Influence of wide band gap oxide substrates on the photoelectrochemical properties and structural disorder of CdS nanoparticles grown by the successive ionic layer adsorption and reaction (SILAR) method

The photoelectrochemical properties of nanoheterostructures based on the wide band gap oxide substrates (ZnO, TiO2, In2O3) and CdS nanoparticles deposited by the successive ionic layer adsorption and reaction (SILAR) method have been studied as a function of the CdS deposition cycle number (N). The incident photon-to-current conversion efficiency (IPCE) passes through a maximum with the increase of N, which is ascribed to the competition between the increase in optical absorption and photocarrier recombination. The maximal IPCE values for the In2O3/CdS and ZnO/CdS heterostructures are attained at N ≈ 20, whereas for TiO2/CdS, the appropriate N value is an order of magnitude higher. The photocurrent and Raman spectroscopy studies of CdS nanoparticles revealed the occurrence of the quantum confinement effect, demonstrating the most rapid weakening with the increase of N in ZnO/CdS heterostructures. The structural disorder of CdS nanoparticles was characterized by the Urbach energy (EU), spectral width of the CdS longitudinal optical (LO) phonon band and the relative intensity of the surface optical (SO) phonon band in the Raman spectra. Maximal values of EU (100–120 meV) correspond to СdS nanoparticles on a In2O3 surface, correlating with the fact that the CdS LO band spectral width and intensity ratio for the CdS SO and LO bands are maximal for In2O3/CdS films. A notable variation in the degree of disorder of CdS nanoparticles is observed only in the initial stages of CdS growth (several tens of deposition cycles), indicating the preservation of the nanocrystalline state of CdS over a wide range of SILAR cycles

Band-gap and sub-band-gap photoelectrochemical processes at nanocrystalline CdS grown on ZnO by successive ionic layer adsorption and reaction method

Cadmium sulfide nanoparticle (NP) deposition by the successive ionic layer adsorption and reaction (SILAR) method on the surface of mesoporous ZnO micro-platelets with a large specific surface area (110 ± 10 m2g− 1) results in the formation of ZnO/CdS heterostructures exhibiting a high incident photon-to-current conversion efficiency (Y) not only within the region of CdS fundamental absorption (Ymax = 90%; 0.1 M Na2S + 0.1 M Na2SO3), but also in the sub-band-gap (SBG) range (Ymax = 25%). The onset potentials of SBG photoelectrochemical processes are more positive than the band-gap (BG) onset potential by up to 100 mV. A maximum incident photon-to-current conversion efficiency value for SBG processes is observed at larger amount of deposited CdS in comparison with the case of BG ones. The Urbach energy (EU) of CdS NPs determined from the photocurrent spectra reaches a maximal value on an early deposition stage (EU = 93 mV at SILAR cycle number N = 5), then lowers somewhat (EU = 73 mV at N = 10) and remains steady in the range of N from 20 to 300 (EU = 67 ± 1 mV). High efficiency of the photoelectrochemical SBG processes are interpreted in terms of light scattering in the ZnO/CdS heterostructures

Влияние редокс-потенциала электролита на фотоэлектрохимические свойства оксоиодида висмута

Photoelectrochemical behavior of semiconductor bismuth oxoiodide BiOI in solutions containing [Fe(CN)6]3-/[Fe(CN)6]4-or I3-/I- redox system has been studied. The effect of photocurrent sign inversion with the electrode potential change has been observed. It has been shown that this effect is governed by both thermodynamic conditions (shifting of the conduction band position and equilibrium potential EOx/Red) and by kinetic factor (the capture rate of photogenerated charged particles by oxidizing or reducing agents). Photoelectrochemical reactions are diffusion-controlled, and photocurrent is a function of the surface concentrations of oxidized and reduced forms of the redox system. Specific structure of BiOI coating consisting of thin plate-like crystallites (40-60 nm thick) provides good contact of its surface with the reaction medium and high efficiency of photoinduced processes.Изучено фотоэлектрохимическое поведение полупроводникового оксоиодида висмута BiOI в растворах, содержащих [Fe(CN)6]3-/[Fe(CN)6]4- либо I3-/I- редокс-систему. Обнаружен эффект инверсии знака фототока при изменении электродного потенциала. Показано, что данный эффект определяется как термодинамическими условиями - изменением положения зоны проводимости BiOI и равновесного потенциала EOx/Red, так и кинетическим фактором - скоростью захвата фотогенерированных носителей тока адсорбированными частицами окислителя или восстановителя. Фотоэлектрохимический процесс является диффузионно-лимитированным и величина фототока в значительной степени является функцией поверхностных концентраций окисленной и восстановленной форм редокс-системы. Специфическая структура BiOI покрытия, состоящего из тонких пластинчатых кристаллитов (40-60 нм), обеспечивает хороший контакт их поверхности с реакционной средой и высокую эффективность фотоиндуцированных процессов

МИКРОСКОПИЧЕСКИЕ И РЕНТГЕНОСПЕКТРАЛЬНЫЕ ИССЛЕДОВАНИЯ МАССИВОВ СТОЛБИКОВ НИКЕЛЯ В МАТРИЦЕ ДИОКСИДА КРЕМНИЯ

Ni rods distributed in silicon dioxide matrix formed on silicon wafers have been characterized by means of scanning electron microscopy and X–ray absorption near edge structure (XANES) spectroscopy. Ni rods have been obtained by electrochemical deposition of the metal onto a silicon dioxide matrix pores formed with the tracking technique. Latent tracks have been obtained by SiO2 film irradiation  with heavy gold ions at the Hahn–Meitner–Institute (Berlin, Germany). Scanning electron microscopy has established the peculiarities of pore filling with metal and the specificity of Ni rod formation and their morphology (surface and cleavages). High intensity synchrotron radiation of the Helmholtz Zentrum Berlin has been used in the ultrasoft X–ray range for electron energy structure studies of the Ni rods with the XANES technique. The specific phase composition of the surface layers has been investigated using Si, Ni and O atom local surrounding analysis performed based on synchrotron XANES technique data including the rod/matrix interface. Possible Ni silicide formation has been demonstrated for a certain rod array formation mode in which partial SiO2 matrix destruction occurs and the metal contacts with the silicon wafer. Natural oxidation specificity has also been studied for the Ni rod/SiO2 heterostructure surface.Методом растровой электронной микроскопии (РЭМ) и спектроскопии ближней тонкой структуры края рентгеновского поглощения (X−ray absorption near edge structure — XANES) проведена диагностика массивов столбиков никеля, распределенных в матрице SiO2 на подложке кремния. Столбики Ni получены методом электрохимического осаждения металла в поры матрицы диоксида кремния, сформированные трековым методом. Латентные треки получены путем облучения слоя SiO2 тяжелыми ионами золота на ускорителе Института Хан−Майтнер (Берлин, Германия). Методом РЭМ установлены особенности заполнения пор металлом, показана специфика образования столбиков Ni, их морфология (поверхность и сколы). Электронно−энергетическое строение массивов столбиков Ni исследовано методом XANES с помощью высокоинтенсивного синхротронного излучения ультрамягкого рентгеновского диапазона накопительного кольца BESSY II Гельмгольц−центра г. Берлина. Путем анализа локального окружения атомов кремния, никеля и кислорода по данным синхротронного метода XANES изучена специфика фазового состава поверхностных слоев, включая границу раздела столбик−матрица. Возможное образование фазы силицида никеля показано лишь при определенных режимах формирования массивов столбиков: в случае частичного разрушения матрицы диоксида кремния и при контакте металла с подложкой Si. Изучена специфика естественного окисления поверхности гетероструктуры «столбик никеля — диоксид кремния»

Influence of wide band gap oxide substrates on the photoelectrochemical properties and structural disorder of CdS nanoparticles grown by the successive ionic layer adsorption and reaction (SILAR) method

The photoelectrochemical properties of nanoheterostructures based on the wide band gap oxide substrates (ZnO, TiO2, In2O3) and CdS nanoparticles deposited by the successive ionic layer adsorption and reaction (SILAR) method have been studied as a function of the CdS deposition cycle number (N). The incident photon-to-current conversion efficiency (IPCE) passes through a maximum with the increase of N, which is ascribed to the competition between the increase in optical absorption and photocarrier recombination. The maximal IPCE values for the In2O3/CdS and ZnO/CdS heterostructures are attained at N ≈ 20, whereas for TiO2/CdS, the appropriate N value is an order of magnitude higher. The photocurrent and Raman spectroscopy studies of CdS nanoparticles revealed the occurrence of the quantum confinement effect, demonstrating the most rapid weakening with the increase of N in ZnO/CdS heterostructures. The structural disorder of CdS nanoparticles was characterized by the Urbach energy (EU), spectral width of the CdS longitudinal optical (LO) phonon band and the relative intensity of the surface optical (SO) phonon band in the Raman spectra. Maximal values of EU (100–120 meV) correspond to СdS nanoparticles on a In2O3 surface, correlating with the fact that the CdS LO band spectral width and intensity ratio for the CdS SO and LO bands are maximal for In2O3/CdS films. A notable variation in the degree of disorder of CdS nanoparticles is observed only in the initial stages of CdS growth (several tens of deposition cycles), indicating the preservation of the nanocrystalline state of CdS over a wide range of SILAR cycles

Influence of wide band gap oxide substrates on the photoelectrochemical properties and structural disorder of CdS nanoparticles grown by the successive ionic layer adsorption and reaction (SILAR) method

The photoelectrochemical properties of nanoheterostructures based on the wide band gap oxide substrates (ZnO, TiO2, In2O3) and CdS nanoparticles deposited by the successive ionic layer adsorption and reaction (SILAR) method have been studied as a function of the CdS deposition cycle number (N). The incident photon-to-current conversion efficiency (IPCE) passes through a maximum with the increase of N, which is ascribed to the competition between the increase in optical absorption and photocarrier recombination. The maximal IPCE values for the In2O3/CdS and ZnO/CdS heterostructures are attained at N ≈ 20, whereas for TiO2/CdS, the appropriate N value is an order of magnitude higher. The photocurrent and Raman spectroscopy studies of CdS nanoparticles revealed the occurrence of the quantum confinement effect, demonstrating the most rapid weakening with the increase of N in ZnO/CdS heterostructures. The structural disorder of CdS nanoparticles was characterized by the Urbach energy (EU), spectral width of the CdS longitudinal optical (LO) phonon band and the relative intensity of the surface optical (SO) phonon band in the Raman spectra. Maximal values of EU (100–120 meV) correspond to СdS nanoparticles on a In2O3 surface, correlating with the fact that the CdS LO band spectral width and intensity ratio for the CdS SO and LO bands are maximal for In2O3/CdS films. A notable variation in the degree of disorder of CdS nanoparticles is observed only in the initial stages of CdS growth (several tens of deposition cycles), indicating the preservation of the nanocrystalline state of CdS over a wide range of SILAR cycles