Shuttling Photoelectrochemical Electron Transport in Tricomponent

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Shuttling Photoelectrochemical Electrons Transport in Tricomponent CdS/rGO/TiO Nanocomposites 2

Haihua Yang, Stephen V Kershaw, Yu Wang, Xuezhong Gong, Sergii Kalytchuk, Andrey L. Rogach, and Wey Yang Teoh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405227t • Publication Date (Web): 12 Sep 2013 Downloaded from http://pubs.acs.org on September 15, 2013

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The Journal of Physical Chemistry

Shuttling Photoelectrochemical Electrons Transport in Tricomponent CdS/rGO/TiO2 Nanocomposites Haihua Yang, †,‡ Stephen V. Kershaw, † Yu Wang, † Xuezhong Gong, ‡ Sergii Kalytchuk,†,‡ Andrey L. Rogach,† and Wey Yang Teoh*,‡ †

Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City

University of Hong Kong, Kowloon, Hong Kong S.A.R. ‡

Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment,

City University of Hong Kong, Kowloon, Hong Kong S.A.R. * Corresponding E-mail: [email protected]; Tel: (+852) 3442 4627, Address: Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong S.A.R.

ACS Paragon Plus Environment

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ABSTRACT: Composite photoelectrodes consisting of CdS sensitizer, reduced graphene oxide (rGO) transporter and TiO2 acceptor were synthesized in a solvothermal synthesis. Under solvothermal conditions, the dimethyl sulfoxide (DMSO) solvent medium decomposed to form free sulfides, which facilitated the formation of CdS, and at the same time also reduced graphene oxide sheets by forming disulfides moieties. Compared to pure CdS and TiO2, coupling of these materials either as bi- or tricomponent composites (including rGO) allowed efficient interfacial charge separation and prolonged electron lifetimes. In particular in the CdS/rGO/TiO2 tricomposite case, the rGO plays vital roles in alleviating trapped electrons at the heterojunction and serve as a platform for shuttling electrons between CdS and TiO2. Taken into account all of the structure-related charge transport characteristics, including interfacial contacts, the highest quantum efficiency (incident photon-to-current efficiency, IPCE at 460 nm = 12%) was achieved for the CdS/rGO/TiO2 tricomposite, and this was six-fold that of CdS/TiO2.

KEYWORDS: Cadmium Sulfide, Titanium Dioxide, Reduced Graphene Oxide, Charge Transport, Transient Photoluminescence, Water splitting


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1. Introduction Photoelectrochemical solar harvesting forms the basis of excitonic solar cells1 and photoelectrochemical water splitting,2 arguably the two most intensively investigated solar energy conversion systems. The working electrode construct of these photoconversion devices typically consists of a wide bandgap semiconductor and/or narrow bandgap sensitizer.3,4 For reasonable coverage of the solar spectrum, the latter may consist of visible to near-infrared absorbing organic dyes or inorganic metal chalcogenides. These sensitizers should have energy levels (LUMO or conduction band edge, Ecb, respectively) more negative than that of the semiconductor acceptor, to make interfacial photoelectron injection possible. There are two primary advantages of coupling the sensitizers to wide bandgap acceptors, namely, (i) increased stability of the narrow bandgap sensitizers, and (ii) the high density of states of acceptors promotes efficient interfacial charge injection.5 In recent years, increasing interest has been aimed towards the incorporation of graphene in semiconductor nanostructures to enhance photoconversion efficiencies.6 It has been well-established that the high electron conductivity of graphene through its hybridized sp2 carbon bonds is beneficial to interfacial electron extraction and photocharge separation.7 Despite much success, most of these studies have been limited to the coupling of bicomponent composites, that is, the semiconductor photocatalyst and graphene, the latter in the form of reduced graphene oxide (rGO).8 The rGO is a popular variant of the graphene family or monolayer carbon sheet due to its facile and scalable synthesis based on the chemical oxidation-exfoliation of graphite followed by reduction. Charge transport in multicomponent composites involving two semiconductors (a wide and a narrow bandgap component) interfaced with rGO in a photoelectrochemical system has not been fully


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understood to date. Complexities arise from the shuttling of electrons between the two or more semiconductor components via the rGO. In the current work, we synthesized and studied the charge transport characteristics of CdS/rGO/TiO2 nanocomposites prepared by a solvothermal technique. The syntheses of similar structures, bi- and tricomponent composites, have been reported in the literature, focusing on the applications for photocatalytic water splitting,9,10 abatement of environmental pollutants11,12 and organic photosynthesis.13-16 By careful analysis of the physicochemical and photoelectrochemical properties of the composites and their variations including sub-set combinations, and combinations with permutations of direct and indirect coupling, we investigated and compared the electron transport characteristics of the composites. This was further verified by transient charge kinetics measurements. In particular, we identified that the three-component composite enhanced the photon conversion efficiencies across the UV and visible light range, showing the beneficial and concerted effects of the sensitizer (CdS), transporter (rGO) and acceptor (TiO2). Any combination of the bicomponent proved inferior to the CdS/rGO/TiO2 tricomponent composite. To the best of our knowledge, the work is the first to elucidate the charge transport in such multicomponent composites involving rGO.

2. Experimental Details 2.1 Synthesis of CdS/rGO/TiO2 nanocomposites and electrodes fabrication Graphene Oxide (GO) was synthesized using a modified Hummers method.17,18 Graphite powder (Acros Organics) was pre-calcined in a Thermolyne muffle furnace at 900 °C for 1 hour at a ramp rate of 5 °C min-1 to remove impurities and non-graphitic carbons. The calcined graphite powder (0.7 g) was transferred into a round-bottom flask (RBF) and held in an ice bath.


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Sulfuric acid (15.3 mL, Aldrich, 95-97%) and 0.25 mL of nitric acid (Aldrich, 65%) were added to the flask. The suspension was kept below 10 °C and magnetically stirred at 700 rpm for 5 min. Potassium permanganate (2 g, Aldrich, 99%) was added gradually to the suspension over 15 min. The RBF was transferred to a water bath bringing the suspension temperature to 35 °C and held for 30 min before being transferred to a paraffin oil bath. A water-cooled reflux-condenser was attached and 31 mL of Milli-Q water was added before bringing the suspension to 95 °C for another 15 min. An additional 93 mL of warm water was used to dilute the suspension before allowing it to cool to 80 °C. The graphene oxide was separated from the suspension by centrifugation (Thermo) at 10 000 rpm and washed 3 times with Milli-Q water to remove residual acids and organic impurities. The final separated GO was dried at 60 °C for 48 h. The CdS/rGO/TiO2 nanocomposites and their constituents were synthesized by a dimethyl sulfoxide (DMSO)-based solvothermal method.9,19 For the synthesis of CdS and composites, Cd(CH3COOH)2 (0.7 mmol, Aldrich) was dissolved in DMSO (70 mL, Riedel-de Haen). The solution was then transferred into a Teflon-lined autoclave (100 mL) and kept at 180 °C for 12 h. The obtained precipitates were centrifuged and washed repeatedly with acetone and then alcohol. For the samples containing TiO2 and/or rGO, Aeroxide P25 TiO2 (70 mg, Evonik) and/or the above obtained GO (1.75 mg) were respectively mixed in the DMSO solvent by sonication and vigorous stirring, prior to autoclaving under the same conditions as mentioned above. The autoclaved samples were dried at 60 oC for characterization or dispersed in methanol (70 mL) for electrode fabrication. Fluorine-doped tin oxide (FTO) conductive glass (Nippon Special Glass) was used as the transparent conductive oxide substrate onto which composite samples suspended in methanol at 1 mg ml-1 were drop-cast. A total of 5 mL of suspension was drop-cast stepwise in twenty steps.


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The suspensions were sonicated for 2 min prior to each drop-casting step. The electrodes were dried under a gentle stream of nitrogen. Once prepared the final electrodes were rinsed with Milli-Q water. 2.2 Physicochemical and photoelectrochemical characterization The morphology of the samples was observed with a transmission electron microscope (TEM, Philips CM20). High-resolution transmission electron microscope (HRTEM) images were obtained with a Philips CM200 operated at 200 kV. X-ray photoelectron spectra (XPS) were measured in a Physical Electronics PHI-5802 instrument. UV-Vis diffuse-reflectance absorption spectra of the samples were collected on a UV-Vis-NIR spectrophotometer (Shimadzu UV-3600) equipped with an integrating sphere, using Ba2SO4 as the reflectance standard. Time-resolved photoluminescence decays were measured on an Edinburgh Instruments FLS920P spectrometer, with a picosecond pulsed diode laser (EPL-405 nm, pulse width: 49 ps) as the single wavelength excitation light source for time-correlated single-photon counting (TCSPC) measurements. All spectra were obtained at room temperature. The photoelectrochemical (PEC) set-up followed a classic three-electrode system, with a dropcasted photoanode, Ag/AgCl reference electrode and platinum counter electrode. The electrolyte consisted of 0.5 M aqueous sodium sulfate (Na2SO4, Aldrich) deaerated with N2 for 20 min prior to use. A 300 W arc xenon lamp (Newport) was used as the light source. For visible light measurements, a 420 nm cutoff filter (Schott, GG420) was used to filter out the ultraviolet component, suppressing the excitation of TiO2. The photocurrent and open-circuit voltage characteristics were recorded on Solartron Modulab potentiostat. The wavelength-dependent photocurrent response was measured using a focused monochromatic light source provided by a 300 W xenon lamp (Newport) source coupled to an Oriel 1/8m Cornerstone Monochromator


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(Newport, Oriel Instruments USA). The incident photon to charge carrier generation efficiency (IPCE) was calculated by IPCE (%) = [1240/λ (nm)] ×[Isc (A cm-2)/Pi (W cm-2)] ×100, where Pi is the power density of monochromatic light of wavelength λ (nm) incident on the electrode measured using a calibrated silicon photodetector (Newport), and Isc is the current density at 0 V vs. Ag/AgCl. The electron lifetime (τe) was deduced from the decay of the open circuit voltage (VOC) upon termination of irradiation using τe=-(kBT/e)(dVOC/dt)-1, where kB is the Boltzmann’s constant, T is temperature, and e is the elementary charge.20,21

3. Results and discussion 3.1 Composite photoelectrodes design and architecture The solvothermal synthesis of cadmium acetate, graphene oxide (GO) and titanium dioxide (TiO2) in dimethyl sulfoxide (DMSO) at 180 °C resulted in a tricomponent nanocomposite comprising a dense layer of CdS deposited on reduced graphene oxide (rGO) and to a much smaller extent on TiO2 nanoparticles (NPs) (Figure 1A). Under the solvothermal condition, DMSO was decomposed to reduce GO and in the process formed dense distal thiol moieties on rGO, which we verify later from surface XPS measurements. These thiol moieties are the nucleation sites on which CdS NPs were formed on rGO. Although DMSO typically decomposes at 190 °C under ambient pressure, it decomposed at elevated pressure during autoclaving, giving rise to the formation of sulfides. HRTEM reveals the high crystallinity of the multilayer CdS NPs (Figure 1B), whilst EDX analysis confirms the predominant deposition of CdS NPs on rGO sheets (Figure 1D, Cd:S = 1:1 with negligible amount of Ti). Only a small amount of CdS deposited on the TiO2 NPs (Figure 1C, Ti:Cd:S = 42:1:1.1). The dense coverage of CdS on rGO (Figures 1A and F) renders sporadic contact between TiO2 and rGO, which as we show later,


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imparts a strong influence on the mechanism of interfacial charge transport. In the absence of the CdS layer, i.e., in a bicomponent synthesis, better coverage of rGO with TiO2 can actually be achieved (Figure 1E), although this may still be less dense than those typically synthesized by photocatalytic reduction since the absence of thiol moieties in the latter method allows more direct conjugation between the hydroxyl moieties of TiO2 and the carboxylic groups of rGO.22 Analysis by XPS confirms the reduction of GO (brown suspension) to rGO (black suspension) during the solvothermal synthesis. Figure 2A shows the C 1s spectrum of the GO obtained from the Hummers’ chemical oxidation and exfoliation of graphite.17,18 The as-prepared GO contains oxygenated groups such as carboxyl and epoxides, as reflected by the C-C, C-O, C=O, and OC=O bonds. Here, the fraction of oxidized carbon was estimated to be 54%, with the remaining content made up of graphitic carbon (46%). The lack of an sp2 carbon network in the former is known to impede electron conductivity.7 Solvothermal treatment of the GO chemically reduced and restored the graphitic carbon content of the product rGO to 65%, while some oxygenated contents remained but at significantly decreased levels (Figure 2B). In particular, the carboxylic moieties at the edges of rGO are not easily removed by chemical reduction.7 This is comparable to those produced by conventional chemical reduction23 and photocatalytic reduction.24 Whilst the formation of CdS is a clear indication of thiol formation through decomposition of DMSO (Figure 2C, thiol BE S 2p = 161.9 eV),25 the sole S source, further oxidation of the thiols to form disulfides (Figure 2D, BE S 2p3/2 = 165 eV)25,26 chemically reduced GO to rGO. Such a reductive disulfide path has been shown by us, as well as others, in the reduction of γ-Fe2O3 to Fe3O427 and CuO to metallic Cu,28 respectively. Analysis of the S 2p binding energy of solvothermally-treated TiO2 revealed the incorporation of sulfur in TiO2 as cationic dopants, which was absent in the untreated sample (Figure 2C). The


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S 2p peaks at around 169.1 and 170.2 eV are attributed to S4+ and S6+, respectively.29-31 With increasing depth below the surface of TiO2, the atomic concentration of S gradually decreased from 1.0% to about 0.3% in the bulk (Figure 2D). The surface sulfur enrichment and speciation is consistent with the observation by Ohno and co-workers on thiourea impregnated S-doped TiO2, which upon calcination resulted in 1.6% surface S concentration.29 It is interesting to note the diffusion of S into TiO2 lattice, that is, subsurface doping was possible in our case, despite the relatively mild solvothermal conditions. Doping of S in TiO2 is known to be a difficult process.32 We did not attempt to increase the S dopant concentration, but this is in principle possible using precursors such as thiourea and mercaptopropinic acid under parametric solvothermal synthesis conditions. The UV-Vis diffuse reflectance absorption spectra of different samples are presented in Figure 3B, with the corresponding photograph of the as-prepared colloidal samples. As-obtained CdS NPs possess an absorption edge at around 530 nm, corresponding to a bulk bandgap of 2.34 eV. A similar band edge can be seen for CdS when incorporated in composites as CdS/rGO, CdS/TiO2 and CdS/rGO/TiO2. All rGO-containing samples, that is, CdS/rGO, TiO2/rGO and CdS/rGO/TiO2, show broad and enhanced absorption across the measured spectra, which is typical of absorption by π electrons. TiO2 with and without solvothermal treatment in DMSO showed the typical absorption edge at around 410 nm (equivalent to 3.03 eV), which corresponds to the intrinsic bandgap of a TiO2 anatase/rutile mixture (83% anatase and 17% rutile).33,34 The extended visible light absorption of the light pale yellow S-doped TiO2 powder following solvothermal treatment could not be measured by UV-Vis diffuse-reflectance absorption spectroscopy due to limitations of the instrument sensitivity (Figure 3A). This can be better inferred from the monochromatic photoresponse measurements, as shown in the next section.


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Analogous physical mixing of CdS/rGO and TiO2 by sonication resulted in an almost similar absorbance spectrum to that of the tricomponent composite of CdS/rGO/TiO2.

3.2 Photoelectrochemical properties and charge transport The as-prepared samples were fabricated as electrodes by direct drop-casting onto a transparent conductive electrode. The photocurrent conversions of these electrodes under full arc xenon irradiation are shown in Figure 4A, where both the components of CdS and TiO2 are photoexcited. In all cases, anodic photocurrents, typical of n-type semiconductors were measured. Despite the S-doping in TiO2, no significant difference in photoresponse was measured for TiO2 NPs with or without solvothermal treatment, at least under full arc irradiation. Incorporation of rGO during solvothermal synthesis enhanced the photoresponse of all electrodes, which in the case of CdS-containing samples was increased by at least two-fold. This is consistent with the observation by Zhang et al.14 on similarly prepared CdS/rGO. The enhancement can be traced to the intimate contact between CdS and rGO transporter, which significantly enhanced the interfacial electron extraction from the CdS conduction band (and trap states). By comparison, only a modest 50% enhancement can be measured for TiO2 when incorporated with rGO due to the less intimate contact between TiO2 with thiolated-rGO as discussed earlier. This is in contrast to the ten-fold enhancement in photocurrent reported for rGO/TiO2 synthesized by the photoreduction of GO.35 A synergetic enhancement of photocurrent can be measured for the tricomponent composite of CdS/rGO/TiO2, where the photocurrent is higher than that of the combinations of CdS/rGO and TiO2/rGO under full arc irradiation (Figure 4A). A post-solvothermal mixture of analogous CdS/rGO + TiO2 by sonication only shows a comparable photocurrent even when compared to


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CdS/rGO. The synergetic enhancement infers more efficient charge separation and extraction when the three components are held in intimate physical contact. The observation is more obvious under visible light irradiation where despite the inactivity of the TiO2 subcomponent, the CdS/rGO/TiO2 shows a higher photocurrent than CdS/rGO alone. The same effect could not be applied to CdS/rGO + TiO2, where the inactivity of TiO2 resulted in significantly lower photocurrent compared to CdS/rGO. It is interesting to note that while untreated TiO2 did not show any sub-bandgap photocurrent response under visible light excitation (λ ≥ 420 nm), cathodic photocurrent was measured for the solvothermally-treated TiO2 (Figure 4B). Non-metal dopants such as N, S and C are known to introduce impurity energy states above the valence band edge of TiO2, thereby allowing subbandgap excitation.36,37 Here, the cathodic photocurrent induced by S-doping shows p-type semiconductor behavior, which is only evident under sub-bandgap excitation. The S dopant on TiO2 exists as reversible S6+ and S4+ species, where under sub-bandgap excitation, we consider that S6+ may reversibly release two holes to form S4+, which is more compatible with the tetravalent nature of the Ti species. Kudo and coworker also reported a similar type of subbandgap cathodic behavior of Rh-doped SrTiO3.38 At this juncture, it is not possible to rule out the presence cathodic photocurrent under the full arc irradiation of the solvothermal-treated TiO2 since the cathodic current, even if took place, would have been overwhelmed by the intrinsic anodic photocurrent. Further insightful information can be gathered from the onset potentials of the electrodes (Table 1), which reflect the quasi-Fermi levels of the semiconductor materials under steady-state excitation.33,39 Here, the more negative onset potential of rGO/TiO2 (-0.25 V vs Ag/AgCl) compared to TiO2 (-0.15 V vs Ag/AgCl) indicates more efficient interfacial extraction of


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photoelectrons from trap sites below the TiO2 conduction band in accordance with a trap filling mechanism.40-42 This is consistent with the general agreement that rGO is a good electron transport material that enhances charge separation.22 By the same analogy, the introduction of rGO in CdS alleviated the charge trapping and recombination as reflected by the more negative onset potential (-0.30 V vs Ag/AgCl) compared to that of CdS alone (-0.25 V vs Ag/AgCl). Perhaps the most interesting observation is the highly negative onset potential of CdS/TiO2, which under full arc irradiation excited the two semiconductor components in a Z-scheme manner.43 This is reflected by a highly negative onset potential (-0.60 V vs Ag/AgCl) due to the extraction of photoelectrons at energy levels closer to the CdS conduction band (-0.69 V vs Ag/AgCl)14,15 than in all other cases. However, this does not warrant significant enhancement in photocurrent efficiency since charge trapping at the CdS-TiO2 interface may be more significant than that with rGO (to be substantiated by electron lifetime measurements in Section 3.3). As such, the enhancement in photocurrent density was only marginally increased compared to pure CdS but less than that of CdS/rGO. The onset potential of CdS/TiO2 (-0.40 V vs Ag/AgCl) under visible light reflected the closer proximity to the TiO2 conduction band due to the interfacial injection from the CdS to the TiO2 acceptor in the non-excited state. The onset potential for the tricomponent composite, CdS/rGO/TiO2, was measured to be -0.45 V vs Ag/AgCl, which is more negative than both CdS/rGO and CdS/TiO2 under visible light irradiation. Coupled with the high photocurrent density, the onset potential of CdS/rGO/TiO2 reflects the efficient charge separation arising from interfacial photoelectron transport from CdS to TiO2 through the rGO transporter. By comparison, the less negative onset potential (-0.40 V vs Ag/AgCl) of TiO2 + CdS/rGO is attributed to the less efficient charge separation across the two weakly contacted components.


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Figure 5 shows the monochromatic excitation photoresponse represented in incident photon-tocurrent efficiencies (IPCE) for different electrode samples. In general, drastic enhancement can be observed in all samples involving solvothermally synthesized CdS/rGO, with significant photon conversion in the visible light range, consistent with the respective absorbance spectra. Although deposition of CdS on TiO2 (IPCE460

nm

= 2%) improves the photoconversion

efficiencies of CdS (IPCE460 nm < 1%), this was far exceeded by CdS/rGO (IPCE460 nm = 8.5%). The rGO efficiently extracts and transports the photoelectrons from CdS thereby enhancing the net charge separation. The tricomponent composite of CdS/rGO/TiO2 exhibits the highest photoconversion efficiency (IPCE460

nm

= 12%), which is higher than the combinations of

rGO/CdS and rGO/TiO2. As mentioned earlier, the synergetic IPCE may arise from the beneficial charge separation across rGO and TiO2 upon photoexcitation of CdS (Scheme 1). Note: Although the extrinsic excitation of solvothermally-treated TiO2 may contribute to counteracting cathodic photocurrent (up to 500 nm), the contribution is relatively minor where IPCE values at λ ≥ 420 nm are less than 1%. It is postulated that photoelectrons injected from CdS into rGO are likely to be trapped and detrapped temporarily at the TiO2 shallow traps before reaching the back of the electrode. In principle, such de-trapping only requires low energy photons i.e., infrared photon energies or higher, that can be adequately provided by the visible light, since these trap states are just below the conduction band edge level.44,45 The TiO2 + CdS/rGO sample did not produce the same synergetic effects of CdS/rGO/TiO2 due to the absence of intimate contact between the mixed components. The lower IPCE of TiO2 + CdS/rGO above 400 nm compared to pure CdS/rGO is due to the lower volume fraction of photoactive components, i.e., CdS, in the overall construct since a proportion of the drop-cast mass was TiO2.


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3.3 Electron lifetime and transient photoluminescence The electron lifetimes (τe) of the samples were measured as a function of the rate of VOC decay (Figure 6).20,21 Solvothermally-prepared single components of CdS and TiO2 are characterized by short electron lifetimes under full arc irradiation due to rapid carrier recombination, compared to those conjugated with rGO. This is consistent with the discussion that conjugated rGO is efficient in interfacially extracting the photoelectrons from TiO2 and CdS, thereby enhancing charge separation. In fact, as shown in Figure 6A, the electron lifetimes of these electrodes predominantly reflected that of the rGO. TiO2 with or without DMSO solvothermal treatment showed similar electron lifetimes. The highest electron lifetime can be observed for CdS/TiO2, which in the absence of rGO resulted in the trapping of electrons at the CdS-TiO2 interface. As such, the high electron lifetime did not give rise to higher photocurrent. Similar electron trapping at heterojunctions had also been reported for the interface of CdX (X=Se, Te) sensitizers and TiO2 nanotube acceptors.46 Here, similar electron lifetimes were measured for λ ≥ 420 nm (Figure 6B) confirming the same limitation of the two-way electron transport at the heterojunction, i.e., from TiO2 to CdS under full-arc irradiation (Z-scheme), and from CdS to TiO2 under visible light. Figure 7 shows the time resolved photoluminescence decay of the solvothermally-prepared samples under pulse excitation wavelength at 405 nm. In general, samples containing CdS show broad photoluminescence emission (though weak when in combination with the other materials) peaking at around 670 nm (Figure S1). This is at longer wavelength than corresponding to the bulk bandgap of CdS and longer than from any CdS nanoparticles, so it is probably not bandedge or exciton recombination, but more likely recombination from carriers in traps/defect states. In the PL decays measured at this wavelength, a longer decay tail is clearly seen for rGO


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containing samples (particularly for those with rGO and CdS coupled), demonstrating that the photoelectrons transfer to and reside on the rGO, thereby prolonging the times to recombination of the photogenerated electron and hole and consequent PL emission. This is in agreement with the conclusions inferred from the electron lifetime measurements. It is interesting to compare the decay (and relative PL strength) for the two cases, CdS/rGO + TiO2 and for CdS/rGO/TiO2. In both cases the responses show a slow decay tail but the emission from the former is far weaker than for the CdS/rGO/TiO2 tricomposite where all materials are fully coupled. The slow decay is however proportionately faster where both CdS and TiO2 are both in contact with the rGO. Since photoelectrons from CdS sensitizer are efficiently extracted and trapped on the rGO, the slow decay in CdS/rGO + TiO2 infers less efficient interfacial charge transfer to the TiO2 acceptor, relative to CdS/rGO/TiO2. The results corroborate our earlier hypothesis (Section 3.2) on the limiting photoelectron transfer from CdS/rGO to the weakly contacted TiO2, resulting in inefficient photoelectrochemical collection of the photoelectrons. By the same token, and reaffirming our earlier discussion, the more efficient interfacial shuttling between the tricomponents of CdS/rGO/TiO2 facilitated efficient charge separation and further photoelectron extraction.

5. Conclusions Tricomponent CdS/rGO/TiO2 nanocomposites consisting of a sensitizer, transporter and acceptor, respectively, have been prepared by solvothermal synthesis. The tricomponent composite shows high quantum efficiency (IPCE) across the UV-visible light spectrum compared to any of the single or bicomponent constructions, highlighting a synergetic charge separation and an efficient charge transport channel. We showcase the importance of rGO as a


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mediator transporter of photoelectrons between CdS and TiO2, which otherwise are trapped at the CdS-TiO2 interface. The alignment of energy bands and trap states of the CdS sensitizer, rGO transporter and TiO2 acceptor is vital in governing the shuttling of photoelectrons between multicomponent semiconductors, beyond the combinations shown here. The work is of fundamental relevance to the design of complex and efficient photoelectrodes for energy harvesting applications, including that of photovoltaic solar cells and water splitting.

ACKNOWLEDGMENTS The authors acknowledge the financial assistance of Research Grant Council of Hong Kong through the Early Career Scheme (CityU 104812) and CityU Seed Grant (Project 7003051). S.K. acknowledges the funding support of Ability R&D Energy Research Centre.

SUPPORTING INFORMATION AVAILABLE: Photoluminiscence spectra of solvothermally-prepared samples, as well as untreated TiO2. This information is available free of charge via the Internet at http://pubs.acs.org


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Table 1. Onset potential of different solvothermally-prepared samples. Irradiation sources were provided by a 300 W xenon arc lamp (full arc), and that filtered with a 420 nm cutoff filter (visible light). Onset potential (V vs. Ag/AgCl) Full arc

Visible light

TiO2

-0.15

--

CdS

-0.25

-0.25

CdS/TiO2

-0.60

-0.40

CdS/rGO

-0.30

-0.30

TiO2/rGO

-0.25

--

CdS/rGO/TiO2

-0.45

-0.45

TiO2+CdS/rGO

-0.40

-0.40


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Ti

Ti

0

6 . 33

nm

C

Cu

Cd S S Cd Cd Cd

Ti

CdS S

Cd

Cd Cd Cu

S Cd

Cd

Figure 1. TEM image of solvothermally synthesized (A) CdS/rGO/TiO2 tricomponent composite, with (B) HRTEM of Area II, and the accompanying EDX analysis of (C) Area I (Ti:Cd:S – 42:1:1.1), and (D) Area II (Cd:S – 1:1, with negligible amounts of Ti). Also shown are bicomponent composite samples of (E) TiO2/rGO and (F) rGO/CdS.


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Figure 2. High-resolution XPS C 1s spectra of (A) GO prepared by the Hummers method of chemical oxidation-exfoliation, and (B) rGO after chemical reduction by DMSO solvothermal treatment. (C) The XPS S 2p spectra of as-prepared solvothermally-treated CdS/rGO/TiO2, rGO, TiO2 and that of untreated TiO2. Also shown is (D) the depth profile of the overall S content in solvothermally-treated TiO2 as a function of Ar+ sputtering duration, with the inset showing the corresponding decrease in the XPS S 2p peak area with increasing sputtering time.


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A rGO

GO

TiO2

(untreated)

TiO2

CdS

TiO2/CdS TiO2/rGO

CdS/rGO CdS/rGO/ TiO2

TiO2 + CdS/rGO

B

Figure 3. (A) Photographs of the different solvothermally-prepared colloidal samples, as well as the untreated TiO2, and (B) the corresponding UV-Vis diffuse-reflectance absorbance spectra of drop-casted sample electrodes.


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Figure 4. Photocurrent response versus time at 0 V vs. Ag/AgCl under (A) full arc and (B) visible light (λ ≥ 420 nm) irradiation, for electrodes fabricated from solvothermally-prepared samples listed on frame (B), as well as untreated TiO2. Sample TiO2 + CdS/rGO was prepared by sonication mixing of individual solvothermally-prepared samples of TiO2 and CdS/rGO.


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Figure 5. The incident photon to charge carrier generation efficiency (IPCE) of solvothermallyprepared samples listed on the frame, as well as untreated TiO2.


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Figure 6. Electron lifetimes as a function of Voc of solvothermally-prepared samples listed on the frames, as well as untreated TiO2, under (A) full arc irradiation, and (B) visible light irradiation (λ ≥ 420 nm).


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Figure 7. Time resolved photoluminescence emission decays of various solvothermally-treated samples at 670 nm emission wavelength (excitation wavelength = 405 nm). The zero delay time and instrumental rise/decay times were determined from the scattered light response for a dilute non-emissive colloidal scattering solution.


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Scheme 1: Simplified diagram of the interfacial charge transport pathways in tricomponent composite CdS/rGO/TiO2 under (A) UV or full arc, and (B) visible light irradiations. Solid lines show the excitation and charge transfer paths, while broken lines show the charge recombination paths.


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TOC Figure


33

Shuttling Photoelectrochemical Electron Transport in Tricomponent

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