Hybrid Microwave Annealing Synthesizes Highly Crystalline Nanostructures for (Photo)electrocatalytic Water Splitting

ConspectusHydrogen is regarded as an ideal energy carrier for the hydrogen economy that could replace the current hydrocarbon economy in order to achieve global energy security and mitigate climate change. For this purpose, H2 has to be produced from renewable sources (e.g., solar and wind) without producing global-warming CO2.(Photo)­electrolysis of water into H2 and O2 is one of the most promising technologies for the production of renewable H2, which requires (photo)­electrocatalysts of high efficiency, chemical robustness, and scalability. An essential attribute required for high-efficiency (photo)­electrodes is high crystallinity with few defects to facilitate charge transfer without recombination. To this end, fabrication of photoelectrodes is usually completed with high temperature thermal annealing in a furnace. However, conventional thermal annealing (CTA) always results in undesirable crystal sintering, which reduces the surface area, and damage to the transparent conducting oxide (TCO) substrate. An emerging alternative method, hybrid microwave annealing (HMA), offers the beneficial effect of the high-temperature annealing (crystallinity) while minimizing its negative effects of sintering and TCO damage, enabling the fabrication of efficient (photo)­electrodes for water splitting.HMA combines direct microwave heating with additional heating from an effective microwave absorber (called a susceptor), thereby avoiding a nonuniform temperature distribution between the interior and exterior of the synthesized material. More importantly, an extremely high temperature of the entire sample can be reached in only a few minutes. Compared with CTA, HMA has several advantages in the preparation of (photo)­electrodes: (i) formation of a high-purity phase; (ii) high crystallinity with fewer defects; (iii) preservation of the original nanostructure; (iv) less damage to the TCO substrate for photoelectrodes; (v) smaller nanocrystals and uniform dispersion of catalyst particles. Overall, HMA is a convenient, ultrafast, and energy-economical technology for the synthesis of efficient (photo)­electrodes.In this Account, we discuss recent progress made in our laboratory on HMA for preparing photoanodes (Fe2O3, BiVO4, ZnFe2O4, and Fe2TiO5), photocathodes (Cu2O and CuFeO2), and a graphene-based electrocatalyst (MoS2/graphene composite), which exhibit distinctive behavior and efficient performance in (photo)­electrocatalytic water splitting. In particular, we have advanced the HMA technique further to synthesize hematite-based photoanodes with core–shell heterojunction nanorods (Nb,Sn:Fe2O3@FeNbO4 and Ta,Sn:Fe2O3@FeTaO4) by solid–solid interface reaction, which simultaneously achieves multiple doping effects (Nb or Ta, Sn) to improve the photoelectrocatalysis of water splitting. Thus, this Account focuses on the synthetic aspects of HMA, which may offer new research opportunities for the synthesis of other metal oxide (photo)­electrode materials and hybrid electrocatalysts in the fields of solar energy conversion and storage, secondary batteries, and H2 fuel production

Large-Scale, Surfactant-Free, Hydrothermal Synthesis of Lithium Aluminate Nanorods: Optimization of Parameters and Investigation of Growth Mechanism

Lithium aluminate nanorods were successfully synthesized from Al2O3 nanoparticles and lithium hydroxide by a simple, large-scale hydrothermal process without any surfactant or template. The various reaction parameters were optimized to achieve the maximum yield. The as-obtained nanorods had orthorhombic β-lithium aluminate structure with edges in the range of 40−200 nm and lengths of 1−2 μm confirmed by SEM, TEM, XRD, and NMR. Upon calcination at 1273 K for 12 h it transformed to γ-lithium aluminate, yet maintained the initial morphology, demonstrating the thermal stability. The ratio of lithium hydroxide to aluminum oxide showed a significant effect on the morphology as Li/Al = 1 gives “microroses”, whereas Li/Al = 3 and Li/Al = 15 gave “microbricks” and “nanorods”, respectively. Investigation of the mechanism showed that the nanorods were formed via a “rolling-up” mechanism. As we used all-inorganic raw materials and a simple synthetic procedure under mild conditions, the scale-up of this process for large-scale production should be very easy

Large-Scale, Surfactant-Free, Hydrothermal Synthesis of Lithium Aluminate Nanorods: Optimization of Parameters and Investigation of Growth Mechanism

Lithium aluminate nanorods were successfully synthesized from Al2O3 nanoparticles and lithium hydroxide by a simple, large-scale hydrothermal process without any surfactant or template. The various reaction parameters were optimized to achieve the maximum yield. The as-obtained nanorods had orthorhombic β-lithium aluminate structure with edges in the range of 40−200 nm and lengths of 1−2 μm confirmed by SEM, TEM, XRD, and NMR. Upon calcination at 1273 K for 12 h it transformed to γ-lithium aluminate, yet maintained the initial morphology, demonstrating the thermal stability. The ratio of lithium hydroxide to aluminum oxide showed a significant effect on the morphology as Li/Al = 1 gives “microroses”, whereas Li/Al = 3 and Li/Al = 15 gave “microbricks” and “nanorods”, respectively. Investigation of the mechanism showed that the nanorods were formed via a “rolling-up” mechanism. As we used all-inorganic raw materials and a simple synthetic procedure under mild conditions, the scale-up of this process for large-scale production should be very easy

Large-Scale, Surfactant-Free, Hydrothermal Synthesis of Lithium Aluminate Nanorods: Optimization of Parameters and Investigation of Growth Mechanism

Lithium aluminate nanorods were successfully synthesized from Al2O3 nanoparticles and lithium hydroxide by a simple, large-scale hydrothermal process without any surfactant or template. The various reaction parameters were optimized to achieve the maximum yield. The as-obtained nanorods had orthorhombic β-lithium aluminate structure with edges in the range of 40−200 nm and lengths of 1−2 μm confirmed by SEM, TEM, XRD, and NMR. Upon calcination at 1273 K for 12 h it transformed to γ-lithium aluminate, yet maintained the initial morphology, demonstrating the thermal stability. The ratio of lithium hydroxide to aluminum oxide showed a significant effect on the morphology as Li/Al = 1 gives “microroses”, whereas Li/Al = 3 and Li/Al = 15 gave “microbricks” and “nanorods”, respectively. Investigation of the mechanism showed that the nanorods were formed via a “rolling-up” mechanism. As we used all-inorganic raw materials and a simple synthetic procedure under mild conditions, the scale-up of this process for large-scale production should be very easy

An Undoped, Single-Phase Oxide Photocatalyst Working under Visible Light

A novel photocatalyst, PbBi2Nb2O9 has been discovered that shows high activities for degradation of organic pollutants, generation of photocurrent, and water decomposition into O2 or H2, all under visible right irradiation (λ ≥ 420 nm). This is the first example of an undoped, single-phase oxide photocatalyst that shows such reactivity. Its quantum yields are much higher than those for most of the previously reported materials, especially in water decomposition to generate oxygen (29%). Since it is an oxide, there is much less concern for stability under light irradiation

Barium Substituted Lanthanum Manganite Perovskite for CO₂ Reforming of Methane

Barium substituted lanthanum manganite La1–xBaxMnO3 (x = 0.10–0.50) of a single phase perovskite structure was used as catalysts for CO2 reforming of CH4 for the first time. The optimal level of Ba substitution (Ba/Mn = 0.10, 0.15) produced La1–xBaxMnO3 of high surface area, uniform particle dispersion, and highly ordered pores. The optimally substituted perovskite catalysts showed much improved reducibility of Mn3+/Mn4+ to Mn2+ to provide oxygen vacancies and rapid migration of lattice oxygen from the bulk toward the surface. The ability of donating lattice oxygen to the catalytic cycle seems responsible for the facilitated decomposition and dissociation of CH4 and CO2, which led to high conversions, excellent syngas selectivity, and stability with little coke formation. The addition of oxygen to the dry reforming reaction showed improved conversion and selectivity to syngas by making catalysts less prone to coke formation. To the best of our knowledge, the results represent the first example of Mn-based reforming catalysts that perform better than more common Ni-based catalysts of the same structure

An Undoped, Single-Phase Oxide Photocatalyst Working under Visible Light

A novel photocatalyst, PbBi2Nb2O9 has been discovered that shows high activities for degradation of organic pollutants, generation of photocurrent, and water decomposition into O2 or H2, all under visible right irradiation (λ ≥ 420 nm). This is the first example of an undoped, single-phase oxide photocatalyst that shows such reactivity. Its quantum yields are much higher than those for most of the previously reported materials, especially in water decomposition to generate oxygen (29%). Since it is an oxide, there is much less concern for stability under light irradiation

Exposed Crystal Face Controlled Synthesis of 3D ZnO Superstructures

We report a method for synthesizing exposed crystal face controlled 3D ZnO superstructures under mild conditions (at room temperature or 90 °C under 1 atm) without organic additives. The exposed crystal faces of the building blocks of the 3D structures were controlled by varying the reactant concentrations and the reaction temperatures. On the basis of the experimental results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures (structures I, II, III, and IV) under the different growth conditions. The optical properties of the 3D ZnO superstructures were probed by UV−vis diffuse reflectance spectroscopy. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the 3D superstructures varied according to the exposed crystal faces, which could be controlled by this method (structure I > structure IV > structure III > structure II)

Photoelectrochemical Nitrate and Nitrite Reduction Using Cu₂O Photocathodes

Nitrate in wastewater streams causes eutrophication, and nitrate removal is of great importance for environmental protection. Electrochemical nitrate reduction has the advantage of directly converting nitrate to benign or useful chemicals, but it typically requires a considerable overpotential. In this study, photoelectrochemical nitrate reduction is investigated using a Cu2O photocathode, where photoexcited electrons in the conduction band inherently have an overpotential of >1.6 V for nitrate reduction. The Cu2O photocathode is found to reduce nitrate to nitrite selectively with a high Faradaic efficiency (>85%). More importantly, as the surface of Cu2O is particularly catalytic for nitrate reduction, nitrate reduction on Cu2O kinetically suppresses photocorrosion of Cu2O without the need for additional catalyst or protection layers. In addition to nitrate reduction, nitrite reduction on Cu2O is examined to compare the effects of nitrate and nitrite reduction kinetics on the photocurrent generation and photocorrosion of Cu2O photocathodes

Precipitating Metal Nitrate Deposition of Amorphous Metal Oxyhydroxide Electrodes Containing Ni, Fe, and Co for Electrocatalytic Water Oxidation

We report here a facile, one-step precipitating metal nitrate deposition (PMND) method to prepare amorphous metal oxyhydroxide films containing Fe, Co, and Ni as efficient electrocatalysts for water oxidation. The unique synthesis technique allows easy control of the metal composition over a wide range on various substrates. A series of unary and binary metal oxyhydroxides of 30 compositions are synthesized by PMND on fluorine-doped tin oxide (FTO) substrate as water oxidation electrocatalysts. The activity of the metal oxyhydroxide films is represented by a volcano plot as a function of a single experimental descriptor, i.e., the fraction of hydroxide in the surface oxygen species. The optimum compositions for binary metal oxyhydroxide (NiFe, NiCo, and CoFe) are determined on conductive substrates of FTO, nickel foam (NF), nickel mesh (NM), and carbon felt (CF), and the best NiFe (2:8) electrocatalyst on NF exhibits a water oxidation current density of 100 mA/cm2 with only 280 mV of overvoltage, which outperforms conventional noble metal catalysts like IrOx and RuOx in an alkaline medium. Finally, we demonstrate a tandem PV–electrolysis system by using a c-Si PV module with a power conversion efficiency of 13.71% and an electrochemical cell composed of NiFe (2:8)/NF anode and a bare NF cathode with a conversion efficiency of 71.8%, which records a solar-to-hydrogen conversion efficiency of 9.84%