Photoinduced Charge Transfer in Hybrid Structures Based on

Article Cite This: J. Phys. Chem. C 2019, 123, 14790−14796

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Photoinduced Charge Transfer in Hybrid Structures Based on Titanium Dioxide NPs with Multicomponent QD Exciton Luminescence Decay Ekaterina Kolesova,† Vladimir Maslov,† Farrukh Safin,† Finn Purcell-Milton,‡ O. Cleary,‡ Yurii Volkov,†,§,∥,⊥ Yurii K. Gun’ko,†,‡,⊥ and Anna Orlova*,† †

ITMO University, St. Petersburg 197101, Russian Federation School of Chemistry, The University of Dublin, Trinity College, Dublin 2, Ireland § School of Medicine, The University of Dublin, Trinity College, Dublin 2, Ireland ∥ Department of Histology, Cytology and Embryology, First Moscow State Sechenov Medical University, Moscow 119991, Russian Federation ⊥ CRANN Institute and AMBER Centre, The University of Dublin, Trinity College, Dublin 2, Ireland

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‡

S Supporting Information *

ABSTRACT: Hybrid structures based on TiO2 nanoparticles and quantum dots (QDs) are effective electron-transfer systems. Here, we report the results of the investigation of photoinduced electron-transfer efficiency in these multilayered hybrid structures, taking into account the resulting multiexponential PL decay of QDs and ROS generation efficiency by the structures. We demonstrate the inhomogeneity of electron-transfer rate in the structures and discuss some reasons for this phenomenon. In this work, for the first time, we estimate electron-transfer efficiency in the TiO2/QDs hybrid structures using ROS generation under visible light. We show that the photoinduced electron transfer is the main reason for QD quenching in hybrid TiO2-based nanoparticles and propose a way to improve the efficiency of these structures.

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nonradiative channels of trap states on the QDs surface.12−14 Recently, an approach combining these two popular ideas of QD blinking has also been proposed.15 Clearly, when one forms a hybrid structure based on QDs as energy/charge donors, these processes should be taken into account because they lead to QD multicomponent PL decay, which reflects the difference in nonradiative rate between QDs fractions with different characteristic PL lifetimes. Hybrid structures based on colloidal CdSe/ZnS QDs and titanium dioxide NPs are outstanding examples of the QD systems possessing a strong application potential in the treatment of infectious diseases because of their antibacterial activity under visible light. Thus, it has been shown that titanium dioxide and especially its nanostructured form features remarkable antibacterial efficiency.16,17 Ultraviolet excitation is needed to activate titanium dioxide NPs as an antibacterial agent, and therefore, this limits their wide application since UV light is harmful for other living systems.

INTRODUCTION The first papers concerning synthesis of bright photoluminescent (PL) colloidal quantum dots (QDs), published by Bawendi’s group,1,2 began an era of intense research on QDs and their applications in photovoltaics, sensors, and biomedical applications,3−5 owing to their superb optical properties. At the same time, the complexity of QD systems was becoming more clearly understood, as their physical properties dependent upon a range of parameters including stabilizing molecules, environment, surface charge, etc., were extensively investigated.6,7 Multicomponent QD exciton luminescence decay at room temperature is well documented for QD ensembles and single QDs. The reasons for this phenomenon have been discussed since the first report of QD blinking, i.e., random PL intensity fluctuations, was published.8 In the framework of the QD-charged states model, Auger recombination efficiency significantly increases with the addition of an extra charge carrier to a QD. Therefore, random charging of QDs by extra carriers (electrons or holes) is the main reason for multiexponential PL decay of QDs at room temperature.9−11 According to another approach, QD multiexponential PL decay is caused by opening and closing of © 2019 American Chemical Society

Received: March 15, 2019 Revised: May 19, 2019 Published: May 22, 2019 14790

DOI: 10.1021/acs.jpcc.9b02481 J. Phys. Chem. C 2019, 123, 14790−14796

Article

The Journal of Physical Chemistry C

Formation of TiO2/QDs Hybrid Structures. To form the structures, colloidal semiconductor I type CdSe/ZnS QDs with an average core diameter of 5.5 nm obtained by hightemperature organometallic synthesis were utilized.47 The surface of QDs is stabilized by trioctylphosphine oxide (TOPO) molecules as a result of synthesis. Synthesis of titanium dioxide nanoparticles was carried out by the method proposed in ref 48. Here, we use spherical TiO2 nanoparticles with 2 nm diameter stabilized by oleic acid molecules. The size of the hybrid structure components was estimated from SEM images presented in the Supporting Information (section S1). We formed our multilayer hybrid structures using the modified Langmuir−Blodgett technology. Briefly, colloidal solutions of TiO2 nanoparticles or QDs were dropped onto the water surface, and then they were left for 15 min for solvent evaporation; finally, nanoparticles layers on the water− air interface were transferred to the dielectric substrates. The details of hybrid structure formation are given in the Supporting Information (section S2). The formed hybrid structures contain three QDs layers and three TiO2 nanoparticles layers. Investigation of samples with atomic force microscopy has confirmed that each QDs or TiO2 layer consists of one to two monolayers of components (section S5).49 Figure 1 shows a schematic depiction of formed structures.

Therefore, sensitizing TiO2 NPs via the use of QDs is an effective means to remove this restriction because QDs can efficiently absorb visible light and transfer photoexcited electrons to TiO2 NPs.18−29 Despite the fact that numerous publications describe the properties of TiO2 NP/QD structures,30−32 currently there are no reports that demonstrate stronger or even similar reactive oxygen species (ROS) generation under visible excitation as exhibited by free TiO2 NPs under UV irradiation.33−35 The reasons for this discrepancy between the expected and actual ROS generation efficiency by TiO2 NPs/QDs structures can be explained if we could estimate the impact of electron transfer (ET) on the structures’ functionality directly from experimental data. Today the pump−probe spectroscopy36−38 and evaluation of PL quenching of QDs39,40 are the most commonly used approaches for finding electron-transfer efficiency in hybrid structures based on QDs. Recently, it has also been demonstrated that time-resolved infrared (TRIR) spectroscopy can be effectively applied to estimation of electron transfer in TiO2 conduction band.41 It should be pointed out that the evaluation of QD photoluminescence quenching is an indirect method for estimation of energy or charge transfer efficiency because coupling of QDs with other species often gives rise to new nonradiative channels of ET from the QDs.30,31 At the same time, even direct methods of analysis of the ET such as pump−probe and TRIR spectroscopy are not useful for the estimation of ET impact on hybrid structure functionality. In TiO2/QD structures, ROS generation under visible light makes it possible to evaluate the realistic potential of hybrid structures. Similar approaches have been used to estimate FRET efficiency in QD based hybrid structures.42,43 In our work, we have produced and studied multilayer hybrid structures based on CdSe/ZnS QDs and TiO2 NPs that demonstrate a synergistic effect under visible light because of efficient photoinduced ET from the QDs to TiO2 NPs. We show that the ET rate calculated from the fast-decay QD fraction is four times higher than that found from the slowdecay QD fraction. This finding is in a good agreement with the charged QD blinking model, which attributes these QD fractions to negative trion (X−) QDs, i.e., QDs with an extra electron, and neutral (X0) QDs, respectively.44−46 Here, using ROS generation for our structures under visible light we clearly demonstrate that QD blinking is the main reason for relatively low functionality of hybrid structures based on CdSe/ZnS QDs and TiO2 NPs.

Figure 1. Sketch representation of TiO2/QDs multilayer hybrid structures.

Characterization of TiO2/QDs Hybrid Structures. The morphology of TiO2/QDs hybrid structures are investigated using luminescent confocal microscope LSM-710 (Zeiss, Germany). The PL kinetics of samples is analyzed using a time-correlated single photon counting spectrometer MicroTime100 (PicoQuant, Germany). The PL decay was approximated by a biexponential function y = A 1

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EXPERIMENTAL SECTION Materials. Cadmium oxide (Sigma-Aldrich, USA), selenium powder (Sigma-Aldrich, USA), zinc oxide (SigmaAldrich, USA), 2-ethylhexanoic acid (Sigma-Aldrich, USA), octadacene (Sigma-Aldrich, USA), oleylamine (Sigma-Aldrich, USA), hexadecylphosphonic acid (Sigma-Aldrich, USA), trioctylphospine (Fluka, Sweden) were used for synthesis of colloidal CdSe/Zns QDs. To synthesize TiO2 nanoparticles, titanium(IV) chloride (Sigma-Aldrich, USA), benzyl alcohol (Sigma-Aldrich, USA) and oleic acid (Fluka, Sweden) were used. Hexane (Vekton, Russia) and isopropyl alcohol (Lenreactiv, Russia) were used for formation structures. As a chemical sensor for investigation of ROS generation, the sensor p-nitrosodimethylaniline (Sigma-Aldrich, USA) was used. All chemical reagents were used without further purification.

( ) + A exp(− ), where τ is PL decay time, A is t

exp − τ

1

2

t τ2

i

i

amplitude. Layers of free QDs formed under the same conditions as the hybrid structures are used as a reference sample. Investigation of ROS Generation by the Hybrid Structures. ROS generation is investigated using a chemical sensor, p-nitrosodimethylaniline (RNO). The RNO absorption band at 440 nm is decreased when interacting with ROS species.50 Samples of hybrid structures were formed on cell windows (the cell photo is given in Supporting Information) filled up with 2.5 mL of sensor solution (RNO concentration was 2 × 10−5 M) for ROS generation experiments. As a source of external radiation, 365 nm emission of a mercury lamp and a 460 nm LED are used. 14791


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RESULTS

Photoluminescent Properties of TiO2/CdSe/ZnS QD Hybrid Structures. TiO2/CdSe/ZnS QD multilayered hybrid structures have been formed on dielectric slides by a modified Langmuir−Blodgett technology, described in detail in the Experimental Section. Figure 2 shows a PL confocal and local PL spectrum of the TiO2/QD hybrid structures.

Figure 3. Decay of CdSe/ZnS QDs luminescence. (1) Free QD layers on the glass slide. (2) TiO2/QD hybrid structures. (3) Fittings of (1) and (2) by an exponential function y = A1

( ) + A exp(− ). t

exp − τ

1

2

t τ2

Table 1. PL Decay Times (τ) and Amplitudes (A) of QDs in the Samples sample type layers of free QDs TiO2/QD hybrid structures

Figure 2. PL image of TiO2/QDs multilayer hybrid structures (425 × 425 μm2area, excitation wavelength 405 nm, 20x/0.75 objective lens. Inset shows a local PL spectrum of the TiO2/QD hybrid structures from the whole scanning area.

( ) + A exp(− ). The fitting pat

1

2

A1 (%)

τ 2 (ns)

A2 (%)

12 ± 2 26 ± 2

3.4 ± 0.5 2.5 ± 0.5

88 ± 2 74 ± 2

the ratio of fractions with the two lifetimes. For both QDs fractions, quenching of QDs luminescence is observed during the formation of hybrid structures. Taking into account the relative positioning of the energy levels of QDs and TiO2 NPs, electron transfer is the only possible cause of quenching. The analysis of both QDs fractions lifetimes has clearly shown that the fraction with longer lifetime (τ1) is quenched more strongly by TiO2 NPs than the second fraction (with τ2). This result supports our suggestion to consider multicomponent PL decay of QDs for correct estimation of electron-transfer efficiency in the structures based on QDs. Here, we have analyzed QD luminescence kinetics under the assumption that all QD fractions are characterized by the same radiative rate and different nonradiative rates, which compete differently with ET process in our structures. Electron-Transfer Efficiency in TiO2/QDs Hybrid Structures. Here, we estimate ET efficiency based on PL quenching of the QDs in the hybrid structures working with the assumption that ET is the dominant channel competing with emissive deactivation routes.51 The quantum yield of PL of ith QDs fraction can be estimated from their PL decay time as51,52

The formation of hybrid structures by successive deposition of TiO2 NP layers and CdSe/ZnS QDs makes it possible to obtain samples with a relatively uniform QD distribution, as can be seen from Figure 1. The PL image of the free QD layers obtained by a similar method is shown in Supporting Information (section S3). The resulting PL spectrum of the hybrid structures (Figure 2 inset) demonstrates no change from the PL spectrum of the solution of free QDs (see section S4 in Supporting Information) as expected. Here the PL properties of the TiO2/QD hybrid structures have been analyzed using PL kinetics of the CdSe/ZnS QDs. Figure 3 shows the PL decay curves of the TiO2/QD hybrid structures and free QD layers that are utilized as a reference sample. The analysis of the PL decay of QD hybrid structures and free QDs both has shown to be of biexponential character. Experimental PL decay curves are fitted by a biexponential function y = A1 exp − τ

τ1 (ns) 18 ± 0.5 12 ± 0.5

t τ2

rameters, characteristic PL decay times of the samples and their amplitudes, are shown in Table 1. The data presented in Table 1 demonstrate that the QD PL kinetics in the multilayered TiO2/QDs hybrid structures can be fitted by a biexponential function with 12 and 2.5 ns characteristic lifetimes. The biexponential character of QDs PL decay indicates the presence of two fractions with different lifetimes. Assuming that all QDs in the ensemble have the same characteristics of the radiative transition and, therefore, the same radiative rate (kr), the two observed QDs fractions are characterized by different efficiencies of nonradiative processes. The amplitudes (Ai) given in Table 1 show only

ϕi = k rτi ϕQDs =

kr k r + k nr

(1)

where ϕi in percentages, kr and knr in reverse seconds, and τi in nanoseconds are PL quantum yield, radiative and nonradiative rate, and PL decay time of the ith QD fraction, respectively. We suppose here that the QDs radiative rate equals kr = 4 × 107 s−1 for both QDs fractions,53 and it does not change in the 14792


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The Journal of Physical Chemistry C Table 2. PLQY of QDs and the Rates of Exciton Relaxation in the Samplesa QD fractions

φQDs1 (%)

φTiO2/QDs1 (%)

kr, ×107 s−1

knr1, ×107 s−1

kET3, × 107 s−1

EET3 (%)

⟨EET⟩4 (%)

with τ1 with τ2

72 ± 2 14 ± 2

48 ± 2 10 ± 2

4 4

1.5 ± 0.2 24.6 ± 0.9

2.8 ± 0.2 11.4 ± 0.5

34 ± 3 28 ± 3

29 ± 3

a

The indices 1−4 correspond to the formulas that are used to estimate these values.

Figure 4. (a) Sketch representation of ROS generation experiments. Absorption band of RNO at different external radiation dose is shown in the inset; (b) and (c) are the dependence of optical density of RNO absorption band at 440 nm on radiation dose in samples with TiO2/QDs hybrid structures and their components under 365 and 460 nm, respectively. The solid lines are drawn for visual guidance.

It makes efficient ET from τ2 QD fraction possible despite the high nonradiative rate. It apparently demonstrates that it is fundamentally wrong to ignore the QDs fraction with high efficiency of nonradiative processes as an electron donor.54,55 The average ET efficiency can be estimated taking into account the concentration of QDs in each fraction

TiO2/QDs structures. We suppose also that quenching of QDs luminescence in the structures is completely associated with the photoinduced electron transfer from QDs to titanium dioxide NPs. Therefore, the ET rate in the structures can be estimated as kET =

(1 − φTiO2/QDs)k r φQDs

− kr (2)

⟨E ET⟩ =

Assuming that the structure formation changes neither the CdSe/ZnS QDs radiative rate nor the Auger rates, the electron-transfer efficiency can be estimated as E ET =

kET k r + k nr + kET

i 2 i ∑i (E ET )A i ∑i E ET Ai

(4)

where Ai is the amplitude of the ith fraction of QDs and EiET is electron-transfer efficiency of these fractions. The average efficiency of photoinduced electron transfer estimated using formula 4 is 29% (see Table 1) and is 160% higher than the ET efficiency estimated using the average lifetime, i.e., 11% (see section S9 in the Supporting Information). It should be pointed out that an estimation of ET efficiency using PL quenching of QDs may give incorrect information owing to the formation of new additional nonradiative channels involving trap states in the QDs.56 For this reason, we suggest that ET efficiency in TiO2/QD hybrid structures should be estimated through the efficiency of reactive oxygen

(3)

Using the formulas 1−3 (formula derivation is presented in the Supporting Information, section 8) parameters of the structures can be estimated assuming equal distances between QD and TiO2 NPs in our samples. Therefore, here we operate with average nonradiative and ET rates, and Table 2 presents the parameters of our structures. Data presented in Table 2 clearly demonstrate that the ET rate for QD fraction with τ2 is 4 times that for τ1 QD fraction. 14793


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The formula 7 is correct for structures with the equal component concentrations, i.e., for systems with one acceptor per one donor. The number of ROS molecules generated in the samples is proportional to the change of optical density of the sensor absorption band

species (ROS) generation by these structures. We believe that ROS generation in our TiO2/QD hybrid structures can occur solely due to the photoinduced electron transfer from the QD to the TiO2 nanoparticles under visible light. Hence, ROS concentration is proportional to the ET efficiency and therefore ET efficiency in a TiO2/QD hybrid structure can be estimated based on the ROS generation under visible light irradiation. This approach also enables the estimation of trap state effects upon the PL quenching of QDs in the hybrid structures produced. ROS Generation by TiO2/QDs Hybrid Structures. It is well-known that TiO2 nanoparticles are able to generate ROS under UV radiation.57 Here, ROS generation by the samples is investigated using the chemical sensor p-nitrosodimethylaniline (RNO), which is traditionally used to investigate photocatalytic properties of TiO2 nanoparticles and hybrid structures based on them.58 The sketch representation of ROS generation experiments (for the experimental details see the Experimental Section) and dependence of optical density of RNO absorption band at 440 nm on radiation doses under UV and visible light in samples with TiO2/QDs hybrid structures and their components are shown in Figure 4. The strong decrease of RNO concentration under UV radiation in the presence of the TiO2/CdSe/ZnS QD hybrid structures and with TiO2 NPs (see Figure 4b) confirms ROS generation by these samples. Parts b and c of Figure 4 demonstrate also that the CdSe/ZnS QDs are unable to generate ROS under UV or visible light. At the same time, our hybrid structures demonstrate effective ROS generation under visible light (see Figure 4b) owing to ET from the QDs to the TiO2 NPs. Estimation of Electron-Transfer Efficiency Based on ROS Generation by TiO2/QDs Hybrid Structures. The number of ROS molecules generated by TiO2 nanoparticles can be expressed as NROS = NeE ROS

direct direct NROS = NedirectE ROS ∼ ΔDRNO sens sens NROS = NesensE ROS ∼ ΔDRNO

Ndirect e

where and are estimated from formulas 6 and 7, respectively, and EROS is the ROS generation efficiency of TiO2 NPs. Formulas 5−8 allow estimating ET efficiency in TiO2/QDs hybrid structures using TiO2 NPs layers as a reference directly from ROS generation, if the following experimental conditions are met: (i) equal TiO2 NPs surface concentration in samples of the structures and TiO2 NPs layers; (ii) equal concentration of chemical sensor used for registration of ROS generated by the structures and TiO2 NPs layers; (iii) equal concentrations of water and oxygen molecules in the immediate environment of the structures and TiO2 NPs layers. Taking into account these model assumptions, ET efficiency can be estimated as E ET =

(5)

direct (1 − TλQDs)W λsens ΔDRNO

(9)

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CONCLUSIONS We have shown that the combination of titanium dioxide NPs and CdSe/ZnS QDs in a multilayered hybrid structure results in synergistic effect of ROS generation under visible light. We demonstrate that ET efficiency based on RNO photobleaching in the sample with TiO2/QDs hybrid structures is almost equal to the estimate based on PL quenching of QDs in structures (29% and 26%, respectively). This result clearly confirms that photoinduced electron transfer from QD to TiO2 nanoparticle is the dominant channel of QDs’ PL quenching of in our TiO2/QD hybrid structures. It is pointed out that blinking is the main reason for relatively low functionality of the TiO2/ QD hybrid structures under visible light. Our finding that the ET rate of QD fraction with shorter lifetime (τ2) exceeds that of the QD fraction with τ1 is in good agreement with the charged QD-blinking model, where the former is the fraction of QDs with extra electron and the latter is the neutral QD fraction.9 Therefore, replacement of type I CdSe/ZnS QDs by alloyed QDs characterized by Auger recombination suppression59,60 should significantly increase ET efficiency, especially

(6)

TiO

where (1 − Tλ 2) is the portion of incident light at λ is the number of wavelength absorbed by TiO2 NPs and Wdirect λ photons with energy hc/λ incident on the NPs layers per second. In the case of sensitized ROS generation, the number of electrons transferred to the TiO2 NPs layers per second due to electron transfer from QDs to TiO2 NPs can be estimated as Nesens = NeQDsE ET = (1 − TλQDs)W λsensE ET

sens (1 − TλTiO2)W λdirect ΔDRNO

The electron-transfer efficiency in our structures estimated using formula 9 is 26 ± 3%. Detailed information on the experimental conditions and calculations is given in the Supporting Information (section S10). It should be pointed out that use of formula 9 suggests no changes of nonradiative rate in QDs. We believe that this condition is satisfied in our structures because of organic shells on the QDs and TiO2 NPs. Therefore, there is no direct contact between QDs and TiO2 NPs surfaces, and as a result, no influence of the NPs on nonradiative rate in QDs.

where Ne is the number of electrons in TiO2 conduction band generated during irradiation and EROS is the efficiency of ROS generation by the TiO2 nanoparticles. The number of electrons generated in TiO2 NPs layers per second due to direct absorption of UV light can be estimated as Nedirect = (1 − TλTiO2)W λdirect

(8)

Nsens e

(7)

where NQDs is the number of electrons generated in QDs layers e as a result of incident light absorbance, EET is the electrontransfer efficiency from QD to TiO2 NP, (1 − TQDs λ ) is a portion of incident light at λ wavelength absorbed by QDs, and is the photon number with energy hc/λ incident on the Wsens λ QDs layers per second. 14794


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for anion QDs. This will allow QDs to reveal their full potential as effective electron donors and in the biomedical perspective to reach the expected antibacterial efficiency using TiO2/QD hybrid structures under visible-light irradiation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02481. Sample characterization, details of electron-transfer efficiency estimations, Figures S1−S10, and Tables S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Finn Purcell-Milton: 0000-0002-3591-9477 Yurii K. Gun’ko: 0000-0002-4772-778X Anna Orlova: 0000-0002-6465-7672 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Government of Russian Federation, Grant 08-08. REFERENCES

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