KEYWORDS: Binder-free; Phase-selectively reduced titanium dioxide film; Microwave covalent coating; Condensation reaction; Volatile organic compounds degradation ABSTRACT: A binder-free attachment method for TiO 2 on a substrate has been sought to retain high active photocatalysis. Here, we report a binder-free covalent coating of phase-selectively disordered TiO 2 on a hydroxylated silicon oxide (SiO 2 ) substrate through rapid microwave treatment. We found that Ti-O-Si and Ti-O-Ti bonds were formed through a condensation reaction between the hydroxyl groups of the disordered TiO 2 and Si substrate, and the disordered TiO 2 nanoparticles themselves, respectively. This covalent coating approach can steadily hold the active photocatalytic materials on the substrates and provide long-term stability. The binderfree disordered TiO 2 coating film can have a thickness (above 38 μm) with high surface integrity with a strong adhesion force (15.2 N) against the SiO 2 substrate, which leads to the production of a rigid and stable TiO 2 film. This microwave treated TiO 2 coating film showed significant volatile organic compounds degradation abilities under visible light irradiation. The microwave coated selectively reduced TiO 2 realized around 75% acetaldehyde degradation within 12 hours and almost 90% toluene degradation after 9 hours, also retains stable photodegradation performance during the cycling test. Thus, the microwave coating approach allowed the preparation of the binder-free TiO 2 film as a scalable and cost-effective method to manufacture the TiO 2 film that shows an excellent coating quality and strengthens the application as a photocatalyst under severe conditions.
Currently, the energy crisis and environmental pollution are becoming serious global concerns. Due to the above worldwide issues, human civilization development has been relatively restricted, and public health faces severe detrimental impacts from various pollutants [1] . For tackling energy and pollution problems, titanium dioxide (TiO 2 ) has attracted great interest due to the excellent photocatalytic activity since the discovery of TiO 2 water splitting to hydrogen gas phenomena under near ultra-violet (UV) light by Fujisma and Honda at 1972 [2] .
As one of the most promising photocatalysts, TiO 2 showed favorable heterogeneous photocatalytic properties and has been applied in various fields, such as environmental pollutants degradation [3, 4] , antibacterials for sterilization in the hospital system or food industry [5] , selfcleaning coatings [6] , light-driven water splitting, and CO 2 reduction [7] , among others [8] .
Moreover, the inert, non-toxic, and economic nature makes TiO 2 more favorable in the development and industrial process without the introduction of environmental issues [9] . During the ongoing COVID-19 coronavirus pandemic, TiO 2 as a potential, environment-friendly antivirus agent will gain more concerning in the practical applications [10] [11] [12] .
Recently, researchers found that the disordered surface on TiO 2 can effectively reduce the bandgap and dramatically increases visible light absorption [8, 13] . Our group has reported a visible-light-driven high photocatalytic performance of the phase-selectively disordered blue P25 TiO 2 nanomaterials, which are disordered rutile (R d ) phase and ordered anatase (A o ) crystalline phase Li-P (R d /A o ) TiO 2 , and ordered crystalline rutile (R o ) and disordered anatase (A d ) Na-P (R o /A d ) TiO 2 , respectively [8, 14, 15] . The selectively disordered phase that has many hydroxyl groups can enhance visible light absorption, and the unchanged crystalline phase can provide high charge separation [8] .
Meanwhile, most current applications and research of the TiO 2 needs to be loaded or coated on the substrates as a loading medium (silicon dioxide, conductive glasses, activated carbon or polymer, etc.) [16] [17] [18] . Multiple events can easily cause a detachment problem of the loaded active catalysts such as 1) wind, rain, and other natural processes in ambient conditions ; 2) shearing force in the agitated reaction chamber; 3) high-temperature annealing process and catalytic reaction conditions, and so on. For solving the detachment problems, several deposition methods have been developed, such as plasma or thermal spray [17] , sol-gel process [19] , and sinter coating [20] . However, the abovementioned approaches require high energy, high temperature, and complex processes [21] . The conventional TiO 2 deposition strategies lack the robust interfacial binding ability for fulfilling long-term or severe environmental applications, which will result in easy detachment from the substrate with a loss of activity.
Several types of the binder for TiO 2 coating have been tested, such as tetraethylorthosilicate (TEOS) and Poly(vinylidene fluoride (PVDF) [22, 23] . Even though the binding ability with long-chain binders is enhanced between TiO 2 and substrates, the efficiency in the TiO 2 applications can decrease due to the large portion of the non-active binder composition. Therefore, the formation of a strong adhesive bond of TiO 2 to substrates without a binder has become a significant issue in practical applications.
To produce the binder-free TiO 2 film, several important questions must be addressed: 1) Is it possible to form covalent bonds between TiO 2 nanoparticles and any substrates, and also covalent bonds between TiO 2 nanoparticles by themselves? 2) What are the best functional groups to form the covalent bonds between TiO 2 and the substrate; and also, what is the best way to activate the functional groups? 3) If the functional groups are hydroxyl groups, what is the J o u r n a l P r e -p r o o f best way to form the chemical bonds? 4) Finally, is it possible to form the chemical bonds through a condensation reaction between hydroxyl groups by microwave irradiation?
Herein, we report a binder-free adhesion method for hydrophilic TiO 2 film on substrates via a condensation reaction between hydroxyl groups of the phase-selectively reduced TiO 2 nanoparticles and the silicon oxide substrate by microwave treatment. It is expected that the phase-selectively reduced TiO 2 nanoparticles that have many hydroxyl (-OH) functional groups can be chemically attached to the hydrophilic SiO 2 substrate via covalent ether bonding (-O-) through a few minutes of microwave treatment due to the rapid and uniform oscillation among polar moieties (especially hydroxyl groups). In this approach, a strong binding force exists among reduced TiO 2 nanoparticles to maintain good film integrity. The covalent coating methods can hold the active photocatalyst material steady on the substrates and provide longterm stability, resulting in high photocatalytic performance.
Degussa P25 TiO 2 was purchased from Sigma-Aldrich ( ≥ 99.5% trace metals basis, nanopowder and 4.26 g/mL at 25 °C) and directly used; Lithium (Li) granules (1-6 mm) 99% (metal basis) was supplied by Alfa Aesar (United States); Sodium (Na) lump, hydrochloric acid (HCl), nitric acid (HNO 3 ) and ethylenediamine anhydrous (EDA) were obtained from Sigma Aldrich, Samchun, OCI Company Ltd., and Tokyo Chemical Industry Co., Ltd., respectively.
J o u r n a l P r e -p r o o f
The selective reduction TiO 2 synthesis method was reported in our previous research work [8, 14, 15, 24] . First, 1 g of TiO 2 nanoparticles (Degussa P25, 20-40 nm) were added into the 300-ml three-neck round-bottom-flask (RBF), which contained 100 ml EDA. The RBF was connected with a condenser and placed in the ice-bath over a stirring plate, with N 2 gas flowing into the RBF for about 10 mins to keep a low concentration of O 2 /moisture. Then, 0.69 g Li metal granules were weighed and quickly transferred into the suspension. The reduction process was conducted under an anhydrous N 2 atmosphere at room temperature by using the Schlenk line with continuous stirring for 3 days. After the reaction, 35% HCl solution was added dropwise into the RBF to neutralize the remaining Li metal by forming LiCl. Finally, the mixture was washed several times with secondary deionized water and dried by a vacuum rotary evaporator.
The Li-EDA reduced P25, R d /A o TiO 2 was stored in a vacuum oven. Similarly, Na EDA treated P25 TiO 2 , R o /A d TiO 2 was prepared by a similar method to that mentioned above.
SiO 2 substrate (Silicon Single-side Polished Wafers, 300nm Oxidation Layer, 100 mm size) was first cut to 10*10 mm pieces and flowed by soaking in 36% HNO 3 for 30 min and then washed several times with deionized water. After the substrate was dried, O 2 plasma treatment was executed for 30 s (50 Hz, 200 W, 50 cc/min O 2 flow). The water contact angle was measured to characterize the surface hydrophilicity of the SiO 2 substrate.
The prepared R d /A o and R o /A d TiO 2 were suspended in ethanol for 30 min in a sonication bath (2 and 6 mg/ml for thin and thick coating cases, respectively). The suspension viscosity is temperature sensor which can detect the temperature evolution of the substrate. Then, the temperature was controlled at 80 ºC by instrument program. The microwave irradiation time was set at 2 and 5 min (output power: 700W) for thin and thick film coating cases to ensure that the ethanol solvent was fully removed. At first, a glass container with 8 cm diameter was put at the center of the microwave oven, and its cap was punched and installed a soft tube which was connected to the outside of the chamber through the microwave oven top hole. Then, we put the 10*10 mm square hydroxylated SiO 2 substrate into the glass container followed by dropping the TiO 2 suspension (0.2ml and 0.5ml for thin and thick coating samples, respectively) on the substrate. The cap of the glass container is tightened to prevent the evaporated ethanol vapor release to the oven chamber. To create good microwave treatment uniformity during the coating process, a slow manual rotation from top of the oven through the connected soft tube was given to the reaction container during the treatment, which is like turntable function. For the thick film coating treatment, two cycles of treatments were conducted. At first, the 0.2 ml suspension was spread on the substrate and followed by 2 min microwave irradiation and then the additional 0.3ml TiO 2 suspension was added followed by the further 3 min microwave irradiation. After microwave irradiation treatment, the TiO 2 nanoparticles were tightly coated on the SiO 2 substrates. The unbound TiO 2 powder was removed with a compressed air gun.
The SmartLab JD3643N diffractometer performed the powder X-ray diffractometer (XRD) for crystal structure characterization. The water contact angle (WCA) of treated SiO 2 substrates was determined by using SEO PHX300. X-ray Photoelectron Spectrometer (XPS) spectra were J o u r n a l P r e -p r o o f obtained from Thermo ESCALAB250 with a twin-crystal, micro-focusing monochromator. And, before the XPS measurements, the samples are loaded in the chamber and initiated the vacuum.
For the sake of XPS measurement accuracy, the top few layers of samples are sputtered away to avoid the typical surface carbon contamination. JEOL JSM7600F SEM was utilized to obtain the coated film surface and cross images. The microwave coated TiO 2 film binding force was analyzed by a Mecmesin MultiTest 2.5 Tester. The electron paramagnetic resonance (EPR) analysis was conducted by X-band CW-EPR, QM09 under room temperature with 2.97 mW, 100
KHz modulation frequency and 1G modulation amplitude conditions. FTIR spectra were measured with a Bruker Vertex 70/80 FTIR spectrometer.
The photocatalytic activity was evaluated by degrading the common VOCs (acetaldehyde and toluene) under visible light in a transparent Tedlar gas bag. TiO 2 film samples (300 mg catalyst loading amount) were placed in the 3-liter Tedlar bag. The acetaldehyde and toluene gas (JC Gas, 1000 ppm in N 2 ) and O 2 gas were injected through a humidifier into the Tedlar bag.
Acetaldehyde concentration was detected by a gas detector (Gastek, MODEL GV-100) and toluene gas concentration was determined through gas chromatograph-FID (Younglin Instrument, YL6500). A 100 W white LED lamp (Giolite) was used as a visible light source (wavelength: 420-680 nm). The average light intensity on the catalyst was 0.6 W cm -2 . After the absorption-desorption equilibrium was achieved, the light lamp was illuminated. The acetaldehyde and toluene gas were diluted to 100 ppm (initial concentration) with airflow. The VOCs removal ratio was calculated as
where A and A 0 are acetaldehyde concentration (ppm) before and after the light irradiation, respectively.
The reduced TiO 2 (R d /A o and R o /A d ) were selectively disordered rutile and anatase phase, respectively, while kept intact in another crystalline phase. catalyst films were applied to degrade the VOC gas (acetaldehyde and toluene).
The crystal structure of three TiO 2 (R d /A o and R o /A d TiO 2 , and pristine P25) were first characterized by XRD, as shown in Fig. 2 The interface between the crystalline phase and reduced disordered region in R d /A o and R o /A d TiO 2 can efficiently separate the excitons and profoundly enhance photocatalytic performance [8] . The Ti-O-Ti bond can be broken down in the reduction process of raw P25 TiO 2 , and the coordinated state of Ti 4+ decreases to Ti 3+ . Fig. 2(b) shows EPR data among the two kinds of disordered TiO 2 , exhibiting the unpaired electron in the compounds. This is indicative that the reduced TiO 2 samples produce Ti 3+ by catching the electron of the alkali metal. Moreover, the J o u r n a l P r e -p r o o f unbonded O after forming Ti 3+ will capture the protons in the environment to neutralize the negative charge, producing Ti-OH. The Ti 3+ EPR peak is located around 350 and 361 mT, and shows the highest intensity in the R o /A d TiO 2 sample (+19.8 & -13.8 a.u.) while the pristine P25 sample has no response. Compared with the XRD data, the Ti 3+ amount follows the order of To further characterize the TiO 2 chemical state, XPS analysis was conducted, results are shown in Fig. 3 . The two main peaks in Ti2p spectra were assigned to Ti2p 3/2 and Ti2p 1/2 by referring to the Zhao et. al (2014) report [26] .In terms of Ti2p, the peak position had been a lower shift to 457.95 eV in R o /A d but did not change in R d /A o TiO 2 , when compared with pristine P25.
The decrease of 0.26 eV binding energy in Ti2p of R o /A d , whose anatase portion is major, was due to the destruction Ti-O-Ti bond in the TiO 2 lattice. As shown in the Ti2p deconvolution spectra (Fig. 3a) it can be depleted through mild thermal treatment due to the relatively weak binding affinity between the attached functional group and their objects [27] . Further, the restoration phenomena of plasma-treated sample to the hydrophobic surface property with increasing time or temperature have been found frequently (Supplementary Figure 7) , so-called "aging" of the surface [28] [29] [30] . In the chemical etching approach, the surface defect can be produced by acid oxidation, thus converting to the hydroxyl group. Moreover, the hydroxyl group generated by the wet chemical etching approach is more stable than plasma surface modification [31] . Based on the above knowledge, wet chemistry (acid etching) and dry chemical method (O 2 plasma) are incorporated to produce the permanently hydroxylated silicon dioxide (SiO 2 ) surface.
Therefore, for producing hydroxylation of the SiO 2 substrate, a soak in the HNO 3 solution can yield plenty of hydroxyl groups (Si-OH) by the etching of the substrate surface [32] . For covalent bonding TiO 2 nanoparticles on the substrates, the SiO 2 substrates need to be hydroxylated first, to conduct a condensation reaction among Si-OH and Ti-OH between the reduced TiO 2 and hydroxylated SiO 2 substrate. The hydroxylation process of the SiO 2 substrates was performed in two steps: 1) HNO 3 etching (E-Sub.), and 2) O 2 plasma treatment. The WCA and XPS analysis of the processed substrate in each step was performed as shown in Fig. 4 . After nitric acid etching on the initial SiO 2 substrate surfaces, the WCA was decreased from 69.8° to 43.1° (Fig. 4a) , and the binding energy of Si2p and O1s was shifted lower to 0.3 and 0.4 eV, respectively (Fig. 4 b, c) . The above changes in E-Sub indicated the Si-OH was produced J o u r n a l P r e -p r o o f through the breakage of the Si-O bonding of the SiO 2 substrate surface. To further convert the Si-O dangling bond into Si-OH and increase hydrophilicity, O 2 plasma was applied to E-Sub. As shown in Fig. 4a top image, the water drop was fully spread on the hydroxylated SiO 2 substrate, and no WCA can be measured. The XPS Si2p and O1s spectra of the hydroxylated SiO 2 substrate also were further shifted to lower binding energy due to the reduction by radicals in the O 2 plasma cloud. 
The microwave oven has been applied in many organic synthesis reactions due to its effectiveness in inducing a condensation reaction between molecules which have permanent dipoles [33] [34] [35] . Polar molecules will be aligned in the electric field, and their alignment direction can be changed along with the electric field. The commercialized SIENO MAS-II microwave oven can generate 2.45 GHz microwaves, and it subsequently induces extreme agitation (4.9×10 9 times) to the polar molecules. Therefore, the hydroxyl groups in the prepared To investigate whether the phase change happened in the reduced R d /A o , R o /A d , and P25 TiO 2 films after microwave treatment, the thin and thick microwave coated TiO 2 films were analyzed by XRD (Fig. 7) . As shown in the XRD spectra Fig. 7b and c, the amorphous, J o u r n a l P r e -p r o o f disordered rutile in R d /A o TiO 2 and disordered anatase in R o /A d TiO 2 were not changed, maintaining the amorphous states. It was also observed that the TiO 2 peak intensities were increased in the thicker coating than the thin coating cases. Besides, the SiO 2 substrate peak intensity also follows the same trends, which decreased in the thick sample cases. Therefore, we provided the SiO 2 substrate XRD pattern to track the substrate diffraction peaks. In Fig. 7 (d) SiO 2 substrate XRD pattern, the main peak at 69.15º belongs to (100) planes of Si. And the other lower intensity peaks are associated with the silicon oxide species [38] . Besides, the reflection peak at 33º of pristine SiO 2 substrate in Fig. 7d is associated with the forbidden (002) reflection of Si surface through multiple diffractions and Renninger effect [39, 40] . It can appear under XRD azimuth (π angle) rotation measurement after the substrate surface crystal was damaged during wet and dry chemical hydroxylation processes treatments. Subsequently, it disappears in the further TiO 2 film microwave coating treatments process with the soft thermal heating. Also, the peaks near 62º and 67º in Fig. 7 a, b and d arose from the Cu and W in the X-ray sources. In   Fig. 7c , due to the utilization of Ni filter during XRD measurement, the Si (004) peak at around 69º became broaden and the 62º and 67º peaks from X-ray sources were disappeared [41] . respectively. The blue and red arrows in b and c are inserted to indicate that the disordered phase did not recrystallize. TiO 2 diffraction peaks' intensity in the thick film coated cases is stronger compared with the thin film cases which indicated the corresponding higher sample loading.
After obtaining the thick-coated samples, SEM images were collected again to characterize the surface state and thickness (Fig. 8) . After the microwave treatment coating, the loosely attached or uncoated TiO 2 particles were removed by an airflow gas gun. According to the top view images of the coated surface, the surface integrity of R d /A o (Fig. 8b) and R o /A d TiO 2 (Fig.   8c ) is retained better than P25 in the thick film (Fig. 8a) . Besides, the R d /A o TiO 2 achieved 38.1 J o u r n a l P r e -p r o o f μm thickness with dense packing, and R o /A d TiO 2 realized 22.2 μm, as shown in (Fig. 8e and f) .
The P25 microwave coated film has a lower thickness value of about 9.6 μm (Fig. 8d) , which is mainly due to weak interconnection bonds between particles to hold on the substrate surface. The film to retain a relatively better packing quality compared to the R o /A d TiO 2 film case ( Fig. 5   and 8) . Normally, the crystalline phase will lose its regular lattice structure after transforming to J o u r n a l P r e -p r o o f the disordered amorphous state. In this work, even though the disordered TiO 2 phase is expected to form covalent bonds for maintaining film integrity and mechanical strength, a high portion of disordered phase may hinder the contact between TiO 2 particles and substrate for bond formation.
In the pristine commercial P25 TiO 2 powder, the composition is around 70% crystalline anatase and 30% crystalline rutile. Therefore, R d /A o and R o /A d TiO 2 powder have about 30% and 70% disordered phase, respectively. Amorphous solid is known as that a solid which lacks long-range order, non-spherical and shape inhomogeneity. The irregularity stacking of disordered phases will result in random packing and less inter-particles connection [42] . Further, the disordered amorphous state TiO 2 region may reduce the covalent Ti-O-Ti interconnection formation.
Therefore, due to a high proportion of amorphous anatase phase existence (70%), the loose stacking of the MW-coated R o /A d TiO 2 film resulted with a 7.1 N adhesion force on the SiO 2 substrate. The above discussion can be supported by the Brunauer-Emmett-Teller (BET) measurements data [7, 14] , as listed in the Supplementary (Supplementary Figure 6) . Moreover, MW-coated P25 had the lowest value (0.9 N) adhesion forces due to the difficulty to form covalent bonding between TiO 2 particles with SiO 2 substrate, and TiO 2 particles themselves in the coated layer.
The covalent binding formation after microwave irradiation in the selectively reduced R d /A o and R o /A d TiO 2 MW-coated film can significantly enhance the adhesion ability and maintain better integrity of the coated film on the substrate. Interestingly, the remaining crystalline phases can act as an efficient charge separation site but also causes dense stacking in the MW-coated film to induce the covalent interconnection by condensation reactions. Fig. 11 . The adhesion force characterization by adhesive tape removal testing.
To estimate the photocatalysis activity of the binder-free microwave coated TiO 2 film, the degradation of typical VOC compounds, acetaldehyde and toluene, have been performed under 420-680 nm visible light generated by a 100 W white LED lamp. The microwave covalently coated R d /A o TiO 2 film always shows a higher degradation percentage compared with the traditional dip-coating film sample (Fig. 12a) . The acetaldehyde has degraded 20% and 40% by the MW R d /A o film after 2 hours and 4 hours, respectively. The covalently coated R d /A o TiO 2 film could maintain good integrity in the gas reaction chamber, while traditional dip-coating samples inevitably detached and lost catalytic activity. We use our group's previous films in this setup, which provided a 3.46 mmol/g/h H 2 generation rate due to photocatalytic water splitting (higher than most of the previous works of literature) by R d /A o J o u r n a l P r e -p r o o f TiO 2 and 4.0 μmol/g/h CH 4 production from CO 2 reduction (higher than current even metaldoped P25 TiO 2 ) by R o /A d TiO 2 [8, 14] . Besides, the R d /A o MW TiO 2 film exhibited faster degradation speed than P25 MW TiO 2 film (Fig. 12b) . To further confirm the versatile photodegradation strength, we collected the toluene gas degradation by the R d /A o MW TiO 2 film under visible light. It can be seen that the R d /A o MW TiO 2 film stepwise reduced the toluene amount under visible light irradiation and realized around 90% degradation after 9 hours, as shown in Fig. 12c . To investigate the photodegradation active species, we conducted the EPR analysis to detect hydroxyl radical (•OH) reactive oxygen species. The R d /A o MW TiO 2 film can generate more active •OH than P25 TiO 2 for participating in VOCs degradation reactions (Fig.   12d) . Moreover, we determined the recycling test of R d /A o MW TiO 2 film toluene degradation to examine photocatalytic stability. As presented in Fig. 12e , the toluene degradation amount of the R d /A o MW TiO 2 film was around 80% after 7 hours irradiation, which is confirmed with 5 recycling tests. By utilizing the scalable, efficient, and robust microwave covalent coating technique, a long term good photocatalytic property will be realized. Furthermore, the microwave covalent binder-free coating strategy can be applied in other catalysts to maintain photocatalytic activity.
The binder-free and the visible-light-driven phase-selectively reduced TiO 2 (R d /A o and R o /A d ) films covalently bonded on the SiO 2 substrate were successfully prepared through a microwave irradiation method. This is the first known successful attachment of a phase-selectively reduced TiO 2 photocatalyst on the universal SiO 2 substrates through a rapid microwave coating method, showing a highly adhesive covalently coated film. The covalent bonding of Ti-O-Si between J o u r n a l P r e -p r o o f reduced TiO 2 and the hydroxylated SiO 2 substrate was formed through a condensation reaction between Ti-OH and Si-OH under microwave processing. Furthermore, similarly, Ti-O-Ti bonding between the disordered TiO 2 nanoparticles was formed to give a higher thickness packing film. By the condensation reaction strategy using the microwave, binder-free R d /A o TiO 2 MW-coated film with a high thickness (38 μm) was achieved while maintaining surface integrity.
The covalent bond formation of Ti-O-Si and Ti-O-Ti in R d /A o TiO 2 MW-coated film produced the firm adhesion TiO 2 film on the SiO 2 substrate. Further, the VOC degradation performance using the MW-coated R d /A o TiO 2 was realized with 40% removal efficiency within 4 hours, which is faster to decompose acetaldehyde than normal dip-coating films. This microwave treated TiO 2 coating film showed significant volatile organic compounds abilities under visible light irradiation. The microwave coated selectively reduced R d /A o TiO 2 film realized around 75% acetaldehyde degradation within 12 hours and almost 90% toluene degradation after 9 hours, also retains stable photodegradation performance during the cycling test. This study proposes a scalable, cost-effective, and rapid method to manufacture good and long-term stable binder-free 