Hernan Sanchez Casalongue

Mn3O4−Graphene Hybrid as a High-Capacity Anode Material for Lithium Ion Batteries

We developed two-step solution-phase reactions to form hybrid materials of Mn(3)O(4) nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Selective growth of Mn(3)O(4) nanoparticles on RGO sheets, in contrast to free particle growth in solution, allowed for the electrically insulating Mn(3)O(4) nanoparticles to be wired up to a current collector through the underlying conducting graphene network. The Mn(3)O(4) nanoparticles formed on RGO show a high specific capacity up to ∼900 mAh/g, near their theoretical capacity, with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn(3)O(4) nanoparticles grown atop. The Mn(3)O(4)/RGO hybrid could be a promising candidate material for a high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for the design and synthesis of battery electrodes based on highly insulating materials.

Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials

Ni(OH)(2) nanocrystals grown on graphene sheets with various degrees of oxidation are investigated as electrochemical pseudocapacitor materials for potential energy storage applications. Single-crystalline Ni(OH)(2) hexagonal nanoplates directly grown on lightly oxidized, electrically conducting graphene sheets (GS) exhibit a high specific capacitance of approximately 1335 F/g at a charge and discharge current density of 2.8 A/g and approximately 953 F/g at 45.7 A/g with excellent cycling ability. The high specific capacitance and remarkable rate capability are promising for applications in supercapacitors with both high energy and power densities. A simple physical mixture of pre-synthesized Ni(OH)(2) nanoplates and graphene sheets shows lower specific capacitance, highlighting the importance of direct growth of nanomaterials on graphene to impart intimate interactions and efficient charge transport between the active nanomaterials and the conducting graphene network. Single-crystalline Ni(OH)(2) nanoplates directly grown on graphene sheets also significantly outperform small Ni(OH)(2) nanoparticles grown on heavily oxidized, electrically insulating graphite oxide (GO), suggesting that the electrochemical performance of these composites is dependent on the quality of graphene substrates and the morphology and crystallinity of the nanomaterials grown on top. These results suggest the importance of rational design and synthesis of graphene-based nanocomposite materials for high-performance energy applications.

Ultrasmall Reduced Graphene Oxide with High Near-Infrared Absorbance for Photothermal Therapy

We developed nanosized, reduced graphene oxide (nano-rGO) sheets with high near-infrared (NIR) light absorbance and biocompatibility for potential photothermal therapy. The single-layered nano-rGO sheets were ∼20 nm in average lateral dimension, functionalized noncovalently by amphiphilic PEGylated polymer chains to render stability in biological solutions and exhibited 6-fold higher NIR absorption than nonreduced, covalently PEGylated nano-GO. Attaching a targeting peptide bearing the Arg-Gly-Asp (RGD) motif to nano-rGO afforded selective cellular uptake in U87MG cancer cells and highly effective photoablation of cells in vitro. In the absence of any NIR irradiation, nano-rGO exhibited little toxicity in vitro at concentrations well above the doses needed for photothermal heating. This work established nano-rGO as a novel photothermal agent due to its small size, high photothermal efficiency, and low cost as compared to other NIR photothermal agents including gold nanomaterials and carbon nanotubes.

LiMn1−xFexPO4 Nanorods Grown on Graphene Sheets for Ultrahigh‐Rate‐Performance Lithium Ion Batteries

Olivine-type lithium transition-metal phosphates LiMPO4 (M=Fe, Mn, Co, or Ni) have been intensively investigated as promising cathode materials for rechargeable lithium ion batteries (LIBs) owing to their high capacity, excellent cycle life, thermal stability, environmental benignity, and low cost. However, the inherently low ionic and electrical conductivities of LiMPO4 seriously limit Li + insertion and extraction and charge transport rates in these materials. In recent years, these obstacles have been overcome for LiFePO4 by reducing the size of LiFePO4 particles to the nanoscale and applying conductive surface coatings such as carbon, which leads to commercially viable LiFePO4 cathode materials. Compared to LiFePO4, LiMnPO4 is an attractive cathode material owing to its higher Li intercalation potential of 4.1 V versus Li/Li (3.4 V for LiFePO4), providing about 20% higher energy density than LiFePO4 for LIBs. [14–19] Importantly, the 4.1 V intercalation potential of LiMnPO4 is compatible with most of the currently used liquid electrolytes. However, the electrical conductivity of LiMnPO4 is lower than the already insulating LiFePO4 by five orders of magnitude, making it challenging to achieve high capacity at high rates for LiMnPO4 using methods developed for LiFePO4. [14–19] Doping LiMnPO4 with Fe has been pursued to enhance conductivity and stability of the material in its charged form. Recently, Martha et al. have obtained improved capacity and rate performance for carbon-coated LiMn0.8Fe0.2PO4 nanoparticles synthesized by a high-temperature solid-state reaction. Graphene is an ideal substrate for growing and anchoring insulating materials for energy storage applications because of its high conductivity, light weight, high mechanical strength, and structural flexibility. The electrochemical performance of various electrode materials can be significantly boosted by rendering them conducting with graphene sheets. Recent work has shown improved specific capacities and rate capabilities of simple oxide nanomaterials (Mn3O4, Co3O4, and Fe3O4) grown on graphene as LIB anode materials. However, it remains a challenge to grow nanocrystals on graphene sheets in solution for materials with more sophisticated compositions and structures, such as LiMn1 xFexPO4, which is a promising but extremely insulating cathode material for LIBs. Herein we present a two-step approach for synthesis of LiMn1 xFexPO4 nanorods on reduced graphene oxide sheets stably suspended in solution. Fe-doped Mn3O4 nanoparticles were first selectively grown onto graphene oxide by controlled hydrolysis. The oxide nanoparticle precursors then reacted solvothermally with Li and phosphate ions and were transformed into LiMn1 xFexPO4 on the surface of reduced graphene oxide sheets. With a total content of 26 wt% conductive carbon, the resulting hybrid of nanorods and graphene showed high specific capacity and unprecedentedly high power rate for LiMn1 xFexPO4 type of cathode materials. Stable capacities of 132 mAhg 1 and 107 mAhg 1 were obtained at high discharge rates of 20C and 50C, which is 85% and 70% of the capacity at C/2 (155 mAhg ), respectively. This affords LIBs with both high energy and high power densities. This is also the first synthesis of LiMn0.75Fe0.25PO4 nanorods that have an ideal crystal shape and morphology for fast Li diffusion along the radial [010] direction of the nanorods. Figure 1 shows our two-step solution-phase reaction for the synthesis LiMn0.75Fe0.25PO4 nanorods on reduced graphene oxide (for experimental details, see the Supporting Information). The first step was to selectively grow oxide nanoparticles at 80 8C on mildly oxidized graphene oxide (mGO) stably suspended in a solution. Controlling the hydrolysis rate of Mn(OAc)2 and Fe(NO3)3 by adjusting the H2O/N,N-dimethylformamide (DMF) solvent ratio and the reaction temperature afforded selective and uniform coating of circa 10 nm nanoparticles of Fe-doped Mn3O4 (Supporting Information, Figure S1a; X-ray diffraction data in Figure S1b) on the mGO sheets without free growth of nanoparticles in solution. Importantly, our mGO was made by a modified Hummers method (Supporting Information), with which a sixfold lower concentration of KMnO4 oxidizer was used to afford milder oxidation of graphite. The resulting mGO sheets contained a lower oxygen content than Hummers GO (ca. 15% vs. ca. 30% measured by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy) and showed higher electrical conductivity when chemically reduced than [*] H. Wang, Y. Liang, H. Sanchez Casalongue, Y. Li, G. Hong, Prof. H. Dai Department of Chemistry Stanford University, Stanford, CA 94305 (USA) E-mail: hdai@stanford.edu Y. Yang, L. Cui, Prof. Y. Cui Department of Materials Science and Engineering Stanford University, Stanford, CA 94305 (USA) E-mail: yicui@stanford.edu [] These authors contributed equally to this work.

In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction

An iridium oxide nanoparticle electrocatalyst under oxygen evolution reaction conditions was probed in situ by ambient-pressure X-ray photoelectron spectroscopy. Under OER conditions, iridium undergoes a change in oxidation state from Ir(IV) to Ir(V) that takes place predominantly at the surface of the catalyst. The chemical change in iridium is coupled to a decrease in surface hydroxide, providing experimental evidence which strongly suggests that the oxygen evolution reaction on iridium oxide occurs through an OOH-mediated deprotonation mechanism.

On the chemical state of Co oxide electrocatalysts during alkaline water splitting

Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

A New Approach to Solution‐Phase Gold Seeding for SERS Substrates

Surface-enhanced Raman scattering (SERS) vastly improves signal-to-noise ratios as compared to traditional Raman scattering, making sensitive assays based upon Raman scattering a reality. However, preparation of highly stable SERS-active gold substrates requires complicated and expensive methodologies and instrumentation. Here, a general and completely solution-phase, seed-based approach is introduced, which is capable of producing gold films for SERS applications on a variety of substrates, not requiring surface modification or functionalization. SERS enhancement factors of ≈10(7) were observed. Moreover, solution-phase gold film deposition on highly complex surfaces, such as protein-coated bioassays, is demonstrated for the first time. Protein bioassays coated with such SERS-active gold films are combined with bioconjugated single-walled carbon nanotube Raman labels, affording highly sensitive detection of the cancer biomarker, carcinoembryonic antigen in serum, with a limit of detection of ≈5 fM (1 pg mL(-1) ).

Different Reactivity of the Various Platinum Oxides and Chemisorbed Oxygen in CO Oxidation on Pt(111)

We have used X-ray photoelectron spectroscopy and polarization-resolved O K-edge X-ray absorption spectroscopy to investigate the reactivity of various oxygen covered Pt(111) surfaces, which emerge under high temperature and pressure conditions, toward CO. We find that the reactivity of the O/Pt(111) system decreases monotonically with increasing oxygen coverage. Of the three surface oxygen phases, viz., chemisorbed oxygen (O(ad)), a PtO-like surface oxide, and α-PtO2 trilayers, Oad exhibits the highest reactivity toward CO, whereas α-PtO2 trilayers exhibit the lowest. Pt(111) surfaces fully terminated by α-PtO2 trilayers are inert to CO. Here it is proposed that the reactive phase is either O(ad) or PtO-like surface oxide phase on bare non-CO poisoned Pt regions with PtO2 as majority spectator species.