How do I add an extra column to a NumPy array?

Given the following 2D array:

a = np.array([
    [1, 2, 3],
    [2, 3, 4],
])

I want to add a column of zeros along the second axis to get:

b = np.array([
    [1, 2, 3, 0],
    [2, 3, 4, 0],
])