DIRECT SYNTHESIS OF IMPROVED SUPERHYDROPHOBIC CARBON NITRIDE CO-PRODUCTS, AND IMPROVED SUPERHYDROPPBIC CARBON NITRIDE CO-PRODUCTS THEREOF

Abstract

The present invention is concerned with a method of direct synthesis of co-products of at a first co-product and a second co-product. The first co-product is superhydrophilic carbon nitride thin film and the second co-product is superhydrophilic carbon nitride powder. The method has a step of using a guanidine carbonate salt as a precursor material. The present invention is also concerned with carbon nitride co-products. The carbon nitride co-products has a first co-product of superhydrophilic carbon nitride thin film and a second co-product of superhydrophilic carbon nitride powder. The superhydrophilic carbon nitride thin film has chemical formula of CN.sub.x, wherein x is 0.86-1.04, and the superhydrophilic carbon nitride powder has a chemical formula of g-C.sub.3N.sub.4.

Claims

1. A method of direct synthesis of co-products of at a first co-product and a second co-product, wherein the first co-product is superhydrophilic carbon nitride thin film and the second co-product is superhydrophilic carbon nitride powder, comprising a step of using a guanidine carbonate salt as a precursor material.

2. A method as claimed in claim 1, wherein the superhydrophilic carbon nitride thin film has a chemical formula of CN.sub.x, and the superhydrophilic carbon nitride powder has a chemical formula of g-C.sub.3N.sub.4, wherein x is 0.86-1.04.

3. A method as claimed in claim 1, wherein the superhydrophilic carbon nitride thin film has a water contact angle of 0-5.

4. A method as claimed in claim 3, wherein the water contact angle of 4.5.

5. A method as claimed in claim 1, wherein the superhydrophilic carbon nitride thin film on its surface has an oxygen-carbon ratio of 0.01-0.63.

6. A method as claimed in claim 5, wherein the superhydrophilic carbon nitride thin film on its surface has an oxygen-carbon ratio of 0.63.

7. A method as claimed in claim 1, wherein the guanidine carbonate salt has a chemical formula of NH.sub.2C(NH)NH.sub.2.Math.H.sub.2CO.sub.3).

8. A method as claimed in claim 1, wherein the method makes use of chemical vapor deposition (CVD).

9. A method as claimed in claim 1, comprising the steps of: providing a reaction tube acting as a reaction chamber defining opposite open lateral ends, with one end receiving a flow of gas and the opposite end allowing, after reaction, the flow of gas to exit, wherein the opposite end is filled with a one-way valve for preventing backflow, placing a predetermined amount of the guanidine carbonate salt on the bottom of the reaction tube, providing a growth substrate and putting the growth substrate in the reaction tube such that there is a clearance of 1-5 cm between the growth substrate and the guanidine carbonate salt, subjecting the reaction chamber to heat in a furnace, subjecting the reaction chamber to the flow of gas therethrough and allowing the reaction to take place for a predetermined amount of time at a predetermined temperature, and allowing annealing to complete and collecting the first co-product superhydrophilic carbon nitride thin film on the growth substrate and the second co-product superhydrophilic carbon nitride powder at the opposite end of the reaction tube.

10. A method as claimed in claim 9, wherein the flow of gas is dry and consists of nitrogen, oxygen, argon and carbon dioxide.

11. A method as claimed in claim 9, wherein reaction tube has a diameter tube of 8-15 cm.

12. A method as claimed in claim 9, wherein the amount of guanidine carbonate salt placed in the reaction tube is 0.5 g to 1.5 g.

13. A method as claimed in claim 9, wherein the guanidine carbonate salt is located in the center of the heating zone of the furnace, and the growth substrate is located downstream of the heating zone in the reactive tube.

14. A method as claimed in claim 9, wherein the flow of gas has a rate in the range of 50 sccm to 200 sccm.

15. A method as claimed in claim 9, wherein the reaction takes place with an initial ramping time of 30 to 60 min, following by a subsequent annealing time of 1 to 6 hrs at 450-600 C.

16. Carbon nitride co-products of a first co-product of superhydrophilic carbon nitride thin film and a second co-product of superhydrophilic carbon nitride powder, wherein the superhydrophilic carbon nitride thin film has chemical formula of CN.sub.x, wherein x is 0.86-1.04, and the superhydrophilic carbon nitride powder has a chemical formula of g-C.sub.3N.sub.4.

17. Carbon nitride co-products as claimed in claim 16, wherein the superhydrophilic carbon nitride thin film has a water contact angle of 0-5.

18. Carbon nitride co-products as claimed in claim 17, wherein the water contact angle of 4.5.

19. Carbon nitride co-products as claimed in claim 16, wherein the superhydrophilic carbon nitride thin film on its surface has an oxygen-carbon ratio of 0.01-0.63.

20. Carbon nitride co-products as claimed in claim 19, wherein oxygen-carbon ratio is 0.63.

Description

BRIEF DESCRIPTION TO THE DRAWINGS

[0020] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0021] Some embodiments of the present invention will now be explained, with reference to the accompanied drawings, in which:

[0022] FIGS. 1a to 1c include a schematic diagram, a graph and images showing the synthesis of carbon nitrides, in which FIG. 1a illustrates the experimental setup and the heating process for the carbon nitrides synthesis, FIG. 1b illustrates the thermogravimetric analysis (TGA) for decomposition of guanidine carbonate as precursor and the polycondensation of carbon nitrides during heating process, and FIG. 1c are digital images showing final products of CN.sub.x thin film and powder taken out from CVD synthesis chamber after cooling.

[0023] FIGS. 2a to 2e include images and graphs illustrating superhydrophilicity of CN.sub.x thin films and membranes, in which FIG. 2a are images of snapshots series showing evolution of contact angle (CA) between surface of CN.sub.x thin films and water droplet over time [Scale bar=2 mm], FIG. 2b is a graph showing the time-dependent CA of water droplet on surface of CN.sub.x thin films, FIG. 2c is an AFM topographic image of CN.sub.x thin films deposited at 500 C. which contained needle-shaped texture on 2D surface [Scale bar=500 nm], FIG. 2d is an image of the 3D layout of zoom-in area corresponding to the blue square dash-line in FIG. 2c, and FIG. 2e is a graph showing the time-dependent CA of water droplet on surface of membranes made from CN.sub.x powder.

[0024] FIGS. 3a to 3c are graphs illustrating crystalline structure and chemical bonding of carbon nitrides powder and thin films, in which FIG. 3a and FIG. 3b show the X-ray diffraction (XRD) spectrum of powder and thin films deposited on glass substrates at different annealing temperature, respectively, with the red lines marked for the peaks of triazine structure and blue dashed line marked for the peaks of heptazine structure, and FIG. 3c shows the fourier-transform infrared (FT-IR) spectrum of powder and thin films deposited on glass substrates at different annealing temperature.

[0025] FIG. 4 is a graph showing X-ray photoelectron spectroscopy (XPS) characterizations of CN.sub.x thin films deposited on glass substrate at different annealing temperature. The XPS spectrum peak fitting of carbon 1s core-level (left) and nitrogen 1s core-level (right) of CN.sub.x thin films deposited on glass substrate at 450, 500, 550 and 600 C.

[0026] FIGS. 5a to 5d are graphs and an image illustrating photo-electrocatalytic performance of superhydrophilic carbon nitride, in which FIG. 5a shows the polarization curves, FIG. 5b shows the corresponding Tafel slope, and FIG. 5c are nyquist plots of electrochemical impedance spectroscopy of prepared CN.sub.x@CC electrode under dark and light (15 W) condition, and FIG. 5d is a digital picture showing the generation of H2 bubbles on electrode.

[0027] FIGS. 6a to 6b illustrate the optimization of substrate position, in which FIG. 6a are digital images and FIG. 6b are optical microscope (OM) images of CN.sub.x thin films deposited on glass substrates dispensed at different locations in the test tube. The sample in position 2 which was 3-5 cm away from precursor shows the cleanest surface and uniform transparent yellow thin film on glass substrate.

[0028] FIGS. 7a to 7e are images showing the preparation of membranes from powder sample, in which FIG. 7a are digital pictures of powder samples annealed at different temperature, FIG. 7b shows that about 10 mg of selected powder sample at first was dispersed in 10 mL dimethylformamide (DMF), FIG. 7c shows that the vial contained sample solution was put into a sonication bath at 37 kHz80% power in 1 hrs for well dispersion, and FIG. 7d shows that the solution was then filtrated through a hydrophilic PTFE filter to form a uniform membrane on the filter, and FIG. 7e are pictures showing the membranes were dried thoroughly in oven at 60 C. for further measurement.

[0029] FIG. 8 are backlight OM images showing the comparison of CN.sub.x thin films deposited at different annealing temperatures.

[0030] FIGS. 9a to 9d are scanning electron microscope (SEM) images of the cross-sectional morphology of CN.sub.x thin film deposited at 450, 500, 550 and 600 C., respectively.

[0031] FIGS. 10a to 10b are atomic microscope (AFM) topographic images of the CN.sub.x thin films deposited at 450 and 550 C., respectively [scale bar 500 nm].

[0032] FIGS. 11a and 11b are the basic unit of graphitic carbon nitrides (g-C.sub.3N.sub.4), in which FIG. 11a shows heptazine and FIG. 11b shows triazine.

[0033] FIG. 12 shows X-ray photoelectron spectroscopy (XPS) characterizations of g-C3N4 powder sample synthesized at different annealing temperature. The XPS spectrum peak fitting of carbon 1s core-level (left) and nitrogen 1s core-level (right) of g-C3N4 synthesized at 450, 500, 550 and 600 C.

[0034] FIG. 13 shows an absorption spectrum of CN.sub.x thin films deposited at different annealing temperature. The absorption edge at 420-440 nm light agreed with the optical bandgap of graphitic structure.

[0035] FIGS. 14a to 14d are the top-view SEM images of CN.sub.x thin films deposited on glass substrates at (a) 450 C., (b) 500 C., (c) 550 C. and (d) 600 C., respectively.

[0036] FIG. 15 are images showing hydrophilicity of the sample (550 C. CN.sub.x) on glassy carbon electrode without and under light. Scale bar=2 mm. The CA is collected when the droplets make first contact.

[0037] FIGS. 16a to 16b are graphs showing photo-electrocatalytic performance of CN.sub.x sample synthesized at different temperature (450, 500, 550 and 600 C.), in which FIG. 16a illustrates polarization curves and FIG. 16b illustrates corresponding Tafel slope of prepared CN.sub.x@CC electrodes under dark and light (13 W) condition.

[0038] FIG. 17 is a graph showing the XPS signal of N 1s of g-C.sub.3N.sub.4 powder synthesized at 550 C. before and after subtracting the signal from the inelastic scattering of electrons using Shirley method.

[0039] FIG. 18 is a graph of X-ray diffraction showing the carbon nitride thin film and powder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0040] Superhydrophilicity benefits many applications working in the water/humid-involved environments, from the self-cleaning outdoor window surfaces to the high performance photocatalysts and membranes. The present invention provides for direct synthesis of superhydrophilic CN.sub.x which can significantly improve the catalytic efficiency of this metal-free photocatalysts and has a profound application prospect in the field of photocatalysis and photo-electrocatalysis.

[0041] In existing technologies, superhydrophilicity carbon nitrides can be achieved by using strong acids or bases to functionalize the surface. Another way is using plasma treatment for embedding O-based functional groups which has strong polarization toward water molecules. However, these methods always cause serial damage to the original surface and degrade the durability of the materials. Meanwhile, most of superhydrophilic coatings are polymers without photocatalytic behavior.

[0042] The present invention for direct synthesis of superhydrophilic carbon nitrides exempts all chemical waste and cost in post-treatment processes in achieve superhydrophilicity on surface of a metal-free photocatalyst material, as well as shorten the preparation time and yield the manufacturing efficiency. Moreover, this method can directly grow superhydrophilic CN.sub.x thin film on both hydrophilic and hydrophobic substrates. The new superhydrophilic CN.sub.x coating also has potential application in smart window, that surface can own both self-cleaning and photocatalyst function. Experiments leading to the present invention show that superhydrophilic CN.sub.x thin film can be grown directly on different substrate including hydrophilic (glass, FTO glass) and hydrophobic substrate (carbon cloth), and superhydrophilic CN.sub.x powder. The prese invention enables for a large-scale synthesis and improving its photo-electrocatalytic performance.

[0043] The present invention discloses a method for direct synthesis of co-products, including superhydrophilic carbon nitride thin films (CN.sub.x, x0.86-1.04) and graphitic carbon nitride powder (g-C.sub.3N.sub.4) using chemical vapor deposition (CVD) and their application for metal-free photocatalyst in hydrogen evolution reaction (HER). The superhydrophilicity is obtainable on different grown substrate and enhance the photo-electrocatalytic of carbon nitride thin films as well. The process uses guanidine carbonate salt as precursor sealed by quartz wool in a test tube with the grown substrate, and then anneals at appropriated temperature with constant dry air supply.

[0044] Surface wetting greatly impacts on the performances of many photocatalysts in water/humid-involved medium. Carbon nitrides and its isotopes, as emerging metal-free low cost photocatalysts for water splitting, usually require strong chemical or irradiation treatments to obtain highly hydrophilic surfaces, which can undermine their photocatalytic performances. The present invention has identified an alternative and improved method for the direct synthesis of superhydrophilic carbon nitride thin films (CN.sub.x, x0.86-1.04) and graphitic carbon nitride powder (g-C.sub.3N.sub.4) by using chemical vapor deposition (CVD). Experiments have shown that less than 5 (or 1-5) contact angle with water is accessible on both surface of as-grown CN.sub.x thin films and the membranes made from the g-C.sub.3N.sub.4 powder. The present invention has illustrated the remarkable wetting property attributed to the spontaneous hydrophilic functionalization group (e.g., OH, NO.sub.x, O) supplied by a constant multi-elemental air flow. The abundant CN triple bonds also promote needle-shaped nanostructures on the 2D surfaces, which enhances their chemical wettability. The present invention with respect the direct synthesis of superhydrophilic carbon nitride can be applied or embodied in photocatalysis applications. Below are further illustrations including experiments, results and discussion concerning the present invention.

EXPERIMENTS

[0045] FIG. 1a illustrates, schematically, the general experimental setup. Guanidine carbonate salt (NH.sub.2C(NH)NH.sub.2.Math.H.sub.2CO.sub.3) as precursor was placed in the end of a quartz test tube. An amorphous glass substrate was positioned separately at a certain distance from the precursor so that there is provided a sufficient clearance. The substrate position was optimized to obtain the transparent uniform yellow thin film after synthesis (please also see FIGS. 6a to 6b). The test tube (reaction tube) in which the reaction took place was sealed with quartz wool and put into a tube furnace for providing heating. The quartz wool acts as a one-way valve for preventing the back flow of residue/contaminants into the test tube during synthesis. During synthesis, the tube furnace was heated at a rate of 10-20 C./min to a desired temperature ranging from 450 to 600 C. and annealed for 1-6 hours and then naturally cooled down to room temperature. Before initiating the heating process, an air flow of 100 ml/min was introduced for 15-30 minutes to eliminate the humidity in the tube furnace. This air flow was maintained constant until the synthesis was completed.

[0046] Chemical reactions in the synthesis process were characterized by thermogravimetric analysis (TGA) in FIG. 1b. The derivations of weight loss vs. temperature showed two distinguished peaks at 200 C. and 305 C. The first peak at 200 C. indicated the decomposition of guanidine carbonate into guanidium (C(CH.sub.2).sub.3+) and carbonate (H.sub.2CO.sub.3) molecules. Additionally, the (C(CH.sub.2).sub.3+) ions were linked together to form melamine and released ammonia (NH.sub.3). When the temperature ramped above 200 C., the derivative curve fell to zero and gradually increased thereafter, indicating that the decomposition was completed. This was followed by an acceleration in the polymerization of guanidine ions to form volatile melamine as the temperature increased. The rise of derivative curve was also relevant with the increase of weight loss due to sublimation of melamine as well as NH.sub.3 release. The derivation of weight loss to temperature reached the second peak at 305 C., corresponding to the maximum melamine sublimation rate.

[0047] The temperature was ramped up to 450, 500, 550 and 600 C. and held for two hours in separated synthesis experiments, before naturally cooling to room temperature (see Methods). Since the precursor decomposition was complete at 305 C., further annealing at higher temperatures was primarily employed to sublime melamine onto the downstream glass substrate and to promote the polycondensation of melamine ions into larger melem ions, which serve as the primary units of polyheptazine (g-C.sub.3N.sub.4). Finally, the transparent yellow thin film deposited on glass substrate and the yellow solid powder at the end of test tube were collected after annealing for further characterization (FIG. 1c).

[0048] The wettability of CN.sub.x thin films as well as membranes made from powder samples (see Methods and FIG. 7) were unravelled by the CA measurement with water droplet (FIGS. 21-2e). Unexpectedly, both CN.sub.x thin film and the membranes exhibited superhydrophilic behaviors. From the CA of water droplets on the surface of CN.sub.x thin films (FIGS. 2a to 2b), two groups with distinct wetting behaviors were identified. The first group comprises samples synthesized at 450 and 550 C., while the second group consists of samples synthesized at 500 and 600 C. Upon initial contact, the first group exhibited a water CA of approximately 25-30, while the second group had a low CA of 10-15. Two seconds after initial contact, the water droplets on the 450 C. and 550 C. films maintained a CA of approximately 15-20, while the 500 C. and 600 C. films showed a CA with water below 10. The different contrasts on thin film sample under optical microscope (OM) (FIG. 8) were also correlated with their wetting property. Relation between CA value and surface morphology can be described by Wenzel's equation as below:

[00001]$\begin{array}{cc}\cos m=R\cos y& \left(1\right)\end{array}$

where R is the ratio between the actual solid surface area and the nominal surface area; .sub.y and .sub.m are the Young contact angle and measured CA of actual solid surface with water, respectively. According to Wenzel's equation (1), an increase in surface roughness leads to an increase in chemical wettability. The difference in CA between the 600 C. and 500 C. films of 1-2 is negligible, suggesting that their surfaces had similar morphology and chemistry. Additionally, the initial CA of the 550 C. film (28.75) was higher than that of the 450 C. film (26.25). After 2 seconds, the droplets on the surface stabilized, and the final CA of the 550 C. film (15) was significantly lower than the final CA of the 450 C. film (21.25). This phenomenon can be explained by the relation between CA and surface porosity (fp) described by Cassie-Baxter's equation (2):

[00002]$\begin{array}{cc}{f}_p=1-\left[\left(\cos {}_m+1\right)/\left(\cos {}_y+1\right)\right]& \left(2\right)\end{array}$

The final variation in CA between the 450 C. and 550 C. films was attributed to the fact that the 450 C. film had a higher porosity than the 550 C. film, as evidenced by the cross-sectional scanning electron microscope (SEM) images (FIGS. 9a to 9d). The cross-sectional SEM image of the 450 C. film revealed a high level of structural porosity, and the porosity decreased as the thickness decreased by increasing the polycondensation temperature while using the same annealing time of 2 hours, which was consistent with the final CA measurement value (FIG. 2b). Next, the surfaces of CN.sub.x thin films were characterized by AFM (FIGS. 2c to d and FIGS. 10a to 10b) and the surface roughness as well as CA value of each thin film sample were summarized in Table 2. Specially, the surface of CN.sub.x thin films synthesized at 500 C. were composed of 1D needle-shaped structures. (FIGS. 2c to 2d). The 1D structure on the 2D surface greatly increased the surface roughness resulting in the enhancement of hydrophilicity according to the Wenzel's equation (1), which lined up with the smallest CA with water of the 500 C. thin films. Besides, due to the high thermal expansion of glass substrate at 600 C., the CN.sub.x thin films suffered heavy delamination from underlying substrate (FIG. 9d), preventing the accurate observation of surface and thickness. Moreover, the membranes exhibited the lowest CA with water of 0.5-1 (FIG. 2e). Although the trend of water CA with surfaces largely agreed with theories, the experimental results exceeded the calculated values. These results implied that the annealing temperature modulated the surface morphology and surface chemistry, which could collectively influence the hydrophilicity of both the CN.sub.x thin films and the powder samples. Table 1 shows a comparison of Comparison of conventional methodologies and present invention with respect to O/C ratio and CA value.

[0049] To further understand the physical structure and chemical bonding in the CN.sub.x thin films and membranes, X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) measurements were employed on both powder and thin film samples. The yellow powder samples were identified as polyheptazine g-C.sub.3N.sub.4 (FIGS. 11a to 11b), referring to the diffraction peaks at 2=12.8 and 27.5 (FIG. 3a), which corresponded to (100) and (002) planes of the polyheptazine layered structure, respectively. The (100) peak corresponded to in-plane distance of 0.68 nm between the neighbouring heptazine units, and the (002) peak indicated the spacing of 0.32 nm between the graphitic layers. In particular, the 450 C. powder exhibited additional peaks at 2=22 and 25.2, which were attributed to the diffraction from (100) and (101) planes of triazine based g-C.sub.3N.sub.4 (FIGS. 11a to 11b). XRD analysis revealed that the g-C.sub.3N.sub.4 powder synthesized at 450 C. consisted of both triazine and heptazine phase, while the powder synthesized at higher temperature only contained the heptazine phase. This is because the heptazine networks are more stable than the triazine networks, and thus, the products formed heptazine networks at higher annealing temperatures. In contrast, the CN.sub.x films exhibited lower crystallinity in which the (002) peak of graphitic structure was barely observed under the broad diffraction peak from the amorphous glass. An additional peak at 44.1 was observed, which corresponded to the diffraction from (101) planes of graphite layered structure (FIG. 3b).

[0050] In FIG. 3c, the FT-IR result of powder and thin films samples had a distinct peak at 807 cm.sup.1 referring to the vibration of heptazine groups, which agreed with the XRD result. The CN hexagonal rings were represented by high-intensity multi-peak in the range of 1200-1400 cm.sup.1 which was correlated to the stretching of CN and CN bonds. The peak at 1115 cm.sup.1 was correlated to CO stretching which evidenced to the presence of oxygen-based functional groups. In addition, the peak in 1635 cm.sup.1 was correlated with NH bending, and the broad peak range of 3000-3300 cm.sup.1 was related to NH stretching. From the FT-IR result can be seen that our direct growth samples contained sufficient O-based groups and NH groups, which mainly contributed to their superhydrophilic behavior. Notably, the co-existence of O-based groups and NH groups was rare because oxygen would react selectively with hydrogen rather than binding to the surface. This limited the proportion of O-based groups embedded to the surface. However, this limitation was absent in our synthesis because these functional groups were spontaneously implanted during the polycondensation of samples, rather than being embedded after synthesis. The chemical bonds in thin films were similar to these in powder samples except for the higher proportion of CN bonds peaking at 2170 cm.sub.1, which responded to the 1D texture on CN.sub.x thin films (FIGS. 2c to 2d). Moreover, the large amount of functional groups implanted also caused corrugation to the surface itself.

[0051] FIG. 4 displays the X-ray photoelectron spectroscopy (XPS) characterization for the chemical bonds on surface of CN.sub.x thin films. The highest peak of C 1s core-level in 600 C., 500 C. and 450 C. thin films was centered at 284.7 eV with the half-maximum full width of 1.5 eV, corresponding to the tetrahedral CC sp3 bonding. In contrast, the 550 C. thin film exhibited the peak of CC bonding in at 283.9 eV, which was equivalent to the planar-triangle sp.sup.2 bonding. The peak at 287 eV indicated that the sp.sup.2 C atom was coordinated with three N atoms, forming the main bonding in polyheptazine graphitic structure. The lower number of N atoms coordination presented in 600 C., 500 C., and 450 C. thin films, contributed to the peak at 285.4 eV, which corresponded to the (CN) bonding. Especially, the 500 C. thin film contained a significant amount of (CN) bonding, which was assigned to the peak at 286.4 eV and consistent with its 1D texture (FIGS. 2c to 2d). These findings can be elucidated by the thermal effects on the equilibrium between decomposition and polycondensation. The polycondensation of 2D layered structure was enhanced at temperature of 550 C., while higher or lower temperatures led to dominant decomposition into 3D tetrahedral carbon.

[0052] In particular, the superhydrophilic thin films deposited at 600 C. and 500 C. contained CO bonds, indicated by the peak at 281 eV, and the peak at 395 eV was assigned to the NH bond energy in (NH4.sup.+) groups. In contrast, 550 C. and 450 C. thin films presented only bonds between C atoms in CN hexagonal rings with amine groups (NH.sub.x, x=1, 2), as well as bonds between N and H in NH.sub.x, corresponding to the peaks at 288.5 eV and 400 eV, respectively. This observation further supported the conclusion that O-based and NH.sub.x functional groups co-existed in the CN.sub.x thin films, and that the superhydrophilicity was mainly contributed by the rich O-based functional groups embedded on thin films deposited at 500 C. and 600 C.

[0053] For comparison, the XPS results of g-C.sub.3N.sub.4 powder samples were provided in FIG. 12. Unlike thin films, the powder products were located at the higher temperature position in the CVD synthesis process (see FIG. 1a). The higher thermal energy in polycondensation of powder promoted a stronger polymeric binding, as evidenced by the dominance of bonds between C atom with one (285.8 eV) or three (286.9 eV) N atoms in the graphitic structure. In contrast, at the lower annealed temperature like 450 C., the decomposition reaction of precursor was slower and the polymerization of abundant melamine in precursor was not totally completed, as evidenced by the lighter yellow colour of powder sample (FIG. 7a). The conjugated bonds in CN hexagonal rings were determined by the peaks at 401.7 eV and 402.7 eV. Moreover, the functional groups were demonstrated by the peak of CO (290 eV), NH in the amines (400 eV) and NO in the nitrates (403-405 eV). All the g-C.sub.3N.sub.4 powder samples contained large O-based functional groups, which was directly relevant to the superhydrophilic membranes made from powder samples. The variety of functional groups in both thin films and powder samples resulted from the multi-elemental dry air flow during annealing process. The constant flow partially removed NH.sub.3 gas, which was released from decomposition of precursor, and maintained a constant ratio of N.sub.2, O.sub.2, CO.sub.2 and Ar during the polycondensation of CN.sub.x thin films and g-C.sub.3N.sub.4 powder. Therefore, the different functional groups implanted to surface can be simply controlled by temperature without affecting intrinsic optical properties of g-C.sub.3N.sub.4 structure (FIG. 13) except their morphologies. The thin film synthesized at 500 C. and 600 C. showed their surface roughness increase (FIGS. 14a to 14d) that agreed with their chemical hydrophilicity increase (see FIG. 2b).

[0054] The atomic percentage (at %) of carbon (C), nitrogen (N) and oxygen (O) in CN.sub.x thin films were strongly influenced by annealing temperature (Table 3). Overall, the N at % increased proportionally with annealing temperature, while the O at % decreased, and the C at % approximately maintained at 42%, except for the 450 C. thin film. The low N at % of 450 C. thin film was due to the low thermal energy, where the decomposition reaction was stronger than polymerization, as in above-mentioned TGA result. The superhydrophilic 500 C. thin film had high O concentration contributed by rich O-based functional groups (O, OH and NO.sub.x). Meanwhile, the C/N ratio in 550 and 600 C. thin films were similar to the g-C.sub.3N.sub.4 powder. On the other hand, annealing temperature was less effective in controlling the at% in g-C.sub.3N.sub.4 powder (Table 4). The graphitic powder has C/N ratio of 1.05-1.15 with small amount of oxygen intercalation (4.84-7.39 at %).

[0055] The wettability of CN.sub.x thin films was highly consistent with their chemical bonding. The superhydrophilicity in 600 C. and 500 C. thin films were associated with the rich O-based functional groups (indicated by CO and CO bonds) on the surface, resulting high attraction to water molecules. Meanwhile, the 550 C. is the ideal temperature for the polycondensation of heptazine structure, and 450 C. is for the combined structure of triazine and heptazine (See FIGS. 3a to 3b). The well-established structure of 450 C. and 550 C. samples had fewer active sites to attach O-based functional groups except the NH.sub.x groups which persisted in melamine. In contrast, the sample synthesized at 500 C. was in the intermediate state of transforming triazine to heptazine, and sample synthesized at 600 C. started decomposing heptazine to tetrahedral carbon, therefore, they had high level of defect/active sites for O-based functional groups embedding. Moreover, the 1D texture on 2D surface of 500 C. thin film was relevant to the abundance of CN bonds which contributed to the 1D structure alignment (FIGS. 2c to 2d) which significantly increase its chemical wettability as well. Further, the superhydrophilicity of the g-C.sub.3N.sub.4 membranes was subjected to CO groups in powder samples, according to FT-IR and XPS results (FIGS. 3c and FIG. 4). Apart from that, CA of 450 C., 500 C. and 600 C. g-C.sub.3N.sub.4 membranes with water degraded to nearly 0 within 2 secs, meanwhile the CA of 550 C. membrane made a quick drop from 30 to 6. The differences were related to the appearance of NO.sub.x and OH functional groups in the powder formed at 450 C., 500 C. and 600 C., instead, the NH.sub.x groups in 550 C. membrane (FIG. 12) had lower polarization toward the hydrogen bonding with water molecules than the NO.sub.x and OH groups.

[0056] Superhydrophilicity can benefit the photoelectrocatalytic performance by increasing ion transfer and gas separation. In addition, the superhydrophilicity can be further enhanced under light irradiation (FIG. 15). The HER performance of superhydrophilic CN.sub.x thin film directly deposited on carbon cloth (CN.sub.x@CC) was examined through linear sweep voltammetry (FIG. 5a) under 15 W Xenon arc lamb irradiation (see Methods). The CN.sub.x@CC showed an overpotential of 373.7 mV and a Tafel slope of 177.59 mA dec-1 under dark condition (FIG. 5b). Noted that CN.sub.x is an important photocatalysts with a narrow bandgap, its activity great improved under light irradiation with a reduced overpotential of 130.1 mV and a lower Tafel slope of 75.07 mA dec-1, surpassing that the values in previous work. The electrochemical impedance spectroscopy (FIG. 5c) revealed the electron transfer kinetics of the electrode, low ohmic resistances were realized under both dark and light condition, which was ascribed to the superhydrophilic of the synthesized CN.sub.x. Besides, the superhydrophilic surface reduces the bubbles size and promotes the detachment of bubbles from the electrode. FIG. 5d shows that H.sub.2 bubbles generated on the electrode are small and can be released quickly, which effectively avoided dead surface area and increased current density. To investigate the relationship between hydrophilicity and photoelectrochemical performance, the electrocatalytic performance of CN.sub.x@CC grown under different synthesis temperature is tested and provided in FIGS. 16a to 16b. The 450 C. CN.sub.x@CC shows a lowest overpotential of 277.0 mV under light condition, correspondingly, it has the worst hydrophilicity, as depicted in FIGS. 2a to 2b. With the increase of hydrophilicity, the photocatalytic performance was improved. It is evidenced that the superhydrophilicity of catalyst is feasible for the improvement of the HER performance.

METHODS

Samples PreparationSynthesis of Carbon Nitrides

[0057] About 0.6 g of guanidine carbonate (linear formula NH.sub.2C(NH)NH.sub.2.Math.H.sub.2CO.sub.3, 99%, Merck 593-85-1) was placed at the end of a quartz test tube (length: 155 mm, diameter: 12 mm). A glass microscope slide or carbon cloth (CC) was positioned 1 cm away from the guanidine carbonate. The test tube was sealed with quartz wool and put into a tube furnace nameplated STF 15/180. The furnace was heated up at the speed of 10 C./min from room temperature to the desired temperature and annealed for 2 hours afterwards. The dry air flow (78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide) of 100 ml/min was supplied constantly from 30 minutes before the heating process until the end of synthesis process.

Samples PreparationPreparation of Graphitic Carbon Nitride (g-C.SUB.3.N.SUB.4.) Membrane

[0058] About 10 mg of g-C.sub.3N.sub.4 powder dispersed in 10 ml of dimethylformamide (DMF) solvent (99.8%, Merck 68-12-2) was placed into ultrasonic bath for an hour (37 kHz, 80% power) before being filtrated on hydrophilic polytetrafluoroethylene (PTFE) membrane with pore size of 0.1 m to form g-C.sub.3N.sub.4 membranes (see FIGS. 7a to 7e).

Characterization TechniquesThermogravimetric Analysis (TGA)

[0059] The TGA measurement was carried out using the Thermogravimetric Analyser (TA Instruments Q500). About 15 mg of guanidine carbonate salt was put to an alumina pan hanging in the TGA chamber. A dry air flow of 20 ml/min was supplied to the TGA chamber in whole measuring process to imitate the synthesis conditions. At first the chamber was equilibrated at 100 C. for 30 min to remove humid absorption. Then the chamber was ramping up of 10 C./min to 600 C. and isothermal for 30 min before naturally cooling down to room temperature.

Characterization TechniquesContact Angle Measurements

[0060] The wetting behaviors of the samples were examined by contact angle (CA) measurement, using sessile drop technique (Drop shape analyser DSA25S, KRSS GmbH, Germany). It was performed under ambient conditions (20 C. in temperature, 50% in humidity). A water droplet of 1-5 l was deposited on a substrate and CA was measured within two seconds. The CA analysis from the recorded videos was processed by the software of ImageJ.

Characterization TechniquesX-Ray Diffraction (XRD)

[0061] XRD results of thin films and powder were carried out by the Smartlab X-ray diffractometer (RIGAKU, Japan), which scanned over the sample in the 2 range of 10-60, with the resolution of 0.02.

Characterization TechniquesFourier Transform Infrared Spectroscopy (FT-IR)

[0062] FTIR spectra of sample were recorded using FT/IR-4700 (JASCO, Japan) spectrophotometer, which scanned over the wavenumber range of 600-4000 cm.sup.1, with resolution of 1 cm.sup.1 in the transmittance mode.

Characterization TechniquesX-Ray Photoelectron Spectroscope (XPS)

[0063] XPS was applied to measure the chemical binding energy as well as the atomic ratio (at %) using an Al X-ray source (Thermo-Scientific, ESCALAB 250Xi). All the peaks were measured under high vacuum (10-8 Torr). The raw XPS data has been corrected using Shirley method to subtract signal from the inelastic scattering of electrons before analysis (See FIG. 15).

Characterization TechniquesScanning Electron Microscope (SEM)

[0064] SEM (Philips FEG SEM XL30, USA) was used to examine the cross-section morphology of the thin film. High-level resolution at different magnifications is obtain by operating at different accelerating voltages.

Characterization TechniquesAtomic Force Microscope (AFM)

[0065] AFM measurement was carried out using AFM5300E system (Hitachi, Japan). The tapping mode was applied for observation of the topography, using a gold-coated Si cantilever (NSG30, Nanotips, f=340 KHz, C=1 N/m).

Characterization TechniquesPhotoelectrochemical Measurement

[0066] The photoelectrochemical measurements were performed by a three-electrode system in 0.5 M H2SO4 electrolyte. The in-situ grown CN.sub.x@CC is used as the work electrode, Ag/AgCl and Pt are acted as the reference and counter electrode, respectively. The HER performance was tested by linear sweep voltammetry (LSV, 10 mV s1) and electrochemical impedance spectroscopy (EIS) on CHI760E electrochemical workstation. Photocurrents were obtained using a 300 W Xenon arc lamp under an output power of 15 W. All the polarization curves were obtained without IR correction.

CONCLUSIONS

[0067] The present invention has demonstrated the synthesis of superhydrophilic CN.sub.x thin films and g-C.sub.3N.sub.4 membranes using direct CVD method. The superhydrophilicity of our products breaks the previous limitation of the as-grown CN.sub.x products. The effect of different temperature conditions on the thermal polycondensation of CN.sub.x in dry air medium has been revealed, suggesting the dominant incorporation of O-based or N-based functional groups can be spontaneously achieved by CVD annealing, followed by the relative change in chemical wettability. Moreover, enriched functional groups as well as (CN) bonds collectively promoted the formation of 1D texture that greatly enhanced the surface area and porosity of CN.sub.x thin films. The superhydrophilic CN.sub.x thin films and the g-C.sub.3N.sub.4 powder are ideal candidates for a variety of applications working in water/humid environments.

[0068] Superhydrophilicity benefits many applications working in the water/humid-involved environments, from the self-cleaning outdoor window surfaces, to the high performance photocatalysts and membranes in various studies. The contribution of superhydrophilicity to the surface applications can be categorized into two factors: first, by increasing the attraction to water molecules which directly speed up the water splitting reactions or water transportation through the membrane; second, by self-cleaning effect preventing the contaminations of catalytic surface or/and fouling effect on the membrane applications. Therefore, the present invention of increasing hydrophilicity without interfering their intrinsic properties of target materials is advantageous to many applications.

[0069] Specifically, the present invention has contributed to a new method for direct synthesis of the superhydrophilic CN.sub.x thin films on hydrophobic substrate and highly crystalline superhydrophilic powder of g-C.sub.3N.sub.4 using the chemical vapor deposition (CVD). The CA with water of the CN.sub.x thin films and the membranes made from g-C.sub.3N.sub.4 powder reaches below 5, which is inaccessible via direct growth before. In addition, different surface textures and water wettability could be tuned by controlling the thermal condensation temperature of 450 C. to 600 C. The new superhydrophilic CN.sub.x structure has shown their potential as metal-free photocatalyst for hydrogen evolution reaction (HER).

[0070] It should be understood that certain features of the invention, which are, for clarity, described in the content of separate embodiments, may be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the content of a single embodiment, may be provided separately or in any appropriate sub-combinations. It is to be noted that certain features of the embodiments are illustrated by way of non-limiting examples. For example, one application of the present invention is the provision of superhydrophilic CN.sub.x thin film which can be used as metal-free photocatalyst for hydrogen evolution reaction (HER). The superhydrophilic CN.sub.x thin films and powder has potential to be applied as coating for example on smart windows which have both self-cleaning and photocatalytic properties.

TABLE-US-00001 TABLE 1 Comparison of conventional methodologies and present invention with respect to O/C ratio and CA value CA value Conventional methods O/C ratio (deg.) Plasma-enhanced chemical vapor deposition 0.34 81.9 (S.A. Almad Kamal et. al. Applied Surface 0.39 103.9 Science 328 (2015) 146-153) 0.44 120.8 0.08 156.6 Plasma-enhanced pulsed laser deposition ~0 66-77 (M.E. Ransey et. al. Thin Solid Films 360 (2000) 82-88) Present invention 0.01-0.63 0-5

TABLE-US-00002 TABLE 2 Surface roughness and contact angle with water of CN.sub.x then films deposited at different annealing temperature Thin films surface 450 C. 550 C. 450 C. 450 C. AFM 10.4 4.3 2.5 Roughness (RMS) Contact angle 26.25 12.75 28.75 15.5 () 0 sec 1.sup.st sec 22 8.25 19 8.5 2.sup.nd sec 21.25 6.75 15 5.5

TABLE-US-00003 TABLE 3 Elemental atomic ration of g-C.sub.3N.sub.4 powder synthesized at different annealing temperatures Synthesis temperature Element 600 C. 550 C. 500 C. 450 C. C (at %) 48.69 49.30 48.65 50.9 N (at %) 45.17 43.30 46.22 44.26 O (at %) 6.14 7.39 5.14 4.84 C/N ratio 1.08 1.14 1.05 1.15

TABLE-US-00004 TABLE 4 Ratio of thin film carbon/nitrogen atom (C/N) of samples/ embodiments from experiments leading to present invention Samples C/N ratio 1. CN.sub.x film on glass 1.04 2. CN.sub.x film on FTO glass 0.86 3. CNx film on carbon cloth 0.92 4. g-C.sub.3N.sub.4 membrane on PTFE membrane 1.05

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