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Appl. Sci. 2018, 8(8), 1402; doi:10.3390/app8081402

Communication
Conformal Titanyl Phosphate Surface Passivation for Enhancing Photocatalytic Activity
School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Korea
Received: 30 July 2018 / Accepted: 17 August 2018 / Published: 19 August 2018

Abstract

:
A conformal titanyl phosphate passivation with the thickness of ca. 5 nm on the surface of TiO2 nanoparticles for enhancing the photocatalytic degradation of organic pollutants and hydrogen production is described. The phosphate anion species bound to the surface of TiO2 promote the favorable kinetics of photocatalytic activity and influence the catalytic reaction pathway. By using a facile surfactant-assisted sol-gel process, the surface defects of TiO2 associated with deep traps was reduced and passivated by the phosphate anion species to form the titanyl phosphate. The strong bonds between the titanyl phosphate shell and TiO2 core provided a long-term photochemical stability in aqueous electrolytes with enhanced photocatalytic activities. The titanyl phosphate contributed to the production and stabilization of hydroxyl radicals on the surface of photocatalyst, which facilitated the efficient photooxidation of the organic pollutants. Further, enhancing the photocatalytic hydrogen production was achieved by the titanyl phosphate modified TiO2 (TP-TiO2). Consequently, the conformal titanyl phosphate passivation enhanced photocatalytic activity of TiO2. Comparing to the bare TiO2 nanoparticles, approximately two-fold higher photocatalytic H2 production rate was achieved by the TP-TiO2.
Keywords:
titanyl phosphate; passivation of defects; stability; hydrogen production; photodegradation of pollutant

1. Introduction

Titanium dioxide (TiO2) is the most extensively studied metal oxide semiconductor used for diverse solar energy conversions including photovoltaics, photodegradation of organic pollutants, and photocatalytic hydrogen production [1,2,3,4,5,6,7,8,9,10,11]. As for the promising heterogeneous photocatalyst, numerous efforts have been conducted to enhance the photocatalytic activity of TiO2 nanoparticles by modifying their surface properties [12]. Despite numerous advantages of TiO2, the tremendous intrinsic defect states in TiO2 can deteriorate the charge transfer performance of photocatalysis. The deeply localized defects in TiO2 can act as undesired charge trap centers and promote the rapid charge recombination [13,14,15]. These defect states and deep traps induce the crucial charge losses [16,17].
In recent years, numerous efforts have been conducted to enhance the photocatalytic activity of TiO2 nanoparticles by modifying their surface properties with phosphate anion (PO43-) [18,19,20,21,22]. The surface modification using the phosphate species can generate the negative electrostatic field at the interface. This preferred surface electrostatic field facilitates the efficient charge separation and prolongs the photo-induced charge migration [20,22]. In addition, the negative electrostatic field can activate the formation of hydrogen bonds between phosphate anions and water molecules. Thereby, the phosphate species bound on TiO2 can promote the generation of hydroxyl radicals, resulting in the highly efficient photocatalytic degradation of organic pollutants [23]. However, the photochemical conversion efficiency can be weakened since the direct hole oxidation pathway can be hindered by the hydroxyl radicals. The adsorbed phosphate anion on the surface of TiO2 also can increase the resistance of electron transfer at the interface [24,25]. Therefore, a rational design of the phosphate modification is strongly required for efficient TiO2 photocatalysis.
In this study, the TiO2 nanoparticles were conformally modified by the titanyl phosphate passivation layer with a thickness of ca. 5 nm. The use of a facile surfactant-assisted sol-gel process with titanium isopropoxide, phosphoric acid, and polyethylene glycol as a surfactant facilitated the formation of titanyl phosphate because the phosphate anion can strongly bind the surface Ti atom with the oxygen bridge, inducing the Ti-O-P-O bonds [26]. The titanyl phosphate passivated the surface defects in TiO2 and reduced the deep traps associated with Ti3+. Consequently, the titanyl phosphate modified TiO2 (TP-TiO2) shows significantly enhanced photocatalytic activities through the photocatalytic degradation of organic pollutant (rhodamine B) and hydrogen production experiments.

2. Materials and Methods

2.1. Material Synthesis

The facile surfactant assisted sol-gel process was conducted by using titanium(IV) isopropoxide (TTIP, 97%, Aldrich), polyethylene glycol (PEG, Mn 300, Aldrich), phosphoric acid (85 wt%, Aldich) and methyl alcohol (anhydrous, Aldrich). The titanium precursor solution with concentration of 0.35 M was prepared by dropping the TTIP in the methyl alcohol solvent slowly (with the rate of 1 μL/s) during the vigorous stirring over 600 rpm. After fully dissolving, 3.5 M PEG was immediately injected in the precursor. Then, 0.1 M phosphoric acid was mixed with the solution. The thermal annealing process at 750 °C for 2 h was carried out to obtain the TP-TiO2. For the bare TiO2, the same experimental protocol was utilized without the phosphoric acid.

2.2. Material Characterization

The particle morphologies were confirmed by X-ray diffraction (XRD) by using Siemens diffractometer D500/5000 in Bragg-Brentano geometry under Cu Kα radiation and scanning electron microscopy (SEM) by using JSM-7000F. High resolution-transmission electron microscopy (HR-TEM) images and elemental mapping images were obtained from a JEOL (JEM-2100F, Japan) electron microscope with an energy dispersive spectrometer (EDS). XPS characterization was performed with ESCA2000 from VG Microtech.

2.3. Photocatalytic Activity

Photocatalytic degradation of organic pollutant was demonstrated by using 0.01 M rhodamine B (Rh. B) aqueous solution. After achieving adsorption/desorption equilibrium (by keeping the 0.05% TiO2 solution for 2 h in dark), 1 sun (AM 1.5G 100 mW/cm2) irradiation was employed. During the light irradiation and vigorous stirring with 600 rpm, we extracted 1 mL of sample solution at each interval. After centrifuging the solutions, the concentration changes of supernatant were characterized by using a UV-VIS-NIR Spectrophotometer (UV-3600, SHIMADZU).
Photocatalytic hydrogen production was performed in a 100 mL Quartz reactor, of which openings were sealed with the silicon rubber septum. 100 mg of catalyst was dispersed in 50 mL of 1 M KHCO3 solution. During the 1 sun (AM 1.5G 100 mW/cm2) irradiation, the suspension solution was stirred continuously. To analyze the H2 production, we extracted 1 mL of gas from the reactor by using a glass syringe and injected the collected gas in a gas chromatography (7890A GC system, Agilent Technologies, Santa Clara, USA). Here, Ar gas was exploited as a carrier gas. The extraction and injection of generated gas were repeated at intervals.
In this study, a solar simulator (PEC-L01, PECCELL, Yokohama, Japan) was utilized as the light source. AM 1.5G filter was exploited for the 1 sun illumination with the full spectrum (350−1100 nm). The light intensity was calibrated by using a silicon reference cell (Fraunhofer ISE, Certificate No. CISE269).

2.4. Photoelectrochemical Measurement

The current density to potential (J-V) curves were obtained via the linear sweep voltammetry (LSV) mode using the three electrodes system with the standard calomel electrode as the reference electrode, Pt foil as the counter electrode and the working electrode. The measurement was carried out using a potentiostat (CHI 705E, CH Instruments, Austin, TX, USA). 1 M KHCO3 aqueous solution was utilized as the electrolyte. The solar simulator (PEC-L01, PECCELL) was also utilized as the light source. The working electrode was prepared by the following manner [27]: 3 wt% of solution was prepared by dispersing the photocatalyst powders in 1 mL of ethanol solvent. After adding 0.5 mL of ethyl cellulose solution (10 wt% aq.), the vigorous stirring was conducted for 8 h at 80 °C. Then, 0.16 mL of α-terpineol was added. To make a paste, the stirring process was continued for 16 h more. The doctor blade method was used to coat the paste uniformly on the pre-cleaned FTO glass. Then, the annealing process was conducted at 500 °C for 30 min.

3. Results and Discussion

3.1. Morphology Characterization

The importance of crystallinity in the metal oxide photocatalysts has been widely studied. The photochemical energy conversion efficiency can be determined by their crystallinity [28,29]. In the XRD patterns (Figure 1a), the crystalline TiO2 in anatase/rutile mixed crystalline phase (JCPDS# 21-1272 for anatase, JCPDS# 21-1276 for rutile) was observed for bare TiO2 nanoparticles. Considering the phase transition behavior of TiO2, [30] the annealing process at 750 °C resulted in the anatase/rutile mixed crystalline phase. However, the rutile phase peaks were diminished in TP-TiO2 while the anatase phase and strong titanyl phosphate (JCPDS# 35-802) peaks were observed, even the same annealing process was conducted at 750 °C for TP-TiO2. The suppressed phase transformation between anatase and rutile is attributed to the phosphate anions, of which good affinity to Ti atoms hindered the phase transformation of TiO2 [31]. Even the phosphate anions induced these significant changes in crystallinity, no significant changes in morphology were not characterized in SEM images (Figure 1b and c). However, a slight increase in the size of overall particles was observed in TP-TiO2. The estimated particle size was avg. 51 nm (± 27) for bare TiO2 and avg. 58 nm (± 18) for TP-TiO2. This presumably resulted from the phosphate anions. HR-TEM images of TP-TiO2 (Figure 1d and e) demonstrate that the surface of TP-TiO2 was conformally passivated by the titanyl phosphate layer of which thickness was ca. 5 nm. The fringe pattern in Figure 1e and the reduced fast Fourier transform (FFT) pattern in the insert image indicate that the core TiO2 is composed of the anatase phase. The anatase (101) was clearly defined by the distinct lattice space of 0.35 nm. For further determination of this TiO2-core @ titanyl phosphate-shell structure, the elemental mapping of Ti, O, and P atoms was performed using TEM-EDS (Figure 1f). As shown in the high angle annular dark field scanning (HAADF) TEM image, the mapping was focused on the surface passivation layer. This elemental mapping image shows that all elements (Ti, O and P atoms) are well dispersed in the passivation layer, which implies that the surface of TP-TiO2 is conformally passivated by titanyl phosphate.

3.2. Chemical Modification

For the sake of understanding the chemical modification, XPS characterization was performed as shown in Figure 2. The C 1s XPS spectra of bare and TP-TiO2 were utilized to reference the measured XPS results, where the binding energy of major C-C peaks are 284.8 eV (Supplementary Figure S1). The positive shift of Ti 2p1/2 and Ti 2p3/2 peaks of TP-TiO2 were observed in the Ti 2p XPS spectra (Figure 2a). The shift in Ti 2p can be attributed to the ameliorated surface states [32,33]. This positive shift of Ti 2p demonstrates that the Ti-O-P-O bonds were successfully formed on the surface of the TiO2 core particle, and resulted in the reduction of trap states originated from Ti3+ defects. The O 1p XPS spectra are displayed in Figure 2b. Two dominant peaks of bare TiO2 at 530.1 and 531.6 eV are assigned to the lattice oxygen (OL) and surface oxygen (OS), respectively. Here, the lattice oxygen indicates Ti-O-Ti chain. However, the TP-TiO2 exhibited four peaks at 530.1, 531.1, 531.6, and 532.9 eV. The newly arising peak at 531.1 eV, corresponding to Ti-O-P bond, demonstrates the formation of titanyl phosphate on the surface of TiO2 [34]. Moreover, the sharper OS peak centered at 531.6 eV is also attributed to the OH species or O vacancies that originated from the titanyl phosphate groups [35]. Considering the disordered structure of surface titanyl phosphate layer in the TEM image (Figure 2e), the phosphate anion groups can induce -OH groups or O vacancies on TP-TiO2. In addition, the shoulder peak at 532.9 eV, can be attributed to chemisorbed oxygen species (OC) such as CO2 or H2O [36]. During the sol-gel process, the structure determining agent (i.e. PEG) can be bounded to the phosphate anion species via hydrogen bonding. Therefore, the carbon or oxygen related species (including CO2 or carboxylic groups) can be remained on the surface of TP-TiO2 after performing the annealing process. As shown in C 1s XPS (Figure S1), the slight increase in C-O (ca. 286.5 eV) and C=O (ca. 288.5 eV) peaks were observed in TP-TiO2, which implies that the oxygen species can be adsorbed onto TP-TiO2 [7]. P 2p XPS is shown in Figure 2c, where TP-TiO2 has two sharp peaks at 133.0 and 133.9 eV corresponding to P 2p3/2 and P 2p1/2, respectively. These peaks can be assigned to the monodentate coordinated phosphate anion (PO43-), when it comes to the binding energies of Na2HPO4 (133.1 eV) and NaH2PO4 (133.9 eV) [23]. Additional T-P bond was not observed in the XPS spectrum.

3.3. Photocatalytic Performance

The photocatalytic activity was investigated via the photodegradation experiment using rhodamine B aqueous solution. In Figure 3a, TP-TiO2 shows outstanding photocatalytic activity comparing to bare TiO2. In order to calculate their rate constants, natural logarithmic plots were prepared as shown in Figure 3b. By using the first order reaction rate equation (C/C0=exp(- kt)), the rate constants (k) are calculated [27]. TP-TiO2 shows approximately 3-folds higher k value than bare TiO2. Furthermore, Figure 3c shows the performances of photocatalytic hydrogen production. Since the surface phosphate groups can promote the formation of hydroxyl radicals in water, 1 M KHCO3 aqueous solution was exploited as the electrolyte to stabilize the H2O2 production and induce the 2-electron pathway for water splitting [23,37,38,39,40]. Significantly higher photocatalytic hydrogen production rate (86.7 μmol h−1g−1 for TP-TiO2 and 46.1 μmol h−1g−1 for bare TiO2) was achieved. Moreover, TP-TiO2 also shows much stable photocatalytic activity than bare TiO2 due to the conformal passivation of titanyl phosphate. After continuous operation for 24 h, negligible changes in the production rate was observed in TP-TiO2, whereas visible changes were shown by bare TiO2. TEM image of TP-TiO2 after 24 h of operation (Supplementary Figure S2) shows the long-term stability of titanyl phosphate layer. The surface modification using the phosphate species can generate the negative electrostatic field, which facilitates the efficient charge migration. The negative electrostatic field also can activate the formation of hydrogen bond between phosphate anion and water molecule. This promotes the adsorption of water molecules at the surface of photocatalysts and the generation of hydroxyl radical, resulting in high efficient photocatalytic properties [20,22,23]. Moreover, the charge recombination was suppressed since the charge trap states of TiO2 were ameliorated by the surface titanyl phosphate passivation layer. It is noteworthy that no significant changes in the UV-Vis absorbance spectra were observed (Supplementary Figure S3). This implies that the surface passivation layer with the disordered structure does not influence the light harvesting property of photocatalyst [41]. To investigate the efficient charge migration properties, the photoelectrochemical (PEC) response was characterized using the three electrodes system, of which working electrode was comprised of the TiO2 photocatalysts (Supplementary Figure S4). 1 M KHCO3 aqueous solution (pH 7.4) was also used as the electrolyte. The significantly enhanced photocurrent density value of TP-TiO2 was resulted from not only the modified surface catalytic kinetics, but also the suppressed charge recombination and promoted charge migration [41]. Additionally, the external quantum efficiency (EQE) spectra were investigated to demonstrate the enhanced performance of TP-TiO2 in Supplementary Figure S4b.

4. Conclusions

The TiO2 photocatalysts were conformally modified by the titanyl phosphate passivation layer with a thickness of ca. 5 nm. The use of a facile surfactant-assisted sol-gel process facilitated the formation of titanyl phosphate on the surface of TiO2 nanoparticles. The phosphate anions are strongly bound onto the surface Ti atoms with oxygen bridges, inducing the Ti-O-P-O bonds. Thereby, the surface titanyl phosphate bonds resulted in the reduction of trap states originated from Ti3+ defects. These strong bonds enable the long-term photochemical performance in liquid electrolytes with enhanced photocatalytic activities. Moreover, the titanyl phosphate contributed to the production and stabilization of hydroxyl radicals on the surface of TiO2 nanoparticles, which facilitated the efficient photooxidation of the organic pollutants. Furthermore, the significantly enhanced photocatalytic hydrogen production performance was observed by the titanyl phosphate modified TiO2. Consequently, high photocatalytic activity was achieved by introducing the titanyl phosphate surface passivation layer. The demonstrated titanyl phosphate modification enables the conformal passivation of TiO2 photocatalysts with tunable properties, which is conducive to diverse photochemical conversion applications.

Supplementary Materials

The following are available online at http://www.mdpi.com/2076-3417/8/8/1402/s1, Figure S1: C 1s XPS spectra of bare and TP-TiO2, Figure S2: TEM image of TP-TiO2 after carrying out the photocatalytic H2 production experiment for 24 hours, Figure S3: UV-Vis absorbance spectra and Tauc plot (insert) of bare and TP-TiO2, Figure S4: Current density to potential (J-V) curves, EQE and IQE spectra of bare and TP-TiO2 under on/off chopped 1 sun irradiation.

Acknowledgments

The authors acknowledge the support of the NRF of Korea (NRF-2018R1D1A1B07050875).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) XRD pattern of bare and TP-TiO2. SEM images of bare (a) and TP-TiO2 (b). (d) and (e) HR-TEM images of TP-TiO2. The inserted image is a reduced FFT pattern. (f) HAADF TEM image and EDS elemental mapping of Ti, O and P atoms for TP-TiO2.
Figure 1. (a) XRD pattern of bare and TP-TiO2. SEM images of bare (a) and TP-TiO2 (b). (d) and (e) HR-TEM images of TP-TiO2. The inserted image is a reduced FFT pattern. (f) HAADF TEM image and EDS elemental mapping of Ti, O and P atoms for TP-TiO2.
Applsci 08 01402 g001
Figure 2. (a) Ti 2p; (b) O 1s; and (c) P 2p XPS spectra of bare and TP-TiO2.
Figure 2. (a) Ti 2p; (b) O 1s; and (c) P 2p XPS spectra of bare and TP-TiO2.
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Figure 3. (a) Concentration changes in rhodamine B aqueous solution by photodegradation; (b) Natural logarithmic plots for the study of the kinetics of rhodamine B photodegradation profiles; (c) Photocatalytic hydrogen production and cycling performance during 24 h of continuous reaction in the presence of 1 M KHCO3.
Figure 3. (a) Concentration changes in rhodamine B aqueous solution by photodegradation; (b) Natural logarithmic plots for the study of the kinetics of rhodamine B photodegradation profiles; (c) Photocatalytic hydrogen production and cycling performance during 24 h of continuous reaction in the presence of 1 M KHCO3.
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