Femtosecond laser ultrafast photothermal exsolution

Exsolution, as an effective approach to constructing particle-decorated interfaces, is still challenging to yield interfacial films rather than isolated particles. Inspired by in vivo near-infrared laser photothermal therapy, using 3 mol% Y2O3 stabilized tetragonal zirconia polycrystals (3Y-TZP) as host oxide matrix and iron-oxide (Fe3O4/γ-Fe2O3/α-Fe2O3) materials as photothermal modulator and exsolution resource, femtosecond laser ultrafast exsolution approach is presented enabling to conquer this challenge. The key is to trigger photothermal annealing behavior via femtosecond laser ablation to initialize phase transition from monoclinic zirconia (m-ZrO2) to tetragonal zirconia (t-ZrO2) and induce t-ZrO2 columnar crystal growth. Fe-ions rapidly segregate along grain boundaries and diffuse towards the outmost surface, and become ‘frozen’, highlighting the potential to use photothermal materials and ultrafast heating/quenching behaviors of femtosecond laser ablation for interfacial exsolution. Triggering interfacial iron-oxide coloring exsolution is composition and concentration dependent. Photothermal materials themselves and corresponding photothermal transition capacity play a crucial role, initializing at 2 wt%, 3 wt%, and 5 wt% for Fe3O4/γ-Fe2O3/α-Fe2O3 doped 3Y-TZP samples. Due to different photothermal effects, exsolution states of ablated 5 wt% Fe3O4/γ-Fe2O3/α-Fe2O3-doped 3Y-TZP samples are totally different, with whole coverage, exhaustion (ablated away) and partial exsolution (rich in the grain boundaries in subsurface), respectively. Femtosecond laser ultrafast photothermal exsolution is uniquely featured by up to now the deepest microscale (10 μm from 5 wt%-Fe3O4-3Y-TZP sample) Fe-elemental deficient layer for exsolution and the whole coverage of exsolved materials rather than the formation of isolated exsolved particles by other methods. It is believed that this novel exsolution method may pave a good way to modulate interfacial properties for extensive applications in the fields of biology, optics/photonics, energy, catalysis, environment, etc.

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Introduction
Exsolution refers to a phenomenon that metal or metal/oxide or pure oxide particles exsolve from their bulk oxide hosts to uniformly distribute on their surfaces via a series of physical/chemical processes (e.g.ion diffusion, reduction, particle growth [1]), potentially to be used as functional interfaces for catalytic and energy applications [2].Exsolution is often triggered by furnace annealing/sintering in a reductive gas [3][4][5], also available by other methods such as ionic liquid-assisted voltage-driven [6], straindriven [7], electrical-potential-driven [8], quenching treatment [9], thermal shock [10], twice fiber laser treatment [11]), ion irradiation [12], plasma [13] irradiation, and intense pulsed light (IPL) momentary photothermal (>1800 • C within 20 ms [14,15]) treatment.New pathways for exsolution have been opened by non-contact high-energy light/beam irradiation.To this regard, pulsed lasers such as nanosecond laser with a heating rate of 10 5 • C•s −1 [16] and femtosecond (fs) laser with a heating rate of ∼10 14 • C•s −1 [17] are better choices.Fs lasers can also trigger ultrafast quenching at the rate of 10 12 K•s −1 ∼ 10 14 K•s −1 [18], shockwave formation, and high-temperature and high-pressure environment [19] promising for synergistic ultrafast-heating/quenching and thermalshock exsolution.Up to now, the highest exsolution density is 1700 Fe 0 particles•µm −2 exsolved from SrTi 0.65 Fe 0.35 O 3 film with merely a 2 nm deficient layer for exsolution [20].However, for most exsolution methods and exsolved materials, the density of exsolved particles is much lower, often in the order of tens of particles•µm −2 , seldomly in the order of hundreds of particles•µm −2 [21].The challenge lies in the shallow deficient layer for exsolution and the risk of particle growth and enlargement.
Generally, the higher the heating rate, the more difficult to record the exsolution dynamics to clarify exsolution mechanism, while the ultrafast quenching capacity of fs laser ablation may help to get some hints.Regarding oxide hosts, perovskites are predominant, with few attempts on hard-to-process refractory oxides such as 3 mol% Y 2 O 3 stabilized tetragonal zirconia polycrystals (3Y-TZP) [22,23].Exsolution and simultaneous reduction of Fe 3+ to Fe 2+ in N 2 /H 2 at 1 723 K for 2 h is more effective and efficient than direct sintering method in need of 30 wt% Fe 2 O 3 for zirconia darkening [24], which can be used as solar light absorbers [25].
Inspired by in vivo laser photothermal therapy (PTT) based on local temperature increase upon laser irradiation of injected photothermal materials [26] (e.g.iron oxide nanomaterials), we propose fs laser ultrafast photothermal exsolution method.Using 3Y-TZP ceramics as exsolution oxide hosts, three kinds of iron-oxide dopants, including Fe 3 O 4 , γ-Fe 2 O 3 , and α-Fe 2 O 3 , play the role of both photothermal enhancer and exsolution resource (figure 1(a)).Doping 2 wt%, 3 wt% and 5 wt% iron oxides is adopted to gradually enhance photothermal effect to arouse annealing behavior upon fs laser ablation (figure 1(b)).Surface morphologies of all original samples are shown in figure S1.Structural evolution (figure 1(c)) from quasi-periodic microgrooves (Sq = 1.859 µm) to much wider grooves (Sq = 3.424 µm), and finally to patched cracked-structures (Sq = 0.837 µm) offers obvious evidence for gradually enhanced thermal effect with increasing Fe 3 O 4 dopant weight percentage from 2 wt% to 5 wt%.Dopantdependent FeO x exsolution is unveiled, whose mechanism is discussed through analysis of structural morphology and composition/phase change.

Results
Cross-sectional energy dispersive spectrometer (EDS) elemental mapping of unablated/ablated 5 wt%-Fe 3 O 4 -3Y-TZP samples clearly demonstrates the exsolution of Fe-element on the top surface, while top-view EDS mapping indicates the whole coverage of Fe element on the surface (figure 1(d)).A 10 µm thick deficient layer is identified, up to now the deepest, an order of magnitude elevation compared with conventional exsolution methods [20].It merely took 2-3 min for 10 µm thick Fe 0 exsolution, much more efficient than 2 nmthick Fe-deficient layer exsolution from thin SrTi 0.65 Fe 0.35 O 3 film by one-hour annealing in vacuum (∼10 −5 Torr) at 800 • C [20].The experiment was performed in air, so this method is also advantageous for its environmental friendliness.Colordarkening/bluing occurs to all 5 wt% exsolved samples, similar to that of 2 hour 1400 • C-1500 • C sintered 1.5 mol% Fe 2 O 3 doped 3Y-TZP [27].Besides outmost surface exsolution, nanoscale exsolution also takes place in bulk, beneath the defect structures created by shockwaves (figures 1(d) and S2).Exsolution initialization is dopant dependent at 2 wt%, 3 wt%, and 5 wt% for Fe 3 O 4 /γ-Fe 2 O 3 /α-Fe 2 O 3 samples (figure 1(e)), respectively, due to their different photothermal transition capacities.Thermal annealing and exsolution simultaneously occur.Color change and formation of patched microstructures are main exsolution indicators while surface microparticles are additional indicators (figure S3).
X-ray photoelectron spectroscopy (XPS) and EDS elemental quantification (figures 2(a)-(c)) reveal a dramatic and relatively lower increase of Fe atomic percentage, respectively, indicating that Fe-element exsolves on the outmost surface but becomes deficient underneath.Theunissen et al reported the possibility of 30 mol% ∼ 34 mol% YO 1.5 surface enrichment after ⩾1000 • C heating treatment [28].In our case, it is speculated that rapid Y-segregation to the outmost surfaces and removal by laser ablation should occur.An apparent rise in oxygen element is witnessed for all ablated samples from EDS quantification, even including nonexsolved samples, so oxygen incorporation should be induced by laser ablation, preferring to aggregate in the defect regions created by shockwaves (figure S2).Deconvoluted XPS analysis of Zr/Y/O/Fe states gives some hints on what happens during fs laser ablation.For the 5 wt%-Fe 3 O 4 sample, Zr 3d band energy shifts to a higher value (figure 2(d)) due to oxygen incorporation [29], which trend is not observed for the 5 wt%-γ-Fe 2 O 3 sample where a shift towards a lower band energy is detected for Y 3d peak (figure 2(e)) due to formation of positively charged oxygen vacancies [30].The shift of both Zr 3d and Y 3d peaks is observed for ablated 5 wt% α-Fe 2 O 3 samples (figure 2(f)) but the shift amplitude (0.16 eV) is much lower than that of the singular peak shift of Fe 3 O 4 /γ-Fe 2 O 3 samples (figures 2(d) and (e)), indicating different surface chemistries aroused by interaction between fs laser and different iron oxides.Deconvolution of Fe 2p spectra shows that Fe 2+ /Fe 3+ coexist in all exsolved samples and dominant by Fe 2+ .Deconvolution of O 1s spectra indicates that fs laser induces great decrease in the density of oxygen vacancies, ruling out the possibility of oxygen vacancy enrichment for ZrO 2 darkening [31].It also indicates a large amount of oxygen vacancies exist in unablated samples, which can not only reduce the energy for elemental segregation [21], but also enable electron localization near Fe-cations to reduce Fe 3+ into Fe 2+ upon annealing [32].
Oxygen vacancy can induce activation of adsorbed oxygen molecules with the aid of Fe 2+ ions, resulting in the generation of Fe 3+ and reactive oxygen species (ROS) (such as O 2 − , O 2 2− , and O − ) and O 2− [33], which in turn, can release electrons for Fe 3+ /Fe 2 reduction.From thermodynamic perspective, most 3d transition metals including Fe are more prone to be reduced for exsolution at temperatures above 600 • C because of a negative value of Gibbs free energy change [34].Even if formed, pure Fe clusters or atoms will quickly become oxidized [35].Considering the dominance of Fe 2+ in XPS spectra for all ablated 5 wt% and 2 wt% samples (figures 2(d)-(f) and S4), Fe 3+ reduction into Fe 2+ reaction should take place.Dissociated oxygen molecules [36] and ROS will diffuse inside the bulk and aggregate in proximity to structural defects (figure S2) and may also react with segregated/exsolved Fe ions to form amorphous and crystalline iron-oxides.
FIB cutting and EDS mapping of 5 wt%-doped samples were performed to see exsolution states, as shown in figure 3.More details can be found in figures S5-S8.Fe mainly distributes on the outmost surface and in the subsurface structural boundaries of ablated 5 wt%-Fe 3 O 4 /α-Fe 2 O 3 samples, respectively, but almost totally disappears in the γ-Fe 2 O 3 sample (figure 3), which well explains why color appearances are different (figure 1(e)).In combination with O/Y/Zr mapping, it can be concluded that Fe exists in the form of FeO x .On the outmost surface of ablated 5 wt%-Fe 3 O 4 sample, exsolved FeO x is amorphous (as indicated by HRTEM images in figure S8 and broad peak at 675 cm −1 in Raman spectra shown in figure S9) but turns into crystalline Fe 3 O 4 at the bottom of the exsolution layer, forming t-ZrO 2 /FeO x interface (figure S8).FeO x distributes among gaps of ZrO 2 nanocrystals, on top of which an ejected ZrO 2 particle with an FeO x tail (figure 3(c)) is discovered, suggesting the occurrence of ultrafast quenching of splashed FeO x molten materials [37].The melting temperature of ZrO 2 is 2750 • C, much higher than those of Fe/Fe 3 O 4 (bulky: 1538 • C vs 1596 • C [21]), so the molten layer of iron-oxide can capture ZrO 2 particles in different shapes.Underneath the outmost surface, exsolved FeO x strips are still detectable along crystal boundaries, inferring that Fe ions exsolve in grain boundaries, then diffuse quickly upward to the outmost surface [38], and finally aggregate on the outmost surface.A small amount of exsolved FeO x is found among fine-grained regions (figures 3(b) and S10) because of their blocking effect [39].Except for the sporadic exsolved FeO x in subsurface grain boundaries (figure 3(f)), no distinct FeO x is found from the ablated 5 wt%-γ-Fe 2 O 3 sample (figures 3(d) and (e)), inferring the occurrence of exsolution but the exsolved layer should be ablated away.Large-scale FeO x network in the subsurface (figures 3(g) and (h)) indicates the ongoing occurrence of exsolution, already reaching the outmost surface (figure 3(i)).
XRD spectra shown in figures 4(a)-(c) reveal that fs laser annealing induces ubiquitous phase transition from m-ZrO 2 to t-ZrO 2 for all ablated samples, irrespective of iron-oxide dopant composition and percentage.Taking 5 wt%-Fe 3 O 4 sample for instance, unablated sample contains large t-ZrO 2 and small m-ZrO 2 nanocrystals (figure 4(d)).After laser ablation, equiaxed m-ZrO 2 grains almost disappear and instead, t-ZrO 2 columnar grains grow, inferring that thermal effect should be strong enough to make temperature in influential regions to be above the m-to-t-ZrO 2 phase transition point of 1170 • C [40].The grown t-ZrO 2 columnar grains correspond well to the Fe-deficient layer, inferring that exsolution is a spontaneous behavior during thermal-induced columnar growth of t-ZrO 2 (figure 5), highlighting the importance of phase change for exsolution [3].
Fe element mainly distributes in t-ZrO 2 crystals, seldomly in m-ZrO 2 crystals (figure 4(d)).Upon laser ablation and subsequent photothermal enhancement of FeOx dopants, m-to-t-ZrO 2 crystal transition first occurs, and then comes the crystal fusion towards large columnar grains (figure 4(e)), which is evident by the vertical and inclined columnar grains formed on ablated 3 wt%-γ-Fe 2 O 3 and 3 wt%-α-Fe 2 O 3 samples (figures 4(f) and S11).Since m-ZrO 2 and t-ZrO 2 crystals are randomly located, upon laser annealing treatment, some m-ZrO 2 crystals completely transform into t-ZrO 2 and then fuse with other nearby t-ZrO 2 crystals into columnar t-ZrO 2 grains while some others are not.For exsolution triggering, strong thermal effect is necessary, whose lifetime should be long enough for segregated Fe ions to sufficiently diffuse on the surfaces.Since crystal grows along the direction of maximum heat flow [41], the molten states can be either homogeneous or a little chaotic.With respect to ablated 2 wt% and 3 wt%-Fe 3 O 4 samples, phase transition occurs but no distinct columnar grain grows (figure S12), gentle or medium thermal effects are induced, but not strong enough to induce the growth of columnar grains and exsolution.Taking exsolved 5 wt%-Fe 3 O 4 sample for instance, ultrafast laser photothermal exsolution (figures 5(a)-(c)) is the opposite process of 'heterogeneous doping' realized by sputtering metal-film deposition and 700 • C annealing in air to induce metal diffusion into the host oxide [42].In comparison, our method is much simpler and more sufficient.The final product can be classified into exsolution layer, columnar grain growth layer and sub-influence layer.Compared with equiaxed grains, the boundaries of columnar t-ZrO 2 grains offer rapid diffusion channels (figure 5(b)), enabling Fe-ions quickly segregate from inner 3Y-TZP crystals towards the outmost surfaces, which was also promoted by ultrafast quenching process.Beneath the columnar grains and among fine grained region in the columnar grains, many nanoscale ironoxide exsolved regions are formed at m-ZrO 2 /t-ZrO 2 interfaces (figures 4(d) and S8).This phenomenon infers the occurrence of thermal shockwave exsolution [10] and highlights the importance of columnar grains for Fe 3+ /Fe 2+ ions' segregation and diffusion upward to the subsurface, otherwise they will be blocked and aggregate there.From an atomic point of view, the breaking of ionic bonds during laser ablation produces segregated iron ions, which migrate upward and exsolve on the surface in the molten state, and then exsolve into the outmost amorphous/crystalline iron-oxide layer upon ultrafast quenching (figures 5(d) and (e)).
Compared with other exsolution methods, this technique is so far the only one that can induce both ultrafast heating and quenching processes.Such unique property in combination with its ease to operate, environmental friendliness, high efficiency, flexible patterning capacity, security, and photothermal manipulation, thermal/shockwave synergistic effects, makes this technique very appealing for novel functional interface development.Of great interest, ultrafast quenching can freeze the exsolution dynamics, allowing us to witness the emergence of exsolved material networks in the subsurface (figures 3 and S5-S7).Jeong et al summarized diverse energy applications of emerging exsolution materials, including solid oxide fuel cells, oxygen reduction/evolution reaction (ORR/OER) catalysts for metal-air batteries and water splitting, gas sensor and reforming catalysts [51].Such applications should be also suitable for our method in light of the feasibility of fiber laser induced exsolution of perovskite oxides [11].In comparison, fiber laser needs twice treatment with the 1st round to create defects for triggering exsolution upon the 2nd laser irradiation, whereas our method can realize iron-oxide exsolution in one step and the scan speed is fast (200 mm•s −1 ) enough to ensure large-scale (cm 2 order) interfacial exsolution for practical applications.Employing higher power lasers accompanied by further increasing scan speed to m•s −1 can make this technique more efficient and practical for industrial applications, rather than being limited in the lab-scale.From material perspective, black and darkblue ZrO 2 can also be potentially applied as solar absorbers [24,52], pigments and dyes [53], and in decoration, implant dentistry [54], photocatalyst [55] fields; while from technique perspective, the applications of our method is as well compatible with those of fs laser ablation/structuring, patterning, and material synthesis within the scope of optics/photonics [56][57][58][59][60][61], energy [62], photoelectronic [63], flexible electronics [64], wettability [65,66], biomimetic [67], bubble/gas manipulation [68], stimulus-responsive interfaces [69], miniaturized robotics [70], fundamental studies [71][72][73][74], etc.

Conclusion
In this work, fs laser ultrafast photothermal exsolution is demonstrated using iron-oxide doped 3Y-TZP and the dopantdependent composition/concentration exsolution behaviors are presented.The key for triggering exsolution is using iron-oxide to arouse the thermal-annealing behavior, the growth of columnar grains for Fe-ion to get segregated in the grain boundaries which offer open spatially 3D channels for them to diffuse very fast towards the outmost surface.Photomechanical exsolution also occurs, mainly among t-ZrO 2 nanocrystals and t-ZrO 2 /m-ZrO 2 interfaces where grain boundaries are complex for segregated ions to diffuse.Compared with conventional exsolution methods in need of reductive gas and high temperature, this environment-friendly exsolution technique is more advantageous and meanwhile this new exsolution technique is as well highly efficient and simple, thus paving a facile way for interface modulation.

Materials, sample preparation and laser processing
3Y-TZP powders (TZ-3YB-E, Tosoh, Japan) were used as exsolution oxide host, while Fe 3 O 4 (99.0%purity, Aladdin, China), γ-Fe 2 O 3 (98.0%purity, Aladdin, China), and α-Fe 2 O 3 (99.5% purity, Aladdin, China) were adopted as exsolution source and photothermal modulator.Three dopant percentages of 2 wt%, 3 wt%, and 5 wt% of three iron oxides were mixed with 3Y-TZP powders by ball milling in ethanol medium, then granulated after drying at 80 • C. Dry pressing and coldisostatic pressing were used to prepare disc-shaped thin plates which were then sintered at 1450 • C for 3 h.Weight ratio higher than 5 wt% has already darkened 3Y-TZP educts, making it hard to identify the exsolution phenomenon, so higher weight ratio is not investigated.A femtosecond laser was focused on specimens in air.The pulse duration, wavelength and repetition rate are 400 fs, 1030 nm, and 400 kHz.The laser power was set at 8 W and the scanning speed was kept at 200 mm s −1 using a galvanometer.Line-by-line scanning method was adopted with line interval of 5 µm.After laser ablation, all samples were cleaned in ethanol by ultrasonic bath.

Sample characterization
The phase compositions of unablated and ablated samples were analyzed using x-ray diffraction (XRD, D8 ADVANCE Da Vinci, Bruker) and Raman spectroscopy (Renishaw inVia Qontor, Renishaw).Surface/cross-sectional structure morphology and elemental distribution were characterized by scanning electron microscopy (SEM, MIRA3, TESCAN) equipped with EDS and EBSD detectors.Accelerating voltage was 20 kV for EBSD and 30 kV for transmission Kikuchi diffraction (TKD), while the step size were 80 nm and 30 nm, respectively.Surface chemistry was examined using x-ray photoelectron spectrometer (XPS, NEXSA, Thermo Fisher Scientific).Surface roughness and three-dimensional structural mapping of laser-etched samples were characterized by laser confocal microscopy (LEXT OLS5100, Olympus).Focused ion beam (FIB, GAIA3, TESCAN) cutting was adopted to prepare TEM samples to see cross-sectional information by transmission electron microscopy (TEM, Talos F200X G2, Thermo Fisher Scientific).

Figure 1 .
Figure 1.Femtosecond laser ultrafast photothermal exsolution.(a) and (b) Schematic diagram of iron-oxide based PTT and PTT-inspired laser photothermal exsolution based on 2 wt%, 3 wt% and 5 wt% of iron oxide photothermal materials (Fe 3 O 4 , γ-Fe 2 O 3 , and α-Fe 2 O 3 ) with the oxide host of 3Y-TZP.(c) Evidence of laser induced annealing effect with increasing dopant percentage, as evidenced by an increase-decrease roughness of ablated Fe 3 O 4 -doped 3Y-TZP samples.(d) Side/top-view SEM and EDS images of unablated and ablated 5 wt% Fe 3 O 4 -3Y-TZP product showing outmost Fe-element exsolution accompanied by the appearance of a Fe-deficient layer and FeOx dots in the bulk.(e) SEM and optical images of all ablated samples showing that exsolution occurs simultaneously with surface color darkening or bluing.SEM images of all original samples are shown in figure S1 and more details of ablated 5 wt% Fe 3 O 4 -3Y-TZP can be found in figure S2.

Figure 5 .
Figure 5. Exsolution mechanism taking ablated 5 wt%-Fe 3 O 4 -3Y-TZP for instance.(a)-(c) Schematic diagrams of final exsolved state, Fe-ions blocking/diffusion process, and its overview.(d) and (e) Atomic viewpoint of exsolved amorphous-crystalline FeOx layer starting from original state to final state, mainly confined in the ablated and phase-transition regions, beneath which FeOx aggregate due to blocking effect by complex grain boundaries.

Table 1 .
Comparison of exsolution methods and their advantages/disadvantages.