Vapor phase–grown MgO nanoparticles

The MgO nanoparticle cubes described in this review were synthesized by combustion reactions and ranged in size between 1 and 1000 nm. The absence of solvents during nucleation and growth prevented substantial particle aggregation and sintering. In the past, it has been demonstrated that this material represents a well-suited model system for surface science studies that aim at bridging the information gap between atomic level understanding on ordered MgO single crystal surfaces [Reference Sterrer, Berger, Diwald, Knözinger, Sushko and Shluger19, Reference Schneider, Franke, Gurrath, Röckert, Berger, Bernardi, Meyer, Steinrück, Lytken and Diwald20] and metal oxide particle powders as technologically relevant materials components. Two particle systems were used: polydisperse MgO cubes, which were collected by Mg combustion in air [Reference Spoto, Gribov, Ricchiardi, Damin, Scarano, Bordiga, Lamberti and Zecchina15, Reference Altman, Agranovski and Choi21, Reference Gajdardziska-Josifovska and Sharma22, Reference Scarano, Bertarione, Cesano, Spoto and Zecchina23, Reference Baumann, Schneider, Sternig, Thomele, Stankic, Berger, Grönbeck and Diwald24], with sizes ranging from 1 to 1000 nm, and monodisperse MgO nanocubes synthesized by chemical vapor synthesis [Reference Berger, Diwald, Jupille and Thornton18, Reference Siedl, Koller, Sternig, Thomele and Diwald25] at reduced pressures exhibiting defined morphologies with an average edge length around 4 nm. As an additional prerequisite for studies on model systems, we annealed all particle powders in high vacuum as well as pure oxygen atmosphere in order to desorb and decompose adsorbed surface contaminants. These measures were taken to eliminate potential interferences of residual adsorbates with water adsorption and subsequent surface reactions.

Despite the growing number of publications on oxide nanocrystals—predominantly on colloidal particle systems—that address the geometrical shape of crystalline polyhedra and that deal with the estimation of relative and absolute surface areas of different crystal facets, there are only rare cases of nanoparticle systems that have clean particle surfaces and well-defined particle morphologies at the same time. Vapor phase synthesis of metal oxide nanoparticles has the principal advantage to provide materials of high purity and, therefore, of better defined surface compositions than liquid phase synthesis [Reference Berger, Diwald, Jupille and Thornton18].

Water thin film on metal oxides

Adsorption and adsorption-induced chemical processes that occur at the metal oxide–water vapor or metal oxide–liquid water interfaces are typically accompanied by microstructural changes of nanoparticle ensembles [Reference Elser, Berger, Brandhuber, Bernardi, Diwald and Knözinger26, Reference Baumann, Elser, Auer, Bernardi, Hüsing and Diwald27]. Despite their great importance for materials science and in particular for the stability of oxide nanomaterials, the initial reactions of water vapor with solid oxide substrates (e.g., the processes of dissociative H₂O chemisorption versus H₂O physisorption), the thickness, and the structural properties of the thin liquid films are largely unexplored [Reference Bjorneholm, Hansen, Hodgson, Liu, Limmer, Michaelides, Pedevilla, Rossmeisl, Shen, Tocci, Tyrode, Walz, Werner and Bluhm6]. Thin water films under ambient conditions represent an intermediate case between liquid–solid and liquid–vapor interfaces that jointly determine its properties. For both natural and engineered nanomaterials, most solid surfaces in the ambient and at water partial pressures in the range between 1 and 30 Torr [Fig. 2(a)] are covered by several monolayers of water [Fig. 2(b)]. This was demonstrated for clean MgO (100) surfaces, which were investigated with FT-IR spectroscopy in atmospheres of variable relative humidity [Reference Foster, D’Agostino and Passno28]. As shown in Fig. 2(b), at a relative humidity of 20% and above, a ∼1–2 monolayer thick water layer already forms. In fact, hydroxylation and water adsorption start at a relative humidity (RH) <0.1% and significant amounts of molecular water become stabilized during the initial stages of water adsorption. Related observations underline the importance of OH–H₂O interaction, which is stronger than H₂O–H₂O bonds [Reference Bjorneholm, Hansen, Hodgson, Liu, Limmer, Michaelides, Pedevilla, Rossmeisl, Shen, Tocci, Tyrode, Walz, Werner and Bluhm6].

Figure 2:

The composite plot underlines that the typical water vapor pressures present in air and at room temperature (a) lead to the instantaneous coverage of almost every oxide surface with water (b). In comparison with the results of a Fourier Transform Infrared (FT-IR) study, which was performed on clean MgO (100) single crystal surfaces [Reference Foster, D’Agostino and Passno28], the significant H₂O uptake of MgO nanocube powders after 20 min of air exposure is demonstrated in (c). A weight increase of 20% measured for nanocube ensembles with edge lengths between 5 and 20 nm corresponds to the formation of water multilayers at the nanocube surface. Adapted with permission from Ref. Reference Bjorneholm, Hansen, Hodgson, Liu, Limmer, Michaelides, Pedevilla, Rossmeisl, Shen, Tocci, Tyrode, Walz, Werner and Bluhm6 (Copyright 2016 American Chemical Society) and from Ref. Reference Foster, D’Agostino and Passno28 (Copyright 2005 Elsevier).

Surface science typically addresses water adsorption at the level of single molecules up to monolayer surface coverage. On two-dimensional metal oxides, surface studies reveal that the structure of interfacial water, i.e., in the first hydration layers, is distinct from bulk water [Reference Fenter and Lee29]. By means of in situ X-ray scattering techniques, it was found that the organization of hydration layers is limited to 1–2 nm, which corresponds to only a few water correlation lengths. For such hydration shell dimensions, related structures are highly dependent on characteristics of the metal oxide surface, such as local charge, spatial arrangement, and coordination of ions, and nature and abundance of additional adsorbates [Reference Zhang, Fenter, Cheng, Sturchio, Bedzyk, Předota, Bandura, Kubicki, Lvov, Cummings, Chialvo, Ridley, Bénézeth, Anovitz, Palmer, Machesky and Wesolowski30].

MgO nanocube powders with specific surface areas as large as 300 m²/g represent a model system for adsorption studies on oxide nanomaterials [Reference Schneider, Franke, Gurrath, Röckert, Berger, Bernardi, Meyer, Steinrück, Lytken and Diwald20, Reference Sternig, Koller, Siedl, Diwald and McKenna31]. Previous temperature-programmed desorption measurements and thermogravimetric analysis revealed that—depending on the exposure time—the nanoparticle powder can adsorb water between 10 and 20% of weight. H₂O desorption features at T = 303 K and 573 K were observed and attributed to desorption of weakly bound surface water at T = 303 K and to the decomposition of surface Mg(OH)₂ [Reference Hayun, Tran, Ushakov, Thron, van Benthem, Navrotsky and Castro32] into MgO [Reference Susman, Pham, Datye, Chinta and Rimer33] and gas phase water at T = 573 K. An estimate for the number of water molecules per Mg²⁺–O²⁻ surface unit as a function of MgO nanocube size reveals that the number of water layers can be between 2 and 7 monolayers for MgO nanocubes with sizes of 5 nm and 20 nm, respectively [Fig. 2(c)].

Thin water films as a two-dimensional solvent for enhanced chemical growth

Thin water films can act as a two-dimensional solvent and give rise to the spontaneous and spatially controllable growth of hydroxide fibers in ambient air and at room temperature [Reference van Santen34, Reference Gheisi, Sternig, Redhammer and Diwald35]. Gheisi et al. recently reported the formation of Mg₃(OH)₅Cl·4H₂O fibers on MgO nanocubes sequentially exposed to a SiCl₄/O₂ atmosphere at cryogenic temperatures and—in a subsequent step—to air or a water-saturated O₂ atmosphere. This remarkable transformation shows that water thin films promote reactions that are confined at the metal oxide surface (Fig. 3).

Figure 3:

Thin water films covering highly dispersed metal oxides can give rise to the spontaneous and spatially controllable growth of hydroxide fibers out of the MgO nanocube agglomerates shown in the SEM image in the left upper panel. Surface chlorinated MgO nanocubes transform upon contact with water vapor at room temperature into magnesium hydroxychloride Mg₃(OH)₅Cl·4H₂O fibers. The formation of hydroxide fibers under ambient conditions can be applied to a variety of microstructural evolution processes that involve metastable solids and superficial water acting both as a reactant and as a reaction medium for the hydration of metal oxide particles. Figure adapted from [Reference van Santen34, Reference Gheisi, Sternig, Redhammer and Diwald35].

Residual water and transformations of metal oxide nanoparticles in high vacuum

Imaging of metal oxide particles with scanning electron microscope and helium ion microscope (HIM) requires powder sample immobilization on conductive substrates. This can be achieved by pressing the sample into a soft metal that, if not cleaned and prepared prior to sample immobilization, is typically covered with water originating from the ambient atmosphere. Under such conditions, residual and surface adsorbed water molecules can act as a solvent and lead to a partial conversion of MgO into Mg(OH)₂ [Reference Schwaiger, Schneider, Bourret and Diwald36]. HIM studies of MgO cubes obtained by Mg combustion (polydisperse sample with sizes ranging from 4 nm to a few micrometers) and pressed into an untreated indium foil have revealed that the samples prepared at the ambient and stored 4 days in high vacuum showed significant swelling, characteristic with a volume increase due to the formation of an Mg(OH)₂ shell. On the contrary, samples prepared using an indium foil that was dried in high vacuum (10⁻⁵ mbar, 1 day) before MgO cube immobilization showed very limited swelling. Interestingly and similar to the observations made in liquid water (see section Metal oxide immersion into condensed water), it was found that the smallest MgO cubes reacted faster than the larger ones, as shown by their larger relative swelling (in relation to their initial size). The fact that the ambient water condensed at the indium foil surface was sufficient to reactively attack the MgO cubes pressed into the foil and this under high vacuum conditions is remarkable [Reference Schwaiger, Schneider, Bourret and Diwald36]. It demonstrates that spurious amounts of water can react with metal nanostructures even under high vacuum conditions and should be taken into consideration by anyone working with water sensitive metal oxide samples (Fig. 4) [Reference Baer, Engelhard, Johnson, Laskin, Lai, Mueller, Munusamy, Thevuthasan, Wang, Washton, Elder, Baisch, Karakoti, Kuchibhatla and Moon37].

Figure 4:

(a) Proposed mechanism of superficial H₂O-induced transformation of MgO cubes into MgO with Mg(OH)₂ coatings. This reaction step is associated with characteristic volume expansion effects that are consistent with the observations described above. (b and c) High-magnification HIM images of MgO cubes embedded in an indium foil after 1 h (b), and 4 days in the HIM chamber at a chamber pressure P < 3 × 10⁻⁷ mbar (c). Significant volume increase, rounding of the edges, and partial fusion of the particles were observed (see arrows in micrographs). (d and e) Curves showing the edge length increase factor of MgO cubes (L ₂/L ₁) as a function of the edge length L ₁, where L ₁ and L ₂ are the edge lengths before and after 4 days storage in the vacuum chamber of the HIM, respectively. (d) The indium foil was dried under vacuum before being used as a substrate. (e) The indium foil was used without any further treatment. The dashed line corresponds to a full transformation of MgO into Mg(OH)₂ (i.e., L ₂/L ₁ = 1.15). The dotted line corresponds to zero expansion (L ₂/L ₁ = 1). Adapted from Ref. Reference Schwaiger, Schneider, Bourret and Diwald36.

Self-assembly processes in thin water films

Water-mediated forces play an important role in nanoparticle synthesis and structure formation at the mesoscale [Reference Shen, Chun, Rosso and Mundy38]. As an example, oriented attachment of nanoparticles represents a key process in the solution-phase growth of hierarchically organized nanostructures. The underlying self-assembly in aqueous dispersions results from the subtle balance between colloidal (i.e., hydration, electrostatic, and dispersion forces) and hydrodynamic forces [Reference Shen, Chun, Rosso and Mundy38, Reference Welch, Woehl, Park, Faller, Evans and Browning39]. Experimentally, oriented attachment of particles in bulk water has been reported for a variety of metal oxides such as TiO₂ [Reference Niederberger and Cölfen12, Reference Cölfen and Antonietti13], ZnO [Reference Li, Nielsen, Lee, Frandsen, Banfield and De Yoreo40], iron oxide, or FeOOH [Reference Soltis and Penn41]. Complementary molecular dynamics simulations suggest that oriented attachment preferentially involves surfaces that have a higher propensity to hydroxylate and to dissociate water [Reference Raju, van Duin and Fichthorn42]. Therefore, hydroxylation and hydrogen bonding seem to govern particle alignment effects that are followed by coalescence.

Our group recently reported the use of water vapor to induce the self-organization of MgO nanocubes into regularly staggered one-dimensional structures (Fig. 5) [Reference Thomele, Bourret, Bernardi, Bockstedte and Diwald43]. Exposure to water vapor (partial pressure: 30 mbar) and subsequent vacuum annealing at 1173 K results in the reorganization of the MgO nanocubes into one-dimensional structures, that is, MgO nanobars. This transformation evolves via an initial alignment of the MgO cubes [Fig. 5(a)], the formation of intermediate elongated Mg(OH)₂ structures, and their reconversion into staggered MgO cube structures by thermal annealing [Fig. 5(c)]. The particles spontaneously orient themselves to form one-dimensional arrays, which, instead of forming perfect stacked cube assemblies, such as observed for metal nanocubes in solution [Reference Gao, Arya and Tao44], form staggered MgO nanocube assemblies, where the cubes are offset from each other. Ab initio Density Functional Theory (DFT) modelling identifies surface energy changes [Fig. 5(d)] associated with the cube surface hydration and hydroxylation to promote the uncommon staggered assembly of the cubes [Reference Thomele, Bourret, Bernardi, Bockstedte and Diwald43]. This first observation of metal oxide nanoparticle self-organization occurring outside a bulk solution may pave novel routes for inducing texture in ceramics and represents a great test bed for new surface science concepts.

Figure 5:

(a) Illustration of water adsorption–induced arrangement of MgO nanocubes into nanobars. (b and c) TEM micrographs of the MgO nanocubes before (b) and after hydroxylation and and dehydroxylation (c), where the arrows indicate the experimentally observed MgO nanocube bars. (d) Stabilization energy of the hydroxylated stacks relative to the dehydroxylated nanocube versus displacement δ along the surface diagonal for different water pressures at T = 300 K. Bottom: Geometries of the hydroxylated straight and displaced stacks—oxygen ions are red spheres and magnesium ions are light red spheres. Oxygen ions bound in OH groups or in water are blue spheres and hydrogen atoms are white spheres. The hydroxylated nanocube stacks are stable within the range of water vapor partial pressure indicated below the geometries. Adapted from Ref. Reference Thomele, Bourret, Bernardi, Bockstedte and Diwald43.

Thin water films for sintering

Thin water films can also influence sintering processes where a loose metal oxide nanoparticle powder is converted into a dense nanocrystalline material. Sintering is crucial in ceramics processing [Reference Castro, Gouvêa and Wakai45, Reference Gheisi, Niedermaier, Tippelt, Lottermoser, Bernardi and Diwald46, Reference Bordia, Kang and Olevsky47] and can be used to consolidate metal oxide nanoparticle systems with enhanced physicochemical properties [Reference Lu48, Reference Lu49]. It is a surface elimination process that starts with an assembly of particles in close contact. The initial interfaces or interfacial spots range from point of contact to highly deformed interfaces. With sintering, the contacts grow in size and there is extensive loss of surface area in the initial stage [Reference Castro, Gouvêa and Wakai45, Reference Phokha, Klinkaewnarong, Hunpratub, Boonserm, Swatsitang and Maensiri50]. The determining role of interfacial energies with regard to the densification of consolidated particle systems is well accepted [Reference Phokha, Klinkaewnarong, Hunpratub, Boonserm, Swatsitang and Maensiri50, Reference Folly, Soriano, Sinnecker and Novak51]. Since a new type of interface is being created during neck formation, sintering has an energy cost that is proportional to the grain boundary energy [Reference Hayun, Tran, Ushakov, Thron, van Benthem, Navrotsky and Castro32, Reference Castro, Gouvêa and Wakai45, Reference Castro, Tôrres, Pereira and Gouvêa52]. It was recently found that water adsorbed from ambient on MgO crystalline nanocubes (4 nm) can significantly affect their morphology during vacuum annealing depending on the reactor pressure (Fig. 6). While dynamic vacuum annealing at 1173 K (continuous pumping at pressure <10⁻⁵ mbar during annealing) does not affect the particle size and shape [Fig. 6(a)], semi-dynamic annealing [Fig. 6(b)], where dynamic pumping is replaced with a constant static O₂ pressure, at T = 373 K leads to larger cubic particles (size ranging from 4 nm to ∼15 nm) with rounded edge features. The latter phenomenon is due to the increased abundance of step edges that are quite different from the clear-cut sharp edges specific to the parent 4 nm MgO cubes typically obtained from annealing in dynamic vacuum. Annealing at a constant O₂ pressure and without continuous removal of water molecules leads to a significant change in morphology of the particles that become facetted and show significant particle fusion [Fig. 6(c)]. These experiments show that adsorbed water can lead to significant coarsening of the particles. The changes are attributed to enhanced mass transport in the thin water film that acts as a two-dimensional solvent for Mg²⁺ hydration and transport, promoting particle sintering and coarsening at high temperatures.

Figure 6:

Schematic representation of how different annealing protocols affect the growth of MgO nanocubes synthesized in the Mg combustion flame. Adapted with permission from Ref. Reference Thomele, Gheisi, Niedermaier, Elsässer, Bernardi, Grönbeck and Diwald53 (Copyright 2018 WILEY VCH). DVA stands for dynamic vacuum annealing and indicates that annealing was performed under dynamic pumping with a base pressure of p< 10⁻⁵ mbar.

Instead of using high temperatures and residual water to sinter hydroxylated metal oxide particles, thin water films can also be used to promote the low-temperature sintering of TiO₂ nanoparticles and prepare conducting nanocrystalline TiO₂ films. Such films are of great importance due to their uses in many technological applications such as solar cells, photoelectrodes, photocatalysts, and gas sensors [Reference Berger, Monllor-Satoca, Jankulovska, Lana-Villarreal and Gómez54, Reference Sun, Haidry, Fatima, Li and Yao55]. They are usually prepared via high-temperature sintering, typically performed at 450 °C, and are thus not compatible with the use of heat-sensitive substrates such as flexible polymeric films. Annealing a mixture of TiO₂ nanoparticles with Ti(IV) precursors such as titanium tetraisopropoxide [Ti(OⁱPr)₄] in water vapor at 100 °C was found to be an efficient route for the preparation of conducting crystalline porous TiO₂ with enhanced photoelectrochemical properties (Fig. 7). While annealing ethanolic solution of Ti(OⁱPr)₄ under dry conditions at 100 °C yields an amorphous solid, annealing under water vapor at 100 °C yields rutile [Fig. 7(b)]. By using different titanium precursors, it is possible to obtain either rutile or anatase. Such a report shows the beneficial influence of water thin films in the consolidation and preparation of functional ceramics and represents an alternative to high-temperature annealing that is usually necessary for these materials.

Figure 7:

(a and b) SEM photographs of the TiO₂ films produced by mixing P25 particles and Ti(OⁱPr)₄ before (a) and after annealing in water vapor at 100 °C (b). The undefined matrix seen in (a) is the amorphous Ti(OⁱPr)₄ film that embeds the TiO₂ particles within. After sintering/crystallization, one can clearly see the TiO₂ crystalline particles. (c) X-ray diffraction (XRD) patterns of the films prepared by heating a solution of at 100 °C for 12 h in a dry oven (1) or by treating the following chemicals in water vapor at 100 °C for 12 h: Ti(OⁱPr)₄ (2), TiOSO₄ (3), and TiCl₄ (4). Adapted with permission from Ref. Reference Zhang, Yoshida and Minoura56 (Copyright 2003 WILEY VCH).

Water-assisted crystallization

In addition to promoting sintering, water thin films can also be used to crystallize amorphous metal oxide substrates via dissolution/precipitation processes. For example, some groups have reported on the use of water vapor to crystallize anodized amorphous TiO₂ nanotubes into anatase at moderate temperatures (i.e., lower than 100 °C) [Reference Lamberti, Chiodoni, Shahzad, Bianco, Quaglio and Pirri57]. The anodic oxidation of Ti foil into TiO₂ nanotubes has led to a vast field of research [Reference Roy, Berger and Schmuki58]. Unfortunately, the as-produced TiO₂ structures after anodization are amorphous and need an additional annealing step in order to be used as functional TiO₂ photocatalysts, typically done at temperatures around 450 °C. Instead of using thermal energy only to crystallize TiO₂, Lamberti et al. [Reference Lamberti, Chiodoni, Shahzad, Bianco, Quaglio and Pirri57] recently reported that incubation of amorphous anodized TiO₂ nanotubes for 30 min in water vapor at 90 °C converts amorphous TiO₂ into anatase (Fig. 8). Although at present the mechanism is unclear, a dissolution precipitation reaction is expected to occur due to the changes in the TiO₂ structure morphology that was observed. It is likely that a thin water film hydroxylates the TiO₂ surface, similarly to what occurs at room temperature on MgO surfaces. Dissolution of the Ti(III/IV) species as [Ti(OH)₆]²⁻ in the water film then leads to a supersaturated solution [Eq. (1)], eventually yielding crystallization of anatase via a sequential dehydration step [Eq. (2)] [Reference Huo, Wang, Zhang, Cao and Chu59].

(1)$${\rm{Ti}}{{\rm{O}}_x} + 4{{\rm{H}}_{\rm{2}}}{\rm{O}} + \left( {1 - {x \over 2}} \right){{\rm{O}}_2} \to {\rm{Ti}}\left( {{\rm{OH}}} \right)_6^{2 - } + 2{{\rm{H}}^ + }\left( {1 \lt x \le 2} \right)\quad ,$$

(2)$${\rm{Ti}}\left( {{\rm{OH}}} \right)_6^{2 - } + 2{{\rm{H}}^ + } \to {\rm{Ti}}{{\rm{O}}_2} + 4{{\rm{H}}_{\rm{2}}}{\rm{O}}\quad .$$

Figure 8:

(a) Proposed crystallization mechanism from hydroxylated TiO₂. From Ref. Reference Chen and Zhou62. (b) Scheme of the experimental setup used for the crystallization of the amorphous TiO₂ nanotubes in water vapor. (c) X-ray diffraction (XRD) data showing the evolution of the main anatase X-ray diffraction peak [(101) crystal face] during the water vapor exposure. (d and e) SEM images of the TiO₂ tubes before (d) and after 240 min of water vapor annealing at 90 °C (e). (b–e) Adapted with permission from Ref. Reference Lamberti, Chiodoni, Shahzad, Bianco, Quaglio and Pirri57. (f) SEM image of the TiO₂ tubes after 240 min annealing at 90 °C in liquid water. Adapted with permission from Ref. Reference Krengvirat, Sreekantan, Mohd Noor, Negishi, Kawamura, Muto and Matsuda60 (Copyright 2013 Elsevier).

A significant change in the amorphous TiO₂ tubular morphology was observed during water vapor annealing, which was converted into solid anatase wires [Reference Lamberti, Chiodoni, Shahzad, Bianco, Quaglio and Pirri57]. This is quite different from what occurs during annealing in liquid water at 90 °C, where the smooth amorphous TiO₂ tube surface is instead replaced with a rough polycrystalline surface composed of granular anatase nanoparticles [Fig. 8(f)] [Reference Krengvirat, Sreekantan, Mohd Noor, Negishi, Kawamura, Muto and Matsuda60]. As expected, the photocatalytic activity of the water vapor–crystallized TiO₂ substrate was much higher than that of the as-synthesized amorphous TiO₂ nanotubes, as shown by the higher decomposition rate of methylene blue under UV illumination of the crystallized TiO₂ substrate [Reference Lamberti, Chiodoni, Shahzad, Bianco, Quaglio and Pirri57]. It is interesting to note that the substrates crystallized in water vapor were also slightly more photoactive than the TiO₂ tubes annealed at 450 °C under dry conditions. This is also consistent with a previous work showing that annealing similar TiO₂ nanotubes under water vapor in an autoclave at 180 °C yielded a much higher photo-electrochemical activity than the same substrate annealed at 180 °C and atmospheric pressure (10 times increase in the photocurrent under similar conditions) [Reference Liu, Liu, Zhang, Zhai and Jiang61].