Heterogeneously Doped Research Articles

Heterogeneous doping via charge carrier transport improves Photoelectrochemical H2O oxidative H2O2 synthesis

Photoelectrochemical (PEC) water splitting into H2O2 and H2 has attracted significant attention due to its low cost and sustainability. However, slow charge carrier transport and water oxidation kinetics limit the selectivity for H2O2 production and solar conversion efficiency. Herein, we propose a heterogeneous doping approach that combines surface gradient doping with bulk doping to improve the charge carrier transport in the photoelectrode. Inducing gradient Nb and homogeneous Mo doping into BiVO4 photoanode (G-Nb/Mo:BVO) can promote charge separation and carrier transfer, leading to the high selectivity for H2O2 generation and suppression of O2 production. Consequently, the heterogeneously doped G-Nb/Mo:BVO photoanode presents an average Faraday efficiency (FE) of over 80% for H2O2 production in a wide potential of 0.6–1.8 VRHE with the maximum FE of 83.7% at 1.2 VRHE under AM 1.5G illumination. More importantly, H2O2 production rate can reach 1.23 μmol min−1 cm−2 at 1.23 VRHE, representing the best H2O2 production rate reported for the photoelectrodes. Density functional theory calculations prove that Mo and Nb co-doping increases the reactivity of BiVO4 and improves 2e− water oxidation reaction activity and selectivity. This work demonstrates that heterogeneous doping provides a cost-effective strategy to break performance trade-offs by improving charge carrier separation and transport in light absorbers, and modulating the selectivity and activity of water oxidation reaction to generate H2O2, which can be extended to other photocatalytic reactions.

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta3N5 thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta3N5 as a leading performer among visible‐light‐responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Efficient Separation of Photoexcited Charge at Interface between Pure CeO2 and Y3+-Doped CeO2 with Heterogonous Doping Structure for Photocatalytic Overall Water Splitting.

Enhancement of photoexcited charge separation in semiconductor photocatalysts is one of the important subjects to improve the efficiency of energy conversion for photocatalytic overall water splitting into H2 and O2. In this study, we report an efficient separation of photoexcited charge at the interface between non-doped pure CeO2 and Y3+-doped CeO2 phases on particle surfaces with heterogeneous doping structure. Neither non-doped pure CeO2 and homogeneously Y3+-doped CeO2 gave activities for photocatalytic H2 and O2 production under ultraviolet light irradiation, meaning that both single phases showed little activity. On the other hand, Y3+-heterogeneously doped CeO2 of which the surface was composed of non-doped pure CeO2, and Y3+-doped CeO2 phases exhibited remarkable photocatalytic activities, indicating that the interfacial heterostructure between non-doped pure CeO2 and Y3+-doped CeO2 phases plays an important role for the activation process. The role of the interface between two different phases for activated expression was investigated by selective photo-reduction and oxidation deposition techniques of metal ion, resulting that the interface between two phases become an efficient separation site of photoexcited charge. Electronic band structures of both phases were investigated by the spectroscopic method, and then a mechanism of charge separation is discussed.

Understanding the Enhancement of Ionic Transport in Heterogeneously Doped Zirconia by Heterointerface Engineering

Zirconia-based ceramics have been the most promising oxide electrolyte material with high ionic conductivity for solid oxide fuel cell (SOFC) applications. Even though yttria-stabilized zirconia (Y...

Heterogeneous Doping to Improve the Performance of Thin-Film Hematite Photoanodes for Solar Water Splitting

Ti-doped, undoped, and Zn-doped hematite (α-Fe2O3) thick (∼1 μm) films were found to be n-type, weak n-type, and p-type, respectively. Heterogeneous doping profiles were generated in 30 nm thick hematite stacks on F:SnO2-coated glass substrates with 25 nm thick SnO2 underlayers in order to investigate the effect of different doping profiles on photoelectrochemical performance and compare with homogeneously doped counterpart photoelectrodes. Among the homogeneously doped photoelectrodes, the Ti-doped sample displayed the highest plateau photocurrent but also the highest onset potential, whereas the Zn-doped one had the lowest onset potential and the lowest plateau photocurrent. Heterogeneously doped photoelectrodes displayed both high plateau photocurrent and low onset potential, with the highest performance achieved for the specimen with Ti-doped, undoped, and Zn-doped layers at the bottom, center, and top parts of the stack, respectively. This demonstrates the potential of heterogeneous doping to improve...

Atomic-Scale Quantitative Analysis of Lattice Distortions at Interfaces of Two-Dimensionally Sr-Doped La2CuO4 Superlattices.

Using spherical aberration corrected high-resolution and analytical scanning transmission electron microscopy, we have quantitatively studied the lattice distortion and the redistribution of charges in two-dimensionally strontium (Sr)-doped La2CuO4 superlattices, in which single LaO planes are periodically replaced by SrO planes. As shown previously, such structures show Tc up to 35 K as a consequence of local charge accumulation on both sides of the nominal SrO planes position. This is caused by two distinct mechanisms of doping: heterogeneous doping at the downward side of the interface (space–charge effect) and “classical” homogeneous doping at the upward side. The comparative chemical and atomic-structural analyses reveal an interrelation between local CuO6 octahedron distortions, hole spatial distribution, and chemical composition. In particular we observe an anomalous expansion of the apical oxygen–oxygen distance in the heterogeneously doped (space–charge) region, and a substantial shrinkage of the apical oxygen–oxygen distance in the homogeneously doped region. Such findings are interpreted in terms of different Jahn–Teller effects occurring at the two interface sides (downward and upward).

General scalable strategy toward heterogeneously doped hierarchical porous graphitic carbon bubbles for lithium-ion battery anodes.

Novel carbon nanostructures, e.g., carbon nanotubes (CNTs), graphene, hierarchical porous graphitic carbon (HPGC), and ordered mesoporous carbon (CMK-3), have been significantly forwarding the progress of energy storage and conversion. Advanced electrodes or hybrid electrodes based on them are springing up one after another. To step further, a generic synthetic approach to large scale hierarchical porous graphitic carbon microbubbles (HPGCMBs) is developed by zinc powder templated organic precursor impregnation method. The facile technique features scalable (yield: once more than 200 mg), in situ heteroatom's doping (doping ratio: more than 26%) and hierarchical-pore-creating traits (pore volume: 1.01 cm(3) g(-1)). Adjustable graphitic content, doping species and amount are readily realized through varying the organic precursors. Rationally, good conductivity, fast kinetics, and abundant ion reservoirs are entirely achieved. To be applied in practice, state-of-the-art anodes for lithium-ion batteries are fabricated. Benefiting from the large specific surface area, rich heteroatoms, and hierarchical pores, the HPGCMBs electrodes exhibit excellent electrochemical properties. Besides superior storage capability of more than 1000 mAh g(-1) at 100 mA g(-1), stable cycling and excellent retention of 370 mAh g(-1) at large rate of 10 A g(-1) are achieved in the meantime.

Beneficial Lattice Strain in Heterogeneously Doped Ceria

Oxygen ion conduction in heterogeneously doped films composed of alternating layers of pure Y2O3 and pure CeO2 was reported recently, with conduction coming predominantly from vacancies trapped in interfacial space charge regions in CeO2. Here, we expand this concept to films composed of CeO2 heterogeneously doped with Y2O3, Gd2O3, or La2O3 in order to study the effects of heterodopant identity on the oxygen ion conductivity. For all samples, the thickness of the entire structure and that of the individual dopant layers were kept constant. The dopant oxides used in this work adopt the cubic bixbyite crystal structure with pseudo-fluorite lattice parameters that are, relative to CeO2, smaller (Y2O3), larger (La2O3), and nearly equal (Gd2O3). The total conductivity of the films increased with increasing lattice parameter of the dopant oxides. An electrostatic Gouy–Chapman model is not sufficient to explain this behavior because all of the dopants theoretically contribute identical interfacial oxygen vacancy...

Microstructure and ionic conductivity of SrTiO3 heterogeneously doped YSZ composite ceramics

To improve the ionic conductivity of ZrO 2 stabilized by 8 mol% of Y 2 O 3 (8YSZ), the SrTiO 3 heterogeneously doped 8YSZ composite ceramics with different volumetric fractions of SrTiO 3 were prepared through powder sintering at 1400 °C and 1450 °C in air. The phase compositions, microstructures, electrical conductivities and ion transference numbers of the composites were investigated by X-ray diffraction, scanning electron microscopy, complex impedance and Hebb–Wagner polarization method, respectively. The composite ceramics only consist of cubic YSZ and SrTiO 3 . The deviations of the typical diffraction peak positions in XRD spectra and the variations in unit cell parameters of both SrTiO 3 and YSZ in composites have occurred due to the elemental inter-diffusion between both phases. The grain boundaries between these two heterogeneous phases do not exhibit any presence of a segregated impurity phase, and the heterogeneous doping of SrTiO 3 can suppress the growth of YSZ grains at higher doping levels. The apparent conductivities of the composites show a peak at 5 vol.% of SrTiO 3 addition. The activation energies for the oxygen ion transport of 8YSZ doped with 0–15 vol.% of SrTiO 3 all fall into the range of 101.71–104.93 kJ/mol. The ion transference numbers of the composites with 1–15 vol.% of SrTiO 3 are all higher than 0.99, demonstrating that the electrical conductivity is almost pure ionic in nature. By analyzing the effects of SrTiO 3 additions on the normalized conductivities of the composites, the SrTiO 3 /YSZ heterogeneous interfacial effects on the conductivity improvement can be proposed. • SrTiO 3 heterogeneously doped 8YSZ composites are prepared by sintering in air. • Elemental diffusions between SrTiO 3 and YSZ occur after powder sintering. • The SrTiO 3 /YSZ heterogeneous interface can enhance the conductivity of composites. • Electrical conductivities of YSZ doped with 1–15 vol.% of SrTiO 3 are ionic in nature.

Heterogeneously doped polyethylene glycol as nano-composite soft matter electrolyte

We have applied the concept of heterogeneous doping [1] to prepare and examine composite electrolytes, consisting of silica particles, low molecular weight polyethylene glycol solvents and lithium perchlorate salt. These “soggy sand” electrolytes combine high ionic conductivities (on the order of mS cm −1) and high Li transference numbers (typically 60–80%) with improved mechanical properties. They were characterized using differential scanning calorimetry, dc-polarization and ac-impedance spectroscopy, zeta potential measurements and viscosimetry. Oxide, size and concentration as well as solvent molecular weight were varied to better understand the influence of ceramic oxide fillers on the ion conduction in these systems. As regarding the filler content, we observe that both conductivity and transference number of Li + start increasing already at low volume fractions of oxide particles, reach a maximum and subsequently decrease to low values. The percolating network is – after initial partial coarsening – found to be stable within the time periods of the measurements.

Heterogeneously doped nanocrystalline ceria films by grain boundary diffusion: Impact on transport properties

Heterogeneous doping of nanocrystalline ceria films, by controlled in-diffusion along grain boundaries, is explored as a means for modifying the space charge potential and the inhomogeneous distribution of defects in the space charge layer known to control the electrical properties of nanocrystalline electroceramics. Nanocrystalline cerium oxide thin films were grown by pulsed laser deposition and modified by a novel doping technique. Thin diffusion sources were deposited and cations such as Ni2+ and Gd3+ were in-diffused at temperatures of 700–800 °C along columnar grain boundaries normal to the surface; the resulting diffusion profiles were examined by Time-of-Flight SIMS. The properties of these modified films were compared with as-deposited samples, with in-diffusion resulting in decreased electrical conductivity. It is proposed that the variation in conductivity results from a redistribution of charge carriers in the space charge layers due to a change in the space charge potential.

Comparison of Heterogeneously and Homogeneously Doped Polyphenylacetylene Using IR and XPS

Infrared (IR) and x-ray photoemission spectroscopy (XPS) studies have been made on heterogeneously iodine-doped polyphenylacetylene (PPA) polymer films. Similarities and differences between the doping processes (heterogeneous and homogeneous) are presented. Results point to a surface doping process in the heterogeneously doped films vs. a bulk process in the homogeneously doped materials (doped in solution). The counter anion moiety in the films appears to be the polyiodide I5− ion, as contrasted to the bulk powder (homogeneously doped) polymer in which the species of I2, I3− in the presence of a predominance of I5− ion coexist.

The effect of heterogenous doping on heterogeneous catalysis: Dehydrohalogenation of tertiary butyl chloride

The presence of AgCl heterogeneously doped with γ-Al 2O 3 increases the rate of dehydrohalogenation of tertiary butyl chloride significantly. The dependences of the catalytic activity on temperature, grain size and composition of the catalyst are quantitatively analyzed in terms of the concept of “heterogeneous doping”. The mechanism is interpreted in terms of Ag +-vacancy accumulation in the space charge region of silver chloride.

Heterogeneous catalysis with composite electrolytes

We have found that the rate of elimination of HCl from (CH 3) 3 CCl is considerably enhanced by using heterogeneously doped AgCl(AgCl:γ-Al 2O 3) instead of the pure halide as a catalyst. Similar effects are known for AgCl homogeneously doped with higher-valent cations. This enhancement supports the model of “heterogeneous doping” for composite electrolytes developed recently. Kinetic measurements as a function of surface area, composition and temperature are presented and interpreted phenomemologically and mechanistically.

Combined homo-hetero doping for enhancement of ionic conductivity

Electrical conductivity of Na 0.005Ca 0.995F 1.995 and Y 0.008Ca 0.992F 2.008 samples containing 2 mol% CeO 2− x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaF 2 homogeneously doped with NaF. However, heterogeneous doping with CeO 2− x decreased the conductivity of YF 3-doped CaF 2. This suggests that preferential adsorption of F − ions to the CeO 2− x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaF 2 heterogeneously doped with CeO 2− x . By the synergetic effect of homogeneous and heterogeneous doping, the conductivity of polycrystalline CaF 2 can be enhanced by 4.5 orders of magnitude at 630 K.

Ionic Transport in Heterogeneously and Homogeneously Doped Thallium (I)‐Chloride

AbstractThe effect of alumina admixtures on the ionic conductivity of TlCl is studied. In agreement with the model of “Heterogeneous Doping” a change of the intrinsic anionic conduction (Cl ‐vacancies) into a Tl+‐conduction (Tl+‐vacancies) is induced at lower temperatures (≦ 300 K). The experimental conductivity data as a function of temperature, concentration and grain‐size can be quantitatively described with an interface defect concentration considerably lower than for AgCl and AgBr. Likewise, for pure polycrystalline TlCl space charge regions enriched in cation vacancies around grain boundaries are assumed to provide an increase of the conductivity. Similarities and differences between heterogeneous and the classical homogeneous doping are clearly reflected by experiments using PbCl2 as a dopant for both TlCl and TlCl: Al2O3