Templated dewetting: designing entirely self-organized platforms for photocatalysis

Noble metal dewetting on self-organized TiO2 nanotubes – nanoscopic design of photocatalysts towards green H2 generation.


Introduction
Dened metal (M) particles of small size scale decorating an oxide surface are of wide technological interest and nd application in catalysis (chemical, electro-and photo-), photonics, plasmonics, and sensing among other areas. 1,2 One way to produce such particles or ensembles is by "dewetting" a thin metal lm, as illustrated in Fig. 1a and b. For virtually all metal/oxide combinations, a thin metal (some nmto hundreds of nm) deposited by any classic method (chemical or physical vapour deposition, sputtering, evaporation, etc.) will, upon heating to elevated temperatures (as a rule of thumb half the metal's melting point), break up into patches and "ngery networks", and nally aggregate into distinct individual patches or particles (Fig. 1b). 3,4 Except for this purely thermally driven dewetting, there are also other mechanisms such as electrotransport, where metal atom migration is caused by an applied electric eld. Applied gradients that inuence surface diffusion can give to dewetting a direction such as in thermotransport where the mass transport takes place along a temperature gradient across the metal lm. 5 By modication of the initial metal layer thickness, surface wettability, chemical or physical inhomogeneity of the lm/ substrate ensemble, and specically by a dened pre-patterning of the substrate ( Fig. 1c and d), it is possible to steer the geometry of the dewetting process into highly dened arrangements (Fig. 1e) an example is given in Fig. 1f, where the SEM image shows a gold layer that was dewetted on a selforganized titania nanotube (NT) surface forming highly monodisperse single Au nanoparticles (NPs) of $60 nm in diameter in each and every titania cavity.
It is interesting to note that the same driving forces that can be benecially explored to produce these highly dened metal particles in nanotube arrays were historically rstly reported as an undesired phenomenon. Particularly the occurrence of solidstate dewetting of thin metal lms or interconnects in microelectronic and integrated systems may lead to metal rupture or discontinuities in a metal contact and thus can result in total device failure. [5][6][7][8][9] The underlying forces that separate a thin lm into small islands can also lead to agglomeration of very small metal particles (for example, nm sized catalyst particles that are predecorated on a substrate) into coarser aggregates or patchesin catalysis coarsening of the catalyst is, of course, undesired as it causes activity degradation. 10 In any of these examples the overall driving force for dewetting is the minimization of the free surface energy of the metal lm, of the substrate and of the metal-substrate interface. Given that the thinner the metal lm the higher its surface-tovolume ratio and thus its surface energy, the driving force for dewetting increases dramatically when the lm thickness decreases (i.e., the thinner the metal lm the lower the activation energy for metal atom surface mobility). This is a key reason why dewetting can occur at temperatures that are well below the lm melting point, that is, the lm can dewet while remaining in the solid state.
While a range of factors (discussed in Section 3) inuence initiation and growth of discontinuities even in "ideal" thin lms, most metal deposition conditions lead to a meta-stable state for the as-deposited lms, as they are formed under conditions for which the atomic motion is limited and thus an equilibrated lattice may not be achieved. 3,4 Fig. 1 Sketch illustrating dewetting of a metal film (a, b) on a flat surface and (c-e) on a patterned surface of TiO 2 nanotubes (i.e., template-guided dewetting); (f) SEM image of arrays of single-Auparticle-per-cavity formed by dewetting a Au film on a highly regular self-organized anodic TiO 2 nanocavity layer. He joined the group of Prof. Patrik Schmuki at the University of Erlangen-Nuremberg, Germany, rstly in 2012 as a visiting PhD student, and then in 2014 as a postdoctoral fellow and group leader in the frame of the ERC funded project "APhotoReactor". His scientic interests are related to the development of photocatalytic materials for solar energy conversion processes and environmental applications. His current research activity is focused on self-assembling pathways to synthesize functional metal/semiconductor assemblies with nanoscale precision.
Nhat Truong Nguyen studied Chemical Engineering at Ho Chi Minh University of Technology, Vietnam, and obtained his MSc degree in Material Engineering from University of Science and Technology, Korea, in 2012. He is currently a PhD student in the group of Prof. Schmuki at the University of Erlangen-Nuremberg, Germany. His research interests include the synthesis and functionalization of semiconductor nanomaterials for energy conversion applications.
Key elements for initiation of dewetting in lms on ideal surfaces are defects (inhomogeneities) such as holes, edges, impurities, and grain boundaries. However, dewetting can be intentionally brought about at specic sites by using a prepatterned substrate; that is, dewetting can be driven by a dened substrate surface topography rather than by random intrinsic inhomogeneities.
Aer giving some examples (below) the present perspective will give a brief overview of the general features of the dewetting process, and of key parameters and key possibilities to generate desired dewetting patterns. Emphasis will then be on metal/ oxide combinations, and particularly on forms of dewetting that lead to combinations of metal/oxide structures with synergetic effects that can be used in various applications. In particular we discuss pathways to exploit dewetting phenomena for chemistry, (photo-)electrochemistry, catalysis, and some other applications. We will discuss the dewetting of (noble) metals on highly regular self-organized anodic TiO 2 nanotubes that yields strongly enhanced photocatalytic properties of M/TiO 2 combinations. In this context, even more complex assemblies can be fabricated introducing additional self-ordering principles, such as controlled dealloying, spinodal decomposition, site-selective functionalization and others.

Some examples
Historically, a large effort in dewetting research addressed the agglomeration of metal lms, metal silicides and metalloids on silicon and SiO 2 (silicon-on-insulator (SOI) structures) with the goal of suppressing dewetting [5][6][7][8][9] (an example is shown in Fig. 2 where the agglomeration of Au lines leads to break up of an interconnect 11 ).
However, a steadily increasing number of works demonstrate that the spontaneous dewetting of metal thin lms can also be exploited to a technological advantage. For example, in 2001 Liu et al. found that 5-10 nm-thick co-deposited layers of Au and Pd that had undergone thermal dewetting on Si wafers, agglomerated into dened metal islands that could then be used as catalytic sites for the growth of arrays of amorphous silicon nanowires. 12 Early studies on solid-state dewetting demonstrated that the initial metal lm thickness determines key structural parameters of the dewetted metal islands (e.g., their size, spacing and density). 13 Liu et al. adjusted the thickness of the deposited Au-Pd lm to control size and distribution of the Au-Pd islands and gained in turn ne control over the morphology and coverage of the formed Si nanowires.
Meanwhile, a similar strategy was applied by Chhowalla et al. to the growth of vertically aligned carbon nanotubes from dewetted nickel lms. 14 They demonstrated that the structure, degree of ordering and mechanism of formation of the carbon nanotubes can be easily controlled by tuning the initial thickness of the Ni lms.
The ne control over size and distribution of the dewetted lms was exploited to assemble non-volatile memory devices based on arrays of dewetted Si-nanocrystal that showed tuneable stored charge density. 15 Other work showed that dewetting of metals also represents a powerful tool for the fabrication of a large palette of micro-and nano-structured assemblies such as catalysts and electrodes, 16 sensors, 17 nanocrystals for magnetic elements, 18,19 and biomimetic and plasmonic platforms. [20][21][22] Another key direction of efforts was devoted to nd a reliable pathway to maximize the self-ordering degree of dewetting. Among several experimental parameters that have been explored (e.g., physico-chemical properties of the metal lm, thermal treatment conditions, etc.), the topography of the substrate was identied as the most inuential factor. Some pioneering works on "templated dewetting" reported on metal lms that were dewetted on pre-patterned substrates, such as grating structures, 23 and arrays of pits and mesas 24 in order to produce metal structures with a high degree of control over periodicity and arrangement, and to understand the underlying mechanism. 25 For this, patterned SiO 2 /Si substrates were used due to the availability of well-established lithographic tools that allow for patterning of the substrate with nanoscale precision.

Some key concepts of solid-state dewetting
In principle dewetting involves the formation of a hole in a thin lm that reaches the substrate and the subsequent recession of the lm (Fig. 2) such an initiation process can be observed for solid and liquid thin lms. Solid lms can be composed of, e.g., polymers, metalloids or metals. Metal lms can be amorphous or crystalline. Basically also single crystals on atomically at surfaces are subjected to initiation and spreading of rupture as a consequence of self-induced spinodal (stochastic) instabilities. 4,[26][27][28][29][30][31][32] In practice, however, distinct (deterministic) initiation sites for lm rupture are brought about by inherent defects or inhomogeneities 3such as impurities, thickness variations and particularly grain boundaries in polycrystalline metal lms. 11,[33][34][35][36][37] Once a lm rupture event occurs, thin solid lms deposited on a foreign substrate are generally unstable (except for cases of perfect wettability). In particular, any metal/oxide combination does have a driving force for dewetting by the reduction of the total free energy provided namely by a reduction of the interfacial area between the lm and its substrate, that nally results in agglomeration of the lm into three-dimensional (3D) islands. The thermodynamic driving force can be dened as: 3,4 where g i is the surface energy density of the material (with i ¼ A (lm), B (substrate)) and g AB the interfacial energy density. The solid state dewetting process itself proceeds from the spontaneous formation of voids or holes at specic defects ( Fig. 3a) and via a ux (J) of material A leaving the dewetting zone by capillary-driven surface diffusion; 11,33,35 i.e., J is driven by the local surface curvature of the receding metal lm. Due to mass conservation, the metal rst accumulates into a rim at the dewetting front. According to its crystallographic properties the rim may be smooth or faceted and its height depends on the dewetting velocity and the kinetics of diffusion of adatoms on the surface of the lm. As the rim is receding, a valley is generally formed behind the rim due to a Rayleigh instability ( Fig. 3a and b). When the bottom of the valley reaches the underlying substrate, a pinch-off mechanism occurs leaving a line (wire) of material A behind (mass shedding 35,38 - Fig. 3b). In absence of stabilizing factors, the retracting rim at a certain length becomes unstable too (Fig. 3c). As a result, large-scale ordered arrays of elongated structures (generally called "ngers") are formed as the dewetting front recedes ( Fig. 3c and  d). Then, nally, the ngers break into 3D islands by a beading mechanism similar to a Rayleigh-Plateau transition, 39 as sketched in Fig. 3d.
Regarding the initiation site, as mentioned, thin lms deposited by evaporation and sputtering provide a number of relevant inhomogeneities. 40 Most important, these lms are typically polycrystalline in nature with average grain sizes of tens or hundreds of nanometres. The grain boundaries and particularly triple junctions are typical initiation sites for lm rupture that trigger the formation of holes. 33 The key parameters that generally determine the morphology of the dewetted state and the kinetics of dewetting for a given A/B couple are: The initial lm thickness h. The density of formed holes scales inversely with the lm thickness. 13,41 The dewetting temperature T dewet decreases with h.
Capillary energies and lm surface curvature. Capillary energies drive material retraction from the edges of the holes. The rate of material retraction scales with the difference of lm surface curvature at the lm edge and away from the hole (i.e., where the lm is virtually at). 34 The treatment temperature T primarily affects the kinetics of the system by the adatom mobility that increases exponentially with increasing T. In experimental observation, metastable lms dewet forming metal islands when heated up to temperatures that allow for sufficiently high surface mobility of the constituent atoms. As a result, there is a characteristic temperature for thin metal lms at which dewetting can be observed, that is, T dewet is empirically found to be between the Hüttig and Tammann temperatures that are 0.3 and 0.5 of the melting point of the metal, respectively (T Hüttig is the temperature at which atoms at defects become mobile, and T Tammann is the temperature at which atoms in the bulk metals start to diffuse). 42 T dewet is also found to decrease with decreasing metal lm thickness, as a lower thermal budget (i.e., lower energy barrier) is required to initiate lm breakup. 3,13 In general, the higher the metal melting temperature (T m ), the higher T dewet . However, T dewet depends also on the grain structure evolution of the metal lm. Grain growth in high-purity metallic lms can occur at temperatures as low as 0.2T m . In contrast, grain growth in pure diamond cubic materials occurs only when T $ 0.8T m or higher. 3,43 The metal surface self-diffusivity can be affected by inclusion of dopants, 5,6,44 or by the environment (annealing atmosphere, vacuum, etc.). [45][46][47][48] The thermodynamic driving force E S . This parameter essentially depends on the couple A/B (although it may be modied by a foreign adsorption or by strain). The interface tensions g x are material specic. [49][50][51][52] Strain affects the kinetics. Films are oen in a state of mechanical stress and can also experience strain effects when annealed on a substrate due to differential thermal expansion. 53 Srolovitz et al. found that when holes form in a lm in a state of strain, the strain in the material adjacent to the hole can partially relax. 54 The decrease in strain energy associated with the presence of the hole makes hole formation more likely, and thus it can favour dewetting.
The nature and number of defects. Defects in the lm (pinholes and edges, dislocations, thickness variations, impurities) act as nucleation centres for spontaneous void opening. 55 Typically the higher the kinetic energy used for a metal lm deposition (and the deposition rate), the larger the density of defects in the lm. 40 Also, the thinner the lm (e.g., few nanometres), the larger the density of pinholes. A large density of holes accelerates the kinetic of dewetting.
The crystallographic features. Although the concepts of capillary forces and surface curvature have in principle no proper physical meaning in the presence of singular at facets, the crystallographic orientations of the dewetting edges and the faceting of the rim can inuence the stability conditions of the dewetting front. [56][57][58] These features play an important role in the morphology of the nal dewetted state as well as on the dewetting kinetics. Experiments showed that, e.g., edges with different in-plane orientations retract at different rates, and differently faceted rims may induce various dewetting morphologies. 59,60 Triple line pinning. The specic properties of the triple line, related for instance to local adsorption and/or local pinning ascribed to defects, can affect the local mobility of the dewetting front.
However, from an experimental point of view it is generally difficult to discriminate these parameters. Some examples of parameter effects are shown in Fig. 4. [61][62][63] In summary, an ideal polycrystalline lm on a smooth substrate undergoes full dewetting, forming islands of metal, the shape, size, spacing and density of which are relatively homogeneous throughout the surface of the initially continuous lm.
However, common thin lms show intrinsic defects from which dewetting preferentially initiates. Hence, extensive material agglomeration starts not only at grain boundaries but also at pre-existing holes and lm edges. One consequence is that the kinetics of the process is no longer governed only by the hole incubation time. 3,11,13,64 A second consequence, which is more relevant in the frame of this perspective, is that the intrinsic defects in metal lms can cause a certain loss of selfordering degree of dewetting so that the metal may agglomerate into islands of irregular shape and size and with random distribution. Nevertheless dened defects (pre-patterned substrates) can be used benecially to achieve a highly controlled dewetting process as discussed in the next section.

Templated dewetting
The idea of templated dewetting is to impose an initial periodic perturbation in the lm curvature to control breakup and the subsequent metal morphological evolution. An important factor that determines if dewetting can be controlled by the substrate to take place in an ordered manner is the topography features compared to lm thickness, e.g., in a ripple structure, as in Fig. 5a, the thickness of the deposited layer needs to be in the range of the ridge height, and the wavelength of the ripples and distances covered by atomic motion need to be of similar magnitude. 23,24,[65][66][67][68] For two-dimensional (2D) patterned surfaces (Fig. 5) the control over the NP size can be achieved if the introduced topography has an articial curvature modulation with a shorter length scale than the natural instability on a at surfacethis results in a decay into smaller particles. 24 In this case the curvature-induced diffusion mechanism dominates over other agglomeration dynamics (e.g., driven by capillary instability or grain growth) as long as the ridges can act as diffusion barriers that trap the metal into the valleysunder these conditions agglomeration is limited and the surface coverage stagnates.
Interestingly, it was observed that dewetting on corrugated surfaces occurs at lower temperatures compared to at substrates. It was thus proposed that the ripple patterned topography provides a gradient of chemical potential in the direction normal to the direction of the ripples (with minima in the valleys and maxima at the peaks) that triggers curvature induced diffusion (J in Fig. 5a). 67,69 Fine control of the NP size can thus be achieved by lm thickness and substrate topography. 42,68,[70][71][72][73][74][75][76][77] Another key to ordered dewetting is the match between the geometry of the trenches (e.g., the angle at their bottom) and the crystallographic features of the dewetted metal crystals (as shown in Fig. 5a). 23,25 Evidently, if too-thick metal lms (i.e., larger metal amounts) are deposited that ll the trenches completely, one may essentially lose the corrugation effect during the early stage of dewetting, as a virtually even surface is created. 67

Some examples of templated dewetting
A pioneering study on template-guided dewetting of metal using diperiodic substrates is the work of Giermann and Thompson. 24 They explored the formation of ordered metal nanoparticles by dewetting Au lms on square-arrays of pyramidal pits formed on SiO 2 /Si surfaces (Fig. 5b). Different topographical geometries were used that had various spatial period and pit-to-mesa width ratios. It was found that for specic ranges of relative Au lm thickness and topographic dimension, dewetting resulted in arrays of nanometre-scale Au particles embedded in each pit, with highly-uniform periodic spacing, of nearly monodisperse size and controlled crystallographic orientationi.e., Au dewetted in a completely complementary manner with respect to the substrate topography.
Another remarkable work is that of Kushida et al. In this study, polycrystalline Pt lms were evaporated and then annealed onto oxidized Si surfaces that had been previously lithographically patterned with sawtoothed grating structures. 23,66 Owing to the specic angle at the bottom of the trenches, it was found that for a certain thickness of the metal lm and by a suitable annealing treatment the initially conformal Pt lm decomposed, forming within each groove one-dimensional (1D) Pt crystals that had a preferential crystallographic orientationthe Pt crystals grew with the (111) plane parallel to the faces of the grooves.
Such a template-guided grain growth was discussed as driven not only by the metal lm but also by the formation of stabilized Pt crystals within the trenches (Fig. 5a). In contrast, a similar annealing of Pt carried out on smooth SiO 2 surfaces led to irregular Pt particles having no specic preferential crystallographic orientation.

Other interesting aspects
Choi et al. found that both the initial metal thickness and temperature of thermal treatment provide control over dewetting. 70 However, not only does the former lead to better size control of the dewetted NPs, but also annealing at too-high temperature (used in principle to cause crystal coarsening) may lead to a loss of metal by evaporation.
Kojima and Kato developed a technique to form periodically arranged metal NPs by electron-beam-induced dewetting. 63 The main advantages of the technique are that one can select the region where to generate the particles, and that such a region has sharp boundaries (i.e., between dewetted and non-dewetted areas).
Dewetting can also be triggered particularly using nanosecond pulsed lasers. 76,77 This method allows also for dewetting of high melting point metals and avoids substrate deterioration (e.g., by thermal oxidation). Ruffino and Grimaldi provide an overview of different heat sources (annealing, ion or electron beam, laser irradiation) that can be used to cause dewetting.
Oh et al. found that on a topographic substrate (arrays of pits) one particle per pit is obtained when the spacing between the pits is similar to the average distance between the NPs dewetted on a smooth surface (from a metal lm of similar thickness). 71 Yang et al. explored templated-dewetting of co-sputtered metal lms. 42 The use of different metals in controlled amounts leads to simultaneous alloying and dewetting, and forms e.g. ordered alloyed Au-Ag particles with ned composition. They also showed that further control over size and spacing of the metal particles is obtained by using the effect of gravity and by an iterative deposition-dewetting approach.
Overviews of various other metal/substrate combinations that have been investigated are also available in the literature. 4,78 It is however remarkable that while templated dewetting has been studied from a mechanistic perspective in detail, the resulting potential for applications is only considered in comparably few works, such as the fabrication of Si nanowire arrays, 70 magnetic nanocrystals, 71 optical platforms (exhibiting, e.g., enhanced plasmonic properties, surface localised plasmon resonance (SLPR) and surface-enhanced Raman scattering (SERS)), 42,73 and electrocatalysts. 74 In the following section we will deal with using dewetting of (noble) metals on self-organized TiO 2 nanotubes, where such a combination is entirely application driven by the functional features of noble metal decorated TiO 2 , namely for photocatalysis.

TiO 2 nanotube surfaces for selfordering dewetting
We have outlined above how the use of periodic surfaces can lead to controllable dewetting to form dened arrays of metal NPs on a regular substrate. This approach is and has been studied using patterned SiO 2 /Si substrates due to the availability of well-established lithographic tools that allow for patterning the substrate (with large scale uniformity) to virtually any geometry and length scale. 24,68 The use of self-organizing patterns for templated dewetting only recently received wider attention, this because emerging self-organizing electrochemical techniques applied to namely Al, Ti and Ta provided reliable tools to fabricate patterned metal oxide substrates with a sufficiently high degree of ordering. 79,80 TiO 2 nanotube arrays that can reach the highest degree of self-ordering were reported by Yoo et al. in 2013 ( Fig. 6a and b). 16 These nanotubes are almost ideally hexagonally ordered and are of a suitable short aspect ratio to provide a periodic surface for ideal ordered dewetting (Fig. 6c).
Below, we briey introduce the fabrication process and geometry of these anodic TiO 2 nanotubes (that we refer to also as TiO 2 nanocavity arrays). Then we introduce noble metal dewetting on these arrays and show the highly benecial use of these metal/oxide assemblies in photocatalysis.

Highly-ordered anodic TiO 2 nanotube arrays
A direct, scalable and versatile approach to form self-ordered titania structures (but also of other metal oxides 81 ) is self-organizing electrochemical anodization of a Ti metal substrate. [82][83][84] Vertically aligned regular TiO 2 nanostructures with dened tubular geometry can be formed and the self-ordering degree, morphology, and physicochemical properties of the TiO 2 nanotubes can be adjusted by choosing an adequate set of electrochemical parameters. 85,86 Several literature reviews are available that discuss in detail the formation and properties of these anodic nanotubes 82-84,87therefore we will keep this part very brief.
The key to a high degree of self-ordering, as shown in Fig. 6a and b, is the use of electrochemical conditions that during anodization lead to high rate of oxide growth combined with high rate of oxide dissolution. 88,89 This can be achieved by anodizing Ti metal in hot H 3 PO 4 /HF electrolytes. The tube growth conditions can be adjusted to form a short tube length that resembles a nanocavity. The resulting arrays of TiO 2 nanotubes can be formed over large surfaces (some cm 2 ) and present a virtually ideal hexagonal ordering ( Fig. 6a and b). 16 Onto these TiO 2 nanocavity surfaces one can deposit metal lms conformally, e.g., by sputtering, and then trigger dewetting by thermal treatment (Fig. 1c-f and 6c). For the subsequent dewetting to be controllable, the regular tube geometry together with an optimized cavity spacing and height relative to the deposited metal thickness is essential.
Tube arrays of a poor degree of self-ordering lead to metal dewetting in a highly imperfect fashion (Fig. 6d). Fig. 6e shows for comparison also the dewetting result on a at substrate. FFT (Fast Fourier Transform) conversion of the SEM images (insets in Fig. 6b-d) is the most direct method to characterize the regularity of the TiO 2 cavities and offers a clear comparison between different self-organized nanotube structures.

Metal/TiO 2 structures for photocatalysis
Why are metal particles (particularly noble metal) on TiO 2 so important? Because they provide a highly synergistic platform for photocatalysis. [90][91][92][93][94][95] In general, a photocatalytic process is based on the interaction of light with a semiconductor immersed in a suitable reaction environment. Photons of sufficient energy promote electrons from the valence band (VB) to the conduction band (CB); this creates electron-hole pairs (e À -h + ). Then, holes and electrons can be separated, reach the surface of the semiconductor and be captured by reactants in the environment. Holes can be used to oxidize suitable species while electrons cause reduction reactions. 91 Photocatalysis has gained much attention in recent decades, mainly in view of using solar energy for degradation of pollutants, and generation of energy carriers such as hydrogen gas 96 and hydrocarbons. 97 The most relevant TiO 2 -mediated photocatalytic reactions are: HOc radical generation Hydrocarbons mineralization In particular, the photocatalytic generation of H 2 from renewable sources (e.g., water or water-alcohol mixtures) is of high interest. 98 For this, the most-used semiconductor is TiO 2 . The reason is that titania, particularly in its anatase polymorph, offers a number of benecial features such as an adequate alignment of its CB energy relative to the electrochemical potential of H 2 generation from water, an outstanding (photo-) chemical stability, low cost and large availability. 99 In titania, under open circuit conditions, aer photoexcitation and charge carrier separation, electrons and holes can generate cathodic and anodic sites at different parts of the TiO 2 surface. The cathodic sites are directly responsible for reduction reactions. In the case of the water splitting reaction, CB electrons can reduce water at these cathodic sites and form H 2 as outlined in reaction (2). On the other hand, reactions (3)-(5) take place at the anodic sites and are mediated by VB holes.
However, the transfer of charge carriers from VB and CB to the redox species in the environment is kinetically hindered. [90][91][92][93]95,100 Thereby, charge carrier can recombine (heat is released) resulting in loss of photo efficiency. Organics, such as alcohols (e.g., methanol, ethanol), are commonly added to the reaction environment since they can be easily oxidized by VB holes (as so-called sacricial agents reacting eventually towards CO 2 ). [101][102][103] This reduces the charge recombination, and therefore improves the lifetime of CB electrons and thus leads to an enhanced H 2 evolution. 91,104 More importantly, to reach reasonable H 2 generation rates the surface of TiO 2 needs to be modied by depositing small amounts of suitable charge transfer cocatalysts. Typical cocatalysts for TiO 2 are noble metals such as Au, Pt and Pd. 93,105 The noble metal particle at the TiO 2 surface forms a Schottky-type junction that increases dramatically the overall photocatalytic efficiency by trapping the conduction band electrons (this limits charge recombination), and by mediating their transfer to the environment, e.g., H 2 O. Additionally, some noble metals such as Pt aid the recombination reaction of H 0 atoms to H 2 gas (cocatalysis). 99 Most commonly, TiO 2 photocatalysts are based on nanoparticle slurries or compacted nanoparticle layers, and are decorated by noble metal particles using colloidal solutions or by (photo-)reduction from metal ion solutions. 106,107 Owing to the nature of these methods, the metal NPs are decorated at the TiO 2 surface in a fairly inhomogeneous way, i.e., a site-unspe-cic manner. In the following sections we outline efforts during the past three years to use noble metal dewetting principles on TiO 2 nanotube arrays such as in Fig. 1d-f specically to design novel photocatalytic platforms.

Factors to optimize the photocatalytic efficiencies
The TiO 2 surface geometry in the form of highly regular TiO 2 nanocavities is not only key for controlled dewetting of the cocatalytic noble metal into NPs of desired size, density and placement, but can also offer an adjustable and in this case ideal "reaction vessel geometry" for UV-based photocatalysis and for reactions (2)-(5) as discussed above.
As outlined in Fig. 7a, the depth of the cavity (i.e., the length of the tube sidewalls) is in the order of the UV light penetration depth in titania. The thickness of the TiO 2 tube walls is $10-20 nm, which is thus comparable to the solid-state diffusion length of holes, and allows for their efficient transfer to the environment. The virtual volume of the reaction phase in each cavity (with an inner diameter of $80-100 nm) matches well the typical diffusion lengths of generated HOc radicals in the liquid phase. 16 On these nanotubes the sputtering-dewetting conditions can be adjusted (see below) to obtain also other M/TiO 2 congurations, as illustrated in Fig. 7b and c. Fig. 7b illustrates the results of dewetting very thin noble metal lms (with a nominal thickness of 1-2 nm) on the surface of the nanocavities. This leads to a low NP loading on TiO 2 , which allows for a maximized free TiO 2 surface and light absorption by the semiconductor (the shadowing effect ascribed to the cocatalyst decoration is negligible).
Dewetting of thicker conformal metal lms may on the one hand limit the free TiO 2 surface (necessary for hole transfer to the environment) and also the photon ux to the semiconductor (owing to the shadowing effect), but on the other hand may provide the required density of M/TiO 2 junctions at the TiO 2 surface for efficient electron trapping and transfer to the environment.
From a metal/semiconductor junction viewpoint, the principle is that the particle spacing (i.e., decoration density) needs to be adjusted to an optimum value so that the width of space charge layer (W) induced by neighbouring M NPs overlap with each other (see the model in Fig. 7b). 108,109 W is dened as: where 3 denotes the dielectric constant, 3 0 the vacuum permittivity, q the charge of the electron, N d the donor concentration (for an n-type semiconductor), U the applied potential, U  the at-band potential, k the Boltzmann constant, and T the absolute temperature. For annealed TiO 2 nanotubes, and other anodic anatase layers, values of 3 $ 20-80 and N d $ 5 Â 10 18 to 5 Â 10 19 cm À3 are typically reported. U s is the difference between the at-band potential of TiO 2 and the work function of Au, i.e., U s $ 0.7 V. 110 With typical values of TiO 2 , W is in the order of $15-30 nm. 111,112  From a practical point of view, the most effective M cocatalyst decoration, that can be obtained by controlled metal sputtering-dewetting (and other self-ordering tools illustrated below), must be then attributed to a minimized noble metal amount that provides at the same time an optimum of key geometrical and thus electronic features of the semiconductor, namely, the ratio between free TiO 2 surface and the area coated with cocatalyst NPs (both necessary for hole and electron transfer, respectively), light harvesting vs. shadowing effect, and the density of induced Schottky junctions. [111][112][113][114] The metal decoration can also be adjusted to deposit (by shallow-angle sputtering) the cocatalyst NPs only at the mouth of the tubes (illustrated in Section 5.5). As sketched in Fig. 7c, the site-selective decoration can induce a gradient of the semiconductor Fermi level (E F ) in the tube walls along the length of each TiO 2 cavity.
The absorption depth (into TiO 2 nanotubes) of light with an energy in the band-gap region of anatase is a few mm, 115,116 and anatase tubes provide an electron diffusion length in the range of several 10 mm. 117 As in a classic photocatalytic conguration the tube mouths (tube/environment interface) are directly irradiated, the site-specic noble metal deposition at the upper part of the nanocavities can be the most efficient geometry. An electron harvesting (tube bottom)/charge-transfer activity (tube top) combination can be established to signicantly contribute to an overall H 2 evolution enhancement: the transfer of electrons that are generated in the tube bottom towards the metal/ TiO 2 coupled zone (photocatalytically active zone) may be facilitated by the fact that a benecial electronic junction is formed (i.e., a gradient of E F along the TiO 2 tube walls). 118 Nevertheless, this conguration provides also direct light irradiation of the TiO 2 /M/environment interface, and charge carriers formed in its close proximity can thus be effectively transferred to reactants. 119-122

Orderly-dewetted Au nanoparticles/TiO 2 nanocavities
A combination of particular interest for photocatalytic H 2 generation is Au/TiO 2 . Au, compared to Pd and Pt, has a lower melting point ($1064, 1555 and 1768 C, respectively). Temperatures as low as 400-450 C provide the required activation energy for Au surface diffusion so that Au crystals can grow through mass transport 123 and Au lms dewet into equilibrium structures.
An additional advantage is that Au lms do not react with oxygen, that is, they can be dewetted in air. 6,46 This is important for two aspects: (i) the dewetted particles maintain their metallic state (Au 0 ), which is essential to form effective Au/TiO 2 Schottky junctions; 124 (ii) a thermal treatment at 450 C (in air) not only leads to Au dewetting but also converts the as-formed amorphous anodic nanocavities into crystalline TiO 2 composed of mainly anatase (i.e., the most photo-active titania polymorph 125,126 ).
A simple way to control the size/density of the dewetted Au NPs and their self-ordering degree is to adjust the deposited Aulayer thickness t Au relative to the topographical features of the nanocavities (in general, the amount of deposited noble metal is expressed as "nominal thickness"). Additionally, the sputtering conguration can be calibrated with respect to the geometry of the cavity.
As shown in Fig. 8a for relatively thin sputtered lms (<10 nm), the metal can be deposited mainly at the rims and bottoms of the cavities with a sputtering direction normal to the periodic TiO 2 surface (when avoiding rotation or tilting of the substrate). In this case the as-deposited Au coating is not continuous as almost no metal is found along the inner sidewalls of the cavities. Fig. 8b-f shows the different geometries that can be obtained from 0.5-50 nm-thick Au lms. 16,127 Films with t Au $ 0.5-1 nm already partially dewet in the as-deposited state, i.e., without annealing, forming discontinuous lms with nm-sized cracks (T dewet is possibly < room temperature). However, a clear change of morphology is observed aer the thermal treatment. Films with t Au $ 0.5 dewet forming Au NPs that are round in shape and with average diameter of 2 nm (the NP size/distribution is homogeneous throughout the TiO 2 surface - Fig. 8b).
Thicker Au lms (with t Au up to $10 nm) show a clear interaction with the periodic titania substrate. Au layers of 2-3 nm split into circular arrangements of $5 nm-sized NPs that decorate the rim of the nanocavities (Fig. 8c). Dewetted 5 nmthick Au lms (Fig. 8d) form $5-6 nm NPs that are arranged in a hexagonal network (mirroring the hexagonal-packing of the TiO 2 cavities). Each Au NP is located atop the cavity triple-point, i.e., where the sidewalls are shared by three adjacent cavities (Fig. 6b). This strong metal-substrate interaction occurs because not only do the edge of the rims provide a positive excess of chemical potential, 24 but also because t Au (5 nm) is comparable in size to the width of the cavity sidewalls (10 nm). 68 A different result is observed at the bottom of the TiO 2 cavities. These locations provide a smooth surface with low curvature and no sharp edges. As a consequence Au layers with t Au < 5 nm are more likely to dewet as on a at ideal surface, and the NP size and density depends only on the Au initial thickness (as reported for smooth substrates 3,68 ).
When the Au layers are thicker, one observes an inversion of this trend. For t Au $ 10 nm (Fig. 8a and e), the Au lms dewetted at the top of the rims with a loss of self-ordering degree. The hexagonal arrangement is lost, the NP spacing is inhomogeneous and their size distribution becomes broad. The situation is opposite in the nanocavities. 10 nm-thick Au lms split in 3-4 particles of uniform size (10-12 nm) that are conned closed to each other at the very bottom of the cavity.
A remarkable result is found for 20 nm-thick Au lms which provide ideal conditions for maximized self-ordering, leading to arrays of $50 nm-sized single Au cocatalytic NPs per each photocatalytic TiO 2 cavity (Fig. 1f and 6c). 16 The fabrication process is highly reliable and the arrays are lled with almost 100% success rate over large surfaces (some cm 2 ).
In line with the concept outlined in Section 4, this result is ascribed to the synergistic interplay between the geometry of both the TiO 2 cavities and Au lm. 24,63 In spite of the orthogonal sputtering conguration, as-deposited 20 nm-thick Au lms coat the periodic substrate virtually in a conformal way (Fig. 8a) the lm is continuous along the TiO 2 surface (with only small uctuations in its thickness) which is key to controllable dewetting. Initially, the sidewalls of the cavities act as pre-dened locations for the rupture of the metal lm, and the TiO 2 rims are exposed to the ambient. 65,67 Then, Au dewetting proceeds independently in each cavity and the negative excess of chemical potential causes complete Au retraction from the sidewalls towards the very bottom of the cavity. 24 The effect of the highly-ordered TiO 2 surface is remarkable and can be assessed comparing such a result to compact anodic TiO 2 lms loaded with a similar amount of Au and dewetted accordingly (Fig. 6e). Notably, the Au NPs embedded in the periodic TiO 2 substrate are not only much smaller than those on a at surface but are also monodisperse in size (sharp size distribution), and their spacing (particle-to-particle distance) is one order of magnitude smaller than on a at substrate.
This means that this approach can provide Au NPs at the TiO 2 surface with a fully tuneable decoration density (typically much higher than obtained on smooth TiO 2 ). The ne control over the Au/TiO 2 structures is a key prerequisite for their use not only as an efficient photocatalyst (see below) but potentially also as functional electrodes, high-density memory devices, plasmonic platforms and SERS-based sensors.
Au lms thicker than >20 nm agglomerate in a random fashion and with large size distribution (from few nm to few mm as shown in Fig. 8a and f). These results are, in terms of particle size/spacing distribution, similar to those observed for Au dewetted on at surfaces. In line with these ndings, 24 the loss of self-organization is ascribed to the excessive metal initial thickness relative to the topography features of the substrate. 67 The photocatalytic efficiency of the Au/TiO 2 systems was explored in terms of H 2 generation from water-ethanol mixtures under monochromatic UV light irradiation (325 nm). 16,127 The highest hydrogen efficiency (in terms of H 2 evolution rate r H 2 ) is found for 2 nm-thick Au lm dewetted on the TiO 2 nanocavities (Fig. 8g). These arrays lead to a r H 2 of $6.3 mL h À1 that is more than 10 times higher than that of Au/TiO 2 structures formed on a at anodic oxide, and ca. 2 times higher than that of a similar sample that was not subjected to the dewetting step 120the latter result conrms the contribution of dewetting to the photocatalytic enhancement.
Worth noting, for dewetted Au on compact oxide the H 2 generation increases with increasing the Au loading (Fig. 8gstriped columns). Conversely, a remarkably lower amount of Au is required on the TiO 2 tube arrays for maximizing the photoactivity. Interestingly, Au lms which are either thinner or thicker than 2 nm lead (on the tubes) to a dramatic reduction of the H 2 evolution rate, in line with discussion in Section 5.3.
Moreover, repeated photocatalytic runs and photocurrent measurements under external bias-free conditions showed the Au/TiO 2 systems to be highly stable, and neither signicant poisoning nor deterioration of the cocatalyst/catalyst took place with their prolonged use. 16

Adding dealloying to form nanoporous Au/TiO 2 nanocavities
An approach to maximize the cocatalyst specic area is dealloying, i.e., to maximize the Au/environment interface. This can of course be used in the context of dewetted particles on TiO 2 tubes too by suitable dewetting (and then dealloying) a cocatalyst/sacricial metal combination on the TiO 2 nanocavities ( Fig. 9a and b).
Dealloying is widely explored as a nanoscale processing tool to fabricate ultra-high surface area metals for various applications (catalysis, sensing, optical applications). As such, it consists of the selective dissolution of the more (electro-) chemically active element of a single-phase alloy. Typically it leads to the formation of a nanoporous continuous metal sponge that can be almost entirely composed of the more noble element. As a consequence, the activity of the metal per loaded mass can be dramatically improved. [128][129][130][131] Key parameters are the composition and structure of the initial metal alloy. A simple sequential sputtering approach of two (or more) metals combined with thermal dewetting is an efficient approach to form an alloy at the surface of the TiO 2 substrate. The sputtered metals can be selected so that one is less noble than the other(s). The thermal treatment then not only forms the metal alloy precursor of desired composition necessary for the subsequent dealloying step, but also splits the metal alloy into ne NPs of controllable size and distribution. 42 A suitable alloy for this purpose is Au-Ag. The relatively low melting point of these two metals (Ag melts at $962 C) is a crucial advantage since a single optimized thermal treatment at $400 C leads simultaneously to Au/Ag alloying and dewetting. Moreover, Ag is less noble than Au and can be selectively dissolved. Additionally, Au and Ag form alloys in any composition, i.e., with no miscibility gap. These are key prerequisites in the dealloying step to achieve controllable porosication of the metal cocatalyst. 132,133 In the case of pure Au, a 20 nm-thick sputtered lm is found to dewet into arrays of single-Au-NP-per-cavity ( Fig. 1f and 6c). As proposed by Giermann et al., 25 one of the conditions for maximized ordering during dewetting is that the capacity of each cavity of the periodic substrate matches the volume of metal deposited at the surface of (within) the cavity.
Remarkably, when an adequate metal loading is deposited over the TiO 2 surface (a 10 nm-thick Au lm followed by deposition of an additional 20 nm-thick Ag lm), the deposited double-metal layer (having an overall nominal thickness of 30 nm) is found to dewet accordingly (in Ar, 400 C), and split with $100% success rate into a single 50-60 nm-sized alloyed Au-Ag NP in each cavity (Fig. 9a).
However, for this combination of metals on TiO 2 (compared to the case of pure Au/TiO 2 ) the thermal treatment must be optimized. A most efficient solution is a sequential annealing approach. Firstly, the pristine nanocavity layer is annealed in air (450 C)the presence of oxygen in the annealing atmosphere leads to oxide crystallization into anatase TiO 2 with minor content of rutile and low density of oxygen vacancies. [134][135][136] Then, to effectively form an alloy that can be orderly dewetteddealloyed, a thermal treatment in an inert atmosphere (Ar) is necessary.
Both dewetting and alloying occur through a mass transport mechanism and thus a certain surface mobility of both Au and Ag atoms is needed (this is granted by the inert atmosphere). In contrast, when annealing in air Ag/Au lms on a regular TiO 2 surface, Au agglomerates into particles (segregates) while Ag is le behind in the form of irregular strands/patches. This negatively affects the result of dealloying (no porosication) and can limit the photocatalytic performance of the metal/oxide systemswe illustrate below how the porosication of Au impacts the photocatalytic enhancement.
On the other hand, Au-Ag NPs that are alloyed-dewetted by argon-annealing into the TiO 2 cavities can be successful dealloyed by an adequate etchant (e.g., concentrated HNO 3 ). This leads to the highly-dened nanoporous Au/TiO 2 assemblies as shown in Fig. 9b. 132,133,137 The importance of adequate dewetting conditions is remarkable since no porosication is expected to take place for Au-Ag lms that are not properly alloyed. Au-Ag lms deposited on TiO 2 surfaces were subjected to dealloying either without any preliminary alloying-dewetting step or aer air-annealing. In the rst case, the result of etching is metal patches of irregular shape that are randomly distributed over the oxide surface. This structure is formed since no dewetting took place but only Ag dissolution (Fig. 9c). In the second case, single Au NPs per cavity are formed (note however that the success rate is lower compared to argon-dewetting). Here, Au underwent ordered dewetting (in line with our results using pure Au lms) and the Ag that was le behind was then dissolved by the etchant (Fig. 9d). Most importantly, neither the rst nor the second Au structure shows porosication, this conrming the importance of using a proper sequence of alloying-dewetting-dealloying.
A comparison of the insets in Fig. 9a and b shows that each single alloyed Au-Ag deposit turns into a porous particle. The pores are few nm in diameter and the average size of each particle is somewhat retained, i.e., $50-60 nm. A key for this is that Au and Ag elements are homogeneously alloyed and orderly dewetted, as conrmed by the XPS data in Fig. 9e and f. The shi of Au and Ag XPS peaks aer dewetting and dealloying conrms that both these steps lead to signicant change of the chemical surroundings in the metal NPs. The Au XPS peaks of the nanoporous Au/TiO 2 nanotube arrays show a binding energy of 83.9 eV that is similar to that reported in the literature for similar systems, i.e., dealloyed porous Au. 129 Differently, the same process carried out on a at oxide surface leads to irregular metal patches that can be as large as several hundreds of nm (Fig. 9g). For these large patches the dealloying step leads only to some surface pores and thus the increase of metal specic surface area is almost negligible.
In practice, this alloying-dewetting-dealloying approach has great potential to fabricate nanoporous metal or alloy/TiO 2 structures, the composition and geometry of which can be tuned by a simple bulk processes, e.g., by adjusting metal loading, relative amount and deposition sequence, sputtering conguration, temperature of dealloying and its duration. 120 Various Au/TiO 2 assemblies were explored in view of their photocatalytic H 2 evolution ability from water-ethanol mixtures under 325 nm UV light irradiation (in Fig. 10a and b). The results show that the key parameters for efficient H 2 generation (i.e., maximized r H 2 ) are the metal (Ag and Ag) loading, their relative amounts and deposition sequence.
A 1 nm-thick Au lm (along with a 2 nm-thick Ag lm) leads, through dewetting-dealloying, to porous Au/TiO 2 showing a high photocatalytic performance (r H 2 ) owing to an optimized catalyst structure in terms of the (adequate) surface density of metal/oxide junctions and the (minimized) oxide shading effect. However, a remarkable contribution to the photocatalytic enhancement is provided by the dealloying step. Au poros-ication takes place even on particularly small metal particles ($5-10 nm in size - Fig. 10c). The H 2 generation rate of a dewetted-dealloyed sample is almost doubled (r H 2 $ 7.5 mL h À1 - Fig. 10d) compared to Au/TiO 2 layers formed from pure Au lms under otherwise identical conditions (r H 2 $ 4.0 mL h À1 ). 127 Moreover, not only the metal loading but also its placement on the tubes can be adjusted by an adequate sputtering conguration. Metal layers that are deposited by a classic sputtering geometry (sputtering direction normal to the tube arrays) and then alloyed-dewetted-dealloyed form Au porous NPs either in a mixed or full crown or ground position. Whether the Au NP placement is at ground or crown position depends on the nominal metal loading (sketched in Fig. 8a), and a clearly lower r H 2 is obtained for mixed ground/crown position compared to the only crown position (Fig. 10b), which is well in line with the concepts outlined in Section 5.3.
However, the metal lms can also be deposited site-specically. For this, the tube layer substrates can be placed parallel to the direction of sputtering (shallow angle sputtering), in order to deposit the metal (Au/Ag) lm only on the crown position (i.e., the very top of the tubes)then dewetting and dealloying steps follow that are carried out in otherwise identical conditions, and form dewetted-dealloyed porous Au NPs exclusively at the crown position (SEM images in Fig. 10b).
A side-effect of sputtering at a shallow angle is that the amount of Au that is actually deposited on the tubes is less than when sputtered in a normal conguration. For a nominally 5 nm-thick Au lm sputtered at a shallow angle, a loading of $0.10 mg cm À2 was measured. For comparison, 1 and 2 nmthick Au lms deposited by normal angle sputtering lead to loadings of Au NPs of $0.07 and 0.13 mg cm À2 (mixed crown/ ground position). The photocatalytic data in Fig. 10b clearly illustrate that the sample with crown only decoration and fabricated by shallow angle sputtering (labelled as "5*") delivers the largest amount of H 2 . These results demonstrate the importance of a proper "positioning" of a catalytic particle if one targets the use of a minimal Au amount for achieving a maximum photocatalytic H 2 generation performance.

Pt/TiO 2 nanocavitiesoptimizing dewetting and oxide crystallization
The benecial effect of Au in the photocatalytic H 2 evolution is mainly ascribed to its ability to capture electrons from the CB of TiO 2 and mediate their transfer to the reaction phase. For this reaction Pt is an even more efficient cocatalyst since it not only acts (as Au) as an "electron sink" but also can promote the Fig. 10 (a, b and d) Photocatalytic H 2 evolution rate (r H 2 , 5 h-long runs) measured for (a) different Ag/Au ratios (all samples were prepared by sputter-coating 5 nm-thick Au films and different amounts of Ag, followed by dewetting and a 2 h-long dealloying step at 15 C), (b) different amounts of Au (nm) with constant Ag/Au ratio of 2 : 1 (all samples were dewetted and then subjected to a 2 h-long dealloying step at 15 C -SEM images are relative to the samples labelled in (b) as "5*" and "2"), and (d) different Au/TiO 2 photocatalysts prepared by depositing a 1 nm-thick layer of Au (the plot highlights that an optimized combination of dewetting-dealloying can lead to a $4 times increase of the H 2 generation rate); (c) SEM and TEM (inset) images of TiO 2 NTs decorated with porous Au-Ag NPs (by dewetting-dealloying of 1 nm-thick Au and 2 nm-thick Ag films). Fig. (a-d) are reproduced with permission from ref. 120. recombination of H 0 surface species to H 2 . 138 This is the reason why Pt is frequently found to be more active than other noble metals (under comparable deposition conditions). 105 Pt has a higher melting point (1768 C) compared to Au (1064 C), meaning that higher temperatures are required to reach sufficient surface diffusion for Pt and dewetting. 3 Dewetting of Pt on TiO 2 surfaces can be observed at temperatures > 500 C; 119,121,139 in this case the thermal treatment needs to be carried out under inert conditions (or in a reductive gas, as also reported in the literature 17,140 ). If the treatment is carried out in oxygen-containing atmospheres Pt can dewet partially due to the possible reaction of Pt metal with oxygen that leads to the formation of surface platinum oxide 139this may limit surface diffusion and hinder dened dewetting. 45 In pure nitrogen Pt can be dewetted (Fig. 11a-e). Fig. 11a shows the typical result of sputter-coating the TiO 2 nanocavity substrate with a 5 nm-thick Pt lm. In line with the results using Au (Fig. 8), the as-sputtered Pt lm is found to coat preferentially the top of the sidewalls, and its thickness gradually decreases towards the bottom of the rims (as clear from the contrast in the SEM image). When these lms are dewetted (in N 2 at 600 C), globular Pt NPs are formed that decorate the sidewalls and top of the tubes.
In line with the theory of dewetting, that is that particle size and spacing scale with the initial metal lm thickness, 3 Pt lms of 2, 5, 7 and 10 nm in thickness form by dewetting NPs with average size of $5-20, 5-30, 10-40 and 15-50 nm, respectively (Fig. 11b-e). The smaller NPs are typically round in shape, show narrow size distribution, and are ordered at the tube tops in a hexagonal arrangement (Fig. 11c). Thicker metal lms split into irregular Pt islands that are several tens of nm large (Fig. 11e).
Nevertheless, the photocatalytic efficiency (r H 2 ) of these structures is signicantly lower than expected. This is clear from the photocatalytic data in Fig. 11f where the H 2 generation rate of this sample is compared with that of a tube layer that was coated by an identical Pt lm (5 nm) and then subjected to annealing in air at 450 C (no dewetting). The latter structure leads to a 3 times higher H 2 evolution rate. Please note also that tube layers rstly annealed in N 2 at 600 C and then decorated with Pt (5 nm) lead to a negligible H 2 evolution. The reason is the different crystallographic features of TiO 2 NTs annealed under various conditions. Particularly, annealing treatments were found to greatly affect the degree of crystallinity of tubes and the relative amount of formed anatase to rutile with respect to the total amount of crystalline TiO 2 (the XRD data and renement method are discussed in ref. 139). Specifically, it was found that annealing in N 2 forms an oxide with high degree of crystallinity ($30 wt%) but relatively low anatase content ($23%), and it also generates oxygen vacancies in the oxide (and a consequent photoactivity decay due to charge carrier trapping/recombination in the semiconductor 134,141 ).
The solution to this problem is a multiple-step annealing rstly in N 2 at 600 C (dewetting) and then in air at 450 C that leads to both high anatase relative content ($30%) and degree of crystallization ($29 wt%). Additionally, the XRD patterns of the structures subjected to multiple annealing (shown in ref. 139) show the characteristic reections of Pt that can be ascribed to Pt grain growth during dewetting.
This example illustrates typical considerations when designing a dewetting experiment for functional use (photocatalysis). Here the two concepts, namely, the optimized Pt dewetting and oxide crystallization can be benecially combined. The result is dened Pt NP-decorated TiO 2 tube arrays where the oxide shows both a high degree of crystallization and a high relative content of anatase phase, which are necessary for photocatalytic enhancement. Also remarkable is the comparison of these photocatalytic data with the results on Au dewetting (outlined above): by using a similar sputteringdewetting strategy, Au-modied TiO 2 tube layers lead to a maximized photocatalytic activity of $6-7 mL H 2 h À1 , 16,120,127 while the Pt/TiO 2 structures lead to a $3 times higher H 2 evolution rate, i.e., $20 mL h À1 .
The air-crystallization step was further explored exposing dewetted layers to air annealing at various T (350-550 C range), conrming that annealing at 450 C is, in the view of photocatalytic applications, the most optimized condition. Rened XRD data show for these samples that: (i) air annealing at 350 C does not form anatase TiO 2 but only rutile ($26 wt%); (ii) air annealing at 550 C forms anatase and leads to a high degree of crystallinity of the oxide (the total content of crystalline TiO 2 is 64.5 wt%), but it causes also the formation of a large amount of rutile ($52 wt%). 142 The formation at 550 C of relatively large amounts of rutile can be due to the thermal oxidation of the Ti metal substrate. In line with previous works, this occurs rstly by rutile formation at the Ti/TiO 2 interface and then it proceeds (with higher annealing temperatures and/or longer thermal treatments) up the tube walls and toward the tube tops. [143][144][145][146] The SEM cross-sectional images in Fig. 12a-d further conrm that rutile forms (from the Ti metal substrate) as a layer of some hundreds of nm underneath the anodic tube layer. The air treatment at 450 C leads to a $150 nm thick rutile lm (Fig. 12b), which is $3 times thinner than that formed at 550 C (i.e., $500 nm - Fig. 12d). Therefore, the absence of anatase phase in the layers treated at 350 C and the predominant content of rutile in the oxides crystallized at 550 C are the most plausible reasons for their low H 2 generation yield.
The thermal treatment in O 2 -containing atmospheres affects not only the crystallinity of the oxide but also the oxidation state of Pt, 147 and therefore the overall photocatalytic efficiency. 124,148 In fact, Pt/TiO 2 samples that were annealed aer dewetting in air at too high T (>450 C) or in pure O 2 at 450 C lead to a poor H 2 evolution efficiency (Fig. 12e).
XPS data ( Fig. 13a and b) show that aer air annealing, the noble metal at the oxide surface is present as metallic Pt (i.e., Pt 0 ). 148,149 On the contrary, for Pt/TiO 2 structures annealed in pure O 2 a broad shoulder (at $76-80 eV) appears that can be attributed to the formation of PtO (PtII) and PtO 2 (PtIV). 148 The formation of platinum oxide is clearer when exposing tube layers coated by a relatively thick (25 nm) Pt lm to O 2 annealing for 5 h. An even more pronounced shoulder can be seen that is in line with larger amount of formed Pt oxides -tting of this data reveals a good match with the Pt 4f reference signals of PtII and PtIV oxides. 150,151 Thus, the low H 2 generation efficiency of Pt/TiO 2 structures annealed in pure O 2 can be ascribed to the formation of Pt oxides that can limit the ability of the cocatalyst in electron trapping and transfer. 124,[150][151][152] Moreover, the formation of Pt oxide can explain why Pt lms were found not to dewet by annealing in air or oxygen, that is, the formation of surface oxide can reduce Pt surface diffusion. 45 The effect of the amount of Pt cocatalyst on the H 2 evolution efficiency was also explored. The photocatalytic results, as observed in the case of sputter-dewetted Au/TiO 2 layers, 16,120,127 show a clear enhancement of the H 2 generation when the amount of cocatalyst is increased up to a certain amount (in this case 5 nm), while a larger amount of cocatalyst (Pt lm thickness $ 7-15 nm) leads to a signicantly lower photocatalytic activity (Fig. 13c). It is evident that the trend of photocatalytic results is ascribed to an optimum of critical factors such as the density of induced Pt/TiO 2 Schottky junctions, the free TiO 2 surface vs. the TiO 2 area coated with Pt NPs, and the light harvesting vs. shadowing effect.

Summary and outlook
Electrochemically-grown, highly periodic oxide structures, particularly self-organized anodic TiO 2 nanotube arrays, are Fig. 13 (a, b) XPS data: (a) high-resolution Pt 4f XPS spectra of Pt (reference) and of TiO 2 nanotube layers decorated with sputtercoated Pt films (5 and 25 nm-thick) and exposed to different thermal treatments in N 2 , air and O 2 (the arrows indicate the shoulder ascribed to Pt(II) and Pt(IV) oxides); (b) Pt 4f high-resolution XPS spectrum (experimental data) of a TiO 2 nanotube sample sputter-coated with a 25 nm-thick Pt film and then treated at 600 C in N 2 and 450 C in O 2 (the plot shows also the fitting curve and deconvoluted doublets accounting for Pt 0 , PtO ads , Pt(II), and Pt(IV) oxides). (c) Photocatalytic H 2 generation rate (r H 2 ) of Pt/TiO 2 nanotube layers formed by sputteringdewetting of various Pt loadings. Fig. (a-c) were reprinted (adapted) with permission from ref. 139 ideal surfaces for template-guided solid-state dewetting of thin metal lms. By this approach, dened metal/oxide assemblies can be formed with nanoscale precision that have advanced functionalities, namely for photocatalysis and green hydrogen generation, ascribed not only to the inherent physico-chemical features of TiO 2 but also to intimate metal-oxide interaction (metal-semiconductor coupling) and to their synergisticallyachieved "double" self-ordering nature.
In this perspective we discussed the possibilities, limitations and solutions of using an ensemble of multilevel self-ordering principles to reach hierarchical nanoscopic designs. The approach is not only low cost but also scalable, and with high throughput, being completely based on self-ordering processes.
In the frame of photocatalytic applications, owing to the high cost of noble metal cocatalyst, a key challenge is to limit the use of Au and Pt cocatalysts. Dewetting work nicely demonstrates that the ideal conditions for an efficient photocatalytic process are not established by the use of a specic amount of noble metal, but rather by how effective is the cocatalyst/semiconductor junction design in order to satisfy a set of critical factors, such as the density of M/oxide junctions, free-vs. shaded-TiO 2 surface, light harvesting vs. shadowing effects, and the M/ and TiO 2 /environment interface.
In other words, more than the cocatalyst amount as such, critical factors that need to be optimized for reasonable photocatalytic efficiency are the cocatalyst geometry and metal/TiO 2 design, the control of which is only poor when using common photocatalyst syntheses and cocatalyst deposition methods.
There is still an enormous potential regarding the tailoring of TiO 2 nanotube geometry, structure, wettability and doping, and the metal dewetting process will even provide a higher degree of designed functionalities. For example, additional selfordering processes (spinodal decomposition, site-selective functionalization) and post-treatments (annealing in reactive atmosphere) can be interlaced to form more complex hierarchical assemblies (such as core-shell and nano-sponge structures, oxide-metal-molecular complexes).