Extraordinary Enhancement of UV Absorption in TiO2 Nanoparticles

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C: Physical Processes in Nanomaterials and Nanostructures 2

Extraordinary Enhancement of UV Absorption in TiO Nanoparticles Enabled by Low-Oxidized Graphene Nanodots Hyewon Yoon, Daehan Kim, Minsu Park, Jin Kim, Jungmo Kim, Werayut Srituravanich, Byungha Shin, Yeonwoong Jung, and Seokwoo Jeon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03329 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry

Extraordinary Enhancement of UV Absorption in TiO2 Nanoparticles Enabled by Low-Oxidized Graphene Nanodots Hyewon Yoon1, Daehan Kim1, Minsu Park1, Jin Kim1, Jungmo Kim1, Werayut Srituravanich2, Byungha Shin1, Yeonwoong Jung3,*, and Seokwoo Jeon1,* 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology, Daejeon, 34141, Republic of Korea 2

Department of Mechanical Engineering, Chulalongkorn University, Pathumwan, Bangkok

10330, Thailand 3

NanoScience Technology Center, Materials Science and Engineering, Electrical and Computer

Engineering, University of Central Florida, Orlando, FL 32826, USA *

To whom all correspondence should be addressed:

[email protected] [email protected] Corresponding Author *

To whom all correspondence should be addressed:

ACS Paragon Plus Environment

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[email protected] [email protected]

ABSTRACT

Titanium oxide (TiO2) exhibits intrinsically strong absorption of ultraviolet (UV) light, which has been utilized in a variety of applications such as environmental, healthcare, and energies. Accordingly, it is greatly demanded to precisely tune and further improve the UV absorption of TiO2 to significantly broaden its versatility. Herein, we report an extraordinary enhancement of UV absorption in TiO2 nanoparticles (NPs) incorporated with graphene nanodots (GNDs) of low oxygen concentration. Chemically-bonded TiO2 NPs/GNDs composites exhibit highly tunable UV absorption, achieving over 243% enhancement of molar extinction coefficient at 336 nm. We identify the drastic improvement is a result of the direct charge transfer from the lowest unoccupied molecular orbitals (LUMOs) of GNDs to the conduction bands (CBs) of TiO2, enabled by wide/direct band gaps in GNDs with a small amount of oxygen. Also, the significantly improved power conversion efficiency (PCE~16.74 %) and UV stability of the TiO2 NPs/GND composites reveals their high promise for applications benefiting from TiO2 NPs/GNDs composites such as solar cells and photolysis.


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1. Introduction Titanium dioxide (TiO2) has been widely used for a variety of applications including healthcare and environmental purification/sterilization which employ its outstanding UV absorption and photo-catalytic properties.1-3 Accordingly, precisely tailoring and significantly enhancing the UV absorption of TiO2 will lead to vast opportunities in terms of improving the performances of existing technologies as well as perceiving new applications. In this respect, substantive efforts have been made on reducing the band gap of TiO2 to better harness the visible range of the solar spectra, thereby to enhance the light absorption, which has often resulted in insignificant improvements in UV range.4-7 Even the enhanced UV absorption has often been limited to the wavelength range close to the visible spectrum, far away from the ~280 to 400 nm (UV-A and UV-B) which is technologically more relevant.8 Therefore, it is critically demanded to develop reliable approaches to control the UV absorption of TiO2 to tailored spectra ranges beyond the conventional approaches to adjust its intrinsic band gap engineering. Some studies with organicmetal oxide hybrid systems suggest that the enhancement of the optical absorption is possible by tailoring the charge transfer process in between the organic and the metal oxide constituents. For instance, enhanced visible light absorption has been demonstrated in the ligand-to-metal charge transfer (LMCT) sensitization where dye molecules (organics) and TiO2 (metal oxide) have been utilized.7, 9-10 In this case, the electrons directly excited from the adsorbed dyes are transferred to the conduction band (CB) of TiO2. The studies imply that it is also possible to enhance UV absorption in generic organic-metal oxide systems by fulfilling certain conditions; (1) The high (low) level of the highest (lowest) occupied (unoccupied) molecular orbital, i.e., HOMO (LUMO) should be greater than the valence (conduction) bands of the metal oxide, respectively. (2) The acceptors and donors in metal oxides should possess high oxidation states and the


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organic should possess suitable HOMO/LUMO levels. This consideration implies that the UV absorption of TiO2 can be further enhanced by incorporating appropriate organic material with discrete band gaps and HOMO/LUMO levels greater than the conduction/valence bands of TiO2. Recently, graphene nanodots (GNDs) or graphene quantum dots (GQDs) have been suggested to efficiently enhance the optical absorption of TiO25, 11-14 when incorporated with it, owing to their distinguishable optical characteristics (e.g., presence of band gap energies). However, the improvement of UV absorption achieved so far has been limited to the absorption edge in the visible range (> 400 nm in wavelength). GNDs with high oxygen concentration (> 20 %) have exhibited non discrete band gaps13, 15, which has resulted in the narrowed band gap of TiO2 by unintended carbon doping or the presence of additional energy states inside the band gap of TiO2. This observation indicates that GNDs with wide/discrete band gaps and well-aligned energy levels is critically demanded to enhance the UV absorption of TiO2. We have previously identified that GNDs synthesized via a controlled oxidation process employing graphite intercalation compounds (GICs) exhibit wide and discrete band gaps at ~3.1 eV.16-17; the GICsbased oxidation process enables to efficiently obtain GNDs with graphitic domains of sub nanometers, leading them to possess discrete band gaps.16 In this article, we report that GNDs possessing low oxygen concentration significantly enhance the UV absorption of TiO2 nanoparticles (NPs). The TiO2 NPs incorporated with GNDs of ~6% oxygen concentration (confirmed by Auger electron spectroscopy (AES)18)

exhibits

extraordinary enhancement in absorption in the extended UV range; specifically from the molar extinction coefficient of 138.30 M-1cm-1 for pristine TiO2 NPs to 371.22 M-1cm-1 after incorporation of GNDs at the absorption peak of 336 nm. We identify that this unprecedented enhancement originates from the efficient charge transfer process resulting from the suitable


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band structure formed by interfacing GNDs of discrete band gaps with TiO2 NPs. We also demonstrate the outstanding UV blocking effect and stable operation of perovskite solar cells (PSCs) under UV irradiation with the TiO2 NPs/GNDs composites. The study suggests that these new nanocomposites hold tremendous potential for a variety of applications, ranging from solar cells and water purification to UV blocking film or sunscreens.

2. Methods 2.1.GNDs of low oxygen concentration GNDs of low oxygen concentration were fabricated via the previously developed graphite intercalation compounds (GICs)-based method.17 For the preparation of GICs, 300 mg of potassium sodium tartrate tetrahydrate (KNaC4H4O6·4H2O) were vigorously mixed with 20 mg of graphite powders using a pestle. Then the mixture was heated in a heating mantle under 250 o

C for 24 hr. The as-prepared GICs were then exfoliated in water with sonication after cooling.

The exfoliated particles in water were then microcentrifuged with 30,000 MWCO (molecular weight cut off) microfilters, which lead to the formation of GNDs of < 10 nm in size. Finally, the GNDs solution was then dialyzed using 3.5 kDa dialysis pack over 3 days. 2.2. Decoration of metal oxides with GNDs As-prepared GNDs and TiO2 NPs (or ZnO NPs) were vigorously mixed using the pestle, and were subsequently delivered into deionized water. The solution containing the mixture was then heated under a vigorous stirring until the complete evaporation of the water.


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2.3. Synthesis of Emulsion Simple oil-in-water (o/w) phase emulsion was synthesized by mixing 23.5 g of mineral oil, 1.6 g of Span 60 (nonionic detergent), 8.4 g of Myrj (nonionic detergent) and 0.2 g of sorbic acid (preservative) in a beaker. The mixture was then boiled at 62 oC in a water bath under a vigorous stirring. During the boiling and the stirring, 61.3 g of water was slowly added into the mixture. 2.4.Synthesis of UV responsive polymer The UV responsive polymer was prepared by using the previously reported method.19 44 g of polydimethylsiloxane (PDMS) and 4.4 g of curing agent were gently mixed, followed by the addition of 1,3,3-trimethylindolino-6’-nitrobenzopyrylospiran (SP) (20 mg) dissolved in 0.2 mL of methylene chloride solution. After the incorporation of the SP into the PDMS, the SP/PDMS mixture was poured into a petridish, followed by a curing at 70 oC for 2 hrs. 2.5. Photocatalytic measurement Pristine TiO2 NPs and TiO2 NPs/GNDs composites (100 mg, respectively) were added into 100 mL of MB solution (2.7 × 10 mol, Sigma Aldrich). After mixing in the MB solution, the samples were stirred in dark for 6 hr to establish adsorption/desorption equilibrium states. Then, the mixtures were poured into two separate petridishes and were kept under UV lamp (365 nm, Hg lamp, 400 W) for the photo degradation tests. 2.6. Fabrication of perovskite solar cells FTO (TEC 8) was washed using acetone, ethanol, and deionized water. Then the prepared compact TiO2 solution (diluted titanium diisopropoxide bis(acetylacetonate) in ethanol) was


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spin-coated on FTO with a rotation speed of 4500 rpm for 30 sec, followed by annealing at 500 ˚ C for 30 min. Then, 2mg/ml of rutile TiO2 NPs or rutile TiO2 NPs/GNDs composites solution (solvent: water) was spin-coated at 6000 rpm and dried at 150 ˚C to form mesoporous electron transport layers (ETL). In the typical type of PSCs for UV stability test, 0.025 M of SnO2 solution (solvent: ethanol) was spin-coated, followed by annealing at 300 ˚C for 30 min to form ETL. Afterwards, perovskite layers were spin-coated via two-step spin-coating conditions at 1000 and 4000 rpm for 10 and 30 sec, respectively, with 120 µL chlorobenzene anti-solvent dripped 5 sec before the end of the spin-coating process. The spin-coated thin film is heat-treated at 100 ˚C for 10 minutes to obtain a black color perovskite thin film. Prepared doped spiroOMeTAD solution (spiro-OMeTAD powder 72.3 mg, Li-TFSI (synthesized using 520 mg in acetonitrile 1 mL) 17.5 µL, 4-tert-Butylpyridine 28.8 µL, chlorobenzene 1 mL) is deposited using spin-coating (3000 rpm, 30 sec). Lastly, Au electrode is deposited using a thermal evaporator about 60 nm. 2.7. Instruments High resolution transmission electron spectroscopy (HR-TEM) images were obtained by Tecnai F20 (200 kV) and Tecnai G2 F30 (300 kV) with copper coated holey carbon supper film (300 mesh). The elemental and compositional analysis was conducted by Auger electron spectroscopy (AES, PHI 710 Scanning Auger Nanoprobe, source electron beam of 5 kV) and X-ray photoelectron spectroscopy (XPS, Al Kα, K-alpha, Thermo VG Scientific), respectively. The absorption spectra were obtained by UV-Vis spectrometer equipped with an integrating sphere (SolidSpec-3700). Fluorescence decay times were acquired by Time-correlated single photon


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counting (TCSPC) method from a high sensitivity spectrophotofluorometer with TCSPC (Fluorolog3). The electronic band structures were determined by ultraviolet photoelectron spectroscopy (UPS, Sigma Probe, Thermo VG Scientific). Raman characterizations were performed using LabRAM HR UV/Vis/NIR Raman spectrometer (excitation wavelength at 514 nm). For charaterization of PSCs, Solar simulator (McScience) was used to measure the power conversion efficiency of a perovskite device after 1 Sun calibration with Si reference cell and aperture mask of 0.1 cm2 active area was used. The QEXL Solar Cell Quantum Efficiency / IPCE / Spectral Response Measurement System (PV measurement) are used for external quantum efficiency measurement after calibration with Si photodiode. Vilber UV lamp (VL-4LC) is used to investigate UV-light stability of perovskite solar cells.

3. Results & Discussion 3.1. TiO2 NPs/GNDs composites The schematic of the synthesis process and the structure of the TiO2 NPs/GNDs composites is illustrated in Figure 1a. The GNDs of low oxygen concentration used in this study were prepared by exfoliating GICs.16-17 Rutile TiO2 NPs of < 100 nm in diameter were mixed with the GNDs of a few nanometers in size. The as-prepared GNDs and TiO2 NPs were mixed/sonicated in water followed by heating and stirring, which result in TiO2 composites chemically bonded to GNDs. The Transmission electron microscope (TEM) images confirm that small flakes of GNDs are attached the TiO2 NPs, further evidenced by the corresponding fast Fourier transformation (FFT) patterns which reveals the hexagonal spots of the GNDs. (Fig. 1bi-ii and S1). The elemental mapping images obtained by the energy dispersive x-ray spectroscopy (EDS) in TEM


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shows the spatial distribution of the constituent elements in the TiO2/GND composites, further confirming the presence of carbon atoms on the TiO2 surface (Fig. 1biii and iv). Prior to the quantification of the enhanced UV absorption, efficient UV blocking with the TiO2/GND composites was demonstrated using UV responsive photochromic polymers (Fig. 1c) previously developed by Y-S. Nam et al.19; Each of TiO2 NPs, GNDs and TiO2/GNDs were separately mixed in prepared emulsions and these mixtures were uniformly coated onto glass slides using a 60 μm bar coater. The glasses separately coated with the pristine TiO2 NPs, GNDs, and TiO2 NPs/GNDs were located onto the prepared UV responsive polymer. Upon exposure to UV light (365 nm), only the area coated with the TiO2 NPs/GNDs is observed to retain its original color while the areas coated with the other materials turn into purple, indicating their outstanding UV blocking. 3.2. Absorption enhancement The optical absorption of the TiO2 NPs/GNDs was characterized by systematically varying the weight ratio of TiO2 NPs and GNDs in water. The absorption spectra plots in Fig. 2a show that the optical absorption of the TiO2 NPs/GNDs increases with increasing the amount of GNDs throughout the entire test range of 250 nm – 600 nm. The absorption drastically increases up to the weight ratio of 1:0.07, after which, the enhancement becomes insignificant accompanying the pronounced increase in the UV-B region. However, the drastic absorption enhancement already becomes insignificant but additional occurs over the mixing ratio of 1:0.025 when larger sized TiO2 particles (~5μm) are utilized (Fig. S2). The phenomenon is explained as follows. Initially, the small amount of GNDs is uniformly attached on the surfaces of the TiO2 NPs, which is reflected in the drastic absorption increase. Once the surfaces of the TiO2 NPs are mostly


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covered with GNDs, the enhancement becomes less pronounced even with further incorporation of GNDs. Furthermore, additional absorption occurs in the deep UV region (< 350 nm) by excessive GNDs which cannot be attached to the surfaces of the TiO2 NPs. In the same manner, since the larger sized TiO2 particles have less surface areas than the TiO2 NPs, the drastic absorption enhancement is already done with very small amount of GNDs. Particularly, the optical density (OD) of the TiO2 NPs/GNDs with the mixing ratio of 1:0.07 reaches up to 1.64 at absorption maximum (336 nm), which correspond to 2.83 times higher than that of pristine TiO2 NPs (~0.58, Fig. S3a). In the case of the molecular extinction coefficient, the coefficient of the composites with the mixing ratio of 1:0.07 reaches up to 371.22 M-1cm-1 (2.43 times higher than the pristine TiO2 NPs (138.30 M-1cm-1), Fig. S3b). Consistent with the absorption spectra, the color change of the photochromic polymer is observed to be most pronounced with the increasing mixing ratio (Fig. 2b). The observed tendency of the enhanced absorption with the increasing amount of GNDs and a presence of the saturation point indicate that the interaction between the TiO2 NPs and the GNDs is chemical rather than physical. In order to verify this hypothesis, we characterize the UV/Vis absorption of the materials under varying pH conditions. Individual TiO2 NPs and GNDs are separately protonated under varying pH conditions in individual beakers prior to their mixing. The protonation of TiO2 NPs and GNDs hinders the formation of chemical bonding between them. Therefore, the protonated TiO2 NPs and GNDs forms a physically blended mixture but not chemically bonded mixture, presenting the suppression of the enhanced optical absorption (Fig. 2c). Meanwhile, completely/chemically mixed TiO2 NPs/GNDs composites present absorption characteristics nearly insensitive to pH conditions, since the chemical bonding has been readily established through the mixing process (Fig. S5). We also characterized the optical absorption of


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the physically mixed composites formed by a simple grinding method with a pestle and a mortar (Fig. 2d). Although optical absorption is observed to a certain extent owing to the π-electron system of the physically adsorbed GNDs,20-21 the physically mixed composites exhibit less pronounced enhancement. The chemical bonding characteristics of the GNDs in the composites (Fig. 2e) are also characterized by Raman spectroscopy. The Raman peaks assigned to Eg mode in the pristine and the physically mixed composites are observed to be at 445 cm-1 while the peak in the chemically mixed composites is red shifted to 426 cm-1.22 The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) characterizations more directly reveal the bonding nature of TiO2 NPs and GNDs in their chemically mixed composites (Fig. 2f, Fig. S6 and S7). Unlike the pristine TiO2 NPs, both the physically and chemically mixed composites exhibit new peaks assigned to C-O bonding, with much higher intensities observed in the chemically mixed ones. The C-O bonding peak with relatively small intensity in the physically mixed composite is attributed to the small amount of the oxygen functional groups contained in pristine GNDs. The significantly increased C-O bonding peak intensity in the chemically mixed composite clearly evidences the presence of the chemical bonding of the oxygen in TiO2 NPs with the carbon in GNDs. 3.3. Charge transfer absorption To better understand the nature of the chemical bonding in TiO2 NP/GND composites, we preformed control experiments using Zinc Oxide (ZnO) NPs instead of TiO2 NPs as ZnO NPs are also commonly used for UV blocking and photocatalysis applications. We prepared ZnO NPs/GND composites by chemically mixing ZnO NPs and GNDs using the same process adopted for the TiO2 NPs/GNDs composites preparation. In contrast to the significant absorption enhancement observed in TiO2 NPs/GND, ZnO NPs/GNDs composites do not exhibit noticeable


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enhancement despite the similar band gap energies of rutile TiO2 and ZnO (Fig. 3a-b and S8). Moreover, ZnO NPs/GNDs composites do not exhibit efficient UV blocking effects compared to pristine ZnO NPs under the photopolymer tests (Fig. 3c). It has been well known that transition metals or transition metal oxides such as TiO2 incorporated with organic ligands or dye molecules can present unique charge transfer processes known to be ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT). In LMCT, when light is incident on transition metals (or, metal oxides) modified with other molecules (adsorbates), charge transfer from the HOMO of the adsorbates to the conduction bands (CB) of the transition metals (or, metal oxides).7, 10 In order for the LMCT process to occur, metal or metal oxides should be readily reducible and possess higher oxidation states than acceptor levels, while adsorbates should be readily oxidized. To better understand the underlying charge transfer mechanism in TiO2 NPs/GNDs, we investigate the band structures of TiO2 NPs, ZnO NPs and GNDs used in this study. Ultraviolet photoelectron spectroscope (UPS) characterizations determine the work functions and the valence band maxima of TiO2 NPs, ZnO NPs, and GNDs are to be 4.68, 4.64, 4.36 eV, and 18.17, 18.04, 17.98, respectively (Fig. 3d). Based on the band gaps of GNDs determined in our previous study16 and the calculated band gaps of TiO2 and ZnO NPs from UV/Vis spectra, we construct the band alignments of three materials. The band structures confirm that both TiO2 and ZnO NPs possess lower acceptor levels than HOMO levels in GNDs. One noticeable difference is that the 3d orbital of Zn atom is completely filled while Ti possesses empty states (Fig. 3e left scheme), indicating the possibility of single oxidation state in Zn in comparison to the various oxidation states present in Ti. Therefore, the LMCT process is not available with ZnO due to the absence of empty or partially filled d orbitals in Zn (Fig. 3e right). The results support that the extraordinary enhancement of


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optical absorption only observed with TiO2 NPs/GNDs is a result of the LMCT process involving charge transfer from the HOMO level of GNDs to the CB of TiO2 NPs. Moreover, while majority of electrons in GNDs are directly excited to the CB of TiO2, some of them still can transfer to the excited states of GNDs. This analysis explains that the simultaneous enhanced of the absorption from the UV-B range in GNDs is possible, consistent with our observation in Figure 2a. In accordance with the LMCT process, fluorescence lifetime characterizations were employed to better understand the carrier transfer dynamics in TiO2 NPs/GNDs. The fluorescence lifetime is observed to increase in TiO2 NPs/GNDs compared to pristine TiO2 NPs, which is attributed to the increased amount of charges in the CB of TiO2 (Fig. 3f and Table. S1). Moreover, the fluorescence lifetimes of each TiO2 NPs, ZnO NPs and their GNDs incorporated composites were identified with three exponential decays. The fast decay time of the TiO2 NPs increases from 0.0294 ns to 0.0782 ns after mixing with GNDs, while the ZnO NPs present very similar decay times irrespective of GNDs incorporation (before: 0.0538 n, after: 0.0622 ns). We compare the LMCT-induced optical absorption of TiO2 NPs/GNDs composites with other graphitic materials (Fig. 4a) such as graphene oxides (GOs), graphene oxides nanodots (GONDs), and reduced graphene oxides nanodots (rGONDs). The GOs were prepared by modifying the Hummer’s method,23 and GONDs and rGONDs were synthesized using the previously reported methods.16, 24-25 These three different graphitic materials were also mixed with TiO2 NPs in the manner as the TiO2 NPs/GNDs composites preparation. The absorption spectra reveal all these three TiO2 NPs/graphitic materials exhibit significantly lower extent of enhancement. We have previously reported that GNDs prepared from GICs contain extremely low concentration of oxygens compared to GNDs synthesized by acidic oxidations,16 and indeed confirmed the small amount of oxygen in the GNDs used in this study by AES analysis (Fig.


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S9). Moreover, we have revealed by reflection electron energy loss spectroscopy (REELS)26 that the GNDs of low oxygen concentration possess discrete band gaps while the GNDs prepared from acidic oxidations suffer from high densities of defects introduced by the harsh oxidation. In this case, the energy levels of the defect states in the GNDs are located below the CB edge of TiO2, which makes the LMCT process difficult (Fig. 4b). 3.4. Application to perovskite solar cells To find further evidence for the LMCT process in TiO2/GNDs enabled by the band structure of the low-oxidized GNDs, we applied the TiO2 NPs/GNDs composites as a mesoporous electron transport layer (ETL) to perovskite (CH3NH3PbI3) solar cell devices (PSCs). As shown in Figure 5a and 5b, the pristine rutile TiO2 NPs and TiO2 NPs/GNDs composites are utilized as ETL between the perovskite layer and compact TiO2 (c-TiO2, anatase) layer for each device. The measured current (J)-voltage (V) characteristics of the pristine TiO2 NPs employed PSCs (TPSC) and TiO2 NPs/GNDs composites employed PSCs (TG-PSC) are shown in Figure 5c. The TG-PSC achieved power conversion efficiency (PCE) of 16.74 %, whereas the T-PSC only achieved 14.71 % even though the TG-PSC shows relatively lower EQE in UV region due to their strong UV absorption (Fig. S10). It is known that fast electron extraction from the active layer to the electron transport layer improves the performance of solar cells. As the HOMO and LUMO levels of the low-oxidized GNDs are located between the VB and CB of CH3NH3PbI3 and rutile TiO2 NPs by having a wide and discrete band gap, the GNDs facilitates the electron extraction and transport from CH3NH3PbI3 to TiO2 (Fig. 5d).27 As mentioned above, the enhanced UV absorption of TiO2 can provides great benefit in various practical applications including solar cells and photocatalysis (Fig. S11-S13). In this regard, we


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finally applied the TiO2 NPs/GNDs composites to perovskite solar cells (PSCs) as a UV protection layer. Recently, it has been reported that the performance of PSCs are reduced under UV light.28-29 Accordingly, we employed the TiO2 NPs/GNDs composites as a UV absorber layer for enhancing PSCs’ stability under UV irradiation by protecting the PSCs with the strong UV absorbance of TiO2 NPs./GNDs composites. The spray coated TiO2 NPs and TiO2 NPs/GNDs composites on slide glasses were located at the front of the PSC device (Fig. 5e and S14, Tin oxide was employed as ETL rather than TiO2 which is generally used materials for PSC devices). The aging test results under UV irradiation (365 nm) according to the measured J-V curves are depicted in Figure 5f and S15. Although the PSC devices with TiO2/GNDs absorber layers exhibit lower EQE than the devices without an absorber layer and with pristine TiO2 absorber layer (Fig. S16), the PSCs with TiO2/GNDs absorber layer show significantly enhanced stability against UV irradiation. After aging with UV irradiation, the PSCs with TiO2/GNDs layer showed only 15 % decrease after 100 hours whereas the reference PSC device without an absorber layer and the PSCs with pristine TiO2 absorber layer degraded 92 % and 70 %, respectively. The result is also benefiting from the strong UV absorption of the TiO2/GNDs composites and indicating the promise of the composites for PSC devices. Furthermore, it is expectable that highly efficient and stable PSCs can be achieved by utilizing the TiO2/GNDs composites as both ETL and UV protection layer (UV absorber layer).

4. Conclusions In summary, we report the significantly enhanced UV absorption of TiO2 NPs incorporated with low-oxidized GNDs and investigate their underlying charge transfer mechanism and


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chemical bonding natures. The GNDs prepared from GICs via mild oxidation processes form chemical bonding with TiO2 NPs as well as present direct band gaps. Accordingly, the direct charge transfer to TiO2 following LMCT process leads to significantly enhanced UV absorption in TiO2 NPs/GNDs composites, reaching up ~2.43 times higher extinction coefficient compared to pristine TiO2 NPs. The excellent UV blocking effect and improved performance and UV stability of PSCs by employing the composites were also verified, which further implies their great promise for a variety of applications such as solar cells, purifications, sunscreens, and screening films.

FIGURES


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Figure 1. TiO2 NPs/GNDs composites and their enhanced UV light absorption. (a) A schematic illustration for the fabrication of TiO2 NPs/GNDs composites. (b) i, ii: HRTEM images of TiO2 NPs/GNDs composites (inset: FFT patterns of GNDs on TiO2 surface), iii, iv: EDS mapping images (iii-carbon, iv-titanium). (c) Top: Chemical structure of a photochromic molecule upon UV/vis light and its corresponding structural/color change. Bottom: Comparison of UV block effects in pristine TiO2 NPs, GNDs, and TiO2 NPs/GNDs composites using the UV responsive photochromic polymer (spyropyran-PDMS composite). Color changes of the corresponding samples before UV irradiation (mid) and upon 365 nm UV irradiation (right).

Figure 2. Absorption enhancement induced by chemical modification of GNDs. (a) UV/Vis absorption spectra of TiO2 NPs/GNDs composites with various mixing ratio. (b) Images to demonstrate the UV blocking effect of the corresponding TiO2 NPs/GNDs composites before (top) and after (bottom) UV irradiation at 365 nm. (c) UV/Vis absorption spectra of TiO2


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NPs/GNDs composites under various pH conditions. (d) UV/Vis absorption spectra of physically (grinded) and chemically (solution-processed) mixed TiO2 NPs/GNDs composites. (e) Raman spectra of physically and chemically mixed composites. (f) XPS O1s spectra of physically (left) and chemically (right) mixed composites.

Figure 3. Charge transfer induced absorption at the interface of TiO2 NPs and GNDs. (a) UV/Vis absorption spectra of pristine TiO2 NPs, GNDs, and TiO2 NPs/GNDs composites in water. The dotted line (overlapped) indicates the summation of the spectra corresponding to TiO2 NPs and GNDs. (b) UV/Vis absorption spectra of pristine ZnO NPs and ZnO NPs/GNDs


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composites. The dotted line (overlapped) indicates the summation of the spectra corresponding to ZnO NPs and ZnO NPs/GNDs composites. (c) Images to demonstrate the UV blocking effect of pristine TiO2 NPs and ZnO NPs, and their composites of TiO2 NPs/GNDs and ZnO NPs/GNDs before (top) and after (bottom) UV irradiation at 365 nm. (d) UPS spectra of TiO2 NPs, ZnO NPs and GND which reveal the work function (top) and the valence band maximum of the corresponding materials (bottom). (e) Schematic illustrations of the atomic 3d orbitals of titanium (Ti) and zinc (Zn) and the corresponding charge transfer process occurring at the interfaces of TiO2 (and ZnO) and GNDs, respectively. (f) Fluorescence lifetimes of TiO2 NPs, ZnO NPs and their composites with GNDs.

Figure 4. Optical absorption enhancement induced by charge transfer between GNDs and TiO2. (a) UV/Vis absorption spectra of TiO2 NPs incorporate with various graphitic materials (GOs: Graphene oxides, GONDs: Graphene oxide nanodots, rGONDs: Reduced graphene oxide nanodots) with identical mixing (weight) ratios of 1:0.2. (b) Schematic illustrations of the charge transfer process occurring in between TiO2 NPs and GNDs of low (left) and high (right) defect densities.


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Figure 5. Application of the TiO2 NPs/GNDs to perovskite solar cells (a, b) Schematic illustrations of perovskite solar cells without GNDs (left, T-PSC) and with GNDs (right, TGPSC). (c) J-V curves of T-PSC and TG-PSC devices before applying absorber layers. J-V curves


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(d) Electronic band structures of CH3NH3PbI3 and TiO2 NPs/GNDs composites. (e) The image and the structure of PSCs with UV absorber layers composed of pristine TiO2 NPs or TiO2 NPs/GNDs composites. (f) The aging test result normalized by the initial PCE of the PSCs upon UV irradiation (365 nm). ASSOCIATED CONTENT Supporting Information. Further information regarding characterization tools, supplementary table and figures is available. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This research was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2016M3A7B4900118). Also, this work was supported by the Center for Advanced Soft-Electronics as Global Frontier Project (CASE-2013M3A6A5073173) and Nano Materials Technology Development Program (2012M3A7B4049807) through the NRF funded by the Ministry of Science, ICT and Future Planning.

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Extraordinary Enhancement of UV Absorption in TiO2 Nanoparticles

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