Carbon Dot Loading and TiO2 Nanorod Length Dependence of Properties in Carbon Dot_TiO2
Carbon Dot Loading and TiO2Nanorod Length Dependence of Photoelectrochemical Properties in Carbon Dot/TiO2Nanorod Array Nanocomposites
Juncao Bian,?Chao Huang,?Lingyun Wang,?TakFu Hung,?Walid A.Daoud,?and Ruiqin Zhang*,??Department of Physics and Materials Science and Centre for Functional Photonics(CFP),City University of Hong Kong,Kowloon, Hong Kong SAR,China
?School of Energy and Environment,City University of Hong Kong,Kowloon,Hong Kong SAR,China
*Supporting Information
based on C dots.
1.INTRODUCTION
Since the pioneering work of Honda and Fujishima on water splitting with Pt?TiO2nanocomposites,photoelectrochemical (PEC)properties of TiO2have been extensively investigated since it is photostable,nontoxic,cost-e?ective,and abun-dant.1?7Among the various TiO2nanostructures,one-dimen-sional TiO2including nanorods,nanotubes,and nanowires have been con?rmed to be superior to other dimensional TiO2 nanostructures as they can enhance the charge separation and o?er a pathway for oriented charge carrier transport.8?11 However,the grain boundaries in polycrystalline TiO2 nanowires or nanotubes cause electron scattering or trapping, which limit their application in optoelectronics.8,10In contrast, single crystal TiO2nanorods have less defect related barriers blocking the charge carrier transport,5,9and the length of nanorods is a key factor in?uencing the charge collection e?ciency.Recently,Yang and co-workers reported that,with increasing the length of the single crystalline rutile TiO2 nanorods to about 1.8μm,the photocurrent density was close to the maximum.5
On the other hand,bulk TiO2,with the band gaps of3.0eV (in rutile phase)and3.2eV(in anatase phase),can only absorb the light in the UV part,which makes up less than4%of the total solar light reaching the surface of earth.4To enhance the light absorption ability of TiO2in the visible region,various routes such as doping,3,7,12,13using surface plasmons of Au14,15and Ag,and coupling with narrow-band gap semiconduc-tors17?20and dyes21were employed.Recently,carbon dots(C dots)have become a star in the area of quantum dots(QDs)as they are of low-toxicity and eco-friendly compared with the traditional toxic metal-based QDs.22?28With many carboxylic acid moieties on their surface,C dots can be well dispersed in water and are suitable for subsequent functionalization with various organic and inorganic species.22,25,26The C dots typically exhibit size and excitation wavelength dependent photoluminescence(PL)properties and have great potential in bioimaging,26light-emitting diodes(LEDs),29solar cells,30 catalysis,24,31?34and PEC cells.35
In this work,C dots were used as sensitizer to enhance the PEC performance of TiO2nanorods arrays(TNRA).It is well recognized that photoabsorption,charge transport in semi-conductor,and charge separation are three main factors in?uencing the e?ciency of PEC cells.Hence,the e?ect of C dot loading on the surface of TNRA was investigated to optimize the light absorption and charge separation properties, and the length of TNRA was investigated to optimize the PEC performance from the aspect of charge transport.It was found that C dots can enhance the photoresponse of TNRA in the
Received:December23,2013
Accepted:March6,2014
Published:March6,2014
visible range with the incident photon to current conversion e ?ciency (IPCE)being more than 1%.The related mechanisms are discussed.2.EXPERIMENTAL DETAILS All reagents were purchased from Acros unless otherwise stated.
Highly ordered single-crystal TNRA were fabricated on FTO glasses
by a modi ?ed hydrothermal growth method.9
15mL of concentrated HCl (37%)was mixed with 15mL of ultrapure water (18.2M Ω·cm),
and the mixture was stirred for 5min.
Then,0.8mL of titanium butoxide was added.
After vigorously stirring for another 5min,10mL of the solution was transferred to a 23mL Te ?on liner,and precleaned FTO glass (2×3.3cm 2,square resistance:<10Ω,Zhuhai Kaivo
Optoelectronic Technology Co.,Ltd.)was placed against the wall of
the Te ?on liner with the conducting side facing down.Then,the
autoclaves were put into an oven and kept at 150°C for di ?erent
durations to obtain the TNRA with di ?erent lengths.The autoclaves
were cooled down by water for 15min.Then,the FTO glasses with
TNRA were ultrasonically cleaned by ultrapure water for 5min to
remove the surface remnant.C dots were synthesized by a hydrothermal
method.261.05g of
citric acid (Sigma Aldrich)and 335μL of ethylenediamine were
dissolved in 10mL of ultrapure water.The mixtures were vigorously stirred for 10min.They were then transferred to a 23mL Te ?on-lined
autoclave and kept in an oven (200°C)for 5h.After the reaction,the
autoclave was cooled down by water for 15min.Then,after dialysis
(M =1000),rotary evaporation,and freeze-drying processes,C dot powders were collected.For TNRA/C dot nanocomposites,TNRA on
FTO glasses were immersed into C dot solutions with di ?erent
concentrations for 20h in the dark.Then,they were taken out,rinsed
with ultrapure water,
and dried o ?with N 2.The TNRA/C dot nanocomposites were then annealed in a rapid annealing furnace for
20min at 200°C under N 2atmosphere (60Torr).The photoluminescence spectra of C dots were investigated by a
Varian Fluorescence Spectrophotometer.Raman spectra of C dots and TNRA/C dot nanocomposites were
obtained from
the micro Raman
system (RM 3000Ranishaw,Laser:514.5nm)with confocal microscopy.The UV ?vis absorption spectrum of C dots was measured
on a Varian 50Conc UV ?visible spectrophotometer.
UV
?vis absorption spectra of TNRA/C dots were carried out on a
UV ?vis-NIR (UV 3600,Shimadzu)spectrophotometer equipped with integrating sphere.High resolution transmission electron microscope
(HRTEM)images of C dots and TNRA/C dot nanocomposites were
measured
from a HRTEM (JEOL 2100F).Top-down and cross-
section
morphologies of TNRA were obtained from scanning electron microscopy (SEM,JEOL JSM-6335F).X-ray photoelectron spectros-copy
(XPS)was performed on XPS (PHI Model 5802)with Al K α
radiation and was calibrated by C1s.
Photoelectrochemical (PEC)properties of TNRA/C dot nano-composites were measured in a PEC cell.The photoresponse of the photoanodes were recorded by a three-electrode electrochemical workstation
(CHI 760E),in which FTO supported TNRA/C dot
nanocomposites,Pt wire,and Ag/AgCl were used as working
electrode,counter electrode,and reference electrode,respectively.Solutions
containing 0.1M NaSO 4and 0.01M Na 2S were used as
supporting electrolyte and sacri ?cial reagent to keep the stability of the C
dots.Other supporting electrolytes including KNO 3,KCl,and
NaOH and sacri ?cial reagents including methanol and ethanol were
also used to investigate their in ?uences on the PEC properties
of the
TNRA/C dot nanocomposites.Prior to the measurement,N 2was
purged
into the electrolyte for 20min to remove O 2.
A 300W Xe lamp
(Beijing NBET
Technology Co.,Ltd.)was used as light source (100
mW/cm 2).Incident photon to current conversion e ?ciency (IPCE)was measured under monochromatic light,which was realized by the Xe
lamp illuminating through a monochromator.The illumination
intensity of the monochromatic light was measured by a luminometer.
Impedance spectra were
carried out in the dark and under illumination at
0V vs Ag/AgCl in the frequency range of 0.1?105Hz with AC
voltage of 10mV.3.RESULTS AND DISCUSSION
Figure 1a shows the UV ?vis absorption spectrum of as
prepared C dots.Two peaks at around 236and 340nm are found,which may be attributed to the π?π*transition.22The insets in Figure 1a depict the solutions of C dots under lamplight and UV light,and obviously,a blue light emission
was
Figure 1.(a)UV ?vis,(b)HRTEM image (inset:size diagram),and (c)photoluminescence spectra of C dots.(d)XPS spectrum of C dots on the surface of TNRA.Insets in (a)are the C dots solution (0.005mg/mL)under lamplight (left)and UV light (360nm)(right)illumination.
observed under UV light excitation.Figure 1b shows the
HRTEM image of C dots.Some of the C dots show the lattice
fringes with interplane distance of about 0.34nm,correspond-
ing to the (002)distance of graphitic carbon,24,36and the
amorphous C dots without obvious lattice fringes are also
observed.By randomly measuring the size of 100C dots,the size of the C dots is calculated to be 3.3±0.4nm.Excitation-
dependent photoluminescence (PL)spectra of the C dots are
given in Figure 1c.It indicates that the photoexcited charge
carriers can be generated by the visible light.This phenomenon
is attributed to their complex surface states.26,37,38To demonstrate the C dots loading on the surface of TNRA,XPS spectrum of TNRA/C dot nanocomposites was measured.
Typical peaks of C dots (Figure 1d)and TiO
2(Figure S1,
Supporting Information)were detected,indicating successful
loading of C dots on the surface of TNRA.From the C1s peaks,three peaks corresponding to C ?C,
C ?N,and C O were ?tted at 284.6,285.9,and 288.1eV,respectively,39and no Ti ?C bond was found in Figure S1,Supporting Information,
indicating that the C dots were physically adsorbed on the
surface of TNRA.TNRA with di ?erent lengths were obtained by controlling
the growth time at 150°C.Figure 2a ?j shows the top-down
and cross section morphology of TNRA growing for 3,5,6,9,
and 12h,separately.The corresponding average lengths of the TNRA were measured to be about 0.66,1.0,1.3,2.3,and 3.2μm.It is also found that the diameter of the TNRA increases as
they grow for longer time.XRD pattern (Figure S2,Supporting Information)shows that the TNRA are oriented to the [001]direction,which is in good accordance with a previous report.9Figure 3a,b shows the typical HRTEM images of TNRA/C dot nanocomposites.Obvious C dots on the edge of the TiO 2nanorod (Figure 3a)and the lattice fringe with the direction di ?erent from that of the TiO 2phase (Figure 3b)are observed,
indicating that the C dots are successfully loaded on the surface of the TiO 2nanorod.The Raman spectra of the C dots and the
TNRA/C dot nanocomposites are shown in Figure 4.No obvious D band at 1353cm ?1and G band at 1586cm ?1are observed maybe due to the low intensity.28The peaks centered at 447and 612cm ?1are attributed to the rutile TiO 2phase.40
To study the photoanodic activity of the TNRA/C dot nanocomposites dependence on the C dot loading,TNRA with the length of 0.66μm were immersed into C dot solutions with di ?erent concentrations.Figure 5a shows the light absorption spectra of the bare TNRA and TNRA/C dot nanocomposites.Remarkable absorption plus scattering enhancement in the visible range can be observed when the concentration of C dots is 0.2mg/mL.Further increase in the C dot concentration only
slightly improves the light absorption plus scattering.This phenomenon reveals that the loading of the C dots on the
surface of TNRA are nearly saturated.
Photocurrent density versus potential curves of the TNRA/C
dot nanocomposites in Figure 5b shows that the photocurrent density is enhanced with increasing C dot loading until 0.4mg/mL and then decreases with further increase of the C dot loading.The enhancement in photocurrent demonstrates that the C dots are good sensitizers for TiO 2,and
photoexcited
Figure 2.Cross sectional and top-view SEM images of TNRA growing for 3h (a,b),5h (c,d),6h (e,f),9h (g,h),and 12h (i,j).The scale bars are 1μ
m.Figure 3.(a,b)HRTEM images of C dots on the surface of TiO 2nanorod.The circles in (b)highlight the lattice fringes of C
dots.Figure 4.Raman spectra of C dots and TNRA/C dot nanocomposites.
Figure5.(a)100-transmission-re?ection(%)spectra,(b)photocurrent density versus potential curves,and(c)IPCE curves of TNRA/C dot nanocomposites.The nanocomposites were obtained through immersing the TNRA(0.66μm)into C dot solutions with concentrations of0.2,0.4, 0.6,and0.8mg/mL.(d)Curve of IPCE at500nm versus C dot concentration.
Figure6.(a)Curves of photocurrent density versus growing time of the TNRA/C dots.The TNRA growth for di?erent times was immersed into
0.4mg/mL C dot solution.(b)IPCE plot of TNRA growing for6h with and without C dot loading.(c)Nyquist plot curves of TNRA and TNRA/
C dot nanocomposites measured in the dark and under light.(d)Photocurrent densities of TNRA/C dots running for1h for three times.
charge carriers can be transferred to TNRA and collected.Considering the continuous increase in light absorption with increasing the C dot concentration,the decrease in the enhancement may be attributed to the C dot aggregation on the surface of TNRA.41,42As the concentration of the C dots further increases,more C dot aggregations are formed on the surface of TNRA due to their surface functional groups like carboxyl and amido,26and photoexcited charge carriers recombine at the boundary of the C dots,leading to the decrease of the photocurrent density.41,42It is noted that,when the bias is more negative than ?0.6V,the change of the photocurrent densities is di ?erent from that under the bias more positive than ?0.6V.It may be caused by the more negative potential of ?0.6V.Further investigation is needed to clarify the phenomenon.To further evaluate the PEC properties of the TNRA/C dot nanocomposites,IPCE spectra were measured versus incident light wavelength under monochromatic light.Calculation of the IPCE referred to the following equation:λ=I J IPCE 1240/()light where I ,λ,and J light are photocurrent density,wavelength of incident light,and illumination intensity,respectively.35As shown in Figure 5c,it is evident that the IPCE is enhanced in the visible range,indicating that the incorporation of C dots remarkably improves the photoresponse of the TiO 2-based PEC cells in the visible range.It should also be noted that the IPCE values in the visible light range are di ?erent from one to another,especially at 600?700nm.This phenomenon may be attributed to the complex surface states a ?ecting the band gap of the C dots.26,37The IPCEs of TNRA/C dots illuminated under 500nm are plotted versus the C dot concentrations in Figure 5d.The IPCE ?rst increases until 0.4mg/mL and then decreases,which is well consistent with the change in the photocurrent densities under potential more positive than ?0.6V.For the photogenerated electrons collection,the electrons should transport across the TNRA along the axial direction.Therefore,the e ?ect of TNRA length on the photoresponse of the TNRA C dots nanocomposites is also studied,as shown in Figure 6a.It is found that the photocurrent density is increasingly enhanced when the length of the TNRA changes from 0.68to 1.3μm.Then,a photocurrent density decrease is observed.This phenomenon is a compromise between the charge generation and charge collection.At ?rst,with the increase in the length of TNRA to 1.3μm,more C dots are loaded on the surface of the TNRA assuming that the density of the C dots on unit area of TNRA is identical.The increased C dot loading raises the photocurrent density.As the length increases from 1.3to 2.3μm,although more C dots are loaded,the increase in the length plays a major role.The charge carriers
generated in the topper part of the TNRA should di ?use a
longer distance to reach the FTO ?lm,and the recombination rates of the charge carriers during the transport are largely increased,leading to the decrease of the photocurrent density.5
Among the samples,the TNRA (1.3μm)immersed in 0.4mg/
mL C dots have the best PEC performance.The photocurrent density is enhanced about 25.2%compared with the bare TRNA under the bias potential of 0V.Their IPCE is shown in
Figure 6b.The IPCE values in the range of 420to 700nm are 1.2?3.4%.Impedance spectroscopy is a powerful tool to characterize the electric properties of semiconductor nanocomposites and has been widely employed in PEC systems,and the diameter of the Nyquist plot re ?ects the charge transfer resistance between the electrode and electrolyte.43,44Figure 6c presents
a
Figure 7.Photocurrent densities of TNRA/C dots measured under di ?erent monochromatic light in (a)di ?erent supporting electrolytes,(b)
di ?erent supporting electrolytes with Na 2S as sacri ?cial reagent,and (c)di ?erent sacri ?cial reagents with NaSO 4as supporting reagent.The monochromatic light from 420to 700nm is used with an increase of 20nm/20s.(d)Mechanism of C dots in PCE properties enhancement of
TNRA.
decreased semicircle diameter after the loading of C dots measured in the dark and under illumination,con?rming that the C dots can enhance the carrier mobility at the interface of TNRA and electrolyte.Figure6d shows the photocurrent density of the TNRA/C dot nanocomposites running for3 times as the photoanode.It is found that the photocurrent density exhibits a slight decrease in the second time and is nearly stable in the third time.This degeneration may be caused by the partly detached C dots from the surface of TNRA due to the poor adhesion force.The stability of the TNRA/C dots should be improved.In situ growth of C dots on the surface of TNRA will be carried out in the future work.
To investigate the in?uence of the supporting electrolyte on the PEC performance of the TNRA/C dots nanocomposites, photocurrent densities are measured in other electrolytes including KNO3,KCl,and NaOH.It is found that,without the sacri?cial reagent,the photocurrent densities measured by using the electrolytes including Na2SO4,KNO3,KCl,and NaOH are about2orders of magnitude smaller than that measured with sacri?cial reagent,as shown in Figure7a,b. Hence,it is con?rmed that the HOMO of the C dots is higher than the water oxidation potential,which blocks the?ow of the photogenerated holes from C dots to the solution.Without using Na2S as sacri?cial reagent,the holes will accumulate in the HOMO of C dots as a result of small photocurrent due to the large electron?hole recombination rate.The di?erences of the photocurrent densities among di?erent electrolytes may be caused by the di?erent conductivities of di?erent anions and changed surface states by the interactions between the anions and the surface functional groups of C dots.26,37Moreover, other frequently used sacri?cial reagents like ethanol and methanol are also investigated,45as shown in Figure7c.It is found that Na2S is the best sacri?cial reagent.It indicates that the Na2S is more easily oxidized than ethanol and methanol, well consistent with the previous report.45
On the basis of the aforementioned analysis,a possible mechanism for the C dots enhancement in PEC properties of TNRA was proposed,as shown in Figure7d.When excited by the visible light,the electrons in the HOMO level of the C dots get enough energy and jump to the LUMO level of the C dots. Then,they are transferred to the conduction band of TiO2. After that,the electrons are transported through the TiO2 nanorod and FTO?lm.Afterward,the electrons are conducted through the external circuit to the counter electrode and react with H+to form H2.The holes left in the HOMO of C dots react with the S2?to form S22?.4
The main advantages of C dots for its application in the PEC enhancement of TNRA are their elemental abundance,low cost,and low toxicity compared with the toxic metal-based quantum dots(CdSe,CdS,and PbS),expensive noble metals including Au,Ag,and Pt,and expensive dyes.However,the stability of the C dots needs further improvement,and the enhanced e?ciency in the visible light range is still too low. Chemical doping and surface modi?cations are potential approaches to tune the surface states and band gap of the C dots.46,47
4.CONCLUSIONS
We demonstrated that the PCE properties of C dots decorated TNRA are dependent on the C dots loading and the length of TNRA.Photocurrent density curves showed that the photo-current density reached a maximum when concentration of C dots was0.4mg/mL,and a further increase in the C dots concentration decreased the photocurrent,which may be caused by the surface aggregation of C dots.A compromise existed between charge transport and charge collection as the TNRA length increased.The IPCEs of the TNRA/C dots nanocomposites in the visible range were up to1.2?3.4%by immersing the TNRA(1.3μm)in0.4mg/mL C dots solution. Impedance spectroscopy con?rmed that the incorporation of C dots on the surface of TNRA can improve the charge transfer process between the electrode and electrolyte.Na2SO4is a better choice of supporting electrolyte compared with KNO3, KCl,and NaOH,and Na2S is better than ethanol and methanol as sacri?cial reagent.This work can serve as a guidance for the design of highly e?cient photoanode for PEC cells based on
TNRA/C dot nanocomposites.
■ASSOCIATED CONTENT
*Supporting Information
XPS spectrum and XRD pattern of TiO2nanorod arrays with a growth time of9h.This information is available free of charge
via the Internet at https://www.sodocs.net/doc/a28067001.html,.
■AUTHOR INFORMATION
Corresponding Author
*E-mail:aprqz@https://www.sodocs.net/doc/a28067001.html,.hk.Tel:+852********.Fax:+852 ********.
Author Contributions
The manuscript was written through contributions of all authors.All authors have given approval to the?nal version of the manuscript.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
The work described in this paper is supported in part by a grant from the Research Grants Council of Hong Kong SA[Project No.CityU103812].We are grateful to Professor Andrey L. Rogach,Dr.Chunyi Zhi,Dr.Yu Wang,and Dr.Sergii
Kalytchuk for their help with the experiment.
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