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Visible-light-sensitive Photocatalysts. Nanocluster-grafted Titanium Dioxide for Indoor Environmental Remediation Masahiro Miyauchi, Hiroshi Irie, Min Liu, Xiaoqing Qiu, Huogen Yu, Kayano Sunada, and Kazuhito Hashimoto J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02041 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 11, 2015

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The Journal of Physical Chemistry Letters

Visible-light-sensitive Photocatalysts. Nanoclustergrafted Titanium Dioxide for Indoor Environmental Remediation Masahiro Miyauchi,*,† Hiroshi Irie,ĵ Min Liu,‡ Xiaoqing Qiu,‡ Huogen Yu,‡ Kayano Sunada,‡ Kazuhito Hashimoto,*,‡,§

†

Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1

Ookayama, Meguro-ku, Tokyo 152-8552, Japan. ĵ

Clean Energy Research Center, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi

400-8511, Japan. ‡

Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1

Komaba, Meguro-ku, Tokyo 153-8904, Japan. §

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1

Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan

Corresponding Authors

1

ACS Paragon Plus Environment


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* Dr. Masahiro Miyauchi ([email protected]) * Prof. Kazuhito Hashimoto ([email protected])

ABSTRACT

Photocatalytic degradation of organic compounds requires photoexcited holes with strong oxidative power in the valence band (VB) of semiconductors. Although numerous types of doped semiconductors, such as nitrogen-doped TiO2, have been studied as visible-light-sensitive photocatalysts, the quantum yields of these materials were very low because of the limited oxidation power of holes in the nitrogen level above the VB. Recently, we developed visiblelight-sensitive Cu(II) and Fe(III) nanocluster-grafted TiO2 using a facile impregnation method and demonstrated that visible-light absorption occurs at the interface between the nanoclusters and TiO2, as electrons in the VB of TiO2 are excited to the nanoclusters under visible-light irradiation. In addition, photogenerated holes in the VB of TiO2 efficiently oxidize organic contaminants, and the excited electrons that accumulate in nanoclusters facilitate the multielectron reduction of oxygen. Notably, Cu(II) and Fe(III) nanocluster-grafted TiO2 photocatalyst has the highest quantum yield among reported photocatalysts and has anti-viral, self-cleaning, and air purification properties under illumination by indoor light fixtures equipped with white fluorescent bulbs or white light-emitting diodes.

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TOC GRAPHICS

CB Voltage vs. NHE (pH=0)

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H2O2

0

eO2

+ 1.0

+ 2.0

Cu(II) or Fe(III) Nanoclusters

visible light

CO2

+ 3.0

VB

h+

Organic

TiO2

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TEXT: The bandgap energy required for photocatalysis depends on the redox potential of the target chemical reactions. For example, the potential of the conduction band (CB) to drive water splitting must be more negative than 0 V (versus normal hydrogen electrode (NHE) at pH= 0) for proton reduction, whereas that of the valence band (VB) must be more positive than + 1.23 V for water oxidation by a thermodynamic consideration.1 In contrast, the photocatalytic degradation of organic contaminants for environmental purification requires a larger bandgap than that needed for water splitting. Specifically, the potential of holes in the VB must be more positive than + 2.6 V (vs NHE at pH= 0) to generate •OH radicals or drive the direct oxidation of organic molecules, such as acetaldehyde.2 In addition, because photogenerated electrons react with oxygen molecules in the air, the CB potential must be more negative than -0.05 V. Therefore, to decompose organic contaminants under atmospheric conditions, the bandgap of the semiconductor must be wider than 2.6 eV. Further, the required bandgap value is typically larger than the theoretical minimum, because an overpotential is required for decomposition reactions to proceed. For these reasons, visible-light-sensitive photocatalysts used for environmental purification applications can only utilize photons below a wavelength of 480 nm, which although represents only part of the solar light spectrum, does overlap with blue light emitted from common room light fixtures, such those equipped with white fluorescent light bulbs and lightemitting diodes (LED). Although titanium dioxide (TiO2) has suitable CB and VB positions to drive redox reactions for environmental purification,3-4 it can only be activated by ultraviolet (UV) light at wavelengths shorter than 400 nm and is nearly inactive under room light.

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Nitrogen-doped TiO2 was previously reported as a visible-light-sensitive photocatalyst with the potential for environmental purification applications under indoor light illumination.5 Although nitrogen-doped TiO2 absorbs blue light, its quantum efficiency (QE) under these conditions is several fold lower than that under UV irradiation.6-7 Since the nitrogen energy levels were isolated above the VB of TiO2 (Figure 1 (a)), the photogenerated holes in the nitrogen levels had less oxidation power and slower mobility as compared to the holes in the VB, which consisted of the oxygen 2-p orbital of TiO2.

Visible-light-sensitive photocatalysts based on the doping of semiconductors are inefficient for indoor environmental purification application.

In contrast to doping technique, the surface modification of TiO2 is another potential strategy to improve photocatalytic activity. For example, visible light sensitizers like CdS or metal chloride complex have been grafted onto the surface of TiO2 (Figure 1 (b)).8-12 In these systems, excited electrons in the CB of sensitizers are injected into the CB of TiO2 for charge separation (detail explanation was described in our supporting information). However, the oxidation power of holes in sensitizers is limited, since the ionization potential of sensitizers is smaller than that of TiO2. Therefore, the oxidation power of sensitized system under visible light is lower than that of pristine TiO2 under UV light. On the other hand, our group recently developed efficient visible-light-sensitive photocatalysts by the simple grafting of nanoclusters onto TiO2 with strong oxidative activity (Figure 1(c)).13-15

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The nanoclusters were smaller than 5 nm and were composed of amorphous Cu(II) or Fe(III) oxides (Figure 2). The photocatalytic reactions driven by these materials proceed via two novel processes, namely visible-light-induced interfacial charge transfer (IFCT) between the metal oxide nanoclusters and TiO2, and the multi-electron reduction of oxygen molecules mediated by the cocatalytic promoter effect of the nanoclusters, as illustrated in Figure 1 (c). The IFCT process is theoretically feasible between a semiconductor and ligand under photon irradiation,16 and visible-light absorption through IFCT has been experimentally observed.17-19 Our group firstly developed efficient visible-light-sensitive photocatalysts that utilize IFCT for indoor applications aimed at environmental remediation. In addition to mediating the IFCT process, Cu(II) and Fe(III) nanoclusters promote the multi-electron reduction of oxygen molecules in air.14-15 In a typical semiconductor, photoexcited electrons react with oxygen molecules in air to generate O2- radicals through single-electron reduction reactions with a redox potential of -0.05 V (vs NHE at pH= 0). Thus, to drive the single-electron reduction of oxygen, the semiconductor CB must be higher than -0.05 V. In contrast, Cu(II) and Fe(III) nanoclusters promote the multielectron reduction of oxygen molecules to generate hydrogen peroxide. The redox potential of this multi-electron reduction reaction, which is represented in the following equation (1), is +0.68 V (vs NHE at pH =0),20-22 O2 + 2H+ + 2e- o H2O2

(1)

From a thermodynamic point of view, the multi-electron reduction of oxygen is easier than a single-electron reduction process; however, the excited electrons must be accumulated at a specific reaction site to drive the multi-electron reaction. Notably, Cu(II) or Fe(III) nanoclusters promote the multi-electron process due to their nano-scale size and amorphous nature, which provides high structural flexibility even in excited states. On the basis of two findings, i. e. 6


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visible-light absorption of Cu(II)- and Fe(III)-nanocluster-grafted TiO2 by IFCT and the cocatalyst effect of this material for the multi-electron reduction of oxygen, we developed an efficient visible-light-sensitive photocatalyst for environmental remediation, particularly for indoor applications. Figure 3 shows optical absorption spectra for Cu(II) or Fe(III) nanocluster-grafted TiO2. These spectra were much different from those of bulk CuO and Fe2O3, which have bandgaps of 1.3 and 2.0 eV.23-24 Cu(II)- and Fe(III)-grafted TiO2 exhibited visible-light absorption between approximately 400 and 500 nm, attributable to the IFCT process, whereas the optical absorption above 650 nm in Cu(II)-grafted TiO2 was assigned to d-d transition of the copper d-orbital. Action spectrum analysis demonstrated that the optical absorption at 400 to 500 nm (blue light) was responsible for driving the strong oxidation of organic compounds, whereas photons with wavelengths longer than 600 nm did not contribute to any photocatalytic activities.15 Figure 4 shows the evolution of carbon dioxide (CO2) by photocatalytic oxidation of gaseous 2-propanol mediated by Cu(II)- and Fe(III)-grafted TiO2 under blue light. Both photocatalytic materials completely decomposed 2-propanol into CO2 with markedly higher activities than that of nitrogen-doped TiO2. When the Cu(II) or Fe(III) nanoclusters were grafted onto the surface of TiO2 powder, electrons in the VB of TiO2 could be excited to the unoccupied orbital of the Cu(II) or Fe(III) nanoclusters through the interface formed between these materials. The photogenerated holes in the VB of TiO2 have strong oxidative power for the decomposition of organic compounds such as aldehydes and alcohols, and the excited electrons in the nanoclusters are capable of reducing oxygen molecules in air through a multi-electron process. We have characterized the local structure and reaction mechanisms of Cu(II)- and Fe(III) nanocluster-grafted TiO2 using various spectroscopic analyses. The Cu(II) and Fe(III) 7



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nanoclusters were less than 5 nm in size (Figure 2) and did not show any peaks in X-ray diffraction pattern owing to the amorphous nature of these structures. Analyses of the local structures of Cu(II)- and Fe(III) nanoclusters by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) revealed that Cu(II) nanoclusters have a distorted CuO structure, in which the apical oxygen approaches the Cu(II) atom, forming a fivecoordinate square pyramid that results from distortion near the interface of nanoclusters.14 For the Fe(III) nanoclusters, the local structure resembles that of FeO(OH) or Fe2O3,15 and Mössbauer spectrum analysis additionally revealed a clear quadrupole split, indicating that the Fe(III) nanoclusters have a larger structural degree of freedom owing to their amorphous structure.25 The excited states of Cu(II) nanoclusters were also investigated by X-ray absorption analyses before and after visible-light irradiation in aerobic and anaerobic atmospheres.14 Figure 5 shows the XANES spectra of Cu(II) nanocluster-grafted TiO2 before light irradiation, under visiblelight irradiation in a gas mixture of N2 (in the absence of O2) and 2-propanol, and following the photocatalytic reaction followed by upon exposure to an air (O2) atmosphere. The absorbance at approximately 8988 eV increased under the visible-light irradiation, indicating that Cu(I) species formed in nanoclusters. Visible-light irradiation initiates IFCT, which involves the direct transfer of electrons in the VB of TiO2 to Cu(II) nanoclusters, resulting in the formation of Cu(I) species. During the IFCT, the holes produced in the VB are consumed during the oxidation of 2-propanol. After exposure to air, the XANES spectrum appeared similar to that obtained before light irradiation, indicating that the Cu(I) was converted back to Cu(II). The reversibility of these changes was confirmed, demonstrating that the produced Cu(I) can reduce oxygen and is oxidized to Cu(II) in the process. Nosaka et al. conducted electron spin resonance (ESR)

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analyses, which is also powerful tool to discuss the formation of excited charge carriers in heterogeneous semiconductors.26 The ESR signal of Cu(II) species was decreased under visible light irradiation in the absence of oxygen, owing to the change of Cu(II) into Cu(I) species. In contrast, the ESR signal was not changed in the presence of oxygen even under visible light irradiation. Further, the ESR signal of holes in VB of TiO2 was clearly observed under visible light irradiation, while holes' signal was not appeared in the presence of 2-propanol. These results revealed that the Cu(II) nanoclusters act as efficient cocatalyst for oxygen reduction, while photogenerated holes in VB of TiO2 oxidize 2-propanol under visible-light irradiation. Nosaka et al. also observed hydrogen peroxide (H2O2) production from Cu(II) nanoclusters grafted tungsten oxide under visible light irradiation, owing to the multi-electron reduction of oxygen.26 They also suggested the formation of H2O2 from Cu(II)-TiO2, however, the generated H2O2 molecules were strongly adsorbed onto the TiO2 surface.26 Very recently, atomic force microscopy (AFM) analysis of ultrathin-patterned CuO film, which was deposited on single crystals of rutile TiO2 by a pulsed layered deposition (PLD) and lithography method, revealed that the photocatalytic reaction sites under blue LED irradiation are located at the interface between the ultrathin CuO film and bulk TiO2.27 Photocatalytic reaction sites were examined by a probe photo-reduction of silver ions (Ag+) into metallic Ag particles. Patterned CuO/TiO2 samples were immersed in aqueous solution of silver nitrate (AgNO3) and the CuO film was then irradiated with blue light emitting diode (LED) light for 1 h. Figure 6 shows an AFM image captured at the edge of the patterned CuO film (6-nm thick) on a single crystal of rutile TiO2 after photodeposition of Ag nanoparticles under blue LED. Ag particles were selectively deposited on the ultrathin CuO film near the interface with bulk TiO2, suggesting that Ag deposition occurred predominantly through an IFCT process. Under blue

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LED irradiation, IFCT absorption occurs at the interface between CuO and TiO2, and electrons in the valence band of TiO2 would be excited to the conduction band of ultrathin CuO. Near the edges of the CuO film, photogenerated holes in TiO2 can laterally diffuse to the bare substrate region, where they initiate the water oxidation. In contrast, photogenerated holes located far from the CuO film edge are not able to access the aqueous medium and recombine with photogenerated electrons. Thus, the reduction reaction of Ag+ ions is greatly enhanced near the edge of patterned CuO films owing to efficient charge separation. These results strongly suggest the IFCT occurs between nano-sized CuO and bulk TiO2 under irradiation with blue light, and the design of nano-sized cluster is effective to increase reaction sites. By optimizing the synthesis conditions for nanocluster-grafted system, the photocatalytic performance of TiO2-based catalysts has greatly improved.25, 28 Previous study reported that rutile phase had superior photocatalytic performance to that of TiO2 in the anatase phase, owing to high crystallinity and efficient hole mobility of rutile TiO2.29 Table 1 lists the photocatalytic performances of various optimized nanocluster-grafted systems under visible-light irradiation and that of nitrogen-doped TiO2 as a comparison. The performance parameters have the following relationship, Reaction rate (Rco2) = Absorbed photon number (Rap) uQuantum efficiency (QE)

(2)

Rate of incident photons (Rip) was 1.30 u 1016 quanta/sec for all samples, as the same blue light source was used for all experiments. Notably, although the absorbed photon number in nitrogendoped TiO2 was much larger than that of the nanocluster-grafted TiO2 samples, its QE was markedly lower (3.9%). In contrast, the QEs of the nanocluster-grafted TiO2 systems exceeded 50 % under visible-light irradiation and, correspondingly, their reaction rates were much higher

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compared to nitrogen-doped TiO2. In the case of nitrogen-doped TiO2, photoexcited charge carriers are generated at doped sites in bulk and subsequently diffuse to the surface where they participate in photocatalytic reactions. In contrast, all excited electrons in nanocluster-grafted TiO2 systems are generated in the nanoclusters, which act as efficient reduction cocatalysts; thus, these systems exhibit high QE values even under visible-light irradiation. For comparison, the performance of Cu(II) nanoclusters grafted tungsten oxide (Cu(II)-WO3) has been listed in Table 1, since WO3 is also one of the efficient visible-light-sensitive photocatalysts to date.13, 30 It is noteworthy that the QEs of Cu(II)-TiO2 and Fe(III)-TiO2 are higher than that of Cu(II)-WO3, and reaction rates of nanoclusters grafted TiO2 are comparable to that of Cu(II)-TiO2. Although WO3 exhibited high reaction rate under visible light irradiation, WO3 is steep yellow color and is not abundant as natural resources. From the practical point of view, TiO2 based photocatalysts have more economical advantages and be applicable to transparent coating, rather than WO3 based photocatalysts. Notably, nanocluster-grafted TiO2 systems also have superior UV light-induced photocatalytic activity to that of pure TiO2 (see our supporting information, Figure S1). Using ESR and Kelvin probe force microscope (KPFM) analyses, we also recently demonstrated that amorphous Ti(IV) nanoclusters effectively promote the oxidation of organic compounds.31 The holes generated by visible-light excitation were also shown to be trapped at Ti(IV) clusters, leading to better charge separation efficiency. To date, Fe(III) and Ti(IV) nanocluster-grafted TiO2 has the highest QE and reaction rates for the decomposition of gaseous 2-propanol among reported photocatalysts.

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Quantum efficiency of nanocluster-grafted TiO2 exceeds 90 % under visible-light irradiation.

The presently described nanocluster grafting technique for TiO2 is also applicable for various other semiconductor photocatalysts. For example, several studies have reported that Cu(II) or Fe(III) nanoclusters improve the photocatalytic activities of WO3,13, 32 ZnO,33-34 SrTiO3,35-36 Bi2O3,37 SnO2,38 BiOCl,39 and Nb3O8-.40 Furthermore, Yu et al. demonstrated that Cu(II) nanoclusters function as general cocatalysts for Ag-based compounds, including AgCl, Ag3PO4, AgBr, AgI, Ag2CO3, and Ag2O.41 In addition to pristine metal oxides and metal halides, the efficiencies of metal-doped semiconductors, which are generally limited because the doped levels act as recombination centers, were also improved by the grafting of Cu(II) or Fe(III) nanoclusters. For example, the photocatalytic activities of Ti3+ self-doped TiO2,42 Nb5+-doped TiO2,43 and W5+ and Ga3+-codoped TiO2,44 Cd2+-doped ZnO,33 Ce3+-doped ZnO,34 Mo3+ and Na+-codoped SrTiO335 were largely enhanced by the grafting of Cu(II) or Fe(III) nanoclusters. These results suggest that the excited electrons in dopants are transferred to the surface-grafted nanoclusters to facilitate oxygen reduction. Unfortunately, the photocatalytic performances of these doped semiconductors are lower than those of nanocluster-grafted undoped TiO2 listed in Table 1, as only limited charge transport occurs between the doped level and surface-grafted nanoclusters. However, Liu et al. recently reported that Fe(III)-doped TiO2 grafted with Fe(III) nanoclusters, in which the energy levels were matched between the doped Fe(III) and surface Fe(III) nanoclusters to improve charge transfer,25 has comparable reaction rates to that of Fe(III)and Ti(IV)-grafted TiO2 (0.69 Pmol/ h; Table 1).25 Taken together, these findings indicate that

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certain metal-doped semiconductors, which are generally considered to be inactive due to their impurity states, become efficient visible-light photocatalysts after the grafting of co-catalysts. Thus, the grafting of nanoclusters can largely extend the possible candidate materials for the development of efficient visible-light-active photocatalysts. Regarding the variety of nanoclusters to drive IFCT, we have investigated various chemical compositions as nanoclusters cocatalyst onto TiO2, resulting that the Cu(II) and Fe(III) are the most efficient to oxidize organic compounds. In this perspective, though we mainly introduced Cu(II) and/or Fe(III) grafted TiO2, the opposite IFCT process was reported in chromium ion grafted TiO2 (Cr(III)-TiO2), which induces electrons excitation from occupied orbital of Cr(III) nanoclusters to the conduction band of TiO2.45 These results also suggest that the concept of the present IFCT system is applicable not only to the specific oxidation reaction, but also to various chemical reaction by grafting of metal oxide nanoclusters. One of the disadvantages of nanoclusters grafted TiO2 is their chemical stabilities, which would be worse than doped semiconductors such as nitrogen doped TiO2. The Cu(II) or Fe(III) clusters were corroded under strong acid or alkaline condition. But the Cu(II) and Fe(III) nanoclusters are stable under neutral water media and also exhibit long term durability under light irradiation in ambient air. Turnover number of the Cu(II)-TiO2 is greater than 60, which is calculated on the basis of generated CO2 product versus nanoclusters amount.28 Liu et al. also reported that the high performance of Fe(III) grafted TiO2 system could be maintained under repeated light irradiation in air for 1 year, and estimated the turnover number of this system to be more than 80.25 On the basis of these results, we can safely conclude that that our nanoclusters grafted TiO2 is very promising for practical indoor remediation application under aerobic ambient condition.

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Nanocluster-grafted TiO2 is composed of abundant elements and synthesized by simple solution process, and is practically applicable for indoor remediation applications such as anti-pathogenic, self-cleaning, and air purification.

The above-described nanocluster-grafted TiO2 photocatalysts are composed of abundant and safe elements, such as iron, copper, titanium and oxygen, and are easily fabricated by a simple wet chemical method. These materials are potentially suitable for various indoor applications due to their inherent properties, which include self-cleaning and anti-pathogenic functions. One of the key roles of Cu(II) or Fe(III) nanoclusters is to drive multi-electron reduction of oxygen molecules. Thus, our photocatalyst is working under the condition with abundant oxygen molecules such as ambient air atmosphere. In particular, nanocluster-grafted TiO2 is applicable for air purification to improve indoor air quality, as it can be activated under room light and is able to completely oxidize 2-propanol or acetaldehyde, which are volatile organic compounds (VOC) that are linked to sick building syndrome. The light intensity used to evaluate the photocatalysts listed in Table 1 and Figure 4 corresponded to an illuminance of only 300 lx, which is comparable to those of white fluorescent lights and white LED lights. For the performance tests, the initial concentration of 2-propanol was set to 300 ppm, which is much higher than the VOC concentrations typically encountered in indoor environments. In addition to air purification, the other important property of photocatalysts used for indoor applications is anti-fogging and self-cleaning functions based on super-hydrophilic conversion

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reactions.46 Previous studies investigated the mechanism of UV light induced super-hydrophilic conversion on pure TiO2.47-48 These previous studies imply that the photogenerated holes cause surface structural change of TiO2 to increase its surface energy, while photogenerated electrons are consumed through oxygen reduction reaction. Therefore, the requirements for superhydrophilic conversion are photogenerated holes in valence band of TiO2 with strong oxidative power and efficient reduction of oxygen molecules in air. The present nanocluster-grafted TiO2 meets these requirements and is predicted to show efficient super-hydrophilic conversion under visible-light irradiation. For the development of such films, although the crystal structure of rutile TiO2 efficiently promotes the IFCT process, the rutile structure has high thermodynamic stability and does not readily form small particles with high dispersibility in water for the coating of transparent thin films. To overcome this limitation, we used highly dispersive single crystalline rutile nanorods grafted with Cu(II) to produce a material with visible-light-induced super-hydrophilicity.29 Figure 7 shows an SEM image of the thin film of Cu(II) nanoclustergrafted rutile TiO2 nanorods and evaluation of the hydrophilic conversion property of the material under visible-light irradiation. Before exposing the thin films to visible-light irradiation, the film surface was coated with oleic acid as a model contaminant. Under visible-light irradiation, the thin film of Cu(II)-grafted rutile nanorods became super-hydrophilic, whereas the bare rutile nanorod film did not show any detectable hydrophilic conversion. The oleic acid on the surface of Cu(II)-grafted TiO2 film was oxidized by holes in the VB of TiO2, resulting in the super-hydrophilic state. Although the experimental condition of Figure 7 involves both oxidation of oleic acid and surface structural change of TiO2, the photoinduced hydrophilic property of clean surface without the coating of oleic acid in our supporting information (Figure S2). On the clean surface, Cu(II) grafted TiO2 also exhibited super-hydrophilic conversion under visible light

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irradiation. Super-hydrophilic conversion of the Cu(II) grafted TiO2 under visible light irradiation probably proceeds under the same manner with that of pure TiO2 under UV irradiation. The most important function of a visible light-inducible photocatalyst used for indoor applications is anti-pathogenicity (anti-bacterial and anti-viral). Exposure to infectious pathogens with multiple-drug resistance is becoming a more frequent occurrence in our daily environment and can severely impact human health. Recently, several infectious diseases, such as Ebola hemorrhagic fever, bird flu, and Severe Acute Respiratory Syndrome (SARS), reached pandemic proportions. Further, malaria and dengue fever still adversely impact many populations inhabiting tropical areas. The outer membranes of bacteria and viruses are composed of lipids and protein and can be decomposed by photogenerated holes in the VB of photocatalysts. For practical applications, materials that provide sustained anti-pathogenic effects even under dark conditions are required, as indoor light instruments are typically switched off during the night. Sunada et al. reported that copper compounds with Cu(I) species (CuI, Cu2S, or Cu2O) possess both anti-bacterial and anti-viral properties, even under dark conditions.49 The Cu(I) species denature various protein molecules in the microbial pathogens, thereby causing de-activation and/or killing of the bacteria and viruses. Based on this property, Liu et al. investigated the effect of pretreatment of Cu(II)-grafted TiO2 with strong light irradiation to form Cu(I) species, and demonstrated that the pretreated sample exhibited sustained anti-viral activity, even under dark conditions.28 In addition to pretreatment with UV-light irradiation, we speculated that TiO2 grafted with nanoclusters containing Cu(I) and Cu(II) species might have efficient visible-light photocatalytic activity, as well as sustained anti-pathogenic activity under dark conditions.50 To this end, we synthesized and grafted CuxO nanoclusters (x = 1 or 2) containing Cu(I) and Cu(II) 16


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species onto TiO2 particles and examined the deactivation properties of this material for bacteriophage QE (model substitution of human influenza virus), Escherichia coli (E. coli, Gram-negative bacterium), and Staphylococcus aureus (S. aureus, Gram-positive bacterium), respectively (Figure 8). Surprisingly, CuxO nanocluster-grafted TiO2 displayed a 4-log reduction (99.99% reduction) of bacteriophage after 1 h of contact exposure under dark conditions. The anti-viral property of this material was further improved under visible-light irradiation, as a 7.5log reduction of bacteriophage was achieved after 40 min. Similar microbial inactivation trends were found for both E. coli and S. aureus, demonstrating the high anti-pathogenic properties of CuxO nanocluster-grafted TiO2 in indoor environments. We also investigated the ability of CuxO nanoclusters to degrade DNA and proteins, which are essential components of viruses and bacteria, and found that supercoiled plasmid DNA was converted to open circular form under both visible-light irradiation and dark conditions.50 These experimental observations suggest that CuxO nanocluster-grafted TiO2 is able to destroy critical biomolecules of bacteria and viruses, leading to their death and inactivation. The evaluation of the anti-pathogenic properties of CuxO nanocluster-grafted TiO2 was conducted at an illuminance of 800 lx using a commercial 10 W cylindrical white fluorescent light equipped with a UV-cutoff filter, which is commonly used as an indoor lighting source in office buildings, public places, and homes. Photocatalytic oxidation only proceeds at outer membranes of bacteria and viruses, thus the developed photocatalysts are safe for human and do not affect our skin. Nanocluster-grafted TiO2 has been produced commercially (by Showa Denko K. K.) and has been coated on various products, such as tiles (TOTO, Ltd.) and plastic films (Panasonic Corp.). To evaluate the practical application of this photocatalytic material, we conducted field tests of these coated products in public spaces, including hospitals and airports. Among various buildings,

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international airports are particularly important areas for facilitating the spread of human pathogens, as infected human passengers have the potential to come in contact with individuals travelling to worldwide destinations. In a recent field test, we installed nanocluster-grafted TiO2 photocatalysts products in a washroom at Noi Bai International Airport in Hanoi, Vietnam, as illustrated in Figure 9. Specifically, tiles and films coated with visible-light-sensitive photocatalysts were installed in one washroom, and as a control, the photocatalytic products were not installed in an identical washroom also located in the terminal building. Anti-bacterial and deodorization (reduction of ammonia on tiles) effects were monitored in both washrooms for approximately one month (Figure 10). The field test confirmed that films and tiles coated with nanocluster-grafted TiO2 exhibited excellent anti-bacterial and deodorization functions, even in an indoor environment, with greater than 90% of bacteria and ammonia levels being decreased. In summary, this perspective article has introduced recent progress in the development of nanocluster-grafted TiO2 as an efficient visible-light-sensitive photocatalyst for indoor environmental remediation applications. Nanocluster-grafted TiO2 exhibits excellent oxidation activities for organic contaminants under illumination from indoor light fixtures, making this a promising material with new functions and properties, including air purification, self-cleaning, and anti-pathogenic effects, for applications in private dwellings and public places, such as hospitals, airports, metro stations, and schools, and as air filters, respiratory face masks, and antifungal fabrics, among numerous others. From a scientific point of view, IFCT and multielectron reduction processes are key phenomena for the development of these materials. The excited holes in this system are maintained at deep VB and therefore have strong oxidative power for decomposing organic compounds. The concept of grafting nanoclusters is not limited to the specific semiconductors, but is potentially applicable for various metal oxides, metal

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halides, and metal sulfides. This technique is expected to become a widely used strategy to enhance photocatalytic oxidation activities for organic contaminants. From an industrial standpoint, the simplest structure is often the best for practical applications. Nanoclusters can be facilely grafted onto TiO2 particles or films by a simple wet chemical method, which is readily applicable for large-scale production processes. Further, Cu(II) and Fe(III) nanocluster-grafted TiO2 is composed of abundant and safe elements. The concept of a “ubiquitous strategy” for industrial activities has attracted considerable recent attention.51 Ubiquitous means existing or being everywhere especially at the same time and becomes important concept for achieving sustainable development on earth. In the past few decades, photocatalysts have been mainly limited to outdoor applications aimed at self-cleaning. However, the unique properties of nanocluster-grafted TiO2 have the potential to be applied to various products for creating safe and secure indoor environments, which will be a big demand and be essential for human life.

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Table 1. Photocatalytic performance of various materials under visible-light irradiation. Sample

TiO2

Nitrogendoped TiO2

Cu(II)WO313

Cu(II)TiO228

Fe(III)TiO225

Fe(III)-Ti(IV)TiO231

Rip (quanta/sec)

1.30×1016

1.30×1016

1.30×1016

1.30×1016

1.30×1016

1.30×1016

R ap (quanta/sec)

0

4.10×1015

3.91×1015

5.82×1014

7.48×1014

7.48×1014

Rco2 (μmol/h)

0

0.16

0.66

0.40

0.40

0.69

QE (%)

-

3.9

17.0

68.7

53.5

92.2

Rip, rate of incident photons; Rap, absorbed photon number in photocatalyst; Rco2, CO2 generation rate; and QE, quantum efficiency. The QE values were determined by the CO2 generation rate by 2-propanol oxidation versus absorbed photon numbers by considering that the 2-propanol oxidation involves 6 electrons reaction to produce one CO2 molecule.

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Voltage vs. NHE (pH=0) (O2 + e- ĺ O2-)

-0.05 V

CB

e-

CB

ee-

CB

0

+0.68 V (O2 + 2H+ + 2e- ĺ H2O2)

O2

+ 1.0

+ 2.0

h+

+2.55 V

(H2O ĺ 䞉OH + e- + H+)

H2O2

e-

h+

CO2

+ 3.0 VB

VB

VB

h+

Organic

+ 4.0 (a) N-doped TiO2

(b) Sensitized TiO2

(c) Nanoclusters grafted TiO2

Figure 1. Electric band structures for nitrogen doped TiO2 (a), sensitizer modified TiO2 (b), and Cu(II) or Fe(III) nanoclusters grafted TiO2 (c), respectively.

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(a) Cu(II)

TiO2 10 nm

(b) Fe(III)

TiO2 10 nm

Figure 2. TEM images of Cu(II) (a) and Fe(III) nanocluster (b)-grafted TiO2.

22


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(a)

(b)

$EVRUSWLRQ

$EVRUSWLRQ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu(II)-TiO2

Fe(III)-TiO2

TiO2

TiO2

:DYHOHQJWKQP

:DYHOHQJWKQP

Figure 3. UV-Vis absorption spectra for Cu(II) (a) and Fe(III) nanocluster (b)-grafted TiO2.

23



20 18

Fe(III)-TiO2

16

CO2 concentration(Pmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

Cu(II)-TiO2

14 12 10

Nitrogen doped TiO2

8 6 4

TiO2

2 0 0

50

100 Irradiationtime(h)

150

200

Figure 4. Photocatalytic oxidation activities for 2-propanol under visible-light irradiation.

24


0.4 0.3 0.2 0.1

Normalizedabsorbance/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalizedabsorbance/a.u.

Page 25 of 36

0 8970

1.0 0.8 0.6 0.4 0.2 0 8960 8980 9000 9020 9040 Photonenergy/eV

8980

8990

(a) (b) (c) (d) (e)

9000

Photonenergy/eV Figure 5. Reversible XANES spectra of Cu(II)/TiO2 (0.27 wt%) under visible-light irradiation in the presence of a gas mixture of N2 and 2-propanol, followed by exposure to air (O2) in the dark. (a) Before (as-prepared), (b) during the photocatalytic reaction, (c) after the reaction, followed by exposure to air, (d) during the second reaction, and (e) after the second reaction, followed by exposure to air.

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height 65 >QP@ nm

Ag

CuO: 6 nm

2¡P Pm

Page 26 of 36

TiO2 5 Pm[¡P u 5 Pm 0

Figure 6. AFM image of the edge of an ultrathin patterned CuO film deposited on single crystals of rutile TiO2 after photodeposition of Ag particles under blue LED irradiation.

26


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100

Watercontactangle(degree)

1 Pm

(b)

90

(a)

80 70

TiO2

60 50 40 30 20

Cu(II)-TiO2

10 0 0

50 100 Irradiationtime(h)

150

Figure 7. SEM image of a thin film of Cu(II)-grafted rutile nanorods (a) and evaluation of its visible-light induced super-hydrophilic property. Before light irradiation, the surface of the thin films was contaminated with oleic acid.

1

(a)

0

0

control

control

Ͳ1

(b)

0

control

Log(N/N0)

Ͳ3

dark

Ͳ4 Ͳ5 Ͳ6

Ͳ2

dark Ͳ3 Ͳ4

visible light

Ͳ7 Ͳ8 20

40 Time(min)

60

80

Ͳ2 Ͳ3

dark

Ͳ4

visible light

Ͳ5 0

(c)

Ͳ1 Log(N/N0)

Ͳ1

Ͳ2 Log(N/N0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


visible light

Ͳ5 0

100 200 Time(min)

300

0

100 200 Time(min)

300

Figure 8. Anti-pathogenic effect of CuxO-grafted TiO2 films against (a) bacteriophage QE, (b) E. coli, and (c) S. aureus. Visible-light irradiation was performed using a white fluorescent bulb equipped with a UV-cutoff filter (light intensity: 800 lx).

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Floor map of Noi Bai International Airport Terminal Film applied

Toilet

Photocatalyst installation

Toilet

Control test

Tile

(without Photocatalyst)

Figure 9. Field test of products (film and tile) coated with nanocluster-grafted TiO2 in washrooms at Noi Bai International Airport, Vietnam.

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㻞㻡㻜㻜㻝㻜㻜㻜㻜㻜㻜㻜㻜

㻡㻜㻜㻜㻜㻜㻜㻜

(a)

(b)

95% of bacteria were inactivated.

㻜 photocatalyst

without photocatalyst

Ammonia 䠄ng/100cm2)

Bacteria 䠄cfu/100cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


㻞㻜㻜㻜㻝㻡㻜㻜 92% of ammonia was reduced.

㻝㻜㻜㻜㻡㻜㻜㻜

photocatalyst


(c)


photocatalyst

Figure 10. Results of field tests for anti-bacterial and deodorization effects of nanoclustergrafted TiO2. (a) Number of bacteria on tiles, (b) amount of ammonia on tiles, and (c) antibacterial effect of nanocluster-grafted TiO2 films.

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AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Biographies Masahiro Miyauchi received a B.E. (1993) and M.E. (1995) from the Tokyo Institute of Technology, and a Ph.D. from The University of Tokyo (2002). He worked as a research scientist with TOTO, Ltd. (1995-2006) and as a senior research scientist at the National Institute of Advanced Industrial Science and Technology (2006-2011). He is currently an Associate Professor at the Tokyo Institute of Technology (2011-present). His current research interests include materials chemistry and photo-electrochemistry, with a particular focus on photocatalysis and solar cell technology. Further information on Dr. Miyauchi’s research activities can be found at http://scholar.google.co.jp/citations?hl=en&user=f8vRP-QAAAAJ

Hiroshi Irie received his B.E. (1992) and M.E. (1994) from Tokyo Institute of Technology. He served at Sumitomo Metal Industries, LTD. as a research engineer (1994-1997). He received his Ph.D. degree from the University of Tokyo (2000). He was a research staff member at Kanagawa Academy of Science and Technology (2000-2001). He joined Research Center for Advanced Science and Technology at the University of Tokyo as a research associate in 2001. He was transferred to Department of Applied Chemistry at the same University in 2004, and was promoted to a lecture and Associate Professor in 2006 and 2008, respectively. Now, he belongs to Clean Energy Research center at University of Yamanashi from 2009. His current research

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interests are creations of solar energy conversion materials as water-splitting photocatalyts and thermo-electric materials for recycling exhausted heat energy.

Min Liu received his B. S. (2004) and M. S. (2007) from Hunan Normal University and his Ph. D. degree from Institute of Electrical Engineering, Chinese Academy of Sciences (2010). He worked as a project researcher in The University of Tokyo (2010-2015). He is currently a postdoctoral researcher in The University of Toronto. His current research focuses on electrochemical CO2 reduction and water splitting.

Xiaoqing Qiu graduated from East China Institute of Technology in 1996, and received his M.E. from Wuhan University in 2004. He served at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences as a research assistant (2004-2005). He received his Ph.D. degree from Chinese Academy of Sciences (2008). He joined Research Center for Advanced Science and Technology at the University of Tokyo (Prof Hashimoto’s group) as a NEDO researcher in 2009. He is currently a Professor in Fuzhou University from 2012. His current research interests include nano materials, energy conversion materials, and heterogeneous catalysis.

Huogen Yu received his B.E. (2001), M.E. (2004) and Ph.D. (2007) from Wuhan University of Technology in China. He served as a postdoctor at the University of Tokyo from 2008 to 2010. He is currently a Professor in Wuhan University of Technology from 2010. His current research

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interests include the design, synthesis and mechanism of high-performance photocatalytic materials for solar energy conversion and environmental purification.

Kayano Sunada received her Ph. D. from The University of Tokyo (2003). She was working as a researcher in Kanagawa Academy of Science and Technology (2002-2006) and a Project Associate Professor in The University of Tokyo (2006-2015). She came back to the former institute, Kanagawa Academy of Science and Technology, in spring this year. Her research interests are antibacterial and antiviral effects of photocatalytic materials and the practical application to environmental cleanup using photocatalysis.

Kazuhito Hashimoto is currently a professor of chemistry at the University of Tokyo. After he received his BS and MS degrees from the University of Tokyo, he obtained a research position at the Institute for Molecular Science (Okazaki, Japan) in 1980. In 1989, he was invited as a lecturer in the Department Applied Chemistry at the University of Tokyo, where he was promoted to an associate professor in 1991. When he became a full professor in 1997, he opened his laboratory at the Research Center of Advanced Science & Technology. He also succeeded the chair of the Department of Applied Chemistry in 2003 and opened the laboratory at this department, too. His current research interests are development of functionalized materials for energy conversion and environmental sustainability.

Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT This work was performed under the management of the Project “To Create Photocatalyst Industry for Recycling-Oriented Society” supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan. This work was also supported by a grant from the Japan Science and Technology Agency (JST). The field test project conducted at the airport was supported by the Civil Aviation Bureau of the Ministry of Land, Infrastructure, Transport and Tourism (MEXT) in Japan, and the Airports Corporation of Vietnam (ACV). We also acknowledge Mr. Greg Newton for the critical reading of the manuscript.

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(10) Zang, L.; Lange, C.; Abraham, I.; Storck, S.; Maier, W. F.; Kisch, H. Amorphous Microporous Titania Modified with Platinum (IV) Chloridea New Type of Hybrid Photocatalyst for Visible Light Detoxification. J. Phys. Chem. B 1998, 102, 10765-10771. (11) Baker, D. R.; Kamat, P. V. Photosensitization of TiO2 nanostructures with Cds Quantum Dots: Particulate Versus Tubular Support Architectures. Adv. Funct. Mater. 2009, 19, 805-811. (12) Daskalaki, V. M.; Antoniadou, M.; Li Puma, G.; Kondarides, D. I.; Lianos, P. Solar Light-Responsive Pt/CdS/TiO2 Photocatalysts for Hydrogen Production and Simultaneous Degradation of Inorganic or Organic Sacrificial Agents in Wastewater. Environ. Sci. Technol. 2010, 44, 7200-7205. (13) Irie, H.; Miura, S.; Kamiya, K.; Hashimoto, K. Efficient Visible Light-Sensitive Photocatalysts: Grafting Cu(II) Ions onto TiO2 and WO3 Photocatalysts. Chem. Phys. Lett. 2008, 457, 202-205. (14) Irie, H.; Kamiya, K.; Shibanuma, T.; Miura, S.; Tryk, D. A.; Yokoyama, T.; Hashimoto, K. Visible Light-Sensitive Cu(II)-Grafted TiO2 Photocatalysts: Activities and X-Ray Absorption Fine Structure Analyses. J. Phys. Chem. C 2009, 113, 10761-10766. (15) Yu, H.; Irie, H.; Shimodaira, Y.; Hosogi, Y.; Kuroda, Y.; Miyauchi, M.; Hashimoto, K. An Efficient Visible-Light-Sensitive Fe(III)-Grafted TiO2 Photocatalyst. J. Phys. Chem. C 2010, 114, 16481-16487. (16) Hush, N. S. Homogeneous and Heterogeneous Optical and Thermal Electron Transfer. Electrochim. Acta 1968, 13, 1005-1023. (17) Creutz, C.; Brunschwig, B. S.; Sutin, N. Interfacial Charge-Transfer Absorption:ௗ Semiclassical Treatment. J. Phys. Chem. B 2005, 109, 10251-10260. (18) Creutz, C.; Brunschwig, B. S.; Sutin, N. Interfacial Charge-Transfer Absorption:ௗ 3. Application to SemiconductoríMolecule Assemblies. J. Phys. Chem. B 2006, 110, 25181-25190. (19) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. Design of All-Inorganic Molecular-Based Photocatalysts Sensitive to Visible Light: Ti(IV)-O-Ce(III) Bimetallic Assemblies on Mesoporous Silica. J. Am. Chem. Soc. 2007, 129, 9596-7. (20) Yeager, E. Electrocatalysts for O2 Reduction. Electrochim. Acta 1984, 29, 1527-1537. (21) Wang, Y.; Balbuena, P. B. Ab Initio Molecular Dynamics Simulations of the Oxygen Reduction Reaction on a Pt(111) Surface in the Presence of Hydrated Hydronium (H3O)+(H2O)2:ௗ Direct or Series Pathway? J. Phys. Chem. B 2005, 109, 14896-14907. (22) Mustain, W. E.; Prakash, J. A Model for the Electroreduction of Molecular Oxygen. J. Electrochem. Soc. 2007, 154, A668-A676. (23) Serin, N.; Serin, T.; Horzum, ù.; Çelik, Y. Annealing Effects on the Properties of Copper Oxide Thin Films Prepared by Chemical Deposition. Semicond. Sci. Technol. 2005, 20, 398. (24) Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.; Grätzel, M. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. J. Am. Chem. Soc. 2010, 132, 7436-7444. (25) Liu, M.; Qiu, X.; Miyauchi, M.; Hashimoto, K. Energy-Level Matching of Fe(III) Ions Grafted at Surface and Doped in Bulk for Efficient Visible-Light Photocatalysts. J. Am. Chem. Soc. 2013, 135, 10064-10072. (26) Nosaka, Y.; Takahashi, S.; Sakamoto, H.; Nosaka, A. Y. Reaction Mechanism of Cu(II)Grafted Visible-Light Responsive TiO2 and WO3 photocatalysts Studied by Means of Esr Spectroscopy and Chemiluminescence Photometry. J. Phys. Chem. C 2011, 115, 21283-21290.

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