Hollow spherical assembly polymer, carbon material as well as preparation method and application of hollow spherical assembly polymer and carbon material

Abstract

The invention discloses a hollow spherical assembly polymer, a carbon material and a preparation method and application of the hollow spherical assembly polymer and the carbon material, and belongs to the technical field of materials. The hollow spherical assembly polymer comprises a shell formed by assembling a plurality of small monomers and a cavity defined by the shell; the small monomers are polymer blocks with morning glory-shaped or spherical crown-shaped structures. By constructing an oil-in-water microemulsion template and adjusting the interaction type of a surfactant and a polymer precursor, a hollow spherical assembly material with various sizes and morphologies assembled by different monomers is controllably synthesized, so that the hollow spherical assembly material meets the application requirements of different fields, and the limitations of tedious steps, difficulty in morphology regulation and control and the like in the prior art are solved; meanwhile, one-pot synthesis is adopted in the method, secondary template removal is not needed, preparation is simple, and the prepared hollow spherical assembly polymer material and the hollow carbon material obtained by directly roasting the material have potential application in the fields of catalysis, biomedicine and the like.

Description

Technical Field

[0001] The present application relates to a hollow spherical assembly polymer, a carbon material, and a preparation method and application thereof, belonging to the technical field of materials. Background Art

[0002] Compared with traditional solid spherical materials, hollow spherical materials have many unique features. One of its main features is the cavity and nanopores separated by a shell-like structure, which provides optional loading positions for active sites, affects the kinetic process of the reaction, and also provides an adjustable microenvironment for reactants. Another remarkable feature of hollow spherical materials is their buffering effect, which can reduce the damage to the materials caused by external forces such as vibration and impact during the reaction process, improve the stability and reliability of the materials, and make them suitable for applications where mass diffusion is limited. In addition, hollow spherical materials provide a larger effective surface area, thus increasing the loading capacity of active substances. Due to these attractive features, hollow spherical materials show great potential in many application fields such as catalysis, adsorption, and sensing.

[0003] Among many hollow spherical materials, complex hollow structures, such as hollow spherical assemblies composed of different monomers, often exhibit unique and excellent properties completely different from those of the assembly units. Moreover, the surface of the hollow spherical assembly can be subjected to various chemical functionalizations, thus having a wide application prospect in catalysis, drug delivery, energy storage, etc.

[0004] Currently, the main method for synthesizing hollow spherical assemblies is to use materials such as graphene and carbon nanotubes as precursors and achieve the construction of spherical assemblies through modification and secondary assembly. Generally, these methods require the prior preparation of nano-carbon materials, such as carbon nanotubes and carbon nanosheets, and then secondary assembly through various methods to obtain a spherical structure; in addition, most methods require the use of organic solvents to effectively pre-disperse the nano-carbon materials; the prepared materials usually also need to undergo post-treatment processes such as etching. The processes of these methods are relatively cumbersome, and it is difficult to precisely control the morphology. Therefore, developing innovative synthesis strategies to controllably construct hollow spherical assemblies with diverse structures helps to enrich and improve the database of hollow structure materials and explore new application fields. Summary of the Invention

[0005] To solve the foregoing technical problems, the present application provides a technical solution for constructing a hollow spherical assembly polymer. By constructing an oil-in-water microemulsion template and adjusting the interaction type between the surfactant and the polymer precursor, a hollow spherical assembly material assembled from different monomers with diverse sizes and morphologies can be controllably synthesized to meet the application requirements in different fields. This technical solution solves the limitations in the prior art such as cumbersome steps and difficult morphology control. At the same time, this method is a one-pot synthesis without the need for secondary template removal, and the preparation is simple. The obtained hollow spherical assembly material has potential applications in fields such as catalysis and biomedicine.

[0006] The present application adopts the following technical solutions: According to the first aspect of the present application, there is provided a hollow spherical assembly polymer, including a shell assembled from a number of small monomers and a cavity surrounded by the shell; The small monomers are polymer blocks in the shape of morning glory or spherical crown.

[0007] Optionally, the radial dimension of the small monomer is 100~800 nm, and the axial length is 90~700 nm.

[0008] Optionally, the particle size of the hollow spherical assembly polymer is 0.4~5 μm.

[0009] Optionally, the particle size of the hollow spherical assembly material is any value selected from 0.4 μm, 1 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 5 μm or a range value between any two values.

[0010] Optionally, the diameter of the small monomer is any value selected from 100 nm, 200 nm, 700 nm, 800 nm or a range value between any two values.

[0011] Optionally, the longitudinal length of the small monomer is any value selected from 90 nm, 100 nm, 200 nm, 700 nm, 800 nm or a range value between any two values.

[0012] According to the second aspect of the present application, there is provided a method for preparing a hollow spherical assembly polymer, characterized by including the following steps: S1. Add an oil-phase solvent to a mixed solution containing a surfactant, a phenolic compound, and an aqueous-phase solvent, and perform ultrasonic treatment to obtain an oil-in-water emulsion; S2. Add an amine reagent and an aldehyde compound to the oil-in-water emulsion to obtain a reaction feed solution, and react to obtain the hollow spherical assembly polymer.

[0013] Optionally, in step S1, the surfactant is selected from at least one of dodecyl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide, octadecyl trimethyl ammonium bromide, sodium laurate, sodium dodecyl sulfate, Pluronic F127, P123, F108, and F68.

[0014] Optionally, in step S1, the phenolic compound is selected from at least one of 2-aminophenol, 3-aminophenol, 4-aminophenol, 3-aminobenzenethiol, resorcinol, hydroquinone, and phloroglucinol.

[0015] Optionally, in step S1, the aqueous phase solvent is selected from at least one of water and alcohol.

[0016] Optionally, the alcohol is selected from at least one of methanol, ethanol, ethylene glycol, and propylene glycol.

[0017] Optionally, in step S1, the oil phase solvent is selected from at least one of toluene, xylene, 1,3,5-trimethylbenzene, cyclohexane, 1-octane, and dodecane.

[0018] Optionally, in step S2, the amine reagent is selected from at least one of ammonia water, ethylenediamine, and triethanolamine.

[0019] Optionally, in step S2, the aldehyde compound is selected from at least one of formaldehyde, glyoxal, succinaldehyde, and benzaldehyde.

[0020] Optionally, in step S1, in the oil-in-water emulsion, the concentration of the surfactant is 10 - 50 g / L.

[0021] Optionally, the concentration of the surfactant in the oil-in-water emulsion is selected from any value of 10 g / L, 20 g / L, 30 g / L, 40 g / L, 50 g / L or the range value between any two values.

[0022] Optionally, in step S1, in the oil-in-water emulsion, the concentration of the phenolic compound is 2 - 50 g / L.

[0023] Optionally, in the mixed solution, the concentration of the phenolic compound is selected from any value of 2 g / L, 5 g / L, 10 g / L, 20 g / L, 30 g / L, 40 g / L, 50 g / L or the range value between any two values.

[0024] Optionally, the volume ratio of the oil phase solvent to the aqueous phase solvent is 0.01 - 0.1:1.

[0025] Optionally, the volume ratio of the oil phase solvent to the aqueous phase solvent is any value of 0.01:1, 0.03:1, 0.05:1, 0.07:1, 0.1:1 or the range value between two values.

[0026] Optionally, in step S2, the concentration of the amine reagent in the reaction feed liquid is 10 to 100 μL / mL.

[0027] Optionally, in step S2, the concentration of the amine compound in the reaction feed liquid is selected from any value of 10 μL / mL, 20 μL / mL, 30 μL / mL, 40 μL / mL, 50 μL / mL, 60 μL / mL, 70 μL / mL, 80 μL / mL, 90 μL / mL, 100 μL / mL or the range value between any two values.

[0028] Optionally, in step S2, the molar ratio of the phenolic compound to the aldehyde compound is 0.25 to 2:1.

[0029] Optionally, in step S2, the molar ratio of the phenolic compound to the aldehyde compound is selected from any value of 0.25:1, 0.5:1, 1:1, 1.5:1, 2:1 or the range value between any two values.

[0030] Optionally, in step S1, the conditions of the ultrasonic treatment include: temperature 5 to 40 °C, time 10 to 60 min.

[0031] Optionally, in step S1, the temperature of the ultrasonic treatment conditions is selected from any value of 5 °C, 15 °C, 25 °C, 35 °C, 40 °C or the range value between any two values.

[0032] Optionally, in step S1, the time of the ultrasonic treatment conditions is selected from any value of 1 min, 5 min, 10 min, 20 min, 30 min, 50 min, 60 min or the range value between any two values.

[0033] Optionally, in step S2, the reaction conditions include: reacting under stirring, the stirring speed is 100 to 1000 rpm, the reaction temperature is 20 to 70 °C, and the reaction time is 0.5 to 24 h.

[0034] Optionally, in step S2, the temperature of the reaction conditions is selected from any value of 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C or the range value between any two values.

[0035] Optionally, in step S2, the time of the reaction conditions is selected from any value of 0.5 h, 1 h, 3 h, 6 h, 12 h, 24 h or the range value between any two values.

[0036] Optionally, in step S2, the stirring speed of the reaction conditions is selected from any value of 100 rpm, 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm or a range value between any two values.

[0037] According to the third aspect of the present application, a carbon material is provided, and the carbon material is obtained by calcining the above-mentioned hollow spherical assembly polymer or the hollow spherical assembly polymer obtained according to the above preparation method.

[0038] Optionally, the particle size of the carbon material is 0.4 - 5 μm.

[0039] Optionally, the pore size of the carbon material is 0.1 - 20 nm.

[0040] Optionally, the specific surface area of the carbon material is 10 - 1000 m 2 / g.

[0041] Optionally, the atmosphere for calcination is selected from inert atmospheres, preferably at least one of nitrogen, argon or helium.

[0042] Optionally, the temperature for calcination is 300 - 900 °C.

[0043] Optionally, the temperature for calcination is selected from any value of 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C or a range value between any two values.

[0044] Optionally, the time for calcination is 1 - 8 h.

[0045] Optionally, the time for calcination is selected from any value of 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h or a range value between any two values.

[0046] Optionally, the calcination is carried out by raising the temperature to the required calcination temperature at a heating rate of 1 - 10 °C / min, and then holding the temperature.

[0047] According to the fourth aspect of the present application, an application of the above-mentioned hollow spherical assembly polymer, the hollow spherical assembly polymer obtained according to the above preparation method or the above-mentioned carbon material in the fields of catalysis, adsorption and biomedicine is provided.

[0048] According to the fifth aspect of the present application, a catalyst for the selective hydrogenation reaction of phenylacetylene is provided, and the catalyst comprises a carrier and an active component supported on the surface of the carrier; The carrier is selected from the carbon material as claimed in claim 7; The active component is palladium metal particles; The loading amount of the active component is 1 wt% of the carbon material.

[0049] According to the sixth aspect of the present application, a method for the selective hydrogenation of phenylacetylene is provided, including the following steps: In a hydrogen atmosphere, a raw material containing phenylacetylene, a hydrogenation catalyst, and a solvent is placed in a closed container and reacted to obtain styrene; The hydrogenation catalyst is selected from the catalyst for the selective hydrogenation reaction of phenylacetylene described in claim 9.

[0050] Optionally, the molar amount of phenylacetylene, the mass of the catalyst, and the volume ratio of the solvent are selected from 1 to 10 mol: 1 to 50 g: 5 to 50 L.

[0051] Optionally, the solvent is selected from at least one of water, methanol, ethanol, ethylene glycol, acetone, or ethylene oxide.

[0052] Optionally, the reaction conditions include: the reaction temperature is 30 to 100 °C, and the reaction pressure is 1 to 20 bar.

[0053] The beneficial effects of the present application include: 1. The hollow spherical assembly material provided by the present application is a multi-level assembled phenolic resin polyhedron material.

[0054] 2. The preparation method of the hollow spherical assembly material provided by the present application is simple and easy to implement, with high yield and good stability, meeting the requirements of large-scale production.

[0055] 3. The prepared micro-nano structured phenolic resin has good dispersibility. Different morphologies of polymers can be obtained by adjusting the types of surfactants and oil phase solvents. Description of the Drawings

[0056] Figure 1 It is a transmission electron microscope image of the hollow spherical assembly material prepared under the conditions of Example 1 of the present application. Among them, Figure a is the hollow spherical assembly material, and Figure b is the small monomer constituting the material.

[0057] Figure 2 It is a transmission electron microscope image of the hollow spherical assembly material prepared under the conditions of Example 2 of the present application. Among them, Figure a is the hollow spherical assembly material, and Figure b is the small monomer constituting the material.

[0058] Figure 3 Figures a and b in are transmission electron microscope images of the hollow spherical assembly material prepared under the conditions of Example 3 of the present application at different scales.

[0059] Figure 4In the figure, a and b are transmission electron microscope images of the hollow spherical assembly materials prepared under the conditions of Example 4 of the present application at different scales. Detailed implementation mode

[0060] The present application will be described in detail below with reference to the examples, but the present application is not limited to these examples.

[0061] Unless otherwise specified, the raw materials in the examples of the present application are all purchased through commercial channels.

[0062] Unless otherwise specified, the test methods are all conventional methods, and the instrument settings are all the settings recommended by the manufacturer.

[0063] In the examples of the present application, transmission electron microscope analysis (TEM) was carried out using Hitachi 7800 of Japan and FEI Tecnai F20 of the United States for analysis.

[0064] Example 1 Dissolve 0.05 g of F127 and 0.05 g of dodecyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Then add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%) successively. Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material composed of multiple spherical crown-shaped small monomers.

[0065] Figure 1 This is a transmission electron microscope image of the hollow spherical assembly material prepared under the conditions of Example 1 of the present application. It can be seen from the figure that the material is composed of spherical crown-shaped small monomers, with a particle size of about 1457 nm, the diameter of the small monomer is about 722 nm, and the longitudinal length of the small monomer is about 668 nm.

[0066] Example 2 Dissolve 0.05 g of F127 and 0.05 g of octadecyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Then add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%) successively. Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material composed of multiple morning glory-shaped small monomers with adjacent receptacle parts.

[0067] Figure 2 This is a transmission electron microscopy image of the hollow spherical assembly material prepared under the conditions of Example 2 of this application. It can be seen from the figure that the material is composed of morning glory-shaped small monomers, with a particle size of about 1245 nm, a diameter of the small monomers of about 419 nm, and a longitudinal length of the small monomers of about 250 nm.

[0068] Example 3 Dissolve 0.05 g of F127 and 0.05 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material composed of multiple morning glory-shaped small monomers with their petal-like parts connected.

[0069] Figure 3 This is a transmission electron microscopy image of the hollow spherical assembly material prepared under the conditions of Example 3 of this application. It can be seen from the figure that the material is composed of morning glory-shaped small monomers, with a particle size of about 1881 nm, a diameter of the small monomers of about 183 nm, and a longitudinal length of the small monomers of about 92 nm.

[0070] Example 4 Dissolve 0.05 g of F127 and 0.05 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 4-methylstyrene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material composed of multiple morning glory-shaped small monomers with their receptacle-like parts adjacent.

[0071] Figure 4 This is a transmission electron microscopy image of the hollow spherical assembly material prepared under the conditions of Example 4 of this application. It can be seen from the figure that the material is composed of morning glory-shaped small monomers, with a particle size of about 408 nm, a diameter of the small monomers of about 205 nm, and a longitudinal length of the small monomers of about 124 nm.

[0072] Example 5 Dissolve 0.05 g of F127 and 0.05 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material.

[0073] Example 6 Dissolve 0.05 g of F108 and 0.1 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material.

[0074] Example 7 Dissolve 0.05 g of F127 and 0.15 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material.

[0075] Example 8 Dissolve 0.05 g of F127 and 0.02 g of cetyltrimethylammonium bromide in 10 mL of water and 10 mL of ethanol. Add 0.1 g of 3-aminophenol. After dissolution, add 1 mL of 1,3,5-trimethylbenzene. After ultrasonic treatment for 20 min, an oil-in-water emulsion is obtained. Sequentially add 0.1 mL of ammonia water (25 wt.%) and 0.14 mL of formaldehyde aqueous solution (37 wt.%). Stir at 30 °C at a speed of 400 rpm for 12 hours. Centrifuge to collect the precipitate, wash it three times with water and ethanol, and dry it in an oven at 40 °C to obtain a hollow spherical assembly material.

[0076] The above are only several embodiments of the present application, and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the relevant art can make some changes or modifications within the scope of the technical solution of the present application by using the disclosed technical content, which are all equivalent to equivalent embodiments and fall within the scope of the technical solution.

Claims

1. A hollow spherical assembled polymer, characterized in that, It includes a shell assembled by a number of small monomers and a cavity surrounded by the shell; The small monomers are polymer blocks in the shape of morning glory or spherical crown.

2. The hollow spherical assembly polymer according to claim 1, wherein The radial dimension of the small monomers is 100 - 800 nm, and the axial length is 90 - 700 nm; The particle size of the hollow spherical assembly polymer is 0.4 - 5 μm.

3. The preparation method of the hollow spherical assembly polymer according to claim 1 or 2, characterized in that, It includes the following steps: S1. Add an oil-phase solvent to a mixed solution containing a surfactant, a phenolic compound, and an aqueous-phase solvent, and perform ultrasonic treatment to obtain an oil-in-water emulsion; S2. Add an amine reagent and an aldehyde compound to the oil-in-water emulsion to obtain a reaction feed solution, and react to obtain the hollow spherical assembly polymer.

4. The preparation method according to claim 3, wherein In step S1, the surfactant is selected from at least one of ammonium dodecyl trimethyl bromide, ammonium tetradecyl trimethyl bromide, ammonium hexadecyl trimethyl bromide, ammonium octadecyl trimethyl bromide, sodium laurate, sodium dodecyl sulfate, Pluronic F127, P123, F108, and F68; In step S1, the phenolic compound is selected from at least one of 2-aminophenol, 3-aminophenol, 4-aminophenol, 3-aminobenzenethiol, resorcinol, hydroquinone, and phloroglucinol; In step S2, the amine reagent is selected from at least one of ammonia water, ethylenediamine, and triethanolamine; In step S2, the aldehyde compound is selected from at least one of formaldehyde, glyoxal, succinaldehyde, and benzaldehyde.

5. The preparation method according to claim 3, characterized in that, In step S1, in the oil-in-water emulsion, the concentration of the surfactant is 10 - 50 g / L; The concentration of the phenolic compound is 2 - 50 g / L; In step S2, the concentration of the amine reagent in the reaction feed solution is 10 - 100 μL / mL; In step S2, the molar ratio of the phenolic compound to the aldehyde compound is 0.25 - 2:

1.

6. The preparation method according to claim 3, wherein In step S1, the conditions of ultrasonic treatment include: temperature 5 - 40 °C, time 10 - 60 min; In step S2, the conditions of the reaction include: reacting under stirring, the stirring speed is 100 - 1000 rpm, the reaction temperature is 20 - 70 °C, and the reaction time is 0.5 - 24 h.

7. A carbon material, characterized in that, The carbon material is obtained by calcining the hollow spherical assembly polymer described in claim 1 or 2 or the hollow spherical assembly polymer obtained by the preparation method according to any one of claims 3 to 6.

8. The application of the hollow spherical assembly polymer described in claim 1 or 2, the hollow spherical assembly polymer obtained by the preparation method according to any one of claims 3 to 6, or the carbon material described in claim 7 in the fields of catalysis, adsorption, and biomedicine.

9. A catalyst for the selective hydrogenation reaction of phenylacetylene, characterized in that, The catalyst includes a carrier and an active component supported on the surface of the carrier; The carrier is selected from the carbon material described in claim 7; The active component is metal palladium particles; The loading amount of the active component is 1 wt% of the carbon material.

10. A method for the selective hydrogenation of phenylacetylene, characterized in that, It includes the following steps: In a hydrogen atmosphere, place a raw material containing phenylacetylene, a hydrogenation catalyst, and a solvent in a closed container, and react to obtain styrene; The hydrogenation catalyst is selected from the catalyst for the selective hydrogenation reaction of phenylacetylene described in claim 9.

Images