One-Pot Synthesis of Uniform Cu2O and CuS Hollow Spheres and Their Optical Limiting Properties

One-Pot Synthesis of Uniform Cu2O and CuS Hollow Spheres and

Their Optical Limiting Properties

Jining Gao,?Quanshui Li,?Huabo Zhao,?Lianshan Li,?Chunling Liu,?Qihuang Gong,?and

Limin Qi*,?

Beijing National Laboratory for Molecular Sciences,State Key Laboratory for Structural Chemistry of Unstable and Stable Species,College of Chemistry,and State Key Laboratory for Mesoscopic Physics, School of Physics,Peking Uni V ersity,Beijing100871,People’s Republic of China

Recei V ed May23,2008.Re V ised Manuscript Recei V ed July28,2008

Uniform hollow spheres of Cu2O and CuS were successfully synthesized by chemical transformation of in situ formed sacri?cial templates containing Cu(I)in aqueous solutions.The shell thickness of these hollow spheres can be adjusted through the choice of the bromide source used for the formation of intermediate templates.Speci?cally,thick-shell hollow spheres(about130-180nm in shell thickness) were obtained by using CuBr solid spheres as the templates,which were formed by the reduction of CuBr2with ascorbic acid;on the other hand,thin-shell hollow spheres(about20-25nm in shell thickness) were obtained by using spherical aggregates consisting of the Cu+,Br-,and(C4H9)4N+ions as the templates,which were formed by the reduction of CuCl2with ascorbic acid in the presence of(C4H9)4NBr. In both cases,crystalline Cu2O hollow spheres were directly obtained at room temperature,while amorphous Cu2S hollow spheres were?rst obtained at room temperature and transformed into well-crystallized CuS hollow spheres after a hydrothermal treatment at160°C.The optical limiting properties of the thin-shell hollow spheres of Cu2O and CuS were characterized by using nanosecond laser pulses. Strong optical limiting responses were detected for both the Cu2O and CuS hollow spheres,which make these semiconductor hollow spheres promising materials for applications in the protection of human eyes or optical sensors from high-power laser irradiation.

Introduction

Hollow structures have received considerable attention recently because of their potential applications in catalysis, controlled delivery,lightweight?llers,low-dielectric-constant thin?lms,photonic crystals,con?ned-space chemical reac-tors,and biomedical diagnosis and therapy.1-6Various methodologies have been developed to generate micro-and nanostructures with hollow interiors,which involve templates such as polymer and silica spheres,7emulsion droplets,8 polymer/surfactant micelles,9and bubbles,10as well as chemical processes based on the Kirkendall effect,11Ostwald ripening,12or chemically induced self-transformation.13 Among them,the sacri?cial template-directed chemical transformation method based on the Kirkendall effect has been demonstrated to be an effective approach.Reactive sacri?cial templates,which act as both reactive precursors and templates,are free not only from the template removal problem but also from the limited morphologies of hard templates,because the precursors can be fabricated through all the techniques developed for materials morphosynthesis.11 In this regard,a variety of inorganic hollow structures have been successfully prepared by employing reactive sacri?cial templates.14-19Furthermore,it is worth noting that hydro-thermal template techniques have been successfully used for the synthesis of a variety of inorganic hollow spheres in the presence of different templates owing to the speci?c physical properties of the hydrothermal techniques.20 Semiconductor transition-metal oxides and chalcogenides have been of much interest because of their excellent properties

*To whom correspondence should be addressed.E-mail:liminqi@https://www.wendangku.net/doc/c34475253.html,. Fax:+86-10-62751708.

?College of Chemistry.

?School of Physics.Email:clliu@https://www.wendangku.net/doc/c34475253.html,(C.Liu).

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and wide-range potential applications.In particular,as a p-type semiconductor with a band gap of2.17eV,cuprous oxide (Cu2O)is a promising material with potential applications in solar energy conversion,catalysis,and sensing.21-23Copper sul?des have the ability to form various stoichiometries,at least ?ve phases of which are stable at room temperature:i.e., covellite(CuS),anilite(Cu1.75S),digenite(Cu1.8S),djurlite (Cu1.95S),and chalcocite(Cu2S).17Their complex structures and valence states result in some unique properties and promising applications in numerous?elds,such as solar cells,optical ?lters,superconductors,and chemical sensors.24-27Many recent efforts have been devoted to the synthesis of cuprous oxide and copper sul?de micro-and nanostructures with modulated morphologies and architectures.Morphology-controlled synthesis of Cu2O particles has been realized by different methods.22,23,28-31In particular,Cu2O nanocages have been obtained via a Pd-catalyzed solution route,which involves the formation of octahedral Cu2O nanocrystals and a subsequent spontaneous hollowing process.29Hollow Cu2O spheres were formed by reductive conversion of aggregated CuO nanocrystallites using N,N-dimethylformamide as a reducing agent.30Multishelled Cu2O hollow spheres with single-crystalline shell walls have also been synthesized with the assistance of multilamellar vesicles.31On the other hand, copper sul?de micro-and nanostructures with different compositions and morphologies,including nanoparticles, nanoplates,and complex structures,have been successfully synthesized.16,17,24,27,32-34Speci?cally,copper sul?de me-socages were produced through chemical transformation by employing Cu2O as a reactive sacri?cial template.16,17 Micrometer-sized CuS hollow spheres were prepared on the basis of a template interface reaction between CuCl and sulfur liquid droplets,34while nanosized CuS hollow spheres were synthesized by using surfactant micelles as soft templates.24However,it remains a challenge to achieve the facile synthesis of hollow spheres of both Cu2O and CuS by employing a general sacri?cial template.

Herein,a facile,one-pot,aqueous solution route was reported for the controlled synthesis of uniform hollow spheres of cuprous oxide and copper sul?des by chemical transformation of in situ formed sacri?cial templates contain-ing Cu(I).The shell thickness of these hollow spheres can be adjusted through the choice of the bromide source used for the formation of intermediate templates.In particular, monodisperse,well-crystallized,thin-shell Cu2O and CuS hollow spheres with a shell thickness of about20-25nm were produced by using(C4H9)4NBr as the bromide source. It was revealed that these Cu2O and CuS hollow spheres exhibited remarkable optical limiting effects,which could make them useful for protecting human eyes or optical sensors from high-power laser irradiation.

Experimental Section

Hollow spheres of a variety of copper-based oxide and sul?de materials including Cu2O,Cu2S,and CuS were achieved by the formation of intermediate Cu(I)-containing templates followed by their transformation into the?nal hollow structures in aqueous solutions.Notably,thick-shell and thin-shell hollow spheres were obtained by employing CuBr2and(C4H9)4NBr as the bromide source,respectively.In a typical procedure for the synthesis of thick-shell oxide and sul?de hollow spheres,an aqueous solution was ?rst prepared by mixing20mL of water,0.4mL of PVP (poly(vinylpyrrolidone);MW)30000,Beijing Chemical Co.) solution(1.0wt%),and0.2mL of CuBr2solution(0.1M).Then 1.0mL of ascorbic acid solution(0.1M)was added to the solution, and the mixture was stirred for3min and became turbid shortly, indicating the formation of CuBr particles.For the synthesis of oxide hollow spheres,1.0mL of NaOH solution(0.2M)was added to the above turbid precursor solution under stirring,and the resulting solution was then kept for15min at room temperature without stirring.The resultant Cu2O products were collected by centrifuga-tion,washed with water several times,and dried in air.For the synthesis of sul?de hollow spheres,0.4mL of Na2S solution(0.1 M)was added to the turbid precursor solution under stirring,which was then kept for30min at room temperature without stirring, resulting in the formation of poorly crystallized Cu2S hollow spheres.This brown reaction mixture was then transferred into25 mL stainless-steel autoclaves lined with poly(tetra?uoroethylene) (PTFE,Telfon)and maintained at160°C for24h,leading to the formation of well-crystallized CuS hollow spheres.The resultant Cu2S and CuS products were also collected by centrifugation, washed with water several times,and dried in air.On the other hand,the synthesis of thin-shell oxide and sul?de hollow spheres was realized under similar conditions except for the use of2.0mL of(C4H9)4NBr solution(0.1M)and0.2mL of CuCl2solution(0.1 M)instead of0.2mL of CuBr2solution(0.1M).However,it should be noted that loose aggregates containing Cu+,Br-,and(C4H9)4N+ ions were formed instead of CuBr solid spheres in the precursor solution before the addition of NaOH or Na2S.

The solid products were characterized by scanning electron microscopy(SEM;Hitachi S4800,5.0kV),transmission electron microscopy(TEM;JEOL JEM-200CX,160kV),X-ray diffraction (XRD;Rigaku Dmax-2000),and UV-vis spectroscopy(Perkin-Elmer Lambda35).Energy-dispersive X-ray(EDX)spectroscopy was performed with a Horiba EMAX-2000EDX system attached to the SEM microscope.The optical limiting effects of the hollow spheres were investigated by the?uence-dependent transmittance measurement.The hollow spheres were dispersed in ethanol,and the sample cell was5mm thick.The laser source is a Q-switched

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YAG laser (Surelite-III,Continuum Corp.),which delivers the fundamental laser pulses with a pulse duration of 5.6ns and a wavelength of 1064nm.To avoid thermal effects and make every pulse meet new particles,the lasers irradiated the suspensions every 15s.

Results and Discussion

Formation of Thick-Shell Hollow Spheres.For the synthesis of thick-shell hollow spheres of cuprous oxide (Cu 2O)and copper sul?des (Cu x S),CuBr solid spheres were ?rst prepared as sacri?cial templates.Figure 1shows the typical SEM and TEM images of the CuBr product obtained by the reaction between CuBr 2and ascorbic acid in the presence of 2g L -1PVP.It can be seen that the product consists of uniform spheres with an average diameter of about 900nm,and these microspheres usually exhibit relatively rough surfaces composed of nanoparticles smaller than 50nm.The XRD pattern of the obtained microspheres is shown in Figure 2a.All the diffraction peaks can be indexed to the γ-CuBr crystal of the cubic structure (Joint Committee on Powder Diffraction Standards (JCPDS)card no.06-0292),suggesting the formation of pure crystalline CuBr microspheres.The electron diffraction (ED)pattern of a single CuBr sphere is shown in Figure 1d,which shows clear diffraction rings corresponding to γ-CuBr,indicating the polycrystalline nature of the CuBr spheres.

In the present system,ascorbic acid was selected as the reducing agent and CuBr 2was used as both the copper and bromine sources.Being a weak reducing agent,ascorbic acid can reduce Cu 2+to Cu +ions in aqueous solution,which

would combine with the Br -ions in the solution to form the CuBr precipitates (K sp ) 6.27×10-9)at room temperature.35It was observed that the CuBr microspheres were not very stable in aqueous solutions and the presence of excess ascorbic acid in the aqueous solution helped to prevent the CuBr microspheres from oxidation.After the excess ascorbic acid is washed off,the CuBr microspheres can be oxidized relatively easily;hence,special care should be taken when these microspheres are separated from the solution.

It was found that PVP played an important role in the formation of spherical CuBr particles.To clearly show the effect of PVP on the morphology of the CuBr particles,the synthesis was carried out at different PVP concentrations (Figure S1,Supporting Information).When there was no PVP in the starting solution,only polyhedral CuBr products with relatively smooth surfaces were obtained.When 0.004g L -1PVP was added to the solution,aggregated polyhedral particles were obtained,and nearly spherical particles with rough surfaces were produced when the PVP concentration was increased to 0.04g L -1.Spherical particles were generated only when the PVP concentration was above 0.4g L -1.As a well-known capping or stabilizing agent,PVP molecules with long chains can be adsorbed to the CuBr particle surfaces via physical and chemical bonding.When a smaller amount of PVP was added to the reaction system,the capping effect of PVP toward CuBr particles was weaker or insuf?cient,and thus,particles with polyhedral shapes similar to that in the absence of PVP were formed.On increasing the amount of PVP,the CuBr particles were encircled completely by PVP molecules and CuBr nanoc-rystals with smaller and nearly spherical shapes were formed,which preferred to aggregate into large spheres.This unique role of PVP for the formation of spherical structures was frequently reported;examples include the formation of monodisperse CdS spheres by the aggregation of smaller nanoparticles.36

The in situ formed CuBr microspheres can be used as a general sacri?cial template for the controlled synthesis of Cu 2O and Cu x S hollow spheres by directly adding NaOH and Na 2S solutions to the turbid solution containing CuBr microspheres,respectively.Since the thus obtained hollow spheres of Cu 2O and Cu x S have relatively thicker shells compared with the hollow spheres obtained by using Cu(I)-containing aggregates as the template as described below,hereafter the former hollow spheres are denoted as thick-shell hollow spheres and the latter hollow spheres are denoted as thin-shell hollow spheres.Upon the addition of NaOH aqueous solution at room temperature,the white dispersion of the CuBr spheres became yellow quickly.The XRD pattern of the products (Figure 1b)indicates the formation of pure Cu 2O crystals of cubic structure (JCPDS card no.05-0667).As shown in Figure 3,after reacting with NaOH,the CuBr solid spheres were almost completely converted into Cu 2O hollow spheres with diameters mainly ranging from 800to 1200nm and a shell thickness of about 130-170

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Figure 1.SEM (a -c)and TEM (d)images of CuBr spheres obtained in the presence of 2g L -1PVP.The inset shows the ED pattern corresponding to a single

sphere.

Figure 2.XRD patterns of CuBr spheres (a)and the templated thick-shell hollow spheres of Cu 2O (b)and CuS (c).

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nm.The ED pattern of a single hollow sphere exhibits clear rings ascribed to the Cu 2O crystal,indicating the polycrys-talline nature of the hollow Cu 2O spheres.The diameter of these hollow spheres is basically comparable to that of the original CuBr spheres,suggesting that the in situ formed CuBr spheres actually acted as a reactive sacri?cial template for the formation of the thick-shell hollow Cu 2O spheres.Similarly,thick-shell hollow Cu x S spheres can also be obtained by adding Na 2S solution to the reaction solution containing the CuBr spheres.When the reaction between Na 2S and the CuBr spheres was carried out at room temperature,well-de?ned hollow spheres of copper sul?de with rough surfaces were obtained (Figure S2,Supporting Information).The obtained hollow spheres were rather uniform with a diameter of about 1.0-1.2μm and a shell thickness of about 150-180nm.However,the XRD pattern of the as-prepared hollow spheres suggests that they are poorly crystallized or basically amorphous.The EDX result indicates that the molar ratio of Cu to S in these hollow spheres is nearly 2:1,indicating that they predominantly consist of amorphous Cu 2S (Figure S3,Supporting Informa-tion).

It has been documented that the copper -sulfur system can exist in the amorphous Cu 2S and crystalline covellite (CuS)phases with several phases of varying stoichiometries between these two ideal compositions and the transformation from amorphous Cu 2S to crystalline CuS can occur due to the spontaneous disproportion from the Cu(I)phase to the Cu(II)phase.37Accordingly,a hydrothermal treatment of the reaction solution containing the amorphous Cu 2S product was carried out to produce crystalline copper sul?de products.After being hydrothermally treated at 160°C for 24h,the amorphous Cu 2S transformed into crystalline CuS of the covellite phase (JCPDS card no.06-0464),as indicated by the XRD pattern shown in Figure 1c.Moreover,the EDX analysis of the product indicates that it has a Cu:S ratio of 52.9:47.1,which is near the stoichiometric ratio of CuS with a slight Cu excess (Figure S3,Supporting Information),indicating the formation of the crystalline CuS phase with the possible coexistence of a small amount of Cu 2S or other intermediate sul?de species as amorphous impurities.Figure 4shows typical SEM and TEM images of the hydrothermally treated copper sul?de hollow spheres.It can be seen that the obtained CuS product exclusively consists of uniform hollow spheres with rough surfaces,which look like the Cu 2S hollow spheres obtained without hydrothermal treatment.The hollow CuS spheres show a shell thickness of about 150-180nm,which is similar to that of the original Cu 2S hollow

spheres,but have a diameter of about 1.0-1.1μm,which is a little smaller than the diameter of the original Cu 2S hollow spheres,indicating a slight condensation for the hollow spheres after the crystallization from the amorphous phase.The ED pattern of a single CuS hollow sphere exhibits clear rings ascribed to the covellite-phase CuS,suggesting that the hollow spheres are actually polycrystalline and consist of primary CuS nanocrystals.It may be noted that the diameter of both the Cu 2S and CuS hollow spheres is basically comparable to that of the original CuBr solid spheres (～0.9μm in diameter),suggesting that the CuBr microspheres still played the role of sacri?cial templates for the formation of the Cu x S hollow spheres.Therefore,it may be concluded that the in situ formed CuBr microspheres can be employed as a general sacri?cial template for the fabrication of hollow microspheres of both cuprous oxide and copper sul?des.

Formation of Thin-Shell Hollow Spheres.It is interesting to ?nd that uniform thin-shell hollow spheres of Cu 2O and Cu x S with a shell thickness of about 20-25nm can be readily obtained if Cu(I)-containing aggregates instead of CuBr solid spheres are employed as the template.In this case,(C 4H 9)4NBr and CuCl 2were used as the bromide and copper sources,respectively,rather than the use of CuBr 2as both the bromide and copper sources.After the reductant ascorbic acid was added to the solution,the solution became turbid,but no solid precipitates could be separated from the solution by simple centrifugation,indicating the formation of Cu(I)-containing aggregates rather than CuBr solids.Then thin-shell Cu 2O and Cu x S hollow spheres can be produced upon the addition of NaOH and Na 2S to the solution,respectively.As shown in Figure 5a,b,uniform Cu 2O hollow spheres with a diameter of about 570nm and a shell thickness of about 20nm were obtained in a large yield when NaOH was added to the reaction solution at room temperature.The corresponding XRD pattern suggests that the hollow spheres are pure crystalline Cu 2O hollow spheres (Figure 6a).

On the other hand,poorly crystallized Cu 2S hollow spheres with a shell thickness of about 25nm were obtained when Na 2S was added to the reaction solution at room temperature (Figure S4,Supporting Information).After being hydrother-mally treated at 160°C for 24h,these Cu 2S hollow spheres transformed into well-crystallized CuS,which was demon-

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Figure 3.SEM (a)and TEM (b)images of thick-shell Cu 2O hollow spheres.The inset in (a)shows a broken hollow sphere.The inset in (b)shows the ED pattern corresponding to a single hollow

sphere.

Figure 4.SEM (a -c)and TEM (d)images of thick-shell CuS hollow spheres.The inset shows the ED pattern corresponding to a single hollow sphere.

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strated by the XRD pattern shown in Figure 6b.However,the apparent amorphous background in the XRD pattern suggests that there also possibly coexists a small amount of Cu 2S or other intermediate sul?de species as amorphous impurities in the product.As shown in Figure 5c,d,uniform CuS hollow spheres with a diameter of about 570nm and a shell thickness of about 25nm were obtained in a large yield.The polycrystalline nature of the hollow spheres was indicated by the related ED pattern.Furthermore,the EDX analysis of the Cu x S hollow spheres before and after the hydrothermal treatment suggests that the molar ratio of Cu to S is nearly 2:1and 1:1for the products before and after the hydrothermal treatment,respectively (Figure S5,Sup-porting Information),con?rming the transformation from amorphous Cu 2S to crystalline CuS upon the hydrothermal treatment.

These results suggest that (C 4H 9)4NBr played a key role in the formation of the thin-shell hollow spheres of copper oxide and sul?des.Our preliminary experiment showed that,in the absence of (C 4H 9)4NBr,the reaction between ascorbic acid and CuCl 2was not visible and no precipitates were obtained,which could be attributed to the relatively large solubility of CuCl (K sp )1.72×10-7).35If KBr was added to the CuCl 2solution as the bromide source,CuBr spheres could be obtained upon the addition of ascorbic acid,similar to the case of the direct use of CuBr 2as both the copper and bromide sources.Moreover,if (CH 3)4NBr or (C 2H 5)4NBr was used as the bromide source,CuBr spheres could still be obtained upon the addition of ascorbic acid.In contrast,if

(C 4H 9)4NBr was used as the bromide source,CuBr spheres could not be obtained although the reaction solution also became turbid,indicating the formation of Cu(I)-containing aggregates.These results suggest that,compared with the (CH 3)4N +and (C 2H 5)4N +ions,the (C 4H 9)4N +ions with relatively longer and more hydrophobic alkyl groups could lead to the formation of spherical aggregates consisting of Cu +,Br -,and (C 4H 9)4N +ions.It has been reported that (C 4H 9)4NBr can assemble rapidly into aggregates like vesicles in water when mixed with some metal compounds such as PdCl 2,KAuCl 4,AgNO 3,and K 2PtCl 4.38Similarly,in the current situation,spherical aggregates consisting of Cu +,Br -,and (C 4H 9)4N +ions could form upon mixing (C 4H 9)4NBr,Cu 2+ions,and ascorbic acid,which subsequently acted as both sacri?cial templates and the copper source for the formation of thin-shell hollow spheres of Cu 2O and Cu x S.Our preliminary experiments showed that both bromide ions and copper(I)were essential for the formation of the aggregate templates.For example,the mixed solutions containing the Cu 2+ions did not become turbid when (C 4H 9)4NCl instead of (C 4H 9)4NBr was used,and the mixed solution also remained clear when ascorbic acid was not added in the presence of (C 4H 9)4NBr.As expected,no hollow spheres were obtained from these clear solutions after the addition of either NaOH or Na 2S.To summarize,a possible formation process of the copper-based hollow spheres with different shell thicknesses via sacri?cial templates is sche-matically illustrated in Figure 7.In brief,when CuBr 2was used as both the copper and bromide sources,CuBr solid spheres formed,which subsequently acted as sacri?cial templates for the formation of thick-shell hollow spheres of Cu 2O and Cu x S;on the other hand,when CuCl 2was used as the copper source and (C 4H 9)4NBr was used as the bromide source,spherical aggregates consisting of Cu +,Br -,and (C 4H 9)4N +ions formed,which subsequently acted as sac-ri?cial templates for the formation of thin-shell hollow spheres of Cu 2O and Cu x S.The shell thickness of the hollow spheres could be related to the content of Cu(I)ions in the spherical https://www.wendangku.net/doc/c34475253.html,ly,a relatively smaller content of Cu(I)ions in the loose aggregates would result in a much thinner shell for the ?nal hollow spheres.

Optical Properties of the Thin-Shell Hollow Spheres.The optical properties of both Cu 2O and CuS are of interest because of their semiconductor characteristics.Considering that the obtained thin-shell hollow spheres can be readily dispersed in water and ethanol with a good stability,which

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Figure 5.SEM (a,c)and TEM (b,d)images of thin-shell hollow spheres of Cu 2O (a,b)and CuS (c,d).The insets show the related ED

patterns.

Figure 6.XRD patterns of thin-shell hollow spheres of Cu 2O (a)and CuS

(b).

Figure 7.Schematic illustration of the formation of thick-shell (a)and thin-shell (b)copper-based hollow spheres via sacri?cial templates.

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Chem.Mater.,Vol.20,No.19,2008Synthesis of Uniform Cu 2O and CuS Hollow Spheres

makes them relatively ideal systems for optical investigation,the thin-shell hollow spheres of crystalline Cu 2O and CuS were selected for the measurements of the UV -vis absorp-tion and optical limiting properties.Figure 8displays the UV -vis absorption spectra of the thin-shell Cu 2O and CuS hollow spheres suspended in water.As shown in Figure 8a,the Cu 2O hollow spheres show two humplike absorptions around 355and 445nm,which considerably blue-shifted from the absorption of bulk Cu 2O (～570nm)possibly as a result of quantum size effects.29,39-41In a previous paper a quantum-con?nement threshold was deduced to be 14nm for Cu 2O nanocrystallites,30which is slightly less than the shell thickness of the present thin-shell hollow spheres (～20nm).This can be rationalized by considering that the polycrystalline shell of the hollow spheres actually consists of primary nanocrystallites considerably smaller than 14nm.Figure 8b presents the absorption spectrum of the CuS hollow spheres,which shows an absorption peak at ～415nm,which is reminiscent of the absorption for CuS nanowires (～400nm)42and for CuS hollow nanospheres (～470nm).24Moreover,the thin-shell CuS hollow spheres show an increased absorption in the near-IR region,which is char-acteristic of covellite (CuS).37It is noted that the long absorption tail could also be attributed to the scattering effects of the hollow particles;24however,since such an absorption tail in the near-IR region was not observed for the thin-shell Cu 2O hollow spheres with a similar sphere size and shell thickness,it might be more appropriate to ascribe the absorption tail to the inherent property of CuS rather than to the scattering effects.

In recent years,there has been increasing interest in materials that exhibit strong optical limiting properties for their potential applications in the protection of sensors and human eyes from intense laser radiations.An ideal optical limiter exhibits a high,linear transmission at low ?uence,but its output ?uence is greatly attenuated at high ?uence.The optical limiting properties of carbon nanotubes have been

investigated extensively,and they have been shown to be good candidates for optical limiting applications.43The suspensions of thin-shell Cu 2O and CuS hollow spheres in ethanol were found to exhibit remarkable optical limiting properties with nanosecond laser pulses at 1064nm using carbon nanotubes as the reference.As shown in Figure 9,the suspensions of the Cu 2O hollow spheres exhibit linear absorption with approximately ?at transmittances when the incident ?uence is below 0.29J/cm 2,above which the transmittances are signi?cantly attenuated with increasing incident ?uence,exhibiting obvious nonlinear optical re-sponses or optical limiting effects.The transmittances of the suspensions formed by the CuS hollow spheres are continu-ously attenuated with increasing incident ?uence from very low ?uence,suggesting that the dramatic optical limiting effects occur at an incident ?uence much lower than that for the Cu 2O hollow spheres.The limiting threshold,known as the input ?uence at which the transmittance decreases to half of the linear transmittance,can be used to compare the optical limiting properties of different materials.Here the linear transmittances for both Cu 2O and CuS hollow spheres were determined by the linear transmittance spectra obtained from the normal optical absorption.The thresholds of Cu 2O and CuS are measured to be 1.78and 0.51J/cm 2,respec-tively.At 1064nm the carbon nanotubes have exhibited good optical limiting effects,whose thresholds were reported to be about 1.5and 2.5J/cm 2for single-wall carbon nanotubes (SWNTs)43and multiwall carbon nanotubes (MWNTs),44respectively.It is obvious that the threshold of the Cu 2O suspension lies between those for SWNT and MWNT suspensions while the threshold of the CuS suspension is considerably smaller than those for SWNT and MWNT suspensions.Therefore,both the Cu 2O and CuS suspensions exhibit good optical limiting effects,with the CuS suspension showing relatively stronger optical limiting effects.To the best of our knowledge,this is the ?rst time that the optical limiting effects of the Cu 2O suspension have been reported.It may be noted that,in our previous study,strong optical limiting properties have been observed for the CuS hollow

(39)Yin,M.;Wu,C.;Lou,Y.;Burda,C.;Koberstein,J.T.;Zhu,Y.;

O’Brien,S.J.Am.Chem.Soc.2005,127,9506.

(40)Ng,C.H.B.;Fan,W.J.Phys.Chem.B 2006,110,20801.

(41)Xu,Y.;Chen,D.;Jiao,X.;Xue,K.J.Phys.Chem.C 2007,111,

16284.

(42)Roy,P.;Srivastava,S.K.Cryst.Growth Des.2006,6,1921.

(43)(a)Vivien,L.;Lancon,P.;Riehl,D.;Hache,F.;Anglaret,E.Carbon

2002,40,1789.(b)Wang,Q.;Qin,Y.;Zhu,Y.;Huang,X.;Tian,Y.;Zhang,P.;Guo,Z.-X.;Wang,Y.Chem.Phys.Lett.2008,457,159.(44)Zhan,H.;Chen,W.;Wang,M.;Zheng,C.;Zou,C.Chem.Phys.Lett.

2003,382,

313.

Figure 8.UV -vis absorption spectra of thin-shell hollow spheres of Cu 2O (a)and CuS

(b).

Figure 9.Fluence-dependent transmittance of thin-shell hollow spheres of Cu 2O (4)and CuS (2)at a wavelength of 1064nm.

6268Chem.Mater.,Vol.20,No.19,2008Gao et al.

spheres with a diameter of about200nm and a shell thickness of about30nm.24Compared with those CuS hollow spheres, the current thin-shell CuS hollow spheres with a diameter of about570nm and a shell thickness of about20-25nm showed somewhat weaker optical limiting properties,which could be partly attributed to the differences in the diameter and shell thickness of the CuS hollow spheres.However, the relationship between the optical limiting property and the diameter and shell thickness of the CuS hollow spheres remains to be invesitigated in more detail.

In general,the origins of optical limiting effects can be attributed to two-photon absorption,free carrier absorption, and nonlinear scattering.As Cu2O is a direct band gap semiconductor with a band gap of～2.17eV and the incident photon energy is1.17eV(1064nm),which is smaller than the band gap but larger than half of the band gap,two-photon absorption is generally a nonlinear optical process and could be the main mechanism of the optical limiting effects.The incident photon energy is also smaller than the band gap of CuS,so two-photon absorption could also occur;however, since CuS is an indirect band gap semiconductor,there is certain linear absorption at1064nm,so the free carrier absorption induced by the linear absorption would also play an important role in the optical limiting effects.It is known that,for the objects with sizes comparable to the wavelength, the nonlinear scattering due to the bubbles and microplasma is signi?cant.45The bubbles are formed by heat transfer from particles to the surrounding solvent,and their contribution to the nonlinear scattering is obvious within several nano-seconds.46The microplasma are generated by the ionization with the intensive laser,and the rapid expansion of the microplasma will strongly scatter the incident laser pulse.47 In the current situation,the thin-shell Cu2O and CuS hollow spheres could provide rather dense initiation sites in the polycrystalline shell for the formation of bubbles and microplasma during nanosecond-pulse irradiation,thus en-hancing the nonlinear scattering and the optical limiting effects.Moreover,the?eld enhancement along the interfaces between the inner vacuum and the Cu2O or CuS shell with some properties of an electric conductor could also contribute to the enhanced optical limiting properties of the hollow spheres.As for the different optical limiting effects exhibited by the Cu2O and CuS hollow spheres,the stronger free-carrier absorption in the CuS hollow spheres,which is more effective than two-photon absorption,could be responsible for their stronger optical limiting effects.Nevertheless,a more detailed investigation on the optical limiting properties of the obtained semiconductor hollow spheres is required for elucidating the exact origination of the remarkable optical limiting effects,which is currently under way.

Conclusions

A facile one-pot synthesis of uniform hollow spheres of Cu2O and Cu x S(e.g.,Cu2S and CuS)has been realized by chemical transformation of in situ formed sacri?cial tem-plates containing Cu(I)in aqueous solutions.The shell thickness of these hollow spheres can be adjusted through the choice of the bromide source used for the formation of intermediate templates.When CuBr2was used as both the copper and bromide sources,CuBr solid spheres formed by the reduction of CuBr2with ascorbic acid in the presence of PVP,which subsequently acted as sacri?cial templates for the formation of thick-shell hollow spheres of Cu2O and Cu x S (about130-180nm in shell thickness).On the other hand, when CuCl2was used as the copper source and(C4H9)4NBr was used as the bromide source,spherical aggregates consisting of Cu+,Br-,and(C4H9)4N+ions formed,which subsequently acted as sacri?cial templates for the formation of thin-shell hollow spheres of Cu2O and Cu x S(about20-25 nm in shell thickness).In both cases,crystalline Cu2O hollow spheres were directly obtained at room temperature,while poorly crystallized Cu2S hollow spheres were?rst obtained at room temperature and transformed into well-crystallized CuS hollow spheres after a hydrothermal treatment at160°C.A strong optical limiting response has been detected for the thin-shell hollow spheres of both Cu2O and CuS,which could open up new applications for semiconductor hollow spheres in the?eld of optical limiting,such as the protection of human eyes or optical sensors from high-power laser irradiation.

Acknowledgment.This work was supported by NSFC (Grants20673007,20633010,10674010,10521002,and 50521201),MOST(Grant2007CB936201),and SRFDP(Grant 20070001018).

Supporting Information Available:SEM images of CuBr products obtained at different PVP concentrations and thin-shell Cu2S hollow spheres,SEM and TEM images and XRD pattern of thick-shell Cu2S hollow spheres,and EDX spectra of thick-and thin-shell Cu x S hollow spheres(PDF).This material is available free of charge via the Internet at https://www.wendangku.net/doc/c34475253.html,.

CM801407Q

(45)King,S.M.;Chaure,S.;Doyle,J.;Colli,A.;Ferrari,A.C.;Blau,

https://www.wendangku.net/doc/c34475253.html,mun.2007,276,305.

(46)Vivien,L.;Riehl,D.;Delouis,J.F.;Delaire,J.A.;Hache,F.;Anglaret,

E.J.Opt.Soc.Am.B2002,19,208.

(47)Mansour,K.;Soileau,M.;Vanstryland,E.J.Opt.Soc.Am.B1992,

9,1100.

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Synthesis of Uniform Cu2O and CuS Hollow Spheres

从实践的角度探讨在日语教学中多媒体课件的应用

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Unit1 Americans believe no one stands still. If you are not moving ahead, you are falling behind. This attitude results in a nation of people committed to researching, experimenting and exploring. Time is one of the two elements that Americans save carefully, the other being labor. "We are slaves to nothing but the clock,” it has been said. Time is treated as if it were something almost real. We budget it, save it, waste it, steal it, kill it, cut it, account for it; we also charge for it. It is a precious resource. Many people have a rather acute sense of the shortness of each lifetime. Once the sands have run out of a person’s hourglass, they cannot be replaced. We want every minute to count. A foreigner’s first impression of the U.S. is li kely to be that everyone is in a rush -- often under pressure. City people always appear to be hurrying to get where they are going, restlessly seeking attention in a store, or elbowing others as they try to complete their shopping. Racing through daytime meals is part of the pace

新视野大学英语第三版第二册课文语法讲解 Unit4

新视野三版读写B2U4Text A College sweethearts 1I smile at my two lovely daughters and they seem so much more mature than we,their parents,when we were college sweethearts.Linda,who's21,had a boyfriend in her freshman year she thought she would marry,but they're not together anymore.Melissa,who's19,hasn't had a steady boyfriend yet.My daughters wonder when they will meet"The One",their great love.They think their father and I had a classic fairy-tale romance heading for marriage from the outset.Perhaps,they're right but it didn't seem so at the time.In a way, love just happens when you least expect it.Who would have thought that Butch and I would end up getting married to each other?He became my boyfriend because of my shallow agenda:I wanted a cute boyfriend! 2We met through my college roommate at the university cafeteria.That fateful night,I was merely curious,but for him I think it was love at first sight."You have beautiful eyes",he said as he gazed at my face.He kept staring at me all night long.I really wasn't that interested for two reasons.First,he looked like he was a really wild boy,maybe even dangerous.Second,although he was very cute,he seemed a little weird. 3Riding on his bicycle,he'd ride past my dorm as if"by accident"and pretend to be surprised to see me.I liked the attention but was cautious about his wild,dynamic personality.He had a charming way with words which would charm any girl.Fear came over me when I started to fall in love.His exciting"bad boy image"was just too tempting to resist.What was it that attracted me?I always had an excellent reputation.My concentration was solely on my studies to get superior grades.But for what?College is supposed to be a time of great learning and also some fun.I had nearly achieved a great education,and graduation was just one semester away.But I hadn't had any fun;my life was stale with no component of fun!I needed a boyfriend.Not just any boyfriend.He had to be cute.My goal that semester became: Be ambitious and grab the cutest boyfriend I can find. 4I worried what he'd think of me.True,we lived in a time when a dramatic shift in sexual attitudes was taking place,but I was a traditional girl who wasn't ready for the new ways that seemed common on campus.Butch looked superb!I was not immune to his personality,but I was scared.The night when he announced to the world that I was his girlfriend,I went along

新视野大学英语读写教程第一册课文翻译及课后答案

Unit 1 1学习外语是我一生中最艰苦也是最有意义的经历之一。虽然时常遭遇挫折，但却非常有价值。 2我学外语的经历始于初中的第一堂英语课。老师很慈祥耐心，时常表扬学生。由于这种积极的教学方法，我踊跃回答各种问题，从不怕答错。两年中，我的成绩一直名列前茅。 3到了高中后，我渴望继续学习英语。然而，高中时的经历与以前大不相同。以前，老师对所有的学生都很耐心，而新老师则总是惩罚答错的学生。每当有谁回答错了，她就会用长教鞭指着我们，上下挥舞大喊：“错！错！错！”没有多久，我便不再渴望回答问题了。我不仅失去了回答问题的乐趣，而且根本就不想再用英语说半个字。 4好在这种情况没持续多久。到了大学，我了解到所有学生必须上英语课。与高中老师不。大学英语老师非常耐心和蔼，而且从来不带教鞭！不过情况却远不尽如人意。由于班大，每堂课能轮到我回答的问题寥寥无几。上了几周课后，我还发现许多同学的英语说得比我要好得多。我开始产生一种畏惧感。虽然原因与高中时不同，但我却又一次不敢开口了。看来我的英语水平要永远停步不前了。 5直到几年后我有机会参加远程英语课程，情况才有所改善。这种课程的媒介是一台电脑、一条电话线和一个调制解调器。我很快配齐了必要的设备并跟一个朋友学会了电脑操作技术，于是我每周用5到7天在网上的虚拟课堂里学习英语。 6网上学习并不比普通的课堂学习容易。它需要花许多的时间，需要学习者专心自律，以跟上课程进度。我尽力达到课程的最低要求，并按时完成作业。 7我随时随地都在学习。不管去哪里，我都随身携带一本袖珍字典和笔记本，笔记本上记着我遇到的生词。我学习中出过许多错，有时是令人尴尬的错误。有时我会因挫折而哭泣，有时甚至想放弃。但我从未因别的同学英语说得比我快而感到畏惧，因为在电脑屏幕上作出回答之前，我可以根据自己的需要花时间去琢磨自己的想法。突然有一天我发现自己什么都懂了，更重要的是，我说起英语来灵活自如。尽管我还是常常出错，还有很多东西要学，但我已尝到了刻苦学习的甜头。 8学习外语对我来说是非常艰辛的经历，但它又无比珍贵。它不仅使我懂得了艰苦努力的意义，而且让我了解了不同的文化，让我以一种全新的思维去看待事物。学习一门外语最令人兴奋的收获是我能与更多的人交流。与人交谈是我最喜欢的一项活动，新的语言使我能与陌生人交往，参与他们的谈话，并建立新的难以忘怀的友谊。由于我已能说英语，别人讲英语时我不再茫然不解了。我能够参与其中，并结交朋友。我能与人交流，并能够弥合我所说的语言和所处的文化与他们的语言和文化之间的鸿沟。 III. 1. rewarding 2. communicate 3. access 4. embarrassing 5. positive 6. commitment 7. virtual 8. benefits 9. minimum 10. opportunities IV. 1. up 2. into 3. from 4. with 5. to 6. up 7. of 8. in 9. for 10.with V. 1.G 2.B 3.E 4.I 5.H 6.K 7.M 8.O 9.F 10.C Sentence Structure VI. 1. Universities in the east are better equipped, while those in the west are relatively poor. 2. Allan Clark kept talking the price up, while Wilkinson kept knocking it down. 3. The husband spent all his money drinking, while his wife saved all hers for the family. 4. Some guests spoke pleasantly and behaved politely, while others wee insulting and impolite. 5. Outwardly Sara was friendly towards all those concerned, while inwardly she was angry. VII. 1. Not only did Mr. Smith learn the Chinese language, but he also bridged the gap between his culture and ours. 2. Not only did we learn the technology through the online course, but we also learned to communicate with friends in English. 3. Not only did we lose all our money, but we also came close to losing our lives.

新大学日语简明教程课文翻译

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新视野大学英语3第三版课文翻译 Unit 1 The Way to Success 课文A Never, ever give up! 永不言弃！ As a young boy, Britain's great Prime Minister, Sir Winston Churchill, attended a public school called Harrow. He was not a good student, and had he not been from a famous family, he probably would have been removed from the school for deviating from the rules. Thankfully, he did finish at Harrow and his errors there did not preclude him from going on to the university. He eventually had a premier army career whereby he was later elected prime minister. He achieved fame for his wit, wisdom, civic duty, and abundant courage in his refusal to surrender during the miserable dark days of World War II. His amazing determination helped motivate his entire nation and was an inspiration worldwide. Toward the end of his period as prime minister, he was invited to address the patriotic young boys at his old school, Harrow. The headmaster said, "Young gentlemen, the greatest speaker of our time, will be here in a few days to address you, and you should obey whatever sound advice he may give you." The great day arrived. Sir Winston stood up, all five feet, five inches and 107 kilos of him, and gave this short, clear-cut speech: "Young men, never give up. Never give up! Never give up! Never, never, never, never!" 英国的伟大首相温斯顿·丘吉尔爵士，小时候在哈罗公学上学。当时他可不是个好学生，要不是出身名门，他可能早就因为违反纪律被开除了。谢天谢地，他总算从哈罗毕业了，在那里犯下的错误并没影响到他上大学。后来，他凭着军旅生涯中的杰出表现当选为英国首相。他的才思、智慧、公民责任感以及在二战痛苦而黑暗的时期拒绝投降的无畏勇气，为他赢得了美名。他非凡的决心，不仅激励了整个民族，还鼓舞了全世界。 在他首相任期即将结束时，他应邀前往母校哈罗公学，为满怀报国之志的同学们作演讲。校长说：“年轻的先生们，当代最伟大的演说家过几天就会来为你们演讲，他提出的任何中肯的建议，你们都要听从。”那个激动人心的日子终于到了。温斯顿爵士站了起来——他只有5 英尺5 英寸高，体重却有107 公斤。他作了言简意赅的讲话：“年轻人，要永不放弃。永不放弃！永不放弃！永不，永不，永不，永不！” Personal history, educational opportunity, individual dilemmas - none of these can inhibit a strong spirit committed to success. No task is too hard. No amount of preparation is too long or too difficult. Take the example of two of the most scholarly scientists of our age, Albert Einstein and Thomas Edison. Both faced immense obstacles and extreme criticism. Both were called "slow to learn" and written off as idiots by their teachers. Thomas Edison ran away from school because his teacher whipped him repeatedly for asking too many questions. Einstein didn't speak fluently until he was almost nine years old and was such a poor student that some thought he was unable to learn. Yet both boys' parents believed in them. They worked intensely each day with their sons, and the boys learned to never bypass the long hours of hard work that they needed to succeed. In the end, both Einstein and Edison overcame their childhood persecution and went on to achieve magnificent discoveries that benefit the entire world today. Consider also the heroic example of Abraham Lincoln, who faced substantial hardships, failures and repeated misfortunes in his lifetime. His background was certainly not glamorous. He was raised in a very poor family with only one year of formal education. He failed in business twice, suffered a nervous breakdown when his first love died suddenly and lost eight political

新视野大学英语读写教程2-(第三版)-unit-2-课文原文及翻译

Text A 课文 A The humanities: Out of date? 人文学科：过时了吗？ When the going gets tough, the tough takeaccounting. When the job market worsens, manystudents calculate they can't major in English orhistory. They have to study something that booststheir prospects of landing a job. 当形势变得困难时，强者会去选学会计。当就业市场恶化时，许多学生估算着他们不能再主修英语或历史。他们得学一些能改善他们就业前景的东西。 The data show that as students have increasingly shouldered the ever-rising c ost of tuition,they have defected from the study of the humanities and toward applied science and "hard"skills that they bet will lead to employment. In oth er words, a college education is more andmore seen as a means for economic betterment rather than a means for human betterment.This is a trend that i s likely to persist and even accelerate. 数据显示，随着学生肩负的学费不断增加，他们已从学习人文学科转向他们相信有益于将来就业的应用科学和“硬”技能。换言之，大学教育越来越被看成是改善经济而不是提升人类自身的手段。这种趋势可能会持续，甚至有加快之势。 Over the next few years, as labor markets struggle, the humanities will proba bly continue theirlong slide in succession. There already has been a nearly 50 percent decline in the portion of liberal arts majors over the past generatio n, and it is logical to think that the trend is boundto continue or even accel erate. Once the dominant pillars of university life, the humanities nowplay li ttle roles when students take their college tours. These days, labs are more vi vid and compelling than libraries. 在未来几年内，由于劳动力市场的不景气，人文学科可能会继续其长期低迷的态势。在上一代大学生中，主修文科的学生数跌幅已近50%。这种趋势会持续、甚至加速的想法是合情合理的。人文学科曾是大学生活的重要支柱，而今在学生们参观校园的时候，却只是一个小点缀。现在，实验室要比图书馆更栩栩如生、受人青睐。 Here, please allow me to stand up for and promote the true value that the h umanities add topeople's lives. 在这儿，请允许我为人文学科给人们的生活所增添的真实价值进行支持和宣传。

新大学日语课文翻译。

第10课 日本的季节 日本的一年有春、夏、秋、冬四个季节。 3月、4月和5月这三个月是春季。春季是个暖和的好季节。桃花、樱花等花儿开得很美。人们在4月去赏花。 6月到8月是夏季。夏季非常闷热。人们去北海道旅游。7月和8月是暑假，年轻人去海边或山上。也有很多人去攀登富士山。富士山是日本最高的山。 9月、10月和11月这3个月是秋季。秋季很凉爽，晴朗的日子较多。苹果、桔子等许多水果在这个季节成熟。 12月到2月是冬季。日本的南部冬天不太冷。北部非常冷，下很多雪。去年冬天东京也很冷。今年大概不会那么冷吧。如果冷的话，人们就使用暖气炉。 第12课 乡下 我爷爷住哎乡下。今天，我要去爷爷家。早上天很阴，但中午天空开始变亮，天转好了。我急急忙忙吃完午饭，坐上了电车。 现在，电车正行驶在原野上。窗外，水田、旱地连成一片。汽车在公路上奔驰。 这时，电车正行驶在大桥上。下面河水在流动。河水很清澈，可以清澈地看见河底。可以看见鱼在游动。远处，一个小孩在挥手。他身旁，牛、马在吃草。 到了爷爷居住的村子。爷爷和奶奶来到门口等着我。爷爷的房子是旧房子，但是很大。登上二楼，大海就在眼前。海岸上，很多人正在全力拉缆绳。渐渐地可以看见网了。网里有很多鱼。和城市不同，乡下的大自然真是很美。 第13课 暑假 大概没有什么比暑假更令学生感到高兴的了。大学在7月初，其他学校在二十四日左右进入暑假。暑假大约1个半月。 很多人利用这个假期去海边、山上，或者去旅行。学生中，也有人去打工。学生由于路费等只要半价，所以在学期间去各地旅行。因此，临近暑假时，去北海道的列车上就挤满了这样的人。从炎热的地方逃避到凉爽的地方去，这是很自然的事。一般在1月、最迟在2月底之前就要预定旅馆。不然的话可能会没有地方住。 暑假里，山上、海边、湖里、河里会出现死人的事，这种事故都是由于不注意引起的。大概只能每个人自己多加注意了。 在东京附近，镰仓等地的海面不起浪，因此挤满了游泳的人。也有人家只在夏季把海边的房子租下来。 暑假里，学校的老师给学生布置作业，但是有的学生叫哥哥或姐姐帮忙。 第14课 各式各样的学生 我就读的大学都有各种各样的学生入学。学生有的是中国人，有的是美国人，有的是英国人。既有年轻的，也有不年轻的。有胖的学生，也有瘦的学生。学生大多边工作边学习。因此，大家看上去都很忙。经常有人边听课边打盹。 我为了学习日本先进的科学技术和日本文化来到日本。预定在这所大学学习3年。既然特意来了日本，所以每天都很努力学习。即便如此，考试之前还是很紧张。其他学生也是这

新视野大学英语5课文翻译(全)

教育界的科技革命 如果让生活在年的人来到我们这个时代，他会辨认出我们当前课堂里发生的许多事情——那盛行的讲座、对操练的强调、从基础读本到每周的拼写测试在内的教学材料和教学活动。可能除了教堂以外，很少有机构像主管下一代正规教育的学校那样缺乏变化了。 让我们把上述一贯性与校园外孩子们的经历作一番比较吧。在现代社会，孩子们有机会接触广泛的媒体，而在早些年代这些媒体简直就是奇迹。来自过去的参观者一眼就能辨认出现在的课堂，但很难适应现今一个岁孩子的校外世界。 学校——如果不是一般意义上的教育界——天生是保守的机构。我会在很大程度上为这种保守的趋势辩护。但变化在我们的世界中是如此迅速而明确，学校不可能维持现状或仅仅做一些表面的改善而生存下去。的确，如果学校不迅速、彻底地变革，就有可能被其他较灵活的机构取代。 计算机的变革力 当今时代最重要的科技事件要数计算机的崛起。计算机已渗透到我们生活的诸多方面，从交通、电讯到娱乐等等。许多学校当然不能漠视这种趋势，于是也配备了计算机和网络。在某种程度上，这些科技辅助设施已被吸纳到校园生活中，尽管他们往往只是用一种更方便、更有效的模式教授旧课程。 然而，未来将以计算机为基础组织教学。计算机将在一定程度上允许针对个人的授课，这种授课形式以往只向有钱人提供。所有的学生都会得到符合自身需要的、适合自己学习方法和进度的课程设置，以及对先前所学材料、课程的成绩记录。 毫不夸张地说，计算机科技可将世界上所有的信息置于人们的指尖。这既是幸事又是灾难。我们再也无须花费很长时间查找某个出处或某个人——现在，信息的传递是瞬时的。不久，我们甚至无须键入指令，只需大声提出问题，计算机就会打印或说出答案，这样，人们就可实现即时的"文化脱盲"。 美中不足的是，因特网没有质量控制手段；"任何人都可以拨弄"。信息和虚假信息往往混杂在一起，现在还没有将网上十分普遍的被歪曲的事实和一派胡言与真实含义区分开来的可靠手段。要识别出真的、美的、好的信息，并挑出其中那些值得知晓的, 这对人们构成巨大的挑战。 对此也许有人会说，这个世界一直充斥着错误的信息。的确如此，但以前教育当局至少能选择他们中意的课本。而今天的形势则是每个人都拥有瞬时可得的数以百万计的信息源，这种情况是史无前例的。 教育的客户化 与以往的趋势不同，从授权机构获取证书可能会变得不再重要。每个人都能在模拟的环境中自学并展示个人才能。如果一个人能像早些时候那样"读法律"，然后通过计算机模拟的实践考试展现自己的全部法律技能，为什么还要花万美元去上法学院呢？用类似的方法学开飞机或学做外科手术不同样可行吗？ 在过去，大部分教育基本是职业性的：目的是确保个人在其年富力强的整个成人阶段能可靠地从事某项工作。现在，这种设想有了缺陷。很少有人会一生只从事一种职业；许多人都会频繁地从一个职位、公司或经济部门跳到另一个。 在经济中，这些新的、迅速变换的角色的激增使教育变得大为复杂。大部分老成持重的教师和家长对帮助青年一代应对这个会经常变换工作的世界缺乏经验。由于没有先例，青少年们只有自己为快速变化的"事业之路"和生活状况作准备。