Colloidal TiO2 Suspensions Research Articles

Adsorption‐mediated nonlinearity of critical cracking thickness in drying nanoparticle–polymer suspensions

AbstractWe examine the critical cracking thicknesses of as‐dried colloidal TiO2 suspensions containing water‐soluble polymeric additives, above which cracks spontaneously propagate. In contrast to common observations, the critical cracking thickness increases in a nonlinear, stepwise manner as increasing the concentration of the polymeric additive in the suspension. The critical thickness diverges when the polymer content increases above a certain threshold. We demonstrate that the first and second critical cracking thickness increments are attributable to the onset of polymer adsorption and a subsequent transition in the conformations of the adsorbing polymer chains, respectively. The critical thickness profiles with respect to normalized polymer adsorption amounts at different particle diameters converged into a single master curve. These results illustrate nontrivial evaporation processes in which the various roles of polymeric additives delay cracking into thicker films.

Analysis of dependent scattering mechanism in hard-sphere Yukawa random media

The structural correlations in the microscopic structures of random media can induce the dependent scattering mechanism and thus influence the optical scattering properties. Based on our recent theory on the dependent scattering mechanism in random media composed of discrete dipolar scatterers [B. X. Wang and C. Y. Zhao, Phys. Rev. A 97, 023836 (2018)], in this paper, we study the hard-sphere Yukawa random media, in order to further elucidate the role of structural correlations in the dependent scattering mechanism and hence optical scattering properties. Here, we consider charged colloidal suspensions, whose effective pair interaction between colloids is described by a screened Coulomb (Yukawa) potential. By means of adding salt ions, the pair interaction between the charged particles can be flexibly tailored and therefore the structural correlations are modified. It is shown that this strategy can affect the optical properties significantly. For colloidal TiO2 suspensions, the modification of electric and magnetic dipole excitations induced by the structural correlations can substantially influence the optical scattering properties, in addition to the far-field interference effect described by the structure factor. However, this modification is only slightly altered by different salt concentrations and is mainly because of the packing-density-dependent screening effect. On the other hand, for low refractive index colloidal polystyrene suspensions, the dependent scattering mechanism mainly involves the far-field interference effect, and the effective exciting field amplitude for the electric dipole almost remains unchanged under different structural correlations. The present study has profound implications for understanding the role of structural correlations in the dependent scattering mechanism.

Enhanced photoactivity of stable colloidal TiO2 nanoparticles prepared in water by nanosecond infrared laser pulses

A simple laser ablation technique was used to prepare a stable colloidal TiO2 suspension in pure water. A transparent TiO2 aqueous solution was obtained within a few minutes and its photoactivity for the degradation of methylene blue was measured to be higher than that of commercial TiO2 nanoparticles. SEM analysis revealed that the average size of the nanoparticles increased from 20 to 40 nm as the laser power was raised from 0.5 to 2 W. The variation in size, however, had little influence on the resulting photodegradation rate under the given condition. Instead, the photodegradation rate is related to the number of colloidal TiO2 particles in the aqueous solution, which increases proportionally to the ablation time. As the TiO2 particle density increases, however, the photoactivity is measured to be gradually reduced due to the formation of TiO2 aggregates. Thus, the optimum ablation time is 10-30 min under our ablation condition. Our results show that well-dispersed small TiO2 nanoparticles of about a few tens nm can be readily formed by laser ablation within only a few minutes and can be used as highly efficient photocatalysts for photocatalytic remediation of water.

Role of Water Oxidation Catalyst IrO2 in Shuttling Photogenerated Holes Across TiO2 Interface

Iridium oxide, a water oxidation cocatalyst, plays an important role in mediating the hole transfer process of a UV-irradiated TiO2 system. Spectroscopic identification of trapped holes has enabled their characterization in colloidal TiO2 suspension and monitoring of the transfer of trapped holes to IrO2. Titration of trapped holes with potassium iodide yields an estimate of three holes per particle during 7 min of UV irradiation of TiO2 suspension in ethanol containing 5% acetic acid. The hole transfer to IrO2 occurs with a rate constant of 6 × 105 s–1. Interestingly, IrO2 also catalyzes the recombination of trapped holes with reduced oxygen species. The results discussed here provide a mechanistic and kinetic insight into the catalytic role of IrO2 in the photogenerated hole transfer process.

Novel Synthesis of the TiO2(B) Multilayer Templated Films

TiO2(B) mesoporous thin films were grown in two steps on the F-doped SnO2 conductive glass substrates. In the first step, a small amount of H3PO4, corresponding to 0.15−0.375 wt % P on TiO2 basis, was introduced into concentrated HCl which was subsequently used for hydrolysis of titanium ethoxide. The hydrolyzed colloidal TiO2 suspension was further mixed with a 1-butanol solution of the amphiphilic triblock copolymer Pluronic P123. The obtained precursor mixture was used for dip coating of FTO substrates. To achieve over 1 μm thick films, dip coating (followed by a thermal treatment at 350 °C/2 h) was repeated several times to produce multilayer films. The films consisted of amorphous TiO2 with small amounts of anatase and TiO2(B). The amorphous part was converted into the TiO2(B) in a simple firing step at 500−550 °C. The formation of TiO2(B) phase was accompanied by a significant increase of the film thickness. The films demonstrated unique behavior during the electrochemical lithium insertion that wou...

Primary Processes During the Photodeposition of Ag Clusters on TiO2 Nanoparticles

In-situ transient laser flash photolysis absorption studies were carried out on transparent colloidal TiO2 suspensions in the presence of Ag+ ions and polyvinyl alcohol irradiated by an XeF laser (351nm wavelength) to investigate the early stages of Ag photodeposition on the TiO2 nanoparticles in the nanosecond and millisecond timescales. Photon fluxes of 5 × 10-5 and 1 × 10-6 mol photon/L were used; these were equivalent to approximately 60 and 2 photons/TiO2 particle, respectively. Metallic silver clusters were detected immediately after the laser pulse using both photon fluxes, with calculated quantum yields of 0.3 and 0.5, respectively. In both cases, the metallic silver clusters formed had the same characteristic absorption maxima, assigned to silver clusters containing ≥12 silver atoms. The formation of these larger than expected silver clusters, even at low photon fluxes, is believed to be a result of an electron transfer within the TiO2 aggregates, similar to an antenna effect. It is proposed that the silver deposits form an Ohmic contact on the TiO2 particle thus allowing the continuous electron transfer through the metallic silver nuclei and hence the further reduction of adsorbed Ag+ ions. This study reports the first observation of an antenna type effect for a photocatalytic reduction reaction.

Modification of TiO2 for Enhanced Surface Properties: Finite Ostwald Ripening by a Microwave Hydrothermal Process

The effect of microwave modification of colloidal TiO2 suspensions under extended periods of treatment is presented. The nanoparticulate TiO2 is compared and contrasted to similar convection hydrothermally treated TiO2 and a commercial titania product, namely Degussa P25. Microwave-treated samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy to determine their physicochemical characteristics. Comparative surface area analyses were performed by N2 adsorption and calculated from a Brunauer-Emmett-Teller (BET) isotherm. The complementary techniques of XRD and TEM showed good correlation between observed and calculated particle sizes (by application of the Scherrer equation), with the material being highly crystalline anatase TiO2, as identified by XRD and Raman. This investigation identified that extended periods of microwave hydrothermal treatment do not greatly enhance the crystallinity and primary grain size. Treatment of >180 min has a negative effect on crystallite growth; however, treatment up to this time had a significant effect on the material's surface area. The limiting regime of Ostwald ripening for hydrothermal treatment is discussed in relation to the mechanism of microwave treatment, that is, rapid heating to temperature and extremely rapid rates of crystallization. The effect of these property modifications are further discussed in relation to photocatalytic and photoelectrochemical applications of TiO2 nanoparticles.

Efficient microwave hydrothermal preparation of nanocrystalline anatase TiO2 colloids

Titanium dioxide nanocrystals are an important commercial product used primarily in white pigments and abrasives, however, more recently the anatase form of TiO2 has become a major component in electrochemical and photoelectrochemical devices. An important property of titanium dioxide nanocrystals for electrical applications is the degree of crystallinity. Numerous preparation methods exist for the production of highly crystalline TiO2 particles. The majority of these processes require long reaction times, high pressures and temperatures (450–1400 °C). Recently, hydrothermal treatment of colloidal TiO2 suspensions has been shown to produce quality crystalline products at low temperatures (<250 °C). In this paper we extend this idea utilising a direct microwave heating source. A comparison between convection and microwave hydrothermal treatment of colloidal TiO2 is presented. The resulting highly crystalline TiO2 colloids were characterised using Raman spectroscopy, XRD, TEM, and electron diffraction. The results show that the microwave treatment of colloidal TiO2 gives comparable increases in crystallinity with respect to normal hydrothermal treatments while requiring significantly less time and energy than the hydrothermal convection treatment.

Study of Interfacial Charge-Transfer Complex on TiO2 Particles in Aqueous Suspension by Second-Harmonic Generation

We report the first direct observation of an interfacial charge-transfer complex using second-harmonic spectroscopy. The second-harmonic spectrum of catechol adsorbed on 0.4 micron-sized TiO2 (anatase) colloidal particles in aqueous suspension reveals a charge-transfer band centered at 2.72 eV (456 nm). In addition, the adsorption isotherm of catechol on the colloidal TiO2 suspension was obtained and gave an excellent fit to the Langmuir adsorption model. From this, we infer the free energy of the adsorption to be ΔG° = −6.8 kcal/mol.

Picosecond dynamics of an IR sensitive squaraine dye. Role of singlet and triplet excited states in the photosensitization of TiO2 nanoclusters

The photosensitization properties of a novel squaraine dye is probed using picosecond laser flash photolysis. The dye which exists in a protonation equilibrium with a pKa of 4.9 has a strong absorption in the IR region. The protonated form of the dye is highly fluorescent and is readily quenched in colloidal TiO2 suspensions. The association constant as determined from the quenching studies is 1.25×103 M−1. The singlet excited state of the protonated form of the dye participates in the charge injection process with a heterogeneous electron transfer rate constant of 8.3×109 s−1. The unprotonated form of the dye injects electrons from both the singlet and triplet excited state with electron transfer rate constants of 1×1010 s−1 and 1.25×109 s−1, respectively.

Photochemistry of squaraine dyes: excited states and reduced and oxidized forms of 4-(4-acetyl-3,5-dimethylpyrrolium-2-ylidene)-2-(4-acetyl-3,5-dimethylpyrrol-2-yl)-3-oxocyclobut-1-en-1-olate

Singlet and triplet excited states of 4-(4-acetyl-3,5-dimethylpyrrolium-2-ylidene)-2-(4-acetyl-3,5-dimethylpyrrol-2-yl)-3-oxocyclobut-1-en-1-one (SQ) have been investigated by picosecond and nanosecond laser flash photolysis. Both singlet and triplet excited states of SQ have distinct absorption bands in the visible region. The fluorescence yields and singlet lifetimes were higher in CH2Cl2(ϕf= 0.08, τs= 222 ps) than in polar solvents such as CH3CN (ϕf= 0.04, τs < 100 ps). The triplet excited states of the dye were characterized by both direct excitation (ϕT= 0.02 in CH2Cl2) and T–T sensitization models. The oxidized (SQ.+) and reduced (SQ.–) forms of the dye exhibited absorption maxima at 505 nm (in CH2Cl2) and 435 nm (in acetonitrile). The results of the photoelectrochemical reduction of SQ in colloidal TiO2 suspension are also presented.

Photoelectrochemistry in colloidal systems. Part 2.—A photogalvanic cell based on TiO2 semiconductor colloid

Thiazine and oxazine dyes have been photoelectrochemically reduced in colloidal TiO2 suspensions with bandgap excitation. The one-electron reduction of the dye, which resulted in the production of the semireduced radical ion, with quantum yield up to 0.1, further disproportionated to yield a stable leuco dye. The efficiency of the interfacial electron-transfer process is found to be dependent on the standard reduction potential of the dye. The feasibility of employing a colloidal semiconductor system in the operation of a photogalvanic cell is demonstrated. A photogalvanic power conversion efficiency of 0.002% was achieved with thionine and colloidal TiO2 upon bandgap excitation. The use of TiO2 colloid as a carrier for the deposition of a photoactive or an electroactive species on the electrode surface is also discussed.