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The utilization of solar light to induce a photocatalytic reaction in the presence of wide band gap semiconductors requires the modification of their surface and optical properties. The Vis-to-UV up-conversion process (UCP) that occurs in lanthanide ions could be employed to induce TiO2. Thus, having in mind solar-forced photocatalysis, nanostructured TiO2 co-modified with Tm and Li was prepared via a hydrothermal method. In addition to the exhaustive surface characterization of the as-prepared samples, the partial density of states (PDOS) was computed to better understand the electronic structure and physicochemical properties of the prepared photocatalysts. It was demonstrated that Tm ions were beneficial for phenol degradation in the aqueous phase under visible irradiation, whereas the co-presence of Tm and Li ions was profitable for enhanced luminescence. Although action spectra revealed that excitation of Tm-TiO2 in the range from 400 to 490 nm led to the photodegradation of phenol and 2-propanol, the UCP was not responsible for the photocatalytic activity under Vis irradiation. Therefore, it is proposed that the formation of sub-band gap states from the lanthanide 4f level played a key role in the phenol degradation process.

Commercial titania sample (P25) was homogenized first (HomoP25), and then crystalline phases (anatase and rutile) were isolated from it by chemical dissolution methods followed by samples’ purification by washing and annealing. The photocatalytic activities were tested for anaerobic methanol dehydrogenation in two reaction systems: (1) under UV/vis polychromatic irradiation with in situ platinum (Pt) deposition, and (2) under UV/vis monochromatic irradiation (action spectra) for ex situ platinum deposition. The properties of bare and modified titania samples were investigated by XRD, DRS, STEM, BET and zeta potential measurements. It was found that platinized rutile was more active than platinized anatase and platinized HomoP25, due to ability of photoabsorption of more photons (narrower band gap). Moreover, in HomoP25 sample, rutile was platinized first, probably due to positively charged surface allowing favorable adsorption of chloroplatinate ions. More than 10 times lower content of Pt (

Commercial titania photocatalysts were modified with silver and gold by photodeposition, and characterized by diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). It was found that silver co-existed in zero valent (core) and oxidized (shell) forms, whereas gold was mainly zero valent. The obtained noble metal-modified samples were examined with regard to antibacterial (Escherichia coli (E. coli)) and antifungal (Aspergillus niger (A. niger), Aspergillus melleus (A. melleus), Penicillium chrysogenum (P. chrysogenum), Candida albicans (C. albicans)) activity under visible-light irradiation and in the dark using disk diffusion, suspension, colony growth (“poisoned food”) and sporulation methods. It was found that silver-modified titania, besides remarkably high antibacterial activity (inhibition of bacterial proliferation), could also decompose bacterial cells under visible-light irradiation, possibly due to an enhanced generation of reactive oxygen species and the intrinsic properties of silver. Gold-modified samples were almost inactive against bacteria in the dark, whereas significant bactericidal effect under visible-light irradiation suggested that the mechanism of bacteria inactivation was initiated by plasmonic excitation of titania by localized surface plasmon resonance of gold. The antifungal activity tests showed efficient suppression of mycelium growth by bare titania, and suppression of mycotoxin generation and sporulation by gold-modified titania. Although, the growth of fungi was hardly inhibited through disc diffusion (inhibition zones around discs), it indicates that gold does not penetrate into the media, and thus, a good stability of plasmonic photocatalysts has been confirmed. In summary, it was found that silver-modified titania showed superior antibacterial activity, whereas gold-modified samples were very active against fungi, suggesting that bimetallic photocatalysts containing both gold and silver should exhibit excellent antimicrobial properties.

CdS quantum dot-decorated KNbO3 composite photocatalysts co-modified with Bi2S3 QDs were designed and synthesized by a combination of the hydrothermal method with a linker-assisted adsorption route, using starch and thioglycolic acid as capping agents, which facilitated the attachment of modifiers to the surface of potassium niobate. The quantum dots were successfully deposited onto the surface of the perovskite-type KNbO3 with a good dispersion and a stable heterostructure was formed. The as-prepared photocatalysts were subsequently characterized by UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) specific surface area, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) emission spectroscopy. The obtained KNbO3-based composites showed greatly improved photocatalytic performance for the degradation of phenol in the aqueous phase under visible light irradiation (λ > 420 nm) over pristine KNbO3. The highest photocatalytic performance and enhanced stability were observed for the ternary 30% CdS–5%Bi2S3 quantum dot-decorated KNbO3 composite, which could be related to the enhanced visible-light absorption ability, efficient charge separation in the three-level electron transfer heterojunction, improved stability and appropriate amounts of composite components. The formation of a surface layer of CdO decreased the visible light photoactivity of the CdS QD-decorated KNbO3 photocatalysts. The main phenol oxidation intermediates were benzoquinone, catechol, hydroquinone, and 1,2,4-benzenetriol, which underwent further photooxidation to form non-cyclic organic acids. Action spectral analysis proved the better photocatalytic activity of the ternary CdS/B2S3 QDs co-decorated KNbO3 composite compared to the binary CdS QDs decorated KNbO3 sample and revealed that irradiation ranging from 420 to 520 nm was responsible for the visible light photoactivity.

Titanium dioxide (TiO2) nanoparticles (NPs) modified with neodymium (Nd) in the range between 0.1 and 1.0 mol% were prepared via the hydrothermal method. The samples obtained were characterized by diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), X-ray fluorescence (EDX), Brunauer–Emmett–Teller (BET) method, X-ray powder diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). The photocatalytic activity of the obtained samples was evaluated by photodegradation of phenol in aqueous solution under ultraviolet–visible (UV–Vis, λ > 350 nm) and visible (Vis, λ > 420 nm) irradiation. Experimental results showed that the photocatalysts exhibited high photocatalytic activity under Vis light. The sample showing the highest photoactivity under Vis irradiation was in the form of anatase; its surface area equalled 124 m2/g (1.16 times larger than that of pristine TiO2). The average crystal size was 10.9 nm, and it was modified with 0.1 mol% of Nd3+ (28% of phenol was degraded after 60 min of irradiation). The photocatalytic tests of phenol degradation in the presence of scavengers confirm that e− and were responsible for the visible light degradation of organic compounds in the aqueous phase. Action spectra analysis revealed that although Nd-modified TiO2 could be excited under visible light in the range of 400–480 nm, the up-conversion process is not responsible for the degradation of phenol under Vis irradiation.

Octahedral anatase particles (OAPs) were prepared by hydrothermal reaction (HT) with various experimental conditions, including different amounts of titanate nanowires (TNWs), different water volumes and different pH values, to obtain products with high contents of OAPs. The properties of photocatalysts were investigated by XRD, SEM, TEM, XPS and time-resolved microwave conductivity (TRMC). Photocatalytic activities for oxidative decomposition of acetic acid (CO2 system) and anaerobic dehydrogenation of methanol (H2 system) were tested. It was found that a larger amount and concentration of TNWs, as well as higher pressure during HT, resulted in the formation of smaller crystallites with higher density of mobile electrons. Enhanced photocatalytic activity, achieved for samples with the best morphology (higher content of OAPs), correlated with slower TRMC signal decay, i.e., slower recombination of charge carriers (e−/h+) probably due to lower content of deep electron traps. It was found that properties of platinum nanoparticles, deposited in-situ in the H2 system in the absence or presence of pre-sparged oxygen, and their connection with OAPs were decisive factors for photocatalytic activity.

Different TiO2 photocatalysts, i.e., commercial samples (ST-01 and P25 with minority of rutile phase), nanotubes, well-crystallized faceted particles of decahedral shape and mesoporous spheres, were used as supports for deposition of Pt nanoparticles (NPs). Size-controlled Pt NPs embedded in TiO2 were successfully prepared by microemulsion and wet-impregnation methods. Obtained photocatalysts were characterized using XRD, TEM, X-ray photoelectron spectroscopy (XPS), Brunauer&ndash, Emmett&ndash, Teller (BET) specific surface area, DR/UV-vis and action spectrum analysis. The effect of deposition method, amount of Pt precursor and TiO2 properties on size, distribution, and chemical states of deposited Pt NPs were investigated. Finally, the correlations between the physicochemical properties and photocatalytic activities in oxidation and reduction reactions under UV and Vis light of different Pt-TiO2 photocatalysts were discussed. It was found that, regardless of preparation method, the photoactivity mainly depended on platinum and TiO2 morphology. In view of this, we claim that the tight control of NPs&rsquo, morphology allows us to design highly active materials with enhanced photocatalytic performance. Action spectrum analysis for the most active Pt-modified TiO2 sample showed that visible light-induced phenol oxidation is initiated by excitation of platinum surface plasmon, and photocatalytic activity analysis revealed that photoactivity depended strongly on morphology of the obtained Pt-modified TiO2 photocatalysts.

Graphene, graphene oxide, reduced graphene oxide and their composites with various compounds/materials have high potential for substantial impact as cheap photocatalysts, which is essential to meet the demands of global activity, offering the advantage of utilizing “green” solar energy. Accordingly, graphene-based materials might help to reduce reliance on fossil fuel supplies and facile remediation routes to achieve clean environment and pure water. This review presents recent developments of graphene-based semiconductor photocatalysts, including novel composites with faceted particles, photonic crystals, and nanotubes/nanowires, where the enhancement of activity mechanism is associated with a synergistic effect resulting from the presence of graphene structure. Moreover, antimicrobial potential (highly needed these days), and facile recovery/reuse of photocatalysts by magnetic field have been addresses as very important issue for future commercialization. It is believed that graphene materials should be available soon in the market, especially because of constantly decreasing prices of graphene, vis response, excellent charge transfer ability, and thus high and broad photocatalytic activity against both organic pollutants and microorganisms.

A series of Y 3+ , Pr 3+ , Er 3+ and Eu 3+ modified TiO 2 photocatalysts were obtained via sol–gel (SG) and hydrothermal (HT) methods. Samples prepared this way were characterized by X-ray powder diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), scanning transmission microscopy (STEM), BET surface area method and luminescence spectroscopy. The photocatalytic activity of the synthesized samples was evaluated by the degradation of phenol in aqueous solution under visible and ultraviolet light irradiations. Phenol in aqueous solutions was successfully decomposed under visible light ( λ > 420 nm) using TiO 2 modified with RE ions. Luminescence properties of the samples as well as XRD and XPS analyses, indicate that RE are rather in the form of their oxides than in the form of cations in the crystal structure of TiO 2 . Photocatalysts prepared by SG method possessed higher amount of RE 2 O 3 , fewer of OH − groups and Ti 3+ species on the surface layer than powders obtained by HT method. Action spectra analysis showed that Pr 3+ -modified TiO 2 could be excited under visible light in the 420–250 nm range. Furthermore, photocatalysts obtained by HT method showed higher photocatalytic activity and lower intensity of luminescence emission than photocatalyst prepared by SG method.

Enhancing the efficiency of solar light utilization and understanding the acquired mechanism of photocatalytic reactions are the critical goals in the field of heterogeneous photocatalysis. To meet growing demands, the up-conversion particles (UCPs) have become an attractive perspective to extend the absorption spectrum of UV-active photocatalysts. The systematic experimental and theoretical study of the effects of UCPs on the photocatalytic activity of TiO2, and the investigation of its excitation mechanism are vital for understanding and optimizing the photocatalytic properties. Accordingly, hydrothermal process has been employed to manufacture visible light active NaYF4-Er-Yb-TiO2 photocatalyst in the present study. The UCPs have been located at the TiO2 crystal boundaries, where Er3+ and F− surface doping has been also observed. The predicted electronic structures of UCPs-TiO2 indicate that the formation of new Er-4f states within the band gap of TiO2 is the key factor of photocatalytic performance at Vis irradiation. Luminescence spectroscopy measurements reveal up-conversion energy transfer from Yb3+ to Er3+ ions in UCPs particles. Nevertheless, UCPs-TiO2 is not active under NIR irradiation, and the photocatalytic activity under Vis irradiation is not caused by OH, but rather by other forms of reactive oxygen species, such as O2 − and OOH.

One of the challenges in the field of heterogeneous photocatalysis is the development of new type of photoactive materials activated by low-power and low-cost irradiation sources, and to understand the mechanism of photocatalytic reaction in the presence of those materials. Photocatalysis based on Er-TiO2 has become an attractive process to remove contaminants from aquatic environments. Unfortunately, there is no available literature presenting the systematic experimental and theoretical study of the effects of erbium ions on the photocatalytic activity of TiO2 and visible light excitation mechanism of this system. Accordingly, to obtain solar-responsive photocatalysts, nanostructured Er-TiO2 was prepared via hydrothermal method. The physicochemical characterization of samples indicate that modified photocatalysts contain both doped erbium and the surface adsorbed erbium species (e.g., Er2O3, Er(OH)x). Predicted electronic structures of Er-doped TiO2 indicate that the formation of new Er-4f states within the band gap region of TiO2 might be crucial for observed photocatalytic effects towards Vis response. However, the up-conversion process is not responsible for Vis activity and efficient O2 − generation. Moreover, Er modification of TiO2 results in an inhibition of the growth of titania crystallites, and thus an increase in the specific surface area, which are also beneficial for an efficient degradation of pollutants.

Plasmonic photocatalysts have been extensively studied for the past decade as a possible solution to energy crisis and environmental problems. Although various reports on plasmonic photocatalysts have been published, including synthesis methods, applications, and mechanism clarifications, the quantum yields of photochemical reactions are usually too low for commercialization. Accordingly, it has been proposed that preparation of plasmonic photocatalysts with efficient light harvesting and inhibition of charge carriers’ recombination might result in improvement of photocatalytic activity. Among various strategies, nano-architecture of plasmonic photocatalysts seems to be one of the best strategies, including the design of properties for both semiconductor and noble-metal-deposits, as well as the interactions between them. For example, faceted nanoparticles, nanotubes, aerogels, and super-nano structures of semiconductors have shown the improvement of photocatalytic activity and stability. Moreover, the selective deposition of noble metals on some parts of semiconductor nanostructures (e.g., specific facets, basal or lateral surfaces) results in an activity increase. Additionally, mono-, bi-, and ternary-metal-modifications have been proposed as the other ways of performance improvement. However, in some cases, the interactions between different noble metals might cause unwanted charge carriers’ recombination. Accordingly, this review discusses the recent strategies on the improvements of the photocatalytic performance of plasmonic photocatalysts.

The monodispersed TiO2 microspheres were synthesized by a sub-zero sol-gel method followed by calcination. The precursor was prepared by hydrolysis of tetrabutyl titanate at −10 °C. The influence of the post-calcination temperature on the particle morphology and the crystal properties was investigated. The microspheres have possessed excellent thermal stability without anatase-to-rutile transformation even at 900 °C, probably due to the stabilized microstructure of a honeycomb mesh. The photocatalytic activity for oxidative decomposition of methyl orange depends on the morphology and the crystalline properties, being the highest for highly crystallized and mesoporous sample, prepared at 700 °C calcination.

Titania photocatalysts have been intensively examined for both mechanism study and possible commercial applications for more than 30 years. Although various reports have already been published on titania, including comprehensive review papers, the morphology-governed activity, especially for novel nanostructures, has not been reviewed recently. Therefore, this paper presents novel, attractive, and prospective titania photocatalysts, including zero-, one-, two-, and three-dimensional titania structures. The 1D, 2D, and 3D titania structures have been mainly designed for possible applications, e.g., (i) continuous use without the necessity of particulate titania separation, (ii) efficient light harvesting (e.g., inverse opals), (iii) enhanced activity (fast charge carriers’ separation, e.g., 1D nanoplates and 2D nanotubes). It should be pointed out that these structures might be also useful for mechanism investigation, e.g., (i) 3D titania aerogels with gold either incorporated inside the 3D network or supported in the porosity, and (ii) titania mesocrystals with gold deposited either on basal or lateral surfaces, for the clarification of plasmonic photocatalysis. Moreover, 0D nanostructures of special composition and morphology, e.g., magnetic(core)–titania(shell), mixed-phase titania (anatase/rutile/brookite), and faceted titania NPs have been presented, due to their exceptional properties, including easy separation in the magnetic field, high activity, and mechanism clarification, respectively. Although anatase has been usually thought as the most active phase of titania, the co-existence of other crystalline phases accelerates the photocatalytic activity significantly, and thus mixed-phase titania (e.g., famous P25) exhibits high photocatalytic activity for both oxidation and reduction reactions. It is believed that this review might be useful for the architecture design of novel nanomaterials for broad and diverse applications, including environmental purification, energy conversion, synthesis and preparation of “intelligent” surfaces with self-cleaning, antifogging, and antiseptic properties.

Ruthenium(II) complex with the phosphonate anchoring groups of [Ru(tbbpy)

Decahedral anatase particles (DAPs) with eight equivalent (101) facets and two (001) facets were prepared by the gas-phase process. Monometallic and bimetallic photocatalysts were prepared by photodeposition of silver and copper on DAP. It was found that the method of metal deposition (sequential/simultaneous) is crucial for resultant properties and thus for photocatalytic performance. The fastest hydrogen evolution during metal deposition was observed for copper deposited on premodified DAP with silver (DAP/Ag/Cu), probably due to partial coverage of silver with fine clusters of Cu and thus facilitation of proton adsorption and reduction on well-dispersed Cu nanoclusters. Although DAP/Ag/Cu exhibited the fastest rate of hydrogen evolution, single-modified DAP with silver exhibited the best performance for oxidative decomposition of organic compounds under vis irradiation.

TiO2 nanotubes arrays (NTs), obtained via electrochemical anodization of Ti foil, were modified with monometallic (Cu, Bi) and bimetallic (AgCu) nanoparticles. Different amounts of metals’ precursors were deposited on the surface of NTs by the spin-coating technique, and the reduction of metals was performed via gamma radiolysis. Surface modification of titania was studied by EDS and XPS analysis. The results show that AgCu nanoparticles exist in a Agcore-Cushell form. Photocatalytic activity was examined under UV irradiation and phenol was used as a model pollutant of water. Over 95% of phenol degradation was achieved after 60min of irradiation for almost all examined samples, but only slight difference in degradation efficiency (about 3%) between modified and bare NTs was observed. However, the initial phenol degradation rate and TOC removal efficiency was significantly enhanced for the samples modified with 0.31 and 0.63 mol% of Bi as well as for all the samples modified with Cu and AgCu nanoparticles in comparison with bare titania nanotubes. The saturated photocurrent, under the influence of simulated solar light irradiation, for the most active Bi- and AgCu-modified samples, was over two times higher than for pristine NTs. All the examined materials were resistant towards photocorrosion processes that enables their application for long term processes induced by light.