Pump-probe-alternating photothermal interferometry for two-component gas sensing.

We demonstrate a high-sensitivity acetylene/methane gas sensor based on hollow-core fiber photothermal interferometry (PTI) with a pump-probe-alternating technique. This technique utilizes two distributed-feedback lasers as pump and probe beams alternatively for two gas components to facilitate photothermal phase modulation and detection through time-division multiplexing. With a 2.5-cm-long hollow-core conjoint-tube fiber, noise-equivalent concentrations of 370 ppb and 130 ppb are demonstrated for methane and acetylene, respectively. Noise characteristics of the PTI system are analyzed and experimentally tested. The proposed technique eliminates the need for an additional laser in the traditional PTI setup, enabling the construction of a sensitive yet more cost-effective multi-gas component detection system.

Amplified Photothermal Phase Modulation for Carbon Dioxide Detection by Operating a Dual-Mode Interferometer at Destructive Interference.

Photothermal interferometry is a highly sensitive spectroscopic technique for trace gas detection. However, the performance of the state-of-the-art laser spectroscopic sensors is still insufficient for some high-precision applications. Here, we demonstrate optical phase-modulation amplification by operating a dual-mode optical fiber interferometer at destructive interference for ultrasensitive carbon dioxide detection. With a 50 cm long dual-mode hollow-core fiber, amplification of photothermal phase modulation by a factor of nearly 20 is achieved, which enables carbon dioxide detection down to 1 parts-per-billion with a dynamic range of over 7 orders of magnitude. This technique could be readily used to improve the sensitivity of phase modulation-based sensors with a compact and simple configuration.

NiFe layered double hydroxide nanosheet array for high-efficiency electrocatalytic reduction of nitric oxide to ammonia.

Here, we demonstrate that under ambient conditions, a nickel-iron layered double hydroxide nanosheet array can exhibit a promising NORR performance, delivering a maximal faradaic efficiency of 82% and a corresponding yield rate of 112 µmol h-1 cm-2, along with high stability for over 30 h. This superior performance is further confirmed as a proof-of-concept for a Zn-NO battery, in which a peak power density of 1.8 mW cm-2 and a large NH3 yield rate of 32 µmol h-1 cm-2 are observed. Theoretical analyses indicate that NiFe-LDH exhibits effective NO activation capacity and slow hydrogen evolution kinetics.

Heteroatom coordination induces electric field polarization of single Pt sites to promote hydrogen evolution activity.

Herein, we reported a kind of single Pt site (Pt-SA) stabilized on an MXene support (Pt-SA/MXene) via the formation of Pt-O and Pt-Ti bonds to effectively catalyze the hydrogen evolution reaction (HER). Due to the local electric field polarization derived from its unique asymmetric coordination, Pt-SA/MXene displays remarkably higher catalytic HER activity in an alkaline electrolyte. In detail, the Pt-SA/MXene electrocatalyst only needs a low overpotential of 33 mV to reach a current density of 10 mA cm-2 and maintains the performance over 27 h. Besides, Pt-SA/MXene also has a competitive mass activity, 23.5 A mgPt-1, at an overpotential of 100 mV, which is 29.4 times greater than that of the commercial Pt/C counterpart. Density functional theory (DFT) calculations revealed that the polarized electric field could efficiently tailor the electronic structure of Pt-SA/MXene and reduce the energy barrier of adsorption/desorption of the H* intermediate step, further improving its HER catalytic activity.

Isolated copper single sites for high-performance electroreduction of carbon monoxide to multicarbon products.

Electrochemical carbon monoxide reduction is a promising strategy for the production of value-added multicarbon compounds, albeit yielding diverse products with low selectivities and Faradaic efficiencies. Here, copper single atoms anchored to Ti3C2Tx MXene nanosheets are firstly demonstrated as effective and robust catalysts for electrochemical carbon monoxide reduction, achieving an ultrahigh selectivity of 98% for the formation of multicarbon products. Particularly, it exhibits a high Faradaic efficiency of 71% towards ethylene at -0.7 V versus the reversible hydrogen electrode, superior to the previously reported copper-based catalysts. Besides, it shows a stable activity during the 68-h electrolysis. Theoretical simulations reveal that atomically dispersed Cu-O3 sites favor the C-C coupling of carbon monoxide molecules to generate the key *CO-CHO species, and then induce the decreased free energy barrier of the potential-determining step, thus accounting for the high activity and selectivity of copper single atoms for carbon monoxide reduction.

Ethane detection with mid-infrared hollow-core fiber photothermal spectroscopy.

We report a compact mid-infrared (MIR) photothermal spectroscopic ethane (C2H6) sensor with a hollow-core negative-curvature-fiber (HC-NCF) gas cell. The HC-NCF supports low-loss transmission of an MIR pump (3.348 µm) and a near-infrared (NIR) probe (1.55 µm). The pump and probe laser beams are launched into the gas cell from the opposite ends of the HC-NCF, allowing independent MIR pump delivery and NIR fiber-optic probe circuitry. The use of Fabry-Perot as the probe interferometer simplifies the sensor design and suppresses the common-mode noise in the lead in/out single-mode fiber. With a 14-cm-long HC-NCF, an ethane sensor system with the limit of detection (LOD) of 13 parts-per-billion (ppb) is achieved with 1 s lock-in time constant. The LOD goes down to 2.6 ppb with 410 s average time, which corresponds to noise equivalent absorption (NEA) of 2.0×10-6 and is a record for the hollow-core fiber MIR gas sensors. The system instability is 2.2% over a period of 8 hours.

Estimation of the spatial distribution of Frenkel defects in NiFe2O4 by simulation of HAADF-STEM images.

Accurate determination of the atomic spatial configuration of Frenkel defects is important for understanding the mechanism and fully utilizing these defects to optimize the material properties. In this study, aberration-corrected scanning transmission electron microscopy (STEM) was used to identify the Fe vacancies and Fe Frenkel defect pairs, which have not been previously investigated, in NiFe2O4 (NFO). The spatial distribution of these point defects is determined by comparing the experimental and simulated images, where the experimental image intensities are consistent with the calculated image intensities. We confirmed the stabilities of the observed point defect configurations and calculated their electronic structures using density functional theory. A comprehensive understanding of the relationship between the Frenkel defect spatial configurations and electronic properties is obtained, which provides an alternative method to regulate the NFO performance.

Oxygen Gas Sensing with Photothermal Spectroscopy in a Hollow-Core Negative Curvature Fiber.

We demonstrate a compact all-fiber oxygen sensor using photothermal interferometry with a short length (4.3 cm) of hollow-core negative curvature fibers. The hollow-core fiber has double transmission windows covering both visible and near-infrared wavelength regions. Absorption of a pump laser beam at 760 nm produces photothermal phase modulation and a probe Fabry-Perot interferometer operating at 1550 nm is used to detect the phase modulation. With wavelength modulation and first harmonic detection, a limit of detection down to 54 parts per million (ppm) with a 600-s averaging time is achieved, corresponding to a normalized equivalent absorption of 7.7 × 10-8 cm-1. The oxygen sensor has great potential for in situ detection applications.

Trifunctional Single-Atomic Ru Sites Enable Efficient Overall Water Splitting and Oxygen Reduction in Acidic Media.

Development of cost-effective, active trifunctional catalysts for acidic oxygen reduction (ORR) as well as hydrogen and oxygen evolution reactions (HER and OER, respectively) is highly desirable, albeit challenging. Herein, single-atomic Ru sites anchored onto Ti3 C2 Tx MXene nanosheets are first reported to serve as trifunctional electrocatalysts for simultaneously catalyzing acidic HER, OER, and ORR. A half-wave potential of 0.80 V for ORR and small overpotentials of 290 and 70 mV for OER and HER, respectively, at 10 mA cm-2 are achieved. Hence, a low cell voltage of 1.56 V is required for the acidic overall water splitting. The maximum power density of an H2 -O2 fuel cell using the as-prepared catalyst can reach as high as 941 mW cm-2 . Theoretical calculations reveal that isolated Ru-O2 sites can effectively optimize the adsorption of reactants/intermediates and lower the energy barriers for the potential-determining steps, thereby accelerating the HER, ORR, and OER kinetics.

Nitrogen doping and titanium vacancies synergistically promote CO2 fixation in seawater.

The electrocatalytic generation of useful chemicals from CO2, H2O, and sustainable energy resources offers a promising strategy for the carbon cycle. However, the current CO2 electrolysis system is mainly operated in artificial electrolytes (e.g. ionic liquids and inorganic salt solutions), of which the high cost and impractical working conditions hinder its large-scale development. In this case, seawater represents an attractive alternative due to its abundance and good conductivity. Herein, we show that N-doping and titanium vacancies (VTi) can be introduced in Ti3C2 MXene nanosheets via a facile NH3-etching pyrolysis approach. These nanosheets demonstrate impressive CO2 reduction reaction (CO2RR) performances in seawater with a remarkable 92% faradaic efficiency and a partial current density of -16.2 mA cm-2 for CO production, being close to those of noble metal electrodes. Mechanistic studies reveal that the existence of N dopants and VTi synergistically modulates the electronic structure of the active Ti site, on which the free energy barriers for the key *COOH formation and desorption of *CO are greatly reduced, thereby leading to a notable CO2RR improvement. This study provides an opportunity for developing an active and cost-effective CO2 electrolysis system by using seawater as the electrolyte.

Modeling and performance evaluation of in-line Fabry-Perot photothermal gas sensors with hollow-core optical fibers.

We study photothermal phase modulation in gas-filled hollow-core optical fibers with differential structural dimensions and attempt to develop highly sensitive practical gas sensors with an in-line Fabry-Perot interferometer for detection of the phase modulation. Analytical formulations based on a hollow-capillary model are developed to estimate the amplitude of photothermal phase modulation at low modulation frequencies as well as the -3 dB roll-off frequency, which provide a guide for the selection of hollow-core fibers and the pump modulation frequencies to maximize photothermal phase modulation. Numerical simulation with the capillary model and experiments with two types of hollow-core fibers support the analytical formulations. Further experiments with an Fabry-Perot interferometer made of 5.5-cm-long anti-resonant hollow-core fiber demonstrated ultra-sensitive gas detection with a noise-equivalent-absorption coefficient of 2.3×10-9 cm-1, unprecedented dynamic range of 4.3×106 and <2.5% instability over a period of 24 hours.

Mode-phase-difference photothermal spectroscopy for gas detection with an anti-resonant hollow-core optical fiber.

Laser spectroscopy outperforms electrochemical and semiconductor gas sensors in selectivity and environmental survivability. However, the performance of the state-of-the-art laser sensors is still insufficient for many high precision applications. Here, we report mode-phase-difference photothermal spectroscopy with a dual-mode anti-resonant hollow-core optical fiber and demonstrate all-fiber gas (acetylene) detection down to ppt (parts-per-trillion) and <1% instability over a period of 3 hours. An anti-resonant hollow-core fiber could be designed to transmit light signals over a broad wavelength range from visible to infrared, covering molecular absorption lines of many important gases. This would enable multi-component gas detection with a single sensing element and pave the way for ultra-precision gas sensing for medical, environmental and industrial applications.

Oxygen Doping Induced by Nitrogen Vacancies in Nb4 N5 Enables Highly Selective CO2 Reduction.

Surface vacancy engineering holds great promise for boosting the electrocatalytic activity for CO2 reduction reaction; however, the vacancies are generally unstable and may degrade into the inactive phase during electrolysis. Stabilizing the vacancy-enriched structure by heteroatoms can be an effective strategy to get a robust and active catalyst. Herein, a nitrogen-vacancy enriched Nb4 N5 on N-doped carbons is constructed, which is thereafter stabilized by a self-enhanced oxygen doping process. This oxygen-doped complex is used as an effective CO2 catalyst, which exhibits a maximum CO Faradaic efficiency of 91% at -0.8 V (vs reversible hydrogen electrode, RHE) and long-term stability throughout 30 h of electrocatalysis. Density function theory calculations suggest that the incorporation of oxygen in Nb4 N5 facilitates the formation of *COOH and thus promotes the CO2 reduction.

Boosting hydrogen evolution activity of vanadyl pyrophosphate nanosheets for electrocatalytic overall water splitting.

Herein, (VO)2P2O7 nanosheets function as a highly-active electrocatalyst for the hydrogen evolution reaction with an ultralow overpotential of 30 mV at 10 mA cm-2 in basic media, being close to Pt/C. Furthermore, as a bifunctional electrocatalyst, (VO)2P2O7 not only exhibits high activity but also good stability for overall water splitting.

Structural Characterization and Identification of Graphdiyne and Graphdiyne-Based Materials.

Graphdiyne (GDY) is a two-dimensional (2D) carbon allotrope consisting of sp2- and sp-hybridized carbon atoms. It and GDY-based materials have tremendous application potentials in the fields of catalysis, energy, sensor, electronics and optoelectronics because of their excellent chemical and physical properties. Thus, the explorations to synthesize high-quality GDY and GDY-based materials and to reveal the relationship between their structures and properties are of significance, in which their structural characterization and identification are a crucial step. In this review, we focus on advanced structural characterization techniques and results on GDY, GDY derivatives, GDY composites and doped GDY, including scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS). This review can provide a systemic understanding of the structural characterization and identification of GDY and GDY-based materials and help their development for high-performance applications.

Random Multiple Scattering Enhanced Photoacoustic Gas Spectroscopy with Disordered Porous Ceramics.

Light-gas interaction can be enhanced by using disordered porous materials because multiple random scattering increases light intensity near the surface of the material. Here we report signal enhancement of photoacoustic gas spectroscopy with disordered porous ceramics. The amplitude and frequency characteristics of photoacoustic signal due to gas absorption in disordered materials are modeled theoretically. Experiment with a porous Al2O3 sample demonstrates photoacoustic signal enhancement of ∼4 times at 5 kHz.

Integrated FP/RFBG sensor with a micro-channel for dual-parameter measurement under high temperature.

An integrated sensor via overlapping a micro Fabry-Perot (MFP) cavity with a micro-channel on a regenerated fiber Bragg grating (RFBG) is constructed for dual-parameter sensing of temperature, strain, and gas pressure under a high temperature (600°C). The MFP is fabricated by using a 157 nm micro-machining on H2-loaded bendinsensitive fiber. A fiber Bragg grating (FBG) is inscribed at the same position of the MFP using 248 nm laser exposure, and then successfully regenerated after a required annealing process which enhances the strain sensitivity of MFP more than three times. The micro-channel created on the MFP is used to improve gas pressure sensitivity of the MFP nearly 100 times. Since the MFP and RFBG have different sensitivities to gas pressure, strain, and temperature, the sensor head could be used to perform dual-parameter measurement by simultaneous measurement of high temperature and strain, and high temperature and gas pressure.

Laser-machined microcavities for simultaneous measurement of high-temperature and high-pressure.

Laser-machined microcavities for simultaneous measurement of high-temperature and high-pressure are demonstrated. These two cascaded microcavities are an air cavity and a composite cavity including a section of fiber and an air cavity. They are both placed into a pressure chamber inside a furnace to perform simultaneous pressure and high-temperature tests. The thermal and pressure coefficients of the short air cavity are ~0.0779 nm/°C and ~1.14 nm/MPa, respectively. The thermal and pressure coefficients of the composite cavity are ~32.3 nm/°C and ~24.4 nm/MPa, respectively. The sensor could be used to separate temperature and pressure due to their different thermal and pressure coefficients. The excellent feature of such a sensor head is that it can withstand high temperatures of up to 400 °C and achieve precise measurement of high-pressure under high temperature conditions.

Eagle's syndrome associated with lingual nerve paresthesia: a case report.

Eagle's syndrome is characterized by a variety of symptoms, including throat pain, sensation of a foreign body in the pharynx, dysphagia, referred otalgia, and neck and throat pain exacerbated by head rotation. Any styloid process longer than 25 mm should be considered elongated and will usually be responsible for Eagle's syndrome. Surgical resection of the elongated styloid is a routine treatment and can be accomplished using a transoral or an extraoral approach. We report a patient with a rare giant styloid process that was approximately 81.7 mm. He complained of a rare symptom: hemitongue paresthesia. After removal of the elongated styloid process using the extraoral approach, his symptoms, including the hemitongue paresthesia, were alleviated. We concluded that if the styloid process displays medium to severe elongation, the extraoral approach will be appropriate.

Rare giant epidermal cyst in the infratemporal fossa and middle cranial fossa.

A 50-year-old man presented with a rare giant crossing cranium-temporal combined epidermal cyst. Physical examination found left facial numbness and temple severely numbness with light pressure. Horizontalis craniocerebral computed tomography demonstrated a mass lesion of 3.0 × 2.0 cm in the middle cranial fossa area; sagittal craniocerebral magnetic resonance scanning demonstrated a mass consisting of 2 leaves (the upper one, 4.0 × 3.0 cm; the lower one, 2.0 × 1.5 cm). Computed tomography angiography showed that the blood supply of the lesion came from superficial temporal artery and middle cranial fossa artery. The clinical diagnosis was neurilemmoma. Surgery revealed a pearly cyst consisting of 2 leaves (connected by a narrowed bridge located at the articular fossa of temporal bone) was 6 × 3 × 3 cm. Histologic examination disclosed disintegrated keratinizing epithelium layer, keratinizing epithelium layer, and stratified squamous epithelium layer from inner to outer and found no hair follicles or sebaceous gland with the diagnosis of epidermoid cyst. Surgery was successfully performed, and the patient was discharged home with severer left facial numbness relatively and left jaw slight opening. The present case suggests that epidermoid cysts can be seen in any location, even giant crossing cranium-temporal combined lesion, and the blood supply should be considered as a factor judging its pathogenesis.