Grazing Incidence X-ray Diffraction Research Articles

Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells

Thermal annealing alters the morphology of organic donor-acceptor bulk-heterojunction thin films used in organic solar cells. Here, we studied the influence of thermal annealing on blends of amorphous regio-random (RRa) and semi-crystalline regio-regular (RR) poly (3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester. Since the P3HT:PCBM blend is one of the most studied in the OPV community, the existing research provides a solid foundation for us to compare and benchmark our innovative characterization techniques that have been previously under-utilized to investigate bulk heterojunction organic thin films. Here, we combine advanced novel microscopies and spectroscopies, including polarized light microscopy, photo-deflection spectroscopy, hyperspectral photoluminescence imaging, and energy resolved-electrochemical impedance spectroscopy, with structural characterization techniques, including grazing-incidence wide-angle x-ray scattering, grazing-incidence x-ray diffraction, and Raman spectroscopy, in order to reveal the impact of thermal annealing on the microstructural crystallinity and morphology of the photoactive layer in organic solar cells. Coupled transfer matrix and drift-diffusion simulations were used to study the impact of the density of states on the solar cells’ device performance parameters, namely the short-circuit current (J SC), open circuit voltage (V OC), fill factor (FF), and power conversion efficiency (PCE).

Correlation between the micro-shot peening coverage and surface microstructural evolution and fluoride-induced corrosion behavior in Monel 400 alloy

This study examines the influence of micro-shot peening (MSP) on Monel 400 alloy, focusing on surface microstructural evolution and fluoride-induced corrosion behavior compared to its solution annealed (SAed) counterpart. MSP treatments were conducted at various surface coverages (100%, 200%, 500%, 1000%, 1500%, and 2000%). Characterization encompassed microhardness, surface roughness, grazing incidence x-ray diffraction (GI-XRD), field emission scanning electron microscopy (FE-SEM) with electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). With increasing the MSP coverage to 2000%, the surface crystallite size decreased from approximately ∼30 µm in the SAed sample to 60 nm. TEM analysis emphasized dislocation slip and formation of dislocation structures as the primary mechanisms for the grain refinement. However, the 2000% coverage exhibited microcrack formation, compromising its hydrofluoric (HF) corrosion resistance. Electrochemical tests revealed severe localized corrosion in all MSP-treated samples except the 1500% coverage, which exhibited a minimum passive current density of 0.2–1.0 µA/cm2. Inductively coupled plasma optical emission spectroscopy (ICP-OES) of immersion solutions showed lower copper and nickel ion concentrations (<150 ppm) in the 1500% MSP-treated sample, indicating its enhanced corrosion resistance. X-ray photoelectron spectroscopy (XPS) identified NiO, NiF2, CuO, CuF2, and Cu(OH)2 phases in SAed and 1500% MSPed samples, with a higher concentration of copper oxide (CuO) in the corrosion film of the 1500% MSP-treated sample, contributing to its superior corrosion resistance.

Structure of liquid-vapor interfaces: Perspectives from liquid state theory, large-scale simulations, and potential grazing-incidence x-ray diffraction.

Grazing-incidence x-ray diffraction (GIXRD) is a scattering technique that allows one to characterize the structure of fluid interfaces down to the molecular scale, including the measurement of surface tension and interface roughness. However, the corresponding standard data analysis at nonzero wave numbers has been criticized as to be inconclusive because the scattering intensity is polluted by the unavoidable scattering from the bulk. Here, we overcome this ambiguity by proposing a physically consistent model of the bulk contribution based on a minimal set of assumptions of experimental relevance. To this end, we derive an explicit integral expression for the background scattering, which can be determined numerically from the static structure factors of the coexisting bulk phases as independent input. Concerning the interpretation of GIXRD data inferred from computer simulations, we extend the model to account also for the finite sizes of the bulk phases, which are unavoidable in simulations. The corresponding leading-order correction beyond the dominant contribution to the scattered intensity is revealed by asymptotic analysis, which is characterized by the competition between the linear system size and the x-ray penetration depth in the case of simulations. Specifically, we have calculated the expected GIXRD intensity for scattering at the planar liquid-vapor interface of Lennard-Jones fluids with truncated pair interactions via extensive, high-precision computer simulations. The reported data cover interfacial and bulk properties of fluid states along the whole liquid-vapor coexistence line. A sensitivity analysis shows that our findings are robust with respect to the detailed definition of the mean interface position. We conclude that previous claims of an enhanced surface tension at mesoscopic scales are amenable to unambiguous tests via scattering experiments.

Understanding the Retention of Vaping Additives in the Lungs: Model Lung Surfactant Membrane Perturbation by Vitamin E and Vitamin E Acetate.

Deviations from the normal physicochemical and functional properties of pulmonary surfactants are associated with the incidence of lung injury and other respiratory disorders. This study aims to evaluate the alteration of the 2D molecular organization and morphology of pulmonary surfactant model membranes by the electronic cigarette additives α-tocopherol (vitamin E) and α-tocopherol acetate (vitamin E acetate), which have been associated with lung injury, termed e-cigarette or vaping-use-associated lung injury (EVALI). The model membranes used contained a 7:3 molar ratio of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol) to which α-tocopherol and α-tocopherol acetate were added to form mixtures of up to 20 mol % additive. The properties of the neat tocopherol additives and DPPC/POPG (7:3) mixtures with increasing molar proportions of additive were evaluated by surface pressure-area isotherms, excess area calculations, Brewster angle microscopy, grazing incidence X-ray diffraction, X-ray reflectivity, and atomic force microscopy. The addition of either additive alters the essential phase balance of the model pulmonary surfactant membrane by generating a greater proportion of the fluid phase. Despite this net fluidization, both tocopherol additives have space-filling effects on the liquid-expanded and condensed phases, yielding negative excess areas in the liquid-expanded phase and reduced tilt angles in the condensed phase. Both tocopherol additives alter the stability of the fluid phase, pushing the eventual collapse of this phase to higher surface pressures than the model membrane in the absence of an additive.

Structural, chemical, and metal-oxide-semiconductor characteristics of RF magnetron sputtered thulium oxide passivation layer on 4H-silicon carbide substrate

Thulium oxide (Tm2O3) passivation layer (PL) deposited on 4H-silicon carbide (SiC) that was subjected to postdeposition annealing at 700 °C in forming gas-oxygen-forming gas ambient was reported for the first time in this work. X-ray photoelectron spectroscopy characterization revealed the success of annealing process in minimizing the concentration of oxygen vacancies and also curbed the presence of TmO suboxide bonding in the annealed Tm2O3 PL. In addition, XPS characterization has also disclosed the incorporation of nitrogen ions into the Tm2O3 lattice through the annealing process in which grazing incidence X-ray diffraction has also revealed the shift of detected peaks with regards to Tm2O3 phases to small diffraction angles. Although X-ray reflectivity measurements have disclosed the formation of thicker SiO2 interfacial layer (IL) for the annealed Tm2O3 PL, the acquisition of a higher dielectric constant (k) by the annealed PL when compared to as-deposited PL has suggested that the formation of SiO2 IL has minimal effect toward the k value of annealed PL. Metal-oxide-semiconductor characteristics of the annealed Tm2O3 PL on 4H-SiC was systematically reported in this letter.

Structural Parameters and Behavior in Simulated Body Fluid of High Entropy Alloy Thin Films.

The structure, composition and corrosion properties of thin films synthesized using the Pulsed Laser Deposition (PLD) technique starting from a three high entropy alloy (HEA) AlCoCrFeNix produced by vacuum arc remelting (VAR) method were investigated. The depositions were performed at room temperature on Si and mirror-like polished Ti substrates either under residual vacuum (low 10-7 mbar, films denoted HEA2, HEA6, and HEA10, which were grown from targets with Ni concentration molar ratio, x, equal to 0.4, 1.2, and 2.0, respectively) or under N2 (10-4 mbar, films denoted HEN2, HEN6, and HEN10 for the same Ni concentration molar ratios). The deposited films' structures, investigated using Grazing Incidence X-ray Diffraction, showed the presence of face-centered cubic and body-centered cubic phases, while their surface morphology, investigated using scanning electron microscopy, exhibited a smooth surface with micrometer size droplets. The mass density and thickness were obtained from simulations of acquired X-ray reflectivity curves. The films' elemental composition, estimated using the energy dispersion X-ray spectroscopy, was quite close to that of the targets used. X-ray Photoelectron Spectroscopy investigation showed that films deposited under a N2 atmosphere contained several percentages of N atoms in metallic nitride compounds. The electrochemical behavior of films under simulated body fluid (SBF) conditions was investigated by Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy measurements. The measured OCP values increased over time, implying that a passive layer was formed on the surface of the films. It was observed that all films started to passivate in SBF solution, with the HEN6 film exhibiting the highest increase. The highest repassivation potential was exhibited by the same film, implying that it had the highest stability range of all analyzed films. Impedance measurements indicated high corrosion resistance values for HEA2, HEA6, and HEN6 samples. Much lower resistances were found for HEN10 and HEN2. Overall, HEN6 films exhibited the best corrosion behavior among the investigated films. It was noticed that for 24 h of immersion in SBF solution, this film was also a physical barrier to the corrosion process, not only a chemical one.

Heavy-ion irradiation effects of high-entropy A2Ti2O7 pyrochlore with multi-elements at a site

In this work, the irradiation response of A2Ti2O7 (A represents rare earth elements of Gd, Dy, Ho, Er, Yb and Nd) series ceramics were systematically studied under 3 MeV Au2+ ions. The structural changes pre- and post-irradiation were characterized using X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy. After heavy ion irradiation, all the samples undergo structural transformations from ordered pyrochlore to disordered fluorite and then to amorphous phase. The sample surface became fully amorphous when exposed to the maximum irradiation dose. The 5RETO high-entropy pyrochlore exhibits the lowest rate of amorphization during the process of structural transformation. The cation radius ratio and mixed entropy are the primary factors influencing amorphization rate. The cation radius ratio shows a strong positive correlation with the amorphization rate, while the mixed entropy demonstrates a weak negative correlation with the amorphization rate. The cation radius ratio can thus serve as a reliable indicator for predicting the radiation resistance of high-entropy pyrochlore ceramics.

In situ cell for grazing-incidence x-ray diffraction on thin films in thermal catalysis.

A cell for synchrotron-based grazing-incidence x-ray diffraction at ambient pressures and moderate temperatures in a controlled gas atmosphere is presented. The cell is suited for the in situ study of thin film samples under catalytically relevant conditions. To some extent, in addition to diffraction, the cell can be simultaneously applied for x-ray reflectometry and fluorescence studies. Different domes enclosing the sample have been studied and selected to ensure minimum contribution to the diffraction patterns. The applicability of the cell is demonstrated using synchrotron radiation by monitoring structural changes of a 3nm Pd thin film upon interaction with gas-phase hydrogen and during acetylene semihydrogenation at 150 °C. The cell allows investigation of very thin films under catalytically relevant conditions.

Microstructure evolution of γ-Al2O3/FeAl tritium permeation barrier coatings under 6.4 MeV Fe3+ ion irradiation

γ-Al2O3/FeAl coatings for tritium permeation barrier (TPB) application in fusion reactors were fabricated via electro-chemical deposition and selective oxidation. The TPB coatings consisted of a γ-Al2O3 scale (200 nm), an upper interlayer of FeAl (20 μm) and a lower interlayer of Fe3Al (10 μm). Their potential in-reactor performance was investigated based on 6.4 MeV Fe3+ ion irradiations at 400 °C, up to ion fluences of 1.0 × 1019 ions·m−2 and 1.0 × 1020 ions·m−2, at depth regions entailing the oxide scale and the top interlayer. Combined grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM) analysis confirmed the overall phase stability and grain structure stability in γ-Al2O3/FeAl coatings after irradiation. Cavities dominated the damage microstructure for both γ-Al2O3 and FeAl phases, but were not observed at the sub-layer interfaces. The underlying mechanisms for defect production were discussed. Finally, a brief comparison between heavy-ion and fission neutron damage effects in Al2O3-based TPB coatings is made, aiming to bring new insights on the coating integrity and performance reliability during fusion reactor operation.

Confronting positions: para- vs. meta-functionalization in triindole for p-type air-stable OTFTs

The 5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole core, namely triindole, is well-known for its prominent hole-transporting properties and air stability. The functionalization of this core is also rather versatile, which allows the modulation of its properties by anchoring targeted scaffolds to different positions, e.g. 3,8,13 (para with respect to the nitrogens), 2,7,12 (analogously meta) or the nitrogen heteroatoms. Therefore, triindole excels as a pivotal semiconductor to be exploited in long-lasting organic thin-film transistors (OTFTs). This report aims to shed light on the effect of functionalizing whether para or meta positions with sulfurated moieties, in the pursuit of an enhanced performance in OTFTs. Remarkably, meta-substituted derivatives outshone their para- counterparts in terms of thermal, optical, intermolecular arrangement and semiconductor properties, claiming mobility values up to 2 × 10−3 cm2 V−1 s−1 and a shelf lifetime beyond the analyzed period of 5 months. Analysis of the thin films by grazing incidence X-ray diffraction (GIRXD) and atomic force microscopy (AFM) revealed that the meta-substitution also induces a higher degree of order and better morphology, further corroborating the potential of this structural approach.

Icephobic Gradient Polymer Coatings Deposited via iCVD: A Novel Approach for Icing Control and Mitigation.

Materials against ice formation and accretion are highly desirable for different industrial applications and daily activities affected by icing. Although several concepts have been proposed, no material has so far shown wide-ranging icephobic features, enabling durability and manufacturing on large scales. Herein, we present gradient polymers made of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) deposited in one step via initiated chemical vapor deposition (iCVD) as an effective coating to mitigate ice accretion and reduce ice adhesion. The gradient structures easily overcome adhesion, stability, and durability issues of traditional fluorinated coatings. The coatings show promising icephobic performance by reducing ice adhesion, depressing the freezing point, delaying drop freezing, and inhibiting ice nucleation and frost propagation. Icephobicity correlates with surface energy discontinuities at the surface plane resulting from the random orientation of the fluorinated groups of PFDA, as confirmed by grazing-incidence X-ray diffraction measurements. The icephobicity could be further improved by tuning the surface crystallinity rather than surface wetting, as samples with random crystal orientation show the lowest ice adhesion despite high contact angle hysteresis. The iCVD-manufactured coatings show promising results, indicating the potential for ice control on larger scales and various applications.

An experimental investigation on nanomechanical and nanotribological behavior of tantalum nitride coating deposited on Ti6Al7Nb alloy

In this investigation, TaN coating was deposited on ASTM-F1295 grade Ti6Al7Nb alloy by radio frequency magnetron sputtering. Structural, surface morphological, and compositional analyses were conducted using Grazing Incidence-XRD, FESEM, EDS and XPS. The results affirmed that uniform and sub-stoichiometric TaN coating was deposited with the polycrystalline nature of TaN, exhibiting a cubic-NaCl-like structure. Nanoindentation tests revealed that the hardness and modulus of TaN coating decrease with increasing indentation load. Moreover, scratch test under progressive loading conditions uncovered critical loads corresponding to cohesion and adhesion failures. Nanotribological tests were conducted on deposited TaN against different counter bodies (steel and ceramic ball) disclosed fine abrasive grooves on the coating and demonstrate anti-wear properties.

Impact of the Crystal Structure of Interlayer on the Properties of Zr‐Doped Hafnia‐Based Ferroelectric Capacitor

In this work, the effect of interlayer on the ferroelectric properties of Hf0.5Zr0.5O2 (HZO)‐based metal–ferroelectric–metal (MFM) capacitor is investigated. In detail, a 1 nm thick interlayer of amorphous Al2O3 and fluorite‐structured ZrO2 is inserted in the HZO ferroelectric film. Grazing incidence X‐ray diffraction (GIXRD) analysis reveals that the amorphous structure of Al2O3 (vs. the fluorite‐structured ZrO2) effectively blocks the grain boundary growth of HZO layer. In contrast, X‐ray photoelectron spectroscopy (XPS) analysis demonstrates that ZrO2 with a fluorite structure like HZO induces fewer oxygen vacancies at the ferroelectric–insulator interface, and thereby, it provides fewer defects in the HZO layer, achieving fatigue‐free endurance characteristics and a higher remanent polarization (2Pr) of 4.6 μC cm−2.

Effect of bias voltage on the structural properties of WN/NbN nanolayer coatings deposited by cathodic-arc evaporation

In this work, WN/NbN nanolaminate coatings were synthesized by cathodic-arc physical vapor deposition (CA-PVD) technique on a stainless-steel substrate. The paper reports the microstructure, cross-sectional morphology, surface roughness, and adhesion strength changes caused by variations in the absolute values of the negative substrate bias voltage, Us , in the 50-200 V range. Synthesized coatings were analyzed by Grazing incidence X-ray diffraction (GI-XRD), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), and Daimler-Benz test. The phase analysis revealed that multilayer coatings had complex polycrystalline microstructure. They consisted of face-cantered cubic (fcc) β-W2N, fcc δ-NbN, and hexagonal ε-NbN phases. The total thickness and surface roughness had a descending trend with an increase in the absolute value of the negative bias voltage. Moreover, the WN/NbN coating deposited at Us = -50 V demonstrated the best adhesion strength to the substrate, suitable for protective coatings.

Effect in variation of the cationic precursor temperature on the electrical and crystalline properties of MnS growth by SILAR

The crystallographic, optical, and electrical properties of manganese sulfide thin films depend on the control of the temperature precursors in the synthesis process, as shown by the results of this work. MnS thin films were deposited on glass substrates using the SILAR method and over an additional layer of CdS synthesized by chemical bath deposition (CBD) to acquire a p-n heterojunction. SILAR is an inexpensive method performed with a homemade robot in this case. Temperature in the solution precursors varied from 20 to 80 °C in four experiments. The morphology and structure of MnS and FTO/CdS/MnS thin films were studied through scanning electron microscopy (SEM) and grazing-incidence X-ray diffraction (GIXRD); the results indicate that materials showed a polycrystalline behavior, a diffraction peak of α- MnS cubic phase was observed with lattice constants values, ranging from 4.74 to 4.75 Å.Additionally, Raman spectroscopy showed a signal corresponding to the transversal optical phonons of MnS at a wavenumber near 300 cm−1. UV–vis spectroscopy showed optical bandgap values of 3.94, 4.0, 4.09, and 4.26 eV for thin films obtained at 20°, 40°, 60°, and 80 °C.respectively. Results indicated 80 °C as an optimal cationic precursor process temperature, achieving optical transmittance T% and good film quality according to SEM and GIXRD for the synthetization of MnS. The current–voltage (I–V) characterization in the heterojunction showed a characteristic diode curve with an open circuit voltage (VOC) of 300 mV under illumination, which indicated that the manganese sulfide behaves as p-type material contributing with positive charge carriers, while CdS behaves as n-type material.

A grazing incidence diffraction setup for Langmuir trough experiments at the beamline P08 at PETRA III synchrotron

A grazing incidence diffraction setup for Langmuir trough experiments at the beamline P08 at PETRA III synchrotron

Synthesis and characterization of low-friction W-V-N alloy coatings using reactive magnetron sputtering technique for tribological applications

In recent years, self-lubricating hard coatings have garnered significant interest across various industries such as cutting tools, molds, and manufacturing because of their ability to reduce friction and wear at high temperatures in dry-cutting applications. The present study focuses on synthesis of tungsten-vanadium-nitride (W-V-N) coatings using the reactive magnetron cosputtering technique in an Ar + N2 plasma gas environment. The coating microstructure, surface morphology, wetting behavior, and mechanical properties were characterized by grazing incidence x-ray diffraction, field-emission scanning electron microscopy, atomic force microscopy, energy-dispersive spectroscopy, and nanoindentation. Wear resistance properties of the prepared W-V-N alloy coatings were investigated using a ball-on-disk tribometer at two different temperatures. The findings indicate that all W-V-N coatings, regardless of the vanadium content, exhibit a face-centered cubic structure and form a solid solution of W-V-N. Among the coatings studied, W0.68V0.32N exhibited the highest hardness (14.25 GPa) and Young's modulus (257.53 GPa), as well as an excellent wear resistance. Increasing the vanadium content in the W-V-N coating led to a notable reduction in both the specific wear rate and friction coefficient. Moreover, this reduction was more pronounced with an increase in temperature during the wear test. Improvement in the wear properties can be attributed to the formation of Magnéli phases of vanadium oxides on the surface of the coatings. The ability of the W-V-N coating to reduce friction and wear, combined with its improved mechanical properties, makes it a promising candidate for solid lubricating coatings in tribological applications.

Effect of Dose Rate on Tribological Properties of 8Cr4Mo4V Subjected to Plasma Immersion Ion Implantation

The lack of service lifetime of bearings has become a bottleneck that restricts the performance of aero engines. How to solve or improve this problem is the focus of most surface engineering researchers at present. In this study, plasma immersion ion implantation was conducted; in order to enhance the ion implantation efficiency and improve the wear resistance of 8Cr4Mo4V bearing steel, the dose-rate-enhanced method was adopted during ion implantation. The surface roughness, phase constituents, elemental concentration, hardness, contact angle and wear resistance of samples after ion implantation was determined by atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD), elemental dispersive spectroscopy (EDS), X-ray diffraction, nanoindentation tester, universal friction and wear tester, etc. The results showed that the high-dose-rate method had a significant enhancement influence on ion implantation efficiency. At the dose rate of 2.60 × 1017 ions/cm2·h, the roughness of Ra decreases from 24.8 nm to 10.4 nm, which is decreased by 58.1% for the dose rate of 7.85 × 1017 ions/cm2·h. XRD confirmed that the implanted samples consisted of the Fe(M) and Fe2–3N phase and CrN which depends on the implantation dose rate. Meanwhile, the surface hardness was improved from 11.1 GPa to 16.9 GPa and enlarged the hardened region; more valuably, the surface state of samples via high-dose-rate implantation exhibits hydrophobicity with high roughness which is able to store debris and decrease the abrasive wear during testing; thereby, the wear resistance was greatly enhanced by high-dose-rate plasma immersion ion implantation.

Studies on the structural and magnetic properties of PMMA/Ni nanocomposite system prepared by embedding Ni nanoparticles in the PMMA films

PMMA/Ni nanocomposite films have been prepared by embedding Ni nanoparticles and maintaining their different thickness (5–50 nm) in PMMA (polymethylmethacrylate) film matrix by using ion beam sputtering (IBS) technique. Grazing incidence x-ray diffraction (GIXRD) results reveal Ni is present in metallic form in the FCC phase. X-ray reflectivity (XRR) is used to measure the nominal thickness of the Ni nanoparticle layer. Surface morphology was studied using atomic force microscopes (AFM). SXAS (soft x-ray absorption spectroscopy) studies on PMMA/Ni nanocomposites were done using synchrotron radiation to understand the electronic structure of these films which reveals that Ni is present in metallic state confirming that PMMA as host matrix protects the magnetic Ni nanoparticles from external oxidation. The magneto-optical Kerr effect (MOKE) magnetometer and MOKE microscope were employed to study the magnetic properties and simultaneously record the real time evolution of magnetic domains at 0°, 45°, and 90° azimuthal angles. All films show soft ferromagnetic behaviour and all the films show isotropic behaviour except one with Ni nanoparticle layer thickness of 40 nm. MOKE micrographs witness the gradual growth of magnetic domains with increase in thickness of Ni nanoparticle layer. These PMMA/Ni nanocomposite films may be viewed as potential candidate for futuristic functional material.

Grain size effect of the flexoelectric response in BaTiO3 ceramics

Size effect is a fundamental phenomenon in ferroelectric materials and grain size dependence of the dielectric and piezoelectric properties of BaTiO3 (BTO) ceramics has been observed. However, the dependence of flexoelectric response on grain size has not been reported, thus far. In this work, BTO ceramics with grain sizes ranging from 0.59 to 8.90 μm were prepared by a two-step sintering method. We found that with increasing grain size, the flexoelectric coefficient of BTO ceramics increases from less than 20 μC/m (grain size 0.59–0.69 μm) to more than 300 μC/m (grain size 8.90 μm), but the grain size dependence of the flexoelectric response is different from that of the dielectric and piezoelectric properties. Observation by piezoresponse force microscopy reveals that the surface regions of BTO ceramics are spontaneously polarized. Strong inhomogeneous strain is measured by grazing incidence x-ray diffraction and the resultant flexoelectric effect is enough to polarize the surface regions. Fitting of the flexoelectric data indicates that the grain size effect of the flexoelectric response can be well explained by the polarized surface layer mechanism.