Morphological and photoelectrochemical characterization of core-shell nanoparticle films for dye-sensitized solar cells: Zn-O type shell on SnO2 and TiO2 cores

Core-shell type nanoparticles with SnO2 and TiO2 cores and zinc oxide shells were prepared and characterized by surface sensitive techniques. The influence of the structure of the ZnO shell and the morphology ofnanoparticle films on the performance was evaluated. X-ray absorption near-edge structure and extended X-ray absorption fine structure studies show the presence of thin ZnO-like shells around the nanoparticles at low Zn levels. In the case of SnO2 cores, ZnO nanocrystals are formed at high Zn/Sn ratios (ca. 0.5). Scanning electron microscopy studies show that Zn modification of SnO2 nanoparticles changes the film morphology from a compact mesoporous structure to a less dense macroporous structure. In contrast, Zn modification of TiO2 nanoparticles has no apparent influence on film morphology. For SnO2 cores, adding ZnO improves the solar cell efficiency by increasing light scattering and dye uptake and decreasing recombination. In contrast, adding a ZnO shell to the TiO2 core decreases the cell efficiency, largely owing to a loss of photocurrent resulting from slow electron transport associated with the buildup of the ZnO surface layer.     

Efficient synthetic access to cationic dendrons and their application for ZnO nanoparticles surface functionalization: new building blocks for dye-sensitized solar cells

A new concept for the efficient synthesis of cationic dendrons, 4-tert-butyl-1-(3-(3,4-dihydroxybenzamido)benzyl)pyridinium bromide (17), 1,1'-(5-(3,4-dihydroxybenzamido)-1,3-phenylene)bis(methylene)bis(4-tert-butylpyridinium) bromide (18), N1,N7-bis(3-(4-tert-butyl-pyridium-methyl)phenyl)-4-(3-(3-(4-tert-butyl-pyridinium-methyl)phenyl-amino)-3-oxopropyl)-4-(3,4-dihydroxybenzamido)heptanediamide tribromide (19), and N1,N7-bis(3,5-bis(4-tert-butyl-pyridium-methyl)phenyl)-4-(3-(3,5-bis(4-tert-butyl-pyridinium-methyl)phenylamino)-3-oxopropyl)-4-(3,4-dihydroxybenzamido)heptanediamide hexabromide (20), and their facile binding to zinc oxide (ZnO) nanostructures is introduced. Dendrons containing highly reactive benzylic bromides reacted readily with 4-tert-butyl-pyridine and resulted in cationic dendrons. Furthermore, these permanently positively charged dendrons were equipped with a catechol anchor group. This enabled ZnO surface functionalization by simple immersion. The adsorption of 17, 18, 19, and 20 on the colloidal nanoparticles was monitored by Langmuir isotherms. The highest obtained experimental loadings correspond to 99.5%, 98.6%, 99.1%, and 42.5% of the particle surface for 17, 18, 19, and 20, respectively. These results indicate insufficient adsorption of the largest molecule 20 leading to reduced colloidal stability of the nanoparticles, while an enhanced stability after grafting with 17, 18, and 19 was observed. Mesoporous films suitable for the use as electrodes in dye-sensitized solar cells (DSSCs) were prepared. Subsequently, the films were functionalized with 18, 19, or 20 and sensitized with zinc-5,15-bis-[2',6'-bis-{2',2'-bis-(carboxy)-ethyl}-methyl-4'-tert-butyl-pheny]-10,20-bis-(4'-tert-butylphenyl)porphyrin-octasodium-salt. UV-vis absorption spectra confirmed that 18, 19, and 20 are suitable for the stable electrostatic attachment of the dye. Current-voltage characteristics of complete cells demonstrated that increasing positive functionalization of the ZnO surface leads to decreased open circuit voltages (V(oc)). All V(oc) values were around 0.4 V with a maximum for the 18 functionalized ZnO film of 0.45 V. The maximum cell efficiency obtained (0.31%) is rather high, considering the narrow spectral absorption of the dye and the rather thin ZnO films used. Finally, incident photon to current efficiency (IPCE) measurements confirmed photoinduced electron injection from the dye. These features are important assets for applications in particle technology and even facilitated advanced devices like a supramolecular DSSC complete with a subsequent layer of negatively charged porphyrins.     

可见光响应Bi2WO6薄膜的制备与光电化学性能

采用非晶态配合物-提拉法在ITO导电玻璃基底上制备得到Bi2WO6薄膜. 采用FE-SEM、XRD、Raman、DRS、光电流响应谱、IPCE等手段, 研究了Bi2WO6薄膜的形貌、结构、光电性能以及薄膜结构与光电性能的关系. 结果表明, 450 ℃以上煅烧可以得到Bi2WO6结晶薄膜, 薄膜由沿(131)晶面趋向生长的Bi2WO6纳米颗粒组成, 颗粒的粒度随煅烧温度的升高而增大, 同时颗粒之间的间距也相应增大. ITO/Bi2WO6薄膜电极在可见光(λ>400 nm)照射下可以产生光电流, 光电流强度与光强度线性相关; 光电流强度和光电转换量子效率受Bi2WO6薄膜结构的影响, 通过控制薄膜的煅烧温度等制备条件, 可以提高薄膜光电极的光电转换量子效率.     

Preparation of ZnO nanoparticles and characteristics of dye-sensitized solar cells based on nanoparticles film

ZnO nanoparticles were synthesized by hydrolization method and the effects of zinc nitrate concentration and reaction temperature on the resulted particle properties were studied. The transmission of the as-prepared and calcined films and their optical band gaps are measured and calculated respectively. Furthermore, as an application of the ZnO nanoparticles film, dye-sensitized solar cells based on it were successfully fabricated and the cell performances were characterized. The short circuit current for ZnO nanoparticles film DSSCs is 1.35 mA cm^?2, which indicates good value of the prepared film using this technique.     

Synthesis and characterization of ZnO and ZnO:Ga films and their application in dye-sensitized solar cells

Highly crystalline ZnO and Ga-modified zinc oxide (ZnO:Ga) nanoparticles containing 1, 3 and 5 atom% of Ga3+ were prepared by precipitation method at low temperature. The films were characterized by XRD, BET, XPS and SEM. No evidence of zinc gallate formation (ZnGa2O4), even in the samples containing 5 atom% of gallium, was detected by XRD. XPS data revealed that Ga is present into the ZnO matrix as Ga3+, according to the characteristic binding energies. The particle size decreased as the gallium level was increased as observed by SEM, which might be related to a faster hydrolysis reaction rate. The smaller particle size provided films with higher porosity and surface area, enabling a higher dye loading. When these films were applied to dye-sensitized solar cells (DSSCs) as photoelectrodes, the device based on ZnO:Ga 5 atom% presented an overall conversion efficiency of 6% (at 10 mW cm(-2)), a three-fold increase compared to the ZnO-based DSSCs under the same conditions. To our knowledge, this is one of the highest efficiencies reported so far for ZnO-based DSSCs. Transient absorption (TAS) study of the photoinduced dynamics of dye-sensitized ZnO:Ga films showed that the higher the gallium content, the higher the amount of dye cation formed, while no significant change on the recombination dynamics was observed. The study indicates that Ga-modification of nanocrystalline ZnO leads to an improvement of photocurrent and overall efficiency in the corresponding device.     

Preparation of ZnO nanoparticles in a reverse micellar system and their photoluminescence properties

ZnO nanoparticles with spherical morphology and narrow size distribution were obtained by calcination of Zn(OH)2 nanoparticles, which were prepared in a polyethylene glycol mono-4-nonylphenyl ether (NP-5)/cyclohexane reverse micellar system and incorporated into polyurea (PUA) via an in situ polymerization of hexamethylene diisocyanate (HDI). The resulting ZnO nanoparticles demonstrated a near-UV emission and a green emission, the intensity ratio of which depended on calcination conditions. For the nanoparticles studied, the calcination atmosphere influenced remarkably the photoluminescence properties such as intensity ratio of the near-UV emission to green emission, rather than the size, morphology, and crystallinity of the ZnO nanoparticles. The green emission decreased by calcination in O2 flow but increased by calcination in N2 flow, as compared with the case calcined in air flow. This finding suggests that the green emission is enhanced with the increase of the number of oxygen vacancies of the ZnO nanoparticles and thus the photoluminescence properties of the nanoparticles were successfully controlled by the calcination condition, without changing the size and morphology.     

Photoelectrochemical study on photosynthetic pigments-sensitized nanocrystalline ZnO films

Hetero-structures formed by quantum-sized ZnO nanocrystals and photosynthetic pigments were prepared by adsorbing either chlorophyll a, carotenoids or their mixture onto a film of organic-capped ZnO nanoparticles. Photoelectrochemical measurements were comparatively performed on both bulk and nanocrystalline ZnO films after dye-covering in order to probe the photosensitization process occurring at the hetero-junction. The photoconversion process was found to be greatly enhanced at the nanocrystalline electrodes upon sensitization with a dye mixture. The sensitization process is discussed on the basis of the aggregation state of chlorophyll a, and of the specific photoprotective action played by carotenoids.     

Synthesis of ZnO nanoparticles on a clay mineral surface in dimethyl sulfoxide medium

Nanocrystalline ZnO particles have been prepared with different methods using zinc cyclohexanebutyrate as precursor in dimethyl sulfoxide (DMSO) medium via alkaline hydrolysis. A series of preparations were carried out in the presence of layered silicates (kaolinite and montmorillonite). It was revealed by different measurement techniques that the presence of the clay minerals has a stabilization influence on the size of the ZnO nanocrystals. UV-vis absorption spectra show a blue shift when the nanoparticles are prepared in the presence of the clay minerals. The average particle diameters calculated from the Brus equation ranged from 2.6 to 13.0 nm. The UV-vis spectra of the synthesized nanoparticles did not show any red shift after 2-3 days, demonstrating that stable ZnO nanocrystals are present in the dispersions. The presence of the ZnO nanoparticles was also proven by fluorescence measurements. A number of the nanoparticles are incorporated into the interlamellar space of the clays, and an intercalated structure is formed as proven by X-ray diffraction (XRD) measurements. The size of the nanoparticles in the interlamellar space is in the range of 1-2 nm according to the XRD patterns. Transmission electron microscopy and high-resolution transmission electron microscopy investigations were applied to determine directly the particle size and the size distribution of the nanoparticles.     

In situ forced hydrolysis-assisted fabrication and photo-induced electrical property in sensor of ZnO nanoarrays

ZnO nanowhiskers are successfully fabricated on an anhydrous zinc acetate coated substrate by its in situ forced hydrolysis at the initial stage without pre-existing ZnO seeds or catalyst. HR-TEM clarified the formation mechanism that in situ forced hydrolysis of an anhydrous zinc acetate layer to ZnO nanocrystals at the initial stage promoted growth of ZnO nanowhiskers by heterogeneous nucleation and growth. ZnO nanowhiskers films show high transmittance over 80% in the visible range and bandgap energy of 3.29 eV. Porous semiconductor ZnO films show good photo-induced electrical properties after various concentrations of DNA molecules labeled with photoactive dye molecules were adsorbed. In situ forced hydrolysis-assisted technique at low temperature can be useful for the fabrication of optoelectronic devices with low cost and without using expensive catalyst.     

Hydrothermal synthesis and humidity sensing property of ZnO nanostructures and ZnO-In(OH)3 nanocomposites

Prism- and raspberry-like ZnO nanoparticles and ZnO-In(OH)(3) nanocomposites were prepared by template free hydrothermal method. XRD investigations and microscopic studies showed that pill-like In(OH)(3) particles with body-centered cubic crystal structure formed on the surface of ZnO nanoparticles resulting in increased specific surface area. TEM-EDX mapping images demonstrated that not only nanocomposite formation took place in the course of the synthesis, but zinc ions were also built into the crystal lattice of the In(OH)(3). However, only undoped In(OH)(3) was found on the surface of the pill-like particle aggregates by XPS analyses. The raspberry- and prism-like ZnO particles exhibit strong visible emission with a maximum at 585 and 595 nm, respectively, whose intensity significantly increase due to nanocomposite formation. Photoelectric investigations revealed that photocurrent intensity decreased with increasing indium ion concentration during UV light excitation, which was explained by increase in visible fluorescence emission. QCM measurements showed that morphology of ZnO and concentration of In(OH)(3) had an influence on the water vapor sensing properties.     

Bi2MoO6纳米薄膜的制备及其光电性能

采用非晶态配合物法在ITO导电玻璃上制备了Bi2MoO6薄膜. 采用扫描电子显微镜(SEM)、X射线衍射(XRD)、激光拉曼光谱(LRS)、紫外-可见漫反射谱(DRS)、光电流响应谱、光电转换量子效率(IPCE)等技术研究了Bi2MoO6薄膜的制备工艺、形貌、结构与薄膜光电性能的关系. 结果表明, 500 ℃、1 h焙烧后的Bi2MoO6薄膜为γ-Bi2MoO6晶相, 沿(131)晶面方向生长, 薄膜厚度约为69 nm. 随着焙烧温度的升高和焙烧时间的延长, Bi2MoO6薄膜的平均颗粒度增大, 并且在525 ℃焙烧出现β-Bi2MoO6和γ’-Bi2MoO6晶相. Bi2MoO6薄膜具有可见光响应活性, 在可见光照射下可以产生光电流, 优化条件下的Bi2MoO6薄膜在400 nm的光电转换量子效率可以达到2.14%. 薄膜的光电响应和光电转换量子效率受薄膜形貌及结晶状态影响, 可以通过控制薄膜的制备条件来提高薄膜的光电转换量子效率.     

Organic–inorganic nanocomposites of (2‐hydroxypropyl)cellulose as a precursor of nanocrystalline zinc oxide layers

The sol–gel method of synthesis of the hybrid nanocomposite films of ZnO/(2‐hydroxypropyl) cellulose (HPC) on silica glass is presented. The sol phases were prepared for different weight ratios of zinc acetate dihydrate to HPC in the presence of triethylamine (TEA). Raman spectrum of the mixture of ZnAc and HPC indicates coordinating interaction between zinc ion and HPC. The generation of ZnO nanoparticles in the HPC matrix proceeds in situ through the annealing of the gel phase at a temperature of 160°C. Identification of ZnO nanoparticles in the HPC matrix is done by using photoluminescence (PL), UV–Vis, and Raman spectroscopy. The films of ZnO/HPC nanocomposite are transparent in the visible light and show a higher energy value of absorption edge compared with ZnO in the bulk. Nanocrystalline films of ZnO were obtained by the calcination of ZnO/HPC nanocomposite at 500°C. ZnO films possess a good transparency for the visible light and high absorbance for UV light. Nanocrystallite sizes of ZnO particles were estimated from the X‐ ray lines broadening. The properties of ZnO layers were studied by the evaluation of PL, X‐ray investigation and atom force microscope (AFM) scanning, and the optical absorption edge. Copyright © 2008 John Wiley & Sons, Ltd.     

Design of an organic chromophore for p-type dye-sensitized solar cells

A successful model for the design of efficient dyes for p-type dye-sensitized solar cells (DSSCs) is presented. As an example, a novel and efficient organic dye containing a triphenylamine chromophore has been synthesized and successfully applied in a p-type DSSC. The highest incident photon-to-current conversion efficiency (IPCE) of 18% in the visible region has been obtained, which is the highest value so far in p-type DSSCs. This is remarkably high, considering that only 600 nm thin NiO mesoporous films were used as p-type DSSC electrodes.     

Preparation of cellulose-ZnO hybrid films by a wet chemical method and their characterization

Zinc oxide (ZnO) nanoparticles were immobilized on the surface of regenerated cellulose films by a wet chemical method in which the controlled hydrolysis of a Zn(II)-amine complex leads to the formation of ZnO nanoparticles. Cellulose-ZnO materials were characterized by spectral, thermal and optical methods. Scanning electron microscope and atomic force microscope analyses confirmed the formation of ZnO nanoparticles on the surface of the regenerated cellulose film and X-ray diffraction patterns showed the ZnO had the wurtzite structure. The reported method is very simple, and can immobilize the nanoparticles without the aid of a binder or dendritic side group and without high temperature treatments like calcination. ZnO immobilized on biopolymers like cellulose has many potential applications such as strain sensors, biomedical sensors, flexible display devices and optoelectronics.     

旋涂水溶液前驱体制备氧化锌薄膜及其性质

通过旋涂法, 采用Zn(OAc)2·2H2O和聚环氧乙烷(PEO)的水溶液为前驱体在不同的热处理温度下制备了ZnO薄膜. PEO的加入增加了溶液的成膜性, 其较低的热分解温度有利于制得纯净的ZnO薄膜. 文中考察了在不同热处理温度下制备的ZnO薄膜的形貌、结晶性、带隙(Eg)以及电导性. 原子力显微镜(AFM)测试表明在热处理温度为400、450和500 ℃制备的ZnO薄膜的粗糙度均方根值分别为3.3、2.7和3.6 nm. 采用透射电子显微镜(TEM)测试发现ZnO薄膜中含有大量纳晶粒子. 通过测试ZnO薄膜的UV-Vis吸收光谱, 根据薄膜位于373 nm处的吸收带边计算得到ZnO的带隙为3.3 eV. 通过对薄膜的电流-电压(I-V)曲线的测试计算得到在热处理温度为400、450和500 ℃制备的ZnO薄膜的电阻率分别为3.3×109、2.7×109和6.6×109 Ω·cm. 450 ℃时制备的ZnO薄膜的电阻率最小, 主要是由于较高的热处理温度有利于提高薄膜的纯度、密度和吸附氧. 而纯度较高、密度较大的薄膜电阻率比较小; 吸附氧含量增加, 晶界势垒增大, 电阻率增大. 因此在纯度和吸附氧的双重作用下450 ℃时制备的ZnO薄膜的电阻率最小, 而500 ℃时制备的ZnO薄膜的电阻率最大.     

The fabrication of TiO<Subscript>2</Subscript> nanoparticle/ZnO nanowire arrays bi-filmed photoanode and their effect on dye-sensitized solar cells

Porous TiO₂ nanoparticles coated on ZnO nanowire arrays (TiO₂ NP/ZnO NW) as photoanode for dye-sensitized solar cell (DSSC) has been fabricated and investigated to improve the power conversion efficiency. The TiO₂ NP/ZnO NW photoanode consists of single crystalline ZnO NWs synthesized via hydrothermal method and porous TiO₂ NP film covered on the surface of ZnO NW arrays by screen printing technique. The effect of TiO₂ NPs thickness of the bi-filmed photoanode on the cell performance has been investigated, and TiO₂ NP/ZnO NW DSSC with NP thickness of ~5 μm exhibits the best efficiency of 4.68%, higher than 1.16% of ZnO NW DSSC and 3.18% of TiO₂ NPs DSSC, prepared and tested under identical conditions. The efficiency increase is attributed to the enlarged photocurrent, due to the greatly enhanced surface area for dye absorption and light harvesting efficiency resulted from TiO₂ NPs, and improved open-circuit voltage, due to reduced electron recombination by providing direct conduction pathway along ZnO NWs.     

Mechanical,thermal and barrier properties of pectin/cellulose nanocrystal nanocomposite films and their effect on the storability of strawberries (Fragaria ananassa)

In this work, two formulations of pectin/cellulose nanocrystals/glycerol nanocomposites were employed as packaging to extend storage life of strawberries. The effects of incorporating cellulose nanocrystals extracted from bleached Kraft wood pulp on the mechanical, thermal, and barrier properties of pectin‐based nanocomposites were evaluated. Nanocomposite films with different filler levels of cellulose nanocrystals (1, 2, 4 and 8% w/w) were prepared by casting. Compared with the neat film of pectin, improvements in the mechanical properties of the nanocomposites were observed, but these films became fragile. To improve the film flexibility, glycerol was added as a plasticizer and then new variations in the mechanical, thermal, and barrier properties of these nanocomposites were evaluated. The effects of nanocomposite films on storability of strawberries were compared with Poly vinyl chloride packaging films. The Poly vinyl chloride film and the nanocomposites showed similar behavior regarding weight loss by the strawberries, especially in the initial days of storage. The results show that pectin/cellulose nanocrystals/glycerol nanocomposites could be considered as a viable packaging alternative for replaced the Poly vinyl cloride film. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation and properties of polymer/zinc oxide nanocomposites using functionalized zinc oxide quantum dots

Poly(methyl methacrylate) (PMMA)/zinc oxide (ZnO) or carbazole polymer (PCEM)/ZnO nanocomposites, which are composed of high molecular weight PMMA or PCEM with narrow molecular weight distributions and ZnO nanoparticles, were successfully prepared by atom transfer radical polymerization (ATRP) initiated by 2-bromo-2-methylpropionyl (BMP) group (ZnBM) introduced onto the ZnO nanoparticle surfaces. Introduction of the BMP group onto the ZnO surfaces was achieved by esterification of OH group of the ZnO surfaces. The chemically attached OH group-having ZnO nanoparticles (ZnHM) were fabricated by sol-gel reaction from zinc acetate dihydrate, followed by treatment of the ZnO nanoparticles with 2-hydroxypropionic acid (HPA). The ZnHM nanoparticles showed one UV absorption and two emission bands: UV emission peak and broad visible emission band, while the ZnBM exhibited broad UV absorption and no emission spectra. The PMMA/ZnO nanocomposites displayed UV absorption and photoluminescent (PL) band with blue emission on the basis of the ZnHM nanoparticles, where the ZnO nanoparticles dispersed homogeneously in the PMMA matrix. The PCEM/ZnO nanocomposites depicted UV emission peak due to the carbazole unit in the UV range, but no visible emission. Thermal properties of the PMMA/ZnO nanocomposites were improved by dispersion of the ZnO nanoparticles into the PMMA, but the PCEM/ZnO nanocomposites showed no improvement of the thermal properties.     

Hybrid junctions of zinc(II) and magnesium(II) phthalocyanine with wide-band-gap semiconductor nano-oxides: spectroscopic and photoelectrochemical characterization

The optical properties of zinc phthalocyanine (ZnIIPc) and magnesium phthalocyanine (MgIIPc) in DMSO and DMF solutions have been extensively investigated, and the photoelectrochemical behaviors of layer-by-layer hybrid junctions formed of the two metallo(II) phthalocyanines (MIIPcs) and wide-band-gap colloidal semiconductors, namely, ZnO and TiO2 nanocrystals (NCs), have been probed. Different experimental conditions, such as the Pc center metal ion, dye concentration, and solvent identity, were investigated in order to elucidate their effects on the photoelectrochemical performances of the prepared heterojunctions. Finally, thermal treatment of either dye and NC films and control of the NC shape and surface chemistry were also studied and, interestingly, were found to be critical in affecting the performance of photochemical sensitization processes, occurring at the dye/oxide and oxide/solution interfaces.     

敏化染料分子长径比对太阳能电池性能的影响

采用同一系列但分子长径比不同的3种染料:2-氰基-3-[2-[4-{2-[4-N,N-二(4-甲基苯基)氨基苯基]乙烯基}-苯基氨基)-嘧啶-5-取代基]-丙烯酸(MTPA-Pyc)、2-氰基-3-(4-{2-[4-N,N-二(4-甲基苯基)氨基苯基]乙烯基}-苯基)-丙烯酸(MTPAcc)和2-氰基-3-[4-N,N-二(4-甲基苯基)氨基苯基]-丙烯酸(MTPAc),研究了在不同吸附溶剂中3种染料分子在Ti O2上的吸附量和聚集态,探讨了敏化染料分子长径比对染料敏化太阳电池性能的影响.结果表明,MTPAcc具有最合适的分子长径比,其在Ti O2表面的吸附量及应用的光电性能最高;吸附溶剂的极性增大有利于提高染料的吸附量,但也会影响染料分子的聚集态.当以四氢呋喃为吸附溶剂时,MTPAcc在Ti O2表面的吸附量大且不发生聚集,对应的敏化太阳能电池器件在所有结果中表现最好,在490 nm处的单色光光电转化效率(IPCE)极值达到84%,总光电转化效率(η)达到5.72%.