Controlling surface oxygen vacancies in Fe-doped TiO2 anatase nanoparticles for superior photocatalytic activities

Journal Pre-proofs Full Length Article Controlling surface oxygen vacancies in Fe-doped TiO2 anatase nanoparticles for superior photocatalytic activities Gyeongtak Han, Joo Yeon Kim, Ki-Jeong Kim, Hangil Lee, Young-Min Kim PII: DOI: Reference:

S0169-4332(19)33733-X https://doi.org/10.1016/j.apsusc.2019.144916 APSUSC 144916

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 September 2019 14 November 2019 30 November 2019

Please cite this article as: G. Han, J. Yeon Kim, K-J. Kim, H. Lee, Y-M. Kim, Controlling surface oxygen vacancies in Fe-doped TiO2 anatase nanoparticles for superior photocatalytic activities, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144916

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Controlling surface oxygen vacancies in Fe-doped TiO2 anatase nanoparticles for superior photocatalytic activities

Gyeongtak Hana, Joo Yeon Kimb, Ki-Jeong Kimc, Hangil Leeb,*, Young-Min Kima,d,* a

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of

Korea b

Department of Chemistry, Sookmyung Women's University, Seoul 04310, Republic of Korea

c

Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang 790-784,

Republic of Korea d

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon

16419, Republic of Korea

*Corresponding authors. E-mail addresses: [email protected] (H. Lee), [email protected] (Y.-M. Kim).

ABSTRACT Introduction of defect structures into Fe-doped TiO2 nanoparticles (Fe@TiO2 NPs) has been shown to endow NPs with improved photocatalytic properties. However, current strategies for the preparation of defect-containing NPs require high temperature or complicated treatments, which can induce unwanted phase transition. In this paper, we report a facile method to introduce surface oxygen vacancies into anatase-type Fe@TiO2 NPs without altering the crystalline phase via simple pH treatments at moderate temperatures. Furthermore, we present the effects of pH on the formation of surface oxygen vacancies. The optimized treatment under basic conditions is

 

revealed to promote the formation of oxygen vacancies on the surface of anatase Fe@TiO2 NPs and effectively reduces the particle size by more than 25%, thereby causing a significant enhancement in the photocatalytic activities of the NPs (e.g. ~3.5 times better in photocatalytic degradation rate of 4-CP as compared to acid-treated Fe@TiO2). Comprehensive structural and chemical characterizations reveal that the point defects are predominantly formed on the surface of anatase NPs, and their population can be maximized by use of basic pH conditions. Our results pave a way toward the facile and efficient engineering of surface defect structures on catalytic metal oxide NPs for the design of high-performance photocatalysts. Keywords: Surface oxygen vacancy; Fe-doped TiO2 NPs; pH effect; Atomic defects; Photocatalytic activity.

1. Introduction Metal oxide nanoparticles (MO NPs) have been shown to display better photocatalytic properties than those of metals and have, thus, been used as catalysts for several decades. MO NPs such as TiO2, SnO2, and CeO2 NPs have been shown to be efficient catalysts and have been synthesized in various forms (for example, different sizes and shapes). Because MO NPs are not only used as catalysts but also used in solar cells and photodetectors, research into these materials continues, in particular, to increase their efficiency [1–12]. TiO2 NPs are particularly attractive in this role because of their low cost and usefulness in the visible-light range, as well as their good charge transport properties [13–16]. Globally, the stable supply of energy is a concern, and it is necessary to develop new catalyst materials or to increase the efficiency of existing catalyst materials. Thus, many research groups have attempted to modify the techniques used to synthesize MO NPs, for example, by doping with metals or anions, to increase the catalytic efficiency [17,18]. Chemical modification has recently  

been used to transform white-colored TiO2 NPs into black TiO2 NPs, which show significantly improved catalytic properties. In detail, the improved properties of the modified black TiO2 NPs can be mainly ascribed to the high concentration of oxygen vacancies and surface defects, which introduces new energy bands near the Fermi level with a decreased band gap [19–21]. The introduction of defects has also been used for other TiO2 NPs to improve their catalytic properties. Moreover, the surface morphologies of TiO2 NPs have been shown to vary with the surface area, which depends on the concentration of defect structures including oxygen vacancies and impurities [22–25]. Recently, one of the most popular applications of TiO2 NPs and its nanocomposites has been the photocatalytic decomposition of pollutants, and their applications in the environmental field have been actively studied [26-30]. One of the most frequently used pollutants to compare the efficiencies of photocatalytic materials such as TiO2 NPs is 4-chlorophenol (4-CP). The photocatalytic decomposition of organic pollutants using metal oxide NPs is recognized as a very important process in the purification and treatment of industrial wastewater [31–34]. As is well known, TiO2 NPs are one of the most efficient photocatalysts, being stable and inexpensive photosensitive materials [35]. The effective photoexcitation of TiO2 NPs requires the application of light with energy equal to or higher than its band gap energy. It is generally reported that anatase, which has a band gap (Eg) of 3.2 eV, is a more efficient photocatalyst than rutile [6–8]. However, the overall photoreactivity of TiO2 NPs is also determined by various other factors such as the electron/hole generation rate and the formation of OH- radicals [36,37]. In the current work, to enhance the photocatalytic activity of TiO2 NPs, we have been pursuing a strategy of doping them with metallic Fe (an environmentally safe material) and, subsequently, modifying the pH level, which can increase the concentration of defect structures [38–40], thus concurrently altering the catalytic activities of the TiO2 NPs. To this end, we synthesized TiO2 NPs, which have been well-studied recently, doped with metallic Fe (denoted as Fe@TiO2 NPs),  

and have tracked how the catalytic properties of these fabricated NPs are affected by changes in the pH during synthesis. Subsequently, we have systematically compared their catalytic activities with those of TiO2 NPs. We have also characterized the atomic and electronic structures of the Fe@TiO2 NPs using scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDX), high-resolution photoemission spectroscopy (HRPES), and X-ray absorption spectroscopy (XAS). We specifically aimed to compare the catalytic properties of the Fe@TiO2 NPs treated at different pH conditions and to determine the driving force of each catalyst with the ultimate purpose of deriving the pH condition that best increases the catalytic efficiency of these NPs. To estimate the efficiency of the photocatalytic decomposition of organic pollutants, the degradation of 4-CP and the reaction of OH radicals with benzoic acid (BA) on irradiation with a 300-W Xe lamp were measured. As a result, we found that the Fe@TiO2 NPs shows significantly increased photocatalytic activity and stability when treated under basic conditions. The change in pH (which changes the charge environment in the synthetic solution) was revealed to be closely related to the formation of an oxygen-vacancy-mediated defect structure on the NP surfaces, which is directly responsible for enhancing the photocatalytic properties.

2. Experimental 2.1. Sample preparation TiO2 NPs were synthesized from tetramethylammonium hydroxide (TMAOH) and titanium isopropoxide (TTIP) [41]. After fabricating the TiO2 NPs, the desired amounts (1 wt %) of the Fe dopant was added in the form of Fe(NO3)3∙9H2O (99%), expressed as mole fractions with respect to the TiO2 NPs, i.e., (Fe/(Fe+ TiO2)). The pH of the prepared solution was tuned using HNO3 (acid) or KOH (base), and the target pH was maintained for 3 h. After the formation of homogeneous and transparent solutions had been confirmed, the solutions were transferred to  

Teflon-lined autoclaves, which were sealed and heated at 220 °C for 7 h in a convection oven. The resulting three different Fe@TiO2 NPs (Fe@TiO2-A; acid treatment, Fe@TiO2-N; neutral treatment, and Fe@TiO2-B; base treatment) were filtered and washed with double distilled water (DDW) to remove any residues (see Scheme 1). All substances were purchased from Sigma– Aldrich.

Scheme 1. Steps used to fabricate pH-modified Fe-doped TiO2 NPs.

2.2. Characterizations The morphologies and atomic structures of the three Fe@TiO2 NPs were imaged using annular dark field (ADF) imaging mode with a detector angle range of 45–180 mrad and a probe forming angle of 23 mrad in an aberration-corrected STEM (JEM-ARM200CF, JEOL Ltd.) operated at 200 kV. All the acquired ADF-STEM images were denoised using the Wiener filtering method as implemented in a commercial program (HREM Filter Pro, HREM Research Ltd.). EDX and EELS spectrometers installed in the STEM equipment were used to obtain elemental distribution maps and core-loss electron energy loss spectra of the samples. Surface area measurements (Brunauer–  

Emmett–Teller, BET) were taken by using an Autosorb-iQ 2ST/MP (Quantachrome). HRPES and XAS experiments were performed at the 8A1 beamline of the Pohang Accelerator Laboratory (PAL) [25]. We used 4-chlorophenol (4-CP, Sigma-Aldrich, ആ 99%) and benzoic acid (BA, Sigma-Aldrich, ആ 99%), as target contaminants. The required amount of the three different Fe@TiO2 nanoparticles (0.4 g/L; pH treatements) was prepared in a reactor containing an aqueous solution of each substrate (30 mL, 100 µM). The suspensions were magnetically stirred in the dark for 15 minutes to establish absorption–desorption equilibrium. Then, the reactor was irradiated by a 300-W Xe arc lamp (Newport) with a cutoff filter for UV illumination (the wavelength was 320 nm). Solution samples including 4-CP (or BA) were collected using a 1-mL syringe every 15 min, filtered through 0.45 µm PTFE filter (Whatman), and then, analyzed. The concentrations of 4-chlorophenol and pHBA were measured by high-performance liquid chromatography (HPLC, Shimadzu UFLC LC-20AD pump) equipped with a diode array detector and a Shim-pack GIS column (4.6 mm × 250 mm).

 

3. Results and discussion

Fig. 1. ADF-STEM images of (a) Fe@TiO2-A, (b) Fe@TiO2-N, and (c) Fe@TiO2-B NPs, respectively. Note that A, N, and B indicate the pH treatment of the Fe@TiO 2 NPs in acidic (pH = 1.5), neutral (pH = 7), and basic (pH = 13.5) conditions, respectively. (d) Surface areas of the

three NPs determined from the BET measurements. Scale bars: 50 nm. The morphologies of Fe@TiO2 NPs treated at different pH conditions were observed in ADFSTEM imaging mode, as shown in Figs. 1(a)–(c). Three different pH treatments using acidic (pH = 1.5), neutral (pH = 7), and basic (pH = 13.5) solutions were applied to synthesize the modified Fe@TiO2 NPs, which are denoted as Fe@TiO2-A, N, or B, respectively. From the ADF-STEM images, we see that most of the NPs have a similar round shape. However, the Fe@TiO2-B NPs are reduced in size compared to the other samples. Indeed, the average particle size of the Fe@TiO2-B NPs was measured to be approximately 34.9 ± 6.7 nm, whereas the other two Fe@TiO2-A and N NPs have similar particle sizes, about 42.3 ± 7.3 and 39.9 ± 6.6 nm, respectively. This result indicates that basic pH treatment of the starting material in the synthesis of Fe@TiO2 NPs is effective in reducing the NP size, which would increase the formation of  

defect structures such as atomic defects and the surface area of the NPs. Complementary bright field and high-resolution TEM investigations on the NPs show the similar shape and size distributions (Fig. S1). The BET surface area measurements (Fig. 1(d)) show the exceptional increase in the surface area in the Fe@TiO2-B NPs compared to those in the other samples, which is consistent with the notable size reduction of the NPs determined from the STEM measurements. The results of the BET measurements for the three tested samples are listed in Table 1. Table 1. Surface areas of the three tested samples. NPs

Fe@TiO2-A

Fe@TiO2-N

Fe@TiO2-B

Surface area (m2/g)

122.08

124.16

189.11

To identify the possible polymorphic phases of the three Fe@TiO2 NP sample, atomic arrangements of the samples were imaged using high-magnification ADF-STEM measurements, as shown in Fig. 2. In this imaging mode, because the contrast in the resulting image is roughly scaled to Z»n, where Z is the atomic number and n » 1.6–2 [42,43], Ti appears as bright dots whereas O shows very weak contrast. Figs. 2(a)–(c), thus, show the well-defined positions of the Ti atoms in TiO2 structure. From these results, we confirmed that all the NPs have the same atomic structure, which was identified as anatase TiO2. X-ray diffractions performed on the three Fe@TiO2 NP samples also identify all the three samples as anatase TiO2 phase (Fig. S2). The atomic model of anatase TiO2 aligned in the [001] direction shown in Fig. 2(d) is exactly matched with the experimental STEM image (Fig. (e)). Because the pH affects phase stability of the TiO2 NPs substantially, often unexpectedly resulting in poor photocatalytic properties [44,45], the

 

Fig. 2. Atomic structure images of the three different Fe@TiO2 NPs treated under (a) acidic (pH = 1.5), (b) neutral s (pH = 7), and (c) basic (pH = 13.5) conditions, respectively. The inset images show low-magnification ADF-STEM images of the three different samples, and the boxes denote the regions of interest corresponding to the atomic structure images. (d) Atomic

model of anatase TiO2 aligned in the [001] orientation and (e) a representative magnified ADF-STEM image exhibiting an exact match with the anatase structure in which the positions of Ti (light blue) in this direction appear as bright dumbbell-like structures and those of O (red) show very weak contrast.  structural stability of the anatase TiO2 phase [46] over the whole pH range is remarkable and will

play an important role in sustaining a desirable catalytic properties during pH adjustment because the anatase phase is known to be more efficient as photocatalyst than rutile phase [6–8]. Note that a very thin amorphous surface layer (less than 1 nm) is often commonly observed in the NPs regardless of synthetic or post-synthetic treatment at different pHs. This indicates that the thin amorphous surface layer has an insignificant effect in changing the photocatalytic activities of the NPs.

 

Fig. 3. STEM-EDX elemental mapping data of the three different Fe@TiO2 NPs treated under (a) acidic (pH = 1.5), (b) neutral (pH = 7), and (c) basic (pH = 13.5) conditions, respectively. For the elemental mapping, Ti K (red, 4.508 keV), O K (blue, 0.525 keV), and Fe K (green, 6.398 keV) X-ray peaks were chosen. To detect the small amount of Fe, the characteristic Xrays were acquired using a high-efficiency dual-type silicon drift detector for 30 min. Scale

bars: 50 nm. To probe the presence of Fe ions doped into the three TiO2 NPs, we performed STEM-EDX mapping at the nanoscale, as shown in Fig. 3, in which the constituent elements of Ti (red), O (blue), and Fe (green) were mapped out. As shown by the elemental mapping data, all the nanoparticles are composed of TiO2, and a small amount of Fe ions are dispersed into the TiO2 matrix, regardless of the pH treatment. Notably, the segregation of excess Fe (green) was observed in all the samples (see Fig. S3 in the Supplementary data) in the large-scale view, which is ascribed to the formation of ferric oxide (Fe2O3) as a secondary phase. From this result, we inferred that the amount of soluble Fe ions added to the TiO2 matrix should be much smaller than the amount

 

Fig. 4. STEM-EELS analysis probing the change in the electronic structure. ADF-STEM images showing the morphologies of the three different Fe@TiO2 NPs treated under (a) acidic (pH = 1.5), (b) neutral (pH = 7), and (c) basic (pH = 13.5) pH conditions, respectively. (d)–(f) Ti L2,3 and O K electron energy loss near-edge structures (ELNES) spectra obtained at surface (red) and core (blue) parts of the NPs corresponding to (a)–(c), respectively. Note that the simultaneously acquired low-loss EELS including zero loss peak was used to compensate for the small energy drift in the spectrometer and a plural scattering contribution in the core-loss EELS. Scale bars: 50 nm. (1 wt%) used in this experiment. A previous study [47] revealed that dopant Fe for TiO2 can effectively stabilize the anatase structure, and the amount of soluble Fe ion should be below 1 wt%, otherwise Fe2O3 is likely to be formed as a secondary phase.

 

From the atomic structure and elemental mapping data, we realized that the three Fe@TiO2 NPs have the same crystal structure (anatase, space group = I41/amd) both at the surface and the interior of the NPs, and the Fe dopants are evenly dispersed in the TiO2 matrix. The only notable physical change is the apparent reduction in the average particle size of the Fe@TiO2-B NPs compared to those of the other samples. The reduction in the NP size is one factor known to improve the photocatalytic activity because the exposed surface area containing active sites for the catalytic reaction is increased. However, to understand the changes in the catalytic properties of the Fe@TiO2 NPs with respect to the pH, information regarding the change in electronic and chemical nature associated with atomic defect disorder is required. Thus, we probed all the Fe@TiO2 NP samples using STEM-EELS technique to obtain electron energy loss near-edge structures (ELNES) of Ti L2,3 and O K edges. The fine electron probe used for the ADF-STEM imaging was employed to acquire the separated signals of the respective surfaces and the interior of the NPs, as shown in Fig. 4. The intensity and positions of the peaks in the ELNES spectra provide information about the unoccupied density of states (DOS) above the Fermi energy level, which are sensitive to changes in bonding and charge state of an atom. ELNES spectra arise from the transition of a core level electron to unfilled states under the dipole selection rule [48,49]. The L edges of a transition metal are, therefore, attributed to the transitions of the 2p core electrons to the final states with s and 3d orbital components, and the shapes and positions of the peaks vary sensitively with the change in oxidation state and local bonding environment of the transition metal [50,51]. In particular, the L3 and L2 edges of Ti4+ in anatase TiO2 are observed as characteristic white lines after the onset energy of 456 eV, and each white line typically shows a clear splitting of two peaks: t2g and eg. This peak splitting becomes less pronounced when the Ti valence state of +4 is reduced to a lower valence state of +3 or +2 by charge injection on the formation of oxygen vacancy (Vo). The Vo can give rise to rearrangement of the excess electrons among the nearest neighboring Ti atoms around the Vo site, forming shallow donor levels just  

below the conduction band consisting of Ti 3d orbitals and further creating localized donor states that are located at 0.75–1.18 eV below the conduction band of anatase TiO2 [52,53]. It has been shown that the donor levels associated with Vo can even overlap the bottom of the conduction band in highly oxygen-deficient titania, thus resulting in the disappearance of the t2g peak first in the Ti L2,3 edges and causing a chemical shift in the onset energy toward a lower energy (red shift) in the ELNES spectrum of Ti as the concentration of the Vo increases [53]. For O K ELNES, a split prepeak starting at the onset energy of ca. 529 eV is typically observed in titania owing to the Ti 3d-O 2p hybridization; thus, the same reasoning discussed above can be applied to the prepeak splitting feature. In the presence of oxygen vacancies or local defect structures inside the titania, the split prepeak becomes broader, finally overlapping as a single peak with increasing vacancy content [54]. In the STEM-EELS experiments, the shapes of the Ti L2,3 and O K edge spectra obtained from the core parts of the three Fe@TiO2 NPs (shown as blue spectra in Figs. 4(d)–(f)) show typical spectral features corresponding to anatase TiO2 [55]. However, characteristic changes in the edge spectra taken at surface regions (see red spectra in Figs. 4(e) and (f)) are conspicuous, except for that of the Fe@TiO2-A NP sample (Fig. 4(d)), which shows almost the same spectral features in the spectra for both the surface and core regions. This result suggests that the Fe@TiO2 NP sample has a substantial Vo content in the surface region when considering a purely ionic scenario where the substitution of a trivalent cation like Fe3+ into the Ti4+ site can extrinsically induce oxygen vacancies for charge compensation [53]. However, after the acidic treatment, we found that the ELNES structures of the Ti and O atoms are not different on the surface or inside of the NPs (Fig. 4(d)). From this result, we can deduce that the formation of surface oxygen vacancies, which act as a +2 charge, can be suppressed because of the enhanced protonation in a strongly acidic solution whereby the positive charge is increased by the gain of a proton (H+). On the other hand, we can also expect that the strong deprotonation, where negative charge is obtained by the loss of a proton  

(H+), in the basic treatment can further promote the creation of surface oxygen vacancies on the oxygen-deficient Fe@TiO2 NPs, thus resulting in a significant change in the electronic structures of the surface Ti and O atoms. The experimental Ti and O ELNES spectra for the Fe@TiO2-B NPs (Fig. 4(f)) support this argument because of the complete disappearance of the splitting peaks in the Ti L2,3 and O K edges and the perceptible chemical shift toward a lower energy of 1.3 eV in the Ti L2,3 edge. For the Fe@TiO2-N NPs treated at neutral pH (pH = 7.0), the oxygen defect density appears to be slightly higher than that of the acid-treated sample, Fe@TiO2-A (Fig. 4(d)), as judged from the Ti and O ELNES spectra (Fig. 4(e)). Based on the STEM-EELS experiments, it is predicted that the Vo density decreases in the following order: base, neutral, and acid treated Fe@TiO2 NPs, and this order of defect density can be translated into the order of photocatalytic activity because the high Vo density enhances the photochemical reactions on the TiO2 NP surface. Utilizing multiple linear square fitting approach to map out spatial distribution of spectral phases, it is unequivocally demonstrated that the point defects are predominantly formed on the surface of the anatase NPs, and the defect population can be maximized by use of basic pH condition, thereby resulting in the formation of the core-shell nanoparticles with different electronic properties (Fig. S4 in the Supplementary data). It is worth noting that bandgaps of the three Fe@TiO2 NPs were measured to be reduced by ~1 eV as compared to bare TiO2 (3.1 eV) in mesoscale X-ray photoemission spectroscopy analysis and the base-treated one showed the largest bandgap reduction (Fig. S5 in the Supplementary data). The core-level spectra (Ti 2p) of the three different types of Fe@TiO2 NPs were obtained by HRPES (left panel of Figs. 5(a)–(c)) to determine the differences between their electronic structures in the occupied state region. These spectra all contained two distinctive features: peaks corresponding to Ti 2p at 459.4 eV (Ti4+) and 458.1 eV (Tix+), which arises because of the formation of defect structures [56,67]. When comparing the three TiO2-containing samples (Fe@TiO2-A NPs, Fe@TiO2-N NPs, and Fe@TiO2-B NPs), we focused on the changes in the  

Fig. 5. HRPES results (Ti 2p) and XAS spectra (Ti L2,3-edge) for (a) Fe@TiO2-A NPs, (b) Fe@TiO2-N NPs, and (c) Fe@TiO2-B NPs. B. E. and P. E. stand for binding energy and photon energy, respectively.

intensity of the Tix+ peak, which indicate the formation of the defect structure. The intensities of the peaks related to defect structures obtained from Fe@TiO2-B NPs were observed to be larger than those of the other samples (see Fig. 1). The changes in the two peaks were, again, found to be correlated with the changes in the catalytic properties, a pattern also found for the BET results shown in Fig. 5. The Tix+ species have been shown to be related to the defect structures in TiO2 NPs as a result of charge compensation. Table 2 shows the intensity ratio of the defect structure for each of the three spectra shown in Fig. 5. In addition, to obtain more detailed characterization of the electronic structures even in the unoccupied state region, we also obtained the Ti L-edge XAS for the three Fe@TiO2 NP samples using STXM. First, the shape of the eg orbital located at ca. 460 eV for the Ti L2,3-edge XAS spectra indicates the presence of a typical anatase TiO2 structure in all Fe@TiO2 NPs [17].  

However, in the case of the Fe@TiO2-B NPs, the ratio of the intensities of the t2g (457.4 eV) and eg (459–460 eV) peaks increases below those of the other samples (Fe@TiO2-A and Fe@TiO2N), which indicates the presence of a weak crystal field or an increase in the number of undercoordinated Ti atoms. In other words, these differences are due to the different dopants or vacancies, which produce different defect structures in the nanoparticles. The small doublets at 456.0 and 456.6 eV in these figures correspond to the Ti3+ state, and it is well known that metal doping enhances the surface defect structure [18]. Table 2 Ratio of the intensity of the defect structure peak to the intensity of the defect and metal peaks for each of the three tested samples.

NPs

Intensity ratio Fe@TiO2-A Fe@TiO2-N

Fe@TiO2-B

ITix+/ITi4+

0.315 ↑

0.154

0.177

Interestingly, we found a correlation between the BET analysis, which indicates changes in the surface area, and the HRPES and XAS results, which indicate changes in the intensities of the defect structure peaks. From Tables 1 and 2, the increase in the surface area coincided with increases in the intensities of the defect structure peaks. That is, a positive correlation was found between the surface area and the concentration of defect structures. Our analyses of the experimental results for the samples (three tested samples) suggest that there is a close relationship between the defect structure concentration and the photocatalytic activity. These results are explained in detail below. We were able to make several conclusions based on these experimental results. First, a higher proportion of defect states was present in the Fe@TiO2-B NP sample compared to the proportion present in the other Fe@TiO2 NPs, as indicated by the STEM-EELS, HRPES, and XAS results (see Figs. 4 and 5). In addition, from the BET experiment (see Fig. 1), we confirmed that the  

defect concentration is proportional to the surface area. The concentration of defect structures was also shown to be positively associated with the quality of the catalyst. Secondly, the doping, in combination with pH modulation, appeared to be directly involved in modifying the TiO2 NP surface structure by further increasing the concentration of defect structures and, in this way, enhancing catalysis. Third, another explanation for these results is active radical formation on the surface of the TiO2 NPs. The reaction of OH ions with the holes of TiO2 NPs has been shown to generate more OH radicals in basic conditions than in acidic or neutral conditions [58,59]. Our experiments also indicate that the Fe@TiO2 NPs synthesized under basic conditions (pH = 13.5) exhibited the best catalytic properties. The experimental results, which will be shown later, clearly indicate an improvement in the catalyst characteristics after pH modulation. In particular, OHradicals, which increase the rate of catalytic reactions, and the durability and reactivity of the doping material, appeared to be crucial. Thus, we determined that the increase in the number of

Fig. 6. The doped Fe L-edge XAS spectra of 1 wt% Fe@TiO2 NPs and (b) relative intensities of Fe L3-edges depending on the pH treatment, which were normalized to Fe L3-peak for Fe@TiO2-A sample for comparison.

 

defect structures is the most important driving force for improving the catalyst characteristics, and pH modulation can further enhance the photocatalytic activity. As mentioned above, we believe that the doped Fe metals formed Fe2O3 on the TiO2 NPs. To identify the electronic states induced by the transition metal dopants in more detail, we acquired Fe L-edge XAS spectra, as shown in Fig. 6(a). The two sharp peaks shown in Fig. 6(a) at 706.7 eV (marked A) and 708.5 eV (marked B) can be clearly matched to the Fe3+ L3-edge of Fe2O3 [60,61]. Therefore, we confirmed that the doped Fe metal has an electronic state consistent with that of Fe2O3. In particular, as shown in Fig. 6(b), we found that Fe@TiO2-B has a relatively large amount of doped Fe on the TiO2 NPs, which means that the number of defects on the surface of TiO2 NPs is relatively large. Putting all the results obtained from the multiple spectroscopic measurements together, we expected that the Fe@TiO2-B NPs would have the most enhanced catalytic properties of the three NPs (see Fig. 7).

Fig. 7. (a) Photocatalytic degradation of 4-CP and (b) radical-induced formation of phydroxybenzoic acid (p-HBA) from BA reacted with three different Fe@TiO2 NPs. [Fe@TiO2] = 0.4 g/L, [BA]0 = 10 mM. The photocatalytic degradation property of bare TiO2 NP is included for comparison. Note that all numerical values used for the plots are listed in Table S1 and S2 in the supplementary data.  

Several investigators have studied photocatalytic decomposition of phenol and chlorinated phenolic compounds in aerated aqueous suspensions of TiO2 upon illumination with near-UV light [62,63]. The catalyst dosage, initial concentration of pollutants, pH [64], UV light intensity [65], and concentration of charge trapping species [66] are the main parameters affecting the degradation rate of 4-CP in TiO2 NPs. Hence, the photodegradation process with the three different Fe@TiO2 NPs was also investigated in the presence of metal ions as electron scavengers [67]. The reaction rates in the presence of the metal ions were higher than that obtained when oxygen alone was used. Fig. 7(a) shows the 4-CP degradation measurements for three tested Fe@TiO2 NPs, showing that 4-CP is degraded by all samples, although to different degrees. In order to clarify the photocatalytic decomposition reaction of the three tested samples, the photocatalytic decomposition reaction was performed after the dark process (without UV irradiation) for the first 15 minutes and then irradiated with UV light. Thus, the wavelength of light provided on UV light irradiation is sufficient for the NPs to form electron–hole pairs. We found that Fe@TiO2-B NPs degraded the 4-CP solution effectively after 90 min UV light illumination because of the increased number of defect structures (active sites) in this sample. In other words, the UV light irradiating the surface of the Fe@TiO2-B NPs effectively produced electrons and holes by a reduction and oxidation process during the photocatalytic reaction owing to the higher density of oxygen vacancies over the larger surface area (compared to that of the other tested Fe@TiO2 NPs). These electrons and holes react with the aqueous 4-CP solution and, thus, produce OH radicals that are capable of effectively degrading the 4-CP solution [68,69]. The degradation rate of the three tested Fe@TiO2 NPs by 4-CP was observed to be a function of time. In general, because changes in contaminant concentrations caused by environmental effects follow a linear function (first-order reaction), we used the well-known first-order reaction formula (Eq. (1)) and fitted the experimental data to this equation.  

ln(C/C0) = – kt

(1)

In Eq. (1), C and C0 are the absorbance of the 4-CP at time t and 0, respectively. k is the rate constant (min−1), and t is the time (min.). When ln(C/C0) was plotted with respect to time (Fig. 7(a)), a linear relationship was obtained. As observed from the first-order reaction plot shown in Fig. 7(a), the rates of change when using the Fe@TiO2-B NPs are remarkably faster than those of the other NPs. This indicates that the photocatalytic activity of the Fe@TiO2-B NPs is better than that of the other NPs. This result confirms that the photocatalytic properties of the Fe@TiO2-B NP sample was enhanced by the increased defect density. Additionally, we also tested the OH radical formation of the three tested NPs via the radical reaction with BA, as shown in Eq. (2). ௛௩

଺ ହ െ  ൅൉  ሱሮ  െ ଺ ହ െ 

BA

Fe@TiO2 NPs





(2)

p-HBA



When the OH radicals formed on the surface of the NPs react with BA, as shown in Eq. (2), phydroxybenzoic acid (p-HBA) is formed. By comparing the amounts of p-HBA formed with time, the above-mentioned influence of the OH radicals on the catalytic reaction can be analyzed comparatively [70,71]. As shown in Fig. 7(b), the radical formation reactions of the nanoparticles synthesized under different pH conditions show large differences. As expected, the Fe@TiO2-B NP sample was the most reactive. The results are also consistent with the results of the 4-CP degradation experiments. Remarkably, it is evident that the trend in the photocatalytic activities is the same as the order of the changes in the surface area and the atomic defects, as confirmed by the BET measurements and STEM-EELS investigation (see Figs. 1 and 4). This is direct proof that a high density of OH radicals should be present on the surface of the Fe@TiO2 NPs when the concentration of the surface defect structures increases. Table 3 shows the k values obtained from the 4-CP degradation and p-HBA concentration (a proxy for OH radical concentration) data  

obtained from the results shown in Fig. 7. From the photocatalytic reactivity results (see Fig. 7), we confirmed that the photocatalytic activities of Fe@TiO2-B NPs are better than those of the other samples. Summarizing the results so far, we can represent conceptual illustration of the bandgap structure and the photocatalytic decomposition mechanism of the Fe@TiO@-B NP as shown in Fig. 8(a). 

Fig. 8. (a) Conceptual illustration of bandgap structure and photocatalytic decomposition mechanism of Fe@TiO2 NP treated under basic condition. Bandgap of TiO2 is significantly reduced from 3.1 eV to 2.1 eV by Fe doping (Fig. S5). Furthermore, the basic treatment (pH = 13.5) to Fe@TiO2 effectively introduce a high density of oxygen vacancies on the surface, thus forming an electron-rich shell. These surface oxygen vacancies can create localized donor states at 1.3 eV below conduction band minimum (Fig. 4), which can thus propel photocatalytic activities in the decomposition

process of 4–CP and the radical reaction with BA. (b) Recovery rate (reproducibility) of the Fe@TiO2-B NPs for 4-CP degradation over five cycles.

 

Table 3 Values of the calculated rate constants (k) for 4-CP degradation experiments and the concentration of OH radicals ([p-HBA]) of the three tested samples. Fe@TiO2-A

Fe@TiO2-N

Fe@TiO2-B

k (min ): 4-CP

0.0063

0.0109

0.0224

p-HBA (µM)

11.35

17.16

27.27

-1

Finally, MO NPs exhibiting catalytic properties must be able to maintain their catalytic ability after re-use because economic efficiency must be considered for practical applications in various fields [72,73]. Therefore, the 4-CP degradation experiments using the Fe@TiO2-B NPs that showed the best performance among the tested Fe@TiO2 NPs were repeated several times to evaluate the performance stability for cyclic use, as shown in Fig. 8(b). The reaction rate constants were only reduced by ca. 4.9 % in the Fe@TiO2-B sample after five re-uses, demonstrating the robustness of the photocatalytic properties of the Fe@TiO2-B NPs in cyclic use and suggesting their practical applicability. Therefore, our results confirm that the concentration of defects facilitates the formation of OH radicals. This is the most important factor that influences the catalyst quality and, importantly, can be enhanced by pH treatment under basic conditions.

4. Conclusion In this study, we have identified the significant effect of pH treatment on the catalytic properties of Fe-doped TiO2 nanoparticles (Fe@TiO2 NPs). We found that Fe@TiO2 treated under strongly basic conditions (pH = 13.5), i.e., Fe@TiO2-B NPs shows the best catalytic performance among the tested samples. From a systematic investigation based on microscopy and spectroscopy analyses, we revealed that the basic pH treatment of the Fe@TiO2 NPs is remarkably effective in forming atomic defects, such as oxygen vacancies, on the surface, as well as reducing the size of the NPs compared to those of Fe@TiO2 NPs treated at neutral or acidic pH. At basic pH, the strong  

deprotonation effect via the negative charge obtained by the loss of protons, promotes the creation of surface oxygen vacancies on the oxygen-deficient Fe@TiO2 NPs, thus giving rise to a significant change in the electronic structures of the surface Ti and O atoms. Therefore, it is obvious that the change in the pH of the starting material solution directly alters the final density of surface atomic defects and the size (or surface area) of the Fe@TiO2 NPs. This can be used as a facile and effective means to enhance the catalytic performance of metal doped TiO2 NPs. From the 4-CP degradation measurements, we found that the catalytic activity of Fe@TiO2-B NPs is better than those of the other samples. We also found that OH radicals were formed in the greatest quantity on the surface of the Fe@TiO2-B NPs, which is direct evidence for the high concentration of defect structures on the surface. Overall, we believe that our work provides new insights into the understanding of the photocatalytic phenomena in metal-doped TiO2 NPs on the basis of the multiple structural and chemical information obtained at the atomic to mesoscopic scale, and we have suggested a pragmatic approach to tailor the density of surface oxygen vacancies for the preparation of high-performance TiO2 photocatalysts.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or person al relationships that could have appeared to influence the work reported in this paper.

Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) funded by the Government of Korea (MSIP) (No. 2017R1A2A2A05001140). Y.-M.K. acknowledges financial support by the Creative Materials Discovery Program (NRF-2015M3D1A1070672) through an NRF grant and the Institute for Basic Science (IBS-R011-D1).

 

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi: https://doi.org/10.1016/j.apsusc.2019.XXXXXX.

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