Present and Future of Phase?Selectively Disordered Blue TiO2 for Energy and Society Sustainability

Yongguang Luo, Hyoyoung Lee,3,4

ABSTRACT Titanium dioxide (TiO2) has garnered attention for its promising photocatalytic activity, energy storage capability, low cost, high chemical stability, and nontoxicity. However, conventional TiO2 has low energy harvesting efficiency and charge separation ability, though the recently developed black TiO2 formed under high tem?perature or pressure has achieved elevated performance. The phase?selectively ordered/disordered blue TiO2 (BTO), which has visible?light absorption and efficient exciton disassociation, can be formed under normal pres?sure and temperature (NPT) conditions. This perspective article first discusses TiO2 materials development mile?

KEYWORDS Blue TiO2 (BTO); Phase?selective disordering; Visible?light?driven photocatalyst; Charge separation; Energy and society sustainability

1 Introduction

Modern society has achieved great science and technol?ogy explosion to date but faces severe energy demands and environmental concerns to realize sustainable development. One crucial issue in the twenty?first century is finding ways to convert and store renewable energy efficaciously while tackling climate change and environmental pollution caused by unsustainable human activity. In that context, titanium dioxide (TiO2) has received a lot of attention for its photo?catalytic activity, energy storage capability, low cost, high chemical stability, and nontoxicity. The initial discovery of the photocatalytic potential of TiO2dates back to the end of the 1920s [1]. Researchers observed that aniline dyes faded and fabrics degraded in the presence of TiO2, oxygen gas (O2), and ultraviolet (UV) light. However, the academic community did not show strong scientific enthusiasm about the phenomenon at that time due to a lack of interest in renewable energy and environmental stewardship.

That began to change after Fujishima and Honda reported the discovery of water photoelectrolysis into hydrogen (H2) by rutile TiO2under UV irradiation in 1969 [2]. The Honda-Fujishima water splitting finding was refined and called “natural photosynthesis” byNaturein 1972 [3]. Now, TiO2is one of the most promising photocatalyst materials. Its valence band (VB) and conduction band (CB) positions offer more diverse catalysis reaction potential than available with many other transition metal oxides and dichalcogenides [4]. Furthermore, the heterogeneous photocatalysis of TiO2enables smoother industrial recycling than is available for homogeneous photocatalysts.

TiO2has three main polymorphs, anatase, rutile, and brookite. The anatase and rutile phases of TiO2are the most frequently studied and synthesized in laboratories and industry, whereas brookite, as a natural phase, is rarely investigated as a photocatalyst due to difficulties in synthe?sizing it [5]. The anatase phase is reported to have higher photocatalytic performance than the rutile phase because it has better bulk charge transportation and a smaller recom?bination portion of the exciton [6, 7]. After several decades of developments, various TiO2synthesis approaches have been established through gas?phase reactions, solution?based methods, and alcoholysis from titanium tetrachloride (TiCl4) [8], titanium oxysulfate (TiOSO4) [9], and Ti(OC4H9)4[10]. To date, one of the famous TiO2photocatalyst products is commercial Degussa P?25 (P25) (Degussa Co., Ltd), which has been frequently applied as a benchmark photocatalyst [11, 12]. P25 TiO2contains a unique hybrid of anatase and rutile phases in a roughly 3:1 ratio and exhibits good per?formance in many photocatalytic systems [13]. TiO2is also used in other industries: energy (energy production and stor?age), environment (degrading pollution in the air, wastewa?ter, and indoors), human health and food (antibacterial, anti?virus sterilization), cosmetics (sunscreen against UVA and UVB), and self?cleaning and antifogging products [14, 15]. The antivirus potential of TiO2will certainly draw atten?tion during the struggle with the COVID?19 coronavirus pandemic [16].

TiO2still faces several hindrances to its photocatalytic performance. First, pristine TiO2can absorb sunlight only in the UV region (5%) due to its large electronic bandgaps (anatase, 3.2 eV; rutile 3.0 eV), which results in extremely low photocatalysis quantum efficiency that fails to meet the needs of industrial applications. Second, the separated charges (electrons and holes) formed after photoexcitation of a photocatalyst can recombine and disappear, giving sub?sequent photoluminescence. This exciton recombination process reduces the number of active electrons and holes on the photocatalyst surface, which is detrimental to any photocatalysis reaction. Therefore, research is needed to boost light absorption efficiency and block charge recom?bination to maintain a high exciton dissociation capability. Many attempts have been made to attain a broader range of light absorption by using non?metallic elements (C, N, and S) [17] and transition metal doping [18, 19] to tune the TiO2electronic structure. However, only a few researchers have tried to develop advanced TiO2by targeting both vis?ible?light absorption and high charge separation efficiency. Those efforts produced “Black TiO2” [20] and “Blue TiO2” [21] in 2011 and 2016, respectively. In addition, the phase?selectively disordered blue TiO2(BTO) offers high H2gen?eration performance through its three unique phase-interface configurations.

Next, we summarize several historical milestones in the development of TiO2materials and then systematically illus?trate the development logic and discovery of BTO, includ?ing its mild synthesis conditions, robust reducing agent design, and phase?selective disordering. The phase selec?tivity of BTO results from its unique structure, which we disclose on the crystalline dimension level, and the reduction power of alkali metal amines. Its ordered?disordered phase junctions, type II band alignment structure, and a surface rich in hydroxyl groups explain the high exciton dissociation efficiency, visible?light absorption, and superior photoca?talysis of BTO. Particularly, we further present the explora?tory attempts of BTO in various energy and environmen?tal aspects. Finally, we suggest future research avenues to explore the potential of BTO further.

2 Milestones in TiO2 and the Development of BTO

The research community has long worked to exploit the energy efficiency and activity of TiO2in versatile applica?tions. Events of considerable significance in the recent his?tory of TiO2development are shown in Fig. 1a. The first fundamental finding in TiO2photocatalysis for energy con?version was a report of water electrochemical photolysis which generates the absolute clean energy gas, H2, using a rutile TiO2semiconductor electrode in 1972 by Fujishima and Honda et al. However, the applied rutile TiO2has lower charge transportability than anatase TiO2, even though it has better light absorption efficiency to generate more charges. The underlying reasons that anatase has better charge transportation than the rutile phase are the higher VB maximum energy level of the anatase phase (Fig. 1b) [7], its preferred crystalline surface orientation [22], and its longer exciton (electron and hole pair) lifetime [23]. To use the advantages of both the anatase and rutile phases, the commercially available phase?mixed P25 TiO2has been widely used as a standard photocatalyst. It has been proved a better activity than the single?phase TiO2since the 1990s [13, 24, 25]. Extending the TiO2light absorption range was the main challenge after the development of P25. In 2011, Xiaobo Chen et al. found that TiO2phase disorder engi?neering through hydrogenation enhanced its light absorp?tion capability into the visible and infrared ranges [20]. That hydrogenated black TiO2mostly answered concerns about the photocatalysis energy efficiency of TiO2. Moreo?ver, the photocatalytic activity of black TiO2is boosted by suppressing exciton recombination through the middle VB by means of localized holes generated in the disordered sur?face. During approximately four decades of research, TiO2material development has produced decent UV-visible?light absorption, acceptable photocatalytic activity, and reason?able charge generation. Nevertheless, TiO2still requires more development to be practical for energy production and photocatalysis applications. After thoroughly investigating the recent achievements in black TiO2material design, we found that most black TiO2synthesis approaches require high temperatures (400-900 °C) or a high H2atmospheric pressure (20-70 bar) [26]. Furthermore, the black TiO2core/shell structure produces a back reaction that diminishes its photocatalytic power because of its sole surface reaction interface. The issues that remain to be addressed since the development of black TiO2are developing an industrially suitable manufacturing process with mild conditions and further strengthening exciton dissociation and catalytic reac?tion efficiency.

3 NPT Synthesis of BTO and Its Phase?selective Specialty

The high?temperature and H2atmospheric pressure synthe?sis conditions of most black TiO2are energy?intensive and potentially explosive, an unfavorable manufacturing choice in both laboratories and industry. Therefore, it is essential to find a suitably, potent reducing agent or system. Birch reduction agents, an alkali metal in liquid ammonia, can reduce arenes into cyclohexadiene rather than cyclohexane [27]. Among the Birch reduction processes, the electride salts that form by mixing an alkali metal (M) and ammonia (NH3) as [M(NH3)x]+e?have strong reducing power. There?fore, we supposed that producing such vital electride salts as a reduction species would contribute to TiO2reduction. We found that a lithium ethylenediamine (Li?EDA) solu?tion reduced the rutile phase of P25 TiO2while keeping the anatase phase intact, which resulted in a unique blue TiO2product (BTO(I)) [21].

The superior photocatalysis performance of BTO stimu?lated us to investigate the origin of its phase selectivity fur?ther. As shown in Fig. 2a, the free electron of M?EDA elec?trides can attack the firm Ti-O bond and produce a reduced Ti3+state. The evidence for Ti3+and oxygen vacancy (OV) were provided by X?ray photoelectron spectroscopy and electron paramagnetic resonance in our previous reports from 2015 to 2019, respectively. Besides, the reduced TiO2is generally presented in a disordered amorphous physical state with a black appearance. The blue color of BTO is caused by the coexistence of an ordered crystalline anatase (Ao) phase and a disordered amorphous rutile (Rd) phase. The successful reduction of TiO2by a Li?EDA solution in normal pressure and temperature (NPT) conditions indicates its mighty reducing power. It successfully replaced the high pressure and temperature hydrogenation reduction approach.

Fig. 1 a Milestones in TiO2 material development and b the corresponding band structure of each typical TiO2 configuration [3, 20, 21, 25, 28, 32]. The TiO2 nanoparticles illustration figures in (a) are adapted with the permission from Ref. [28]. Copyright (2019) American Chemical Society

After successfully preparing ordered anatase (Ao)/disor?dered rutile (Rd) TiO2from P25 with the Li?EDA solution under NPT conditions, we set out to design disordered anatase (Ad)/ordered rutile (Ro) TiO2from P25. With the curiosity of other alkali metal EDA reduction phenomena, we applied Na and K EDA solutions to reduce the P25. Interestingly, the Na/K?EDA solution selectively reduced the P25 TiO2reverse from the Li?EDA. Figure 2b shows that P25 TiO2turns to Rd/Ao(BTO(I)) through Li?EDA reduction and Ro/Ad(BTO(II)) through Na/K?EDA reduc?tion. Furthermore, the anatase and rutile phases TiO2were individually treated by Li?EDA and Na/K?EDA, respectively. The pure white rutile TiO2becomes black RdTiO2in a Li?EDA environment, and the anatase sin?gle?crystalline form becomes black or gray AdTiO2after the Na/K?EDA reduction. Based on that initial finding, which we were the first to report, the TiO2architecture can be widely enriched to extend its potential applications. Because of their blended ordered and disordered phase structure, BTO(I) and BTO(II) have high potential as pho?tocatalysts with effective heterojunctions and visible?light absorption. In addition, the M?EDA?reduced Rdand Adcan be used to anchor hybrid material systems and maintain a steady structure through covalent combinations.

To investigate the best amines for dissolving alkali met?als and reducing TiO2, we selected various liquid amine derivatives, including monoamines with different alkyl chain lengths (Numb. 1-4 in Fig. 2c) and diamines with diverse alkyl chain lengths and positions (Numb. 5-8 in Fig. 2c). The various M?amine solutions produced diverse forms from the P25 that were colored from blue to gray. Among them, the shortest alkyl chain diamine solution, Na?EDA, exhibited the best reduction results, producing a deep blue color and entirely vanished anatase crystalline phase, as shown in the detailed XRD characterization in Ref. [28]. EDA’s effects result from its effective diamine structure and higher polar?ity than the long alkyl chain amines, which contribute to its high alkali metal solubility. This newly developed, powerful reducing system (M?EDA) can be readily extended to the reduction of other metal oxides or metal sulfides and defect design objectives.

Next, we examined the crystallography of anatase and rutile TiO2at the atomic level to find the origins of the phase selectivity. Beginning with facet information about the rutile (110) and anatase (101) phases [28-30], we found the gap distance in the unit lattice to be around 2.96 ? × 2.96 ? for rutile (110) and 2.86 × 3.79 ?2for anatase (101), as shown in Fig. 2d. The diameters of Li, Na, and K atoms in the EDA environment are 2.7, 3.1, and 3.9 ?, respectively, as shown in the inserted table in Fig. 2d. Clues about phase selectivity can be drawn from that lattice and atomic size information. Na and K, which are larger than Li, are relatively close to the anatase (101) lattice unit dimensions but more massive than the rutile (110) lattice gaps. Therefore, Na and K can attack Ti-O?Ti bonds in the anatase phase and break the anatase crystalline into a disordered state. On the other hand, Li atoms can effectively attach to the rutile (110) lattice units, rather than the wider lattice spaces of the anatase (101), and thus successfully reduce only the rutile TiO2phase. In that way, the intrinsic adaptability of Li?EDA to the rutile phase and Na/K?EDA to the anatase phase determine the selectiv?ity of the disordering results.

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Fig. 2 NPT synthesis of BTO and its phase selectivity. a M?EDA electrides reduce pristine TiO2 to BTO under NPT conditions. b Different starting TiO2 phases are selectively reduced/disordered by M?EDA solutions. c Amine solvent investigation to synthesize BTO. Adapted with permission from Ref. [28]. d Proposed mechanism for the BTO phase?selective phenomenon. Adapted with permission from Ref. [28, 29]. e Water contact angle measurements (SEO PHX300) of the original P25 TiO2 and the phase?selectively reduced BTO. f Structure and appearance stability characterization by X?ray powder diffraction (SmartLab JD3643N) and digital photo images

Furthermore, the M?EDA treatment process is easy to scale up and highly repeatable through the alkali metal stepwise feeding. Using a hydrophilic material is necessary to provide good interfacial contact in many photocatalysis and other real?world applications. As shown in Fig. 2e, a water drop fully spreads on the BTO film, which indicates that BTO is more hydrophilic than pristine P25 TiO2. The excellent hydrophilicity of BTO originates from the enriched surface hydroxyl (OH) groups that appear after the M?EDA reduction. Pristine P25 TiO2has a hydrophobic surface, with a 130° water contact angle, due to the absence of hydro?philic functional groups on its intact TiO2surface. Material stability is another concern for practical applications. BTO has maintained its original disordered/ordered structure and appearance for almost 2.5 years under ambient conditions, as represented in Fig. 2f. Thus, BTO has many advantages, from low?cost production to high potential for many practi?cal applications.

4 Explored and Potential Applications of BTO

BTO exhibits strong visible?light (380-740 nm) absorption ability with a narrow optical bandgap (Fig. 1b), efficient photoinduced exciton disassociation with a heterojunction structure [21], and excellent hydrophilicity and stability (Fig. 2e, f). Our group has applied BTO to promote green energy and social sustainability in the field of hydrogenation [21], algae elimination from aquatic ecosystems [31], carbon dioxide (CO2) reduction [28, 32], and visible?light?driven organic synthesis (C-H arylation) [33]. Next, we describe those BTO applications and then propose strategies and directions for further designs and applications of BTO.

4.1 Explored Photocatalytic Aspects of BTO

Hydrogen has been deemed a perfect blue energy source that could solve the energy crisis in the twenty?first century. For example, its heating value (141.72 MJ kg?1) is three times higher than gasoline (46.4 MJ kg?1) [34], and it is an extremely abundant material that produces zero pollu?tion and has reproducible capabilities through the water. Solar?driven photohydrogenation has received much atten?tion because of its high sustainability. BTO, as a typical semiconductor material, can be used as a robust hydrogena?tion photocatalyst and has shown a remarkable performance enhancement over P25 and most other reported TiO2mate?rials [21]. The extended light absorption spectrum of BTO covers all solar illumination, which maximizes the quantum efficiency of its photocatalysis process. However, it is not enough to have a favorable light?harvesting ability; a desir?able hydrogenation photocatalyst must also produce effec?tive charge separation through a specific structure designa?tion. As shown in Fig. 3a, BTO retains discrete catalytic redox reaction sites for the reduction of water to hydrogen and methanol sacrificial agent oxidation. The right?side gray Rdis responsible for absorbing enough light irradia?tion and generating the photoinduced electron and holes. Afterward, the adjacent Aoaccepts electrons to trigger water splitting. Compared with the conventional core-shell struc?ture of black TiO2, BTO eliminates the need for electrons to migrate from the core to the interface of the shell and water. Therefore, it greatly reduces the potential for charge recombination. Furthermore, the type II band alignment configuration of BTO assists in exciton dissociation and keeping the effective charges. The open ordered/disordered structure of BTO realized a superior H2production rate of 13.89 mmol h?1g?1with 0.5 wt% Pt and 3.46 mmol h?1g?1without the Pt co?catalyst.

Algae blooms happen regionally in various brine and river systems, mainly due to water eutrophication induced by human activities, and they damage public health and the social economy [35]. Severe overgrowth of algae can kill aquatic creatures by consuming the limited oxygen dissolved in the water. TiO2, as a typical photocatalyst, can gener?ate reactive oxygen species (ROS), which mainly consist of a hydroxyl radical (·OH) and superoxide anion radicals (·O2?), through photo irradiated hot carriers that attack water molecules. The ROS then remove algae. However, conven?tional TiO2has low efficiency in generating sufficient ROS for algae elimination. Based on our previously obtained photocatalytic hydrogenation experience, we applied BTO to remove Chlamydomonas green algae (Fig. 3b) [31]. We expected that the powerfully wide range of light absorption and effective charge separation properties of BTO would produce an efficient ROS amount. The algae removal test was conducted under both UV and solar light with various types of TiO2. The BTO wiped out all the algae cells within 2-2.5 h, which was the most rapid among the kinds of TiO2tested. Thus, BTO has meaningful roles to play in realizing a sustainable society.

The photoreduction of CO2into chemical fuels under solar or visible light is supposed to be an excellent way to target both energy and environmental concerns. This so?called artificial photosynthesis strategy has been under study for a while, but desirable conversion selectivity and production yield are still lacking [36]. Moreover, it is quite hard to crack the C = O bonds in the CO2molecule because the dissociation energy demand is high (around 750 kJ mol?1) [37]. The ideal photocatalyst for the CO2reduction reaction (CO2RR) needs a specific configura?tion with an optical band position (especially the CB) that is close to the CO2reduction potential, such as the ? 0.24 VNHEof CO2to CH4or the ? 0.52 VNHEof CO2/CO, and also efficient charge separation with good electron transport. BTO has those structures, so we conducted CO2reduction experiments using BTO(II) under visible light [28] and BTO hybrid materials (BTO(I)/WO3?Ag) under solar light [32]. The BTO(II) reached unprecedented CH4production levels (3.98 μmol g?1h?1), with the highest yield among all the metal (Pt, Ru, W, and Ag)?doped P25 TiO2materials tested. The evident CO2RR ability was conferred by the excellent match between the CB position of BTO(II) (? 0.24 VNHE) and the CO2to CH4potential and the efficient visible?light absorption by the Adwith rapid charge?carrier disassociation (Fig. 3c). Even though BTO(II) offered excellent CO2RR performance, another critical issue for CO2RR, product selectivity, also has to be addressed. Consequently, we designed and constructed BTO(I)/WO3?Ag, a combination material intended to build a particularZ?scheme band structure, as presented in Fig. 3d. The assembledZ?scheme band alignment can maximize the effective potential between a high CB and low VB and then strengthen the catalytic redox power. Notably, the CB position (? 1.55 VNHE) of BTO(I) is close to the CO2to CO potential (Fig. 1b), which contributes to CO production, and higher than the CB of WO3(0.74 VNHE) used to construct theZ?scheme band alignment. In addition, the low difference between the VB of BTO(I) (1.14 VNHE) and the CB of WO3facilitates the flow of excited WO3electrons to BTO(I), thereby reinforcing the number of effective hot electrons. The decorated Ag nano?particles serve as an electron reservoir that can initiate photoelectron production by means of the localized sur?face plasmon resonance effect and further enhance visible?light absorption. When tested, this BTO basedZ?scheme composite produced absolute CO selective?production of 1166.7 μmol g?1h?1at the excellent photocatalytic elec?tron reaction pace of 2333.4 μmol g?1h?1. All in all, BTO showed vigorous CO2RR strength in producing CH4or CO with high output and selectivity, which means it can be an attractive way to tackle global warming and energy deficiency together.

Fig. 3 Explored photocatalytic applications for BTO. a Unique three?phase?interface BTO(I) robust H2 photogeneration from water. Adapted with permission from Ref. [21]. b Efficient Chlamydomonas green algae disinfection by BTO(I) under solar irradiation. Adapted with permis?sion from Ref. [31]. c Visible?light?driven CO2 reduction (CO2RR) to CH4 by BTO(II). Adapted with permission from Ref. [28]. d BTO(I)/WO3?Ag combination with a Z?scheme band structure for high?selectivity CO2RR to CO. Adapted with permission from Ref. [32]. e BTO(I) photocatalytic activity in C-H arylation organic synthesis. Adapted with permission from Ref. [33]

Light?driven chemical synthesis is also an essential field that requires promising photocatalysts to boost synthesizing efficiency [38]. C-H arylation for organic synthesis was cho?sen as a typical study case to show the photocatalytic activity of BTO (Fig. 3e) [33]. The phase?mixed BTO absorbs light in the visible range through its Ti3+defect?rich disordered state. It maintains good adsorptivity of an organic reactant and charges separation via its ordered crystalline phase. First, a charge transfer complex (4) formed on the Aosite of BTO(I) from the aryl diazonium compound (2). Then, under visible?light irradiation, photogenerated electrons flowed to the anatase CB due to the type II band alignment and efficiently separated from the holes. An aryl intermedi?ate radical (5) was produced after the single electron transfer process from Aoto (4). As arylation proceeded, after initia?tion by aryl radical (5), the resulting radical (7) intermedi?ate was oxidized by the hot hole carrier from the AoVB and gave the desired product (3) after deprotonation of the product (8). Moreover, BTO offers high reusability through direct filtration, and it maintained consistent yield (63%) performance when five batches were examined under six?fold scaled?up conditions. This application of BTO to pho?tocatalytic chemical synthesis will enrich the role of TiO2in industrial chemical synthesis and contribute to further product cost reductions.

4.2 Potential Application and Design Commentary of BTO

Currently, ammonia (NH3) synthesis from nitrogen gas (N2) is an essential approach to supplying nitrogen to plants and humans by industry manufacturing. The Haber-Bosch pro?cess for NH3synthesis (N2+ H2→ NH3), which has been used in industry for more than a century, urgently needs to be replaced due to its high consumption of fossil fuels, which results in enormous greenhouse gas (CO2) emissions and extremely harsh operating conditions (400-500 °C, 100-200 bar with an iron?based catalyst) [39]. Therefore, photocatalysis nitrogen fixation that can use sustainable solar energy and eliminate CO2emissions has attracted growing attention. However, it remains challenging to design an effi?cient photocatalyst to convert N2to NH3under NPT condi?tions with a high production rate and clear mechanism [40]. It has been reported since 1977 that TiO2generated NH3and other gasses under UV irradiation with an N2source, but the process offered minimal yields and low selectiv?ity [41]. After several decades of progress, the yields from TiO2?driven photosynthesis of NH3have been enhanced by hundreds?fold [40]. However, most related studies still apply only UV light because of the narrow light absorption region of conventional TiO2, which hinders the application range and produces low solar coulombic efficiency. As illus?trated in Fig. 4a, by tracking the advanced TiO2photocata?lyst design milestones, it has high credits to investigate the N2fixation to NH3by taking phase?selective disordering and visible?light harvesting advantages of BTO for target?ing maximized NH3production yield and selectively under mild NPT conditions.

In recent years, volatile organic compounds (VOCs), which vaporize easily at room temperature, have become major hazardous pollutants in the air through speedy indus?trialization and urbanization. Some studies show that indoor atmospheres can have 2-10 times more VOCs than outdoor environments [42]. Therefore, VOCs’ health concerns, such as cancer, headaches, and dizziness, are serious among peo?ple who spend most of their time in buildings or enclosed spaces. Among the various VOCs, toluene, benzene, and aldehydes (formaldehyde and acetaldehyde) are the most common and toxic species [43]. Photodegradation of VOCs is inevitably regarded as the best and most economical choice for dealing with VOCs in the air. The carbon-car?bon bonding and carbonyl groups in VOC molecules are comparatively stable, requiring sufficiently hot carriers from a powerful photocatalyst to be decomposed. BTO is expected to actively cause full VOC degradation into CO2and H2O by effectively generating photoinduced charges and inhibiting exciton recombination under solar and indoor LED lamplight (Fig. 4b). Additionally, the hydroxyl?rich character of the disordered portion of BTO can specifically support the covalent coating and binding process on various substrates and objects (such as air conditioner filters, indoor walls, and subway carriages) and thereby provide versatile application choices.

Microbial pathogens, which include various bacteria and viruses, are major health concerns to humans world?wide. They occasionally cause serious infectious disease pandemics, such as those caused by the novel corona?virus (COVID?19), severe acute respiratory syndrome coronavirus (SARS), swine influenza virus (H1N1), and Middle?East respiratory syndrome coronavirus (MERS) [16]. For the sake of human health, society needs effec?tive microbial disinfection systems with enough versatility to attack airborne, waterborne, and foodborne pathogenic species. Practically, various microbicidal processes have already been adopted, such as UV disinfection, antibi?otic sterilization, thermal treatments, and nanofiltration. However, the current approaches possess significant limi?tations; for instance, some microbes have already evolved antibiotic or UV resistance [44], and thermal, and filtration operations can cause energy exhaustion and are incompat?ible in many spaces. The microbial pathogens inactivation by TiO2photocatalyst can trace to 1994 after the Sjogren et al. finds the inactivation ability to bacteriophage MS2 on TiO2[45]. Besides, TiO2could be a good option for microorganism disinfection that is low cost, requires mini?mal energy consumption, and is harmless and eco?friendly [46, 47]. In the TiO2photocatalysis microorganism disin?fection process, the ROS generated from photocatalytic processes after light irradiation plays the major roles [16]. To further boost the microbial pathogens inactivation per?formance of TiO2, we need to strengthen the producing amount of ROS species. In the authors’ group previous reports, BTO can generate a higher amount of ROS spe?cies under UV, visible or solar light illumination, which is represented by the higher peak intensity of BTO than pristine TiO2in electron paramagnetic resonance analysis [31, 48]. Therefore, BTO could act as a broad?spectrum antimicrobial agent and outperform pristine TiO2by gen?erating sufficient antibacterial and viricidal ROS at differ?ent band positions, as depicted in Fig. 4c. Harmful bacteria and viruses in living spaces could be effectively deacti?vated under mild conditions by using BTO and solar or visible light.

Fig. 4 Potential applications and design commentary of BTO. a Photo?driven N2 reduction to NH3 to replace the conventional Haber-Bosch approach. b Photodegradation of volatile organic compounds (VOCs), especially in indoor atmospheres. c Adapting to visible?light?induced microbicidal processes. d Exploring BTO as electrode material in an energy storage system by taking advantage of its electro?conducting Ti3+ species, oxygen vacancy, and stability

Because TiO2has superior stability, high safety, and good economic value, it has been investigated and considered as an anode or cathode candidate in various ion battery systems, including single?valent alkali?ion batteries (LIBs, SIBs, and KIBs) [49], multivalent magnesium ion batteries (MIBs) [50] and aluminum ion batteries (AIBs) [51]. Also, researchers have noticed that Ti3+self?doped black anatase TiO2has better rate capability than pristine white anatase in LIBs [52], and the associated OV of black TiO2resulted in high?performance magnesium ion (Mg2+) storage [50]. Nevertheless, the synthesis of black TiO2requires a high?temperature reduction process, and their black TiO2prod?ucts remain in a majority crystalline phase and only acquire a small portion Ti3+; even the OV and Ti3+was suggested as main contributions to the advances. Therefore, we pro?pose BTO (including the Adand Rdsynthesized by M?EDA) as an encouraging candidate for battery system electrodes (Fig. 4d). Our M?EDA reduction approach, along with the production of BTO under NPT conditions, can almost completely disorder anatase (Na?EDA) and rutile (Li?EDA) TiO2and deliver sufficient OVs and electro?conducting Ti3+species to enhance energy storage performance.

5 Prospects and Summary

The science and technology exploitation has been speeding up in modern society than any other historical era. Based on the invention of BTO, the research progress towards energy and society sustainability can be promoted from diverse aspects. The forthcoming flourishing research suggestions based on the account of BTO achievements are suggested below (Fig. 5).

1. Design and synthesize a BTO specific morphology and structure in a different dimension (0, 1, 2, 3D). Nano?structured materials are essential for photoelectrochemi?cal devices because of their exposed active surfaces, obviously upgraded kinetics, and versatile adaptations [53]. The potential of BTO could be widely explored by investigating it in 0D (quantum dots), 1D (nanowires, nanotubes, nanoribbons, and nanorods), 2D (nanoplates, nanodisks, and nanosheets), and 3D (nanoflowers, nano?coils, and ordered mesoporous framework) forms.

Fig. 5 Future research suggestions based on the unique properties of BTO to improve energy and social sustainability

2. Construct graphene/carbon composites with BTO for use in flexible and wearable energy devices to advance their mechanical and electron flow properties.

3. Synergize BTO with other typical transition metal dichalcogenides and single or dual metal atoms to further boost its photocatalytic performance in terms of yield, selectivity, and long?term stability.

4. Couple BTO applications with external fields (elec?tricity, magnetism, plasmonic energy, microwaves, or polarized light). The external fields are expected to influ?ence the photocatalytic process in several ways, such as inducing polarization in reactant molecules (like CO2, N2, and VOCs) to assist in the dissociation of molecules, prompting chiral molecule pure enantiomer synthesis, and altering the hot carrier migration pathways of the photo?catalyst under an electromagnetic wave, so on.

5. On the frontier of space science, one of the ultimate goals is to build an environment in which humans could live. Space applications of BTO could lead to a bright future for sustainable human civilization. Currently, the International Space Station is equipped with the “Photocatalytic Oxidation Reactor System” (PORS) for VOC removal during the potable water purification step [54]. And the Kennedy Space Center has developed a visible?light?responsive Ag?doped TiO2catalyst PORS in 2016 for better water purification system [55]. BTO, as an advanced photocatalyst, has shown superior pho?tocatalytic activity than most noble metal?doped TiO2and will enable the efficient acquisition of clean?living necessities (food, water, and air) in the Space living area. Furthermore, researchers have found that up to 10 wt% of TiO2exists in the regional area of Moon’s crust, which can further serve to assist the future human exploration of the Moon [56].

Herein, we have described milestones in TiO2mate?rial design, including the development of BTO. Then, we explained our M?EDA phase?selective disordering mecha?nism and the unique advances offered by BTO in visible?light absorption and exciton disassociation. We continued by discussing applications already achieved and pro?spective advances from those. Last, we proposed several potential new prospects for BTO that target energy and social sustainability. Relying on the structure specialty and superior accomplishments, the unique NPT?synthesized BTO could offer more socially beneficial applications and approach to commercial, robust visible?light?driven ver?satile photocatalyst if its potential is fully explored by the research community.

AcknowledgementsThis work was supported by the Institute for Basic Science (IBS?R011?D1) and partially supported by the Korea Evaluation Institute of Industrial Technology (20004627) and the INNOPOLIS Foundation (2019?DD?SB?0602).

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