Cellulose-based photocatalytic paper with Ag 2 O nanoparticles loaded on graphite fibers

TiO2 nanoparticle is the most extensively used photocatalyst for photocatalytic paper. However, TiO2 nanoparticle is active only under UV light that accounted for less than 5% of the solar light energy. There is a need to develop a photocatalytic paper with full spectrum photoactivity. Herein, a full spectrum photocatalytic paper was fabricated by incorporating cellulose fibers with graphite fibers which were pre-loaded with Ag2O nanoparticles. X-ray diffraction (XRD) testing and scanning electron microscopy (SEM) examination confirmed the loading of the Ag2O nanoparticles onto the graphite fibers. The photocatalytic activity of the paper was evaluated with the degradation of methyl orange (MO) under ultraviolet, visible or near-infrared light irradiation. It was found that the degradation rate of MO could reach 32%, 19% and 19% under UV, visible and NIR light, respectively after 3 h of irradiation. Meanwhile, the photocatalytic paper was very stable and can be reused repeatedly. Antibacterial tests showed that the photocatalytic paper could inhibit Escherichia coli (E. coli) growth under office lighting condition.


INTRODUCTION
Photocatalytic paper is a specialty paper with cellulose fibers as the matrix and photocatalysts fixed on the cellulose fiber surfaces for photodegradation of organic pollutants and for sterilization. 1,2Photocatalytic paper as a composite material offers advantages over conventional nanoparticle-based photocatalyst systems that require an extra separation process to remove the contaminants in an aqueous solution. 3,4Fixed on the paper substrate, the nanoparticles are much less likely entering the environment and the human body to cause health-related problems. 5The photocatalytic paper has obtained a considerable development since its conception was proposed more than twenty years ago. 6There is no doubt that photocatalytic paper will find many important applications in photocatalytic field.
Various forms of photocatalysts have been developed in recent years. 7However, the photocatalysts used for photocatalytic paper are limited to TiO 2 or ZnO nanoparticles.Matsubara et al. first reported the conception of TiO 2 photocatalytic paper in 1995, which was used to degrade acetaldehyde in the vapor phase under the illumination of a weak fluorescent light. 8Ichiura et al. published a series of works about the photocatalytic paper, which solved the strength problems of paper met in photocatalytic degradation of organic pollutants in liquid by compositing zeolite and titanium dioxide. 9,10Ghule et al. reported the growth of ZnO nanoparticulate coating on paper support and studied its antibacterial property. 11aruah et al. embedded zinc oxide nanorods in porous matrix sheet structure of paper and demonstrated the paper's photocatalytic performance and sterilization effect. 12Martinez et al. discussed a color-reaction of Au loaded on cellulose fibers as a biosensor, and the color change of dye could be observed by naked eye, widening the application of photocatalytic paper in biological testing. 13][16][17] Because both TiO 2 and ZnO are wide band gap semiconductors, they cannot absorb photons for stimulating the separation of electrons and holes, and thus cannot form reactive oxygen species under visible and NIR light irradiations. 6It is therefore meaningful to fabricate photocatalytic paper that can work at visible and NIR light regions because the visible and NIR light accounts for more than 90% of the whole solar light energy. 7ilver oxide was first known as an excellent candidate for UV and visible light driven photocatalyst as a narrow band gap semiconductor. 18Wang et al. reported that methyl orange (MO) could be degraded completely with the help of Ag 2 O nanoparticles under visible light irradiation in 160 min. 19A composite photocatalyst, Ag@Ag 2 O nanoparticles, were fabricated slightly later and found to exhibit even higher efficiency in decomposition of MO under visible light irradiation. 20Later, Jiang et al. investigated the UV, visible and NIR photocatalytic activity of Ag 2 O nanoparticle prepared with a facile precipitation method, and found its good photocatalytic performance in photodegradation of MO and excellent stability under visible and near-infrared lights. 21 In this study, cellulose paper containing Ag 2 O nanoparticle loaded graphite fibers was prepared by applying typical paper making techniques.The Ag 2 O nanoparticles functioned as a full spectrum photocatalyst and an antibacterial agent to provide photocatalytic and antibacterial activities, 22,23 while the graphite fibers functioned as a conductor to facilitate the separation of photo-generated electron holes and reduce the photodegradation of cellulose fiber as a catalyst carrier. 24he Ag 2 O nanoparticle loaded graphite fiber could also act as an interweaving material to form paper with cellulose fibers.The natural cellulose fibers formed a fiber network by hydrogen bonding of the hydroxyl groups of interwoven fibers.To the best of our knowledge, this is the first time to report a cellulose based Ag 2 O photocatalytic paper of full spectrum photocatalytic and antibacterial activities.

Synthesis of Ag 2 O nanoparticles
Ag 2 O nanoparticles were synthesized according to the method reported by Jiang et al. 21AgNO 3 solution (0.2M) was added dropwise to a solution of 0.3M NaOH and 0.1M NH 3 •H 2 O, with mixing provided by a peristaltic pump.The resulting mixture was mixed for additional 30 min and then aged at 50 ℃ for 2 h.The suspension was then filtered and washed with deionized water to remove the unreacted chemicals.The obtained solid was dried in a vacuum oven at 50 ℃ for 12 h.It should be noted that all the above operations were carried out in darkness.

Preparation of Ag 2 O-loaded graphite fibers and photocatalytic paper
To prepare Ag 2 O-loaded graphite fibers, i.e., Ag 2 O@graphite fibers, 0.2 g graphite fibers were mixed with a solution of 0.3M NaOH and 0.1 M NH 3 •H 2 O, and then AgNO 3 solution was added dropwise.And then Al 2 (SO 4 ) 3 was as the fixing agent and mixing was continued for 30 min.The mixture was aged for 3 h at 50 ℃ and filtered.The filter cake was washed 3 times with deionized water, and dried in a vacuum oven at 50 ℃ for 12 h.The mass ratio of Ag 2 O to graphite fiber was about 1:4.
The prepared Ag 2 O@graphite fibers were mixed with aspen BCTMP fibers at a mass ratio of 20% based on cellulose fibers to form a paper stock, and 0.5wt% PAE was added based on the cellulose fibers.0.05 grams of the above mixture was dispersed with 500 ml of deionized water under magnetic stirring, and filtered on a sub-microporous filter of 4 cm diameter to form a wet paper sheet which was dried at 50 ℃ .The dried paper sheet was used for subsequent photocatalytic and antibacterial experiments.
The paper sheet was characterized with a Bruke D8 Advance powder X-ray diffractometer (XRD) with Cu Kαradiation (λ= 0.15406 nm) to record their XRD patterns.Quanta 200 field-emission scanning electron microscope (SEM) was used to observe the morphology of the paper sample.

Degradation experiment of MO
3][4] In a typical process, 20 ml of 0.02 g/L MO aqueous solution were added to a 50 ml beaker, and a square piece of the photocatalytic paper with a side length of 20 mm was dipped in the MO solution.And then the sample in the beaker was irradiated under either UV, visible or NIR light.A 500 W mercury lamp with a nominal wavelength of 365 nm was used as the UV source, and a 350 W Xe arc lamp with an UV filter to cut-off the light of wave-length below 380 nm was used as the visible source. 4In addition, a 250 W infrared lamp with a cut-off filter (λ<720 nm) was employed as the near infrared source. 25At designated irradiation time intervals, samples were taken from the solution for testing.The concentration of MO was determined by UV-vis spectroscopy (Hitachi UV-300) at 465 nm.

Antibacterial activity of photocatalytic paper
The microbicidal properties of the photocatalytic paper with a diameter of 7 mm were monitored by using a strain of bacterium Escherichia coli (E.coli).The broth (15 g/L Tryptone, 15 g/L Ager, 1.5 g/L NaCl) was disinfected at 120 ℃ for 30 min in an autoclave.E. coli bacteria were inoculated to the broth, which was incubated for 24 h at 37 ℃ in a rotary shaker as the germiculture.To evaluate the antibacterial activity, both the conventional cellulose paper and photocatalytic paper samples were carefully placed at the center of the germiculture. 26,27The level of antibacterial activity was assessed as the area of the inhibition zone, i.e. the area around the paper samples absent of E. coli cells. 12,28,29

Characterization
Three papers, i.e., blank paper composed of sole cellulose fibers, graphite fiber containing paper and Ag 2 O@graphite fiber containing paper, were prepared and characterized by x-ray diffraction (XRD) technique and scanning electronic microscopy (SEM) for comparison.Fig. 1 shows the XRD patterns of the blank paper, Ag 2 O@graphite fiber-containing photocatalytic paper, graphite fiber-containing paper and Ag 2 O nanoparticles.In the XRD patterns of the blank paper, the diffraction peak at 2θ = 22.5° can be indexed as the (020) reflection of α-cellulose, i.e. cellulose fibers.This peak also appeared in the XRD of the photcatalytic paper, indicating the presence of BCTMP fibers in the sample.The XRD pattern of the as-synthesized Ag 2 O nanoparticles exhibits sharp and symmetric peaks at 2θ = 33.02°,38.25°, 55.22° and 65.87°, which are assigned to (111), ( 200), (220), and (311) planes of cubic Ag 2 O, respectively. 21,30The XRD pattern of graphite fibers shows a special diffraction peak at 2θ = 26.06°,demonstrating its graphite-like crystalline structure. 31,32In the XRD pattern of the Ag 2 O@graphite fiber-containing photocatalytic paper, typical diffraction peaks that belong to cellulose fibers, graphite fibers and Ag 2 O occur, indicating that Ag 2 O and graphite fibers were embedded in the networks of cellulose fibers successfully.
Fig. 2 shows the SEM images of the blank paper, graphite fibers, Ag 2 O nanoparticles, Ag 2 O@graphite fiber-containing photocatalytic paper.As shown in Figs.2a1 and a2, the cellulose fibers were connected together to form a porous structure.The cellulose fibers were slightly curved and fluffed on their surfaces.The SEM images of graphite fibers, as shown Figs.2b1 and  2b2, reveal that graphite fibers were straight and smooth characterized as a cylinder structure.The Ag 2 O nanoparticles shown in Fig. 2c1 and 2c2 indicate that they occurred as aggregates of micro-scale.From the SEM images shown in Figs.2d1 and 2d2 one can find that the graphite fibers inlayed the network of cellulose fibers.The cellulose fibers formed a dense network due to their flexibility and hydrogen bonding.Most of fine Ag 2 O nanoparticles were well distributed on the graphite fibers as small aggregates, while a small amount of Ag 2 O nanoparticles gathered on the surface of cellulosic fibers.

Photocatalytic activity
The photocatalytic activity of the prepared Ag 2 O@graphite fiber-containing photocatalytic paper and Ag 2 O nanoparticles were evaluated by photodegradation of MO under UV, visible and near-infrared light irradiations.The results are shown in Fig. 3.The degradation rate of MO was calculated by (1−C/C 0 ) × 100%, where C is the concentration of MO at the irradiation time t and C 0 is the original concentration of MO.To evaluate the stability of the as-prepared photocatalytic paper in the photodegradation of MO, the as-prepared photocatalytic paper was used repeatedly, and each cycle lasted 3 h.The results are shown in Fig. 4. The degradation rates of MO were reduced to 20.7% for UV light photocatalysis, 5.4% for visible and 6.9% for NIR light photocatalysis after four cycles.The reduction in efficiency was probably associated with the washing and drying treatment before each photocatalytic cycle, in which part of the Ag 2 O nanoparticles loaded on the graphite fibers were lost.Another possibility is that when used repeatedly, organic substances might accumulate in the micro pores of the paper to interfere with the catalytic activity of the Ag 2 O nanoparticles. 33

Antibacterial effect
5][36] Herein the antibacterial activity of the as-prepared Ag 2 O@graphite fiber-containing photocatalytic paper against E. coli was also investigated.For comparison, the antibacterial activities of the blank paper, graphite fiber-containing paper and Ag 2 O@graphite fiber-containing photocatalytic paper were tested in the dark and office lighting conditions, respectively.The results are shown in Fig. 5.As shown in Fig. 5, evident inhibition zones were formed around the Ag 2 O@graphite fiber-containing photocatalytic paper, i.e., no bacterial colonies were found around the photocatalytic paper both in the dark and in the office lighting conditions, indicating the strong bactericidal effect of the photocatalytic paper.In contrast, no inhabitation zone was found either around the blank paper or around the graphite fiber-containing paper, implying that the antibacterial activity of the photocatalytic paper was due to the loading of the Ag 2 O nanoparticles.

Radical-trapping
To demonstrate the reactive oxygen species (ROS) generated in the presence of the Ag 2 O@graphite fiber-containing photocatalytic paper for photocatalyzing the degradation of MO and inhibiting the growth of E. coli with light irradiation, radical-trapping experiments were conducted, in which ammonium oxalate (AO), benzoquinone (BQ) and tert-butyl alcohol (TBA) were used as the scavengers of electron holes (h + ), superoxide anions (•O 2 − ) and hydroxyl radicals (•OH), respectively. 37− and •OH radicals have redox properties to decompose bacterium cell wall, 38 resulting in a better antibacterial capability of the Ag 2 O nanoparticles in the paper under illumination than in the dark.The most important ROS under UV light irradiation was •OH, while the least important one was h + .However, under visible and NIR light irradiations, h + became the most important ROS.

CONCLUSIONS
Cellulose-based photocatalytic paper active in UV, visible and NIR regions was successfully fabricated by loading Ag 2 O nanoparticles onto graphite fibers and incorporating the Ag 2 O loaded graphite fibers into cellulose paper in the papermaking process.The loading of the Ag2O nanoparticles onto the graphite fibers was confirmed by XRD diffraction and SEM observation.The photocatalytic activities of the paper were evaluated with the degradation of methyl orange (MO) under ultraviolet, visible and near-infrared light irradiation.It was found that the degradation rate of methyl orange reached 32%, 19% and 19% under UV, visible and NIR light, respectively after 3 h of irradiation.The photocatalytic paper could be reused several times with some loss in the photocatalytic efficiency in each cycle, due to loss of the Ag 2 O nanoparticles in the washing process of each cycle.The synthesized photocatalytic paper also showed strong antibacterial properties towards E. coli in both dark and office lighting conditions.

Fig. 1 .
Fig. 1.XRD patterns of Ag 2 O@graphite fiber-containing photocatalytic paper, blank paper, graphite fibers and Ag 2 O nanoparticles.For the photocatalytic paper, the molar ratio of Ag 2 O to graphite fibers is 1:4; The mass ratio of Ag 2 O@graphite fibers to cellulose fiber is 1:5 (the same blow in the other figures).

Fig. 3 .
Fig. 3. Photocatalytic activities of Ag 2 O nanoparticles and Ag 2 O@graphite fiber-containing photocatalytic paper.The added amount of Ag 2 O nanoparticles in photocatalytic test is 20 mg.The amount of Ag 2 O in the paper sample is 2.19 mg.Obviously, Ag 2 O nanoparticles show high full spectrum photocatalytic activity at the extensively used charge level.The MO was completely degraded within 90 min under UV light irradiation, and around 90% of MO was degraded under visible light irradiation in 180 min at the catalysis of as-prepared Ag 2 O nanoparticles.More importantly, around 80% of MO was degraded within 180 min under NIR light irradiation.These results demonstrate the success in synthesis of Ag 2 O-based full spectrum photocatalyst.After the Ag 2 O nanoparticles is incorporated into paper with the graphite fibers, it is possible that the Ag 2 O nanoparticles confer a full spectrum photocatalytic activity as well as an antibacterial property to the as-prepared photocatalytic paper.The photocatalytic activity of the Ag 2 O@graphite fiber-containing photocatalytic paper confirms the above conjecture.After 3 h of UV light irradiation, the degradation rate of MO catalyzed by the Ag 2 O@graphite fiber-containing photocatalytic paper was nearly 32%.The corresponding MO degradation rate under 3 h of NIR light irradiation is 19%.The low photocatalytic activity of Ag 2 O@graphite fiber-containing photocatalytic paper was due to its low Ag 2 O content in photocatalytic tests.The mass of Ag 2 O in the paper was nominally 2.19 mg that was far less than the 20 mg for the pure Ag 2 O catalyst.In fact, the loading of the Ag 2 O nanoparticles onto the graphite

Fig. 4 .
Fig. 4. Repeated photodegradation experiments of Ag 2 O@ graphite fiber-containing photocatalytic paper under the irradiation of (a) UV light, (b) visible light and (c) NIR light.

Fig. 5 .
Fig. 5. Results of antibacterial experiments carried out using Ag 2 O@graphite fiber-containing photocatalytic paper, graphite fiber-containing paper and blank paper after incubation for 24h (a) in the dark and (b) in ambient lighting conditions.

Fig. 6 .
Fig. 6.Photocatalytic activity of Ag 2 O nanoparticles under (a) UV, (b) visible and (c) NIR light irradiation using AO, BQ, and TBA as scavengers of h + , •O 2 -and •OH, respectively.The MO degradation curves in Fig. 6 show that the three scavengers have more or less inhibitory effect for the photocatalytic activities under UV, visible and NIR light irradiation, indicating the occurrence of three ROS produced by Ag 2 O nanoparticles in the photocatalytic process.These results confirmed the hypothesis that a combined function of photogenerated h + , •O 2 − and •OH radicals occurred in the oxidative degradation process of MO under full-spectrum light irradiation.Both •O 2 − and •OH radicals have redox properties to decompose bacterium cell wall, 38 resulting in a better antibacterial capability of