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Herein, we report a feasible method for forming barrel-like hybrid Cu(OH)2-ZnO structures on α-brass substrate via low-potential electro-oxidation in 1 M NaOH solution. The presented study was conducted to investigate the electrochemical behavior of CuZn in a passive range (−0.2 V–0.5 V) and its morphological changes that occur under these conditions. As found, morphology and phase composition of the grown layer strongly depend on the applied potential, and those material characteristics can be tuned by varying the operating conditions. To the best of our knowledge, the yielded morphology of barrel-like structure has not been previously observed for brass anodizing. Additionally, photoactivity under both UV and daylight irradiation-induced degradation of organic dye (methyl orange) using Cu(OH)2-ZnO composite was explored. Obtained results proved photocatalytic activity of the material that led to degradation of 43% and 36% of the compound in UV and visible light, respectively. The role of Cu(OH)2 in improving ZnO photoactivity was recognized and discussed. As implied by both the undertaken research and the literature on the subject, cupric hydroxide can act as a trap for photoexcited electrons, and thus contributes to stabilizing electron-hole recombination. This resulted in improved light-absorbing properties of the photoactive component, ZnO.

Developing ultraefficient electrocatalytic materials for the hydrogen evolution reaction (HER) with low content of expensive platinum group metals (PGMs) via low-energy-input procedures is the key to the successful commercialization of green water electrolysis technologies for sustainable production of high-purity hydrogen. In this study, we report a facile room-temperature synthesis of ultrafine metallic Ir nanoparticles on conductive, proton-intercalated TiO2 nanotube (H-TNT) arrays via galvanic displacement. A series of experiments demonstrate that a controlled transformation of the H-TNT surface microstructure from neat open-top tubes to disordered nanostripe bundles (“nanograss”) is highly beneficial for providing an abundance of exposed Ir active sites. Consequently, for nanograss-engineered composites, outstanding HER activity metrics are achieved even at very low Ir(III) precursor concentrations. An optimum Ir@TNT cathode loaded with 5.7 μgIr cm−2 exhibits an overpotential of −63 mV at −100 mA cm−2 and a mass activity of 34 A mgIr−1 at −80 mV under acidic conditions, along with excellent catalytic durability and structural integrity. Density functional theory (DFT) simulations reveal that the hydrogen-rich TiO2 surface not only stabilizes the deposited Ir and weakens its H binding strength to a moderate intensity, but also actively takes part in the HER mechanism by refreshing the Ir catalytic sites near the Ir|H–TiO2 interface, thus substantially promoting H2 generation. The comprehensive characterization combined with theory provides an in-depth understanding of the electrocatalytic behavior of H-TNT supported PGM nanoparticles and demonstrates their high potential as competitive electrocatalyst systems for the HER.

TiO2 coatings modified by Bi2O3 particles were successfully formed by plasma electrolytic oxidation (PEO) process of titanium in supporting electrolyte (10 g/L Na3PO4·10H2O) with the addition of Bi2O3 particles. The morphology, crystal structure, chemical composition, and optical properties of formed PEO coatings were investigated by scanning electron microscopy, energy-dispersive spectroscopy, X-ray fluorescence, X-ray diffraction, X-ray photoelectron spectroscopy, UV–Vis diffuse reflectance spectroscopy, and photoluminescent techniques. The photodegradation of methyl orange under simulated sunlight condition was used to determine the potential of TiO2/Bi2O3 coatings in photocatalysis. The photocatalytic activity (PA) of TiO2/Bi2O3 coatings is higher than that of pure TiO2. PA of TiO2/Bi2O3 coatings is highly dependent on the amount of Bi2O3 particles in the supporting electrolyte and the highest PA was observed for TiO2/Bi2O3 coating formed in supporting electrolyte with the addition of 1.2 g/L of Bi2O3 particles. Enhanced PA is related to the reduction of photo-generated electron/hole recombination rate as a result of TiO2 and Bi2O3 coupling.

MgAl and MgAl/ZnO coatings were prepared by plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy in aluminate electrolyte (5 g/L NaAlO2) without and with addition of ZnO particles in various concentrations. The MgAl coating was partially crystallized and contained MgO and MgAl2O4 phases. The addition of ZnO particles to aluminate electrolyte had no significant effect on the surface morphology of formed coatings, while the Zn content increased with ZnO particle concentrations. X-ray diffraction confirmed the incorporation of ZnO particles in the coatings. The photodegradation of methyl orange (10 cm3 of 8 mg/L) was used to measure the photocatalytic activity (PA) of MgAl and MgAl/ZnO coatings. The PA of MgAl coating after 8 h of irradiation was around 58%, while the PA of MgAl/ZnO coatings formed in aluminate electrolyte with the addition of ZnO particles in concentrations of 4 g/L, 8 g/L, and 12 g/L were around 69%, 86%, and 97%, respectively.

The effect of selected transition metals (Mn, Ni, Co) modified TiO2 coatings formed by plasma electrolytic oxidation of titanium on their surface morphology, phase and chemical composition, optical absorption, and photocatalytic degradation of methyl orange (MO) was investigated. Coatings were formed in an alkaline electrolyte with varying concentrations of MnO, NiO, or Co3O4 powders. The morphology, thickness and phase structure of all formed coatings are nearly identical and independent of the content of transition metal ions embedded in TiO2. The X-ray diffraction and X-ray photoelectron spectroscopy results indicated that anatase TiO2 coatings were loaded with MnO, NiO, or Co3O4 from electrolytes. When up to 2.0 g/L of MnO, NiO, or Co3O4 is added to the electrolyte, the photocatalytic activity (PA) of the forming coatings is higher than for pure TiO2. The coatings formed in electrolyte with the addition of 0.75 g/L MnO, 1.5 g/L NiO, or 2.0 g/L Co3O4 had the highest PA. Increased PA is associated with a decrease in photogenerated electron/hole recombination rate, according to UV–Vis diffuse reflectance spectroscopy and photoluminescence measurements.

The development of PEO surface treatments for Nb (niobium) provides a possibility to produce functionalized coatings with a combination of new compositions and phases. In the frame of this work, PEO coatings on pure Nb are produced in three electrolytes containing aluminate, phosphate and silicate. Furthermore, the influence of the electrolyte composition on the PEO process and the microstructure, composition and properties of the coatings were studied. It was observed that most of the coating forming species did not participate in the plasma reactions directly, while the chemical-, electrochemical- and thermal stimulated reactions are dominating the coating formation. The results show that all the coatings are mainly composed of the Nb$_2$O$_5$ phase. An additional mixture of oxides (Al$_2$O$_3$ and AlNbO$_4$) is found in the coating formed in aluminate-containing electrolyte, and the coating formed in phosphate-containing electrolyte contains a small amount of Nb$_2$(PO$_4$)$_3$ and amorphous phosphate. No additional crystalline phase other than the amorphous silica phase is detected in the coating formed in silicate-containing electrolyte. Additionally, different morphology of the coatings is revealed and the most uniform coating is produced in silicate-containing electrolyte, which further improves its corrosion resistance. This coating also shows the highest photocatalytic activity due to the combination of Nb2O5 and silica. Since all PEO coatings on pure Nb show superior corrosion resistance and photocatalytic activity, they might be interesting for a wide range of applications ranging from transportation, biomedical implants to environmental protection.

The synthesis of Eu3+-doped MgAl oxide coatings containing MgO and MgAl2O4 was accomplished through plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte with the addition of Eu2O3 particles in various concentrations. Their morphological, structural, and above all, photoluminescent (PL) and photocatalytic activity (PA) were thoroughly investigated. PL emission spectra of MgAl oxide coatings feature characteristic emission peaks, which are ascribed to f–f transitions of Eu3+ ions from excited level 5D0 to lower levels 7FJ. The charge transfer state of Eu3+ or direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold are both attributed to the PL peaks that appear in the excitation PL spectra of the obtained coatings. The fact that the transition 5D0 → 7F2 (electrical dipole transition) in Eu3+-doped MgAl oxide coatings is considerably stronger than the transition 5D0 → 7F1 (magnetic dipole transition) indicates that Eu3+ ions occupied sites with non-inversion symmetry. Because of oxygen vacancy formation, the Eu3+-doped MgAl oxide coatings had a higher PA in the degradation of methyl orange than the pure MgAl oxide coating. The highest PA was found in Eu3+-doped MgAl oxide coating formed in an aluminate electrolyte with 4 g/L of Eu2O3 particles. The PA, morphology, and phase of Eu3+-doped MgAl oxide coatings did not change after several consecutive runs, indicating outstanding stability of these photocatalysts.

In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water solution of sodium tungstate (Na2WO4∙2H2O) with the addition of the pure and Ce-loaded zeolites clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized with respect to their morphologies and chemical and phase compositions using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray diffraction. The prepared coatings contained γ-alumina, WO3, and metallic tungsten. The surface morphologies of the obtained coatings strongly depended on the PEO processing time, the roughness of all coatings increased with PEO time, while porosity decreased with PEO processing time as a result of microdischarge coalescence and growth. All coatings contained elements originating from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for 10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for various applications.

This paper discusses ZrO2:Ho3+ and ZrO2:Ho3+/Yb3+ coating formation by the plasma electrolytic oxidation (PEO) of zirconium in alkaline electrolyte containing Ho2O3 and Ho2O3/Yb2O3 particles, respectively. The morphology, chemical and phase composition, as well as down- and up-conversion photoluminescence (PL) of the coatings, were investigated. Down-conversion PL emission spectra of ZrO2:Ho3+ excited by 280 nm radiation are composed of broad PL band associated with ZrO2 host and bands corresponding to f–f transitions of Ho3+. The main PL emission bands of Ho3+ at around 540 nm and 550 nm are assigned to 5F4→5I8 and 5S2→5I8 transitions, respectively. The down-conversion PL intensity of ZrO2:Ho3+ excited by 450 nm radiation (transition 5I8→5G6) strongly depends on the concentration of Ho3+ incorporated into ZrO2 coatings. That is, it depends on the duration of the PEO process and concentration of Ho2O3 particles in electrolyte. PL intensity increases with rising Ho3+ concentration in ZrO2 coatings, reaching a maximum Ho:Zr ratio of about 18⋅10−3 and shows concentration quenching for higher values. Excitation of ZrO2:Ho3+/Yb3+ coatings by 980 nm laser diode results in the appearance of a strong green (5F4,5S2→5I8), weaker red (5F5→5I8), and NIR (5F4,5S2→5I7) up-conversion emissions. An increase of the PL intensity with increasing Yb3+ concentration indicates an efficient transfer of energy from excited Yb3+ ions to Ho3+ ions.

In the present work, down- and up-conversion photoluminescence properties of Er3+/Yb3+ doped ZrO2 coatings formed by plasma electrolytic oxidation of zirconium in electrolyte containing Er2O3 and Yb2O3 particles were investigated. Down-conversion PL analysis shows that emission spectra of ZrO2:Er3+/Yb3+ coatings excited with 280 nm radiation are composed of broad PL band related to ZrO2 host and bands assigned to f-f transitions of Er3+. The main PL emission bands of Er3+ at around 548 nm and 560 nm are related to 4S3/2→4I15/2 transition. PL excitation spectra monitored at 548 nm feature broad band in the region from 250 nm to 350 nm which is associated with the electron transfer transition from 2p orbital of O2− to 4f orbital of Er3+ and transitions of ZrO2. On the other hand, bands in PL excitation spectra ranging from 350 nm to 535 nm are related to 4f transitions of the Er3+ from the ground state 4I15/2 to higher energy levels. Down-conversion PL intensity decreases with increasing concentration of Yb3+ in coating due to energy transfer from Er3+ to Yb3+. ZrO2:Er3+/Yb3+ coatings show intense green (4S3/2→4I15/2) and red (4F9/2→4I15/2) up-conversion PL emission under the excitation with a 980 nm diode laser. With increasing Yb3+ concentration red up-conversion PL intensity increases more rapidly with respect to green emission, because red up-conversion PL intensity strongly depends on Yb3+ concentration, i.e. 4F9/2 state of Er3+ is directly excited by energy transfer from excited Yb3+.

Al2O3 coatings doped with Sm2+ and Eu2+ ions are synthesized using plasma electrolytic oxidation (PEO) process. Photoluminescence (PL) spectra of formed coatings excited by 260 nm are composed of broad PL bands associated with Eu2+ ions in Al2O3 host with a maximum at around 405 nm and sharp bands in red region corresponding to 5D0 → 7FJ (J = 0, 1, 2) transitions of Sm2+ ions. Comparison of the emission PL spectra of Sm2+, Eu2+ and Sm2+/Eu2+ doped Al2O3 shows that the emission PL intensity of Sm2+ ions in Al2O3 host is about one order of magnitude lower than that of Sm2+/Eu2+ doped Al2O3 indicating that Eu2+ is a very efficient sensitizer for Sm2+ PL enhancement. It is also evident that an addition of Sm2+ to Eu2+ doped Al2O3 coatings causes a decrease of the Eu2+ emission, indicating that non-radiative energy transfer takes place from Eu2+ ions to Sm2+ ions.

Plasma electrolytic oxidation (PEO) process of Al–Zn–Si alloy in water solution containing 4 g L-1 sodium metasilicate at constant current density of 400 mA cm−2 was investigated. The species present in PEO micro-discharges and their ionization stages were identified using optical emission spectroscopy technique. The obtained PEO spectrum consists of atomic/ionic lines originating from the elements present both in the substrate (Al, Zn) and the electrolyte (Na, O, H). Apart from atomic and ionic lines, AlO band at 484.2 nm was also detected. Plasma electron number density diagnostics was performed from the Hβ line shape. The electron temperature of 4000±400 K was estimated by measuring the relative line intensities of zinc atomic lines at 481.05 and 636.23 nm. In addition, surface morphology, chemical and phase composition of oxide coatings were investigated by SEM-EDS and XRD. Oxide coating morphology is strongly dependent of PEO time. The elemental components of PEO coatings are Al, Zn, O and Si. The oxide coatings are partly crystallized and mainly composed of gamma phase of Al2O3. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. 171035]

The preparation and properties of zeolite-containing oxide coatings obtained by plasma electrolytic oxidation are investigated and discussed. Pure and Ce-exchanged natural (clinoptilolite) and synthetic (13X) zeolites are immobilized on aluminum support from silicate-based electrolyte. Obtained coatings are characterized with respect to their morphology, phase and chemical composition, photocatalytic activity and anti-corrosion properties. It is observed that all mentioned properties of obtained coatings are dependent on processing time and type of immobilized zeolite. Coatings with Ce-exchanged zeolite show higher photocatalytic activity and more effective corrosion protection than those with pure zeolite. The highest photocatalytic activity is observed for coatings processed in a pulsed DC regime for 30 min containing Ce-exchanged 13X zeolite, followed by those containing Ce-exchanged clinoptilolite. Pronounced anti-corrosion properties feature almost all samples containing Ce-exchanged 13X zeolite.

The influence of various electrolysis parameters, such as the type of cathode, composition of the electrolyte and electrolysis time, on the morphology, structure and hardness of copper coatings has been investigated. Morphology and structure of the coatings were analyzed by scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray diffraction (XRD), while coating hardness was examined by Vickers microindentation test applying the Chicot–Lesage (C–L) composite hardness model. Depending on the conditions of electrolysis, two types of Cu coatings were obtained: fine-grained mat coatings with a strong (220) preferred orientation from the sulfate electrolyte and smooth mirror bright coatings with a strong (200) preferred orientation from the electrolyte with added leveling/brightening additives. The mat coatings showed larger both measured composite and calculated coating hardness than the mirror bright coatings, that can be explained by the phenomena on boundary among grains. Independent of electrolysis conditions, the critical relative indentation depth (RID) of 0.14 was established for all types of the Cu coatings, separating the zone in which the composite hardness can be equaled with the coating hardness and the zone requiring an application of the C–L model for a determination of the absolute hardness of the Cu coatings.

Recently the successful formation of PEO coatings on zinc alloy in a phosphate aluminate electrolyte was shown. The produced composite coatings contain various mixtures of ZnO and ZnAl2O4. In frame of the current study, the properties of the formed coatings including adhesion/cohesion, wear, corrosion and photocatalytic activity were analysed to identify possible applications. However, the coatings show internal porosity and a sponge-like structure. Thus the cohesion within the coating is quite low. Pull-off tests have demonstrated clear rupture within the PEO layer at strength values as low as 1 MPa. The photocatalytic activity is limited, in spite of the formation of a higher amount of ZnO at shorter treatment times. Interestingly, the composite coatings of ZnO and higher amounts of ZnAl2O4 spinel showed a higher activity, but not sufficient for fast and effective catalytic cleaning applications.

Copper electrodeposition on (111)-oriented Si substrate was performed by the pulsating current (PC) regime at various average current densities in the range of 15&ndash, 70 mA&middot, cm&minus, 2, obtained by varying either the frequency (30, 50, 80 and 100 Hz for the current density amplitude of 100 mA&middot, 2) or the current density amplitude (120 and 140 mA&middot, 2 at 100 Hz). The produced Cu coatings were examined by SEM, AFM and XRD techniques. The morphology of the coatings changed from those with large grains to fine-grained and globular, while the crystal structure changed from the strong (220) to the strong (111) preferred orientation by increasing the average current density. The mechanical characteristics of coatings were examined using Vickers micro-indentation tests, applying the Chicot&ndash, Lesage (C&ndash, L) composite hardness model for the analysis of microhardness. The maximum microhardness was obtained for the Cu coating produced at an average current density of 50 mA&middot, 2, with a current density amplitude of 100 mA&middot, 2 and a frequency of 100 Hz. This copper coating was fine-grained and showed the smallest roughness in relation to the other coatings, and it was obtained in the mixed activation&ndash, diffusion control between the end of the effect of the activation control and the beginning of the dominant effect of diffusion control.

This paper presents a new method for improvement of ultraviolet light driven photocatalytic activity of ZnO by immobilizing ZnO particles into Al2O3:Ce3+ coatings which emit intense radiation in the near ultraviolet region when excited by middle ultraviolet radiation. When Al2O3:Ce3+/ZnO coatings are illuminated by ultraviolet light in photocatalytic reactor, emitted ultraviolet radiation originating from Ce3+ ions synergistically participates in photocatalytic reactions on ZnO. Al2O3:Ce3+/ZnO coatings were formed by plasma electrolytic oxidation of aluminum in electrolyte containing 2 g/L of ZnO particles and up to 8 g/L of CeO2 particles. Structural, optical, photoluminescent, and photocatalytic properties of formed surface coatings were examined. All formed coatings feature virtually the same thickness, morphology, phase composition, and absorption properties. Concentration of incorporated Ce3+ ions into Al2O3 host and concentration of immobilized ZnO particles are the only parameters by which the coatings differ from each other resulting in difference in photocatalytic activity. Al2O3:Ce3+/ZnO coatings show better photodegradation of methyl orange with respect to Al2O3/ZnO coatings. The best photocatalytic activity is observed for the coating formed in electrolyte containing 4 g/L of CeO2 particles.

Anatase TiO2 coatings modified by Sb2O3 particles were formed by plasma electrolytic oxidation of titanium in 10 g/L Na3PO4⋅10H2O with addition of Sb2O3 particles. The photodegradation of methyl orange under simulated sunlight was used to probe the photocatalytic potential of TiO2/Sb2O3 coatings. The photocatalytic activity (PA) of obtained coatings is higher than pure TiO2 for adding up to 2.0 g/L of Sb2O3 particles to electrolyte. The highest PA was observed for coatings formed in electrolyte with the addition of 0.75 g/L Sb2O3. Increased PA is related to a decrease of photogenerated electron/hole recombination rate as a result of TiO2 and Sb2O3 coupling.

In this paper, Ce3+ doped Al2O3 coatings were prepared by plasma electrolytic oxidation (PEO) processing of aluminum in electrolyte which contained CeO2 particles. The morphology, chemical composition, crystal structure, and photoluminescent (PL) properties of obtained coatings were investigated. The excitation PL spectra of Al2O3:Ce3+coatings show broad band in the range from 250 nm to 340 nm, which can be attributed to the transitions from 4f ground state of Ce3+ ions to the field splitting levels of 5d state. The emission PL spectra, obtained under 285 nm excitation, feature broad 5d→4f emission band in the ultraviolet/visible spectral range with the maximum peak located at around 345 nm. PL intensity depends of the content of Ce3+ ions incorporated into PEO coatings, i.e. PEO time and concentration of CeO2 particles in electrolyte. In addition, Ce3+ sensitized PL of Tb3+ions in Al2O3:Ce3+/Tb3+ coatings formed by PEO was examined. The emission PL spectrum of Al2O3:Ce3+/Tb3+ coating exhibits two distinct regions: the first region is related to Ce3+ PL band and the second region features a series of characteristic peaks of Tb3+ ions corresponding to 5D3→7FJ (J = 4, 5, and 6) and 5D4→7FJ (J = 3, 4, 5, and 6) transitions. A strong green 5D4→7F5 emission of Tb3+ ions upon excitation of Ce3+ is initiated by efficient energy transfer from Ce3+ to Tb3+ ions.

Al2O3 coatings doped with Eu ions were prepared by plasma electrolytic oxidation (PEO) process in electrolyte containing Eu2O3 particles. The analysis of the photoluminescence (PL) spectra showed that Eu3+ ions were reduced to Eu2+ during PEO in Al2O3 host. The most intense PL was obtained under excitation of 260 nm. PL emission spectra consist of two distinct broad bands with corresponding maxima at around 410 nm and 500 nm related to 4 f65d1 → 4f7 electronic transitions of Eu2+ ions. Sharp emission band in orange-red region of Eu3+ ions attributed to f-f transitions 5D0 →7FJ were not observed in PL emission spectra. Two PL bands in emission spectra indicate existence of two different Eu2+ sites in Al2O3 host. Relative ratio of these two broad bands depends on excitation wavelength as well as concentration of Eu2+ ions incorporated into Al2O3 coatings.

CdS particles modified anatase TiO2 photocatalysts were formed on titanium substrate by plasma electrolytic oxidation for 2 min in supporting electrolyte (10 g/L Na-3 center dot PO4 center dot 12H(2)O) with addition of CdS particles in concentrations up to 8 g/L. Content of CdS particles incorporated into TiO2 coatings depends of CdS particles concentration in supporting electrolyte, while surface morphology, phase structure and absorption properties of formed coatings were not significantly influenced by the addition of CdS particles. In contrast to pure TiO2 coatings, TiO2/CdS coatings exhibit enhanced photocatalytic activity (PA) in the degradation of methyl orange, used as a model organic pollutant, under simulated solar irradiation. The highest PA was observed for TiO2/CdS coating formed in supporting electrolyte with addition of 0.4 g/L of CdS particles. Photoluminescence measurements indicate that enhanced PA is related to the reduction of the recombination rate of photogenerated electron/hole pairs as a result of TiO2 and CdS coupling. This is the peer-reviewed version of the article: Stojadinović, S., Tadić, N., Radić, N., Grbić, B.,& Vasilic, R. (2018). CdS particles modified TiO2 coatings formed by plasma electrolytic oxidation with enhanced photocatalytic activity. Surface and Coatings TechnologyElsevier., 344, 528-533. [https://doi.org/10.1016/j.surfcoat.2018.03.080] The published version: [https://cer.ihtm.bg.ac.rs/handle/123456789/2445]

Tb3+ doped ZrO2 coatings were formed on zirconium substrate by plasma electrolytic oxidation (PEO) in electrolyte containing Tb4O7 powder. The evolution morphology, chemical composition, and crystalline structure of formed coatings as a function of PEO time are probed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The concentration of Tb incorporated into coatings increases with the time of PEO processing. Obtained coatings are crystallized and composed of monoclinic and tetragonal phases of ZrO2. The ratio of monoclinic and tetragonal phases, as well as crystallite size, is controlled by PEO time and concentration of Tb in coatings. Photoluminescence (PL) emission spectra of Tb doped ZrO2 coatings feature two distinct regions. The first region is related to ZrO2 PL band with a maximum positioned at about 490 nm, while the second region features several sharp emission bands which can be attributed to f-f transitions of Tb3+ from excited level 5D4 to lower levels 7FJ (J = 2,3,4,5, and 6). PL excitation spectra of Tb doped ZrO2coatings are characterized by broad band region from 250 nm to 350 nm with a maximum at around 280 nm originating from 4f8 → 4f75d1 transition of Tb3+ ions. The evolution of PL emission spectra shows that with increasing PEO time, i.e. increasing concentration of Tb in coatings, sharp emission bands of Tb3+ increase, while PL intensity of broad emission band of ZrO2 host decreases. These results indicate the existence of energy transfer from ZrO2 host to Tb3+ dopant.

Noble metal-based materials are inevitable components of cathodes for the hydrogen evolution reaction (HER) in future water electrolysis systems for clean hydrogen fuel production. However, designing active and durable nanostructured catalysts with low amount of costly noble metals is still a great challenge. Herein, we show that Pd nanoparticles (NPs) can be synthesized on the highly developed surface of anodically grown TiO2 nanotube (TNT) arrays by applying a simple galvanic displacement strategy. In a two-step procedure, air-annealed TNT arrays are first cathodically protonated and then partially reoxidized by Pd(II) ions from a PdCl2 solution while providing a scaffold for the metallic Pd deposit. Structural and electrochemical characterizations reveal that the Pd content and the width of the Pd-populated zone of the tube walls are in correlation with the tube length. The Pd@TNT composites display remarkable HER activity in 1 M HClO4 delivering a current density of −10 mA cm−2 at an overpotential of −38 mV and a Tafel slope of only −13 mV/dec. More impressively, the mass and apparent activity of the Pd@TNTs is superior to even commercial Pt/C at higher current densities. The composites also show stable chronopotentiometric response over 25 h and a negligible HER overpotential increase after potential cycling tests. The exceptional performance of the Pd@TNT cathodes is assigned to the unique semiconducting properties of the three-dimensional, interactive TNT supporting structures that, on the one hand, provide abundance of Pd active sites with optimized atomic hydrogen binding energy for the cathodic HER, but on the other hand, prevent anodic degradation of the Pd catalyst.

Plasma electrolytic oxidation of zirconium in alkaline solution containing Er 2 O 3 powder was used for preparation of ZrO 2 :Er 3+ coatings. Photoluminescence (PL) emission spectra of ZrO 2 :Er 3+ excited by ultraviolet irradiation are composed of broad PL band associated with ZrO 2 host and sharp bands corresponding to f – f transitions of Er 3+ . The strongest green PL emission band of Er 3+ in the range from 540 nm to 580 nm is assigned to 4 S 3/2 → 4 I 15/2 transition. The PL excitation spectra of ZrO 2 :Er 3+ characterize broad band from 250 nm to 350 nm associated with charge transfer state of Er 3+ and the series of peaks in the range from 350 nm to 530 nm which are associated with 4 f transitions of the Er 3+ from ground state 4 I 15/2 to higher levels. Obtained results allowed the identification of down-conversion PL mechanism.

Plasma electrolytic oxidation was used for synthesis of Eu3+ doped monoclinic HfO2 coatings on hafnium substrate. Results of photoluminescence (PL) measurements show the existence of two distinct regions: one that is related to the blue emission originating from oxygen vacancy defects in HfO2 and the other one characterized with a series of sharp orange-red emission peaks related to f–f transitions of Eu3+ from excited level 5D0 to lower levels 7FJ (J = 0, 1, 2, 3, and 4). PL peaks appearing in excitation spectra of obtained coatings are attributed either to charge transfer state of Eu3+ or to direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold. PL of formed coatings increases with PEO time due to an increase of oxygen vacancy defects and the content of Eu3+. Acquired experimental data suggest that hypersensitive electrical dipole transition is much more intense than the magnetic dipole transition, indicating that Eu3+ ions occupy a non-inversion symmetry sites.

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of hafnium. Atoms ionized during the PEO micro-discharging were identified using optical emission spectroscopy. The spectral line shape analysis of the hydrogen Balmer line H β indicated the presence of two types of micro-discharges characterized by electron number densities of around 2.5·10 21 m − 3 and 1.3·10 22 m − 3 . Scanning electron microscopy and X-ray diffraction were employed to investigate surface morphology and phase composition of the PEO coatings obtained. The coatings were crystalline and composed of monoclinic HfO 2 . Diffuse reflectance spectroscopy has shown that HfO 2 coatings have a broad absorption band in the range from 200 nm to 400 nm. Optical band gap of HfO 2 coatings was around 5.4 eV, as estimated from absorption spectra. Photoluminescence measurements show that HfO 2 coatings have broad emission band in the visible region, with a maximum at around 480 nm. The highest photoluminescence was obtained for the excitation wavelength of 270 nm. Intensity of photoluminescence increased with PEO time and is related to an increase of oxygen vacancy defects in HfO 2 coatings formed during the process.

Sm doped Al 2 O 3 coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte with addition of various concentrations of Sm 2 O 3 particles. Photoluminescence (PL) emission spectra of Sm doped Al 2 O 3 coatings exhibit three distinct regions: the first region is related to Al 2 O 3 PL band with a maximum positioned at about 410 nm, the second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm 3+ ions from excited level 4 G 5/2 to lover levels 6 H J ( J = 5/2, 7/2, 9/2), while the third region in the range from 670 nm to 750 nm is characterized by sharp emission bands corresponding to 5 D 0 → 7 F J ( J = 0, 1, 2) transitions of Sm 2+ ions. PL excitation spectrum monitored at 410 nm exhibits maximum at about 260 nm, while PL excitation spectrum monitored at the wavelength of the most intense PL of Sm 3+ ions (transition 4 G 5/2 → 6 H 5/2 at 599 nm) can be divided into two regions. The broad band region from 240 nm to 300 nm with a maximum at about 250 nm is associated with the electron transfer transition from 2p orbital of O 2- ions to 4 f orbital of Sm 3+ ions, while the series of peaks ranging from 300 nm to 550 nm corresponds to direct excitation of the Sm 3+ ground state 6 H 5/2 into higher levels. PL excitation spectrum obtained by monitoring the 688 nm ( 5 D 0 → 7 F 0 ) emission from Sm 2+ ions shows three broad bands centered around 285 nm, 340 nm, and 480 nm, which arise from the 4f 6 → 4f 5 5d 1 transitions of Sm 2+ ions. Evolution of PL emission spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm 2+ , while prolonged PEO process leads to oxidation of Sm 2+ ions and their conversion to Sm 3+ ions.

TiO2/SnO2 coatings are formed by plasma electrolytic oxidation of titanium substrate in supporting electrolyte (10 g/L Na3PO4 center dot 12H(2)O) with addition of various concentrations of SnO2 particles. Photocatalytic activity (PA) of TiO2/SnO2 coatings was evaluated by measuring the degradation of methyl orange under simulated sunlight conditions. It was observed that PA of TiO2/SnO2 coatings strongly depends on the concentration of SnO2 particles in the electrolyte, i. e., the highest PA was observed with addition of 0.3 g/L of SnO2 particles to supporting electrolyte.

Pt-Ba-Al 2 O 3 coatings were obtained by spray pyrolysis deposition on stainless steel foil. The XRD results showed that the coatings consisted from Ba carbonate and Pt (fcc) phases. SEM micrographs demonstrated that the coatings are smooth and compact. The XPS data confirmed that Ba and Pt are in close contact. Pt crystallite size is only slightly influenced by the order of Pt and Ba deposition and the dispersion of Pt is in the narrow range from 28 to 33% for all samples. Catalyst component deposition order affects NOx storage capacity. Simultaneous deposition of Ba and Pt on already formed Al 2 O 3 substrate produces the most efficient catalyst. Pt deposited on top of the Ba layer results in slightly reduced NOx storage capacity, while deposition of Pt on the separated layer with Ba layer on top significantly reduces the ability for NOx storage.

Plasma electrolytic oxidation (PEO) process of Al–Zn–Si alloy in water solution containing 4 g L-1 sodium metasilicate at constant current den­sity of 400 mA cm-2 was investigated. The species present in PEO micro- -discharges and their ionization stages were identified using optical emission spectroscopy tech­nique. The obtained PEO spectrum consists of atomic/ionic lines originating from the elements present both in the substrate (Al, Zn) and the electrolyte (Na, O, H). Apart from atomic and ionic lines, AlO band at 484.2 nm was also det­ected. Plasma electron number density diagnostics was performed from the Hb line shape. The electron temperature of 4000±400 K was estimated by measuring the relative line intensities of zinc atomic lines at 481.05 and 636.23 nm. In addi­tion, surface morphology, chemical and phase composition of oxide coatings were investigated by SEM-EDS and XRD. Oxide coating morphology is strongly dependent of PEO time. The elemental components of PEO coatings are Al, Zn, O and Si. The oxide coatings are partly crystallized and mainly composed of gamma phase of Al2O3.

The growth of thin oxide coatings on the aluminum substrate in water-based sodium tungstate electrolyte by plasma electrolytic oxidation (PEO) is discussed and experimentally illustrated. The growth is carried out using a distinctive ultra-low duty cycle pulsed direct current (DC) power supply. During the PEO processing elements present in micro-discharges are identified using standard optical emission spectroscopy (OES) technique. The spectral line shape analysis of the first two hydrogen Balmer lines shows the presence of two types of micro-discharges. Obtained coatings are also characterized with respect to their morphology and chemical and phase composition. It is shown that coatings are composed of Al, O, and W, featuring low roughness and porosity. Partial crystallization of the coatings resulted in identification of WO3, W3O8, and γ-Al2O3 crystalline phases.

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of zinc foil at constant potential of 480 V in a mixed solution of Na 2 SiO 3 ⋅ 5H 2 O and KOH. Optical emission spectroscopy technique was used to obtain electron temperature and electron number density in microdischarges. The spectral line shape analysis of hydrogen H β line indicates the presence of two types of microdischarges during PEO characterized by electron number densities of around 1.6 × 10 21 m − 3 and 2.2 × 10 22 m − 3 . The electron temperature of (3600 ± 300) K was estimated by measuring the relative line intensities of zinc atomic lines at 334.50 nm and 472.21 nm. Structural and optical properties of films formed by PEO were probed by scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction, diffuse reflectance spectroscopy and photoluminescence. The main constituents of ZnO films are Zn, Si and O, with clearly observed presence of wurtzite type ZnO structure. Photoluminescence measurements show that ZnO films have broad emission band in the visible region. With increasing excitation wavelength the maximum of PL emission spectra shifts to longer wavelengths, indicating the existence of various defects in obtained coatings (single and double ionized oxygen vacancies, zinc vacancies and oxygen interstitial defects).

Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings have been successfully deposited by plasma electrolytic oxidation of niobium in 0.1 M phosphoric acid containing either Eu 2 O 3 or Sm 2 O 3 powder. Surface morphology and chemical composition of obtained coatings are correlated to the mechanism of oxide growth during the PEO process. X-ray photoelectron spectroscopy revealed that Nb and P in the oxide coatings are in pentavalent, while Eu and Sm are in trivalent oxidation state. Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ oxide coatings are found to be in low crystallinity pseudohexagonal Nb 2 O 5 form. Photoluminescent emission spectra of either type of coatings are characterized by sharp emission bands in orange-red region ascribed to f – f transitions of Eu 3+ ions from excited level 5 D 0 to lower levels 7 F J (J = 0, 1, 2, 3, and 4) and Sm 3+ ions from excited level 4 G 5/2 to the lower levels 6 H J (J = 5/2, 7/2, 9/2, and 11/2). It is observed that the intensity of peaks in excitation and emission photoluminescent spectra increases with the concentration of Eu 3+ and Sm 3+ , i.e. time of PEO, but the peak positions remain virtually unchanged. The fact that in Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings electrical dipole transition is much more intense than magnetic dipole transition indicates that Eu 3 + and Sm 3 + ions occupied non-inversion symmetry sites in Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings, respectively.

Eu3+ doped ZrO2 coatings were formed by plasma electrolytic oxidation of zirconium in electrolyte containing Eu2O3 powder. The content of Eu in coatings increases with the time of PEO process. The coatings are crystallized and mostly composed of monoclinic ZrO2 phase. Photoluminescence (PL) emission spectra of Eu3+ doped ZrO2 coatings feature two distinct regions. The first region is related to ZrO2 PL band with a maximum positioned at about 490 nm, while the second region in the orange–red part of the spectrum shows sharp emission bands which are related to f–f transitions of Eu3+ from excited level 5D0 to lower levels 7FJ (J=0, 1, 2, 3, and 4). Excitation PL spectra of Eu3+ doped ZrO2 coatings monitored at 480 nm feature a broad PL band with spectral maximum at about 280 nm. In contrast, the excitation PL spectra monitored at the wavelength of the most intense peak in emission PL spectra can be divided into two regions: (a) the broad band region from 240 nm to 350 nm with a maximum at about 250 nm, which is associated with the electron transfer transition from 2p orbital of O2− ions to 4f orbital of Eu3+ ions and (b) the region in the range from 350 nm to 550 nm with the series of sharp peaks, which correspond to direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold.

Tungsten doped Al2O3 /ZnO coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte (0.1 M boric acid + 0.05 M borax + 2 g/L ZnO) with addition of different concentrations of Na2WO4 center dot 2H(2)O. The morphology, crystal structure, chemical composition, and light absorption characteristics of formed surface coatings are investigated. The X-ray diffraction and X-ray photoelectron spectroscopy results indicate that formed surface coatings consist of alpha and gamma phase of A(2)O(3), ZnO, metallic tungsten and WO3. Obtained results showed that incorporated tungsten does not have any influence on the absorption spectra of Al2O3/ZnO coatings, which showed invariable band edge at about 385 nm. The photocatalytic activity of undoped and tungsten doped Al2O3 /ZnO coatings is estimated by the photodegradation of methyl orange. The photocatalytic activity of tungsten doped A(2)O(3)/ZnO coatings is higher thanof undoped Al2O3 /ZnO coatings; the best photocatalytic activity is ascribed to coatings formed in supporting electrolyte with addition of 0.3 g/L Na2WO4 center dot 2H(2)O. Tungsten in A(2)O(3)/ZnO coatings acts as a charge trap, thus reducing the recombination rate of photogenerated electron-hole pairs. The results of PL measurements are in agreement with photocatalytic activity. Declining PL intensity corresponds to increasing photocatalytic activity of the coatings, indicating slower recombination of electron-hole pairs.

In this paper, we used plasma electrolytic oxidation (PEO) of titanium in water solution containing 10 g/L Na3PO4 center dot 12H(2)O + 2 g/L Eu2O3 powder for preparation of TiO2:Eu3+ coatings. The surfaces of obtained coatings exhibit a typical PEO porous structure. The energy dispersive X-ray spectroscopy analysis showed that the coatings are mainly composed of Ti, O, P, and Eu; it is observed that Eu content in the coatings increases with PEO time. The X-ray diffraction analysis indicated that the coatings are crystallized and composed of anatase and rutile TiO2 phases, with anatase being the dominant one. X-ray photoelectron spectroscopy revealed that Ti 2p spin-orbit components of TiO2:Eu3+ coatings are shifted towards higher binding energy, with respect to pure TiO2 coatings, suggesting that Eu3+ ions are incorporated into TiO2 lattice. Diffuse reflectance spectroscopy showed that TiO2:Eu3+ coatings exhibit evident red shift with respect to the pure TiO2 coatings. Photoluminescence (PL) emission spectra of TiO2:Eu3+ coatings are characterized by sharp emission bands in orange-red region ascribed to f-f transitions of Eu3+ ions from excited level D-5(0) to lower levels F-7(J) (J=0, 1, 2, 3, and 4). The excitation PL spectra of TiO2:Eu3+ coatings can be divided into two regions: the broad band region from 250 nm to 350 nm associated with charge transfer state of Eu3+ and the series of sharp peaks in the range from 350 nm to 550 rim corresponding to direct excitation of the Eu3+ ions. It is observed that the intensity of peaks in excitation and emission PL spectra increases with the concentration of Eu3+, but the peak positions remain practically unchanged. The ratio of PL emission for electric and magnetic dipole transitions indicates highly asymmetric environment around Eu3+ ions. The photocatalytic activity (PA) of TiO2:Eu3+ coatings is evaluated by measuring the photodegradation of methyl orange under simulated sunlight conditions. It is shown that PEO time, i.e., the amount of Eu3+ incorporated into coatings is an important factor affecting PA; TiO2:Eu3+ coating formed after 1 min of PEO time showed the highest PA. This is the peer-reviewed version of the article: Stojadinović, S., Radić, N., Grbić, B., Maletić, S., Stefanov, P., Pacevski, A.,& Vasilic, R. (2016). Structural, photoluminescent and photocatalytic properties of TiO2:Eu3+ coatings formed by plasma electrolytic oxidation. Applied Surface Science, Elsevier Science Bv, Amsterdam., 370, 218-228. [https://doi.org/10.1016/j.apsusc.2016.02.131] The published version: [https://cer.ihtm.bg.ac.rs/handle/123456789/2034]

In this paper, we have investigated photoluminescence (PL) of Sm3+ doped ZrO2 coatings formed by plasma electrolytic oxidation (PEO) of zirconium in electrolyte containing Sm2O3 powder. Obtained PL emission spectra feature two distinct regions. The first region is related to ZrO2 PL band with maximum positioned at about 490 nm, while the second region in the orange-red part of the spectrum shows four sharp emission bands which are related to f–f transitions of Sm3+ from excited level 4G5/2 to the lover levels 6HJ (J=5/2, 7/2, 9/2, and 11/2). Increase of PL with time is related to increase of oxygen vacancy defects and the content of Sm3+ in the coatings formed during the process.

This paper presents the results of our recent investigation focused on the preparation and possible photocatalytic application of iron doped TiO2 coatings formed by plasma electrolytic oxidation of high purity titanium in electrolyte containing FeSO4. The morphology, phase and elemental composition, and absorption band of the coatings were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance spectroscopy. The photocatalytic activity of coatings was evaluated by measuring the degradation of methyl orange under simulated sunlight conditions. It was found that the addition of iron precursor to electrolyte solution improves the photocatalytic properties of obtained coatings with respect to pure TiO2 coating. The incorporation of iron in TiO2 lattice was identified as the main factor affecting the increase of photocatalytic activity. The highest photocatalytic activity was observed when coatings were processed for 2 min with addition of 1 g/L FeSO4 to electrolyte.

Nanocrystalline TiO2-CeO2 powders were synthesized from their TiO2 and CeO2 oxides using mechanical ball milling process. The response surface method is applied to identify optimal parameters for the synthesis of TiO2-CeO2 photocatalyst. Analysis of variance and main effect plot are used to determine the significant parameters and set the optimal level for each parameter. Regression analysis showed good agreement of experimental data with the second-order polynomial model with a coefficients of determination: R2 = 0.991, R2Adj. = 0.940 and R2Pred. = 0.983. Under optimal experimental conditions of TiO2:CeO2 weight percentage ratio 71:29, milling speed 200 rpm, and milling time 115 min the highest photodegradation efficiency was achieved. On the basis of the above statistical analysis, it was found that the band gap energy of TiO2-CeO2 nanoparticles decreases with the increase of the milling speed and milling time with constant TiO2:CeO2 weight percentage ratio. Obtained results suggest that mechanical ball milling process is a rapid, efficient and low energy consumption method to synthesize TiO2-CeO2 photocatalyst.

In this study, WO3 doped TiO2 powders were synthesized via sol-gel method combined with a hydrothermal process. The effect of sintering temperature on mesoporous structure and catalytic activities of these powders were investigated. The physical analysis via X-ray diffraction indicates that prepared samples are a mixture of anatase and rutile TiO2 phases. X-ray peak analysis is used to evaluate the crystallite size and lattice strain by the Williamson-Hall analysis. Considering all the reflections of the anatase phase the lattice strain ranging from c = 9.505 to c = 9.548 is calculated, suggesting that microstrain decreases when calcination temperature increases. N2 adsorption-desorption analysis shows that the surface area and pore volume decrease with increasing temperature and that WOx-TiO2 powders primarily consist of mesopores. Sintering temperature induced a change in textural properties causing a systematic shift towards larger mesopores. Simultaneously, photoactivity in decolorization of methyl orange increases with increasing calcination temperature up to 700°C, followed by significant decrease with its further increase. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. 172022, Grant no. 172026, Grant no. 172015, Grant no. 172018, Grant no. 172001 and Grant no. III 45001]