Chemical Regulation of Carbon Quantum Dots from Synthesis to

DOI:10.1002/asia.201300076

Chemical Regulation of Carbon Quantum Dots from Synthesis to

Photocatalytic Activity

Shengliang Hu,*[a,b]Ruixue Tian,[b]Lingling Wu,[b]Qing Zhao,[b]Jinlong Yang,[b,c]

Jun Liu,[b]and Shirui Cao [b]

Introduction

Carbon quantum dots (CQDs)share the major advanta-geous characteristics of semiconductor quantum dots with-out incurring the burden of intrinsic toxicity or elemental scarcity.They can be obtained without the requirement for stringent,intricate,tedious,costly,or inefficient preparation steps,but they exhibit promising applications in bioimaging,photocatalysis,nonlinear optics,energy conversion,and so on.[1,2]

The studied results suggest that both the size and surface state are key to the properties of CQDs.[1–5]For example,size-dependent photoluminescence (PL)behavior was ob-served by many research groups.[5–7]Accordingly,many ef-forts have been made to obtain a narrow size distribution of CQDs.It is pleasing that bottom-up approaches such as the carbonization of small organic compounds (such as glucose and citric acids)and natural source (such as coffee and orange juice)through hydrothermal routes have achieved significant progress in preparing small-size CQDs.[8–12]How-ever,these methods still cannot realize size control of CQDs.Subsequent to the pioneering work towards the syn-thesis of CQDs by laser ablation,[1]several top-down ap-proaches have been proposed,but they generally involve te-dious size separation.Size control was recently realized by electrochemical oxidation and laser ablation of carbon pow-ders,[3,5]but such methods were limited to special carbon sources and had low yields.Mass production can be ob-tained by heating various carbon sources at reflux in aque-ous nitric acid.[13–15]Nevertheless,it remains unclear how to control size.Herein,we report that the size of CQDs can be tuned simply by adjusting the concentrations of the aqueous nitric acid reaction medium.

CQDs obtained from chemical oxidation of various carbon sources contain many kinds of surface oxygen groups (surface states),and their types determine the PL behavior and quantum yields of CQDs.[1,4,13–15]The photoexcited CQDs could be excellent electron donors and acceptors,[16]but there are few reports that the CQDs alone,like tradi-tional semiconductor quantum dots,showed photocatalytic activity.Although the composites of CQDs and other inor-ganic materials have shown good photocatalytic perfor-mance,[17–21]it was believed that CQDs only acted as visible-Abstract:Carbon quantum dots (CQDs)were synthesized by heating various carbon sources in HNO 3solu-tion at reflux,and the effects of HNO 3concentration on the size of the CQDs were investigated.Furthermore,the oxygen-containing surface groups of as-prepared CQDs were selectively re-duced by NaBH 4,leading to new sur-face states.The experimental results show that the sizes of CQDs can be tuned by HNO 3concentration and then influence their photoluminescent be-haviors;the photoluminescent proper-ties are related to both the size and surface state of the CQDs,but the pho-tocatalytic activities are determined by surface states alone.The different oxygen-containing groups on the sur-face of the CQDs can induce different degrees of the band bending upward,which determine the separation and combination of the electron–hole pairs.The high upward band bending,which is induced by C =O and COOH groups,facilitates separation of the electron–hole pairs and then enhances high pho-tocatalytic activity.In contrast,the low upward band bending induced by C àOH groups hardly prevents the elec-tron–hole pairs from surface recombi-nation and then exhibits strong photo-luminescence.Therefore,both the pho-tocatalytic activities and optical proper-ties of CQDs can be tuned by their sur-face states.

Keywords:carbon ·luminescene ·nanostructures ·photocatalysis ·surface chemistry

[a]Dr.S.Hu

Key Laboratory of Instrumentation Science and Dynamic Measurement

Ministry of Education,Science and Technology on Electronic Test and Measurement Laboratory Taiyuan 030051(China)Fax:(+86)351-3559638E-mail:hsliang@https://www.360docs.net/doc/f513446217.html, [b]Dr.S.Hu,R.Tian,L.Wu,Q.Zhao,Prof.J.Yang,Prof.J.Liu,S.Cao

School of Material Science and Engineering North University of China Taiyuan 030051(China)[c]Prof.J.Yang

State Key Laboratory of New Ceramics and Fine Processing Tsinghua University Beijing 100084

(China)

Supporting information for this article is available on the WWW under https://www.360docs.net/doc/f513446217.html,/10.1002/asia.201300076.

FULL

PAPER

light sensitizers in the photocatalytic process.Since some surface oxygen groups result in the radiative recombination of excitons,[1,21]there should be specific surface groups to transfer photogenerated charges for redox reaction.In this work,the dependence of photocatalytic activity on the sur-face states of CQDs was investigated.

Results and Discussion

For the same reaction time (8h),the sizes of the CQDs gen-erally decreased with increasing HNO 3concentrations (see Figure S1in the Supporting Information),but above 6m the average sizes of CQDs stay about 1nm.Figure 1a–c shows transmission electron microscopy (TEM)images of CQDs

obtained at different HNO 3concentrations.The images

reveal that the sizes and shapes of the as-prepared CQDs

change with increasing HNO 3concentration.To obtain a reli-able size distribution,we recorded about 200nanoparticles and carried out a statistical analysis of the different sizes.The average sizes correspond to 5,3,and 1.3nm,respective-ly (Figure 1d–f).Moreover,the crystallinity of CQDs from low HNO 3concentrations is better than that from high HNO 3concentrations.

It is well known that carbon soot and active carbon con-sist of disordered crystal fragments with very small sizes and

thus show an amorphous structure.At low HNO 3concentra-tion,the defect sites of large carbon particles were preferen-tially oxidized.Therefore,CQDs prepared under these con-ditions showed a spherical shape (Figure 1a)and better crystallinity.With the increase of HNO 3concentration,all sites on large carbon particles are exposed to a high oxida-tion rate,resulting in smaller particles and irregular shapes.This behavior could explain why the sizes and shapes of CQDs can be tuned by varying the HNO 3concentration.The luminescent properties of the as-prepared CQDs

were explored.Emission spectra of CQDs are dependent on excitation wavelength and particle size.Figure 2shows the photoluminescence (PL)and PL excitation (PLE)spectra of CQDs from three different HNO 3concentration.The PL spectra were recorded by excit-ing three samples with the strongest absorption wavelength in the PLE spectra,and the cor-responding PLE spectra were acquired with detection wave-lengths of 504,519,and 539nm,respectively,which also are the strongest emission peaks of three samples.There is a shift of the emission to longer wave-length as the size of CQDs de-creases.For the PLE spectra

shown in Figure 2,there are two peaks in three samples,and

the positions of the strongest peaks change with the sizes of CQDs.For example,the strongest PLE peak of sample 1is 435nm while that of sample 3is 470nm.The samples pre-pared with HNO 3concentrations above 6m have size distri-butions similar to sample 3and exhibit the same optical be-havior.The above optical behaviors could be attributed to the difference of defect types.We have demonstrated by experi-mental results and theoretical analysis that the luminescent centers of CQDs arise from the mixture of sp 2and sp 3bond-ing,and the energy gap depends on the fraction of sp 2-hy-bridized carbon atoms in the mixture,while the changes of size can influence the formation of sp 2or sp 3hybridization and their fraction.[22]Accordingly,the excitons with different

energy,which result in excitation-wavelength-dependent PL behavior,arise from the crystal defects in CQDs.[17]When the produced excitons are trapped by appropriate surface

states,the radiative recombination take places,and then visible light is emitted.However,which surface groups,such as hydroxy,carbonyl,carboxy,and epoxy that arise from the

different surface oxidation degree of carbon atoms,[23,24]are suitable for exciton trapping?In this context,the oxygen-containing surface groups were investigated by IR spectra of typical CQDs and reduced CQDs (R-CQDs)with NaBH 4

.

Figure 1.a–c)TEM images of CQDs from 2(a),4(b),and 6m HNO 3(c).d–f)The corresponding size distribu-

tions.Figure 2.PL and PLE spectra of CQDs from 2,4,and 6m HNO 3.The PL spectra were recorded by exciting three samples with the strongest ab-sorption wavelength in the PLE spectra,and the corresponding PLE spectra were acquired with detection wavelengths of 504,519,and 539nm,respectively.

As shown in Figure 3,the stretching vibrations of àOH,C =O,and C àO àC were observed in both CQDs and R-CQDs.However,the vibrational absorption band of C =O at

1638cm à1shifted to 1676cm à1and its intensity decreased after reduction.Moreover,the stretching vibrations of àOH from CQDs was wider than that from R-CQDs.These changes could be attributed to the existence of many more COOH groups in CQDs.[23]

To further confirm the nature of the functional groups on the surface of both CQDs and R-CQDs,we carried out X-ray photoelectron spectroscopy (XPS)characterization.Figure 4shows the XPS spectra of C1s.The measured spec-trum of CQDs (shown in Figure 4a)can be deconvoluted into four surface components,corresponding to sp 2(C =C)at binding energy of 284.4eV ,C àOH at 286.6eV ,C =O at 288.4eV ,as well as HO àC =O at 289eV .[25,26]In contrast,the high-resolution C1s spectrum of R-CQDs only shows the presence of C =C,C àOH,and HO àC =O.Moreover,the ratio of the intensities (I C =O /I C àO )decreases after NaBH 4re-duction from the original value of 1.02to 0.56,indicating

the decrease of the number of C =O groups on the surfaces of R-CQDs.On the other hand,the O1s spectra of CQDs and R-CQDs further confirm the presence of oxygen-rich groups,such as C àO and C =O (see Figure S2in the Sup-porting Information).However,it can be seen from the de-convoluted spectra that there are fewer C =O groups on the surface of the R-CQDs than on the CQDs.This result sug-gests that the oxygen-related functional groups located on the surface of CQDs and R-CQDs are different.

According to the selective reduction of NaBH 4,[23,27]the following selective reaction takes place (Scheme 1):Most of carbonyl groups were reduced to hydroxy groups,thus sig-

nificantly increasing the number of hydroxy groups on the R-CQDs.[4,23]Therefore,the intensity and wavenumber of vi-brational absorption band of C =O changed in the IR spec-trum of R-CQDs,while their stretching vibrations of àOH became narrow;the relative content of the surface compo-nent (C =O)reduced in XPS results.

Based on the FTIR spectroscopy and XPS results,differ-ent kinds and relative contents of functional groups (C àOH,C =O,COOH)are presented on the surface of CQDs and R-CQDs.To reveal these effects,optical properties of CQDs and R-CQDs were measured (Figure 5).The concentrations of both CQDs and R-CQDs were kept the same to

compare

Figure 3.IR spectra of CQDs and

R-CQDs.

Figure 4.The C1s XPS spectra of a)the CQDs and b)the R-CQDs.The solid dots are experimental data points and the dotted lines are fitting curves for spectra (a)and

(b).Scheme 1.Selective reduction of NaBH 4

.

Figure 5.a)PL behavior of CQDs from different HNO 3concentrations before and after reduction with NaBH 4.b)Absorption spectra of CQDs and R-CQDs.

their PL intensities.Figure5a shows that the PL intensities from R-CQDs are enhanced to a great extent,and their PL peaks were shifted a little comparing with the as-prepared CQDs.The oxygen-containing groups on the surface of CQDs form“surface states”,which affect the energy gap of the surface trap.Therefore,the different kinds and relative contents of the functional groups on the surface of both CQDs and R-CQDs could give rise to the changes in their emission,such as the intensities and position of PL peaks. UV/Vis absorption spectra(Figure5b)show stronger ab-sorption of as-prepared CQDs in the range of300–450nm than that of R-CQDs.This band could be dominated by n–p*transition due to the presence of C=O and N=O groups.[4,14,15]Therefore,the reduction of the intensity of this band also reflects that the surface states on CQDs were changed after the reduction with NaBH4.However,an open question is the roles of the C=O groups in the CQDs.Are they nonradiative recombination traps,or do they have an-other function?

Recently,complex photocatalysts containing CQDs,such as TiO2/CQDs,Ag3PO4/CQDs,Fe2O3/CQDs,and Au(Pt)/ CQDs,have been designed and exhibited good photocata-lytic activity.[15–21]It was believed that the photocatalytic origin of such complex photocatalysts could arise from the photosensitization of CQDs,[17–20]that is,the electron-accept-

ing and electron-transport properties of CQDs provided a convenient way to direct the flow of photogenerated charge carriers.However,some questions remain.Since the surface groups of CQDs act as photogenerated charge traps,[28]why can the photogenerated charges transfer from the surface of CQDs to other nanostructures?Therefore, some surface groups must transmit the photogenerated charges,thus suggesting that CQDs alone also possess pho-tocatalytic activity.To gain more evidence for such proposal, the photocatalytic behavior of as-prepared CQDs and R-CQDs were explored for the degradation of methylene blue (MB)under visible-light irradiation.To the best of our knowledge,this is the first time that CQDs alone have been used as photocatalysts for these reactions.

Figure6a gives a plot of the extent of photodegradation of MB versus time at the different pH values using as-pre-pared CQDs as the photocatalysts.It can be seen clearly that photocatalytic activities of the samples for the MB deg-radation reaction increase with increasing pH value.It should be mentioned that all of reductants and oxidants had been removed from the carbon dot suspensions,and obvious MB photodegradation was not observed after irradiation for 1h in the absence of CQDs.Therefore,the vital role of CQDs in the photocatalytic process is confirmed.The effect of pH value on the photocatalytic activity could be attribut-ed to the different adsorption rates of CQDs for molecular MB with positive charges at the different pH values.[29]H+ impedes the adsorption of CQDs for MB,whereas OHàfavors such adsorption.

In the space charge region,the electric potential(P)of a point(x,y,z)relative to the bulk material can be de-scribed by the Poisson equation[Eq.(1)],[30]@2P

t

@2P

t

@2P

?à

1

e r e0

e1T

where e r,e0are the relative dielectric constant and the vacuum permittivity,respectively;1is the space charge den-sity,which is a function of the x,y,z coordinates and is influ-enced by the surface charge distribution.Due to the termi-nation of lattice periodicity at the nanoparticle surface,the unpaired electrons in the dangling bonds of surface atoms interact with one another or with heteroatoms,thus forming an electronic state with a special energy band.This situation creates a new space charge region near the nanoparticle sur-face.[31]Physical properties,such as energy band structure, free carrier density,and local conductivity,will be changed in this space charge region compared with the bulk,and hence the chemical properties will be also changed.[30,32]

For photochemistry,the energy of the incident photons is initially stored in the semiconductor by photoexcitation,and then it is converted into chemical form by a series of elec-tronic processes and surface reactions.[33]As an MB mole-cule approaches the surface of a CQD,an unfilled molecular orbital interacts with the CQD and shifts downward to lower energy.Meanwhile,the broadened molecular orbital accepts electrons from the CQD,forming a Helmholtz layer on the surface of the CQD.[30]An electric field is set up and the bands are bent up near the surface of the CQD.[30]This bending facilitates separation of the electron–hole pairs and induces faster carrier migration,leading to high photocata-lytic efficiency.[33]Hence,the interaction between the MB molecules and CQDs is indispensable for the efficient pho-toinduced carrier injection and the subsequent degradation reactions.However,the electrostatic interaction

between Figure6.a)Extent of MB photodegradation versus time using as-pre-pared CQDs as the photocatalysts at the different pH values.b)Photoca-talytic activities of CQDs and R-CQDs at the different pH values for MB degradation under visible-light irradiation.Here,C0is the original concentration of MB and C is the residual concentration of MB after ir-radiation.

MB and the surface of CQDs is expected to greatly depend on the pH values,which can alter the charge nature of the surface of CQDs and the ionization of MB by protonation and deprotonation equilibria.[34]Cationic MB tends to absorb on the surface at high pH values due to electrostatic interactions between the negative surface of the CQDs and the MB cations,[35]and their electronic coupling intensity de-termines the charge transfer efficiency.This is why the pH value can influence the photocatalytic activity of CQDs. Figure6b compares the photodegradation of MB for typi-cal CQDs and R-CQDs at different pH values.The as-pre-pared CQDs exhibited higher photocatalytic activities for the MB degradation reaction,and their photocatalytic activ-ities were also affected by the pH value.Figure7shows how the size of the as-prepared CQDs affects the photocatalytic activities.The CQDs from both4and6m HNO3display little differences in photocatalytic activities.Thus,the photo-catalytic activity of CQDs is mainly affected by the surface states.

Usually,the efficiency of photoreaction is expressed by the quantum yield(Q),which is defined in Equation(2):

Q?rateofreactantconsumptionorproductformation

rateofphotonabsorption

e2T

The rate of reactant consumption or product formation is proportional to the rate of charge transfer(k ct)from the CQDs to the reactant molecules,and the rate of photon ab-sorption is proportional to the sum of the rates of the charge transfer(k ct)and electron–hole recombination(k r). In other words,Q can be expressed as in Equation(3)[31]

Q?k ct=ek cttk rTe3TAccording to the relationship between the depletion layer (the space charge region that causes a decrease of free elec-tron carriers and an increase of hole carriers)length(D) and Q[Eq.(4)]:[36,37]

D?eQ=aTtLeQà1Te4Twhere a is the reciprocal absorption length and L is the mi-nority carrier diffusion length,k r can be obtained in Equa-tion(5):

k r?k cte1àaDT=eaDtLaTe5TIt can be seen that the low electron–hole recombination rate is caused by the broad depletion layer and high upward band bending.Therefore,a change of the outer-layer struc-ture of CQDs can change the photocatalytic activity signifi-cantly.Since the different oxygen-containing groups on the surface of CQDs may induce different degrees of band bending,CQDs and R-CQDs exhibit different photocatalyt-ic activities.When the surface of CQDs is mainly terminated by C=O or COOH groups in solution,the high upward band bending is induced,enhancing the hole-mediated photocata-lytic process.Conversely,the surface of R-CQDs mainly ter-minated by OH groups in solution induces low upward band bending,decreasing the hole-mediated process.

Based on the process of photocatalysis,the hole can react with surface-absorbed H2O to produce C OH radicals or di-rectly oxidize the organic substrate into its radicals,as in Equations(6)and(7):[34]

H2Otht!C OHtHte6T

RàHtht!RHtCDR CtHte7TUnder aerobic conditions,the electrons are usually scav-enged by O2to yield superoxide radical anions O2àC [Eq.(8)]:

O2teà!O2àCe8TIn this way,both the O2and organic substrates are activat-ed.The subsequent radical reactions,such as Equations(9)–(11),usually have low or nonexistent barriers,leading to the facile oxidative degradation of organic substrates.

RàHtC OH!R CtH2Oe9T

R CtO2!ROO C O2=H2O

!Degradatione10TRàHtC OH!HROH C O2=H2O

!Degradatione11TTherefore,it can be seen that surface hole transport plays a key role in photocatalysis.Since the presence of many more C=O and COOH surface groups can induce the high upward band bending for CQDs,more hole transport occurs,resulting in high photocatalytic activity.

Based on all of above results and discussion,the following photocatalytic mechanism of CQDs was proposed,and its schematic model is shown in Scheme2.When the absorption of a photon can excite an electron from the highest occupied molecular orbitals(HOMO)to the lowest unoccupied mo-lecular orbital(LUMO)of CQDs,the reductive LUMO electrons(eà)and oxidative HOMO band holes(h+)are produced.[19,33,38]Due to the effects of oxygen-containing groups on the surface of CQDs,a new electric field or upward band bending near the surface of CQDs is formed.

Figure7.Extent of MB photodegradation versus time using as-prepared CQDs from different HNO3concentrations as the photocatalysts.

The different oxygen-containing groups produce the differ-ent electric field intensities (or upward band bending de-grees)and then give rise to different depletion layers.The larger electric field (or higher upward band bending)will separate the electron–hole pair,reducing the rate of recom-bination and promoting hole transport to the surface.Thus,better photocatalytic activity will be produced.Conversely,there is a small effect of the smaller electric field (or lower upward band bending)on the separation of the electron–hole pair,facilitating their recombination.This results in the increase of the emission.The relationship between the PL intensity (I PL )and depletion layer,which depends on the magnitude of electric field or band bending upward,is deter-mined by Equation (12):[39]I PL /I 0exp eàa D T

e12T

where I 0is the incident light intensity and a is the sum of the absorptivities of the incident and emitted photons.It can be seen from Equation (12)that the smaller the value of D ,the stronger I PL can be obtained.This is the reason that the R-CQDs with C àOH groups exhibit better emission than the as-prepared CQDs with C =O and COOH groups.The function of C =O and COOH groups is similar with noble-metal nanoparticle and acceptor molecules on the semiconductor surface.These materials can cause band bending,which provides an internal electric field that facili-tates separation of the electron–hole pairs and induces faster carrier migration.[33]

Conclusions

CQDs were synthesized by heating various carbon sources in aqueous HNO 3at reflux,and their sizes can be tuned by controlling the concentration of the HNO 3solution.The sur-face states of CQDs can be changed by selective reduction of NaBH 4.Studies of PL behaviors and photocatalytic per-formance of CQDs have indicated that their PL behaviors depend on their sizes and surface states,while their photoca-talytic activities are determined by surface states alone.The effective separation of the photogenerated electron–hole pairs is attributed to the high upward band bending induced by the C =O and COOH groups on the surface of CQDs.To

improve photocatalytic activities of CQDs,it is required to tune their surface states.

Experimental Section

Preparation

CQDs were synthesized according to the reported method.[13–15]The carbon source can be active carbon powder and soot derived from the combustion of unscented wood.Typically,carbon source (300mg)was heated at reflux in 2,4or 6m HNO 3(100mL,analytically pure and used as received)for 8h.After the mixture was cooled to room temperature,the brownish-yellow supernatant containing CQDs was obtained after centrifugation and then was neutralized by Na 2CO 3or NaOH.For the re-duction of CQDs,sodium borohydride (NaBH 4,200mg)was mixed with an aqueous solution of CQDs (50mL)and then stirred for 8h at room temperature.Excess reductant was removed by dialysis before the next experiments and analyses were carried out.Photocatalysis Experiments

For the photocatalytic activity comparison,the same amount (10mL,the same concentration of CQDs)of the as-prepared CQDs and R-CQDs was used,and then dispersed in aqueous methylene blue (MB)solution (10mL,30mg L à1).The entire solution was transferred to the same quartz cell with an inner cell edge length of about 2cm and a small capped opening at the top.Before illumination,the contents of the cell were stirred for 30min in the dark to ensure the establishment of an ad-sorption–desorption equilibrium.After that the cell was irradiated with light from a 300W xenon lamp placed 25cm away.Air conditioning of the room was employed to minimize the heat effect.The light intensity reaching the cell was about 550mW cm à2.Visible and IR light were ob-tained by using cutoff filters to remove light of l <420nm.UV/Vis ab-sorption spectra of the samples were taken before and after every 5min of irradiation for up to 30min by removing the cap to withdraw the solu-tion.

Characterization

Fluorescence and absorption spectra were recorded on a Hitachi F4500fluorescence spectrophotometer and a Shimadzu UV-2550UV/Vis spec-trometer,respectively.Diluted supernatants containing CNQs were drop-ped onto copper grids covered with amorphous carbon film to prepare specimens for TEM observation,which was performed in a FEI Tecnai G2F20microscope with a field-emission gun operating at 200kV .The in-frared spectra were obtained on a Thermo Nicolet 360FT-IR spectropho-tometer.The XPS was performed with a Perkin–Elmer PHI-1600X-ray photoelectron spectrometer.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos.50902126,51172214,51272301),a China Post-doctoral Science Foundation funded project (No.2012M510788),Pro-gram for the Top Young Academic Leaders of Higher Learning Institu-tions of Shanxi,the Innovation Method Special Project of Ministry of Science and Technology of China (No.2011M030800),and State Key Laboratory of New Ceramic and Fine Processing,Tsinghua University.

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Received:January20,2013

Published online:February25,2013