Enhanced Oxygen Evolution Reaction Activity in Hematite Photoanodes: Effect of Sb-Li Co-Doping

Co-doping represents a valid approach to maximize the performance of photocatalytic and photoelectrocatalytic semiconductors. Albeit theoretical predictions in hematite suggesting a bulk n-type doping and a surface p-type doping would deliver best results, hematite co-doping with coupled cations possessing low and high oxidation states has shown promising results. Herein, we report, for the first time, Sb and Li co-doping of hematite photoanodes. Particularly, this is also a seminal work for the introduction of the highly reactive Sb5+ directly into the hematite thin films. Upon co-doping, we have a synergistic effect on the current densities with a 67-fold improvement over the standard. Via a combined investigation with profuse photoelectrochemical measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analyses, we confirm the two doping roles of Sb5+ and Li+ as the substitutional and interstitial dopant, respectively. The improvements are attributed to a higher charge carrier concentration along with a lower charge transfer resistance at the surface

Mesoporous Semiconductors: A New Model To Assess Accessible Surface Area and Increased Photocatalytic Activity?

Mesoporous photocatalysts have gained tremendous attention in the past decade by demonstrating that increased surface area and porosity can strongly improve their performance. In fact, all reports on mesoporous semiconductors corroborate this scenario. But is it possible to quantify and compare the reported advantages of the mesopores and the increased surface area between different works? In this contribution, we present a model that can evaluate the improvements in photocatalytic activity achieved by the introduction of mesoporosity independent of synthetic or test conditions. We exemplify this methodology focusing on photocatalytic hydrogen/oxygen evolution with sacrificial reagents, but also include examples of CO2 reduction and electrocatalysis. By correlating the relative increase in surface area to the relative increase in activityin comparison to non-porous counterpartswe show that the origin of mesoporosity can have a pronounced influence on the activity enhancement and that different semiconductor materials behave quite differently. Our model can serve as a starting point for the community to extract and compare key information on mesoporous photocatalysts, to put results into context of existing data, and to compare the performances of various catalytic systems much better

Beware of Doping: Ta₂O₅ Nanotube Photocatalyst Using CNTs as Hard Templates

Nanostructuring constitutes a promising strategy to increase efficiency and stability of contemporary photocatalysts. Here, we report on the synthesis of highly crystalline Ta₂O₅ nanotubes (NTs) by using carbon nanotubes (CNTs) as sacrificial hard templates and elucidate the role of residual Fe nanoparticlesoften used as catalyst for the CNT growthon their photocatalytic performance toward H₂ evolution. We show that, when using as grown CNTs, the resulting Ta₂O₅ NTs contained detectable amounts of Fe and possessed negligible photocatalytic activity. When CNTs were, however, purified from Fe by thermally annealing the CNTs at 2100 °C, the same synthetic procedure yielded pure Ta₂O₅ NTs that showed a 40-fold increase in activity compared to the Fe-containing counterpart. A complementary set of analytical techniques in a combination with additional model experiments indicate that the detrimental effect of the residual Fe on the photocatalytic activity originates from atomic doping and formation of a segregated FeO_<i>x</i> phase within the Ta₂O₅ matrix that can both act as efficient electron traps. Our result highlights that the presence of residual catalyst needs to be taken into account when using CNTs as hard templates and generally illustrates a possible effect of unintentional dopants that are often not considered in preparing functional nanostructures

Continuous Formation of Limonene Carbonates in Supercritical Carbon Dioxide

We present a continuous flow method for the conversion of bioderived limonene oxide and limonene dioxide to limonene carbonates using carbon dioxide in its supercritical state as a reagent and sole solvent. Various ammonium- and imidazolium-based ionic liquids were initially investigated in batch mode. For applying the best-performing and selective catalyst tetrabutylammonium chloride in continuous flow, the ionic liquid was physisorbed on mesoporous silica. In addition to the analysis of surface area and pore size distribution of the best-performing supported ionic liquid phase (SILP) catalysts via nitrogen physisorption, SILPs were characterized by diffuse reflectance infrared Fourier transform spectroscopy and thermogravimetric analysis and served as heterogeneous catalysts in continuous flow. Initially, the continuous flow conversion was optimized in short-term experiments resulting in the desired constant product outputs. Under these conditions, the long-term behavior of the SILP system was studied for a period of 48 h; no leaching of catalyst from the supporting material was observed in the case of limonene oxide and resulted in a yield of 16%. For limonene dioxide, just traces of leached catalysts were detected after reducing the catalyst loading from 30 to 15 wt %, thus enabling a constant product output in 17% yield over time

Silicon Oxycarbide (SiOC)-Supported Ionic Liquids: Heterogeneous Catalysts for Cyclic Carbonate Formation

Silicon oxycarbides (SiOCs) impregnated with tetrabutylammonium halides (TBAX) were investigated as an alternative to silica-based supported ionic liquid phases for the production of bio-based cyclic carbonates derived from limonene and linseed oil. The support materials and the supported ionic liquid phases (SILPs) were characterized via Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption, X-ray photoelectron spectroscopy, microscopy, and solvent adsorption. The silicon oxycarbide supports were pyrolyzed at 300–900 °C prior to being coated with different tetrabutylammonium halides and further used as heterogeneous catalysts for the formation of cyclic carbonates in batch mode. Excellent selectivities of 97–100% and yields of 53–62% were obtained with tetrabutylammonium chloride supported on the silicon oxycarbides. For comparison, the catalytic performance of commonly employed silica-supported ionic liquids was investigated under the same conditions. The silica-supported species triggered the formation of a diol as a byproduct, leading to a lower selectivity of 87% and a lower yield of 48%. Ultimately, macroporous monolithic SiOC-SILPs with suitable permeability characteristics (k1 = 10–11 m2) were produced via photopolymerization-assisted solidification templating and applied for the selective and continuous production of limonene carbonate with supercritical carbon dioxide as the reagent and sole solvent. Constant product output over 48 h without concurrent catalyst leaching was achieved

Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS₂ Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS₂ Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS₂ Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

Atomic Layer Deposition of TiO₂ for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO₂ as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO₂ films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO₂ films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO₂ compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron–hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO₂ compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability