DBHP-Functionalized ZnO Nanoparticles with Improved Antioxidant

27 Feb 2019 - The calculated results showed that DIOS containing DBHP–ZnO nanoparticles have the lowest reaction constant and the longest half-life ...
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DBHP-Functionalized ZnO Nanoparticles with Improved Antioxidant Properties as Lubricant Additives Lina Huang,†,‡ Changhua Zhou,† Yujuan Zhang,† Shengmao Zhang,*,† and Pingyu Zhang*,† †

Engineering Research Center for Nanomaterials, Henan University, Kaifeng 475004, China School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China

‡

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S Supporting Information *

ABSTRACT: In this article, 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid (DBHP)-functionalized ZnO (DBHP−ZnO) nanoparticles were synthesized by decomposing the organometallic precursor Zn(DBHP)2 under alkaline conditions. This in situ surface modification method can induce small-sized ZnO nanoparticles (5 nm) and form strong linkage between DBHP and ZnO nanoparticles. DBHP as an organic compound hindered phenol antioxidant that not only improved the dispersion stability of the prepared DBHP−ZnO nanoparticles in the lubrication oil but also scavenged free radicals produced during the oxidation process of oil. Compared with DBHP, the thermal stability of the prepared composite antioxidant was greatly enhanced by introducing inorganic ZnO nanoparticles, which was proved by the results of the thermogravimetric analysis test. A rotary oxygen bomb test, pressurized differential scanning calorimetry, and free-radical-scavenging method all showed that DBHP−ZnO nanoparticles had better antioxidant properties than DBHP under high temperature in the base oil of di-iso-octylsebacate (DIOS). The activation energy of the oxidation process was used to analyze this result by the model-free methods, including the Flynn−Wall−Ozawa method and the Kissinger equation. The calculated results showed that DIOS containing DBHP−ZnO nanoparticles have the lowest reaction constant and the longest half-life period compared to those of individual DBHP and ZnO nanoparticles, which is attributed to the combined action of the organic−inorganic composites. Besides, DBHP−ZnO nanoparticles as the additive are able to improve the antiwear ability of DIOS to some extent. Therefore, the as-prepared DBHP−ZnO nanoparticles with desired dispersibility as well as better thermal stability and antioxidant ability than DBHP in the DIOS base oil could be a potential high-performance nanocomposite additive for a synthetic lubricant base oil like DIOS.

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INTRODUCTION The demand for synthetic lubricants with superior thermal stability and antioxidant performance grows quickly with the development of engine oils servicing under harsh conditions (high speed and high load).1−3 Commonly used hindered phenol antioxidants and amine-type antioxidants with low molecular weight and poor thermal stability, however, are volatile at an elevated temperature.4 To overcome these disadvantages of organic antioxidants, researchers have tried to synthesize oligomers via polymerization of conventional antioxidants and/or graft organic antioxidants onto the surface of metallic nanoparticles via a surface modification technique.5−9 The resultant oligomers as antioxidants indeed possess improved thermal stability than relevant monomers. Unfortunately, they often suffer harsh synthetic conditions and exhibit a complex composition, which is unfavorable for their application in the industry.10,11 Therefore, a variety of inorganic nanoparticles surface-capped by organic antioxidants could be of special significance for the development of a new generation of high-performance antioxidants. Over the past decade, much attention has been paid to immobilizing antioxidants with low molecular weight on the surface of nanomaterials. For instance, a variety of silane © XXXX American Chemical Society

coupling agents were used as a bridge to mobilize various antioxidants on the surface of silica nanoparticles12−18 and antioxidants such as caffeic acid and gallic acid, both containing reactive groups, were immobilized on the surface of silica or iron oxide nanoparticles.19−22 At present, the inorganic nanomaterials used in these research works do not have the antioxidant ability and there are less reports on the combination of organic and inorganic components both having the antioxidant ability. ZnO nanoparticles exhibits antioxidant activity especially for scavenging free radicals.23−26 The application of ZnO nanoparticles as a lubricant additive, however, is hindered by its poor dispersibility in lubricant base oils. In this sense, it is imperative to modify ZnO nanoparticles with organic antioxidants so as to improve their dispersibility in the lubricant base oil and combine their antioxidant ability with that of organic antioxidants. Therefore, we adopt a simple in situ surface modification method to functionalize small-sized ZnO nanoparticles (5 nm) with 3-(3,5-di-tert-butyl-4-hydroxReceived: January 10, 2019 Revised: February 22, 2019 Published: February 27, 2019 A

DOI: 10.1021/acs.langmuir.9b00093 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 1. Route for Synthesis of DBHP−ZnO Nanoparticles

yphenyl) propionic acid (DBHP), an organic antioxidant,27−31 hoping to acquire desired antioxidant behavior by integrating the inorganic antioxidant with the organic one. Thus, Zn(DBHP)2 was prepared as a precursor for fabricating ZnO nanoparticles and the resultant Zn(DBHP)2 precursor was then decomposed under alkaline conditions to afford DBHPfunctionalized ZnO (DBHP−ZnO) nanoparticles. This article reports the preparation of DBHP−ZnO nanoparticles as well as the evaluation of the thermal stability, antioxidant behavior, and tribological properties of DBHP−ZnO nanoparticles as an additive of di-iso-octylsebacate (denoted DIOS), a synthetic ester base oil.

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of DBHP−ZnO nanoparticles and 4 mL of absolute ethanol were added in a centrifuge tube and cleaned in an ultrasonic cleaning machine for 10 min. The sample was obtained after centrifugal separation and freeze-drying. The same process was repeated by changing acetonitrile as the solvent. The DBHP−ZnO nanoparticles with a modifier content of 30% were selected as the representative for morphology observation and microstructure characterization as well as for the evaluation of thermal stability, antioxidant behavior, and tribological properties. Morphological Observation and Microstructure Characterization. The Fourier transform infrared spectra (FTIR) of the asprepared DBHP−ZnO nanoparticles were recorded with a Thermo Scientific Nicolet Is50-FTIR spectrometer (Thermo Nicolet Corporation), and their 1H nuclear magnetic resonance (1H NMR) spectra were measured with a 400 MHz Avance HD facility (Bruker, Germany; solvent, chloroform-d (CDCl3)). The microstructure and morphology of the products were analyzed with a JEM-2100 transmission electron microscope (TEM; JEOL Corporation; Japan), and their phase ingredients were identified by X-ray diffraction (XRD; D8-Advance, Bruker Optics; Germany; Cu Kα radiation). The composition of DBHP−ZnO nanoparticles was analyzed by X-ray photoelectron spectroscopy (XPS; Thermo Scientific Eescalab 250XI). The thermal stability of DBHP−ZnO nanoparticles from an ambient temperature to 700 °C in a nitrogen atmosphere was determined by thermogravimetric analysis (TGA; METTLER TOLEDO, Switzerland) at a heating rate of 10 °C/min. Moreover, a LUMiSizer stability analyzer (L.U.M. GmbH, Germany) was used to evaluate the stability of DBHP−ZnO nanoparticles dispersion in DIOS, with which the transmitted light as the function of position and time under the centrifugal force was recorded at a centrifugal rotational speed of 4000 rev/min and a temperature of 25 °C for 3600 s. Evaluation of the Antioxidant Behavior and the Tribological Properties. The antioxidant behavior of DBHP−ZnO nanoparticles was tested by the rotary oxygen bomb test (ROBT). Briefly, 50 g of DIOS containing different concentrations of DBHP− ZnO nanoparticles was mixed with 55.6 ± 0.3 g of copper catalyst coil and 5 mL of distilled water. The mixture was placed into a glass sampler and sealed in a rotary bomb that was filled with oxygen at room temperature until a pressure of 620 kPa was achieved. The rotary bomb was then put into a dimethylsilane heating bath at 150 °C. The time required to reduce the pressure by 175 kPa from the highest pressure is used to express the oxidation induction time (OIT). Pressure differential scanning calorimetry (PDSC) tests were carried out with a NETZSCH DSC 204HP instrument (Bavarian;

EXPERIMENTAL SECTION

Materials. Analytical reagents zinc chloride and sodium hydroxide were purchased from Tianjin Chemical Reagent Company (Tianjin, China). Analytical reagents DBHP and tetrabutylammonium hydroxide (TBAOH) were purchased from Saen Chemical Technology Company (Shanghai, China). Analytical reagent 2,2-diphenyl-1picrylhydrazyl (DPPH; its molecular structure is shown in Figure S1) was provided by Sigma-Aldrich. Chemically pure DIOS was supplied by Lanzhou Institute of Chemical Physics of Chinese Academy of Sciences (Lanzhou, China). All of the other reagents are of analytical grade and used without any further treatment. Preparation of DBHP−ZnO Nanoparticles. DBHP−ZnO nanoparticles were prepared by an in situ surface modification method. Briefly, 4 g of zinc chloride was dispersed in 60 mL of distilled water, followed by the addition of 30 mL of sodium hydroxide solution (1.25 mol/L) under stirring to yield zinc hydroxide precipitate. The as-formed zinc hydroxide precipitate was filtrated and dried under vacuum in a freeze-drying machine, followed by washing with distilled water. The as-washed zinc hydroxide precipitate (0.75 g) was placed into a Schlenk flask and mixed with 4 g of DBHP and 24 mL of absolute ethanol, followed by heating at 100 °C for 2 h under sealed conditions and drying in an electric oven at 50 °C for 10 h to afford soil-yellow zinc precursor Zn(DBHP)2. The asdried zinc precursor (0.5 g) was dissolved in 17 mL of absolute ethanol under magnetic stirring while tetrabutylammonium hydroxide (0.05 mmol/L) was added and heated at 60 °C for 12 h. At the end of the reaction, DBHP−ZnO nanoparticles, the product, were separated by centrifugation, washed with distilled water, and dried in a vacuum freeze dryer. Then, DBHP−ZnO nanoparticles were purified to get samples with different surface modifier contents. The procedure is as follows: 0.1 g B

DOI: 10.1021/acs.langmuir.9b00093 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) FTIR spectra of DBHP, Zn(DBHP)2, and DBHP−ZnO nanoparticles. (b) XRD spectrum of DBHP−ZnO nanoparticles. Germany) under the pressure of 3.5 MPa and an oxygen flow rate of 100 mL/min, with which 3.00 ± 0.2 mg of the to-be-tested sample was placed in an aluminum boat. The OIT and oxidation onset temperature (OOT) were measured in isothermal temperature mode (heating rate, 30 °C/min) and programmed temperature mode (the sample was heated from ambient temperature to 260 °C at a heating rate of 10 °C/min), respectively. According to the plots of heat flux versus temperature, the OOT was obtained by considering the baseline with respect to the extrapolated onset temperature of the exothermic process. The antioxidant behavior of DBHP−ZnO nanoparticles was also evaluated with the modified DPPH method reported by Serpen et al.32,33 The purple DPPH radical is a stable free radical and has absorbance at 517 nm. When DBHP−ZnO nanoparticles are used as a free radical scavenger, their reaction with the DPPH radical can be monitored by measuring the change in the absorbance at 517 nm. Typically, 1.6 mg of DPPH was dispersed in 10 mL of absolute ethanol to obtain the DPPH solution with a concentration of 0.16 mg/mL, whereas 10 mg of DBHP−ZnO nanoparticles was dispersed in 10 mL of absolute ethanol to afford DBHP−ZnO dispersion. The resultant DBHP−ZnO dispersion (1 mL) was placed into a colorimetric tube while 1 mL of the DPPH reagent was added to start the radical scavenging reaction. The as-obtained mixed solution was rapidly diluted with ethanol to a volume of 5 mL, and the absorbance of the diluted solution at 517 nm was determined with a UV−vis spectrophotometer (Shimadzu, UV-2600, Japan) at different time intervals (the absorbance is denoted As). Besides, the absorbance of DPPH (denoted Ac) was also detected and used for reference. The DPPH scavenging activity of the sample could be calculated following the equation

peak at 5.07). The carboxyl group of DBHP can be used as a link between the antioxidant and ZnO nanoparticles. After DBHP is grafted onto the surface of ZnO nanoparticles, the resultant DBPH-capped ZnO nanoparticles can be well prevented from agglomeration and endowed with improved dispersibility in the lubricant base oil. The FTIR spectra of DBHP, Zn(DBHP)2, and DBHP−ZnO nanoparticles are shown in Figure 1a. DBHP−ZnO nanoparticles show a strong absorbance peak of zinc oxide at 492 cm−1 as well as a broad O−H bond vibration peak at 3438 cm−1. Besides, DBHP−ZnO nanoparticles do not show the peaks of the hydroxyl group of carboxylic acid at 2696 and 2615 cm−1, and their stretching vibration peak of carboxylic acid at 1703 cm−1 disappears and that of carboxylate at 1556 cm−1 is observed. This means that DBHP is chemically bonded on the surface of ZnO nanoparticles.34,35 The structure and surface composition of the DBHP−ZnO nanoparticles were also analyzed by X-ray photoelectron spectroscopy. The characteristic peaks of C, O, and Zn were observed from the wide scan XPS spectrum in Figure S4. The binding energy of C 1s can be divided into four peaks at 284.2, 284.8, 286.1, and 288.3 eV, which are assigned to sp2 carbon, carbon in C−C bonds, carbon in C−O bonds, and carboxylate carbon (O−CO), respectively.36,37 The peaks at around 1021.8 eV (Zn 2p3/2) and 1044.9 eV (Zn 2p3/2) are related to divalent zinc in the prepared DBHP−ZnO sample. Compared with the values reported in refs38−41, the shift of the position of Zn 2p is due to the change of the chemical environment, which indicates that the organic antioxidant modifier of DBHP is chemically bonded on the surface of ZnO nanoparticles. Figure 1b presents the XRD spectrum of DBHP−ZnO nanoparticles. The diffraction peaks correspond well to those in the standard JCPDS card no. 36-1451, indexed to the hexagonal wurtzite structure of ZnO nanoparticles. Besides, the widened diffraction peaks indicate that the as-synthesized DBHP−ZnO nanoparticles exhibit small particle sizes. The TGA curves of DBHP and DBHP−ZnO nanoparticles are displayed in Figure 2. The initial degradation temperature of DBHP is about 200 °C, and its weight loss in the range of 50− 700 °C is 99.03%. The TGA curve of DBHP−ZnO nanoparticles involves three stages of weight loss. The first stage of weight loss below 200 °C is attributed to the elimination of the adsorbed water;42−44 the second one (about 30%) in the range of 200−430 °C is due to the removal of DBHP grafted on the surface of ZnO nanoparticles; and the third one above 430 °C is due to the combustion of remnants.45,46 Moreover, DBHP−ZnO nanoparticles have a much higher initial degradation temperature than DBHP and

DPPH scavenging activity (%) = [1 − A s /Ac] × 100 The tribological properties of DBHP−ZnO nanoparticles as the additive of DIOS were tested with a UMT-2 test rig in a reciprocating mode (sliding frequency, 2 Hz; sliding time, 30 min). The GCr15 steel ball and 304 stainless steel disk were used to assemble the frictional pair.

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RESULTS AND DISCUSSION Preparation and Characterization of DBHP−ZnO Nanoparticles. DBHP−ZnO nanoparticles were prepared via a facile in situ one-step route (see Scheme 1). DBHP reacts with zinc hydroxide to form zinc precursor Zn(DBHP)2, and the carboxyl hydrogen and phenolic hydroxyl hydrogen of DBHP can participate in the acid−base reaction. Therefore, it is imperative to strictly control the proportion of the reactants to prevent the phenolic hydroxyl hydrogen of DBHP from participating in the acid−base reaction. A comparison of the 1 H NMR spectra of the reactant and the product shown in Figures S2 and S3 indicates that the hydroxyl hydrogen of DBHP remains unchanged after the reaction (see the single C

DOI: 10.1021/acs.langmuir.9b00093 Langmuir XXXX, XXX, XXX−XXX

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concentration of TBAOH in the reaction system is 0.025 mmol/L and DBHP−ZnO nanoparticles are obtained when the concentration of TBAOH is higher than 0.05 mmol/L. Particularly, DBHP−ZnO nanoparticles with a desired morphology are obtained at a TBAOH concentration of 0.05 mmol/L and the morphology of the DBHP−ZnO nanoparticles becomes irregular when the concentration of TBAOH is above 0.05 mmol/L. Figure 4 shows the effect of reaction time on the morphology and size of DBHP−ZnO nanoparticles. The DBHP−ZnO nanoparticles prepared at a reaction time of 6 h have a uniform spherical shape, and their mean diameter is approximately 4.6 nm. As the reaction time extends to 12 h, there is little change in the particle size. However, the size of DBHP−ZnO nanoparticles increases to 6.3 nm as the reaction time reaches 24 h and their particle size distribution broadens. Effect of the DBHP Modifier Content on the Dispersibility of DBHP−ZnO Nanoparticles in the DIOS Base Oil. According to the TGA curves shown in Figure 2b−d, the content (mass fraction) of the DBHP modifier on the surface of as-prepared DBHP−ZnO nanoparticles and the content after 10 min of ultrasonic cleaning with ethanol or acetonitrile are calculated to be 30, 22, and 15%, respectively. TEM images of three samples are shown in Figures 4b and S5a,b. By comparison, DBHP−ZnO nano-

Figure 2. TGA curves of DBHP (a), DBHP−ZnO nanoparticles (b), and DBHP−ZnO nanoparticles after ultrasonic cleaning for 10 min with ethanol (c) or acetonitrile (d).

antioxidants reported (Table S1), which demonstrated that the thermal stability of the prepared organic−inorganic composite antioxidants was greatly enhanced. Effect of TBAOH Amount and Reaction Time on the Morphology of DBHP−ZnO Nanoparticles. The amount of TBAOH has a significant effect on the synthesis of DBHP− ZnO nanoparticles. As shown in Figure 3, almost no DBHP− ZnO nanoparticles can be observed by TEM when the

Figure 3. TEM images and size distribution (inset) of DBHP−ZnO nanoparticles prepared at different concentrations of TBAOH: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 mmol/L. D

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Figure 4. TEM images and size distribution (inset) of DBHP−ZnO nanoparticles prepared after different reaction times: (a) 6 h, (b) 12 h, (c) 18 h, and (d) 24 h.

particles with the highest content of the surface modifier are regular spherical and uniformly dispersed in Figure 4b. The presence of sufficient organic modifiers on the surface prevents the agglomeration of nanoparticles, so clear boundaries can be seen between particles. As to DBHP−ZnO nanoparticles with the organic content of 22%, the phenomenon of clear boundaries between particles can still be seen in some areas (Figure S5a). When the content of the surface modifier is reduced to 15%, agglomeration can be seen from Figure S5b, which may lead to the poor stability. To investigate dispersion stability, the three samples were dissolved in DIOS at a concentration of 2% and their transmission profiles were recorded as the function of position and time under the centrifugal force. As shown in Figure S6a, the dispersion of asprepared DBHP−ZnO nanoparticles in DIOS, with a high DBHP modifier content of 30%, exhibits a high transmittance that remains nearly unchanged in the initial (red) and final (green) stages, which means it has good dispersibility in DIOS. In contrast (Figure S6b), the transmission profile of DBHP− ZnO nanoparticles with the organic content of 22% shows a moderate change. It is obvious from Figure S6c that the sample with the organic content of 15% has low transmission at the position from 109 to 128 mm in the initial stage. With the prolongation of aging time, the curve gradually moves toward high transmittance, which means poor stability during the whole process. The instability index of three samples gradually

increases with the time from 0 to 1000 min and then remains approximately constant (Figure S6d). Moreover, the dispersion of the as-prepared DBHP−ZnO nanoparticles in DIOS has the lowest average instability index, which corresponds well to its highest dispersibility. The reason lies in that the encapsulation with a high content of the DBHP modifier contributes to effectively preventing the agglomeration of DBHP−ZnO nanoparticles and greatly improving its compatibility with the DIOS base oil. Antioxidant Behavior and Tribological Properties of DBHP−ZnO Nanoparticles as the Lubricant Additive in DIOS. The antioxidant behavior of DBHP−ZnO nanoparticles was evaluated by ROBT. The OIT values of pure DIOS and DIOS containing different concentrations of DBHP and DBHP−ZnO nanoparticles are shown in Figure 5. It can be seen that DBHP−ZnO nanoparticles as the lubricant additive can significantly increase the OIT of DIOS. Namely, the OIT increases from 45 to 119 min after 0.25% (mass fraction; the same hereafter) of DBHP−ZnO nanoparticles are added in DIOS. For the same amount of addition, DBHP−ZnO nanoparticles have longer OIT than that of DBHP, demonstrating the good antioxidant ability of DBHP−ZnO nanoparticles. Besides, the OIT tends to rise continuously with the increase of the concentration of DBHP−ZnO nanoparticles and the increased degree of OIT gradually reduces when the amount exceeds the concentration of 0.5%, which is a E

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and/or disproportionation reactions. Interestingly, DPPH can form a stable free radical in the center of the nitrogen atom, which is due to resonance stabilization and steric hindrance of the three benzene rings. The dark purple solution of the DPPH free radical has a strong absorption at 517 nm.48−50 The unpaired electrons in the nitrogen atom, sandwiched in the middle, cannot participate in the coupling reaction with themselves, but they can react with the antioxidant that can donate an electron or a hydrogen atom and become colorless or light yellow after neutralization. Therefore, we choose DPPH to test the antioxidant potential of the as-prepared DBHP−ZnO nanoparticles. During the reaction process, the change of the reaction system was monitored by taking photos and testing UV−vis absorption spectra at different intervals. Relevant results are shown in Figures 7 and S7. For the solution of DPPH with ZnO nanoparticles, the intensity of the absorbance peak at 517 nm and the solution color remain nearly constant during the UV−vis measurement (Figure S7a,c), which means that ZnO exhibits a very weak antioxidant ability. This is also confirmed by the DPPH scavenging activity of ZnO (only 1.9% after 60 min of reaction). After the introduction of DBHP or DBHP−ZnO nanoparticles in the DPPH solution, the intensity of the absorbance peak at 517 nm tends to gradually decrease with the extending reaction time. Namely, DBHP has a DPPH scavenging activity of 35.26% at 15 min and 77.41% at 60 min (Figure 7b) and DBHP−ZnO nanoparticles have a DPPH scavenging activity of 53.15% at 5 min and 90.87% at 60 min (Figure 7d). This indicates that both DBHP and DBHP−ZnO nanoparticles have the ability of scavenging the DPPH free radical and that DBHP−ZnO nanoparticles exhibit higher antioxidant activity than that of DBHP. This is because DBHP−ZnO nanoparticles can enhance the transfer of electrons to the DPPH free radical. These observations are also supported by the changes in the color of the DPPH free radical solution with different additives (Figure 7e,f). Specifically, the DPPH solution with DBHP gradually turns lavender after 45 min of reaction and then it becomes pale yellow after 90 min of reaction. The DPPH solution with DBHP−ZnO nanoparticles turns lavender just after 10 min of reaction and pale yellow after 30 min of reaction and then it remains almost unchanged with the prolonging reaction time. All of these results indicate that DBHP−ZnO nanoparticles have better antioxidant ability than that of DBHP and ZnO nanoparticles. Under the conditions of light, heat, and transition metal catalysis, DIOS undergoes reaction with oxygen forming hydroperoxide as well as a series of free radical chain reactions

Figure 5. Oxidation induction time of DIOS and DIOS containing different concentrations of DBHP or DBHP−ZnO nanoparticles, obtained from ROBT.

common phenomenon in the process of adding antioxidants.9,11,47 Pressure differential scanning calorimetry is commonly used to investigate the antioxidant capacity of oil products. The OIT and OOT are obtained by the PDSC method in isothermal mode and programmed temperature mode, respectively. The OIT values of pure DIOS and DIOS containing different concentrations of DBHP−ZnO nanoparticles at 180 and 190 °C are shown in Figure 6a. The OIT of DIOS significantly increases with the introduction of DBHP−ZnO nanoparticles into the base oil. Namely, DIOS without any additive has a low OIT of 4.1 min at 190 °C and 5.8 min at 180 °C, which could be due to its accelerated oxidation at elevated temperatures. After the introduction of DBHP−ZnO nanoparticles, the OIT increases from 6.2 to 12.9 min at 190 °C and from 12.4 to 24.5 min at 180 °C at the concentration from 0.25 to 3%. This variation trend is consistent with the result of ROBT. The OOT obtained from PDSC is often used as the parameter to describe the oxidative stability, and a higher OOT refers to a better oxidative stability of the base oil. Figure 6b shows the OOT of pure DIOS and DIOS containing different concentrations of DBHP−ZnO nanoparticles. The OOT of pure DIOS is 216.61 °C, corresponding to its poor antioxidant stability. The OOT rises to 224.28 °C after the introduction of 0.25% DBHP−ZnO nanoparticles, and it gradually rises to 226.68 and 228.94 °C with the addition of 1.50 and 3.00% DBHP−ZnO nanoparticles. This demonstrates that the DBHP−ZnO nanoparticles as a potential antioxidant can increase the oxidation resistance of DIOS. Most of free radicals are unstable under normal condition and tend to transform into stable structures through coupling

Figure 6. (a) Oxidation induction time and (b) oxidation onset temperature of DIOS and DIOS containing different concentrations of DBHP− ZnO nanoparticles, obtained from PDSC. F

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Figure 7. UV−vis absorption spectra of DPPH radicals with (a) DBHP and (c) DBHP−ZnO nanoparticles after different reaction periods. Timedependent DPPH scavenging by (b) DBHP and (d) DBHP−ZnO nanoparticles. Optical photographs of DPPH radicals reacting with (e) DBHP and (f) DBHP−ZnO nanoparticles after different durations.

Scheme 2. Antioxidant Mechanism of DBHP−ZnO Nanoparticles in DIOS

producing various oxidation products including acid, alcohol, aldehyde, ester, etc. Traditional organic antioxidants including

hindered phenol and diphenylamine as a radical inhibitor and a scavenger, respectively, can interrupt the free radical chain G

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Figure 8. Kinetic study by model-free methods: (a) Flynn−Wall−Ozawa method and (b) Kissinger method.

Table 1. Kinetic Parameters by the Flynn−Wall−Ozawa Method (β = 10 K/min) DIOS DIOS + DBHP DIOS + DBHP−ZnO

Ea (kJ/mol)

ln A

k (min−1)

t1/2 (min)

R2

154.1917 136.5313 127.0134

37.1255 32.4064 29.8968

0.8079 0.6691 0.6664

0.8578 1.0358 1.0399

0.9951 0.9973 0.9879

growth, thus preventing or postponing the oxidative degradation process of the oil. The DBHP linked on the surface of DBHP−ZnO nanoparticles is a typical hindered phenolic antioxidant, and it works by donating a hydrogen atom to terminate the free radical chain growth (see Scheme 2). It has been reported that the antioxidant ability of nano-ZnO highly depends on its ability to transfer electron density to the free radical in the center of nitrogen.23,24 Moreover, DBHP−ZnO nanoparticles may absorb the electron of the system and create a recombination center through the transition of electrons between the conduction band and the valence band. As a result, DBHP−ZnO nanoparticles exhibit better antioxidant ability than that of DBHP and ZnO nanoparticles. The tribological properties of DBHP−ZnO nanoparticles as a lubricant additive in DIOS are shown in Figure S8. DBHP− ZnO nanoparticles as the additive have little effect on the friction coefficient, as shown in Figure S8a. The wear rate slightly decreases in the early stage of sliding and increases later with the increase in the additive concentration, and the lowest wear rate occurs at an additive concentration of 1% (Figure S8b). Then, variations of the friction coefficient and wear rate with load are shown in Figure S8c,d. It can be seen that DBHP−ZnO nanoparticles can reduce the wear rate under the selected loads, which indicates that DBHP−ZnO nanoparticles as the lubricant additive not only display good antioxidant performance but also improve the antiwear ability of DIOS to a certain extent. Kinetic Study by Model-Free Methods. The oxidation exothermic peak temperatures at heating rates of 5, 10, 15, 20, 25, and 30 °C/min were obtained by PDSC and adopted to calculate the kinetic parameters by model-free methods without preselection of the reaction model. As to the indeterminate temperature and heterogeneous reaction, the kinetic equation can be expressed as r=

dα = k(T )f (α) dt

i E y k(T ) = A expjjj− a zzz k RT {

(2)

where A is the pre-exponential factor, Ea is the activation energy (kJ/mol), and R is the ideal gas constant (8.314 J/mol/ K). In the process of testing, the heating rate (β) has a linear relationship with the temperature, and it can be written as β=

dT dt

(3)

After k(T) and dt calculated from eqs 2 and 3, respectively, are substituated in eq 1, the reaction rate of general nonisothermal decomposition can be written as51 i E y dα A = expjjj− a zzzf (α) β dT k RT {

(4)

In this article, Flynn−Wall−Ozawa (FWO) and Kissinger methods are selected since they have been successfully applied to deal with solid decomposition52−54 and commercial antioxidants.55 The main equations are listed in the following paragraphs. The FWO method can be expressed as i AEa zy zz − 2.315 − 0.4567 Ea log β = logjjjj z ( α ) RG RT k {

(5)

where G(α) is defined as G (α ) =

∫0

α

dα f (α )

(6)

Under different heating rates, the conversion rates at the temperature of each oxidation exothermic peak are approximately the same, which means log(AEa/RG(α)) is a constant. Therefore, the activation energy, Ea, can be obtained from the slope of the curve of log β versus 1/T under the given value of α. The Kissinger equation can be expressed as

(1)

where α represents the extent of the reaction, t is the time of the reaction, T is the absolute temperature (K), and k is the reaction constant related to temperature, according to the Arrhenius formula shown in eq 2.

ln H

ij AR yz E β = lnjjj zzz − a 2 j Ea z RT Tp p k {

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Langmuir Table 2. Kinetic Parameters by the Kissinger Method (β = 10 K/min) DIOS DIOS + DBHP DIOS + DBHP−ZnO

Ea (kJ/mol)

ln A

k (min−1)

t1/2 (min)

R2

154.1757 135.1874 125.1278

37.1028 32.0314 29.3601

0.7928 0.6351 0.6110

0.8741 1.0912 1.1343

0.9917 0.9969 0.9861

According to the method of the Kissinger equation, the activation energy, Ea, can be determined from the slope of ln(β/Tp2) versus 1/Tp curve under different heating rates, where Tp is the temperature of the oxidation exothermic peak. The linear fitting results obtained from plots of the FWO method and Kissinger method for DIOS, DIOS containing 0.3% DBHP, and DIOS containing 1.0% DBHP−ZnO nanoparticles are shown in Figure 8a,b, respectively. From the slope and intercept of the linearly fitted results, the kinetic parameters are obtained and shown in Tables 1 and 2, where the minor differences in the values of kinetic parameters could be attributed to the use of different methods. It can be seen that all correlation coefficient (R2) values exceed 0.98, corresponding to a high degree of correlation. Besides, as determined by the FWO method, the introduction of antioxidant DBHP leads to a decrease in the activation energy. The value of activation energy reflects the degree of temperature sensitivity of a chemical reaction; the reaction with a higher activation energy is more sensitive to temperature and has a faster reaction rate at a higher temperature. DIOS containing DBHP−ZnO nanoparticles has the lowest reaction constant and the longest half-life period, which means that DBHP−ZnO nanoparticles have better antioxidant behavior than that of DBHP, as evidenced by the Kissinger method.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Z.). *E-mail: [email protected] (P.Z.) ORCID

Shengmao Zhang: 0000-0003-4233-1097 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant nos 21671053, 51875172, and 51775168) and the Scientific and Technology Innovation Team of Henan Province University (Grant no. 19IRTSTHN024).

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REFERENCES

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CONCLUSIONS DBHP as a hindered phenolic antioxidant is chosen as the surface modifier and covalently grafted on the surface of ZnO nanoparticles through an in situ surface modification technology to improve the dispersibility and antioxidant performance of ZnO nanoparticles in the lubricant base oil. The ROBT, PDSC, and DPPH scavenging tests demonstrate that DBHP−ZnO nanoparticles exhibit desired antioxidant ability in the DIOS base oil. Kinetic parameters analyzed by the FWO method and Kissinger method indicate that the activation energy value of DIOS can be reduced by combining ZnO nanoparticles with DBHP to afford DBHP−ZnO nanoparticles. In summary, DBHP−ZnO nanoparticles as the lubricant additive exhibit good antioxidant performance and can also improve the antiwear ability of DIOS to a certain extent, showing promising potential as a novel antioxidant serviceable under elevated temperatures. The present approach, hopefully, could provide good guidance to developing nanocomposite antioxidants with integrated dispersibility, thermal stability, and antioxidant ability for lubricant base oils like DIOS.

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tribological properties of DBHP−ZnO nanoparticles in DIOS (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00093. XPS survey scan of DBHP−ZnO nanoparticles, TEM images and transmission profiles of DBHP−ZnO nanoparticles with different organic contents, and I

DOI: 10.1021/acs.langmuir.9b00093 Langmuir XXXX, XXX, XXX−XXX

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