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Article

Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites

by
Seong-Rak Eun
1,
Shielah Mavengere
1,
Bumrae Cho
2 and
Jung-Sik Kim
1,*
1
Department of Materials Science and Engineering, University of Seoul, Seoul 02504, Korea
2
Department of Materials Engineering, Keimyung University, Daegu 42403, Korea
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(10), 1188; https://doi.org/10.3390/catal10101188
Submission received: 14 September 2020 / Revised: 12 October 2020 / Accepted: 12 October 2020 / Published: 15 October 2020
(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)
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Abstract

:
Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO2 solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO2 coated on NaYF4:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO42− because of the doping effect. NaYF4:Yb,Er composites coated with N- and CNS-doped TiO2 exhibited a slight shift of UV-Vis spectra towards the visible light region. Photodecomposition of methylene blue (MB) was evaluated under 254 nm germicidal lamps and a 300 W Xe lamp with UV/Vis cut off filters. The photodegradation of toluene was evaluated on TiO2/NaYF4:Yb,Er and CNS-doped TiO2/NaYF4:Yb,Er samples under UV light illumination. The photocatalytic reactivity with CNS-doped TiO2/NaYF4:Yb,Er surpassed that of the undoped TiO2/NaYF4:Yb,Er for the MB solution and toluene. Photocatalytic activity is increased by CNS doping of TiO2, which improves light sensitization as a result of band gap narrowing due to impurity sites.

Graphical Abstract

1. Introduction

Titanium dioxide (TiO2) inorganic semiconductors have emerged as trending materials for photocatalysis applications [1,2,3]. The band gap of 3.2 eV necessitates ultraviolet (UV) light absorption to cause electron movement from the valence band to the conduction band to progress the photocatalytic reaction. However, these photoreactions only proceed within the UV portion of the solar spectrum, which leaves the broad portions of visible and near infrared (NIR) spectra undetected. Reversing the visible and NIR scattering phenomenon using TiO2 remains an arduous task for researchers. The methods for improving spectral absorbance include doping with metals and non-metals [4,5,6,7] and coupling with other compounds to form nanocomposites [8,9].
Doping TiO2 with elements such as B [10,11], C, N, and S [12,13,14] is among the extensively studied techniques used to narrow the band gap. C and N are organic elements that possess remarkable electronic structures and lower toxicity than metallic elements. Thus, non-metallic doping is advantageous because of the smaller ionic radii that can occupy the interstitial sites of TiO2 [15,16,17]. The extra energy levels imparted by dopants in TiO2 promote absorption of visible light photons [18]. To date, several studies have reported alternative mechanisms for electron–hole reaction pathways to facilitate photocatalysis. Specifically, N-doped TiO2 forms a unique band structure as a substitution at vacancy or interstitial sites in TiO2. As a result of this, the TiO2 band gap of 3.2 eV is narrowed to between 2.8 and 3.06 eV. The doped TiO2 then sensitizes visible light through the low-energy sites occupied by N for photocatalytic reactions. Consequently, N-doped TiO2 is reported to perform better in photodegradation reactions due to suppressed electron recombination [16,19]. However, in tri-doped CNS-TiO2, C, N, and S elements substitute at O sites in TiO2. In other words, the 2p orbitals of C, N, and S interact with O orbitals inside TiO2’s conduction band. Therefore, new energy levels from the CNS doping elements lower the overall band structure of TiO2. The great advantage of multi-element doping is that the sensitization sites increase several times as compared to singly doped element [14,20,21]. However, utilizing N- or CNS-doped TiO2 nanoparticles in aquatic and air purification methods requires a stable substrate to immobilize and prevent release of TiO2 into the environment. There are several immobilizing substrates for practical application of TiO2 nanoparticles, which include stainless steel [22] and glass [23]. However, these substrates only offer support. Their chemical composition has minimal effects in terms of improving the light absorption, which is essential for photocatalytic activity progression. Thus, there is a need to stabilize the nanoparticles in micro-sized compounds such as NaYF4:Yb,Er phosphors, which possess light-harvesting properties in the NIR region.
Although upconversion phosphors have short photoluminescence lifetimes, if coupled with TiO2 they promote light harvesting or photocatalysis, even under visible light photons, due to the heterojunction effect [24,25]. The heterojunctions that exist at interfacial peripheries of the TiO2 catalyst and phosphor support material are associated with modified electronic structure due to defects [24,26]. This phenomenon distinguishes NaYF4:Yb,Er from other compounds, since both the heterojunctions and the light upconversion effects simultaneously improve the optical and photocatalytic performance of the composites.
Coupling TiO2 with NaYF4:Yb,Er phosphor has been reported as a promising approach for utilizing low-energy photons in the NIR region and emitting UV-visible light [27,28]. Over the past decades, NaYF4:Yb,Er has been utilized to convert NIR 980 nm photons to emitted photons at 525–550 nm [26,29,30], and even at lower wavelengths such as 390–420 nm [31]. However, only the TiO2 coating on phosphor has been reported to cause improved photocatalytic efficiencies after a long photoreaction time above 10 h [32]. There is a need to evaluate photocatalysis in consideration of the amount of catalyst in the reactor, the light source intensity, and the concentration of pollutant in an effort to complete the photoreaction rapidly. The effect of N doping [33] or carbon–nitrogen–sulfur (CNS) doping [21] has been reported as a method to promote visible light activation of TiO2, but few studies have been reported that compare the effect of coupling N-doped or CNS-doped TiO2 with NaYF4:Yb,Er phosphor.
This study focuses on investigating the effects of N doping and CNS doping of TiO2 and coupling with NaYF4:Yb,Er upconversion phosphor. The stability of the organic–inorganic molecular bonding was characterized to confirm the existence of dopants in the TiO2/NaYF4:Yb,Er composites. Photocatalytic properties were evaluated with aqueous methylene blue (MB) and toluene pollutant mediums. The photocatalyst samples of N- or CNS-doped TiO2/NaYF4:Yb,Er were activated by UV, visible, and NIR light illumination of the solar spectrum.

2. Results

Figure 1 shows XRD spectra of NaYF4:Yb,Er phosphor composites. The crystallinity is indexed for the phosphor in reference to hexagonal sodium, yttrium, ytterbium, erbium, and fluoride (JCPDS 00–028–1192). The NaYF4:Yb,Er phosphor peaks were centered at (100), (110), (101), (200), (111), (201), (211), and (311). However, doping elements and TiO2 crystal were undetectable in the composites owing to the relatively small amounts coated on the sub-micron phosphor matrix. The XRD peaks were invariant in terms of peak broadening, which normally confirms the effect of N or CNS doping in TiO2.
Figure 2 shows SEM images of NaYF4:Yb,Er (Figure 2a), TiO2 (Figure 2b), TiO2/NaYF4:Yb,Er (Figure 2c), N-TiO2/NaYF4:Yb,Er (Figure 2d), and CNS-TiO2/NaYF4:Yb,Er (Figure 2e). The phosphor particles in Figure 2a are in the ~10 micrometer range, while the undoped TiO2 particles in Figure 2b are agglomerates with a spherical morphology measuring 20–50 nm. Table 1 shows the elemental compositions of F, Na, Y, Er, and Yb. When the TiO2 sol was coated on the phosphor, its spherical morphology was maintained (Figure 2c). Table 2 shows the elemental compositions of O, F, Na, Ti, Y, Er, and Yb. However, with N and CNS doping in TiO2, the TiO2 morphology changed to clustered nanorods on the surfaces of phosphor particles (Figure 2d,e). Table 3 shows the elemental compositions and confirms the presence of N, O, F, Na, Ti, Y, Er, Yb, and the N-doped TiO2 at 0.17 at.% N. The dense nanorods’ TiO2 morphology is presumed to be because of the change in pH after addition of the urea (N) or thiourea carbon-nitrogen-sulfur (CNS) doping reagent, as described in the Materials section. Table 4 shows the elemental compositions of C, N, O, F, Na, S, Ti, Y, Er, and Yb. The EDS spectra are shown in Figures S1, S3, S5, and S7. The EDS elemental mapping data are shown in Figures S2, S4, S6, and S8. Interestingly, the CNS-doped TiO2 shows values of C 46.30 at.%, N 0.79 at.%, and 0.15 at.%. Therefore, the thiourea doping reagent is a rich source for CNS elements.
FTIR spectroscopy results for N-TiO2/NaYF4:Yb,Er and CNS-TiO2/NaYF4:Yb,Er are shown in Figure 3a,b, respectively. The stretching vibrations at 3410 and 1624 cm−1 are assigned to the -OH groups bonded to Ti- atoms and H2O bending, respectively; while the stretching vibrations at 1369 and 1130 cm−1 arise from carboxylate and S = O bonds, respectively, because of surface-adsorbed SO42− species. The stretching vibrations at 1044 and 616 cm−1 are because of hyponitrite and the Ti-O groups, respectively [34].
Figure 4 exhibits UV-Vis absorption spectra of NaYF4:Yb,Er coated with undoped, N-doped, and CNS-doped TiO2. The NaYF4:Yb,Er phosphor absorbs UV light from 200 nm with an edge at 250 nm (due to the NaYF4 host), and in Figure S9 absorption peaks were centered at 524 and 654 nm due to the Er3+ co-activator [35]. However, coating TiO2 on phosphor broadened the absorption spectra. Specifically, the TiO2/NaYF4:Yb,Er composites exhibited UV-Vis absorption between 200 and 400 nm, with peak absorption at 300 nm. The additional N-doped and CNS-doped TiO2 phosphor composites show higher absorption intensities and a slight shift towards the visible region; the red-shift has been referenced by other researchers as being because of the lower energy levels in TiO2 imparted by N or CNS doping elements [20,36].
Figure 5a shows photoluminescence (PL) spectra as obtained after 980 nm NIR irradiation of NaYF4:Yb,Er upconversion phosphor composites. Firstly, the full emission spectra (UV-Vis–NIR emission) in Figure 5a exhibit emissions of visible light photons at 520, 527, 540, and 548 nm. Thus, as a result of simultaneous energy transfer occurring in the excited phosphor, the energy losses resulted in lower energy photons at 652 and 658 nm. The 2H11/2-to-4I15/2 transition is assigned to the emission at 527 nm, while the 4S3/2 to 4I15/2 is assigned to the emission at 540 nm. Additionally, the transitions emitting low-energy photons at 652 nm are from 4F9/2 to 4I15/2 [25]. The overall reduction in emission peaks in the TiO2-coated NaYF4:Yb,Er samples was also observed in previous research [32]. Thus, TiO2 nanoparticles on phosphor act as barriers to emitted light.
Figure 5a was enlarged and labeled (Figure 5b–d) to clearly observe peak variations. Figure 5b shows UV light emission at 384 nm and visible light photons at 407 and 484 nm in the NaYF4:Yb,Er phosphor. However, all of these peaks were suppressed in TiO2/NaYF4:Yb,Er and N/CNS-doped TiO2/NaYF4:Yb,Er composites. The effect of doping on photoluminescence was observed in the enlarged peaks in Figure 5c,d. Specifically, at 520 and 527 nm in Figure 5c, the TiO2/NaYF4:Yb,Er and CNS-doped TiO2/NaYF4:Yb,Er show unchanged emission intensities, while only N-doped TiO2/NaYF4:Yb,Er exhibits further peak suppression. However, at the peaks located at 540 and 548 nm, the CNS-doped TiO2/NaYF4:Yb,Er exhibits the highest photoluminescence, followed by TiO2/NaYF4:Yb,Er and N-doped TiO2/NaYF4:Yb,Er.
Figure 5d shows the enlarged PL emission spectra to clearly show peaks in the NIR region. At 652 and 658 nm, the peak intensity decreases in the order of NaYF4:Yb,Er > TiO2/NaYF4:Yb,Er TiO2 > CNS-doped TiO2/NaYF4:Yb,Er > N-doped TiO2/NaYF4:Yb,Er. The peak suppression and enhancement phenomena represent a multi-energy transfer process. However, some researchers have highlighted that the pH level in urea (the doping reagent in phosphor) tunes the PL emissions for visible and NIR emissions in Y2O3:Yb,Er nanophosphors [37]. Thus, in our work we confirmed the tuning of visible and NIR emissions. In the N-doped TiO2/NaYF4:Yb,Er (urea additive: pH 5.79) and in CNS-doped TiO2/NaYF4:Yb,Er (thiourea additive: pH 5.73) as compared to undoped TiO2/NaYF4:Yb,Er (pH 6.39), the pH values are almost the same, however the most acidic CNS-doped phosphor exhibits the highest PL intensity (at 540 and 548 nm), followed by the TiO2 phosphor and the doped TiO2-coated samples. Upconversion phosphors are characterized by having short lifetimes, as exhibited in the decay curves in Figure S10.
Figure 6 shows the photobleaching variation of the MB solution for NaYF4:Yb,Er phosphor composites under UV light illumination. The maximum peak at 664 nm is the characteristic MB absorbance that is monitored to evaluate intensity variations as a measure of the degradation of the organic compound. In a 4 h reaction period, the CNS-TiO2/NaYF4:Yb,Er absorbance spectra with the lowest intensities at seen at 4 h. Therefore, CNS-TiO2/NaYF4:Yb,Er shows the highest photocatalytic reactivity among the three samples. However, the TiO2 and N-TiO2 upconversion phosphor composites require more time to completely degrade MB solutions (Figure 6b,c).
Figure 7a shows the peak absorbance variations for photodegradation of the MB solution under UV illumination. The photodegradation of the MB solution proceeded to 40% efficiency for the photoreaction mixture with TiO2 only. However, N-doped TiO2 and CNS-doped TiO2 improves the efficiency to 80%. This is owing to the light absorption property related to N- and CNS-doped TiO2. Supporting TiO2 on the NaYF4:Yb,Er upconversion phosphor improved the photocatalytic efficiency up to 90%. Moreover, with N-TiO2 or CNS-TiO2 supported on NaYF4:Yb,Er, the photocatalytic efficiencies were enhanced to completion (100% in 4 h). Thus, N-TiO2 or CNS-TiO2 coupling with NaYF4:Yb,Er phosphor improves the catalytic activity due to the improvement in light absorption by phosphor and N or CNS doping elements. In detail, the NaYF4:Yb,Er phosphor support has light absorption properties due to the NaYF4 host (as exhibited in Figure 4), which coincides with the TiO2 absorption band. Additionally, light absorption for the composite is improved with additional N and CNS doping.
Both CNS doping and N doping of TiO2 further cause improvements in photocatalytic efficiencies. The N or CNS doping of TiO2 incorporates new energy levels in the interstitial and substitution sites. Thus, lower energy levels lower the TiO2 absorption band, which promotes the flow of electrons into the conduction band. As follows, the photocatalytic activity achieves completion in a 4 h cycle for phosphor coated with doped CNS or N-TiO2. Figure 7b illustrates the rate of kinetics for the samples in Figure 7a. As shown in Figure 7b, the reactions have a linear and typical relationship for first-order kinetics. The rate constants are 0.16 min−1 for TiO2, 0.37 min−1 for N-TiO2, 0.41 min−1 for CNS-TiO2, 0.0072 min−1 for NaYF4:Yb,Er, 0.58 min−l for TiO2/NaYF4:Yb,Er, 0.91 min−1 for N-TiO2/NaYF4:Yb,Er, and 1.2 min−1 for CNS-TiO2/NaYF4:Yb,Er. This result shows that MB photodegradation with CNS-TiO2/NaYF4:Yb,Er is the fastest reaction by 7.5 times for TiO2, by 3.2 times for N-TiO2, by 2.9 times for CNS-TiO2, by 2.1 times for TiO2/NaYF4:Yb,Er, and by 1.3 times for N-TiO2/NaYF4:Yb,Er. Therefore, CNS doping of TiO2 and its support on NaYF4:Yb,Er phosphor improves the photocatalytic performances.
Figure 8 exhibits the MB peak absorbance against time for visible light activation. The N-doped TiO2 and CNS-doped TiO2 show improvements in visible light photocatalytic efficiencies as compared to TiO2 only. Furthermore, coupling TiO2 with phosphor and doping the TiO2/NaYF4:Yb,Er causes enhancements of photocatalytic efficiencies. Precisely, TiO2/NaYF4:Yb,Er, N-doped TiO2/NaYF4:Yb,Er, and CNS-doped TiO2/NaYF4:Yb,Er showed 50%, 60%, and 70% efficiencies after 120 min of light irradiation. The undoped TiO2/NaYF4:Yb,Er composite showed significant photocatalytic efficiency as compared to TiO2, N-TiO2, and CNS-TiO2, mainly due to the heterojunction effect that exists between phosphor and TiO2. The CNS-doped TiO2/NaYF4:Yb,Er shows the highest photocatalytic reactivity over visible light illumination. As a result, the effect of doping with N-TiO2/NaYF4:Yb,Er or CNS-TiO2/NaYF4:Yb,Er is clearly exhibited by 10% and 20% efficiency enhancements, respectively, compared with undoped-TiO2/NaYF4:Yb,Er. Visible light sensitization is conceptualized by the low-energy states induced by doping with N or CNS elements. Thus, tri-element doping in TiO2 imparts more impurities in the TiO2 band than N doping only. Figure 8b shows the rate of kinetics for the samples in Figure 8a. The CNS-doped TiO2/NaYF4:Yb,Er exhibited the swiftest reaction with a 9.7 × 10−3 min−1 rate constant. In comparison with other photocatalysts, the CNS-doped TiO2/NaYF4:Yb,Er reaction is 9.7 times greater for TiO2 only, 4.4 times greater for N-TiO2, 2.8 times greater for CNS-TiO2, 1.8 times greater for TiO2/NaYF4:Yb,Er, and 1.2 times greater for N-TiO2/NaYF4:Yb,Er. Therefore, the photocatalytic efficiencies are improved by tri-doping TiO2 and its support on the NaYF4:Yb,Er upconversion phosphor.
Figure 9 shows the MB peak absorbance against time for NIR light activation. Undoped TiO2–phosphor and N-doped TiO2–phosphor composites only show ~15% efficiency with NIR illuminations, but CNS-doped TiO2/NaYF4:Yb,Er exhibits a ~25% improvement. Therefore, NIR irradiations in CNS-doped TiO2/NaYF4:Yb,Er have the highest photocatalytic activity. Under NIR, the Yb3+ in NaYF4:Yb,Er phosphor sensitizes the NIR photons and emits UV-Vis–NIR photons through the Er3+ emission center, as discussed in Figure 5. Visible light photons are sensitized through the low energy levels imparted by N- or CNS doping and electrons are injected into the TiO2 conduction band for photocatalysis. Additionally, the doping elements promote electron–hole generation efficiencies to facilitate photocatalysis. However, the overall photocatalytic efficiencies were low due to the mono-wavelength absorption properties of Yb3+ ions at 980 nm.
Figure 10 exhibits the concentration variations of toluene during photodegradation with TiO2-NaYF4:Yb,Er and CNS/TiO2-NaYF4:Yb,Er powders under UV light illumination. The toluene concentration in the Y-axis corresponds to the concentration of toluene remaining in the Teflon bag after 1 h sampling under UV light illumination. Through the first 1 h sampling cycle, the CNS/TiO2-NaYF4:Yb,Er photocatalyst has 1.5 ppm of toluene less than the undoped TiO2-NaYF4:Yb,Er sample. The 4 h toluene degradation is above 95% for the CNS/TiO2-NaYF4:Yb,Er. This result indicates that the CNS/TiO2-NaYF4:Yb,Er has exceptionally superior photoactivity compared with the TiO2-NaYF4:Yb,Er sample. Thus, CNS doping is essential for improving photocatalytic activity.

3. Discussions

Figure 11 is a schematic diagram depicting the synthesis and photocatalytic test performances. The pH of the coating sol varied due to the interaction of the TiO2 precursor and ethanol without pH modifiers. Thus, the TiO2 sol without the doping reagent showed slightly alkaline conditions at pH 6.39. As a result of this slight alkalinity, the TiO2 morphology existed as spherical particles on phosphor. However, with additional thiourea and urea as doping reagents for N and CNS in the TiO2 sol, the pH decreased to acidic conditions. For instance, in the N-doped TiO2 sol the pH was 5.79, while in the CNS-doped TiO2 sol the pH was 5.73. As a result, the morphologies were changed to nanorods. Hence, the drop in pH promoted the formation of nanorods. The photoluminescence emission peaks were also observed to be tuned by urea or thiourea reagents owing to the drop in pH.
Figure 12 shows the proposed photocatalysis mechanism for the NaYF4:Yb,Er phosphor coated with TiO2 under different doping conditions. The three conditions for TiO2 coating on NaYF4:Yb,Er are illustrated in Figure 12 as TiO2, N-TiO2, and CNS-TiO2. The first condition is TiO2 supported on NaYF4:Yb,Er phosphor, where only the TiO2 energy band is present. The second condition is N-doped TiO2 supported on NaYF4:Yb,Er, where the continuous solid line inside the TiO2 represents N energy levels. The third condition is CNS-doped TiO2 supported on NaYF4:Yb,Er, where the dotted line in the TiO2 band represents discrete energy levels imparted by C, N, or S elements. As illustrated, the upconversion phosphor as the support material contains Yb3+ as the NIR sensitizer in the 2F7/2 state, which transfers energy to the co-activator or Er3+ through the 4F7/2 and 4I11/2 states [35,38,39]. Simultaneously, the Er3+ emits UV-Vis–NIR photons of less than 980 nm. Hence, the upconversion phosphor converts low-energy NIR photons at 980 nm to high-energy photons (Figure 5). Peak wavelengths were observed at 384, 407, 520, 527, 540, 548, 652, 658, and 836 nm. Thus, NaYF4:Yb,Er upconversion phosphor emits UV-Vis–NIR photons.
Photocatalytic activity proceeds under UV-Vis–NIR irradiation. The reactions are related to light sensitization centers [40]. For instance, under UV light activation, the TiO2-NaYF4:Yb,Er composite absorbs light and facilitates photocatalysis. It is noteworthy that the NaYF4 phosphor host also sensitizes UV light photons (Figure 4), which enhances the overall light absorption by the TiO2-NaYF4:Yb,Er composite. Under visible light, N-TiO2-NaYF4:Yb,Er or CNS-TiO2/NaYF4:Yb,Er light is sensitized through the lower energy levels in the N or CNS, which inject electrons into the TiO2 conduction band for photoreaction at the nanorod surface. Under NIR irradiation, the light is sensitized through the Yb3+ and energy is transferred through Er3+. Then, the emitted visible light is absorbed through low-energy impurities in N or CNS elements and through the low energy levels due to heterojunctions between TiO2 and NaYF4:Yb,Er phosphor. Consequently, after sensitization of UV-Vis–NIR light, electrons are injected into the TiO2 conduction band, leaving holes in the valence band. At the surface of TiO2, electrons are adsorbed by O2 molecules while holes are adsorbed by H2O molecules to form superoxide and hydroxyl radicals, respectively. After several intermediate reactions, superoxide or hydroxyl radicals attack and photodegrade the MB or toluene pollutants. The CNS-doped TiO2/NaYF4:Yb,Er outperformed the N-doped TiO2-NaYF4:Yb,Er and TiO2-NaYF4:Yb,Er in UV-Vis–NIR photocatalytic activities in methylene blue due to the existence of low-energy CNS doping elements with improved light absorption properties [20].
This work mainly focused on adding 2.5% urea or thiourea to TiO2 to study the effects of doping with N or CNS. Since the photoluminescence spectra exhibited interesting visible and NIR light tuning, further investigations into the effects of varying urea or thiourea (N or CNS, respectively) doping of TiO2/NaYF4:Yb,Er is recommended. Specifically, the photoluminescence emissions at 540 and 548 nm in CNS-doped TiO2/NaYF4:Yb,Er were enhanced (Figure 5b) compared to the TiO2-NaYF4:Yb,Er and N-TiO2-NaYF4:Yb,Er. This insinuates the possibility that CNS doping improves the emission of visible light. Additionally, the photoluminescence under NIR is limited due to the narrow absorption of Yb3+ ions to the 980 nm wavelengths [39,40]. It is also recommended to evaluate the effects of adding 2 co-sensitizers to improve the absorption of NIR light to achieve significant photocatalytic efficiency.

4. Materials and Methods

4.1. Experimental

The NaYF4:Yb,Er upconversion phosphor was synthesized from a stoichiometric amount of Y:Yb:Er at 77:20:3 mol% from yttrium, ytterbium, and erbium oxides (99.9%, Sigma Aldrich, St. Louis, Missouri, USA) by mixing in a nitric acid solution. The mixture solution was constantly stirred at 120 °C for 30 min to form a transparent sol. In a separate beaker, urea, ammonium hydrogen fluoride, and sodium silicon fluoride (Duksan Pure Chemicals Co., Ansan-si, Kyunggi-do, Korea) were dissolved in distilled H2O by magnetic stirring at 80 °C for 2 h. Then, the multi-component NaYF4:Yb,Er solution was poured into a closed crucible for combustion at 650 °C for 5 min in a box furnace (SK1700-B30, Thermotechno Co., Siheung-si, Gyeonggi-do, Korea).
A flow chart of the TiO2 sol preparation and coating process on NaYF4:Yb,Er is shown in Figure 13. The TiO2 sols were prepared from 10 mL titanium(IV)-butylate (99%, Acros Organics) with anhydrous ethyl alcohol (99.9%, Duksan Pure Chemicals Co., Ansan-si, Kyunggi-do, Korea) and distilled H2O at a volumetric ratio of 5:1. After a 10 min mixing procedure, the 2.5 mol% urea or 2.5 mol% thiourea doping reagents (Daejung Chemicals Siheung-si, Gyeonggi-do, Korea) for N or CNS were added to separate beakers with TiO2 sol. The N- or CNS-doped TiO2 sols were further magnetically stirred at 50 °C for 2 h. For comparison, undoped TiO2 sol was also prepared using similar conditions. The pH for the as-synthesized coating solutions was measured using a Hanna Instruments portable digital meter (HI 8424, Rhode Island, USA). The pH readings for the TiO2 sol, N-doped TiO2 sol, and CNS-doped TiO2 sol were 6.39, 5.79, and 5.73, respectively. Thus, the pH values dropped with urea and thiourea reagents.
The sol–gel coating process was proceeded by dip coating 2 g NaYF4:Yb,Er upconversion phosphor powder samples in 5 mL of TiO2 sol, N-TiO2 sol, or CNS-TiO2 sol. The dispersed NaYF4:Yb,Er phosphor was treated in an ultrasonic bath for 5 min followed by removal of excess ethanol solution through Advantec filter paper (Toyo Roshi Kaisha, Ltd., Tokyo, Japan). Then, the coated phosphors were placed on a quartz petri dish for drying at 100 °C for 10 h. Finally, the undoped TiO2, N-doped TiO2, and CNS-doped-TiO2 coated NaYF4:Yb,Er samples were placed in alumina crucibles with lids for a 2 h calcination process at 450 °C. Only N-doped TiO2 and CNS-doped TiO2 were also calcined using a similar process for the purpose of acting as photocatalytic test controls.

4.2. Characterizations

Crystallinity and morphology were evaluated by X-ray diffractometry (Bruker AXS8 Advanced, D8 Discover, Bruker AXS GmbH, Karlsruhe, Germany) and scanning electron microscopy (SEM, Hitachi S-4300, Hitachi Ltd., Tokyo, Japan). The organic and inorganic molecular bonds were examined by Fourier transform infrared (FTIR; Bruker Vertex 70, Karlsruhe, Germany) spectroscopy at a 50:1 wt. % ratio of KBr/phosphor. UV-Vis diffuse reflectance spectra (DRS) for the synthesized composites were evaluated by UV-Vis–NIR spectroscopy (UV-3150 Shimadzu, Kyoto, Japan) in fast scan speed mode through a 30 nm slit. Photoluminescence (PL) characteristics were examined using a fluorescence spectrophotometer (Hitachi F-4500, Tokyo, Japan). The PL powder samples were excited with a 40 mW, 980 nm infrared dot laser (ILaser Lab Co. Ltd., Seong Dong-gu, Seoul, Korea), and the emitted photons were detected through a 10 nm slit.
Photocatalytic activity was evaluated under three conditions of light illumination. First, UV light from 254 nm photons (G6T5 Sankyo Denki 2-fluorescent lamps, Sankyo Denki Co, Kanagawa, Japan) in a protective chamber was illuminated over a mixture of 100 mg catalyst and 100 mL MB 5 ppm solution. Photodegraded 2 mL samples were collected and labeled with respective light irradiation times throughout the 3 light illumination conditions. Second, visible light (10,000 lux, 300 W Xe Solar Simulators Luzchem SolSim2) was irradiated over a 410 nm cut-off filter to the 100 mg catalyst in 50 mL MB 5 ppm solution. Third, NIR photons (15,000 lux, Luzchem SolSim2, Ottawa, Ontario, Canada) were illuminated on a photoreaction of 100 mg catalyst–50 mL MB 2 ppm solution with 410 and 760 nm cut-off filters. The absorbance variation of UV-Vis–NIR photodegraded MB solutions was analyzed in quartz cell holders of a UV-Vis–NIR spectroscope (UV-3150 Shimadzu, Kyoto, Japan). Photocatalytic activity control experiments were also conducted on N-doped TiO2 only and CNS-doped TiO2 using the same UV and visible light conditions.
Toluene photodegradation was evaluated on 6 g TiO2-NaYF4:Yb,Er and CNS/TiO2-NaYF4:Yb,Er powders. The photocatalytic powders were sparsely dispersed on a borosilicate Petri dish in a Teflon sampling bag. The light illumination source was an 8 W Philips TUV G6T5. At 1 h intervals, 1 mL toluene vapor was withdrawn from the sampling bag and injected into an Agilent Technologies gas chromatography (GC) system (7890 A, Agilent Technologies Inc., California, USA) to determine the extent of degradation in relation to the UV light illumination time.

5. Conclusions

Upconversion phosphor was coupled with undoped TiO2 and N- and CNS-doped TiO2 using a sol–gel coating method. The TiO2 nanocrystalline morphologies on NaYF4:Yb,Er upconversion phosphor changed from spherical to nanorods, most probably due to the use of urea and thiourea doping reagents. N doping and CNS doping in TiO2 were successful, as confirmed by EDS and FTIR through broad molecular bonds of SO42−, carboxylate, and NO3−. The UV-Vis DRS of the photocatalyst powder samples exhibited a slight red-shift with N doping and CNS doping. The MB photocatalytic degradation efficiencies under UV-Vis–NIR illumination sources are enhanced with doping with N or CNS elements. Specifically, N-TiO2 and CNS-TiO2 photocatalysts exhibited lower UV-Vis photocatalytic efficiencies as compared to the N-TiO2 and CNS-TiO2 supported on NaYF4:Yb,Er phosphor. Moreover, toluene degradation efficiencies were improved by CNS doping on TiO2/NaYF4:Yb,Er. The CNS-doped-TiO2/NaYF4:Yb,Er photocatalyst is a plausible candidate for pollutant remediation.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/10/1188/s1, Figure S1: Elemental mapping for NaYF4:Yb,Er, Figure S2: Elemental spectra for NaYF4:Yb,Er, Figure S3: Elemental mapping for TiO2/NaYF4:Yb,Er, Figure S4: Elemental spectra for TiO2/NaYF4:Yb,Er, Figure S5: Elemental mapping for N-TiO2/NaYF4:Yb,Er, Figure S6: Elemental spectra for N-TiO2/NaYF4:Yb,Er, Figure S7: Elemental mapping for CNS-TiO2/NaYF4:Yb,Er, Figure S8: Elemental spectra for CNS-TiO2/NaYF4:Yb,Er, Figure S9: UV-Vis absorption spectra enlarged along the absorbance axis from Figure 4, Figure S10: Decay curve for NaYF4:Yb,Er coated with TiO2, N-TiO2 and CNS-TiO2 monitored at 540 nm under 980 nm excitation.

Author Contributions

Supervision, J.-S.K.; methodology and experimentation, S.-R.E.; writing—original draft preparation, S.M.; writing—review and editing, B.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Land, Infrastructure, and Transport of Korea, grant number 20CTAP-C157721–01, under the Infrastructure and Transportation Technology Promotion Research Program.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD spectra of (a) NaYF4:Yb,Er phosphor and NaYF4:Yb,Er coated with (b) TiO2, (c) N-TiO2, and (d) CNS-TiO2.
Figure 1. XRD spectra of (a) NaYF4:Yb,Er phosphor and NaYF4:Yb,Er coated with (b) TiO2, (c) N-TiO2, and (d) CNS-TiO2.
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Figure 2. SEM images of (a) NaYF4:Yb,Er, (b) TiO2 particles, and NaYF4:Yb,Er phosphors coated with (c) TiO2, (d) N-TiO2, and (e) CNS-TiO2.
Figure 2. SEM images of (a) NaYF4:Yb,Er, (b) TiO2 particles, and NaYF4:Yb,Er phosphors coated with (c) TiO2, (d) N-TiO2, and (e) CNS-TiO2.
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Figure 3. Fourier transform infrared spectra of NaYF4:Yb,Er coated with (a) N-TiO2 and (b) CNS-TiO2.
Figure 3. Fourier transform infrared spectra of NaYF4:Yb,Er coated with (a) N-TiO2 and (b) CNS-TiO2.
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Figure 4. UV-Vis absorption spectra of NaYF4:Yb,Er coated with TiO2, N-TiO2, and CNS-TiO2.
Figure 4. UV-Vis absorption spectra of NaYF4:Yb,Er coated with TiO2, N-TiO2, and CNS-TiO2.
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Figure 5. Photoluminescence spectra of NaYF4:Yb,Er coated with TiO2, N-TiO2 and CNS-TiO2. (a) UV-Vis–NIR emissions and the enlarged spectra for (b) UV light, (c) visible light, and (d) NIR light emissions.
Figure 5. Photoluminescence spectra of NaYF4:Yb,Er coated with TiO2, N-TiO2 and CNS-TiO2. (a) UV-Vis–NIR emissions and the enlarged spectra for (b) UV light, (c) visible light, and (d) NIR light emissions.
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Figure 6. Methylene blue (MB) absorbance variation for NaYF4:Yb,Er phosphor coated with (a) TiO2, (b) N-TiO2, and (c) CNS-TiO2 under UV light activation.
Figure 6. Methylene blue (MB) absorbance variation for NaYF4:Yb,Er phosphor coated with (a) TiO2, (b) N-TiO2, and (c) CNS-TiO2 under UV light activation.
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Figure 7. (a) MB peak absorbance variation as degraded with 254 nm UV illumination. Here, UP is the NaYF4:Yb,Er upconversion phosphor and (b) correlated rate of kinetics.
Figure 7. (a) MB peak absorbance variation as degraded with 254 nm UV illumination. Here, UP is the NaYF4:Yb,Er upconversion phosphor and (b) correlated rate of kinetics.
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Figure 8. (a) MB peak absorbance against the visible light activation time and (b) correlated rate of kinetics.
Figure 8. (a) MB peak absorbance against the visible light activation time and (b) correlated rate of kinetics.
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Figure 9. (a) MB peak absorbance against the NIR light activation time and (b) correlated rate of kinetics.
Figure 9. (a) MB peak absorbance against the NIR light activation time and (b) correlated rate of kinetics.
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Figure 10. Photodegradation for toluene with CNS-TiO2/NaYF4:Yb,Er and TiO2/NaYF4:Yb,Er composites under UV light illumination.
Figure 10. Photodegradation for toluene with CNS-TiO2/NaYF4:Yb,Er and TiO2/NaYF4:Yb,Er composites under UV light illumination.
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Figure 11. Schematic diagram for synthesis and photocatalysis tests.
Figure 11. Schematic diagram for synthesis and photocatalysis tests.
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Figure 12. Proposed photocatalysis mechanism for the NaYF4:Yb,Er phosphor coated with TiO2 under different doping conditions.
Figure 12. Proposed photocatalysis mechanism for the NaYF4:Yb,Er phosphor coated with TiO2 under different doping conditions.
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Figure 13. Flow chart of TiO2 sol preparation and its coating process on NaYF4:Yb,Er.
Figure 13. Flow chart of TiO2 sol preparation and its coating process on NaYF4:Yb,Er.
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Table
Table 1. Elemental analysis for NaYF4:Yb,Er.
Table 1. Elemental analysis for NaYF4:Yb,Er.
ElementMass%Atom%
F29.2260.86
Na10.1917.54
Y35.6315.86
Er5.111.21
Yb19.844.54
Totals100-
Table
Table 2. Elemental analysis for TiO2/NaYF4:Yb,Er.
Table 2. Elemental analysis for TiO2/NaYF4:Yb,Er.
ElementMass%Atom%
O4.158.98
F30.8556.25
Na10.6616.05
Ti4.663.37
Y28.3511.05
Er4.420.92
Yb16.913.38
Totals100-
Table
Table 3. Elemental analysis for N-TiO2/NaYF4:Yb,Er.
Table 3. Elemental analysis for N-TiO2/NaYF4:Yb,Er.
ElementMass%Atom%
N0.070.17
O7.3316.30
F25.6248.01
Na7.5911.75
Ti11.128.27
Y28.4311.38
Er4.270.91
Yb15.573.20
Totals100-
Table
Table 4. Elemental analysis for CNS-TiO2/NaYF4:Yb,Er.
Table 4. Elemental analysis for CNS-TiO2/NaYF4:Yb,Er.
ElementMass%Atom%
C25.9746.30
N0.520.79
O12.7917.12
F19.4121.89
Na5.415.04
S0.230.15
Ti7.943.55
Y14.603.52
Er2.780.36
Yb10.361.28
Totals100-
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Eun, S.-R.; Mavengere, S.; Cho, B.; Kim, J.-S. Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites. Catalysts 2020, 10, 1188. https://doi.org/10.3390/catal10101188

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Eun S-R, Mavengere S, Cho B, Kim J-S. Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites. Catalysts. 2020; 10(10):1188. https://doi.org/10.3390/catal10101188

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Eun, Seong-Rak, Shielah Mavengere, Bumrae Cho, and Jung-Sik Kim. 2020. "Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites" Catalysts 10, no. 10: 1188. https://doi.org/10.3390/catal10101188

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