Etching-based Hollowing of Nanostructures

YE Zuyang，YIN Yadong

（Department of Chemistry，University of California，Riverside，California 92521，the United States）

Abstract The controllable synthesis of hollow nanomaterials has broad application prospects in many fields，such as catalysis，energy conversion and storage，and biomedicine.This account aims to reveal the key effects of etching on the hollowing process of nanostructures.We discuss the precise manipulation of the hollowing process by enhancing the relative stability of the nanoparticle surface in the etchant，mainly focusing on three types of etching strategies，including the hard templating method，redox-assisted etching method，and surface-passivated etching method.Finally，we provide an outlook on the future development of etching-based strategies for the controllable synthesis of hollow nanostructures.

Keywords Hollow nanostructure；Etching；Template；Self-templating；Nanosynthesis；Surface passivation

1 Introduction

Hollow nanostructures，owing to their large surface area，low density，and high loading capacity，have a wide range of applications in the fields of catalysis［1—3］，energy conversion and storage［4—6］，and drug delivery［7—9］.Their controllable synthesis has been the focus of materials research［10—13］.It is difficult to directly obtain hollow nanostructures through one-step synthesis by conventional synthetic methods.Instead，a sacrificial template is typically prepared first，followed by the chemical transformation of the solid template into the target hollow nanostructure.A significant advantage of this approach is that the product can be easily predicted：by independently controlling the reaction parameters，one can fine-tune the morphology of the nanostructures to meet the requirement of the target application［14］.

The hollowing process usually follows an elimination mechanism，where the interior of the solid template is removed during or after the shell formation.A typical removal approach is selective etching，with the classic example of dissolving polystyrene using an organic solvent such as toluene.In fact，in the semiconductor microfabrication process，etching is an essential method to selectively remove unwanted materials from the surface of the wafer to create various geometries with isotropic or anisotropic patterns.In addition，oxidative etching is a valuable mechanism in the controlled synthesis of metal nanocrystals［15，16］.For instance，the presence of an etchant，H2O2，is beneficial for inducing the formation of planar twinned silver seeds and removing relatively unstable nontwinned particles，thereby producing silver nanoplates with high yields［17，18］.

In an etching-based hollowing process，the nanoparticle surface is protected by a“masking”material that can resist etching，allowing the selective removal of the interior.The masking material can be an additionally introduced shell or a self-passivated layer with higher resistance to the etchant.Etching plays an important role in the hollowing transformation，but the precise manipulation of the hollowing process by enhancing the stability of the surface shell towards etchants has not been discussed extensively.Here，we aim to discuss our understanding of the effect of the protective shells on the hollowing of nanostructures during the etching process based on our recent relevant studies.According to different mechanisms of enhancing the etching resistance of the surface layer，we can roughly group the etching-based hollowing approaches into three categories：hard templating method and redox-assisted and surface-passivated etching methods（Table 1）.The hard templating method is the most common for synthesizing hollow nanomaterials of various morphologies and sizes.While conceptually intuitive，this method requires additional surface modification and coating procedures.The latter two are self-templating methods.The redox-assisted etching method generates a protective shellin situduring the redox-based etching and does not require additional coating steps.The surfacepassivated method achieves self-templating through surface passivation，with the hollow product maintaining the same chemical composition as the original template.Finally，we summarize the current progress and offer personal views on potential future research directions in this field.

Table 1 Summary of etching-based hollowing methods

2 Etching-based Hollowing of Nanostructures

2.1 Hard Templating Method

The key to producing hollow-structured nanoparticles through etching is to selectively etch the interior while keeping the outer shell by controlling their different resistances toward etchants.One of the most intuitive ways is to use the hard templating method.As shown in Fig.1（A），nanoparticles with different morphologies are first synthesized as sacrificial templates，which can be easily removed by reacting with different chemical reagents.Then，a shell layer that can resist the etching is coated on the surface of the template to form a core-shell structure.When exposed to the etchant，only the interior of the core-shell structure is etched away，leaving the outer shell and thereby forming a hollow nanostructure.

Fig.1 Schematic illustration showing the typical synthesis procedure of the hard templating method(A)and TEM images of the samples at each preparation step(B—D)[19]

Silica is a widely used hard template due to its simple preparation，uniform and controllable sizes，and low cost.SiO2can be etched away by base or hydrofluoric acid at room temperature.Different etchants can be selected according to the properties of the target shell.We previously prepared TiO2hollow nanostructures using SiO2as a template［19，20］.First，a sol-gel method was adopted to coat TiO2on the SiO2surface for producing SiO2@TiO2core-shell nanostructures using titanium tetrabutoxide（TBOT）as a precursor，hydroxypropyl cellulose（HPC）as a surfactant，and ethanol as the reaction medium［Fig.1（B）］.Amorphous TiO2hollow nanoshells can be obtained by selectively etching away the silica core using aqueous NaOH solution.To improve the crystallinity of the resulting TiO2shells，we pretreated the core-shell nanoparticles at high temperatures before etching［21，22］.For instance，TiO2can gradually crystallize under reflux conditions to form a porous network of anatase particles［Fig.1（C）and（D）］.In addition，silica-protected calcination can be used to further improve the crystallinity of TiO2.Etching the SiO2template with bases typically takes several hours，sometimes requirs high temperatures to ensure the complete removal of SiO2.HF is an alternative etchant that removes the silica core within several minutes.In a recent work to develop a photonic crystal sensor based on resorcinol-formaldehyde（RF）hollow structures［23］，we first coated a uniform RF shell on the pre-synthesized SiO2templates，then self-assembled the SiO2@RF core-shell structures into an ordered array，and finally removed the SiO2core with HF，producing a photonic crystal film composed of highly ordered hollow RF spheres.If NaOH instead of HF was used as the etchant，the crosslinking degree of the RF shell would be changed after soaking for a long time，which also changed the surface properties of photonic crystal films and led to the detachment of films from the substrate.Thanks to the fast reaction of HF，the film could attach to the substrate firmly，and the ordered structure could be well maintained after etching.However，it should be noted that HF is highly corrosive and should be handled with care.

In addition to isotropic hollow spherical shells，one can also prepare hollow structures with various morphologies by selecting anisotropic templates［24—27］.For example，we used the nickel-hydrazine complex nanorod as the sacrificial template，silica as the coating layer，and HCl as the etchant to prepare hollow silica nanotubes［24］.The length and width of the sacrificial template could be precisely tuned by the ratio of hydrazine/nickel species and the amount of polyethylene glycol segments in the surfactants，respectively.This unique nanotube structure could be used as a nanoscale reactor for the space-confined seeded growth of various noble metal nanorods［28，29］.The rigid silica shell，however，does not allow the nanoparticles to grow completely within the nanotube.Alternatively，RF could be employed as the confining layer.As a crosslinked polymer with good molecular permeability，RF allows the diffusion of growth solution into the hollow space while maintaining the shape of the original template［30—32］.We first prepared ellipsoidal FeOOH nanorods with a uniform size and an adjustable aspect ratio by the direct hydrolysis of FeCl3［Fig.2（A）］.After loading gold seeds（1—3 nm）on the FeOOH surface，a layer of RF was coated as a confining shell on FeOOH/Au nanorods［Fig.2（B）］.To create void spaces，F(xiàn)eOOH was selectively etched away with an acidic solution［Fig.2（C）］.Oxalic acid was chosen as the etchant because it could coordinate with Fe3+to accelerate the etching process while providing an acidic environment.The hollow RF nanocapsules with ultra-fine gold seeds decorated on the inner surface obtained by selective etching can be used for the space-confined seeded growth of anisotropic Ag nanorods and Cu nanorods，which showed good performances for applications such as water steam generation and photothermal actuation［26，31］.In addition，a similar strategy was used to prepare Fe3O4-Au@RF yolk-shell nanostructures by converting FeOOH to magnetic Fe3O4and then coating SiO2and RF as the sacrificial template and the confining shells，respectively.The yolk-shell structure can be used to synthesize novel nanomaterials with coupled magnetic and plasmonic anisotropy for various applications，such as sensors［25，33］，actuators［34，35］，and bio-imaging［36，37］.

Fig.2 TEM images of anisotropic hollow nanostructures prepared using the hard templating method[31]

2.2 Redox-assisted Etching Method

The hard templating method，although intuitive，usually requires tedious and time-consuming steps，and additional modification of the template surface is often needed for the uniform coating of the target shell.Besides the pre-coating method，chemical reactions during the etching process can also facilitate the deposition of precipitates as a hard shell to protect the surface，resulting in a hollow structure.The galvanic replacement reaction is a typical example，widely used for making hollow metal structures［38—41］.During the galvanic replacement process，the salt solution of metal with a higher reduction potential etches the other metal template with a lower reduction potential so that the template is oxidized and dissolved in the solution.In contrast，the metal salt etchant is reduced to the metal atom and deposited on the outer surface of the template.The template metal in the inner core continuously diffuses to the surface to participate in the replacement process，which promotes the generation of vacancies inside the nanocrystal，and finally forms a hollow nanostructure.We used Ag triangular nanoplates as sacrificial templates［Fig.3（A）］，HAuCl4as the Au precursor，and polyvinylpyrrolidone（PVP）as the capping ligand to conduct the galvanic replacement reaction［42］.Since the reduction potential of［（0.99 Vvs.standard hydrogen electrode（SHE）］is higher than that of Ag+/Ag（0.80 Vvs.SHE），Au（III）species were reduced to Au atoms and deposited on the Ag surface，while Ag atoms were oxidized and dissolved in solution，according to the reaction 3Ag（s）+→Au（s）+3Ag+（aq）+4Cl-（aq）.Etching preferentially occurred on the（111）facets of the nanoplate during the initial stage of the reaction，resulting in small pits.Further etching took place on the pre-existing pit due to the protection of the deposited gold.In contrast，the silver atoms in the interior were allowed to diffuse out and participate in the galvanic replacement，transforming the solid silver nanoplates into highly porous alloy nanoplates［Fig.3（B）］.Further replacement led to the dealloying of the porous nanostructures，ultimately creating triangular nanoframes［Fig.3（C）］.Using HAuCl4as the Au source，only one Au atom is formed for every three Ag atoms consumed，so the Au/Ag ratio of the obtained alloy structure is very low.The Au/Ag ratio of the intermediate porous nanoplate was about 0.3，while the final triangular nanoframe was 0.65［Fig.3（F）］.Nanostructures of low Au/Ag ratio are very unstable and prone to oxidation.Alternatively，Au（I）can be used to increase the Au/Ag ratio.When we selected Na3Au（SO3）2as the Au source，the reduction potential was significantly reduced（0.111 Vvs.SHE），and the galvanic replacement reaction could be completely suppressed.Epitaxial growth of Au on Ag nanostructures was achieved by adding ascorbic acid（AA）as a reducing agent［43］.Na3Au（SO3）2is highly stable at pH ofca.12.When the pH value was adjusted to 5，the complex became unstable by detaching the sulfite ligand，thus allowing galvanic replacement to occur again［42］.A similar pitting process was observed in the initial stage of the reaction［Fig.3（D）］.Since Au atoms and Ag atoms can be replaced at a ratio of 1∶1 at this time，the Au/Ag ratio increased rapidly during the etching process.It can finally reach about 1.6［Fig.3（F）］.The resulting alloy nanostructures were highly stable and could form high-quality holey Au-Ag alloy nanoplates［Fig.3（E）］.

Fig.3 Hollow nanostructures prepared by galvanic replacement reaction[42]

Other etching processesviaredox reactions can also trigger structural hollowing because of the formation of void spaces due to the different densities between the template and the newly formed precipitated layer［44—46］.For example，we synthesized Au@RF core-shell nanostructures and etched RF with KMnO4aqueous solution to form MnO2-based hollow nanostructures［44］.With the addition of a small volume of KMnO4，the reaction started with the deposition of a flaky MnO2layer on the RF surface［Fig.4（A）］.With the gradual increase of the KMnO4content，cavities began to form inside the RF sphere，producing Au@RF@MnO2yolk-shell structures［Fig.4（B）］.Different from a dense metal coating，the MnO2shell is composed of intersecting spiky crystals，leaving enough gaps for the etchant to diffuse in and the dissolved template to diffuse out，allowing the reaction to proceed uniformly from outside to inside.When more manganese precursor was added，the shell thickness further increased until all the RF polymers were consumed，forming the Au@MnO2yolk-shell structures［Fig.4（C）and（D）］.By comparing the morphologies of yolk-shell structures under different reaction conditions，we found that the outer diameter of the hollow spheres did not increase much（ca.5 nm），while the inner diameter decreased dramatically by about 45 nm as the amount of KMnO4increased［Fig.4（C）and（D）］.This result indicates that the outer surface was relatively stable，and the etchant penetrated from the surface into the interior，so the newly generated MnO2was deposited on the inner surface.This strategy allows the synthesis of hollow nanomaterials with thick shells.

Fig.4 Hollow nanostructures prepared by redox reaction based on the density differences between reducing agents and products[44]

A special scenario is that the etching only removes some elements from the compound templates.In crystalline templates，such etching creates vacancies that may grow into voids as the reaction proceeds，a phenomenon known as the Kirkendall Effect.For example，we synthesized PdP2nanocrystals by reacting Pd nanocubes with trioctylphosphine（TOP）at 250 ℃［45］.The P element in PdP2can be oxidized by air in oleylamine，converting the nanocrystals back to Pd nanocrystals.During this etching process，PdP2reacted with O2to form P2O5which was quickly dissolved in oleylamine and left the nanocrystals.As the reaction proceeded，the P atoms diffused outwards，leaving vacancies inside the nanocrystal.These vacancies accumulate，migrate and coalesce at the elevated temperature，creating large voids and eventually forming hollow structures.Interestingly，inserting and extracting P could be repeated multiple times，leading to increased diameter and decreased shell thickness of hollow nanocrystals，given that the voids were the result of the dominant outward diffusion of P atoms.As shown in Fig.5，after 1，2 and 3 repeated insertions and extractions，we obtained hollow Pd nanocrystals with outer diameters of 20.7，25.3，and 26.4 nm and inner diameters of 7.5，16.0，and 19.2 nm，respectively.The resulting thin-shelled hollow nanocrystals possess more reactive sites and defects，exhibiting enhanced catalytic activity and high durability toward formic acid oxidation.

Fig.5 Synthesis of hollow Pd nanocrystals with thin walls by repeating the cavitation process three times[45]

2.3 Surface-passivated Etching Method

To produce hollow nanostructures with the same composition as the original templates，one can use surface engineering methods to enhance the stability of the outer layer of the template against etching and preferentially etch the interior of the template to achieve hollowing［47，48］.We first reported in 2007 the surfaceprotected etching for synthesizing TiO2hollow structures［47］.The amorphous titania nanospheres synthesized by the sol-gel method were modified with polyacrylic acid（PAA），which could form a crosslinked network on the TiO2surface.When etched with glycol at a high temperature，only the unmodified TiO2with a low crosslinking degree inside the sphere was dissolved，thus forming the hollow TiO2nanostructure.This concept can be extended to synthesizing hollow SiO2nanomaterials［49—52］using PVP as an effective capping ligand to protect the surface SiO2.Later，we found that PVP mainly played two roles in the surface-protected etching of SiO2［53］.First，PVP could passivate the surface of SiO2，making it less likely to be dissolved by OH-ions.Second，PVP could bind to the silica surface through hydrogen bonding，which reduced the condensation of the silica network and made it easier for OH-ions to diffuse through the shell，thereby etching the interior of silica.The polymeric capping ligands enable the efficient pathway for the inward diffusion of the etchant and the outward diffusion of the etching products.When the PVP/SiO2molar ratio increased，the release rate of SiO2under NaOH etching also accelerated（Fig.6），confirming that PVP modification could promote the etching of the inner part of the silica spheres.Combining these two roles ensures the highly selective etching of the interior of the silica nanospheres，making the surface-protected etching a robust method for synthesizing hollow silica nanoshells.This surface protection strategy can also be applied to the solid-state transformation of nanoparticles［54］.

Fig.6 Hollow SiO2 nanostructures prepared based on the surface-protected etching method[53]

In addition to the surface protection method，intrinsic structural differences between the interior and exterior of nanomaterials can also lead to their different resistance toward etching［55—57］.This phenomenon is often known as Ostwald ripening，which describes unstable components being dissolved and then redeposited on the surface of more stable regions［58，59］.During the hollowing process of nanoparticles，the particle’s interior is preferentially etched and dissolved by the etchant and then redeposited on the surface of the original particles.For instance，we found that RF spheres synthesizedviathe modified St?ber method have heterogeneity in structure，where the inner sphere is composed of short-chain oligomers，while the near-surface mainly consists of relatively long-chain oligomers［Fig.7（A）］［55］.Therefore，when etched by solvents such as ethanol and tetrahydrofuran，the internal short-chain oligomers are preferentially removed，resulting in the formation of hollow structures［Fig.7（B）］.Interestingly，when the etching time was prolonged，the interior gradually became empty while the thickness of the shell gradually increased［Fig.7（C）and（D）］.During this process，the dissolved oligomers diffused out and redeposited onto the outer surface of the nanospheres，thus increasing the thickness of the shell.In addition，the newly deposited oligomers helped to protect the shell structure from etching.Further，the transmittance of the colloidal dispersion was found to increase sharply from 10%to 72%in 5 min of starting etching［Fig.7（E）］，indicating that dissolution was dominant at the early stage.Then the transmittance gradually decreased and reached 21%after 360 min，suggesting that the growth played a major role in the later stage.This dissolution-regrowth mechanism，commonly found in many other syntheses where Ostwald ripening may occur［60，61］，could be used to prepare hollow nanoshells with thick walls.

Fig.7 Hollow RF nanostructures prepared based on the dissolution-regrowth mechanism[55]

A similar dissolution-regrowth process was also observed in the synthesis of metal nanoparticles［62］.In the presence of the etchant and additional growth solution，the nanoframes with hollow structures can be prepared by taking advantage of different regrowth rates at various crystal planes.We synthesized Pd nanoframes with well-defined structures by etching solid Pd nanocrystals（Fig.8）［62］.The success of this synthesis mainly relies on fine control over the oxidative etching and regrowth rates.Since the three surface sites（i.e.，corner sites，edge sites，and face sites）are different in surface energy and coordination number，their physical and chemical properties，such as growth rate and reactivity，are different.While Pd was etched with O2at a high temperature，a certain amount of formaldehyde was added to the system as a reducing agent to allow the regrowth of Pd.The growth rate of different sites of Pd nanocrystals followed the sequence of corner site＞edge site＞face site.By tuning the amount of the etchant and the reducing agent，we could control the etching rate of the corner and edge sites to be equal to the regrowth rate，while face site has a lower regrowth rate than that of the etching.Therefore，the etching mainly occured on the face site，and Pd nanoframes could be obtained.We etched Pd nanocrystals with different structures such as cubes，cuboctahedrons，and octahedrons into corresponding nanoframe structures，providing a simple and efficient approach to prepare hollow metal nanoframes with a high proportion of active surface sites.

Fig.8 Pd octahedral nanoframes prepared by maneuvering the rates of oxidative etching and regrowth[62]

3 Summary and Outlook

With the many new opportunities offered by creating voids within solid shells，significant research effort has been devoted to developing new approaches for synthesizing hollow nanostructures.In this account，we focus our discussions on the etching-based hollowing strategies，highlighting the important principle of enhancing the stability of the shell against etching to enable selective removal of the interior of the nanoscale templates.Depending on the different methods of strengthening the etching resistance of the surface shell，we reviewed the hard templating method，redox-assisted hollowing method，and surface-passivated selftemplating method.

The hard templating method is conceptually the most intuitive approach.First，a shell layer that does not react with the etchant is coated on the surface of the template，and then the template is selectively etched away to form hollow structures.In principle，any type of hollow nanostructures can be prepared by the hard templating method，as long as the template surface can be coated with target shells.However，the coating process usually requires extra surface modification procedures，which makes the preparation process cumbersome，less repeatable，and costly.Compared with the traditional templating method，the redox reaction-assisted hollowing method uses the nanostructure itself as the template without additional surface modification and coating procedures.The shell structure is introducedin situduring the etching process，rendering the synthesis steps relatively simple.Since this process mixes redox and etching in one reaction，delicate selection and rational design of templates（reducing agents）and etchants（oxidizing agents）are required.Some unique physical and chemical phenomena，such as galvanic replacement and the Kirkendall effect［63—66］，need to be considered，as they affect the structure of the target product.These phenomena sometimes occur concurrently in a reaction，complicating precise control over the product structure［67］.On the other hand，if these principles can be well exploited，fine control over the morphologies of nanomaterials can be attained in minimal steps［68］.The surface-passivated self-templating method also harnesses the nanostructure itself as the template and does not require tedious pre-coating steps.In addition，the target structure and the template share the same chemical composition.Without a complicated experimental design，welldefined shells with a predictable composition and structure can be obtained after etching.The mechanism of the hollowing process from surface passivation determines the porous nature of the shell structure，which facilitates the effective diffusion of active chemical species and is crucial for their applications as nanoreactors or drug carriers.

Undoubtedly，the strategies for synthesizing hollow nanostructures will be further improved through continued efforts，therefore promoting more interesting and unique applications.We believe future research direction may gradually transfer to fine control over the structural details of the hollow nanoparticles so that they can exert unique advantages in different application scenarios.For example，by controlling the morphology and size of the template，one can prepare complex hollow structures，such as those with anisotropic shapes and chirality，which can conveniently serve as templates for further growing functional materials with complex structures inside their hollow interiors.In addition，by employing different synthetic strategies，the etching and regrowth can be controlled to take place inside or outside the shell，allowing precise tuning of the shell thickness.Apart from the selective etching of internal structures，partial etching of local areas can also be performed to synthesize nanomaterials with multi-walled structures.Furthermore，the future design and synthesis of these hollow nanomaterials should be more application-oriented，with their composition，size，porosity，and morphology aimed at fulfilling the desired functions.