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Cite This: J. Phys. Chem. Lett. 2019, 10, 3024−3028
Surface Bridge Hydroxyl-Mediated Promotion of Reactive Oxygen Species in Different Particle Size TiO2 Suspensions Wanchao Yu,†,‡ Lixia Zhao,*,† Fengjie Chen,†,‡ Hui Zhang,† and Liang-Hong Guo†,‡ †
State Key Laboratory of Environmental Chemistry and Eco-toxicology, Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, P.O. Box 2871, Beijing 100085, China ‡ University of Chinese Academy of Sciences, Beijing 100039, China
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S Supporting Information *
ABSTRACT: Reactive oxygen species (ROS) play an essential role in TiO2 photocatalysis. They arise from the transfer of lightinitiated carriers to the TiO2 surface and react with oxygen or water, in which the TiO2 surface is crucial. However, how the TiO2 surface affects ROS production is unclear. Herein, dynamic generation of ROS in suspensions of TiO2 of different particle sizes was investigated under ultraviolet-light irradiation. It is surprising to find that more ROS were produced more quickly for 100−140 nm TiO2 than for 20−60 nm TiO2. Further research suggested that ROS production was intrinsically correlated with the surface bridging hydroxyls per unit area. More bridging hydroxyls induced lower IEP and more negative charges on the TiO2 surface, which favored the transfer of photogenerated carriers, resulting in the promotion of ROS and photocatalytic activity. This provided insight into designing high-efficiency photocatalysts to solve the problem of small particle sizes causing loss and blockage in wastewater treatment.
R
However, Nosaka et al. reported that there was no obvious correlation between O2•− production and the primary particle size of TiO2.10 The inconsistent results were probably due to batch methods for ROS detection at discrete time intervals, which may cause some errors, and not directly reflect the dynamic formation. More importantly, the particle size can influence the TiO2 surface, but how do the surface properties of TiO2, which are usually modulated by the surrounding environment, affect the transfer of light-initiated charge carriers and thus the ROS production and photocatalytic activity? What are the key factors? These questions remain unanswered. Herein, dynamic generation of ROS including ·OH and O2•− in six different particle sizes of TiO2 suspensions was conducted, and the results were compared using a continuous flow chemiluminescence (CFCL) system that we built ourselves (Figure S1).11 It is surprising to find that more ROS were produced more quickly for 100−140 nm than for 20−60 nm TiO2. The ROS generation was intrinsically related with the bridging hydroxyls on the TiO2 surface. More bridging hydroxyls induced the lower shift of IEP and more negative charges on the TiO2 surface, which favored the separation of photogenerated carriers, resulting in the promotion of ROS and photocatalytic activity. This may
eactive oxygen species (ROS) is a general term for oxygen free radicals typically including hydroxyl radicals (·OH), superoxide ions (O2•−), singlet oxygen (1O2), and hydrogen peroxide (H2O2), which are often generated in many chemical and biological processes.1 Because ROS possess high reactivity, they have been used to decompose harmful substances.2 In the field of photocatalysis, such as TiO2, the main mechanism of photodegradation involves light-initiated generation of ROS and their subsequent redox reactions with chemicals.3 The ROS production mostly arises from the transfer of lightinitiated charge carriers to the surface of TiO2 which then react with oxygen or water, in which TiO2 surface properties are very critical. The surface of TiO2 is usually affected by the physicochemical properties, such as crystal phase, crystal face, particle size, etc.,4−6 which influence the ROS formation and photocatalytic activity. Taking particle size as an example, it is generally assumed that the smaller the particle size, the larger the surface area, the higher the photocatalytic efficiency and ROS formation,7,8 especially when the size is less than 40 nm. However, Almquist and Biswas9 recently found that photocatalytic activity of 169 nm anatase TiO2 was close to that of 25−40 nm anatase TiO2. This result was very interesting and probably was related with the ROS formation. Then Jiang et al.6 and Nosaka et al.10 focused on the effect of particle size on the ROS formation. Jiang et al. did find that with the increase of particle size, the ROS generation increased within 4−30 nm and then reached a plateau until 195 nm.6 © 2019 American Chemical Society
Received: March 26, 2019 Accepted: May 20, 2019 Published: May 22, 2019 3024
DOI: 10.1021/acs.jpclett.9b00863 J. Phys. Chem. Lett. 2019, 10, 3024−3028
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The Journal of Physical Chemistry Letters
Figure 1. CFCL methods for the detection of ROS generated by TiO2 with different particle sizes: (a) detection of O2•− by luminol CL (TiO2, 0.1 mg/mL; luminol, 50 μM; flow rate, 2.2 mL/min) and (b) detection of ·OH by Phth/H2O2/K5Cu(HIO6)2 CL (TiO2, 5.0 μg/mL; Phth, 20 μM; H2O2, 50 μM; K5Cu(HIO6)2, 0.1 mM; flow rate, 2.2 mL/min).
addition, the degradation efficiency of TiO2 in the range of 20−140 nm for Rhodamine B and Methyl Orange was investigated. We can find that the degradation efficiency of 100−140 nm TiO2 was better than that of 20−60 nm TiO2, which was consistent with the formation of ROS (Figure S9). To explore the inherent effect of particle sizes on ROS formation and photocatalytic activity, the physicochemical properties of the used TiO2 suspensions were characterized (Table S1). It was found that the zeta potential of two groups of TiO2 were obviously different. The zeta potential of 20−60 nm TiO2 was positive, while that of 100−140 nm TiO2 was negative. This meant that the surface of 100−140 nm TiO2 has a negative charge and its surface is covered by anionic groups such as Ti−O−. Tsujiko et al.19 reported that the TiO2 covered with Ti−O− could facilitate electron transfer to adsorbed oxygen molecules. To be specific, in contrast to the weak physical adsorption of O2 on the surface of positively charged TiO2, the Ti−O− on the surface of negatively charged TiO2 served as an electron donor, while the O2 molecule adsorbed on the surface acted as an electron acceptor which enhanced the chemisorption of O2 on the surface of TiO2. As a result, the electron transfer from conduction band to O2 was also increased, resulting in the promotion of O2•−. Some researchers20−22 found that the surface negative charges can more likely adsorb the H3O+ by electrostatic force which can draw the photogenerated holes to the TiO2 surface and accelerated the formation of ·OH. Therefore, the negative charges on the surface of TiO2 can promote the separation of electrons and holes and can force the formation of ROS.23 The surface charge of TiO2 particles was a function of solution pH, which was affected by the reactions that occur on the particle surface as shown in eqs 1 and 2.
provide insight into designing high-efficiency TiO2 photocatalysts to solve the problem of small particle size causing loss and blockage in wastewater treatment applications. Six different particle sizes of TiO2 were characterized. The X-ray diffraction patterns of all TiO2 samples showed peaks at about 25.5°, 37.9°, 48.2°, 53.8°, and 55.0°, representing the indices of (101), (004), (200), (105), and (211) planes of anatase phase (Figure S2).12 The Raman spectrum exhibited two Brillouin zones located at 396, 517, and 636 cm−1, which agreed with the Raman peaks of anatase TiO2 (Figure S3).13 Transmission electron microscopy images (Figure S4) showed all TiO2 particles were near-spherical, and the primary particle size, (Figure S5) were about 20 nm (ST01), 40 nm (ST02), 60 nm (ST03), 100 nm (ST04), 120 nm (ST05), and 140 nm (ST06). The purities of samples were about 99%, and other similar trace composition has no effect on ROS generation (Figure S6). The ultraviolet−visible (UV−vis) adsorption spectra (Figure S7) indicated the band gap values slightly increased from 3.14 to 3.20 eV as the particle sizes increased (Table S1). This was different from the quantization size effects of TiO2 nanoclusters, in which the band gap was smaller with the increase of the size, which was probably due to the larger particle sizes, but it was in agreement with a previous report.14 These results confirmed that the used TiO2 particles were in a large range of different sizes but had the same crystal structure, composition, and shape. Then the dynamic formation of two kinds of ROS (O2•− and ·OH) on these TiO2 suspensions were detected using the CFCL method under ultraviolet light irradiation. The CL intensity induced by ROS was measured at a given CL probe to examine the dynamic generation of O2•− and ·OH. For O2•−, luminol was used as a CL emitter. For ·OH, Phth as CL probe to specifically capture ·OH was converted to 5-OHPhth. The latter emitted strong CL when mixed with H2O2 and K5Cu(HIO6)2.11 The concentration of O2•− and ·OH were quantified according to the CL intensity.15−18 Figure 1 shows CL intensity increased because of a substantial amount of ROS formed with the irradiation time. However, the ROS generation on 100−140 nm TiO 2 suspensions was unexpectedly more than that of 20−60 nm, although it increased with the decrease of particle size in the range of both 20−60 nm and 100−140 nm except for 100 nm. To exclude the contingency of the material, the ROS generation values of TiO2 obtained from different companies were determined (Figure S8). More ·OH was still generated for 120 nm TiO2 than for 20 nm, which suggested that the result was not an accidental individual phenomenon. In
TiIV−OH + H+ → TiIV−OH 2+
(1)
TiIV−OH → TiIV−O− + H+
(2)
The pH of the TiO2 suspension being less than the isoelectric point (IEP) resulted in the surface of the TiO2 having a positive surface charge and positive zeta potential. When pH was greater than IEP, the surface has a net negative charge and negative zeta potential.24−26 Therefore, the surface charge of TiO2 particles was usually determined by the IEP and pH, both of which can be affected by particle size.25 With the increase of particle size, TiO2 surface area decreased (Table S1), whereas the surface area affected the solution pH in the dispersion. As the particle surface area decreased, solution pH slightly increased from 5.4 to 5.9 (Figure 2a). This was probably 3025
DOI: 10.1021/acs.jpclett.9b00863 J. Phys. Chem. Lett. 2019, 10, 3024−3028
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The Journal of Physical Chemistry Letters
Figure 2. (a) pH of anatase TiO2 suspensions with different particle sizes. (b) IEP of anatase TiO2 suspensions with different particle sizes. The concentration of TiO2 was 0.1 mg/mL.
Figure 3. (a) FTIR spectra of TiO2 with different particle sizes (TiO2: 0.1 mg/mL). (b) The relationship between I(HObr) − I(HOt) and the particle size of TiO2. (c) The relationship between HObr and O2•− under unit specific surface area. (d) The relationship between HObr and ·OH under unit specific surface area.
TiO2, while larger than the IEP of 4.3−3.3 of 100−140 nm TiO 2 . Therefore, 20−60 nm and 100−140 nm TiO 2 suspensions were positively charged and negatively charged, respectively. Imanishi et al.29 and Tsujiko et al.19 reported that the negatively charged TiO2 can accelerate photoelectron and hole separation and inhibit their recombination. The result of this phenomenon was to promote the production of ROS in TiO2 photocatalytic processes. As a result, 100−140 nm TiO2 generated more ROS than that of 20−60 nm. However, in our experiment, the change of pH was negligible compared with that of the IEP (Figure S10). Therefore, the IEP was the key factor for the ROS generation and its photocatalytic activity. The size effect on dispersion IEP was further investigated through dimensional relationship to the surface hydroxyl of TiO2,30 because the surface acidity may also be related to the density of hydroxyl groups.31 On the surface of TiO2, there are two kinds of hydroxyl groups: bridged hydroxyl group (HObr) and terminal hydroxyl group (HOt).21 The HObr is acidic (pKa = 2.9), whereas the HOt is basic (pKa = 12.7).30,32,33 Therefore, the Ka value is larger along with the amount of
because when TiO2 nanoparticles were dispersed in water, the surface was ordinarily covered by the hydroxyl group and extra hydrogen ions were produced (eq 3). With the decrease of surface area, relatively smaller amounts of hydrogen ions were generated and the solution pH increased. TiIV + H 2O → TiIV−OH + H+
(3)
On the other hand, the effects of six primary particle sizes on the dispersion IEP of TiO2 were investigated. Figure 2b shows that when the particle size of TiO2 increased from 20 to 60 nm, the IEP decreased from 7.8 to 6.2; when the particle size increased from 100 to 140 nm, the IEP decreased from 4.3 to 3.3. The measured IEP was somewhat different from previous reports,24 which is probably because the different IEPs can be obtained because of different synthesis methods even for the same material.27,28 However, there was agreement that with the increase of primary particles, the IEP greatly decreased from 7.8 to 3.3. On the basis of the above results, the solution pH of TiO2 suspensions was less than the IEP of 7.8−6.2 of 20−60 nm 3026
DOI: 10.1021/acs.jpclett.9b00863 J. Phys. Chem. Lett. 2019, 10, 3024−3028
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HObr, and the negative charge on the TiO2 surface increases, which would induce the lower shift of the IEP.30 The surface functional hydroxyl groups of the used TiO2 were characterized by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The spectra exhibited successive growth of the HObr vibration band centered at 3730 cm−1 with the increase of particle sizes, whereas the HOt vibration band at 3670 cm−1 was changed only a little (Figure 3a).34,35 The relationship between the spectral intensity difference of HObr and HOt and the primary particle sizes was studied. It was found that the larger the particle size, the greater the difference in the amounts of HObr and HOt (Figure 3b), which would contribute more negative charges and more production of ROS. Furthermore, the correlation between the amount of HObr and the ROS generation was investigated under the unit specific surface area. Figure 3c,d shows that the amount of ROS was a certain degree of linear enhancement with the increase of HObr. These results further indicated that the HObr on the TiO2 surface was an important active site which was beneficial to the transfer of photogenerated carriers,22 resulting in more ROS (Figure 4) generation and correspondingly better photocatalytic activity (Figure S11).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: (86) 10-62849338. Fax: (86) 10-62849685. ORCID
Lixia Zhao: 0000-0002-2068-4732 Liang-Hong Guo: 0000-0003-1399-5716 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the technical help provided by Drs. Fanglan Geng, Rui Liu, Wei Yan, Shaoyu Lu, and Wanyu Shan. This work was financially supported by National Key Research and Development Program of China (2016YFA0203102) and National Natural Science Foundation of China (Nos. 21677152, 21876184, 21577156, and 91543203).
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REFERENCES
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Figure 4. Schematic illustration of mechanism of ROS formation and the surface hydroxyl groups of TiO2 in aqueous suspension.
In summary, we found that more ROS were produced for 100−140 nm TiO2 than for 20−60 nm TiO2. Further research suggested that the greater amount of HObr than HOt led to the lower shift of the IEP and more negative charge on the surface of 100−140 nm TiO2. As a result, it enhanced the separation of the photogenerated holes and electrons, promoting the amount of ROS production and photocatalytic activity. The results presented herein should not only facilitate a better understanding of the photocatalytic mechanism but also be helpful to provide some significant theoretical guidance for designing TiO2-based photocatalysts with large particle size and high efficiency to solve the problems associated with small particle size of catalysts causing loss and blockage in wastewater treatment.
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Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00863. Detailed experimental procedures and results of characterizations (PDF) 3027
DOI: 10.1021/acs.jpclett.9b00863 J. Phys. Chem. Lett. 2019, 10, 3024−3028
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