Abdullah M. Al-Enizi

Interlaced NiS2–MoS2 nanoflake-nanowires as efficient hydrogen evolution electrocatalysts in basic solutions

The electrocatalytic activity of transition metal sulfides is strongly correlated with the active edge sites and conductivity of the catalysts. Here we developed a hybrid nanowire structure consisting of NiS2–MoS2 interlaced nanoflakes by the thermal conversion of nickel molybdate (NiMoO4) nanowire precursors. The obtained NiS2–MoS2 interlaced nanoflakes have sub-200 nm sizes with abundant active edge sites and defects, as well as synergistic nanoscale interfaces between NiS2 and MoS2 throughout the whole structure. The hetero-nanowires exhibit superior hydrogen evolution reaction (HER) catalytic activity including a low onset potential of 76 mV and a small Tafel slope of 70 mV dec−1 in a 1 M KOH aqueous solution, which substantially exceeds those of most of the metal sulfide HER catalysts including individual NiS2 and MoS2 nanostructures in basic solutions.

Radially oriented mesoporous TiO2 microspheres with single-crystal–like anatase walls for high-efficiency optoelectronic devices

Uniform mesoporous single-crystal TiO2 spheres with radial channels from driving orientation assembly can be used for energy storage. Highly crystalline mesoporous materials with oriented configurations are in demand for high-performance energy conversion devices. We report a simple evaporation-driven oriented assembly method to synthesize three-dimensional open mesoporous TiO2 microspheres with a diameter of ~800 nm, well-controlled radially oriented hexagonal mesochannels, and crystalline anatase walls. The mesoporous TiO2 spheres have a large accessible surface area (112 m2/g), a large pore volume (0.164 cm3/g), and highly single-crystal–like anatase walls with dominant (101) exposed facets, making them ideal for conducting mesoscopic photoanode films. Dye-sensitized solar cells (DSSCs) based on the mesoporous TiO2 microspheres and commercial dye N719 have a photoelectric conversion efficiency of up to 12.1%. This evaporation-driven approach can create opportunities for tailoring the orientation of inorganic building blocks in the assembly of various mesoporous materials.

Myriophyllum-like hierarchical TiN@Ni3N nanowire arrays for bifunctional water splitting catalysts

Inspired by Myriophyllum, a natural plant, we report an efficient electrochemical water splitting device based on hierarchical TiN@Ni3N nanowire arrays. The bifunctional TiN@Ni3N nanowire arrays serve as both hydrogen evolution reaction (HER) and oxygen reaction evolution (OER) catalysts in this device. As a hydrogen evolution catalyst, the TiN@Ni3N nanowire arrays possess an onset overpotential of 15 mV vs. the reversible hydrogen electrode (RHE), a Tafel slope of 42.1 mV dec−1, and an excellent stability of <13% degradation after being operated for 10 h, much better than Pt disks and Ni3N nanosheets in alkaline electrolytes. For oxygen evolution performance, the Myriophyllum-like TiN@Ni3N nanowire arrays exhibit an onset potential of 1.52 V vs. RHE, and a high stability of 72.1% current retention after being measured for 16 h in the potentiostatic mode. Furthermore, a symmetric electrochemical water splitting device was assembled by using the Myriophyllum-like TiN@Ni3N nanowire arrays as two electrodes, possessing a water splitting onset of ∼1.57 V with a current retention of 63.8% after 16 h of operation.

Ultradispersed Palladium Nanoparticles in Three-Dimensional Dendritic Mesoporous Silica Nanospheres: Toward Active and Stable Heterogeneous Catalysts

Immobilization of highly monodispersed palladium nanoparticles in colloidal mesoporous silica supports has been successfully achieved. The Pd nanoparticles with a uniform small size of ∼1.2 nm can be homogeneously distributed in individual mesopore channels of amino group-functionalized three-dimensional dendritic mesoporous silica nanospheres (3D-dendritic MSNSs) with a Pd content of ∼2.8%. The 3D-dendritic MSNSs-based nanoreactors show high activity in Suzuki-Miyaura cross-coupling reactions of bromobenzene with phenylboronic acid, obtaining a yield over 99% with 0.075 mol % Pd catalyst at room temperature in the dark within 12 h. More importantly, the excellent catalytic performance can be maintained with a negligible decrease lasting at least six cycles. It further reveals that the mesoporous frameworks of the colloidal silica supports can be well-preserved after four catalytic runs; meanwhile, the Pd nanoparticles in the mesopore channels also can remain the sizes of 1.5±0.3 nm without significant transfer and aggregation. The unique mesostructure of the 3D-dendritic MSNSs with mesopore channels of short length and large diameter is supposed to be the key role in immobilization of active and robust heterogeneous catalysts, and it would have more hopeful prospects in catalytic applications.

New Insight into the Synthesis of Large-Pore Ordered Mesoporous Materials

Ordered mesoporous materials (OMMs) have received increasing interest due to their uniform pore size, high surface area, various compositions and wide applications in energy conversion and storage, biomedicine and environmental remediation, etc. The soft templating synthesis using surfactants or amphiphilic block copolymers is the most efficient method to produce OMMs with tailorable pore structure and surface property. However, due to the limited choice of commercially available soft templates, the common OMMs usually show small pore size and amorphous (or semicrystalline) frameworks. Tailor-made amphiphilic block copolymers with controllable molecular weights and compositions have recently emerged as alternative soft templates for synthesis of new OMMs with many unique features including adjustable mesostructures and framework compositions, ultralarge pores, thick pore walls, high thermal stability and crystalline frameworks. In this Perspective, recent progresses and some new insights into the coassembly process about the synthesis of OMMs based on these tailor-made copolymers as templates are summarized, and typical newly developed synthesis methods and strategies are discussed in depth, including solvent evaporation induced aggregation, ligand-assisted coassembly, solvent evaporation induced micelle fusion-aggregation assembly, homopolymer assisted pore expanding and carbon-supported crystallization strategy. Then, the applications of the obtained large-pore OMMs in catalysis, sensor, energy conversion and storage, and biomedicine by loading large-size guest molecules (e.g., protein and RNA), precious metal nanoparticles and quantum dots, are discussed. At last, the outlook on the prospects and challenges of future research about the synthesis of large-pore OMMs by using tailor-made amphiphilic block copolymers are included.

Hollow TiO2–X porous microspheres composed of well-crystalline nanocrystals for high-performance lithium-ion batteries

Hollow TiO2–X porous microspheres consisted of numerous well-crystalline nanocrystals with superior structural integrity and robust hollow interior were synthesized by a facile sol-gel template-assisted approach and two-step carbonprotected calcination method, together with hydrogenation treatment. They exhibit a uniform diameter of ~470 nm with a thin porous wall shell of ~50 nm in thickness. The Brunauer-Emmett-Teller (BET) surface area and pore volume are ~19 m2/g and 0.07 cm3/g, respectively. These hollow TiO2–X porous microspheres demonstrated excellent lithium storage performance with stable capacity retention for over 300 cycles (a high capacity of 151 mAh/g can be obtained up to 300 cycles at 1 C, retaining 81.6% of the initial capacity of 185 mAh/g) and enhanced rate capability even up to 10 C (222, 192, 121, and 92.1 mAh/g at current rates of 0.5, 1, 5, and 10 C, respectively). The intrinsic increased conductivity of the hydrogenated TiO2 microspheres and their robust hollow structure beneficial for lithium ion-electron diffusion and mitigating the structural strain synergistically contribute to the remarkable improvements in their cycling stability and rate performance.

Mesoporous TiO2 Mesocrystals: Remarkable Defects-Induced Crystallite-Interface Reactivity and Their in Situ Conversion to Single Crystals.

Oriented self-assembly between inorganic nanocrystals and surfactants is emerging as a route for obtaining new mesocrystalline semiconductors. However, the actual synthesis of mesoporous semiconductor mesocrystals with abundant surface sites is extremely difficult, and the corresponding new physical and chemical properties arising from such an intrinsic porous mesocrystalline nature, which is of fundamental importance for designing high-efficiency nanostructured devices, have been rarely explored and poorly understood. Herein, we report a simple evaporation-driven oriented assembly method to grow unprecedented olive-shaped mesoporous TiO2 mesocrystals (FDU-19) self-organized by ultrathin flake-like anatase nanocrystals (∼8 nm in thickness). The mesoporous mesocrystals FDU-19 exhibit an ultrahigh surface area (∼189 m2/g), large internal pore volume (0.56 cm3/g), and abundant defects (oxygen vacancies or unsaturated Ti3+ sites), inducing remarkable crystallite-interface reactivity. It is found that the mesocrystals FDU-19 can be easily fused in situ into mesoporous anatase single crystals (SC-FDU-19) by annealing in air. More significantly, by annealing in a vacuum (∼4.0 × 10–5 Pa), the mesocrystals experience an abrupt three-dimensional to two-dimensional structural transformation to form ultrathin anatase single-crystal nanosheets (NS-FDU-19, ∼8 nm in thickness) dominated by nearly 90% exposed reactive (001) facets. The balance between attraction and electrostatic repulsion is proposed to determine the resulting geometry and dimensionality. Dye-sensitized solar cells based on FDU-19 and SC-FDU-19 samples show ultrahigh photoconversion efficiencies of up to 11.6% and 11.3%, respectively, which are largely attributed to their intrinsic single-crystal nature as well as high porosity. This work gives new understanding of physical and chemical properties of mesoporous semiconductor mesocrystals and opens up a new pathway for designing various single-crystal semiconductors with desired mesostructures for applications in catalysis, sensors, drug delivery, optical devices, etc.

Carbon-Coated Co3+-Rich Cobalt Selenide Derived from ZIF-67 for Efficient Electrochemical Water Oxidation

Oxygen evolution reaction (OER) electrocatalysts are confronted with challenges such as sluggish kinetics, low conductivity, and instability, restricting the development of water splitting. In this study, we report an efficient Co(3+)-rich cobalt selenide (Co0.85Se) nanoparticles coated with carbon shell as OER electrocatalyst, which are derived from zeolitic imidazolate framework (ZIF-67) precursor. It is proposed that the organic ligands in the ZIF-67 can effectively enrich and stabilize the Co(3+) ions in the inorganic-organic frameworks and subsequent carbon-coated nanoparticles. In alkaline media, the catalyst exhibits excellent OER performances, which are attributed to its abundant active sites, high conductivity, and superior kinetics.

Mesoporous Fe2O3–CdS Heterostructures for Real-Time Photoelectrochemical Dynamic Probing of Cu2+

A three-dimensional (3D) mesoporous Fe2O3-CdS nanopyramid heterostructure is developed for solar-driven, real-time, and selective photoelectrochemical sensing of Cu(2+) in the living cells. Fabrication of the mesoporous Fe2O3 nanopyramids is realized by an interfacial aligned growth and self-assembly process, based on the van der drift model and subsequent selective in situ growth of CdS nanocrystals. The as-prepared mesoporous Fe2O3-CdS heterostructures achieve significant enhancement (∼3-fold) in the photocurrent density compared to pristine mesoporous Fe2O3, which is attributed to the unique mesoporous heterostructures with multiple features including excellent flexibility, high surface area (∼87 m(2)/g), and large pore size (∼20 nm), enabling the PEC performance enhancement by facilitating ion transport and providing more active electrochemical reaction sites. In addition, the introduction of Cu(2+) enables the activation of quenching the charge transfer efficiency, thus leading to sensitive photoelectrochemical recording of Cu(2+) level in buffer and cellular environments. Furthermore, real-time monitoring (∼0.5 nM) of Cu(2+) released from apoptotic HeLa cell is performed using the as-prepared 3D mesoporous Fe2O3-CdS sensor, suggesting the capability of studying the nanomaterial-cell interfaces and illuminating the role of Cu(2+) as trace element.

Photoelectrochemical Conversion from Graphitic C3N4 Quantum Dot Decorated Semiconductor Nanowires

Despite the recent progress of developing graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst, the synthesis of nanostructured g-C3N4 has still remained a complicated and time-consuming approach from its bulk powder, which substantially limits its photoelectrochemical (PEC) applications as well as the potential to form composites with other semiconductors. Different from the labor-intensive methods used before, such as exfoliation or assistant templates, herein, we developed a facile method to synthesize graphitic C3N4 quantum dots (g-CNQDs) directly grown on TiO2 nanowire arrays via a one-step quasi-chemical vapor deposition (CVD) process in a homemade system. The as-synthesized g-CNQDs uniformly covered over the surface of TiO2 nanowires and exhibited attractive photoluminescence (PL) properties. In addition, compared to pristine TiO2, the heterojunction of g-CNQD-decorated TiO2 nanowires showed a substantially enhanced PEC photocurrent density of 3.40 mA/cm(2) at 0 V of applied potential vs Ag/AgCl under simulated solar light (300 mW/cm(2)) and excellent stability with ∼82% of the photocurrent retained after over 10 h of continuous testing, attributed to the quantum and sensitization effects of g-CNQDs. Density functional theory calculations were further carried out to illustrate the synergistic effect of TiO2 and g-CNQD. Our method suggests that a variety of g-CNQD-based composites with other semiconductor nanowires can be synthesized for energy applications.