Surface Reaction Mechanism Research Articles

A detailed surface reaction mechanism to investigate oxidation of methane over nickel catalyst

AbstractWe have developed a kinetically consistent detailed surface reaction mechanism for modeling the oxidation of methane over a nickel‐based catalyst. A one‐dimensional model, LOGEcat based on the single‐channel 1D catalyst model, is used to perform the simulations. The original multi‐step reaction mechanism is thermodynamically consistent and consists of 52 reactions. By thermodynamic consistency, we mean that the equilibrium is achieved with the support of the Arrhenius parameters and does not depend on the thermochemistry of the species involved in the considered reactions. The detailed mechanism developed in this investigation contains 26 reversible reactions. These reactions are obtained with the use of the thermochemistry of the species. The study focuses on ensuring kinetic consistency and this is done with the help of thermodynamic analysis by bringing the thermochemistry of the species in play in order to develop a surface reaction mechanism. The new mechanism can be used to understand the other processes, for example, steam‐ and dry‐reforming of methane over nickel, however, the main focus of the paper is to check the performance of the detailed mechanism for catalytic partial oxidation of methane. The applicability of the mechanism is checked for various reactor conditions in terms of parameters such as temperature and pressure by comparing the results with the available reference data. The detailed mechanism developed in this study is able to accurately express oxidation of methane over the nickel catalyst for the considered reactor conditions.

Selective supply of atomic oxygen to a surface placed in room air using vacuum ultraviolet photolysis of carbon dioxide

Reactive species play important roles in plasma treatments of surfaces, including polymers, metals, water, and living tissues. To elucidate the surface reaction mechanism, it is crucial to measure the effect of each type of reactive species on the surface treatment. Accordingly, we previously developed a method to selectively supply OH or O radicals to a surface placed in room air. Ar/H2O or Ar/O2 gas flowing through a quartz tube is irradiated with a vacuum ultraviolet lamp, causing photodissociation of H2O and O2 to produce OH and O. The resulting gas flows to the surface from the tube end. The O density supplied to the surface, however, is only approximately cm−3 according to our simulation, which is not sufficient to determine the surface treatment effect of O atoms. Therefore, in this study, a new method is proposed, supplying O atoms with a density that is two orders of magnitude higher. Specifically, CO2 gas is used as the medium to be photodissociated instead of O2 gas. The performance of this method is discussed herein using simulation results. The maximum O density supplied to the surface reaches cm−3 according to the simulation. Additionally, the experimental results demonstrate that the newly developed method can supply a sufficient amount of O atoms to selectively measure the effect of O atoms on the surface treatment of polypropylene (PP). In particular, the water contact angle (WCA) is considered as an index of the surface treatment. As a result, O atoms decrease the WCA of the PP surface at rates of the same order of magnitude as OH.

Metal–Organic Framework (MOF) Derivatives as Promising Chemiresistive Gas Sensing Materials: A Review

The emission of harmful gases has seriously exceeded relative standards with the rapid development of modern industry, which has shown various negative impacts on human health and the natural environment. Recently, metal-organic frameworks (MOFs)-based materials have been widely used as chemiresistive gas sensing materials for the sensitive detection and monitoring of harmful gases such as NOx, H2S, and many volatile organic compounds (VOCs). In particular, the derivatives of MOFs, which are usually semiconducting metal oxides and oxide-carbon composites, hold great potential to prompt the surface reactions with analytes and thus output amplified resistance changing signals of the chemiresistors, due to their high specific surface areas, versatile structural tunability, diversified surface architectures, as well as their superior selectivity. In this review, we introduce the recent progress in applying sophisticated MOFs-derived materials for chemiresistive gas sensors, with specific emphasis placed on the synthesis and structural regulation of the MOF derivatives, and the promoted surface reaction mechanisms between MOF derivatives and gas analytes. Furthermore, the practical application of MOF derivatives for chemiresistive sensing of NO2, H2S, and typical VOCs (e.g., acetone and ethanol) has been discussed in detail.

Emerging Applications of 17O Solid-State NMR Spectroscopy for Catalytic Oxides

Recent advances in applying 17O solid-state NMR spectroscopy for catalytic oxides have provided valuable information on the surface structure, active sites, and reaction mechanisms. Here we present a short introduction to 17O solid-state NMR spectroscopy before discussing recent methodological developments related to its applications. After that, a brief review on the applications of 17O NMR for elucidating the structure of oxide catalysts and related mechanisms is given. Specifically, we focus on recent research results in the investigations of oxide nanomaterials and zeolites. Finally, some opinions on remaining challenges and future directions of this emerging field are provided.

Real-space imaging of a phenyl group migration reaction on metal surfaces

The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.

Promoted coke resistance of Ni by surface carbon for the dry reforming of methane

Dry reforming of methane (DRM) is an efficient process to transform methane and carbon dioxide to syngas. Nickel could show good catalytic activity for DRM, whereas the deactivation of nickel surfaces by the formation of inert carbon structures is inevitable. In this study, we carry out a detailed investigation of the evolution and catalytic performance of the carbon-covered surface structure on Ni(100) with a combined density functional theory and microkinetic modeling approach. The results suggest that the pristine Ni(100) surface is prone to carbon deposition and accumulation under reaction conditions. Further studies show that over this carbon-covered reconstructed Ni(100) surface, a carbon-based Mars-van-Krevelen mechanism would be favored, and the activity and coke resistance is promoted. This surface state and reaction mechanism were rarely reported before and would provide more insights into the DRM process under real reaction conditions and would help design more stable Ni catalysts.

Surface reaction mechanisms of CO with Fe-based oxygen carrier supported by CaO and K2CO3 in chemical looping combustion: Case study

The density functional theory was used to explore the CO adsorption mechanisms of pure and CaO (100)- and K2CO3 (001) -supported Fe2O3 clusters. The supporting carriers, CaO (100) and K2CO3 (001), had a strong bond with the Fe2O3 cluster and the binding energies were −3.50 eV and −5.54 eV, respectively. The interaction between the Fe2O3 cluster and supporting carriers lengthened the chemical bonds of the cluster. Compared with the pure and K2CO3-supported Fe2O3 clusters, the adsorption performance of the CaO-supported Fe2O3 cluster was improved, and the adsorption energies at the three adsorption sites were greater than 1 eV. The Fe2O3 cluster was activated by CaO and K2CO3, which extended and weakened the bonds, causing the bonds to easily break. Furthermore, CO was adsorbed on the Fe2O3 cluster surface as carbonates. Therefore, the atoms on the Fe2O3 cluster surface required less energy to overcome the potential migration resistance and traveled shorter distances to form new chemical bonds with CO.

Theoretical studies on the reaction mechanisms of the oxidation of tetramethylethylene using MO3Cl (M=Mn, Tc and Re)

A theoretical study on the reaction mechanisms of the addition of transition metal oxo complexes of the type MO3Cl (M = Mn, Tc, and Re) to tetramethylethylene (TME) is presented. Theoretical calculations using B3LYP/LACVP* and M06/LACVP* (LACVP* is a combination of the 6-31G(d) basis set along with LANL2DZ pseudopotentials on the metallic centres) were performed and the results are discussed within the framework of reaction energetics. The nature of the stability of the reaction mechanisms was equivalent for both theories. However, the M06/LACVP* simulations generally had slightly lower energies and shorter bond lengths compared to the B3LYP/LACVP* computations.Furthermore, it was observed that the reaction does not proceed via the stepwise reaction mechanism due to kinetic and thermodynamic instabilities. Epoxidation was also found to occur via the [2 + 2] concerted reaction mechanism for the MO3Cl (M = Tc and Re) whereas the permanganyl chloride complex epoxidizes TME via the [2 + 1] concerted reaction mechanism on the singlet potential energy surface (PES). Dioxylation was observed to proceed via the [3 + 2] route for the addition of MO3Cl (M = Tc and Re) and TME. Results indicate that all reaction surfaces were unselective except for the permanganyl chloride catalyzed surface which leads to the formation of epoxides exclusively.Changes in temperatures from 298.15 K to 373.15 K, resulted in kinetically and thermodynamically unstable reaction pathways as the activation and reaction energies increased generally. On the singlet PES, the rate constant calculations showed that the [3 + 2] catalyzed surface reaction mechanism leading to dioxylation was faster than the [2 + 2] mechanism in cases where plausible.Theoretical values from the global reactivity parameters, namely the chemical hardness, chemical potential, electrophilic and nucleophilic indices, are in good correlation with the DFT activation and reaction energies at both levels of theories. Thus, as the electrophilic nature of the metal decreases from Mn to Re, so do the activation and reaction energies increase from Mn to Re, indicating that the higher the electrophilicity of the metal centre, the more spontaneous the oxidation reaction.

Surface reaction mechanism of atomic layer deposition of niobium oxide: In situ characterization and first-principle study

We report the surface reaction mechanism of the atomic layer deposition (ALD) of niobium oxide films by combining experimental and theoretical studies. Tert-butylimido tris(dimethylamido)niobium [tBuN=Nb(NMe2)3] and ozone (O3) were studied as the precursor and co-reactant. The in situ quartz crystal microbalance (QCM) analysis predicted that two HNMe2 molecules or one H2NtBu molecule were released at 200 °C per one molecule tBuN=Nb(NMe2)3. Since the QCM alone could not distinguish two possible reaction pathways, we compared two pathways by density functional theory (DFT) calculations. We simulated the chemisorption of the precursor molecule on the –OH terminated T-Nb2O5 substrate. The release of two –NMe2 ligands was favorable due to low activation energies of 0.71 and 0.63 eV for the first and second releases. On the other hand, the release of =NtBu was not favorable due to the high activation energy of 1.50 eV. Therefore, the amido and imido ligands were actor and spectator ligands during the first half-reaction of the ALD of niobium oxide films. The combination of QCM and DFT could successfully reveal the surface reaction mechanism of the ALD process using a heteroleptic precursor.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO2 reduction reaction (CO2RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO2RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO2RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO2RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO2RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Mechanistic Insights into Oxidative Molecular Layer Deposition of Conjugated Polymers

Oxidative molecular layer deposition (oMLD) promises to enable molecular-level control of polymer structure through monomer-by-monomer growth via sequential, self-limiting, gas-phase surface reactions of monomer(s) and oxidant(s). However, only a few oMLD growth chemistries have been demonstrated to date, and limited mechanistic understanding is impairing progress in this field. Here, we examine oMLD growth using 3,4-ethylenedioxythiophene (EDOT), pyrrole (Py), p-phenylenediamine (PDA), thiophene (Thi), and furan (Fu) monomers. We establish key insights into the surface reaction mechanisms underlying oMLD growth. We specifically identify the importance of a two-electron chemical oxidant with sufficient oxidation strength to oxidize both a surface and a gas-phase monomer to enable oMLD growth. The mechanistic insights we report enable rational molecular assembly of copolymer structures to improve electrochemical capacity. This work is foundational to unlock molecular-level control of redox-active polymer structure and will enable the study of previously intractable questions regarding the molecular origins of polymer properties, allowing us to control and optimize polymer properties for energy storage, water desalination, and sensors.

H2 Production from Methane Reforming over Molybdenum Carbide Catalysts: From Surface Properties and Reaction Mechanism to Catalyst Development

Hydrogen, with its high energy content and environmental-friendly properties, is considered an effective energy carrier in addition to fossil fuels. Methane reforming represents a major method of hydrogen production, although the applied catalysts often suffer from coke deposition and metal sintering at high operating temperatures. Transition-metal carbides (TMCs), particularly molybdenum carbides (MoxC), possess features such as Pt-like behaviors, affinity with oxidants such as CO2 and H2O, and a strong metal–support interaction for metal dispersion and stabilization, rendering them great prospective candidates for catalyzing the methane reforming reactions (MRRs). This review focuses on the recent applications and challenges of TMCs in MRRs, with an emphasis on the strategies to improve their performance by (1) engineering the operational conditions, (2) designing a dual M–MoxC (M = Ni or Co etc.) active site, (3) dispersing M–MoxC on supports, and (4) generating a M–MoxC/MoOxCy interface in situ. The present review will provide guidance for the future design of efficient catalysts for H2 production from MRRs.

Selectively efficient removal of micropollutants by N-doped carbon modified catalytic ceramic membrane: Synergy of membrane confinement and surface reaction

Surface-dependent nonradical oxidation of N-doped carbon (NC)-based peroxymonosulfate (PMS) system features high stoichiometric efficiency and target degradation selectivity. However, these are severely limited by the ineffective mass transfer of organics and PMS to NC surface. Herein, we utilize membrane confinement to enhance the interaction between organics/PMS and NC catalyst, and propose the synergy mechanism of surface reaction and membrane confinement towards organics removal. The NC catalyst layer is firstly uniformly loaded throughout the ceramic membrane (NC@CM) via an innovative simultaneous polymerization-coating method. As expected, NC@CM/PMS system significantly expedites the surface reaction dominant oxidation in the filtration-through mode, and its kinetics constant for bisphenol A removal is 3700 times higher than that of NC powder/PMS system. Moreover, it has high selectivity in removal of targeted organics such as BPA (81–100 % removal rate) under various interferences (anions, NOM, water sources) due to the corporation of surface reaction and membrane sieving effect. Mechanism study reveals that the synergy of membrane confinement and surface reaction of NC@CM/PMS system removes large size disturbance and reinforces mass transfer process of reactive species towards target organics, and promotes the generation and transformation of reactive species on the NC surface. Our system can selectively and efficiently remove targeted micropollutants with strong anti-interference capability, which is of great significance in practical application.

Reaction mechanism of atomic layer deposition of zirconium oxide using zirconium precursors bearing amino ligands and water.

As a unique nanofabrication technology, atomic layer deposition (ALD) has been widely used for the preparation of various materials in the fields of microelectronics, energy and catalysis. As a high-κ gate dielectric to replace SiO2, zirconium oxide (ZrO2) has been prepared through the ALD method for microelectronic devices. In this work, through density functional theory calculations, the possible reaction pathways of ZrO2 ALD using tetrakis(dimethylamino)zirconium (TDMAZ) and water as the precursors were explored. The whole ZrO2 ALD reaction could be divided into two sequential reactions, TDMAZ and H2O reactions. In the TDMAZ reaction on the hydroxylated surface, the dimethylamino group of TDMAZ could be directly eliminated by substitution and ligand exchange reactions with the hydroxyl group on the surface to form dimethylamine (HN(CH3)2). In the H2O reaction with the aminated surface, the reaction process is much more complex than the TDMAZ reaction. These reactions mainly include ligand exchange reactions between the dimethylamino group of TDMAZ and H2O and coupling reactions for the formation of the bridged products and the by-product of H2O or HN(CH3)2. These insights into surface reaction mechanism of ZrO2 ALD can provide theoretical guidance for the precursor design and improving ALD preparation of other oxides and zirconium compounds, which are based ALD reaction mechanism.

Encounter-based approach to diffusion with resetting.

An encounter-based approach consists in using the boundary local time as a proxy for the number of encounters between a diffusing particle and a target to implement various surface reaction mechanisms on that target. In this paper, we investigate the effects of stochastic resetting onto diffusion-controlled reactions in bounded confining domains. We first discuss the effect of position resetting onto the propagator and related quantities; in this way, we retrieve a number of earlier results but also provide complementary insights into them. Second, we introduce boundary local time resetting and investigate its impact. Curiously, we find that this type of resetting does not alter the conventional propagator governing the diffusive dynamics in the presence of a partially reactive target with a constant reactivity. In turn, the generalized propagator for other surface reaction mechanisms can be significantly affected. Our general results are illustrated for diffusion on an interval with reactive end points. Further perspectives and some open problems are discussed.

Surface Exchange Reaction of Mixed Conductive La0.65Ca0.35FeO3−δ during Oxygen Evolution and Incorporation as Traced by Operando X-ray Photoelectron Spectroscopy

High oxygen permeability of mixed conductive La0.65Ca0.35FeO3-δ (LCF) is applicable to pure oxygen gas generators and cathodes for solid oxide fuel cells, etc.; however, lower surface exchange reactions at temperatures below 800 °C reduce permeability. To understand the microscopic surface reaction mechanism, operando soft X-ray photoelectron spectroscopy of an LCF film surface was conducted during the evolution and incorporation of oxygen. LCF film was prepared on yttria-stabilized zirconia and a current was applied throughout the film at ∼600 °C. From operando X-ray photoelectron spectra, surface oxide species involved in the surface exchange reaction obviously appeared on the film during the evolution of oxygen from the surface. The number of surface oxide species abruptly decreased during incorporation of oxygen. By applying the current from a negative to positive value, the numbers of surface oxide species and ligand holes near Fe3+ ions on the surface both significantly increased. The results infer that ligand holes in the Fe 3d-O 2p hybrid orbitals correspond to active reaction sites at which surface oxide species change to oxygen molecules. Increasing the number of active reaction sites is key to improving oxygen evolution of mixed conductive oxides.

Kinetically consistent detailed surface reaction mechanism for steam reforming of methane over nickel catalyst

A one-dimensional model, LOGEcat is used to develop a detailed surface reaction mechanism for modeling the steam reforming of methane over a nickel-based catalyst. The focus of the paper is to develop a kinetically consistent surface reaction mechanism. The two terms, kinetically and thermodynamically consistent mechanisms, will be used frequently in this article. Note that when the mechanism is thermodynamically consistent then the thermodynamic data or the thermochemistry of the species is not used and all the reactions in the mechanism are forward reactions (no backward reactions included) which need the Arrhenius parameters (pre-exponential factor, A_{r}; activation energy, E_{r}; temperature exponent, beta _r) explicitly for each reaction. The kinetically consistent mechanisms need Arrhenius parameters only for the forward reactions and does not need these parameters for backward reactions making the backward reactions independent of the kinetics. The rate for the backward reactions is calculated with the help of thermochemistry of the intermediate species involved in the reaction mechanism. Since the thermochemistry of the intermediate species are not easily available, this makes such studies much more complex. The original multi-step reaction mechanism consists of 42 reactions which are thermodynamically consistent. In this study we have modified this reaction mechanism from literature and used only 21 (reversible) direct reactions. This aspect is important because by using only 21 reactions, we use the thermochemistry of the species to achieve thermodynamic equilibrium instead of providing the Arrhenius parameters for forward and backward reactions. We use the same A_{r}, E_{r} and beta _r values as used in the reference paper for the considered 21 reactions which are in equilibrium. However, the value of A_{r}, E_{r} and beta _r for the inverse/backward reactions are not given in the mechanism and the reverse rates are calculated by using the equilibrium and forward rate. Therefore, a sensitivity analysis on thermochemistry of the species is performed for a better agreement with the literature data. The method is presented for ensuring kinetic consistency and bringing thermochemistry of species in play while developing a surface reaction mechanism. The applicability of the mechanism is tested for two different simulation set-ups available in literature considering various reactor conditions in terms of parameters given as temperature and steam-to-carbon (S/C) ratio. Several chemical reaction terms, such as, selectivity, conversion and mathrm {H_2}/{text{CO}} ratio are shown for different parameters in comparison with the available reference data. The detailed mechanism developed in this study is able to accurately express the steam reforming of methane over the nickel catalyst for complete ranges of temperature for both the set-ups and within acceptable limit for S/C ratio considered for the analysis.

Expanding the Scope of Electrosynthesis through Catalyst Design and Operando Spectroscopy

Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. To this end, research group develops heterogeneous catalysts (e.g. nanomaterials, MOFs) geared for the reduction of CO2 and oxidation of biomass platforms to fuels and value-added chemicals. Further, we establish strategies in emergent areas of electrocatalysis such as electrochemically forming and cleaving C-N bonds. Because the design of new catalytic systems is inherently linked to a precise understanding of how these reactions proceed on heterogeneous surfaces, we put considerable efforts in developing methodology for operando probing of these systems with vibrational spectroscopy. In all, I show how using these experiments provides key mechanistic information on surface reaction mechanisms that enhance our understanding of functional interfaces and how the research provides avenues for future materials design within the context of renewable energy electrocatalysis.

(Invited) Atomically-Precise Surface Processes: From Molecular Mechanisms to Realistic Devices

The atomic-level precision in designing surfaces and nanostructures is quickly making its way from the one-off laboratory investigations into chemical manufacturing. However, in order to make the applications feasible, fundamental understanding of the mechanisms of surface reactions leading to the formation of the desired surface structures is needed. A great deal of progress has been made over the years in uncovering reactions behind atomic layer deposition (ALD), but much more limited information is available about atomic layer etching (ALE), although both processes are often required to build the components of present and future microelectronics.This talk will highlight recent work on understanding the reactions for metal and metal oxide deposition on functionalized (and patterned) surfaces and on recent advances in ALE of complex materials, specifically focused on tertiary alloys, such as CoFeB, used in magnetic tunnel junctions. I will outline the work needed to understand the mechanisms of these processes that can be further used to improve the control over atomically-precise manufacturing methods and to reduce the use of hazardous procedures. The relatively well-understood ALE procedures that include oxidation or chlorination as the first half-cycle of ALE of such materials and introduction of a bidentate ligands (such as acetylacetonates) to remove complex materials uniformly and with atomic-level control will be extended to describe the potential use of much milder conditions and reagents. A combination of experimental and computational methods will be used to make this analysis possible.

In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation

Up to now, a great number of catalysts have been reported that are active in the oxidation of volatile organic compounds (VOCs). However, supported noble-metal catalysts (especially Pt-based catalysts) are still the most excellent ones for this reaction. In this study, Pt species supported on γ-Al2O3 and ranging from single-atom sites to clusters (less than 1 nm) and 1–2 nm nanoparticles were prepared and investigated for oxidizing C2H4. The Pt-loaded γ-Al2O3 nanoflakes (PtAl-NF) and Pt-loaded γ-Al2O3 nanowires (PtAl-NW) were successfully prepared. The samples were characterized using XRD, TEM, XPS, HAADF-STEM, and in situ DRIFTS. Based on in situ DRIFTS, a simple surface reaction mechanism was developed. The stable intermediates CO on single-atom Pt, subnanometer Pt particles, and fully exposed Pt clusters could be explained by the strong binding of CO molecule poisoning Pt sites. Moreover, the oxidation of C2H4 was best achieved by Pt particles that were 1–2 nm in size and the catalytic activity of PtAl-NF was better when it had less Pt. Lastly, the most exposed (110) facets of γ-Al2O3 nanoflakes were more resistant to water than the majorly exposed (100) facets of γ-Al2O3 nanowires.