High‐Performance Photodetectors Based on Organometal Halide Perovskite Nanonets

The booming development of organometal halide perovskites has prompted the exploration of morphology‐engineering strategies to improve their performance in optoelectronic applications. However, the preparation of optoelectronic devices of perovskites with complex architectures and desirable properties is still highly challenging. Herein, novel CH₃NH₃PbI₃ nanonets and nanobowl arrays are fabricated facilely by using monolayer colloidal crystal (MCC) templates on different substrates. Specifically, highly ordered CH₃NH₃PbI₃ nanonets with high crystallinity are fabricated on a variety of flat substrates, whereas regular CH₃NH₃PbI₃ nanobowl arrays are produced on a coarse substrate. The photodetection performance of the CH₃NH₃PbI₃ nanonet‐based photodetectors is significantly enhanced compared to the photodetectors based on conventional CH₃NH₃PbI₃ compact films. Particularly, the nanonet photodetectors exhibit a high responsivity (10.33 A W^?1 under 700 nm monochromatic light), which is six times higher than that for the compact CH₃NH₃PbI₃ film devices, fast response speed, and good stability. Owing to the two‐dimensional arrayed structure, the CH₃NH₃PbI₃ nanonets exhibit an enhanced light harvesting ability and offer direct carrier transport pathways. Meanwhile, the MCC template brings about larger grain sizes with enhanced crystallinity. Furthermore, the perovskite nanonets can be formed on a flexible polyethylene terephthalate substrate for the fabrication of promising flexible nanonet photodetectors.     

Templating Synthesis of SnO2 Nanotubes Loaded with Ag2O Nanoparticles and Their Enhanced Gas Sensing Properties

A new kind of SnO₂ nanotubes loaded with Ag₂O nanoparticles can be synthesized by using Ag@C coaxial nanocables as sacrificial templates. The composition of silver in SnO₂ nanotubes can be controlled by tuning the compositions of metallic Ag in Ag@C sacrificial templates, and the morphology of tubular structures can be changed by use of nanocables with different thicknesses of carbonaceous layer. This simple strategy is expected to be extended for the fabrication of similar metal‐oxide doped nanotubes using different nanocable templates. In contrast to SnO₂@Ag@C nanocables as well as to other types of SnO₂ reported previously, the Ag₂O‐doped SnO₂ nanotubes exhibit excellent gas sensing behaviors. The dynamic transients of the sensors demonstrated both their ultra‐fast response (1–2 s) and ultra‐fast recovery (2–4 s) towards ethanol, and response (1–4 s) and recovery (4–5 s) towards butanone. The combination of SnO₂ tubular structure and catalytic activity of Ag₂O dopants gives a very attractive sensing behavior for applications as real‐time monitoring gas sensors with ultra‐fast responding and recovering speed.     

SnO2 gas sensor fabricated by low frequency alternating field electrophoretic deposition

SnO₂ thick film gas sensor has been prepared by applying low frequency (0.1 Hz) AC electric fields to a stable suspension of SnO₂ nanoparticles in acetylacetone. Parallel gold electrodes were used as the deposition substrate. Effect of CO, O₂ and H₂ gas exposure as well as ethanol vapor on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic-based gas sensors.     

Reliable Monolayer‐Template Patterning of SnO2 Thin Films from Aqueous Solution and Their Hydrogen‐Sensing Properties

We demonstrate a novel lithographic technique utilizing a solvent to fabricate a chemically based semiconductor microdevice from an aqueous solution. According to this technique, SnO₂ thin film could be integrated onto predefined sites on a SiO₂/Si wafer. A patterned octadecyltrimethoxysilane self‐assembled monolayer (ODS‐SAM) was prepared by vacuum ultraviolet (VUV) irradiation through a photomask to use as a template for the fabrication of a micropatterned SnO₂ thin film on the SiO₂/Si surface. A Sn‐based thin film was then deposited onto the entire surface of the ODS template from an aqueous solution containing 0.03 mol L^–1 of SnCl₂·2H₂O at 60 °C for 16 h in an ambient atmosphere. The thin film deposited on the methyl‐terminated area of the template was then peeled using an ultrasonic rinse in anhydrous toluene for 30 min, while the film deposited on the silanol area remained intact and undamaged. Rinsing in hydrophilic solvents did not facilitate peeling of the thin film from the methyl‐terminated area. We succeeded by this process in obtaining a high‐resolution, micropatterned Sn‐based thin film on an ODS‐SAM template on Si. The as‐deposited film was composed of fine Sn‐based particles. The sensitivity of this SnO₂ thin film to H₂ gas increases linearly with improving crystallinity. The effectiveness of anhydrous toluene as a rinse in solution lithography is discussed from the viewpoint of the high hydrophobic affinity between the rinse solvent and the terminal groups in the monolayer template.     

Fabrication of Hollow Inorganic Microspheres by Chemically Induced Self‐Transformation

Two contrasting approaches, involving either polymer‐mediated or fluoride‐mediated self‐transformation of amorphous solid particles, are described as general routes to the fabrication of hollow inorganic microspheres. Firstly, calcium carbonate and strontium tungstate hollow microspheres are fabricated in high yield using sodium poly(4‐styrenesulfonate) as a stabilizing agent for the formation and subsequent transformation of amorphous primary particles. Transformation occurs with retention of the bulk morphology by localized Ostwald ripening, in which preferential dissolution of the particle interior is coupled to the deposition of a porous external shell of loosely packed nanocrystals. Secondly, the fabrication process is extended to relatively stable amorphous microspheres, such as TiO₂ and SnO₂, by increasing the surface reactivity of the solid precursor particles. For this, fluoride ions, in the form of NH₄F and SnF₂, are used to produce well‐defined hollow spheroids of nanocrystalline TiO₂ and SnO₂, respectively. Our results suggest that the chemical self‐transformation of precursor objects under morphologically invariant conditions could be of general applicability in the preparation of a wide range of nanoparticle‐based hollow architectures for technological and biomedical applications.     

Direct Conversion of Single‐Layer SnO Nanoplates to Multi‐Layer SnO2 Nanoplates with Enhanced Ethanol Sensing Properties

Direct conversion of single‐layer SnO nanoplates to multi‐layer SnO₂ nanoplates is achieved by annealing in an O₂ ambient at 700 °C. For 50 ppm ethanol, the sensitivities of the multi‐layer SnO₂ nanoplates are more than double that of single‐layer SnO₂ nanoplates, which are also formed from the single‐layer SnO. The higher sensitivity of the multi‐layer nanoplates is attributed to their larger surface/volume ratio. The facile fabrication of interconnected multi‐layer SnO₂ nanoplates at low temperature directly on a Si substrate and sensing chip without the aid of catalysts offers vast advantages over competing methods for the fabrication of high‐sensitivity SnO₂ sensors.     

Self-Standing 3D Hollow Nanoporous SnO2-Modified CuxO Nanotubes with Nanolamellar Metallic Cu Inwalls: A Facile In Situ Synthesis Protocol toward Enhanced Li Storage Properties

SnO₂ is regarded as a prospective anode material candidate for high energy density lithium-ion batteries (LIBs). However, rapid structural degradation and low conductivity always bring about poor cycling stability and electrochemical reversibility, becoming critical dilemmas toward its practical application. To address these issues, herein, a facile multi-step in situ synthesis protocol is developed to tactfully achieve self-standing 3D hollow nanoporous SnO₂-modified Cu_xO nanotubes with nanolamellar metallic Cu inwalls (3D-HNP SnO₂/Cu_xO@n-Cu) via chemical dealloying, heat treatment, electrochemical replacement, and selective etching. The results show that the unique 3D-HNP SnO₂/Cu_xO@n-Cu as a binder-free integrated anode for LIBs exhibits superior Li storage properties with high initial reversible capacity of 3.34 mAh cm⁻² and good cycling stability with 85.6% capacity retention and >99.4% coulombic efficiency after 200 cycles (capacity decay of only 0.002 mAh cm⁻² per cycle). This is mainly attributed to the unique 3D hollow nanoporous configuration design composed of interlinked Cu_xO nanotubes modified by ultrafine SnO₂ nanocrystals (4–10 nm) with two-way mechanical strain cushion and nanolamellar metallic Cu inwalls with boosted electrical conductivity. This work can be expected to offer an original and effective approach for rational design and fabrication of advanced MOx-based anodes toward high-performance LIBs.     

Investigation of charge transport of monolayer polymeric films with field effect tuning and molecular doping for chemiresistive sensing application

Semiconducting and conducting 2D conjugated polymers have been widely investigated due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. Nevertheless, the transport characteristics of field-effect tuning and molecule doping ultra-thin even monolayer conjugated polymers haven't been understood thoroughly. Moreover, a general and facile method for the direct synthesis of large-area conjugated polymer monolayer films with good uniformity and high coverage on the SiO₂/Si substrates is highly desirable and still remains challenging. Here, we have used the simple drop-casting method to successfully fabricate high-coverage and homogeneous poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno [3], [2-b]thiophene) (PBTTT) monolayer films directly on the surface of SiO₂/Si substrate. The electrical transport behaviors were investigated systematically in both field effect doping and molecular doping the systems. 2D hopping transport was observed dominating the conductance of the field effect tuned monolayer PBTTT. While Efros-Shklovskii variable range hopping (ES-VRH) contributed the main transport at higher carrier concentration system doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ). And the onset interlayer transport should effectively reduce the energetic disorder revealed in bi- or tri-layer films. In addition, the F₄-TCNQ doping conductive PBTTT monolayer films displayed superior chemiresistive sensing toward ammonia. The proposed semiconducting and conducting conjugated polymer monolayer-based ideal platform can open new avenues for ultra-thin film organic electronics.     

Cover Picture: A Hierarchically Structured Ni(OH)2 Monolayer Hollow‐Sphere Array and Its Tunable Optical Properties over a Large Region (Adv. Funct. Mater. 4/2007)

The fabrication of hierarchically structured Ni(OH)₂ monolayer hollow‐sphere arrays with the shell composed of building blocks of nanoflakelets is reported on p. 644 by Weiping Cai and co‐workers. The morphology can be easily controlled by the synthesis parameters, and the arrays show a tunable optical transmission stop band. Tuning can be achieved by changing the size or morphology of the hollow spheres. Such arrays may have potential applications in optical devices, photonic crystals, and as sensors for gas detection. The fabrication of a hierarchically structured Ni(OH)₂ monolayer hollow‐sphere array with the shell composed of building blocks of nanoflakelets is demonstrated based on a colloidal monolayer and electrochemical deposition. The morphology can be easily controlled by the colloidal monolayer and deposition parameters. Importantly, such monolayer hollow‐sphere array shows a morphology‐ and size‐dependent tunable optical transmission stop band. This stop band can be easily tuned from 455–1855 nm by changing the size of the hollow spheres between 1000 and 4500 nm, and also fine‐adjusted by changing the deposition time. The array exhibits a nearly incident‐angle‐independent position of the stop band that 3D photonic crystals do not possess. This structure may have potential applications in optical devices, photonic crystals, and sensors for gas detection.     

Synthesis of compositionally different multicomponent metal-oxide films (SnO2) x (ZnO)1 − x (x = 1–0.5)

The study is concerned with high-purity SnO₂ and ZnO powders produced from salt solutions of corresponding metals by low-temperature hydrothermal synthesis. Fragments of SnO₂ and ZnO ceramic targets formed as 1 × 8 cm bars are fabricated by dry pressing. The bars are used to form composite targets for ion-beam sputtering and the fabrication of compositionally different (SnO₂) _x (ZnO)_{1 ? x} (x = 1–0.5) films appropriate for the production of gas sensors or transparent electronic devices. The optical and electrical parameters of the films with different compositions are studied.     

Insights into Fullerene Passivation of SnO2 Electron Transport Layers in Perovskite Solar Cells

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO₂ as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO₂ ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO₂ interface. Through a comparative study using [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C₆₁‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO₂ interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO₂ ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C₆₀ and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO₂ surface.     

Dopant‐Free,Amorphous–Crystalline Heterophase SnO2 Electron Transport Bilayer Enables >20% Efficiency in Triple‐Cation Perovskite Solar Cells

Improving the ohmic contact and interfacial morphology between an electron transport layer (ETL) and perovskite film is the key to boost the efficiency of planar perovskite solar cells (PSCs). In the current work, an amorphous–crystalline heterophase tin oxide bilayer (Bi‐SnO₂) ETL is prepared via a low‐temperature solution process. Compared with the amorphous SnO₂ sol–gel film (SG‐SnO₂) or the crystalline SnO₂ nanoparticle (NP‐SnO₂) counterparts, the heterophase Bi‐SnO₂ ETL exhibits improved surface morphology, considerably fewer oxygen defects, and better energy band alignment with the perovskite without sacrificing the optical transmittance. The best PSC device (active area ≈ 0.09 cm²) based on a Bi‐SnO₂ ETL is hysteresis‐less and achieves an outstanding power conversion efficiency of ≈20.39%, which is one of the highest efficiencies reported for SnO₂‐triple cation perovskite system based on green antisolvent. More fascinatingly, large‐area PSCs (active areas of ≈3.55 cm²) based on the Bi‐SnO₂ ETL also achieves an extraordinarily high efficiency of ≈14.93% with negligible hysteresis. The improved device performance of the Bi‐SnO₂‐based PSC arises predominantly from the improved ohmic contact and suppressed bimolecular recombination at the ETL/perovskite interface. The tailored morphology and energy band structure of the Bi‐SnO₂ has enabled the scalable fabrication of highly efficient, hysteresis‐less PSCs.     

Growth of Aligned Square‐Shaped SnO2 Tube Arrays

Tin dioxide (SnO₂) box beams, or tubes with square or rectangular cross‐sections, are synthesized on quartz substrates using a combustion chemical vapor deposition (CVD) method in an open atmosphere at 850 °C to 1150 °C. The cross‐sectional width of the as‐synthesized SnO₂ tubules is tunable from 50 nm to sub‐micrometer depending on synthesis temperature. Each tubule is found to be a single crystal of rutile structure with four {110} peripheral surfaces and <001> growth direction. Although several growth patterns are observed for different samples, the basic growth mechanism is believed to be a self‐catalyzed, direct vapor–solid (VS) process, where most new material is incorporated into the bottom parts of the existing SnO₂ tubules through surface diffusion. The tubes are readily aligned in the direction perpendicular to the substrate surface to form tube arrays. These well‐aligned SnO₂ tubule arrays with tunable tube size could be the building blocks or templates for fabrication of functional nanodevices, especially those relevant to energy storage and conversion such as nanobatteries, nanofuel cells, and nanosensors. A gas sensor based on a single SnO₂ nanotubes demonstrated extremely high sensitivity to ethanol vapor.     

Photoluminescence from Liquid‐Exfoliated WS2 Monomers in Poly(Vinyl Alcohol) Polymer Composites

While liquid phase exfoliation can be used to produce nanosheets stabilized in polymer solutions, very little is known about the resultant nanosheet size, thickness, or monolayer content. The present study uses semiquantitative spectroscopic metrics based on extinction, Raman, and photoluminescence (PL) spectroscopy to investigate these parameters for WS₂ nanosheets exfoliated in aqueous poly(vinyl alcohol) (PVA) solutions. By measuring Raman and PL simultaneously, the monolayer content can be tracked via the PL/Raman intensity ratio while varying processing conditions. The PL is found to be maximized for a stabilizing polymer concentration of 2 g L^?1. In addition, the monolayer content can be controlled via the centrifugation conditions, exceeding 5% by mass in some cases. These techniques have allowed tracking the ratio of PL/Raman in a droplet of polymer‐stabilized WS₂ nanosheets as the water evaporates during composite formation. No evidence of nanosheet aggregation is found under these conditions although the PL becomes dominated by trion emission as drying proceeds and the balance of doping from PVA/water changes. Finally, bulk PVA/WS₂ composites are produced by freeze drying where >50% of the monolayers remain unaggregated, even at WS₂ volume fractions as high as 10%.     

Amorphous Zinc Stannate (Zn2SnO4) Nanofibers Networks as Photoelectrodes for Organic Dye‐Sensitized Solar Cells

A new strategy for developing dye‐sensitised solar cells (DSSCs) by combining thin porous zinc tin oxide (Zn₂SnO₄) fiber‐based photoelectrodes with purely organic sensitizers is presented. The preparation of highly porous Zn₂SnO₄ electrodes, which show high specific surface area up to 124 m²/g using electrospinning techniques, is reported. The synthesis of a new organic donor‐conjugate‐acceptor (D‐π‐A) structured orange organic dye with molar extinction coefficient of 44 600 M^?1 cm^?1 is also presented. This dye and two other reference dyes, one organic and a ruthenium complex, are employed for the fabrication of Zn₂SnO₄ fiber‐based DSSCs. Remarkably, organic dye‐sensitized DSSCs displayed significantly improved performance compared to the ruthenium complex sensitized DSSCs. The devices based on a 3 μm‐thick Zn₂SnO₄ electrode using the new sensitizer in conjunction with a liquid electrolyte show promising photovoltaic conversion up to 3.7% under standard AM 1.5G sunlight (100 mW cm^?2). This result ranks among the highest reported for devices using ternary metal oxide electrodes.     

Overcoming the Challenges of Freestanding Tin Oxide-Based Composite Anodes to Achieve High Capacity and Increased Cycling Stability

Freestanding electrodes are a promising way to increase the energy density of the batteries by decreasing the overall amount of electrochemically inactive materials. Freestanding antimony doped tin oxide (ATO)-based hybrid materials have not been reported so far, although this material has demonstrated excellent performance in conventionally designed electrodes. Two different strategies, namely electrospinning and freeze-casting, are explored for the fabrication of ATO-based hybrid materials. It is shown that the electrospinning of ATO/carbon based electrodes from polyvinyl pyrrolidone polymer (PVP) solutions was not successful, as the resulting electrode material suffers from rapid degradation. However, freestanding reduced graphene oxide (rGO) containing ATO/C/rGO nanocomposites prepared via a freeze-casting route demonstrates an impressive rate and cycling performance reaching 697 mAh g⁻¹ at a high current density of 4 A g⁻¹, which is 40 times higher as compared to SnO₂/rGO and also exceeds the freestanding SnO₂-based composites reported so far. Antimony doping of the nanosized tin oxide phase and carbon coating are thereby shown to be essential factors for appealing electrochemical performance. Finally, the freestanding ATO/C/rGO anodes are combined with freestanding LiFe_0.2Mn_0.8PO₄/rGO cathodes to obtain a full freestanding cell operating without metal current collector foils showing nonetheless an excellent cycling stability.     

A ferroelectric field effect transistor based on a Pb(Zr<Subscript>x</Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript>/SnO<Subscript>2</Subscript> heterostructure

The possibility of fabricating a ferroelectric FET based on a Pb(Zr_xTi_1?x)O₃/SnO₂ (PZT/SnO₂) heterostructure is investigated. Sb-doped epitaxial SnO₂/Al₂O₃ thin film deposited by YAG laser ablation from a metal target is used as the FET channel. The highest obtained electron mobility in the channel is 25 cm²/(V s) at an electron density of 8 × 10¹⁹ cm^?3. The possibility of growing PZT film directly on SnO₂ film using two different techniques, laser ablation and magnetron sputtering, is demonstrated. Both methods have been used in the fabrication of Au/PZT/SnO₂ capacitor heterostructures, whose top Au electrode is 250×250 μm² in size. These cells demonstrate a capacitance of 1000 pF at a 10-V bias and remnant polarization up to 16 μC/cm². A Au/PZT/SnO₂/Al₂O₃ transistor structure with 94% modulation of the channel current is fabricated. The difference in the channel current under the effect of positive and negative remnant polarization of the undergate ferroelectric is 37%.     

Zn掺杂和Sn空位对p型SnO2电子结构和光学性能的影响

用第一性原理计算了本征SnO2、Zn掺杂SnO2、带Sn 空位(VSn)的SnO2和Zn-VSn共掺杂 SnO2的电子结构和光学性能。结果表明,Zn 原子替位SnO2中的Sn原子后费米能级进入价带,价带顶的空能态由Zn 3d和O 2p态组成,Zn掺杂SnO2显示p型半导体性能。 带Sn空位的Zn掺杂SnO2的相对空穴数量较Zn掺杂SnO2的相对空穴数量有明显增加,Sn空位有助于增加Zn掺杂SnO2的p型导电性。Zn掺杂SnO2在可见光区域有明显的光吸收,带Sn空位的Zn掺杂SnO2的光吸收较Zn掺杂SnO2明显增强,吸收光谱发生蓝移。     

p‐Si/SnO2/Fe2O3 Core/Shell/Shell Nanowire Photocathodes for Neutral pH Water Splitting

Silicon is one of the promising materials for solar water splitting and hydrogen production; however, it suffers from two key factors, including the large external potential required to drive water splitting reactions at its surface and its instability in the electrolyte. In this study, a successful fabrication of novel p‐Si/n‐SnO₂/n‐Fe₂O₃ core/shell/shell nanowire (css‐NW) arrays, consisting of vertical Si NW cores coated with a thin SnO₂ layer and a dense Fe₂O₃ nanocrystals (NCs) shell, and their application for significantly enhanced solar water reduction in a neutral medium is reported. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NW structure is characterized in detail using scanning, transmission, and scanning transmission electron microscopes. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs show considerably improved photocathodic performances, including higher photocurrent and lower photocathodic turn‐on potential, compared to the bare p‐Si NWs or p‐Si/n‐SnO₂ core/shell NWs (cs‐NWs), due to increased optical absorption, enhanced charge separation, and improved gas evolution. As a result, photoactivity at 0 V versus reversible hydrogen electrode and a low onset potential in the neutral solution are achieved. Moreover, p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs exhibit long‐term photoelectrochemical stability due to the Fe₂O₃ NCs shell well protection. These results reveal promising css‐NW photoelectrodes from cost‐effective materials by facile fabrication with simultaneously improved photocathodic performance and stability.     

Room-Temperature Hydrogen Sensor with High Sensitivity and Selectivity using Chemically Immobilized Monolayer Single-Walled Carbon Nanotubes

Although semiconducting single-walled carbon nanotubes (sc-SWNTs) exhibit excellent sensing properties for various gases, commercialization is hampered by several obstacles. Among these, the difficulty in reproducibly fabricating sc-SWNT films with uniform density and thickness is the main one. Here, a facile fabrication method for sc-SWNT-based hydrogen (H₂) sensors with excellent reproducibility, high sensitivity, and selectivity against CO, CO₂, and CH₄ is reported. Uniform-density and monolayer sc-SWNT films are fabricated using chemical immobilized through the click reaction between azide-functionalized polymer-wrapped sc-SWNTs and immobilized alkyne polymer on a substrate before decorating with Pd nanoparticles (0.5–3.0 nm). The optimized sc-SWNT sensor has a high room-temperature response of 285 with the response and recovery times of 10 and 3 s, respectively, under 1% H₂ gas in air. In particular, this sensor demonstrates highly selective H₂ detection at room temperature (25 °C), compared to other gases and humidity. Therefore, the chemical immobilization of the monolayer SWNT films with reproducible and uniform density has the potential for large-scale fabrication of robust room-temperature H₂ sensors.