Density Of Surface Hydroxyl Groups Research Articles

Effect of Purification Methods on Commercially Available Cellulose Nanocrystal Properties and TEMPO Oxidation

Cellulose nanocrystals (CNCs) are attractive for use in polymer composites, biomedical applications, and barrier packaging. In all of these applications they are mixed with other components and compatibility is a major design consideration, as CNCs naturally have a high density of surface hydroxyl groups and primarily disperse well in polar media. Numerous surface modification approaches have been used to address these issues, but challenges remain due to the variability in the commercially available CNC materials. CNCs can be produced from biomass using several extraction methods, most notably acid hydrolysis and biomass extraction, also known as the American Value Added Pulping process. The production method of the CNC material has an impact on both physical and surface properties of CNCs, including size, shape, crystal structure, and zeta potential. In addition, post-treatments can be used to purify the CNC material and further alter these properties. This work studies the properties of CNCs from three different commercial suppliers and after conducting three different post-treatments: dialysis, Soxhlet extraction, and acetone washing to understand the effect of the commercial source and purification on CNC surface properties and modification via 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) mediated oxidation. We show that there is significant variation in CNC physical and surface properties between different commercial suppliers before and after purification. Importantly, we show that for CNCs produced through acid hydrolysis, acetone washing or Soxhlet extraction in ethanol decreases the achievable degree of TEMPO modification, but makes it more consistent between the different commercial suppliers. This has important implications for improving reproducibility in CNC research as well as aiding the expanding fields of applications.

Role of surrounding crystallization media in TiO2 polymorphs coexistence and the effect on AOPs performance

Two TiO2 catalysts were prepared by sol-gel method, with the only singularity of the critical solvent removal step prior to heat-induced crystallization. Their different physico-chemical properties caused very distinct activity in the phenol degradation by several AOPs: heterogeneous photocatalysis and ozonation based processes (catalytic and photocatalytic). The remarkable differences found in structural, morphological and chemical surface properties, for both TiO2 catalysts, were ascribed to the differences in the crystallization environment, driving to diverse titania polymorphs. Titanium dioxide (TiEt-B) prepared with an exhaustive step for organics removal by dialysis, has given place to open-structured metastable brookite/anatase mixtures. This crystal structure has generated a texture governed by meso- and macroporosity with a large surface area, acidity and hydroxyl density. The lower hydrodynamic particle size in the aqueous media increased photo-exposed surface, which has led to higher mineralization degrees in phenol catalytic degradation performance, especially in all light-driven catalytic AOPs studied here.

Adsorption kinetics, conformational change, and enzymatic activity of β-glucosidase on hematite (α-Fe2O3) surfaces

Substantial fractions of extracellular enzymes are intimately associated with soil minerals, which may protect enzymes from denaturation, precipitation, proteolysis, or microbial consumption in soil environments. However, how mineral surface properties affect enzyme-mineral interactions and enzymatic activity of enzymes associated to mineral surface is still unclear. In the present study, adsorption behavior, conformational change, and enzymatic activity of β-glucosidase (BG) on hematite (001) face and (104) face, respectively, were investigated using in situ attenuated total reflectance FTIR spectroscopy and batch experiments. β-glucosidase undergoes greater conformational changes upon adsorption onto hematite (104) face than on hematite (001) face, probably due to the stronger protein-surface interactions on (104) face with the relatively higher surface hydroxyl density. On the other hand, the amount of BG sorbed on hematite (001) face was nearly two times higher than that on hematite (104) at the end of the 150-min adsorption experiments, due to the higher extent of conformational change in β-glucosidase on hematite (104) face. Correspondingly, the initial rate of cellobiose hydrolysis by per gram of β-glucosidase adsorbed on hematite (104) face was 1.7 times higher than that on hematite (001) face. However, when the density of hematite particles was same, the extent of cellobiose hydrolysis was 1.2 times higher on hematite (001) face than that on the (104) face, because of the higher adsorbed amount of β-glucosidase on the former. This study decoupled the effects of mineral surface properties on adsorption kinetics and conformational changes of soil enzymes bound to soil minerals and provided new insights into the correlation between mineral surface properties and catalytical activity of mineral-associated enzymes in soil environments.

Brønsted acid property of alumina-based mixed-oxides-supported tungsten oxide

The effect of alumina-based supports (Al2O3, Al2O3–TiO2, Al2O3–ZrO2, SiO2–Al2O3, and SiO2) on the structure and acid properties of supported tungsten oxide catalysts was investigated. Among the tested supported tungsten oxide catalysts, WO3/Al2O3–TiO2 (Al2O3/TiO2 = 9, Al–Ti9) showed the highest activity for reactions catalyzed by Brønsted acid sites and the largest Brønsted acidity. Structural characterization revealed that Brønsted acid sites on WO3/Al–Ti9 were generated at the boundaries between domains of amorphous monolayer WO3 and Al2O3–TiO2 support and WO3/Al2O3 reported previously, and that Al2O3 and Al–Ti9 with a high density of Lewis acid and surface hydroxyl groups resulted in the formation of Brønsted acid sites. Crystalline WO3 species formed mainly on SiO2–Al2O3 and SiO2 with a low density of Lewis acid sites and a high density of surface hydroxyl groups. These results suggested that a high density of Lewis acid sites and surface hydroxyl groups are important factors to form amorphous WO3 monolayer domains and to generate Brønsted acid sites on alumina-based supports.

β-FeOOH catalyzed peroxymonosulfate for organic pollutant degradation in water: radical and non-radical mechanism

Iron-based catalysts as peroxymonosulfate (PMS) activator are commonly used to oxidize and degrade azo dye. However, there are still some issues such as low catalytic performance, relatively less reactive oxygen species and high metal iron dissolution. As such, a novel iron oxyhydroxide (FeOOHs) with four crystal form was synthesized and employed as efficient PMS activator for AO7 degradation in this study. It was found that the crystal form of catalyst had significant influence on its catalytic performance. β-FeOOH exhibited the best catalytic performance, owing to its smallest lattice size, maximum specific surface area and highest surface hydroxyl density. β-FeOOH/PMS system possessed a great high apparent reaction rate constant (1.3919 h−1), which was about 17, 3, 31, 20, 14, 49 and 23 times higher than that in β-FeOOH/persulfate(PS), Fe0/PMS, Fe0/PS, Fe2O3/PMS, Fe2O3/PS, Fe3O4/PMS and Fe3O4/PS system, respectively. More importantly, radical quenching tests and Electron spin resonance (ESR) analysis revealed that OH., SO4.−, O2.− and 1O2 were involved in the reaction process. The catalytic mechanism proposed that β-FeOOH had the effective electron transfer rate performance, and Fe3+ underwent a reduction to Fe2+ during the β-FeOOH/PMS catalytic oxidation process. Then, the regeneration of Fe2+ on the hydroxylated surface of FeOOH promoted the formation of FeOH+, which was crucial for PMS activation and reactive oxygen species (ROS) generation. The recyclability tests proved good stability and reusability of β-FeOOH. This study provides a kind of new understanding of peroxymonosulfate activation for environmental remediation and advances the study of iron-based oxyhydroxide in heterogeneous catalysis.

Enhanced heterogeneous catalytic ozonation of pharmaceutical pollutants using a novel nanostructure of iron-based mineral prepared via plasma technology: A comparative study

Plasma-treated goethite nanoparticles with high surface area and improved density of surface hydroxyl groups were synthesized from natural goethite (NG) using Argon (PTG-Ar) and Nitrogen (PTG-N2) as plasma environment to enhance the performance of heterogeneous catalytic ozonation process. Synthesized samples were characterized by FESEM, EDX, TEM, XRD, XPS, BET-BJH, FTIR, AAS and pHPZC. Results indicated a significantly different morphology for the prepared samples with negligible change in crystal structure. Furthermore, the catalytic activity and synergy factor of the NG and PTG nanocatalysts were evaluated for degradation and mineralization of Sulfasalazine antibiotic (SSZ) as an environmental hazardous contaminant. The highest removal efficiency was achieved 96.05 % under the optimal operating conditions. The kinetic study confirmed the pseudo-first-order reaction for the degradation process. Moreover, the dissolved ozone concentration and effect of organic and inorganic salts were studied in order to assess the reactive oxidant species (ROSs) and catalyst active sites in the process. The mechanism investigation showed the catalytic ozonation of SSZ was mainly performed by successive attacks of hydroxyl radicals (•OH), superoxide radicals (O2−) and direct ozone molecules. Environmentally-friendly modification of the NG, negligible iron leaching, successive reusability and superior catalytic activity are the major benefits of the PTG nanoparticles.

Optimization of mica surface hydroxylation in water vapor plasma monitored by optical emission spectroscopy

Mica, a mineral consisting of alumino-silicate layers, can be cleaved to obtain atomically smooth surfaces, and it is often used as a model substrate for surface force measurements. Due to mica's chemical inertness, covalent surface modifications are not straightforward. By applying a water vapor plasma treatment, reactive silanol groups can be generated on the surface. Up to now, the optimization of the plasma process was a time-consuming trial-and-error process. Furthermore, no clear correlation between the plasma parameters and the hydroxyl surface density was elucidated. To overcome such limitations, we invoke in-situ optical emission spectroscopy (OES). A correlation between the pressure-normalized OES hydroxyl intensity and the surface density of hydroxyl groups was found, which allows real-time optimization of the plasma conditions. The density of silanol groups was quantified via chemical derivatization X-ray photoelectron spectroscopy (XPS) using a fluorinated monochlorosilane. An apparent upper limit for the hydroxyl surface density of 2.7 SiOH/nm2 was found—the limitation being imposed by packing restraints of the derivatizing reagent. While real surface density may be underestimated by derivatization, the obtained maximum value is an order of magnitude higher than obtained from wet-chemical hydroxylation, and, it is above the hitherto highest reported values for water vapor plasma treatment.

The synthesis of bayberry-like mesoporous TiO2 microspheres by a kinetics-controlled method and their hydrophilic films

Bayberry-like mesoporous TiO2 hydrophilic films with high surface roughness and high density of surface hydroxyl groups.

Redox-Inactive CO2 Determines Atmospheric Stability of Electrical Properties of ZnO Nanowire Devices through a Room-Temperature Surface Reaction.

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO2 in air critically determines the atmospheric electrical stability of hydrothermally grown single-crystalline ZnO nanowires. A series of analyses using atmosphere-controlled electrical characterization of single nanowire devices, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy consistently revealed that atmospheric CO2 reacts substantially with the ZnO nanowire surfaces, even at room temperature, to form an electrically insulative zinc carbonate thin layer. The formation of this layer essentially limits the atmospheric electrical stability of the ZnO nanowire devices. Based on this surface carbonation mechanism, we propose a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment. This approach improves the atmospheric electrical stability to at least 40 days in air.

The Crystallinity and Aspect Ratio of Cellulose Nanomaterials Determine Their Pro-Inflammatory and Immune Adjuvant Effects In Vitro and In Vivo.

Nanocellulose is increasingly considered for applications; however, the fibrillar nature, crystalline phase, and surface reactivity of these high aspect ratio nanomaterials need to be considered for safe biomedical use. Here a comprehensive analysis of the impact of cellulose nanofibrils (CNF) and nanocrystals (CNC) is performed using materials provided by the Nanomaterial Health Implications Research Consortium of the National Institute of Environmental Health Sciences. An intermediary length of nanocrystals is also derived by acid hydrolysis. While all CNFs and CNCs are devoid of cytotoxicity, 210 and 280 nm fluorescein isothiocyanate (FITC)-labeled CNCs show higher cellular uptake than longer and shorter CNCs or CNFs. Moreover, CNCs in the 200-300 nm length scale are more likely to induce lysosomal damage, NLRP3 inflammasome activation, and IL-1β production than CNFs. The pro-inflammatory effects of CNCs are correlated with higher crystallinity index, surface hydroxyl density, and reactive oxygen species generation. In addition, CNFs and CNCs can induce maturation of bone marrow-derived dendritic cells and CNCs (and to a lesser extent CNFs) are found to exert adjuvant effects in ovalbumin (OVA)-injected mice, particularly for 210 and 280 nm CNCs. All considered, the data demonstrate the importance of length scale, crystallinity, and surface reactivity in shaping the innate immune response to nanocellulose.

Understanding the synergetic effect from foreign metals in bimetallic oxides for PMS activation: A common strategy to increase the stoichiometric efficiency of oxidants

Synergistic effect was commonly observed in bimetallic oxide catalysts in sulfate radical-based advanced oxidation processes by forming spinel structures. In this paper, we found the incorporation of Cu demonstrated the most remarkable promotional effect to Co oxides comparing with Fe and Mn in PMS activation, and no spinel structure of mixed metals was found. Moreover, the synergism between oxides of Cu and Co was observed on various supports including MgAl layered double hydroxides, mesoporous MnO2 and ZnO. Under conditions of 0.2 g L−1 catalysts, 2 mM PMS, and 40 ppm 4-chlorophenol (4-CP), CuCo@ZnO/PMS system demonstrated excellent performance that 96.0% 4-CP could be removed, while the removal efficiency of 4-CP in Cu@ZnO/PMS and Co@ZnO/PMS systems was only 12.1% and 45.4%, respectively. The simultaneous incorporation of Co and Cu bimetallic oxides in CuCo@ZnO led to a higher surface hydroxyl density, and this surface hydroxyl density showed a linear relationship to their first order kinetic constant in 4-CP degradation. Moreover, the results of peroxymonosulfate (PMS) decomposition, quantification of sulfate radical (SO4−), EPR, XPS, H2-TPR, and CV indicated that the synergism of Cu/Co also arose from the electron transfer between Cu(I) and Co(III), which altered the redox circle of Co(II)/Co(III) and increased the steady concentration of oxidative radicals. Due to this synergism, the degradation percentage of organic contaminants, stability of heterogeneous catalysts and the stoichiometric efficiency of PMS were all improved. These findings were highly suggestive to the further development of bimetallic heterogeneous catalysts in persulfate based advanced oxidation processes.

Synergetic activation of peroxymonosulfate by MnO2-loaded β-FeOOH catalyst for enhanced degradation of organic pollutant in water

In this paper, manganese dioxide (MnO2) loaded iron oxyhydroxide (β-FeOOH) was synthesized aiming to improve the catalytic performance of β-FeOOH as peroxymonosulfate activator. The β-FeOOH@MnO2/PMS system exhibited excellent performance and its reaction rate constant of Acid Orange 7 (AO7) degradation (0.0533 min−1) was approximately 2.3 times as that in β-FeOOH/PMS system (0.0232 min−1). β-FeOOH@MnO2 possessed superior properties as catalyst than β-FeOOH, owing to the higher surface hydroxyl density with higher specific surface area, redox ability and electronic transmission rate. Moreover, on the basis of the analysis from FTIR and XPS, it was found that the redox reaction of Fe3+/Fe2+ and Mn4+/Mn3+ synergistically activated PMS as well as the generation of FeOH+ and MnOH2+ accelerated activating PMS in the β-FeOOH@MnO2/PMS system. Thus, MnO2 and FeOOH synergistically activated PMS to reactive oxygen species (ROS). And 1O2, O2− and OH were identified as predominant ROS in the β-FeOOH@MnO2/PMS system on the basis of the result from quenching experiments and ESR. As a result, TOC removal rate was increased up to 22.62%. Additionally, β-FeOOH@MnO2 exhibited good stability with low iron dissolution and manganese dissolution. Generally, this study proposed that β-FeOOH@MnO2 was an efficient and environmental catalyst as PMS activator for organic pollutant degradation in water.

Scalable Synthesis of Mesoporous TiO2 for Environmental Photocatalytic Applications.

Increasing environmental concern, related to pollution and clean energy demand, have urged the development of new smart solutions profiting from nanotechnology, including the renowned nanomaterial-assisted photocatalytic degradation of pollutants. In this framework, increasing efforts are devoted to the development of TiO2-based nanomaterials with improved photocatalytic activity. A plethora of synthesis routes to obtain high quality TiO2-based nanomaterials is currently available. Nonetheless, large-scale production and the application of nanosized TiO2 is still hampered by technological issues and the high cost related to the capability to obtain TiO2 nanoparticles with high reaction yield and adequate morphological and structural control. The present review aims at providing a selection of synthetic approaches suitable for large-scale production of mesoporous TiO2-based photocatalysts due to its unique features including high specific surface area, improved ultraviolet (UV) radiation absorption, high density of surface hydroxyl groups, and significant ability for further surface functionalization The overviewed synthetic strategies have been selected and classified according to the following criteria (i) high reaction yield, (ii) reliable synthesis scale-up and (iii) adequate control over morphological, structural and textural features. Potential environmental applications of such nanostructures including water remediation and air purification are also discussed.

Application of the CLAYFF and the DREIDING Force Fields for Modeling of Alkylated Quartz Surfaces.

To extend applicability and to overcome limitations of combining rules for nonbond potential parameters, in this study, CLAYFF and DREIDING force fields are coupled at the level of atomic site charges to model quartz surfaces with chemisorpt hydrocarbons. Density functional theory and Bader charge analysis are applied to calculate charges of atoms of the OC bond connecting a quartz crystal and an alkyl group. The study demonstrates that the hydrogen atom of the quartz surface hydroxyl group can be removed and its charge can be redistributed among the oxygen and carbon atoms of the OC bond in a manner consistent with the results calculated at the density functional level of theory. Augmented with modified charges of the OC bond, force fields can then be applied to a practical problem of evaluation of the contact angle of a water droplet on alkylated quartz surfaces in a carbon dioxide environment, which is relevant for carbon geo-sequestration and in a broader context of oil and gas recovery. Alkylated quartz surfaces have been shown to be extremely hydrophobic even when the surface density of hydroxyl groups is close to the highest naturally observed density of 6.2 OH groups per square nanometer.

The Function of Surface Hydroxyl Groups of Incorporated Alumina on Catalytic Ozonation of Heavy Oil Produced Water

ABSTRACTγ-Al2O3 was incorporated with SiO2 and used as carrier to support Ni and Cu oxides for catalytic ozonation of heavy oil produced water. The change of catalytic activity was in accordance with that of surface hydroxyl density after incorporation or reuse of catalysts. Nitrogen adsorption and X-ray power diffraction analysis indicated that the specific surface area decreased and the mineral phase and crystal structure of metal oxides unchanged after incorporation. The results confirmed that the neutral surface hydroxyl groups of the carrier were the active sites during catalytic ozonation. Increasing of surface hydroxyl could be another way of improving the catalytic efficiency of catalytic ozonation.

Charge separation and ROS generation on tubular sodium titanates exposed to simulated solar light

The research focuses on a few key points concerning the light-driven processes taking place on TiO2 anatase and sodium titanates with tubular morphology, such as the relationship between the morphology and activity for H2 and CO2 production, density of surface hydroxyl groups, ROS (OH and O2−) production and photocatalytic activity, and charge separation at the interface of semiconducting domains and enhancement of activity. One key point discussed is whether the materials with peculiar morphologies (i.e. tubular) are superior to the conventional ones. The experimental evidences show that the main advantage of the tubular morphology of sodium titanate is given by its significantly higher surface area compared to parental anatase. FTIR and XPS progressive analyses evidence that the density of surface hydroxyl groups decreases with the development of the tubular morphology. The radical trapping experiments show that the variation of surface hydroxyl density is, generally, followed by activities for OH and O2− generation, as well as by the photocatalytic production of H2 and CO2 from water/methanol mixture. Consequently, the ROS, formed by action of photogenerated electrons and holes on adsorbed O2 and hydroxyl groups, respectively, play an important role in determining the photocatalytic activity of titania-based materials. The other major aspect revealed by this research is that the charge separation at the interfaces formed between anatase and sodium titanate crystalline phases has remarkable effect on the activity formation rates of H2 and CO2.

Particularities of trichloroethylene photocatalytic degradation over crystalline RbLaTa2O7 nanowire bundles grown by solid-state synthesis route

This is the first report on synthesis and photocatalytic activity for trichloroethylene (TCE) degradation under simulated solar light over RbLaTa2O7 layered perovskites with predominant nanowire or platelet morphologies. SEM images witnessed that the one step thermal treatment at 1200 °C lead to formation of RbLaTa2O7 nanowires with diameter of 80–320 nm and several microns in length associated in bundles and sharp-edged, merged platelets (minor phase). The two-step annealing at 950 °C and 1200 °C resulted in decrease of wires bundle population and increase in that of platelets merged in facetted particles. The RbLaTa2O7 nanowires are made of by well-aligned atomic rows with preferred orientation toward the c-axis, relatively free of defect. High density of hydroxyl groups on the sample calcined in mild conditions (RbLaTa_01) favors the photo mineralization of TCE. In contrast, the activity of RbLaTa_02 annealed in harsh conditions (950 and 1200 °C), poor in surface hydroxyl groups, remained modest. The weak surface basicity directed the reaction mainly to generation of intermediate chlorinated compounds. Pd and Au were supported on RbLaTa2O7 perovskites as an alternative strategy to boost the removal of chlorinated pollutants by combining photocatalytic (mineralization) and catalytic (hydrodechlorination, HDC) processes. The mineralization of TCE to Cl− was drastically hindered in presence of methanol due to quenching of ⋅OH radicals by alcohol. The results suggested that the density of RbLaTa2O7 surface hydroxyl groups is essential for photo mineralization of TCE whereas the surface carbonate is beneficial for the formation of intermediate chlorinated product.

Enhanced catalytic ozonation treatment of dibutyl phthalate enabled by porous magnetic Ag-doped ferrospinel MnFe2O4 materials: Performance and mechanism

Enhanced catalytic ozonation of di-n-butyl phthalate (DBP), a recalcitrant waterborne pollutant, was enabled by a family of novel Ag-doped spinel ferrites (MnFe2O4) materials. A sol-gel approach was used to synthesize the Ag-doped MnFe2O4 catalysts, resulting in a highly porous structure with good magnetic property. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of Ag into the MnFe2O4 structure. The Ag-doped MnFe2O4 significantly accelerated ozonation of DBP, and a Ag doping of 0.5% (relative to Mn) enhanced the apparent rate constants by a factor of three and two under circumneutral conditions, compared to the ozone-alone and undoped MnFe2O4 systems, respectively. Catalyst surface hydroxyl groups were identified as a critical factor for ozone decomposition and the subsequent hydroxyl radical production. Spectroscopic and electrochemical measurements showed that Ag doping enhanced DBP degradation by increasing the density of surface hydroxyl groups and promoting the electron transfer and cycling between the catalyst and ozone. The Ag-doped MnFe2O4 catalysts exhibited high stability and can be used for multiple cycles.

Influencing investigation of metal ions on heterogeneous catalytic ozonation by ceramic honeycomb for the degradation of nitrobenzene in aqueous solution with neutral pH

Abstract Influence of metal ions on heterogeneous catalytic ozonation by ceramic honeycomb for degradation of nitrobenzene in aqueous solution was investigated through the semi-continuous batch experiments to compare apparent reaction rate constant (kapp) and hydroxyl radical ( OH) formation rate ( k · OH ) in the presence of different metal ions including monovalent (Na+, K+ and Ag+), divalent (Zn2+, Ni2+, Co2+, Mn2+, Ca2+, Fe2+, Cu2+ and Mg2+) and trivalent ions (Fe3+, Al3+ and Cr3+). Each metal ion can increase kapp and k · OH in the process of ozonation alone under the present experimental conditions. Except for the positive influence of Mn2+, the others all show the early enhancement effect and the latter inhibition effect on kapp and k · OH , namely existing a critical maximum concentration of metal ion for catalytic ozonation. The ratio of OH initiation from heterogeneous and homogeneous systems decreases with the increasing concentration of metal ion, indicating that the contribution of OH initiation derived from heterogeneous catalytic system decreases. Results confirm that the increasing concentration of metal ion causes the decrease in density of surface hydroxyl groups on ceramic honeycomb, and a novel correlation is established between the density decrease rates of surface hydroxyl group and the standard reduction potentials of metal ions.

A high-performance TiO2 nanotube supercapacitor by tuning heating rate during H2 thermal annealing

TiO2 nanotube is a promising material for supercapacitor electrodes. Thermal annealing has been proved as an effective way enhancing the performance of TiO2 nanotube supercapacitors. However, heating rate, as an important parameter in the thermal annealing, has been overlooked as a factor that can influence the electrochemical performance of TiO2 nanotube supercapacitors. In this paper, we demonstrate that the electrochemical performance of TiO2 nanotube supercapacitors fabricated by anodization process can be significantly improved by tuning heating rate during hydrogen thermal annealing. At the optimal condition, the areal capacitance of TiO2 nanotube supercapacitors increased from 27.36 to 52.40 mF cm−2 with a scan rate of 100 mV s−1, while maintaining a high capacitance retention of 65.9% when the scan rate increased from 10 to 1000 mV s−1. Moreover, outstanding long-term cycling stability with only 4.8% capacitance reduction after 5000 charge–discharge cycles is observed. It is found that the electronic carrier densities, surface hydroxyl group density, as well as crystallite size in TiO2 are maximized at the optimal annealing condition, all of which are accounted for the enhancement of the electrochemical performance.