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Christian Harito, Munawar Khalil, [Leanddas Nurdiwijayanto](https://orcid.org/0000-0003-1594-0196), Ni Luh Wulan Septiani, Syauqi Abdurrahman Abrori, Budi Riza Putra, Syed Z. J. Zaidi, [Takaaki Taniguchi](https://orcid.org/0000-0002-8460-5431), Brian Yuliarto, Frank C. Walsh

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[Facet-controlled growth and soft-chemical exfoliation of two-dimensional titanium dioxide nanosheets](https://mdr.nims.go.jp/datasets/0760584d-a662-44cb-ae9e-698c85fb5b1c)

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Facet-controlled growth and soft-chemical exfoliation of two-dimensional titanium dioxide nanosheetsNanoscaleAdvancesREVIEWOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueFacet-controlledaIndustrial Engineering Department, BINUSEngineering, Bina Nusantara University, Jakabinus.ac.idbDepartment of Chemistry, Faculty of MatheIndonesia, Kampus Baru UI, Depok, Jawa BcResearch Center for Materials NanoarchitMaterials Science (NIMS), 1-1 Namiki, TsukdResearch Center for Advanced Materials, NKomplek PUSPIPTEK, Serpong, South TangeeAutomotive & Robotics Program, ComputSchool of Engineering, Bina Nusantara UnivfResearch Center for Metallurgy, NationalPUSPIPTEK Area, Building No. 470, Setu RIndonesiagInstitute of Chemical Engineering and TechPakistanhDepartment of Engineering Physics, AdvaInstitute of Technology Bandung (ITB), BandiResearch Center for Nanosciences andTechnology Bandung (ITB), Bandung, 40132jElectrochemical Engineering Laboratory, FacUniversity of Southampton, Southampton, UCite this: Nanoscale Adv., 2024, 6,4325Received 29th May 2024Accepted 15th July 2024DOI: 10.1039/d4na00442frsc.li/nanoscale-advances© 2024 The Author(s). Published bygrowth and soft-chemicalexfoliation of two-dimensional titanium dioxidenanosheetsChristian Harito, *a Munawar Khalil, b Leanddas Nurdiwijayanto, cNi Luh Wulan Septiani,d Syauqi Abdurrahman Abrori, e Budi Riza Putra, fSyed Z. J. Zaidi,g Takaaki Taniguchi,c Brian Yuliarto hi and Frank C. WalshjTiO2 remains one of the most popular materials used in catalysts, photovoltaics, coatings, and electronicsdue to its abundance, chemical stability, and excellent catalytic properties. The tailoring of the TiO2structure into two-dimensional nanosheets prompted the successful isolation of graphene and MXenes.In this review, facet-controlled TiO2 and monolayer titanate are outlined, covering their synthesis routeand formation mechanism. The reactive facet of TiO2 is usually controlled by a capping agent. Incontrast, the monolayer titanate is achieved by ion-exchange and delamination of layered titanates. Eachroute leads to 2D structures with unique physical and chemical properties, which expands its utilisationinto several niche applications. We elaborate the detailed outlook for the future use and research studiesof facet-controlled TiO2 and monolayer titanates. Advantages and disadvantages of both structures areprovided, along with suggested applications for each type of 2D TiO2 nanosheets.1. IntroductionTitanium, as the 9th most abundant element in the Earth'scrust, is naturally found in the form of oxide minerals, partic-ularly titania, TiO2. Over the last y years, TiO2 has been uti-lised in many applications involving photocatalysts,photovoltaics, corrosion/UV protection coatings, and elec-tronics while further studies exploring novel uses continue.Graduate Program – Master of Industrialrta, Indonesia. E-mail: christian.harito@matics and Natural Sciences, Universitasarat, Indonesiaectonics (MANA), National Institute foruba, Ibaraki 305-0044, Japanational Research and Innovation Agency,rang 15314, Banten, Indonesiaer Engineering Department, BINUS ASOersity, Jakarta, 11480, IndonesiaResearch and Innovation Agency (BRIN),egency, South Tangerang, Banten 15314,nology, University of the Punjab, Lahore,nced Functional Materials Laboratory,ung, 40132, IndonesiaNanotechnology (RCNN), Institute of, Indonesiaulty of Engineering and Physical Sciences,Kthe Royal Society of ChemistryModication of the titanium oxide morphology into tailorednanostructures is sought by many practitioners since it is ableto amplify functionality due to a larger active surface area,leading to higher reactivity. Many unique properties can only beobserved at the nanoscale regime. For instance, quantumconnement may occur at a nanoscale thickness, tuning in thedensity of states and band gap of nanomaterials.1 In catalysis,the exposed facets (surface orientation) of nanomaterials playa crucial role. Certain facets may have higher catalytic activitydue to their crystallographic orientation, making them moreeffective in promoting chemical reactions. For titania, quantumconnement and surface orientation play a major role in pho-toconversion efficiency.2Since the rise of graphene over the last two decades,3 thepromise of this unique material has accelerated researchinterest in inorganic 2D nanomaterials. The rapid developmentof 2D nanomaterials is not limited to carbonaceous materials.Recently, titanium carbide-based 2D nanosheets, known asMXenes, have receivedmuch attention. Since 2011, an article onthe exfoliation of MXenes (i.e., Ti3AlC2) by HF has received over2500 citations,4 indicating the rapid growth of research. Tita-nium oxide nanosheets, a 2D analogue of MXenes, have alsoshown an academic impact, especially in catalysis; a 2008contribution on anatase TiO2 with exposed facets has been citedover 3000 times.5 Titania itself has enjoyed a huge impact andhas helped transform our knowledge of photoelectrochemicalcells since 1972.6 Titanium oxide-based nanosheets are animportant research topic, which merits a comprehensive reviewNanoscale Adv., 2024, 6, 4325–4345 | 4325http://crossmark.crossref.org/dialog/?doi=10.1039/d4na00442f&domain=pdf&date_stamp=2024-08-16http://orcid.org/0000-0002-9792-5629http://orcid.org/0000-0002-7712-1738http://orcid.org/0000-0003-1594-0196http://orcid.org/0000-0003-0217-6301http://orcid.org/0000-0003-1520-7170http://orcid.org/0000-0003-0662-7923http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fhttps://pubs.rsc.org/en/journals/journal/NAhttps://pubs.rsc.org/en/journals/journal/NA?issueid=NA006017Nanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineto update our fundamental knowledge and awareness of theiruses.2D nanosheets typically have a thickness of a few nano-metres. They can be divided into three categories, namely,exposed facet TiO2 nanosheets, multi-layered nanosheets andmonolayered nanosheets. Exposed facet TiO2 nanosheets arethin non-layered materials with a 3D crystallographic structure(i.e., TiO2 with dominant {001} facets), as shown in Fig. 1a. Thethickness of this type of nanosheets could reach <5 nm whilemaintaining the crystallographic structure of TiO2.7 Meanwhile,layered titanium oxide nanosheets consist of thin layer struc-tures made from TiO6 octahedra, as shown in Fig. 1b. Mono-layered or single-layered titanium oxide nanosheets has beenextensively researched by Sasaki et al.8 who discovered a two-step method to exfoliate titanium oxide (titanate) nanosheetsin 1998. In contrast to research on exposed facets TiO2, whichmainly focuses on photocatalysis, research on the monolayeredtitanate nanosheets extends the exploration of theirfunctionalities/properties, such as dielectric characteristics,together with spin-electronic applications.9 While titaniumoxide nanosheets have potential in electrochemistry,10–12 appli-cations remain exploratory.2D TiO2 nanosheets also offer distinct advantages over other2D materials primarily due to their exceptional chemicalstability and abundant availability. For example, MXenes, whilepromising for various applications, oen suffer from oxidationFig. 1 (a) SEM image of the exposed {001} facet of TiO2 with illustrationand its 2D structure;14 (c) historical timeline from the ground-breaking clafacet TiO2 (ref. 5, 10–13 and 16) and single-layered titania nanosheets.8,1Chemical Society. Panel (b) is adapted with permission from John Wiley4326 | Nanoscale Adv., 2024, 6, 4325–4345and stability issues, limiting their long-term usability in harshenvironments. In contrast, TiO2 nanosheets are highly resistantto chemical degradation, ensuring consistent performance overtime. Meanwhile, although graphene is renowned for itsexceptional electrical conductivity and mechanical strength, itoen lacks the inherent photocatalytic properties of TiO2nanosheets. This makes them less suitable for environmentalremediation and energy conversion applications, such as pho-tocatalytic water splitting and pollutant degradation. Besides,producing high-quality graphene can be expensive and chal-lenging to scale up, whereas TiO2 nanosheets are more cost-effective and accessible in large quantities.Considering their advantages, this review aims to offera comprehensive perspective on both the synthesis techniquesand the distinct material properties of two key types: exposedfacet TiO2 nanosheets and monolayer titanates. Special atten-tion is devoted to their applications, ranging from energystorage solutions such as sodium and potassium ion batteriesto environmental remediation efforts including ion-exchangeprocesses. Moreover, we delve into the advantages and chal-lenges of various synthesis routes, particularly emphasizing thetrend toward non-uorine-based precursors as a safer, moresustainable approach. A forward-looking discussion isincluded, highlighting the potential of these nanomaterialspresent in diverse scientic and industrial sectors. Futureresearch directions aimed at optimizing these materials for(inset);13 (b) TEM image of the chemically exfoliated titania nanosheetsssic research on TiO2 (ref. 6 and 15) to the development of the exposed7–19 Panel (a) is adapted with permission.13 Copyright © 2017 Americanand Sons.14 Copyright © 2010 WILEY-VCH.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fReview Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineelectrochemical applications and potential integrations withother technologies are also considered.2. Synthesis routes and mechanism2.1. Exposed facets titaniaOver the last y years, crystal facet engineering has beenattracting increased attention as one of the most promisingways to enhance both the physical and chemical properties ofsolid-state materials. Exposing specic types of crystal facets ofmaterials has been reported to be responsible not only for theincreased catalytic activity, but also specialised optical andelectronic properties.20–23 A similar approach has been appliedto TiO2, considering its great potential in energy andenvironmental-related applications. Considerable effort hasbeen made to develop a facile and straightforward syntheticprotocol for the synthesis of TiO2 with specic control overparticular crystal facets. Using both experimental and theoret-ical calculations, it is reported that several TiO2 physicochem-ical properties, such as catalytic activity, adsorption capability,surface atomic conguration, optoelectronic properties, andcatalytic selectivity, could be affected by the type and degree ofcrystal facet exposure.24–28 Nevertheless, exposing the desiredcrystal facet of TiO2 during its crystal growth is a very chal-lenging task. For instance, under equilibrium condition, mostof the available anatase TiO2 crystals involve the thermody-namically stable {101} facet due to its low surface free energy(0.44 J m−2).5,29,30 High surface free energy facets, such as {001}(0.90 J m−2) and {010} (0.53 J m−2), quickly diminish duringcrystal growth due to their instability.31In general, the controlled synthesis of titania with well-dened crystal facets can be achieved using different routes,i.e., gas oxidation, epitaxial growth, spray-drying, topotactictransformation, crystallization transformation from amor-phous TiO2 and wet-chemical syntheses such as hydrothermal,solvothermal or non-hydrolytic routes.32–37 However, hydro-thermal and solvothermal synthetic routes are mostly preferredfor the scalable fabrication of two-dimensional TiO2 nano-structures. This is primarily due to their ability to offer severalbenecial advantages, such as low cost and strong ability todirect crystal growth and nucleation by only controlling reactionparameters. Typically, one of the most common strategies forexposing specic types of crystal facets in TiO2 is by the utili-zation of an appropriate capping agent during hydrothermal orsolvothermal reaction.38–41 A capping agent is used to direct TiO2crystal growth in a specic direction as the result of its prefer-ential adsorption in a particular crystal plane. Other reactionparameters, such as the presence of Ti precursors, reactiontime, temperature and the type of solvent, can inuence theexposure of a particular crystal facet.31The TiO2 {101} facet is one of the most common crystal facetsin the anatase phase due to its low surface energy. Nevertheless,the truncated octahedral bipyramid with eight {101} facets andtwo {001} facets is found to be the most common crystal shapeof anatase in the nature-based Wulff construction.29,30 Hence,many efforts have been made to develop synthetic routes for theformation of TiO2 that show only a {101} facet. One of the© 2024 The Author(s). Published by the Royal Society of Chemistryearliest approaches was to slow the reaction rate, which can beachieved by using Ti(III) as the precursor rather than Ti(IV).42–45In this approach, Ti(III) is considered to be oxidized to Ti(IV)before it undergoes hydrolysis under hydrothermal conditions.Consequently, this would signicantly slow the overall reactionrate due to the lack of dissolved oxygen. This approach wassuccessfully applied by Hosono et al. when they preparedanatase TiO2 nanooctahedra with approximately 100% exposureof the {101} facets using TiCl3 as the precursor in the presenceof sodium dodecyl sulfate (SDS) as a capping agent.42 Based onthe result, it was also suggested that SO42− from SDS wasresponsible for the formation of an equilibrium crystal shape.This was proven by the formation of a slightly different slenderpyramidal morphology when H2SO4 was used instead of SDS. Inanother report, a similar approach of utilizing TiCl3 as the Tiprecursor was also reported in the hydrothermal synthesis ofTiO2 with a {101} facet.44 In this approach, H2O2 was added asan oxidizing agent to produce the intermediate Ti(O2)32−. Incontrast, HCl was used to suppress the formation of the rutileTiO2 phase and to induce the crystal growth into the [101]direction. As a result, pyramidal anatase TiO2 with 100%exposure of {101} facets could easily be obtained.Furthermore, the highly exposed (101) facet of the TiO2nanocrystals with octahedral morphology could also be ob-tained by transforming the amorphous one-dimensional TiO2nanober via hydrothermal method at 160 °C.46 Based on theresult, it is reported that such an approach was able to produceuniform octahedral TiO2 nanoparticles with high specicsurface area (SSA) that predominantly exhibit the {101} facetand a small percentage of the {100} facet. Furthermore, Wu andco-workers have also successfully synthesized single-crystallineanatase TiO2 nanobelts with a high degree of surface exposureof the (101) facet by a hydrothermal transformation of the TiO2powder in concentrated NaOH aqueous solution.47 It was foundthat the as-prepared (101)-exposed TiO2 nanobelts exhibiteda lower rate of excitons recombination due to the signicantenhancement in chargemobility, fewer localized recombinationzones due to the reduction of unpassivated surface states, andimprovement in the ability to trap photogenerated electrons. Inanother report, two-dimensional TiO2 with a high percentage ofthe {101} facet could also be obtained by converting both crys-talline and amorphous TiO2 via the chimie-douce (so chem-istry) method. For instance, Peng and co-workers havesuccessfully converted commercial anatase TiO2 powder intoa two-dimensional (101)-exposed anatase TiO2 nanosheet.48Based on their results, it is believed that the bulk anatasecrystals were able to be initially dissolved into several zigzagtitanate chain building blocks in highly basic conditions, whichcould then be recrystallized back into the lepidocrocite struc-ture where the exposure of the (101) surface is mostly preferred.Fig. 2 shows a schematic illustration for the conversion pathwayof commercial bulk anatase to the two-dimensional (101)-exposed anatase TiO2 nanosheet.Another approach that can be used to prepare two-dimensional TiO2 nanocrystals with high exposure of the{101} facets is by selecting the appropriate capping agent. Forexample, Yang and co-workers were able to develop a robust andNanoscale Adv., 2024, 6, 4325–4345 | 4327http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 2 Schematic illustration of the conversion pathway of commercial bulk anatase to two-dimensional (101)-exposed anatase TiO2 nanosheetin the chimie-douce method. Reprinted with permission from ref. 48. Copyright 2008, American Chemical Society.Nanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinestraightforward synthetic protocol for TiO2 nanoleaves usinga hydrothermal method at 140 °C, with titanium(IV) isoprop-oxide and triethylamine (Net3) as the Ti precursor and cappingagent, respectively.49 Based on the result, the as-prepared TiO2nanoleaves were able to be self-assembled into a facet-selectivetwo-dimensional stacking structure along the [101] plane usingZn(II)-porphyrin and the bidentate bipyridine. Recently, two-dimensional NTA was also successfully transformed intoanatase TiO2 nanostructures with up to 95% exposure of the{101} facet using a solvothermal method with tert-butyl alcoholas the solvent.50 According to the report, it was found that thepercentage of the {101} facet of the as-prepared TiO2 nano-crystals was proportional to its photocatalytic ability inhydrogen production. This superiority in catalytic performancewas believed to be primarily due to the ability of the TiO2 {101}facet to serve as reduction sites with enriched electronpopulations.In the literature, the fabrication of two-dimensional TiO2nanostructures with a high exposure of {001} facets is by far themost exploited approach due to their high surface energy. Inmost cases, the synthesis of such material is carried out bypreventing the crystal from growing in the [101] direction at thenaturally occurring TOB shape according to Wulff construction.This can be achieved by making sure the crystal growth iscarried out under the non-equilibrium condition at a kineticallycontrolled regime.51,52 In general, the TiO2 crystal nucleus wouldinitially evolve as a TOB seed. Under equilibrium condition,TiO2 (ref. 33) facets would rapidly be diminished as the crystal4328 | Nanoscale Adv., 2024, 6, 4325–4345prefers to grow into the thermodynamically stable TOB withpredominately {101} facet. This is mainly because the {101}facet has signicantly lower surface energy than the {001} facet.Under non-equilibrium conditions, the high surface energy{001} facets could be stabilized, resulting in the formation ofa metastable two-dimensional TOB crystal with increasedexposure of the {001} facets. Fig. 3 presents the schematicillustration for the TiO2 crystal evolution in both equilibriumand non-equilibrium conditions.Traditionally, a non-equilibrium condition during TiO2crystal growth could be kinetically achieved by controlling thetemperature and ramping rate during the reaction. Forexample, Ahonen and co-workers were able to create a non-equilibrium condition by carrying out rapid heating andquenching of titanium(IV) isopropoxide via high-temperature(1200 °C) gas phase thermal oxidation.53 Based on this result,it was found that such a condition was able to form a well-faceted anatase TiO2 particle with the predominant exposureof the {001} facet. In another report, a similar rapid heating andquenching approach was also carried out using TiCl4 as theprecursor.54 Here, the thermal oxidation process was done byliberating the Ti precursor vapor using argon bubbles andmixing with high-rate oxygen stream, where it was subsequentlysubjected to high temperature (1300 °C), which results in theformation of decahedral single-crystalline TiO2 particles withup to 40% exposure of the {001} facet. Both thermal oxidationtemperature and its ramping rate were crucial in this syntheticmethod. A high exposure of the {001} facet could only be© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 3 Schematic illustration of the TiO2 crystal evolution under equilibrium and non-equilibrium conditions.Review Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineachieved when the annealing temperature was above 500 °Cwith a ramping rate above 16 °C min−1.55,56 This syntheticapproach is also known to result in the formation of the rutileTiO2 phase as a byproduct.53 This method has been widelyconsidered unsuitable for the scalable industrial production ofsuch products.Many recent studies have considered the inuence of variousreaction dynamics for the synthesis of TiO2 crystals with a highexposure of the {001} facet in both aqueous and non-aqueousliquid phase systems. It has been shown that selecting a suit-able titanium precursor, reaction temperature, pressure andsolvent is important. The introduction of capping agents isessential to control the crystal nucleation.21,28 Among thesefactors, the type and amount of capping agent are considered asthe most crucial contributing parameters in ensuring the highexposure of the {001} facet in TiO2. This is primarily due to thekinetics of crystal growth being exponentially proportional tothe crystal surface energy.51 Typically, the specic surface energyFig. 4 SEM and TEM images of the TiO2 nanocrystals with different degrewith permission from ref. 63. Copyright 2017, American Chemical Socie© 2024 The Author(s). Published by the Royal Society of Chemistryof a crystal can be enhanced or reduced by selective adsorptionof a capping agent on that particular crystal facet.30 As a result,the presence of a specic capping agent can signicantlyinuence the nal shape of the crystal. For the case of TiO2 withhigh exposure of the {001} facet, uorine-based capping agentshave been widely utilized due to their strong preferentialinteraction and ability to stabilize the {001} facet.5 During thepast several years, different types of uorine-based cappingagents, such as HF, NH4F, NaF, and [bmim]-[BF4], have beeneffectively used to synthesize TiO2 with a high exposure of the{001} facet.5,57–60 Moreover, the utilization of uorine-based Tiprecursors, such as TiF4 and TiOF4, has also been reported to beable to produce TiO2 with high exposure of the{001} facet due tothe simultaneous in situ generation of the F− species.61,62Furthermore, a study by Liu and co-workers revealed that thevariation in the degree of co-exposure for both {101} and {001}facets could also be simply controlled by the ratio of HF/H2Oduring the solvothermal reaction.63 Based on the result, thees of the {001} facet synthesized at various ratios of HF/H2O. Reprintedty.Nanoscale Adv., 2024, 6, 4325–4345 | 4329http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinepercentage of {001} facet exposure was found to be proportionalto the concentration of HF. Using this synthetic approach, two-dimensional TiO2 nanosheets withz92% exposure of the {001}facet were successfully fabricated and proven to exhibitedexceptional ability as an antibacterial agent due to the presenceof the {101}/{001} surface heterojunction. Fig. 4 shows the SEMand TEM images of the as-prepared TiO2 nanocrystals withdifferent degrees of {001} facet exposure prepared at various HF/H2O ratios.Additionally, a uorine-free hydrothermal route with K-titanate nanowires and urea as the precursors was also intro-duced for the synthesis of two-dimensional TiO2 nanostructureswith the {001} facet.64 In this synthetic approach, it was reportedthat the carbonate ions resulting from the decomposition ofurea were found to be responsible for the formation of a highpercentage of {001} facet (60%). In other reports, other inor-ganic species such as Cl− and SO42− anions were also reportedto be sufficient for directing the formation of TiO2 nanocrystalswith a high exposure of the {001} facet.65,66 Recently, organic-based capping agents have also been explored for a similarapplication. For instance, Khalil and co-workers have also re-ported that an amine-based capping agent, i.e., DETA, couldalso be utilized to expose the {001} facet during the hydro-thermal synthesis of TiO2 with spindle-like morphology.33,67 Inanother study, Chen et al. successfully fabricated hierarchicalsphere microstructures comprising the self-assembled two-dimensional ultrathin TiO2 nanosheet with nearly 100% expo-sure of the (001) facet using a mixture of isopropyl alcohol andDETA as the capping agent.10 Recently, a combination of HF andpolymer-based capping agents, i.e., poly(vinylpyrrolidone)(PVP), has also been reported to be utilized for the synthesis ofTiO2 nanomosaics comprising two-dimensional TiO2 witha high percentage of exposure for {001} facet.68 In this report, itwas believed that the large and bulky polymeric PVP moleculescould serve not only as the linker between TiO2 nanosheets, butalso prevent them from stacking together along the c-axis.2.2. Monolayer titanateSince 1998, Sasaki et al.17 have studied the single layerednanosheets prepared by chemical exfoliation of lepidocrocite-like titanate, in which the solid-state reaction was the mainmethod used to synthesize the parent compound at that time.69Since the reaction occurs in the solid state, a high temper-ature process (800–1500 °C) is usually required to induce thereaction of solid precursors.70 To ensure a uniform reaction,crushing and grinding are usually performed with a mortar andpestle to produce a thorough mixture of precursors, while ballmilling could be used for a larger quantity. To help with thehomogenisation, a small amount of solvent such as alcohol oracetone can be added, in which it will evaporate aer theprecursors are perfectly mixed.70 Instead of using additionalsolvent, pelleting can be performed as an alternative to producea good contact between the precursors. The rate of the solid-state reaction can be controlled by adjusting the temperatureand by considering certain properties of the precursor, such asthe surface area, its reactivity, and morphology. To increase the4330 | Nanoscale Adv., 2024, 6, 4325–4345reactivity, a molten salt is oen used as an additive andsolvent.71The common solid-state reaction of titania and alkali saltprecursors, such as CsNO3, Cs2CO3, and K2CO3 oen results ina brous (monoclinic) titanate structure. In 1987, Grey et al.69discovered a new type of titanate compound using a non-stoichiometric reaction, where the resulting product hasa layered structure of the lepidocrocite-like (orthorhombic)titanates. Fig. 5 shows the crystal structures and scanningelectron microscopy (SEM) images of the brous andlepidocrocite-like titanate compounds.In the Grey et al. method, a TiO2 : CsNO3 molar ratio ofaround 1 : 2.8–3.2 was mixed, followed by heating at 800–1050 °C for 0.5–20 hours, producing a white powder of lepidocrocite-like caesium titanate with the chemical formula of CsxTi2−x/4,x/4O4, where x is about 0.61–0.65 and , represents a tita-nium vacancy. The procedure has been further developed bySasaki et al., who used Cs2CO3 and TiO2 with a molar ratio of 1 :5.3 exhibiting lepidocrocite-like lamellar sheets.73 Besides thecaesium-based precursor, Sasaki et al. also utilised Li2CO3 andK2CO3 in place of Cs2CO3 for the reaction with titania powder.75The reaction of Li–K-based titanates can be enhanced byK2MoO4 molten ux process, which acts as an excellent heattransfer medium. The slow-cooling procedure in the uxprocess yields very large nanosheets of up to 30 microns, whilethe solid-state reaction typically produces nanosheets ofz0.5–1microns.76,77The layered lepidocrocite-like titanate needs to be exfoliatedto produce monolayer nanosheets. Sasaki et al. showed a faciletwo-step ion-exchange method to exfoliate the nanosheets.Firstly, the interlayer caesium or potassium ions were etched withacid and replaced byH+ ions. For complete removal of alkali ions,a repeated acid treatment with a fresh solution was required inwhich 98% of the alkali ions were removed aer three dailycycles.72 As a result, lepidocrocite-like titanate with a high cation-exchange capacity was produced aer the acid ion-exchangereaction, exhibiting a similar smectite clay-like behaviour.Secondly, the exchange of bulky ions with TBA+ or TMA+ ionswas conducted to assist the complete exfoliation of protonatedlayered titanate. The properties of the resulting compoundswere similar to those of smectite clays such as montmorillonite,hectorite, and saponite, in which the basal spacing could beexpanded (swollen) by the intercalation of guest molecules.Depending on the concentration of bulky ions, the titaniananosheets can be in intercalated, exfoliated, or osmotic-swelling states,17 as shown in Fig. 6. An extensive study on theexfoliation of nanosheets has been conducted by Sasaki et al.17By controlling the molar ratio of TBA+ to H+, the state of thetitania nanosheets can be adjusted from intercalation / exfo-liation / swelling. For caesium-based titania nanosheets, theintercalation state occurs when the ratio of TBA+/H+ is less than0.5, as examined by SAXS. The interlayer spacing of nanosheetsincreases as the number of bulky ions increases, leading toinnite interlayer spacing and the induction of exfoliation. Thefully exfoliated state occurs within the TBA+/H+ ratio of 1–5.When the ratio of TBA+/H+ exceeds 5, a multilayer arrangementof lamellar sheets occurs, exhibiting a diffuse double layer© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 5 (a) Crystal structure of the fibrous-like titanate (monoclinic);72 (b) crystal structure of the lepidocrocite-like titanate (orthorhombic) viewedalong the a-axis;73 (c) FESEM image of fibrous-like titanate;73 (d) FESEM image of lepidocrocite-like titanate.74 Panel (a) adapted with permission.72Copyright © 2010, American Chemical Society. Panel (b and c) adapted with permission.73 Copyright © 1995, American Chemical Society. Panel(d) adapted with permission.74 Copyright © 1998, American Chemical Society.Review Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethrough osmotic swelling. During the osmotic-swelling state,the interlayer spacing becomes smaller, leading to sheet coag-ulation as the number of ions increases. One must note that theFig. 6 Schematic representation of the synthesis of single-layer titaniaPublishing.© 2024 The Author(s). Published by the Royal Society of Chemistryratio of TBA+/H+ varies for each type of nanosheets dependingon the stoichiometry and charge density of the layeredcompounds.75,78,79 This chemical exfoliation method maynanosheets via chemical exfoliation process.80 Copyright © 2017 IOPNanoscale Adv., 2024, 6, 4325–4345 | 4331http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineproduce very large nanosheets if gentle stirring or shaking isapplied during the exfoliation process.75Besides the chemical exfoliation method, the exfoliation canbe conducted by mechanical approaches such as supercriticaluid exfoliation81 and ultrasonication assisted ion-exchange.82High energy jets created by the implosion of bubbles duringultrasonication break up the layered nanosheets in a relativelyshort time, although it also reduces the lateral size of thenanosheets. Meanwhile, the supercritical uid method utilisesthe uid expansion to exfoliate the nanosheets. A supercriticaluid is any compound at a temperature and pressure above itscritical point, where the intermediate phase (which can effusethrough solids like a gas and dissolve materials like a liquid)occurs. At the beginning, the layered nanosheets are interca-lated by the supercritical uid. In this state, the exfoliation caneasily occur by applying thermal stress to the intercalatednanosheets. However, the exfoliated nanosheets may berestacked upon cooling down; hence, a faster cooling rate ispreferable. The highest yield of exfoliated nanosheets by thismethod, however, was estimated to be only 10%.81While the chemical exfoliation method uses a top-downapproach from precursors synthesized by a solid-state reac-tion, other researchers synthesised single-layered nanosheetsusing bottom-up approaches such as the electron beam depo-sition (EBD) of titania and oxygen atoms under ultra-highvacuum83 and sol–gel method.18,84 Ti was deposited by e-beamdeposition on (1 × 2)-Pt(110) at room temperature (pO2 = 1 ×10−4 Pa), followed by post-annealing treatment at 700 K andcooling down in oxygen (pO2 = 1 × 10−4 Pa), resulting in titaniaFig. 7 The illustration of (a) the effect of the organic ligand in the cryrestacking by dialysis.87 Copyright © 2015, American Chemical Society.4332 | Nanoscale Adv., 2024, 6, 4325–4345nanosheets with 3.9× 1.6 nm lateral size.83 A sol–gel solution oftitania nanosheets can be synthesised by reacting the titaniumprecursor (i.e., TiF4 and (NH4)2[TiO(C2O4)2]) with an aqueoussolution of KOH or NaOH. The resulting product was a smallmulti-layered nanosheet; hence, a bulky molecule such asTBAOH or TMAOH was still required to exfoliate the nano-sheets.85,86 On the contrary, a sol–gel synthesis of TIP witha large excess of aqueous bulky molecule solution of TMAOHexhibited a high yield of diamond or rhombic-shaped mono-layered nanosheets.18 The bulky molecule served as the reactantfor the acid–base reaction with titanic acid, as well as providingenough ionic charge to maintain the exfoliation of nanosheets.Compared to the chemically exfoliated nanosheets, the sol–gelsynthesis usually produces relatively small nanosheets of lessthan 50 nm in lateral size.Ban et al.87 further developed the sol–gel synthesis by using anorganic ligand (e.g., triethanolamine and lactic acid) to forma titanium complex, hence retarding the nucleation of titaniananosheets while promoting growth in the lateral direction. Thismethod created z100 nm diamond-shaped titania nanosheetsaer several days of reaction in the autoclave. However, theorganic ligand may also cause the restacking of nanosheetsduring evaporation; hence, it should be removed by dialysis.Ban's sol–gel synthesis of large nanosheets is illustrated in Fig. 7.The need to conne the growth of titanate in the lateraldimension has been developed by another group. Sol–gelsynthesis at the hydrophobic/hydrophilic (i.e., hexane/ice)interface can be deployed to create large nanosheets, as illus-trated in Fig. 8.84 These nanosheets contain several smallstallisation of the titania nanosheets and (b) inhibition of nanosheets© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 8 (a) Schematic illustration of the 2-D TiO2 formation on the hexane/ice interface.84 TEM images of a nanosheet consisting of horizontallyagglomerated TiO2 nanodiscs:84 (b) HR-TEM image of nanodiscs, (c) TEM image of nanosheets. Reproduced from ref. 84 with permission fromthe Royal Society of Chemistry.Review Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinenanodiscs with z5 to 15 nm in lateral size, which areagglomerated horizontally. The single-layered nanosheetsstructure was conrmed by atomic force microscopy (AFM),whereas the nanosheets are only z0.5–1 nm in thickness. AerFig. 9 Several routes to synthesize single-layer titania nanosheets: (a) TETEM image of nanosheets made by the sol–gel route,86 (c) TEM imagnanosheets made by the sol–gel route,18 (e) high-resolution STM imag13.6 nm; bias voltage = 0.42 V; IT = 0.9 nA).83 Panel (a) adapted with peWeinheim. Panel (b) adaptedwith permission.86 Copyright © 2013, Americfrom the Royal Society of Chemistry. Panel (d) adapted with permission.© 2024 The Author(s). Published by the Royal Society of Chemistryhydrolysis by HCl, the anatase TiO2 structure was formed, ascharacterised by X-ray diffraction (XRD). A schematic of thesynthesis route to TiNS is outlined in Fig. 9 and it is summa-rized in Table 1.M image of nanosheets made by the solid-state route,14 (b) bright-fielde of nanosheets made by the ice sol–gel route,84 (d) TEM image ofe of nanosheets made by the e-beam deposition route (13.6 nm ×rmission.14 Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,an Chemical Society. Panel (c) reproduced from ref. 84 with permission83 Copyright © 2006 by the American Physical Society.Nanoscale Adv., 2024, 6, 4325–4345 | 4333http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fTable 1 Summary of the synthesis methods for single-layered titania nanosheetsTop-down approachSynthesis of layered nanosheetsExfoliation method and itsadditiveChemicalformula Lateral size Ref.Solid-state reactionCs2CO3 + TiO2 / CsxTi2−x/4,x/4O4 (x z0.7; , = titanium vacancy)Ion-exchange at 25 °C for 2 weeksby 0.00825 to 0.0825 mol L−1aqueous solution of(tetrabutylammonium hydroxide)TBAOHTi0.91O20.36− z0.1–1 mm 17Reaction at 800 °C for 20 h (2 times)Cs2CO3 + TiO2 + MgO / CsxTi2−x/2Mgx/2O4 (x z 0.7)Ion-exchange at 50 °C for 1 weekby 5 wt% aqueous solution ofTBAOH or(tetramethylammoniumhydroxide) TMAOHTi0.825O1.8250.35− z0.1–1 mm 88Reaction at 800 °C for 1 h followed by 2times heating at 950 °C for 20 hK2CO3 + TiO2 + Li2CO3 / KxTi2−x/3Lix/3O4 (x z 0.8); (with K2MoO4 as ux melt)Ion-exchange at 25 °C for 2 weeksby 0.0125 to 0.025 mol L−1aqueous solution of TBAOH orTMAOHTi0.87O20.52− 0.5–2 mm; average z 1 mmfor TBAOH and 10–30 mmfor TMAOH75Reaction at 1200 °C for 10 h, followed byslow cooling (4 °C h−1) until it reaches950 °CNa2CO3 + TiO2 / Na2Ti3O7 Ion-exchange by methylamine at60 °C for 6 d, followed bypropylamine at 60 °C for 6 dTi3O72− z0.1–1 mm (rectangular) 89Reaction at 900 °C for 24 hCs2CO3 + TiO2 / CsxTi2−x/4,x/4O4 (x z0.7; , = titanium vacancy); reaction at800 °C for 20 h (2 times)Ion-exchange by TBA+ ion assistedwith ultrasonication (60–300 W,2–30 min)Ti0.91O20.36− z0.1–0.2 mm 82K2CO3 + TiO2 + Li2CO3 / KxTi2−x/3Lix/3O4 (x z 0.8) (with K2MoO4 as ux melt).Reaction at 927 °C for 10 h (spontaneouscooling)Supercritical DMF exfoliation(400 °C, 15 min)Ti0.87O20.52− z5–20 mm 81Sol–gel followed by ion exchange(NH4)2[TiO(C2O4)2] + KOH /K1.1H0.9Ti2O5$2.6H2O (1 day, 22–80 °C)Ion-exchange by aqueous solutionof TBAOH at 22 °CTi2O52− z10–20 nm 85TiF4 + NaOH / Na0.8Ti1.8,0.2O4$yH2O(y < 1.17) (3 days, 22 °C)Ion-exchange by aqueous solutionof TBAOH at 22 °CNot available z2–5 nm 86Bottom-up approachMethod Chemical formula Lateral size Ref.Reux of Ti(IV)isopropoxide (TIP) + aqueoussolution of tetramethylammoniumhydroxide (TMAOH); (5 min to 24 h, 100 °C)(TMA)xTi2−x/4,x/4O4 (x z 0.7) Diamond shape with a diagonallength of (27.3, 19.1) nm to (7.7,5.5) nm18TIP + organic ligand (e.g., triethanolamine orlactic acid) + tetrabutylammoniumhydroxide (TBAOH) heated in autoclave at80 °C for 1–7 days, followed by dialysis withwater for 2 days(TBA, H)0.7Ti1.825O4$xH2O Diamond shape with z100 nmlateral size87Sol–gel of hexane + TIP + ice granuleinterface, followed by hydrolysis with HClTiO2 z5 mm consist of 5–15 nmnanodiscs84e-beam deposition on (1 × 2)-Pt(110); Ti wasdeposited at room temperature (pO2 = 1 ×10−4 Pa), followed by post-annealingtreatment at 700 K and cooling down inoxygen (pO2 = 1 × 10−4 Pa)TiO2 3.9 × 1.6 nm 83Nanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online3. Properties3.1. Physical properties3.1.1. Exposed facets titania. Both optical and electronicproperties have been widely considered as the most common4334 | Nanoscale Adv., 2024, 6, 4325–4345direct consequences for the exposure of a specic crystal facet in2D TiO2 nanostructures. Typically, this can easily be observed bythe alteration of both bandgap and band edge location.According to recent studies, the exposure of the TiO2 {001} facetwhile diminishing the existence of the {101} facet may© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 10 Slab model for the surface structures of the relaxed stoichiometric TiO2's {111}, {001}, {010} and {101} facets. Reprinted with permissionfrom ref. 100. Copyright ©2013, American Chemical Society.Review Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinecontribute to the reduction of the TiO2 optical band gap.44,90,91For instance, Liu et al. calculated that the optical bandgap ofTiO2 nanostructures with 5%, 60%, and 92% exposure of the{001} facet was found to be 3.33, 3.29, and 3.16 eV, respec-tively.92 A similar bandgap narrowing due to the exposure of the{001} facet was also observed elsewhere.93,94 According to theo-retical calculation using DFT, such reduction was most likelybecause the {101} facet possesses a slightly higher CB than the{001} facet.95–97 Such a phenomenon might also have occurreddue to the presence of oxygen vacancies as a result of the uniquesurface atomic arrangement.98,99 Furthermore, a similar bandstructure alteration was also observed in other high-index fac-ets. For example, using both experimental and theoreticalestimations, Xu and co-workers revealed that TiO2 with the{111} facet exhibited a higher conduction band minimum incomparison to TiO2 with {001}, {101}, and {010} facets.100 It isbelieved that such a phenomenon was partially attributed to thelarge percentage of undercoordinated Ti and O atoms at thesurface of the {111} facet. Fig. 10 presents the slab model for thesurface structure of TiO2 at different crystal facets.Furthermore, the unique arrangement of atoms at thesurface of TiO2 from the exposure of different facets may alsoinuence the efficiency of the charge carrier separation. Tradi-tionally, the prevention of the fast photogenerated electron–hole recombination of TiO2 was typically done by hetero-junction or the application of sacricing agents.101,102 However,recent studies have suggested that crystal facet engineering ofTiO2 could also be used as an effective strategy to avoid suchissue.103–106 For example, the high surface energy {001} facet hasbeen proven to exhibit superior ability in ensuring efficientseparation of photoexcited charge carriers. It is believed that© 2024 The Author(s). Published by the Royal Society of Chemistrysuch a phenomenon was partly due to the presence of surfacedefects, e.g., oxygen vacancies, which could efficiently mediatethe interfacial electron transfer.21 Additionally, a high density ofundercoordinated Ti atoms and large Ti–O–Ti bond angles atthe surface of the {001} facet has also been considered as one ofthe main contributors for such phenomenon.5 Recently, thecontribution of two or more co-existing facets has also beenassociated with amore efficient charge separation. For instance,Yu et al. reported that the co-exposed {001} and {101} facets werefound to exhibit a synergistic effect that was responsible for theenhancement of the photocatalytic activity of the 2D TiO2nanosheet.107 Using DFT calculation, it was revealed that theenhancement in the photoactivity was primarily due to theformation of a surface heterojunction between the {001} and{101} facets as a result of their band alignment. This is possiblesince the positions of CBM and VBM of the {001} facet werefound to be more positive than that of the {101} facet.92 Asa result, the photogenerated electron tends to be thermody-namically transferred to {101}, while the hole preferentiallymoves in the opposite direction.Another physical characteristic that may be inuenced by theexposure of certain facets in 2D TiO2 nanostructures is theircapability in substrate adsorption. It is reported that the specicgeometric structure and atomic arrangement at the surface ofa particular TiO2 facet could affect the interaction between TiO2and various types of substrates, e.g., water, methanol, CO2, orother small molecules.108–110 One of the widely acceptedrationalizations for such a phenomenon was the fact thatcertain facets exhibit different degrees of oxygen vacancy andundersaturated Ti coordination. For example, the surface ofTiO2's {001} facet is widely known to have 100%Nanoscale Adv., 2024, 6, 4325–4345 | 4335http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineundercoordinated Ti-5c atoms and half saturated Ti-6c atoms.In contrast, the {101} facet exhibits half of the under-coordinated Ti-5c atoms and half saturated Ti-6c atoms.31 The{001} facet is also reported to have a larger stoichiometricamount of the surface hydrophilic Ti3+ and surface OH groupsthan the {101} facet.111 As a result, the sorption capacity of the{001} facet for water or other polar molecules is expected to behigher than that of the {101} facet. A similar superiority in thesorption capacity of the {001} facet over different facets was alsoobserved elsewhere for the absorption of Cr2O72− andarsenic.112,113 It is also worth noting that the specic surface areaand particle size may both contribute to the overall sorptioncapacity.3.1.2. Monolayer titanate. The properties of chemicallyexfoliated titania nanosheets are related to its chemicalformula, in which the precursors (i.e., lepidocrocite-like tita-nate) have a general formula of CsxTi2−x/4,x/4O4, where x isaround 0.7 and , is titanium vacancy114,115 for the caesium-based and AxTi2−x/3Lix/3O4 nanosheets, where x z 0.8 for A =K (potassium) and x z 0.75 for A = Rb.17 The detailed crystalstructures of these compounds are shown in Fig. 11. Aer alkaliion removal, the nanosheets are negatively charged with thegeneral formula of Ti0.91O20.36− for nanosheets derived fromCsxTi2−x/4,x/4O4.Fig. 11 The polyhedral representation of the crystal structure of layered lmono-layer titania nanosheets after exfoliation. Adapted from ref. 88 wi4336 | Nanoscale Adv., 2024, 6, 4325–4345The absorption peak wavelength of the Ti0.91O20.36− nano-sheets is blue-shied to around 265 nm, as compared to anataseTiO2 at 377 nm.8 It is known that the molar absorption coeffi-cient or molar extinction coefficient (3) is 2.2 × 104 mol−1dm3 cm−1 at 265 nm.116 This blue shi also occurs in sol–geldiamond-like titania nanosheets, which has the peak around250 nm.18 It was concluded by both researchers that thequantum connement signicantly contributes to the opticalproperties of titania nanosheets especially due to the transitionfrom the 3D to 2D structure. Using spectroscopic ellipsometry,the refractive index of the Ti0.87O20.52− nanosheets was found tobe around 2.1 at 600 nm and the extinction coefficient of a thinlm (k) was nearly zero.117 The titanate nanosheets in thestructure possess diamagnetic properties, which may havealigned itself in the 2D plane perpendicular to the magnetic uxdirection due to the highly anisotropic magnetic suscepti-bility.118 The magnetic susceptibility can be altered via UVphotoreduction of TiIV to TiIV/III nanosheets, which exhibitsparamagnetic properties. It changes the orientation from theorthogonal to parallel direction when exposed to the magneticeld, as depicted in Fig. 12.The electronic band gap energy of Ti0.91O20.36− nanosheetswas 3.84 eV, as estimated by in situ UV-vis spectroscopy,119which is 0.6 eV larger than that of anatase titania.120 Comparedepidocrocite-like titania nanosheets viewed down along the c-axis andth permission from the Royal Society of Chemistry.© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fFig. 12 The orthogonal and parallel magnetic orientation switching of titania nanosheets via photoreduction and oxidation.118 Copyright © 2018,American Chemical Society.Review Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineto anatase TiO2, TiNS has a slightly higher conduction band at−1.27 eV vs. Ag/Ag+ and signicantly lower valence band at2.53 eV vs. Ag/Ag+.119 The exfoliated titania nanosheets hasa larger band gap than its lepidocrocite-like titanate precursor,hence further conrming the effect of quantum connement.Compared to the anatase TiO2, stronger UV light is required toactivate the photocatalytic capability of titania nanosheets. Toovercome the large band gap, metal and non-metal doping areoen introduced for narrowing the band gap. Due to a titaniumvacancy in the structure, co-doping is possible for titaniananosheets.119 One cation is used to replace the interlayer ions,while the other may co-substitute Ti4+ in octahedral sites. Fanet al.121 utilised the photocatalytic properties of titanate titaniananosheets by doping with platinum nanoparticles via photo-reduction of Pt(IV) ions, which is indicated by the colourchanging from white to dark grey. Besides the precious metal,titania nanosheets have been doped by Fe, Ni, Co, Nb, and Mnions for metal doping and nitrogen ions for non-metaldoping.119 Few studies have examined non-metal doping.Thus, an exploration of co-doping by a non-metal dopantshould be conducted for further research.In terms of thermal stability, the monolayered titaniananosheets maintained its structure up to 800 °C before ittransformed to anatase TiO2.119 The stability was reduced withincreasing number of stacks of the titanate layers, in which 10stacks of nanosheets transformed into anatase TiO2 at around400 °C. The 2D structure limits the diffusion of atoms,hampering the 3D formation of the anatase structure. Mean-while, the electrical conductivity depends on the relativehumidity, where it increases by about 5 orders of magnitudefrom 45% to 95% relative humidity.122 Water molecules© 2024 The Author(s). Published by the Royal Society of Chemistryadsorbed on the titanate surface can bridge the electricaltransport in the lateral dimension.3.2. Chemical properties3.2.1. Exposed facets of titania. The unique geometricstructure and atomic arrangement at the surface of 2D TiO2nanostructures with certain exposed facets have also beenassociated with the enhancement of their catalytic reactivity.Recently, the ability to control the surface and electronicproperties via crystal facet engineering of 2D TiO2 nano-structures has attracted much attention as a way to improvetheir performance in various applications, especially in catalysisor light-harvesting devices. It is believed that the presence ofundercoordinated Ti atoms and the number of oxygen vacan-cies at certain crystal facets has a signicant inuence indictating both the kinetics and thermodynamics of the reaction.For instance, TiO2 with a high exposure of the {001} facet hasbeen well-documented to be more reactive towards waterdissociation and more effective for facilitating photoredoxreactions than that with a high exposure of the {101} facet.30,110In another report, Amano et al. have also reported that theperformance of 2D decahedral single-crystalline TiO2 witha high exposure of the {001} facet in hydrogen evolution via thewater splitting reaction was superior to that of the commercialP25 Degussa TiO2 powder.54 Recently, Khalil and co-workershave also proven that the exposure of the {101} facets wasresponsible for the enhancement in the photocatalytic activityof the nano Au–TiO2 heterostructures for the photodegradationof organic pollutants.33 Based on this result, the synergisticeffect between the surface plasmon resonance phenomenonNanoscale Adv., 2024, 6, 4325–4345 | 4337http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineand the exposure of the {001} feature was able to signicantlyincrease the reaction rate by ten-folds. Furthermore, they alsoreported that a similar enhancement in photocatalytic activityby the co-exposure of the TiO2 (101) and (001) facets was alsoobserved in the photocatalytic reduction of bicarbonate usingCdSe–TiO2 nanostructures.67In addition to the enhancement in the catalytic activity, theexposure of certain crystal facets in 2D TiO2 nanocrystals wasalso reported to be responsible in the variation of the catalyticselectivity. For instance, the selectivity of the toluene conversionto benzaldehyde can be enhanced by simply increasing thepercentage of exposure for the {001} facet in the two-dimensional TiO2 nanosheet.62 According to the report, theselectivity for the formation of benzaldehyde could be increasedby up to 93% (yield of 26%) by exposing 50% of the {001} facet.In another report, Liu and co-workers reported that the expo-sure of the {001} facet could also inuence the selectiveadsorption and photocatalytic activity towards azo dyes.123 Itwas revealed that TiO2 with a low exposure of the {001} facet(P25 titania, 5% of exposed {001} facet) showed a preferentialphotocatalytic decomposition of MO. Meanwhile, TiO2 witha high exposure of the {001} facet favors the degradation of MB.In the literature, this selectivity was believed to originate fromthe unique surface atomic conguration of the {001} facet,which results in the alteration of surface characteristics such asthe surface charge, Lewis and Brønsted acidity, and exposedfunctional groups.124,125 It is suggested that the spatial distri-bution of the redox sites due to the preferential separation ofphotogenerated charge carriers at certain crystal facets may alsocontribute to the aforementioned catalytic selectivity.126,1273.2.2. Monolayer titanate. The high reactivity of interlayeralkali metal ions such as Cs+ and K+ is advantageous for the ionexchange reaction with protons that facilitate the exfoliation oftitania nanosheets. The cation exchange capability of chemi-cally exfoliated titania nanosheets is benecial in energy storageapplications; for example, it can be used for lithiation and de-lithiation in a lithium-ion battery.In terms of colloidal stability, a net negative charged on thetitanate surface is formed aer the removal of alkali metal ions,in which it is stable in basic solution with the point of zerocharge at pH 8 and zeta potential of −37 mV at pH 10–13.128 InTBAOH or TMAOH solutions, the colloidal suspension ofchemically exfoliated titania nanosheets is stable for more than6 months. It was observed that sol–gel titania nanosheets aremore stable due to the smaller particle size. A stable colloidalsuspension is convenient for the deposition process, in whichthe controlled deposition of titania nanosheets can be realisedby Langmuir–Blodgett procedure and electrostatic layer-by-layerassembly.129 Alternatively, an amount of titanate can be drop-casted on the surface, yielding a lm with cation-conductingproperties.130 Electrophoretic deposition can also be per-formed to decorate the electrode via the negative surface chargeof chemically exfoliated titania nanosheets.131 The negativesurface charge is also exhibited in sol–gel titania nanosheets.132When an electrophoretic deposition technique combines withmechanical stimulation, small sol–gel titania nanosheets can4338 | Nanoscale Adv., 2024, 6, 4325–4345be inserted within titanate nanotubes to create a hierarchicalstructure132 of titania nanosheets.Modication of the surface functional group of titaniananosheets has been studied.128 In an aqueous solution,chemisorbed and physiosorbed water molecules are attached tothe surface of titanate, leading to a hydroxylated surface, wherethe functionalisation can be performed via these hydroxyl grouptitania nanosheets. Generally, the modication of the hydroxylgroup of titanate can be approached via hydrolysis with silanegroups, esterication with carboxylic acid, peroxo-titaniumcomplex formation by H2O2, acid–base reaction, and forma-tion of admicelles by surfactant.133,134 Silanisation with APTESaltered the zeta potential of titania nanosheets via amino-endgroups, in which the APTES–titania nanosheets have the pointof zero charge at pH 6 and it is stable in acidic solution (pH <4).128 Similar to titanate nanotubes which have a lot of hydroxylgroups on its surface, the chemically exfoliated titania nano-sheets are also highly reactive to H2O2. Reaction with H2O2creates titanium(IV) peroxo-complex, which is indicated bya colour transformation from white to yellow. Interestingly, thecolour reverts back to white aer reacting with azo dyes, indi-cating the release of the oxo group while cutting the azo dyechain.135 The colour transformation does not occur in sol–geltitania nanosheets, which is probably due to the hindrancecaused by the excess of bulky molecules of TMAOH. Furtherstudy is required for the formation of peroxo complexes in sol–gel titania nanosheets.4. Applications4.1. Exposed facets titaniaIn the past several years, the exposure of certain crystal facets inTiO2 has emerged as a highly promising avenue for solvingseveral challenges that hampers the efficiency of conventionalTiO2 in photocatalysis. The exposure of an unusual active crystalfacet in TiO2 has garnered signicant attention as one of manypotential solutions to enhance the photocatalytic performanceby improving the light absorption and charge carrier recombi-nation. For example, Wu and co-workers demonstrated thatsynthesizing rutile TiO2 with a tunable ratio of the {110} and{111} facets was evidently able to enhance the photocatalyticactivity in the hydrogen evolution reaction.136 A tunable ratio ofboth unusual facets was achieved by using seed-mediatedhydrothermal method using NaF as a crystal directing agent.Based on the result, rutile TiO2 with wholly {111} facet photo-catalyst was found to exhibit the most superior photocatalyticactivity towards hydrogen production under the irradiation ofUV light. This was attributed to the exposure of the more reac-tive {111} facet.In another report, it was shown that exposing the (001) facetin anatase TiO2 was also evidently able to provide a signicantincrease in the photocatalytic activity of the Au–TiO2 nano-composite in the photodegradation of a potent organic dyeunder visible light.33 According to this work, it is evident thatanatase TiO2 with nanospindle morphology exhibited a four-time higher photocatalytic reaction rate than TiO2 with thenanocube morphology. Such enhancement in the activity of the© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fReview Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineTiO2 nanospindles was believed to be due to the high exposureof the (001) facet, which is responsible for improving themigration and separation of the generated charge carriers. Asa result, this would allow an efficient prevention of fast elec-tron–hole recombination and lead to a better photocatalyticperformance. Similar enhancements in activity for the photo-catalytic activity was also observed when the (001) exposed TiO2was composited with other materials, such as two-dimensionalgraphene oxides or CdSe quantum dots nanoparticles.67,137Recently, a composite of BiVO4 and anatase TiO2 with co-exposed (001) and (101) facets was also used as photoanodematerials, and exhibited good performance in a photocatalyticfuel cell (PFC).138 In this study, the as-prepared photoanode wasable to exhibit a considerably high photoelectrochemicalresponse with a current density of 29.8 mA cm−2 (at 0.8 V vs.NHE) under the low-intensity illumination of 13 W LED light.Additionally, the photoanode was able to generate electricpower (0.00232 mW cm−2) using rhodamine B (RhB) as fuel. It isbelieved that such enhancement originated from the ability ofthe co-exposed (001) and (101) facets in TiO2 to form an internalsurface heterojunction, in addition to the already existingexternal interfacial heterojunction between BiVO2 and TiO2. Asa result, this would allow a further enhancement and efficientdistribution of photogenerated charge carriers.The exposure of the unusual crystal facet in TiO2 has alsoattracted considerable attention in recent years for the applica-tion of solar energy harvesting, particularly in photovoltaic solarcells. Typically, a solar cell relies primarily upon efficient lightabsorption, charge separation, and transport to maximize theenergy conversion efficiency. To serve such purposes, meso-porous semiconducting materials such as TiO2 are oen used asboth support light absorption layer and ETL. Nevertheless,commercial and conventional TiO2 oen suffer from poorconductivity, inefficient electron mobility, and low diffusion rateof the carrier, leading to low power conversion efficiency. Crystalfacet engineering in TiO2 presents an intriguing avenue forenhancing the performance of solar cells. The recent surge inresearch elucidates the potential of exposing certain facets inTiO2 to revolutionize solar cell technology through their excep-tional properties. For instance, Qaid et al. reported that TiO2nanocrystals with exposed {001} facet prepared with facile HF-and NaF-mediated hydrothermal method exhibited a signicantimprovement in the performance for DSSC.139 Additionally,a similar enhancement in performance was observed when the{001} facet-dominant TiO2 nanoparticles were used as ETL inCH3NH3PbI3 perovskite solar cells.140 According to the experi-ment, it is evident that the exposure of TiO2's {001} facet wasresponsible for the enhancement of the electron injection andsuppression of electron–hole recombination, which resulted inan increase of both photocurrent and open-circuit voltage.The application of exposed facet titania within the eld ofenergy storage has emerged as an exciting frontier over the pastseveral years. Energy storage technologies, such as lithium-ionbatteries and supercapacitors, play a crucial role in achievingefficient energy utilization and management. Recently, manyreports have also highlighted that the exposure of the unusualcrystal plane in TiO2, characterized by its unique atomic© 2024 The Author(s). Published by the Royal Society of Chemistryarrangement and distinctive surface properties, has proven toevidently improve the efficiency, stability, and overall performanceof energy storage devices. For example, a composite of hierarchi-cally porous TiO2 nanosheet with large exposure of the (001) facetand rGO was able to exhibit a superior and stable lithium storagecapacity and high performance as an anode material in lithiumion batteries.141 Based on the result, it is reported that the anodematerial showed an excellent reversible capacity of 250 mA h g−1in a voltage window of 1.0–3.0 V and demonstrated good stabilityeven aer 1000 cycles. In another report, Wang and co-workerscompared the performance of the (001)-faceted TiO2 nanosheetvs. spherical TiO2 nanoparticles as anode material in lithium ionbatteries.142 Here, it is evident that the battery fabricated with the(001)-faceted TiO2 nanosheet exhibited superior storage capacity,enhanced stability, and higher charge/discharge rate compared tothat of the spherical TiO2 nanoparticles. It is believed that suchenhancement was due to the ability of the exposed (001) facet inTiO2 to facilitate an efficient charge diffusion, which led to anincrease in the rate of Li ion insertion/extraction along the c-axisduring the charge–discharge.4.2. Monolayer titanateMost monolayer titania nanosheets are made by top-downapproaches through the exfoliation of layered titanatecompounds. The layered structured of titania also has manyapplications. Having a layered structure, the interlayer cationscan be reversibly exchanged with other cations. The ionexchange properties enable the layered nanosheets to adsorbradioactive ions; hence, it is useful for environmental remedi-ation. Several researchers utilised acid-modied titania nano-sheets for Cs+ ion adsorption, in which the adsorption capacitydid not decrease even aer 5 cycles.143 The adsorption capacityof Cs+ ions reached 329 mg g−1, which is promising for radio-active wastewater treatment. Protonated TiNS was also able toadsorb cationic dyes such as methylene blue with the adsorp-tion capacity up to 3937 mg g−1, following the Langmuirmodel.144 For dye removal, peroxo-modication of the TiNSsurface could be done, changing the colour of titania fromwhiteto yellow.135 With hydrogen peroxide, the Ti(IV)–H2O2 complexwas formed, creating TiOOH moieties on the surface. The per-oxo groups were then able to oxidise dyes into smaller mole-cules. Hence, the dye removal can be performed without theassistance of UV or visible light. The interlayer spacing andsurface charge of acid-modied TiNS also induced size selec-tivity for adsorbing the pharmaceutical compound, uo-roquinolone.145 In neutral and acidic solutions, the acid-modied TiNS was able to be intercalated by positively CIPwith a thickness of 0.41 nm. Selective adsorption was alsoobtainable by surface modication of TiNS.128,146 Boronic acidligands were immobilized on the surface of modied TiNS,resulting in the selective adsorption of IgG up to 1669.7 mg g−1capacity.146 APTES-modied TiNS was deployed a as nano-container of DNA.128 The DNA was intercalated in the layer ofAPTES–TiNS, where it was protected by TiNS from enzymaticcorrosion, acid condition, and UV-vis light irradiation. Thus,DNA could be stored and released on demand.Nanoscale Adv., 2024, 6, 4325–4345 | 4339http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThe ion exchange capacity of TiNS can also facilitate energystorage. During charging and discharging, the intercalation andde-intercalation of cations occur. Layered titania nanosheetswith minimal layer-to-layer interaction and a robust galleryspace enabled the fast and stable intercalation and de-intercalation of large ions such as sodium and potassium ionsin a non-aqueous electrolyte.147 To obtain the minimum layer-to-layer interaction, the titania nanosheets were exfoliated viaa chemical exfoliation method, followed by coagulation witha Mg2+ solution to obtain a randomly stacked nanosheetstructure. At a rate of 3000 mA g−1, the capacity was retained atmore than 80% aer 10 000 cycles for Na+ ion storage, whichwas performed using an electrode thickness of 80 mm. Suchremarkable performances did not occur without the priorexfoliation of titanates. The specic capacity for Na+ ion storagewas 53 mA h g−1 and 188 mA h g−1 without and with priorexfoliation, respectively. Nevertheless, the theoretical capacityof titanate is relatively small compared to that of graphite orSnO2.8 Doeff et al. synthesised the composite of carbon–TiNS byexfoliating the titanate structure, followed by carbonization ofdopamine for the sodium half-cell conguration.148 The hetero-structure of carbon-titania resulted in higher capacity andcapacity retention, while lowering the impedance. The combi-nation of titania nanosheets with SnO2 for the sodium ionbattery should be expected in the near future. The titaniananosheets could also be used as an electrode for electroanal-ysis.121,130,149 The titania nanosheet exfoliated with tetrabuty-lammonium cations was deposited from a colloidal aqueoussolution onto a glassy carbon electrode, creating a lamellarstructure.130 The lamellar titania acted as a sorbent and host forthe hydrophobic redox system and for electrochemical reac-tivity. A future study on the electron transfer, mobility, andbinding of guest species within the lamellar is intriguing. Thenegatively charged TiNS could also act as a host of ferrocene-boronic acid receptor molecules, exhibiting the selectivesensing of fructose while insensitive for glucose.149 Moreover,the cationic diode behaviour was observed using the TiNSdeposit on top of the micron-sized hole of the PET lm.150 Theionic current rectication was possible due to the negativesurface charge of TiNS and tortuous path of ions within thelamellar space.Titanium dioxide is known to show striking photocatalyticactivities, while the high surface area of the 2D nanosheetsincreases the density of active sites. TiNS has a larger band gap(i.e., 3.84 eV) than anatase TiO2 (3.2 eV).120 A strong UV light isneeded to excite the electrons for photocatalysis. Therefore,many researchers combine TiNS with other catalysts to obtaina narrow band gap, while maintaining the high surface area.One group of researchers combined positively charged Zr-EDTAcomplexes with negatively charged TiNS, creating a porousstructure with a surface area of 193 m2 g−1 and a specic porevolume of 0.39 mL g−1.151 The composite of Zr-EDTA–TiNSyielded a band gap of 3.15 eV and was used for degradingmethylene blue (MB) under UV irradiation. The photocatalyticdegradation kinetics of methylene blue was 5-fold higher andreached 98.1%MB removal for the Zr-EDTA–TiNS composite, ascompared to TiNS alone. The photocatalytic mechanism can be4340 | Nanoscale Adv., 2024, 6, 4325–4345described as an articial Z-scheme heterostructure due toohmic contact, facilitating charge transfer between theconduction band of TiNS and valence band of Zr-EDTA. TiNShas also been combined with alkaline Co(OH)2 (ref. 152) andNi(OH)2 (ref. 153) for the photocatalytic reduction of CO2. Thealkaline Co(OH)2 and Ni(OH)2 acted as a CO2 binder, while TiNSadsorbed the sensitiser and became an electron relay thatbridged the sensitiser with Co(OH)2 and Ni(OH)2 active sites.For Ni(OH)2–TiNS, the production rate of CO/H2 was 1801/2093mmol g−1 h−1, while Co(OH)2–TiNS was 56.5/59.3 mmol h−1. Forphotovoltaic application, TiNS was used as an atomic stackingtransporting layer (ASTL) in the lead halide perovskite solarcell.154 The TiNS was stacked into a multilayer thin lm by layer-by-layer deposition, which achieved complete surface coverageaer 5 repetitions. Contrary to the conventional sintered TiO2thin lm, the layer-by-layer deposition of TiNS exhibited nearlynegligible oxygen vacancies. The oxygen vacancies may causeUV instability of the perovskite solar cell. For titania nanosheetsASTL, the power conversion efficiency remained at around 70%of the initial value aer 5 hours of UV irradiation, while severereductions of PCE occurred for the conventional TiO2 thin lm,resulting in only 5% initial value of PCE. Besides photovoltaicapplication, TiNS could also be used for hydrovoltaic devices.155The electricity was generated from water evaporation. The tita-nium vacancy of TiNS enhanced the water–solid interaction.When water molecules ow over the solid surface, themigrationof counterions occurs to generate an electric output. Thehydrovoltaic device based on TiNS produced an open circuitvoltage of 1.32 V for more than 250 h.Coatings of layer-by-layer deposition of TiNS were used toprotect stainless steel car baffle from corrosion.156 The ve-cyclelayer-by-layer deposition of TiNS exhibited a thickness ofaround 10 nm with a corrosion inhibition efficiency of 99.92%and an estimated corrosion rate of 5.32× 10−5 mm per year. The2D structure of TiNS created a tortuous path for iron and oxygendiffusion, hampering the rusting process of iron. Titania nano-sheets have been known as a strong adsorbent of rare earthelements, such as Eu, exhibiting photoluminescence proper-ties.157 Intense red emission was observed at 616 nm under theirradiation of 400 nm UV LED light. It would be interesting tocombine layers of red-emitting TiNS with blue-emitting rare-earth mixed metal oxides, such as BaMgAl11O17: Eu2+ (ref. 158)to create multi-colour luminescent layers for monitoring coatinghealth. As a nanocomposite coating, silk–TiNS enhanced thetribological properties (e.g., hardness, reduced modulus, wear,adhesion, and scratch resistance) of silk coatings.159 The hard-ness and reduced modulus of the silk–TiNS composite werehigher than those of the graphene–silk composite lm. Thereinforcement behaviour also occurred for bulk polymer nano-composites, following micromechanical models such as Halpin–Tsai and Brune–Bicerano, up to few number of nanosheetslayers.160 As discussed in section 3.1.2, TiNS is sensitive tomagnetic ux and UV light, in which the orientation of TiNSwithin a polymer matrix can be adjusted. Hence, stimuli-responsive polymer nanocomposites could be realised by incor-porating TiNS within the polymer. A silk–TiNS multilayer thinlm also exhibited moisture-responsive coating.161 The water© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fReview Nanoscale AdvancesOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinemolecules were adsorbed into the nanosheets, causing swellingand reduction of the refractive index of the lm. In summary, thelayered 2D structures of nanosheets and the photoresponsive,chemically stable, negatively charged TiNS have many existingand potential applications worthy of further investigation incombination with other nanomaterials or polymers.5. Conclusions and outlookIn conclusion, two-dimensional TiNS has emerged as a multi-faceted and promisingmaterial that has captured the attention ofthe scientic community.With signicant implications for elds,ranging from catalysis, electronics, and energy conversion toenvironmental remediation, energy storage, and biomedicalapplications, TiNS offers a transformative potential. Key to thisaspect is the manipulation of their crystal facets and structures,which allow for a tailored set of properties for diverse applica-tions. Synthesis routes involving both exposed facet and mono-layer titania nanosheets have demonstrated unique properties,such as heightened catalytic activity, ion-exchange capabilities,and exceptional optoelectronic behaviours.Within the realm of synthesis, hydrothermal and sol-vothermal methods have proven effective for facet control. Therehas been a shi toward non-uorine-based precursors, primarilydue to the associated safety and environmental considerations.This trend aligns well with the broader scientic movementtoward more sustainable and eco-friendly materials. In contrast,the chemical exfoliation methods use non-uorine precursors,offering a safer yet versatile route to monolayered structures.For applications, TiNS demonstrates a myriad of function-alities. Their ion-exchange properties make them valuablecandidates for environmental applications, such as theabsorption of radioactive ions and organic dyes. The adapt-ability of TiNS in energy storage, particularly sodium andpotassium ion batteries, and their potential in photocatalysis,signal an exciting trajectory for these materials. Compositestructures have shown that TiNS can work in synergy with othermaterials to further enhance their performance in these sectors.As we look to the future, the focus should be on rening anddiversifying non-uorine-based synthesis methods and deepeningour understanding of the relationship between the crystal structureand material properties. Exploring hybrid composites, particularlythrough the integration of TiNS with polymers and other nano-materials, appears to be a promising avenue. Moreover, targetedresearch into nanoengineering for optimizing energy storage andtunable band gaps for photocatalytic applications holds signicantpotential. These endeavours not only serve to advance our tech-nological capabilities, but also usher in an era of increased safety,energy efficiency, and environmental consciousness.Abbreviations[bmim]-[BF4]© 2024 The Aut1-Butyl-3-methylimidazolium tetrauoroborate2D Two dimensional3D Three dimensionalAFM Atomic force microscopyhor(s). Published by the Royal Society of ChemistryAPTES 3-Aminopropyl triethoxysilaneASTL Atomic stacking transporting layerCB Conduction bandCBM Conduction band minimumCIP Charged ciprooxacinDETA DiethylenetriamineDFT Density functional theoryDMF N,N-DimethylformamideDNA Deoxyribonucleic acidDSSC Dye-sensitized solar cellsEBD Electron beam depositionEDTA Ethylenediaminetetraacetic acidETL Electron-transporting layerFESEM Field emission scanning electron microscopyHF Hydrouoric acidIgG Immunoglobulin GITO Indium-doped tin oxideMB Methylene blueMO Methyl orangeMXenes Two-dimensional transition metal carbideNTA Nanotube titanic acidPET Poly(ethylene-terephthalate)PFC Photocatalytic fuel cellPVP Poly(vinylpyrrolidone)rGO Reduced graphene oxideSAXS Small-angle X-ray scatteringSDS Sodium dodecyl sulfateSEM Scanning electron microscopySSA Specic surface areaTBA TetrabutylammoniumTEM Transmission electron microscopyTiNS Titania nanosheetsTIP Titanium isopropoxideTMA TetramethylammoniumTOB Truncated octahedral bipyramidalUV UltravioletUV-vis Ultraviolet-visible lightVBM Valence band maximumXRD X-ray diffractionData availabilityNo primary research results, soware or code have beenincluded, and no new data were generated or analysed as part ofthis review.Conflicts of interestThere are no conicts to declare.AcknowledgementsM. K. would like to acknowledge the nancial support providedby the Directorate of Research and Development, UniversitasIndonesia through Hibah PUTI Q1 2024 (Contract No. NKB-425/UN2.RST/HKP.05.00/2024). C. H. is also supported by a post-doctoral program at the Advanced Functional MaterialsNanoscale Adv., 2024, 6, 4325–4345 | 4341http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4na00442fNanoscale Advances ReviewOpen Access Article. Published on 16 July 2024. Downloaded on 11/29/2024 9:16:51 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineLaboratory, Department of Engineering Physics, Faculty ofIndustrial Technology, Institut Teknologi Bandung (ITB).References1 T. Edvinsson, R. Soc. Open Sci., 2018, 5, 180387.2 D. Varsano, G. Giorgi, K. Yamashita and M. Palummo, J.Phys. Chem. Lett., 2017, 8, 3867–3873.3 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.4 M. 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