Photocatalytic Performance Of TiO2 Research Articles

Effects of Nd-doping on crystal phase, structure and photocatalytic performance of TiO2 powders prepared by sol-gel method

Abstract In this work, 0.3–2 mol% neodymium (Nd) doped titanium dioxide (TiO2) powder was prepared by sol–gel method. The XRD showed that the anatase phase occur for the Nd doped TiO2 powders, and the Nd doping suppressed the original rutile phase for the pure TiO2 powder. SEM and TEM revealed that Nd doping reduced the tendency of severe aggregation and increased the specific surface area compare with pure TiO2 powder, and the Gibbs free energy difference did not reach the critical value of the phase transition driving force, while the adjustment of the Ti-O bond length caused by the addition of Nd3+ impeded the transformation of TiO2 to rutile phase. The photocatalytic activity test results for Methylene blue show that the 0.5 mol% Nd doped TiO2 had the best photocatalytic activity with the degradation efficiency of 96.2%.

Effect of water on formic acid and formaldehyde decomposition on the TiO2 (110) surface

The catalytic and photocatalytic performance of TiO2 in air pollutant degradation is significantly influenced by the adsorption of water. Here, we explore the molecular mechanism of formic acid and formaldehyde decomposition and their interaction with water on the TiO2 (110) surface, employing density functional theory (DFT) calculations and reactive molecular dynamics (RMD) simulations. RMD simulations indicate that due to the competitive adsorption of water at the surface Ti site, formic acid predominantly exists in a monodentate dissociative adsorption form on the TiO2 (110) surface. Moreover, on the TiO2 (110) surface, water molecules inhibit the adsorption and dissociation of formic acid, but they facilitate that of formaldehyde, forming methanediol (*CH2OHO) and dioxymethylene (*CH2O2). DFT calculations suggest that the formation pathway of *CH2OHO or *CH2O2 is energetically favorable, but the formation of formate is difficult. This contrasts with the adsorption of formaldehyde on the anatase surface, where formate species are formed.

Effect of Yttrium doping on enhancing the photocatalytic performance of TiO2/GO nanocomposite

Pure and Yttrium doped Titania/Graphene oxide (YTGO) nanocomposites were synthesized by hydrothermal method. XRD analysis confirmed the crystal structure and anatase phase of TiO2 nanoparticles. The anatase phase of TiO2 has been retained in the composite as confirmed from the XRD pattern. SEM and TEM images revealed that the spherical TiO2 nanoparticles are embedded into sheet like structures of GO. The size of the spherical nanoparticles in the composites is relatively smaller than that of pure sample due to anchoring on the GO sheets. The photocatalytic performance of TiO2 has significantly increased in the Y-doped TiO2 with GO compared to pure TiO2 and TiO2/GO (TGO). Moreover, 0.7 % Y-doped TiO2/GO nanocomposite exhibited superior photocatalytic performance with overall efficiency of 97 % compared to other samples. The 0.7 % Y-doped TiO2/GO nanocomposite shows higher rate constant and shorter half life time compared to other samples. The enhancement of photocatalytic activity of YTGO composites attributed to the combination of increased absorption due to the absorption edge shift and effective separation of photo generated carriers.

Effect of varied amount of photocatalytically active TiO2 loaded on AlON carrier-type photocatalyst

An AlON carrier with light-transmitting performance and through-hole structure was prepared by carbothermal reduction and nitridation (CRN). The TiO2/AlON photocatalyst was prepared by hydrothermal method. Under vertical irradiation of the light source, the degradation rate of MB by TiO2(15%)/AlON photocatalyst was 97.3% after 180 min. And 88.8% MB was mineralized by determining the total organic carbon in the solution. An increase in TiO2 loading can accelerate the efficiency of TiO2/AlON degradation of MB within 0–90 min. At the same time, it will also exacerbate the blockage of AlON carrier and increase the amount of free TiO2. These can all exacerbate light loss, thereby shortening the penetration distance of light in the mixed liquid. Within the range of 90–180 min, the photocatalytic performance of TiO2(15%)/AlON powders was better than that of TiO2(25%)/AlON (97.3% vs 82.5%). In addition, TiO2/AlON photocatalyst was easy to cycle and has stable photocatalytic performance.

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.

Bi-element gradient doped B/N-TiO2-x synthesized by one-step solvothermal method enhances the photocatalysis of Cr(VI)

Studying the influence of non-uniform distribution of bi-element doping on the surface structure and photocatalytic performance of TiO2 in interface photocatalytic processes can be an interesting research topic. Taking B/N bi-element doped TiO2 as an example, this article reports an extremely simple gradient doping method of TiO2 and its enhancement mechanism for high concentration wastewater treatment. The surface enrichment and doping of B caused by electrostatic and atomic radius factors changed the charge properties of the complex during the induction of in-situ reduction of Ti4+. The defects in gap N and Ti3+ lead to the formation of intermediate states, resulting in a narrowing of the band gap of the composite to 2.58 eV. The gradient doping of two atoms endows the optimal photocatalyst with a larger specific surface area (114 m2∙g), stronger photocurrent response (0.179 mA·cm−2), suitable Ti3+/Ov double defect structure, and surface positive charge properties. These characteristics improve the effective treatment of high-concentration pollutants by the preferred B2/N-TiO2-x [e.g., 0.0059 min−1 for 0.5 g·L-1 Cr(VI)] and sustainable treatment (removal efficiency > 90 % within 5 cycles). The synthesis strategy will provide a fast, simple, and safe method for the design and synthesis of new TiO2 based catalysts in the future.

Effects of intrinsic point defects (Tii-Vo) on the photocatalytic performance of bilayer rutile/anatase TiO2

Despite the progress on the photocatalytic performance of TiO2 as a typical photocatalytic material, previous research has overlooked the systematic study of the coexistence of Tii and Vo on the photocatalytic property of rutile or anatase TiO2 with bilayers. Studies on the comparative photocatalytic property of these materials have been rarely reported. Thus, the influences of point defects (Tii-Vo) on the photocatalytic performance of bilayer rutile or anatase TiO2 were investigated by GGA + U calculations under density general function theory framework. Results showed that the bilayer rutile Ti24TiiO47 (1 1 0) system exhibited the highest relative stability, strong carrier activity, reduced carrier recombination, and the longest lifetime. Furthermore, the absorption spectrum is clearly red shifted. This system is beneficial for the dissociation of water to produce H2 and O2. This research provides certain theoretical reference value on the design and fabrication of novel bilayer rutile TiO2(1 1 0) surface photocatalysts.

The effects of water, substrate, and intermediate adsorption on the photocatalytic decomposition of air pollutants over nano-TiO2 photocatalysts.

The photocatalytic performance of nano-TiO2 photocatalysts in air pollutant degradation greatly depends on the adsorption of water, substrates, and intermediates. Especially under excessive humidity, substrate concentration, and intermediate concentration, the competitive adsorption of water, substrates, and intermediates can seriously inhibit the photocatalytic performance. In the past few years, extensive studies have been performed to investigate the influence of humidity, substrate concentration, and intermediates on the photocatalytic performance of TiO2, and significant advances have been made in the area. However, to the best of our knowledge, there is no review focusing on the effects of water, substrate, and intermediate adsorption to date. A comprehensive understanding of their mechanisms is key to overcoming the limited application of nano-TiO2 photocatalysts in the photocatalytic decomposition of air pollutants. In this review, the progress in experimental and theoretical fields, including a recent combination of photocatalytic experiments and adsorption and photocatalytic simulations by density functional theory (DFT), to explore the impact of adsorption of various reaction components on nano-TiO2 photocatalysts is comprehensively summarized. Additionally, the mechanism and broad perspective of the impact of their adsorption on the photocatalytic activity of TiO2 in air treatment are also critically discussed. Finally, several solutions are proposed to resolve the current problems related to environmental factors. In general, this review contributes a comprehensive perspective of water, substrate, and intermediate adsorption toward boosting the photocatalytic application of TiO2 nanomaterials.

Synergistic doping with Ag, CdO, and ZnO to overcome electron-hole recombination in TiO2 photocatalysis for effective water photo splitting reaction.

This manuscript is dedicated to a comprehensive exploration of the multifaceted challenge of fast electron-hole recombination in titanium dioxide photocatalysis, with a primary focus on its critical role in advancing the field of water photo splitting. To address this challenge, three prominent approaches-Schottky barriers, Z-scheme systems, and type II heterojunctions-were rigorously investigated for their potential to ameliorate TiO2's photocatalytic performance toward water photo splitting. Three distinct dopants-silver, cadmium oxide, and zinc oxide-were strategically employed. This research also delved into the dynamic interplay between these dopants, analyzing the synergetic effects that arise from binary and tertiary doping configurations. The results concluded that incorporation of Ag, CdO, and ZnO dopants effectively countered the fast electron-hole recombination problem in TiO2 NPs. Ag emerged as a critical contributor at higher temperatures, significantly enhancing photocatalytic performance. The photocatalytic system exhibited a departure from Arrhenius behavior, with an optimal temperature of 40°C. Binary doping systems, particularly those combining CdO and ZnO, demonstrated exceptional photocatalytic activity at lower temperatures. However, the ternary doping configuration involving Ag, CdO, and ZnO proved to be the most promising, surpassing many functional materials. In sum, this study offers valuable insights into how Schottky barriers, Z-scheme systems, and type II heterojunctions, in conjunction with specific dopants, can overcome the electron-hole recombination challenge in TiO2-based photocatalysis. The results underscore the potential of the proposed ternary doping system to revolutionize photocatalytic water splitting for efficient green hydrogen production, significantly advancing the field's understanding and potential for sustainable energy applications.

The inhibitory mechanism of humic acids on photocatalytic generation of reactive oxygen species by TiO2 depends on the crystalline phase

To enable the practical use of TiO2 for photocatalytic water treatment, it is important to understand how dissolved natural organic matter affects its photocatalytic activity and the mechanisms involved in generating reactive oxygen species (ROS). In this study, we systematically investigated the inhibitory effects of humic acids (HA) on 4-chlorophenol (4-CP) degradation using two common TiO2 crystalline phases (anatase and rutile) individually. HA strongly hindered the photocatalytic activity of anatase, with an Ra (the ratio of the first-order rate constant for 4-CP degradation in the absence vs. presence of 30 mg/L HA) of 2.30 (±0.13). This ratio was significantly higher than that of rutile (Rr = 1.21 ± 0.08), which was less susceptible to this inhibitory effect and even exhibited improved photocatalytic activity at HA concentrations below 20 mg/L. Similar trends were observed for various HA sources, corroborating the crystalline-phase-dependent effect of HA. Adsorption experiments, Fourier transform infrared spectroscopy, and photoelectrochemical analyses suggested that the adsorption mechanisms and hole-scavenging effect of HA on the two TiO2 polymorphs did not differ. Importantly, scavenger, probe, and electron spin resonance experiments revealed that the difference in inhibitory effects of HA originate from the distinct ROS generation mechanisms for the two polymorphs. The oxygen reduction pathway for OH generation over anatase was hindered by surface-adsorbed HA, while the water oxidation pathway for OH generation over rutile was less affected. Furthermore, surface-adsorbed HA boosted O2− generation on rutile, increasing 4-CP degradation efficiency. This mechanistic insight into NOM-TiO2 interactions informs materials selection and strategies for higher TiO2 photocatalytic performance in different water matrices.

Synthesis of Tio2 photocatalyst with tunable optical properties and exposed facet for textile wastewater treatment

TiO2 semiconductor plays a paramount important as an active photocatalyst mainly due to its photo-induced electron transfer properties. The metastable anatase in particular are widely accepted to have the most photoactive activity in catalytic application as opposed to rutile TiO2. This is majorly due to the significantly lower recombination rate of electron pair possesses by anatase TiO2. Unfortunately, high recombination rate in anatase TiO2 hinders its application as a functional photocatalyst. Therefore, it is crucial to further enhance the photocatalytic activity of anatase TiO2, by modifying its optical properties, morphological properties, and crystal facet. The study aims to provide a benign synthesis method for TiO2 with a co-exposed {001}/{100} anatase TiO2 facet. The synthesis was conducted via one-pot reflux condensation method. A concise evaluation upon the effect of different synthesis heating duration, temperature and doping materials towards the crystal phase, morphology, and photocatalytic performance of TiO2 were conducted. Anatase TiO2 with high percentage of the high-energy {001} facet have the ability to boost the adsorption and photocatalytic performance as opposed to anatase TiO2 dominated with a thermodynamically stable {101} facet. The incorporation of non-metal dopant (HF) acts as surface modifier which leads to the enhancement of {001} facet percentage. This study further evaluates the functionalities of HCl in paving way towards the formation of mid-gap state, hence improving Anatase TiO2 optical properties. The reduction of band gap of the anatase TiO2 can clearly be identified from the shifts in the adsorption band towards higher wavelength. From the findings, it was found that the synergistic effect upon co-doping anatase TiO2 with the two non-metal dopants plays an important role in altering the ratio of exposed facet hence leading towards improving the photocatalytic efficiency of the nanomaterial. To evaluate the photocatalytic performance of the synthesized TiO2, Remazol Brilliant Blue (0.01 g/L) was chosen as the target pollutant. The enhanced photodegradation of co-doped F/Cl-TiO2 was evaluated and it was found that 87.21% and 43.21% of Remazol Brilliant Blue had successfully been degraded within 120 min with irradiation of broad spectrum and blue light (475 nm) respectively. This is mainly attributed to the co-exposed {001}/{100} anatase TiO2 facet presence in the F/Cl-TiO2 sample.

STUDY ON PHOTOCATALYTIC PERFORMANCE OF TiO2 FILM PREPARED ON Q235A STEEL AND ITS DEGRADATION OF ORGANIC POLLUTANT IN SIMULATED WASTEWATER

TiO2 film is prepared on the surface of Q235A substrate from an acid solution ([Formula: see text]) to research its degradation of methylene blue organic pollutant in wastewater. The titanium oxysulfate is used as the titanium precursor to provide the source of titanium. The preparation of TiO2 film is mainly a polymerization reaction of titanium oxysulfate, which makes Ti2O[Formula: see text] and OH− combine to produce [Ti2O5(OH)2]n precipitation deposited on the cathode surface by electrochemical reaction. TiO2 film is formed after high temperature heating. The nitrate ion which derives from potassium nitrate in the electrolyte is reduced to hydroxide ion during the electrochemical reaction process, resulting in the formation of titanium compounds and titanium hydroxide. The TiO2 film prepared from solution containing 0.2[Formula: see text]mol/L KNO3 has the best photocatalytic performance which can effectively degrade the methylene blue organic pollutant in wastewater. However, when the KNO3 in the solution is 0.25[Formula: see text]mol/L, excessive hydroxide ions inhibit the hydrolysis of titanium salts and accelerate the oxygen evolution resulting in the decrease of degradation efficiency.

An investigation of the role of samarium on the sol stability, particle growth, optical, and photocatalytic performance of TiO2

An investigation of the role of samarium on the sol stability, particle growth, optical, and photocatalytic performance of TiO2

A novel macro-meso-micro intimate heterogeneous structure constructed 3DOM TiO2/rGO/TiN composite for efficiently solar photocatalysis degradation of organic dye pollutants

The new three-dimensionally ordered macroporous titanium dioxide/reduced graphene oxide/titanium nitride (3DOM TiO2/rGO/TiN) ternary nanocomposites are designed based on the pore structure of 3DOM for efficient photocatalytic dye degradation. A series of structural, optical, and photoelectrochemical characterizations demonstrate that the remarkably improved photocatalytic performance is attributed to the combined effects of macro-meso-micro architecture, abundant reactive sites, improved light response range, and efficient photogenerated charges-holes pair separation and transport. By optimizing the loading amounts of rGO and TiN, 3DOM TiO2/10%rGO/5%TiN nanocomposite exhibits the highest photocatalytic degradation activity, which is 7.4 times higher than 3DOM TiO2. Additionally, the 3DOM TiO2/rGO/TiN ternary photocatalyst shows excellent acid-base resistance and reusability. The mechanism by which 3DOM structure, 2D rGO, and TiN synergy enhance the TiO2 photocatalytic performance was fully revealed comprehensively. This work provides an effective strategy for designing TiO2-base macro-meso-micro intimate heterogeneous structure for photocatalysis degradation of dyes.

Modulation of the Structure of the Conjugated Polymer TMP and the Effect of Its Structure on the Catalytic Performance of TMP–TiO2 under Visible Light: Catalyst Preparation, Performance and Mechanism

The photocatalytic activity of titanium dioxide (TiO2) is largely hindered by its low photoresponse and quantum efficiency. TiO2 modified by conjugated polymers (CPs) is considered a promising approach to enhance the visible light responsiveness of TiO2. In this work, in order to investigate the effect of CP structural changes on the photocatalytic performance of TiO2 under visible light, trimesoyl chloride-melamine polymers (TMPs) with different structural characteristics were created by varying the parameters of the polymerisation process of tricarbonyl chloride (TMC) and melamine (M). The TMPs were subsequently composited with TiO2 to form complex materials (TMP-TiO2) using an in situ hydrothermal technique. The photocatalytic activity of TMP-TiO2 was evaluated by the degradation of rhodamine B (RhB). The results showed that the trend of the structure of the TMP with the reaction conditions was consistent with the visible light responsiveness of TMP-TiO2, and TMP (1:1)-TiO2 had the best photocatalytic activity and could degrade 96.1% of the RhB. In conclusion, our study provided new insights into the influence of the structural changes of TMPs on the photocatalytic activity of TMP-TiO2 under visible light, and it improves our understanding of how conjugated polymers affect the photocatalytic activity of TiO2 under visible light.

The impact of Carbon Quantum Dots (CQDs) on the photocatalytic activity of TiO2 under UV and visible light

In this study, the impact of carbon quantum dots (CQDs) on the photocatalytic performance of TiO2 in aqueous medium was investigated under UV and visible light. CQDs were synthesized through a solvothermal procedure, and the effect of synthesis conditions on their optical properties were optimized. The CQDs/TiO2 nanocomposites were prepared via a hydrothermal method using CQDs with the extreme band-gaps (i.e. CQDs450 and CQDs550). The physicochemical properties of CQDs and CQDs/TiO2 samples were studied by various characterization techniques. According to the High-Resolution Tunneling Electron microscopy (HRTEM) micrographs, the synthesized spherical CQDs had diameters smaller than 5 nm. The Diffuse Reflectance Spectroscopy (DRS) analysis revealed that incorporation of CQDs into nanocomposite samples causes a remarkable shift in absorption edge to the visible region, in comparison with pure TiO2. According to the experimental data, the nanocomposite sample containing CQDs with the higher absorption edge, i.e. CQDs550/TiO2, showed a superior performance in photocatalytic removal of methylene blue (MB) under visible light. In this regard, the removal percent of MB on CQDs550/TiO2 and CQDs450/TiO2 nanocomposites, with 4%wt CQDs, under visible light (λ > 420 nm) were 92.1 % and 53.4 % after 120 min of irradiation at pH = 11 and 150 mgcat./L catalyst dosage, which are respectively 2.4 and 1.4 times higher than that on pure TiO2. However, under UV irradiation the observed photocatalytic performances were in the reverse order. Finally, two different reaction mechanisms have been proposed to explain the obtained results on CQDs/TiO2 samples, based on possible electron transfer between TiO2 and CQDs nanoparticles under UV and Visible lights.

Comparative Photocatalytic Activity and Total Organic Carbon Removal Efficiency of TiO2 And ZnO for Reactive Black 5 Photodegradation

AbstractThe photocatalytic performance of TiO2 and ZnO was comparatively studied for photocatalytic degradation of reactive black 5 (RB5) under artificial UVC irradiation. The effect of initial solution pH, catalyst concentration, initial RB5 concentration, purging oxygen rate, stirring rate, and process temperature on the degradation process by TiO2 and ZnO was investigated. In addition, total organic carbon (TOC) analysis has been employed for each parameter to detect the presence of organic intermediates and the photodegradation performance of catalysts simultaneously with the photocatalytic degradation process. The RB5 photodegradation was more rapid in the aqueous medium by TiO2 under UVC irradiation compared to the catalysis by ZnO. On the other hand, the slower photocatalysis by ZnO resulted in generating more organic intermediates. The structural and morphological analyses of photocatalysts TiO2 and ZnO were performed using XRD and FE‐SEM techniques. The UV‐vis spectroscopy measurements supported that the TiO2 demonstrated evidently better photocatalytic activity than ZnO for RB5 degradation.

Influence of calcination temperature of TiO2 nanowires via hydrothermal method for photocatalytic degradation

TiO2 nanowires were prepared by hydrothermal method using tetrabutyl titanate as raw material and then calcined at various temperatures. The samples were characterized by Xray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The photocatalytic performance of TiO2 was analyzed by the degradation of Rhodamine B (RhB). The formation mechanism of TiO2 nanowires was revealed in this study. The results showed that the calcination temperature had a great influence on the crystal structure and morphology of TiO2 nanowires. The TiO2 sample calcined at 650 °C exhibited higher photocatalytic activity due to the enhanced crystallinity and the nanowire structure with large specific surface area. As the calcination temperature exceeded 650 °C, the nanowire structure collapsed, leading to a decrease in the photocatalytic degradation efficiency

Study on the design and photocatalytic performance of TiO2@g–C3N4–Ag/Ag2O ternary heterojunction nanomaterials

In this work, we designed and prepared the TiO2@g–C3N4–Ag/Ag2O (TCNA) ternary nanoparticle materials in a simple and low-cost hydrothermal method, in which, small Ag/Ag2O nanoparticles were uniformly distributed on a layer of thin g-C3N4 nanosheets coated TiO2 nanospheres. By carefully changing the ratios of surface distributed Ag/Ag2O nanoparticles, the photocatalytic degradation performance of the prepared TCNA can be easily changed. Under the modified catalytic device, we found that a high degradation rate of about 99% at 90 min in degrading RhB can be obtained by TCNA-2 reached, which is 4.7 and 3.6 times higher than the photodegradation rate constants of pure TiO2 and TiO2@g-C3N4 composites, respectively. Furthermore, the prepared TCNA nanomaterials also showed high photocatalytic stability and good cycling properties. The scavengers study indicated that the ·O2− and ·OH were the main active groups of the obtained ternary photocatalysts. Subsequently, we proposed a suitable mechanism for exposing the high photocatalytic performance of our TCNA ternary nanomaterials, which may have a positive influence on the design and preparation of other high-performance ternary photocatalytic nanomaterials.

Interesting molecular adsorption strategy induced surface charge redistribution of commercial TiO2: Boosting 9 times photocatalytic hydrogen production performance

A higher conduction band position and more photogenerated electrons are distinctly important for the high-efficiency photocatalytic hydrogen evolution performance. However, raising conduction band position will enlarge the band gap and reduce the production of photogenerated electrons. Herein, a super simple and efficient ammonia molecular adsorption strategy was proposed and designed to promote efficient hydrogen production by a simple ammonia molecules adsorption on commercial TiO2 surface. The experimental and theoretical calculation results indicated that aniline can be successfully adsorbed on TiO2 surface by a straightforward mechanical grinding method, and the photocatalytic hydrogen production performance of TiO2 can be improved by more than 9 times after the adsorption of aniline. This is because the adsorbed aniline can provide electrons to the TiO2 surface and lead to a charge redistribution and accumulation on the surface of TiO2. Excessive electrons make the conduction band of TiO2 bend upward, thus improving the photogenerated electron reduction ability and the photocatalytic performance of TiO2. In addition, the adsorption of aniline enables TiO2 to show light absorption in the visible region, generating more photogenerated electrons to participate in the photocatalytic water splitting reaction. In this work, the molecular adsorption strategy is not only simple to operate, but also can improve the photoreduction ability and surface charge quantity of materials. It provides a simple and efficient new method for photocatalyst modification in addition to the traditional modification methods such as doping compounds.