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Article

Is Black Titania a Promising Photocatalyst?

by
Marcin Janczarek
1,*,
Maya Endo-Kimura
2,
Kunlei Wang
2,
Zhishun Wei
3,
Md Mahbub A. Akanda
2,
Agata Markowska-Szczupak
4,
Bunsho Ohtani
2 and
Ewa Kowalska
2,*
1
Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Berdychowo 4, 60-965 Poznan, Poland
2
Institute for Catalysis, Hokkaido University, N21 W20, Sapporo 001-0021, Japan
3
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, China
4
Department of Chemical and Process Engineering, West Pomeranian University of Technology in Szczecin, 71-165 Szczecin, Poland
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1320; https://0-doi-org.brum.beds.ac.uk/10.3390/catal12111320
Submission received: 15 September 2022 / Revised: 21 October 2022 / Accepted: 24 October 2022 / Published: 27 October 2022
(This article belongs to the Special Issue Role of Defects and Disorder in Catalysis)
Browse Figures
Versions Notes

Abstract

:
Five different (commercial and self-synthesized) titania samples were mixed with NaBH4 and then heated to obtain black titania samples. The change in synthesis conditions resulted in the preparation of nine different photocatalysts, most of which were black in color. The photocatalysts were characterized by various methods, including X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), photoacoustic and reverse-double beam photoacoustic spectroscopy (PAS/RDB-PAS). The photocatalytic activity was tested for oxidative decomposition of acetic acid, methanol dehydrogenation, phenol degradation and bacteria inactivation (Escherichia coli) under different conditions, i.e., irradiation with UV, vis, and NIR, and in the dark. It was found that the properties of the obtained samples depended on the features of the original titania materials. A shift in XRD peaks was observed only in the case of the commercial titania samples, indicating self-doping, whereas faceted anatase samples (self-synthesized) showed high resistance towards bulk modification. Independent of the type and degree of modification, all modified samples exhibited much worse activity under UV irradiation than original titania photocatalysts both under aerobic and anaerobic conditions. It is proposed that the strong reduction conditions during the samples’ preparation resulted in the partial destruction of the titania surface, as evidenced by both microscopic observation and crystallographic data (an increase in amorphous content), and thus the formation of deep electron traps (bulk defects as oxygen vacancies) increasing the charge carriers’ recombination. Under vis irradiation, a slight increase in photocatalytic performance (phenol degradation) was obtained for only four samples, while two samples also exhibited slight activity under NIR. In the case of bacteria inactivation, some modified samples exhibited higher activity under both vis and NIR than respective pristine titania, which could be useful for disinfection, cancer treatment and other purposes. However, considering the overall performance of the black titania samples in this study, it is difficult to recommend them for broad environmental applications.

Graphical Abstract

1. Introduction

Titania (titanium(IV) oxide; TiO2) has been intensively studied for various photocatalytic applications during the last fifty years, including environmental purification, solar energy conversion and organic synthesis [1,2,3,4]. Although its high photocatalytic activity, stability, abundance, and low toxicity, are highly appreciated, the broad application is still limited since titania can work only under UV irradiation and the recombination of charge carriers (e−/h+) causes energy loss [5,6,7]. Therefore, various methods have been proposed to prepare novel materials with: (i) high quantum yields of photocatalytic reactions under UV, and (ii) vis response, e.g., via doping, surface modifications, coupling and nanoarchitecture design (nanotubes, nanowires, faceted nanoparticles, photonic crystals, etc.) [8,9,10,11,12,13,14,15,16,17]. Similar methods have been proposed for both purposes, but contrary results can be found in the literature, showing both an increase and a decrease in the obtained photocatalytic activity after titania modification. For example, though many reports have confirmed that the deposition of noble metals improves the overall performance (both increased UV activity and vis response originating from plasmon features) [14,18,19,20,21,22,23], some papers suggest that noble metals might work as a charge carrier recombination center [24,25,26]. However, it should be pointed out that the properties of the photocatalyst must be optimized (While commonly only several different photocatalysts are compared.), since deposits of noble metals on the titania surface might work as a shield, limiting photon absorption. Similarly, in the case of titania doping, though the dopants usually cause the narrowing of the bandgap (and thus vis response), they could also work as a recombination center under UV, and thus the improvement of overall performance is questionable [27]. Moreover, the stability of doped materials could be also a problem as doping may be only temporary, as exemplified by the continuous loss of dopant in the titania structure, correlating with a drop in vis activity [28].
Defective titania materials, i.e., a special group of doped titania samples, in which no other elements (except hydrogen in some cases) are introduced into the titania structure, are probably the newest kind of modified titania photocatalysts, though the preparation and characterization of defective/non-stoichiometric TiOx have been proposed several decades ago [29,30,31]. It has already been shown that defective titania photocatalysts are perspective materials for light-induced processes, such as photocatalysis, photovoltaics and fuel cells [32,33,34,35,36]. Since Chen et al. reported black hydrogenated titania nanocrystals with a narrower bandgap in 2011 research in this field has accelerated [37]. To date, the bandgap engineering of titania, which is a crucial approach to optimizing TiO2 light harvesting capability, has mainly focused on the introduction of external species to the titania structure [38]. However, this strategy might bring many challenges, such as intrinsic stability problems and the formation of charge carrier recombination centers, which, as mentioned above, is detrimental to the overall photocatalytic performance. Furthermore, the high price of modifiers such as noble metals can limit their commercial applicability [35].
The above-mentioned hydrogenated black titania with a narrowed bandgap (ca. 1.5 eV) was reported to boost the full absorption spectrum of sunlight and photocatalytic activity [37]. The functional character of black titania is based on the effect of self-structural modifications such as Ti3+/oxygen vacancies or incorporation of H-doping [36]. The photocatalytic effect of black titania was also confirmed in other studies [39,40,41,42,43,44]. However, it has also been shown in many reports that the modification of photo-absorption properties (vis response) does not necessarily mean the improvement of photocatalytic properties in the considered range of irradiation [45,46,47]. The existence of conflicting results about black titania as a photocatalyst shows that the correlation between defects’ properties (content and type) and photocatalytic activity requires clarification.
It has also been known that black titania absorbs visible to near-infrared (NIR) light, which could be utilized for photothermal therapy of cancer cells [48]. Although white titania has also been investigated as a photocatalyst for cancer therapy because of its high photocatalytic activity (resulting in the efficient generation of reactive oxygen species), biomedical applications using white titania by UV-light is limited as UV-light is mutagenic, causing shallow light penetration. In contrast, NIR light penetrates the skin well as the transparent bio-window appears in the range from 700 to 1100 nm [49]. Therefore, long-wavelength response titania (e.g., by various modifications, such as PEG-coating [48], epidermal growth factor (EGF)-epidermal growth factor receptor (EGFR) [50] and folic acid-folate receptor [51]) has been applied to utilize NIR light and bind to cancer cells specifically. However, there are no reports on the bactericidal activity of black titania using visible-NIR light. It should be pointed out that NIR itself is a highly promising tool for disinfection due to its antibacterial features, such as the generation of heat and the formation of reactive oxygen species (ROS). It has been shown that NIR might help to eradicate antibiotic-resistant bacteria (ARB) or antibiotic resistance genes (ARGs) and prevent further development of bacterial resistance [52].
In the present study, black titania samples were prepared by the reduction of different types of titania (three commercial titania: P25, ST01 and TIO-2, and two self-prepared faceted samples: decahedral anatase particles (DAP) and octahedral anatase particles (OAP), as shown in Section 3). Sodium tetrahydroboride (NaBH4) was used as a source of hydrogen by thermal decomposition in the solid state [53]:
NaBH4 → NaH + B +1.5H2
NaBH4 → Na + B + 2H2
The prepared samples were evaluated for possible broad application (decomposition of organic compounds, hydrogen evolution and inactivation of bacteria) under UV, vis and NIR irradiation, as well as in the dark.

2. Results and Discussion

The successful preparation of reduced/hydrogenated titania has been easily observed by the intense coloration of the prepared samples, as shown in Figure 1f. Although most samples are black, the color of the ST01-based samples is much lighter (gray). The photoabsorption properties of the prepared samples, examined by diffuse reflectance spectroscopy, are shown in Figure 1a–e. The DRS spectra confirm the visual observation of samples’ coloration, i.e., the black-colored samples absorb a wide range of vis-NIR light with high intensity, whereas the gray-colored samples show lower absorbance in the vis-NIR region. The significant difference in color between the ST01-based samples and the others indicates insufficient reduction (uncompleted reaction), probably resulting from a large difference in surface properties, because the ST01 sample is characterized by a much larger specific surface area (ca. 300 m2 g−1) than the other samples, e.g., ca. 59 m2 g−1 for P25. Moreover, this also suggests that the modification of titania (reduction with NaBH4) is rather on the surface than in the bulk.
The crystal properties of all samples (Figure 2) were largely unchanged after the reduction. All samples contain mainly anatase phase (Rigaku, Tokyo, Japan PDXL PDF card: 5757), as clearly observed by characteristic peaks at ca. 25.31°, 37.79°, 48.04°, etc., relating to the (101), (004) and (200) planes, respectively. Additionally, P25 also contains rutile phase (Rigaku PDXL PDF card: 7814), as could be seen from the peaks at ca. 27.63°, 41.45°, 54.65°, 69.48°, etc., relating to the (110), (111), (211), (301) planes, respectively. In the case of all commercial samples, a slight shift of the diffraction peaks to lower angles has been observed (Figure 2f for P25), indicating the formation of oxygen-deficient TiO2−x [54]. However, the shift is almost undetected in the case of the faceted samples (OAP and DAP), as shown for OAP in Figure 2f. This is not surprising as faceted samples are highly stable, and thus their bulk modification is very difficult, i.e., usually “surface” self-doping is achieved (our own unpublished data and [55]).
To examine if and how the surface of titania has been changed after reduction, XPS analysis was performed, and the obtained data are shown in Figure 3 and Table 1. First, it was confirmed that NaBH4 was not adsorbed on the surface of titania since sodium and boron were not detected in the black titania samples. Interestingly, contrary results were obtained for the commercial (P25) and self-synthesized (faceted: OAP and DAP) samples. The reduction conditions have caused a significant decrease in O/Ti ratio for the faceted samples, i.e., from 6.0 to 2.3 and 3.6 in the case of OAP and DAP, respectively. The enrichment of titania with oxygen is common because of easy water and carbon dioxide adsorption on its surface, and thus much higher values than the stoichiometric 2.0 are not surprising. The significant decrease in oxygen content after chemical reduction is also typical. However, in the case of P25, the opposite result has been observed, i.e., an increase of O/Ti ratio from 2.6 to 5.1 and 4.0 for P25-R1 and P25-R2, respectively. It should be remembered that P25 (despite being commonly used as a standard) is not uniform, and thus even samples taken from the same container could have different compositions, e.g., 76–80% anatase, 13–15% rutile and 6–11% amorphous phase [56,57]. Moreover, the amorphous phase content can change during the preparation of reduced titania [58,59], which might result in easier adsorption of both water and carbon dioxide on the titania surface. For example, it has been proposed that amorphous titania is capable of anchoring water molecules more strongly than crystalline titania [60]. Indeed, the content of hydroxyl groups in reduced P25 is much higher (30.3% and 23.4%) than that on the surface of the pristine sample (8.6%). In contrast, in the case of the faceted samples, the content of hydroxyl groups has decreased significantly, especially in the case of the OAP sample, which might further confirm that the modification of those samples proceeds mainly on the surface rather than in the bulk.
Interestingly, when titanium is considered, there is almost no change in its oxidation state. Accordingly, it is proposed that trivalent titanium should not be considered as the main defect site, contrary to many reports. Therefore, either oxygen deficiency or hydrogenation (TiO2−x:H [61]) is responsible for the observed black/grey coloration of the modified samples.
Additionally, the total density and distribution of electron traps (ETs) were analyzed by photoacoustic (PA) and reverse-double beam PA (RDB-PAS) spectroscopy for the ST01-based samples. The obtained data are shown in Figure 4. The distribution of ETs and total density of ETs (38, 35 and 44 μmol/g for ST01, ST01-R1 and ST01-R2, respectively) are both very similar in the pristine and modified samples, indicating that the content of ETs changed little during the modification, similarly to the XPS results (Ti3+). Only, in the case of the ST01-R2 sample, additional ETs could be observed at ca. 1.8–2.0 eV above the valence band, probably correlating with the bridging proton in titania (Ti–H [61]). Therefore, an additional method (photochemical [62]) has also been applied to investigate the total content of defects in the pristine and modified titania samples. Using this approach, finally, a significant difference between samples could be found, as shown in Table 2. An increase in the density of defects was obvious after titania thermo-chemical reduction (three- to six-fold increase in defect density), especially for the black samples.
Next, it was investigated if such huge differences in defect densities between pristine and modified samples could also cause changes in the sample morphology. Accordingly, STEM images were captured for four different samples, and the obtained data are shown in Figure 5 and Figure 6. Although the differences between associated white and black samples are not huge, it could be observed that the surface (especially in the case of the P25-based samples) was slightly destroyed, as clearly seen by the comparison between images (b) and (d).
All obtained data suggest that defective titania was formed. Therefore, XRD analysis in the presence of an internal standard (NiO) was additionally performed to investigate if the crystalline phase content of titania remained unchanged. Exemplary patterns are shown in Figure 7, and all the obtained data are summarized in Table 3. The normalized patterns clearly demonstrate that the crystallinity of titania was significantly worsened, which corresponds to an increase in the amorphous content. Therefore, it might be concluded that the thermal treatment with NaBH4 caused the partial conversion of crystalline titania into amorphous titania with a high defect density.
The XRD data in the present study (Table 3) show that non-crystalline content increases with the increase of the density of defects. The conversion of titania crystals into an amorphous state might be the result of the introduction of defects (disorder) but the mechanism of this transformation remains unclear [63]. Furthermore, the XRD diffraction patterns (Figure 2) do not indicate the presence of Magnéli phases (TinO2n−1), which might also be potentially responsible for the black color of samples (in the form of a shell) [64].
The photocatalytic activity of the obtained samples was tested for degradation of organic compounds, hydrogen evolution and bacteria inactivation under irradiation with UV or vis, as well as in the dark. First, methanol dehydrogenation (H2 evolution) under UV irradiation was tested, and the obtained results are shown in Figure 8. Since pristine titania is practically inactive in this reaction (high overvoltage of hydrogen evolution), a metallic (usually platinum) co-catalyst is commonly used. Indeed, high photocatalytic activity has been obtained for platinum-modified samples (in-situ platinum photodeposition), reaching 0.7 mM/h of hydrogen evolution for the ST01 commercial titania. The activity of the pristine samples decreases in the following order: ST01 > P25 > DAP > OAP > TIO-2, which correlates well with the specific surface area of these samples (Table 2). Obviously, the larger the specific surface area is, the larger is the number of active sites for hydrogen formation. The lower-than-expected activity for the OAP sample (considering its large specific surface area and perfectly shaped morphology) could result from the low content of surface defects (electron traps; ETs). It should be pointed out that platinum photodeposition is very fast under anaerobic conditions and in the presence of methanol as a hole scavenger, which usually results in the completion of the reaction within a few minutes [65]. At the same, this also means that formed deposits of platinum might easily aggregate, especially in the case of titania samples with a low content of ETs (active sites for metal cations’ adsorption [66] and reduction). Accordingly, to form uniformly distributed deposits of noble metals on the surface of titania, photodeposition in the initial presence of oxygen/air in the system has even been proposed [65].
Surprisingly, the activities of all reduced titania samples are lower than the activities of the respective pristine materials. Considering the P25-based samples, P25-R1, P25-R2 and P25-R3, it is possible to propose that an increase in the degree of reduction treatment (larger amount of NaBH4, higher reduction temperature, or longer time of treatment) contributes to the observed activity decrease. A similar correlation is observed for the ST01 samples, i.e., the higher the reduction temperature, the lower the resultant activity. Interestingly, a decrease in activity correlates with an increase in defect density (Table 2). For example, the activity of the P25-based samples decreases in the following order: P25 > P25-R3 > P25-R2 > P25-R1, which corresponds to the increase in defect density for these samples: 70 < 87 < 97 < 214 μmol g−1. Similarly, in the case of the ST01-based photocatalysts, the most and the least active are ST01 and ST01-R1, which have the lowest (84 μmol g−1) and highest (117 μmol g−1) defect density, respectively. In the case of another reaction system, the oxidative decomposition of acetic acid, analogous results have been obtained, as shown in Figure 8b. As mentioned in the Introduction, contrary results on the defective/hydrogenated/self-doped titania have been published, showing both activity increase and decrease under UV irradiation. For example, Bielan et al. observed a decrease in activity for phenol degradation [67], whereas Zywitzki et al. reported an activity enhancement for hydrogen evolution [68]. It has been proposed that Ti3+ species might also reduce protons to H2 or platinum cations (Pt2+) to form Pt nanoparticles (NPs). Accordingly, smaller and more uniformly distributed Pt NPs (co-catalyst) on the titania surface could be obtained [68]. Unfortunately, in our study, titania modification is detrimental to UV activity for all tested samples both under anaerobic (Figure 8a) and aerobic (Figure 8b) conditions. This should probably not be surprising, considering the color of samples (black), and thus less efficient light penetration through irradiated suspension. It should be underlined that defective samples prepared by Bielan et al. and Zywitzki et al. were much lighter in color than the samples considered in this study, i.e., yellow and blue, respectively. It is hard to find any report on black titania with activity testing under UV irradiation. Many other types of doped titania (not only self-doped) are also commonly tested only under vis irradiation (or solar simulator), despite it being common knowledge that defects could be a recombination center. Accordingly, though in many cases vis response could be obtained, the activity under UV is diminished, and thus the overall performance (considering much higher activity of titania under UV than vis response of modified titania samples) in many cases could be questionable.
Chen et al. observed improved photocatalytic activity of black titania under UV irradiation, but this was in the case of dye degradation [69], and thus another possible mechanism could also be involved in the overall performance, i.e., dye decomposition by a sensitization mechanism. The results presented in this study support the conclusions of Leshuk et al., which showed that the photocatalytic activity of black titania samples (hydrogenated TiO2) under simulated sunlight (mainly vis) was significantly worse than that of pristine control samples [47]. They concluded that the strong visible light absorption of hydrogenated samples is due to oxygen vacancy bulk doping (VO) of titania rather than any significant shift in band edge position or change in the surface structure. It was proposed that the presence of vacancy defects was confined to the core of TiO2 crystals rather than their surface, and regardless, they behave as trap sites and charge recombination centers. In this study, it has been shown that surface atomic defects (Ti3+) were not responsible for the defective character of the samples. The origin of the worsening of the photocatalytic properties could be due to the presence of bulk defects (oxygen vacancies) generated by titania hydrogenation. Furthermore, Kong et al. reported that an increase in the relative concentration of bulk defects compared to surface defects in titania crystals could explain the charge separation deterioration that causes inhibition of photocatalytic activity [70]. Similar conclusions were drawn by Liu et al. for black and gray-colored titania samples prepared by hydrogenation [71]. Improved photocatalytic activity (also under simulated solar radiation) was reported only for gray-colored titania, which was explained as being due to the photocatalytic activity (H2 evolution) not being related to the optical absorption of TiO2. Black titania samples prepared at higher temperatures contain a higher concentration of Ti3+ defects that could act as recombination centers. Conversely, Naldoni et al. proposed that the synergistic effect of bulk defects (VO) and surface disorder could influence the bandgap narrowing of black TiO2 [72]. Furthermore, the increase of amorphous content could also be responsible for the decrease of photocatalytic efficiencies of reduced titania samples, as it is well known that amorphous titania is characterized by a high rate of charge carrier recombination [73,74]. However, Tan et al. prepared TiO2@TiO2–x samples with a crystalline core/amorphous shell that had improved photocatalytic activity [75]. They suggested that the obtained colored titania had an optimal concentration of surface oxygen vacancies that increases charge carriers’ separation efficiency.
However, the main aim of this study (black titania with broad vis/NIR response) was not to improve the performance under UV but to achieve vis and NIR activity. Accordingly, the performance of pristine and reduced titania for oxidative decomposition of phenol was investigated. As these activities are much lower than those under UV irradiation, and thus adsorption of phenol on the titania surface must also be considered, the relative activity data, in respect to “dark activity” (examined under same conditions; the dotted line at a value of 1) are shown in Figure 9. Here, finally, four hydrogenated samples show higher activity than the respective pristine titania materials under vis irradiation, ST01-R1, DAP-R1, TIO-2-R1 and TIO-2-R2. Of course, pristine titania is inactive under NIR conditions, but interestingly, slight activity under NIR has been observed for the ST01-R1, P25-R1 and OAP-R1 samples. Accordingly, it might be concluded that titania hydrogenation can cause both vis and NIR responses. It is proposed that the ST01-R1 sample probably shows the highest vis and NIR activity because of the optimal content (and the type) of defects. Interestingly, this sample exhibits the lowest UV activity among the ST01-based samples, especially for hydrogen generation, as well as the lowest content of ETs. Accordingly, it might be proposed that though the content of high-energy ETs has been decreased, the modification of ST01 results in either: (i) the formation of low-energy ETs (just above the valence band), working as charge carriers’ recombination centers responsible for UV activity loss, but also narrowing bandgap, and thus causing a vis response; and (ii) the adsorbed hydrogen (e.g., TiO2−x:H [61]) might work as a recombination center and vis-absorption initiator (e.g., Ti–H within titania bandgap). It is also worth remembering the ST01-based samples are the only gray (not black) samples.
Next, the bactericidal activity of all samples was examined under vis, NIR and in the dark, and the obtained data are shown in Figure 10. It has been found that titania photocatalysts exhibit a broad spectrum of antibacterial properties, depending on the light conditions. The activities of most samples under vis were higher than those under NIR. The highest enhancement of activity under vis in comparison to pristine samples of three orders of magnitude is achieved for P25-R2 and P25-R3, with the TIO-2-R2, TIO-2-R1, ST01-R2 and DAP-R1 samples showing a ca. one order of magnitude increase. In contrast, there is no activity enhancement for P25-R1 and even a decrease for ST01-R1 and OAP-R1. Although ST01-R2 exhibits high vis activity, ST01-R1 does not, despite its high phenol-oxidation activity. In the case of NIR, a significant activity increase is achieved for both ST01-modified samples. Interestingly, NIR causes an increase in bacterial colonies for the P25-R1 sample. It is well known that in the case of the oxidative decomposition of organic compounds (e.g., phenol), different ROS (superoxide anion radicals and hydroxyl radicals) might be responsible for the reaction mechanism. However, for the inactivation of Gram-negative bacteria, the attack by hydroxyl radical is more important since the superoxide anion radical might be repelled by the anionic bacterial cell wall membrane [52]. Therefore, it is possible that the photocatalytic process under NIR conditions causes some changes in the electrical charge distribution on the bacterial cell wall, which might be crucial for the integrity of the bacterial membrane, or electrolysis of molecules on the bacterial cell surface. Accordingly, this homeostatic imbalance might lead to bacterial death [76]. Therefore, it is proposed that the mechanisms of oxidative decomposition of phenol and microorganisms must be different, and this needs further investigation.

3. Materials and Methods

3.1. Selected Titania Materials

The following titania samples were selected as a base for the preparation of black titania, commercially available: P25 (Nippon Aerosil Co., Yokkaichi, Japan), ST01 (Ishihara Sangyo, Osaka, Japan), TIO-2 (provided by the Catalysis Society of Japan, Tokyo, Japan), and self-prepared, single-crystalline titania materials with different particle morphology: decahedral anatase particles (DAP) [77] and octahedral anatase particles (OAP) [78]. The basic properties of all titania samples are shown in Table 4.

3.2. Preparation of Black Titania Particles

Each titania sample was mixed with NaBH4 as the reducing agent in different mass ratios. Next, the solid mixture was placed in a rotary oven and pre-mixed for 30 min under argon. Then, the sample was heated under an argon atmosphere (different temperatures and calcination times were used; the heating rate was 298 K min−1). After thermal treatment, the samples were washed five times with Milli-Q water and then freeze-dried. The detailed preparation conditions for each sample are shown in Table 5.

3.3. Characterization of Black Titania Particles

The photoabsorption properties of the prepared photocatalysts were measured by diffuse reflectance spectroscopy (DRS) with both BaSO4 and the respective bare titania, used as references, using a JASCO V-670 (JASCO Corp., Tokyo, Japan) spectrophotometer with a PIN-757 integrating sphere. The crystalline properties of the photocatalysts were determined by X-ray powder diffraction (XRD) using a Rigaku XRD SmartLab (Rigaku intelligent X-ray diffraction system SmartLab, Osaka, Japan) using Cu-Kα radiation, equipped with Rigaku PDXL 2 powder diffraction software with Rietveld refinement module and ICDD PDF database. In addition, for quantitative estimation of the crystalline composition, an internal standard method was applied using highly crystalline NiO as a standard. In brief, 40 mg NiO was mixed thoroughly with 160 mg of titania sample by grinding in an agate mortar before being scanned. Since Rietveld analysis gives the composition of each crystal among total crystal content, the composition of the standard (formally 20.0 wt%) is measured to be larger, if the sample contains a non-crystalline phase, the composition is estimated by re-calculation to make the standard composition to be 20.0 wt%.
For morphological observation, scanning transmission electron microscopy was used (STEM, Hitachi HD2000, Tokyo, Japan). The surface characterization was performed using X-ray photoelectron spectroscopy (XPS; JEOL JPC-9010MC, JEOL Ltd., Tokyo, Japan). For the estimation of the density and energy distribution of ETs, photoacoustic spectroscopy (PAS) and reversed double beam photoacoustic spectroscopy (RDB-PAS) were carried out, according to the procedure given in previous reports [79]. The photochemical method for quantitative estimation of defects was also used, according to the procedure reported previously [62].

3.4. Photocatalytic Activity Tests

3.4.1. Methanol Dehydrogenation Test

Ten milligrams of photocatalyst was dispersed in 5 mL of 50 vol% aqueous methanol in a test tube containing a stirring bar, and then hydrogen hexachloroplatinate(IV) (H2PtCl6·6H2O) was added for 2.0 wt% platinum loading. The suspension was purged with argon before irradiation to remove oxygen from the system. The test tubes were sealed with rubber septa and irradiated with UV/vis (λ > 290 nm) using a 400-W high-pressure mercury lamp (Eiko-sha) while subjected to magnetic stirring at 1000 rpm. The reaction temperature was kept at 298 K using a thermostatted water bath. To analyze the progress of the reaction, the H2 content was measured using a TCD gas chromatograph (Shimadzu GC-8A) equipped with Molecular Sieve 5A. Gas samples were taken every 15 min during 1-h irradiation.

3.4.2. Acetic Acid Decomposition Test

Ten milligrams of sample were dispersed in 5 mL of 5 vol% aqueous acetic acid in a test tube with a stirring bar and then irradiated under UV/vis (λ > 290 nm) using a 400-W high-pressure mercury lamp (Eiko-sha) while subject to magnetic stirring at 1000 rpm. To analyze the progress of the reaction, the CO2 content was measured using a TCD gas chromatograph (Shimadzu GC-8A) equipped with a Porapak-Q column. Gas samples were taken every 15 min for 1 h.

3.4.3. Phenol Oxidation Test

Ten milligrams of photocatalyst were dispersed in 5 mL of 20 mg/L aqueous phenol in a test tube containing a stirring bar. The suspension was irradiated with xenon lamp (visible light; with CM1 and Y-45 filter; λ > 420 nm) or halogen lamp (NIR reflection mirror inside; 800 < λ < 1300 nm). A 0.3 mL quantity of suspension was taken from the tube, filtrated, and then the phenol concentration was measured with high-performance liquid chromatography (HPLC) with sampling every 1 h for 3 h. Reference experiments were also performed in the dark, i.e., the test tubes were covered with aluminum foil and left under continuous stirring.

3.5. Bactericidal Test

Ten milligrams of sample were dispersed in 7 mL of Escherichia coli suspension (initial number of bacteria: 1 × 108 cells/mL (by measurement of absorbance: ca. 0.180 at 630 nm)) in a test tube containing a stirring bar. The suspension was irradiated (or kept in the dark) with a xenon lamp (with CM1 and Y-45; λ > 420 nm) or halogen lamp (NIR reflection mirror; 800 nm < λ< 1300 nm) for activity testing under vis and NIR, respectively. Serial dilutions (10−1–10−6) were prepared and 0.3 mL was inoculated on plate count agar media at 0, 0.5, 1, 2 and 3 h. Plates were incubated at 310 K overnight and then colonies were counted.

4. Conclusions

Black titania photocatalysts could be readily prepared by chemical/thermal reduction of both faceted and irregularly shaped titania samples. The faceted titania nanocrystals proved high resistance to deep modification (no shift in the XRD peaks), due to their characteristic crystalline morphology, and thus high stability. Only the ST01-based reduced samples were lighter in color (gray), probably due to their high specific surface area, and thus incomplete reduction/hydrogenation.
Independently of the resultant properties, all modified samples lost their high UV activity for both the oxidative decomposition of acetic acid and the dehydrogenation of methanol. It is proposed that enhanced charge carriers’ recombination, resulting from partial destruction of crystalline titania (formation of an amorphous phase with a large content of bulk defects, consisting of oxygen vacancies but not Ti3+ or/and Ti–H as recombination sites, similar to other doped titania samples) could be responsible for this activity drop. In contrast, under vis and NIR irradiation some samples exhibited slightly improved activity. However, this could not be called a “significant” improvement. Similarly, improved bactericidal response under vis and NIR was achieved for some samples.
In summary, the significant loss of UV activity, and only slight enhancement in vis and NIR response, leads to the conclusion that black titania is not a promising photocatalyst. Despite the possible potential for photodynamic therapy (PDT) to kill bacteria colonizing non-healing wounds and other biomedical purposes (further research would be needed to show if black titania is biosafe), it is concluded that black titania samples should not be considered as highly efficient photocatalysts for broad environmental applications.

Author Contributions

Conceptualization, M.J.; methodology, M.J. and A.M.-S.; investigation, M.J., M.E.-K., K.W., Z.W. and M.M.A.A.; resources, B.O. and E.K.; writing—original draft preparation, M.J., M.E.-K. and E.K.; writing—review and editing, M.J., M.E.-K. and A.M.-S.; visualization, K.W.; supervision, E.K.; funding acquisition, B.O. and E.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Polish Ministry of Education and Science subsidy for Poznan University of Technology.

Data Availability Statement

The data presented in this study are available on request from corresponding author (E.K.)

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a–e) DRS properties of pristine and reduced titania: (a) P25; (b) ST01; (c) TIO-2; (d) OAP; (e) DAP; and (f) the photographs of the hydrogenated titania samples.
Figure 1. (a–e) DRS properties of pristine and reduced titania: (a) P25; (b) ST01; (c) TIO-2; (d) OAP; (e) DAP; and (f) the photographs of the hydrogenated titania samples.
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Figure 2. XRD data of pristine and reduced titania: (a) P25; (b) ST01; (c) TIO-2; (d) OAP; (e) DAP; and (f) magnification of main (101) anatase peak for P25- and OAP-based samples.
Figure 2. XRD data of pristine and reduced titania: (a) P25; (b) ST01; (c) TIO-2; (d) OAP; (e) DAP; and (f) magnification of main (101) anatase peak for P25- and OAP-based samples.
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Figure 3. XPS data after deconvolution of titanium (a,c,e) and oxygen (b,d,f) peaks for pristine (on the left) and reduced (on the right) titania: (a,b) P25 and P25-R1; (c,d) OAP and OAP-R1; (e,f) DAP and DAP-R1.
Figure 3. XPS data after deconvolution of titanium (a,c,e) and oxygen (b,d,f) peaks for pristine (on the left) and reduced (on the right) titania: (a,b) P25 and P25-R1; (c,d) OAP and OAP-R1; (e,f) DAP and DAP-R1.
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Figure 4. Energy distribution of the ETs present in the ST01, ST01-R1 and ST01-R2 samples. The total density of ETs is written above the sample names.
Figure 4. Energy distribution of the ETs present in the ST01, ST01-R1 and ST01-R2 samples. The total density of ETs is written above the sample names.
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Figure 5. STEM images of: (a,b) P25; (c,d) P25-R1.
Figure 5. STEM images of: (a,b) P25; (c,d) P25-R1.
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Figure 6. STEM images of: (a,b) ST01; (c,d) ST01-R1.
Figure 6. STEM images of: (a,b) ST01; (c,d) ST01-R1.
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Figure 7. XRD patterns for TIO-2-based samples: pristine and modified (TIO-2-R1) titania mixed with internal standard (NiO): original data were normalized to the standard.
Figure 7. XRD patterns for TIO-2-based samples: pristine and modified (TIO-2-R1) titania mixed with internal standard (NiO): original data were normalized to the standard.
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Figure 8. Photocatalytic activity under UV irradiation of pristine and reduced titania samples: (a) Methanol dehydrogenation (anaerobic conditions); (b) oxidative decomposition of acetic acid (aerobic conditions).
Figure 8. Photocatalytic activity under UV irradiation of pristine and reduced titania samples: (a) Methanol dehydrogenation (anaerobic conditions); (b) oxidative decomposition of acetic acid (aerobic conditions).
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Figure 9. The relative activity under vis (blue) and NIR (orange) for phenol photodegradation on pristine and reduced titania samples.
Figure 9. The relative activity under vis (blue) and NIR (orange) for phenol photodegradation on pristine and reduced titania samples.
Catalysts 12 01320 g009
Catalysts 12 01320 g010 550
Figure 10. Bactericidal activity of black and pristine titania, shown in the decreased number of cells at 3 h from initial number.
Figure 10. Bactericidal activity of black and pristine titania, shown in the decreased number of cells at 3 h from initial number.
Catalysts 12 01320 g010
Table
Table 1. XPS data for titanium (2p3/2) and oxygen (1s).
Table 1. XPS data for titanium (2p3/2) and oxygen (1s).
SampleComposition (%)O/Ti RatioTitanium Form (%)Oxygen Form (%)
TiOTi3+Ti4+O–HM–OH/C=OTiO2
P2527.672.42.63.696.48.634.656.8
P25-R116.383.75.16.193.930.338.730.9
P25-R220.080.04.03.596.523.434.741.9
OAP14.385.76.02.697.428.442.029.6
OAP-R130.569.52.34.495.61.030.968.1
DAP13.786.36.34.895.239.840.230.1
DAP-R121.978.13.64.895.229.430.141.9
Table
Table 2. Defect density [μmol g−1], estimated by photochemical method.
Table 2. Defect density [μmol g−1], estimated by photochemical method.
Sample NamePristineModified (“Black”)
R1R2R3
P25732119787
TIO-229173135-
ST0184120103-
OAP52184--
DAP20115--
Table
Table 3. Crystalline properties, estimated with an internal standard (NiO).
Table 3. Crystalline properties, estimated with an internal standard (NiO).
Sample NameCrystalline Composition (%)Crystallite Size (nm)
AnataseRutileNCAnataseRutile
P2577.213.29.62539.6
P25-R363.2 11.325.520.323.3
TIO-283.1 016.9 68.9 -
TIO-2-R229.5 070.5 13.5 -
ST0172.8 027.2 7.6 -
ST01-R166.1 033.9 4.6 -
OAP78.0022.027.0-
OAP-R161.8 038.2 24.1 -
DAP90.80.06.267.0-
DAP-R155.5 044.5 63.8 -
Table
Table 4. The properties of titania (pristine) samples.
Table 4. The properties of titania (pristine) samples.
Sample NameComposition (%)Crystallite Size *
(nm)		BETETs **
ARNC(m2 g−1)(μmol g−1)
P2576–8013–156–112559114
TIO-29101768.91732
ST0180027834494
OAP780222712634
DAP9136671619
A—anatase; BET—specific surface area; ETs—total density of electron traps (defects); NC—non-crystalline phase; R− rutile; *—crystallite size of main phase (anatase); **—estimated by RDB-PAS.
Table
Table 5. The preparation conditions of black titania samples.
Table 5. The preparation conditions of black titania samples.
Sample NameMass Ratio of TiO2/NaBH4Calcination Temperature (K)Calcination Time
(min)
P25-R15:159860
P25-R210:152360
P25-R310:15235
TIO-2-R13.333:159860
TIO-2-R25:159860
ST01-R15:159860
ST01-R25:152360
DAP-R15:152330
OAP-R15:15985
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Janczarek, M.; Endo-Kimura, M.; Wang, K.; Wei, Z.; Akanda, M.M.A.; Markowska-Szczupak, A.; Ohtani, B.; Kowalska, E. Is Black Titania a Promising Photocatalyst? Catalysts 2022, 12, 1320. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12111320

AMA Style

Janczarek M, Endo-Kimura M, Wang K, Wei Z, Akanda MMA, Markowska-Szczupak A, Ohtani B, Kowalska E. Is Black Titania a Promising Photocatalyst? Catalysts. 2022; 12(11):1320. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12111320

Chicago/Turabian Style

Janczarek, Marcin, Maya Endo-Kimura, Kunlei Wang, Zhishun Wei, Md Mahbub A. Akanda, Agata Markowska-Szczupak, Bunsho Ohtani, and Ewa Kowalska. 2022. "Is Black Titania a Promising Photocatalyst?" Catalysts 12, no. 11: 1320. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12111320

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