Improving the stability of photodoped metal oxide nanocrystals with electron donating graphene quantum dots

Doped metal oxide nanocrystals are emerging as versatile multi-functional materials with the potential to address several limitations of the current light-driven energy storage technology thanks to their unique ability to accumulate a large number of free electrons upon UV light exposure. The combination of these nanocrystals with a properly designed hole collector could lead to steady-state electron and hole accumulation, thus disclosing the possibility for light-driven energy storage in a single set of nanomaterials. In this framework, it is important to understand the role of the hole collector during UV light exposure. Here we show, via optical absorbance measurements under UV light, that well-defined graphene quantum dots with electron-donating character can act as hole acceptors and improve the stability of the photo-generated electrons in Sn-doped In2O3 nanocrystals. The results of this study offer new insight into the implementation of photo-charged storage devices based on hybrid organic/inorganic nanostructures.


Details on the synthesis of intermediate compounds 3, 4, 5 and HBC-AOM  General Methods and Materials
All the reagents were obtained from Sigma Aldrich, TCI, abcr, BLD pharm, Strem, fluorochem, chempur.All these chemicals were used as received without further purification.All reactions dealing with air-or moisture-sensitive compounds were carried out in a dry reaction vessel under Ar atmosphere.Anhydrous dichloromethane and tetrahydrofuran were obtained from MBRAUN MB-SPS-5 solvent purification system.All the sensitive reactions were performed using standard vacuum-line and Schlenk techniques.
Thin layer chromatography (TLC) was performed on silica-coated aluminum sheets with a fluorescence indicator (TLC silica gel 60 F254, purchased from Merck KGaA).
NMR spectra were recorded on a Bruker AV-II 300 spectrometer operating at 300 MHz for 1 H and at 75 MHz for 13 C at room temperature (23°C).CD 2 Cl 2 (δ( 1 H) = 5.33 ppm, δ( 13 C) = 53.7 ppm) was used as solvents and as internal chemical shift reference.Chemical shifts (δ) are reported in ppm.The following abbreviations are used to describe peak patterns as appropriate: s = singlet, d = doublet, t = triplet, q = quartet, and m = multiplet.
Relative molar masses were determined by gel permeation chromatography (GPC) with an Aligent Technologies 1260 Infinity LC system equipped with two Resipore columns and RI and UV-vis detection.Chloroform was used as eluent with a flow rate of 1 mL min -1 .The measurements were carried out at 40 ºC.The molar masses were calculated relative to polystyrene standards with low dispersity.
UV-visible spectra were measured on an Agilent Cary 5000 UV-vis-NIR spectrophotometer by using 10 mm optical-path quartz cell at room temperature.
Fluorescence spectra were recorded at room temperature on a PerkinElmer Fluorescence Spectrometer LS 55 using a 10 mm fluorescence quartz cell.
The residue was washed by methanol and ethanol and to afford compound 4 as a white solid (410 mg, 82%).

 Synthesis of compound 5
Compound 4 (100 mg, 63 µmol), compound 1 (263 mg, 1.26 mmol), and 10 ml anhydrous oxylene were added into a 25 ml Schlenk flask; the mixture was bubbled with argon for 30 min and then heated to 150 o C for 48 h.After that, the reaction mixture was cooled down to room temperature, and then the solvent was evaporated.The residue of the mixture was precipitated with methanol and the crude product was collected by filtration.The crude product was further purified by recycle-GPC to remove the excess compound 1 and white target compound 5 (156 mg, 88%) was obtained.

 Synthesis of HBC-AOM
A solution of compound 5 (50 mg, 16.56 µmol) in dry DCM (50 ml) was degassed by argon bubbling for 30 min.A suspension of FeCl 3 (214 mg, 1.32 mmol) in nitromethane (1.2 ml) was added to the degassed solution.After stirring at room temperature for 5 h under continuous argon bubbling, the reaction was quenched by the addition of methanol to form brown precipitates.Filtration by suction using a membrane filter and washing intensively with methanol and water gave the target compound as a brown powder (44.8 mg, 90%).

Figure
Figure S2 UV-Vis absorption and emission spectra of compound 5 in CH 2 Cl 2 .

Figure
Figure S4 UV-Vis absorption and emission spectra of HBC-AOM in CH 2 Cl 2 .

Figure S5
Figure S5 Photographs of the dispersions of HBC-AOM (5 mg/mL) in various organic solvents under daylight illumination and excited by 360nm.

Figure S6 .
Figure S6.The optimized molecular geometry and frontier orbitals of HBC-AOM calculated by DFT using B3LYP/6-31G(d) basis set.The C 8 H 17 alkyl chains are replaced by methyl groups for clarity.

Figure
Figure S7 (a) TEM image and (b) X-ray diffraction pattern of ITO NCs used in this work.XRD diffraction pattern (black line) is overlapped to the normalized XRD reference pattern of ITO (red vertical lines, ICSD 98-005-0849 card, ICDS Database).

Figure S8
Figure S8 Absorbance spectra of HBC-AOM/ITO NCs (2.53:1 weight ratio, 1.18 10 4 molar ratio) upon simple mixture and functionalization (i.e.stirring overnight).Panel (a) shows the absorbance of ITO NCs (black line) along with HBC-AOM/ITO NCs mixture (red line) and HBC-AOM /ITO NCs functionalized mixture (blue line).(b) ITO NCs localized surface plasmon resonance (LSPR) of HBC-AOM/ITO NCs functionalized mixture shows a small redshift of 15.4 meV (35 nm) when compared to the LSPR-peak normalized absorbance spectra of ITO NCs and HBC-AOM/ITO NCs mixture.(c) Normalized and energy-shifted ITO NCs LSPR spectra show that neither simple mixture (red dashed line) nor functionalization (blue line) affects the as-synthesized ITO NCs LSPR spectral shape (black line).

Figure S9
Figure S9 Evolution of the peak absorbance of HBC-AOM graphene quantum dots in the HBC-AOM/ITO NCs functionalized mixture upon UV light exposure.Error bars represent the standard deviation at the HBC-AOM peak absorbance (i.e.3.28 eV) of ten subsequent spectra acquired without UV light exposure.The total decrease of the peak absorbance is 4 % over 57 minutes of ŨV light exposure.

Figure S10
Figure S10 Comparison between absorbance spectra of ITO NCs, HBC-AOM GQDs, HBC-AOM GQDs/ITO NCs functionalized mixture and sum between ITO NCs absorbance and HBC-AOM GQDs absorbance -green empty diamonds.Absorbance spectra of ITO NCs and HBC-AOM GQDs are obtained by preparing two solutions having the same concentration of NCs and GQDs of the functionalized mixture (9.1 mg/mL and 5mg/mL in 1306 L of anhydrous toluene,  respectively).The absorbance spectrum of the functionalized mixture was measured after the acquisition of the photodoping series in Figure 2 of the manuscript.Violet shaded area represents the spectral region covered by the UV LED (300 nm central wavelength, 20 nm full width at half maximum).The inset shows the comparison between absorbance spectra in the regions of ITO NCs LSPR.All spectra are measured with a UV-Vis-NIR Varian Cary 5000 spectrophotometer.

Figure
Figure S11 (a) Evolution of the absorbance spectra of ITO NCs upon UV light exposure.Time zero marks the start of UV light exposure.Panel (b) highlight the variation of the LSPR peak absorbance at selected UV exposure times.

Figure S12
Figure S12 Comparison between LSPR dynamics of ITO NCs and of a functionalized mixture of ITO NCs and HBC-AOM GQDs.(a) LSPR peak intensity and (b) LSPR peak energy vs UV exposure time.The dynamic in panel (a) and (b) are normalized with respect to LSPR peak intensity and energy of the as-prepare (i.e.un-photodoped) samples, respectively.