Ti 2p Research Articles

XPS Binding Energy Shifts in 2D Ti3C2Tz MXene go largely Beyond Intuitive Explanations: Rationalization from DFT Simulations and Experiments.

MXenes are prototypes of surface tunable 2D materials with vast potential for properties tuning. Accurately characterizing their surface functionalization and its role in electronic structure is crucial, X-ray photoelectron spectroscopy (XPS) being among the go-to methods to do so. Despite extensive use, XPS analysis remains however intricate. Focusing on the benchmark MXene Ti3C2Tz, Density Functional Theory (DFT) calculations of core-level binding energy shifts (BE.s.) are combined with experiments in order to provide a quantitative interpretation of XPS spectra. This approach demonstrates that BE.s. are driven by the complex interplay between chemical, structural, and subtle electronic structure effects preventing analysis from intuitive arguments or comparison with reference materials. In particular, it is shown that O terminations induce the largest BE.s. at Ti 2p levels despite lower electronegativity than F. Additionally, F 1s levels show weak sensitivity to the F local environment, explaining the single contribution in the spectrum, whereas O 1s states are significantly affected by the local surface chemistry. Finally, clear indicators of surface group vacancies are given at Ti 2p and O 1s levels. These results demonstrate the combination of calculations with experiments as a method of the highest value for MXenes XPS spectra analysis, providing guidelines for otherwise complex interpretations.

γ- WO3 decorated MXene: An advanced nanomaterial for room temperature operable enhanced ammonia sensor

The proliferation of Internet of Things (IoT) applications necessitates the deployment of high-performance ammonia (NH3) sensors. In recent studies, Ti3C2Tx, a novel two-dimensional (2D) material, has been extensively investigated for its potential as a room temperature NH3 sensor. However, pristine MXene-based sensors have exhibited deficiencies in long-term stability and reproducibility. In response, an MXene/γ-WO3 composite has been synthesized using an ultra-sonication method to address these challenges. The resulting composite sensor has demonstrated remarkable performance, attributable to, increased active sites from defects induced by WO3, and the presence of a p-n heterojunction. Notably, electron transfer from WO3 to Ti3C2Tx has been identified as a critical contributing factor to the enhanced characteristics of these nanohybrids, as evidenced by observed alterations in binding energies for the Ti 2p and W 4f core levels. These findings have expanded our understanding of the electrical interactions in Ti3C2/WO3 and their potential applications in diverse domains. The pristine MXene sensor exhibited a response of 59.39% at 300 ppm at room temperature, while the composite MXene/γ-WO3 displayed a response of 92.3%. Additionally, the response recovery time was recorded at 22seconds and 9seconds, respectively. The chemiresistive ammonia sensor's performance was evaluated across a range of concentrations (25–300 ppm) and relative humidity levels (11–94% RH). The composite sensor has demonstrated reproducibility and long-term stability, coupled with high sensitivity to NH3. This manuscript aims to highlight the enhanced ammonia sensing properties and selectivity of MXene/γ-WO3 composite materials.

Spin asymmetry of O 2p –related states in SrTiO3(001)

Atomically clean SrTiO3(001) is characterized by low energy electron diffraction, core level and valence band photoelectron spectroscopy, the latter also with spin resolution. Samples prepared by a sputtering-annealing procedure exhibited in-gap states in the valence band spectra, Ti3+ components in Ti 2p core level spectra and a noticeable spin asymmetry in the 3–9 eV binding energy range, which corresponds to valence states of mainly O 2p character. Upon annealing in oxygen, the spin asymmetry vanishes, accompanied by the intensity decrease of the contribution of titanium low ionization states and of in-gap states, indicating that these three phenomena are mutually connected. The observed spin asymmetry may be generated by indirect exchange mediated by the in-gap states between O 2p orbitals, or by the partial Ti 3d character of these states, which acquire non-zero spin in case of incomplete oxygen coordination.

La3+ doped CaCu3Ti4O12 ceramics: Investigation of its microstructural and dielectric properties against varying sintering durations

This article reports the relationship between microstructure and dielectric properties of the title material were prepared through combination of solid-state and sol–gel methods and then sintered by varying durations. XRD indicates Im-3 space group with a body centred cubic structure, and Rietveld refinement analysis showed good fits for all samples, with R-factors below 10 % and GoF less than 2. The XPS analysis revealed the existence of Ca 2p, La 3d, Cu 2p, Ti 2p, O 1s, and C 1s retained their correct oxidation states. SEM results show grain sizes of 1.10 (±0.25) μm, 1.24 (±0.10) μm, and 1.75 (±0.31) μm for 8, 12, and 16 h of sintering, respectively. EDS analysis confirms the presence of Ca, Cu, Ti, La, and O, indicating the purity of the ceramics. As the sintering duration increases, grain enlargement consistently leads to higher εr values with very low tanδ. At a low frequency of 50 Hz, εr increases with sintering duration, reaching values of 5.65 × 103, 5.90 × 103 and 9.94 × 103. Meanwhile, tanδ remains significantly low (<0.1) at 1 kHz. The Nyquist plots demonstrate deviation from Debye type relaxation with NTCR behaviour. The frequency-dependent AC conductivity follows Jonscher’s power law, with activation energy values of 0.44 eV, 0.46 eV, and 0.49 eV for ceramics sintered for 8, 12, and 16 h, respectively.

Realization of Atomically Uniform ALD-Al2O3 and TiO2 on SiO2 and GeO2 by UV-Ozone Treatment

Recently, Interface Dipole Modulation (IDM) has been observed in amorphous HfO2/1-Monolayer (ML) TiOx/SiO2 stack structures [1]. To fabricate IDM devices, the thickness of the modulation layer, which is at the atomic layer level, must be precisely controlled to produce a uniform film with high-quality. Therefore, Atomic Layer Deposition (ALD) is one of the promising methods. However, it is difficult to form very thin oxide films on thermally oxidized SiO2/Si or GeO2/Ge by ALD because the oxide film surface is chemically inert. In fact, deposition delays, which is called incubation cycles, and island growth have been observed in the 0-20 cycle range for Al2O3 with Trimethylaluminum (TMA) and H2O as precursors [2, 3]. This is thought to depend on the time it takes for -OH bonds to form on the surface, so surface modification with low damage is necessary. Several surface modification methods exist, especially UV-Ozone (UVO) treatment [4], which has been reported to modify the surface of oxide films such as SiO2 and Al2O3 and has the advantage of being environmentally harmless and compatible with CMOS processes. In this study, the effect of surface modification of insulating films by the UVO method on ALD deposition was investigated for the purpose of controlling atomic layer thickness and producing a uniform oxide film.The samples for the measurement were prepared by the following procedure. First, the chemically cleaned Si or Ge substrate was thermally oxidized to form a ~10 nm thick oxide film. Next, after 15 min of UVO treatment in air at room temperature, the oxide films of Al2O3 or TiO2 were deposited using ALD method on two kinds of substrate. The precursor used for Al2O3 deposition and for TiO2 deposition were TMA and Tetrakis(dimethylamido)titanium (TDMAT), respectively. The oxidant is H2O. The samples were evaluated by Atomic Force Microscopy (AFM), Spectroscopic Ellipsometry (SE) and X-ray Photoelectron Spectroscopy (XPS).Al 2p photoelectron spectra from Al2O3 films deposited after UVO treatment were measured by XPS to confirm the composition of Al2O3. As shown in Fig. 1(a), fitting is possible with two peaks of Al 2p 3/2 and 2p 1/2, which can be said to be stoichiometric Al2O3. Next, Fig. 1(b) shows the photoelectron spectrum of the TiO2 film used as the modulation layer in the IDM device. The Ti 2p spectra, as same as the Al 2p spectra, can be fitted with two peaks, and the photoelectron intensity increases almost in proportion to the number of cycles. The thickness of the ALD film was then calculated from the photoelectron intensity ratio from the Al2O3, TiO2 and the lower oxide film. As shown in Fig. 1(c), there is a delay of one-cycle for the deposition of Al2O3 on SiO2, but it can be seen that the film thickness increases in proportion to the increase in the number of cycles. This is thought to have occurred by the following mechanism. First, UVO treatment breaks the Si-O or Ge-O bonds on SiO2 or GeO2 surface. This dangling bond is the starting point for precursor formation during initial layer formation. Therefore, the H2O during the ALD process is thought to have formed a Si-OH or Ge-OH bond and improved reactivity with methyl groups. The growth per cycle (GPC) can be calculated to be about 0.10 nm/cycle, which is in close agreement with the literature value [5]. Furthermore, SE measurements of Al2O3 films deposited on SiO2 at 50 and 200 cycles yielded thicknesses of 4.83 and 19.97 nm, respectively, which were in close agreement with the GPC calculated using XPS. The TiO2 film deposited on SiO2 also experienced a one-cycle delay similar to that of the Al2O3 film, and the GPC could be calculated to be 0.046 nm/cycle. Furthermore, as shown in Fig. 1(d), UVO treatment increased the average film thickness and reduced the surface roughness from the AFM images. These results indicate that UVO treatment is effective for surface modification of SiO2 and GeO2, and it can be concluded that the ALD method can deposit uniform oxide films without delay.This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.[1] N. Miyata, Sci. Rep., 8, pp. 8486, 2018.[2] H. Fukumizu et al., Jpn. J. Appl. Phys. 59 016504, 2020.[3] E. Shigesawa et al., Semicond. Sci. Technol. 33 124020, 2018.[4] D. Guo et al., Chem. Phys. Lett., 429, 124–128, 2006.[5] A.W. Ott et al., Thin Solid Films, 292, pp. 135-144, 1997. Figure 1

(Invited) Role of Oxidant Gas for Atomic Layer Deposition of HfxZr1−XO2 Thin Films on Ferroelectricity of Metal-Ferroelectric-Metal Capacitors

Since the first report of ferroelectricity in HfxZr1−xO2 (HZO) in 2011 [1], a lot of attention has been devoted for future non-volatile memory device applications due to its scalability (< 10 nm), low thermal budget (< 400°C) and matured atomic layer deposition (ALD) technique [2]. The thermal ALD process using H2O or O3 is generally employed for the fabrication of ferroelectric HZO thin films. On the other hand, the effect of the plasma-enhanced ALD process using O2 plasma gas on the crystallinity and ferroelectricity has not been systematically clarified. In this paper, we report the importance of ALD oxidant gases for HZO films using H2O or O2 plasma on the ferroelectricity of metal–ferroelectric–metal (MFM) capacitors.MFM capacitors with the H2O- and O2 plasma-based HZO films were fabricated as follows. A 10-nm-thick HZO films were deposited on a TiN bottom-electrode (BE-TiN) by ALD at 300°C using H2O or O2 plasma as oxidant gases and (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor. The Hf/Zr ratios in both types of HZO films were estimated to be 0.4/0.6 using X-ray photoelectron spectroscopy (XPS). Next, a post-deposition annealing (PDA) process was carried out at 400 °C for 1 min under a N2 atmosphere. Finally, a TiN top-electrode was deposited by DC sputtering.First, the crystallinity and remanent polarization (2P r = P r + − P r −) of the H2O- and O2 plasma-based HZO films were investigated. Nanocrystals with ferroelectric orthorhombic (O), tetragonal (T), cubic (C) phases partially existed in the O2 plasma-based film after the ALD process, while an amorphous structure dominantly existed in the H2O-based film, analyzed by the grazing-incidence X-ray diffraction analysis and cross-sectional transmission electron microscopy images [3]. This could be because of a strong oxidation power and high energy ion/electron bombardment of O2 plasma. After the PDA process, the O phase was formed more in the O2 plasma-based HZO film than the H2O-based film. Therefore, the nanocrystal grains in the as-grown O2 plasma-based HZO film could play an important role as nuclei for the crystallization and formation of O phase [3,4]. From the polarization–electric field hysteresis curves, the PDA-treated O2 plasma-based capacitor exhibited 2P r of 20 µC/cm2, which was 1.5 times higher than that (13 µC/cm2) of the H2O-based capacitor. Thus, these results indicate that the crystallization and O phase formation of HZO films were promoted by using O2 plasma gas as an ALD oxidant, resulted in a higher 2P r.Next, endurance properties of these MFM capacitors were evaluated. In the wake-up state, both capacitors showed almost the same increase rate (~8%) of switching polarization (P sw). In the fatigued state, on the other hand, the O2 plasma-based capacitor exhibited less P sw degradation (~33%) after 106 cycles, whereas the P sw of H2O-based capacitor decreased by ~47%. To clarify these different fatigue properties, the TiOxNy interface reaction layer (IRL) formation at the HZO/BE-TiN interface was analyzed using synchrotron hard X-ray photoelectron spectroscopy (HAXPES). For the HAXPES Ti 2p spectra, the Ti-O peak intensity for the O2 plasma-based capacitor was larger than that of the H2O-based capacitor [5]. Therefore, an oxygen-rich TiON-IRL could be formed at the HZO/BE-TiN interface during ALD process due to the stronger oxidizing power of O2 plasma. Additional oxygen vacancies (VO) were reported to formed in HfO2-based films during field cycling due to the interface reaction between the HfO2-based film and TiN electrode. Moreover, one of the origins of the P sw degradation was thought to be domain pinning caused by VO [6]. Therefore, an oxygen-rich TiON-IRL for the MFM capacitor with the O2 plasma-based HZO film could prevent the interface reaction and formation of additional VO in HZO films, led to superior fatigue properties.In summary, the MFM capacitor with the O2 plasma-based HZO film showed superior 2Pr and fatigue resistance compared to those of the H2O-based one. Based on these experimental results, it is important to consider the selection of an ALD oxidant for fabrication of HZO-based MFM capacitors.This work was partly supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213). The HAXPES measurements were performed under approval of the NIMS Synchrotron X-ray Station (2020A4602 and 2020A4651).[1] J. Müller et al., Appl. Phys. Lett. 99, 112901 (2011).[2] J.-H. Kim et al., ACS Appl. Electron. Mater. 5, 4726 (2023).[3] T. Onaya et al., APL Mater. 10, 051110 (2022).[4] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019).[5] T. Onaya et al., Solid State Electron. 210, 108801 (2023).[6] M. Pešić et al., Adv. Funct. Mater. 26, 4601 (2016).

Influence of anti-ferromagnetic ordering and electron correlation on the electronic structure of MnTiO3

Electron correlation and long-range magnetic ordering have a significant impact on the electronic structure and physical properties of solids. Here, we investigate the electronic structure of ilmenite MnTiO3 using room temperature photoemission spectroscopy and theoretical approaches within density functional theory (DFT), DFT+ U and DFT+dynamical mean-field theory (DMFT). Mn 2p (Ti 2p) core level photoemission spectra, confirming Mn2+ (Ti4+) oxidation state, exhibit multiple satellites which are very similar to that of MnO (TiO2), suggesting similar strength of various interactions in this system. Valence band spectra collected at different photon energies suggest dominant Mn 3d character in the highest occupied band with a wide insulating gap. DFT(+ U) correctly predicts the experimentally observed anti-ferromagnetic (AFM) insulating ground state for MnTiO3 where the requirement of a large U to reproduce the experimental values of magnetic moment and band gap signifies the importance of electron correlation. Magnetically disordered paramagnetic (PM) phase could be well captured within DFT+DMFT, which provides an excellent agreement for the experimental band gap, paramagnetic moment, valence band spectra as well as dominant Mn 3d character in the highest occupied band. The calculated spectral function remains largely unaffected and exhibits sharper features in the magnetically ordered AFM phase. We show that the electronic structure of MnTiO3 in both the PM and AFM phases can be accurately described within DFT+DMFT.

Common anion rule in oxide heterointerfaces: Experimental verification by in situ photoemission spectroscopy

The band alignment at the interface is one of the fundamental parameters for designing electronic devices and artificial functional materials. However, there is no firmly established guideline for oxide heterostructures, limiting the functional design of oxide heterostructures. Here, we provide spectral evidence that the band diagram of oxide heterointerfaces is well described by the Zhong and Hansmann scheme based on the common anion rule [Z. Zhong and P. Hansmann, Phys. Rev. X 7, 011023 (2017)]. By utilizing the elemental selectivity of Ti 2p–3d resonant photoemission for the Ti 3d state near the Fermi level, we directly visualize the presence or absence of charge transfer from the overlayer films to SrTiO3 in prototypical heterointerfaces of SrVO3/SrTiO3 and SrNbO3/SrTiO3. It is found that the charge transfer occurs in SrNbO3/SrTiO3 but not in SrVO3/SrTiO3, as predicted by the Zhong and Hansmann scheme, indicating that the presence or absence, as well as the sign and amount, of interfacial charge transfer is predicted by this scheme. Our findings provide guidelines for designing and controlling the functionalities in oxide nanostructures.

Graphitic carbon nitride sheets sandwiched metal oxides: A novel platform for S-Scheme heterojunction generation for efficient photodegradation of volatile organics

This study reports the preparation of new kind of S-Scheme heterojunction (SSHJ) photocatalyst by distributing two semiconducting metal oxides (titanium dioxide and ZnO) onto the surface of graphitic carbon nitride (G-CN) two dimensional (2D) sheets. The composite photocatalysts, designated as T/Z@ GCN SSHJ, were prepared with different mass ratios of T,Z and G-CN.X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy were used to characterize the microstructure, morphology, chemical composition, electronic states, and optical properties of the T/Z@ GCN SSHJ composite. TEM image of the typical T/Z@ GCN SSHJ composite shows the presence of distribution of particles with sizes in the range 40–80 nm on the lamellar surface of G-CN inferring the formation of a heterostructure Z(0 D) /G-CN(2D)/T(0D) SSHJ composite. The comparison of the core level XPS spectrum of Ti 2p, Zn 2p, C1s, O1s and N1s of T/Z@ GCNSSHJ composite with the binary composites (T+G-CN and Z+G-CN) and pristine components clearly suggests that that Z, T and G-CN are kept together through van der Walls interactions. The T/Z@ GCN SSHJ composite photocatalyst with T: Z: G-CN mass ratio of 1.0: 0.80: 0.12 exhibited enhanced photocatalytic degradation (%) over binary and pristine single components for all the tested volatile organic compounds (VOCs), namely, acetaldehyde (ACD), ethyl acetate (EA), toluene (T) and benzaldehyde (BA). On comparing the % removal of VOCs after 20 minute of light irradiation, BA showed the highest removal % (94.3 %) and the others take the following order: ACD (70.9 %) >EA (56.45 %)> TL (40.12 %). The study explains the enhanced performance of T/Z@ G-CN HJ composite towards VOC photodegradation in terms of S-Scheme based charge transfer and internal electric field driven efficient charge separation. The charge transfer process of T/Z@ GCN HJ composite photocatalysts is discussed through S-Scheme HJ formation and supported with electronic states and band energy levels of the constituent components derived through the results of XPS and DRS analysis. This study gives insight into the design for regulating the hetero-interfaces for enhancing the photocatalysis and expected to further understand into charge transfer mechanism at multi-junction-based photocatalysts.

Structural, Morphological, and Electrochemical Properties of MXene/MoS2-based Supercapacitor

Two-dimensional (2D) materials, inclusive of molybdenum disulfide (MoS2), are auspicious due to their structure providing absorption sites and shorter diffusion paths. However, their sheet restacking affects the functional properties, resulting in the device’s low efficiency. A strategy is proposed to combine MoS2 with MXene material. X-ray Diffraction data revealed peaks at d-spacings of 6.07, 2.72, 2.49, 2.27, 1.82, and 1.53 Å. X-ray photoelectron spectroscopy shows prominent peaks for C 1s and Ti 2p, with no signal for Al detected in the pristine Ti3C2 MXene. This indicates successful etching of the Al layer. Electron Microscopes were used to confirm the samples’ morphology and showed that the hybrid sample has a layered flake-like structure with a small sphere attached to the surface. While the HR-TEM confirmed the layered structure of Ti3C2 after exfoliation from Ti3AlC2, consistent with prior studies. Prior, the electrochemical performances of the MXene/MoS2 supercapacitor in 6M KOH aqueous electrolyte were examined through cyclic voltammetry and galvanostatic charge–discharge. The highest specific capacitance reached was 139.71 Fg-1, attributed to heterostructures of an equally distributed MoS2 and MXene. Furthermore, the device retained about 83% of its initial capacitance after 10,000 cycles of cyclic stability testing. The results demonstrated that the MXene material improved the capacitive performance of MoS2, and conversely. Further enhancements can be expected as a result of a major omission in electrode synthesis, particularly the importance of delamination in MXene preparation.

X-ray photoelectron spectroscopy, structural, and radiation shielding properties for transparent borosilicate glasses

The current work investigates the impact of SrO content on the structural, physical, and shielding properties of xSrO-10TiO2–10SiO2-(65-x) B2O3, where (15 ≤ x ≤ 30) glasses. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and radiation shielding features were utilized to investigate the samples’ features. XRD results affirmed the amorphous behavior. The FTIR values presented many peaks attributed to SiO2 and B2O3. The Sr30B50 sample (3.048 g/cm3) showed the highest density, followed by Sr25B55 (2.936 g/cm3), Sr20B60 (2.720 g/cm3) and Sr15B65 (2.679 g/cm3). The XPS analysis of Ti 2p core level spectra reveals that Ti ions exist in both Ti4+ and Ti3+ oxidation states, with the content of Ti4+ increasing with the SrO ratio in the glass sample. The XPS analysis of O 1s core level spectra revealed that the number of non-bridging oxygen atoms is enhanced with an increase in SrO content, indicating the weakening of the glass structure. Furthermore, neutron and photon shielding properties were calculated theoretically using Phy-X software. The gamma shielding properties were enhanced with increasing SrO, while the neutron attenuation was equivalent to water. For example, the fast neutron removal cross-section (FNRCS) values are 0.101, 0.098, 0.102, and 0.101 cm−1 for Sr15B65, Sr20B60, Sr25B55, and Sr30B50, respectively. The findings suggest that the SrO improved radiation shielding properties while decreasing glass stability.

Unveiling the cation ratio mediated structural modifications in TiO2:GeO2 mixtures for gravitational-wave detectors

Amorphous thin films of Ti doped GeO2 are of interest for coatings of the mirrors in gravitational wave detectors (GWDs) due to their low internal friction (Vajente et al 2021 Phys. Rev. Lett. 127 071101). The addition of Ti to amorphous GeO2 (a-GeO2) enables tailoring of the optical and structural properties of the mixtures. However, the specific modifications that occur in the amorphous network with the addition of Ti are not known. In this work, x-ray photoelectron spectroscopy is used to identify modifications to the bonding of Ge and Ti atoms in mixtures of Ti doped a-GeO2 with different Ti cation content. The formation of (Ti–O–Ge) bonds is evidenced from: (1) the presence of a peak which intensity increases with Ti content and causes a shift to lower binding energy (BE) of the core level O 1s peak; (2) the shift to higher BE of the Ti 2p 3/2 peak and a decrease in the energy split; and (3) the shift to lower BE of the Ge 3d 5/2 peak and increase in the energy split. These changes reflect modifications to the bonding when Ge replaces Ti in Ti–O–Ti bonds and Ti replaces Ge in Ge–O–Ge bonds due to their difference in electronegativity. A decrease in the O–O nearest-neighbour distance due to the incorporation of Ti atom is also observed from the broadening of the valence band spectra. The results show the 0.44 Ti doped a-GeO2 mixture has a balance between the (Ti–O–Ge) and the (Ge–O–Ge) networks, not observed in Ti poor and Ti rich mixtures. This finding could have important consequences in the optimisation of amorphous Ti doped a-GeO2 mixtures for low internal friction coatings of GWDs.

The hydrophilicity mechanism of anatase and rutile (110) TiO2 films based on donor-acceptor complexes

When the surface of TiO2 is exposed to ultraviolet (UV) light, its hydrophilicity increases initially and then contact angle relaxation progresses over time. We investigated the source mechanism and working principles of long-lasting hydrophilicity in an ambient environment. Previously, we reported that the amount of donor-acceptor complexes (DACs) generated by polarizable OH groups with strong charge polarity is critical for maintaining long-lasting hydrophilicity (>90 days) on an anatase surface. In less reactive TiO2 films with low surface energy, a fast relaxation occurs due to the loss of polarizable OH groups. In this research, quantitative four-peak X-ray photoelectron spectroscopy analysis with TiO2 O 1 s spectra surface analysis was used to determine the relative amount of DACs and the binding energy shifting of water adsorbates on single crystal rutile (110) and polycrystalline anatase after UV exposure or plasma treatment. Films treated with oxygen or argon plasma and/or UV were analyzed and compared to suggested mechanisms that can occur on the surface of the TiO2 phase. All treated rutile (110) films except those treated with an Ar plasma exhibited rapid contact angle relaxation during the first 3 days, and the angle increased gradually to >50° over 14 days. In contrast, all anatase films retained long-lasting hydrophilicity, demonstrating a <∼10° contact angle over 56 days. At the first measurement, the binding energies were shifted for rutile (110) treated with O2 plasma and O2 plasma+UV films, demonstrating Ti–OH and DAC values of +0.15 eV each. However, the Ti–OH and DACs of all anatase films were located in the negative binding energy position, indicating that the DACs of rutile (110) were unstable due to a relatively weak bonding. In addition, the IOH/Ibulk intensity ratio of Ti–OH for rutile (110) treated with an O2 plasma increased significantly from 20 % to 37 %. However, the intensity ratio for O2 plasma+UV increased only from 20 % to 23.8 %. This result in combination with the binding energy shifting tendency indicates that relaxed OH groups increased rapidly without UV treatment. When rutile (110) was irradiated with UV, the effect of photoinduced electron-hole pairs shown with Ti 2p spectra was much shorter than that of anatase. Differences in relaxation occurred in both phases due to the charge polarity effect between rutile (110) and anatase, the unstable DACs related to weak OH groups, and the difference in surface characteristics. Using single crystal rutile (110) with a well-defined surface, these experiments validated further the DAC mechanism and theory presented by our group. Specifically, we confirmed that both the amount of DACs and the binding energy position shifting are important for long-term hydrophilicity.

Improved photocatalytic performance of TiO2/carbon photocatalysts: Role of carbon additive

A series of TiO2 – based photocatalysts have been prepared by the incorporation of 10 wt% of various carbon-based nanomaterials as modifying agents to titania. More specifically, commercial TiO2 P25 was modified through a wet impregnation approach with methanol with four different carbon nanostructures: single-walled carbon nanotubes (SWCNTs), partially reduced graphene oxide (prGO), graphite (GI), and graphitic carbon nitride (gCN). Characterization results (XPS and Raman) anticipate the occurrence of important interfacial phenomena, preferentially for samples TiO2/SWCNT and TiO2/prGO, with a binding energy displacement in the Ti 2p contribution of 1.35 eV and 1.54 eV, respectively. These findings could be associated with an improved electron-hole mobility at the carbon/oxide interface. Importantly, these two samples constitute the most promising photocatalysts for Rhodamine B (RhB) photodegradation, with nearly 100% conversion in less than 2 h. These promising results must be associated with intrinsic physicochemical changes at the formed heterojunction structure and the potential dual-role of the composites able to adsorb and degrade RhB simultaneously. Cyclability tests confirm the improved performance of the composites (e.g., TiO2/SWCNT, 100% degradation in 1 h) due to the combined adsorption/degradation ability, although the regeneration after several cycles is not complete due to partial blocking of the inner cavities in the carbon nanotubes by non-reacted RhB. Under these reaction conditions, Rhodamine-B xanthene dye degrades via the de-ethylation route.

Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

Computational and experimental investigation of the structure, morphology, optical and electrical properties of TiO2 and caffeine adsorbed TiO2 in methyl ammonium lead iodide

Effects of caffeine and titanium dioxide (TiO2) in methylammonium lead iodide (MAPbI3/CH3NH3PbI3) as a photon absorber are studied. XRD showed high crystallinity of TiO2:Caffeine:MAPbI3 with larger crystallite size of ∼3.9 nm compared to TiO2:MAPbI3 with ∼3.6 nm. Large grain size of ∼550 µm was obtained for TiO2:Caffeine:MAPbI3 compared to ∼256 µm of TiO2:MAPbI3. Rod-like structures were observed though TiO2:Caffeine:MAPbI3 morphology and was smoother compared to TiO2:MAPbI3. Reduced bandgap energy of ∼2.3 eV was observed from TiO2:Caffeine:MAPbI3 and PL quenching indicated the possibility of reduction of the electron-hole recombination. TiO2:Caffeine:MAPbI3 showed better electrical conductivity compared to TiO2:MAPbI3. Density functional theory calculations showed increased charge transfer/redistribution which accounts for the slight enhanced electrical conductivity for TiO2:Caffeine:MAPbI3. The density of state calculations suggests the formation of unoccupied states and O 2p, Ti 3p, N 2p and C 2p as major contributors to the electronic and electrical properties of TiO2:Caffeine.

The effect of different annealing temperatures on the mechanical and tribological properties of the Ti2AlN MAX phase films

Ti2AlN MAX phase films produced at different N2 flow rates and different annealing temperatures were deposited on 52100 steel substrates using the closed-field unbalanced magnetron sputtering technique. N2 flow rates were changed to 2, 3.5, and 5 sccm. Due to the increasing N2 flow rate, film thickness decreased from 2 to 1.3 µm. Then the produced films were subjected to annealing at temperatures of 700, 750, and 800°C. After the film produced at a flow rate of 2 sccm was annealed at 750°C, the presence of the Ti2AlN MAX phase was determined. The binding energies of Ti 2p, Al 2p, and N 1s determined in the XPS analysis confirmed the existence of the Ti2AlN film. The friction coefficient measured for Ti2AlN MAX phase films was calculated as 0.53, the hardness value as 22.8 GPa, the elastic modulus as 291.4 GPa, and the wear rate as 2.96 × 10−4 mm3/(N·m).

Fabrication of efficient natural dye-sensitized Solar Cells using Mediterranean olive leaves as natural dye sensitizer

The synthesis of synthetic dyes employed in dye-sensitized solar cells (DSSCs) is often associated with substantial costs and environmental concerns. As a result, there is growing interest in utilizing natural dyes as sensitizers for DSSCs. This paper presents a comprehensive investigation into the extraction of natural dyes from Mediterranean olive leaves using a methanol-based solution, with a particular focus on the impact of different drying times during the dye extraction process on the efficiency of fabricated solar cells. The extracted olive leaf dyes were systematically characterized, encompassing their optical properties analyzed through ultraviolet–visible (UV–Vis) and Fourier Transform Infrared (FTIR) spectroscopy, electrochemical behavior, photovoltaic performance, crystallographic and morphological structures, as well as elemental composition. The research findings unveiled distinctive attributes of the olive leaf dyes, attributable to variations in their charge transfer processes to the band edge of the TiO2 semiconductor. These outcomes were substantiated through comprehensive analyses involving UV–Vis, FTIR, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Notably, the olive leaf dye subjected to a longer drying time (referred to as OLDS1) exhibited an energy level of the lowest unoccupied molecular orbital (LUMO) closer to that of the conduction band of TiO2, facilitating efficient electron transfer. Furthermore, X-ray photoelectron spectroscopy revealed alterations in elemental composition upon dye adsorption onto the TiO2 surface, with a notable reduction in the intensities of the Ti 2p and O 1 s peaks. The OLDS1 dye demonstrated lower charge transfer resistance and recombination rates compared to the shorter drying time for both fluorine-doped tin oxide (FTO) and indium-doped tin oxide (ITO)-based natural DSSCs (NDSSCs). Importantly, the choice of substrate material, specifically FTO and ITO, significantly influenced the NDSSC performance. The FTO/TiO2/OLDS1 configuration exhibited remarkable characteristics, including a high fill factor (FF) of approximately 0.73 and an efficiency of 0.31 %, accompanied by a short-circuit current density of 0.662 mA/cm2 and an open-circuit voltage of 0.642 V. These results highlight the crucial role of drying time and substrate selection in the fabrication of NDSSCs utilizing Mediterranean olive leaf extracts, with the FTO/TiO2/OLDS1 configuration emerging as the most promising and efficient combination.

Plasma-assisted annealing of Pt-doped rutile TiO2 nanoparticles for enhanced decomposition and bacterial inactivation under general lighting

Enhanced photocatalytic activity of rutile-based TiO2 materials under general lighting is practically desired. O2 plasma-assisted annealing (PAA) effects on Pt-doped rutile TiO2 nanoparticles were clarified along with its visible-light-driven photocatalytic activity enhancement. The PAA-treated samples were mainly analyzed using optical spectroscopy and x-ray photoelectron spectroscopy (XPS). The photocatalytic activity was assessed by decomposing methylene blue dye and inactivating Bacillus subtilis under general lighting. The PAA treatment changed the O 1s, Ti 2p, and Pt 4f spectra of XPS from those of the pristine sample. This change indicated that the PAA treatment introduced more oxygen deficiency or oxygen vacancies and more oxygen groups adsorbed on the surface. The introduced oxygen vacancies and adsorbed oxygen groups would change the band structure, which primarily narrowed the bandgap energy or broadened the valence band edge, increased the number of electron-trapping sites from the shallow to midgap levels, and enhanced the upward band-bending at the surface. The PAA-induced change in the band structure enhanced the decomposition and bacterial inactivation because it facilitated the separation and concentration of photoexcited carriers. The findings provide a new perspective on enhancing the photocatalytic activities of rutile-based TiO2 nanoparticles under general lighting.

Characterization of photocatalytic hybrid TiO2–WOX thin films deposited via co-sputtering

Photocatalytic hybrids of titanium dioxide (TiO2) and tungsten oxide (WOX) can exhibit high photocatalytic activity under weak ultraviolet (UV) and visible-light irradiation. In this study, hybrid TiO2–WOX thin films deposited via co-sputtering are characterized using radiofrequency sputtering at various compositions, and their photocatalytic activity in the degradation of a methylene blue solution is evaluated. X-ray photoelectron spectroscopy measurements show enhanced W(4f) and decreased Ti(2p) signal intensities with increasing WOX content. Further, chemical shifts toward lower binding energies are observed as the compositions of TiO2 and WOX become equivalent. X-ray diffraction analysis reveal decreased anatase TiO2 peaks and monoclinic WOX peaks with the formation of amorphous structures as the compositions of TiO2 and WOX become equivalent. Atomic force microscopy images show homogeneous and flat structures on the surfaces of the hybrid TiO2–WOX thin films. The deposition rate decreases with decreasing WOX sputter power, whereas the optical bandgap decreases with increasing WO3 content. Furthermore, the photocatalytic activity under UV irradiation improves with increasing TiO2 content, whereas that under visible-light irradiation improves with increasing WOX content. This approach can provide the guidelines to fabricate the high-performance photocatalytic materials that can be used in different condensed thin-film devices.