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Hydrogen (H2) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H2O). However, H2/air mixtures are explosive at H2 concentrations above 4%, thus any leakage of H2 must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H2 sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H2 chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H2 sensors. Furthermore, recent key advances in other types of H2 sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.

Although Li-ion batteries have emerged as the battery of choice for electric vehicles and large-scale smart grids, significant research efforts are devoted to identifying materials that offer higher energy density, longer cycle life, lower cost, and/or improved safety compared to those of conventional Li-ion batteries based on intercalation electrodes. By moving beyond intercalation chemistry, gravimetric capacities that are 2-5 times higher than that of conventional intercalation materials (e.g., LiCoO2 and graphite) can be achieved. The transition to higher-capacity electrode materials in commercial applications is complicated by several factors. This Review highlights the developments of electrode materials and characterization tools for rechargeable lithium-ion batteries, with a focus on the structural and electrochemical degradation mechanisms that plague these systems.

The oxygen, O2, in air interferes with the detection of H2 by palladium (Pd)-based H2 sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air-relative to N2 or Ar. Here, we describe the preparation of H2 sensors in which a nanofiltration layer consisting of a Zn metal-organic framework (MOF) is assembled onto Pd NWs. Polyhedron particles of Zn-based zeolite imidazole framework (ZIF-8) were synthesized on lithographically patterned Pd NWs, leading to the creation of ZIF-8/Pd NW bilayered H2 sensors. The ZIF-8 filter has many micropores (0.34 nm for gas diffusion) which allows for the predominant penetration of hydrogen molecules with a kinetic diameter of 0.289 nm, whereas relatively larger gas molecules including oxygen (0.345 nm) and nitrogen (0.364 nm) in air are effectively screened, resulting in superior hydrogen sensing properties. Very importantly, the Pd NWs filtered by ZIF-8 membrane (Pd NWs@ZIF-8) reduced the H2 response amplitude slightly (ΔR/R0 = 3.5% to 1% of H2 versus 5.9% for Pd NWs) and showed 20-fold faster recovery (7 s to 1% of H2) and response (10 s to 1% of H2) speed compared to that of pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H2). These outstanding results, which are mainly attributed to the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, rank highest in H2 sensing speed among room-temperature Pd-based H2 sensors.

The recent ExecutiveOrder by President Trump attempting to ban temporarily the citizens of seven countries (Iran, Iraq, Libya, Somalia, Sudan, Syria, and Yemen) from entering the United States is having significant consequences within the country and around the world. The Order poses a threat to the health and vitality of science, barring students and scientists from these countries from traveling to the United States to study or to attend conferences. In preventing those members of the international scientific community from traveling beyond U.S. borders without guaranteed safe return, the Executive Order demeans them; in so doing, it demeans us all. Universities and research communities are especially impacted, as major universities have students and often faculty holding passports from one of these seven countries. This temporary ban would affect refugees fleeing war-torn areas, challenging the long-standing notion that the United States is a safe haven for those fleeing persecution and war in addition to being a magnet for talent from every corner of the world. The pages of this journal reflect the geographic, ethnic, and cultural diversity that underpins great science. The ban impacts domestic and global scientific efforts and communities. Science succeeds through the cooperation between collections of individuals and teams around the world discovering and learning from each other. To ensure rapid scientific progress, open communication and exchange between scientists are essential. As scientists, engineers, and clinicians, we have benefited from open interactions and collaborations with visitors and students from all parts of the world as well as through scientific publications and discussions at scientific meetings.

Nanoscience and nanotechnology are poised to contribute to a wide range of fields, from health and medicine to electronics, energy, security, and more. These contributions come both directly in the form of new materials, interfaces, tools, and even properties as well as indirectly by connecting fields together. We celebrate how far we have come, and here, we look at what is to come over the next decade that will leverage the strong and growing base that we have built in nanoscience and nanotechnology.

Author(s): Voiry, Damien; Chhowalla, Manish; Gogotsi, Yury; Kotov, Nicholas A; Li, Yan; Penner, Reginald M; Schaak, Raymond E; Weiss, Paul S

Platinum (Pt)-modified palladium (Pd) nanowires (or Pd@Pt nanowires) are prepared with controlled Pt coverage. These Pd@Pt nanowires are used as resistive gas sensors for the detection of hydrogen gas in air, and the influence of the Pt surface layer is assessed. Pd nanowires with dimensions of 40 nm (h) × 100 nm (w) × 50 μm (l) are first prepared using lithographically patterned nanowire electrodeposition. A thin Pt surface layer is electrodeposited conformally onto a Pd nanowire at coverages, θPt, of 0.10 monolayer (ML), 1.0 ML, and 10 ML. X-ray photoelectron spectroscopy coupled with scanning electron microscopy and electrochemical measurements is consistent with a layer-by-layer deposition mode for Pt on the Pd nanowire surface. The resistance of a single Pd@Pt nanowire is measured during the exposure of these nanowires to pulses of hydrogen gas in air at concentrations ranging from 0.05 to 5.0 vol %. Both Pd nanowires and Pd@Pt nanowires show a prompt and reversible increase in resistance upon exposure to H2 in air, caused by the conversion of Pd to more resistive PdHx. Relative to a pure Pd nanowire, the addition of 1.0 ML of Pt to the Pd surface alters the H2 detection properties of Pd@Pt nanowires in two ways. First, the amplitude of the relative resistance change, ΔR/R0, measured at each H2 concentration is reduced at low temperatures (T = 294 and 303 K) and is unaffected at higher temperatures (T = 316, 344, and 376 K). Second, response and recovery rates are both faster at all temperatures in this range and for all H2 concentrations. For higher θPt = 10 ML, sensitivity to H2 is dramatically reduced. For lower θPt = 0.1 ML, no significant influence on sensitivity or the speed of response/recovery is observed.

Author(s): Chan, Warren WC; Chhowalla, Manish; Glotzer, Sharon; Gogotsi, Yury; Hafner, Jason H; Hammond, Paula T; Hersam, Mark C; Javey, Ali; Kagan, Cherie R; Khademhosseini, Ali; Kotov, Nicholas A; Lee, Shuit-Tong; Li, Yan; Mohwald, Helmuth; Mulvaney, Paul A; Nel, Andre E; Nordlander, Peter J; Parak, Wolfgang J; Penner, Reginald M; Rogach, Andrey L; Schaak, Raymond E; Stevens, Molly M; Wee, Andrew TS; Willson, C Grant; Fernandez, Laura E; Weiss, Paul S

The oxygen, O

Electroluminescence (EL) from nanocrystalline CdSe (nc-CdSe) nanowire arrays is reported. The n-type, nc-CdSe nanowires, 400-450 nm in width and 60 nm in thickness, were synthesized using lithographically patterned nanowire electrodeposition, and metal-semiconductor-metal (M-S-M) devices were prepared by the evaporation of two gold contacts spaced by either 0.6 or 5 μm. These M-S-M devices showed symmetrical current voltage curves characterized by currents that increased exponentially with applied voltage bias. As the applied biased was increased, an increasing number of nanowires within the array "turned on", culminating in EL emission from 30 to 50% of these nanowires at applied voltages of 25-30 V. The spectrum of the emitted light was broad and centered at 770 nm, close to the 1.74 eV (712 nm) band gap of CdSe. EL light emission occurred with an external quantum efficiency of 4 × 10(-6) for devices with a 0.60 μm gap between the gold contacts and 0.5 × 10(-6) for a 5 μm gap-values similar to those reported for M-S-M devices constructed from single-crystalline CdSe nanowires. Kelvin probe force microscopy of 5 μm nc-CdSe nanowire arrays showed pronounced electric fields at the gold electrical contacts, coinciding with the location of strongest EL light emission in these devices. This electric field is implicated in the Poole-Frenkel minority carrier emission and recombination mechanism proposed to account for EL light emission in most of the devices that were investigated.

The preparation by electrodeposition of transverse nanowire electroluminescent junctions (tn-ELJs) is described, and the electroluminescence (EL) properties of these devices are characterized. The lithographically patterned nanowire electrodeposition process is first used to prepare long (millimeters), linear, nanocrystalline CdSe nanowires on glass. The thickness of these nanowires along the emission axis is 60 nm, and the width, wCdSe, along the electrical axis is adjustable from 100 to 450 nm. Ten pairs of nickel-gold electrical contacts are then positioned along the axis of this nanowire using lithographically directed electrodeposition. The resulting linear array of nickel-CdSe-gold junctions produces EL with an external quantum efficiency, EQE, and threshold voltage, Vth, that depend sensitively on wCdSe. EQE increases with increasing electric field and also with increasing wCdSe, and Vth also increases with wCdSe and, therefore, the electrical resistance of the tn-ELJs. Vth down to 1.8(±0.2) V (for wCdSe ≈ 100 nm) and EQE of 5.5(±0.5) × 10(-5) (for wCdSe ≈ 450 nm) are obtained. tn-ELJs produce a broad EL emission envelope, spanning the wavelength range from 600 to 960 nm.

Nanocrystalline cadmium selenide (nc-CdSe) was electrodeposited within a sub-50 nm gold nanogap, prepared by feedback-controlled electromigration, to form a photoconductive metal-semiconductor-metal nanojunction. Both gap formation and electrodeposition were rapid and automated. The electrodeposited nc-CdSe was stoichiometric, single cubic phase with a mean grain diameter of ∼7 nm. Optical absorption, photoluminescence, and the spectral photoconductivity response of the nc-CdSe were all dominated by band-edge transitions. The photoconductivity of these nc-CdSe-filled gold nanogaps was characterized by a detectivity of 6.9 × 10(10) Jones and a photosensitivity of 500. These devices also demonstrated a maximum photoconductive gain of ∼45 and response and recovery times below 2 μs, corresponding to a 3 dB bandwidth of at least 175 kHz.

Nanocrystalline cadmium selenide (nc-CdSe) nanowires were prepared using the lithographically patterned nanowire electrodeposition method. Arrays of 350 linear nc-CdSe nanowires with lateral dimensions of 60 nm (h) × 200 nm (w) were patterned at 5 μm pitch on glass. nc-CdSe nanowires electrodeposited from aqueous solutions at 25 °C had a mean grain diameter, d(ave), of 5 nm. A combination of three methods was used to increase d(ave) to 10, 20, and 100 nm: (1) The deposition bath was heated to 75 °C, (2) nanowires were thermally annealed at 300 °C, and (3) nanowires were exposed to methanolic CdCl(2) followed by thermal annealing at 300 °C. The morphology, chemical composition, grain diameter, and photoconductivity of the resulting nanowires were studied as a function of d(ave). As d(ave) was increased from 10 to 100 nm, the photoconductivity response of the nanowires was modified in two ways: First, the measured photoconductive gain, G, was elevated from G = 0.017 (d(ave) = 5 nm) to ∼4.9 (100 nm), a factor of 290. Second, the photocurrent rise time was increased from 8 μs for d(ave) = 10 nm to 8 s for 100 nm, corresponding to a decrease by a factor of 1 million of the photoconduction bandwidth from 44 kHz to 44 mHz.

Lithographically patterned nanowire electrodeposition (LPNE) is a new method for fabricating polycrystalline metal nanowires using electrodeposition. In LPNE, a sacrificial metal (M(1)=silver or nickel) layer, 5-100 nm in thickness, is first vapor deposited onto a glass, oxidized silicon, or Kapton polymer film. A (+) photoresist (PR) layer is then deposited, photopatterned, and the exposed Ag or Ni is removed by wet etching. The etching duration is adjusted to produce an undercut approximately 300 nm in width at the edges of the exposed PR. This undercut produces a horizontal trench with a precisely defined height equal to the thickness of the M(1) layer. Within this trench, a nanowire of metal M(2) is electrodeposited (M(2)=gold, platinum, palladium, or bismuth). Finally the PR layer and M(1) layer are removed. The nanowire height and width can be independently controlled down to minimum dimensions of 5 nm (h) and 11 nm (w), for example, in the case of platinum. These nanowires can be 1 cm in total length. We measure the temperature-dependent resistance of 100 microm sections of Au and Pd wires in order to estimate an electrical grain size for comparison with measurements by X-ray diffraction and transmission electron microscopy. Nanowire arrays can be postpatterned to produce two-dimensional arrays of nanorods. Nanowire patterns can also be overlaid one on top of another by repeating the LPNE process twice in succession to produce, for example, arrays of low-impedance, nanowire-nanowire junctions.

Despite the fact that polymer surfaces are soft, they are notoriously difficult to pattern over large areas on the nanoscale. Two previously described methods, nanoimprint lithography (NIL) and nanosphere lithography (NSL), can be used to nanopattern polymer surfaces, but both of these methods involve many (>6) processing steps. Laser interference patterning (LIP) is a maskless surface nanopatterning technology that has been around for more than 10 years. In this issue of ACS Nano, Wang and co-workers demonstrate that LIP can form the basis for a simplified nanopatterning scheme that is general for a wide variety of polymer surfaces. As reported in this issue of ACS Nano, laser interference patterning (LIP) has been adapted to the problem of nanopatterning polymer surfaces rapidly and over wafer-scale areas.

Arrays of mesoporous manganese dioxide, mp-MnO(2), nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO(4))(2)-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This "multipulse" deposition program produced mp-MnO(2) nanowires with a total porosity of 43-56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3-5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO(2) nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO(2) nanowires were characterized by a specific capacitance, C(sp), of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These C(sp) values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total C(sp) were deconvoluted as a function of the potential scan rate.

Palladium nanowires prepared using the lithographically patterned nanowire electrodeposition (LPNE) method are used to detect hydrogen gas (H2). These palladium nanowires are prepared by electrodepositing palladium from EDTA-containing solutions under conditions favoring the formation of β-phase PdHx. The Pd nanowires produced by this procedure are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. These nanowires have a mean grain diameter of 15 nm and are composed of pure Pd with no XPS-detectable bulk carbon. The four-point resistance of 50-100 μm segments of individual nanowires is used to detect H2 in N2 and air at concentrations ranging from 2 ppm to 10%. For low [H2]1%, the response amplitude increases by a factor of 2-3 with a reduction in the lateral dimensions of the nanowire. Smaller nanowires show accelerated response and recovery rates at all H2 concentrations from, 5 ppm to 10%. For 12 devices, response and recovery times are correlated with the surface area/volume ratio of the palladium detection element. We conclude that the kinetics of hydrogen adsorption limits the observed response rate seen for the nanowire, and that hydrogen desorption from the nanowire limits the observed recovery rate; proton diffusion within PdHx does not limit the rates of either of these processes.

Nanowires of lead telluride (PbTe) were patterned on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). LPNE involved the fabrication by photolithography of a contoured nickel nanoband that is recessed by approximately 300 nm into a horizontal photoresist trench. Cubic PbTe was then electrodeposited from a basic aqueous solution containing Pb(2+) and TeO(3)(2-) at the nickel nanoband using a cyclic deposition/stripping potential program in which lead-rich PbTe was first deposited in a negative-going potential scan and excess lead was then anodically stripped from the nascent nanowire by scanning in the positive direction to produce near stoichiometric PbTe. Repeating this scanning procedure permitted PbTe nanowires 60-400 nm in width to be obtained. The wire height was controlled over the range of 20-100 nm based upon the nickel film thickness. Nanowires with lengths exceeding 1 cm were prepared in this study. We report the characterization of these nanowires using X-ray diffraction, transmission electron microscopy and electron diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The surface chemical composition of PbTe nanowires was monitored by XPS as a function of time during the exposure of these nanowires to laboratory air. One to two monolayers of a mixed Pb and Te oxide are formed during a 24 h exposure. The electrical conductivity of PbTe nanowires was strongly affected by air oxidation, declining from an initial value of 2.0(+/-1.5) x 10 (4) S/m by 61% (for nanowires with a 20 nm thickness), 55% (for 40 nm), and 12% (for 60 nm).