubbled, (b) 1 ml of air bubbled and the reversibility of the sample, (C) Calibration diagram of the effect of different volumes of air on the LSPR peak (d) and the intensity of absorption
Figure 7: Cause of stability and effect of NaBH4 on dissolved oxygen concentration of WO3 + PdCl2
DO test1
Time temperature indicators (TTI) in smart packaging
After studying the optical and structural properties of prepared WO3-x and with the knowledge that the obtained nanoparticles are highly sensitive to the presence of oxygen, they were used as oxygen sensors and sensitive to the passage of time. The combination of WO3-x and PVA nanoparticles has now been used to build sensors that show the freezing status and heat history of food based on a clear and visible color change based on the WO3-x surface plasmon resonance. Therefore, WO3-x nanoparticles / PVA can be used as indicators that show the solidification status and thermal history of the material by changing the blue color to colorless. In order to investigate the behavior of PVA / WO3-x composition prepared at different temperatures, 3 samples were kept in microtubes at room temperature, freezer and liquid nitrogen. As shown in Figure 3, the sample (WO3-x / PVA) is dark blue at room temperature, but when exposed to -20 C and -196 C, the color of the material turns light blue, possibly due to the presence of polyvinyl Alcohol and the formation of ice crystals. The frozen sample remains in the freezer for at least 2 weeks without any color change with acceptable stability and becomes completely colorless after being exposed to room temperature for only after 1 hour. It is also observed that the sample at a temperature of -196 ° C, i.e. in liquid nitrogen for more than two months can remain stable without any color change, but after 1 hour of leaving the liquid nitrogen, the color change is clearly visible. Therefore, the prepared WO3-x / PVA has a very good ability to detect low temperatures and can be used as temperature and time indicators that show the thermal history of temperature sensitive materials such as perishable foods and biological materials (Figure 10). ). This color change can be used to ensure the quality of seafood, meat and biological materials that are of high value. The important point about these markers is that the discoloration is irreversible, which provides the assurance needed to detect the thermal history of the products. As a result, we report very simple, cost-effective, new, environmentally friendly TTI indicators, as well as without the need for additional readout devices, which can be used to ensure the quality and safety of temperature-sensitive, medicinal and food.
Figure 8: Time temperature indicators (left to right): freshly prepared sample, sample in freezer at -20 C, WO3-x stored at liquid nitrogen at -196 ° C and frozen sample at room temperature
Indicators of freshness food
Cigarette filters were used as a substrate with high adsorption capacity to prepare oxygen markers. Various attempts have been made to achieve stable markers comparable to those reported. In the initial experiments, we were able to create stable markers for only 1 hour, which due to the high sensitivity of WO3-x to oxygen, the color of the markers quickly changed from blue to white, thus showing the presence of oxygen and then lost their effectiveness. Therefore, 10% wt. PVA polymer was used to create indicators for higher stability, which due to the adhesion of the polymer and the construction of a membrane around the filter containing WO3-x, oxygen penetration did not occur as quickly as before and the stability of markers to 10 hours in The open-air increased. However, the degree of stability was not yet within the range of similar indicators, so it was suggested that the indicators be stored and frozen in the freezer. It was observed that the indicators show good stability for up to a week before the indicator comes out of the freezer, but immediately after leaving the freezer, the indicators start to change color and 1 hour after leaving the freezer, completely colorless. Therefore, we can use these indicators as oxygen and time-temperature indicators that can show oxidation damage and thermal history in both food and other temperature-sensitive biological materials.
Figure 9: Indicators containing PVA in ambient conditions with 10-hour stability
Conclusion
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