Bifunctional Cr3+ modified ion exchange resins as efficient reusable catalysts for the production and isolation of 5-hydroxymethylfurfural from glucose

Cr modified readily available cation exchange resins were prepared and explored as heterogeneous bifunctional catalysts for the dehydration of glucose to 5-hydroxymethylfurfural (HMF) in tetraethyl ammonium bromide (TEAB)/water as reaction medium. Excellent HMF isolated yields of up to 70% were achieved using simple crystallization of the reaction medium (TEAB) from ethyl acetate/ethanol which allowed the isolation of HMF in high purity. The best identified catalyst (Amberlyst 15/Cr) exhibited high activity over 4 cycles. The loss of activity was attributed to the decreased number of acidic sites of the catalyst, thus a simple treatment of the catalyst with 10% HCl efficiently restored its activity in the following cycles.

Introduction
Nowadays, biorenery is considered the most promising approach that may gradually replace current dependence on fossil resources, which although cheap and easy to obtain, lack any sustainability in comparison with biorenery. Among other building blocks derived from renewable resources such as ethanol, glycerol, lactic acid and furfural, 5-hydroxymethylfurfural (HMF) has been recognized as a key biorening building block derived from carbohydrates, such as cellulose, glucose, inulin, fructose, etc. Among them, fructose is currently the only carbohydrate that can be readily dehydrated into HMF, typically in high yield and selectivity. 1,2 However, fructose has a low abundance in nature resulting in high cost and very importantly is used in the food sector. In contrast, glucose has much higher abundancy and is readily available from non-food cellulosic biomass, thus being a highly desirable feedstock. 3 However, due to its resistance to dehydration and formal carbonyl isomerization, the development of catalytic systems and processes that could efficiently convert glucose into HMF is more demanding being still an open topic. It is known that generally the conversion of glucose to HMF is a two-step process where rst the glucose is isomerized to fructose and the last undergoes subsequent dehydration to form HMF. The isomerization step is typically enzymatic, base or metal salt catalysed, while dehydration is an acid catalysed process. In the recent years a considerable number of bifunctional catalysts have been reported allowing this one pot transformation.
Some metal salts were reported to effectively promote the isomerization and dehydration of glucose. Among several metal salts such as metal oxides 4-6 and phosphates, 7-9 chlorides and, in particular, chromium chlorides, proved to be highly effective to promote the isomerization and dehydration of glucose. Zhang et al. 10 reported the rst example of CrCl 2 catalysed dehydration of glucose achieving nearly 70% HMF yield using the ionic liquid 1ethyl-3-methylimidazolium chloride at 100 C. Aer this pioneer work, many reports emerged describing the use of chromium chlorides as catalysts for glucose dehydration. Yong et al. 11 reported NHC-CrCl 3 /1-butyl-3-methylimidazolium chloride ([bmim]Cl) system capable of converting glucose into HMF in up to 81% yield. In 2009, Zhao et al. 12 described the microwave accelerated glucose dehydration catalysed by CrCl 3 in [bmim]Cl, yielding HMF in up to 91% yield. CrCl 3 /tetraethylammonium chloride system was reported by Lin et al. 13 that efficiently catalyse the glucose dehydration and was achieved an HMF yield of 71%.
It is known solvent plays a key role in the chromium catalysed dehydration of glucose to HMF, and high conversion and selectivity are usually achieved using ionic liquids as reaction media, as explained by Hensen et al. 14 the presence of highly concentrated and mobile chloride anions from the ionic liquid promotes the (de)protonation reactions important for the glucose isomerization (Scheme 1).
The problematic downstream separation of HMF from highboiling organic solvents or ionic liquids and the toxicity and pollution of chromium in the environment decreases the Besides all the achievements reported so far, the development of a glucose dehydration process that will allow both easy isolation of HMF from the reaction media and recyclability of the catalysts from both economic and environmental point of view is still an open topic. Previously, we reported that tetraethyl ammonium bromide (TEAB)/water is an efficient reaction media for fructose dehydration, which allowed quantitative HMF isolation via simple TEAB crystallization and subsequent recycling of both TEAB and organic solvents. 18,19 Nevertheless, under the optimised conditions for fructose, we were able to achieve only 35% HMF yield from glucose using CrCl 3 as catalysts under homogeneous conditions. Later, we demonstrated that TEAB/water reaction media proved to be highly efficient for two-step integrated chemo-enzymatic production of HMF from glucose. Using this approach 87% isolated HMF yield from glucose was achieved, 20 which to the best of our knowledge still remains among the highest reported yields as stated in a recent review. 21 The constructing an efficient and recyclable one-pot process for glucose transformation to HMF using the already proven TEAB/water reaction media is still challenging. Here in, a series of cheap and readily available acidic cation exchange resins modied with Cr 3+ were prepared and used as bifunctional heterogeneous catalysts in one pot glucose dehydration in TEAB/water reaction media. The previously developed crystallization method for HMF isolation was used allowing both quantitative isolation of HMF from the reaction media and reuse of the catalyst.

Results and discussion
It is known that acidic ion-exchange resins are successfully applied for adsorptive removal of Cr 3+ . 22,23 Encouraged by that, we envisioned the preparation of Cr 3+ modied ion exchange resins as promoters for the glucose dehydration. A series of such catalysts was obtained by adsorption of Cr 3+ over ion-exchange resins in MeOH solutions. The prepared catalysts were characterized by ICP-AES analyses and the Cr loading was calculated in w/w%. Amberlyst 15, Amberlyst 36 and Amberlite IR120 exhibit similar results 4.16%, 4.86% and 4.70% respectively. As it was expected the highest Cr loading of 5.24% was observed for Amberlite IRC748I being a chelating resin, while only 2.41% loading was achieved with IRC86 ( Fig. 1). The catalytic conversion of glucose to HMF was performed using mover para o nal da frase talvez TEAB/water (10% w/w) reaction media. 19 As expected, due to the higher dehydration resistance of glucose compared to fructose, longer reaction times and higher catalyst loading were required. However, even aer 60 min reaction time at 100 C, using Amberlyst 15/Cr 3+ 100% (w/w, glucose/catalyst) loading only 34% yield was achieved. Nevertheless, our previous observations about the importance of small amounts of water (10-15%) in the reaction media to achieve clean conversion were conrmed and outstanding purity of 99% was obtained. In the following experiments the effect of temperature was studied by rising in 10 C steps. The highest HMF yield of 70% with outstanding purity of 98% was achieved at 120 C. Using Amberlyst 15 without Cr 3+ under these conditions the HMF yield was only 24%. Further increase of the temperature to 130 C was not benecial and decreased yield of 55% was observed probably due to the unstable nature of HMF at high temperatures (Fig. 2). Using lower catalyst loading of 50% (w/w) resulted in a lower yield of 59%, thus 100% (w/w) was established as optimal for the further experiments.
Further optimization of the reaction time was performed. However, prolonged reaction times of 75 min and 120 min resulted in lower yields of 68% and 64% and purity of 91% and 92% respectively (Fig. 3).
With the optimized conditions in hands for Amberlyst 15, we studied the catalytic activity of the other Cr 3+ modied Amberlyst 36, Amberlyst IRC86, Amberlite IRC748I and Amberlite IR120 resins (Fig. 4). No direct correlation between Cr loadings Scheme 1 Proposed mechanism of the Cr 2+ promoted glucosefructose isomerization in ionic liquids. 14 Fig. 1 Chromium incorporation in the ion-exchange resins (quantification by ICP-AES). and observed yields have been noticed. Surprisingly the two Cr 3+ modied ion exchange resins with the highest Cr loadings Amberlite IR120/Cr 3+ (4.70%) and Amberlite IRC748I/Cr 3+ (5.24%) provided very low yields of only 10% and 27% respectively. Similar yield of 62% as for Amberlyst 15/Cr 3+ was achieved with Amberlyst 36/Cr 3+ . Amberlyst IRC86/Cr 3+ which exhibited the lowest Cr loading of only 2.41% provided almost identical yield of 68%, thus leading to the idea that very low amounts of Cr are sufficiently catalysing glucose to fructose isomerization. However, considering the all process a good balance between Cr loading and acidity of the catalyst is important. It is worthy to be mentioned that although considerable variation of the yields was observed for different resins, the HMF was isolated in high purities of above 98% in all cases (Fig. 4).
Amberlyst 15/Cr 3+ was selected as the best catalyst to study its reusability under optimized conditions. Minimal mechanical destruction of the catalysts was observed over the full set of cycles, when the stirring was precisely controlled to not overcome 600 rpm. High catalytic activity and repetitive yield of close to 70% was observed over the rst 3 cycles, while low decrease of the yield to 59% was observed over the fourth cycle. However, the yield rapidly decreases to only 25% in the h cycle. The leaching of the Cr 3+ was followed by ICP analysis of the reaction mixture and the recovered catalyst aer each cycle. The leaching in the reaction mixture over the rst cycle was determined to be 29%. However, over the second cycle it dramatically decrease to only 1.5%, which can be attributed to the fact that non-absorbed Cr 3+ was not sufficiently removed during the catalyst pretreatment. The overall leaching of the following cycles 3-5 was determined to be 5% and average of 1.7% per cycle. The same was observed from the ICP analysis of the recovered Amberlyst 15/Cr 3+ (Fig. 5). The Cr 3+ loading decrease from 4.16 to 2.94% over the rst cycle and further slowly drop down to 2.59% over the cycles 2-5. We assumed that the signicant loss of catalytic activity aer the 5 th cycle can be attributed to the decrease of the acid sites of the catalysts since as we already observed that on one hand only small amounts of Cr are sufficient to promote the rst step of the reaction, and on the other hand the determined Cr 3+ leaching was minimal, thus aer the h cycle the catalysts was regenerated using simple treatment with 10% aq. HCl. The use of the regenerated catalyst resulted in 50% yield in the sixth cycle which decreased to 35% in the eight cycle. It is worthy to be mentioned that restoring the catalytic activity using 10% aq. HCl was used as a proof of concept and no optimizations were performed, thus suggesting that full recovering of the catalytic activity may be possible upon further optimization (Fig. 6).    4 Glucose dehydration using different Cr 3+ modified cation exchange resins. Reaction conditions: catalyst (0.7 g), glucose (0.7 g), water (0.7 mL), TEAB (7 g), 120 C, 60 min. Fig. 5  ICP analyses were performed on radial Horiba Jobin-Yvon, Ultima ICP-AES equipped with Czerny-Turner monochromator and 40.68 MHz RF generator.

Catalysts preparation
All the catalysts were prepared as followsa pressure glass reactor tube (Fig. S4 †) (15 mL, Aldrich Z181064) was charged with 3 g of the corresponding resin and 1 g CrCl 3 $6H 2 O, 10 mL MeOH was added and the mixture was heated without stirring at 80 C overnight. Aer cooling down the Cr 3+ modied resins were ltered and washed subsequently with 100 mL of water and 100 mL of MeOH prior dried in open air for 12 h.

Catalysts characterisation
To approximately 100 mg of the corresponding Cr 3+ ion exchange resin were added 0.6 mL 35% aq. HCl and 0.2 mL 69% aq. HNO 3 . Then, the sample was heated at 120 C for 180 min. The obtained homogeneous solution was transferred into a 5 mL volumetric ask and lled with deionized water. 1 mL of the resulting solution was transferred in 100 mL volumetric ask and diluted with deionized water. The ICP-AES analyses were performed using the following conditions: RF power 1050 kW, coolant gas ow rate (Ar) 12 mL min À1 , nebulizer Mira Mist with 3 bar pressure, sample ow rate 1.0 mL min À1 , wavelength 267.716 nm.
General procedure for catalytic dehydration of glucose into HMF A pressure glass reactor tube (Fig. S4 †) (15 mL, Aldrich Z181064) was charged with the catalyst (0.7 g), glucose (0.7 g, 3.885 mmol), water (0.7 mL), tetraethylammonium bromide (TEAB) (7 g) and the resulting mixture was stirred magnetically at 600 rpm using a cross-shaped magnetic stirrer and heated with a thermostatically controlled (0.1 C) oil bath at a certain temperature for a specic time. Upon cooling to room temperature, the mixture was dissolved in absolute ethanol (20 mL) and the catalyst was ltered and washed with absolute ethanol (20 mL). The solvent was evaporated under vacuum and the resulting solid was then dissolved in hot absolute ethanol (3.5 mL). The clear solution was then precipitated with ethyl acetate (200 mL), stirred for 20 minutes and ltered off over a pad of silica (10 g) to yield HMF as a brownish liquid, which solidies in freezer (À12 C). The product purity was analyzed by HPLC and 1 H NMR.

Recycling experiments and catalyst reactivation
The recycling experiments were performed using the described general procedure, the recovered Amberlyst 15/Cr 3+ catalyst was directly used in the next cycle upon drying under vacuum for 1 h (0.1 mbar).
The Amberlyst 15/Cr 3+ catalyst isolated from the h cycle was reactivated using the following procedure: the catalyst and 14 mL of 10% HCl were charged into 50 mL round-bottomed ask and stirred at room temperature for 1 h. The catalyst was ltered, washed with absolute ethanol (15 mL) and dried under vacuum (0.1 mbar, 1 h) and used in the following cycles without further treatment.

Conclusions
In conclusion, a highly efficient approach for the direct transformation of glucose to HMF that addresses both isolation and environmental issues related with the use of Cr 3+ catalysis was described. Five Cr 3+ modied readily available cation exchange resins were prepared and explored as heterogeneous bifunctional catalysts for this transformation. Among them Amberlyst 15/Cr 3+ exhibit the best performance and was successfully recycled over 4 cycles, while signicant decrease of the catalytic activity was observed only in the 5th cycle. However, the loss of catalytic activity could be overcome by simple regeneration of the acidic sites of the catalyst. The already optimized in our previous studies of TEAB/water reaction media proved to be highly efficient allowing easy HMF isolation in high purity, thus providing superior alternative to the commonly used ionic liquids. Fig. 6 Recycling experiments. Reaction conditions: Amberlyst 15/Cr 3+ (0.7 g), glucose (0.7 g), water (0.7 mL), TEAB (7 g), 120 C, 60 min. After 5 th cycle, the Amberlyst 15/Cr 3+ was treated with 10% aq. HCl.


extfieldsofstudy: Chemistry Medicine

text:
Preparation and evaluation of microemulsion-based transdermal delivery of Cistanche tubulosa phenylethanoid glycosides

The primary aim of the present study was to develop a novel microemulsion (ME) formulation to deliver phenylethanoid glycoside (PG) for use in skin lighteners and sunscreens. The oil phase was selected on the basis of drug solubility, while the surfactant and cosurfactant were screened and selected on the basis of their solubilizing capacity and the efficiency with which they formed MEs. Pseudoternary phase diagrams were constructed to evaluate ME regions and five formulations of oil-in-water MEs were selected as vehicles. In vitro skin permeation experiments were performed to optimize the ME formulation and to evaluate its permeability in comparison to that of saline solution. The physicochemical properties of the optimized ME and the permeating ability of PG delivered by this ME were also investigated. The optimized ME formulation was composed of isopropyl myristate (7%, w/w), Cremorphor EL (21%, w/w), propylene glycol (7%, w/w) and water (65%, w/w). The cumulative amount of PG that permeated through excised mouse skin when carried by ME was ~1.68 times that when PG was carried by saline solution only. The cumulative amount of PG in the microemulsion (4149.650±37.3 µg·cm−2) was significantly greater than that of PG in the saline solution (2288.63±20.9 µg·cm−2). Furthermore, the permeability coefficient indicated that optimized microemulsion was a more efficient carrier for transdermal delivery of PG than the control solution (8.87±0.49 cm/hx10−3 vs. 5.41±0.12 cm/hx10−3). Taken together, the permeating ability of ME-carried PG was significantly increased compared with saline solution.

Introduction
Herba cistanche, a tonic herb commonly distributed in desert regions, has been frequently used in traditional Chinese medicine. To date, multiple studies have been conducted to investigate its biopharmaceutical properties, including improvement of memory, sexual potency, free radical scavenging, and anti-aging activity (1). The major component of H. cistanche is phenylethanoid glycoside (PG). Previous studies have revealed that PG inhibits the activity of tyrosinase, the rate-limiting enzyme for the synthesis of melanin in the human body (2). In addition, PG has been reported to be responsible for the scavenging of 2,2-diphenyl-1-picrylhydrazyl radicals (3), and protecting keratinocytes from ultraviolet (UV) radiation. Therefore, this component may have potential in skin depigmentation.
Topical medication has been commonly used in cosmetic fields, with the drug delivered directly onto the body surface (4). Microemulsions (ME) are a novel drug delivery system conventionally composed of oil, surfactant, cosurfactant and aqueous phases (5) and have been reported to improve the rate and absorption of lipophilic drugs (6). In addition, due to its satisfactory solubility, low skin irritation and high permeability, it is expected to be used in topical drug delivery (7,8).
To the best our knowledge, few studies have been conducted where PG was delivered via topical medication using microemulsion. In the present study, an oil-in-water (O/W) nanoemulsion delivery system for transdermal administration was established, and PG was incorporated into the nanoemulsion (9,10). Meanwhile, preparation and characterization of PG-based nanoemulsion was performed by investigating the transdermal release in vitro.

Materials and methods
Reagents. Cremorphor EL and isopropyl myristate (IPM) were purchased from Sigma Aldrich; Merck Millipore (Darmstadt, Germany). Ethanol, oleic acid (OA) and Tween-80 were purchased from Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Ethyl oleate was purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals and solvents used were of analytical reagent grade.
Preparation of PhGs. PhG was prepared as routinely described (11). The air-dried C. tubulosa was powered and extracted by percolation with 80% EtOH. The percolate was evaporated under pressure, followed by filtration of the residual fluid. The filtrate was concentrated and chromatographed on a SP-825 macroporous resin column using 0, 30, 50, 70 and 90% EtOH in water as eluants. The 30-50% EtOH eluants were concentrated and dried to obtain PhG rich fraction. UV spectrophotometry indicated the content of the PhG was 87.6%.
Screening of oils, surfactants and cosurfactants for ME. Solubility tests were conducted to select appropriate components for the preparation of an ME formulation with a high drug-loading capacity. In this process, an excess amount of PG was added into 4 ml of each of the following reagents: IPM, liquid paraffin, OA, Cremophor EL, Tween-80, absolute ethanol, propylene glycol, glycerin and PEG400, respectively. Following this, the resulting mixture was shaken reciprocally at 37˚C for 72 h, followed by centrifugation at 12,830 x g for 10 min. Subsequently, the pellet was resuspended in methanol, and the PG was determined using UV spectrophotometry at 333 nm. Appropriate diluted solutions of oils, surfactants and cosolvents in methanol served as a blank control.
Construction of phase diagram. To obtain a concentration range of components for the existing microemulsion boundary, pseudoternary phase diagrams were constructed using a water titration method as previously described (12). Three phase diagrams were prepared with 1:3, 1:1, and 3:1 weight ratios of surfactant to cosurfactant. The oil phase and the surfactant mixture were then mixed at the weight ratios of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1. The oil and surfactant mixture were mixed homogeneously, and were diluted dropwise with purified water under magnetic stirring (300 rpm) at room temperature until transparent ME was obtained. The concentration of components was recorded in order to complete the pseudoternary phase diagram. Based on these diagrams, the appropriate concentrations of materials were selected for the preparation of PG-based ME.
Preparation of ME and the aqueous solution of PG. A total of 5 MEs (ME1-5) were selected according to the phase diagrams with a surfactant: Cosurfactant weight ratio of 3:1 (Table I). Microemulsion systems were prepared by mixing oil with the mixture of surfactant and cosurfactant, and water was added precisely into oily phases with magnetic stirring (300 r/min) at 37˚C. The systems were equilibrated using gentle magnetic stirring for 30 min followed by dissolving the appropriate amount of PG under ultrasonication. The aqueous solution was prepared with the same PG content dissolved in purified water.
Characterization of microemulsion. The average droplet size and polydisparity index of MEs were evaluated by photon correlation spectroscopy (Malvern Instruments, Ltd., Malvern, UK). All samples were diluted with water at a 1:5 ratio and were filtered through a 0.22 µm filter. The test was performed in a thermostatic chamber at 25˚C in triplicate.
The morphology of PG microemulsions was observed using transmission electron microscopy (JEM-100CXII; JEOL Ltd., Tokyo, Japan). Briefly, one drop of diluted sample was deposited on a film-coated coppergrid, followed by staining with one drop of 2% phosphotungstic acid aqueous solution. The average droplet diameters of the microemulsions were determined by photon correlation spectroscopy instrument (BI-200SM; Brookhaven Instruments Corporation, Holtsville, NY, USA) at room temperature.
The viscosity of the ME vehicles was measured at 25˚C, using the NDJ-8S digital viscometer (Shanghai Precision and Scientific Instrument Co., Ltd., Shanghai, China) with a no. 1 rotor set at 60 rpm. Preparation of skin tissues. Male Kunming mice weighing 20 g were purchased from Experimental Animal Center of Xinjiang Medical University (Xinjiang, China). 10 mice (6 weeks old) were subjected to adaptive feeding for 1 week on 12 h light/12 h dark cycle, at room temperature (24±2˚C), with a humidity of 50±15%. All animals had access to a standard diet and water. The hair was removed with an electrical clipper. Following terminal anesthesia with chloral hydrate (10%) via subcutaneous injection, skin tissues were excised from the abdominal region and the subcutaneous fat and connective tissue were removed. The tissues were placed in a refrigerator at 4˚C overnight. The study protocols were approved by the Ethical Committee of Xinjiang Medical University (Xianjiang, China).
In vitro permeation study. Permeation experiments were conducted as previously described (12). The excised skin was clamped between the donor and the receptor chamber of a vertical diffusion cell with an effective diffusion area of 2.8 cm 2 . The receptor chamber was filled with fresh, normal saline. The diffusion cell was maintained at 37˚C using a recirculating water bath and the solution in the receptor chambers was stirred continuously at 200 rpm. Following this, the test formulation (1.0 ml) was gently placed in the donor chamber. Subsequently, 3 ml of the solution in the receptor chamber was removed and replaced with an equal volume of fresh normal saline at predetermined intervals (0, 2, 4, 6, 8, 10 and 24 h). The sample obtained from the receptor chamber was used to determine PG levels using UV spectroscopy at a wavelength of 333 nm.
The cumulative amount of PG that permeated through the excised mouseskins was calculated according to the following formula: Where Q n was the accumulated amount at a given time t, Cn is the drug concentration of the receiver solution at each sampling time, C i is the drug concentration of the sample, S is the effective diffusion area (S=2.8 cm 2 ), and V 0 and V i are the volumes of the receiver solution and the sample, respectively.
The cumulative amount (Q n ) of PG permeated was plotted as a function of time (t) for each formulation. The permeation rate of PG at a steady-state (J ss , µg/cm -2 /h -1 ) through the mouse skin was calculated from the slope of linear portion of the plots of Qnversus time. The permeability coefficient (K p , cm/h) was calculated according to the following formula: Where K p is the permeability coefficient, J ss is the flux calculated at steady-state, and C 0 represents the drug concentration which remained constant in the vehicle.
Skin retention study. The following sections detail the comparison of the ex vivo permeation behavior, cumulative amount, permeation rate, permeability coefficient and skin retention characteristics between optimized ME formulations (PG content, 2%, w/w) and the aqueous solution.
At the end of the permeation experiment, the residual drug level in the skin was determined using a homogenization method. Following washing with normal saline, the skin was cut into small sections and placed in a glass tube containing 3 ml methanol in an ice bath. The sample was homogenized with a speed of 12000 r/min. rpm for 2 min, and then centrifuged at 12,830 x g horizontally for 30 min at 25˚C. The supernatant was passed through a microporous filter (0.45 µm), and the PG levels in the supernatant were determined by UV spectroscopy at a wavelength of 333 nm.
Stability study. The physical stability of PG-loaded ME was investigated via clarity, particle size analysis and phase separation, which was observed at 2-8˚C and at room temperature for 3 months. Centrifugation was performed at 12,830 x g for 30 min at 25˚C to assess the physical stability of ME. Phase separation and concentration of PG were investigated monthly to judge the optimal storage temperature. Chemical stability was evaluated on drug loaded formulations, stored at 2-8˚C and at room temperature, with the PG content determined by UV spectrophotometry at 333 nm.
Skin sensitivity test. Skin sensitivity tests were performed on rabbits (weighing 2.02.5 kg) to evaluate the irritant potential of topical application of the developed formulation as previously described (13). Nine Holland rabbits (16 weeks old; weighing 2.0-2.5 kg; 5 male and 4 female) were obtained from the Experimental Animal Center of Xinjiang Medical University. The animals were subjected to adaptive feeding for 1 week on 12 h light/12 h dark cycle, at room temperature (24±2˚C), with a humidity of 50±15%. All animals had access to a standard diet and water. Air embolism was used for the sacrifice. The protocols used in the present study were approved by the Ethic Committee of Xinjiang Medical University (Xinjiang, China). The hairs on the back were removed ~24 h prior to administration of the ME formulations. Animals were divided into the following groups: An optimized ME group (n=3), with topical administration of optimized ME containing 2% (w/w) PG; a PG saline group (n=3), with topical administration of saline solution containing 2% (w/w) PG; and control saline group (n=3), subject to topical administration of normal saline. The administration was performed twice a day for 5 days. The animals were observed for any signs of itching or changes in the skin, including erythema, papule, flakiness and dryness (14). Following drug withdrawal, observation for single or multiple administrations was continued for 3 days. The irritation scores of the test area were evaluated by judging the extent of erythema and edema according to the following criteria: 1, no visible reaction; 2, a slight reaction; 3, a moderate reaction and 4, a severe reaction (15,16). Finally, the total irritation scores for each condition were calculated using the following equation: Averaged irritation scores = (erythema reaction + edema reaction) / number of animals Statistical analysis. Data are presented as the mean ± standard deviation. Data analysis was performed using one-way analysis of variance followed by the least significant difference test. Data were analyzed using SPSS software version 19.0 (IBM SPSS, Armonk, NY, USA). P<0.05 was considered to indicate a statistically significant difference.

Results and Discussion
Screening of oils, surfactants and co-surfactants for ME. PG was almost insoluble in liquid paraffin and oleic acid, but moderate solubility was observed in IPM (Table II). IPM increases the diffusion coefficient in skin and results in the increase of the permeation coefficient (17,18). In addition, IPM is commonly selected for the preparation of MEs due to its well-known permeation enhancing properties and biocompatibility. Surfactant selection is critical for the successful formulation of MEs as it contributes to the reduction of interfacial tension by forming a film at the oil-water interface (19). In the present study, PG displayed a higher solubility in Cremophor-EL (40.193 mg/ml; Table II) compared with Tween-80 (28.051 mg/ml; Table II). Therefore, CremophorEL was selected for use in further studies due to its solubility profile and its low toxicity level as a non-ionic surfactant. Furthermore, its aqueous solution demonstrated good solubility of PG. The highest solubility of PG among the cosurfactants was observed in propylene glycol (48.714 mg/ml; Table II). In addition, propylene glycol has the ability to form MEs with IPM and CremophorEL. Taken together, IPM, CremophorEL and propylene glycol were used as the oil phase, surfactant and cosurfactant, respectively, for the formulation of PG-loaded MEs.
Construction of pseudo-ternary phase diagrams. In the present study, phase diagrams were constructed to select the concentration range of components for the microemulsions, and the marked areas indicate the clear O/WMEregion (Fig. 1). A close relationship between the hydration effect of the stratum corneum and the dermal permeation has previously been reported (20), and the thermodynamic activity of drugs in MEs was a significant driving force behind the release and penetration of drugs into the skin (20,21). The thermodynamic activity of drugs in ME with lower surfactant mixture content was demonstrated to be an important driving force for the release and the penetration of the drug into skin (22,23). The decreased concentration of surfactant in dispersed systems may also increase the rate of drug release and its permeation in the skin (24,25). Thus, the water content ranged between 50 and 70%, while that of the surfactant and cosurfactant ranged between 20 and 45%. The microemulsion domain was determined by visual inspection for clarity and fluidity. The rest of the region in the phase diagrams represents turbid and conventional emulsions based on visual observation (Fig. 1). No liquid crystalline structure was observed using the cross polarizer.
The area of the O/W ME region was expanded following the increase of the Km value (Fig. 1). These findings were consistent with previous studies, which demonstrated that a higher level of surfactant reduces interfacial tension of the colloidal solution more effectively, thereby improving the fluidity of the interface and increasing the entropy of the system (21,24). Thus, it is reasonable to speculate that the effect of CremophorEL on ME regions depends on the other ME components, especially the cosurfactant. The combination of short to medium carbon chain length alcohols, including propylene glycol, with single chain surfactants may lower interfacial tension due to increased fluidity at the interface. Additionally, medium carbon chain length alcohols may induce elevation of miscibility of the aqueous and oily phases due to the partitioning behavior between the two phases. Finally, in the presence of a Km ratio of 3:1, the ratio of the surfactant mixture to oil was 1:9 or 2:8. MEs prepared with a ratio of surfactant mixture to oil >2:8 were turbid and instable (Fig. 2). The final compositions of ME1-5 are listed in Table I.  Table III. The disperse systems were macroscopically identical, homogeneous, and transparent, without any precipitates. Mean droplet sizes of these MEs ranged between 30.56 and 79.21 nm (Fig. 3; Table III), with polydispersity-index values ranging between 0.187 and 0.321 (Table III). This indicated a uniform ME with a narrow size distribution. In addition, droplet size increased in proportion with the oil content, but decreased with surfactant mixture content. This phenomenon may be attributed to the expansion of the ME oil core, and the high surfactant concentration forcefully reducing the oil-water interfacial tension to reduce droplet size. Furthermore, ME viscosity was elevated as a result of increased surfactant level and decreased water. The pH values of all MEs fell within the appropriate range of 5.0-7.5 (Table III) and so were compatible with that of the skin tissues. Regarding physical stability, no phase separation, breaking or drug precipitation were observed in any of the tested MEs, indicating that the MEs possessed satisfactory thermodynamic stability against centrifugation. The dye solubility test revealed that a water-soluble dye (methyl orange) distributed uniformly throughout the ME system, which implied that the formed ME was of O/W type (data not shown). The micrographs revealed separate single ME5 droplets with a spherical outline (Fig. 4).
Exvivo skin permeation study. The permeation profiles of all the tested vehicles followed zero-order release kinetics (26,27) and ME5 provided the highest transdermal flux of PG (86.795±0.23 µg/cm -2 /h -1 ; Fig. 5; Table III). In the present study, the effects of oil content and surfactant mixture on the skin permeation of PG were determined. The results indicated that surfactants enhance skin permeation by increasing membrane fluidity, drug solubilization, and the extraction of lipid from the stratum corneum. The permeation rate of ME5 (86.795±0.23 µg/cm -2 /h -1 ) was significantly increased compared with the permeation rates of ME1 (69.57±0.78, P=0.0094), ME2 (44.326±0.84, P=0.0052), ME3 (79.859±0.12, P=0.039) and ME4 (58.332±0.39 µg/cm -2 /h -1 , P=0.0063, Fig. 5; Table III) This was consistent with the results of a previous study, in which the thermodynamic activity of the ME drug cargo may decrease with the increase of surfactant levels (12,28 ME1  4  33  11  52  1  ME2  3  27  9  61  1  ME3  5  30  10  55  1  ME4  5  24  8  63  1  ME5  7  21  7  65  1   ME, microemulsion; IPM, isopropyl myristate. for the drug molecules, and resulted in a faster release with a diffusion controlled release mechanism (29). The permeation enhancing effect of IPM may increase the diffusion coefficient of PG-loaded ME in skin tissues, which may result in the increased permeation coefficient (30). Furthermore, the water content of MEs may enhance permeation as hydration of the stratum corneum contributes to the development and widening of channels in the keratin layer and the distortion of the lipid bilayer (24). In the present study, the water content in ME5 (65%) was higher than in the other MEs (Table I), implying that ME5 might induce a higher permeation rate. In addition, the droplet size might affect the permeation profiles, whereby   Table III. Characteristics of ME formulation 1-5 and the skin permeation parameters of phenylethanoid glycoside when delivered by these formulations.
Comparisons between the optimized ME and the PG aqueous solution are displayed in Table IV. A 1.68-fold increase in the skin permeation rate of PG was observed in the optimized ME group compared with the PG saline group (Fig. 7). The cumulative amount of PG in the optimized ME group (4149.650±37.3 µg·cm -2 ) was significantly increased compared Figure 5. In vitro skin permeation profiles of PG from ME formulations 1-5. PG loading was 1% (w/w). PG, phenylethanoid glycoside; ME, microemulsion.       Table IV], suggesting that optimized ME is a more effective carrier of PG than saline. According to previous reports (16,32), high concentration (2%) of PG in in MEs resulted in a high concentration gradient, which contributed to the permeation of PG into the skin. MEs may act as drug reservoirs, with the drug being released from the inner phase to the outer phase and then onto the skin. Furthermore, the MEs may affect the stratum corneum structure and reduce the diffusional barrier by acting as a permeation enhancer (26,33).
Skin retention study. The optimized ME in the present study demonstrated a higher drug deposition capacity than the PG aqueous solution (174.07±1.51 µg·cm -2 vs. 93.25±1.49 µg·cm -2 ; P=0.029, Table IV). This maybe associated with the faster release of PG from the optimized ME, resulting in elevated drug penetration into the skin, or to the higher solubility of the optimized ME compared with the PG saline solution. Due to this effect, an increased amount of PG is retained in the skin. The optimized ME5 formulation demonstrated higher skin retention than the PG saline solution (30). The optimized ME was also superior to the PG saline solution the terms of skin permeation profiles and skin retention (Table IV). Taken together, the optimized ME delivered PG more effectively than the PG saline solution.
Stability study. Stability studies were performed to detect any changes in pH, droplet size, and drug content. Optimized ME was physically stable (16,29) retaining homogeneity and demonstrating no phase separation after 3 months. No major changes were observed in the droplet size and degradation of PG within 3 months. The centrifuge tests demonstrated that optimized ME is physically stable. No significant changes were recorded in the stored ME, but there was a slight decrease in viscosity. This decrease may be due to loss of water during storage. Three months later, no difference was noticed in the pH, droplet size, drug content and viscosity between ME stored at 2-8˚C and at room temperature (Table V).
Skin sensitivity test. The intensity criterion of skin irritation followed the protocol, and scores of <0.5 meant no irritation, 0.5-3 slight irritation, 3-6 moderate irritation, and >6 severe irritation. Little visible irritation was observed in animals treated with optimized ME. The average response scores of skin irritation for saline solution with 2% (w/w) PG were 0.1±0.05, 0.1±0.13, and 0.1±0.07 at 24, 48 and 72 h, respectively. For the optimized ME, the scores were 0.28±0.02, 0.16±0.11 and 0.15±0.08 at 24, 48 and 72 h, respectively. Despite a slight increase in the scores, optimized ME did not appear to irritate the skin. For these reasons, PG treatment with optimized ME appears to be safe. A novel ME was prepared using IPM as the oil phase, CremophorEL as the surfactant and propyleneglycolas the cosurfactant. This formulation was characterized by a transparent appearance, low viscosity, a spherically uniform distribution, stable physical and chemical properties, as well as good stability. Compared with saline solution, a 1.68-fold increase was noted in the skin permeation rate of PG for optimized microemulsion. The cumulative amount of PG in the microemulsion (4149.650±37.3 µg·cm -2 ) was significantly higher than that of PG in the saline solution (2288.63±20.9 µg·cm -2 ). Furthermore, the permeability coefficient indicated optimized microemulsion was a more efficient carrier for transdermal delivery of PG than the control solution (8.87±0.49 cm/hx10 -3 vs. 5.41±0.12 cm/hx10 -3 ), the skin permeation ability of PG was significantly increased by this ME, which may be due to the special characteristics of MEs.


s2fieldsofstudy:
Chemistry
Materials Science

text:
Interface modi ﬁ cation with PCBM intermediate layers for planar formamidinium perovskite solar cells †

Although two-step methods are convenient for producing perovskite layers in mesoporous perovskite solar cells, they usually lead to intensive recombination for planar perovskite devices based on formamidinium ions. In this study, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was employed as the intermediate layer between TiO 2 electron transport layers and formamidinium perovskite layers. SEM images indicate that this PCBM intermediate layers encourage the growth of grains in formamidinium perovskite layers. XRD patterns show that these PCBM layers eliminate the yellow d -phase, thereby improving crystallization. Via optimizing the concentration of PCBM precursor solutions, the average power conversion e ﬃ ciency of formamidinium planar devices was promoted from 6.8% to 15.1%. Further, it is also noted that devices with optimized PCBM layers are free from the hysteresis e ﬀ ect. Impedance spectra indicate that this improvement comes from the suppression of recombination processes by the PCBM intermediate layers.

Introduction
Organic lead halides have become promising photovoltaic materials due to their broad light absorption, high charge mobility and solution processability. [1][2][3][4] These kinds of materials have a perovskite structure of AMX 3 , where small organic ammonium cations occupy the A sites, Pb 2 + ions occupy the M sites, and halide ions occupy the X sites. 5 Except the widely investigated perovskite materials with methylammonium ions (CH 3 NH 3 + ) at A sites, perovskite materials with formamidinium ions (HC(NH 2 ) 2 + ) become attractive due to their relatively small band gap, which facilities the utilization of solar energy. [6][7][8][9] In conventional perovskite solar cells, mesoporous layers are oen used as scaffolds of perovskite layers, which improve lm formation and facilitate charge collection. 9,10 Recent investigations have indicated that efficient planar perovskite solar cells can be fabricated without these mesoporous layers. 7,11 The planar junction device is attractive for its low hysteresis and simplied device structure, but it needs compact perovskite layers with smooth surfaces. 12,13 High-quality perovskite layers with formamidinium ions (FA + ) are difficult for deposition because they need relatively higher process temperatures that easily translates into the yellow d-phase. [14][15][16] Perovskite layers can be produced by the spin-coating of mixed precursor solutions, termed as a one-step method. This kind of method can produce compact perovskite layers with smooth surfaces by employing an anti-solvent treatment, and several efficient planar solar cells with formamidinium ions have been reported. 17,18 On the other hand, perovskite layers can also be obtained by the reaction between PbI 2 lms and CH 3 NH 3 I (or HC(NH 2 ) 2 I) precursors, termed as a two-step method. Two-step methods are convenient for fabricating mesoporous perovskite solar cells, 19,20 but planar devices usually have relative low reproducibility and efficiency. 21 For two-step methods, pin hole defects and unreacted PbI 2 oen exist in the perovskite layers, which can lead to an intensive recombination process.
Interface modication with intermediate layers is an effective method for improving lm deposition and suppressing the recombination process. [22][23][24] Fullerene derivatives are a kind of extensively used acceptor material, which can be used as an intermediate layer for electron transport. Li et al. developed a triblock fullerene derivative for passivating the TiO 2 surface and reducing the recombination process in CH 3 NH 3 PbI 3 layers. 25 [6,6]-Phenyl-C61-butyric acid is also used as an intermediate layer that eliminates the adsorbed solvent molecules and promotes the charge transfer process at the TiO 2 /CH 3 -NH 3 PbI 3 interfaces. 26,27 Zhou et al. found that the electron transport bilayers of TiO 2 and fullerene dramatically improved the electron injection from CH 3 NH 3 PbI 3 to TiO 2 . 28 CH 3 NH 3 PbI 3 is also deposited on [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-modied TiO 2 electron transport layers by the vacuum evaporation method, which improves charge extraction and passivates defects. 29,30 As perovskite layers with formamidinium ions have similar properties with the ones with methylammonium ions, these previous reports have motivated us to employ fullerene derivatives as intermediate layers for planar formamidinium perovskite solar cells.
In this study, planar formamidinium perovskite solar cells are fabricated by solution-based two-step methods, and PCBM is used to introduce intermediate layers at TiO 2 /perovskite interfaces. By optimizing these intermediate layers, recombination processes are intensively suppressed, yielding efficient planar devices.

Device fabrication
Fluorine-doped tin oxide (FTO, 7 U sq À1 , NSG) was etched by using a 1064 nm laser and cleaned by acetone, ethanol, and deionized water for 10 min in an ultrasonic bath. Compact TiO 2 layers were prepared by spraying 2.0 mL 0.05 M Ti(OPr) 2 (acetylacetonate) 2 in an ethanol solution on FTO substrates at 450 C. PCBM layers were deposited on TiO 2 layers by spin-coating PCBM/chlorobenzene solutions at 4000 rpm for 30 s. Further, the concentrations of PCBM solutions are 10 mg mL À1 , 20 mg mL À1 , 30 mg mL À1 , and 40 mg mL À1 , which are labeled as PCBM10, PCBM20, PCBM30, and PCBM40, respectively. The samples were dried at 80 C for 5 min on a hot plate. Further, 462 mg mL À1 PbI 2 in N,N-dimethylformamide (DMF) solution doped with 26 mg mL À1 CsI was stirred for 10 h at 70 C. 31 The precursor solution was spin-coated on the samples at 4000 rpm for 30 s, followed by annealing at 160 C for 5 min. To produce FA 0.9 Cs 0.1 PbI 3 perovskite layers, 50 mL FAI in isopropanol (50 mg mL À1 ) solution was sprayed on the top of CsI-doped PbI 2 layers for the interface reaction. Aer cooling to room temperature, the samples were washed with isopropanol and annealed at 160 C for 10 min. Hole conductor layers were prepared by spin-coating a solution containing 72.3 mg spiro-MeOTAD, 28.8 mL 4-tert-butylphyridine, and 17.5 mL lithium bis(triuoromethanesulfonyl)imide solution (520 mg mL À1 in acetonitrile) in 1 mL chlorobenzene at 4000 rpm for 20 s. Finally, gold electrodes were deposited on the HTM layers by thermal evaporation.

Characterization
Scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) were investigated by a FEI-MAGELLAN 400 scanning electron microscope. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max-2550 X-ray diffractometer. The UV-vis absorption spectra were recorded with a L5S UV-vis spectrophotometer in the 450-900 nm wavelength range. Photocurrent density-photovoltage characteristics were recorded from 1.15 V to 0 V by a CHI660 electrochemical workstation. The active area of the solar cells is 0.15 cm 2 , which is dened by a mask. The AM1.5 illumination was provided by a 3A class solar simulator (UHE-16, ScienceTech Inc.), which was calibrated to one sun by a KG5-ltered Si reference solar cell (certicated by VLSI Standards Inc., traceable to National Renewable Energy Laboratory). Photoluminescence (PL) spectra were detected by a CCD detector (PIXIS256BR, Princeton Instruments Inc.), and the excitation wavelength was 532 nm provided by a low-noise solid-state laser (MLL-III-532). Impedance spectra were measured under constant illumination condition over a frequency range from 1 MHz to 1 Hz by a CHI660 electrochemical workstation. The applied bias potential was 0.7 V and the constant illumination was provided by a white LED lamp array.

Results and discussion
The device structure of a planar formamidinium perovskite solar cell is shown in Fig. 1a, which is in accordance with the cross-sectional SEM image shown in Fig. 1b. The light-harvester layer is composed of large perovskite grains with a compact stacking structure, and a single perovskite grain contacts the hole and electron transport layers. We study the effect of PCBM intermediate layers on the morphology of formamidinium perovskite layers by means of SEM measurements. Fig. 3a shows the morphology of the TiO 2 electron transport layers as a reference. Aer depositing formamidinium perovskite without any intermediate layers, a compact perovskite layer is formed with an average grain size of 108 nm (Fig. 3b). By inserting PCBM intermediate layers at the TiO 2 /perovskite interfaces, a remarkable increase can be observed on the average grain size of formamidinium perovskite layers, namely, 206 nm, 230 nm, 302 nm, and 316 nm for the PCBM10, PCBM20, PCBM30 and PCBM40 intermediate layers ( Fig. 3c-f), respectively. Therefore, PCBM intermediate layers facilitate the growth of formamidinium perovskite grains. Fig. 4a shows the XRD patterns of formamidinium perovskite layers on different PCBM intermediate layers. Except three diffraction peaks from the FTO substrates, all the main diffraction peaks can be assigned to the trigonal crystal structure of formamidinium perovskite materials, which implies the successful deposition of a-phase formamidinium perovskite layers. [32][33][34] It is also noted that a small diffraction peak at about 22.5 exists in the XRD pattern of the samples without PCBM intermediate layers. This peak can be assigned to the diffraction of the (002) planes in the yellow d-phase of formamidinium perovskite materials. 35,36 As the photovoltaic performance of the yellow d-phase is much lower than that of the black a-phase, the yellow d-phase is believed to be harmful to the solar-to-electric conversion process. It is also found that the diffraction peak of the yellow d- As the PCBM intermediate layers change the surface of the substrates, the spin-coated PbI 2 layers are subsequently modi-ed, which can affect the formation of perovskite layers. The formation of yellow d-phase comes from the low stability of the black a-phase in formamidinium lead iodide materials, which is related to the large tolerance factor of their perovskite crystal structure. The SEM and XRD measurements ( Fig. 3 and 4, respectively) indicate that the PCBM intermediate layers can encourage the crystallization and growth of perovskite layers with the a-phase structure. It implies that the presence of PCBM facilities the formation of the black a-phase and improve its stability. We think that this is the possible reason for restraining the formation of the yellow d-phase by the PCBM intermediate layers. The main interaction between PCBM and formamidinium perovskite is located at the vicinity of the TiO 2 / perovskite interface. Moreover, previous literatures have reported that PCBM can partially diffuse into the perovskite   layers. 42 Therefore, the interaction between PCBM and formamidinium perovskite can extend into the perovskite layers. Fig. 5a shows the typical J-V curves of devices with different intermediate layers under one sun condition, and photovoltaic parameters are listed in Table S1. † The device without a PCBM intermediate layer shows poor performance with the power conversion efficiency (PCE) of 6.6%, open-circuit voltage (V OC ) of 1.08 V, short-circuit current density (J SC ) of 12.3 mA cm À2 , and ll factor (FF) of 0.497. By inserting the PCBM10 intermediate layer, J SC and FF are remarkably increased to 16.7 mA cm À2 and 0.603, respectively. It is noted that the performance improvement is dependent on the concentration of PCBM precursor solutions. For the intermediate layers of PCBM20 and PCBM30, J SC and PCE of the devices increase with the increase of the PCBM concentration. Further increasing the concentration of PCBM solutions to 40 mg mL À1 , J SC and PCE of the devices decrease to 18.1 mA cm À2 and 12.2%, respectively. For the optimized intermediate layer of PCBM30, the corresponding device has a PCE of 15.5% with J SC of 22.7 mA cm À2 , photovoltage of 1.05 V, and FF of 0.653. Therefore, intermediate layers, which are fabricated by 30 mg mL À1 PCBM solution, can remarkably improve the performance of planar devices with formamidinium perovskite materials as the harvester. The improved performance of devices with PCBM30 layers is in accordance with the improvement of crystallization and elimination of the yellow d-phase as evident from the SEM and XRD measurements. Fig. 5b Fig. 5c, the mesoporous device gives remarkably different J-V curves between forward scan and reverse scan, while the planar device with PCBM30 intermediate layer has little difference on the J-V curves with opposite scan directions. Therefore, planar devices with PCBM30 intermediate layers eliminate the hysteresis effect, which usually exists in mesoporous perovskite solar cells. 43 PL spectrum is a facile tool for characterizing the charge transfer at TiO 2 /perovskite interfaces. 42,44 Fig. 6 shows the PL spectra of formamidinium perovskite layers on TiO 2 -coated substrates without and with different PCBM intermediate layers, and the formamidinium perovskite layer on the glass substrate without a TiO 2 layer is used as the reference. For the sample without intermediate layers, the intensity of the PL peak quenches to 7.8% of the intensity of the reference sample, which reects the intensive extraction of photoelectrons from perovskite layers to TiO 2 layers. By inserting PCBM10 intermediate layers between TiO 2 layers and perovskite layers, the intensity of PL peaks increases to 13.8% of the intensity of the reference sample, and the quenching effect of the PL peak slightly attenuates. By increasing the concentration of PCBM solutions, the quenching effect of the PL peak continuously decreases. This result means that PCBM intermediate layers cannot improve the charge transfer process at the interface between the TiO 2 layers and formamidinium perovskite layers.
To reveal the function of PCBM intermediate layers in formamidinium perovskite solar cells, we perform impedance measurements at 0.7 V bias potential under constant light illumination. Fig. 7 shows the Nyquist plots of the devices without and with different PCBM intermediate layers. In the Nyquist plots, two impedance arches can be distinguished. As discussed in previous reports, the high-frequency arch can be attributed to the charge transfer process at the TiO 2 /perovskite interface, and the low-frequency arch is attributable to the recombination process in the perovskite layers. 45,46 By tting the impedance data with an equivalent circuit (Fig. 7), we can extract the charge transfer resistance (R ct ) and recombination resistance (R rec ), which are listed in Table S2. † R ct of the device without an intermediate layer is 3.7 U cm 2 , and it increases aer inserting PCBM intermediate layers in the devices. For PCBM10, PCBM20, and PCBM30 intermediate layers, R ct gradually increases from 22.0 U cm 2 to 29.6 U cm 2 and 40.0 U cm 2 , respectively. It is noted that R ct remarkably increases to 706.4 U cm 2 for the device with the PCBM40 intermediate layer. This increase of R ct reects the slow charge transfer process at the TiO 2 /perovskite interface. It means that PCBM intermediate layers cannot promote the charge transfer process, and PCBM40 layer intensively blocks the charge transfer process. This result is in accordance with the analysis of the PL spectra (Fig. 6). On the other hand, R rec of the device without an intermediate layer is 40.3 U cm 2 , which increases aer the PCBM intermediate layers are inserted. For PCBM10, PCBM20, and PCBM30, R rec remarkably increases up to 631.7 U cm 2 , 839.2 U cm 2 , and 1236.3 U cm 2 , respectively. Aer further increasing the concentration of the precursor solution to 40 mg mL À1 , R rec of the devices slightly decreases to 1215.4 U cm 2 . As large R rec is related with a slow recombination process in the perovskite layers, PCBM layers remarkably reduce the recombination process. This is in accordance with the promotion of crystallization and elimination of the yellow d-phase in the SEM and XRD measurements by inserting PCBM intermediate layers.

Conclusions
In summary, we have fabricated efficient planar junction solar cells with formamidinium perovskite as the light harvester by a two-step method, and the functions of PCBM intermediate layers are investigated. The SEM images indicate that PCBM intermediate layers improve the growth of perovskite grains, and the XRD patterns show that PCBM layers improve the crystallization and eliminate the yellow d-phase in formamidinium perovskite layers. It is found that the function of PCBM intermediate layers is dependent on the concentration of precursor solutions. By using optimized PCBM intermediate layers, the average PCE of planar devices can be increased from 6.8% to 15.1%, and the J-V curves of optimized devices show a marginal hysteresis effect. The impedance and PL measurements indicate that PCBM layers remarkably reduce the recombination process in formamidinium perovskite layers, but thick PCBM layers block the charge transfer process at TiO 2 / perovskite interfaces. The optimized condition comes from the balance between these two effects.


Insights into heterogeneous catalytic activation of peroxymonosulfate by Pd/g-C3N4_ The role of superoxide radical and singlet oxygen

Pd nanoparticles anchored on graphitic nitride carbon (Pd/g-C3N4) hybrid catalytic system was prepared by a simple KBH4 reduction method. Pd/g-C3N4 was very active in the degradation of bisphenol A (BPA) by efficiently activating peroxymonosulfate (PMS). Important influencing factors, such as catalyst dosage, PMS concentration, pH and reaction temperature for the BPA removal were investigated. Pd/g-C3N4-PMS demonstrated a wide effective pH range, namely 3.0–9.0. Radical quenching tests and electron spin response (ESR) analyses revealed that superoxide radical (O2%) and singlet oxygen (O2) instead of sulfate radical (SO4%) and hydroxyl radicals (%OH) were the major oxidized species for the degradation of BPA.

Generally, SO 4 % − and/or %OH generated from PMS/PDS activation are responsible for the degradation of organics. However, it was found that the inhibitory effect of ethanol/methanol (scavenger for both %OH and SO 4 % − ) on the degradation of organics in heterogeneous activation of PMS/PDS was insignificant [14][15][16][17]. In these reports, SO 4 % − and/or %OH were not assigned to be the major oxidized species for the degradation of organics. Instead, non-radical oxidation mechanisms were proposed. For example, Zhang et al. [17] suggested that the outer-sphere interaction between the CuO and PDS caused the electron rearrangement of PDS molecular, which subsequently increased the oxidizing power of PDS. Similar phenomena were also observed in the PDS activation by carbon nanotubes (CNTs) [16] and graphitized nanodiamond [18], in which process reactive complexes were formed due to the interplay of PDS and CNTs, where immediately decomposed upon reaction with the organic compounds. Ahn et al. [19] found that noble metals, such as Pt, Pd, Au and Ag also possess catalytic activity for the activation of PMS. The latter was believed to act as an oxidant that directly accepts electrons from the organic compound with the help of noble metal, and no radical is formed during reaction. Feng et al. [20] claimed that degradation of 1,4-dioxane is due to the surface-bound sulfate radical, which is generated from the activation of PMS on alumina-supported palladium (Pd/Al 2 O 3 ). Therefore, more efforts still need to be made for further understanding the intrinsic catalytic mechanism of PMS activation by Pd.
Graphitic carbon nitride (g-C 3 N 4 ) with characteristic N-coordinating cavities has attracted increasing interest in the field of material and catalytic science [21]. It might be facilely produced at a large scale with low cost. In this work, g-C 3 N 4 was prepared via one-step polymerization of urea, where Pd nanoparticles were anchored into the cavities of g-C 3 N 4 to form Pd nanoparticles modified g-C 3 N 4 hybrid system (Pd/g-C 3 N 4 ). The latter was used as heterogeneous catalyst for the activation of PMS. Bisphenol A (BPA), an endocrine disrupting compound, is released from landfill leachates, paper and plastics plants [22] selected as a target pollutant to evaluate the catalytic activity of Pd/g-C 3 N 4 . Compared with the current technologies (e.g., photo-Fenton [23], electro-Fenton [24] and photocatalysis [25]) for the degradation of BPA, Pd/g-C 3 N 4 -PMS system requires no energy input and leads to no secondary pollution. Important influencing factors for the BPA removal, such as PMS concentration, catalyst dosage, pH value and reaction temperature were investigated. A plausible mechanism was proposed based on X-ray photoelectron spectroscopy (XPS) and electron spin response (ESR) studies and radical quenching tests.

Catalyst preparation
The g-C 3 N 4 support material was prepared by heating urea at 600°C with a ramp rate of 5°C/min for 2 h in a closed alumina crucible placed in a muffle furnace. 2.5 g prepared g-C 3 N 4 were dispersed in 250 mL deionized water and magnetically stirred in an ultrasonic bath for 1 h. Subsequently, 11.8 mL of a solution of PdCl 2 (0.01 M) and NaCl (0.04 M) were added, and the resulting suspension was further stirred in the ultrasonic bath for 1 h. Then, freshly prepared aqueous solution of KBH 4 (3.5 mL, 0.5 M) was added to this slurry and stirred for 24 h. The obtained Pd/g-C 3 N 4 catalyst was separated by vacuum filtration, washed with deionized water, and vacuum dried overnight at 50°C.

Catalyst characterization
Detailed information of chemicals and reagents, catalyst characterization studies, catalytic activity tests and analytical methods are provided in Supporting Information.

Characterization of Pd/g-C 3 N 4
Pd/g-C 3 N 4 shows a similar X-ray diffraction (XRD) pattern with the pristine g-C 3 N 4 (Fig. S1), suggesting that the crystal structure of g-C 3 N 4 is not changed after the introduction of Pd nanoparticles. No diffraction peaks of Pd nanoparticles are observed in the XRD pattern of Pd/g-C 3 N 4 , which might be the result of Pd content below the detection limit of XRD and/or the peak broadening from the small size of Pd nanoparticles formed. Scanning electron microscope (SEM) images of g-C 3 N 4 and Pd/g-C 3 N 4 are shown in Fig. S2. It can be seen that Pd/g-C 3 N 4 exhibits a layered structure and no obvious changes are observed compared with pristine g-C 3 N 4 . The bright regions in the scanning transmission electron microscope (STEM) image of Pd/g-C 3 N 4 (Fig. 1a) point out the presence of Pd nanoparticles, which are highly dispersed on g-C 3 N 4 with estimated size in the 2-4 nm range (inset in Fig. 1a).
The N 2 adsorption-desorption isotherms of g-C 3 N 4 and Pd/g-C 3 N 4 reveal that all the samples exhibit typical IV isotherms with H3-type hysteresis loop in the relative pressure range of 0.8-1.0 (Fig. S3). This indicates the existence of mesoporous structure originated from the stacking of g-C 3 N 4 nanosheets. The BET specific surface area of g-C 3 N 4 and Pd/g-C 3 N 4 are 65 and 59 m 2 ·g − 1 , respectively, indicating that the existence of Pd nanoparticles only little might affect the structure of g-C 3 N 4 . The pore size distribution curves present two peaks at about 2 and 30 nm, which might be beneficial to mass transfer effects during the catalytic process.
The surface composition and chemical oxidation states of Pd/g-C 3 N 4 were determined by XPS (Fig. S4). The elements of C, N, O and Pd are observed in the survey spectrum. The N 1s spectrum was fitted into four peaks at 397.5, 398.3, 399.9 and 403.4 eV, which can be assigned to the sp 2 -hybridized nitrogen (CeN]C), tertiary nitrogen (Ne(C) 3 ), amino functional groups (CeNeH) and the charging effects, respectively [26]. The XPS spectra of Pd 3d before and after reaction are presented in Fig. 1b, in which the peaks at 342.6 and 337.2 eV are assigned to Pd II , while the peaks at 340.6 and 335.3 eV to Pd 0 [27]. After the degradation reaction, the Pd 0 /Pd II ratio decreases from 2.02 to 1.19, illustrating that part of Pd 0 changed to Pd II during the PMS activation catalytic process.

The catalytic performance of Pd/g-C 3 N 4
The adsorption ability of Pd/g-C 3 N 4 toward BPA was investigated first and the removal of BPA by adsorption was found to be insignificant (Fig. 2a). Meanwhile, the degradation of BPA by PMS was negligible due to the low oxidative capacity of PMS. The g-C 3 N 4 presented poor catalytic activity for activation of PMS, which is consistent with the previous report [28]. However, degradation efficiency of BPA was achieved using the Pd/g-C 3 N 4 -PMS catalytic system. It was found that 91% of BPA (20 mg·L − 1 ) can be removed in 60 min with 0.1 g·L − 1 of catalyst loading and 1.0 mM of PMS, which indicates the good catalytic performance of Pd/g-C 3 N 4 for the activation of PMS.
To comprehensively evaluate the catalytic performance of Pd/g-C 3 N 4 -PMS, the influence of PMS concentration, catalyst loading, pH value and reaction temperature were systematically investigated. As shown in Fig.  S5a, the removal efficiency of BPA increases from 74% to 91% in 60 min when the PMS concentration increases from 0.2 to 1.0 mM. However, only 5% enhancement is achieved when PMS concentration further increased to 1.5 and 2.0 mM. It is inferred that the active sites were almost occupied by PMS (1.0 mM). Fig. S5b indicates that catalyst loading has a great influence on the BPA removal rate. The degradation rate constant of BPA increases from 0.038 to 0.463 min − 1 with catalyst loading increasing from 0.1 to 0.4 g·L − 1 . One reasonable explanation might be that more active sites are present for the activation of PMS at higher catalyst loadings. To investigate the influence of pH on the BPA removal rate, the pH of the suspension was maintained at a certain pH value ( ± 0.2) after adjustment with 0.1 M H 2 SO 4 and 0.1 M NaOH at all times. As illustrated in Fig. S5c, the catalytic ability of Pd/g-C 3 N 4 is greatly influenced by the pH of solution. With the pH increasing from 3.0 to 9.0, the degradation rate of BPA increases from 0.034 to 0.114 min − 1 . However, when the pH is further increased to 11.0, the degradation rate of BPA decreases to 0.023 min − 1 . The pKa value of PMS is 9.4 [29], thus the SO 5 2 − is the dominant species when the pH of solution is higher than 9.4. As reported elsewhere [30], the redox potential of SO 5 Fig. 2b. The degradation rate constants of BPA were estimated and found to be 0.0367, 0.0629 and 0.0969 min − 1 at 25, 35 and 45°C, respectively. According to the Van't Hoff's equation, the negative slope (inset in Fig. 2b) indicates that the catalytic reaction process is endothermic and the activation energy (E a ) for degradation of BPA was estimated to be 38.6 kJ·mol − 1 (R 2 = 0.989).

Radical quenching tests
To identify the reactive oxygen species (ROS) generated in the Pd/g-C 3 N 4 -PMS system, radical quenching tests were conducted at 0.1 g·L − 1 catalyst loading and 1 mM PMS concentration with different scavengers added at room temperature. Methanol (MeOH) can scavenge both %OH (k MeOH-%OH = 9.7 × 10 8 M − 1 ·s − 1 ) and SO 4 % − (k MeOH-SO4%− = 2.5 × 10 7 M − 1 ·s − 1 ), whereas tert-butyl alcohol (TBA) only scavenge %OH since it reacts with SO 4 % − over 1000-fold slower than with %OH (k TBA-%OH = 3.8-7.6 × 10 8 M − 1 ·s − 1 ; k TBA-SO4%− = 4.0-9.1 × 10 5 M − 1 ·s − 1 ). To guarantee complete quenching of free radicals, the degradation experiments of BPA with different molar ratios between scavenger and PMS were carried out (Fig. S6). The degradation of BPA is not significantly inhibited by TBA/MeOH even using 1000:1 molar ratio of TBA/MeOH:PMS (Fig. S7). Compared with the control experiment, the degradation rate constant of BPA decreased by 3% and 10% with TBA and MeOH as scavengers, respectively (Fig. 3). Therefore, it can be concluded that %OH and SO 4 % − are not the major oxidizing species for the degradation of BPA. Other ROS might be involved in the case of Pd/g-C 3 N 4 -PMS system. As shown in Fig. 3, the degradation rate of BPA is greatly reduced in the presence of p-benzoquinone (p-BQ), which is a scavenger of superoxide radical (O 2 % − ) with a rate constant of 0.9-1.0 × 10 9 M − 1 ·s − 1 [32]. The BPA degradation is almost completely quenched with the addition of L-histidine, which is a scavenger of singlet oxygen ( 1 O 2 ) [18]. These results demonstrate that O 2 % − and 1 O 2 play a key role for the degradation of BPA in the Pd/g-C 3 N 4 -PMS system. It can be concluded that O 2 % − and 1 O 2 instead of % OH and SO 4 % − are the major oxidizing species in the Pd/g-C 3 N 4 -PMS system and the experimental conditions applied.

Conclusions
Pd/g-C 3 N 4 was prepared via a simple KBH 4 reduction method and Pd nanoparticles (d~2-4 nm) were highly dispersed on g-C 3 N 4 . Pd/g-C 3 N 4 exhibited high heterogeneous catalytic activity for the activation of PMS. At the conditions of catalyst loading of 0.1 g·L − 1 and PMS concentration of 1.0 mM, 91% BPA with initial concentration of 20 mg·L − 1 can be degraded in 60 min. The activation energy for BPA oxidation in Pd/g-C 3 N 4 -PMS was found to be 38.6 kJ·mol − 1 . The ROSs involved in Pd/g-C 3 N 4 -PMS system were identified and the results showed that O 2 % − and 1 O 2 instead of SO 4 % − and %OH were the major oxidative species for the degradation of BPA.