Unravelling the surface structure of ß-Ga2O3 (100).

The present work is on a comprehensive surface atomic structure investigation of ß-Ga2O3 (100). The ß-Ga2O3 single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the ß-Ga2O3 surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the ß-Ga2O3 surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

Solar-based photocatalytic ozonation employing novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst: effect of pH, salinity, turbidity, and temperature on salicylic acid degradation.

This paper proposes the study of a solar-based photocatalytic ozonation process for the degradation of salicylic acid (SA) using a novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst. The incorporation of CuO and Cu2O aims to enhance charge mobility through the formation of p-n heterojunctions with ZnO, whereas the carbon xerogel (XC) was selected due to its eco-friendly nature, capacity to stabilize S-scheme heterojunctions as a solid-state electron mediator, and ability to function as a reducing agent under high temperatures. The characterization of the composites demonstrates that the presence of the XC during the calcination step led to the reduction of a fraction of the CuO into Cu2O, forming a ternary semiconductor heterojunction system. In terms of photocatalysis, the XC/ZnO-CuxO 5% composite achieved the best efficiency for salicylic acid degradation, mainly due to the stabilization of the S-scheme charge transfer pathway between the ZnO/CuO/Cu2O semiconductors by the XC. The total organic carbon (TOC) removal during heterogeneous photocatalysis was 80% for the solar-based process and 68% for the visible light process, after 300 min. The solar-based photocatalytic ozonation process was highly successful regarding the degradation of SA, achieving a 75% increase in the apparent reaction rate constant when compared to heterogeneous photocatalysis. Furthermore, a 78% TOC removal was achieved after 150 min, which is half the time required by the heterogeneous photocatalysis to obtain the same result. Temperature, salinity, and turbidity had major effects on the efficiency of the photocatalytic ozonation process; the system's pH did not cause any major performance variation, which holds relevance for industrial applications.

3D-printed electrochemical cells with laser engraving: developing portable electroanalytical devices for forensic applications.

A new electrochemical device fabricated by the combination of 3D printing manufacturing and laser-generated graphene sensors is presented. Cell and electrodes were 3D printed by the fused deposition modeling (FDM) technique employing acrylonitrile butadiene styrene filament (insulating material that composes the cell) and conductive filament (lab-made filament based on graphite dispersed into polylactic acid matrix) to obtain reference and auxiliary electrodes. Infrared-laser engraved graphene, also reported as laser-induced graphene (LIG), was produced by laser conversion of a polyimide substrate, which was assembled in the 3D-printed electrochemical cell that enables the analysis of low volumes (50-2000 µL). XPS analysis revealed the formation of nitrogen-doped graphene multilayers that resulted in excellent electrochemical sensing properties toward the detection of atropine (ATR), a substance that was found in beverages to facilitate sexual assault and other criminal acts. Linear range between 5 and 35 µmol L-1, detection limit of 1 µmol L-1, and adequate precision (RSD = 4.7%, n = 10) were achieved using differential-pulse voltammetry. The method was successfully applied to beverage samples with recovery values ranging from 80 to 105%. Interference studies in the presence of species commonly found in beverages confirmed satisfactory selectivity for ATR sensing. The devices proposed are useful portable analytical tools for on-site applications in the forensic scenario.

Combined computational and experimental study about the incorporation of phosphorus into the structure of graphene oxide.

Phosphorus-containing graphene-based hybrids are materials with outstanding properties for diverse applications. In this work, an easy route to produce phosphorus-graphene oxide hybrid materials is described, involving the use of variable amounts of H3PO4 and H2SO4 during the reaction of oxidation of a graphitic precursor. The physical and chemical features of the hybrids change significantly with the variation in the acid amounts used in the syntheses. XPS and solid-state 13C and 31P NMR results show that the hybrids contain large amounts of oxygen functional groups, with the phosphorus incorporation proceeding mostly through the formation of phosphate-like linkages and other functions with C-O-P bonds. The experimental findings are supported by DFT calculations, which allow the assessment of the energetics and the geometry of the interaction between phosphate groups and graphene-based models; these calculations are also used to predict the chemical shifts in the 31P and 13C NMR spectra of the models, which show good agreement with the experimentally observed solid-state NMR spectra.

X-ray photoelectron diffraction study of the approximant Al5Co2(001) quasicrystal.

The intermetallic Al5Co2 is defined as a structurally complex material and is considered a low-order quasicrystalline approximant. A single crystal of Al5Co2(001) was obtained by the Czochralski method. The sample was characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (PED). The surface composition was also analyzed by XPS, indicating only Al and Co compounds. In the current research, the crystal structure was qualitatively analyzed using the LEED patterns for different incident beam energies indicating a (1 × 1) termination, also in accordance with some literature works. The structure study was performed by applying the standard software MSCD and showed a (1 × 1) pattern. In addition, four different termination models for this termination were tested. The reliability factor indicated that the best termination belongs to the Al-rich surface layer.

3D-printed sensor decorated with nanomaterials by CO2 laser ablation and electrochemical treatment for non-enzymatic tyrosine detection.

The combination of CO2 laser ablation and electrochemical surface treatments is demonstrated to improve the electrochemical performance of carbon black/polylactic acid (CB/PLA) 3D-printed electrodes through the growth of flower-like Na2O nanostructures on their surface. Scanning electron microscopy images revealed that the combination of treatments ablated the electrode's polymeric layer, exposing a porous surface where Na2O flower-like nanostructures were formed. The electrochemical performance of the fabricated electrodes was measured by the reversibility of the ferri/ferrocyanide redox couple presenting a significantly improved performance compared with electrodes treated by only one of the steps. Electrodes treated by the combined method also showed a better electrochemical response for tyrosine oxidation. These electrodes were used as a non-enzymatic tyrosine sensor for quantification in human urine samples. Two fortified urine samples were analyzed, and the recovery values were 106 and 109%. The LOD and LOQ for tyrosine determination were 0.25 and 0.83 µmol L-1, respectively, demonstrating that the proposed devices are suitable sensors for analyses of biological samples, even at low analyte concentrations.

Kraft lignin-based carbon xerogel/zinc oxide composite for 4-chlorophenol solar-light photocatalytic degradation: effect of pH, salinity, and simultaneous Cr(VI) reduction.

Considering the ever-increasing need for efficient wastewater treatment, this study focused on the development of new kraft lignin-based carbon xerogel/zinc oxide (XCL/ZnO w) photocatalysts. The inclusion of the carbon xerogel is expected to cause an improvement in charge transfer throughout the photoactivation process, consequently enhancing its overall photocatalytic efficiency. Characterization shows that the materials developed are composed of both zinc oxide and carbon xerogel. The addition of the lignin-based carbon xerogel caused a significant morphological modification to the composite materials, resulting in a greater specific surface area. Regarding the photocatalytic efficiency, the optimized composite (XCL/ZnO 1.0) displayed superior efficiency to the pure zinc oxide, especially when calcined at 700 °C, with an increase of 20% in the overall photodegradation capacity for the 4-chlorophenol (4CP) molecule. The XCL/ZnO 1.0 also displayed better performance than its tannin counterpart, previously reported in the literature, obtaining a 60% increase in the apparent reaction rate constant. The XCL/ZnO 1.0 also displayed better performance for the simultaneous hexavalent chrome (Cr (VI)) reduction/4CP oxidation reaction. Salinity and system pH had a significant influence on the efficiency of the 4CP photodegradation, as higher values of salinity and lower pHs caused a decrease in the overall efficiency of the process. At last, chronoamperometry and open-circuit potential tests confirmed the superiority of the XCL/ZnO 1.0 over the pure ZnO, highlighting the beneficial impact of the carbon xerogel on the charge transport dynamics of the composite.

Additively manufactured carbon/black-integrated polylactic acid 3Dprintedsensor for simultaneous quantification of uric acid and zinc in sweat.

For the first time the development of an electrochemical method for simultaneous quantification of Zn2+ and uric acid (UA) in sweat is described using an electrochemically treated 3D-printed working electrode. Sweat analysis can provide important information about metabolites that are valuable indicators of biological processes. Improved performance of the 3D-printed electrode was achieved after electrochemical treatment of its surface in an alkaline medium. This treatment promotes the PLA removal (insulating layer) and exposes carbon black (CB) conductive sites. The pH and the square-wave anodic stripping voltammetry technique were carefully adjusted to optimize the method. The peaks for Zn2+ and UA were well-defined at around - 1.1 V and + 0.45 V (vs. CB/PLA pseudo-reference), respectively, using the treated surface under optimized conditions. The calibration curve showed a linear range of 1 to 70 µg L-1 and 1 to 70 µmol L-1 for Zn2+ and UA, respectively. Relative standard deviation values were estimated as 4.8% (n = 10, 30 µg L-1) and 6.1% (n = 10, 30 µmol L-1) for Zn2+ and UA, respectively. The detection limits for Zn2+ and UA were 0.10 µg L-1 and 0.28 µmol L-1, respectively. Both species were determined simultaneously in real sweat samples, and the achieved recovery percentages were between 95 and 106% for Zn2+ and 82 and 108% for UA.

Selecting 'convenient observers' to probe the atomic structure of CVD graphene on Ir(111) via photoelectron diffraction.

CVD graphene grown on metallic substrates presents, in several cases, a long-range periodic structure due to a lattice mismatch between the graphene and the substrate. For instance, graphene grown on Ir(111), displays a corrugated supercell with distinct adsorption sites due to a variation of its local electronic structure. This type of surface reconstruction represents a challenging problem for a detailed atomic surface structure determination for experimental and theoretical techniques. In this work, we revisited the surface structure determination of graphene on Ir(111) by using the unique advantage of surface and chemical selectivity of synchrotron-based photoelectron diffraction. We take advantage of the Ir 4f photoemission surface state and use its diffraction signal as a probe to investigate the atomic arrangement of the graphene topping layer. We determine the average height and the overall corrugation of the graphene layer, which are respectively equal to 3.40 ± 0.11 Å and 0.45 ± 0.03 Å. Furthermore, we explore the graphene topography in the vicinity of its high-symmetry adsorption sites and show that the experimental data can be described by three reduced systems simplifying the moiré supercell multiple scattering analysis.

Metalation of 2HTCNPP on Ag(111) with Zn: Evidence for the Sitting atop Complex at Room Temperature.

We study the interaction and metalation reaction of a free base 5,10,15,20-terakis(4-cyanophenyl)porphyrin (2HTCNPP) with post-deposited Zn atoms and the targeted reaction product Zn-5,10,15,20-terakis(4-cyanophenyl)porphyrin (ZnTCNPP) on a Ag(111) surface. The investigations are performed with scanning tunneling microscopy at room temperature after Zn deposition and subsequent heating. The goal is to obtain further insights in the metalation reaction and the influence of the cyanogroups on this reaction. The interaction of 2HTCNPP with post-deposited Zn leads to the formation of three different 2D ordered island types that coexist on the surface. All contain a new species with a bright appearance, which increases with the amount of post-deposited Zn. We attribute this to metastable SAT ("sitting atop") complexes formed by Zn and the macrocycle, that is, an intermediate in the metalation reaction to ZnTCNPP, which occurs upon heating to 500 K. Interestingly, the activation barrier for the successive reaction of the SAT complex to the metalated ZnTCNPP species can also be overcome by a voltage pulse applied to the STM tip.

An assessment of biomedical CoCrMo alloy fabricated by direct metal laser sintering technique for implant applications.

CoCrMo alloys have been used for several decades in implantable devices due to their favourable mechanical properties, low wear rate in addition to good biocompatibility and high corrosion resistance. These alloys are conventionally produced via casting and/or forging route, however additive manufacturing techniques being recently employed in their fabrication. In this work, CoCrMo samples were produced by direct metal laser sintering additive manufacturing process. The microstructure and surface composition were examined employing scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The corrosion resistance was measured in 0.14 M sodium chloride solution and in phosphate buffered solution (PBS) both with and without addition of albumin at pH 7.4 and 37 °C. For this, potentiodynamic tests in addition to electrochemical impedance spectroscopy were employed. The studied CoCrMo alloy exhibits a good corrosion resistance in solutions tested being the highest in PBS solution without albumin addition. The XPS analysis showed that the passive film composition and its thickness are not modified by the adsorbed layer. Microstructural analysis revealed occurrence of strain-induced martensitic transformation.

Controlling the Self-Metalation Rate of Tetraphenylporphyrins on Cu(111) via Cyano Functionalization.

The reaction rate of the self-metalation of free-base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free-base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN-Cu-CN bonds, accompanied by self-metalation of the free-base porphyrins following a first-order rate equation. When comparing the non-cyano-functionalized and the tetracyano-functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron-withdrawing cyano groups and explain the experimentally observed effects.

"Inverted" porphyrins: a distorted adsorption geometry of free-base porphyrins on Cu(111).

Based on density functional theory calculations combined with experimental results, we report and discuss an extremely distorted, "inverted" adsorption geometry of free-base tetraphenylporphyrin on Cu(111). The current findings yield new insights into a well-studied system, shedding light on the peculiar molecule-substrate interaction and the resulting intramolecular conformation.

Self-assembly of NiTPP on Cu(111): a transition from disordered 1D wires to 2D chiral domains.

The growth and self-assembling properties of nickel-tetraphenyl porphyrins (NiTPP) on the Cu(111) surface are analysed via scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). For low coverage, STM results show that NiTPP molecules diffuse on the terrace until they reach the step edge of the copper surface forming a 1D system with disordered orientation along the step edges. The nucleation process into a 2D superstructure was observed to occur via the interaction of molecules attached to the already nucleated 1D structure, reorienting molecules. For monolayer range coverage a 2D nearly squared self-assembled array with the emergence of chiral domains was observed. The XPS results of the Ni 2p(3/2) core levels exhibit a 2.6 eV chemical shift between the mono- and multilayer configuration of NiTPP. DFT calculations show that the observed chemical shifts of Ni 2p(3/2) occur due to the interaction of 3d orbitals of Ni with the Cu(111) substrate.

The atomic structure of a bare buffer layer on SiC(0001) chemically resolved.

A chemical-specific photoelectron diffraction structure determination of a carbon rich buffer layer on SiC is reported. In addition to the long-range ripple of this surface, a local buckling in the hexagonal sublattice, which breaks the local range order symmetry, was unraveled.

Electronic and structural study of Pt-modified Au vicinal surfaces: a model system for Pt-Au catalysts.

Two single crystalline surfaces of Au vicinal to the (111) plane were modified with Pt and studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultra-high vacuum environment. The vicinal surfaces studied are Au(332) and Au(887) and different Pt coverage (θPt) were deposited on each surface. From STM images we determine that Pt deposits on both surfaces as nanoislands with heights ranging from 1 ML to 3 ML depending on θPt. On both surfaces the early growth of Pt ad-islands occurs at the lower part of the step edge, with Pt ad-atoms being incorporated into the steps in some cases. XPS results indicate that partial alloying of Pt occurs at the interface at room temperature and at all coverage, as suggested by the negative chemical shift of Pt 4f core line, indicating an upward shift of the d-band center of the alloyed Pt. Also, the existence of a segregated Pt phase especially at higher coverage is detected by XPS. Sample annealing indicates that the temperature rise promotes a further incorporation of Pt atoms into the Au substrate as supported by STM and XPS results. Additionally, the catalytic activity of different PtAu systems reported in the literature for some electrochemical reactions is discussed considering our findings.