Composite Nanostructures of TiO2 and ZnO for Water Splitting Application: Atomic Layer Deposition Growth and Density Functional Theory Investigation

The commercialization of solar fuel devices requires the development of novel engineered photoelectrodes for water splitting applications which are based on redundant, cheap, and environmentally friendly materials. In the current study, a combination of titanium dioxide (TiO₂) and zinc oxide (ZnO) onto nanotextured silicon is utilized for a composite electrode with the aim to overcome the individual shortcomings of the respective materials. The properties of conformal coverage of TiO₂ and ZnO layers are designed on the atomic scale by the atomic layer deposition technique. The resulting photoanode shows not only promising stability but also nine times higher photocurrents than an equivalent photoanode with a pure TiO₂ encapsulation onto the nanostructured silicon. Density functional theory calculations indicate that segregation of TiO₂ at the ZnO surfaces is favorable and leads to the stabilization of the ZnO layers in water environments. In conclusion, the novel designed composite material constitutes a promising base for a stable and effective photoanode for the water oxidation reaction.     

Controlling Photoactivity in Ultrathin Hematite Films for Solar Water‐Splitting

A promising route to increase the performance of hematite (α‐Fe₂O₃) photoelectrodes for solar hydrogen production through water‐splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1 V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm^?2 at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiO_x is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum‐dot and extremely‐thin‐absorber (ETA)‐type solar cells.     

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

Graphene Doping Improved Device Performance of ZnMgO/PbS Colloidal Quantum Dot Photovoltaics

Lead sulfide (PbS) colloidal quantum dots (CQDs) solar cells possess the advantages of absorption into the infrared, solution processing, and multiple exciton generation, making them very competitive as a low‐cost photovoltaic alternative. Employing an n‐i‐p ZnO/tetrabutylammonium (TBAI)–PbS/ethanedithiol (EDT)–PbS device configuration, the present study reports a 9.0% photovoltaic device through ZnMgO electrode engineering and graphene doping. Sol–gel‐derived Zn_0.9Mg_0.1O buffer layer shows better transparency and higher conduction band maximum than ZnO, and incorporation of graphene and chlorinated graphene oxide into the TBAI–PbS and EDT–PbS layer respectively boosts carrier collection, leading to device with significantly enhanced open circuit voltage and short‐circuit current density. It is believed that incorporation of graphene into PbS CQD film as proposed here, and more generally nanosheets of other materials, would potentially open a simple and powerful avenue to overcome the carrier transport bottleneck of CQD optoelectronic device, thus pushing device performance to a new level.     

Hierarchical Nanowire Arrays Based on ZnO Core−Layered Double Hydroxide Shell for Largely Enhanced Photoelectrochemical Water Splitting

Well‐aligned hierarchical nanoarrays containing ZnO core and layered double hydroxide (LDH) nanoplatelets shell have been synthesized via a facile electrosynthesis method. The resulting ZnO@CoNi–LDH core?shell nanoarray exhibits promising behavior in photoelectrochemical water splitting, giving rise to a largely enhanced photocurrent density as well as stability; much superior to those of ZnO‐based photoelectrodes. This is attributed to the successful integration of photogenerated electron–hole separation originating from the ZnO core and the excellent electrocatalytic activity of LDH shell. This work provides a facile and cost‐effective strategy for the fabrication of multifunctional nanoarrays with a hierarchical structure, which can be potentially used in energy storage and conversion devices.     

Ultrathin Cobalt–Manganese Nanosheets: An Efficient Platform for Enhanced Photoelectrochemical Water Oxidation with Electron‐Donating Effect

An ultrathin cobalt–manganese (Co‐Mn) nanosheet, consisting of amorphous Co(OH)_x layers and ultrasmall Mn₃O₄ nanocrystals, is designed as an efficient co‐catalyst on an α‐Fe₂O₃ film for photoelectrochemical (PEC) water oxidation. The uniformly distributed Co‐Mn nanosheets lead to a remarkable 2.6‐fold enhancement on the photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and an impressive cathodic shift (≈200 mV) of onset potential compared with bare α‐Fe₂O₃ film. Furthermore, the decorated photoanode exhibits a prominent resistance against photocorrosion with excellent stability for over 10 h. Detailed mechanism investigation manifests that incorporation of Mn sites in the nanosheets could create electron donation to Co sites and facilitate the activation of the OH group, which drastically increases the catalytic activities for water oxidation. These findings provide valuable guidance for designing high‐performance co‐catalysts for PEC applications and open new avenues toward controlled fabrication of mixed metallic composites.     

Dye-sensitized solar cells based on ZnO nanotetrapods

In this paper, we reviewed recent systematic studies of using ZnO nanotetrapods for photoanodes of dye-sensitized solar cells (DSSCs) in our group. First, the efficiency of power conversion was obtained by more than 3.27% by changes of conditions of dye loading and film thickness of ZnO nanotetrapod. Short-circuit photocurrent densities (Jsc) increased with the film thickness, Jsc would not be saturation even the film thickness was greater than 35 μm. The photoanode architecture had been charactered by good crystallinity, network forming ability, and limited electron-hopping interjunctions. Next, DSSCs with high efficiency was devised by infiltrating SnO2 nanoparticles into the ZnO nanotetrapods photoanodes. Due to material advantages of both constituents described as above, the composite photoanodes exhibited extremely large roughness factors (RFs), good charge collection, and tunable light scattering properties. By varying the composition of the composite photoanodes, we had achieved an efficiency of 6.31% by striking a balance between high efficiency of charge collection for SnO2 nanoparticles rich films and high light scattering ability for ZnO nanotetrapods rich films. An ultrathin layer of ZnO was found to form spontaneously on the SnO2 nanoparticles, which primarily was responsible for enhancing open-circuit photovoltage (Voc). We also identified that recombination in SnO2/ZnO composite films was mainly determined by ZnO shell condition on SnO2, whereas electron transport was greatly influenced by the morphologies and sizes of ZnO crystalline additives. Finally, we applied the composite photoanodes of SnO2 nanoparticles/ZnO nanotetrapods to flexible DSSCs by low temperature technique of "acetic acid gelation-mechanical press-ammonia activation." The efficiency has been achieved by 4.91% on ITO-coated polyethylenenaphtalate substrate. The formation of a thin ZnO shell on SnO2 nanoparticles, after ammonia activation, was also found to be critical to boosting Voc and to improving inter-particles contacts. Mechanical press, apart from enhancing film durability, also significantly improved charge collection. ZnO nanotetrapods had been demonstrated to be a better additive than ZnO particles for the improvement of charge collection in SnO2/ZnO composite photoanodes regardless of whether they were calcined.     

Multifunctional ZnO‐Nanowire‐Based Sensor

A simple fabrication of ZnO‐nanowire‐based device and their implementation as a pH sensor, temperature sensor, and photo detector is reported. The presented multifunctional ZnO multiple‐nanowire sensor platform contains a Au finger structure, which is realized by conventional photolithography on a SiO₂ substrate. The nanowires are grown using thermal chemical vapor deposition. In order to detect the physical signals, changes in electrical signals were measured (conductance and current). For temperature sensing, the current behavior from 90 to 380 K under vacuum conditions exhibit a tunneling behavior between spaced nanowires. For photo sensing, the current response between the “on” and “off” states of light was measured when exposed to different wavelengths ranging from UV to visible light. Finally, for pH sensing the conductance was measured between a pH of 5 and 8.5. The ZnO nanowires were protected from chemical attacks by a thin layer of C₄F₈‐plasma‐based coating.     

Development of flexible piezoelectric nanogenerator: Toward all wet chemical method

This paper reports on the fabrication of conductive polymer composite film incorporating ZnO nanorod arrays for use as a self-powered nanodevice. Hexagonally structured wurzite ZnO nanorods were grown with a preferred orientation along the (0 0 2) direction on a novel conductive poly (3,4-ethylenedioxythiophene)-co-polyaniline (PEDOT-co-PANI) film using a hydrothermal solution method. The PEDOT-co-PANI copolymer film was prepared by using an oxidative chemical in situ polymerization method. A large area (2 × 2 cm²) piezoelectric nanogenerator bimaterial sheet was fabricated by combining a vertically grown ZnO nanorod array, acting as an electrode in conjunction with an Au coated PU film as the other electrode. With applying rolling force on the top electrode, the nanodevice was demonstrated to generate an optimal output current density of approximately 11 nA/cm².     

Controlled Design of Well‐Dispersed Ultrathin MoS2 Nanosheets inside Hollow Carbon Skeleton: Toward Fast Potassium Storage by Constructing Spacious “Houses” for K Ions

The large volume expansion induced by K⁺ intercalation is always a big challenge for designing high‐performance electrode materials in potassium‐ion storage system. Based on the idea that large‐sized ions should accommodate big “houses,” a facile‐induced growth strategy is proposed to achieve the self‐loading of MoS₂ clusters inside a hollow tubular carbon skeleton (HTCS). Meantime, a step‐by‐step intercalation technology is employed to tune the interlayer distance and the layer number of MoS₂. Based on the above, the ED‐MoS₂@CT hybrids are achieved by self‐loading and anchoring the well‐dispersed ultrathin MoS₂ nanosheets on the inner surface of HTCSs. This unique compositing model not only alleviates the mechanical strain efficiently, but also provides spacious “roads” (hollow tubular carbon skeleton) and “houses” (interlayer expanded ultrathin MoS₂ sheets) for fast K⁺ transition and storage. As an anode of potassium‐ion batteries, the resultant ED‐MoS₂@CT electrode delivers a high specific capacity of 148.5 mAh g^?1 at 2 A g^?1 after 10 000 cycles with only 0.002% fading per cycle. The assembled ED‐MoS₂@CT//PC potassium‐ion hybrid supercapacitor device shows a high energy density of 148 Wh kg^?1 at a power density of 965 W kg^?1, which is comparable to that of lithium‐ion hybrid supercapacitors.     

Improved Performance in FeF2 Conversion Cathodes through Use of a Conductive 3D Scaffold and Al2O3 ALD Coating

FeF₂ is considered a promising conversion compound for the positive electrode in lithium‐ion batteries due to its high thermodynamic reduction potential (2.66 V vs Li/Li⁺) and high theoretical specific capacity (571 mA h g^?1). However, the sluggish reaction kinetics and rapid capacity decay caused by side reactions during cycling limit its practical application. Here, the fabrication of Ni‐supported 3D Al₂O₃‐coated FeF₂ electrodes is presented, and it is shown that these structured electrodes significantly overcome these limitations. The electrodes are prepared by iron electrodeposition on a Ni support, followed by a facile fluorination process and Al₂O₃ coating by atomic layer deposition. The 3D FeF₂ electrode delivers an initial discharge capacity of 380 mA h g^?1 at a current density of 200 mA g^?1 at room temperature. The 3D scaffold improves the reaction kinetics and enables a high specific capacity by providing an efficient electron pathway to the insulating FeF₂ and short Li diffusion lengths. The Al₂O₃ coating significantly improves the cycle life, probably by preventing side reactions through limiting direct electrode–electrolyte contact. The fabrication method presented here can also be applied for synthesis of other metal fluoride materials on different 3D conductive templates.     

Metal‐Ion (Fe,V, Co,and Ni)‐Doped MnO2 Ultrathin Nanosheets Supported on Carbon Fiber Paper for the Oxygen Evolution Reaction

Manganese dioxides (MnO₂) are considered one of the most attractive materials as an oxygen evolution reaction (OER) electrode due to its low cost, natural abundance, easy synthesis, and environmental friendliness. Here, metal‐ion (Fe, V, Co, and Ni)‐doped MnO₂ ultrathin nanosheets electrodeposited on carbon fiber paper (CFP) are fabricated using a facile anodic co‐electrodeposition method. A high density of nanoclusters is observed on the surface of the carbon fibers consisting of doped MnO₂ ultrathin nanosheets with an approximate thickness of 5 nm. It is confirmed that the metal ions (Fe, V, Co, and Ni) are doped into MnO₂, improving the conductivity of MnO₂. The doped MnO₂ ultrathin nanosheet/CFP and the IrO₂/CFP composite electrodes for OER achieve a low overpotential of 390 and 245 mV to reach 10 mA cm^?2 in 1 m KOH, respectively. The potential of the doped composite electrode for long‐term OER at a constant current density of 20 mA cm^?2 is much lower than that of the pure MnO₂ composite electrode.     

Highly Conductive Cable‐Like Bicomponent Titania Photoanode Approaching Limitation of Electron and Hole Collection

TiO₂‐based materials are cheap and stable choices for photoelectrochemical devices. However, the activity is still limited by the inefficient charge extraction. Here a highly conductive cable‐like bicomponent titania photoanode, consisting of reduced anatase‐coated TiO₂‐B nanowires, is proposed to simultaneously establish effective electron and hole transport channels separately, which meets the requirements of electronic dynamics for efficient water splitting. A synergistic effect of charge separation from the built‐in electric field is demonstrated with this 1D TiO₂‐B/anatase heterojunction, in which a high electron collection efficiency of up to 97.1% at 0.6 V versus reversible hydrogen electrode is achieved. The efficient electron collection approaching the limitation is also attributed to the large electron conducting region in the photoanode. Moreover, the O‐deficient amorphous layer is found to be more catalytic toward the oxygen evolution reaction through quantifying rate constants for charge recombination and charge transfer. It can reduce onset potential and suppress charge‐carrier recombination simultaneously, prompting surface hole collection efficiency up to 95% at 0.6 V versus reversible hydrogen electrode.     

Beyond Seashells: Bioinspired 2D Photonic and Photoelectronic Devices

The discovery of novel materials that possess extraordinary optical properties are of special interest, as they inspire systems for next‐generation solar energy harvesting and conversion devices. Learning from nature has inspired the development of many photonic nanomaterials with fascinating structural colors. 2D photonic nanostructures, inspired by the attractive optical properties found on the inner surfaces of seashells, are fabricated in a facile and scalable way. The shells generate shining clusters for preying on phototactic creatures through interaction with incident solar light in water. By alternately depositing graphene and 2D ultrathin TiO₂ nanosheets to form 2D–2D heterostructures and homostructures, seashell‐inspired nanomaterials with well‐controlled parameters are successfully achieved. They exhibit exceptional interlayer charge transfer properties and ultrafast in‐plane electron mobility and present fascinating nacre‐mimicking optical properties and significantly enhanced light‐response behavior when acting as photoelectrodes. A window into the fabrication of novel 2D photonic structures and devices is opened, paving the way for the design of high‐performance solar‐energy harvesting and conversion devices.     

Metal oxide sandwiched dye-sensitized solar cells with enhanced power conversion efficiency fabricated by a facile and cost effective method

Developing efficient and cost effective photoanode and counter electro materials have been a persistent objective by a wide community for efficient, cost effective and stable dye-sensitized-solar cells (DSSCs). We have developed a unique and inexpensive way of co-electroplating-annealing method to synthesize metal oxides and employed it to prepare ZnO material on top of doctor bladed TiO₂ as photoanode and CuO on top of reduced graphene oxide (RGO) as counter electrode. By sandwiching these two electrodes with I^-/I₃ redox couple in-between we have shown the improvement of photovoltaic properties with ZnO barrier layer using Pt or cost effective CuO/RGO counter electrodes (CEs). This paper provides a comprehensive guide to prepare those electrodes cost effectively and fabricate metal oxide sandwiched DSSCs providing up to 6.91% power conversion efficiency with 26.5% enhancement compared to the conventional DSSCs with TiO₂ photoanode and Pt CE under AM1.5 simulated solar light. This work also addresses the reduction of fabrication cost of the cells to make them more economically viable as it is     

High‐Performance CO2 Capture through Polymer‐Based Ultrathin Membranes

Thin film composite (TFC) membranes have attracted great research interest for a wide range of separation processes owing to their potential to achieve excellent permeance. However, it still remains challenging to fully exploit the superiority of thin selective layers when mitigating the pore intrusion phenomenon. Herein, a facile and generic interface‐decoration‐layer strategy collaborating with molecular‐scale organic–inorganic hybridization in the selective layer to obtain a high‐performance ultrathin film composite (UTFC) membrane for CO₂ capture is reported. The interface‐decoration layer of copper hydroxide nanofibers (CHNs) enables the formation of an ultrathin selective layer (≈100 nm), achieving a 2.5‐fold increase in gas permeance. The organic part in the molecular‐scale hybrid material contributes to facilitating CO₂‐selective adsorption while the inorganic part assists in maintaining robust membrane structure, thus remarkably improving the selectivity toward CO₂. As a result, the as‐prepared membrane shows a high CO₂ permeance of 2860 GPU, superior to state‐of‐the‐art polymer membranes, with a CO₂/N₂ selectivity of 28.2. The synergistic strategy proposed here can be extended to a wide range of polymers, holding great potential to produce high‐efficiency ultrathin membranes for molecular separation.     

Mitigating Plasmonic Absorption Losses at Rear Electrodes in High‐Efficiency Silicon Solar Cells Using Dopant‐Free Contact Stacks

Although charge‐carrier selectivity in conventional crystalline silicon (c‐Si) solar cells is usually realized by doping Si, the presence of dopants imposes inherent performance limitations due to parasitic absorption and carrier recombination. The development of alternative carrier‐selective contacts, using non‐Si electron and hole transport layers, has the potential to overcome such drawbacks and simultaneously reduce the cost and/or simplify the fabrication process of c‐Si solar cells. Nevertheless, devices relying on such non‐Si contacts with power conversion efficiencies (PCEs) that rival their classical counterparts are yet to be demonstrated. In this study, one key element is brought forward toward this demonstration by incorporating low‐pressure chemical vapor deposited ZnO as the electron transport layer in c‐Si solar cells. Placed at the rear of the device, it is found that rather thick (75 nm) ZnO film capped with LiF_x/Al simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption. Next, these electron‐selective contacts are integrated in c‐Si solar cells with MoO_x‐based hole‐collecting contacts at the device front to realize full‐area dopant‐free‐contact solar cells. In the proof‐of‐concept device, a PCE as high as 21.4% is demonstrated, which is a record for this novel device class and is at the level of conventional industrial solar cells.     

“Layer‐Filter Threshold” Technique for Near‐Infrared Laser Ablation in Organic Semiconductor Device Processing

Although conventional laser ablation (CLA) method has widely been used in patterning of organic semiconductor thin films, its quality control still remains unsatisfied due to the ambiguous photochemical and photothermal processes. Based on industrial available near‐infrared laser source, herein, a novel “layer‐filter threshold” (LFT) technique is proposed, which involves the decomposition of targeted “layer‐filter” and subsequent explosive evaporation process to purge away the upper layers instead of layer‐by‐layer ablation. For photovoltaic device with structure of metal/blend/PEDOT:PSS/ITO/glass, the PEDOT:PSS layer as the “layer‐filter” is first demonstrated to be effective, and then the merged P1–P2 line and metal electrode layer are readily patterned through the “self‐aligned” effect and regulation of ablation direction, respectively. The correlation between laser fluence and explosive ablation efficacy is also investigated. Finally, photovoltaic modules based on classical P3HT:PC₆₁BM and low‐bandgap PBDT‐TFQ:PC₇₁BM systems are separately fabricated following the LFT technique. It is found that over 90% of geometric fill factor is achieved while device performances maintain in a limited change with increased number of series cells. In comparison to conventional laser ablation methods, the LFT technique does not require sophisticated instruments but reaches comparable processing accuracy, which shows promising potential in the fabrication and commercialization of organic semiconductor thin‐film devices.     

On the Relation between Morphology and FET Mobility of Poly(3‐alkylthiophene)s at the Polymer/SiO2 and Polymer/Air Interface

The influence of the interface of the dielectric SiO₂ on the performance of bottom‐contact, bottom‐gate poly(3‐alkylthiophene) (P3AT) field‐effect transistors (FETs) is investigated. In particular, the operation of transistors where the active polythiophene layer is directly spin‐coated from chlorobenzene (CB) onto the bare SiO₂ dielectric is compared to those where the active layer is first spin‐coated then laminated via a wet transfer process such that the film/air interface of this film contacts the SiO₂ surface. While an apparent alkyl side‐chain length dependent mobility is observed for films directly spin‐coated onto the SiO₂ dielectric (with mobilities of ≈10^?3 cm² V^?1 s^?1 or less) for laminated films mobilities of 0.14 ± 0.03 cm² V^?1 s^?1 independent of alkyl chain length are recorded. Surface‐sensitive near edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements indicate a strong out‐of‐plane orientation of the polymer backbone at the original air/film interface while much lower average tilt angles of the polymer backbone are observed at the SiO₂/film interface. A comparison with NEXAFS on crystalline P3AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3AT crystals suggest a close to crystalline polymer organization at the P3AT/air interface of films from CB. These results emphasize the negative influence of wrongly oriented polymer on charge carrier mobility and highlight the potential of the polymer/air interface in achieving excellent “out‐of‐plane” orientation and high FET mobilities.     

Efficiency enhancement of ZnO based inverted BHJ solar cells via interface engineering using C70 modifier

The interface quality of ZnO and the photoactive polymer blend is of utmost importance in the performance of organic-inorganic hybrid photovoltaic devices. The chemically prepared ZnO electron transporting layer often produce surfaces unacceptable for efficient electron extraction and understate the photovoltaic performance. Herein, we propose a facile interfacial modification technique to enhance the charge collection efficiency of ZnO cathode electrode by efficiently bridging the superficial troughs and ridges of ZnO with the photoactive PCDTBT: PC₇₁BM polymer blend. The investigations show that vacuum sublimated C₇₀ interlayer efficiently fills the gaps between ZnO and the polymer blend reducing accumulation of the charges at the interface and thus minimizing the recombination probability. It also plays a very crucial role in passivating ZnO electrode against interfacial traps due to adsorbed chemical species. The inclusion of C₇₀ interlayer into the devices led to a substantial increase in device performance with PCE reaching close to 4%, an increment by a factor of 2 compared to the control devices. Our investigations aim towards showing the efficacy of C₇₀ small molecule in significantly enhancing the PCE of ZnO based BHJ solar cells fabricated and measured in ambient conditions rather than setting benchmark efficiency for the configured device. However, better performances for the devices are conceivable by performing the fabrication and measurement in controlled inert atmosphere.