Synthesis of vacancy-rich titania particles suitable for the additive manufacturing of ceramics

In the last decades, titania (or TiO2) particles played a crucial role in the development of photo-catalysis and better environmentally-friendly energy-harvesting techniques. In this work, we engineer a new generation of TiO2 particles rich in oxygen vacancies using a modified sol–gel synthesis. By design, these vacancy-rich particles efficiently absorb visible light to allow carefully-controlled light-induced conversion to the anatase or rutile crystalline phases. FTIR and micro-Raman spectroscopy reveal the formation of oxygen vacancies during conversion and explain this unique laser-assisted crystallization mechanism. We achieve low-energy laser-assisted crystallization in ambient environment using a modified filament 3D printer equipped with a low-power laser printhead. Since the established high-temperature treatment necessary to convert to crystalline TiO2 is ill-suited to additive manufacturing platforms, this work removes a major fundamental hurdle and opens whole new vistas of possibilities towards the additive manufacturing of ceramics, including carefully-engineered crystalline TiO2 substrates with potential applications for new and better photo-catalysis, fuel cells and energy-harvesting technologies.

www.nature.com/scientificreports/ In this work, we establish how carefully-controlled hydrolytic TiO 2 sol-gel chemistry can provide a precise control over the incorporation of oxygen vacancies at the particles' interface. In turn, these dark-colored oxygen vacancy-rich amorphous TiO 2 particles absorb very efficiently visible light. The increase in oxygen vacancies potentially leads to increase their photo-catalysis and energy-harvesting performances 39,[50][51][52][53][54] . Indeed, we successfully achieve room-temperature conversion to anatase or rutile TiO 2 in air using only a low-power laser. Since the high-temperatures required to crystallize TiO 2 are detrimental for additive manufacturing platforms, this last breakthrough opens new vistas of possibilities towards the additive manufacturing of ceramics and the design of engineered crystalline TiO 2 substrates for new and better photo-catalysis, fuel cells and other energyharvesting technologies.

Synthesis.
As a control experiment, we use a common sol-gel synthesis of amorphous TiO 2 through the hydrolysis and condensation of a metal alkoxides 55 . In this case, we use titanium tetrabutoxide Ti(OBu) 4 as the precursor 56 . In this relatively standard synthesis, the hydrolysis reaction is exothermic and occurs in a matter of seconds 56 . It is followed by the condensation reaction, where the white amorphous TiO 2 nanoparticle aggregate is formed and precipitates to the bottom of the beaker 56 . While the whole process is completed in a few seconds, it is common practice to age the system for up-to 72 h 57 before evaporating the solvent and recuperate a white amorphous TiO 2 powder as shown in Fig. 1a.
To engineer the titania particles' properties, we begin by slowing-down the reaction kinetics in an attempt to synthesize amorphous TiO 2 material rich in oxygen vacancies. Most common alkoxy groups used in the TiO 2 synthesis contain between two carbon atoms (ethoxy) and four carbon atoms (butoxy). Their reactivity towards hydrolysis decreases as the number of carbon atoms in the chain increases 58 . To decrease their reactivity towards water, it is common to dilute the nanoparticles in alcohols or mix with complexing agents 58 . In our case, acetylacetone (acac) is used as a complexing agent in order to chelate the metallic cation on the Ti(OBu) 4 . The reaction between acac and Ti(OBu) 4 occurs trough an interexchange substitution mechanism and it can be represented as follows 58 : As such, the standard hydrolysis and condensation reactions participating in the conventional sol-gel process can be controlled through the hydrolysis and complexation molar ratios. We define these two ratios as r c = [acac]/ [Ti] and r w = [H 2 O]/[Ti], respectively. These ratios can be varied in order to obtain precipitates, opaque gels, transparent gels, stable sols and cluster solutions 58 . To obtain transparent gels, the values we use for the hydrolysis and complexation ratios are summarized in the Table 1.
We rapidly observe that the complexation ratio significantly affects the coloration of the TiO 2 sol-gel system. While a pale-yellowish color is observed using the standard synthesis (r c = 0), the system presents an intense yellow color when the acetylacetone (acac) concentration is increased (r c = 1.96). As shown in Fig. 1b, its color also continues to gradually evolve from yellow to blood-red in a very slow aging process during up-to 8 months. At this time, a burnt-red TiO 2 powder shown in Fig. 1c can be obtained after solvent evaporation. This drastic change in color can be directly attributed to the formation of surface defects or oxygen vacancies and it originates from the charge transfer from the chelant agent (acac) to the Ti 4+ ion 59 58 . These, peroxo groups are also known to significantly enhance the visible light photo-excitation of TiO 2 60-63 .

Results and discussions
Synthesis and properties. In the Fig. 2a, it is possible to observe that the standard (white) amorphous Interestingly, these bands (1128 cm −1 and 1039 cm − 1) present a significant variation in their intensity ratios when both systems are compared. The intensity of the band at 1039 cm -1 is significantly reduced in the white amorphous TiO 2 with more Ti (i.e. r c = 0), while the intensity of the 1128 cm −1 band is much more pronounced in the red amorphous TiO 2 synthesized with more acac (r c = 1.96) 59 . The bands at 2960 cm −1 , 2935 cm −1 and 2873 cm −1 also contain information on the symmetric and asymmetric modes νCH 3 and γCH 2 for Ti(OBu) 4 present in the white amorphous TiO 2 and the Ti-acac complex present in the red amorphous TiO 2 64,65 . These bands are sharper in the white amorphous TiO 2 and less pronounced in the red amorphous TiO 2 , as a direct consequence of the chelation reaction between acac and Ti 64 . In contrast, the doublet at 1585 cm −1 and 1532 cm −1 observed exclusively in the red TiO 2 corresponds to the vibrational modes νC = C and νC = O. The presence of this doublet is another direct consequence from the acac-Ti bonding 66 . Furthermore, the absence of the acac characteristic band at 1620 cm −1 suggests that it reacted completely by chelating the Ti cation 59,66 . The band at 662 cm −1 also appears exclusively in the red amorphous TiO 2 , and corresponds to the modes ν(C-CH3) and ν(Ti-O) of the aromatic ring formed between acac and Ti 65,67 . We can directly confirm from this FTIR analysis the formation of the Ti(OBu) 4 -acac complex, which reduces the chances of the condensation and polymerization reactions. Based on these FTIR observations, the cyclic dimeric structure shown in Fig. 2b can be proposed for the chelation process yielding the red amorphous TiO 2 powder 68 .
This high concentration of oxygen vacancies creates intermediate states within the TiO 2 bandgap as shown in Fig. 2c. These oxygen vacancies can yield very efficient light absorption, while augmenting the Urbach energy of the oxygen vacancy rich TiO 2 69 . We confirmed the light absorption properties of the amorphous red TiO 2 by UV-vis (Suppl. Figure S1). As such, we can now environ a light-assisted conversion process triggered at room temperature and under ambient conditions, having a vast supply of molecular oxygen available to participate in the process. In turn, this reaction facilitates the phase transition thanks to the ionic mobility created with oxygen vacancies 70 . In fact, oxygen molecules act as very efficient photo-excited electron scavengers, trapping the excited electrons from the conduction band into the surface states of the TiO 2 71 . Then, the oxygen molecules are adsorbed at the surface of the red TiO 2 nanoparticles to partially compensate these oxygen vacancies 72 . The presence of oxygen vacancies promotes the formation of Ti 3+ sites in the crystal structure as the electrons left behind by the vacancy are distributed on neighboring Ti sites, reducing them 71,73,74 from Ti 4+ to Ti 3+ . Assisted by continuous irradiation, the adsorbed oxygen molecule passivates the TiO 2 by bridging the metallic ions 75 . A schematic of this process is presented in Fig. 2c.

Laser-induced crystallization of the oxygen vacancy-rich TiO 2 . In this oxygen vacancy-rich (red)
amorphous TiO 2 we synthesize, the combined use of the oxygen vacancies and peroxo groups now offers the potential for laser-assisted crystallization. To demonstrate the rapid laser-assisted crystallization of amorphous red TiO 2 powder, we use a Raman micro-spectroscopy system to record the transient evolution of the Raman signatures over a period of two minutes after opening the 532 nm laser shutter. The Fig. 3 shows the evolution towards a full conversion to anatase using only a 75 Wmm −2 power density (Fig. 3a,c), and to rutile using a 445 Wmm −2 power density (Fig. 3b,d). There, we note that the crystallization process is especially fast due to the higher excitation power and the system can hardly record the start of the conversion.  www.nature.com/scientificreports/ Raman modes can be sensitive to crystal size. However, this is not case of TiO 2 for which grain size has no effect on the Raman spectra [76][77][78] . Defect structures in the TiO 2 strongly affect the Raman spectrum by producing shifts and broadening of some of the Raman peaks 76 . Among those defects, oxygen vacancies are responsible for the non-stoichiometric effects that cause the shift and broadening of the Eg Raman mode 77,78 .
Compared to our conventional (white) crystalline TiO 2 powder crystallized using thermal annealing (see supplementary section Suppl. Figure S2), we observe a significant shift and broadening of the E g mode for the red TiO 2 converted to both anatase and rutile using laser-induced conversion in air. In fact, this E g mode is known to be more sensitive to oxygen vacancies and it is often used as a direct indicator to detect their presence 79 .
After complete conversion to anatase, Fig. 3c, this dominant E g mode normally at 156 cm −1 and 204 cm −1 shifts and broadens significantly compared with conventional (white) TiO 2 after conventional thermal annealing. After the complete conversion to rutile using higher laser power densities, Fig. 3d shows that the E g mode normally at 232 cm −1 peak also shifts to higher energies (251 cm −1 ). In contrast, the higher vibrational peaks normally at 401, 520 and 643 cm −1 for anatase TiO 2 shift to slightly lower wavenumbers for the red anatase TiO 2 (396, 511 and 634 cm −1 respectively). However, this power-temperature dependence of the anatase Raman signatures is consistent with the literature [80][81][82] . Similarly, the vibrational peak normally at 451 cm −1 for rutile TiO 2 also shifts to lower wavenumbers for the red rutile TiO 2 (433 cm −1 ), which is also consistent with the literature [83][84][85] .
The significant shift and broadening of the E g mode using laser-induced conversion in air indicates a high concentration of residual oxygen vacancies concentration after crystallization. Indeed, the temperature-dependent color changes, as well as the promotion and the disappearance of oxygen vacancies during the thermal crystallization can be explained using previous models 86 . These models suggest that temperatures between 300 °C and 500 °C promote the entropy-driven outward diffusion of Ti 3+ defects towards the nanoparticle surface and to produce a black-gray color transition, which is an indicator of the high oxygen vacancy concentration 86 .
The lattice structure for laser-converted red TiO 2 can be directly observed in Fig. 4. All the samples display a well-organized lattice structure and selected-area (electron) diffraction (SAED) analysis confirms the clearlydefined polycrystalline anatase and rutile polymorph structures.
Thermally-induced crystallization of the oxygen vacancy-rich TiO 2 . While this unique laserassisted crystallization to anatase or rutile TiO 2 is only possible for the oxygen vacancy-rich (red) amorphous TiO 2 powder, conventional thermally-induced crystallization to anatase or rutile TiO 2 always remains possible. We chose to perform this experiment for both the red and white amorphous TiO 2 powders shown in Fig. 1a,c to help-us fully understand the complex mechanisms associated with these high oxygen vacancy densities.  www.nature.com/scientificreports/ As expected, our standard (white) amorphous TiO 2 powder yields white anatase and white rutile powders after thermal annealing at 450 °C (anatase) and 800 °C (rutile). However, the oxygen vacancy-rich (red) amorphous TiO 2 powder yields a darker (gray) anatase powder after thermal annealing at 450 °C (anatase) and a white rutile powder after thermal annealing at 800 °C (rutile). Typical examples are shown in Fig. 5.
The grayish color of the anatase is not unusual for this TiO 2 polymorph 86 . In fact, when anatase TiO 2 is found in its natural form, it can vary from indigo-blue to black and steely luster 46 . The synthesis of dark anatase has been previously reported by means of UV irradiation followed by annealing under argon atmosphere 87 or by hydrogenation of anatase powders under high pressure 50 . In both cases the darker color arises as a consequence of the oxygen vacancy-mediated absorption 39,88 . This gray anatase proves better-suited to harvest visible and infrared light, making it more efficient for the photocatalytic reactions 46 . Coupled with platinum, it proves an outstanding material for energy conversion applications including hydrogen generation from water/ethanol solutions 46 . Recently, gray anatase has also been deployed for CO 2 conversion 89 and air quality control applications 90 .
Once again, Raman micro-spectroscopy analysis is used to compare the anatase and rutile signatures of the red and white TiO 2 after the thermal annealing. In Fig. 6a, both the standard (white) and defect-rich (red) amorphous TiO 2 powders show a well-defined anatase phase with peaks at 147, 199, 401, 520, 634 cm −1 after annealing at 450 °C 91 . In contrast, Fig. 6b also shows a well-defined rutile phase with peaks around 147, 232, 451, 611 cm −1 for both powders after annealing 92 at 800 °C.
Once again, the E g Raman modes for anatase (at 147 cm −1 ) and rutile (at 451 cm −1 ) can be used to compare the crystallized TiO 2 samples (Fig. 6). After thermal annealing, Fig. 6c,d shows a slight broadening of the E g peak    Fig. 6e,f shows no clear distinction between the white and red TiO 2 after thermal annealing. This suggests near-complete oxygen vacancy removal after thermal annealing. From these results, we can conclude that higher temperatures can also generate enough energy to break the saturated state of Ti 3+ defects making the gray color fade away significantly decreasing the concentration of oxygen vacancies 86 . The TEM analysis shown in Fig. 7 also confirms the very well-defined crystalline structures for the anatase and rutile TiO 2 powders obtained after thermally-assisted conversion of the standard (white) and defect-rich (red) amorphous TiO 2 powders. SAED analysis confirm that all the samples are polycrystalline anatase and rutile polymorphs.  To move one step closer towards the laser-assisted additive ceramic manufacturing, we used a standard commercial filament-based 3D printer mounted with a low-power 405 nm laser printhead. This procedure has been detailed by our team in previous reports 94 . Using this laser, we used 140 W mm −2 (for anatase) and 215 W mm -2 (for rutile) to form a complex mosaic pattern combining amorphous, anatase and rutile polymorphs originating from the vacancy-rich amorphous TiO 2 94 . Figure 8 shows it is now possible to spatially organize different crystalline phases with high level of precision within complex architectures and patterns. The procedure and the optimal processing parameters are fully described in the supplementary section.
The laser-converted anatase and rutile areas in Fig. 8 are well-defined and the change in color for each crystallized area matches the results obtained during thermal crystallization of the vacancy-rich (red) amorphous TiO 2 , resulting in dark-grey powder for anatase and white powder as previously described. The structure of the crystallized areas is confirmed by Raman micro-spectroscopy and XRD as shown in the supplementary information (Suppl. Figure S2 and S3 respectively) where the characteristics peaks are clearly identifiable for both polymorphs. The high-resolution topographic 3D surface reconstructions from the converted areas shown Fig. 8b,c are obtained using a laser-scanning microscope (LEXT OLS4100 from Olympus) with ImageJ reconstruction. It reveals cracks in the laser-converted regions, both anatase and rutile. These cracks appear as a direct consequence of the rapid densification of the material during the laser-assisted crystallization process 70 . As expected, the cracks are more pronounced in rutile compared with anatase since its unit-cell volume 95 is half that of anatase and it possesses a higher density value due to the increased number of atoms 9,10 .

Conclusion
We report the controlled preparation of oxygen vacancy-rich (red) amorphous TiO 2 nanoparticles by green hydrolytic sol-gel reaction of Ti(OBu) 4 and using acetylacetone (acac) as a chelating agent, significantly slowing-down the reaction kinetics due to the steric inhibition effect in order to promote oxygen vacancies. FTIR spectroscopy is used to differentiate the new bonds due to the chelating agent and suggest a cyclic dimeric structure for the chelation reaction consistent with experimental models reported in the literature. For comparison,  www.nature.com/scientificreports/ thermally-induced crystallization to anatase and rutile TiO 2 is performed at 450 °C and 800 °C respectively for our standard non-chelated (white) and the oxygen vacancy-rich (red) amorphous TiO 2 . It yields gray anatase and white rutile after thermal conversion, which is also expected and consistent with the literature. TEM and SAED results show a well-organized highly-crystalline lattice structure for all the samples. We can exploit these oxygen vacancies to achieve low-energy laser-assisted crystallisation at room-temperature and in ambient environment using this oxygen vacancy-rich (red) amorphous TiO 2 . Using a 532 nm laser with 75 W mm −2 and 445 W mm −2 power densities yields induces complete crystallization to anatase and rutile, respectively. Transient Raman micro-spectroscopy shows that crystallization occurs within the very first seconds of irradiation, and that this effect is permanent and noncumulative. This laser-induced crystallization is directly attributed to the presence of oxygen vacancies and its reactivity towards molecular oxygen and a process schematic is proposed. Moreover, a standard commercial filament-based 3D printer mounted with a low-power 405 nm laser printhead is used to convert selectively to anatase and rutile. We successfully produced a complex polymorphic mosaic meta-structure combining three (3) TiO 2 polymorphs with high level of precision to potentially study and exploit new synergistic effects 96,97 . Most importantly, these laser-induced phase transitions are entirely performed at room-temperature in ambient environment, without any kind of dopant in the TiO 2 prior to photo-activation.
Obviously, these oxygen vacancy-rich nanoparticles strongly absorb visible light. Furthermore, the increase in oxygen vacancies can potentially lead to an increase in the photo-catalysis and energy-harvesting performances of TiO 2 . As it can be easily converted to anatase or rutile TiO 2 in ambient room conditions using only a low-energy laser excitation, we believe this last breakthrough opens new vistas of possibilities towards the additive manufacturing of engineered crystalline TiO 2 substrates for photo-catalysis, fuel cells and energy-harvesting applications.

Materials and methods
In order to prepare the standard (white) amorphous TiO 2 powder, 28.8532 g of ethanol (Product 1590102500 from Sigma-Aldrich) are mixed with 10.8604 g of titanium(IV) butoxide (Product 244112-500G from Sigma-Aldrich) and this solution is stirred for 40 min. Finally, the hydrolysis reaction is triggered by adding dropwise 0.84 mL of deionized water. Precipitation of the amorphous white TiO 2 occurs within the first few seconds after the reaction is started. This mixture is aged for 72 h to form the TiO 2 sol-gel and then the solvent is evaporated at ambient conditions to obtain the white TiO 2 powder shown in Fig. 1a.
In order to prepare the oxygen-rich (red) amorphous TiO 2 , 28.8532 g of ethanol (Product 1590102500 from Sigma-Aldrich) are mixed with 1.4748 g of acetylacetone (Product P7754-1L-A from Sigma-Aldrich). This solution is stirred for 20 min. Then, 10.8604 g of titanium(IV) butoxide (Product 244112-500G from Sigma-Aldrich) are added and stirred for another 40 min. Finally, the hydrolysis reaction is triggered by adding dropwise 0.84 mL of deionized water. The resulting mixture is stirred for 120 min and then aged for 8 months to form the red TiO 2 sol-gel. During the aging time, the ethanol slowly evaporates to precipitate a vitreous red TiO 2 . It is important to notice that both syntheses are carried-out entirely at room temperature.

Data and availability
All data needed to evaluate the conclusions in the paper are present in the paper and/or the supplementary information.