Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (34 results)

  • Article
  • Peer Reviewed
Chemical Sciences (2019)
Crystalline polymorphs have been considered a prevailing phenomenon in inorganic nanocrystals and provide approaches to modulate fundamental properties and innovative advanced applications. As a basic demand for phase engineering, accessible and controllable synthetic methodologies are indispensable for acquisition of high-quality products in expected phases. Phase stability is also a non-negligible issue that determines continuous gains of functionality and long-term sustainability of characteristic features. Maintaining structural stability of metastable phases provides challenges and opportunities for investigations on fascinating properties and intriguing applications of inorganic nanocrystals. Phase engineering is of great significance to acquire metallic (1T) and semiconducting (2H) Mo- and W-based dichalcogenides for hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR), respectively. The catalysts in 1T phase have superior electron transfer kinetics and abundant active sites on both basal planes and edges for HER, while ones in 2H phase are preferentially deployed for CO2RR to utilize edge sites for catalysis and restrain competitive HER activity. In addition, the photocatalytic performance for HER has been enhanced by combining anatase and rutile phases because electron transfer between the two phases during photocatalysis facilitates the separation of charge carriers and thus impedes the recombination of electron-hole pairs. Although ample effort has been devoted to developing phase engineering, principle understanding at an ultrathin scale remains obscure. In this Account, we provide comprehensive insight into work from our group regarding controllable synthesis of inorganic nanocrystals with phase engineering, critical effects on phase stability, and noteworthy studies on phase-related properties and applications. For bulk materials, phase control and transition have a large energy barrier, so they can only be achieved under rigorous conditions. However, at the initial stage of synthesis, especially for nucleation, there are a small quantity of chemical bonds that contribute to regulate phase and structure with ease. In our work, we mainly modulate nucleation and growth at an ultrathin scale to demonstrate facile approaches for phase engineering. This unique perspective makes for a distinct guidance of controllable synthesis and deliberate stabilization of inorganic nanomaterials with phase engineering. We have developed a series of synthetic strategies for phase engineering to fabricate inorganic nanocrystals in a specific phase with controlled size and composition and adjustable morphologies and surface features. Four sorts of models (MoS2, ZrO2, In2O3, and TiO2) are used for demonstrating finely tuned growth at an ultrathin scale. However, phase engineering has been regarded as immature because only one phase in polymorphs is thermodynamically stable generally. Phase stability of metastable nanocrystals has attracted much interest. Our substantial investigations illustrate several crucial factors on phase stability, leading to inspiration for facilitating persistent emergence of characteristics and functionalities. By full use of the features of a specific phase, we spotlight ligand-induced surface interactions on coverage-dependent electronic structures and chemisorption effects at one-unit thickness of TiO2(B) nanomaterials with phase engineering. Meanwhile, an energy conversion system for overall water splitting (OWS) drives forward steps in function-oriented synthesis of MoS2-based nanomaterials with phase engineering. In the last section, we summarize this theme and highlight several promising directions for future development.
    Cover page: Phase Control in Inorganic Nanocrystals through Finely Tuned Growth at an Ultrathin Scale
    • Thesis
    • Peer Reviewed
    UC Davis Electronic Theses and Dissertations (2021)

    Reinforced concrete (RC) has been extensively used for construction of building and bridge infrastructures for over a century, yet the corrosion of steel reinforcement may significantly limit the long-term performance of these structures. Instead of demolition and reconstruction, retrofitting and strengthening of the existing structures to extend their service life and upgrade their load-carrying capacity to accommodate the modern demand is often more cost efficient and practical. In this realm, fiber-reinforced polymer (FRP) composite materials have shown more potential than steel plates because they are corrosion-free, higher in strength and stiffness to weight ratios, lightweight and more durable. In the recent years, using the near-surface mounted (NSM) FRP composites to strengthen the deficient structures has attracted broad research attention, as a promising alternative to the externally bonded (EB) FRP method. However, research on the characteristics of bond between NSM FRP and concrete under fatigue loading is still lacking, and a comprehensive study focusing on the bond level is necessary prior to the practical implementation in the field. This research aims to significantly advance our understandings of the bond development and failure mechanism of NSM carbon FRP (CFRP) reinforcements in concrete under fatigue loading, as well as the influence on the local bond characteristics induced by the key parameters. A total of 84 NSM CFRP-to-concrete joint specimens, with a dimension of 350 × 300 × 150 mm, were constructed and tested under a single shear direct pull-out configuration, including 24 specimens under static loading and 60 specimens under fatigue loading. Investigated variables in the experiment include: different cross‐sectional shape (rod vs. strip), surface treatment of NSM reinforcement (roughened, sand‐coated, sand‐coated and spirally wound), adhesive type (one three-component epoxy and three two-component epoxies), concrete strength (two concrete batches) and fatigue load range (10-50%, 10-60%, and 10-70% of the corresponding static load-carrying capacity Pf). The local bond degradation was observed for all the specimens under fatigue loading. In general, the bond regions closer to the loaded end of the specimen were firstly developed to resist the applied fatigue load. With the gradual local bond degradation within these regions, more bond regions were further developed along the bond line toward the free end during the fatigue cycles. This phenomenon was accompanied by a peak local bond stress migration from the loaded end to the free end of the specimen. The specimen failed when the residual load-carrying capacity was unbale to balance the applied fatigue load. The test results also indicated that the fatigue cycles changed the failure mechanism of CFRP rod specimens from concrete and epoxy breakage under static loading to interfacial debonding between FRP and adhesive by different extents, but the CFRP strip specimens had the same interfacial debonding failure mode in both loading cases. In addition, under the same 10-60%Pf fatigue load range, specimens using CFRP strip with sand-coated and spirally wound surface and three-component epoxy had a better resistance to fatigue degradation and longer fatigue life. Furthermore, a higher fatigue load range (i.e., 10-70%Pf) significantly shorten the fatigue life of specimens, but the concrete strength did not noticeably affect the fatigue bond behavior. An analytical model adopting the finite element method (FEM) was proposed based on a trilinear local bond stress-slip law (representing the distinctive linear elastic, softening and debonding stages), and it provided good agreement with the experimental data under both static and fatigue loading cases. The model well captured the migration of the peak local bond stress toward the free end of the NSM FRP-to-concrete bonded joints during fatigue cycles, and it visualized the bond development and failure procedure because of the local bond degradation. Additionally, the parametric study based on the analytical model confirmed the importance of the local bond strength, as well as the positive effect induced by a higher residual bond strength ratio and a higher Young’s modulus of FRP reinforcements on the fatigue life of specimen. To supplement the experimental study, a three-dimensional (3D) finite element (FE) modeling using the concrete damaged plasticity (CDP) model in ABAQUS was adopted to simulate both NSM CFRP rod and strip joint specimens under the static direct pull-out force. The concrete breakage failure of CFRP rod specimen and interfacial debonding failure of CFRP strip specimen were successfully reproduced in the analysis. The parametric study from the numerical simulation showed that a higher dilation angle and concrete strength could increase the load-carrying capacity of the specimen, but the influence caused by groove dimension was negligible. To sum up, this research conducts a thorough investigation on the fatigue bond characteristics of NSM FRP-to-concrete joint specimens both experimentally and analytically. Model predictions in terms of pull-out force, distribution of FRP strain and local bond stress, and relative slip between FRP reinforcement and concrete are verified by the experimental study. The proposed strategy of FE modeling also provides an efficient alternative to the static direct pull-out test to study the bond performance of NSM CFRP reinforcements in concrete.

      Cover page: Fatigue Bond Characteristics and Modeling of Near Surface Mounted FRP Reinforcements in Concrete
      • Article
      • Peer Reviewed
      LBL Publications (2015)
      Two-dimensional (2D) nanomaterials such as transition metal dichalcogenides (TMDs) and graphene have attracted extensive interest as emergent materials, owing to their excellent properties that favor their future use in electronic devices, catalysis, optics, and biological- or energy-relevant areas. However, 2D nanosheets tend to easily restack and condense, which weakens their performance in many of these applications. Assembling these 2D nanosheets as building blocks for three-dimensional (3D) architectures not only maintains the intrinsic performances of the 2D nanostructures but also synergistically makes use of the advantages of the 3D microstructures to improve the overall material properties. In this critical review, we will highlight recent developments of sundry 2D nanosheet-assembled 3D architectures, including their design, synthesis, and potential applications. Their controllable syntheses, novel structures, and potential applications will be systematically explained, analyzed, and summarized. In the end, we will offer some perspective on the challenges facing future advancement of this field.
        Cover page: Three-dimensional architectures constructed using two-dimensional nanosheets
        • Article
        • Peer Reviewed
        Chemical Sciences (2016)
        A series of nickel phosphate single-walled nanotubes with controllable structure parameters are fabricated by utilizing alkali metal ions with the same charge number but different ion radii. Different electrostatic interactions between the cations and related growing intermediates are demonstrated to play a crucial role in controlling the growing process of nanotubes.
          Cover page: Electrostatic Interaction‐Directed Growth of Nickel Phosphate Single‐Walled Nanotubes for High Performance Oxygen Evolution Reaction Catalysts
          • Article
          • Peer Reviewed
          Chemical Sciences (2015)
          A preliminary study of the growth of 0D, 1D, and 2D nanostructures of uranium oxides with feature sizes from several nanometers down to 1 nm are presented. Cerium is successfully doped into these oxides and its influence on the growth dynamics and electrochemical performance investigated.
            Cover page: Nanocrystals of Uranium Oxide: Controlled Synthesis and Enhanced Electrochemical Performance of Hydrogen Evolution by Ce Doping
            • Article
            • Peer Reviewed
            LBL Publications (2019)
            Structure–activity relationship (SAR) is the key problem of nanoscience, thus to fabricate novel and well-defined nanostructure will provide a new insight on catalyst preparation method. Highly active and low cost electrocatalysts for oxygen evolution reaction (OER) are of great importance for future renewable energy conversion and storage. Herein, NiFe-based layered double hydroxides with laminar structure (NFLS) were successfully fabricated via a one-step hydrothermal approach by using sodium dodecyl sulfate as surfactant. The as-fabricated NFLS showed a well-defined periodic layered-stacking geometry with a scale down to 1-nm. Benefitting from the unique structure, NFLS exhibited an excellent catalytic activity towards OER with current densities of 10 mA·cm−2 at overpotential of 197 mV. The synergistic effect of Ni and Fe plays a key role in electrode reactions. The present work provides a new insight to improve the OER performance by rational design of electrocatalysts with unique structures. [Figure not available: see fulltext.].
              Cover page: Fabrication of NiFe layered double hydroxide with well-defined laminar superstructure as highly efficient oxygen evolution electrocatalysts
              • Article
              • Peer Reviewed
              LBL Publications (2018)
              Electrochemical or photoelectrochemical water splitting is an environmentally friendly technology for H2 production to replace fossil fuels for industrial and social utilization of energy. Metallic transition-metal dichalcogenide (TMDC) nanomaterials are significantly promising for applications in practical catalysis of water splitting to replace state-of-the-art noble-metal-based catalysts. High intrinsic activity and remarkable conductivity are the main advantages of these materials. Structural and compositional design can also be used to improve the catalytic performance.
                Cover page: Metallic Transition-Metal Dichalcogenide Nanocatalysts for Energy Conversion
                • Article
                • Peer Reviewed
                Chemical Sciences (2017)
                Fullerene-like nickel oxysulfide hollow nanospheres with ≈50 nm are constructed by in situ growth on the surface of nickel foam by taking advantage of solvothermal reaction. The as-prepared composite exhibits exhilaratingly high HER and OER performance in 1 m KOH, which opens up a very promising aspect for non-noble metal chalcogenides as bifunctional electrocatalysts.
                  Cover page: Fullerene‐Like Nickel Oxysulfide Hollow Nanospheres as Bifunctional Electrocatalysts for Water Splitting
                  • Article
                  • Peer Reviewed
                  LBL Publications (2016)
                  Transition metal dichalcogenides (TMDs, such as MoS2, WS2 and MoSe2) with lamellar structures have attracted tremendous attention owing to their similar structure to graphite. However, two-dimensional (2D) TMD nanomaterials display a strong tendency to restack and condense, which leads to a marked decrease of performance in their applications. In this study, we report the synthesis of TMDs by using polyoxometalates (POMs) as tungsten or molybdenum precursors. Hierarchical TMDs with novel structures can be formed below 200 °C, including WS2 rods, MoS2 rods and MoSe2 hollow spheres. All of these structures assembled from the corresponding nanosheets. Meanwhile the epitaxial growth of noble metal nanoparticles (NPs) on TMDs could be achieved very easily. The obtained Pt/WS2 and Pt/MoS2 hybrids were highly active and durable HER catalysts. In addition, the Pt NPs on the surface of TMDs could improve the HER activity sharply, which was likely due to the synergistic effect between Pt NPs and MoS2 rods.
                    Cover page: Generalized Synthesis of Hierarchical Transition Metal Dichalcogenide Nanosheets from Polyoxometalates
                    • Article
                    • Peer Reviewed
                    Chemical Sciences (2016)
                    Sub-1 nm, extremely long nickel molybdate nanowires are synthesized based on a good/poor solvent system. The ultrathin nanowires can be hierarchically assembled into flexible, free-standing films with good mechanical properties. Compared with the large-size counterpart, nickel molybdate ultrathin nanowires display promising oxygen evolution reaction catalytic performance derived from the ultrathin feature.
                      Cover page: Sub‐1 nm Nickel Molybdate Nanowires as Building Blocks of Flexible Paper and Electrochemical Catalyst for Water Oxidation
                      Top