Electron-hole Recombination Centers Research Articles

UiO-66 uniformly assembled foldable polyimide films with high transmittance and excellent ultraviolet resistance

As a protective layer for the flexible solar cell, a flexible protective material should be of high transmittance, excellent ultraviolet (UV) resistance and mechanical property. Herein, a foldable transparent polyimide (PI) composite film assembled with the zirconium-based metal-organic frameworks (UiO-66-X, X = H, NH2, NO2) is prepared by the direct dispersion or in-situ growth methods. The results show that the transmittances of UiO-66-X@PI films (U@PI, NU@PI and OU@PI) prepared with the in-situ growth are higher than those of UiO-66-X/PI films prepared with the direct dispersion, and the former's mechanical properties are also better than those of the latter. This may be because that the UiO-66-X in the former are embedded into the PI matrix to construct multipoint bonding network. The U@PI exhibits stronger UV-resistance than the Pristine PI, NU@PI and OU@PI after 5000 equivalent sun hours (ESHs) UV irradiation. The attenuations of average transmittance and tensile strength for U@PI are 2.19 % and 18.68 %, respectively, which are only one half and one-third of those for the Pristine PI (4.59 % and 56.58 %), respectively. And the average transmittance of U@PI has been over that of the Pristine PI after 5000 ESHs UV irradiation. This may be because that the porous UiO-66 increases the UV transmission path and the electron-hole recombination center, thereby inhibits the destructive effect of UV.

First-principles study on the electronic and optical properties of pn-type SnO/MoS[formula omitted] heterojunction tuned by various intrinsic vacancy defects and layer thicknesses

The impact of various intrinsic point defects and layer thicknesses on the electronic structure and optical properties of pn-type SnO/MoS2 heterojunctions has been investigated using first-principles calculations in this study. SnO/MoS2 heterojunctions are indirect bandgap semiconductors, with a narrower bandgap compared to monolayer SnO and MoS2, which decreases with increasing thickness of SnO or MoS2 layers. Intrinsic vacancy defects with different valence states were introduced using the transfer to the real state model method, which ensures that ionized electrons or holes are placed in the real band edge states. The primary intrinsic vacancy defects in the SnO/MoS2 heterojunction are Sn vacancies, and the valence states of the defects significantly influence the defect formation energy. Under different growth conditions, the dominant defects transition between VSn2−, VSn−, VO, and VS as the Fermi level varies. The intrinsic monolayer (1/1) SnO/MoS2 heterojunction exhibits weak n-type doping characteristics, while the introduction of VSn− defects leads to a shift towards p-type doping. As the number of SnO layers increases, VSn2− and VS defects introduce defect levels of different degrees in the bandgap, acting as electron–hole recombination centers and facilitating the transition between p-type doping and metallic conductivity characteristics. Increasing the number of MoS2 layers introduces localized defect energy levels in the bandgap, leading to p-doping or n-doping characteristics. The intrinsic SnO/MoS2 heterojunction primarily absorbs in the near-ultraviolet region, while also exhibiting a certain absorption capacity in the visible and deep ultraviolet regions. The presence of Sn and S defects can enhance the response to visible light absorption.

Periodic hybrid-DFT study on the N-doped TiO2 (001) nanotubes as solar water splitting catalysts: A comparison with the rutile and anatase bulk phases

TiO2 is a routinely used catalyst due to its photocatalytic activity. The main problem of TiO2 is its large band gap. Doping metal and non-metal atoms in TiO2 crystals reduce the band gap. In this work, first-principle calculations by the full-potential numeric atom-center orbital theory were used to investigate detailed electronic properties of N-doped one monolayer (1 ML) and two monolayers (2 ML) (4,4), (8,8) and (16,16) TiO2 (001) nanotubes. The proposed model first validated by comparing its data with the experimental available data for rutile and anatase bulk phases, and then was used to predict the electronic properties of proposed nanotubes. PBE0 hybrid method was used to align the valence and conduction bands and to predict their standard reduction potential. In addition, different routine generalized gradient approximation (GGA) and meta-GGA exchange-correlation functionals were used to show how much their errors are in predicting the band gaps of nanotubes. Hybrid data showed that a 2 ML (8,8) nanotube is a proper candidate for photocatalyst not only for its band gap but also from its formation energy point of view. Contrary to the N-doped rutile and anatase phases, there is no electron-hole recombination center for the N-doped 2 ML (8,8) TiO2 nanotube.

Investigating of the anticancer activity of salen/salophen metal complexes based on graphene quantum dots: Induction of apoptosis as part of biological activity

This research work is the first report on the synthesis and stabilization of [Fe-Salophen] and [Fe-Salen] complexes by two methods of surface modification and anchoring of synthesized Schiff base ligand on the surface of graphene quantum dots (GQDs). The GQDs contain oxygenated functional groups that can act as non-radiative electron-hole recombination centers. Therefore removing these oxygen functional groups may improve quantum yield by reducing or deactivating the surface. In this work, GQDs with the amine functional group were synthesized with a quantum yield of 37.48%. The physicochemical properties of GQDs were investigated by Ultraviolet–visible (UV–Vis) and fluorescence spectroscopies, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Powder X-ray diffraction (PXRD), Transmission electron microscope (TEM). The synthesis of GQDs-[Fe-Salen] and GQDs-[Fe-Salophen] was evaluated by FT-IR, Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and Energy dispersive X-Ray analysis (EDX) analyses. Then, using MTT- assay, annexin V-FITC/PI, DAPI staining and cellular uptake assays, the biochemical activity of these complexes on the MCF7 cell line was investigated. The results shows that GQDs-[Fe-Salen] and GQDs-[Fe-Salophen] affect the survival of MCF7 cancer cells and, by nuclear fragmentation cause 35.77% and 19.41% of early apoptosis in cells, respectively. Also was found cellular uptake of GQDs-[Fe-Salen] is higher than that of GQDs-[Fe-Salophen].

Intrinsic Defects and the Inducing Conduction Mechanism of Langasite-Type High-Temperature Piezoelectric Crystals.

Increasing the crystal resistivity is critically important for enhancing the signal-to-noise ratio and improving the sensing capability of high-temperature piezoelectric sensors based on langasite-type crystals. The resistivity of structural ordered langasite-type crystals is much higher compared to that of the disordered crystals. Here, we selected structural ordered Ca3TaGa3Si2O14 (CTGS) and disordered La3Ga5SiO14 (LGS) as representatives to investigate the microscopic conduction mechanism and further reveal the origin of the different resistivities of the ordered and disordered langasite-type crystals at elevated temperatures. By combining first-principles calculations and experimental investigations, we found that the different conductivity behaviors of the ordered and disordered crystals originate from different types of point defects formed in the crystal and their different contributions to the conductivity. For the disordered LGS crystal, the oxygen vacancies are apt to be formed at high temperatures, promoting the transition of valence electrons and yielding high conductivity. For the ordered CTGS crystal, the dominant TaGa antisite defects can introduce an electron-hole recombination center in the electronic band gap, significantly shortening the carrier lifetime and thus reducing the conductivity. This provides effective guidance to improve the resistivity performance of langasite-type crystals at high temperatures by optimizing the experimental conditions, such as oxygen atmosphere treatment, antisite defect modification, etc.

Enhancing Hole Density and Suppressing Recombination Centers through Illumination in Kesterite Thin Film Solar Cells.

Enhancing carrier density and increasing carrier lifetime are critical for the good performance of thin film solar cells. We apply illumination during the growth of kesterite Cu2ZnSnS4 (CZTS) to enhance hole density and suppress defects of nonradiative electron-hole recombination centers simultaneously. To examine the effect of the injected carriers generated by illumination, we first extend the scheme of detailed balance equations relating free carriers and defects beyond thermal equilibrium conditions by developing an extended Fermi level (EF') to characterize a homogeneous semiconductor with non-equilibrium carriers. On the basis of this scheme, we find that illumination can promote the formation of carrier-providing defects and suppress the formation of carrier-compensating defects. Then, we demonstrate that applying proper illumination during the growth of CZTS will help achieve a higher hole density and simultaneously suppress the formation of the SnZn antisite significantly, which are beneficial for the performance of CZTS solar cells.

Competing Superior Electronic Structure and Complex Defect Chemistry in Quasi-One-Dimensional Antimony Chalcogenide Photovoltaic Absorbers

Antimony chalcogenides Sb2Se3 and Sb2S3 are an emerging class of quasi-one-dimensional van der Waals bonded materials with rapid progress and great promise for low-cost, high-efficiency, and scalable thin-film photovoltaics. Here using first-principles theoretical calculations, we show that antimony chalcogenides Sb2Se3 and Sb2S3 possess competing superior electronic structure and complex defect chemistry, limiting their photovoltaic performance. The high optical absorption, large Born effective charge, and high dielectric constant lead to low exciton binding energy and small hole effective mass, which ultimately benefit the photovoltaic performance. In contrast, quasi-1D antimony chalcogenides exhibit complex defect chemistry. The cation–anion antisites are the dominating native defects in the Sb rich condition, while in the Sb poor condition the anion–cation antisites are dominant. In both cases, the anion vacancies are also easy to form. These native defects introduce midgap transition levels which act as electron–hole recombination centers, limit the dopability, and lower the open circuit voltage. As a result, Sb2Se3 is a p-type semiconductor in Se rich condition, while Sb2S3 is an intrinsic semiconductor without significant free carrier density. Additionally, due to the unique quasi-1D structures and strong electron–lattice coupling, antimony chalcogenides are able to accommodate larger lattice relaxation than conventional 3D bulk crystals, resulting in negative-U behavior in their defect structures. Therefore, the competing superior electronic structure and complex defect chemistry suggest that, while antimony chalcogenides are excellent photoabsorbers, effort shall be made to suppress the formation of the detrimental native defects to further improve their open-circuit voltage, electronic transport, and, thus, power conversion efficiency.

Hydrogen-Anion-Induced Carrier Recombination in MAPbI3 Perovskite Solar Cells.

Identification and passivation of defect-induced electron-hole recombination centers are currently crucial for improving the efficiency of hybrid perovskite solar cells. Besides general intrinsic defects, experimental reports have indicated that hydrogen interstitials are also abundant in hybrid perovskite layers; however, few reports have evaluated the effect of such defects on the charge carrier recombination and device efficiencies. Here, we reveal that under I-poor synthesis conditions, the negatively charged monatomic hydrogen interstitial, Hi-, will form in the prototypical CH3NH3PbI3 perovskite layer, acting as a detrimental deep-level defect, which leads to efficient electron-hole recombination and lowers the cell performance. We further rationalize that Br doping can mitigate the large atomic displacement caused by the presence of Hi- and hence suppress the formation of the deep localized state. The results advance the knowledge of the deep-level defects in hybrid perovskites and provide useful information for enhancing solar cell performance by defect engineering.

Spatially heterogeneous ultrafast interfacial carrier dynamics of 2D-MoS2 flakes

Understanding the ultrafast carrier dynamics of transition metal dichalcogenides is critical for future applications in high-speed electronic and optoelectronic devices. In this work, we used a combination of femtosecond (fs) time-resolved micro-area photoelectron spectroscopy and photoemission electron microscopy (PEEM) to obtain the space-resolved surface photovoltage (SPV) and photoelectron intensity dynamics of two-dimensional (2D) molybdenum disulfide (MoS2) flakes, both of which exhibited high spatial heterogeneity and defect effects. Additionally, surface S vacancy defects were characterized by spatially resolved X-ray photoelectron spectroscopy. The SPV relaxation dynamics indicated that the charge carrier lifetime in the space-charge layer (SCL) ranged from several to tens of nanoseconds and was dominated by the thermionic emission process. The defects, which acted as electron-hole recombination centers, greatly shortened the charge carrier lifetime by almost one order of magnitude. Furthermore, three relaxation processes were observed in the photoelectron intensity dynamics, namely, two fast processes with rates of 3–6 ps (ps) and 60–100 ps, which were largely affected by the defect density, and one slow relaxation process with a rate of 2–5 ns (ns). However, these were not related to the defect density and were attributed to the transportation of electrons towards the bulk. Therefore, our results provide a deeper understanding of the interfacial carrier dynamics of 2D-MoS2 materials and the effects of defects on charge carrier lifetime in the SCL.

Structural phase transition and its consequences for the optical behavior of LaV1−xNbxO4

We present the structural, electronic, vibrational, and photoluminescence properties of polycrystalline ${\mathrm{LaV}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{4}$ $(x=0--0.2)$ samples at room temperature. The substitution of Nb at the V site shows the fascinating structural and optical behavior due to their isoelectronic character and larger ionic radii of ${\mathrm{Nb}}^{5+}$ as compared to the ${\mathrm{V}}^{5+}$. The Rietveld refinement of x-ray diffraction patterns demonstrate that the $x=0$ sample exists in a monoclinic $(P{2}_{1}/n)$ phase, whereas for the $x>0$, both monoclinic and scheelite-tetragonal $(I{4}_{1}/a)$ phases coexist in a certain proportion. Interestingly, a monotonous enhancement in the Raman spectral intensity with Nb substitution is correlated with the substitution induced increase in the scheelite-tetragonal phase. The x-ray absorption measurements reveal that the La ions exist in a trivalent oxidation state, while V and Nb cations possess $5+$ oxidation state in tetrahedral coordination. Moreover, the Fourier-transform infrared (FTIR) spectra indicate that the Nb substitution give origin to some additional IR modes owing to the deformation of the ${\mathrm{VO}}_{4}^{3\ensuremath{-}}$ tetrahedra and mixing of monoclinic and tetragonal phases. The photoluminescence measurements on these samples exhibit broadband spectra and their deconvolution designate the availability of more than one electron-hole pairs recombination center.

Preparation of hydrogen, fluorine and chlorine doped and co-doped titanium dioxide photocatalysts: a theoretical and experimental approach

Titanium dioxide (TiO2) has a strong photocatalytic activity in the ultra-violet part of the spectrum combined with excellent chemical stability and abundance. However, its photocatalytic efficiency is prohibited by limited absorption within the visible range derived from its wide band gap value and the presence of charge trapping states located at the band edges, which act as electron–hole recombination centers. Herein, we modify the band gap and improve the optical properties of TiO2 via co-doping with hydrogen and halogen. The present density functional theory (DFT) calculations indicate that hydrogen is incorporated in interstitial sites while fluorine and chlorine can be inserted both as interstitial and oxygen substitutional defects. To investigate the synergy of dopants in TiO2 experimental characterization techniques such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray and ultra-violet photoelectron spectroscopy (XPS/UPS), UV–Vis absorption and scanning electron microscopy (SEM) measurements, have been conducted. The observations suggest that the oxide’s band gap is reduced upon halogen doping, particularly for chlorine, making this material promising for energy harvesting devices. The studies on hydrogen production ability of these materials support the enhanced hydrogen production rates for chlorine doped (Cl:TiO2) and hydrogenated (H:TiO2) oxides compared to the pristine TiO2 reference.

High-performance and broadband photodetection of bicrystalline (GaN)1-x(ZnO)x solid solution nanowires via crystal defect engineering

Crystal defect engineering is widely used as an effective approach to regulate the optical and optoelectronic properties of semiconductor nanostructures. However, photogenerated electron-hole pair recombination centers caused by structural defects usually lead to the reduction of optoelectronic performance. In this work, a high-performance photodetector based on (GaN)1-x(ZnO)x solid solution nanowire with bicrystal structure is fabricated and it shows excellent photoresponse to ultraviolet and visible light. The highest responsivity of the photodetector is as high as 60, 86 and 43 A/W under the irradiation of 365 nm, 532 nm and 650 nm, respectively. The corresponding response time is as fast as 170, 320 and 160 ms. Such wide spectral responses can be attributed to various intermediate energy levels induced by the introduction of various structural defects and dopants in the solid solution nanowire. Moreover, the peculiar bicrystal boundary along the axial direction of the nanowire provides two parallel and fast transmission channels for photo-generated carriers, reducing the recombination of photo-generated carriers. Our findings provide a valued example using crystal defect engineering to broaden the photoresponse range and improve the photodetector performance and thus can be extended to other material systems for various optoelectronic applications.

Fluorescence “turn-on” of silicon-containing nanoparticles for the determination of resorcinol

A fluorescent nanosensor based on silicon-containing nanoparticles (Si CNPs) with green fluorescence (FL) was prepared by one-step method. The prepared Si CNPs emitted green FL at 470nm under the excitation at 350nm. The FL signal of Si CNPs reveals an obvious enhancement in the presence of resorcinol (RC), due to the passivation of surface trap states of Si CNPs via the binding of OH group of RC with the NH group of Si CNPs, which allowed the formation of new radiative electron-hole recombination centers. This was confirmed by some analytical experiments performed on zeta potential, FL lifetime steady state, and the FTIR spectra. Most importantly, this nanosensor could selectively determine RC with high sensitivity and without interference from hydroquinone (HQ) and catechol (CT) as RC isomers. RC was detected in the linear range 0.05-40μM, with a detection limit of 0.012μM. The synthesized nanosensor was applied to the determination of RC in fresh fruit juice and water samples. The collected results confirmed the feasibility of our approach with high accuracy.

Time-Resolved Spectroscopy of Defect-Induced Effects on Photocarrier Dynamics in SrTiO3 Powder

Photocatalytic water splitting has attracted much attention as an environment-friendly way to produce hydrogen gas as energy resources. Photocatalysts are usually used in powder form in view of their low cost and large surface area. Powder particles are rich in surface defects. It has been widely believed that defects work as electron-hole recombination centers and decrease the photocatalytic activity. Nevertheless, in fact, photocatalytic reactions proceed well with powders. Moreover, it has been reported that introducing more defects can sometimes elongate the photocarrier lifetime and improve the photocatalytic activity. However, the effects of powder defects on photocarrier dynamics have not been fully understood yet. Herein, we studied the effects of defects on photocarrier recombination processes by comparing powder and a defect-less single-crystalline SrTiO3. We also compared photocarrier dynamics in undoped and Na-doped SrTiO3 to investigate the roles of defects on photocatalytic reaction.A broadband time-resolved absorption spectroscopy was conducted to observe the photocarrier dynamics. In the case of the single-crystalline SrTiO3, most of the photoexcited electrons survive as free electrons without being trapped by defects. The absorptions by free electrons hardly decayed in the picosecond region. However, the absorption decayed almost single-exponentially in the nanosecond region, and the intensity decreased to less than one-hundredth the original value by 150 ns. On the other hand, in the case of the powder SrTiO3, a large portion of the photogenerated carriers are trapped by defects. The free and trapped electrons decayed rapidly even within a few picoseconds, where trapped electrons decreased more rapidly than free electrons. This result indicates that defects accelerate the recombination just after the excitation. However, the electrons in the powder decayed not in a single-exponential but a multi-exponential manner, and their decay decelerated gradually. Especially, the decay of the trapped electrons became slower than that of free electrons within ~200 ps. Some of the photocarriers survive into the millisecond region. When the decay of electrons in the single-crystalline and powder SrTiO3 were compared, the lifetime of photocarriers in the single crystal was longer than that in powder in a few nanoseconds. However, the photocarrier lifetime in powder became longer in the nanoseconds to milliseconds region than that in the single crystal.These results suggest that defects in powders initially accelerate the photocarrier recombination, but later they decelerate it. This dualistic property of defects in powder is considered to depend on whether electrons and holes are trapped in the vicinity or far away from each other. Powders contain a lot of defects, and most of the photoexcited carriers in powder are captured by these defects. Since the charge separation is not proceeded enough just after the excitation, electrons and holes will tend to be captured when in the vicinity of one another. The closely trapped electrons and holes can hop to approach each other. Therefore, the recombination will be accelerated in the picosecond region. However, once the electrons and holes escape from this initial recombination process, they can spread and get separated by longer distances. In this case, the electrons and holes need to travel long distances by repeated hopping or tunneling for recombination to come closer. This makes the photocarrier lifetime in powder longer than that in single crystal after the nanosecond region.To further investigate the effects of defects in photocatalysts, we next compared the transient absorption of 4 mol% Na-doped SrTiO3 with that of undoped SrTiO3. We found that the trapped electrons in Na-doped SrTiO3 have longer lifetime than those in undoped one, and the number of surviving electrons in microseconds increased ~15 times. It is well known that substitution of cations by lower valence cations increases oxygen vacancies to keep the charge neutrality. When Na ions are doped into SrTiO3, Sr ions tend to be substituted by Na ions. This results in increasing the number of oxygen vacancies, which works as electron trapping sites. Moreover, the decay of trapped electrons was accelerated by introducing O2 gas. This indicates that the trapped electrons can still keep the reactivity with reactant molecules. Finally, we measured the photocatalytic activities of these photocatalysts. The rate of hydrogen evolution from pure water increased by ~200 times by Na-doping. This drastic enhancement could be accounted for by the increase of trapped electrons.The results obtained in this study demonstrate that the trapped electrons are contributing to enhance the steady-state water splitting reactions. This confirms that appropriate defect control is important on improving the photocatalytic activity, because the long photocarriers lifetime increases the probability of photocarriers to meet with the reactant molecules.

Tuning the Carrier Lifetime in Black Phosphorene through Family Atom Doping

It is highly desirable to control the carrier lifetime in two-dimensional (2D) materials to suit the needs of various device functionalities. In this work, by ab initio nonadiabatic molecular dynamics simulation, we find the single atom doping from phosphorus family elements can sufficiently tune the carrier lifetime in black phosphorene (BP). Instead of forming electron-hole (e-h) recombination centers, the e-h recombination is suppressed by doping compared with the pristine BP. Moreover, it is found the carrier lifetime has a strong correlation with the mass of the doping atoms. A doping atom with larger mass leads to a longer lifetime. With the heaviest family element Bi doping, the carrier lifetime increases from 0.29 to 5.34 ns. This trend can be understood from the reduction of the nuclear velocity due to the heavy doping atom. We propose this conclusion can be extended to other monoelemental 2D semiconductors, which provides important guidance for the future design of functional nanodevices.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

Individual effect of charge balance defects on the photocatalytic activity of Cr3+-modified TiO2

Loss in the photocatalytic activity of Cr3+: TiO2 powders upon hydrogen annealing shows that charge-compensating oxygen vacancies VO and Cr6+ ions can have opposite effects on the kinetics of photocatalytic decolorization of methyl orange solution. The former ones inhibit this reaction acting as electron–hole recombination centers, and the latter ones promote it playing the role of active sites. This conclusion is verified by an XPS technique for a Cr3+-doped TiO2 sample and by 121Sb Mossbauer spectroscopy for a co-doped (Cr3+,Sb5+): TiO2 sample.

Suppression of Electron-Hole Recombination by Intrinsic Defects in 2D Monoelemental Material.

The Shockley-Read-Hall (SRH) model, in which the deep trap defect states in the band gap are proposed as nonradiative electron-hole (e-h) recombination centers, has been widely used to describe the nonradiative e-h recombination through the defects in semiconductor. By using the ab initio nonadiabatic molecular dynamics method, we find that the SRH model fails to describe the e-h recombination behavior for defects in 2D monoelemental material such as monolayer black phosphorus (BP). Through the investigation of three intrinsic defects with shallow and deep defect states in monolayer BP, it is found that, surprisingly, none of these defects significantly accelerates the e-h recombination. Further analysis shows that because monolayer BP is a monoelemental material, the distinct impurity phonon, which often induces fast e-h recombination, is not formed. Moreover, because of the flexibility of 2D material, the defects scatter the phonons present in pristine BP, generating multiple modes with lower frequencies compared with the pristine BP, which further suppresses the e-h recombination. We propose that the conclusion can be extended to other monoelemental 2D materials, which is important guidance for the future design of functional semiconductors.

Synthesis and Photocatalytic Properties of Tantalum Nitride with an Inverse Opal Structure

Certain oxynitrides (LaTiO2N and TaON) and nitrides (Ta3N5) demonstrate potential as visible-light-driven photocatalysts1. These materials are typically synthesized from oxide precursors by thermal ammonolysis at temperatures up to 950 oC. Accordingly, these oxynitrides and nitrides intrinsically possess an abundance of anion vacancies or defects which act as electron-hole pair recombination centers, thereby limiting photocatalytic performance. In this work, we introduce photonic band gap engineering as a strategy for suppressing electron-hole pair recombination in tantalum nitride thin films, thereby achieving improved photocatalytic performance. Briefly, a series of inverse opal (IO) Ta3N5 thin films with different macropore sizes were fabricated by the colloidal crystal template approach. This kind of approach has been successfully made on TiO2 photocatalysts2. Three batches of monodisperse polymethylmethacrylate (PMMA) colloids of diameters ranging from 230 to 430 nm were first synthesized by the surfactant-free emulsion polymerization of methyl methacrylate. PMMA colloidal crystal thin films were then grown on silica glass slides using a flow-controlled vertical deposition technique. As the meniscus of the suspension moved down the glass slide by pumping up a peristaltic pump, colloidal crystal layer of several microns were deposited. Several drops of TaCl5 solution were placed softly on the edge of the colloidal crystal and immersed into the crystal by capillary phenomena. Subsequently, the interstitial voids in the colloidal crystal films (synthetic opals) were filled with a tantalum hydroxide sol. Ammonolysis of the resulting composite films under an ammonia stream of 200 ml/min at 700ºC yielded IO Ta3N5 thin films, shown in Fig.1. The IO Ta3N5 films possess a 3-dimensionally ordered macroporous structure with periodicity on the scale of visible-light wavelengths, resulting in pseudo photonic band gaps at visible wavelengths. By matching the red-edge of the PBG location with the absorption edge of Ta3N5, electron-hole pair recombination is suppressed and the photocatalytic activity dramatically improved. In this experiment, strong reflection due to [111] photonic bandgap was clearly observed at 543nm for the IO amorphous Ta2O5 film and at 581nm for the IO Ta3N5 film, respectively. Appearance of the IO Ta3N5 film was reddish orange in color but it gave a strong yellow reflection in a certain angle, seen in Fig. 2. Since the optical bandgap of Ta3N5 was estimated to be 2.1eV (=590nm), the IO tantalum nitrides with both optical and bandgaps properly matched could be fabricated. Photocatalystic H2 evolution activity of the IO Ta3N5 under UV or visible ray irradiation will be appeared at the presentation, togather with a comparison with that of Ta3N5 powder. References Tsuyoshi Takata, Chengsi Pan and Kazunari Domen; Sci. Technol. Adv. Mater., 16 (2015) 033506 (18pp).Waterhouse et al., Sci. Rep., 3 (2013) 2849 (5pp). Figure 1

Synthesis of Zn-doped TiO2 nano-particles using metal Ti and Zn as raw materials and application in quantum dot sensitized solar cells

Zn-doping in TiO2 gives an n-type doped material and can tailor the optical band gap and Fermi level of TiO2. In the present work, using metal Ti and Zn as raw materials, Zn-doped TiO2 nano-particles are synthesized via chemical co-precipitation and calcination process. The effects of Zn-doping on the morphology, crystal structure, optical behavior of TiO2 films, and the performance of CdS/CdSe sensitized cells are systemically investigated, we focus on improving the power conversion efficiency of the cells based on Zn-doped TiO2 photoanodes. The results show that the solar cell assembled with the Zn0.1Ti0.9O2 film exhibits a short circuit current density of 12.32 mAcm−2, open-circuit voltage of 0.604 V, and a power conversion efficiency of 3.03%. Its power conversion efficiency is 1.5-fold higher than that of the cell assembled with the pure TiO2 film, which is ascribed to reducing electron recombination and enhancing electron transport in the TiO2 film by the optimal Zn doping. Further increasing the amount of Zn doping in the TiO2 film, the performance of the cell decreases, which is attributed to introducing a large number of ZnO clusters and producing more surface defects in the TiO2 film that act as electron–hole recombination centers.