Modification of N-doped TiO2 photocatalysts using noble metals (Pt, Pd) - a combined XPS and DFT study

A Facile Strategy for the Preparation of N-Doped TiO2 with Oxygen Vacancy via the Annealing Treatment with Urea

Although titanium dioxide (TiO2) has a wide range of potential applications, the photocatalytic performance of TiO2 is limited by both its limited photoresponse range and fast recombination of the photogenerated charge carriers. In this work, the preparation of nitrogen (N)-doped TiO2 accompanied by the introduction of oxygen vacancy (Vo) has been achieved via a facile annealing treatment with urea as the N source. During the annealing treatment, the presence of urea not only realizes the N-doping of TiO2 but also creates Vo in N-doped TiO2 (N-TiO2), which is also suitable for commercial TiO2 (P25). Unexpectedly, the annealing treatment-induced decrease in the specific surface area of N-TiO2 is inhibited by the N-doping and, thus, more active sites are maintained. Therefore, both the N-doping and formation of Vo as well as the increased active sites contribute to the excellent photocatalytic performance of N-TiO2 under visible light irradiation. Our work offers a facile strategy for the preparation of N-TiO2 with Vo via the annealing treatment with urea.

Visible-Light-Active N-Doped TiO2 Photocatalysts: Synthesis from TiOSO4, Characterization, and Enhancement of Stability Via Surface Modification

This paper describes the chemical engineering aspects for the preparation of highly active and stable nanocomposite photocatalysts based on N-doped TiO₂. The synthesis is performed using titanium oxysulfate as a low-cost inorganic precursor and ammonia as a precipitating agent, as well as a source of nitrogen. Mixing the reagents under a control of pH leads to an amorphous titanium oxide hydrate, which can be further successfully converted to nanocrystalline anatase TiO₂ through calcination in air at an increased temperature. The as-prepared N-doped TiO₂ provides the complete oxidation of volatile organic compounds both under UV and visible light, and the action spectrum of N-doped TiO₂ correlates to its absorption spectrum. The key role of paramagnetic nitrogen species in the absorption of visible light and in the visible-light-activity of N-doped TiO₂ is shown using the EPR technique. Surface modification of N-doped TiO₂ with copper species prevents its intense deactivation under highly powerful radiation and results in a nanocomposite photocatalyst with enhanced activity and stability. The photocatalysts prepared under different conditions are discussed regarding the effects of their characteristics on photocatalytic activity under UV and visible light.

One-Pot Synthesis of Hexamethylenetetramine Coupled with H2 Evolution from Methanol and Ammonia by a Pt/TiO2 Nanophotocatalyst

Utilization of solar energy for photocatalytic H₂ evolution coupled with value-added chemical synthesis is a promising avenue to address energy and environmental crises. Here, we report the hexamethylenetetramine (HMT) synthesis and H₂ evolution from methanol and ammonia in one pot using a nanophotocatalyst of the conventional semiconductor TiO₂ (P25) loaded with Pt (Pt/P25). The addition of ammonia inhibits byproduct ethylene glycol formation, promotes H₂ evolution, and obtains HMT with high selectivity (>99.0%). The Pt valence state is regulated by calcination and reduction treatment, indicating that Pt/P25 is a stable catalyst for the photocatalytic synthesis of HMT from methanol and ammonia. The optimized formation rates of H₂ and HMT are 71.53 and 11.39 mmol g_cat ^-1 h^-1, respectively. This work provides a green and sustainable pathway for the photocatalytic HMT synthesis coupled with H₂ evolution under mild conditions.

Photodegradation of 2,4-D (dichlorophenoxyacetic acid) with Rh/TiO2; comparative study with other noble metals (Ru, Pt, and Au)

In this work the effect of noble metal on the photodegradation of 2,4-dichlorophenoxyacetic acid herbicide using TiO₂ as support was studied. The metals and concentration were: Rh, Ru, Pt and Au and 1, 0.98, 1.89, and 1.91 wt% respectively. Rhodium was taken as reference for this experiment. The samples were characterized by X-Ray Diffraction (XRD), UV-vis absorption spectra, N₂ physisorption (BET Specific Surface Area), High Annular Angle Analysis Darkfield (HAADF) and Transmission Electron Microscopy Scanning (STEM), H₂ chemisorption, optical emission spectroscopy with inductive coupling plasma analysis (ICP-OES), solid fluorescence, X-ray Photoelectron Spectroscopy (XPS) and OH quantification. The presence of the anatase crystalline phase was mostly confirmed in all samples. The band gap decreased with the presence of metal (from 3.24 to 2.92 eV). The specific area was a function of the metal particle size. The metal particle diameter showed the following sequence Pt > Ru > Au > Rh. By XPS, TiO₂ does not manifest changes in oxidation states, but when impregnated with metals, only Pt shows the highest abundance of any oxidized state (Pt²⁺). The presence of metal reveals less electron–hole recombination compared with titanium oxide. The results of photocatalytic activity showed that Pt and Rh are the two metals with the highest mineralization (99.0 and 98.3%, respectively).

The optimum catalyst for the photocatalytic degradation of 2,4-D was Rh (1%)/TiO₂ by UV radiation. The Rh presents a strong metal-support interaction and improves the photocatalytic properties of TiO₂, modifying its band gap energy.

Oxygen vacancy and nitrogen doping collaboratively boost performance and stability of TiO2-supported Pd catalysts for CO2 photoreduction: a DFT study

The regulation of interfacial charge transfer, optimization of active sites, and maintenance of stability are effective strategies for improving catalytic performance. The effect of the oxygen vacancy (V_O) and nitrogen doping on these parameters for CO₂ photoreduction on Pd₁₀/TiO₂(101) was studied using density functional theory calculations. The results demonstrate that introduction of the V_O could trigger reversed electron transfer, making the V_O and Pd atoms the active center for CO₂ reduction. However, the V_O is repaired by the dissociated O atom. The combined effect of the V_O and N is related to the position of N. Although the substitutional N (N_S) can delocalize electrons at the V_O, it cannot improve the activity and stability. The interstitial N (N_i) located below the V_O forms N_i-Ti bonds with two Ti atoms adjacent to the V_O. This can delocalize the electrons near the V_O, and the five-fold-coordinated titanium (Ti_5C) replaces the V_O as the active center, thus enhancing the reactivity and protecting the V_O. Further research indicates that the co-modification of the V_O and N_i improves photoexcited electron transfer and distribution, which would in turn promote CO₂ reduction. The results of this study propose that surface defect engineering holds great promise for boosting CO₂ photoreduction by integrating functions of electron density modulation and catalysis.

Insoluble matrix proteins from shell waste for synthesis of visible-light response photocatalyst to mineralize indoor gaseous formaldehyde

HCHO is the most concerned indoor air pollutant that photocatalytic degradation is a feasible approach. To achieve efficient and complete degradation of HCHO under visible light irradiation, heteroatoms are usually doped in TiO₂. But using natural materials as a dopant instead of expensive and toxic chemicals to fertilize TiO₂ remains challenging. This paper proposes a sustainable and green approach to synthesize an efficient N, Ca co-doped TiO₂ photocatalyst (TIMP) by using the insoluble matrix proteins (IMPs) extracted from abalone shell. TIMP-0.8 achieves near completely degradation HCHO within 45 min under visible light at ambient temperature and exhibits superior stability after 7 cycles. TIMP-0.8 has monodispersity with smaller diameter, high porosity, abundant defects and high adsorption affinity for surface hydroxyls compared with pure TiO₂. With the assistance of IMPs, the rate-determining step of HCHO degradation changes from -COOH oxidation to spontaneous decomposition of HCO₃^-, significantly facilitating the elimination and mineralization of HCHO. Overall, IMPs from abalone shell are natural surfactant, bio-templet, and dopant for TiO₂ modification, contributing to desirable visible-light photocatalytic performance for HCHO degradation. This paper provides new insight for high-value utilization of waste shell and photocatalytic indoor purification.

Photocatalytic Conversion of Organic Pollutants in Air: Quantum Yields Using a Silver/Nitrogen/TiO2 Mesoporous Semiconductor under Visible Light

This research studies the photocatalytic conversion of methanol (25–90 µmol/L range) as a volatile organic compound (VOC) surrogate into CO2, using a N/Ag/TiO2 photocatalyst under visible light irradiation in a Photo-CREC Air unit. The N/Ag/TiO2 mesh supported photocatalyst is prepared via the solvothermal method. While the bare-TiO2 is inactive under visible light, the N/Ag/TiO2 2 wt.% loaded stainless-steel woven mesh displays 35% quantum yields, with 80% absorbed photons and 60% methanol conversion in a 110 min irradiation period. Results obtained are assigned to silver surface plasmon resonance, silver and nitrogen species synergistic impacts on band gap, and their influence on particle agglomerate size and semiconductor acidity. The determined quantum yields under visible light in a Photo-CREC Air unit, are the highest reported in the technical literature, that these authors are aware of, with this opening unique opportunity for the use of visible light for the purification of air from VOC contaminants.

Synthesis of N-Doped TiO2 for Efficient Photocatalytic Degradation of Atmospheric NOx

Titanium oxide (TiO2) is a potential photocatalyst for removing toxic NOx from the atmosphere. Its practical application is, however, significantly limited by its low absorption into visible light and a high degree of charge recombination. The overall photocatalytic activity of TiO2 remains too low since it can utilize only about 4–5% of solar energy. Nitrogen doping into the TiO2 lattice takes advantage of utilizing a wide range of solar radiation by increasing the absorption capability towards the visible light region. In this work, N-doped TiO2, referred to as TC, was synthesized by a simple co-precipitation of tri-thiocyanuric acid (TCA) with P25 followed by heat treatment at 550 degrees C. The resulting nitrogen doping increased the visible-light absorption and enhanced the separation/transfer of photo-excited charge carriers by capturing holes by reduced titanium ions. As a result, TC samples exhibited excellent photocatalytic activities of 59% and 51% in NO oxidation under UV and visible light irradiation, in which the optimum mass ratio of TCA to P25 was found to be 10.

Effect of Synthesis Conditions of Nitrogen and Platinum Co-Doped Titania Films on the Photocatalytic Performance under Simulated Solar Light

Platinum and nitrogen co-doped titania films of different surface morphologies obtained via a sol-gel process have been tested for tetracycline hydrochloride photocatalytic decomposition under simulated solar light. Titania crystallization to anatase is shown by XRD for all films. A shift of the bandgap edge toward the visible region in absorption spectra and, consequently, a narrowing of the bandgap is observed for some films doped with nitrogen and/or exposed to UV pretreatment. The surface peculiarities of the samples are presented by an SEM and TEM investigation. The surface saturation by Pt and N with a homogeneous distribution of Pt ions on the surface as well as bulk as established by XPS and EDS data can be achieved with a certain synthesis procedure. The influence of the platinum content and of the pretreatment procedure on the state and atomic surface concentration of incorporated nitrogen and platinum is studied by XPS analysis: substitutional and interstitial nitrogen, non-metal containing fragments, Pt0, Pt2+ and Pt4+ ions. The photocatalytic activity of the films is ruled by the presence of Pt2+ ions and N rather than Pt0. The formation of the polycrystalline titania structure and Pt0 nanoparticles (NPs) is confirmed by TEM and electron diffraction images. The mechanism of primary photocatalytic processes is proposed.

Origin of photocatalytic activity enhancement in Pd/Pt-deposited anatase N-TiO2- experimental insights and DFT study of the (001) surface

In pursuit of the ideal photocatalyst, cheap and stable semiconductor TiO₂ is considered to be a good choice if one is able to reduce its band gap and decrease the recombination rate of charge carriers. The approach that offers such improvements for energy conversion applications is the modification of TiO₂ with nitrogen and noble metals. However, the origin of these improvements and possibilities for further design of single-atom catalysts are not always straightforward. To shed light on the atomic-scale picture, we modeled the nitrogen-doped (001) anatase TiO₂ surface as a support for palladium and platinum single-atom deposition. The thermodynamics of various synthesis routes for Pd/Pt deposition and nitrogen doping is considered based on density functional theory (DFT)-calculated energies, highlighting the effect of nitrogen doping on metal dimer formation and metal-support interaction. XPS analysis of the valence band of the modified TiO₂ nanocrystals, and the calculated charge transfer and electronic structure of single-atom catalysts supported on the (001) anatase TiO₂ surface provide an insight into modifications occurring in the valence zone of TiO₂ due to nitrogen doping and Pd/Pt deposition at the surface. DFT results also show that substitutional nitrogen doping significantly increases metal-support interaction, while interstitial nitrogen doping promotes only Pt-support interaction.

In situ formation of defect-engineered N-doped TiO2 porous mesocrystals for enhanced photo-degradation and PEC performance

N-Doped oxygen defective N/TiO_2-x mesocrystal nanocubes were successfully prepared by a facile strategy in our system. Crystal topotactic transformation from NH₄TiOF₃ mesocrystals facilitated the formation of a porous structure of TiO₂. Meanwhile, the introduction of N dopants and oxygen vacancies (OVs) was also achieved during this process. The as-prepared products exhibit much higher photoelectrochemical (PEC) and photocatalytic degradation performance under visible light illumination. It is suggested that the promising catalytic properties result from the synergistic effect of doping, OVs and the amazing porous mesocrystal structure of N/TiO_2-x .

Photocatalytic Degradation of Bisphenol-A using N, Co Codoped TiO2 Catalyst under Solar Light

Advanced oxidation processes (AOPs) including heterogeneous photocatalysis has proven as one of the best technique for waste-water treatment. Photocatalytic process using semiconductor like TiO₂ based heterogeneous photocatalysis is a promising method for the treatment of toxic pollutants. In the present study, visible-light photoactive cobalt and nitrogen co-doped TiO₂ nanoparticles were synthesized via wet impregnation method. The photocatalysts were characterized using X-ray diffraction (XRD), Raman Spectra, Fourier Transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM), UV-vis spectrophotometer and X-ray photoelectron spectrophotometer (XPS). The photocatalytic activitiy of prepared (N, Co)-codoped TiO₂ on the mineralization of Bisphenol-A (BPA) under visible light irradiation was studied and the results were compared to commercial TiO₂ (Degussa P25). The results demonstrated that 1.5% Co and 0.5% N – codoped TiO₂ samples revealed higher activity than commercial TiO₂. Total organic carbon (TOC) removal was observed to be 97%, which indicate the complete mineralization of BPA. GC-MS analysis was carried to find out the possible intermediates formed and reaction pathway.

Facile Sol-Gel-Derived Craterlike Dual-Functioning TiO2 Electron Transport Layer for High-Efficiency Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) are considered promising materials for low-cost solar energy harvesting technology. An electron transport layer (ETL), which facilitates the extraction of photogenerated electrons and their transport to the electrodes, is a key component in planar PSCs. In this study, a new strategy to concurrently manipulate the electrical and optical properties of ETLs to improve the performance of PSCs is demonstrated. A careful control over the Ti alkoxide-based sol-gel chemistry leads to a craterlike porous/blocking bilayer TiO₂ ETL with relatively uniform surface pores of 220 nm diameter. Additionally, the phase separation promoter added to the precursor solution enables nitrogen doping in the TiO₂ lattice, thus generating oxygen vacancies. The craterlike surface morphology allows for better light transmission because of reduced reflection, and the electrically conductive craterlike bilayer ETL enhances charge extraction and transport. Through these synergetic improvements in both optical and electrical properties, the power conversion efficiency of craterlike bilayer TiO₂ ETL-based PSCs could be increased from 13.7 to 16.0% as compared to conventional dense TiO₂-based PSCs.

PAMAM templated N,Pt co-doped TiO2 for visible light photodegradation of brilliant black

This study examined the photocatalytic degradation of an azo dye brilliant black (BB) using non-metal/metal co-doped TiO₂. N,Pt co-doped TiO₂ photocatalysts were prepared by a modified sol-gel method using amine-terminated polyamidoamine dendrimer generation 0 (PG0) as a template and source of nitrogen. Structural, morphological, and textural properties were evaluated using scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM/EDX), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), Fourier transform infrared (FTIR), Raman spectroscopy (RS), photoluminescence (PL) and ultra-violet/visible spectroscopy (UV-Vis). The synthesized photocatalysts exhibited lower band gap energies as compared to the Degussa P-25, revealing a red shift in band gap towards the visible light absorption region. Photocatalytic activity of N,Pt co-doped TiO₂ was measured by the reaction of photocatalytic degradation of BB dye. Enhanced photodegradation efficiency of BB was achieved after 180-min reaction time with an initial concentration of 50 ppm. This was attributed to the rod-like shape of the materials, larger surface area, and enhanced absorption of visible light induced by N,Pt co-doping. The N,Pt co-doped TiO₂ also exhibited pseudo-first-order kinetic behavior with half-life and rate constant of 0.37 and 0.01984 min^-1, respectively. The mechanism of the photodegradation of BB under the visible light irradiation was proposed. The obtained results prove that co-doping of TiO₂ with N and Pt contributed to the enhanced photocatalytic performances of TiO₂ for visible light-induced photodegradation of organic contaminants for environmental remediation. Therefore, this work provides a new approach to the synthesis of PAMAM templated N,Pt co-doped TiO₂ for visible light photodegradation of brilliant black.

Synergistic Effect of Fluorinated and N Doped TiO₂ Nanoparticles Leading to Different Microstructure and Enhanced Photocatalytic Bacterial Inactivation

This work focuses on the development of a facile and scalable wet milling method followed by heat treatment to prepare fluorinated and/or N-doped TiO₂ nanopowders with improved photocatalytic properties under visible light. The structural and electronic properties of doped particles were investigated by various techniques. The successful doping of TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS), and the atoms appeared to be mainly located in interstitial positions for N whereas the fluorination is located at the TiO₂ surface. The formation of intragap states was found to be responsible for the band gap narrowing leading to the faster bacterial inactivation dynamics observed for the fluorinated and N doped TiO₂ particles compared to N-doped TiO₂. This was attributed to a synergistic effect. The results presented in this study confirmed the suitability of the preparation approach for the large-scale production of cost-efficient doped TiO₂ for effective bacterial inactivation.