Surface Modification Of TiO₂ Nanoparticles And Their Application In Photocatalytic Removal Of Typical Toxic Pollutants

The toxic contaminants in the wastewaters mainly contain organic pollutants (such as dyes, and phenols) and inorganic (such as Cr(â…¥)) pollutants, which can be removed via the oxidation and reduction methods, respectively. TiO₂ photocatalysis has been intensively investigated for the elimination of environmental toxic pollutants, because the UV-illuminated TiO₂ would generate the photo-induced electron/hole (e–/h+) pairs, of which holes and the successive generated (?)OH play an important role in the photocatalytic oxidation of organic pollutants, while electrons are critical to the photocatalytic reduction of heavy metal ions. However, TiO₂ photocatalysis suffers the mainly drawbacks of the narrow absorption wavelength range (only 5% of the spectrum of the incoming sunlight in the near ultraviolet region) and low quantum yield. In addition, there are very limited studies on the photocatalytic reduction of TiO₂, possible due to the lack of suitable probes that respond to the electron transfer. Therefore, the goals of the present work are to improve the photocatalytic ability of TiO₂ for the removal the typical pollutants including dyes, aniline, phenolic and Cr(â…¥), and so on, and to study the related reaction mechanisms.(1) The novel Fe(OH)3/TiO₂ and Cu(OH)2/TiO₂ nanocomposites were synthesized through a modified wet precipitation process, and they showed much improved photocatalytic activity by using methyl orange (MO) as a model compound of organic pollutants. The half time of MO (10 mg L(?)(?)) during its photocatalytic degradation at pH 6.0 under UV illumination was decreased from 332 min for unmodified neat TiO₂ to 63 min for Fe(OH)3/TiO₂ and 65 min for Cu(OH)2/TiO₂, respectively. The high photocatalytic activity of M(OH)X/TiO₂ was further observed in a wide composition range with various M/Ti atomic ratios in photocatalysts and in a wide pH range of the MO solution from 3 to 7. The characterizations with XRD, FTIR, BET, UV-vis DRS, and TGA revealed that the enhancing effect of M(OH)X/TiO₂ is mainly attributed to the existence of the hetero-junction as the efficient trapping of the photogenerated electron and the enriched surface hydroxyl groups which accept photogenerated holes to yield more (?)OH radicals.(2) Synergistic effects of cupric ions and fluoride ions were investigated on the photocatalytic degradation of phenol in TiO₂ suspensions at pH 3 under UV irradiation. Under all tested conditions, the phenol photodegradation was observed to follow a pseudo-first-order reaction in kinetics. The rate constant k of the phenol photodegradation was evaluated as 0.010 min(?)(?) in the absence of Cu²⁺ and F(?) ions, and was increased up to 0.014 min(?)(?) by adding 0.4 mmol L(?)(?) Cu²⁺, and to 0.022 min(?)(?) by adding 5 mmol L(?)(?) F(?) ions. The coexistence of both Cu²⁺ and F- ions increased the k value further to 0.041 min(?)(?), being 410% of that without any addition of Cu²⁺ and F(?) ions. The much enhanced mineralization of phenol induced by the coexistence of Cu²⁺ and F(?) ions was also confirmed by the monitoring of degradation intermediates. With the aid of detecting the (?)OH radicals and H2O₂ generation, the observed significant synergistic effects were attributed to the shielding effect of fluoride on the charge separation efficiency and the efficient trapping of the photogenerated electron by the Cu²⁺ adsorbed on TiO₂ surface.(3) Photocatalytic degradation of colorless aniline and phenolic pollutants was investigated over TiO₂ under visible light irradiation, which was confirmed to proceed via a charge-transfer-complex (CTC)-mediated pathway. The correlation between the chemical structure and the degradation rate of these pollutants was established experimentally and theoretically. It was found that an electron-donating substituent in benzene ring, which raises the highest occupied molecular orbital and lowers the ionization potential of the organic compound, is favorable to the CTC-mediated photodegradation of the pollutant, but an electron withdrawing substituent has a reversed effect. The addition of sacrificial electron acceptors was adopted to enhance the degradation and mineralization of the aromatic pollutants. The increased degradation rate by 3-10 times suggests that the CTC-mediated photocatalytic technique has promising applications in the removal of colorless organic pollutants in the presence of sacrificial electron acceptors.(4) Visible-light photoreduction of toxic Cr(â…¥) over TiO₂ was achieved through surface modification with small molecular weight organic acids (SOAs) as sacrificial organics. Although no photoreduction of Cr(â…¥) was observed in TiO₂ dispersions being irradiated with visible light (λ> 420 nm), when a small amount of colorless SOAs was added into the TiO₂ dispersion, a CTC complex was formed between TiO₂ and SOA, which was sensitive to visible light irradiation and induced the photooxidation of SOA and photoreduction of Cr(â…¥). It was observed that about 95% of added Cr(â…¥) (0.2 mmol L(?)(?)) was removed in the visible-light illuminated TiO₂ (1.0 g L(?)(?)) dispersions at pH 3.0 within 2 h by adding 0.2 mmol L(?)(?)(?)tartaric acid. The SOA-induced photoreduction of Cr(â…¥) proceeded via a CTC-mediated path, being governed by chemical structures of sacrificial SOAs. A higher energy of the highest occupied molecular orbital or lower ionization potential of SOAs is favorable to electron-transfer within TiO₂-SOA complex, thereby accelerating the photoreduction of Cr(â…¥). The Cr(â…¥) removal was further enhanced by increasing SOA concentration and/or decreasing solution pH.(5) It developed a novel redox-responsive boron-dipyrromethene fluorescent probe (i.e., 8-(3,4-dinitrophenyl)-BODIPY, DN-BODIPY) for elucidating the inherent photocatalytic reduction ablitiy of TiO₂-based systems. The loaded Au or coated graphene on TiO₂ significantly enhanced the photocatalytic reduction of DN-BODIPY, due to the electron transfer from the conduction band (CB) of TiO₂ to Au or graphene on the photocatalysts surface and then to DN-BODIPY. Moreover, DN-BODIPY was used to monitor the electron transfer on the surface of single TiO₂ and Au/TiO₂ particle by utilizing single-molecule fluorescence microscopy. The mapping of the fluorescence bursts revealed that the reduction of DN-BODIPY over Au/TiO₂ is more easily to occur on the surface of Au nanoparticles rather than uncovered TiO₂ surface. A single-particle kinetic analysis suggested the photocatalytic activity of Au/TiO₂ was controlled not only by the substrate concentration and excitation intensity but also by the Au particle size, and that these factors are intricately interrelated. These findings would provide valuable information for designing highly efficient photocatalysts.