Location Effect in a Photocatalytic Hybrid System of Metal-Organic Framework Interfaced with Semiconductor Nanoparticles

Qi-chao ShangXin-zuo FangacHai-long JiangacQun Zhang

a.Hefei National Laboratory for Physical Sciences at the Microscale,School of Chemistry and Materials Science,University of Science and Technology of China,Hefei 230026,China

b.Department of Chemical Physics,Synergetic Innovation Center of Quantum Information&Quantum Physics,University of Science and Technology of China,Hefei 230026,China

c.Department of Chemistry,CAS Key Laboratory of Soft Matter Chemistry,Collaborative Innovation Center of Suzhou Nano Science and Technology,University of Science and Technology of China,Hefei 230026,China

Key words:Metal-organic framework,Photocatalysis,Ultrafast spectroscopy,Transient absorption

I.INTRODUCTION

Metal-organic frameworks(MOFs),a class of porous coordination polymers formed by metal ions and organic ligands,have shown great promise in photoelectrochemical applications due to their unique properties such as high specific surface areas,outstanding porosity,and structural diversity[1?3].MOF-based heterogeneous composites,in which metal or semiconductor nanoparticles(NPs)are interfaced with a MOF material,have been exploited to optimize the desired performance in photocatalysis[4?10].Recently,in a prototypical metal-MOF hybrid system,i.e.,platinum(Pt)NPs interfaced with UiO-66-NH2,we revealed that the Pt location(i.e.,either incorporated inside or supported onto the MOF)can significantly in fluence the photocatalytic efficiency for hydrogen production[6].In this case the MOF functions as an electron donor while the Pt NPs serve as a co-catalyst,facilitating electron transfer and promoting charge separation therein.During the carrier-transfer process within heterostructures,MOFs could act as either electron donors[4?6]or electron acceptors[7?10].Notably,it would be more bene ficial when the MOF behaves as an electron acceptor in the hybrid system given that the MOF offers extensive active sites for photocatalysis[11].Recent efforts have been made to develop semiconductor-MOF hybrid systems,in which the MOF receives electrons donated by the interfaced semiconductor component[7?10].

It is interesting to see if the location effect observed in metal-MOF system also manifests in semiconductor-MOF system. A useful way to examine this is to track in real time the interface electron dynamics in the system by ultrafast optical spectroscopy[12?16].To this end,we have performed a set of femtosecond(fs)time-resolved transient absorption(TA)measurements on a typical semiconductor-MOF system with TiO2NPs being incorporated inside or supported onto Cu3(BTC)2(BTC=benzene-1,3,5-tricarboxylate),forming the two composites denoted TiO2@Cu3(BTC)2and TiO2/Cu3(BTC)2,respectively.Analysis of our fs-TA results revealed that the interface states formed between TiO2and Cu3(BTC)2can serve as an effective relay for electron transfer,whose efficiency is highly dependent on the relative location of the two components.It is such a subtle location effect that leads to their different photocatalytic performances in CO2reduction.

II.EXPERIMENTS

A.Materials

Polyvinylpyrrolidone(PVP,MW=29000)and 1,3,5-benzenetricarboxylic acid(H3BTC)were purchased from Sigma-Aldrich.Cuprous oxide(Cu2O)was purchased from Alfa Aesar. Tetrabutyl titanate,N,N-dimethylformamide(DMF),and ethanol were purchased from Sinopharm Chemical Reagent Co.,Ltd.All of the chemicals were used as received without further purification.

B.Synthesis of the samples

1.Synthesis of TiO2

1 mL tetrabutyl titanate was added into a Pyrex vial.The vial was then placed into an oven and kept at 85?C.The precursor,tetrabutyl titanate,was hydrolyzed smoothly and the resulting white powders were dehydrated to yield the amorphous sample of TiO2NPs.

2.Synthesis of Cu3(BTC)2

1.48 g PVP was dissolved in 10 mL pure water and heated at 70?C under magnetic stirring.8 mL of DMF containing 21 mg H3BTC was added into the PVP solution,which was heated for 10 min.Then,2 mL of DMF containing 5.6 mg Cu2O was added and further heated at 70?C for 30 min.The product was washed with ethanol three times to remove PVP.The resulting product was dispersed in ethanol again.After 2 h the supernatant was taken out and centrifuged to obtain the Cu3(BTC)2microcrystals.

3.Synthesis of TiO2@Cu3(BTC)2

5 mg Cu3(BTC)2was activated by evacuation at 130?C for 1 day in a Pyrex reaction tube.After being cooled to room temperature,Cu3(BTC)2was immersed with 1 mL tetrabutyl titanate under evacuation for another day.Subsequently,the sample was removed from the Schlenk flask and washed with ethanol to remove tetrabutyl titanate on the surface.Finally,the sample was dried to yield the TiO2@Cu3(BTC)2composites.

4.Synthesis of TiO2/Cu3(BTC)2

5 mg Cu3(BTC)2was dispersed in 1 mL ethanol,and then 50 L tetrabutyl titanate was added into ethanol under stirring.The mixture was dehydrated quickly to yield the TiO2/Cu3(BTC)2composites.

C.Characterization

1.Routine characterizations

The concentrations of Ti and Cu were measured with a Thermo Scientific Plasma Quad III inductively coupled plasma mass spectrometry(ICP-MS)with the samples being dissolved in HCl/HNO3(3:1,V:V).The sample morphology of TiO2,Cu3(BTC)2,TiO2@Cu3(BTC)2,and TiO2/Cu3(BTC)2was examined by transmission electron microscopy(TEM,JEOL-2010).Prior to electron microscopy characterizations,a drop of the aqueous suspension of particles was placed onto a piece of carbon-coated copper grid or silicon wafer and dried under ambient conditions. Powder X-ray diffraction(XRD)was carried out on a Japan Rigaku Smart Lab rotation anode X-ray diffractometer equipped with graphite monochromatized Cu Kαradiation(λ=1.54 ?).The X-ray photoelectron spectroscopy(XPS)characterization was performed on a Thermo Scientific ESCALAB 250 X-ray photoelectron spectrometer,using nonmonochromatized Al KαX-ray as the excitation source.The expected charging of samples was corrected by setting the C 1s binding energy of the adventitious carbon to 284.5 eV.The steady-state UV-Vis diffuse re flectance spectra(270?510 nm)were recorded on a Shimadzu Solid Spec-3700 spectrophotometer(reference:BaSO4).

2.Ultrafast spectroscopy

The fs-TA measurements were performed under ambient conditions in an integrated Helios spectrometer(Ultrafast Systems LLC)with pump and probe beams derived from an amplified femtosecond laser system(Coherent).The 320-nm pump pulses(～50 nJ/pulse at the sample cell)were delivered by a travelling-wave optical parametric amplifier system(TOPAS-800-fs),which was excited by a Ti:sapphire regenerative amplifier(Legend Elite-1K-HE;800 nm,35 fs,3 mJ/pulse,and 1 kHz repetition rate)seeded with a mode-locked Ti:sapphire laser system(Micra 5)and then pumped with an Nd:YLF laser(Evolution 30).The pump power was adjusted through a series of neutral-density filter wheels.The stable white-light continuum(WLC)probe pulses(450?650 nm)were generated by attenuating and focusing the 800-nm beam(split from the regenerative amplifier,～400 nJ/pulse)onto a sapphire crystal plate.To correct the pulse-to-pulse fluctuation of the WLC,the WLC was split into a probe beam and a reference beam.The time delays between the pump and probe pulses were controlled by a motorized optical delay line.The instrument response function(IRF)was determined to be～100 fs by measuring solvent responses under the same experimental conditions(with the exception of a higher excitation power).A mechanical synchronized chopper operating at 500 Hz was used to modulate the pump pulses such that the fs-TA spectra with and without the pump pulses can be recorded alternately.The temporal and spectral profiles(chirp corrected)of the pump-induced differential transmission of the WLC probe light(i.e.,absorbance change)were visualized by an optical fiber-coupled multichannel spectrometer(with a complementary metaloxide-semiconductor sensor)and further processed by the Surface Xplorer software equipped with the Helios system.The samples were well dispersed in a mixture of spectrum-pure acetonitrile(CH3CN)and ethylene glycol.The solution was contained in a 0.7-mL sealed quartz cuvette under a continuous magnetic stirring condition,ensuring that the photoexcited volume of the sample was kept fresh during the fs-TA measurements.

3.Photocatalytic tests

The photocatalytic CO2reduction tests were carried out in a 100-mL optical reaction vessel under stirring at ambient temperature using a 300-W Xe lamp as the light source.In addition,a 400-nm short-wave-pass cutofffilter was used.The catalyst(10 mg)was dispersed in CH3CN(19 mL)with 1 mL triethanolamine(TEOA)being added as a sacrificial reagent.The mixture was bubbled with CO2under stirring for 40 min.The product was measured by a gas chromatograph(Shimadzu GC-2014,nitrogen as carrier gas),during which 200μL of the product was injected and quantified by a calibration plot against the internal CO standard.

III.RESULTS AND DISCUSSION

The two composites with an identical mass ratio of TiO2over Cu3(BTC)2(i.e.,～0.2)were prepared based on the pre-synthesized Cu3(BTC)2product.TEM images(FIG.1)revealed that the MOF morphology(i.e.,octahedron-shaped Cu3(BTC)2microcrystal with a lateral size of～800 nm)was well maintained upon being integrated with TiO2NPs.As for TiO2@Cu3(BTC)2,one can see that the surfaces of Cu3(BTC)2were very smooth(FIG.1(a))and TiO2NPs were well con fined within the Cu3(BTC)2(FIG.1(b),which is an enlarged view of the selected region in FIG.1(a)).In contrast,as for TiO2/Cu3(BTC)2,the surfaces of Cu3(BTC)2were rough and covered with TiO2NPs(FIG.1(c)and(d)).The excellent encapsulation of TiO2NPs inside MOF for TiO2@Cu3(BTC)2and nearly uniform coverage of TiO2with MOF for TiO2/Cu3(BTC)2clearly indicated the different locations of TiO2NPs for the two composites.FIG.2 shows the comparison of the size distribution of TiO2NPs in TiO2@Cu3(BTC)2and TiO2/Cu3(BTC)2,which shows no significant difference,precluding the possible impact of size distribution of TiO2NPs on the interface electron dynamics in the hybrid systems.Moreover,the observation of nearly identical powder XRD patterns(FIG.3)suggested that the crystalline structure of Cu3(BTC)2was well maintained for the two composites.

FIG.1 Typical TEM images for the two hybrid systems under investigation:(a,b)TiO2@Cu3(BTC)2and(c,d)TiO2/Cu3(BTC)2.Panels(b)and(d)are the enlarged views of the selected regions(red rectangles)in panels(a)and(c),respectively.

FIG.2 Comparison of the size distribution of TiO2NPs in TiO2@Cu3(BTC)2and TiO2/Cu3(BTC)2.

FIG.4 exhibits the valence-band(VB)XPS spectra recorded on the two composites and the two bare samples of TiO2and Cu3(BTC)2.The VB edges for TiO2and Cu3(BTC)2were determined to be at 2.74 and 2.25 eV,respectively,below the Fermi level.Notably,both composites turned out to hold an identical VB edge at 1.92 eV below the Fermi level,suggesting that the different loading locations of TiO2NPs in the composites would not bring on significant modification for the electronic structures and properties of Cu3(BTC)2.The survey and high resolution XPS spectra recorded on the two composites can be found in FIG.S1 in the supplementary materials.

FIG. 3 Powder XRD patterns recorded on TiO2@Cu3(BTC)2andTiO2/Cu3(BTC)2,showingno obvious difference.

FIG.4 Valence-band XPS spectra recorded on the two composite samples of TiO2@Cu3(BTC)2and TiO2/Cu3(BTC)2 as well as the two bare samples of TiO2and Cu3(BTC)2.

To track the real-time photoexcited electron dynamics in the two hybrid systems,we performed fs-TA spectroscopy characterization.A routine pump-probe con figuration with an ultraviolet(UV)pump(320 nm)and a white-light continuum probe(450?650 nm)was adopted.The 320-nm photoexcitation(～3.88 eV)is suited to induce interband transition in TiO2(bandgap～3.2 eV).FIG.5(a)displays the fs-TA spectra taken at several representative probe delays for the two composite samples(lower panels)and two reference samples(upper panels).Bare Cu3(BTC)2exhibits nearly indiscernible fs-TA signal as compared to bare TiO2,echoing to their steady-state UV-Vis absorption spectra(see FIG.S2 in the supplementary materials).Markedly,the negative probe bleach(PB)signal of bare TiO2is completely reversed to the positive photoinduced absorption(PA)signal upon being interfaced with Cu3(BTC)2.As for bare TiO2,the PB evolution probed in 450?650 nm,a red-shifted region relative to the 320-nm pump,tracks via stimulated emission the population of photoexcited electrons in its conduction band(CB)[13,15]. As for the two hybrid systems,however,the PA evolution observed in the same region should not correspond to the excited-state absorption(i.e.,upward probing)from the CB electrons on the TiO2side,but rather re flects that of the electrons transferred rapidly from the CB of TiO2to the interface states formed between TiO2and Cu3(BTC)2.There should occur a sort of efficient TiO2-to-Cu3(BTC)2interface electron-transfer process,given that the photoexcited component is dominantly TiO2instead of Cu3(BTC)2.The mechanistic picture is illustrated in FIG.5(b).

FIG.5(a)Representative fs-TA spectra(pump at 320 nm)for the two hybrid systems(lower panels)and for the two reference systems(upper panels).(b)Schematic illustration of the interface electro-transfer processes in the semiconductor-MOF hybrid system.

FIG.6 Representative fs-TA kinetics(pump at 320 nm,probe at 560 nm)for the two hybrid systems as well as for the two reference systems.

To gain more insights into the interface electron transfer and the subsequent relaxation processes in the hybrid systems,we turn our attention to the involved PA kinetics.FIG.6 exhibits a set of representative fs-TA kinetics data taken at 560 nm.As for bare TiO2,the negative PB signal builds up within the instrument response function(IRF)of～100 fs and then recovers with an average lifetime of～110 ps.The fast buildup(<100 fs)re flects the initial generation and cooling of hot excitons,while the following recovery(～110 ps)accounts for the subsequent exciton trapping and recombination[12,17].Also,the early-time build-up for the two hybrid systems occurs within the IRF,suggesting that the interface electron-transfer process therein is extremely fast(indicated by a grey,wiggly arrow in FIG.5(b)).Strikingly,the subsequent relaxation,which should be linked to the event that the promptly accumulated interface-state electrons are redistributed into the electron acceptor states of Cu3(BTC)2(indicated by a red,wiggly arrow in FIG.5(b)),exhibits a pronounced difference:τ1=(182±16)ps(69.1%)andτ2=(1006±323)ps(30.9%)(or(769±292)ps on average)for TiO2@Cu3(BTC)2,whileτ1=10.1±1.1 ps(51.2%)andτ2=(157±7)ps(48.8%)(or(148±6)ps on average)for TiO2/Cu3(BTC)2.A roughly 5-fold acceleration(in terms of average lifetime)was detected for TiO2/Cu3(BTC)2relative to TiO2@Cu3(BTC)2.This evidences that the interface states formed between semiconductor and MOF can act as an effective relay for semiconductor-to-MOF electron transfer,whose efficiency is highly dependent on the relative location of the two components. In the current case,such a relay effect in the system with TiO2NPs being supported onto Cu3(BTC)2(i.e.,TiO2/Cu3(BTC)2)was proven more efficient than in the system with TiO2NPs being incorporated inside Cu3(BTC)2(i.e.,TiO2@Cu3(BTC)2),suggesting that the coupling between the interface states and the electron acceptor states of MOF is stronger in TiO2/Cu3(BTC)2than in TiO2@Cu3(BTC)2.

FIG.7 CO conversion rates for TiO2,TiO2@Cu3(BTC)2,and TiO2/Cu3(BTC)2in the recycling tests of photocatalytic CO2reduction.The product mole yields were calibrated against the 15-mg photoactive TiO2.Signals for by-products were indiscernible.

Considering that the higher efficiency in interface states-mediated electron transfer would promote electron-hole separation(on the photoexcited semiconductor side)and electron redistribution(on the electron-accepting MOF side),we anticipated that TiO2/Cu3(BTC)2could achieve a better photocatalytic performance than TiO2@Cu3(BTC)2.This expectation was verified by a set of photocatalytic CO2reduction tests,in which TiO2/Cu3(BTC)2was found to stand out among others(FIG.7).Notably,the two composites were found to possess an identical mass ratio of TiO2NPs over Cu3(BTC)2(i.e.,～0.2)and a similar size distribution of TiO2NPs(FIG.2),as discussed above.On the basis of these observations,it is safe to conclude that the difference in photocatalytic performance for the two composite samples should not simply be due to the slight differences in the loading distribution and density of TiO2NPs,but rather related to the different interface electron-transfer efficiencies for the two samples.

IV.CONCLUSION

We have presented an ultrafast spectroscopy investigation addressing the subtle location effect in a prototypical semiconductor-MOF hybrid system with TiO2nanoparticles being incorporated inside or supported onto Cu3(BTC)2,denoted TiO2@Cu3(BTC)2and TiO2/Cu3(BTC)2,respectively.By tracking in real time the interface electron dynamics in the hybrid system,we find that the interface states formed between TiO2and Cu3(BTC)2can act as an effective relay for electron transfer,whose efficiency rests on the relative location of the two components.Such a subtle location effect is responsible for the performance difference in photocatalytic CO2reduction using the two hybrid systems.The mechanistic insights into the involved interface electron-transfer effect provide a helpful perspective for rational design of MOF-based hybrid systems for photoelectrochemical applications.

Supplementary materials:XPS spectra and steadystate UV-Vis absorption spectra of the samples are available in the supplementary materials.

V.ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology of China(No.2016YFA0200602),the National Natural Science Foundation of China(No.21573211 and No.21633007),and the Fundamental Research Funds for the Central Universities of China(No.WK2340000063).

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