Z. Phys. Chem. 2019; 233(1): 117–165 Review Richard Hinterding* and Armin Feldhoff Two-Dimensional Oxides: Recent Progress in Nanosheets A Retrospection on Synthesis, Microstructure and Applications https://doi.org/10.1515/zpch-2018-1125 Received January 26, 2018; accepted February 27, 2018 Abstract: Two-dimensional (2D) materials have been widely investigated for the last few years, introducing nanosheets and ultrathin films. The often superior electrical, optical and mechanical properties in contrast to their three-dimen- sional (3D) bulk counterparts offer a promising field of opportunities. Especially new research fields for already existing and novel applications are opened by downsizing and improving the materials at the same time. Some of the most promising application fields are namely supercapacitors, electrochromic devices, (bio-) chemical sensors, photovoltaic devices, thermoelectrics, (photo-) catalysts and membranes. The role of oxides in this field of materials deserves a closer look due to their availability, durability and further advantages. Here, recent progress in oxidic nanosheets is highlighted and the benefit of 2D oxides for applications discussed in-depth. Therefore, different synthesis techniques and micro structures are compared more closely. Keywords: nanosheet; oxide; thermoelectric; two-dimensional; ultrathin film. 1 Introduction With the enablement of measuring functional properties of two-dimensional (2D) graphene sheets and the discovery of electrons being able to behave like Dirac-type fermions without restmass [1], new interest in 2D materials was awaken. Actual research considers all elements of the periodic table and their *Corresponding author: Richard Hinterding, Leibniz University Hannover, Institute of Physical Chemistry and Electrochemistry, Callinstraße 3A, D-30176 Hannover, Germany, e-mail: [email protected] Armin Feldhoff: Leibniz University Hannover, Institute of Physical Chemistry and Electrochemistry, Callinstraße 3A, D-30176 Hannover, Germany 118 R. Hinterding and A. Feldhoff combinations with the aim to synthesize materials with improved functional- ity. This research includes 2D oxides and got triggered by sophisticated analyti- cal methods and new models, which led to a significant boost in attention and acceleration of investigations regarding this topic. The determining features of 2D materials are their molecular thickness with structural resemblance to graphene. Since graphene is a monolayer sheet of carbon atoms with lateral sizes up to mil- limeters, the ratio between lateral and axial dimensions in 2D materials is usually between 2 and 5 orders of magnitude [2]. Despite the monolayer in graphene, molecular thickness in so-called nanosheets also provides a significant change in functional properties in comparison to their corresponding 3D bulk materials [3]. These shifts in properties show huge potential for a wide range of applications and have been investigated intensively in the recent years. Accompanying the novel interest in 2D materials, several reviews have been published in the last few years covering various aspects of 2D materials. While some of them discuss 2D materials in general [2, 4–6], others focus on specific functional properties like electrical capacity [7], charge transport [8] or dielectrics [9]. Furthermore, synthesis methods like the liquid exfoliation process [10] or special application fields as biosensing [11] are covered in-depth. Reviews on 2D oxides and hydroxides in particular are either a subcategory in the generalized reviews, do not cover the most recent research because new methods have been established since the publication [12] or focus on exfoliated materials exclusively [13]. The work from ten Elshof et al. [13] is also recommended for further insights into hydroxides, since they are only peripherally mentioned here. This review focuses on novel 2D oxide nanosheets and provides an overview of the currently researched compounds, the state-of-the-art synthesis routes, the microstructures within the 2D compounds leading to their extraordinary proper- ties and the resulting application fields. 2 Elemental compositions Two-dimensional oxide materials have been researched comprehensively in the past few years, which includes research of practical work in synthesis and the- oretical calculations of properties with ab initio methods. The Tables 1–5 offer an overview of the by now most investigated oxidic compounds. Therefore, the tables are oriented to the order of appearance from specific atoms in the periodic table. While this order is easily maintained with binary oxides, ternary and quar- ternary compounds are listed separately. The tables do not claim to be complete, but they surely give an insight on the versatility of research in 2D oxide materi- als. Moreover, specific application fields as supercapacitors, (photo-) catalysis, Two-Dimensional Oxides: Recent Progress in Nanosheets 119 Tab. 1: Selected oxidic compounds containing alkali metals with approved synthesis methods and possible application fields. Elemental composition Synthesis method Application field Reference NaCo2O4, Na0.7CoO2 Template, pyrolysis Thermoelectrics [14, 15] 2− CsW11O36 Ion intercalation Electrochromic devices [16] x− Rb4−xW11O35 Ion intercalation Electrochromic devices [17] K2W6O19, K0.3WO3 CVD Electrochromic devices [18] Tab. 2: Selected oxidic compounds containing earth-alkali metals with approved synthesis methods and possible application fields. Elemental composition Synthesis method Application field Reference BeO Oxidation Gas sensors, catalysis [19, 20] MgO Laser deposition, pyrolysis Adsorbent, catalysis [21–23] Mg(OH)2 Template Adsorbent [24] CaO Template Adsorbent [15] − Ca2Nam−3NbmO3m+1 Ion intercalation Catalysis, supercapacitors [25–28] − CaNb3O10 Ion intercalation Photocatalysis [29] 2− CaNaTa3O10 Ion intercalation Catalysis [30] 2− CaNb2TiO10 Ion intercalation Catalysis [30] 2− A2Ta2TiO10 (A = Ca, Sr) Ion intercalation Catalysis [30] 2− SrA2TiO10 (A = Ta, Nb) Ion intercalation Catalysis [30] − SrNb2O10 Ion intercalation Catalysis [31] thermoelectrics, electrochromic devices or (bio-) chemical sensors play a decisive role as motivation for further investigations. They will be discussed later on in the Section Applications. It should be stated, that several charged compounds appear in the tables, which result from ion intercalation. Their characteristics will be discussed in the Sections Synthesis and Microstructures. Alkali and earth-alkali metals (see Tables 1 and 2) play a minor but not unimportant role in two-dimensional materials in comparison to transition metals, which has various explanations. First of all, there is a numerical reason with simply less existing elements of alkali and earth-alkali metals than transition metals, but this is not the main reason. Binary bulk oxides of alkali and earth-alkali metals find unfrequent usage in nowaday applications and are either a side component of larger production chains or of little use for industrial processes. Since the bulk materials already find limited merit, the properties of their nanosheets need to be extraordinary and not only improved to increase the production value. While this is not the case for the binary oxides of these groups, especially the smaller elements as lithium, sodium and potassium have a special role in the synthesis of 120 R. Hinterding and A. Feldhoff Tab. 3: Selected ternary and quartenary oxidic compounds containing transition metals with approved synthesis methods and possible application fields. Elemental composition Synthesis method Application field Reference − ANb2O7 (A = La, Pr) Ion intercalation Catalysis [32] − AWO6 (A = Ta, Nb) Ion intercalation Catalysis [33] − NbMoO6 Ion intercalation Catalysis [34] − TiAO5 (A = Ta, Nb) Ion intercalation Catalysis [35] − Ti2NbO7 Ion intercalation Catalysis [35] Ba5Ta4O15 Solvothermal Photocatalysis [36] NiFe2O4 Template Supercapacitors [37] NiCo2O4 Solvothermal Batteries, catalysis [38, 39] ZnCo2O4 Solvothermal Supercapacitors [40] CoCr2O4/C Calcination Electrocatalysis [41] LaNiO3 Sol-gel Supercapacitors [42] FeVO4 CVD Batteries [43] −1.8 Bi0.2Sr0.8Ta2O7 Ion intercalation Luminescence [44] Bi2MoO6 Solvothermal Photocatalysis [45, 46] Tab. 4: Selected oxidic compounds containing metals and non-metals of a higher main group with approved synthesis methods and possible application fields. Elemental composition Synthesis method Application field Reference Al2O3 Rapid heating Undefined [47] Bi2O3 Solvothermal Photocatalysis [48, 49] SnO2 Template Supercapacitors [50–52] SnO2/ZnO Template Gas sensors [53] In2O3/SnO2 (ITO) Laser deposition Electronics [54] Graphene oxide Various Template [55] CeO2/PdO doped Self assembly Catalysis [56] nanosheets. As further explained in the Section Synthesis, layered materials are required for top-down synthesis techniques and their delamination is crucial for gaining nanosheets. Their small ionic radii and the singular charge provides the possibility of ion exchange with H+ ions in acidic solutions. This method gets com- monly used in the liquid exfoliation method, wherefore alkali metals are welcomed within the layered materials as in KNb3O8, RbLaNb2O7, Rb4W11O35 or K0.45Mn1−xRuxO2 to exercise this technique [17, 32, 72, 90]. Despite of this, earth-alkali metals in par- ticular are used in combination with Ruddlesden-Popper phases with a composi- * tion of AA21[]nn−+13BnO (A = alkali, A* = earth-alkali, B = transition metal) [30] due to high research interest