Titanium

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Materials and Interfaces

Facile and green preparation of high UV-blocking lignin/titanium dioxide nano composites for developing natural sunscreens Jue Yu, Lan Li, Yong Qian, Hongming Lou, Dongjie Yang, and Xueqing Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04101 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Industrial & Engineering Chemistry Research

Facile

and

green

preparation

of

high

UV-blocking

lignin/titanium dioxide nano composites for developing natural sunscreens Jue Yu,† Lan Li,† Yong Qian,*,†,‡ Hongming Lou,†,‡ Dongjie Yang,† and Xueqing Qiu*,†,‡ †School

of Chemistry and Chemical Engineering, Guangdong Engineering Research

Center for Green Fine Chemicals, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China ‡State

Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China E-mail: [email protected] (Y. Qian); [email protected] (X. Q. Qiu).

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ABSTRACT: Lignosulfonate (LS), one type of industrial lignin and natural macromolecular sun blocker, was applied to modify TiO2 via one-pot hydrothermal esterification. The reaction mechanism and composite structure of LS@TiO2 were investigated by DLS, TEM, AFM, TG, UV, FTIR, XPS, XRD, ESR and contact angle measurements. The results show that the esterification modification occurred between the carboxyl groups of lignin and the hydroxyl groups on the surface of TiO2. 3.4 wt% LS with thickness of 13 nm was effectively coating on the normal TiO2 and the LS@TiO2 composites were nano scale with average size of 119 nm. Hydroxylated TiO2 particles were more reactive and the contents of LS on TiO2 that processed by 0.1 M and 1 M HCl and increased to 5.0 wt% ([email protected]) and 6.0 wt% (LS@TiO2-1M), respectively. Chemically coated LS not only improved the dispersibility of TiO2 in the substrates, but also significantly boosted its UV-blocking ability. Therefore, TiO2@LS nano composites were applied as sole active in the pure cream and the sunscreen performance was compared with that of TiO2. The sun protection factor (SPF) values of creams containing 5, 10 and 20 wt% of LS@TiO2 were 16, 26 and 48, respectively. It was 30%~60% higher than those of the creams containing same amount of TiO2. The sunscreen performance of LS@TiO2 improved with LS content. SPF value of pure cream containing 10 wt% of LS@TiO2-1M reached 50+. The possible UV-blocking enhancement mechanism of LS@TiO2 was also revealed. Preparation of LS@TiO2 nano composites provides a facile and green way for value-added utilization of lignin biomass and high-efficient UV-blocking of nano TiO2 materials. KEYWORDS: Lignin, Titanium dioxide, Nano composites, Hydrothermal esterification, Natural sunscreen.


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 INTRODUCTION Titanium dioxide (TiO2) is the most commonly employed photocatalyst in water splitting, pollutant degradation, CO2 reduction and other fields.1-6 It suffers from the fact that it can utilize only 3% of solar radiation owing to its large bandgap of 3.0-3.2 eV.7 Therefore, strong efforts with high costs are being made to shift its photocatalytic activity to the visible spectral region.8-11 Fortunately, due to the strong UV rays scattering ability through its high refractive index and/or by absorbing UV rays because of its semiconductive properties, TiO2 provides good UV protection and is widely used in cosmetics, polymers and building materials.12-16 The annual consumption of nanoscale TiO2 in coatings and cosmetics is estimated to 60 000 tons.17 Compared with organic UV absorbers, inorganic UV-blockers are more favorable in practical uses due to their high photo and chemical stability, wide shielding range and low toxicity.18-20 However, the desirable property is always based on vast dosage, although the Scientific Committee on Cosmetic Products and Non-Food Products (SCCNFP) concludes that TiO2 is safe for use in cosmetic products at maximum concentration of 25%. Such high amount of dosage not only increases the cost but also imposes uncomfortable feeling and poor transparency.21-23 Therefore, TiO2 is often reduced to nano size to increase the UV blocking efficiency and improve the dispersivity by surface modification. Typically used TiO2 nanoparticles are 10-20 nm in size. The strong attraction forces between crystals cause them to form tightly-bound 30-150 nm aggregates and represent the smallest particles actually occur in a sunscreen formulation. Surface modification of TiO2 includes inorganic and organic coatings. Inorganic coating is realized by using metal oxides such as zinc oxide (ZnO), silicon oxide (SiO2), aluminum oxide (Al2O3) or zirconium oxide (ZrO2) to precipitate on the surface of TiO2 and establish a barrier between TiO2 and surrounding substrate.24-28 Inorganic coating can effectively weaken the photocatalytic activity of TiO2, but it is difficult to improve its



dispersibility, especially in cosmetic and polymeric products. Organic coating of TiO2 is usually conducted by using coupling agents or reacting with the hydroxyl groups on its surface.29-32 Surfactants or polymers that coated on the surface endow TiO2 with good compatibility with most substrates, and some additional functions as well.33 The esterification reaction is an effective organic coating method, which is simple and suitable for laboratory preparation and large-scale production. Organics that reported for the esterification coating of TiO2 include fullerene, 3-hydroxytyramine, sulfanilic acid, salicylic acid and L-lactic acid, etc.34-38 However, most of the coating materials, either inorganic or organic type, do not blocking UV radiation, which weakens the UV-blocking performance of TiO2. New coatings that could both improve the dispersibility and enhance the UV-blocking performance of TiO2 are encouraged to be developed. Lignin is the most abundant botanical aromatic polymer.39-41 Natural phenylpropane skeleton and functional groups such as phenolic hydroxyls, double bonds and carbonyls endow lignin with good UV-absorbing and antioxidant properties.42-45 The sulfonate products of lignin exhibit good dispersibility and have been developed as dye dispersant, concrete water-reducer and pesticide adjuvant.46-48 Lignin and its derivatives are also tired as anti-UV agents in polymers.49,

50

However, the mechanical performance of

hydrophobic polymers is usually negatively influenced due to the poor compatibility. Lignin reverse micelles, which had better compatibility with polymers, were thus prepared and provided better UV protection.51 After Ugartondo et al. and Tortora et al. demonstrate that lignin has low cell cytotoxicity and its micelles present good biocompatibility, more and more efforts are put in to lignin-based healthcare products, with the hope that pushing lignin into high end use.52, 53 We added lignin into pure cream as sunscreen active, and got a sun cream with sun protection factor (SPF) of 8.66.43 It is also found that lignin had synergistic effect with chemical sunscreens, which could provide long-time UV protection, especially after UV irradiation.42 Lignin is also tried to modify physical sunscreens including TiO2. Both Hasegawa et al. and Chen et al.


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prepared lignin/TiO2 hybrids by sol-gel method.54, 55 During the hydrolysis process, the precursor of TiO2 was bound with the oxygen-containing functional groups in lignin and formed stable complexes. Nair et al. combined lignin with TiO2 by a ball milling method and found that the composites prepared in polar solvents present better thermal stability than those obtained by dry grinding.56 It is believed that there were chemical interactions between TiO2 and lignin during the wet milling process, which also inspired us. Morsella et al. prepared the lignin/TiO2 composites by UV irradiation in THF solution.57 Coated lignin effectively reduced the photocatalytic activity of TiO2. However, the coating amount of lignin was too thin that lignin itself was easily degraded and the overall sunscreen performance was unsatisfactory. Herein, we took advantage of the carboxyl groups of lignosulfonate (LS), one type of industrial lignin, and the hydroxyl groups on the surface of TiO2, to coat LS onto TiO2 by one-pot hydrothermal esterification method. Compared with the lignin/TiO2 composites prepared by physical electrostatic adsorption, the LS@TiO2 nano composites formed by hydrothermal esterification would be more stable. The LS@TiO2 exhibited good dispersibility and excellent UV-blocking performance. Pure cream blended with 10 wt% of LS@TiO2 nano composites achieved SPF 50+, which has great potential in skincare and polymeric products.

 EXPERIMENTAL SECTION Materials. Lignosulfonate (LS) was the acidification product of sulfonated alkali lignin supplied by Shenhua Forest Products Co., Ltd. (Tongdao, Hunan, China). The physicochemical parameters of LS were summarized in Table S1 in the Supplementary Information. Rutile titanium dioxide (TiO2) was purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. Deionized water (resistivity ≥



18 MΩ•cm-1) was obtained from a Millipore water purification system. Pure cream and BB cream were commercial Nivea 153 refreshingly soft moisturizing cream (75 mL), Loreal lucent miracle BB cream (60 g mL) purchased from the supermarket.

Hydroxylation of TiO2. 1 g TiO2 was added into 100 mL HCl solution (0.1 or 1 mol•L-1) with the aid of ultrasound. The dispersion was stirred for 2 h. Hydroxylated TiO2 particles were obtained by centrifuging, washing with DI water and freeze-drying. The products were named as TiO2-0.1M and TiO2-1M, respectively.

Synthesis of LS@TiO2 composites. 1 g of TiO2 was added to 100 mL of 0.1 g•L-1 LS solution and dispersed under ultrasonic processing for 10 min. The mixture was then heated at 120 °C for 4 h to form a LS-modified TiO2 dispersion. After centrifugation, the LS@TiO2 solids were collected and washed with DI water for three times, and the LS@TiO2 composites were obtained after drying under 50 °C. [email protected] and LS@TiO2-1M were prepared by the same procedure.

Evaluation of UV-blocking performance of the LS@TiO2 composites. The UV-blocking properties of the LS@TiO2 composites were evaluated by their sunscreen performance. LS@TiO2 composites to be tested were added into pure NIVEA cream by different mass ratios, and the mixtures were stirred in the dark room for 24 h to form uniform sun creams. In vitro SPF determination was conducted according to the previous work.42 Specifically, clean transparent quartz slide with thickness of 2 mm was taped with 12.5 cm2 of 3M transpore tape. Homemade sun cream was evenly dotted on the tape and then carefully rubbed with the aid of finger cots to ensure a coating density of 2 mg•cm-2. Sample was dried in the dark for 20 min before measurement. A UV-Vis spectrometer (UV-2600, Shimadzu, Japan) equipped with an integrating sphere was used to measure the UV transmittance of the sample in the range of 290-400 nm. SPF value was calculated by the equation below.


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400

400

290

290

SPF=  E S /  E ST

(1)

Where Eλ =CIE erythemal spectral effectiveness, Sλ =solar spectral irradiance, Tλ =spectral transmittance of the sample.

Other characterizations. The molecular weight of LS was determined by aqueous gel permeation chromatography (GPC, Waters Corp., Ltd., USA) on the Ultrahydragel 120 and 250 columns. The mobile phase was 0.1 mol•L-1 NaNO3 solution with pH of 8.0-8.5. The contents of the functional groups of LS were measured by an automatic potentiometric titrator (905 Titrando, Metrohm AG, Switzerland). The internal standard substrate was P-hydroxybenzonic acid and dissolved in water for use. The content of methoxyl groups was measured by headspace gas chromatography (HS-GC). The GC was running under 100 °C with 3.8 mL•min-1 high-purity nitrogen as the carrier gas. The flame ionization detector was running under 270 °C with 25 mL•min-1 hydrogen and 300 mL min-1 air. Before the HS-GC measurement, LS would be dissolved in hydroiodic acid (57%) in a sealed headspace test vial. After being heated at 130 °C for 2 hours, NaOH solution (6 mol•L-1) was injected into the vial and then the measurements started. Size distributions of TiO2 before and after modification were obtained by dynamic light scattering (DLS) (ZetaPALS, Brookhaven Instrument Corp., USA). Experimental samples were filtered by a membrane with pore size of 0.22 μm before measurement. Morphologies of LS@TiO2 particles were characterized by FEI transmission electron microscopy (TEM, TF-20, USA). Thermogravimetry (TG) of the powders were performed using a TG 209 equipment (Netzsch Group, Germany). 5-10 mg samples were placed in an aluminum oxide crucibles and heated from 30 to 700 °C at a rate of 10 °C•min-1 in air atmosphere. The crystal structures were investigated using a SmartLab SE X-ray diffractometer (Rigaku Co., Ltd., Japan) equipped with a nickel filter. The Fourier transform infrared spectra (FTIR) in the range of 4000-400 cm-1 were obtained by the FTIR spectrometer (Vector 333, Bruker, Germany). Disks were prepared



by mixing 3 mg of sample with 300 mg KBr and then being pressed at 6 MPa for 1 min. X-ray photoelectron spectroscopy (XPS) was obtained from an Ultra Axis DLD multifunction electronic spectrometer (Kratos Corp. UK). The water contact angles of TiO2 and LS@TiO2 were detected by a static contact angle analyzer (JC2000C1, Zhongchen Digital Technic Apparatus Inc., China). 10 mg of TiO2 or LS@TiO2 was dispersed in 1 mL ethanol and disk was prepared by spin-coating 100 μL of the dispersion on a 1.5 cm ×1.5 cm slide glass at 1000 rpm for 60 s. The morphology and roughness of LS@TiO2 were analyzed by atomic force microscope (AFM, Park Systems Corp., Korea). The samples were prepared by dropping the dispersions on the silicon slices and then observed by a tapping mode. UV-Vis reflectance spectra of TiO2 and LS@TiO2 were recorded using a spectrophotometer (UV-2600, Shimadzu, Japan). The dispersion of LS@TiO2 nano composites in pure cream were observed by Hitachi scanning electron microscope (SEM, SU8220, Japan). The electron spin resonance spectra were obtained by Bruker ECS-EMX X-Band spectrometer equipped with ER4119HS cavity at room temperature (ESR, Bruker, Germany).

 RESULTS AND DISCUSSION Modifying TiO2 via lignin coating. TiO2 is the most commonly used physical UV blocker and white pigment in cosmetic and coating products. However, aggregation induced poor dispersity limits the full play of its function. After hydrothermal esterifying modification by LS, TiO2 particles turned into ochre and the LS@TiO2 can be readily dispersed in water, as shown in Figure 1a. TiO2 aqueous dispersion is white with some blue light caused by nano scattering, while LS@TiO2 dispersion is yellowish, which is much lighter and more acceptable than its solid state. The size distributions of TiO2 before and after modification were analyzed by DLS. As shown in Figure 1b, the average sizes of TiO2 and LS@TiO2 are 106 and 119 nm, respectively. It means a thickness of 13


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nm LS was coated on the surface of TiO2, which is confirmed by the TEM images. As shown in Figure 1c-d, the modified TiO2 shows core-shell like structure. The coated LS presents as fuzzy corona, and the thickness of LS shell is almost the same as the DLS result. The effect of coated LS on the physicochemical properties of TiO2 was analyzed. The TG spectra of TiO2 and LS@TiO2 are shown in Figure 2a. The weight loss of TiO2 is mainly due to the remove of the surface bound water, while that of LS@ TiO2 is derived from the loss of surface water at the first stage and the loss of LS at the second stage. The weight decrement of LS@TiO2 in the stage of 100-300 °C is smaller than that of TiO2, indicating that the content of hydroxyl and bound water on the surface of TiO2 were reduced after esterfying modification.58 The weight reductions of both TiO2 and LS@TiO2 tend to be stable at after 500 °C, which are 2.06% and 5.35%, respectively. The average content of coated LS in LS@TiO2 particles was calculated according to the following equation.59 Wcont 

' Wloss  Wloss  100 ' 100  Wloss

(2)

where Wcont = the content of lignin coating, Wloss = the total weight loss of the modified particles, W’loss = the total weight loss of TiO2. The LS content in TiO2@LS was calculated to be 3.4 wt%. The XRD spectra in Figure S1 in the supplementary information show that the crystal form of TiO2 is rutile and there is no change after modification. The FTIR spectra of TiO2, LS and LS@TiO2 were determined. As shown in Figure 2b, characteristic peaks of the functional groups of LS appear in the spectrum of LS@TiO2, such as stretching vibrations of methyl and methylene at 2930 cm-1 and 2839 cm-1, and vibrations of the aromatic ring at 1514cm-1 and 1423 cm-1. It confirms that LS was successfully coated on TiO2. The broad peak of hydroxyl absorption of TiO2 in the range of 3200-3700 cm-1 decreases evidently, while a new peak attributed to C-O stretching



vibrations of ester bond appears at 1119 cm-1, indicating that LS was coated on the surface of TiO2 by the dehydration condensation between the hydroxyl groups on the surface of TiO2 and the carboxyl groups of LS. To confirm this hypothesis, XPS characterization of LS@TiO2 was performed. The survey and O1s spectra of LS@TiO2 are shown in Figure 2c and 2d. The O1s spectra show a chemical linkage between LS and TiO2. Peaks at 529.3 eV and 531.5 eV correspond to Ti-O-Ti bonds and surface hydroxyl groups respectively,60 while those at 529.8 eV and 532.9 eV are classified as C-O-Ti bonds and C-O bonds.61 The presence of C-O-Ti bonds demonstrates that LS was coated on the surface of TiO2 by esterification based dehydration condensation. To verify the important role of the surface hydroxyl groups of TiO2 in the coating process, TiO2 was hydroxylated and then modified with LS. A variety of methods have been used for hydroxylation of TiO2 surface, such as UV irradiation, acid treatment and H2O2 treatment.62-64 When acid treatment is applied, the SO42- or PO43- of sulfuric acid or phosphoric acid are easy to bond with the O- on the surface of TiO2, thus affecting the further modification.65 Therefore, HCl was used in this work. The hydroxylated TiO2 treated with 0.1 and 1 mol L-1 HCl solutions were named as TiO2-0.1M and TiO2-1M. As shown in Figure S2, the average sizes of TiO2-0.1M or TiO2-1M are 107 nm and 106 nm, which are almost the same as that of untreated TiO2. Their FTIR spectra in Figure S3 show that the hydroxyl peak at around 3400 cm-1 increases with the acid concentration. It indicated that more surface hydroxyls were formed. Results of water contact angle also proved that the particles were more hydrophilic after being treated with HCl. As shown in Figure 3, the contact angles of TiO2, TiO2-0.1M and TiO2-1M are 39.8°, 31.3° and 27.1°, verifying the effectiveness of HCl treatment. TiO2-0.1M and TiO2-1M were then coated by LS to get [email protected] and LS@TiO2-1M, and the LS content were calculated to be 5.0 wt% and 6.0 wt%. With the increment of hydroxyl group content on the surface of TiO2, more LS could be reacted and thus coated on the surface of TiO2, which is also another evidence of the important role of hydroxyl groups during the surface


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modification of TiO2. Also, more coating content of LS further increased the hydrophily of the composites, the contact angles of LS@TiO2, [email protected] and LS@TiO2-1M decrease to 26.5°, 22.0° and 19.2°, respectively. It benefited the dispersing of LS@TiO2 composites due to the enhanced electrostatic repulsion among sulfonates in LS. UV-blocking performance of lignin-modified TiO2. TiO2 is the most commonly used physical sunscreens. LS is not only a macromolecular dispersant, but also a natural sun blocker. Coating LS on the surface of TiO2 can not only improve its dispersity, but also enhance the UV-blocking property. Therefore, the sunscreen performance of LS@TiO2 was investigated and compared with that of LS, TiO2, and their physical mixture. The color of cream matrix mixed with 10 wt% LS@TiO2 is faint yellow, which is lighter than the color of commonly used BB cream and therefore is acceptable, as shown in Figure S4. Figure 4 shows the typical UV transmittance of pure cream blended with 10 wt% of LS, TiO2, LS@TiO2 and the physical mixture in UVA and UVB areas. The transmittance of LS is higher than that of TiO2. The transmittance of LS@TiO2 is much lower than that of TiO2 in both UVA and UVB areas, while that of their physical mixture is much higher, indicating the poor UV-blocking performance. The SPF values of sun creams with different dosages of LS, TiO2 and LS@TiO2 are calculated and listed in Table 1. At the same dosage, SPF values of LS@TiO2 based sun cream is higher than the addition of those of LS and TiO2 based sun creams, which reaches 47.77 when the dosage of LS@TiO2 is 20 wt%. Higher dosage of LS would demulsify the sun cream.43 However, same amount of nano composites dispersed well in the pure cream. In addition, 3.4 wt% content of LS could significantly boost the sunscreen performance of TiO2. Natural lignin exhibited synergistic effect with TiO2, which has been revealed in lignin-chemical sunscreen systems.43 To enhance the synergistic effect of between LS and TiO2, [email protected] and LS@TiO2-1M with 5.0 wt% and 6.0 wt% of LS were applied. As shown in Table 2, SPF value of pure cream blended with 10 wt% of [email protected] is 38.87, while that



containing 10 wt% of [email protected] reaches 55.56, which is higher than that of containing 20 wt% of LS@TiO2. It means that sun cream with only 10 wt% dosage of lignin modified physical sunscreen can reach SPF 50+. Similar performance cannot be obtained until adding more than 30 wt% of sole physical or chemical sunscreens. UV-blocking enhancement mechanism of lignin coated TiO2. There are several factors affecting the UV-blocking performance of TiO2, such as structure, particle size and dispersibility.66 As shown in Figure 2 and 3, the thickness of coated LS is 13 nm, which only increases 12% of the size and have little influence on the nano scattering property of TiO2. Inversely, coating right amount of LS significantly enhanced the UV-blocking ability. As shown in Figure 5, the smooth surface of TiO2 become burr shape after coating LS and the surface roughness increase from 18 to 50 nm, which is beneficial to scatter UV rays. The UV-Vis reflectance spectra of TiO2 and LS@TiO2 are shown in Figure 6. In the visible light region, TiO2 diffuses almost all of the incident light, exhibiting an extremely high whiteness, whereas LS@TiO2 absorbs part of visible light and is light brown (Figure 1a). However, in the UV region, especially in UVA and UVB areas, the reflectance of LS@TiO2 is much higher than that of TiO2 due to the strong scattering of roughly coated LS. In addition, the coating of LS effectively eases the aggregation of TiO2 and benefits its dispersing in the cream. As shown in Figure S5, LS@TiO2 nano composites are well dispersed in the pure cream, while same dosage of TiO2 nano particles highly aggregate. Therefore, coating LS on the surface of TiO2 is more conducive to its uniform distribution in the substrate, and better reflection, scattering and absorption of UV light. As a commonly used physical sunscreen, TiO2 shields UV light by scattering, reflecting and absorbing, while the UV resistance of lignin mainly comes from absorption of its aromatic rings.43 After the surface modification, coated LS made the TiO2 particles have a stronger UV absorbance. On the other hand, another redox reaction happened during the hydrothermal esterification process. The possible redox reaction is presented


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as equation 3.

(3) TiO2 particles were reduced and forming Ti3+ self-doping. As illustrated in Figure 7, TiO2 gives no response in ESR testing, indicating the complete Ti4+ state in the TiO2 particles. LS responds clearly since it was oxidized to semiquinone state.67 The physical mixture of LS and TiO2 presents radical response in the same area as LS and the g value remains 2.0003, while the radical response of LS@TiO2 move to high magnetic field and the g value decreases to 1.9990. The smaller g value of 1.9990 indicates that Ti3+ defects generated predominantly on the surface of TiO2.68 The original ESR data is shown in Figure S6. The Ti3+ defects not only broadened the absorbing spectrum, but also strengthened the UV absorbing ability of TiO2. On the other hand, the semiquinone structure of LS was easily transformed into hydroquinone and quinone structures, which also strengthened the UV absorbing ability.69 The possible UV-blocking enhancement mechanism of LS@TiO2 is shown in Figure 8.

 CONCLUSION Lignosulfonate (LS), one type of industrial lignin with broad-spectrum UV-blocking property and good dispersibility, was coated on the surface of TiO2 by one-pot hydrothermal esterification method. The esterification reaction occurred between the carboxyl group of lignin and the hydroxyl group on the surface of TiO2, which made LS combine with TiO2 tightly. HCl treated TiO2 had more hydroxyl groups and the LS



coating amount increased nearly 1 time under the same modification procedure. Coating LS effectively boosted the UV-blocking performance of TiO2. The SPF values of nano LS@TiO2-based sun creams were 30-60% higher than those of the sun creams with same dosage of nano TiO2. The UV-blocking enhancement was more obvious when LS content increased. Pure cream blended with 10% of LS@TiO2 with 6.0 wt% LS resulted in an SPF value of 56, which provided enough protection even for prolonged exposure to UV irradiation. The LS boosting mechanism was also revealed. Coated LS itself absorbed UV irradiation and eased the aggregation of the TiO2 in the substrate. The surface roughness of TiO2 significantly increased after coating LS, which allowed more scattering and reflection of UV rays. During the hydrothermal esterification process, TiO2 particles were reduced and forming Ti3+ self-doping. Ti3+ defects not only broadened the absorbing spectrum, but also strengthed the UV absorbing ability of TiO2. On the other hand, LS was oxidized into the semiquinone structure, which was easily transformed into hydroquinone and quinone structures and could further enhance the UV absorbing ability. The synthesis of LS@TiO2 nano composites is facile and green and has a wide range of raw materials, allowing it to be scale produced. It has a broad prospect in cosmetics and polymeric materials.

ASSOCIATED CONTENT Supporting Information. Physicochemical parameters of LS; XRD and FTIR spectra of TiO2 and LS@TiO2; Particle size distributions of TiO2 treated by 0.1M and 1M HCl; Photograph and SEM images of pure cream blended with 10 wt% of TiO2 and LS@TiO2, and commercial BB cream; ESR spectra of LS, TiO2, LS@TiO2 and LS/TiO2 physical mixture.

 AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Author * E-mail: [email protected]; [email protected].

Notes The authors declare no competing ﬁnancial interest.

 ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (21878113, 21606089, 21436004), State Key Laboratory of Pulp and Paper Engineering (201701), Guangdong Province Science and Technology Research Project of China (2017B090903003), and Guangzhou Science and Technology Research Project of China (201704030126, 201806010139). We also thank Professor Shiping Zhu of McMaster University Canada for his invaluable suggestion and helpful discussion.

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Captions of Tables and Figures Table 1. SPF values of pure creams blended with different amounts of LS, TiO2 and LS@TiO2. Table 2. SPF values of LS@TiO2-based sun creams with different LS content at dosage of 10 wt%. Figure 1. (a) Photographs of solid particles and 1 g•L-1 dispersions of TiO2 and LS@TiO2. (b) Size distributions of TiO2 and LS@TiO2 water dispersions, the particle contents were both 1 g•L-1. (c-d) TEM images of dispersed LS@TiO2. Figure 2. (a) TG spectra of TiO2 and LS@TiO2. (b) FTIR spectra of TiO2, LS and LS@TiO2. (c-d) XPS survey and O 1s spectra of LS@TiO2. Figure 3. Photographs of water contact angle of (a) TiO2, (b) TiO2-0.1M, (c) TiO2-1M, (d) LS@TiO2, (e) [email protected] and (f) LS@TiO2-1M. Figure 4. Typical UV transmittance of pure cream blended with 10 wt% of LS, TiO2, LS@TiO2 and LS/TiO2 physical mixture in UVA and UVB areas. The weight ratio of LS and TiO2 in both LS@TiO2 and physical mixture was 3.4: 96.6. Figure 5. 2D and 3D AFM topography of (a) TiO2 and (b) LS@TiO2. Figure 6. UV-Vis reflectance spectra of TiO2 and LS@TiO2 in the range of 190-700 nm. Figure 7. ESR spectra of LS, TiO2, LS@TiO2 and LS/TiO2 physical mixture. The weight ratio of LS and TiO2 both LS@TiO2 and the physical mixture was 3.4: 96.6. Figure 8. Mechanism of UV-blocking enhancement of LS-modified TiO2.


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Table 1.

a

Sample

SPF (5 wt%)

SPF (10 wt%)

SPF (20 wt%)

LS

3.22±0.21

6.21±0.32

/a

TiO2

10.19±0.52

16.37±0.29

35.99±3.23

LS@TiO2

15.57±0.49

26.49±1.47

47.77±3.27

SPF cannot be measured due to demulsification of the cream at a high lignosulfonate dosage.



Page 26 of 35

Table 2 Sample

LS content (wt%)

SPF

LS@TiO2

3.4

26.49±1.47

[email protected]

5.0

38.87±1.14

LS@TiO2-1M

6.0

55.56±2.06


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Figure 1.



Figure 2.


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Figure 3.



Figure 4.


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Figure 5.



Figure 6.


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Figure 7.



Figure 8.


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For Table of Contents Use Only Synopsis: Sustainable natrual lignin/titanium dioxide nano composites with good dispersity and high UV-blocking property were obtained via one-pot hydrothermal esterification.


Titanium

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