160 combinatorial chemistry & high throughput screening ...€¦ · 2department of materials...

160 Combinatorial Chemistry & High Throughput Screening, 2011, 14, 160-172

1386-2073/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd.

Application of Parallel Synthesis and High Throughput Characterization in Photocatalyst Discovery

Song Sun1, Jianjun Ding

1, Jun Bao

1, Zhenlin Luo

1 and Chen Gao

*,1,2

1National Synchrotron Radiation Laboratory & School of Nuclear Science and Technology, University of Science and

Technology of China, Hefei, Anhui 230029, China

2Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui

230026, China

Abstract: The last decade has seen significant progresses in the application of combinatorial approaches and high throughput

screening in photocatalyst discovery. This paper aims at providing a comprehensive review on the parallel synthesis and

high throughput characterization of photocatalysts, including the development of instrumentation, strategy of experiment,

preparation of libraries, high throughput screening technique and data analysis. The review ends with a summary of the

remaining challenges and prospects on combinatorial photocatalyst discovery.

Keywords: Photocatalyst, combinatorial synthesis, high throughput screening.

1. INTRODUCTION

Semiconductor photocatalysis is one of the most attractive techniques for water splitting and decomposition of pollutants to mineral compounds. The reaction can be summarized briefly as follows. When a photocatalyst absorbs photons having energy higher than the band gap of the semiconductor, electrons are excited from the valence band to the conduction band with simultaneous generation of holes in the valence band. The photo-generated electron and hole can recombine or migrate toward the photocatalyst surface where they can react with electron donor or acceptor species adsorbed on the surface of photocatalyst. The energy level at the bottom of the conduction band and the energy level at the top of the valence band determine the reduction ability of electrons and the oxidation ability of holes respectively [1-3]. A good photocatalyst should have not only a relatively narrow band gap but also an acceptable position of conduction band and valence band. Over the past decades, TiO2-based photocatalysts have been widely studied because of their relatively high photocatalytic activity, chemical stability, low production costs and environmental friendliness [2, 3]. However, TiO2 is only active under UV irradiation due to its large band gap energy (3.2 eV), which results in a low efficiency to make use of solar light. Some researchers devoted to prepare TiO2-based photocatalysts sensitive to visible light by surface modification [4], metal ion (Ce, In, Ag, etc.) doping [5], nonmetal ion (N, C, S etc.) doping [6], coupling with other narrow band-gap semiconductors [7], etc. Recently, many efforts have been carried out on the development of new materials to exhibit a strong photocatalytic activity in water splitting and organic contaminant degradation under the visible light irradiation. The representative material systems include In0.9N0.1TaO4,

*Address correspondence to this author at the National Synchrotron

Radiation Laboratory & School of Nuclear Science and Technology,

University of Science and Technology of China, Hefei, Anhui 230029,

China; Tel: (+86) 551-360-2031; Fax: (+86) 551-514-1078;

E-mail: [email protected]

Ag2ZnGeO4, (Ca1-xZnx)(N1-xOx), CaIn2O4, Fe86.1Ti9.6Sr4.3Ox, etc. Their structure, physical chemical property and visible-light photocatalytic activity were investigated in detail [8-12]. However, Some difficulties remain to be solved for the discovery of good photocatalysts, because the properties of photocatalysts are highly sensitive to the change of dopant composition, stoichiometric ratio and processing conditions etc. [1, 3, 5]. Therefore, although the physical mechanism of photocatalysis is relatively well understood, there is no reliable theory to guide the search of good photocatalysts. Despite several decades of intensive studies, only a few of commercial photocatalysts have been discovered through the conventional one-at-a-time synthesis and characterization.

Since Schultz and co-workers [13] published a combinatorial approach to material science, combinatorial chemistry has been demonstrated high efficiency for searching candidate functional materials in the fields of luminescence [14-16], ferroelectric/dielectric [17], drug [18] and catalysis [19], etc. and has become an important branch of materials science [20]. Recent years have witnessed the increasing application of combinatorial methods in the photocatalysis field, including the parallel synthesis of photocatalysts and high throughput screening for photocatalytic activity. As shown in Fig. (1), the first key step for the combinatorial discovery of new photocatalysts is library design. For parallel synthesis of photocatalysts, thin-film deposition by the laser molecular beam epitaxy (MBE), pulsed laser deposition (PLD) or chemical vapor deposition (CVD) technique combined with physical masks and solution-based synthetic methods are the most commonly mentioned [21-34]. In solution-based parallel synthesis, sol-gel method is feasible to obtain nanoparticles with different surface properties by modification of precursors, aging time and temperature, and calcination temperature [21-25]. Solution combustion method, which has the advantage of instantaneous high temperature, makes it very suitable for efficient synthesis of photocatalysts and combinatorial study [26, 35]. Accordingly, commercial and home-made instruments for parallel synthesis have been developed in recent years [21, 36, 37]. After the high throughput screening

Photocatalyst Discovery Combinatorial Chemistry & High Throughput Screening, 2011, Vol. 14, No. 3 161

of photocatalyst library by direct or indirect measurement of reactants and products, some excellent photocatalysts may be obtained or some clues from the “lead photocatalyst” from the first generation library can guide the combinatorial research for next generation of libraries.

In this paper, we aim at providing a comprehensive review on the parallel synthesis and high throughput characterization of photocatalysts and focus on some new instruments and screening techniques reported in the literatures. A summary of the remaining challenges and prospects for combinatorial photocatalyst discovery is also given.

2. COMBINATORIAL SYNTHESIS OF PHOTO-CATALYST LIBRARIES

Photocatalysts can be synthesized in either thin films or powders. Combinatorial methods have been developed to allow synthesis of photocatalysts libraries in both forms.

2.1. Synthesis of Thin Films

The deposition equipment with a combinatorial masking strategy can be used to synthesize thin-film “spatially addressable” libraries. The masking strategy, including the choice of mask form and masking schemes, is the key to the art of making a thin-film library and determine the efficiency of library [13, 19, 20, 38-40]. Matsumoto and Ohsawa [27-29] demonstrated successful application of the combinatorial approach in the preparation of photocatalysts of [(SrTiO3)1 x

(LaAlO3)x] (0<x<1) and Ti1-xMxO2 (M=transition metal) epitaxial thin film by a PLD system equipped with a movable mask. Fig. (2) shows the principle of fabricating a binary composition spread of [(SrTiO3)1 x(LaAlO3)x] (0<x<1) (20 nm thickness) with the aid of reflection high energy electron diffraction (RHEED) monitoring. The KrF excimer laser pulses were introduced to the “A” target with the mask moving from one side to another side, which created “A” compositional gradient on one substrate. Subsequently, the mask moved to the opposite direction with synchronized switching “B” target so that the layer-by-layer growth of “A” and “B” was obtained after several runs. The growth conditions were optimized by adjustment of the substrate temperature, oxygen partial pressure, the energy

density and the frequency of the laser pulse. After annealing in atmospheric oxygen at 400

oC to compensate for the lack

of oxygen in the films, [(SrTiO3)1 x(LaAlO3)x] exhibited a step and terrace surface structure from the AFM images and the RHEED results. Ohsawa et al. [30] synthesized epitaxial Sr(VxCryTi1-x-y)O3 (0 x+y 0.05) thin films on two different single crystal substrates, LaAlO3 (001) and Nb-doped SrTiO3 (001) with different film thickness using combinatorial laser molecular beam epitaxy (MBE) with a specially patterned slide masking plate, as shown in Fig. (3). Ternary composition spreads can be deposited by rotating the substrate 120

o in between depositions of each component

with a simple mask of trapezoid and triangle shape moving along one axis [30]. For creating the ternary composition spreads efficiently, some researchers designed the different masking strategy in experiment and computer simulation [41, 42]. Chemical vapor deposition (CVD) was also proved to be a powerful synthesis tool for photocatalyst thin-film library [32-34]. Parkin et al. [32] synthesized a compositionally gradient N-doped TiO2 film containing 247 positions on a 13 19 grid shown in Fig. (4) via a combinatorial atmospheric pressure CVD. The interference fringes of this film demonstrated the variation in thickness. Yellow tint is deeper in bottom-left section and become lighter across and along the film, which is attributed to the decreasing of N-doping concentration. PLD, MBE and CVD methods have the advantage on the synthesis of combinatorial film rapidly with variations in thickness, phase and composition. In contrast, sol-gel films are not acceptable for combinatorial study because they are inherently homogenous, and their combinatorial synthesis requires a series of films with doping or composite concentration varying in order to ascertain the relationship between the photocatalytic performance and the composition.

Automated electrochemical synthesis was first applied in creating combinatorial libraries of photocatalysts by McFarland’s group [36, 43-45]. Fig. (5) shows the working mode of automated electrochemical deposition system, which consists of a set of computer-controlled x-y-z stages, a pair of electrodes (a coiled Pt wire for the counter electrode and a coiled Ag wire for reference electrode) and library substrate. The substrate is mounted between an aluminum bottom plate and a perforated Teflon block, which seal the wells using independent O-rings. The film deposition occurs

Fig. (1). Combinatorial approaches to synthesis and screening of photocatalysts.

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162 Combinatorial Chemistry & High Throughput Screening, 2011, Vol. 14, No. 3 Sun et al.

in each well automatically by moving the electrodes and applying the desired voltages. Zn1-xCoxO (0.000 x 0.068), WnOmMx (M = Ni, Co, Cu, Zn, Pt, Ru, Rh, Pd, and Ag) and mesoporous ZnO libraries were synthesized successfully by automated serial electrochemical deposition [43-45].

2.2. Synthesis of Powders

Conventional sol-gel synthesis method [46] was developed to prepare photocatalyst libraries in powder form by some specific instruments [21-25]. Maier’s group [21, 22] synthesized the potential photocatalysts in 45 transparent

HPLC glass flasks by means of a synthesis robot shown in Fig. (6). In sol procedure, the library was agitated by micro-titerplate-orbital shaker to ensure rapid and intensive mixing of the reagents. After generation, the library was aged and subsequently calcined at a set temperature to yield the powders. The shortcoming of the combinatorial sol-gel synthesis is that some gel photocatalysts on the flask bottom after the calcination remains clear gels, some shows severe cracks, others are powdery. Therefore the photocatalytic activity may be affected by this state [21]. Once a “lead photocatalyst” was obtained by this method, the bulk sample can be synthesized to confirm its activity.

Fig. (2). Schematic illustration of fabricating a binary composition spread by the PLD method [27]. (©

2005 IOP Publishing Ltd.)

Fig. (3). (a, b) Schematic diagram of thin film deposition profile with the specially designed slide mask for ternary composition spreads.

Thickness gradients for (c) SrTiO3, (d) SrV0.05Ti0.95O3, (e) SrCr0.05Ti0.95O3 were synthsized by repeating the deposition [30]. (©

2005 Elsevier)

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for a 10 12 array of thin films. The library substrate is sealed

underneath a perforated Teflon block, and each of the 120 cells is

filled with a unique electrolyte composition [44]. (©

2005 American

Chemical Society)

Parallel solution combustion synthesis technique developed by Gao et al. [35] provided another effective tool for the synthesis of photocatalyst libraries in powder form. A substrate-net-mask microreactor array system was designed and its schematic diagram as well as a photograph was shown in Fig. (7a, b) respectively. The corundum substrate with a predrilled mini-well array in which solution reactants are placed, is covered by a copper net and a metal mask in sequence. To prevent the synthesized powders in different wells flying out and mixing with each other, the mask is tightened firmly on the substrate with four screws so that the copper net covers the substrate tightly. The powders can be taken out for the further characterization through the process shown in Fig. (7c, d). Compared with the conventional solid-

Fig. (6). The photograph of experimental setup for synthesis and

screening of photocatalysts. (A) Array of lamps; (B) bath of frosted

glass; (C) library of 45 HPLC flasks arranged in five columns and

nine rows; (D) orbital shaker [21]. (©

WILEY-VCH Verlag GmbH

2001)

state reaction, solution combustion synthesized powders are generally more homogeneous, have fewer impurities and higher surface area [26, 35]. Noticeably, the solution combustion reaction is a complex chemical process and could be affected by the type of fuel, fuel-to-oxidizer ratio, furnace temperature, etc. [35, 47, 48]. To achieve an optimal reaction condition, these factors should be considered and tested beforehand. Generally, using a lower ignition temperature can effectively decrease the reaction strength. Gao’s group [26] successfully synthesized the photocatalyst library of ABO3-type oxides (A = Y, La, Nd, Sm, Eu, Gd,

Fig. (4). Digital image of compositionally graded N-doped TiO2 film synthesized by a combinatorial APCVD technique [32]. (©

the Owner

Societies 2009)

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Dy, Yb; B = Al, In) by parallel solution combustion synthesis technique. A certain mount of C2H5NO2 as fuel and the different precursor solutions of nitrates were dispensed into the wells in a stoichiometric ratio of combustion reaction. Taking the YInO3 as an example, it can be represented by the equation:

The solutions in wells were fully mixed in an ultrasonic cleaner and then placed in furnace for evaporating the excess solvent. Solution combustion reactions took place when the temperature reached the ignition temperature. They found that the yields of this combustion reaction were 90% after the calcination and there was no residual organic or carbon found in the final products from the element analysis results.

Worth mentioning is that a drop-on-demand inkjet delivery system developed by Gao et al. [37] facilitate the combinatorial solution synthesis of the powder photocatalyst libraries. Fig. (8) shows the schematic diagram of the drop-on-demand inkjet delivery system. There are eight independent inkjet heads and each head consists of a

piezoelectric disk, a stainless steel diaphragm and a sapphire nozzle. Each inkjet head is fixed in the frame above the substrate and connected to a suspension reservoir through a tube. When a high voltage electric pulse is applied to the piezoelectric disk to produce a mechanical vibration, one drop is ejected out from the nozzle. During the inkjet

delivery process, software automatically coordinates the motion of the x-y stage via motor controller and the ejection of the heads to ensure that a certain amount of drops were dropped into right wells. Noticeably, not only soluble compounds but also ultrafine/nano insoluble oxides obtained by using a wet ball-milling technique can be used as precursors in inkjet combinatorial synthesis.

3. HIGH THROUGHPUT CHARACTERIZATION OF PHOTOCATALYTIC ACTIVITY

After being synthesized, photocatalyst libraries are ready to undergo a high throughput screening for the discovery of

Fig. (7). (a) Schematic diagram and (b) photograph of the substrate-net-mask microreactor array system for parallel solution combustion

synthesis [35]. (c) A plastic substrate with the same predrilled shallow (2 mm in depth) well array was assembled with the library and then

flipped over so that the synthesized powder materials drop into the shallow wells. (d) After removal of the corundum substrate, a metal plate

was used to compact the powders. (© 2005 American Chemical Society)

3Y(NO)3 + 3In(NO)3 + 10C2H5NO2 3YInO3 +25H2O + 20CO2 + 14N2

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promising photocatalyst. Woodhouse and Parkinson [49] had reviewed some techniques for the screening of semiconductors for water photoelectrolysis with sunlight. Recently, some probe reactions such as the change of color, fluorescence intensity, and pH value of reactant or product in photocatalytic process have been exploited for rapid screening of photocatalyst [23-26, 31-34, 50-53]. Besides it, some groups used the commercial available instruments or home-made tools or robots for high throughput characterization of photocatalytic activity [31, 21-23, 44, 54-58].

3.1. High Throughput Characterization by pH Imaging Method

Fig. (9a) illustrates one principle of water splitting by TiO2 photocatalyst [31]. The photo-generated electron-hole pairs migrate toward the photocatalyst surface, where electrons reduce protons to hydrogen molecules, and holes initiate oxidation reaction with water to give oxygen. If the proton reduction is replaced by some other reductive reaction that consumes photogenerated electrons, such as Fe

3+ + e

Fe2+

, proton concentration in water increases. Therefore, photocatalytic activity can be evaluated by measuring the pH value in water near the photocatalyst surface. Suzuki et al. [28, 31] evaluated photocatalyitc activities of the libraries, TiO2 doped with twelve transition metals at nine concentration levels, by the two-dimension pH image sensor to screen the change of pH value. The photocatalysts are placed upside down on the electrolyte gel, through which the protons diffuse and adsorb to the surface of pH sensors (Fig. 9b). They found that doping with cobalt at 5.4-9.0 atom % made the TiO2 active under visible light irradiation. The pH imaging method is suitable for preliminarily screening of photocatalysts for water splitting, since the energy of electrons required for reducing protons to hydrogen is more than that for reducing ferric ions.

Fig. (9). (a) Principle of photocatalytic water splitting; (b) Diffuse

of protons from the photocatalyst surface to the Si3N4 surface of

two-dimensional pH image sensor [31]. (©

2002 Elsevier)

Fig. (8). Schematic diagram of the drop-on-demand inkjet delivery system [37]. (©

2004 American Chemical Society)

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3.2. High Throughput Characterization by Color Change

Parkin et al. [32-34] reported a simple method for the rapid parallel screening of photocatalytic activity over multiple positions of self-cleaning films using an intelligent ink comprising of a redox dye resazurin and the sacrificial electron donor glycerol within aqueous hydroxyl ethyl cellulose polymer media. This intelligent ink can be photocatalytically degraded via the photo-reductive conversion of resazurin to resorufin with a color change from blue to pink, and the subsequent photo-reduction of the resorufin with a slower change from pink to colorless. By a red-green-blue extractor, the analyses of color values recorded by digital images of the intelligent ink on library can rapidly identify the active and inactive composition point. Fig. (10) shows the consecutive images of compo-sitionally gradient N-doped TiO2 film for photocatalytic

degradation. A color map shown in Fig. (11) represents the photocatalytic ability of the 13 19 grid by recording the minimum irradiation time taken to degrade resazurin to resorufin.

Some researchers [26, 50, 53] applied the organic dyes as initial probe pollutant and identified the photocatalytic activity by observation of the solution color variation. Gao et al. [26] synthesized the ABO3 (A = Y, La, Nd, Sm, Eu, Gd, Dy, Yb; B = Al, In) library by parallel solution combustion reaction as described in Section 2.2. As shown in Fig. (12), according to the fading rate of MB solution, two novel photocatalysts cubic YInO3 and perovskite YAlO3 were identified rapidly. Scale-up experiments confirmed that the two photocatalysts, especially the YInO3, had excellent photocatalytic activity for toluene oxidation and water splitting under visible-light irradiation.

Fig. (10). Consecutive images of photocatalytic degradation of an intelligent ink on a compositionally gradating N-doped TiO2 film [32].

(©

the Owner Societies 2009)

Fig. (11). Color map of the UVA-irradiation time taken to reach the minimum in the green component in digital color (red-green-blue) at

crossings in 13 19 grid drawn with intelligent ink on an underlying compositionally gradating N-doped TiO2 film. The numbers are

indicative of a maximum formation of the resorufin (pink) from the original redox resazurin dye (blue) [32]. (©

The Owner Societies 2009)

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3.3. High Throughput Analysis by Fluorescence Change

Another high throughput screening method based on the fluorescence change was investigated by Zhou et al. [23-25]. NH2(CH2)6NH2 was chosen as probe molecule because it can react with fluorescamine to generate a substance that gives strong fluorescence light under UV irradiation as the following equation [23, 24].

The activity of photocatalysts is evaluated by the consumption of NH2(CH2)6NH2 and corresponding change of fluorescence intensity collected by CCD camera during the photodegradation reaction. Fig. (13) shows the fluorescence

imaging system. Zhou et al. [23] revealed that the addition of Nb2O5 or WO3 to TiO2 resulted in significant improvement in catalytic activities, while no remarkable improvement was observed when ZrO2 was coupled with TiO2. The activity results of TiO2-Nb2O5-WO3 libraries displayed in Fig. (14) present that when TiO2 is co-doped with 20–30% of Nb2O5 and 10–20% of WO3, the photocatalysts shows the superior photocatalytic activity to any other compositions.

Fluorescence imaging and spectroscopy was also successfully documented by Potyrailo et al. [51] and Corma et al. [52].

Fig. (12). Photos of ABO3 library containing MB solution before (a) and after being exposed to the sunlight for 150 min (b) [26]. (©

2009

American Chemical Society)

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3.4. High Throughput Screening by General Chemical

Method

Ohsawa et al. [30] selected the photocatalytic reduction of Ag

+ in an AgNO3 aqueous solution to deposit Ag metal on

the Sr(VxCryTi1-x-y)O3 (0 x+y 0.05) ternary composition gradient film. From the snapshots in Fig. (15), Ag was

photodeposited firstly on SrV0.05Ti0.95O3 composition and spread to other composition region. By estimating the amount of Ag deposited on the film using EPMA, the activity of library was evaluated.

Maier’s group [21, 22] designed an experimental setup described above in Fig. (6) for parallel synthesis and high

Fig. (14). Performance of TiO2-WO3-Nb2O5 library for 1,6-hexamethylenediame photodegradation [23]. (©

2005 Elsevier)

Fig. (15). Photos of ternary composition spread thin film in AgNO3 solution during UV illumination [30]. (©

2005 Elsevier)

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throughput screening of photocatalytic water purification and hydrogen production. This setup fixes the library on a flat which is exposed to homogeneous irradiation of UV or visible light. Model pollutant and other reactants are added in each member of the library. During the photocatalytic process, the whole library is agitated at the maximum shaker speed. Four fans installed in the chamber are used to reduce the ambient temperature for preventing the thermal effect. After the reaction proceeded for some time, the photocatalytic activity is evaluated by testing the pollutant concentration or hydrogen with HPLC and GC respectively. By this method, the libraries of TiO2, SnO2 and WO3 doping with different metals or salts (1 mol%) were characterized for degradation of 4-chlorophenol under visible light irradiation as shown in Fig. (16). Maier et al. [21] firstly discovered the positive effects of doping Tb, Mn, and Pr on TiO2. Furthermore, a new lead structure, an aluminum-lead-bismuth-oxide has been discovered for hydrogen production from aqueous methanol solutions over a diverse synthesized library of mixed metal oxides [22]. As shown in Fig. (17), active materials of hydrogen production were marked in

black, which compose of either bismuth or lead. Gray samples are indicative of no significant hydrogen production and white stands for samples that have not been synthesized. To improve the photocatalytic activity, further generation-libraries were prepared and characterized. The visible-light photocatalyst Al40Bi40Pb20Ox was identified. On the basis of it, a high active Te1Al39.6Bi39.6Pb19.8Ox(Cl) photocatalyst for hydrogen production was found by the combinatorial approach.

3.5. High Throughput Analysis by Atomic Force Microscope

Atomic force microscope (AFM) is an effective tool for high throughput screening of the photocatalysts. The photodecomposition of pentacene on the [(SrTiO3)1 x(LaAlO3)x] (0<x<1) composition spread film library was directly observed and quantitatively analyzed for each composition by AFM [27]. One monolayer of pentacene which was used as a model pollutant was deposited on [(SrTiO3)1 x(LaAlO3)x] (0<x<1) thin-film

Fig. (16). 4-CP conversions X for (a) TiO2-based, (b) SnO2-based, and (c) WO3-based mixed oxides (1 mol %) after 2.5 h of irradiation.

Each library member is identified by its position in the library, given by a column (A~E) and a row (1~13, 1~13, 1~15, respectively). The

table contains the doping salts added to the corresponding positions during the sol-gel preparation. The library members showing 4-CP

conversions >5% are shaded grey in the table [21]. (©

WILEY-VCH Verlag GmbH & Co. KGaA 2001)

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library at room temperature by K-cell evaporation in UHV. Before UV irradiation, pentacene exhibits a stable large dendritic islands pattern with the height of 1.5-1.6 nm in AFM images. As shown in Fig. (18), after the library was irradiated under UV light for 4 h, the surface of the film showed rugged features and the amount of decomposed pentacene can be estimated by evaluating the total area of pentacene islands before and after UV irradiation. The coverage of pentacene showed a linear increase with x. For x=0, the pentacene film was almost totally decomposed. And for x=1.0, no decomposition of pentacene occurred even after the long UV irradiation, indicating that LaAlO3 is inactive for photocatalytic decomposition of pentacene under UV irradiation. This work gives some insight into the utilizing of common instruments on combinatorial study. A

comparable technique, UV-imaging technique was developed as a useful tool for optimizing photocatalytic thin films by Barkschat et al. [59]. Additionally, infrared microscopy, infrared and Raman spectroscopy allow direct spectral analysis of interference reaction with the change of IR or Raman bands. Gremlich [60] has reviewed the use of optical spectroscopy in combinatorial chemistry and proposed the development of high throughput monitoring of chemical reactions with the automated accessories. However, these potential applications of scientific instruments have not been fully realized or attracted enough attentions. Especially for photocatalysis, mechanism investigation as well as parallel synthesis and high throughput characterization of activity can be expected with promising developments of instruments and smart strategy.

Fig. (17). Result of the screening of binary 50:50 mixed oxides; active materials are represented by black pixels; white pixels correspond to

excluded samples; reaction conditions: 24 h irradiation, 200 μmol catalyst per sample, 250 μL methanol/water mixture (1:1, v/v) [22]. (©

2007

Elsevier)

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4. SUMMARY

As this review demonstrates, combinatorial methodologies provide a low-cost and high efficient tool for the screen of better photocatalysts. Both thin films and powders can be prepared and characterized by common chemical synthesis methods in connection with smart strategy of experiment and improved instruments. However, more conventional analytical techniques and synthesis methods should be improved and modified to meet the requirements of extensive studies on the diversity of photocatalysts and the complexity of photocatalysis mechanism. Development and commercialization of appropriate experimental instruments is a key step towards the breakthrough. Secondly, the data mining is also an important factor to dig the underneath mechanism from the relationship between the photocatalytic performance and the composition, structure and physical chemical property of photocatalysts. With these breakthroughs, we can safely expect that parallel synthesis and high throughput characterization will further accelerate the discovery of superior photocatalysts.

ACKNOWLEDGEMENTS

This work was supported by National Nature Science Foundation of China (50721061, 20903084), Anhui Provincial Natural Science Foundation (090416226) and Chinese Academy of Sciences.

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Received: May 31, 2010 Revised: August 5, 2010 Accepted: January 24, 2011

160 combinatorial chemistry & high throughput screening ...€¦ · 2department of materials...

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