How can I increase bicep *width*?

I am happy with my bicep peak, but I am having trouble increasing the width of my biceps. My arms look rather big from the side but when you look at them head on they are quite narrow/skinny. I have heard that hammer curls improve width, but I haven't seen much of an improvement in the last couple of months. 

Currently, I work arms once a week (as I do every muscle group) on Tuesdays. My routine usually consists of 3 bicep workouts followed by 3 tricep workouts. I don't have a set routine because I like to switch it up and hit different workouts each week. I will give a sample workout below, again, this is different each week. 

    Biceps:
      Warmup: Alternate dbell curls, 8 reps w/ light weight
        Alternate dbell curls,       4 x 8 (to failure)
        Alternate dbell hammercurls, 4 x 8 (to failure)
        Preacher curls,              4 x 8 (to failure)
    Triceps:
      Warmup: 15 bench dips
        Rope Pulldowns,   4 x 10 (to failure)
        Skull Crushers,   4 x 8  (to failure)
        Tricep Kickbacks, 4 x 10 (to failure)

Note: When I am doing any excercise I make sure my form is always as good as possible and my reps are generally slow and controlled with an emphasis on negative resistance (i.e. letting the weight down slowly. 

All sets are to failure because I am trying to build mass.


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**Edit:** Elaborating on the improvement I have seen in arm growth in general - About 3 months ago my arms measured around 15.5" and have increased to 16+ (They vary, sometimes up to 16.5 without a pump). My arms have been getting bigger, triceps and bicep peak are increasing, but bicep width remains stagnant.