Designed functions of oxide/hydroxide nanosheets via elemental replacement/doping

Partial replacement of one structural element in a solid with another of a similar size was conducted to impart functionality to the solids and modify their properties. This phenomenon is found in nature in coloured gemstones and clay minerals and is used in materials chemistry and physics, endowing materials with useful properties that can be controlled by incorporated heteroelements and their amounts. Depending on the area of research (or expected functions), the replacement is referred to as "isomorphous substitution", "doping", etc. Herein, elemental replacement in two-dimensional (2D) oxides and hydroxides (nanosheets or layered materials) is summarised with emphasis on the uniqueness of their preparation, characterisation and application compared with those of the corresponding bulk materials. Among the 2D materials (graphene, metallenes, transition metal chalcogenides, metal phosphate/phosphonates, MXenes, etc.), 2D oxides and hydroxides are characterised by their presence in nature, facile synthesis and storage under ambient conditions, and possible structural variation from atomic-level nanosheets to thicker nanosheets composed of multilayered structures. The heteroelements to be doped were selected depending on the target application objectively; however, there are structural and synthetic limitations in the doping of heteroelements. In the case of layered double hydroxides (single layer) and layered alkali silicates (from single layer to multiple layers), including layered clay minerals (2 : 1 layer), the replacement (commonly called isomorphous substitution) is discussed to understand/design characteristics such as catalytic, adsorptive (including ion exchange), and swelling properties. Due to the variation in their main components, the design of layered transition metal oxide/hydroxide materials via isomorphous substitution is more versatile; in this case, tuning their band structure, doping both holes and electrons, and creating impurity levels are examined by the elemental replacement of the main components. As typical examples, material design for the photocatalytic function of an ion-exchangeable layered titanate (lepidocrocite-type titanate) and a perovskite niobate (KCa2Nb3O10) is discussed, where elemental replacement is effective in designing their multiple functions.

Cost-effective preparation of layered tantalum oxynitrides for visible light-driven photocatalysis

Layered oxynitrides are promising materials for visible light photocatalysis. However, the conventional method for the synthesis of oxynitrides using ammonia as a nitrogen source is dangerous. In this work, we successfully synthesized two layered tantalum oxynitrides, K_1.35LaTa₂O_6.65N_0.35 and K_1.4Ca₂Ta₃O_9.6N_0.4, via a topochemical nitridation process using urea as a solid nitrogen source. Employing different characterization methods, we determined the structure and composition of layered oxynitrides. Furthermore, using Pt as a co-catalyst, these two layered oxynitrides showed excellent photocatalytic performances under visible light irradiation. In contrast to ammonia, urea process provides easy access for the synthesis of layered oxynitrides and sheds new light on the design of effective visible light-driven photocatalysts.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets

Bandgap tunable lanthanum niobium oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- and oxide [Ca₂ Nb₃ O₁₀ ]^- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Photocatalytic Hydrogen Generation from Aqueous Methanol Solution over n-Butylamine-Intercalated Layered Titanate H2La2Ti3O10: Activity and Stability of the Hybrid Photocatalyst

The stability of platinized n-butylamine-intercalated layered titanate H2La2Ti3O10 during the process of photocatalytic hydrogen production from aqueous methanol under UV irradiation has been thoroughly investigated by means of XRD, CHN, TG, 13C NMR, BET, SEM and GC-MS analysis. It was revealed that n-butylamine completely abandons the interlayer space and transforms into n-butyraldehyde within 3 h of the reaction, while the particle morphology and specific surface area of the photocatalyst are preserved. The resulting solid phase contains carbon in at least two different oxidation states, which are attributed to the intermediate products of methanol oxidation bound to the perovskite matrix. The activity of the photocatalyst formed in this way is stable in time and strongly depends on the medium pH, which is not typical of either the parent H2La2Ti3O10 or TiO2. An approximate linear equation φ ≈ 29−2∙pH holds for the apparent quantum efficiency of hydrogen production in the 220–340 nm range at 1 mol. % methanol concentration. In the acidic medium, the photocatalyst under study outperforms the platinized H2La2Ti3O10 by more than one order of magnitude. The variation in methanol concentration allowed a maximum quantum efficiency of hydrogen production of 44% at 10 mol. % to be reached.

Photocatalytic Hydrogen Production from Aqueous Solutions of Glucose and Xylose over Layered Perovskite-like Oxides HCa2Nb3O10, H2La2Ti3O10 and Their Inorganic-Organic Derivatives

Nowadays, the efficient conversion of plant biomass components (alcohols, carbohydrates, etc.) into more energy-intensive fuels, such as hydrogen, is one of the urgent scientific and technological problems. The present study is the first one focused on the photoinduced hydrogen evolution from aqueous D-glucose and D-xylose using layered perovskite-like oxides HCa2Nb3O10, H2La2Ti3O10, and their organically modified derivatives that have previously proven themselves as highly active photocatalysts. The photocatalytic performance was investigated for the bare compounds and products of their surface modification with a 1 mass. % Pt cocatalyst. The photocatalytic experiments followed an innovative scheme including dark stages as well as the control of the reaction suspension's pH and composition. The study has revealed that the inorganic-organic derivatives of the layered perovskite-like oxides can provide efficient conversion of carbohydrates into hydrogen fuel, being up to 8.3 times more active than the unmodified materials and reaching apparent quantum efficiency of 8.8%. Based on new and previously obtained data, it was shown that the oxides' interlayer space functions as an additional reaction zone in the photocatalytic hydrogen production and the contribution of this zone to the overall activity is dependent on the steric characteristics of the sacrificial agent used.

Nitrogen-doped, proton-exchanged Dion-Jacobson layered niobate perovskites for photocatalytic hydrogen generation in solar light

Wide band gap semiconductor niobate photocatalysts with Dion-Jacobson layered perovskite structure were nitrogen-doped via simple gas-solid reaction to extend their absorption into the visible light range. Nitrogen doping was performed using ammonia as precursor, resulting in decreased band gaps of doped AB₂Nb₃O₁₀ compounds (A = Cs, Rb, K; B = Ca, Sr) down to 2.5 eV. The resulting materials were investigated concerning their chemical and electronic structures. Nitrogen-doped AB₂Nb₃O₁₀ crystals showed a clear red shift in absorption. Photocatalytic performance tests for the doped materials evaluated the capability of H₂ production under simulated solar irradiation. The addition of carbonates to the gas-solid reaction turned out to be advantageous for the reduction of defects and the preservation of photocatalytic activity of nitrogen-doped layered niobates AB₂Nb₃O₁₀.

Developing sustainable, high-performance perovskites in photocatalysis: design strategies and applications

Solar energy is attractive because it is free, renewable, abundant and sustainable. Photocatalysis is one of the feasible routes to utilize solar energy for the degradation of pollutants and the production of fuel. Perovskites and their derivatives have received substantial attention in both photocatalytic wastewater treatment and energy production because of their highly tailorable structural and physicochemical properties. This review illustrates the basic principles of photocatalytic reactions and the application of these principles to the design of robust and sustainable perovskite photocatalysts. It details the structures of the perovskites and the physics and chemistry behind photocatalytic reactions and describes the advantages and limitations of popular strategies for the design of photoactive perovskites. This is followed by examples of how these strategies are applied to enhance the photocatalytic efficiency of oxide, halide and oxyhalide perovskites, with a focus on materials with potential for practical application, that is, not containing scarce or toxic elements. It is expected that this overview of the development of photocatalysts and deeper understanding of photocatalytic principles will accelerate the exploitation of efficient perovskite photocatalysts and bring about effective solutions to the energy and environmental crisis.

Photocatalytic Activity of n-Alkylamine and n-Alkoxy Derivatives of Layered Perovskite-like Titanates H2Ln2Ti3O10 (Ln = La, Nd) in the Reaction of Hydrogen Production from an Aqueous Solution of Methanol

Two series of hybrid inorganic-organic derivatives, obtained via the modification of protonated Ruddlesden–Popper phases H2Ln2Ti3O10 (Ln = La, Nd) with intercalated n-alkylamines and grafted n-alkoxy groups, have been systematically investigated in relation to photocatalytic hydrogen production from a model of 1 mol % aqueous solution of methanol for the first time. Photocatalytic measurements were performed both for bare samples and for their composites with Pt nanoparticles as a cocatalyst using an advanced scheme, including dark stages, monitoring of the volume concentration of the sample in the reaction suspension during the experiment, shifts of its pH and possible exfoliation of layered compounds into nanolayers. It was found that the incorporation of organic components into the interlayer space of the titanates increases their photocatalytic activity up to 117 times compared with that of the initial compounds. Additional platinization of the hybrid samples’ surface allowed for achieving apparent quantum efficiency of hydrogen evolution of more than 40%. It was established that the photocatalytic activity of the hybrid samples correlates with the hydration degree of their interlayer space, which is considered a separate reaction zone in photocatalysis, and that hydrogen indeed generates from the aqueous methanol solution rather than from organic components of the derivatives.

Synthesis of n-Alkoxy Derivatives of Layered Perovskite-Like Niobate HCa2Nb3O10 and Study of Their Photocatalytic Activity for Hydrogen Production from an Aqueous Solution of Methanol

A series of hybrid inorganic–organic niobates HCa2Nb3O10×ROH, containing n-alkoxy groups of primary alcohols (R = Me, Et, Pr, Bu, Hx, and Dc) grafted in the interlayer space, has been studied for the first time in relation to photocatalytic hydrogen generation from a model 1 mol % aqueous solution of methanol under ultraviolet irradiation. Photocatalytic activity was measured both for bare samples and for their composites with Pt nanoparticles as a cocatalyst. The advanced measurement scheme allowed monitoring the volume concentration of a sample in a suspension during the experiment, its pH, and possible exfoliation of layered compounds into nanolayers. In the series of n-alkoxy derivatives, the maximum rate of hydrogen evolution was achieved over a Pt-loaded ethoxy derivative HCa2Nb3O10×EtOH/Pt. Its apparent quantum efficiency of 20.6% in the 220–350 nm range was found not to be caused by changes in the light absorption region or specific surface area upon ethanol grafting. Moreover, the amounts of hydrogen released during the measurements significantly exceeded those of interlayer organic components, indicating that hydrogen is generated from the reaction solution rather than from the hybrid material.

Study of n-alkylamine Intercalated Layered Perovskite-Like Niobates HCa2Nb3O10 as Photocatalysts for Hydrogen Production From an Aqueous Solution of Methanol

A series of hybrid niobates HCa₂Nb₃O₁₀×RNH₂, containing n-alkylamines (R = Me, Et, Pr, Bu, Hx, Oc) intercalated into the interlayer space, has been thoroughly studied concerning the photocatalytic hydrogen production from a model aqueous solution of methanol for the first time. All the hybrid photocatalysts were synthesized by the conventional ceramic technique followed by protonation and intercalation of n-alkylamines. The products were characterized using XRD, Raman, IR and diffuse reflectance spectroscopy, TGA, CHN-analysis and SEM. Photocatalytic measurements were conducted according to an advanced scheme taking into account possible changes in the photocatalyst concentration because of sedimentation, pH shifts and exfoliation of the samples into nanoplatelets. Special attention was also paid to the feasible improvement of the photocatalytic activity of the samples via their modification with Pt nanoparticles as a cocatalyst. In the series of amine derivatives, the highest rate of hydrogen generation was demonstrated by the Pt-loaded HCa₂Nb₃O₁₀×BuNH₂ reaching apparent quantum efficiency of 13% in the 220–340 nm range. The initial HCa₂Nb₃O₁₀ showed comparable efficiency of 8.3% that is greater than for other amine derivatives. It was demonstrated that for the investigated samples the photocatalytic activity correlates with their ability of water intercalation.

Recent Advances in Niobium-Based Materials for Photocatalytic Solar Fuel Production

The search for renewable and clean energy sources is a key aspect for sustainable development as energy consumption has continuously increased over the years concomitantly with environmental concerns caused by the use of fossil fuels. Semiconductor materials have great potential for acting as photocatalysts for solar fuel production, a potential energy source able to solve both energy and environmental concerns. Among the studied semiconductor materials, those based on niobium pentacation are still shallowly explored, although the number of publications and patents on Nb(V)-based photocatalysts has increased in the last years. A large variety of Nb(V)-based materials exhibit suitable electronic/morphological properties for light-driving reactions. Not only the extensive group of Nb2O5 polymorphs is explored, but also many types of layered niobates, mixed oxides, and Nb(V)-doped semiconductors. Therefore, the aim of this manuscript is to provide a review of the latest developments of niobium based photocatalysts for energy conversion into fuels, more specifically, CO2 reduction to hydrocarbons or H2 evolution from water. Additionally, the main strategies for improving the photocatalytic performance of niobium-based materials are discussed.

Layered Titanate H2Nd2Ti3O10 Intercalated With n-Butylamine: A New Highly Efficient Hybrid Photocatalyst for Hydrogen Production From Aqueous Solutions of Alcohols

A layered perovskite-type oxide intercalated with n-butylamine is reported as an efficient photocatalyst for hydrogen production from aqueous solutions of alcohols for the first time. The hybrid photocatalyst H₂Nd₂Ti₃O₁₀×BuNH₂ was synthesized by solid-state ceramic method followed by protonation, intercalation of methylamine and subsequent substitution by n-butylamine. The product was characterized by powder XRD, TGA, STA-MS, DRS, IR, and Raman spectroscopy, CHN analysis, SEM. Intercalation of n-butylamine caused a dramatic increase in photocatalytic activity of H₂Nd₂Ti₃O₁₀ in the reaction of hydrogen evolution from aqueous solutions of methanol, ethanol, and n-butanol under UV radiation. While the non-intercalated Pt-loaded H₂Nd₂Ti₃O₁₀ showed a maximum quantum efficiency of only 2% in the 220-340 nm range, the efficiency for hybrid samples reached 23% under the same conditions and after variation of experimental parameters even 52% efficiency was achieved. This effect may be associated with the significant expansion of the interlayer space, which is considered as a separate reaction zone.

The impact of nitrogen doping and reduced-niobium self-doping on the photocatalytic activity of ultra-thin Nb3O8− nanosheets

Ultra-thin [Nb₃O₈]⁻ nanosheets with N doping, reduced-Nb doping and N/reduced-Nb codoping were fabricated by combining chemically controlled syntheses and liquid exfoliation, which enable comparative studies on the doping effect for photocatalytic H₂ evolution.